The Greenhorn Cyclothem of the Cretaceous Western Interior Seaway: Lithology Trends, Stacking Patterns, Log Signatures, and Application to the Eagle Ford of West Texas
The Cenomanian-Turonian Greenhorn Cyclothem of the Cretaceous Western Interior Seaway has been extensively studied in Colorado and adjacent states, thereby making it an ideal dataset for generating sequence model that links shallow-marine siliciclastic sediments to pelagic carbonates. The Greenhorn Cyclothem, also known as the T6-R6 cycle, consists of a transgressive lower portion and a regressive upper portion that, in axial portions of the basin has a carbonate-rich member (Bridge Creek Limestone) that was deposited near peak transgression, from latest Cenomanian into the Lower Turonian. In the 1970s, Erle Kauffman recognized that the stratigraphic stacking pattern was the product of Waltherian stacking of facies during the T6–R6 cycle. The pelagic carbonates of the Bridge Creek represent the most distal, siliciclastic sediment starved, facies in the basin.
In this paper, I elaborate on Kauffman's cyclothem concept by showing how stratigraphic stacking patterns might differ between distal and proximal areas. I also suggest that distinctive wireline log signatures should be observable in each of these areas, and illustrate the applicability of the conceptual model using wireline log data from Kansas, Colorado and New Mexico.
The Eagle Ford Formation of Texas was deposited at the same time as a the Greenhorn Cyclothem further north, and herein I illustrate how the stratigraphy of the Eagle Ford in West Texas outcrops might be interpreted using the conceptual model derived for the Greenhorn Cyclothem further north. In particular, the interbedded limestones and marls of facies C (BP designation) can be interpreted to represent peak transgression at the Cenomanian-Turonian boundary. In both the Colorado area and West Texas, deviations from the idealized model are useful for deciphering the impact of different forcing mechanisms (e.g., subsidence, siliciclastic sediment supply) on sequence development.
AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015