Significance of Organic Carbon and Bulk Nitrogen Fluctuations Across the Cenomanian-Turonian Boundary, Eagle Ford Formation, Maverick Basin, Texas

Abstract

A stratigraphic record of organic matter deposition spanning late Cenomanian through at least early Turonian time was reconstructed from a drill core recovered from the Maverick Basin, South Texas. The strata (Eagle Ford Fm) record 1) the depositional evolution in an anoxic/euxinic shelf setting following a hiatus, represented by the contact with the underlying Buda Fm, 2) a shift to more oxygenated conditions after the initiation of the OAE-2 (as defined by a positive shift in δ13CTOC), and 3) variably oxygenated (poikiloaerobic) conditions throughout much of the post-OAE-2 succession. Approximately 170 ft of TOC-rich (average 3%, maximum 6.5%) lower Eagle Ford (LEF) strata are subdivided into two sub-equal zones: a lower zone with variable δ13CTOC values (ranging from −28.9 to −25.9‰), and an upper zone with a largely unidirectional δ13CTOC trajectory that upwardly trends to less depleted values (~−28.1 to ~−26.8‰). Interestingly, the abrupt shift between the two zones coincides with the apex in TOC/N ratio, suggesting that a shift in organic matter source or degradation occurred at this time. The bulk organic characteristics of the upper Eagle Ford (UEF) are significantly more variable. Redox-sensitive trace elemental results show that the OAE-2 began at the LEF-UEF boundary, when the concentration of sedimentary molybdenum (a proxy for euxinia) greatly diminished. The OAE-2 interval (lowermost portion of UEF) spans a thickness of ~70 ft, and yields a δ13CTOC range of ~4‰. The upper 230 ft of core (UEF and lower Austin Fm) is defined by highly variable TOC (maximum 4%), an upwardly-increasing trend in TOC/N, and variable δ13CTOC (range −28 to −26‰). The majority of bulk rock δ15Ntotal values fall between −5 and −2‰, which is a typical range for strata of this age. The most interesting feature of the nitrogen isotopic record is an abrupt decrease in δ15Ntotal of several permil at the apex of the δ13CTOC record in the OAE-2 interval. Following the depletion of nitrate inputs from others sources, this decrease may represent a unique set of water mass conditions under which nitrogen fixation by diazotrophic cyanobacteria was the sole process responsible for bringing nitrogen into the system. The significance of the new record will be discussed with respect to the many existing records of Cenomanian-Turonian paleoceanographic change.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015