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. The San Juan Mountains formably by Telluride Conglomerate (Burbank, are made up principally of igneous rocks, which com­ 1940, p. 200; Luedke and Burbank, 1962) of prob­ prise probable Upper Cretaceous and ,lower Terti­ able early T·ertiary age. ary laccolithic intrusives along the west side of the Erosion following early Tertiary deposition cut CRITICAL REVIEW OF THE SAN JUAN PENEPLAIN, SOUTHWESTERN COLORADO I 3 deeply into the sedimentary rocks and exposed re­ litic to quartz latitic ash-flow field formed around sistant Precambrian rocks in the cores of the a source area near Creede. Cauldron subsidence ac­ Needle Mountains and Sawatch uplifts and, locally, companied many of the recurrent ash-flow erup­ along the top of the Uncompahgre arch. Fluvial tions, and at least six overlapping cauldrons have deposits of the Telluride Conglomerate and Blanco been recognized within an area about 50 miles long Basin Formation accumulated locally on the ero­ and 15-25 miles wide. The youngest of these, the sion surface, particularly near the Precambrian San Luis Peak caldera, north of Creede, and the rocks exposed in the cores of the Laramide uplifts. Creede caldera, south of Creede, bear on consid­ The ages of the Telluride Conglomerate and Blanco eration of the San Juan peneplain as their form is Basin Formation have not been established defi­ still reflected in the present topography. Potassium­ nitely, but the Blanco Basin has been correlated on argon age determinations (Steven and others, 1967) the basis of lithologic and stratigraphic similarities indicate that the oldest unit in the central San with the early Eocene San Jose Formation in the Juan sequence, the La Garita Quartz Latite, was central part of the San Juan Basin (Kelley, 1955, p. erupted in late Oligocene about 27.8 million years 86; Van Houten, 1957, p. 386; and Simpson, 1948, ago. p. 382). A series of summit volcanoes, consisting largely Major volcanic activity broke out in the San Juan of viscous rhyolite and quartz latite flows, domes, region following deposition of the Telluride Conglo­ and related breccias called Fisher Quartz Latite by merate and the Blanco Basin Formation. Inasmuch Larsen and Cross (1956, p. 172), formed along the as major volcanism ended near the close of Oligo­ crest of the San Juan volcanic field. Two of the cene time (Steven and others, 1967), it must have largest areas of Fisher Quartz Latite are associated begun some time in the Eocene or early Oligocene. with the Creede and San Luis Peak calderas and re­ In the western San Juans, the earliest volcanic ac­ sulted from the last of a long sequence of related tivity resulted in deposition of rhyodacitic pyroclas­ eruptions that earlier led to the extensive ash tic breccias and tuff, reworked volcanic debris, and flows and cauldron subsidences. A similar accumula­ local lava flows of the San Juan Formation to a tion to the southeast near Summitville (Steven and thickness of 3,000 feet over an estimated 2,400 Ratte, 1960a) does not show any such clear-cut square miles (Luedke and Burbank, 1963, p. C41). affinities to the underlying rocks and structures. Eruption of this great mass of material led to sub­ Potassium-argon age determinations (Steven and sidence of the general vent area between Silverton others, 1967) indicate that the type Fisher Quartz and Lake City to form a volcanic depression nearly Latite along the south margin of the Creede caldera 30 miles long and 15 miles wide. Alternating lava­ was erupted near the end of the Oligocene--about flow and pyroclastic eruptions deposited the Sil­ 26 million years ago-whereas one of the youngest verton Volcanic Group mostly within this depres­ flows at Summitville was erupted significantly more sion, and volcanic activity culminated in great ash­ recently-about 20.5 million years ago. flow eruptions of the Potosi Volcanic Group Fluvial gravels accumulated here and there within (Luedke and Burbank, 1963). The Silverton and the San Juan region late in the period of volcanic Lake City cauldrons, nested within the earlier San activity, and these have been called Los Juan volcanic depression, subsided in response to the Pinos Gravel, Bridgetimber Gravel, and Bayfield Potosi eruptions. Gravel. According to Atwood and Mather (1932, In the eastern San Juans, volcanic activity be­ p. 92, 101) the Bayfield Gravel is slightly older gan at a few widely scattered centers and in­ than the Bridgetimber and Los Pinos Gravels, creased to widespread eruption and accumulation of which presumably are lateral equivalents. They be­ andesitic to rhyodacitic lavas and breccias of the lieved that all the gravels accumulated within and Conejos Formation, which forms the largest vol­ along the flanks of the volcanic pile in either Plio­ canic accumulation in the San Juans. The com para.. cene or early Pleistocene time. Atwood and Mather tive age relations of the Conejos and San Juan (1932, p. 100-101) and Cross and Larsen (1935, Formations are not known, but both directly overlie p. 94-95) interpreted the Los Pinos as a member prevolcanism Telluride and Blanco Basin fluvial de­ of the Hinsdale Formation resting on the San Juan posits, and they may have been erupted at about the peneplain, whereas Larsen and Cross ( 1956, p. 186) same time. considered it to be a lateral equivalent of their Volcanism shifted to the central San Juans during Fisher Quartz Latite and, thus, to underlie the the last major period of activity, and a great rhyo- peneplain. The correlation of these widely sepa- 14 SHORTER _CONTRIBUTIONS TO GENERAL GEOLOGY

rated patches of gravels without more specific the mountains. Baltz (1965, p. 2073) postulated evidence than was cited seems hazardous, and the that the San Luis Valley is an eastward-tilted fault equivalencies that have been inferred are tenuous. block that was depressed and alluviated after most (For further discussion see "Postpeneplain Alluvia­ of the volcanic activity in the San Juans, an inter­ tion and Volcanism.") pretation that seems plausible although not yet The last volcanic rocks erupted in the San Juans proved. are widely scattered basaltic flows and locally as­ THE PENEPLAIN CONCEPT sociated rhyolitic lavas; these have been included together in the Hinsdale Formation (Larsen and The San Juan peneplain was conceived when most Cross, 1956, p. 193). Potassium-argon age determi­ physiographers enthusiastically supported the cycle­ nations (Steven and others, 1967) have shown that of-erosion concept as proposed by W. M. Davis. the rhyolite at the base of the succession in the Ancient peneplains, the natural end products of such central San Juan .Mountains is 22-23 million years cycles, were projected across many mountain old, whereas the overlying basalt flows are 12-15 ranges, particularly those with accordant ridge million years old, and that, in this vicinity at least, crests or high-level areas of low to moderate relief. the two rock types should not be included in the The concept was applied widely throughout the same formation. No evidence is now available to Southern Rocky Mountains (Davis, 1911; Lee, 1922; establish the age of the widespread basalt flows in Little, 1925; Mather, 1925; Blackwelder, 1909; Van the eastern San Juans relative to the dated flows Tuyl and Lovering, 1935), and the San Juan in the central San Juans, except to note that they peneplain was but one of many. all are late in the local sequence in which they According to Atwood and Mather (1932, p. 20- occur, and all rest unconformably on the interme­ 21), the San Juan peneplain was cut from a high diate to silicic volcanic rocks that make up the volcanic plateau during the interval between the great bulk of the volcanic field. Fisher and Hinsdale eruptive periods. As described The San Luis Valley along the east side of the by them, the peneplain's development progressed San Juan region is a great alluvium-filled segment through a typical Davisian erosion cycle, from of the Rio Grande trough, a complex graben zone youth, through maturity, to old age; but before all that extends northward from southern New Mexico the local monadnocks had been reduced to base to the vicinity of Leadville in central Colorado. The level the entire region was upwarped, and rejuve­ San Luis Valley is a down thrown block between nated streams have since carved out the present the San Juan Mountains on the west and the rugged terrain. Sangre de Cristo Mountains on the east. The details The concept of a summit peneplain was first de­ of structure are obscured by alluvial fill, but phyio­ veloped along the south flank of the San Juan graphic evidence for faulting is most conspicuous Mountains northeast of Durango, where numerous along the east margin of the valley, where coales­ accordant ridge crests and relatively flat summit cing alluvial fans are steeper and smaller than the areas clearly reflect an old erosion surface. The broad low fans from the west, and the steep front original description by Atwood and .Mather (1932, of the mountains is marked by truncated gullies p. 22) characterizes the area well: and faceted spurs. The buried structure along the west side of the valley and the relation of volcanic The divide between the Florida and Animas Canyons, for example, is a gently undulating upland surface 1 to 2 activity to this structure are not known. A deep miles in width, rising gradually from an altitude of 9,000 oil test (Powell, 1958, p. 275-280) drilled in the feet at the south to 12,000 feet 10 miles to the north, in valley about 29 miles northeast of Del Norte (pl. 1) the vicinity of the Needle Mountains. Neighboring divides was bottomed below sea level after penetrating on each side, which separate similar southward-trend valleys, more than 5,000 feet of alluvial fill and then nearly have similar altitudes increasing regularly toward the cen­ ter of the mountains. In the foothill zone numerous hog­ 3,000 feet of volcanic material without reaching back ridges rise to altitudes of 8,000 to 9,000 feet, so any of the Mesozoic, Paleozoic or Precambrian rocks arranged that a surface projected across their summits that constitute the higher peaks of the Sangre de rises gently toward the mountains and coincides with the Cristo Mountains which range from 12,000 to more intercanyon uplands. Outlying mesas and plateaus, such as than 14,000 feet in altitude. These pre-Tertiary Bridge Timber Mountain and the Mesa Mountains, display remarkably smooth tops, which likewise slope gently away rocks underlie the volcanic rocks of the San Juan from the San Juan Mountains. These outliers carry the Mountains at altitudes generally from 8,000 to restored graded surface downward to an altitude of about 10,000 feet along the north and southwest sides of 7,000 feet at the Colorado-New Mexico State line. CRITICAL REVIEW OF THE SAN JUAN PENEPLAIN, SOUTHWESTERN COLORADO 15

