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Critical Review of the San Juan Peneplain Southwestern Color~ Do Critical Review of the San Juan Peneplain Southwestern Color~ do GEOLOGICAL SURVEY PROFESSIONAL PAPER 594-I Critical Review of the San Juan Peneplain Southwestern Colorado By THOMAS A. STEVEN SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL -PAPER 594-I The volcanic and geomorphic history of the San Juan Mountains indicates no peneplain cycle of erosion between the end of ma;·or volcanism and the present time UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page Albstract ----------------------------------------­ I 1 Discussion of the San Juan peneplain-Continued Introduction -------------------------------------­ 1 3. The peneplain remnants -------------------- 18 Alcknowledgments --------------------------------- 2 4. Postpeneplain deformation -------------------- 9 General geology of the San Juan region ___________ _ 2 5. Postpeneplain alluviation and volcanism ______ _ 10 The peneplain concept ----------------------------­ 4 6. Postpeneplain development of drainage_------- 12 Discussion of the San Juan peneplain -------------- 6 Possible character of the late Tertiary landscape ___ _ 13 1. Subsidence of the prevolcanism erosion surface _ 6 Summary ----·------------------------------------ 14 2. Volcanism and volcano-tectonic deformation Catalog of peneplain remnants ---------------------- 14 in the central San Juan Mountains _______ _ 7 References cited ----------------------- ___ -------- 18 ILLUSTRATION PLATE 1. Geologic map, San Juan region, Colorado, showing relations of the San Juan Peneplain of Atwood and Mather------------------------------------------------------------------------------ __ . In pocket iii SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY CRITICAL REVIEW OF THE SAN JUAN PENEPLAIN, SOUTHWESTERN COLORADO By THOMAS A. STEVEN Abstract volcanologic ideas then in vogue, both represented The San Juan peneplain has been described as a wide­ significant advances. spread surface of low relief cut in late Tertiary time on The present report is concerned primarily with the San Juan Mountains of southwestern Colorado. A re­ only one of the aspects of geology discussed by the view of the geologic relations of many local flat areas that earlier authors, the concept of a widespread sub­ were interpreted to be remnants of this surface indicates that in most of these areas the flatness reflects somewhat summit peneplain of late Tertiary age which At­ modified original constructional surfaces, stripped surfaces wood and .Mather called the San Juan peneplain. on resistant layers of rock, or exhumed surfaces of diverse When proposed, this conceptual surface was perhaps origin. Bedrock structures now delineated by geologic map­ one of the most thoroughly "documented" pene­ ping show little or no similarity to many of the structures plains of its day, and since then it has been ac­ that have been postulated ,on the basis of the peneplain concept. The detailed volcanic and geomorphic history of cepted by most authors dealing with the general the central San Juan Mountains indicates no peneplain geologic history of the region. A wide variety of cycle of erosion between the end of major volcanism and geologic problems in adjacent areas have been in­ the present time. Rather, the original surface of volcanic terpreted in the light of relations postulated for the accumulation was only slightly modified from the end of peneplain. major volcanic activity in late Oligocene and early Miocene time until regional epeirogenic uplift of the Southern Rocky Subsequent, more detailed work done with the Mountains in the late Cenozoic caused widespread erosion advantage of increased geologic knowledge and and canyon cutting. newer concepts has resulted in major revisions of many of the older ideas. The concept of a San Juan INTRODUCTION peneplain, like that of many another peneplain pos­ tulated during the early part of the century, is no In 1932 Atwood and Mather published a mono­ longer tenable. This report is inevitably critical, graphic report on the physiography and Quaternary and in large part negative, but it is in no way in­ geology of the San Juan Mountains, southwestern tended to supplant the physical description of the Colorado, which summarizes the geologic history landscape that forms the fundamental core of the and discusses in detail the various processes that Atwood and Mather monograph. led to the present spectacular landscape of the re­ gion. Their report is the only comprehensive de­ As described by Atwood and Mather (1932, p. scription of the landscape that is available, and it 20-26), the San Juan peneplain was cut in late Tertiary time, after all but the final stage of vol­ still serves as the standard reference on the sub~ ject. The fieldwolik leading to their report was canic activity had occurred in the San Juan Moun­ done largely from 1909 to 1924, almost contempo­ tains. Erosion reduced a high volcanic plateau to a raneously with the study on