End-Mississippian Paleotopography and Interpreted Drainage Pattern: Eastern Colorado: ABSTRACT

Abstract

Eastern Colorado is the site of Pennsylvanian production from Morrowan, fluvial-deltaic and estuarine valley-fill sandstones. At least seven sequences of transgression-regression have been recognized. During regressions, erosional down-cutting carved valley systems on the flood-plain surface. During transgressions, the valleys were filled. The underlying pre-Pennsylvanian unconformity influenced the location of the Morrowan valley systems. Using 1263 well logs, this paleotopographic surface has been defined by detailed correlation of the Mississippian System and by contouring the isopach values of each formation at its erosional subcrop locale. Cross-sections have been prepared to show: (1) that regional formation thicknesses are reasonably constant, therefore, isopach irregularities at the subcrop are mainly due to erosional effects, and (2) that a paleotopographic valley-ridge system was incised on the eastern Colorado shelf at end-Mississippian time. A trellis type drainage pattern, probably controlled by parallel fracturing and jointing of the subaerially exposed carbonate shelf, was established that drained southeastward toward the proto-Anadarko Basin. A major erosional valley, trending mainly north-south, may have existed in far western Kansas that served as a catch-all for the eastern Colorado, karst valley-ridge system.