IGCP Project No. 381

South Atlantic Mesozoic Correlations

SAMC NEWS No. 2 - July 1995

Dear Colleague,

In this second issue of SAMC News you will find the agenda for the inaugural project meeting in Uberaba (24-25 July 1995) and extended abstracts for the oral presentations, together with some additional contributions; announcements for a working group and a workshop in New Zealand; calls for papers for two thematic volumes; information about the First Annual SAMC Conference to be held in Aracaju, Brazil (2-5 September 1996) and details about the proposed pre- and post-conference field trips (see p. 25-26).

We are pleased to welcome a number of new participants to the project (see enclosed list). On behalf of IGCP (UNESCO/IUGS) we extend our good wishes to you as you begin what we trust will be a long and rewarding association. We hope you will take advantage of your affiliation in every way - participating actively in project meetings whenever you can; keeping informed about research activities; and calling upon us or any coordinator to assist you at any time.
Between meetings, the project newsletter, SAMC News, is your best source of news and information about SAMC and its participants. It will be distributed to all project participants and is also available online via Internet at URL http://www.nhm.ac.uk/paleonet/Newsletters/IGCP.Html

IGCP Project 381

South Atlantic Mesozoic Correlations (SAMC)

Key Objectives :

To promote interdisciplinary geological correlative studies in order to establish a well-defined standard stratigraphical scale for the Mesozoic sedimentary basins of the South Atlantic, and to further improve our understanding of the nature and sequence of major geological events during the formation of the South Atlantic and the global impact of these events, thus also contributing significantly to the exploration of hydrocarbon and mineral resources in the marginal basins.

Project proposers:

Eduardo A. M. Koutsoukos (PETROBRAS-Cenpes/Divex/Sebipe, Rio de Janeiro, Brazil)
Peter Bengtson (Geologisch-Palaeontologisches Institut der Universitaet Heidelberg, Germany)

"IGCP is interdisciplinary, covering all specialities of geology, geophysics and geochemistry. IGCP maintains active interfaces with disciplines related to the geological sciences such as marine sciences, atmospheric sciences and biological sciences."

The project newsletter will be issued regularly (every four to six months) as a forum for exchanging information and promoting the integration and collaboration among project participants. Progress reports and research results by individuals and working groups, state-of-the-art reports, information on new collaborative programmes, discussions of scientific novelties within the scope of the project and announcements of forthcoming meetings will be among the topics covered by the newsletter. Please distribute this information and copies of the enclosed registration forms to your colleagues. If you are receiving this newsletter and have not yet registered, you can do so by filling in the enclosed registration form.

We invite all participants of SAMC to send us, from time to time, announcements, descriptions and short reports of current research work, information on projects being planned, underway, or recently completed, which relate to the scope of SAMC, literature reviews, etc. to be published in the next issues of the newsletter. Comments and discussions of problems within the areas of interest will be especially welcome. For all written communications please submit a printout of the text and a diskette containing the text file in Microsoft Word, WordPerfect or Nisus format (Macintosh or IBM/Windows). Alternatively, texts may be submitted by e-mail to: koutsoukos@cenpes.petrobras.gov.br Please do not send text for the newsletter by fax.

The IGCP Logotype and Project Publications

We wish to emphasize that all publications deriving from project research carried out by participants should be identified with the "IGCP/IUGS/UNESCO-hammer" logotype and the name and/or number of the project, with further reference in the Acknowledgments stating that "This paper is a contribution to IGCP Project 381, South Atlantic Mesozoic Correlations". This will greatly facilitate for official bodies to judge the output and success of the project.

References of papers and abstracts in preparation, in press, or recently published will be compiled and listed in each issue of SAMC News. Please report them for inclusion in our newsletters and annual reports, by sending to the SAMC Secretariat a list of your papers in press and four offprints of published papers relating to the project (of which two will be forwarded to the IUGS/IGCP Secretariat).
Please note that we are reminded by the IGCP Scientific Board to report only those publications that acknowledge the project.

Funding

Financial support for the scientific activities of the project should be sought by participants from governmental and other funding agencies, in particular where collaborative research has been set up. Please note that funding of research activities within the project is the responsability of individual participants or groups, not of IGCP/Unesco, the project leaders or coordinators.

Limited IGCP support is provided particularly to facilitate participation in meetings, workshops and field excursions by participants from developing countries who contribute actively to the project.

CALL FOR PAPERS

As a contribution to the aims of SAMC we are planning thematic volumes with collections of papers addressing specific issues relevant to the project. It is intended that these thematic sets be published as special issues of internationally disseminated journals.

We propose the following volumes, to be published by the end of 1997:

If you are interested in submitting a paper, you are invited to send an abstract in English with a maximum of 2000 words on four letter-sized (A4) pages. Illustrations such as maps, charts and tables, may be included for clarity. Papers will be selected for the thematic volumes by the editorial committee after analysis of the abstracts.

Preliminary deadline for submission of abstracts: 31 December 1995
Send your abstract, by letter, fax or electronic mail, to the SAMC Secretariat.

SAMC Secretariat

Please send your registration and enquiries about IGCP Project 381 (SAMC) to either one of the following secretaries:

(for English-speaking participants) Marcio R. Mello PETROBRAS-Cenpes/Divex/Segeq Cidade Universitaria, Quadra 7 - Ilha do Fundao BR-21949-900 Rio de Janeiro, RJ - Brazil Tel.: +55-21-5986460, Fax: 5986799 E-mail: bv77@c53000.petrobras.anrj.br

(for French-speaking participants) Mitsuru Arai PETROBRAS-Cenpes/Divex/Sebipe Cidade Universitaria, Quadra 7 - Ilha do Fundao BR-21949-900 Rio de Janeiro, RJ - Brazil Tel.: +55-21-5986440, Fax: 5986795 E-mail: bvb5@c53000.petrobras.anrj.br

AGENDA

for the inaugural SAMC meeting at Uberaba, Minas Gerais, Brazil (24-25 July 1995), in conjunction with the 14th Brazilian Palaeontological Congress (23-29 July 1995)

Objectives of the meeting:

Introduction of the project to the Latin American scientific community promoting integration and collaboration among participants. Presentation of status reports and research results. Proposal for and organization of working groups. Identification of key areas and research objectives. Schedule for project meetings (workshops and general meetings).

DAY 1 (24 July) Selected talks by project participants on relevant scientific topics for future interdisciplinary collaboration, main research in progress (as a means of a 'getting-to-know' what everyone is working on and his/her fields of interest), and state-of-the-art reports.

Each speaker (underlined if more than one) will be given 15 to 25 minutes, with 5 minutes at the end for questions.

8:30-9:00: Welcoming Remarks. Presentation of the objectives and proposal of the chronogram for the meeting.

9:00-11:30

1. Eduardo Olivero (CADIC, Ushuaia, ARGENTINA): 'Molluscan Cretaceous biostratigraphy of the James Ross Basin (Antarctica) and the Austral Basin of Tierra del Fuego (Argentina)'
2. Eduardo A. Musacchio (U.N.P.S.J.B., Comodoro Rivadavia, ARGENTINA): 'Correspondences of Jurassic and Cretaceous stratigraphic events from central Patagonia with the Neuquean model'
3. Peter Szatmari, Marta C. M. Guerra & Mônica A. Pequeno (PETROBRAS-CENPES, Rio de Janeiro, BRAZIL): 'Origin of Cretaceous rift basin in Brazil: an overview'
4. Paulo S. Milhomem (PETROBRAS-CENPES, Rio de Janeiro, BRAZIL): 'Ostracod biostratigraphy of Brazilian Lower Cretaceous rift systems: state of the art and perspectives'
5. Dermeval A. do Carmo (UFRGS, Porto Alegre, BRAZIL): 'Taxonomy of the ostracodes from the Alagamar Formation (Lower Cretaceous), Potiguar basin. Biostratigraphic and palaeoecologic correlation with basins of the Brazilian continental margin'

11:30-12:00: open forum for discussion and questions.

12:00-13:30: Lunch.

13:30-16:30

6. Peter Bengtson (Heidelberg Universitaet, GERMANY): 'Ammonite and inoceramid biostratigraphy of northeastern Brazil: a status report'
7. Eduardo A.M Koutsoukos (PETROBRAS-CENPES, Rio de Janeiro, BRAZIL): 'Upper Cretaceous to Danian foraminiferal biostratigraphy of northeastern Brazil: a progresss report'
8. René Rodrigues (PETROBRAS-CENPES, Rio de Janeiro, BRAZIL): 'Chemical stratigraphy of Cretaceous sequences in Brazil: state of the art and perspectives'
9. Dimas Dias Brito (UNESP, Rio Claro, SP, BRAZIL): 'The northern part of the South Atlantic ocean was an arm of the Tethys early in its opening history'
10. Ismar Carvalho (UFRJ, Rio de Janeiro, BRAZIL): 'Working Group Proposal for IGCP 381: Cretaceous Continental Ecosystems'
11. Nick Cameron (Geo Insight, Chesham, UK): 'The West African framework'

16:30-17:30: open forum for discussion and general remarks.

DAY 2 (25 July)

Discussion of topics for the action groups. If necessary, we may extend the meeting a further day.

1. Summary of main objectives.

2. Brainstorming among participants according to area of work.

Suggested topics of discussion for the action groups:

We propose the following Working Groups (WG) to be erected initially:

Dr. Ismar S. Carvalho (Dept. de Geologia, UFRJ) will present a proposal for a WG on Cretaceous Continental Ecosystems (see the announcement on page 23).

INAUGURAL MEETING OF IGCP PROJECT 381 (SOUTH ATLANTIC MESOZOIC CORRELATIONS) Uberaba, Brazil, 24-25 July 1995

EXTENDED ABSTRACTS

Molluscan Cretaceous biostratigraphy of the James Ross Basin (Antarctica) and the Austral Basin of Tierra del Fuego (Argentina)

Eduardo B. Olivero - CADIC, Av. Malvinas Argentinas s/n, 9410 Ushuaia, Tierra del Fuego, Argentina.

The Austral basin of southern South America and the James Ross basin of the northern Antarctic Peninsula preserve a similar Upper Jurassic-Cenozoic marine sedimentary pile of about 6-7 km in thickness. The paleontological and geofisical evidence suggests that at least during the Late Cretaceous-Paleogene these basins were situated on the eastern side of a continuous land mass that was disrupted in the Cenozoic with the dramatic tectonic events that lead to the final separation of South America and the Antarctic Peninsula. The present geological evidence indicates, however, that the Cretaceous evolution of these basins differs in a number of facts. In Patagonia and part of Tierra del Fuego, most of the deposits of the Austral basin were deposited in platform settings, with a source area in the Deseado Massiff to the north and east of the basin. During the Early Cretaceous the southern and western sides, however, were characterized by a back-arc marginal basin, partly floored by ophiolites and filled with volcaniclastic, deepmarine flysh deposits derived from a Pacific facing volcanic arc. The geometry of the basin changed markedly in the Late Cretaceous with a compressional event that produced the tectonic invertion of the marginal basin and the formation of a retroarc foreland basin in front of the rising cordillera. The closure of the marginal basin is reflected in the strong deformation of the early Cretaceous rocks, characterized by an Alpine tectonic style of deformation. In Antarctica, the James Ross basin represent deposition to the rear of a magmatic arc located along the present Antarctic Peninsula. The Cretaceous part of this back-arc basin is characterized by deep-marine and slope Aptian to Turonian-Coniacian deposits followed by platform deposits in the Late Cretaceous and Paleogene. The geological structure of the area is relatively simple and it is characterized for the most part by gently tilted, homoclinal strata and high angle faults. In the past decade,intensive stratigraphic research by American, Argentinian, and British institutions has resulted in the recognition that James Ross basin preserves a superb, richly fossiliferous marine succession that it is almost continuous from the Aptian to the Danian. Recent studies on the molluscan biostratigraphy of the Aptian-Coniacian by Francisco A. Medina (CIRGEO, CONICET) and Late Cretaceous by the author, resulted in the recognition of the following, age significant molluscan faunas.

