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AAPG Bulletin

Abstract


Volume: 68 (1984)

Issue: 5. (May)

First Page: 574

Last Page: 597

Title: Model for Sandstone-Carbonate "Cyclothems" Based on Upper Member of Morgan Formation (Middle Pennsylvanian) of Northern Utah and Colorado

Author(s): Steven G. Driese (2), R. H. Dott, Jr. (3)

Article Type: Meeting abstract

Abstract:

The upper member of the 200 m (660 ft) thick Morgan Formation (Middle Pennsylvanian) consists of 5-25 m (16-82 ft) thick, very fine-grained quartz sandstone units that are interbedded repetitively with 0.5-11 m (1.6-36 ft) thick, oolitic, bioclastic, peloidal, and micritic carbonate units. Similar repetitive sequences occur widely in western North America. A modified Markov chain analysis verified the presence of 2 kinds of repetitive, regressive-dominant, shoaling-upward vertical sequences. The carbonate sequence consists of, from bottom to top: (a) a knife-sharp contact between sandstone and the overlying carbonate unit, (b) planar-bedded bioclastic grainstones or cross-bedded oolitic grainstones, (c) fragmented-fossil and intact-fossil wackestones, (d) peloidal and eva oritic(?) wackestones and mudstones, (e) peloidal and ostracod-gastropod grainstones and packstones, (f) planar-bedded to low-angle cross-bedded fossiliferous calcareous sandstones, commonly comprising the transitional contact between carbonate and overlying sandstone. The sandstone sequence consists of, from bottom to top: (f) planar-bedded to low-angle cross-bedded fossiliferous calcareous sandstone, (g) medium-scale trough and wedge cross-bedded sandstones punctuated by bioturbated zones dominated by Ophiomorpha nodosa, (h) large to very large-scale wedge and planar-tabular cross-bedded sandstones, (i) deformed cross-bedded sandstone erosionally truncated by the next overlying carbonate unit.

Both sequences together help define a new conceptual depositional model that may explain the origin of many other widespread interbedded quartz sandstone-carbonate successions deposited on the western edge of the North American craton during Pennsylvanian and Permian time. One depositional phase employs virtually instantaneous transgression, followed by dominantly progradational subtidal carbonate-shelf to shoal conditions. The other involves progradation of sandstones across intertidal and subtidal carbonate environments. Eustatic changes related to Gondwana glaciation were probably the primary control on sedimentation, but localized shelf subsidence may also have been important. Although modern analogs are exceedingly rare, one possibility is the Qatar Peninsula in the Arabian Gulf, where siliciclastic eolian dunes are presently prograding into the Arabian Gulf across extensive subtidal to supratidal carbonate sediments. The quartz sandstone-carbonate "cyclothems" defined by this study have potential as targets for hydrocarbon exploration. Both eolian dune sandstones and dolomitized shelf carbonate strata are locally important reservoir rocks in the subsurface in parts of the western Overthrust belt in Utah and Wyoming.

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