EARLY PERMIAN WIND PARAMETERS RECORDED IN THE LYONS FORMATION, MANITOU SPRINGS, COLORADO

The Permian of western North America is characterized by the initiation of drying conditions that are in part related to the development of Pangean monsoonal circulation. At Manitou Springs, Colorado, the Permian Lyons Formation is a moderately well sorted to well sorted-subrounded, fine to medium quartsoze sandstone exhibiting large-scale (up to 9m) crossbedding with internally graded ripple laminae beds. The large-scale crossbedding is bounded by thoroughgoing, relatively planar surfaces inferred as bounding surfaces. These relationships are consistent with an eolian interpretation inferred by other workers. Cross-bedding orientations were analyzed to test if monsoonal circulation influenced the surface winds that mobilized the Lyons dunes. Restoring bounding surfaces to horizontal, the mean down dip direction of crossbeds (N = 75) is 221 ± 4.5° SW with a standard deviation of 16°. Correcting restored planar crossbeds to paleo-orientations, the mean down dip direction is 246 ± 4.5° SW. Given that the Lyons Formation was between 5-10° N latitude at the time of deposition, the surface wind direction reported here is consistent with zonal circulation.

Disaggregation of the Lyons Formation allows for robust grain size distribution analysis which is then used to estimate wind speed. Grain size distributions show a fine skewed histogram with a 5% mud tail. In thin section, the clay is most commonly observed as intragranular cement or coatings but not as individual grains, thus removing the finest 5% should capture a better representation of the grain size distribution during transport. Histogram parameters of the coarsest 95% are well sorted (σ¹ = 0.47ϕ) and nearly symmetrical. Wind speed critical thresholds at the grain surface are calculated between 31.5 and 21.5 cm/s for the coarsest (D₀) and median (D₅₀) grain sized, respectively. Assuming a relatively flat and normally rough bed surface, these thresholds equate to wind speeds between 35 to 24 km/hr for entrainment and 28 to 21 km/hr under saltation for an elevation 3m above the surface. Because we assume no entrainment inhibiting factors, such as moisture content, sorting and vegetation, the estimates are minimums and suggest that at least 21 km/hr surface winds for dune migration.