GEOLOGY AND ORIGIN OF SOUTH PARK, COLORADO
J. T. Stark, J. H. Johnson, C. H. Behre, Jr., W. E. Powers, A. L. Howland, Don B. Gould, 1949. "GEOLOGY AND ORIGIN OF SOUTH PARK, COLORADO", Geology and Origin of South Park, Colorado, J. T. Stark, J. H. Johnson, C. H. Behre, Jr., W. E. Powers, A. L. Howland, Don B. Gould, others
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South Park in central Colorado is a basin of some 900 square miles with a relatively level floor, depressed in comparison with the enclosing hills to the south and even more so as compared with the Mosquito and Front Ranges that border it elsewhere. The chief purpose of this study, which comprised three field seasons and a party ranging from 3 to 15 persons each season, was to determine the origin of the Park. Broadly speaking this proved to be partly a matter of tectonics but largely one of erosion and deposition in Tertiary and Quaternary times. Details of structure, igneous history and petrology, Tertiary and earlier stratigraphy, and Pliocene and Pleistocene development are significant by-products of the investigation.
The rocks underlying the Park are pre-Cambrian and younger, representing every period except the Silurian and possibly the Triassic and Jurassic. The pre-Cambrian rocks are metamorphosed sediments and lavas (now schists and gneisses) and intrusive rocks; these are best exposed on the east side of the Park. The Paleozoic section resembles that of the Mosquito Range, and the Mesocoic section that east of the Front Range. Among these sediments, the Pennsylvano-Permo-Triassic(?) series of redbeds and the Dakota sandstone are most conspicuous.
Tertiary sediments contain vertebrate and invertebrate fossils and plant remains, and fairly definite correlation is possible. The Tertiary section extends through the Denver (Eocene) sandstones and conglomerates, Oligocene Balfour and Antero arenites and lutites with scanty limestones (collectively called “lake beds”), Miocene (?) Wagontongue clastics, Pliocene (?) Trump fanglomerates, and three sets of Pleistocene gravels.
From the Laramide Revolution onward, igneous activity was important. Intrusions, chiefly acid, include large and small plugs, sills of varying sizes, and narrow dikes, such as the Eshe porphyry and Chalmers granite. But extrusive rocks are also conspicuous, especially in Eocene, Oligocene, and Miocene times. Great rhyolitic flows and tuffs occur in the northern part of South Park; trachytic, andesitic, and basaltic extrusive rocks are more prominent in the south, Buffalo Peaks being part of one conspicuous series of such flows. The flows, flow breccias, and tuffs are mostly intercalated with the “lake beds”.
The sedimentary rocks younger than the pre-Cambrian and older than the Miocene (?) are folded into a large south-plunging basin, the southern part of which is truncated by younger rocks and complicated by flows. The eastern synclinal limb is broken and covered by a great thrust—the Elkhorn fault—which brings pre-Cambrian up in the hanging wall, the fault plane dipping gently east. This is one of the major Rocky Mountain thrust faults, but the comparable Mosquito fault and the somewhat similar but smaller London reverse fault appear near the western margin of the Park. These thrusts, all, unlike the chief thrusts of the Northern Rocky Mountains, from the east, and other faults and folds have been mapped in considerable detail.
The outstanding physiographic features of South Park are partly terraces and pediments, partly terminal and lateral moraines and outwash deposits of glacial origin. The terraces and pediments are in part pre-glacial. These are related to the later history of the Park.
During the Laramide Revolution, in a late stage of which the Elkhorn thrust developed, the synclinal basin was formed. In this the earlier Tertiary (but post-Denver) sedimentary rocks were deposited, clearly in part in fresh water. During this time, lava, ash, and tuff also accumulated. The basins of accumulation apparently resulted from ponding of one large stream or stream system flowing generally south, in consequence of the repeated and thick lava dams formed near the southern end of the Park. No block faulting could be found that might have been an alternate cause of the lake basin of those times. Then came folding of the “lake beds”, resulting in uplift, trenching by the same stream system, and deposition of the Miocene (?) Wagontongue beds. Uplift south of the Park led to a deflection of the drainage toward the east and the development of the Trump fanglomerate in Pliocene or less probably in the earliest Pleistocene. Between Wagontongue and Trump deposition a graded south-sloping surface was developed, now preserved at the southerly end of South Park in an excellent terrace and in the hilly to mountainous upland east of the Park. Correlation of this and later surfaces by elevation alone is hazardous because of late faulting; but the dating of a given surface remnant locally is fairly exact in view of the paleontologically established ages of the “lake beds” truncated by such surfaces.
Later five broadly separable pediments, the Como surfaces, were developed, presumably in pre-glacial times, not earlier as suggested by Lovering and Van Tuyl. Subsequent glaciation contributed drift of three stages and moraines and outwash of varying prominence, the latest (Wisconsin?) being the most distinct.