Abstract

Five distinct limestone facies have been defined by studying six shallow cores through a Late Pennsylvanian (Missourian) carbonate bank in Wise County, North-Central Texas. The limestone facies designated according to their characteristic fossils or structures are: (1) fusulinid-peloidal grainstone facies; (2) algal-echinodermal wackestone facies; (3) algal wackestone-packstone facies; (4) laminated wackestone facies; and (5) mudstone facies.

The algal wackestone-packstone facies formed the bank core and the algal-echinodermal wackestone facies was developed on the bank fringe. The fusulinid-peloidal grainstone facies was formed on the seaward-side flank of the bank near wave base. The laminated wackestone facies was developed on the landward-side flank of the bank and served as a hardground for accumulation of carbonate sediments. The mudstone facies was formed locally within a well-protected environment or on the sea-side flank below the wave base. The superposition of these facies in the cores suggests a landward migration of environments which was caused by marine transgression. A local depositional break is marked by the formation of red soil and limestone nodules at the top of two cores studied.

The primary porosity ranged from 25 to 55 percent, depending on the carbonate mud content in the sediments, but it was almost totally eliminated by diagenetic processes. Future petroleum exploration in the Upper Pennsylvanian carbonate banks of the Fort Worth basin should concentrate on the seaward-side flanks of carbonate buildups like the Chico Ridge bank where favorable porosity and permeability are likely to be present.