APTIAN: Australiceras Assemblage Zone and Peltocrioceras Assemblage Zone. ALBIAN: Ptychoceras Assemblage Zone; Anopea Assemblage Zone; and Tetragonites Faunule. CENOMANIAN: Mariella Assemblage Zone; Hypoturrilites Faunule; Calycoceras Assemblage Zone; and Scaphites Faunule. TURONIAN: Hyphantoceras Faunule. CONIACIAN-SANTONIAN: Madagascarites Faunule. SANTONIAN: Baculites aff. kirki Assemblage. CAMPANIAN: Natalites rossensis Assemblage; Natalites spp. Assemblage; Grossouvrites occultus-Maorites sp. Assemblage; Neograhamites morenoi-N. taylori Assemblage; Karapadites aff. centinelaensis Assemblage; and Neograhamites sp.- Mesopuzosia sp. Assemblage. CAMPANIAN- MAASTRICHTIAN: Neograhamites kiliani Assemblage; Gunnarites-Neograhamites Assemblage; Gunnarites-Jacobites Assemblage. MAASTRICHTIAN: Maorites aff. tuberculatus Assemblage; Pachydiscus ootacodensis Assemblage; Pachydiscus riccardi Assemblage; and Pachydiscus ultimus Assemblage.
Most of the elements of these assemblages are also present in the platform deposits of the Austral basin of Santa Cruz (Argentina) and Ultima Esperanza (Chile) allowing for the stratigraphic correlation of the Aptian-Maastrichtian strata of Patagonia and the Antarctic Peninsula. On the contrary, the paleontology of the deep marine deposits of the southern marginal basin in Patagonia and Tierra del Fuego is inadequately known and the timing of the opening and closing of the marginal basin and the initiation of the foreland basin stage is poorly constrained because of the scarcity of fossil evidence. So far, the marine deposits of the marginal basin were assigned to the Late Jurassic-Early Cretaceous on the basis of the record of Belemnopsis patagoniensis (Favre) and B. madagascariensis (Besairie) and Favrella sp. in Navarino and Hoste islands (southern Chile). In the northern part of Chilean and Argentinian Tierra del Fuego, the following Aptian-Albian and Late Cretaceous faunas were recorded. Aucellina andina Feruglio; A. radiatostriata Bonarelli; Sphaera? striata (Richter); Inoceramus sp., and Parahibolithes fuegensis Stolley were recorded in the Aptian-Albian platform deposits. Maorites spp., Gunnarites spp., and Baculites duharti nicken were recorded in the Campanian-Maastrichtian foreland deposits.
Recent studies by the author in the Beagle Channel area of Tierra del Fuego demonstrates, however, that marginal basin deposits are as young as late Albian on the basis of the record of Birostrina concentrica (Parkinson) and Inoceramus carsoni McCoy. In the same area, the subsequent foreland basin deposits bear two distinctive inoceramidae horizons that by comparison with the Antarctic fauna can be assigned to the Turonian-Coniacian and Santonian-Campanian, respectively.

Correspondences of Jurassic and Cretaceous stratigraphic events from central Patagonia with the Neuquean model

Eduardo A. Musacchio - Laboratorio de BioestratigrafÁa de la U.N.P.S.J.B., Burmeister 3834, 9000 Comodoro Rivadavia, Argentina.

The Jurassic and Cretaceous marine deposits of the Neuquén basin - at the west margin of Gondwana - include an excellent fossil record. The well known biostratigraphy of this basin is mainly based on ammonoids and other planktonic fossil groups. The intercalated continental deposits can be gauged, at their limiting-tops, with relative accuracy. In addition, different marker-species of non-marine ostracods, charophytes and palinomorphs, are present in the lacustrine units, and provide valuable chronologic information. In the last years, several of these non-marine markers are being found in various Patagonian extra-Andean sedimentary environments. Many cases of shared species were documented in different stratigraphic units belonging to the intracontinental San Jorge Gulf basin. Some of these units were formerly considered poor, or lacking of fossil guide invertebrates. They are now better understood in terms of geologic history and correlations.

The Neuquén and the San Jorge Gulf basins, in spite of their respective individuality, occasionally share a limited set of comparable tectonic conditions during their respective Jurassic and Cretaceous histories. Similar lithogenetic events, both in time as space, are not frequent. However, they may take place when the tectonic or the physiographic frameworks, surpass the geologic-provincial boundaries. Such is the case of Early Jurassic volcanic rocks. An approximately NNW-SSE belt, including different types of volcanic rocks related to Liassic rifting, embraces the north of Santa Cruz and the north central part of the Chubut Province. A corresponding Liassic volcanism is well exposed in the southern part of the Neuquén basin. On turn, both basins share a common late Pliensbachian-early Toarcian Pacific transgression.
During the Aptian, a postectonic lacustrine sequence, represented by the Pozo D-129 Formation in the Chubut Province, includes a Flabellochara harrisi assemblage. Both, units and microfossils, can be correlated with lacustrine to brackish deposits of the Rayoso Group in the Neuquén basin.
Finally, the set of Maastrichtian-Danian Atlantic transgressions, belonging to the Colorado Embayment, can be recognized both in sub-Andean places (Loncoche and Jagüel/Roca Formations in the west-central part of Argentina) and the centre of Chubut (Lefipán Formation).

Micropaleontology based on non-marine calcareous microfossils:

Eight biostratigraphic units were previously proposed. They were based on approximately 70 species of ostracods and 40 species of charophytes known to the author. At present, a number of new taxa and units can be added. All the microfossils come from outcrop samples. The early biostratigraphic units are listed below (references: Musacchio, 1993, "Use of the global time scale in correlating non marine Cretaceous rocks in southern South America"; Cretaceous Research 14: p. 113-126, and references therein).

JURASSIC. In the Chubut River medium valley, two Jurassic Darwinula assemblages, include carapaces of lacustrine ostracods. They can be well compared with markers from Chinese faunas, respectively of late J1? to J2 and late J2 to J3 age. In Chubut, they are present in sequences having features, respectively, of sin-rift and post-rift style.

LOWER CRETACEOUS. The Dorsoacuminata Zone, with Cypridea (Ulwellia) spp. and Mesochara stipitata, lies interbedded in middle Hauterivian marine strata from Neuquén. On turn, the Atopochara triquetra Zone, including species of the ostracods Cypridea (Ulwellia) , Clinocypris, Wolburgiopsis and Teriosynoecum (among others), rests over marine strata of middle Hauterivian age. The levels bearing the later non-marine ostracods are considered facies of a marine unit. The later (Agrio Formation) is formed by biorudites and calcareous mudstones bearing ammonoids of late Hauterivian-early Barremian age, in the corresponding section.

"MIDDLE CRETACEOUS." The Aptian harrisi Zone is present in a main stratigraphic unit belonging to the San Jorge Gulf basin. This unit rests unconformably over different Neocomian (in part of rift-style) sequences. Some markers of this zone are widely distributed in America. They can be recognized in different lacustrine patagonian deposits, as in the Rayoso Group in Neuquén and the "Alagoas stage" in Santos and Potiguar basins of Brasil (Musacchio & Viviers, 1994). In Central Chubut, besides the charophytes Flabellochara harrisi (Peck) and Porochara mundula (Peck), the ostracods Cypridea (C.) diminuta Vanderpool, 1926, Cypridea (C.) amerikana Musacchio, 1975 and Cypridea (C.) craigi Musacchio are also present. The later assemblage shows micropaleontologic affinities with several similar cases of Aptian age in the Rocky Mountain domain and Gulf Region in North America, previously described by Peck and other authors. This fact suggests the existence of species interchange between South and North America at this time, as well as the presence of a common tecto-sedimentary activity in the west margin of the main continent. The resultant geographical design of the Barremian to Aptian tectonism should be responsible for possible land geographic communications.

UPPER CRETACEOUS. At this time, an ample Atlantic embayment related with the Colorado basin, reached the area early settled by the Neuquén basin during the J-K1. The marine deposits, including the K-T boundary, formed part of a sedimentary cycle (Malalhueyano sensu Groeber). This cycle overlies, thick and well represented non-marine deposits. In addition, non-marine deposits are well documented in proximal facies of the "Malalhueyan" Maa-Dan marine sequence. Related with the above mentioned continental deposits, three zones or non marine microfossils were already proposed: First, the triebelli Zone, which characterizes a wide blanket set of lacustrine sediments (Anacleto Formation), plenty of the ostracod Ilyocypris triebelli. They are followed by brackish and marine Maastrichtian deposits (respectively, Allen and Jagüel Formations), in part bearing the decurrens Zone. The ostracods "Ovocytheridea" rionegrensis Musacchio, 1973, and Procytheroptheron sp. Musacchio, 1973, belonging the assemblage of I. triebeli, resemble other African taxa at the generic level. These two species, and charophytes of Gobichara seem of interest for correlations with Campanian deposits of other Atlantic South American basins. Likewise, they seem useful in the palaeogeographic reconstruction task of the non marine and brackish facies related to the patagonian Colorado Embayment. The second one is the grambasti Zone. It was proposed for brackish deposits of the Loncoche Formation. They are, in turn, facies of the Jagüel Formation. The grambasti Zone includes different continental as well as brackish non-marine ostracods and charophytes, interbedded with Maastrichtian foraminifers belonging the decurrens Zone. Finally, the lacustrine deposits bearing the Peckichara varians Zone overlie, continuously, Danian marine levels. The same was previously assigned to the Paleocene. The diversity observed in Late Cretaceous assemblages diminishes in the Paleocene, but a sharp change in species and genera was not observed.

Origin of Cretaceous rift basin in Brazil: an overview

Peter Szatmari, Marta C. M. Guerra & Mônica A. Pequeno - PETROBRAS Research Centre (Cenpes/Divex), Cidade Universitária, Quadra 7, 21949-900 Rio de Janeiro - RJ, BraziL.

The Cretaceous breakup of western Gondwana followed the opening of the Central Atlantic by about 80 Ma. This delay may be attributed to the thick lithosphere along the boundaries of the separating continents, consequent on the long union of South America and Africa as parts of Gondwana during the Paleozoic. Rifting may have started by the formation of conjugate fracture zones between South America and Africa, as proposed by LePichon, creating the nearly orthogonal South Atlantic and Equatorial Atlantic margins. This was followed by the clockwise rotation of South America, away from Africa and towards the free subductional margin in the west. This rotation was probably, at least in part, due to lithospheric thinning and uplift over the Parana hotspot. Physical modelling of rifting by such differential rotation in the laboratory reproduces, and throws light on the origin of, the distinctive structural features of Brazil's Cretaceous rift basins.

During the early phases of rotation South America's southern tail (Argentina and Chile) swung to the NW, pushing back the subductional margin in the south, while the rest of the continent rotated clockwise still in partial contact with Africa. Thus the rifts between South America and Africa propagated nearly simultaneously northwards along the South America's Eastern margin, and westwards along the Equatorial margin. The reactivation of major Precambrian shear zones conditioned the location of the early Cretaceous rift basins. Several basins formed in a transtensional regime, owing to the obliquity of the rotationally defined direction of extension to basin outlines controlled by continental-scale fracturing and modified by the reactivation of ancient zones of weakness. Along the Equatorial margin, Precambrian shear zones were reactivated by N-S compression in the west, N-S extension in the east, reflecting the rotation of the continent. In the easternmost segment of the Equatorial margin, N-S extension opened up the Pendencia Graben in the Potiguar basin during Neocomian time. Subsequently, in Alagoas time, extension propagated westwards, overprinting earlier fault characteristics. Rotation of South America increased the angle between Africa's Equatorial and South Atlantic margins, forcing open the Benue trough that later, during Aptian-Albian time, developed into a left lateral transtensional basin.
Left lateral strike slip without extension predominated between Natal and Recife (Niger Delta-Cameroon) where pre-Albian sediments are absent. Extension was nearly orthogonal along the Eastern margin between Salvador and Campos. To the south, this N-S segment is connected to the Santos-Pelotas basins by the Rio de Janeiro transfer zone. To the north, the Salavador-Campos segment is connected with the strike slip segment north of Recife through a complex system of transtensional basins including those of Sergipe-Alagoas (Gabon), where extension reached its maximum during Alagoas time. These transtensional basins mark the Atlantic margin of the NE-Brazilian microplate, which is bordered to the north by the E-W trending Proterozoic Pernambuco shear zone. Rotation of the microplate during Early Cretaceous Neocomian time was intermediate between those of Africa and of South America, causing the N-S trending Tucano Rift and the ENE-trending Jatoba Rift to form, respectively along the western and northern margins of the microplate. As the Pendencia Graben of the Potiguar basin, so the Tucano and Jatoba rifts were also abandoned from Alagoas time as intraplate stresses decreased and extension became concentrated along the future mid-oceanic ridges.

