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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 52 (2002), Pages 501-516

Styles of Rudist Buildup Development along the Northern Margin of the Maverick Basin, Pecos River Canyon, Southwest Texas

Kerans, Charles

ABSTRACT

The Pecos River Canyon exposures of upper Albian (Cretaceous) limestones provide a spectrum of styles of rudist reef development. These rudist buildups range in style from isolated mounds with up to 15 ft (5 m) of local relief to hydrodynamic accumulations within tidal inlet fills and along the foreshore-upper shoreface transition. Rudist buildups also range widely in terms of faunal content, and in rock fabric and porosity-permeability characteristics. This study presents an analysis of these buildups and a means for predicting style and reservoir quality of the buildups by placing them in a sequence stratigraphic framework.

The study area occurs along the lower 60 mi of the Pecos River Canyon area where 35 measured sections have been used to construct a detailed stratigraphic framework of the upper Albian section as it is traced from platform interior facies (Fort Lancaster or Segovia Formations) through intrashelf-basin margin (Devils River Formation) into the intrashelf basin (Salmon Peak Formation). A stratigraphic hierarchical analysis suggests that the interval included within the study, bounded below by the Fort Terrett Formation and above by the Del Rio and Buda Formations, represents a single composite sequence that in turn is made up of the six latest Albian high-frequency sequences.

Considering an ideal model for a single high-frequency sequence, it can be shown that the transgressive systems tract (TST) is dominated by mud-rich facies that contain low-relief pancake-shaped buildups with a core colonizing community of chondrodontid clams and a capping facies of radiolitid rudist rudstone/bafflestone. In contrast, highstand systems tract (HST) deposits contain large accumulations of caprinid rudists that are clustered as hydrodynamic piles within tidal inlets in tide-dominated settings and as lags along the upper shoreface of wave-dominated sequences. The high-energy highstand setting of the caprinid buildups makes the reservoir quality of these facies far superior to their chondrodont-radiolitid counterparts in the mud-dominated TST. The comparison of highstand rudist accumulations from early transgressive sequence set (TSS) sequences to later highstand sequence set (HSS) sequences within the upper Albian composite sequence illus trates a different trend. High-frequency sequences in the TSS have highstand caprinid rudist complexes with greater dip dimension, coarser grain size, and higher interparticle porosity than their wave-dominated HSS counterparts. Thus knowledge of setting within the sequence/systems tract framework allows prediction of rudist buildup type, gross external geometry, and rock fabric/reservoir quality.


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