Using Trace Fossils to Determine the Role of Oceanic Anoxic Event II on the Cretaceous Western Interior Seaway Paleoenvironment

by Jacob Frederick Grosskopf

Institution: Louisiana State University
Department: Geology & Geophysics
Degree: PhD
Year: 2015
Keywords: seaway; anoxic event; Cretaceous
Record ID: 2057892
Full text PDF: http://etd.lsu.edu/docs/available/etd-04012015-135304/


Did well-oxygenated benthic conditions exist in all parts of the Cretaceous Western Interior Seaway (CWIS) during the deposition of Bed 63 at the onset Oceanic Anoxic Event II (OAE II), as suggested by findings from two cores from the central portion of the CWIS? This problem was approached by categorizing the bioturbation that is prevalent through Bed 63 into oxygen-related ichnofabrics (ORI) in order to describe benthic oxygen conditions. These data were collected from outcrop and core among seven different sites along WestEast and NorthSouth transects that traversed portions of the CWIS with differing depositional settings and proximities to the global ocean during OAE II. ORI results from both transects indicated oxygenated benthic conditions for the duration of Bed 63 throughout all parts of the CWIS. Were these well-oxygenated benthic conditions consistent for the duration of OAE II in the CWIS? This problem was approached by examining ORIs from synchronous horizons, represented by laterally-extensive limestone beds found through the OAE II interval along the WestEast transect. Among the six sites, it was determined that the limestone beds indicate well-oxygenated benthic conditions at the onset of and early into the OAE II interval, while ORIs slightly dropped up-section. Also, variable ORIs were recognized in thick limestone beds, which may have been controlled by precessional-scale orbitally-forced long-term climate effects. However, competing phases of climate-forced carbonate deposition limited the ability to determine the temporal variability among the ORIs. Additionally, an alternate interpretation was established for the onset of OAE II at Antonio Canyon outcrop, near Langtry, TX, in southern portion of the CWIS. The onset of OAE II at this locality was correlated to CenomanianTuronian Boundary GSSP reference section and time-equivalent Portland Core using recognizable patterns in CWIS lithology, and ÷, GRS, total organic carbon, and ä13Corganic and ä13Ccarbonate curves from the Antonio Canyon section. More work is needed to locate the CenomanianTuronian Boundary and the end of OAE II in the Antonio Canyon outcrop.