Abstract
Sandstone in the Lower Pennsylvanian (Morrowan-Atokan) portion of the Fountain Formation in the Manitou Springs, Colorado, area was derived from Precambrian granite gneiss and deposited in associated medial alluvial-fan, foreshore, shoreface, and offshore environments constituting a wave-dominated fan delta. The sandstone facies are arranged in six vertically stacked progradational sequences. The regressive marine portions of the sequences rest on a transgressive lag conglomerate and range from 5 to 9 m thick. The alluvial deposits capping each sequence are up to 20 m thick. The different sandstone facies of the fan delta system show variations in mineralogical composition. Because all of the sandstone facies share a common first-cycle provenance and burial history, differences in composition largely reflect differences in depositional-process control on composition. Foreshore sandstones possess maximum compositional maturity; offshore sandstone is most immature. Apparently feldspar is winnowed out of the foreshore sand, bypasses the shoreface, and is concentrated in offshore, storm-deposited sand. Alluvial and shoreface sandstone are intermediate in composition with QFR content of 55:35:10 and 60:35:5, respectively.
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