The structural backbone of southeastern Colorado is formed by the Front Range, the Wet Mountains, and a buried ridge, the Apishapa-Sierra Grande uplift, which extends southeastward from the Wet Mountains. The Las Animas arch, of later origin, plunges off this buried ridge to the northeast. Parts of three major structural basins are present in the area--the Hugoton embayment of the Anadarko basin, the Denver basin, and the Raton basin.

Southeastern Colorado was first covered by Paleozoic seas in late Cambrian time, when coarse clastics (Lamotte sandstone) derived mainly from the north were deposited in front of a low landmass that extended from Union County, New Mexico, northwestward to the vicinity of Canon City. The seas gradually encroached on this landmass, depositing additional upper Cambrian (Bonneterre dolomite) and lower Ordovician (Arbuckle group) rocks farther southwest. Near the close of early Ordovician time, gentle upwarping, possibly related to the development of the Transcontinental arch of Eardley, raised the central part of southeastern Colorado above sea-level. Seas of middle and late Ordovician age lapped upon the eastern and western slopes of this upwarping, depositing the sediments of the Simpso group and the Viola limestone. After the close of Ordovician time, uplifting, probably with some faulting, along the present trend of the Front Range, Wet Mountains, and Ahishapa-Sierra Grande uplift permitted the removal by erosion of most of the Cambrian and Ordovician rocks from that area.

In Mississippian time the seas advanced from the Anadarko basin, lapping on the gently sloping landmass. During Meramec time the seas probably completely covered the landmass and at the end of Meramec time began a withdrawal into the Anadarko basin. During Chester time the seas appear to have been restricted to the Hugoton embayment in the southeasternmost part of Colorado.

At the beginning of Pennsylvanian time, the Morrow seas advanced upon the flanks of the low landmass of eastern Colorado, bringing clastic material from the southeast. Near the end of Morrow time major uplifting with faulting elevated the Apishapa-Sierra Grande uplift, the Wet Mountains, and the Front Range, which supplied clastic material to transgressing seas during the remainder of Pennsylvanian time. A cross-flexure marking the earliest beginnings of the Las Animas arch seems to have occurred near the end of Missouri time. During early Permian time the seas gradually covered the Apishapa-Sierra Grande landmass, and during late Permian time the shore line remained fairly stable, until the seas receded at the close of the period.

End_of_Article - Last_Page 2613------------