Extrapolating from these well-defined remnants, downward in pace with uplift and was thus able to Atwood and Mather (1932, p. 21-23) found many maintain its ancestral course. Although Atwood isolated high-level flat to gently rolling areas that and Mather did not call attention to the fact, the seemed also to represent fragments of an old ero­ Rio Grande shows similar relationships; it heads in sion surface. Most of these are simply high-level the western San Juan Mountains and flows east­ surfaces with altitudes generally accordant with ward across the center of the range to the east others nearby, but a few are veneered with patches margin at Del Norte. of gravel, and others by basaltic lava flows of the Accelerated downcutting resulting from the Hinsdale Formation. Mountain tops that project postulated arching of the San Juan region at the above the general level of the high, flat areas were end of the peneplain cycle was believed responsible believed to represent monadnocks. Integrating all for the deposition of the great alluvial fans around these features, Atwood and .Mather assembled a the margins of the mountains. These fans are par­ conceptual lowland plain locally studded with resi­ ticularly apparent along the west edge of the San dual hills and drained by sluggish meandering Luis Valley, where Los Pinos Gravel is shown (At­ streams. wood and Mather, 1932, pl. 2) as overlapping the This conceptual plain was contoured at 500-foot in­ edges of the San Juan peneplain. The Hinsdale tervals (Atwood and Mather, 1932, pl. 2), and the Formation was believed to have accumulated at representation obtained is summarized on plate 1 about the same time, as recounted by Atwood and of this report. The surface is irregular, as they de­ Mather (1932, p. 26): scribed it (Atwood and Mather, 1932, p. 24): While this process of sedimentation was continuing ex­ As thus restored the peneplain surface is nowhere smooth tensive flows of basaltic lava of Hinsdale age flooded hun­ and horizontal. Considerable portions of it in the central dreds of square miles of country. For the most part the part of the San Juan region now stand more than 13,000 lava floods were poured out on the newly deposited beds feet above the sea. In the foothills and near the margin of gravel and sand in the piedmont and valley regions. of the range its present altitude is generally less than In part, however, the lavas spread over bare rock surfaces 9,000 feet. Viewed in the large the peneplain surface arches in the eastern half of the domed area. gently over the San Juan Mountains. The surface of this dome is, however, complicated by numerous minor undula­ The concept of a San Juan peneplain as out­ tions, so that certain points upon the peneplain stand more lined above required corollary tectonic concepts. than 2,000 feet above other points only 4 or 5 miles away. The area underlain by the volcanic rocks was be­ To account for the gross and local irregularities lieved to have subsided concurrently with accumula­ on their peneplain surface, Atwood and Mather tion. According to Atwood and Mather (1932, p. (1932, p. 24-25) appealed in part to local residual 19), "The surface beneath the Tertiary volcanic hills or monadnocks, but to a far greater extent to rocks in the San Juan Mountains is at the present later deformation. The broad domal form of the time a great saucer tipped downward toward the surface, with the central area 12,000-13,000 feet east." above sea level and the marginal parts 7,000- This subsidence was required to depress the bulk 9,000 feet, was taken to indicate broad crustal of the volcanic rocks now remaining in the San warping and uplift, and the smaller scale "arches," Juan Mountains to below base level of peneplana­ "sags," and "basins" were taken to indicate more tion; despite this required subsidence, Atwood and local deformation. An abrupt discordance of the Mather (1932, p. 19-21) envisaged that the top of surface remnants near the center of the mountains the volcanic pile stood 3,000-4,000 feet above its was ascribed to offset along the Cannibal fault, and surroundings and that a significant but unspecified a similar discordance near the Gunnison River, to thickness of this pile was removed during the pene­ the Cimarron fault. plain cycle of erosion. Following the peneplain Drainage radiates outward from the higher parts cycle, the area that had previously subsided was of the San Juan Mountains, presumably reflecting domed to the form now shown by the reconstructed an outward flow of consequent streams from the center of the updomed area. Atwood and Mather peneplain surface (pl. 1). This local doming appar­ (1932, p. 25) called special attention to the Animas ently was superimposed on a postulated general River, however, which starts near the north margin uplift of Southwestern United States that took of the mountains and flows southward across the place at about the same time. dome. This relationship was interpreted to indicate Salient features of the San Juan peneplain, as that the Animas River was sufficiently active to cut visaged by Atwood and Mather, that are discussed 289-087 0-68-2 I6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY in the following review may be summarized as incompatible with the bedrock relations and should follows: therefore be abandoned. 1. The area subsided concurrently with accumula­ 1. SUBSIDENCE OF THE PREVOLCANISM EROSION SURFACE tion of the volcanic rocks, and the underlying The area subsided concurrently with accumulation erosion surface "is at the present time a great of the volcanic rocks, and the underlying erosion sur­ saucer tipped downward toward the east" (At­ face "is at the present time a great saucer tipped wood and Mather, 1932, p. 19). downward toward the east" (Atwood and Mather, 1932, p. 19). 2. The peneplain cycle of erosion, during the in­ terval between the Fisher and Hinsdale erup­ This generalization is supported in part by the tive periods, removed a significant thickness of gross distribution and altitudes of exposure of the rocks from above the level of the San Juan volcanic rocks, but it is an oversimplification that peneplain. does not take into account many of the diverse 3. The principal evidence for the peneplain is the elements in the structure of the San Juans. It is existence throughout the San Juan .Mountains difficult at the present state of knowledge to sepa­ of many isolated high-level flat to gently roll­ rate the effects .of irregular underlying topography, ing areas, which are interpreted as remnants of regional and local tectonic warping, volcano-tectonic the peneplain. Some irregularities projecting subsidence and resurgence of caldera areas, and above the conceptual surface are interpreted as possible basining caused by the load of volcanic monadnocks. rocks, as postulated by Atwood and Mather. Evidence for basining apparently depends on ex­ 4. The broad domal form of the peneplain is the posures of volcanic rocks at relatively low altitudes result of subsequent crustal uplift; small irreg­ along an east-trending belt through the middle of ularities on the surface, other than monadnocks, the San Juan Mountains. These low exposures are resulted from smaller scale deformation. Two best displayed in the Silverton-Lake City area in abrupt discordances are due to major faulting. the western San Juans (alts near 9,000 ft) and in 5. Great marginal alluvial fans indicate accelerated the Creede area in the central San Juans (alts near downcutting that followed regional arching at 8,400 ft). At these places, however, more re~ent the end of the peneplain cycle. The Hinsdale geologic work has established the existence of Formation (chiefly basaltic lava flows) was major cauldron complexes, and these complexes ac­ erupted at about the same time and spread count in part at least for the low position of the widely over the peneplain and marginal fans. rocks. 