Middle Pennsylvanian cyclic sedimentation in the Rocky Mountain region
Strongly cyclical sediments, dominated by evaporites, were deposited in the Paradox and Powder River basins and Central Colorado trough during the Middle Pennsylvanian. Striking similarities between these widely separated facies suggest a common depositional control. Primary focus of this paper is on the Paradox basin cycles; however, a comparative analysis with the other sequences is made. The Paradox cycles, which span most or all of the Desmoinesian, consist of about 40 evaporite-carbonate-siliciclastic units, 30 of which contain halite. Each cycle represents a depositional response to a rapid rise followed by a gradual fall in sea level. Nearly continuous subaqueous deposition may have provided one of the world's most complete and detailed records of Desmoinesian sea level. A critical element in each cycle is time-synchronous black shales that pass through a shelf-carbonate facies to deep-basin evaporites. Recognition that black shales are elements of a prodelta(.) facies allows correlation of climatic change on the adjacent landmass (Uncompahgre uplift) with sea level events in the basin. Chemical and siliciclastic changes were also accompanied by changes in polynomorph biofacies, which reflect climatic conditions. An observed tripartite division of biofacies corresponds to the initial restricted evaporite phase of deposition, subsequent evaporite maximum, and finally a return to normal marine conditions. This change in polynomorph flora within each cycle indicates climatic control of the cyclic deposition observed. Variations in the geochemical, mineralogic, and biologic parameters of individual cycles provide fingerprints that offer promise of cycle-to-cycle correlation with Desmoinesian cycles in the other basins and thus would strengthen the theory of a common glacio-eustatic control of cyclic sedimentation.
- Research Organization:
- Geological Survey, Denver, CO
- OSTI ID:
- 6962963
- Report Number(s):
- CONF-8609129-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Vol. 70:8; Conference: AAPG Rocky Mountain Section meeting, Casper, WY, USA, 7 Sep 1986
- Country of Publication:
- United States
- Language:
- English
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