LATE CENOZOIC TECTONIC AND GEOMORPHIC EVOLUTION OF THE SOUTHERN FRONT RANGE, COLORADO

Late Cenozoic erosion in the southern Front Range developed a paleotopography ("early Miocene terrain") that beveled Oligocene igneous rocks, and preceded mountain erosion and Great Plains deposition of the middle and upper Miocene Ogallala Formation. This terrain ranged from a rounded upland in mountain cores, outward to nearly flat marginal areas. Uplift and erosion in mid-Miocene cut a hilly topography ("incised terrain") into the older terrain, and fed debris to the Ogallala. During latest Miocene and Pliocene times, uplift of the western Great Plains and adjacent mountains ended alluviation and caused erosion to cut a "steep-walled canyon terrain" in the mountains, and broad valleys in the plains. To illustrate: Mount Evans centers a broad dome with relics of the early Miocene terrain near its summit; its flanks are deeply fretted by hilly incised terrain and its margins by steep-walled canyons. To the south, an ovid area (Kenosh-Tarryall block) was strongly uplifted, and is marked by broad summit areas and block-faulted walls, all of which display relics of early Miocene terrain modified by incised terrain valleys. The broad South Park area to the S and SW was structurally stable, and according to Steven, Evanoff, and Yuhas (1997) was graded to sea level for nearly all of Oligocene and Miocene time. South Park was uplifted in Pliocene and Pleistocene but resulting steep-walled canyons have only now eroded headward to its outer margins. The Pikes Peak area was complexly uplifted concurrently with the Kenosha-Tarryall block (Shawe and Steven, this volume). It is a composite uplift with a sharply defined, strongly uplifted inner core, surrounded by a broader plateau-like platform. Remnants of older terrains are still preserved throughout the uplift and form the present summit of Pikes Peak (similar to Kenosha-Tarryall block and Mount Evans). Complex drainage changes took place concurrently with incised terrain erosion. Ancestral South Platte River was diverted from a channel extending from SE South Park through the Florissant Lake Beds and S along Fourmile Creek, to a new linear course from South Park NNE 110 miles to Greeley. The older course of the river was incised antecedently along the outer flank of the rising Pikes Peak block.