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.

Along the volcanic front of the Southern Volcanic Zone of the Andes between 33°S and 42°S the continental crust increases in thickness from south to north, and erupted lavas define several along-arc geochemical trends. These geochemical trends are the integrated effects of processes occurring in the subducted oceanic crust, the overlying mantle, and the continental crust. Major- and trace-element abundances and isotopic ratios are used to distinguish the effects of crustal and mantle processes. In the relatively thin crust region south of 37°S, lavas evolve dominantly by low-pressure crystallization and incorporate insignificant amounts of continental crust. Abundance ratios of most incompatible elements and isotopic ratios in these lavas reflect components derived from the mantle and subducted oceanic crust. Compared to basalts from north of 37°S, basalts from 37 to 42°S have higher CaO but lower Na 2 O and incompatible element contents. These differences are consistent with a north to south increase in degree of mantle melting, perhaps controlled by variable fluxing from the subducted slab (Rb/Cs ratios decrease from north to south), or variations in the thickness of the mantle column that undergoes melting (thicker crust in the north leads to a thinner mantle column and lower degree of melting). In the thicker crust region north of 37°S crustal contamination exerts a greater control on lava compositions. In the interval from andesite to rhyolite, upper crustal contamination causes Rb, Cs, and Th enrichment and isotopic variability. The evolution from basalt to basaltic andesite in this region occurs in the lower crust and crustal contamination causes enrichment in Rb, Cs, and Th and increased La/Yb. The most probable lower crustal protolith for the contaminant is a young, arc-derived garnet granulite.