PALEOCLIMATE RECONSTRUCTIONS FROM AEOLIAN CROSS-STRATA: A NEW METHOD APPLIED TO THE CEDAR MESA SANDSTONE

A new method is presented to reconstruct the wind regime from aeolian rocks based on the spatial distribution of stratification types and cross-strata dip directions. Wind direction(s) are extracted by finding incidence angles (angle between the crestline and the wind) that are in agreement with the outcrop evidence of stratification types. In contrast, the current standard method plots a number of measured cross-strata dip directions on a rose diagram with the mean taken as the net transport direction. This mean direction is commonly taken as the prevailing paleowind direction. However, this is only true for transverse dunes under unidirectional flow.

Incidence angles are defined as transverse (70-90°), longitudinal (0-10°), and oblique (10-70°) (Sweet & Kocurek, 1990). Each incidence angle defines a characteristic secondary flow configuration on the lee face (Walker & Nickling, 2003), which in turn gives rise to specific surface processes and resultant lee-face stratification (Hunter, 1977): 1) grainfall; 2) grainflow or avalanching; and 3) wind ripples.

The uppermost sequence of the Permian (Wolfcampian) Cedar Mesa Sandstone in SE Utah was chosen to develop and test the new methodology because of the extensive previous work and high-quality exposures. Cross-strata dip measurements taken by numerous workers clearly document net transport to the SE (Poole, 1962; Loope, 1984; Peterson, 1988; Langford & Chan 1989; Stanesco & Campbell, 1989; Mountney, 2006; Langford et al., 2008).

Results indicate that wind blew from the WNW and N during the Permian in SE Utah, confirming that two transverse crestline configurations existed within the overall wind regime. The average of these two directions is the SE dip direction found by all previous workers, although the wind never blew to the SE. This method sets a new standard for producing specific and accurate paleowind reconstructions that can be used to interpret past climates and validate paleoclimate models.