6. The generally radial drainage pattern of the pres­ Atwood and Mather ( 1932, p. 25) emphasized a ent San Juan Mountains reflects outward flow general eastward tilting of the San Juan region to­ of consequent streams from the center of the ward the San Luis Valley, a tilting that seems indi­ domed San Juan peneplain area. The Animas cated by the generally higher structural levels of River is an exception that cut downward rap­ the rocks in vicinity of the Silverton and Lake City idly enough to keep pace with uplift and main­ cauldrons, in the western San Juans, relative to tain its course across the crest of the dome. structural levels of the rocks along the east mar­ DISCUSSION OF THE SAN JUAN PENEPLAIN gins of the mountains near Del Norte. At least part of this difference in structural level reflects post­ The premise of this paper is that the geomorphic volcanism deformation, for the tilting is most history of an area must be compatible with the obvious along the east margin and the Conejos geologic relations determined from the bedrock. In River area of the San Juan Mountains, where the paragraphs that follow, each of the six salient welded ash-flow tuffs, sedimentary beds in the Los features summarized above is discussed in the light Pinos Gravel, and basalt flows in the Hinsdale For­ of the geologic structure and history determined mation dip eastward and project under the alluvial from detailed and reconnaissance bedrock studies, many of which were done since the idea of the San fill of the valley; also, the tilting seems closely related Juan peneplain was conceived. The discussion in­ to subsidence of the San Luis Valley. The base of the cludes an analysis of the geologic relations of volcanic rocks around the north, west, and south­ nearly all the local surfaces interpreted by Atwood west sides of the San Juans (pl. 1) shows no indi­ and Mather as peneplain remnants. The conclusion cation of a general eastward tilt, but it ranges is that the concept of the San Juan peneplain is irregularly in altitude from 8,000 to 10,000 feet CRITICAL REVIEW OF THE SAN JUAN PENEPLAIN, SOUTHWESTERN COLORADO I 7 from the Cochetopa Creek area westward to to later deformation almost concurrent with post­ Telluride, and from the Piedra River area south­ volcanism foundering of the San Luis Valley. eastward to New Mexico. In the Needle Mountains 2. VOLCANISM AND VOLCANO-TECTONIC DEFORMATION area, where exposed Precambrian rociks stood high IN THE CENTRAL SAN JUAN MOUNTAINS prior to volcanic activity, the base of the overlap­ The peneplain cycle of erosion, during the interval ping volcanic rocks rises to an altitude of more between the Fisher and Hinsdale eruptive periods, than 12,000 feet. A broad gentle syncline extends removed a significant thickness of rocks from above eastward across the central San Juans from the the level of the San Juan peneplain. headwaters of the Rio Grande to the margin of the The above assumption is invalidated by the rec­ mountains at Del Norte, and in combination with ognition of many original constructional volcanic the irregularly high position of the base of volcanics features in the central San Juan Mountains. Begin­ to the north, west, and south gives a basinlike struc­ ning with Burbank's (1933, p. 156-162) pioneer ture to much of the volcanic field. Inasmuch as the recognition of the Silverton cauldron, nested vol­ syncline jnvolves basaltic lavas of the Hinsdale cano-tectonic subsidence structures have been found Formation, it probably formed largely in postvol­ in two main areas in the western and central San canism time, and the basining may thus have had Juan Mountains (pl. 1). Of especial interest to the little to do with accumulation of the volcanic present review are the Creede and San Luis Peak rocks. calderas (Steven and Ratte, 1960b, 1965); these are Many of the structures in the volcanic rocks of the youngest and most prominent of the cauldron the central San Juan Mountains near Creede are structures that have so far been recognized in the related to local caldera subsidence or postsubsi­ central San Juans, and the original form of these dence doming, and no regional eastward tilting features is still reflected in the present topography. seems indicated. Layers of welded ash-flow tuffs The Creede caldera is particularly well preserved, dip gently eastward over some fairly extensive and the later history of its development is well areas, but the same layers are flat or dip gently understood. in other directions in nearby areas, and all are cut by faults of local volcano-tectonic origin. The The cauldron complex of the central San Juan Wason Park Rhyolite, a distinctive welded tuff Mountains developed concurrently with the erup­ (Steven and Ratte, 1964, p. D59), lies at the same tions that deposited the surrounding ash-flow tuffs. altitudes-between 10,800 and 11,200 feet--over The last of these eruptions were of the Nelson areas of several square miles or more in both the Mountain Quartz Latite in the northern part of the Clear Creek area, 15 miles west of Creede, and field and the Snowshoe Mountain Quartz Latite in the headwaters of Alder Creek, 13 miles east­ the central part of the field (Steven and Ratte, southeast of Creede. Clearly there is little differ­ 1964, 1965). The San Luis Peak caldera subsided in ence in gross structural level of this formation over response to the Nelson Mountain and earlier erup­ an east-west span of more than 25 miles across tions, and the Creede caldera in response to the Snowshoe Mountain eruptions. The cores of both the central San Juans. calderas were resurgently domed following subsi­ The north margin of the San Juan Mountains, dence, and the uplifted blocks still constitute prom­ from the divide between Cochetopa and Cebolla inent high points in the present topography. Creeks westward, shows many layers of differen­ tially welded ash-flow tuffs inclined a few degrees San Luis Peak in the core of the San Luis Peak northward toward the Gunnison River. Although caldera has an altitude of 14,014 feet and is the irregular in detail, the underlying surface on Pre­ highest peak in the central and eastern San Juan Mountains. All the exposed rock beneath this peak cambrian and Mesozoic rocks is also inclined north­ is Nelson Mountain Quartz Latite, which has an ward, generally parallel to the welded-tuff layers. aggregate thickness of more than 3,500 feet in the Thus, a general basining and eastward tilting of core of the caldera. Related welded tuffs outside the surface beneath the central and eastern parts the caldera range from 0 to 1,000 feet in thickness, of the volcanic pile seems to have occurred. How­ but are only a few hundred feet thick in most ever, the gross structure is complicated by so many places, and form high benches at altitudes above local structures of diverse age and origin that it is 12,000 feet along the Continental Divide east and difficult to determine which aspects are due to west of San Luis Peak. No way is known to de­ basining concurrent with accumulation and which termine how much rock has been eroded from the 18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY tops of many of these benches. It seems to have been est lacustrine beds in the Creede Formation are at minor, however, as remnants of local Fisher Quartz an altitude of about 10,000 feet, but fluvial beds Latite volcanoes rest on the Nelson Mountain tuffs in a tributary valley northwest of Creede are pre­ west of the San Luis Peak caldera and indicate served at altitudes up to 10,700 feet. The sill of that the original top of the tuffs was only slightly the outlet probably was not below the top of the higher than the tops of nearby benches not covered hard Wason Park Rhyolite, which is near the pres­ by Fisher lavas. ent altitude of 11,000 feet. Although not conclusive, La Garita .Mountain, which lies between the all these factors suggest that the top of the Creede southeast margin of the San Luis Peak caldera and Formation fill probably was at least at the level area marked (9) on plate 1, is a high fault block of the top of the Wason Park Rhyolite and perhaps of La Garita Quartz Latite that was uplifted early significantly higher, but that it was not as high as during the period of volcanic eruptions from the the position of the San Juan peneplain, which in central San Juan source area. Younger ash-flow this vicinity was drawn across the tops of the high units, including the Nelson Mountain Quartz La­ benches adjacent to the graben that provided the tite, overlapped the margins of the block but may outlet (Atwood and Mather, 1932, pl. 2). never have covered the higher parts, which still The domes and viscous flows of Fisher Quartz stand at altitudes of 13,488-13,895 feet. The orig­ Latite along the south side of the Creede caldera inal rough structural topography formed early in still stand as rounded mountains extending above the volcanic history of the central San Juan area the general level of the surrounding terrain. Fisher thus has persisted in modified form to the present Mountain and Beautiful Mountain (pl. 1, area 11) time. are local domal accumulations of quartz latite whose In the development of the well-displayed Creede present rounded summits have been lowered from caldera south of Creede, final subsidence left a the original level of the flow tops by an unknown nearly circular flat-floored basin below the present amount. However, the present high areas at least bed of the Rio Grande, which ranges in altitude reflect the original constructional highs. Areas ad­ from 8,400 to 8,800 feet. Steep walls extending up jacent to these domal protrusions may never have to present altitudes of about 10,500-13,000 feet attained the height postulated for the San Juan surrounded most of the east, north, and west mar­ peneplain in this vicinity by Atwood and Mather gins, and tangential grabens extended outward from (1932, pl. 2). the southwest and northeast margins (pl. 1). One Although many of the original constructional of the graben faults west of the Creede caldera forms still recognized in and adjacent to the central coincides in part with the Cannibal fault of At­ San Juan caldera complex were preserved by burial wood and Mather ( 1932, pl. 2); the faulting here and were exhumed only recently, others were not, is definitely related to caldera subsidence or post­ and the deep erosion required by the peneplain subsidence doming, however, and