FRANCZYK, K. J., and JANET K. PITMAN, U.S. Geological Survey, Denver, CO, and D. C. BREW, Ft. Lewis College, Durango, CO

ABSTRACT: Proximal and Distal Facies of Clastic-Dominated Pennsylvanian Strata Along the Eastern Margin of the Paradox Basin, Colorado

The exposures of the Pennsylvanian Hermosa Group on the eastern margin of the Paradox basin at Ouray and Hermosa Mountain, Colorado, provide excellent examples of how variations in proximity to the western margin of the ancient Uncompahgre highland affects component lithologies, geometries, and paleocurrent patterns. The Hermosa Group in both areas consists of stacked coarsening-upward sequences that reflect repeated fluctuations in relative sea level. In the more proximal section (425 m thick) exposed at Ouray, the lower parts of sequences consist of a fossiliferous marine carbonate unit, generally less than 3 m thick, deposited during marine transgressions. As relative sea level slowed and subsequently fell, prograding delta complexes deposited fine-grained clastic material over the carbonates. With continued fall in relative sea level, braided rivers eroded much of the delta front and distributary channel deposits. Subsequent fluvial aggradation produced coarse-grained to pebbly sandstone units that are typically thick (>20 m), tabular, cliff forming, and yield radial (southwest to north) paleocurrent indicators. These fluvial deposits are abruptly overlain by marine carbonates deposited during the transgressive phase of the overlying sequence. Sequence thickness ranges from 5 to 30 m.

Contrasts between the Ouray sequences and those exposed in the more distal Hermosa Mountain section (840 m thick) include the following: (1) the middle, fine-grained pro-delta and delta-front parts of the sequence are thicker; (2) fluvial deposits at the top of a sequence are thinner, finer grained, less laterally extensive, of both braided and meandering origins, and contain only southwesterly paleocurrent indicators; (3) erosion at the base of the fluvial sequence, although present, appears less extensive;

and (4) sequence thickness ranges from 36 to 45 m.

The Uncompahgre highlands contributed quartz, potassium and sodium feldspar, metamorphic and sedimentary lithic fragments, and abundant mica to the Hermosa Group's alluvial and deltaic complexes. At Hermosa Mountain, the mineralogy appears fairly consistent throughout the Hermosa Group except feldspar is absent to rare in the lower 168 m. This change in feldspar content could reflect a non-Uncompahgre source, as other workers have suggested, or alternatively, variations and changes in drainage areas within the Uncompahgre Ongoing studies will determine if a similar pattern occurs in the Ouray section.

Brachiopod assemblages in the Hermosa Mountain section indicate that the basal part of the Hermosa Group was deposited close to the Atokan-Desmoinesian boundary. The uppermost part of the Hermosa Group is, at the youngest, earliest late Desmoinesian in age. The gradational contact between the Hermosa Group and the overlying Cutler Formation in this area suggests a late Desmoinesian age for the basal part of the Cutler. In the Ouray section, most of the Hermosa Group and the basal part of the overlying Cutler is probably also Desmoinesian, but the basal part of the Hermosa Group is alluvial, nonfossiliferous, and gradational with the underlying conglomeratic Molas Formation and, therefore, is undated.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.