Microbialites in the Codó Formation, Parnaíba Basin, Brazil

Abstract

The Codó Formation, located in the Parnaíba Basin, Northeast Brazil, contains a unique stratigraphic
sequence up to 20-m-thick, composed of well-preserved carbonate microbialites deposited
in a closed Upper Aptian lacustrine basin during the initial break up phases of the South American
and African Continents. The depositional system of microbialites is associated with a shallow, low
energy, subaqueous and hypersaline environment,occasionally exposed to meteoric and/or capillary
conditions. Based on macroscopic and microscopic
analyses, four microbialite facies were identified. These microbialite facies, designated as stromatolite,
laminae, massive carbonate and spherulite, could be related to changes in paleo-depth of the
lacustrine environment. Clumped isotope measurements of selected fabrics yield precipitation paleotemperatures
with an average value of 37.5°C.The d18O values of the bulk carbonate (-6.8ppmto -1.5ppm VPDB) imply precipitation from water with calculated d18O values between -1.6ppm and1.8ppm VSMOW, possibly reflecting precipitation
from variable modified meteoric waters, whereasthe δ13C values of the bulk carbonate (-15.5ppmto -7.2ppm VPDB) indicate a significant input ofcarbon derived from aerobic or anaerobic respirationof 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 paleo-environment with alternating episodes ofcontracting and expanding lake levels, which led to the development of specific microbialite faciesassociations. Our evaluation confirms previous studies
that the Codó Formation was deposited underwarm and arid paleo-climate conditions with high evaporation.