Ostracode biostratigraphy of Brazilian Lower Cretaceous rift systems: state of the art and perspectives

Paulo da Silva Milhomem - PETROBRAS Research Centre (Cenpes/Divex), Cidade Universitária, Quadra 7, 21949-900 Rio de Janeiro - RJ, Brazil.

The use of ostracodes in the biostratigraphic study of sedimentary basins, particularly in support of petroleum exploration activities, is especially important for the zonation of fluvial-deltaic-lacustrine sequences, where other calcareous microfossils with similar biostratigraphic resolution potential do not occur.

In the Brazilian marginal basins, ostracodes have been traditionally used for the biozonation of non-marine deposits. As a result of this, a detailed biochronostratigraphic framework has been developed to give support to exploration efforts focused on the Lower Cretaceous rift systems. These microfossils are also recovered from basins in West Africa, where they occur at similar stratigraphic ranges (Grekoff & Krömmelbein, 1967). Although references to Lower Cretaceous Brazilian ostracodes extend back to the work of Jones (1860), studies relating to the description of the microfauna were more extensively undertaken from the end of the 50's on, as a result of the continuous development of exploration activities carried out by PETROBRAS.
By means of the description of the microfauna and the evaluation of the stratigraphic distribution of diagnostic species, many authors have contributed to the development of a biostratigraphic framework for the Brazilian Lower Cretaceous sequences. The work of Krömmelbein (1962) has been considered a mark to the settling of more intensive biostratigraphic studies based on ostracodes. This author performed the first systematic study of these microfossils aiming at the development of a biostratigraphic scheme for the Reconcavo and Tucano basins. The work of Krömmelbein (op. cit.) was followed by the proposals of Weber (1964), Viana (1967), Moura (1969) and Viana et al. (1971), the latter having settled the formal biostratigraphic framework addopted in these basins, which is composed of 9 biozones and 26 subzones.
Regarding other Brazilian marginal basins, many works have brought up variably detailed biostratigraphic schemes, such as the ones of Troelsen (1966), for the Sergipe-Alagoas basin, Moura (1987), for the Campos basin, and Praça & Hashimoto (1991), for the Potiguar basin. From the works of Silva Telles (1992) and Picarelli et al. (1993), it becomes apparent that the continuity of taxonomic and stratigraphic studies will lead to the refinement of the integrated biostratigraphic framework presented by Brito et al. (1984) for the Upper Jurassic-Lower Cretaceous of Brazil. In the context of such studies, the observed differences on the ostracod assemblages reveal the need of developing formal and diferentiated biostratigraphic schemes, attending to the particular biochronostratigraphic context of each basin. As exemplified for the Potiguar basin by Picarelli et al. (op. cit. ), the correlation among different frameworks may be accomplished by a comparative analysis of the ostracod fauna and, eventually, with the aid of correlative palynological data.
For the near future, biochronostratigraphic studies will focus on Barremian to Aptian sequences, strengthening the necessity for the development of reference sections for the Jiquiá (Barremian-Aptian) and Alagoas (Aptian-Albian) local stages.
Specific research projects for the refining of biochronostratigraphic frameworks refer not only to detailing previous schemes but also to characterize the qualitative and quantitative responses (bioevents) of ostracod assemblages to tectonic and/or climatic depositional controls. These projects are part of multidisciplinary studies comprising sismostratigraphy, sequence stratigraphy, ciclostratigraphy, isotope stratigraphy, geochemistry and sedimentology, which aim to develop predictive geological models in the context of a high resolution stratigraphy.
Regarding predictive geological models, regional biochronocorrelations based on the integration of ostracod and palynological data give support to reconstructing the Lower Cretaceous paleogeography in Brazil. Evaluating the synchronism of sedimentary sequences in different basins would draw a general picture of the earliest history of the opening of the South Atlantic, helpful to understanding the distribution of source and reservoir rocks in the rift systems.
This integration of biostratigraphic data is especially important since the criteria for correlating Brazilian local stages (Viana et al. ,1971) to the International Chronostratigraphic Chart might rely on palynological studies. Although the first Lower Cretaceous biostratigraphic framework was developed on the basis of quantitative palynology (Müller, 1962), the usefulness of ostracodes restricted the efforts towards the later refinement of palynological schemes. The chronostratigraphic positioning of the Brazilian Lower Cretaceous rift systems might be also determined by correlating the ostracode fauna with coeval associations in basins where the non marine sequences are calibrated by intervening, accurately dated marine episodes.
In the Brazilian marginal basins, the development of paleoecological models for the distribution of Lower Cretaceous ostracod assemblages extends back to the studies which accomplished the description of the microfauna, as far as they have referred to the occurrence and variability of diagnostic species. However, only a limited number of works have focused on the subject, such as the one of Tolderer-Farmer (1989).
In the context of the exploration efforts carried out by PetrobrÊs, it is readily apparent the importance of further developing the study of the Lower Cretaceous ostracod fauna as part of integrated stratigraphic projects and with the objective of developing a refined and well established chronostratigraphic framework which can support the proposal of predictive geological models.

References

BRITO, H.S.; VIANA, C.F. & PRAÇA, U.M., 1984. Atlas dos Ostracodes não Marinhos (Neojurássico/Eocretáceo) do Brasil. Petrobrás-CENPES/DIVEX/SEBIPE, Internal Report, 2 vols.
GREKOFF, N. & KRöMMELBEIN, K., 1967. étude comparée des ostracodes Mésozoiques Continentaux des Bassins Atlantiques: Serie de Cocobeach, Gabon et Série de Bahia, Brésil. Rev. Inst. Fr. Pet., Paris, 22 (9), p. 87-103.
JONES, T.R., 1860. Fossil Entomostraca from Montserrate (Brazil): Quart. Jour. Geol. Soc. London, 16, p. 266-268.
KRÖMMELBEIN, K., 1962. Zur Taxonomie und Biochronologie Stratigraphish wichtiger Ostracoden-Arten aus der oberjurassich?-unterkretazischen Bahia-Série (Wealden Fazies) NE-Brasiliens. Senckenbergiana Lethaea, Frankfurt am Main, 43 (6), p. 437-528.
MOURA, J.A., 1969. Revisão da Coluna Bioestratigráfica do Recôncavo-Tucano. Petrobrás-RPBA/DIREX, Internal Report, no. 1250, Salvador.
MOURA, J.A., 1987. Biocronoestratigrafia da seqüência não marinha do Cretáceo Inferior da Bacia de Campos, Brasil: Ostracodes. 10 Congresso Brasileiro de Paleontologia (Rio de Janeiro, 1987), Proceedings, 2, p. 717-731, Rio de Janeiro.
MÜLLER, H., 1962. Summary report on palynological results worked out in the Petrobrás Paleontological Laboratory Salvador, Bahia, since September-1957. Petrobrás-RPBA, Internal Report , 30 pp.
PICARELLI, A.T.; LANA, C.C.; GUZZO, J.V.P. & MILHOMEM, P.S., 1993. Correlação e Refinamento Bioestratigráfico da Se qüência Rifte das Bacias do Recôncavo/Tucano e Potiguar, com base em Palinologia Quantitativa e Ostracodes. Petrobrás/CENPES/DEBAR/DEXBA, Internal Report.
PRAÇA, U. M. & HASHIMOTO, A.T., 1991. Análise Bioestratigráfica da Formação Pendência - Bacia Potiguar, parte emersa. PetrobrÊs/CENPES/DIVEX/SEBIPE, Internal Report.
SILVA-TELLES, Jr., A.C., 1992. Novo zoneamento das coquinas da Formação Lagoa Feia (NeojiquiÊ da Bacia de Campos) com base em ostracodes - aspectos evolutivos. In: XXXVII Congresso Brasileiro de Geologia (São Paulo, 1992), Boletim de Resumos Expandidos., 2, p. 489.
TOLDERER-FARMER, M.; COIMBRA, J.C.; MOURA, J.A. & GILSON, H.N., 1989. Reconstrução paleoambiental da Bacia do Recôncavo com base em ostracodes - Um estudo preliminar. Petrobrás-CENPES/SEBIPE, Internal Report, 111 pp, Rio de Janeiro.
TROELSEN, J.C., 1966. Zonas-concorrentes, baseadas na amplitude-local aproximada de ostracóides diagnósticos, seção não marinha, bacias de Sergipe e Alagoas. Petrobrás-RPNE. Internal Report, 47 pp., 22 plates, Maceió.
VIANA, C.F. - 1967 - Microfósseis do Creteaceo do Nordeste Brasileiro eÁfrica Ocidental. In: XXI Congresso Brasileiro de Geologia (Curitiba, 1967), Anais., p.19-28.
VIANA , C.F; GAMA, Jr, E.G.; SIMÃO, I.A.; MOURA, J.A.; FONSECA, J.R. & ALVES, R.J., 1971. Revisão estratigráfica da Bacia do Recôncavo/Tucano. Bol.Tec. Petrobrás, 14 (3/4): 157-192.
WEBER, R.T., 1964. Zoneamento por meio de Ostracodes nas Bacias do Recôncavo,Tucano e Jatobá. Petrobrás-RPBA/SETEX, Internal Report no. 754, Salvador.

Taxonomy of the ostracodes from the Alagamar Formation (Lower Cretaceous), Potiguar basin. Biostratigraphic and palaeoecologic correlation with basins of the Brazilian continental margin

Dermeval Aparecido do CARMO - Curso de Pôs-Graduação em GeociÃncias, Instituto de Geociências, UFRGS, Caixa Postal 15001, 91501-970 Porto Alegre, RS, BRAZIL. E-mail: dermeval@if1.ufrgs.br

This report deals with a doctorate research, currently in progress, on the ostracode fauna from the Alagamar Formation (Aptian-lower Albian), Potiguar basin, northeastern Brazil. The study is based on the taxonomy, biostratigraphy and palaeoecology of ostracodes recovered from approximately 1,200 core samples of 15 boreholes drilled in the Potiguar basin.

List of studied intervals:

Boreholes Cores Intervals (m)
1 1-CES-7-RN #1 2076.5-2087.5
2 7-MA-11-RN #1-7 1340-1456
3 7-SO-5-RN #1-9 1212-1291
4 3-UB-27-RN #1-10 2376-2538
5 1-MB-1-RN #1 1708-1716.6
6 4-PE-2-RN #1-2 1697-1724.4
7 1-RCA-1-RN #1 1409-1424
8 1-MA-9-RN #1-2 1200-1211.5
9 7-MA-7-RN #1-3 1314-1331.7
10 3-RNS-42-RN #1-8 1534-1594
11 1-ES-2A-RN #1/2 775-793/912-921
12 1-SDM-1-RN #1 1276-1285
13 1-UMB-1-RN #1 1595-1596.5
14 7-MA-7-RN #1 1314-1370
15 MA-4-RN #1 1301-1310

It is intended, through detailed microbiostratigraphic analyses, to characterise the ostracode assemblages and the biozone sequence, to carry out comparative studies with other microfossil groups already studied and correlate this stratigraphic interval with coeval sequences on other basins of the Brazilian continental margin.

Ammonite and inoceramid biostratigraphy of northeastern Brazil: a status report

Peter Bengtson - Geologisch-Paläontologisches Institut der Universität Heidelberg, Im Neuenheimer Feld 234, D-69120 Heidelberg, Germany.

The northeast Brazilian coast comprises a series of Mesozoic basins containing fossiliferous marine sequences. They are, from north to south: the Barreirinhas, Piauí-Ceará, Potiguar, Pernambuco- Paraíba, Alagoas, Sergipe, Camamu, Almada, Jequitinhonha and Cumuruxatiba basins. Extensive exposures occur in the Potiguar, Pernambuco-Paraíba, and Sergipe basins; however, only the Sergipe basin exhibits a nearly complete marine (Aptian-Maastrichtian) sequence, containing abundant representatives of the two stratigraphically most important macrofossil groups, ammonites and inoceramid bivalves. The Camamu and Almada basins and the chiefly offshore basins of Barreirinhas, PiauÁ-CearÊ, Jequitinhinha and Cumuruxatiba have yielded only scattered finds of macrofossils,which nevertheless, in some cases, have proved to be of stratigraphical importance.

