ABSTRACT: Depositional Environments and Regional Significance of Pennsylvanian Caddo Limestone, Noah-Central Texas

C. Kenneth Schwartz, Jr.

The Caddo limestone (Atokan-Desmoinesian) is a regionally extensive carbonate unit present throughout much of north-central Texas. Attaining a maximum thickness of over 600 ft, the Caddo is one of the

more prolific oil-and-gas-producing formations of this region. Optimum carbonate deposition occurred within the Pennsylvanian epeiric seas in a north-south belt that approximates the bend flexure.

Stratigraphic analysis, regional relationships, and a systematic wholecore study revealed three prominent elongated regions of carbonate development. The geometry and position of these regions, herein termed (from south to north) the Cross Plains, the Breckenridge, and the Graham buildups, delineate the depositional trends of the Concho platform.

Originally, three separate formations (Eastland Lake, Caddo Pool, and Parks formations) defined these buildups. Elaborate stratigraphic subdivisions within the Caddo are not mappable with any degree of confidence beyond the type locality. By investigating the Caddo as a single stratigraphically related unit, important information concerning the stratigraphic relationships and depositional environments is now more precisely understood.

Six major rock types are identified within the Caddo on a regional scale. These include (1) algal wackestones to packstones, (2) crinoidal grainstones, (3) oolitic grainstones, (4) fusulinid packstones to grainstones, (5) spiculitic mudstone to wackestone, and (6) black shale.

Regional study indicates that the Caddo limestone exhibits two prominent phases of carbonate development. Initial carbonate deposition revealing no break in slope between the Concho platform and the subsiding Fort Worth basin, and wide, irregular facies tracts indicate ramp deposition. Subsequently, a carbonate platform with a characteristic narrow shelf margin developed.

Knowledge of platform morphology and spatial distribution of regional depositional facies of the Caddo limestone will provide additional insight for evaluating reservoir possibilities within north-central Texas.

AAPG Search and Discovery Article #90096©1990 AAPG Southwest Section Meeting, Wichita Falls, Texas, March 11-13, 1990