Cyclicity, Dune Migration, and Wind Velocity in Lower Permian Eolian Strata, Manitou Springs, CO
Pike, James Daniel
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The Permian of western North America is characterized by a long-term drying trend, due in part to supermonsoonal conditions related to the assembly on Pangea. This trend is reflected in the gradational contact of the Pennsylvanian alluvial-fluvial Fountain Formation and the early Permian eolian Ingleside Formation. The Ingleside Formation at Manitou Springs is characterized by two major eolian depositional intervals, punctuated by fluvial-alluvial facies. The shift from eolian to alluvial-fluvial depositional systems represents a major pulse of humidity during an otherwise arid period. Eolian facies are moderately-sorted, subrounded, fine-grained, sub-arkosic sandstone. Cross stratification is up to 9 meters, and strata exhibits internal ripple laminae. Eolian cross-strata are periodically truncated by parallel to sub-parallel zones consisting of laterally continuous massive to weakly planar stratified muddy sandstones. Rhizoliths are locally present within these zones, and zones demonstrate abundant clay and carbonate cementation in thin section. Zone thicknesses range from true surfaces with no thickness up to 1.7 meters. These zones are highest-order present in the stratigraphy, and are inferred to be Super Bounding Surfaces (SBS), which likely reflect deflation of the erg field to an elevated water table due to changes in sedimentary flux, aerodynamic and/or environmental conditions. Following erosion to an elevated water table, deposition occurred in a stabilized and/or wet sand flat system. Fourteen SBS zones were identified in the eolian strata at Red Rock Canyon Open Space. SBS zones separate distinct intervals of eolian migration and deposition. Mean paleowind directions for the measured intervals range from 230 to 254 degrees, indicating northeasterly to easterly winds. Wind directions are consistent with an equatorial circulation pattern, and are likely influenced by the lack of highlands to the west (Woodland Park Trough). Paleowind threshold velocities range from 24.1 Km/hr to 32.4 Km/hr using D10 grain sizes and a height of 1.5 meters above bedding surfaces. As SBS are low porosity, low permeability zones, they likely act as fluid baffles and partition reservoir intervals from one another. Additionally, these zones can act as pathways along which fluids can flow.