Sergipe Basin. Approximately 70 ammonite genera with well over 100 species are known from this basin. The current ammonite zonation serves as a standard for the northwestern South Atlantic macrofossil sucession; it comprises 16 zones (chiefly Oppel zones) ranging from the upper Aptian to the middle Coniacian. Preliminary study suggests that at least 5 upper Cenomanian, 25 Turonian and 29 lower Coniacian species and subspecies of inoceramids are present. Inoceramids are normally less common in the Aptian-middle Cenomanian and Santonian-Maastrichtian beds.
Aptian - The ammonite zonation is based on representatives of the early douvilleiceratid lineage Cheloniceras -Eodouvilleiceras. Inoceramids occur but have as yet not been determined.
Lower-middle Albian - This interval corresponds to the Douvilleiceras and Oxtropidoceras zones, which can probably be further subdivided. Inoceramids are common and in need of taxonomic revision.
Upper Albian - This substage is subdivided into two ammonite zones, named after the genera Elobiceras and Mortoniceras, respectively. Correlation with the type areas is hampered by provincialism, and only a broad assignment to the M. inflatum "standard zone" is possible. The uppermost Albian Stoliczkaia dispar "standard zone" seems to be missing (as in Nigeria). Inoceramids are rare; all forms need taxonomic revision. Lower Cenomanian - A diverse ammonite fauna occurs in the lower Cenomanian Graysonites lozoi-Hypoturrilites betaitraensis Zone. The material derives from the only known ammonite-bearing exposure of this age along the western South Atlantic margin. The fauna is dominantly Tethyan, with close relationships with the Texas-Mexico region but also with southern and western Africa and Algeria-Tunisia. The zone correlates with the Mantelliceras mantelli "standard zone". No inoceramids have been found in these beds.
Middle-upper Cenomanian - The Acompsoceras spathi-Dunveganoceras and Acanthoceras jukesbrownei-Eucalycoceras pentagonum zones correspond broadly to the middle Cenomanian Acanthoceras rhotomagense "standard zone". The uppermost Cenomanian contains the cosmopolitan ammonite Euomphaloceras septemseriatum, which is diagnostic of the Metoicoceras geslinianum "standard zone". Inoceramids have not been found in the middle Cenomanian. Above the septemseriatum zone, correlation with the international "standard zonation" becomes difficult, in particular the positioning of the Cenomanian-Turonian boundary. On the basis of associated taxa, the Vascoceras harttii-Pseudaspidoceras footeanum ammonite zone is tentatively correlation with the uppermost Cenomanian Neocardioceras juddii "standard zone" and the lower part of the Turonian Watinoceras spp. Zone. Principal inoceramids of the upper Cenomanian belong to the Inoceramus (I.) pictus and (I.) tenuistriatus lineages.
Lower-middle Turonian - This interval comprises the ammonite zones of Watinoceras amudariense-Kamerunoceras seitzi and Mammites nodosoides-Kamerunoceras turoniense. In the lower Turonian the Mytiloides mytiloides, M. hercynicus and M. latus lineages predominate, co-occurring with largely endemic species of Sergipia. To this can be added the diagnostic, radially ornamented lower Turonian Cladoceramus mauryae and C. alatus. Two inoceramid zones have been established for the lower Turonian. Both ammonite and inoceramid studies suggest depositional breaks in the mid-Turonian and beginning of the late Turonian. The rare occurence of Mytiloides hercynicus and early members of the Inoceramus (I.) cuvieri lineage suggest that basal middle Turonian strata are locally present in Sergipe.
Upper Turonian - The ammonites define the Subprionocyclus-Reesidites Zone. Common inoceramid groups include the Inoceramus (I.) apicalis, I. perplexus-I. vancouverensis and Mytiloides striatoconcentricus lineages.
Lower-middle Coniacian - This interval is represented by the Barroisiceras onilahyense-Forresteria and Solgerites armatus-Prionocycloceras lenti ammonite zones. The Turonian-Coniacian boundary beds and the lower Coniacian show the highest diversity of inoceramids, including members of the M. striatococentricus, M. dresdensis, M. fiegei, M. lusatiae, I. vancouverensis, I. winkholdioides, "I." rotundatus, and Cremnoceramus? waltersdorfensis lineages. The bivalve Didymotis is a common associate. These beds mark the end of the carbonate-dominated depositional cycle in Sergipe. In most onshore areas these follows a significant gap, caused by seaward tilting of the basin, whereas offshore sedimentation was more continuous.
Campanian - Sedimentation in Sergipe resumed in the Santonian with chiefly terrigenous clastic deposits. A single ammonite identified as Pseudoschloenbachia sp. and inoceramids of the I. balticus group suggest an early Campanian age.
Potiguar Basin. The few ammonites and inoceramids described from the Potiguar basin derive from the Turonian, upper Coniacian and probably Campanian. No viable zonation exists.
Pernambuco-Paraíba Basin. A basal sandstone unit has yielded sparse inoceramids and a single ammonite, probably a Pseudoschloenbachia, which suggests a Santonian or early Campanian age. Inoceramus balticus found near the base of the overlying carbonate point a Campanian age. The bulk of the carbonates is Maastrichtian, as evidenced, for example, for an abundant pachydiscid fauna. No systematic biostratigraphic work has been done and many previous descriptions are in need of taxonomic revision.
Alagoas Basin. The marine record of Alagoas is extremely scanty; an isolated occurrence of silicified oolites has yielded late Cenomanian Pseudocalycoceras cf. harpax and Kamerunoceras sp., which is in agreement with global eustatic patterns. A correlation problem exists in that similar beds and faunas have not been found in the adjacent Sergipe basin.
Camamu Basin. Marine limestones on top of the non-marine Cretaceous sequence have yielded the diagnostic ammonite Elobiceras, which indicates a late Albian age. No inoceramids are known from this basin.

For many basins in northeastern Brazil there exist detailed biostratigraphic schemes based on micropalaeontological work by PetrobrÊs. Owing to facies discrepancies and problems of preservation, these schemes cannot always be applied to outcrop sequences. For the Sergipe Basin, work on the integration of macrofossil and microfossil schemes has only just begun.

A logical next step in the macrobiostratigraphical work will be to refine the existing zonations through study of successive assemblages obtained by careful bed-by-bed collecting. It is particularly important to study the stage boundaries to ensure as good a correlation as possible with the international "standard zonation". The Cenomanian-Turonian boundary is currently the subject of such a detailed study. Most of the Sergipe sequences have the advantage of yielding both inoceramids and ammonites and can therefore be seen as potential reference sections for several middle and upper Cretaceous stage boundaries in the South Temperate Realm.
For the Potiguar, Pernambuco-Paraíba and Camamu basins detailed macrobiostratigraphic schemes have yet to be established.

Upper Cretaceous to Danian foraminiferal biostratigraphy of northeastern Brazil: a progress report

Eduardo A. M. Koutsoukos - PETROBRAS Research Centre (Cenpes/Divex), Cidade Universitária, Quadra 7, 21949-900 Rio de Janeiro, RJ, BRAZIL.

The final structural detachment of the South American and African plates in the late Coniacian-early Santonian resulted in a major oceanographic event with the establishment of a deep-ocean circulation regime. Simultaneously there was a rapid change from a dominantly dry to a more humid and warm climate, probably caused by the development of a high pressure atmospheric cell at low latitudes over the widening Atlantic watermasses (Parrish & Curtis, 1982; Chang et al., 1988; Koutsoukos & Bengtson, 1993). This climatic turnover caused increased siliciclastic influx to the basin, due to increased continental runoff. Thick successions of shales and, subordinately, tubiditic sandstones were deposited. Facies-constant stratigraphical sequences, where no major environmental change has taken place over a long time-interval, commonly occur within the post-Coniacian siliciclastic deposits. The study of these strata, from both outcrop and subsurface sections in Sergipe and Pernambuco-Paraíba basins, has allowed the selection of foraminiferal zonal markers and the precise recognition of their first appearance and extinction levels, which provide potentially isochronous zonal boundaries (mostly Oppel zones - sensu Hedberg, 1976, p. 56 - characterized both by distinctive assemblages and by some clustering of events) for northeastern Brazil (Koutsoukos & Bengtson, 1993; Koutsoukos, in press, a, b).

The upper Coniacian to upper Santonian, i.e. the base of the Upper Cretaceous siliciclastic-dominated system, is represented by two planktonic foraminiferal zones, Dicarinella concavata-Marginotruncana sinuosa and D. asymetrica zones, respectively. It is important to note the last appearences (LA's) in the zonal assemblage of M. renzi, M. undulata and Ventrilabrella austinana, together with the first appearence (FA) of Contusotruncana ex gr. fornicata at or near the Coniacian-Santonian boundary. The Campanian is subdivided with planktonic foraminifera into two zones: the Contusotruncana patelliformis-Globotruncanita elevata-stuartiformis plexus Zone (FA's of C. patelliformis, G. stuartiformis s.l. and Globotruncanella havanenesis; LA of Heterohelix reussi); and the Globotruncana orientalis-G. ventricosa Zone (FA's of G. orientalis, G. ventricosa, G. bulloides, G. linneiana, Globotruncanita subspinosa and Rugoglobigerina ex gr. rugosa).
The Maastrichtian is represented by three planktonic foraminiferal zones:
the C. ex gr. fornicata-Globotruncana linneiana Zone (LA's of C. ex gr. fornicata, G. bulloides, G. linneiana and G. subspinosa; FA's of Gansserina gansseri, Globotruncanella petaloidea and Globotruncana aegyptiaca in the zonal assemblage); the G. gansseri-Globotruncanita stuartiformis Zone (LA's of G. gansseri, Globotruncana rosetta, G. ventricosa, G. stuartiformis and C. patelliformis); and the Contusotruncana contusa-G. aegyptiaca Zone (Total Range of C. contusa and LA's of nearly all associated Late Cretaceous species). The uppermost Cretaceous strata are represented by the Plummerita hantkeninoides-Pseudotextularia intermedia-Racemiguembelina fructicosa Subzone (Pernambuco-Paraíba Basin; Koutsoukos, in press, a, b). This is correlated with the Kassabiana falsocalcarata Total-Range Zone of northeastern Tunisia (Solakius et al., 1984). It is characterized by the occurrence of nearly all known latest Maastrichtian foraminifera species, such as Contusotruncana contusa (locally an Acme-Zone), C. navarroensis, Globigerinelloides prairiehillensis, G. subcarinata, Globotruncanella pschadae, Globotruncana aegyptiaca, Globotruncanita pettersi, G. stuarti, Hedbergella holmdelensis, H. monmouthensis, Kassabiana falsocalcarata (Total Range), Pseudoguembelina costulata, P. palpebra, Pseudotextularia elegans, P. intermedia, P. nuttalli, Plummerita hantkeninoides (Total Range), Racemiguembelina fructicosa, R. powelli, Rugoglobigerina reicheli, R. ex gr. rugosa and R. scotti. Particularly significant at this horizon is the occurrence of P. hantkeninoides, whose range is assumed to be restricted to the last 200,000 years of the Maastrichtian (Stinnesbeck & Keller, 1995).
The overlying deposits contain an abundant reworked Cretaceous fauna and the FA's of very rare lowermost Paleocene specimens belonging to the Danian foraminiferal zones P 0 and P a, such as Chiloguembelina waiparaensis, Eoglobigerina eobulloides, E. fringa, E. simplicissima, Guembelitria irregularis, Parasubbotina cf. pseudobulloides, Parvularugoglobigerina eugubina (P. longiapertura morphotypes), Praemurica taurica, Woodringina claytonensis, W. hornerstownensis, together with Guembelitria cretacea, a Cretaceous survivor which continues into the lower Paleocene up to the lower part of the P 2 Zone in NE Brazil.
The succeeding strata yield, successively, the first appearances of diagnostic Danian taxa:
Studies are currently in progress to detail the microfaunal changes (at taxonomic and community structure levels) of the benthic foraminifera and ostracodes (the latter work with Gerson Fauth, Heidelberg Universität) across the K/T boundary in the Pernambuco-Paraíba Basin. An integrated biostratigraphy of microfossils and macrofossils, based on a integrated geological survey of key-sections, is still in need for the Upper Cretaceous sequences of northeastern Brazil.