preceded the pos­ concept is precluded. At least some of the area ap­ tulated peneplain cycle of erosion. parently never reached the height postulated for After subsidence, the core of the Creede caldera the peneplain. was strongly domed, and the center of the core, 3. THE PENEPLAIN REMNANTS now marked by Snowshoe Mountain, was uplifted The principal evidence for the peneplain is the ex­ to well above a present altitude of 12,000 feet, but istence throughout the San Juan Mountains of many the margins remained near 8,000 feet. Viscous lava isolated high-level fiat to gently rolling areas, which of Fisher Quartz Latite was erupted locally around are interpreted as remnants of the peneplain. Some the margin of the caldera, chiefly toward the south irregularities projecting above the conceptual surface (pl. 1, area 11), where the lavas heaped up near the are interpreted as monadnocks. vents to form Fisher .Mountain, Beautiful Moun­ Either from personal knowledge derived from tain, and other volcanic domal mountains. Stream field examinations, from reviewing published ac­ and lake sediments, ash beds, and travertine of counts, or from discussions with colleagues, I have the Creede Formation were deposited elsewhere in been able to classify by origin most of the peneplain the structural moat around the caldera margin, remnants shown by Atwood and Mather (1932, pl. filling the closed basin to the lowest place on the 2). This was done as an initial step in reviewing rim, which was along the graben extending south­ the San Juan peneplain concept, and the informa­ eastward from the caldera. The actual height of tion obtained is fundamental to much of this re­ this outlet is not known; the topographically high- view. The classification that emerges from this CRITICAL REVIEW OF THE SAN JUAN PENEPLAIN, SOUTHWESTERN COLORADO I9 analysis is shown below; numbers refer to locali­ the surface, other than monadnocks, are the result ties on plate 1, and detailed discussions of the of smaller scale deformation. Two abrupt discordances remnants are given in the catalog at the end of are due to major faulting. this report. The broad dome, like the peneplain itself, results from the integration of many high-level surfaces Classification of peneplain remnants of diverse origins. No independent evidence has [Asterisk preceding number indicates that classification applies to only part of locality) been cited in its support, and considerable evidence Stripped surfaces on resistant rock layers­ can be cited against it. (See "Subsidence of the On welded tuffs: 1-4, *6, 7, *10, *11, 13, 24. Prevolcanism Surface," and "Volcanism and Vol­ On sedimentary beds: 5, *6, *15, 16-18, *19, *21. cano-tectonic Deformation in the Central San Juan On sills: *15, 23. Mountains.") In particular, the evidence cited in­ Exhumed older erosion surfaces: 14, *21, *25. Slightly modified original constructional surfaces: 8, *9, dicating that much of the deformation responsible *11, *25. for the basinlike structure of the volcanic field is Irregular topography (not a plain): 10, 12, *15, *19, *25. late, perhaps coincident with subsidence of the San Unknown origin: *9, *19, 20, *22, *25. Luis Valley, makes this conceptual broad dome un­ Most of the surfaces that were interpreted to be likely. peneplain remnants reflect layers of resistant rock The smaller scale deformation, on the other hand, of one kind or another from which softer material postulates specific features that can be dealt with has been stripped. Several factors militate against in terms of local geological relations. In part, the the hypothesis that these surfaces truly represent detailed arches and sags on the San Juan peneplain a late Cenozoic peneplain: (pl. 1) coincide with the structure in the underly­ ing rocks, but in all these places the so-called pene­ A. The remnants represent many different geo­ plain remnants expose hard layers of rock from logic horizons, from slightly modified original which less resistant materials have been stripped. constructional surfaces to exhumed remnants The structures reflected by these stripped surfaces of the early Paleozoic surface cut on hard vary widely in age, and differential erosion of de­ Precambrian rocks. Most of the remnants can formed layers having different resistances seems be explained readily without postulating re­ to have been more important than erosion to a par­ gional peneplanation. ticular base level at a particular time. B. Only those stripped surfaces that conform to the envisaged configuration of the peneplain Stripped surfaces are especially evident in the were included as remnants; those nearby but southeastern San Juan Mountains, where the so­ at different levels were excluded. Stripped called peneplain remnants are developed on in­ surfaces at many levels commonly follow the clined layers of differentially welded tuffs. (See 24, tops of particularly resistant rock layers, and "Catalog of Peneplain Remnants.") These surfaces the resulting bench-and-tableland topography are generally parallel to the underlying layers and is typical of many terrains underlain by low­ are in part covered by Los Pinos Gravel and basalt dipping strata of widely varying resistance. flows of the Hinsdale Formation; all were tilted eastward toward the San Luis Valley, beginning C. Many of the inclined remnants are stripped sur­ late in the period of volcanic activity and ending faces on tilted layers of resistant rock. Tilt­ some time following the Hinsdale eruptions. Simi­ ing in some areas clearly preceded stripping, larly, most of the remnants along the north side and regional base level had little control on of the San Juan Mountains are stripped surfaces local topographic form. on hard layers of welded tuffs. (See under 1, 2, 3, The origins of many individual remnants consid­ 4, and 6, "Catalog of Peneplain Remnants.") These ered alone neither preclude nor require a former stripped surfaces dip gently north from the San peneplain; however, considered in the context of Juan Mountains and south from the West Elk bedrock geology, they appear to depend more on Mountains toward a low area along the Gunnison local control of erosion by resistant rock layers River and reflect the tops of warped resistant beds than on erosion to a regional base level as required in the underlying layered units. by the peneplain concept. Differential erosion of interlayered hard and soft 4. POSTPENEPLAIN DEFORMATION rocks also is reflected by the peneplain remnants The broad domal form of the peneplain is the result around Rico (Cross and Ransome, 1905) and in the of subsequent crustal uplift; small irregularities on La Plata Mountains (Cross and others, 1899; Eckel, I 10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

1949) northwest of Durango (pl. 1). Here, lacco­ and synclines occur in adjacent areas, but these lithic intrusives, possibly of latest Cretaceous a,ge structures do not conform to the postulated configu­ (Shoemaker, 1956, p. 162), domed the adjacent ration of a deformed San Juan peneplain. sedimentary rocks. The peneplain remnants are al­ The "arches," "sags," and "faults" that presum­ most all stripped surfaces on inclined hard sand­ ably deform the San Juan peneplain throughout the stone beds or, in a few places, on igneous sills central part of the San Juan .Mountains (pl. 1) also related to the laccolithic centers. (See under 15, 16, were derived from integration of surface remnants 17, and 18, "Catalog of Peneplain Remnants.") Dom­ of diverse origin. Any consideration of the pene­ ing was much earlier than the erosion, and the plain in this area, however, is inextricably involved stripped surfaces appear to have formed with their with details of volcanic history and has been pre­ present inclinations. sented under item 2 in the section "Volcanism and In other places the "arches" and "sags" on the Volcano-Tectonic Deformation in the Central San San Juan peneplain (pl. 1) are not reflected in the Juan Mountains." underlying bedrock. The "sag'' along the trend of The abrupt discordances of the peneplain surface the upper San Miguel River coincides with the south­ (pl. 1) were attributed by Atwood and Mather to ma­ east end of the Placerville anticline, a broad low fold jor postpeneplain faulting. Their Cannibal fault co­ in the southeastern part of the Paradox fold-and­ incides in part with one of the graben faults fault belt of the Colorado Plateau (Bush and others, extending northwest from the Creede caldera, and 1960, p. 468, fig. 45; 1961). The "arch" between this graben fault is definitely related to caldera the San Miguel River and Rico also has no counter­ subsidence and resurgence and preceded the postu­ part in the bedrock, for Bromfield (1961) and lated peneplain. The Cimarron fault does not offset Bromfield and Conroy (1963) showed sedimentary the volcanic rocks (Larsen and Cross, 1956, pl 1), beds beneath this area to be flat or to dip gently and according to W. R. Hansen (oral commun., northward everywhere except adjacent to intrusive 1965) is clearly prevolcanism in age. igneous bodies. In summary, the record of deformation indicated The steep northwestern slope of the so-called by bedrock structures is clearly incompatible with Summitville arch (Atwood and Mather, 1932, pl. 2), major doming and faulting and with smaller scale which extends northeastward through area 25 (pl. deformation of the peneplain inferred by Atwood 1), also appears not to be of structural origin. As allld Mather. described later under 25 in the section "Catalog of 5. POSTPENEPLAIN ALLUVIATION AND VOLCANISM Peneplain Remnants," several of the small rem­ Great marginal alluvial fans indicate accelerated nants of the San Juan peneplain along the top of downcutting that