ABSTRACT
Within the Greater Rocky Mountain Region, ventifacts have not been extensively studied, and little is known about various local relationships between rock type, wind regime, and associated ventifaction styles. The Killpecker Sand Dunes area of southwest Wyoming hosts several significant ventifact sites—including the site of this study, which focuses on Cretaceous Almond Formation outcrops along the south margin of the dune field. The ventifacts appear to be the product (at least in part) of Holocene winds, based on the freshness of ventifacted surfaces and their alignments with local wind directions. The ventifaction occurs on both fragments and in-situ bedrock sandstones exposed within and marginal to the dune field. This study, begun in 2020, demonstrates how ventifaction can be used to recognize local wind regimes within broader areas of atmospheric circulation.
Analysis of regional wind regimes as they affect sand movement, and thus the formation of ventifacts, indicates a dominant release of wind energy from the southwest. Yet the most intense ventifaction in the study area at Killpecker dunes has been produced by northeast winds. It seems likely that topographic controls on winds in the study area, as well as local sand supply variations, have resulted in this strong ventifaction by northeast winds. This supports the idea that ventifacts may sometimes be a useful guide to understanding the history of a local wind regime, especially where there is no meteorological station.
Ventifacts at the Killpecker dunes commonly have clusters of features that differ with increasing angles of sand impact. The angle of sand impact is the sum of the angle of descent of saltating sand (average 14 degrees) and the dip of the rock surface into the flow. Flat- to shallow-dipping (0 to 15 degrees) surfaces typically tend to polish, with a dominance of flute and scallop ventifaction features. In addition, terraces and scour pits around concretions on rock surfaces are common at this angle of exposure. Rock surfaces with moderate dip angles (16 to 30 degrees) into the sand flow direction tend to have an abundance of grooves. Where the rock surface dips at a steep angle (31 to 90 degrees), fretting is the dominant ventifaction feature.
In the study area there are two ventifacted rock types. Type A ventifacted rocks are brownish (Munsell 10YR 5/4) laminated, commonly cross-bedded, mostly fine- to medium-grained sandstones of the Almond Formation. The interpreted depositional environment is a mixture of nonmarine, deltaic, and shallow marine sediments. Type B ventifacted rocks are commonly massive, gray, (Munsell 7.5YR 6/1) fine-grained, and highly calcareous. There are common shelly (molluscan) sandstones and thin limestones with occasional cross-bedding reflecting a more marine shoreline setting of this portion of the Almond Formation. Ventifacted surfaces are better polished on Type B sandstones than the Type A sandstones, and ventifaction features are more sharply delineated. Exposed shell fragments deflect or modify the shapes of flutes and grooves.
Many large and small rocks, particularly flat-bedded sandstones, have broken in place—possibly due to the accumulated effects of the sharp diurnal temperature fluctuations at Killpecker—followed by ventifaction of daughter fragments.
