Sedimentary Facies and Palaeoenvironmental Records of an Intracratonic Basin Lake: Aptian Lacustrine Crato Formation, Jatobá Basin, NE Brazil

Neumann, Virginio H.; Rocha, Dunaldson; Vortisch, Walter; Gratzer, Reinhard; Lima, Mario; Barbosa, Jose A.; Fambrini, Gelson

The purpose of this study was to characterize the Aptian lacustrine sequence of the Jatobá Basin in Northeastern Brazil. Emphasis was placed on the carbonate sedimentation within the Crato Formation of the Santana Group. We analyzed lithofaciological, petrographic, paleontological, geochemical, and paleoenvironmental aspects of samples collected in twelve outcroppings, in addition to the well. Forty-seven sections were selected and studied: twenty-nine from the stratigraphic well, and eighteen from the outcroppings. Of these samples, thirty-eight represent carbonates and nine from sandstone and silt rock layers. Study of the well core permitted identification, in the lacustrine sequence, of three facies associations: a) association of deltaic facies consisting of three lithofacies, b) association of terrigenous lacustrine facies, comprised of eleven lithofacies, and c) association of carbonate facies, consisting of four groups of microfacies. The combination of carbonate facies is made up of the following lithofacies: clay-carbonate rhythmites, where two microfacies were individualized; limestone layers consisting of eight microfacies; the marls and, finally, the bioclastic limestones, also represented by only one microfacies. Twelve species of ostracods were identified, belonging to nine genera from four distinct families. Of particular note was the Ilyocyprididae family with six species, followed by the Cyprideidae family with four species, along with the Darwinulidae and Limnocytheridae families, each with one specie. The results of X-ray diffraction showed a prevalence of calcite in the limestone layers. Expansive clays were identified in the shales, with a predominance of interstratified illite-smectite followed by illite, kaolinite, and chlorite. Calcite is sometimes present, essentially as cement. From the biostratigraphic and isotopic studies associated with the faciologic studies of the Crato formation, it can be suggested that the Aptian Paleolake of the Jatobá Basin is best characterized as a region typified by an elevated lake with low salinity, having the characteristics of a hydrologically closed lake, that has passed through short periods of time as an open lake. Indications are that freshwater and brackish water predominated (oligohaline to mesohaline), with high levels of organic matter, along with intense bacterial activity (eutrophic lake), where a broad diversity of ostracod species developed, confirming its Aptian age.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013