Sedimentological and Stratigraphic Analysis of Aptian Non-Marine Carbonates of the Southern Campos Basin, Brazil
The establishment of Aptian sedimentological and stratigraphic models is one of the most exciting challenges in the study of South Atlantic Passive Margin Basins because they provide an understanding of the transitional phase of the South Atlantic opening. The early Aptian stratigraphic interval overlies the Gondwana break up unconformity and lies beneath a thick layer of late Aptian evaporites. The Aptian sediments have an average thickness of 200m and are characteristic of deposition in half-graben and sag basins, in a restricted shallow-water environment. The facies distribution normally shows clastic alluvial fans close to the shoreline and shallow-water carbonates basinward. High-energy depositional facies are associated with sub-aqueous sites at the zone of ancient wave base, close to the platform edge.
Combined sidewall core data, FMI and Gamma Ray obtained from several wells drilled on a nearshore to offshore transect, as well as modern and ancient outcrop analogues, were used to classify facies, construct a facies model and identify high-frequency sediment cycles. Shallow-water carbonates are dominated by microbial facies that accumulated in emergent and sub-aqueous settings. Unequivocal open marine facies have not been seen.
This early to middle Aptian (117 to 123Ma) interval forms one regressive - transgressive 3rd order cycle. Within this, three 4th order depositional sequences are identified that shallow-upwards. High frequency cycles (5th order) are also interpreted and these normally start with a sub-aqueous facies association and culminate with emergent facies with dessication cracks and tepee structures. Occasionally subaerial features such as breccias are observed; and these are most common nearshore or on structural highs.
These carbonate sediments accumulated in a shallow and restricted, non-marine environment, during the transitional phase of the South Atlantic opening. However, facies distribution and geometry are controlled mostly by extensional tectonics and relative sea-level changes, which establish a basis for facies prediction.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009