Facies Architecture and Sequence Stratigraphic Context of a Fluvial Succession, Barbalha Formation (Aptian), Araripe Basin, Brazil

Abstract

The Aptian Barbalha Formation record deposition in a fluvial and lacustrine environment accumulated in a sag basin developed during the early post-rift in the Araripe Basin, northeastern Brazil. The development of depositional sequences in this unit reflects variation in the accommodation-to-sediment supply (A/S) ratio. Two depositional sequences, showing an overall fining-upward trend, are preserved within the succession. The sequences are bounded by regional subaerial unconformities formed during negative A/S ratio, and may be subdivided in Low-accommodation Systems Tracts (LAST) (positive A/S ratio close to zero) and High accommodation Systems Tracts (HAST) (A/S ratio between 0.5 and 1). Variation in A/S ratios must be related to tectonic subsidence and uplift of the basin, despite the post-rift setting. Sequence 1 is characterized by amalgamated, multistorey and multilateral, braided fluvial channel sandbodies, defining a LAST. These are interlayered with crevasse splay and floodplain deposits towards the top, passing to open lacustrine deposits, defining a HAST. The end of the first fining-upward cycle is marked by organic-rich shales interbedded with microbial carbonate laminae, coprolites, ostracodes, fish remains and carbonaceous plant debris. The thin (< 10 m thick), laterally persistent black shales are a significant marker in the basin. Total organic carbon in these shales is up to 24 wt.%, and they are correlative to organic-rich layers in the Ceará and in the Potiguar Basins, comprising potentially important source rocks. Sequence 2 overlies the organic-rich lacustrine deposits. At the base, this sequence is composed of amalgamated, multistorey and multilateral braided fluvial channel sandbodies (LAST), similar to Sequence 1, overlain by anastomosed fluvial deposits capped by lacustrine deposits, both grouped in a HAST. The second lacustrine cycle culminated with the precipitation of laminated (“papery”), micritic limestones of the Crato Member (basal Santana Formation). Paleocurrent data on fluvial deposits of sequences 1 and 2 shows a consistent pattern indicating paleoflow to the SE. Sedimentological evidence indicates humid to sub-humid climatic conditions during deposition of sequences 1 and 2.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015