Early Tertiary Fanglomerate, Big Horn Mountains, Wyoming

Abstract

Great accumulations of coarse bouldery gravel along the east base of the central Big Horn Mountains are composed almost wholly of debris derived from the pre-Cambrian core of the range. The name "Moncrief gravel" is proposed for this deposit. It has previously been described as Pleistocene glacial material, late Tertiary to earliest Quaternary bench gravels, or the coarse phase of an early Tertiary basin fill. The gravel was found to be gradational into fine-grained early Tertiary beds and to be separated from the pre-Tertiary rocks of the mountains by thrust faults. For these reasons the Moncrief gravel is identified as an early Tertiary, probably Eocene, fan deposit, formed as the Big Horn Mountains were progressively uplifted and thrust eastward during the Laramide Revolution. Early Tertiary glaciation may have played a part, but this is largely speculation.