ERG reconstruction: The entrada sandstone (Jurassic) of northern Utah and Colorado

Abstract

To a large extent, the Jurassic Entrada erg (sand sea) of northern Utah and Colorado can be reconstructed by the coupling of field relationships with current hypothesis and observations from modern ergs. Stratigraphic relationships show that the Entrada erg occupied a cratonic basin characterized by sabkha, continental and shallow-marine deposits. Paleopole positions indicate that the erg was situated between 15 and 25°N latitude, a position conducive for erg development as a trade wind feature in a hot, arid climate. Storm deposits suggest that this setting may have been conducive to events such as typhoons. Entrada paleowind directions suggest an overall erg circulation pattern formed by the trade winds, sea breeze effects, and frictional drag of wind over land. Four erg facies occur largely with respect to position within the basin. A central erg facies, characterized by the vertical repetition of cosets of large-scale cross-strata and thin horizontal deposits, represents downwind migrating and climbing, large dunes and interdune areas. Interdune deposits show a wide variety of sedimentary structures reflecting deposition on wet, damp, and dry surfaces. A coastal erg facies shows a less well-developed erg with smaller, more irregular dunes. An inland-margin erg facies consists of deposits of wind ripples, small dunes, and wadis that flowed into the erg basin. Locally, a sheet sand facies is present. The geometric relationships of the hierarchy of bounding surfaces indicate that dunes of the central erg facies were compound crescentic (draas), and interdune areas were generally enclosed. The distribution of stratification types, and foreset dips dispersion similarly suggest crescentic dunes. Direct estimates of draa wavelength, angle of climb, and some interdune and dune dimensions can also be made. Correlation of these field measurements with relationships found in modern ergs yield additional bedform dimensions. These estimates suggest erg bedforms similar to those of the Rub'al Khali, Saudi Arabia, but of smaller scale such as those of the Peski Karakumy, U.S.S.R.