Page 188 - Carbonate Facies in Geologic History

Middle Pennsylvanian Beds of the Carbonate Shelf Facies, Paradox Basin 175

Fig. VI-2. Facies of Desmoinesian (Mid Pennsylvanian) strata in Paradox basin of Utah and
Colorado. Open shelf, tidal shelf and shoal sands are largely composed of cyclic carbonate
strata

these lie on a shelf southwest of the basin axis. Figure VI-2 shows the asymmetri-
cal Paradox basin with its thick mid-Pennsylvanian central evaporite sequence
(paradox Formation) which grades northeastward to even thicker arkosic sand-
stones derived from the Uncompahgre uplift. These Desmoinesian beds are more
than 2000 m thick in the basin and thin to 300- 700 m of shelf carbonate strata
within a relatively short distance (Hermosa Formation). The shelf strata consist of
depositional cycles separated by dark shales and shaly dolomite which can be
traced basinward into the evaporites (Fig. VI-3). The bioherms occur toward the
top of the cycles in at least three levels in the formation. Earliest Desmoinesian
bioherms lie farthest to the southwest on this shelf. As the basin filled progres-
sively in later Desmoinesian time, younger bioherms developed farther northeast
across the basin. The trend of the outcropping bioherms is northwest, parallel to
the strike of the buildup trends on the southwest side of the basin. In general this
applies also to the oil fields developed in younger buildups. Wengerd's (1963) map
(Fig. VI-4) shows them to be long, straight, narrow, and somewhat stacked verti-
cally. E.g., bioherms in the middle level are from 150 to 300 m long and 20 m
thick.