References

CHANG, H. K., KOWSMANN, R. O. & FIGUEIREDO, A. M. F. de, 1988. New concepts on the development of east Brazilian marginal basins. Episodes, 11 (3), p. 194-202.
HEDBERG, H. D., 1976. International Stratigraphic Guide: a guide to Stratigraphic Classification, Terminology and Procedure. 200 pp., John Wiley & Sons, New York.
KOUTSOUKOS, E. A. M., in press, a. The Cretaceous-Tertiary boundary at Poty, NE Brazil: event stratigraphy and paleoenvironments. Compte-rendu de 12 Colloque de Stratigraphie et de Paléogéographie de l'Atlantique Sud (Angers, 16-20 Juillet 1994), Recueil des Communications. Bull. Centres Rech. Explor.-Prod. Elf Aquitaine.
KOUTSOUKOS, E. A. M., in press, b. Phenotypic experiments into new pelagic niches in early Danian planktonic foraminifera: aftermath of the K/T boundary event. In: M. B. Hart and E. Kauffman (Eds), Recovery from Global Mass Extinction Events, Special Publication of the Geological Society of London.
KOUTSOUKOS, E.A.M. & BENGTSON, P., Towards an integrated biostratigraphy of the upper Aptian-Maastrichtian of the Sergipe Basin, Brazil. Docum. Lab. Géol. Lyon, n 125, p. 241-262
PARRISH, J. T. & CURTIS, R. L., 1982. Atmospheric circulation, upwelling, and organic-rich rocks in the Mesozoic and Cenozoic Eras. Palaeogeogr., Palaeoclim., Palaeoecol., 40 (1-3), p. 31-66..
STINNESBECK, W. & KELLER, G., 1995. The Cretaceous-Tertiary boundary in southern low-latitude regions: preliminary study in Pernambuco, northeastern Brazil. Comments. Terra Nova, 7.

Chemical stratigraphy of Cretaceous sequences in Brazil: state of the art and perspectives

René RODRIGUES - PETROBRÁS Research Centre (Cenpes/Divex), Cidade Universitária, Quadra 7, 21949-900 Rio de Janeiro - RJ, BraZil.

Introduction. In the present study we have made a first attempt to establish a chemical stratigraphy in the Cretaceous sequences of the Brazilian marginal basins. We will show the possibility of using chemical and isotopic data in order to recognize major palaeoceanographic events in these basins and to learn more about some aspects of the evolution of the South Atlantic Ocean.

Results and Discussion. The Brazilian marginal basins are directly related to the rifting of the African and South American plates. Thick sequences of lacustrine and fluvial deposits were accumulated during the Early Cretaceous rifting phase. Following this, tectonic activity was restricted to subsidence (Estrella et al., 1984) while intermittent marine transgressions took place in the lower Aptian. The sedimentary record of these marine transgressions is characterized by the presence of chemical fossils derived from marine algae and dinoflagellates (of marine origin) and less negative d13C values, when compared to the underlying sequences with biomarkers and carbon isotopic signatures of freshwater to saline lacustrine environments (Rodrigues, 1989, 1995; Mello et al., 1991).
Isolation by topographical barriers and arid climatic conditions led to the deposition of a thick succession of carbonate and evaporitic sediments for most of the Aptian (Asmus, 1975). The 87Sr/86Sr ratio in the evaporites and limestones show isotopic values characteristic of salts precipited from sea water, but with some enrichment in 87Sr radiogenic as compared to the strontium ratio of coeval salts deposited elsewhere.
Widespread marine bituminous black shales were deposited about the Aptian-Albian transition, mostly in the northeastern and equatorial marginal basins. Southwards this same interval is mostly represented by limestones and/or sandstone/limestone sequences. This black shale interval yields high total organic carbon content (TOC up to 25%), high hydrogen index (IH up to 980 mg HC/g COT) and is besides caracterized by specific biomarkers compounds. These are interpreted as the result of a global oceanic dysoxic/anoxic event, probably related to the well known late Aptian-early Albian episode and, as such, it can be used for correlation purposes.
Carbon and oxygen isotope stratigraphy through the Albian was established on bulk carbonates from the Campos basin, in the southeast Brazilian coast (Takaki & Rodrigues, 1984). The isotopic data indicate deposition in a warm hypersaline, shallow restricted marine environment. For the upper Albian-lower Cenomanian, the less negative d18O and the more negative d13C values suggest an increase of marine circulation, colder waters and a decrease of organic productivity.
At the Cenomanian-Turonian boundary more widespread marine bituminous black shales were deposited from the equatorial to the southeast Brazilian marginal basins. In order to test the application of d13C shifts to cronostratigraphic correlations of dysoxic/anoxic events, a detailed isotope study was carried out in a shelf carbonate sequence from northeastern Brazil, considered as coeval to the black shale deposition episode (Takaki & Rodrigues, 1993). An abrupt and positive d13C variation of 2.5 o/oo, in both organic and inorganic fractions provides an excellent tool to correlate the nearshore carbonate sequences with deeper offshore, condensed pelitic sequences. In addition, the more negative d18O values suggest that the dysoxic/anoxic event took place under mild climatic conditions, associated with a higher relative water temperature. Other smaller d13C changes were also observed following the oxygen depletion event, lending further support to its application in stratigraphic correlations. These d13C fluctuations, turning gradually less positive in the Turonian and younger sequences, might reflect a decrease in organic productivity of the ocean, probably linked to the progressive cooling of oceanic waters as shown by the increasing d18O values.
Perspectives. These selected data from various Brazilian marginal basins, hundred of kilometers apart from one another, provide a good picture of our future research needs. We believe that the South Atlantic Mesozoic Correlation IGCP Project 381 could provide the best opportunity to improve our knowledge of these areas. In conclusion, as we have available appropriate sections to be studied, the main actual need is of more multidisciplinary work being done on this subject.

References

ESTRELLA, G, et al., 1984. The Espirito Santo Basin-Brazil. Source rock characterization and petroleum habitad. Petroleum Geochemistry and Basin Evaluation, A.A.P.G. Mem. 35, p. 253-271.
MELLO, M.R.; MOHRIAK, W.U.; KOUTSOUKOS, E.A.M. & FIGUEIRA,J.C.A., 1991. Brazilian and West African oils: generation, migration, accumulation and correlation. In: 13th World Petroleum Congress (Buenos Aires), Proceedings, p. 1-12.
RODRIGUES, R., 1989. Estudo geoquímico dos óleos dos poços 1-BAS-60 e 1-BAS-85, Bacia do Espirito Santo. Petrobrás-CENPES/DIVEX/SEGEQ, Internal Report no.673-12638, .
RODRIGUES, R., 1995. A ocorrência de evidências marinhas (biomarcadores e isótopos estáveis do carbono) na seqüéncia sedimentar Pré-Alagoas da Bacia do Espirito Santo. Petrobrás-CENPES/DIVEX/SESTRA, Technical Report no. 001/95, Rio de Janeiro.
TAKAKI, T. & RODRIGUES, R., 1984. Isótopos estáveis do carbono e oxigênio dos calcários como indicadores paleoambientais- bacias de Campos, Santos e Espirito Santo. In: XXXIII Congresso Brasileiro de Geologia (Rio de Janeiro), Proceedings, p. 4750-4762.
TAKAKI, T. & RODRIGUES, R., 1993. Isotopic evidence of the Cenomanian-Turonian anoxic event in the Sergipe-Alagoas Basin, Brazil. In: Third Latin American Congress On Organic Geochemistry (Manaus, 1992), Extended Abstract, p. 141-143.

The northern part of the South Atlantic ocean was an arm of the Tethys early in its opening history

Dimas Dias Brito - Departamento de Geologia Sedimentar, UNESP, Av. 24-A no. 1515, 13506-900 Rio Claro, SP, BRAZIL.

The water mass of the northern South Atlantic, from the Walvis-São Paulo Ridge to the north, formed a long and narrow arm of the Tethys Sea during Aptian-Albian time in the Cretaceous, when Atlantic African and Brazilian basins were entering their passive drift phase. This water mass, here termed "the Tethyan South Atlantic", was at that time the most restricted portion of the Tethys and influence of temperate Austral water was minor.

The shallow areas were occupied by cyanobacteria, which formed oncoidal packstones and grainstones that are commonly associated with oolitic limestones. In these shallow waters the prevailing extreme conditions of high temperature and hypersalinity excluded coraline and rudistid reefs and large foraminifera such as orbitolinids, discyclinids and alveolinids. On the other hand, the open sea realm was largely dominated by a typically Tethyan planktonic biota, including: a) calcareous dinoflagellates (Pithonelloideae: Pithonella sphaerica, P. ovalis, P. trejoi, P. cf. P. perlonga, Bonetocardiella conoidea; Orthopithonelloideae: Carpistomiosphaera sp., Colomisphaera spp.; Obliquipithonelloideae: Cadosina fusca; Crustocadosina callosa); b) calpionellids (Collomiellidae: Collomiella recta, Collomiella mexicana); c) planktonic crinoids [saccocomids/ roveacrinids: Lombardia arachnoidae, Lombardia perplexa, Microcalamoides diversus (forma c), rare Eothrix alpina] foraminifera (Favusella washitensis, rare talmaninellids/ rotaliporids/ praeglobotruncanids), calcareous nannoplankton (abundant Nannoconus truitti and other nannoconids). The majority of these organogenic elements are photodocumented in Dias Brito (1995) and constitute the essence of the resulting Brazilian Albian chalkstones which are very similar to those of the same age or younger (Cenomanian-Coniacian) from the Gulf of Mexico and the European continent. Without doubt the same global water mass connected these far distant basins during the late Aptian to the late Albian.
From the end of the Albian (Vraconian), all the South Atlantic experencied large oceanographic changes so that the Tethyan South Atlantic water masses largely lost their distinctive character except in the Barreirinhas, Potiguar, Sergipe-Alagoas and Pernambuco-Paraíba basins. In these four basins both shallow and deep water carbonates continued for part of the Late Cretaceous. This interpretation is based on detailed microfacies study of Albian fine-grained carbonates from the early drift phase of the Brazilian marginal basins, including the shallow carbonate platform megasequence and the open-marine megasequence. Almost 2,000 thin sections were investigated from core and cutting samples from a dozen of wells drilled by PetrobrÊs along the Brazilian continental margin.

References

Dias Brito, D., 1995, Atlas dos carbonates pelágicos do Cretáceo médio do Brasil (microfácies e constituintes biogênicos, com ênfase aos pitonelóideos). In: Calcisferas e microfácies em rochas carbonáticas pelágicas mesocretáceas. Tese de Doutoramento, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil, 688 pp.

The West African framework Nick Cameron - Geo Insight, Clalkyfold, Grove Lane, Chesham, Bucks HP5 3QQ, UK.

One reason for the interest in IGCP 381 is the need to understand the processes creating the abundant hydrocarbon resources between the Walvis Ridge and the Fang Fracture Zone region, and to determine whether the geology of this region could recur elsewhere in the South Atlantic. Regional studies in West Africa between Angola and Gabon indicate the geology of this region is unique and that it cannot recur elsewhere. Five primary factors account for the unique morphology and infill of the West African Atlantic margin basins. They are:

52% of reserves are directly related to play suites controlled by facies and structures related to the Atlantic Hinge. Superimposed on these reserves are additional plays related to maturation and tectonism induced by the Tertiary progradation of the Zaire Fan across the Hinge. Altogether 70% of the total West African reserves are contained within the confines of the Zaire Fan. Only the onshore Kwanza basin reserves and the Pre-salt plays in Gabon are unrelated to the Atlantic Hinge or the Zaire Fan.

This paper is introduced at this time to a predominantly paleontological grouping to provide an indication of the economic importance of IGCP 381 and the scale of the new understanding of the area's Petroleum Systems that must emerge with the completion of the Project in 2000. Our future looks very exciting.

ADDITIONAL CONTRIBUTIONS

Cretaceous foraminifera from the Austral basin, Argentina

N. Malumián & Carolina Náñez - Dirección Nacional del Servicio Geológico, Benito Correa 1194, (1107) Buenos Aires, Argentina.

