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Modern and Ancient Alluvial Systems
Sedimentology and Tectonic Setting of Early Tertiary Quartzite Conglomerates, Northwest Wyoming
Quartzite cobble and boulder conglomerates are widely scattered through the Paleocene-early Eocene stratigraphic record of the western Bighorn Basin, Wyoming. Paleocurrent indicators, in conjunction with the timing of tectonic events in northwest Wyoming, demonstrate that accumulation of these conglomerates was instigated by episodic orogenic activity in the Jackson Hole area, over 100 km west of the Bighorn Basin. Deformation resulting from intrabasinal tectonic pulses also controlled the periodicity of gravel influxes to the Bighorn Basin as well as the depositional settings in which the quartzite conglomerates formed. Different modes of deposition are distinguished on the basis of conglomerate outcrop pattern and distinctive suites of depositional facies.
The oldest (earliest Paleocene) conglomerate is dominated by massive or horizontally bedded cobble conglomerate that was deposited by a braided river that flowed through an area of dissected, upturned strata along the western margin of the Bighorn Basin. Unusually thick sets of planar cross-stratified conglomerate are also common and are attributed to deep and prolonged floodstages, reflecting humid conditions and valley confinement of the stream system. In contrast, the youngest (early Eocene) conglomerate, though dominated by massive or horizontally bedded conglomerate, lacks abundant large scale cross-bedded conglomerate and was deposited on an extensive braidplain. Facies change markedly over short distances due to lateral spread of flow on the braidplain and decrease in paleoslope toward the Tertiary structural axis of the Bighorn Basin.
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