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Dark-Brown Laminated Deposits in Shallow-Water Carbonates During the OAE2: Shallow Water “Black-Shales”?


The Oceanic Anoxic Event 2 (OAE2), spanning the Cenomanian-Turonian boundary, was characterized by the extensive deposition of organic-carbon rich black shales in ocean basins worldwide. The sedimentological, geochemical and paleontological aspects of deep-water expressions of OAE2 have been intensively studied in the last few decades, whereas much less attention has been paid to coeval shallow-water deposits. The Southern Apenninic Platform preserves a continuous record of shallow-water carbonates through the OAE2, offering the unique opportunity of looking at the archive of paleoenvironmental changes in a carbonate platform that grew at tropical latitudes far from the influence of a any continental block. Here we present the results of a detailed facies, petrographic (optical microscope and SEM) and geochemical study on two key shallow-marine sections, using biostratigraphy and carbon-isotope stratigraphy to establish the time-framework corresponding to OAE2. The most characteristic sedimentological features found in the studied sections are represented by laminated, dark-brown, cm-thick limestones interbedded with muddy lagoonal carbonates. These laminated deposits are fine-grained and relatively enriched in organic carbon respect to the background lagoonal deposits. They cluster together forming two discrete intervals in the sections, which occur in distinct stratigraphic positions with respect to the carbon-isotope profile of OAE2. The first cluster slightly predates and straggles the onset of OAE2, whereas the second one occurs in the so-called plateau phase of the positive carbon-isotope excursion where d values are relatively constant. Petrographic and geochemical analyses suggest a bacterial origin for these more organic-rich beds. The good correlation between these deposits and deep-water black shales interval suggest that they are different expressions of the same environmental perturbations occurring in two disparate physiographic settings. Moreover, our study suggests that relatively high-TOC facies during the OAE2 could also be preserved in very shallow carbonate-platform settings.