Lithofacies, Stratal Architecture, and Depositional Model of an Inner Platform Transgressive Rudist Complex, Basal Edwards Formation, West- Central Texas

A. Matthew Oyedeji and Peter F. Holterhoff
Department of Geosciences, Texas Tech University

The Fredricksburg Group of west-central Texas is divisible into a northern, open platform facies belt (an extension of the East Texas Basin) and a southern, inner platform facies belt developed atop the Concho Arch. Exposures of the Fredricksburg Group on the inner platform contain a distinctive encrusted, Trypanites hardground representing a sequence boundary separating the underlying “Walnut – Comanche Peak” sequence from the overlying Edwards Formation. The basal Edwards Formation at the inner platform study section south of Sweetwater, Texas contains a thin lowstand marl overlain by a prominent 3 to 4 meter thick rudist complex representing the transgressive systems tract of the Edwards sequence.

Three major carbonate lithofacies are developed within this transgressive unit. Ooid – mollusk grainstones contain a variable mixture of coated grains and dominantly fine to some coarse mollusk grains. Porosity in this facies averages 17%. Ooid – peloid grainstones also contain mollusks, green algae and composite grains, with porosity averaging 10%. Rudist rudstones are dominated by very coarse rudist debris with a medium to coarse skeletal sand matrix. Rudist rudstones have up to 30% porosity.

This transgressive rudist complex is bounded above and below by planar contacts, giving the unit a tabular external morphology. However, welldeveloped platform – stepping oblique and sigmoidal bedding surfaces produce a complex internal architecture. The three lithofacies are characteristic of specific portions of the clinothems. Ooid – mollusk grainstones compose the bulk of the toe- sets, downlapping onto the lowstand marl and grading upward into the rudist rudstone. The rudstone facies is the thickest portion of the clinothem and grades upward into the flat – lying ooid – peloid grainstones. This complex clinoform architecture and lithofacies/rock property partitioning would greatly reduce connectivity and fluid flow through this apparently tabular flow unit analog. The basal Edwards rudist complex was deposited as an aggrading and back-stepping shoal system developed at the transition from the southern inner platform facies belt to the northern open platform. The ooid – mollusk grainstones were deposited as mound flank aprons and shoal top washover lobes downlapping onto the inner platform marl facies. Rudstones are loose accumulations of rudists set up as a mosaic of mounds or patches backstepping over the washover and flank grainstones. The ooid – peloid grainstones were deposited as shoal top sand flats that are the ultimate source of ooids in the toe set grainstones. The entire transgressive rudist - shoal complex is overlain by cycles of small isolated rudist mounds and muddy lagoonal lithologies representing the highstand systems tract of the Edwards sequence.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009