Sedimentary Cycles in the Nugget Sandstone, Northeastern Utah

Abstract

The Nugget Sandstone is one of the classic eolian sequences of the Colorado Plateau region. North of Vernal, Utah, the Nugget contains sedimentary cycles consisting of three units: (I) red mudstone, (2) siltstone, and (3) sandstone. The thinly bedded facies of the Nugget is characterized by complete cycles (1-2-3) in which the redbeds of units I and 2 are dominant. The crossstratified facies consists of alternations (2-3), and the medium- to large-scale cross-stratified sandstone of unit 3 is so well developed that it gives the facies its name. The cycles formed during wet-dry fluctuations in the climate. Units I and 2 represent small lakes, inland sebkhas, and mudflats; unit 3 represents eolian dunes. During deposition of the Nugget the general climate became more arid, and eolian dune deposition dominated, broken only rarely by local ephemeral lakes during moist periods.