Recent discoveries of oil and gas in Pennsylvanian rocks of far-western Kansas have directed attention again to the prospects in southeastern Colorado. The structural backbone of southeastern Colorado is formed by the Front Range, the Wet Mountains, and a buried ridge, the Apishapa-Sierra Grande uplift, extending 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.

The Permian and Pennsylvanian rocks of western Kansas thicken westward into the Denver basin mainly by the addition of a thick wedge of lower Pennsylvanian rocks and then thin rapidly by overlap on the Front Range. Rocks of Morrow age in western Kansas can be traced across the Denver basin into the Glen Eyrie formation; rocks of Atoka, Des Moines, Missouri, and Virgil age are represented in the Fountain formation. Most of the Permian subdivisions of western Kansas lose their identity in easternmost Colorado, but the larger units can be delimited by key beds--the Stone Corral dolomite, the Blaine formation, and the Day Creek dolomite. By means of these key beds it

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can be established that Permian beds of Kansas above the Cedar Hills sandstone are represented by the Lykins formation of the southern Front Range; the Cedar Hills, Salt Plain, and Harper formations are represented by the Lyons sandstone; and beds of Sumner and Wolfcamp age probably are included in the upper part of the Fountain formation. (The author's interpretation of the correlation and age designation of formations in Colorado and western Kansas is not necessarily accepted by the U. S Geological Survey.)

Maps showing the thickness and distribution of coarse clastics of the Pennsylvanian and Permian rocks suggest that the Apishapa-Sierra Grande uplift, the Wet Mountains, and the Front Range were relatively low-lying land masses at the beginning of Pennsylvanian time. The Morrow seas advanced upon the flanks of these low-lying land masses, bringing clastic material from the southeast. Near the end of Morrow time major uplifting and 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 beginning of the Las Animas arch seems to have occurred near the end of Missouri time. During early Permian time the seas gradually covered th Apishapa-Sierra Grande land mass; during late Permian time the shore line remained fairly stable until the seas receded at the close of the period.

The possibilities for production of oil and gas from the porous limestones of both Missouri and Des Moines age in Colorado appear to be good, particularly if reef-like developments can be found fringing the more positive elements in the basins. In addition there is always present the chance of oil and gas accumulations in the basal sandstones of Des Moines, Atoka, and Morrow age and in the coarse arkosic sandstones that are overlapped along the positive elements.

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