followed regional arching at the end this "arch" are somewhat modified original flow of the peneplain cycle. The Hinsdale Formation (chiefly tops in a local pile of quartz latite lavas that stood basaltic lava flows) was erupted at about the same above the surrounding terrain cut on older rocks. time and spread widely over the peneplain and mar­ The abrupt northwest margin of the "arch" thus ginal fans. appears to reflect an outer constructional slope on Alluvial deposits now represented by the Los this local volcanic accumulation. Pinos, Bridgetimber, and Bayfield Gravels are scat­ The pronounced arch postulated on the peneplain tered widely around the margins of the San Juan in the southeastern San Juan Mountains near the volcanic field. These deposits were believed by At­ Colorado-New Mexico boundary was obtained by wood and Mather (1932, p. 84-101) to be approxi­ integrating surface remnants of diverse origin. mate stratigraphic equivalents; the Bayfield Gravel Those remnants on the east flank reflect stripped was thought to have been deposited locally on the surfaces on hard layers of welded tuffs tilted east­ San Juan peneplain just prior to doming, and the ward toward the San Luis Valley. The west flank of Bridgetimber and Los Pinos Gravels were thought the arch, however, is represented by a single rem­ to represent alluvial fans deposited slightly later nant near the State line south of Pagosa Springs. As around the margins of the updomed San Juan mapped and described by Wood, Kelley, and Mac­ region. Alpin (1948) and Dane (1948), this remnant is the Available data are insufficient for detailed con­ stripped surface of an igneous sill that intruded the sideration of the equivalence or nonequivalence of Animas Formation of Late Cretaceous and Paleocene all the scattered remnants of gravel that have been age; structural contours and cross sections shown referred to these foundations. The available evi­ by these later workers indicate that broad anticlines dence does suggest, however, that many of these CRITICAL REVIEW OF THE SAN JUAN PENEPLAIN, SOUTHWESTERN COLORADO I 11

gravel deposits accumulated as the result of loeal have been cited as evidence for the existence of a rather than regional conditions, and that at least San Juan peneplain (Kelley, 1957, p. 161). Both some of the earlier correlations are demonstrably Atwood and Mather (1932, p. 26) and Larsen and wrong. Cross (1956, p. 206-207) suggested that the Hins­ As described under item 1 in the "Catalog of dale Formation was erupted from many scattered Peneplain Remnants," W. R. Hansen (oral commun., centers throughout the San Juan Mountains. Lar­ 1964), found that gravel deposits near the Black sen and Cross cited several major centers of accu­ Canyon of the Gunnison River referred by Atwood mulation and postulated the existence of more. In and Mather (1932, p. 86) to the Bayfield Gravel are more recent fieldwork, both P. W. Lipman and I actually interlayered with an extensive sequence of have found several small vent areas not connected ash-flow tuffs. Thus, the gravels here are signifi­ with any of the larger remnants of Hinsdale lavas. cantly older than the postulated peneplain cycle of The flows that were erupted from these multiple erosion and represent deposition from streams that sources covered the existing surfaces around the reestablished themselves throughout the area be­ local vents, and no widespread plain is required to tween periods of volcanic activity. account for the present distribution of flows. Detailed work by Butler (1946) in the Tusas­ No evidence is known to indicate that all the Tres Piedras area, New Mexico, just south of the late basalts and rhyolites previously included in the lower Conejos River area, shown on plate 1, has Hinsdale were erupted during any given brief in­ shown the Los Pinos Gravel there to be an assem­ terval of time, and in the central San Juan area blage of members of limited lateral extent that (near 8, fig. 1) they definitely were not. In a series accumulated concurrently with local volcanic activ­ of remnants extending about 25 miles north from ity. Much of the debris in the Los Pinos Gravel in the upper Rio Grande, a distinctive white rhyolite the Tusas-Tres Piedras area was derived from con­ welded tuff underlies a series of basalt flows. Potas­ temporaneous volcanism, rather than from erosion sium-argon determinations have shown that the of preexisting rocks. rhyolite was erupted 22-23 million years ago, Larsen and Cross (1956, p. 185-186) reported whereas the basalts were erupted 12-15 million similar relations in the area to the north, along the years ago (Steven and others, 1967). It seems east flank of the San Juan Mountains in Colorado equally plausible that rocks in other remnants of (pl. 1). In the Green Ridge-Chiquita Park area the so-called Hinsdale would have been erupted at between Monte Vista and Alamosa Creek, the Los widely different times during the latter part of the Pinos consists in large part of lava flows and pyro­ Tertiary. clastic rocks, which grade laterally southward into Remnants of the so-called Hinsdale Formation typical fluvial sediments of the formation. Larsen in the northern part of the San Juan region com­ and Cross considered the Los Pinos to be equivalent prise two large areas, the Alpine Plateau (north­ in age to their so-called Fisher Quartz Latite at east of area 3, pl. 1) and Cannibal Plateau (south­ Summitville, and thus older than the peneplain west of area 6, pl. 1), as well as several smaller cycle of erosion postulated by Atwood and Mather. patches. At the north edge of the Alpine Plateau Richmond (1965) has shown that the type mass, W. R. Hansen (oral commun., 1964), has Bridgetimber Gravel near Durango, Colo., actually found remnants resting on stripped surfaces on consists of two deposits of different ages and that welded ash-flow tuffs. According to J. C. Olson both are probably of fluvioglacial origin. Nearby (oral commun., 1964), the small remnant south of deposits mapped as Bayfield Gravel are probable cor­ area 4, plate 1, rests largely on a stripped surface relatives of the type Bridgetimber, or are srrtall on a densely welded ash-flow tuff, but southward it overlaps older rocks and extends onto a stripped accumulations of diverse origin. surface on Precambrian rocks. Olson also has As shown by Larsen and Cross (1956, pl. 1), the mapped some of the northernmost tongues of Hins­ Hinsdale Formation consists largely of basaltic lava dale connected with the Cannibal Plateau mass flows containing some rhyolite which occur in scat­ (due west of area 6, pl. 1) and has found these to tered patches throughout the northern, central, and rest on stripped surfaces on densely welded ash­ eastern parts of the San Juan Mountains. The lava flow tuffs. I found the same relationship on a re­ flows are at about the same levels as nearby smooth connaissance traverse across the east margin of the surfaces interpreted as remnants of the San Juan area of the so-called Hinsdale Formation due east peneplain (Atwood and Mather, 1932, pl. 2) and of Lake City. The surface covered by these remnants I 12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

is therefore typical of the surfaces found in many the Hinsdale patches abut and wedge out southward ash-flow fields, where erosion of differentially against the local accumulation of relatively young welded layers forms widespread bench-and-table quartz latitic lavas centered in the Summitville lands. area that stood as much as 2,000 feet higher than Farther south, in the vicinity of Spring Creek the adjacent terrain. (See under 25, "Catalog of Pass (northwest and southwest of area 8, pl. 1), Peneplain Remnants.") The Hinsdale thus appears the Hinsdale Formation (Larsen and Cross, 1956) to have flowed out over a local piedmont plain at rests in part on the slightly modified original top the northern base of a highland area of domal of the Nelson Mountain Quartz Latite and in part quartz latitic flows, and the general topography on stripped surfaces of older welded-tuff layers and had no resemblance to a peneplain. lava flows. Unbroken Hinsdale covers an erosion sur­ The largest and laterally most persistent expo­ face that bevels faults along the west side of the sures of Hinsdale Formation (Larsen and Cross, graben that extends northwestward from the 1956), are in the southeastern part of the San Juan Creede caldera, indicating that the faulting was Mountains near the Conejos River and northward older than the pre-Hinsdale period of erosion, rather along the east margin of the mountains (pl. 1). In than younger as interpreted by Atwood and Mather this same area the Los Pinos Gravel forms a wide­ (1932, p. 24, pl. 2). To the east, this remnant of spread layer between the Hinsdale flows and the Hinsdale Formation projects toward an older fault­ older rocks. Atwood and Mather ( 1932, p. 21, 25) line scarp 1,400 feet high along the east side of attributed these relations to a general downwarp­ the graben, but the actual margin of the Hinsdale ing of the San Juan peneplain in this area, fol­ mass is obscured by glacial and landslide debris. lowed by marginal accumulation of gravel as allu­ This east border fault is locally covered by un­ vial fans, which in turn were covered by Hinsdale broken lava flows of Fisher Quartz Latite that poured flows. As noted earlier, more recent work (Butler, down across the scarp, and it too must have been 1946; Larsen and Cross, 1956; and P. W. Lipman, active earlier than the presumed peneplain cycle of oral commun., 1966), showed that most of the debris erosion. The scarp was thus in existence when the in the Los Pinos was supplied by contemporaneous Fisher Quartz Latite was erupted, about 26 million