ABSTRACT

Sedimentologic, stratigraphic, and petrologic analysis of the Upper Jurassic-Lower Cretaceous Morrison and Cedar Mountain formations of Utah and Colorado provides information on the timing and nature of early Cordilleran foreland-basin development. The Morrison Formation can be subdivided into three depositional facies assemblages: (1) lower progradational shallow marine, lacustrine, fluvial, and eolian facies deposited during Oxfordian-Kimmeridgian retreat of the Stump-Sundance sea; (2) a middle assemblage containing sandy-gravelly braided fluvial deposits, which are overlain by meandering fluvial channel and overbank facies; and (3) an upper assemblage of laterally stable, low-sinuosity, fluvial channel facies deposited during Tithonian-early Neocomian (?) time. The upper part of this assemblage shows evidence of alteration and early diagenesis related to development of an Early Cretaceous unconformity. The overlying Cedar Mountain Formation is subdivided into two facies assemblages: (1) the Neocomian Buckhorn Conglomerate was deposited by northeast-trending, sandy-gravelly braided rivers that were incised into the underlying Morrison Formation; (2) an upper assemblage containing laterally stable, low-sinuosity, fluvial channel facies deposited during late Neocomian-Albian time. The base of the unit contains a thick calcrete zone that formed during an unconformity following Buckhorn deposition.

Morrison and Cedar Mountain formation sandstones contain three petrofacies: a feldspar-rich lower Morrison petrofacies (%QmFLt = 70, 19, 11), and chert-rich upper Morrison and Buckhorn/Cedar Mountain petrofacies (%QmFLt = 54, 5, 41 and 69, 4, 27, respectively). Sandstone composition and paleocurrent data indicate a Cordilleran source area composed of Proterozoic, Paleozoic, and Mesozoic sedimentary rocks.

The Morrison Formation and Buckhorn Conglomerate were deposited in the back-bulge depozone of the Late Jurassic Cordilleran foreland-basin system and onlapped a flexural forebulge located in central Utah. Late Neocomian eastward migration of the forebulge uplifted areas in eastern Utah, producing an unconformity, while the foredeep in central Utah underwent flexural subsidence. The upper part of the Cedar Mountain Formation represents overfilling of the foredeep and deposition above the forebulge.