The Austral or Magallanes Basin, of retroarch setting, located at the southern tip of South America, has an almost complete Cretaceous clastic sequence deposited in a temperate area. The stratigraphic distribution of its foraminifers is related to major global eustatic sea level cycles and to local diastrophic phases. It lacks larger foraminifera, while carinate planktonic ones are scarse and occurring only in the Turonian and in the Santonian-Campanian.

The basin infill begins with basal sandstones (Springhill Fm.) and anoxic-disaerobic black shales of Hauterivian-Barremian age (Río Mayer and Pampa Rincón Fms.) containing low diversity calcareous foraminiferal assemblages. During the Aptian-Cenomanian a mostly turbiditic sedimentation took place (Arroyo Alfa and Nueva Argentina Formations), associated with intense uplift and formation of the "Palaeo Andes" in the west and south of the basin, containing flysch type foraminiferal assemblages. During the Late Cretaceous, shelf foraminiferal faunas occur; flysch type assemblages are known, but restricted to the depocenters up the Santonian-lower Campanian. Since the upper Campanian, the northwestern portion of the foreland basin was under a prograding deltaic sedimentation, being progressively filled with shallow sediments containing mostly agglutinated foraminiferal assemblages of very low diversity.
Three major cycles are recognised, which comprise extended paleoecologic conditions and contain definite foraminiferal assemblages.
First Cycle. Rinconian-Pratian local Chilean stages. It includes black shales and assemblages of very low diversity, mostly lacking agglutinated foraminifers. The austral character is given by the strong affinities with Madagascar and South Africa and the occurrence of Astacolus microdyctiotos Espitalié & Sigal Marginulina pristipellis Ludbrook, one of the most typical groups of forms of the Southern Hemisphere Early Cretaceous. The following assemblages are recognized:
a) Reinholdella assemblage (Berriasian-Valanginian; Springhill Fm.). Lenticulina muensteri (Roemer) seems to be the most abundant species.
b) Lenticulina nodosa - Astacolus gibber assemblage (Late Valanginian-Hauterivian; Pampa Rincón Fm., Esperanzian stage). A low diversity assemblage dominated by the eponymous species and Pseudopolymorphina martinezi (Cañón & Ernst). The latter seems to be one of the few endemic species to the Austral basin for the present cycle and restricted to the Valanginian.
c) Epistomina caracolla assemblage (early? Barremian; Río Mayer Fm.), chacacterized by the dominance of E. caracolla (Roemer) and the apparent earliest occurrence of planktonic foraminifera in the basin.
Second Cycle. Tenerefian and Peninsulian stages. Its main feature is the irruption and abundance of planktonic elements. Affinities with Australia and South Africa remains, but to a lesser extent than occur in the first cycle. It has a more restricted geographical distribution within the basin and contains:
a) Horizons with abundant radiolarians (Aptian). Poor preserved foraminifera and frequent radiolarians characterise these horizons.
b) Globigerinelloides gyroidinaeforms assemblage (early-middle Albian, Nueva Argentina Fm.). Abundant calcareous plankton, either foraminifers or calcareous nannoplankton, characterise the assemblage. The most conspicuous foraminifera are: Osangularia utaturiensis (Sastry & Sastri), O. schoenbachi (Reuss), Gavelinella barremiana Bettenstaedt - G. australiana Ludbrool, G. sp. (forms with large boss), Anomalina? sp. and Globigerinelloides gyroidinaeforms Moullade. The anoxic environment remains at least in parts of the basin, probably due to the high productivity of surface waters.
c) Lingulogavelinella-Tritaxia assemblage (middle Albian-Cenomanian?; Arroyo Alfa Fm.). This is the most diversified early Cretaceous assemblage. It is well developed in the Tierra del Fuego Island, where mainly three groups of species can be recognised:
1. Abundant cosmopolitan arenaceous as Spiroplectammina annectens (Parker & Jones), Tritaxia gaultina gaultina Morozova and Marssonella oxycona (Reuss).
2. Species which seem to be restricted to the Southern Hemisphere as Lingulogavelinella indica (Scheibnerova) and L. africana Lambert & Scheibnerova.
3. Apparent endemic species, in abundance order: Valvulineria fueguina Malumián & Masiuk, Dorothia mordojovichi Cañón & Ernst, Lingulogavelinella magallanica Malumián & Masiuk, and the rare Notoconorbina leanzai Malumián & Masiuk. In organic-rich, platform areas, a poor fauna of lenticulinids and some lingulogavellinids occurs.
At the end of this cycle A. microdictyotos-M. pristipellis disappears, and the biggest faunal break took place.
Third Cycle. Lazian and Riescoian stages. In Tierra del Fuego Island (Cabeza de León Fm.) normal marine shelf calcareous benthic foraminiferal faunas occur. The following zones were recognised:
a) Pseudospiroplectinata ona Malumián & Masiuk Zone (Turonian), which contains frequent Gavelinella (Berthelina) spp., and lacks typical elements of the Lingulogavelinella-Tritaxia assemblage. It includes an horizon with "Praeglobotruncana cf. oraviensis Scheibnerova".
b) Notoplanulina rakauroana Finlay Zone (Coniacian-early Campanian), show affinites with the Australian Carnavon basin, and its middle part includes a horizon with Marginotruncana. In deeper facies, the late Coniacian is characterised by Whiteinella baltica Douglas & Rankin, the Santonian, by abundant Archaeoglobigerina bosquensis Pessagno, and the Santonian-early Campanian by Costellagerina pilula (Belford).
c) Gaudryina healyi Finlay Zone (middle Campanian-early Maastrichtian), corresponds to one of the last Cretaceous transgressions which brought calcareous foraminifera to the Tierra del Fuego Island.
d) "Agglutinated foraminiferal zone" (late Campanian-Maastrichtian), an informal zone containing poor preserved agglutinated foraminifera.
In Maastrichtian open oceanic facies, the following planktonic foraminifera occur: Rugotruncana circumnodifer (Finlay), Rugoglobigerina rugosa (Plummer), Archaeoglobigerina australis Huber, A. mateola Huber and Hedbergella monmouthensis (Olsson).
Since the Campanian, the northern area of the basin (Santa Cruz province) has assemblages dominated by agglutinated foraminifera, varying from flysch-type to marshy faunas. In the northwest, the following assemblages are recognised:
a) Uvigerinammina jankoi assemblage (Santonian-Campanian; Cerro Toro Fm.), a flysch type fauna.
b) Miliammina telemaquensis assemblage (base of the Tres Pasos Fm.), represents the change to a shallower marine environment.
c) Haplophragmoides cf. H. walteri assemblage, a scarce, low diversity agglutinated foraminiferal fauna.
d) Reticulophragmium sp. Assemblage (late Campanian-Maastrichtian; Cerro Cazador Fm.), consisting mainly of Cribrostomoides? sp. 1506, Trochammina wetteri Stelck & Wall, and the eponymous species. It woul correspond to a hyposaline marsh and is also recognised in the central northern area of the basin.
e) Coryphostoma incrassata assemblage (Maastrichtian), separated from the Reticulophragmium sp. assemblage by a glauconitic level, represents the last Cretaceous transgression. It is characterized by C. incrassata gigantea Wicher, Dorothia biformis Finlay and Quasispiroplectammina spectabilis (Grzybowski), corresponds to a shallow, normal marine environment, and is also recognized in the central northern area of the basin. This is one of the few assemblages not described up to now.
It the northeastern area of the basin (well SC-1) monospecific or low diversity assemblages from marginal marine habitats occur.
a) "Spiroplectammina-Textularia" sp. sssemblage (Campanian-early Maastrichtian), of very low diversity. The eponymous species also occurs in the central northern part of the basin from levels equivalent to the Cabeza de Le¢n Fm., and in the northwest, from the Mata Amarilla Fm.
b) Neobulimina aspera assemblages (Maastrichtian). A poor, of a peculiar white, delicate preservation, calcareous assemblage, with Gyroidinoides sp., Anomalinoides sp. cf. A. piripaua (Finlay), Patellina sp. and N. aspera (Cushman & Parker). It unconformably overlies the "S.-T." sp. assemblage and is also recognised in the central northern area of the basin.
c) Cribrostomoides ex. gr. H. rugosus Cushman & Waters assemblage (Maastrichtian), equivalent to the "Agglutinated foraminiferal zone" of the Tierra del Fuego Island.
Although it had generally been assumed that the Late Cretaceous foraminiefral faunas from the Austral Basin were different to those from northern Patagonia, it is noticed that the Maastrichtian assemblages from the northern Austral basin do have species in common to northern Patagonia. At least in the northern part of the basin, a major turnover occurs between the Maastrichtian and Danian foraminiferal assemblages, as it was observed in northern Patagonia.

Maastrichtian foraminiferal assemblages and the K/P boundary in Patagonia, Argentina (*)

Carolina NÁNEZ & N. MALUMIÁN - Dirección Nacional del Servicio Geológico y CONICET, Benito Correa 1194, 1107 Buenos Aires, ARGENTINA. (*) Contribution to IGCP Projects 301 and 381.

The Late Cretaceous flooding of the Argentine margin during a period of tectonic quiescence and low magmatic activity, widely increased the areas under marine influence, mainly of inner-middle platform environments.

In northern Patagonia, the sea covered large areas (Neuquén-Colorado basins). The best exposures are in the western area (Neuquén basin), which for the Maastrichtian-Paleocene has been compared to the Persian Gulf due to its tectonic setting, climate and sedimentary processes. The Campanian-Maastrichtian marine deposits overlie a thick continental sequence of Late Cretaceous age. Peak marine influx ocurred during the middle-late Maastrichtian, followed by a regression during the Danian.
The more widely distributed foraminiferal assemblage was originally described from the Neuquén basin (Jagüel Fm.). The benthic foraminifera are dominated by mainly infaunal, calcareous species, including abundant buliminids. The endemism is high: eight species, some of them very abundant, are apparently endemic to northern Patagonia. At the Bajada del Jagüel section (Añelo area), planktonic foraminifera are abundant, dominated by the shallow-water taxa Heterohelix globulosa (Ehrenberg) and Guebelitria cretacea Cushman, whereas Rugoglobigerina rugosa (Plummer), R. macrocephala Brönnimann, Globigerinelloides multispinatus (Lalicker) and Rugotruncana sp. cf. R. subcircumnodifer (Gandolfi) are very rare.
A geographically more restricted assemblage representing a shallower environment, is known from the basal levels of the Jagüel Fm., mainly composed of Glandulina acuminata Costa and "Discorbis correcta" Carsey; other conspicuous species are Favolagena ardolinoi Malumián, Náñez & Caramés, Patellina subcretacea Cushman & Alexander, Pseudopolymorphina leopolitana (Reuss) and Pseudopolymorphina sp.
A peculiar assemblage, from the lowermost sample of the Bajada del Jagüel section, contains abundant Quasispiroplectammina spectabilis (Grzybowski), being one of the few records of abundant agglutinated foraminifera for the Neuquén-Colorado basins.
In the eastern area of the Neuquén-Colorado basins a peculiar, undescribed up to now, poorly preserved fauna was found in the Aguada Cecilio and Arroyo Barbudo fms. It has a very low diversity and suggests a marginal marine, mainly hyperhaline environment. The more freguent species recorded from the Aguada Cecilio Fm. are: Quinqueloculina sp., "Pararotalia" sp., Fissoelphidium? sp., scarce Gavelinella camachoi Bertels and "Discorbis correcta Carsey". Few levels contain dominant agglutinated foraminifera: Haplophragmoides sp. and Trochammina ex gr. globigeriniformis (Parker & Jones). In the Laguna del Indio Muerto section (Arroyo Barbudo Fm.), monospecific assemblages of "Pararotalia" sp. occur. The most diversified assemblage found contains, in abundance order: Gavelinella? neuquense Bertels, Alabamina kaaschieteri Bertels, Angulogavelinella? sp., "Discorbis" sp., Paralabamina lunata (Brotzen), Gavelinella camachoi (Bertels), Glandulina acuminata Costa, "Eoponidella" sp., among others, and very few specimens of Guembelitria cretacea, suggesting a very shallow marine environment.
In the Austral Basin, the Maastrichtian transgression bears dominantly calcareous foraminiferal assemblages, contrasting with the underlying Campanian agglutinated assemblages. The Coryphostoma incrassata assembalge (upper glauconitic part of the Cerro Cazador Fm.) contains: Coryphostoma incrassata (Reuss), C. decurrens (Ehrenberg), C. incrassata gigantea Wicher, Bulimina arkadelphiana Cushman & Parker, Bulimina cushmani Sandidge, Dorothia biformis Finlay, Haplophragmoides spp., Lenticulina turbinata (Plummer), L. muensteri (Roemer), Pyramidulina affinis Reuss, Psammosphaera fusca Schulze, Pullenia cretacea Cushman, Quasispiroplectammina spectabilis (Grzybowski), Reophax texanus Cushman & Waters, Rhabdammina sp., Saccammina placenta Grzybowski, Saracenaria sp. cf. S. triangularis d'Orbigny, Spiroplectammina adamsi Lalicker, Tritaxia rugulosa (ten Dam & Sigal), Trochammina sp.; among the scarce species: Anomalinoides sp. cf. A. rubiginosa Cushman, Bulimina gonzalezi Cañón & Ernst, Matanzia varians (Glaessner), Quadrimorphina allomorphinoides (Reuss), Ramulina sp., Vaginulina subgracilis Cushman. Another, poorly defined, Maastrichtian assemblage is the Neobulimina aspera assemblage.
At the Cretaceous-Paleogene (K/P) boundary, the benthic foraminiferal faunas of northern Patagonia show a major turnover. Most of the Maastrichtian endemic species become extinct, and the Danian assemblage is mainly of Midway type. The K/P boundary was identified at several sections in the Neuquén basin. The most complete transition seems to be at Cerros Bayos, La Pampa province, where poorly preserved planktonic foraminifera of the P a Zone seem to be present; the P 0 Zone was not recorded up to now. In the east, although Danian and Maastrichtian faunas are known from the Arroyo Barbudo Fm., the K/P boundary has not been identified yet.
The position of the K/P boundary in the Austral basin has been difficult to be identified, due to the unsuitable environments for planktonic foraminifera. It coincides with the Cerro Cazador/Cerro Dorotea fms. boundary, and a complete change in the benthic foraminiferal faunas occurs.
The assumed differences between the northern (Neuquén-Colorado basins) and southern Patagonia (Austral basin) Late Cretaceous assemblages had been explained as the result of oceanographic or topographic barriers. However, after the identification of Maastrichtian calcareous assembalges in the Austral basin, these differences seem to be slighter than it was accepted before. Comparing the Maastrichtian faunas, it is observed that several species are common to both basins.

