Petrographical, petrophysical, and biostratigraphical investigation of the Caddo Limestone (Pennsylvanian) Stephens County, Texas



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Texas Tech University


The Pennsylvanian Caddo Limestone (Desmoinesian) of Stephens County, Texas, is a thick-hydrocarbon bearing carbonate unit. The uppermost Caddo is an algal-mound structure formed by the sediment-baffling effect of phylloid algae. Two cores south and southeast of Breckenridge, Texas were investigated petrographically and using geophysical logs. Within the cored intervals, multiple shallowing-upward mounding cycles can be recognized. Six depositional lithofacies were recognized and were associated with six depositional environments. These associations are: (1) Sponge Spiculite - Deep Water, (2) Fusulinid - Crinoid - Komia Packstone/Grainstone - Debris Bed or Capping Unit, (3) Phylloid Algal Wackestone - Mound Core, (4) Skeletal Wackestone - Near Flank Beds, (5) Carbonate Clast - Wackestone - Intermound, and (6) Skeletal Ooid Grainstone and Fusulinid - Crinoid – Komia Packstone/Grainstone - Capping Unit. The Caddo algal mounds follow a typical mounding pattern: (1) Deep Water, (2) Debris (stabilization of the deep water substrate), (3) Mound Core - Near Flank- Intermound (upward mound growth and lateral mound migration, and (4) a Capping Environments (termination of mound growth). The algal mounds original depositional fabric is heavily modified by four stages of diagenesis: (1) syndiagensis, (2) marine, (3) multiple phases of meteoric, and (4) deep (burial). Meteoric diagenesis had the largest modifying effect in the original depositional fabric.

Three major relative sea level fluctuations occurred within the Caddo algal mounds, producing three mounding cycles. Correlation of cored wells and geophysical logs in the studied area lead to the determination of a progressive step wise westward termination of the Caddo algal mounds were a result the migration of the fore bulge (Bend Arch) associated with tectonic loading of the Ouachita Thrust Belt. Progradation of prodeltaic and basinal terrigenous sediments derived from the Ouachita thrust belt across the Caddo algal mounds in one event. The termination of the Caddo algal mounds, by drowning is time transgressive getting progressively younger in the westward direction.