Paleoenvironments and tectonic significance of the Upper Jurassic Morrison/Lower Cretaceous Cloverly formations, Bighorn Basin, Wyoming

Abstract

<p>The Upper Jurassic Morrison and Lower Cretaceous Cloverly formations of the Bighorn Basin, Wyoming consist of mostly fluvial and lacustrine sediments deposited in a foreland basin east of the developing Sevier fold-thrust belt. Sediment source areas include the fold-thrust belt in eastern Idaho, arc volcanic vents in central Idaho, intraforeland basin uplifts and volcanic vents, and the craton. The Morrison Formation, in the northern part of the Bighorn Basin is transitional with the tide dominated marine deposits of the upper SunDance; The Morrison records deposition in a seasonally arid to semiarid coastal plain, changing upsection from a marginal lagoonal and tidal flat or lacustrine setting to a fluvial setting. The Morrison and Cloverly formations are separated by a regional unconformity. Two members are recognized in the Cloverly: (1) the Little Sheep Mudstone Member and (2) the overlying Himes Member. The Little Sheep Mudstone is part of an extensive clay playa mudflat which interfingers with ephemeral and perennial stream deposits. The Himes Member can be separated into two informal units. The lower Himes is a volcaniclastic-rich, low sinuosity channel sequence which drained western volcanic highlands in the Yellowstone region. It is restricted to the northern reaches of the Bighorn Basin. The upper Himes (informally referred to as the Greybull interval) unconformably overlies the lower Himes or Little Sheep Mudstone. The upper Himes is an estuary complex which marks the beginning of the Early Cretaceous marine transgression. Three distinct channel types are recognized in the upper Himes sequence and represent deposition in (1) an upper estuary, (2) tidal creeks and (3) small meandering fluvial channels. The upper estuary channels are characterized by sand bodies which are usually 15m or more thick and consist of fine to medium grained, well-sorted quartz arenites. The sands within this interval were derived from the east, possibly the craton.</p>