Abstract 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 (CaCO 3 ) 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. Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. Jones SEPM Special Publication No. 93, Copyright © 2009 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-137-7, p. 171–186.

Abstract The Jurassic sequence of northeastern Mexico includes Oxfordian through Tithonian-aged marine sediments that were deposited in eight environments: (1) sabkha, (2) nearshore, (3) lagoonal, (4) shoal, (5) inner-shelf, (6) restricted-shelf, (7) open-shelf, and (8) pelagic. Lithologic correlations relate the sections studied with the type sections of coeval rocks described from the Sierra de Parras. Based on their lithic characteristics they are assigned to the Minas Viejas, Zuloaga, La Gloria, La Caja, La Casita, and San Angel Formations. Three biohorizons are established: (1) the first stratigraphic occurrence of the ammonite Idoceras in the early Kimmeridgian, (2) the first appearance of calpionellids in the middle Tithonian, and (3) the first occurrence of Calpionella elliptica Cadish at the Jurassic/Cretaceous boundary. Two lowerings of sea level are identified in the sections studied, the first in early Kimmeridgian and the second in the early Tithonian. These two drops of sea level are separated by a sea-level rise in the middle Tithonian. Eustatic changes of sea level are associated with tectonism in the area. Active transform faulting in the Gulf of Mexico region resulted in the juxtaposition of Oxfordian-Kimmeridgian shallow- and deep-water facies. The translations along transcurrent faults result in a SE-NW displacement of about 300 kilometers, which caused the formation of the Sabinas Basin of northeastern Mexico. Late Jurassic sea-floor spreading in the central Gulf of Mexico is regarded as the driving mechanism. The Oxfordian-Kimmeridgian and late Tithonian paleogeographies of northeastern Mexico are reconstructed, and parallel lineal facies belts are envisioned which account for the sporadic distribution of the Jurassic shallow-water facies in northeastern Mexico.