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

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. 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 knifesharp 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 evaporitic(?) 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 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 progradationof sandstones across intertidal and subtidal carbonate environments. Eustatic changes related to Gondwana glaciation were probably the primary control on sedimentation. 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.--Modified journal abstract.