Edwards Formation, surface and subsurface, Central Texas




Rose, Peter R.

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In the subsurface of south-central Texas the Lower Cretaceous Edwards Formation consists of about 400 to about 600 feet of porous dolomite and limestone that accumulated on the Comanche shelf as shallow marine, intertidal and supratidal deposits. The Edwards thickens southwestward to about 1000 feet near the Maverick basin and thins northeastward by facies change to zero in the North Texas-Tyler basin. In between, on the San Marcos platform, it is separated into the Kainer Member (new) below and the Person Member (new) above by a thin, widely traceable argillaceous layer called the Regional Dense Bed, the base of which is the conformable Person-Kainer boundary. The Edwards is conformable with the Glen Rose Formation below and disconformable with the Georgetown Formation above. The overlying Georgetown, Del Rio, and Buda formations consist chiefly of lime mud and represent marine open shelf deposition. To the northeast the Kainer grades into the classic Walnut/Comanche Peak/Edwards sequence of the north Texas type area, and the Person grades into the lower Duck Creek Member of the Georgetown. To the southwest the Kainer and Person members pass into the Devils River Formation. At the surface in the eastern Edwards Plateau of central Texas the Edwards Formation consists of about 400 to about 650 feet of dolomite and limestone similar to that of the subsurface Kainer and Person members; it thickens southward from the Central Texas platform and grades into rudist limestone of the Devils River Formation. The Edwards is separated into the Fort Terrett Member (new, after Lozo and Smith) below and the Segovia Member (new) above, separated by a thin, widely traceable marly layer called the Doctor Burt Bed, at the base of the Segovia. The Glen Rose-Edwards, Edwards-Del Rio, and Del Rio-Buda boundaries are disconformable, but the Fort Terrett-Segovia boundary is conformable. The Georgetown is absent, probably by erosion and non-deposition, and thin Del Rio and Buda formations consist of marine open shelf deposits. Southeastward across the Edwards Plateau the lower Doctor Burt Bed acquires miliolid and rudistid limestone at the expense of marl so that only a thin marly layer remains at the top as the Edwards goes into the subsurface; this is the Regional Dense Bed. From northeast to southwest Edwards and equivalent units form two complete carbonate facies complexes. The lower (Fredericksburg) complex reflects extremely shallow water, high salinities and low subsidence rates. The upper (Lower Washita) complex is a facies assemblage more like the standard carbonate model, with low-angle clinoforms along the subsiding basin margins and increasing dolomite toward the Central Texas platform. Upper Washita open shelf units filled in and then blanketed Lower Washita topography. Dolomite is confined chiefly to restricted shallow marine, intertidal, and supratidal deposits, which are controlled chiefly by positive tectonic elements. Collapse breccias were caused by solution and removal of gypsum shortly after deposition. Crystalline limestone is related to Cretaceous exposure, present weathering, and alteration beneath dissolving gypsum. Pulverulite is related to contemporary weathering. The subsurface Edwards is porous toward the top, related to early exposure of mobile fault blocks and to the unconformity at the top of the formation. The surface Edwards is porous throughout; cavernous porosity and permeability at the base produces a widespread, effective aquifer. Ground water probably is enhancing porosity now. Through Fredericksburg and Washita time facies tracts shrank back onto higher parts of the Central Texas platform as it was progressively inundated during deposition of three units of successively deeper-water sediments.