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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 49 (1999), Pages 310-321

Sedimentological Features Within the Edwards Group of West-Central Texas

Brian E Lock (1), and Jamsie L. Roberts (2)

(1) Department of Geology, The University of Southwestern Louisiana, Lafayette, LA 70504.

(2) Austin, TX.

ABSTRACT

Road-cuts along Interstate Highway 10, west of San Antonio, display numerous excellent exposures of Lower Cretaceous limestones, particularly of the Edwards Group. These strata were deposited on a shallow-water platform bounded by the shoreline of the Llano island (sabkhas and tidal flats) to the north and east, and by deeper water platform and shelf-edge rudistid reefs to the south and east.

Shoaling-upward cycles seen in the vicinity of Kerrville and Junction include subtidal, highly burrowed facies passing up into higher energy intertidal grainstones. This sequence is capped by supratidal, commonly stratiform dolomite with gypsum nodule molds, local de-dolomite and erosional truncation. These cycles are typically meter-scale, and the dolomites can reasonably be interpreted as products of penecontemporaneous sabkha processes.

Further west, near Sheffield, shoaling-upward cycles are an order of magnitude thicker, with oyster wackestones coarsening upwards to gastropod grainstones. Cycle boundaries are abrupt, but not necessarily erosional.

Other types of stratigraphic breaks (diastems) are more subtle, occurring without obvious cyclicity. Hardgrounds - with a) oysters cemented on them, b) blackened surfaces, c) borings and d) overlain by rounded clasts - indicate submarine exposure, while other, hummocky erosion surfaces are interpreted as paleokarst, resulting from subaerial exposure.

In addition to the stratiform dolomite associated with the supratidal (sabkha) surfaces described above, lenses of dolomite seen in the road-cuts near Sonora are interpreted to be related to shallow subsurface hydrologic pathways, consistent with seepage-reflux models for dolomitization.


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