Depositional Sequences ("Cycles") in Fredericksburg Rocks (Middle Albian, Cretaceous) of North-Central Texas

David L. Amsbury

Fredericksburg division is an allostratigraphic (physically defined time-stratigraphic) unit of sub-series rank. It is regionally coincident with the 100-m thick Fredericksburg Group. The division contains shallowing-upward depositional sequences at all scales; detailed stratigraphy demonstrates that each has limited lateral extent. A much better understanding of depositional history my be gained by study of the sequences than by study of lithologic units alone.

From the base upward, the Fredericksburg includes the Paluxy Sand, Keys Valley Marl Member of the Walnut Formation, and the Comanche Peak Limestone. Deposition of Paluxy Sand, spread southward across hardgrounds partly by marine currents, was interrupted by periods of caliche soil formation, deposition of oolite and other carbonate sediment over sand mounds, and widespread episodes of clay influx. The Keys Valley Marl Member of the Walnut Formation comprises shale with rippled gryphaeid beds, passing upward into marl containing a diverse marine molluscan fauna, and capped by a gryphaeid biostrome that grades westward into quartz-sand oolitic grainstone. Except for the oolite, water depth may have been similar for all facies. Lenses of Comanche Peak Limestone, capped locally by hardgrounds, pinch out northward into upper Walnut shale. An oncolite shoal trending west-northwest from Lake Belton formed below one prominent surface; this surface became the substrate for a 35-m buildup o Edwards grainstone and rudist rock.

Recognition of depositional sequences provides and excellent tool for discrimination among fluctuations in relative sea level, changes in water-mass properties, and influxes of sand and clay on this stable shelf.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.