--> Paleogeographic and Climatic Controls of Rift and Sag Stages in Eastern Brazilian Basins, Azambuja, Nilo C.; Regali, Marilia; Neves, Igor, #90100 (2009)

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Paleogeographic and Climatic Controls of Rift and Sag Stages in Eastern Brazilian Basins

Azambuja, Nilo C.1
 Regali, Marilia1
 Neves, Igor1

1HRT, Rio de Janeiro, Brazil.

Seven main paleogeographic, lithological and palynological variations are observed during Lower Cretaceous rift and sag phases.

1- The pre-rift phase, Dom João stage, comprises fluvio-eolian sandstones and red beds deposited in ephemeral lakes. The palynoflora is dominated by rimulate gymnospermous pollen grains that indicate arid growth situations.

2- The succeeding phase, Rio da Serra and Early Aratu local stages, is characterized by the initiation of deep lake sedimentation. The palynoflora, represented by pteridophytic trilete spores and gymnospermous bisaccate pollen grains, implies predominance of more humid conditions.

3- During the Early Aratu and Early Buracica stages a large-scale rift filling occurred, with extensive fluvio-deltaic and shallow-lacustrine deposition. The palynoflora consists of a diversity of inaperturate, polyplicate, and rimulate pollen grains and subordinate trilete spores; collectively these indicate a semi-arid environment, in this time appears Tucanopollis the first angiosperm.

4- This fourth phase, Late Buracica and Early Jiquiá stages, reflects a continuation of deltaic and shallow-lacustrine sedimentation. The palynoflora is characterized by diminution of rimulates concomitant with increasing polyplicate and with first appears of new genus of angiosperms (Stellatopollis and Afropollis), decrease bisaccate pollen grains (the latter linked to the Pinaceae) and increase of cicatricosisporites. This assemblage connotes resumption of more humid conditions.

5- During the succeeding phase, Latest Jiquiá stage, coquina beds and black shales were accumulated in shallow lakes. Rimulate pollen grains, indicative of semi-arid conditions, dominate the palynoflora.

6- A regional unconformity marks the base of this phase, dated as Early Alagoas local stage, and characterizes the end of the main rift phase. This succession consists of conglomerates at the base and succeeded by black shales. The palynoflora is marked by the entrance of angiospermous pollen grains (monocolpates mainly), representing a return to milder and humid conditions.

7- During the Late Alagoas stage were deposited sandstones, shales, carbonates (microbiolities) and up to 2000m of evaporites. The palynoflora is dominated by rimulate and polyplicate (gymnospermous), and some tricolpates (angiospermous) pollen grains that are together suggestive of aridity. Marine incursions took place and ultimate the effective emplacement of the
South Atlantic Ocean.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil