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Late Cretaceous (Cenomanian-Coniacian) calcareous nannofossil biostratigraphy and paleoecology of the Western Interior Basin, USA
Analysis of calcareous nannofossil biostratigraphy and paleoecology is performed for eight sites within the central (Mesa Verde National Park, CO; Rock Canyon Anticline, Pueblo, CO; Rt. 36, Cuba, KS) and southern (Lozier Canyon, Terrell Co., TX; industry drillcores, Webb Co., TX; ACC #1 Core, Austin, TX) Western Interior Basin (WIB). A study of nannofossil first and last occurrences from these sites in the WIB and coeval successions in the literature results in a high-resolution optimum sequence of events for the Late Cenomanian through Coniacian using the statistical method of Ranking and Scaling (RASC). Additional taxonomic work on the transitional forms in the Eprolithus-Lithastrinus lineage provides reliable criteria for distinguishing two taxa, Eprolithus moratus and Lithastrinus septenarius, for more accurate use in this sequence. The RASC optimum sequence is contrasted with the placement of key nannofossil bioevents in the revised 2012 edition of the geologic timescale (Ogg and Hinnov 2012) and the widely used CC (Perch-Nielsen, 1985) and UC (Burnett 1998) zonations. Inconsistencies between the RASC sequence and zonation schemes are shown to be the result of revised taxonomy and species concepts, as well as changes to the placement of the Turonian/Coniacian boundary in the new timescale. ^ This high-resolution, quantitatively derived framework allows for a detailed investigation of nannofossil assemblages through the Cenomanian-Turonian Boundary Event, or Oceanic Anoxic Event 2 (OAE2). Prevailing theories suggest increased carbon burial during OAE2 was the result of deep water anoxia, driven either by elevated primary productivity at the surface or enhanced preservation due to sluggish, stratified oceans (Schlanger and Jenkyns 1976; Schlanger et al. 1987; Arthur et al. 1988; Erba 2004; Snow et al. 2005). Analysis of nannofossil fertility proxies (Watznauria, Biscutum, Zeugrhabdotus), total organic carbon (TOC) values, and carbon isotope records indicate oligotrophic surface water conditions existed through the latest Cenomanian and earliest Turonian, suggesting a highly stratified water column, not higher productivity, resulted in anoxic conditions during OAE2. If productivity was not the primary cause of increased carbon burial in the WIB it is probable that increased terrestrial runoff and incursion of warm, saline Tethyan waters from the south led to salinity stratification.^
Corbett, Matthew J, "Late Cretaceous (Cenomanian-Coniacian) calcareous nannofossil biostratigraphy and paleoecology of the Western Interior Basin, USA" (2013). ETD collection for University of Nebraska - Lincoln. AAI3559477.