Abstract

Jurassic rocks in the southern part of the Jurassic Western Interior basin consist largely of clastic materials, with considerably smaller quantities of carbonates and evaporites. The rocks were deposited in nonmarine environments and in marine waters that, at times, had moderately to highly elevated salinities and tended to be inimical to life. A magmatic arc bordered the southwest side of the continent and influenced sedimentation in the Western Interior by episodically providing voluminous quantities of volcanic ash to the region. The arc also produced elevated terrain and a rain shadow that at least partly determined the climate in the Western Interior. The climate was warm and dry and played an important role in sedimentation by fostering precipitation of evaporites, formation of extensive eolian dune fields, and development of a large saline-alkaline lake at the end of the period. The lithologic succession includes at least 8 local or region-wide unconformities, and the system is separated from both the Triassic and Cretaceous Systems by additional region-wide unconformities. All except one of these surfaces record the effects of tectonic adjustments in the region that probably occurred continuously but were made more noticeable by the slight but perceptible structural discordances at the unconformities. Some of the unconformities can be related to global sea-level fluctuations that have been proposed in the literature, but the known structural activity in the region, the poor age calibration of the beds, and questions concerning the validity and documentation of proposed global sea-level curves make it difficult to confidently relate most of the rock sequences to global eustasy.

Lower Jurassic rocks, not present east of the Colorado Plateau, were deposited in large ergs represented by the Wingate Sandstone and the younger Navajo Sandstone and their correlative beds. Eolian sedimentation was continuous in the northern part of the region, but farther south in the central part of the Colorado Plateau, it was disrupted after deposition of the Wingate by a flood of fluvial sand (Kayenta Formation and correlative beds) that was shed westward off the ancestral Rocky Mountains in about the middle of the epoch. The Mogollon slope on the northeast side of the magmatic arc in southern Arizona, also was a source of fluvial clastic debris shed onto the southwestern part of the Colorado Plateau.

Middle Jurassic rocks were deposited during five marine transgressive-regressive cycles in which an interior seaway that was mostly in Idaho and Wyoming expanded and contracted, there by moving the strandline back and forth across the northern and northwestern parts of the region. Eolian sands, evaporites consisting of gypsum or anhydrite and rare halite, and marine limestones form distinctive deposits of the epoch. Rapid subsidence in the Utah-Idaho trough along the west side of the region probably was caused by thrust loading farther west in the western Elko highlands during the early phase of the Elko orogeny. The principal source area for fluvial clastics transported into the southern part of the region during most of this time was the Mogollon slope farther south. In contrast, the principal source area for eolian clastics was to the north in shoreface sands along the southern shoreline of the Sundance seaway. Most of the clastic material in the Utah-Idaho trough probably was derived from the western Elko highlands. During the latest Middle Jurassic (late Callovian), thrust faulting ceased in the Elko orogenic belt and the Utah-Idaho trough or foredeep was inverted to a highland area called the eastern Elko highlands. This area then became an important source of sediment for the region farther east.

Deposition of Upper Jurassic strata began with two additional marine transgressions during Oxfordian and possibly earliest Kimmeridgian time. These were succeeded by continental sedimentation of the Morrison and related formations throughout the entire Western Interior basin. The Mogollon slope as well as the western and eastern Elko high lands were the principal source areas for clastic sediment. Uplift continued in the eastern Elko highlands as a continuation of the later or crustal rebound phase of the Elko orogeny, and thrust faulting, this time in the eastern Elko highlands, evidently did not begin until the Sevier orogeny in late Early Cretaceous time.