CYCLICITY, DUNE MIGRATION AND WIND VELOCITY IN LOWER PERMIAN EOLIAN STRATA, MANITOU SPRINGS, COLORADO

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. Eolian facies are well-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 1.5 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 a climbing 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. At least 14 potential SBS zones exist at Manitou Springs.

Laterally extensive truncation of underlying eolian foresets and the irregular nature of basal surfaces suggest that SBS zones are erosional; as such, they separate coeval eolian strata. Mean paleowind directions for the measured intervals range from 219 to 275 degrees, indicating northeasterly to easterly winds. Wind direction is likely controlled by the study area’s position between highlands to the west, and the ocean to the east. Paleowind speed can be calculated using grain size as a proxy; larger grain sizes require faster winds to entrain and deposit grains. Paleowind threshold velocities range from 20.5 Km/hr to 22.8 Km/hr using D₅₀ grain sizes and a height of 1 meter above bedding surfaces. While variation in these wind parameters is present, no discernable trends exist through time.