Santonian microflora of Rio Guanaco Formation (continental facies) from Rio Turbio profile, Santa Cruz province, Argentine. Southern hemisphere correlation

Elba Diana Poethe de Baldis - INGEO . FCEFyN, Universidad Nacional de San Juan, Av. I. de la Roza y Meglioli, 5400 San Juan, ARGENTINA.

This is the first contribution to the study of Rio Guanaco Formation continental microflora. Spores, gymnosperms, and angiosperms pollen grains are presented. Pediastrum, Potamogetonaceaepites and spores reflect a lacustrine paleoenvironment meanwhile angiosperms and gymnosperms were transported from adjacent or near areas.

Among spores the genus Matonisporites with M. phebopteroides Couper and M. cf. equiexinus Couper dominates; Cicatricosisporites and Triporoletes are well represented.
Saccate gymnosperms show a good diversification representes by the genera Microcachrydites, Podocarpidites, Phyllocladidites, Trisaccites and Vitreisporites. Corollina itunensis and Corollina spp. are frequent.
Angiosperms are well represented beeing tricolpates the most common forms. Tricolpites reticulata Cookson, Psilatricolpites inargutus (Mc intyre) and P. cf inspissatus Mc Intyre are frequent. Rousea patagonica Archangelsky, described for Paleocene deposits of Chubut province and R.zamaloai n.sp. appear frequently.
Among triporates Confossia vulgaris Stough appears commonly; this form typifies a palynological Santonian zone from southeastern Brazil (Santos Basin). Penetetrapites mollis Hedlund and Psiladicolpites laevis Regali both from Albian sediments from USA and Brazil respectively extend their biochrons to Santonian in southwestern Argentina. Tricolporites cf. apoxyexinus Partridge from the palynological Santonian zone of southern Australia is present but very rare. If well coincidences with Australian and Brazilian palynological zones are not neatly defined, mainly due to paleogeographical and paleoclimatological factors, future studies will enlarge this attempt of correlation.

Santonian dinoflagellates of brackish-water facies from Rio Guanaco Formation (San Martin-C profile), Santa Cruz province, Argentina, and their relation with the Austral counterpart of the Williams Suite of temperate paleolatitudes

Elba Diana Poethe de Baldis - INGEO . FCEFyN, Universidad Nacional de San Juan, Av. I. de la Roza y Meglioli, 5400 San Juan, ARGENTINA.

Twenty four species belonging to fourteen genera of Santonian age are described from the brackish-water facies of Rio Guanaco Formation, in Estancia La Herradura, between Lago Viedma and Lago San Martin, southwestern Argentine. The association shows a great percentage of peridineacean forms in relation to gonyaulacacean ones; applying the Harland index (1973) (gonyaulacacean/peridineacean rate) a value of 0.30 is obtained, which indicates a near shore, low salinity paleoenvironment supported by the high percentage of continental forms (69.5%).

The most representative forms are: Amphidiadema rectangularis (Cookson & Eis.), A. nucula (Cookson & Eis.), Isabelidinium cf. magnum (Davey), Manumiella cretacea (Cookson), M. lata (Cookson & Eis.) and Palaeohystrichosphora infusorioides Defl., Chattangiella cf chetiensis (Vozz.) and Ch. cf. ditissima (Mc Intyre) are present frequently but with smaller size than the original ones. Ceratioid cysts are represented by Odontochitina cribropoda Defl. & Cookson, O. operculata and O. porifera Cookson.
Two new forms of Gymnodinium are described: G. sp.1 and G. sp. 2, but they are very scarce, they only allow generic assignment.
Two very primitive forms of Deflandrea, transitional to Alterbidinium are presented: Deflandrea argentina n. sp. and Deflandrea sp. Gonyaulacacean cyst belong to Callaiosphaeridium sp., Cyclonephelium vannophorum Davey, Heterosphaeridium conjunctum Cookson & Eis. and Hystrichosphaeridim cf. tubiferum (Ehr.), but are present in small amounts.
The age of the association, formerly assigned to the Santonian-Campanian is now restricted to Santonian, based on the correlation with Australian palynological zones (Helby, Morgan & Partridge, 1987). Most of dinoflagellates can be placed in the Odontochitina porifera and Isabelidinium cretaceum zones of the Isabelidinium Superzone of lower to upper Santonian age.
On the other hand the studied association would represent the Austral counterpart of the Williams Suite (Lentin & Williams, 1980) created for Campanian peridineacean dinoflagellates of temperate paleolatitudes of the Northern Hemisphere.

REPORTS

Demande de souscription du Département des Sciences de la Terre de l'Université Nationale de Côte d'Ivoire au Projet PICG-SAMC

Pour le Département des Sciences de la Terre, Le Coordonnateur.
DIGBEHI, Zéli Bruno. Maître-Assistant au Départament des Sciences de la Terre - 22 bp 582, Abidjan 22, IVORY COAST. Tél. 225-442934, Fax: 225-440412.
Objectif principal
Promotion et insertion des différents résultats de recherches en sédimentologie et en biostratigraphie réalisés sur le bassin sédimentaire ivoiren, dans le cadre global des corrélations à grande échelle que se propose de réaliser le project PICG-SAMC n 381.
Contexte structuro-sedimentaire et stratigraphique du bassin Ivoirien
Le cadre géologique du bassin ivoiren est connu depuis le début du siècle grâce à de nombreux holotypes et modèles d'évolution structurale et sédimentaire de plus en plus précis (SPENGLER & DELTEIL, 1966; DELTEIL, 1977; CHARPY & NAHON, 1978; TASTET, 1979; EMERY, 1980). Plus récemment, AFFIAN (1986) a mis en évidence l'influence des structures profondes sur la répartition des sédiments de la marge. Les profils Equamarge ont permis à BLAREZ (1986) de comparer la marge ivoirienne à un méga-pull-a-part. DIGBEHI (1987) l'a comparée au précontinent de Californie du Sud en particulier au Crétacé Inférieur.
La stratigraphie générale du bassin est connue grâce à de nombreuses études essentiellement basées sur les foraminifères. Les données palynologiques sérieuses datent des années 60 (JARDINE & MAGLOIRE, 1965). Il a fallu attendre les annés 80 pour voir se réaliser de rares études réalisées essentiellement sur la partie onshore du bassin (REYRE & TEA, 1981; SIMON & AL., 1984; MEDUS & PARADIS, 1987). Dans le même temps, les nombreux résultats de géologues pétroliers restent hélàs consignés dans des rapports confidentiels. De plus, faute d'une collection de référence et de figurations des formes-repères pour le bassin, les déterminations taxonomiques rencontrées dans la bibliographie sont parfois subjectives, ce qui rend aléatoires les corrélations, même locales.
Perspectives
La jeune Equipe de biostratigraphes de Côte d'Ivoire a décidé depuis quelques années, d'entreprendre une analyse biostratigraphique (foraminifères, ostracodes, palynomorphes) des échantillons de forages (carottes et déblais) du domaine public dans le triple but de:
- dresser une échelle biostratigraphique propre au bassin sédimentaire cãtier ivoirien afin de la comparer d'abord à celle de ses homologues ouest-africains, puis à l'échelle plus globale, à celle des autres péri-atlantiques;
- retracer l'histoire paléobiogéographique de la marge ivoirienne, en relation avec les différentes phases de l'ouverture atlantique notamment dans de Golfe de Guinée septentrional;
- proposer un atlas des formes-repères des différents étages du Crétacé Inférieur (à différencier) au Mio-Pliocène; mettant ainsi à la disposition de tout chercheur en biostratigraphie, un document au moins "consensuel" de base.
Les travaux réalisés au Départament des Sciences de la Terre de l'Université Nationale de Côte d'Ivoire, en collaboration avec la Petroci, se rapportent essentiellement aux données de la sédimentologie, de la micropaléontologie (foraminifères, ostracodes et palynomorphes). Quelques résultats sont déjà publiés dans des revues scientifiques spécialisées, certains sont sous presse et d'autres sont en cours de rédaction.
Resultats publies et travaux en cours
DIGBEHI, AFFIAN & YACE, 1993. Contribution à l'étude sédimentologiques de la série marine albo-cénomanienne du bassin onshore de Côte d'Ivoire ivoirien. Ann. Sci. Nat. et Biol. de l'Univ. Nat. de Côte d'Ivoire, XXV B (1992-1993), p. 35-40.
DIGBEHI, CARBONNEL & DUFAURE, 1993. Contribution à l'étude microfaunistique (Ostracodes et Foraminifères) à la reconstitution paléogéographique du bassin onshore de Côte d'Ivoire à la limite Paléocène-Eocene. Rev. Micropal., 36 (3), p.23-246
DIGBEHI & BOURROUILH, sous presse. Séquences de plate-forme carbonatée-bassin en zone transformante: exemple du Cénomanien des marges sud-aquitaine, vallée d'Ossau, Pyrénées atlantiques. Bull. Soc. Géol. France.
DIGBEHI, TEA, AKA, 1994 (communication). Etude palynologique de la formation d'Eboco Essai de reconstitution paléogéographique à la limite Maastrichtien-Paléocène. 12é Coll. Micropal. Angers, 16-20 Juillet 1994.
TEA, YAO, DIGBEHI, BOBLAI (communication). Quelques palynomrphes du Crétacé Supérieur du bassin offshore de Côte d'Ivoire. Implications Paléoenvironmentales. Récent Coll. de Palynologie de Bruxelles, 6-10 mars 1995.
DIGBEHI & BOURROUILH (sous 2ème correction). Étude comparée des systèmes transformants St-Paul/Romanche et San Andreas dans les marges de Côte d'Ivoire et de la Californie du Sud. Journal of Afric. Earth of Sciences.
DIGBEHI, YACE & YAO (sous 2ème correction). Palynoflore et foraminifères planctonique des séries albo-cénomaniennes du bassin de Côte d'Ivoire Cah. de Micropal.
YAO, DIGBEHI, TEA &BOBLAI (en cours). Analyse palynlogique de la série marine campano-maastrichtienne du bassin offshore ivoirien.
DIGBEHI, SIEDOU, AKA & DOBLAI (en cours). Caractéristiques sédimentologique et microfaunistique du Campanien et du Maastrichtien du bassin sédimentaire de Côte d'Ivoire. Implications paléogéographiques.