volcanism rather than by erosion of preexisting years ago, and has persisted, partially buried at rocks. According to P. W. Lipman, the Los Pinos times, to the present. covered an erosion surface of low relief, probably Southeast of the Creede caldera (near area 10, pl. a pediment, cut across the underlying ash-flow 1) the Hinsdale Formation (Larsen and Cross, 1956) tuffs and related rocks. (See No. 24, "Catalog of forms a tabular cap on top of the Wason Park Rhyo­ Peneplain Remnants.") Also, the contact between lite, a widespread sheet of welded ash-flow tuff, and the ash-flow tuffs and the underlying Conejos For­ also forms a string of minor flow remnants down mation and probably the contact between the Los an old north-trending valley. (See under 10, "Cata­ Pinos and the overlying Hinsdale flows are similar log of Peneplain Remnants.") The topography be­ erosional surfaces. These unconformities thus may neath this Hinsdale was irregular, with local relief represent recurrent marginal pedimentation that of at least 800 feet within a mile, and there was did not extend to the areas of irregular topography actually a valley where Atwood and Mather (1932, toward the center of the volcanic pile. pl. 2) showed a north-trending ridge (pl. 1). The 6. POSTPENEPLAIN DEVELOPMENT OF DRAINAGE drainage was northward toward the subsided area The generally radial drainage pattern of the present around the Creede caldera and represents local San Juan Mountains reflects outward flow of con­ rather than regional canyon cutting. sequent streams from the center of the domed San The patches of Hinsdale Formation (Larsen and Juan peneplain area. The Animas River is an excep­ Cross, 1956), northwest of area 25, plate 1, rest tion; it cut downward rapidly enough to keep pace with uplift and to maintain its course across the crest generally on Piedra Rhyolite (Larsen and Cross, of the dome. 1956, pl. 1), which consists of interlayered densely welded to nonwelded ash-flow tuffs. Larsen and The radial drainage pattern of the San Juan Cross' geologic map (1956, pl. 1) indicates that the ·Mountains does indicate outward flow from a cen­ contact between the Hinsdale and the Piedra here tral high area, but this high area includes the main is nearly flat over wide areas, but that locally it cuts source areas for the volcanic accumulations, and abruptly across contours as if jumping from the represents an original constructional feature of the top of one layer to the top of another. A few of volcanic field. The two exceptions to the general CRITICAL REVIEW OF THE SAN JUAN PENEPLAIN, SOUTHWESTERN COLORADO I 13 radial drainage, the Animas River and the JRio facts as known. The factors considered most perti­ Grande, owe their anomalous courses across the cen­ nent in reconstructing such a picture are these: tral high area to the influence of volcano-tectonic 1. Only the central, near-source and probably collapse structures. thicker parts of the San Juan volcanic field re­ The main tributaries in the headwaters of the main. The north, west, and south margins have Animas River, from the vicinity of Silverton up­ been removed by erosion, and the east margin stream, closely follow the faulted margin of the has subsided and been covered by alluvial de­ Silverton cauldron. Erosion along this zone of more posits in the San Luis Valley. broken rock was particularly effective and enabled 2. Late-volcanism and postvolcanism deformation, the Animas River to extend its headwater tributar­ probably broadly coincident with sinking of the ies across the highest part of the mountains in this San Luis Valley segment of the Rio Grande vicinity. Downstream from Silverton the Aninnas trough, is far more widespread than has pre­ River appears to be a typical radial stream except viously been realized and is responsibile for that it has superimposed its course across hard much of the broad eastward-tilted basin struc­ Precambrian rocks along the west flank of the ture of the central part of the volcanic field, as Needle Mountain dome. well as for many of the smaller structures. The Rio Grande heads in the western San Juan 3. Relict structural-constructional volcanic forms Mountains and flows eastward across the range to can be recognized most readily in the central the San Luis Valley at Del Norte. Its course across part of the volcanic field. More rapid erosion to­ the central San Juans follows the general trend of ward the margins of the field has obscured or an east-trending broad syncline and in detail is destroyed these forms elsewhere, particularly in closely controlled by the Creede caldera and related the more highly elevated and deeply dissected tangential grabens. Volcano-tectonic collapse cre­ western San Juans. ated a closed basin that extends across most of the 4. Preservation of original forms can be attributed central San Juan Mountains. The lower parts of in part, to burial and relatively late removal of this depression were filled to the point of overfl.ow cover, but some domal volcanoes that erupted by sediments of the Creede Formation, and the late in the period of volcanic activity appear to drainage that spilled eastward toward the San Luis have survived as topographic highs since they Valley formed the Rio Grande as we now know it. originated in late Oligocene or early Miocene The drainage basin that forms the headwaters and time. major source of water for the Rio Grande was thus 5. In the central part of the volcanic field the areas formed fundamentally by volcano-tectonic collapse, of so-called Hinsdale Formation now are found and the course of the river is closely controlled by largely along the edges of former shallow val­ bedrock structures. Since outflow was established, leys cut into the constructional surface, and they the erosional history has been largely one of exca­ probably represent only minor marginal rem­ vation. The Rio Grande has cut down to its present nants of formerly much more widely distributed level around the margin of the Creede caldera and lavas, largely sheetlike flows of basalt. has exhumed the domed core of harder rocks. Ex­ 6. Those remnants of the late Tertiary landscape that cept for a few deep canyons and many radial gul­ can be recognized have a bench-and-table-land lies, the original form of this dome is strikingly aspect typical of many volcanic areas having displayed in the modern topography. nearly flat lying formations made up of rocks of widely contrasting resistance to erosion. POSSIBLE CHARACTER OF THE LATE TERTIARY 7. The San Juan volcanic field was not elevated LANDSCAPE alone but was involved in the general epeiro­ If the concept of a peneplain cycle of erosion as genic uplift of the whole Southern Rocky envisaged by Atwood and Mather (1932) is unten­ Mountains area in late Cenozoic time. Erosion able, as I believe it to be, what was the character appears to have been minor until the effects of of the landscape of the San Juan area in late Ter­ widespread uplift caused rejuvenation of the tiary time? The evidence available is too scanty and streams. The resulting accelerated erosion was too dispersed to support a well-documented pic­ regional in extent. ture, but from the few basic elements that can be When integrated for that part of the San Juan established, it is possible to reconstruct a concep­ volcanic field that remains, these elements suggest tual picture of the general terrain that fits the a somewhat modified original surface of volcanic I 14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY accumulation. The central, near-source area was a uplift in late Cenozoic time caused widespread ex­ bench-and-tableland terrain of relatively low to cavation and canyon cutting by rejuvenated moderate relief, surmounted by the exposed tops streams. of still mostly buried older hills of diverse origin SUMMARY and by widely scattered late domal volcanoes as When placed in the context of the geologic his­ much as several thousand feet high. The shallow tory required by bedrock geology, many aspects of valleys were partly covered by sheetlike flows of the concept of a San Juan peneplain become un­ basalt. Most of the exposed rocks belonged to the convincing, and in places the evidence contradicts youngest volcanic units, and locally, as around the the concept itself. The regional and local deforma­ Creede caldera, parts of the terrain had actually tions required before and after the postulated pene­ been built up by filling of the lower parts of vol­ planation cannot be supported from bedrock evi­ cano-tectonic collapse structures with stream and dence. Instead, detailed studies have disclosed lake sediments. The stream systems seem to have great volcanic cauldron complexes in the central been poorly established in their headwater areas and western parts of the San Juan Mountains, and and in part were caught in resistant basalt flows. in the central San Juans the rough terrain left by Evidence for canyon-cutting until late Tertiary caldera collapse is in part reflected in the present (post-Hinsdale) time is lacking, and the stream topography. Segments of this area appear to have gradients appear to have been low and base levels been below the level of posulated peneplanation high. since the end of volcanic activity. In addition, sig­ Evidence for recurrent erosion is more apparent nificant areas of only slightly modified original near the southeast margin of the volcanic field than depositional surfaces still exist near the center of it is in the central area just discussed. Repeated the volcanic accumulation. episodes of pedimentation alternated with major The upper course of the Rio Grande is structur­ episodes of volcanic accumulation, and in particu­ ally controlled, and the origin of the river is closely