Major Upper Triassic Lithogenetic Sequences in Colorado Plateau Region

Abstract

Three major lithogenetic sequences of regional extent are recognized in the Upper Triassic Series of the Colorado Plateau region. The sequences, which correspond only partly with named formations, are useful in describing the gross lithologic character and the origin of the Upper Triassic Series. The lowest of the three, the bentonitic sequence, consists of variegated bentonitic claystone and clayey sandstone and thin widespread sandstone and conglomerate units. The sediment in the sequence was derived predominantly from a volcanic source area south of the Colorado Plateau, the Mogollon highland, and was deposited in streams, on floodplains, and in lakes. The overlying redbed sequence consists of reddish-brown or reddish-orange, horizontally stratified siltstone and minor amounts of limestone and sandstone. The sediments were predominantly quiet-water deposits, probably lacustrine, derived from several source areas, the most important of which may have been the Uncompahgre and Front Range highlands of Colorado. The next overlying sequence consists of light-brown, fine- to medium-grained, conspicuously cross-stratified sandstone. It is considered to be of eolian origin; the sediment probably was derived from reworking of upwind arenaceous sedimentary units on the northwest.