bedrock geology by lowland plain studded with residual hills or monad­ Whitman Cross, E. S. Larsen, Jr., and associates nocks; and peneplanation was followed by doming, (Larsen and Cross, 1956). Both final reports clearly which raised the center of the San Juan region indicate an interdependence and cross-fertilization 3,000 feet or more above its margins. This dome of ideas, and in terms of the physiographic and was then dissected by fluvial and glacial agencies; Il I2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY this dissection led to the present rugged mountain­ region, and a great mass of middle Tertiary vol­ ous terrain. canic rooks and related intrusive rocks throughout In considering the specific geologic relations of the central part of the region. The San Luis Valley most of the individual peneplain remnants (see At­ on the east side of the San Juan region is filled wood and Mather, 1932, pl. 2), by comparing bed­ with Tertiary and Quaternary alluvial deposits de­ rock structures with structures required by the rived from the adjacent mountains. peneplain concept, and by reconstructing the de­ The Precambrian rocks, although highly varied, tailed volcanic and geomorphic history of many for present purposes can be considered as a unit of local areas, it has become evident that no wide­ crystalline metamorphic and igneous rocks under­ spread peneplain ever existed over major segments lying most of the sedimentary and volcanic rocks of the San Juan region in late Tertiary time. Those and exposed in the cores of anticlinal uplifts. In aspects of the late Cenozoic history of the South­ general, the Precambrian rocks are resistant and ern Rocky Mountains and Colorado Plateau that stand as rugged mountains where uplifted, as in the were deduced on the basis of such a surface need to Needle Mountains and southern Sawatch Range. be reviewed and revised. The distribution of the various sedimentary rocks is controlled for the most part by tectonic features. ACKNOWLEDGMENTS Paleozoic rocks are present mainly in the south­ Accumulation of data leading to this review western and northeasternmost parts of the region, began in 1952-53 when fieldwork in the Summit­ but were eroded from a late Paleozoic uplift-the ville mining district first convinced me that some­ Uncompahgre-San Luis highland-in the central thing was awry with the concept of a San Juan and northern parts of the area. Mesozoic rocks peneplain as previously described. Data collection cover most Paleozoic rocks and overlap their eroded has been carried on since then as an adjunct to edges onto the older highland, except around the my other fieldwork in the central and eastern San flanks of the Needle Mountains uplift. Juan Mountains, and during this time I have rarely All these rocks were deformed during the Lara­ Inissed an opportunity to discuss the problem with mide orogeny in Late Cretaceous and early Terti­ any of my colleagues who had any interest in, or ary time. An arch marked by the Uncompahgre information on, the subject. I acknowledge all the Plateau extends ~outheastward into the San Juan help and encouragement I have thus received and region and is there overlapped by younger volcanic particularly thank those who contributed unpub­ rocks. The Needle Mountains uplift projects south­ lished information for my use. Particular mention westward from the main Uncompahgre arch and is made of the help received from J. C. Ratte, forms a prominent dome northeast of Durango. P. W. Lipman, W. S. Burbank, R. L. Smith, R. A. Elsewhere, the sedimentary rocks were warped into Bailey, J. C. Olson, D. C. Hedlund, W. R. Hansen, broad shallow folds, including the broad San Juan C. S. Bromfield, and A. P. Butler, all of the U.S. basin of northern New Mexico and southern Colo­ Geological Survey. rado. GENERAL GEOLOGY OF THE SAN JUAN REGION Fluvial and related sediments were deposited in basins that developed in latest Cretaceous and Four major groups of rock units are exposed in early Tertiary time, and some of these deposits the San Juan region (pl. 1) , and all need to be contain abundant volcanic debris that has been in­ considered in a review of the San Juan peneplain. terpreted to indicate nearby volcanic accumulation, Precambrian rocks are exposed in the Needle perhaps associated with some of the laccolithic cen­ Mountains northeast of Durango, along the Gunni­ ters (Burbank and Lovering, 1933, p. 297-298; son River and its tributaries, and in the southern Shoemaker, 1956, p. 162). Direct evidence for an part of the Sawatch Ra;nge in the northeastern part age between Late Cretaceous and middle Tertiary of the region. These are overlain by sedimentary for one of the laccolithic centers can be seen at rocks of Paleozoic, Mesozoic, and early Tertiary age Ouray, where the intrusives cut Mancos Shale of along the east margin of the Colorado Plateau from Late Cretaceous age and are in part overlain uncon­ Montrose to Pagosa Springs, allld in places along the north margin of the region.
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