55-5 A Sequence Stratigraphic Perspective of the OAE2 in the Denver Basin

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:

In the Denver Basin, the  positive δ¹³C isotope (OAE2) anomaly, associated with the Cenomanian/ Turonian boundary, occurs within the basal portions of the Bridge Creek Member of the Greenhorn Formation.  While this geochemical anomaly is recognized in numerous outcrop and core studies, in this and other basins, petrophysical (wire-line log) data has never previously been used as a proxy to regionally map this anomaly within a basin.  In this study, three wells with cores that span and contain the positive δ¹³C geochemical anomaly of the OAE2 were tied to their respective geophysical logs. The petrophysical characteristics of OAE2 interval on these wells was then used as a proxy to define and correlate the OAE2 interval, as well as adjacent strata, on a grid of well-log cross-sections across the study area in Denver Basin.

The results of this study indicate that the Bridge Creek Member of the Greenhorn Formation can be sub-divided into three (3) unconformity-bounded depositional sequences, herein termed the Lower, Middle, and Upper Bridge Creek Beds.  Within this framework, the OAE2 interval spans the Lower and Middle Bridge Creek Beds.  Regional truncation of underlying Harland strata documents the unconformity at the base of the OAE2.  Across most of the western portions of the Denver Basin the OAE2 interval is 20 to 40 feet (6.1 to 12.2 meters) thick.  In the eastern portions of the Denver Basin the OAE2 interval is generally less than 20’ (6.1 meters) thick.  However, in portions of Yuma, Phillips, Washington, Sedgwick, and Logan Counties in Colorado, as well as in Cheyenne and Kimball Counties in Wyoming, the OAE2 interval is absent due to the sequential truncation of the Middle Bridge Creek Beds, and  Lower Bridge Creek Beds by an unconformity at the base of the Upper Bridge Creek Beds.  Regional truncation beneath the Middle Creek Beds also documents the presence of an unconformity within the OAE2 interval.

In summary this research indicates that the OAE2 interval within the Denver Basin is a unconformity-bounded succession, which is not internally conformable. The presence of these unconformities at the base, top, and within the OAE2 clearly impacts any discussions on the onset, termination, duration, completeness, and causal mechanisms of the OAE2 in the Denver Basin and elsewhere.

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

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