Tectonic Implications of the Dakota and Lowermost Tropic Formations, Southwestern Utah: ABSTRACT

The middle Cretaceous (Albian-early Turonian) Dakota and lowermost Tropic formations in southwestern Utah form a wedge-shaped deposit of conglomerate, sandstone, mudstone, shale, and coal that thickens from a few meters in the Kaiparowits Plateau to more than 600 m in the Kolob terrace, a distance of 200 km. Four major facies assemblages are recognized: from the base upward, gravelly braided stream, coastal plain, mixed-energy shoreface, and offshore marine. The lowermost conglomerate unit was deposited by braided streams, that aggraded gravels within southeast-trending paleovalleys. A thin pebble lag occurs above and can be traced between conglomerate-filled valleys. The coastal-plain facies assemblage, characterized by fining-upward sandstone to coal sequences, was deposited by east-trending meandering streams with extensive interfluvial peat swamp and lacustrine environments. Five regionally correlatable (eustatic), transgressive-regressive marine sequences are recognized in the upper two facies assemblages, recording the initial incursion of the Cretaceous seaway. Temporally equivalent strata thicken from 8 m of offshore marine shale in the east to more than 200 m of stacked shoreline deposits in the west.