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ABSTRACT: High-Frequency Climatic Cycles: Transgressive System Tract Turbiditic Deposits in a Rift Succession, Alagoas Basin/Brazil

Arienti, Luci Maria , Petrobras, Rio de Janeiro, RJ, Brazil

The Aptian rift succession of the Alagoas Basin is represented by four main facies: fan deltas associated with the basin border fault; axial fan deltas; turbiditic channel and overbank deposits (Morro do Camaragibe Outcrop); lobe and interlobe deposits (Tabuleiro dos Martins oil/gas field); and lacustrine deposits, represented by calcilutites, black shales and evaporites. During humid periods, very-high siliciclastic input into the basin produced fan deltas and turbidites, representing a transgressive system tract. The shales associated with the turbiditic sandstones contain terrestrial organic matter, such as coal debris and palynomorphs. In contrast, during the arid periods algal lacustrine shales were deposited, containing up to 17% of TOC, associated with algal mats, mudcracks and evaporites, representing the lowstand system tract. The succession was divided into two 3rd order sequences: Maceio and Pocoo/Ibura. The Maceio Sequence was divided into three 4th order sequences: Alpha, Beta and Gamma, each about 200 m thick, internally divided into 5th and 6th order sequences. The 3rd order sequences were mainly controlled by tectonic activity, while the 4th and 5th order ones were controlled by climate, which regulated the water level and the sediment influx into the basin. This rift succession has the main reservoir potential generated during the transgressive system tract, with the deposition of thick turbiditic sandstone deposits. Lowstand sediments are characterized by absence of sand-rich units.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia