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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 63 (1979)

Issue: 3. (March)

First Page: 409

Last Page: 410

Title: Edwards Formation, North-Central Texas--Stratigraphy, Depositional Geometry, and Diagenesis: ABSTRACT

Author(s): David L. Amsbury

Article Type: Meeting abstract


Oolitic and skeletal carbonate grainstone (Moffat Mound) is present as a nearly straight, WNW-ESE-trending belt within the upper Fredericksburg (Cretaceous Comanche) of north-central Texas. The grainstone body with its associated rudist reefs is 2 to 4 mi (3 to 6 km) wide, up to 120 ft (36 m) thick, and at least 50 mi (80 km) long. It separates normal-marine carbonate wackestone and marlstone on the north from tidal-flat dolomite on the south. Detailed tracing of key beds and hardground surfaces through closely spaced, measured sections permitted separation of the body and its finer grained equivalents into several approximate time-stratigraphic subunits. The belt began as a series of discontinuous, relatively thin rudist reefs on a broad shoal, developed into the narrow, linear, carbonate-sand buildup, and then merged with widespread rudist patch reefs to end local Fredericksburg deposition.

Early multiple aragonite(?) and calcite cements filled nearly all intergranular pores and most early leached-grain

End_Page 409------------------------------

pores in grainstones; most muddy marine rocks interbedded with tidal-flat dolomites were dolomitized. Both the grainstones and dolomites are very porous (up to 35 and 42%, respectively) and permeable (up to 160 and 1,430 md, respectively) because of widespread leaching of cements.

The grainstone-dolomite complex forms a large, tabular body of rock which now is continuously permeable to water and which is enclosed above, below, and along one side by impermeable rocks. The linear carbonate-sand body and associated reefs controlled the distribution of original porosity, and it now marks the northeastern boundary of permeable rocks.

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