lar the widespread sheet flows of Hinsdale basalts related to the volcanic history; a widespread pene­ adjacent to lower Alamosa Creek and the Conejos plain is precluded in any· significant segment of its River covered a nearly flat plain. No evidence was drainage basin. seen, however, for deep erosion or canyon cutting Most of the larger summit areas of low relief during any but the last (post-Hinsdale) period of designated by Atwood and Mather (1932, pl. 2) as erosion, and the recurrent pediments seem to rep­ remnants of the San Juan peneplain can be ac­ resent somewhat modified surfaces of accumulation counted for by local factors, and they need never rather than classic peneplains. have been integrated into a single surface at any With collapse of the San Luis Valley segment of time but the present. Equally important, many high­ the Rio Grande trough, and following general epei­ level surfaces of low relief were not included as rogenic uplift of the whole Southern Rocky Moun­ remnants, although most could easily have fit into tains region in late Tertiary time, the rejuvenated other plausible ancient landscapes. streams cut deep canyons, removed most of the The geologic history determined from bedrock widespread flows of basalt in the former lowlands, geology indicates that the initial surface of rela­ tively low relief resulting from accumulation of the and exhumed much of the buried volcanic terrain volcanic materials persisted with only minor modi­ in the central San Juans near the Creede caldera. fication until late Tertiary time, when general The modern canyons are the only ones for which evidence exists, and they represent the first major epeirogenic uplift of the Southern Rocky Moun­ tains caused regional rejuvenation and canyon deep erosion since the end of volcanic activity. Some of the interfluve areas still have been little affected, cutting. The extent of late Cenozoic erosion differed considerably from place to place, depending on local and many original constructional hills persist as conditions, but in general the erosion did not pro­ topographic highs, even though considerably modi­ gress beyond the canyon-cutting stage. fied. The general geomorphic history of the San Juan CATALOG OF PENEPLAIN REMNANTS Mountains is thus not unique but seems closely simi­ Key No. Coordinates Discussion 1 6-C,D The tops of Fitzpatrick Mesa, Black lar to that of other ranges in the Southern Roclky Mesa, and Soap Mesa are stripped Mountains that were low and partly covered by surfaces on, and parallel to, warped middle Tertiary sedimentary rocks until regional layers of welded ash-flow tuffs (W. R. CRITICAL REVIEW OF THE SAN JUAN PENEPLAIN, SOUTHWESTERN COLORADO I 15

Key No. CoMdinates Discussio-n Key No. CoMdinates Discussio-n 1 6-G,D Hansen, oral commun., 1964). The 5 6-E,F to D. C. Hedlund (oral commun., (Con.) (Con.) Bayfield Gravel shown by Atwood (Con.) (Con.) 1964), reflects a capping of hard and Mather (1932, pl. 2) in this area quartzitic sandstone of Jurassic age. is interlayered with the ash-flow 6 5, 6-E, F J. C. Olson (oral commun., 1964) re­ sequence. Only the higher mesa sur­ ported that the southernmost of these faces were interpreted as peneplain two remnants is underlain by a thick remnants, although similar stripped unit of welded ash-flow tuff and that surfaces on lower resistant layers the surface is generally parallel to are numerous in the area. the tuff layering. The northernmost 2 5, 6-C, D According to Cross, Howe, and Irving remnant is underlain largely by the (1907, p. 13): "In the northeast cor­ same hard quartzitic sandstone unit ner of the [Ouray] quadrangle is a discussed under 5, above. Much of small area representative of the the area embracing 4, 5, and 6 is un­ plateau country that is more promi­ derlain by flat-lying sedimentary nent farther east in the Lake City strata and ash-flow tuff units, and quadrangle. The gently undulating it has a typical bench-and-tableland surface is common where the surface topography reflecting differential ero­ flows of the Potosi formation [welded sion of interlayered hard and soft ash-flow tuffs] have any considerable rocks. lateral extent. In this case the San 7 4, 5-C This remnant coincides in large part Juan [Formation] forms Park Mesa with the so-called intrusive American [Trident Mesa, or High Mesa], but Flat Latite (Cross and others, 1907, the horizon of the Potosi is just p. 11). Luedke and Burbank (1961) above it." Thus several different hori­ have shown this to be an ash-flow zons are represented by the stripped tuff. The peneplain remnant largely surfaces in this area. Only the larger exposes hard, densely welded tuff of these local surfaces were inter­ from which the overlying soft tuff preted to be peneplain remnants. has been stripped. In places the sur­ Higher mesas of more limited extent face is on hard welded tuffs in the were called monadnocks, and lower Potosi Volcanic Group (Luedke and benches were ignored. Identical hori­ Burbank, 1963) that unconformably zons within the Potosi were shown as overlies the American Flat Latite. tops of monadnocks along the Trident 8 4-E Reconnaissance connected with my own Mesa Ridge but were included as detailed work near Creede has peneplain remnants along the ridges shown these two remnants to be to the east. stripped surfaces on densely welded 3 5, 6-C, D The longer, north-trending peneplain tuff in the Nelson Mountain Quartz remnants east and southeast of Tri­ Latite (Steven and Ratte, 1964), the dent Mesa (see 2) all coincide with youngest ash-flow unit to have been areas shown by Larsen and Cross deposited in this area. Younger lavas (1956, pl. 1) as Alboroto Rhyo­ of Fisher Quartz Latite overlie the lite-a formation which in adjacent Nelson Mountain tuffs just north of areas consists of differentially welded ash-flow tuffs (W. R. Hansen, oral these remnants, and the position of commun., 1964; J. C. Olson and D. C. the contact indicates that the rem­ Hedlund, oral commun., 1964). The nants are only slightly reduced from discussion under 2 gives the topo­ the original constructional surface of graphic relationships of the rem­ the area. Only a small thickness of nants near this point. nonwelded to partially welded tuff 4 6-E Carpenter Ridge (west of Cebolla seems to have been removed. Stripped Creek) and the Big Mesa area (east surfaces on lower resistant layers, of Cebolla Creek) are underlain by particularly the Wason Park Rhyo­ welded ash-flow tuffs (Olson, J. C., lite, are excellently developed in this oral commun., 1964). The peneplain area. The surface remnant several remnants coincide with stripped sur­ miles north is a slightly modified faces on hard, densely welded layers; original flow top in the Fisher Quartz different horizons are represented in Latite. different remnants, and a unit that underlies a mesa cap in one place 9 4-F,G My own detailed and reconnaissance may underlie a lower bench in an­ studies have shown most of these other. remnants to be stripped surfaces on 5 6-E,F This remnant coincides with the top hard welded ash-flow tuff layers in of Huntsman Mesa and, according the Nelson Mountain Quartz Latite, I 16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Key No. Coordinates Discussion Key No. Coordinates Disc·ussion 9 4-F,G the youngest volcanic unit to have 14 3, 4--C, D poses hard Precambrian rocks over (Con.) (Con.) been deposited in this area. (Con.) (Con.) most of its area, but one small patch The northeast-trending remnant is of a of Devonian Ouray Limestone re­ different origin, and I lack specific mains, and indicates that it is largely evidence concerning it. an exhumed remnant of an early 10 3-F,G This remnant is interpreted from a Paleozoic surface. A photograph of series of small patches of basalt of this surface remnant is shown by the Hinsdale Formation. I have Atwood and Mather (1932, pl. 21A). mapped this area in detail, and the 15 3,4-B As mapped by Cross and Hole (1910), basalt in part forms a mesa capping the northwest end of this remnant is on top of the Wason Park Rhyolite, a stripped surface in part on a sheet­ a widespread densely welded ash­ like igneous intrusion and in part on flow tuff, and in part forms minor a resistant bed in the Dakota Sand­ flow remnants in a north-trending stone which overlies the intrusion. The valley. The pre-Hinsdale topography east- and north-trending se.gments had little resemblance to a peneplain, of the remnant expose a variety of as local relief was at least 800 feet rock units, but they show little re­ within a mile. semblance to a plain. Modern large­ 11 3-F The remnants to the north and to the scale topographic maps (Hermosa south of the point represent the tops Peak and Engineer Mountain quad­ of Fisher and Beautiful Mountains, rangles, 7.5-minute series, U.S. Geol. respectively. Both of these mountains Survey, 1960) show no flat areas; are domal lava flows of Fisher Quartz slopes are rarely less than 400 feet Latite, and the so-called peneplain per mile; and the ridge crests do remnant marking the top of Fisher not appear to be accordant. Mountain may reflect a somewhat 16 3,4-A As mapped and described by Cross and modified original flow top (Steven, Ransome (1905), the three remnants 1964, p. D125). The surface of much of the San Juan peneplain west of of the immediately adjacent areas Rico are stripped surfaces on in­ may never have attained the height clined resistant sandstone beds in of the San Juan peneplain shown by the lower part of the Dakota Sand­ Atwood and Mather (1932, pl. 2). stone. The westerly dips are part of Some of the small remnants to the east the domed structure around the Rico are surfaces of Fisher Quartz Latite laccolithic center, and doming pre­ flows, and the others are stripped