Resume des activites de l'équipe du laboratoire PETROCI, C&0circ;te d'Ivoire

N'DA L., Victor (Coordinateur national au niveau de la Côte d'Ivoire)- Societé Nationale d'Opérations Pétrolières (PETROCI), B.P.V. 194, Abidjan, IVORY COAST.

Les activités de l'équipe du laboratoire PETROCI sont essentiellement axées sur les analyses géologiques. L'objectif étant de recueillir des donnés biostratigraphiques et sédimentologiques indispensables à la recherche et au développement des gisements d'hydrocarbures.

Le bassin sédimentaire de la Côte d'Ivoire, constitue son principal terrain d'investigation. Ce bassin se situe au sud du territoire national, le long de sa façade maritime, entre les frontières ghanéennes et libériennes. Il s'étend sur environ 300 km d'Est en Ouest et fait partie des bassins côtiers Ouest-africains. La marge d'Abidjan constitue sa partie Orientale et la marge de San-Pedro, sa partie Occidentale.
Le bassin ivoirien est dans son ensemble une marge passive transformante typique, consécutive à l'ouverture de l'Atlantique Sud. Un accident majeur Est-ouest (Faille des lagunes) sépare la marge d'Abidjan en deux parties. Le secteur Nord, essentiellement terrestre, est peu épais; tandis que le secteur Sud, offshore est constitué de colonnes sédimentaires atteingnant plusieurs milliers de mÉtres.
Les sondages pétroliers profonds réalisés dans sa partie Est (marge d'Abidjan) ont permis d'obtenir un certain nombre de donnés stratigraphiques. Les différentes formations sédimentaires traversées par les forages ont été datées gr°ce à la Micropaléontologie, à la Palynologie et à l'étude des nannofossiles calcaires.
Les formations d'âge Albien moyen constituent les plus vieilles unités sédimentaires mises en évidence par ces analyses biostratigraphiques. Ces dernières sont exclusivement portées sur le Crétacé Supérieur afin d'en établir une biozonation et de faire des corrélations avec d'autres bassins.

ANNOUNCEMENTS

Working Group Proposal for IGCP 381: Cretaceous Continental Ecosystems Ismar de Souza CARVALHO - Departamento de Geologia, Instituto de Geociências, UFRJ, Cidade Universitária - Ilha do Fundão, 21910-240 Rio de Janeiro, RJ, BRAZIL.

During the Cretaceous Period climatic changes and geographic differentiation created numerous continental settings and, consequently, a diverse biota. Cretaceous strata of continental ecosystems may be studied in the geological record of most intracratonic Brazilian basins (such as Paraná, Sanfranciscan, Parnaíba, Amazonas, Solimões and Parecis), in the onshore northern marginal basins and in other South American basins that originated during the intensive Cretaceous tectonic activity.

Despite the extensive outcrop and core record, study of the continental deposits has been usually regarded as of subordinate importance owing to the presumed absence of significant mineral resources. This state of affairs has caused information to be the generated mainly in a punctual and essentially descriptive way, which makes it impossible to achieve a global comprehension of the environmental and biological interplay phenomena.

Objectives. The principal objective of this WG is to integrate information from different research branches which have, as a common goal, the study of Cretaceous continental environments. In this way, multidisciplinary studies, covering stratigraphy, petrography, geochemistry and palaeontology, will lead to a better reconstruction of the depositional settings and evolution of the Cretaceous continental ecosystems.

Work Stages. For the development of the studies in Brazil, the following data will be initially evaluated:

a. Aspects of the terrestrial biota in the northern inland basins (Souza, Uiraúna, Mangabeira, Cedro, Padre Marcos, Iguatú, Icó, Lima Campos, Malhada Vermelha, Rio Nazaré and Araripe). Definition of the geological setting for the occurrences of microfossils, ichnofossils, invertebrates, vertebrates and plant remains. The primary goal is the evaluation of the biostratigraphic ranges and applicability of the different fossil groups for palaeoecologica interpretations.
b. Distribution area of Cretaceous outcrops in the inland northern basins (Araripe, Cedro, Jatobá, Mirandiba and São José do Belmonte). The presence of vertebrate ichnofossils (dinosaur footprints) in this region, in rocks previously considered to be of Silurian-Devonian age, implies a wider distribution of Cretaceous deposits and a most peculiar regional geological history. It is proposed to carry out palynological studies and to establish geological and stratigraphical cross-sections for these outcrops, together with geological mapping of the occurrences.
c. Fresh-water ecosystems in the Sanfranciscan basin. Integration of palaeontological information from fresh-water deposits recovered in the Sanfranciscan basin. The most common fossils are ostracods, conchostraceans, plant and fish remains. It is planned to evaluate the potencial of these fossil groups for palaeoenvironmental analyses and in the biostratigraphic zonation of the basin.
d. Parnaíba and São Luís basins. The Cretaceous fauna and flora recovered from these areas reveal particular aspects of Gondwana history. There are few terrestrial specimens reported, and little information is available regarding their stratigraphy. It is proposed to evaluate the taphonomic characteristics of the fossil occurrences, their context in the local and regional settings and the geological mapping of the area. New species will be described.
e. Biogeographic affinities of the terrestrial fauna and flora between Brazil and Africa. It is known that several African species are morphologically identical to Brazilian ones, although they have been given different names. Integrated biogeographic studies of different research groups will lead to an adequate standardization of the systematics. A further goal is to verify the existence of tectonic "controls" in the regional variations of the continental ecosystems.

Final Remarks. This proposal is open to discussion and it may be extended to include any other area or sedimentary basin within the geographic framework of IGCP Project 381.

4th Symposium on the Brazilian Cretaceous Where: University of Rio Claro (UNESP), São Paulo, Brazil. When: August 1996. Duration: 4 days.

For more information write to: Dimas DIAS BRITO - Departamento de Geologia Sedimentar, UNESP, Av. 24-A no. 1515, 13506-900 Rio Claro, SP, BRAZIL.

High Latitude Southern Ocean Palaeontology Workshop

Expressions of Interest.

Where: Victoria University of Wellington, New Zealand. When: May 1996. Duration: 3 days.

Aims.

The primary aim of the workshop is to increase our practical understanding of the southern high latitude biotas and in particular the biostratigraphic relations between them. The major output of the workshop will be a compilation of the biostratigraphic zonations for as many fossil groups as possible.
It is recognised that in the time available for the workshop such a document would have to be considered to be very tentative. However, documentation of areas of agreement and disagreement between the various fossil groups should provide a guide for future research. This document should be published.
The end of 1996 marks the beginning of the Cape Roberts Antarctic Drilling project which makes this workshop extremely timely. The document to be produced at this meeting should provide a firm foundation for the Cape Roberts Palaeontology.

Accomplishing the Aims.

The workshop is entirely open and it is hoped that as wide a cross section of workers as possible (foraminifera, nannofossils, diatoms, radiolaria, dinoflagellates, spores and pollen) will attend. Participants will be encouraged to bring slides, photographs and samples for group discussion.
The workshop will open with a plenary session which will outline the aims of the workshop. The participants will then break up into separate disciplines to review material and discuss zonations. Microscopes will be available for each group to use in its review. A "reporter" will deliver the reviews to a plenary session on the final day. This session should work towards the production of the final workshop document. The reporters from each group together with the workshop conveners will be responsible for the final form of the document and its eventual publication.

Interested? Please send expressions of interest to the co- ordinators:

Dr Mike Hannah - Antarctic Research Centre/Research School of Earth Sciences, Victoria University of Wellington, Post: PO Box 600, Wellington, New Zealand. Fax: 64-4-495 5186; E-mail: michael.hannah@vuw.ac.nz
Dr I. Raine - Institute of Geological and Nuclear Sciences, P.O. Box 30-368, Lower Hutt, New Zealand. E-Mail: i.raine@gns.cri.nz

We are aware that there may be problems in getting to New Zealand, and we would be interested in hearing from people that would be interested in attending the workshop if it were held in another (?Northern Hemisphere?) location.

FIRST ANNUAL CONFERENCE OF IGCP PROJECT 381, SOUTH ATLANTIC MESOZOIC CORRELATIONS

Aracaju, Sergipe, Brazil 2-5 September 1996

The 1996 meeting will focus on ongoing research and the state-of-the-art of geological correlations and the bio-chronostratigraphic frameworks currently adopted for the South Atlantic Mesozoic sequences. The problems identified will prompt the proposal of clearly defined research topics and specific research targets for the forthcoming year(s).

Registration fees: Regular member (before 1 April 1996) $70, (after) $100
Student member (attach proof) (before 1 April 1996) $35, (after) $50
Payment by check, money order, or traveller's checks in US dollars.
If you are interested in participating and presenting a paper, or for more information, please fill in the enclosed form and send it to E. Koutsoukos or the Project Secretariat (addresses below).
Deadline for submission of an abstract for either oral or poster presentation is 1 May 1996.
Send your abstract in Microsoft Word, WordPerfect or Nisus format, on either Macintosh or IBM/Windows-compatible diskettes. Alternatively, it may be submitted by electronic mail to: >a Href=mailto:koutsoukos@cenpes.petrobras.gov.br> koutsoukos@cenpes.petrobras.gov.br. Please do not send it by fax. The abstracts should be written in English or French with a maximum of 1,000 words on two letter-sized (A4) pages.
Deadline for the submission of complete manuscripts for the proceeedings will be December 1996. These are expected to be published no later than September 1997, i.e., before the next SAMC Conference in Cameroon.

FIELD EXCURSIONS

Cretaceous sections are widespread in northern Brazil. Field trips will be organized to visit some of these sites, as well as to sample and investigate a typical rift-phase marginal basin (Recôncavo, Bahia) and the Cretaceous marine outcrop record (Sergipe, Pernambuco-Paraíba and Potiguar basins).

Pre-Conference Field Trip:

28-31 August (PR1): 'Cretaceous Marine Record and the K-T Boundary in Northeastern Brazil:

Sergipe, Pernambuco-ParaÁba and Potiguar Basins' Estimated cost: US$ 350.- Limited to 25 participants.

Post-Conference Field Trips:

6-9 September (PO1): 'Lower Cretaceous Rift Record in Northeastern Brazil: Reconcavo Basin' Estimated cost: US$ 400.- Limited to 25 participants.

10-12 September (PO2): 'Continental Cretaceous in Northeastern Brazil: Araripe Basin' Estimated cost: US$ 300.- Limited to 25 participants.
Estimated costs include van transportation, all meals from dinner on the first day through lunch on the last one, hotels (double occupancy) and an informal guidebook. All trips must be prepaid by 1 July 1996. Overbooked trips will be refunded; under booked trips (minimum of 10 persons) will be cancelled.

Outline of Next Project Meetings The locations for the next annual project meetings are as follows (provisionally):

1 Cameroon (1997), in conjunction with the 13th African Micropalaeontological Colloquium; For 1997 a European regional meeting is also being planned, to be held in Germany, simultaneously with the 2nd European Round Table on the Palaeontology and Stratigraphy of South America. For further information please write to Peter Bengtson (Geologisch-Paläontologisches Institut der Universität Heidelberg, Im Neuenheimer Feld 234, D-69120 Heidelberg, Germany, Tel.: +49-6221-568293 , Fax: 563940 or 565503, E-mail: Peter.Bengtson@urz.uni-heidelberg.de)
2 Argentina (1998);
3 Nigeria (1999);
4 South Africa (2000).
These are all key areas for the understanding of the geological links between West Antarctica, South America and Africa, and between the Austral and Tethyan realms.

Languages The official languages at SAMC meetings will be English, French, Spanish and Portuguese, i.e. the main languages of the countries bordering the South Atlantic.

Back to SAMC Index

Link to the Heidelberg Paleo-Page

This Page is maintained by Peter Bengtson and Jens Seeling

Last Update 30 January 1996