ORIGIN AND DISTRIBUTION OF THE MISSISSIPPIAN – PENNSYLVANIAN BOUNDARY UNCONFORMITY IN MARINE CARBONATE SUCCESSIONS WITH A CASE STUDY OF THE KARST DEVELOPMENT ATOP THE MADISON FORMATION IN THE BIGHORN BASIN, WYOMING.
Date of this Version
The causal mechanism of the widespread unconformity that encompasses the Mississippian – Pennsylvanian boundary remains poorly understood. This unconformity, first thought to be restricted to North America, is now known to be present in other regions of the globe. Possible causes for the unconformity include (1) sea level draw down from the onset of glaciation at start of the late Paleozoic ice age and (2) increased tectonic activity from the formation of the supercontinent of Pangea. Thus the origin of the unconformity is still poorly constrained.
This study examines possible causal mechanisms for the widespread unconformity that encompasses the Mississippian – Pennsylvanian boundary through examination of published stratigraphic records from nine paleotropical sites as well as field study of the karst development on top of the Madison Formation formed at the unconformity interval as an outcrop analog for examination of key reservoir properties to aid in the assessment of equivalents in the subsurface. The nine paleotropical sites looked at included, 1) Arrow Canyon, NV, USA, 2) U.S. Midcontinent, 3) Madison Platform, Big Horn Basin, WY, USA 4) Bechar Basin, Algeria, 5) Palentian Zone, Cantabrian
Mountains, Northwest Spain, 6) Central Taurides, Turkey, 7) Donets Basin, Ukraine, 8) Southern Ural Mountains, Russia and 9) South China Platform.
Results from the sites show the development of a sequence boundary and or a shallowing of facies across the boundary, such global synchroneity of stratigraphic patterns suggests that the unconformity encompassing the Mississippian – Pennsylvanian boundary was as a result of global eustatic fall consistent with ice buildup at the onset of the late Paleozoic ice age. Additionally, in areas where tectonism was prevalent, eustatic signals were masked. Also, the heterogeneity and spatial complexes formed as a result of the karst development on at the Mississippian – Pennsylvanian boundary in the Madison Formation of Bighorn Basin Wyoming have shown to be consistent with subsurface examples in China, thus suggesting the viability of the Madison paleokarst as an outcrop analog. Finally, since this interval serves as hydrocarbon reservoir in some parts of the US Midcontinent and the Rocky Mountain region, proper understanding of the widespread distribution of the unconformity as well as reconstruction of the karst feature developed at the Mississippian – Pennsylvanian boundary will aid in global exploration of hydrocarbon.
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