Geology and Origin of South Park, Colorado

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At the beginning of Laramide orogeny, a blanket of undisturbed Cretaceous and minor older Mesozoic sedimentary rocks 1,500 to 3,000 m thick covered the Southern Rocky Mountain province, and the last of a series of Cretaceous seas was starting to withdraw northeastward across the region. Beneath the blanket was an older and inhomogeneous terrane that in some places consisted of the eroded stumps of late Paleozoic mountain ranges made up of Precambrian rocks, and in other places of piles of sedimentary rocks thousands of meters thick on the sites of late Paleozoic basins. At the onset of Laramide orogeny in Late Cretaceous time, most of the buried mountain ranges were re-elevated, and adjoining Laramide basins, in part inherited from the late Paleozoic basins, began to subside and receive orogenic sediments. In addition, two anticlines of mountain-range proportions-the Sawatch and Uinta-rose from the sites of late Paleozoic basins.Orogeny began in the southwest part of the province before marine deposition ended in the northeast. The late Paleozoic San Luis highland of southwestern Colorado and northern New Mexico was re-elevated in late Campanian or middle Montana Cretaceous time, as indicated by orogenic sediments in the San Juan basin to the south and the Raton basin to the east. The Sawatch anticline, which diverges northward from the rejuvenated San Luis highland, rose at about the same time, as indicated by the age (72 to 70 m.y.) of fault-controlled porphyries on the flanks. The Front, Park-Sierra Madre, and Medicine Bow Ranges, on the site of the late Paleozoic Front Range highland, rose after the marine Fox Hills Sandstone was deposited over their sites 67.5 m.y. ago. Uplift and erosion were rapid. By 66 to 65 m.y., and before the close of Cretaceous time, 3,000 m of sedimentary rocks had been eroded from at least parts of these ranges, and streams were carrying detritus from Precambrian rocks to bordering basins.Once started, uplift of mountain units continued through Paleocene and into 1 on July 2, 2015 memoirs.gsapubs.org Downloaded from 2 O. TWETO Eocene time, as indicated by nearly continuous Upper Cretaceous to Eocene sedimentary sequences in the interiors of bordering basins. Uplifts grew laterally as they rose vertically. Consequently, the major uplifts of today are areally larger than those that supplied the first orogenic sediments, and their border structures are younger than those sediments. The crystalline rock body of the interior of the Laramide San Luis highland supplied sediments to adjoining basins in Cretaceous time, but the sedimentary rock flank of the uplift at the site of the Sangre de Cristo Range was not uplifted and deformed until Paleocene and early Eocene time. Similarly, an interior part of the Front Range supplied Precambrian rock detritus to bordering basins very late in Cretaceous time, but the flank structures of the range, which involve the early orogenic sediments, developed in Paleocene and Eocene time. The Laramie Range prong of the northern Front Range probably did not...

Section: Sawatch and Mosquito Ranges And Elk Mountainsmentioning

confidence: 99%

Laramide (Late Cretaceous-Early Tertiary) Orogeny in the Southern Rocky Mountains

Tweto

1975

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130

“…This regional system can be traced to the south along Currant Creek, and it clearly joins the Use fault zone north of the Arkansas River. North of the Thirtynine Mile volcano the postvolcanic fault can be traced almost to Hartsel, where it appears to join the South Park fault as mapped by Stark and others (1949).…”

Section: South Parkmentioning

confidence: 99%

Neogene Tectonism in South-Central Colorado

Taylor¹

1975

“…The youngest sedimentary rocks involved in Laramide deformation and truncation by the surface establish a lower limit on the age of the surface. In eastern South Park, continental sedimentary and interbedded andesitic rocks, which are termed the Denver Formation (Johnson, 1937b;Stark and others, 1949) or South Park Formation (Sawatzky, 1967;DeVoto, 1971), have been folded and overthrust by Precambrian to Mesozoic rocks of the Front Range along the Elkhorn thrust. Stark and others (1949) and Sawatzky (1964Sawatzky ( , 1967 showed that the prevolcanic surface ("Elkhorn Upland") truncates the Elkhorn thrust and the folded strata of the South Park Formation.…”

mentioning

confidence: 99%

Geomorphic and Tectonic Implications of the Post-Laramide, Late Eocene Erosion Surface in the Southern Rocky Mountains

Epis¹,

Chapin²

1975

180

116

A late Eocene erosion surface of low relief, which extended throughout south-central Colorado, provides a post-Laramide, pre-Oligocene, regional structural datum. The age and geomorphic character of the surface are documented for an area of more than 10,400 km 2 in the southern Front Range, Rampart Range, South Park, Thirtynine Mile volcanic field, southern Mosquito Range, upper Arkansas River valley, southern Sawatch Range, and adjacent Great Plains to the east. The surface truncated middle Eocene and older rocks, which were deformed during the Laramide orogeny, and deeply beveled crystalline Precambrian rocks across wide areas. Size, shape, and distribution of the overlying Wall Mountain Tuff and associated gravel units show that the surface sloped gently southward and eastward and merged with the western Great Plains.Correlation of deposits on the surface indicates that it was uplifted 1,500 to 3,000 m and disrupted by block faulting of basin-and-range style in Miocene and later time. Many erosion surfaces now at various levels in the mountainous terrain of the area are faulted segments of this late Eocene surface. 45 on June 29, 2015 memoirs.gsapubs.org Downloaded from EPIS AND CHAPÍN It is suggested that erosion surfaces of similar geomorphic character and age developed over a much larger region of the Southern Rocky Mountains province and of the adjoining Basin and Range province to the south and southwest. INTRODUCTIONEarlier studies of the Thirtynine Mile volcanic field of central Colorado showed that the volcanic pile was underlain by an erosion surface of relatively low relief (Cross, 1894; Chapin and Epis, 1964; Epis and Chapin, 1968; Steven and Epis, 1968). Additional field, petrographic, and radiometrie-age data have provided a better understanding of the stratigraphic sequence of units in the volcanic field and of their regional correlation in outlying areas (Epis and Chapin, 1974). It is now possible to document the age and geomorphic character of the prevolcanic surface across an area of more than 10,000 km 2 and to show that the surface was extensively uplifted and block faulted in Miocene and later time.