Title: Post-Paleocene Tertiary rocks and Quaternary volcanic ash of the Wet Mountain Valley, Colorado
Abstract: The Wet Mountain Valley in south-central Colorado is a tectonic basin bounded by the Sangre de Cristo Range and the Wet Mountains.Eocene deposits are prevolcanic arkosic coarse Echo Park Alluvium that we correlate with Farisita Conglomerate, which may be a facies of the Huerfano Formation.We believe that these deposits of the Echo Park occupy the lower part of a fill in the Wet Mountain Valley and surely extend southward into Huerfano Park where they merge with the Huerfano Formation and its partial lateral equivalent, the Farisita Conglomerate.The Huerfano and Farisita are interpreted by us as sedimentary facies representing distinctive lithologies from separate source areas intermixed to varying degrees by stream action.The onset of volcanism in early Oligocene time is marked between Rosita and Huerfano Park by the Devils Hole Forruation, here containing arkosic tuffaceous sandstones and pumice conglomerates as originally defined, but now assigned to the early Oligocene.The Devils Hole interfingers with the oldest rocks of the Deer Peak Volcanics, an Oligocene unit here named to include the Deer Peak stock and lava flows, lahars, and volcanic sediments derived from this center.Other similar volcanic rocks are present at the Rosita and Silver Cliff centers along the east flank of the Wet Mountain Valley, and small amounts of locally derived volcanic material interfinger with distantly derived rocks along the southern margin of the Thirtynine Mile volcanic field and in the Wet Mountain Valley as far south as Hillside and Goat Creek.The uppermost Tertiary deposit in the Wet Mountain Valley is a salmon-pink basin-fill alluvium, the Santa Fe(?) Formation of Miocene and Pliocene age.Camel bones, Camelops huerfanensis (Cragin), discovered before 1889 and assigned, according to then-current usage, to the Pliocene (now early Pleistocene), have been long used as a guide for age assignment of Tertiary beds of Huerfano Park.These can now be confidently assigned to the type 0 Pearlette ash in a terrace gravel of Yarmouth age rather than to the Tertiary deposits.1