UPPER ALBIAN RUDIST BUILDUPS OF THE EDWARDS FORMATION, CENTRAL TEXAS

Rudist-associated shallow-water carbonates form important hydrocarbon reservoirs in the Middle East, Mexico, and the United States, and outcrops of such carbonates are common in central Texas, USA. The current study integrates outcrop observations with GPR surveys, laboratory dielectric measurements, and petrography to understand lateral and vertical facies relations of rudist-bearing lithofacies in the Cretaceous Edwards Formation (Fredericksburg Group) that are partially exposed in spillways and gullies near Lakes Georgetown and Belton in Texas. They provide insights into the subsurface distribution and heterogeneity of hydrocarbon bearing carbonate systems globally.

The exposures demonstrate a wide spectrum of rudist buildups, which vary not just in faunal content, carbonate fabric, and porosity-permeability characteristics, but also in their depositional styles, that show both biohermal and biostromal character. GPR data revealed four types of mounds that have distinct three-dimensional geometries, and lithofacies, that record both seaward to leeward changes and vertical changes from below wavebase to within wavebase. The lowermost mounds are high relief circular caprinid mounds that mark the aggradational part of the Edwards lithostratigraphic unit at our study locality. Mound morphologies identifiable on GPR profiles included structures that are much smaller in dimension than the caprinid mounds outcropping along the eastern side of the Lake Georgetown spillway. Each subsurface caprinid mound of 15–20 m basal diameter appeared to be constructed of amalgamated mound-shaped sub-units that are relatively geoelectrically similar, and individually 1–2 m thick with basal diameters of 8-10 meters. Well cemented, low relief toucasid-foraminiferal mounds and grainstones cap these caprinid-dominated buildups. Weakly cemented, circular, flat-topped radiolitid-caprinid-toucasid mounds developed above the toucasid mounds and low relief, often sucrosic-dolomitized mounds constitute the most shallow water rudist accumulations.

GPR provides an unparalleled view of the three-dimensional growth, vertical accretion, and progradation of these mounds through time, and the geoelectrical signatures provide insight on porosity and permeability of the lithofacies in outcrop.