MICROSPHEROID ACCUMULATIONS AND GEOCHEMICALCHARACTERISTICS OF A CENOMANIAN-TURONIAN ANOXIC BASIN:THE RECORD OF THE INDIDURA FORMATION, NE MEXICO

摘要

Cyanobacteria and/or bacteria are a major part of the biomass, although their recognition as significant constituents of the sedimentary record has largely b overlooked in rocks other than Precambrian and Paleozoic stromatolites. Their extremely small size has been one of the major obstacles in the recognition of such constituents, which can be observed properly only at high-resolution SEM imaging. Here we present evidence of accumulation of cyanobacterial "microspheroids" as predominant components of sediments of the Cenomanian–Turonian deposits in the "Sierra de Parras", northeastern Mexico, during an interval of predominantly dysoxic to anoxic conditions. The stratigraphic section includes a sequence of limestones and marls with well-defined rhythms at the decimeter to millimeter scale. This facies shows internal structures that are arranged in nearly even-parallel "varve-like" dual laminae less than 3 mm thick. A few scattered planktonic foraminifera and radiolaria occur in the dark laminae, while the light laminae are composed almost entirely of microspheroids. Total carbonate (CaCO3) content varies from 43.0% to 78.3%, and TOC is relatively high, between 0.3% and 3.6% (consistently higher than 1.6%), suggesting an environment favorable for preservation of organic matter. Inorganic-element concentrations (Mo, V, Cr) suggest that the sequence at Parras accumulated in a dysoxic to anoxic environment in which microbial communities were predominant, as also revealed by petrographic and SEM analyses. Microfacies reveal that compositional differences in the laminae are associated with varying abundance of cyanobacterial "microspheroids". The distinctive laminae are the result of recurring cycles of calcareous cyanobacteria blooms, which remained dominant throughout the sedimentary sequence. Organic-carbon-rich black shales and limestones of the Parras region further document unique paleoceanographic situations during the early Late Cretaceous, when strong intermittent dysoxic or anoxic bottom conditions developed at the site of the Parras deposits and were associated with rhythmical production of cyanobacteria.