Characterization of Lacustrine Carbonate Microbialite Facies Associations of the Lower Cretaceous Codó Formation (Northeast Brazil)

Bahniuk, Anelize; McKenzie, Judith A.; Anjos, Sylvia; Barros, Almério; Vasconcelos, Crisogono

The Codó Formation, located in the Parnaiba Basin of Northeast Brazil, contains a unique stratigraphic sequence of up to 20-m-thick, well-preserved carbonate microbialites deposited in a closed Upper Aptian lacustrine basin during the initial break up and separation of South America from Africa. Detailed characterization of the microbialite facies yields diverse microbially produced structures, which are systematically related to specific depositional states associated with a shallow, low energy, subaqueous, hypersaline environment, occasionally exposed to meteoric and/or capillary conditions. Based on macroscopic and microscopic analyses, four microbialite facies, which express detailed textures and exceptional microbial fossil content, were defined. These microbialite facies, designated as stromatolite, laminae, massive and spherulite, can be related to changes in paleo-depth of the lacustrine environment as reflected by 87Sr/86Sr cycles, which have also been observed in the directly overlying evaporite units of the Codó Formation. Clumped isotope measurements of selected fabrics yield precipitation paleo-temperatures with an average value of 37.5°C. The δ18O values of the bulk carbonate (-6.8 to -1.5 ‰ VPDB) imply precipitation from water with calculated δ18O values between -1.6 and 1.8 ‰ VSMOW, possibly reflecting precipitation from variably modified meteoric waters, whereas the δ13C values of the bulk carbonate (-15.5 to -7.2 ‰ VPDB) indicate a significant input of carbon derived from aerobic or anaerobic respiration of organic matter, suggesting precipitation in a semi-enclosed or isolated water body. Combined, our data allow us to interpret the evolution of the Codó Formation as occurring in a closed lacustrine paleoenvironment with alternating episodes of contracting and expanding lake levels, which led to the development of specific microbialite facies associations with varying conditions. Our evaluation confirms previous studies that the Codó Formation was deposited under warm and arid paleoclimate conditions with high evaporation. In summary, our study demonstrates that new insights can be achieved to better evaluate paleoenvironmental conditions and early diagenetic processes involved in ancient microbialite formation.

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