Slope-derived deposits from the Cretaceous Interior Seaway, northwestern Colorado

Abstract

Two types of mass-movement deposits are interbedded within the dark marine Upper Cretaceous Mancos Shale, in outcrops exposed on and near the slopes of Mount Garfield, 12 km east of Grand Junction, Colorado. These deposits are inferred to have formed at or seaward of the shallow-marine shelf edge, about 200 km from the contemporaneous shoreline. The first type is a thick (tens of meters), laterally restricted (200–400 m) deposit composed of rip-up clasts of Mancos Shale, rare sandstone and siltstone blocks, and a silty mudstone matrix. These deposits overlie abrupt, bedding-parallel basal contacts, and are interpreted to record submarine debris flows. The second type of deposit consists of landward-rotated blocks of Mancos Shale tens of meters in scale, adjacent to a listric fault scar, and overlying a similar sharp, planar base. These are interpreted as slump blocks. An associated third type of deposit consists of one or more 4- to 5-m-thick beds composed of laterally extensive, centimeter-scale, sandstone-shale laminations that are rippled and extensively burrowed and that may represent distal turbidite deposits. The mass-movement deposits are interpreted as rare examples of the sedimentary record of the offshore slope environment in the Cretaceous Interior Seaway. A twice-repeated vertical succession, with basal sandstones, intermediate mudstones, and overlying slump blocks or debris-flow deposits, records fluctuations in relative sea level, tectonism, and sedimentation rate during the Claggett transgression.