--> ABSTRACT: Stratigraphic Architecture of a Greenhouse Carbonate Ramp, Cretaceous (Albian-Cenomanian) of Pecos River Canyon, Texas, by C. Kerans; #91021 (2010)

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Stratigraphic Architecture of a Greenhouse Carbonate Ramp, Cretaceous (Albian-Cenomanian) of Pecos River Canyon, Texas

KERANS, CHARLES

Milankovitch climate models predict that Greenhouse periods (ex. Cretaceous) can be characterized by low-amplitude (1-10 m) high-frequency (20-400 ky) eustatic shifts, distinguishing them from high-amplitude (30-200 m) icehouse settings. The rock record of this greenhouse eustatic signal should be enhanced on shallow-water carbonate ramps where vast areas lie within the depth range effected by small-scale eustatic shifts. The greenhouse Milankovitch signature on ramps is expected to include weakly developed 1D vertical cycle-stacking patterns and well-developed 2D facies-offset patterns.

The late Albian-earliest Cenomanian shallow-water shelf to intrashelf basin transect along the Pecos River Canyon provides a superb test of this predicted stratigraphic signature. Continuous exposure of shallow-water ramp to intrashelf basin environments along the canyon walls allows comparison of 1D, 2D, and 3D effects of greenhouse eustatic base level shifts on deposition. Widely-spaced (2-6 km) sections along the 60 km dip-oriented transect provide a seismic-scale architecture of composite- and high-frequency- sequence-scale cycles with detailed 3040 m by 2 km window study areas provide insight on the 1D and 2D stacking of high-frequency cycles.

The stratigraphic hierarchy of the Pecos Canyon area includes one late Albian to earliest Cenomanian composite sequence made up of four Albian and 2 Cenomanian high-frequency sequences (HFS). The HFS are the most clearly defined scale of cyclicity and are recognized by sharp flooding surfaces and lateral offset of HFS ramp-crest facies of between 5 and 15 km. High-frequency cycles nested within the HFS are commonly amalgamated and are only clearly defined using detailed 2D window maps where lateral facies shifts augment the minimal facies diversity of the ideal vertical cycle successions. One-dimensional analysis of greenhouse cycles that overlook this strong tendency towards 2D facies shifts may significantly underestimate reservoir-scale lateral heterogeneities.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.