55-9 Insights into the Presence, Distribution, Thickness, and Duration of the OAE2, across Texas

Session: Sequence Stratigraphic, Geochemical, and Geochronologic Correlation of the Cenomanian-Turonian Ocean Anoxic Event 2 (OAE2) in the Cretaceous Western Interior Seaway (KWIS) and the Gulf Coast

Presenting Author:

Authors:

Abstract:

At the Turonian GSSP in Rock Canyon (Pueblo, Colorado), the distinctive positive δ¹³C isotope signature associated with the OAE2 occurs over a 20’ (6.1 m) interval at the base of the Bridge Creek Member of the Greenhorn Formation.   Similarly, in Lozier Canyon (Terrell County, Texas), the distinctive positive δ¹³C isotope signature associated with the OAE2 occurs over a 30’ (9.1 m) interval at the base of Upper Eagle Ford Formation .   At both localities, the base of OAE2 coincides with by a sharp drop in TOC, Uranium, & Gamma Ray (GR) values due to a change from organic-rich carbonate mudstone (below) to organic-poor interbedded limestone and mudstone (above).  

Research on a recent USGS borehole in the vicinity of Dallas, Texas also contains the distinctive positive δ¹³C isotope signature associated with the OAE2.  However, unlike at Rock Canyon and Lozier Canyon, in this borehole, the OAE2 is 235’ (71.62m) thick.   Even more remarkable is the fact, that based on the ammonite work on the adjacent outcrops, the portion of the OAE2 which is present in this Dallas core is restricted to the basal Late Cenomanian “Scip” (Sciponoceras gracile) Ammonite Zone, within the OAE2, which occurs in the basal 6’ (2m) of the  OAE2 at Rock Canyon (Pueblo, Colorado).   The Dallas area outcrop and core data also documents the presence of an Early Turonian unconformity at the top of the OAE2,.   Recent research has found that the OAE2 is 12’ (3m) thick in Waco area, and absent in the Austin area due to truncation by this Early Turonian Unconformity at the top of the OAE2.

The research in Texas indicates that the OAE2 is an unconformity-bounded sequence.  The onset of the OAE2 in Texas also coincides with an abrupt onset of siliciclastic input into the East Texas Basin.  The presence, distribution, thickness and duration of the OAE2 in the East Texas Basin, however, appears to be mainly controlled by the unconformity at the top of the OA2. The recognition of this, and other unconformities, within the Eagle Ford Group suggests that mudstone-prone successions within shallow epicontinental seaways may not be always be conformable.  This fact impacts any attempt to define astronomically-calibrated cycles, or sedimentation rates, within these successions.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.