Paleocurrent Control of Facies Heterogeneity in Microbial Buildups and Stromatolites; Late Carboniferous (Moscovian) Hermosa Group, Southeastern Utah
Gary L. Gianniny, Amanda P. Peterson, Daniel J. Powers, and Shannon M. Boesch
Fort Lewis College
The facies and fabric heterogeneity within ancient microbial carbonates are an important control on reservoir-scale variations in permeability and porosity. Three sequences within the exquisitely exposed outcrops of Late Carboniferous (Moscovian) limestones in the Goosenecks of the San Juan River, Southeast Utah, demonstrate that predictable variation in facies and microbial fabrics are shaped by trade wind-driven marine currents, tides, and possibly storms. Within a fourth order sequence of the third order Akah sequence, 5-15 meter-thick thrombolitic mounds prograded shoreward to the modern west and northwest. We infer these were formed by currents and wave action from the southeast, producing clinoform partitioning within buildups. These grain-dominated boundstones may be analogous to grainstone aprons which prograde shoreward from the windward margin of modern metazoan reef systems. Lower in the Akah sequence, thinner beds of 2.5-3.5m high digitate stromatolites form isolated 0.5-1.5m wide clusters of club shaped stromatolites inclined to the southeast (186 of 309 samples). This is consistent with asymmetrical growth into the current as documented in the modern of the Bahamas (Dill, 1997), and the ancient (Hoffman, 1967). Thirdly, the intricately digitate stromatolites of the underlying Barker Creek Sequence are the source of reworked, cemented microbial clasts which fill in 0.1 to 0.25m troughs between stromatolitic heads. These reworked and in situ stromatolitic facies overlie a very fine quartz sand, peloidal grainstone with bi-directional current ripples. Although both the thrombolitic mounds and the club-shaped stromatolites were shaped by currents from the southeast, they responded differently. The thrombolitic mounds prograded downwind to the west, while the club-shaped stromatolites grew into the current towards the southeast. This current direction is most consistent with easterly trade wind-driven oceanic currents. Thinner beds of digitate stromatolites, in shallower settings with more exposure, experienced early cementation and were reworked by tides and/or storms.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013