ceded erosion. surfaces on older welded ash-flow 17 2,3-A This long narrow northwest-trending tuffs. peneplain remnant corresponds almost 12 3,4-F This remnant is the top of Snowshoe exactly to .a remnant of Dakota Sand­ Mountain, the highest part of the stone mapped by Cross and Ran­ domed core of the Creede caldera. The some (1905). rounded summit area has been re­ 18 2-A As described by Cross, Spencer, and duced in height an unknown amount Purington (1899, p. 2) : "The south­ but probably was never covered by a ern third of the La Plata quadrangle younger unit, and it has projected belongs physiographically to the above adjacent areas ever since dom­ eastern extension of the Mesa Verde, ing in mid-Tertiary time. The surface a plain normally some 2,000 feet around most of the margin of the above the level of the Dolores Creede caldera probably was never Plateau. This high level is maintained as high as the level of the San Juan as a plateau by the preservation of peneplain shown by Atwood and massive Cretaceous sandstones which Mather (1932, pl. 2). have been entirely removed over the 13 3, 4-D, E The remnant just west of the point is large area of the Dolores and Great a stripped surface on a hard welded Sage plains [to the north]." All the ash-flow tuff in the Potosi Volcanic peneplain remnants in this vicinity Group (Luedke and Burbank, 1963). are within the area mapped by Cross, Extensive stripped surfaces at the Spencer, and Purington as Mesa­ same and other horizons exist in an verde Formation, and all are parallel area with striking bench-and-table­ to the underlying inclined beds that land topography just north of the dip radially off the La Plata dome. Rio Grande in this vicinity. Doming resulted from igneous intru­ 14 3, 4-C, D As mapped by Cross, Howe, Irving, and sion and clearly preceded erosion. Emmons (1905), this remnant ex- The area was mapped in more detail CRITICAL REVIEW OF THE SAN JUAN PENEPLAIN, SOUTHWESTERN COLORADO I 17

Key No. Coordinates Discussion Key No. Coordinates Discussion 18 2-A by Zapp (1949), and his map empha­ 22 2-C largely of resistant sandstone beds (Con.) (Con.) sizes the restriction of these pene­ (Con.) (Con.) in the Mesaverde Group, the Pictured plain remnants to areas underlain by Cliffs Sandstone, the Farmington the Mesaverde Group. Sandstone Member of the Kirtland 19 2-A,B The remnants of the San Juan pene­ Shale, and the McDermott Forma­ plain on the east flank of the La tion. Although generally accordant, Plata Mountains are partly within most ridge crests are narrow and the area mapped by Eckel (1949). In are unconvincing as evidence of a part, these appear to reflect dip former plain. slopes on inclined sedimentary beds The large three-pronged remnant to that were domed by igneous intru­ the northeast is in the lower part sion. Only the remnant southwest of of a sequence, of Cretaceous sedi­ the point (the Wild Oat Mesa area) mentary rocks that Larson and Cross has sufficiently low relief to qualify (1956, pl. 1) mapped as a unit. Pub­ as a possible representative of a lished data are inadequate to deter­ plain. The remnants north and east mine specifically which formations of the point have irregular surfaces underlie the remnant, but its position with many steep-sided hills project­ at and near the base of the Cretace­ ing above them; these remnants seem ous section suggests that the Dakota unconvincing as evidence for a for­ Sandstone is exposed under at least mer peneplain. part of it. 20 2-B,C This is the area in which Atwood and 23 1-E,F This remnant of the San Juan pene­ Mather (1932, p. 22) first conceived plain is the top of Archuleta Mesa, of a San Juan peneplain, and their which Wood, Kelley, and MacAlpin description has been given. (See "The (1948) and Dane (1948) described as Peneplain Concept.") The surface a nearly flat sill of augite andesite bevels several different formations, in the Animas Formation of Late largely sedimentary units of late Cretaceous and Paleocene age. The Paleozoic and Mesozoic age, and it softer sedimentary rocks have been seems quite likely that it truly rep­ stripped from the hard, resistant resents a remnant of an old erosion igneous rock. surface of undetermined age or -ex­ 24 1-G, H, I Most of the remnants flanking the tent. Conejos River in this vicinity are 21 2,3-C The geologic map of Cross, Howe, Irv­ within an area mapped in 1966 by ing, and Emmons (1905) indicates P. W. Lipman (oral commun., 1966). that the remnants of the San Juan The underlying rocks are largely peneplain in the vicinity of this point differentially welded ash-flow tuffs are either stripped dip slopes on re­ that form widespread thin sheets sistant lower Paleozoic sedimentary generally 100 feet or less thick. These rocks or exhumed remnants of the tuffs dip as much as 10° E., and most early Paleozoic surface on which of the peneplain remnants reflect these formations were originally laid stripped surfaces on inclined layers down. Photographs of two of the of densely welded tuff. Only the dip slopes (Lime Mesa and Stag higher inclined benches at any one Mesa) are shown as figures 1 and locality were included as peneplain 2 in the Cross, Howe, Irving, and remnants. Emmons report. The nearby smooth Unconformities representing erosional surface remnants on Precambrian surfaces of low relief (pediments?) rocks either have scattered patches separate the ash-flow tuffs from the of Cambrian Ignacio Quartzite on underlying andesitic to rhyodacitic them or are clearly on projection lavas and breccias of the Conejos from nearby remnants with such Formation and from the overlying patches. The inclined beds were up­ Los Pinos Gravel. A more obscure warped as part of the Needle Moun­ erosional surface separates the Los tains dome, which formed before the Pinos from the overlying basaltic Tertiary volcanic rocks were erupted. lavas of the Hinsdale Formation. The 22 2-C The linear remnants of the San Juan unconformity at the base of the ash­ peneplain extending east from Du­ flow tuffs represents the greatest ero­ rango are all accordant narrow ridge sion. The upper two unconformities crests along a sharp south-dipping appear to represent only minor bevel­ monocline ( Zapp, 1949, sheet 2; ing of the original surfaces of deposi­ Barnes, 1953). The ridges consist tion. The lowest unconformity is I 18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Key No. Coordinates Discussion tion of Colorado and southern Wyoming, in Ore deposits 24 1-G, H, I definitely below any of the strati­ of the Western States (Lindgren volume): New York, graphic positions proposed for the Am. Inst. Mining Metall. Engineers, p. 272-316. (Con.) (Con.) San Juan peneplain in this vicinity. Bush, A. L., Bromfield, C. S., Marsh, 0. T., and Taylor, 25 2,3-G These small remnants of the San Juan R. B., 1961, Preliminary geologic map of the Gray Head peneplain are of several origins. The quadrangle, San Miguel County, Colorado: U.S. Geol. two small areas east of the point Survey Mineral Inv. Field Studies Map MF-176. are exhumed fragments of an erosion. Bush, A. L., Marsh, 0. T., and Taylor, R. B., 1960, Areal surface that once separated the Cone­ geology of the Little Cone quadrangle, Colorado : U.S. jos Formation from overlying rela­ Geol. Survey Bull. 1082-G, p. 423-492 [1961]. tively young quartz latitic lavas. This Butler, A. P., Jr., 1946, Tertiary and Quaternary geology of surface was not a plain but appears the Tusas-Tres Piedras area, New Mexico: Cambridge to have had a hilly topography with a Mass., Harvard Univ., unpub. Ph. D. thesis. relief of 2,000 feet within a few Cross, Whitman, and Hole, A. D., 1910, Description of the miles (Steven and Ratte, 1960a, p. 9 Engineer Mountain quadrangle [Colo.]: U.S. Geol. Sur­ and pl. 1). The younger lavas built vey Geol. Atlas, Folio 171. a local pile of viscous flows, domes, Cross, Whitman, Howe, Ernest, and Irving, J.D., 1907, De­ and related breccias with a rough scription of the Ouray quadrangle [Colo.]: U.S. Geol. local topography that stood above the Survey Geol. Atlas, Folio 153. adjacent terrain (Steven and Ratte, Cross, Whitman, Howe, Ernest, Irving, J. D., and Emmons, 1960a, p. 31-36). The two remnants W. H., 1905, Description of the Needle 'Mountains quad­ to the northwest are somewhat modi­ rangle [Colo.]: U.S. Geol. Survey Geol. Atlas, Folio 131. fied original flow tops in the younger lavas, but the origin of the remnant Cross, Whitman, and Larson, E. S., Jr., 1935, A brief review to the north is not clear. Much of the of the geology of the San Juan region of southwestern area adjacent to this local accumula­ Colorado: U.S. Geol. Survey Bull. 843, 138 p. tion was probably well below the Cross, Whitman, and Ransome, F. L., 1905, Description of level postulated for the San Juan the Rico quadrangle [Colo.]: U.S. Geol. Survey Geol. peneplain in this vicinity. Atlas, Folio 130. Cross, Whitman, Spencer, A. C., and Purington, C. W., 1899, REFERENCES CITED Description of the La Plata quadrangle [Colo.]: U.S. Atwood, W. W., and Mather, K. F., 1932, Physiography and Geol. Survey Geol. Atlas, Folio 60. Quaternary geology of the San Juan Mountains, Colo­ Dane, C. H., 1948, Geology and oil possibilities of the eastern rado: U.S. Geol. Survey Prof. Paper 166, 176 p. side of San Juan Basin, Rio Arriba County, New Mexico: Baltz, E. H., 1965, Stratigraphy and history of Raton basin U.S. Geol. Survey Oil and Gas Inv. (Prelim.) Map 78. and notes on San Luis basin, Colorado-New Mexico: Davis, W. M., 1911, The Colorado Front Range, a study in Am. Assoc. Petroleum Geologists Bull., v. 49, no. 11, p. physiographic presentation: Am. Assoc. Geographers 2041-2075. Annals, v. 1, p. 21-84. Barnes, Harley, 1953, Geology of the Ignacio area, Ignacio Eckel, E. B., 1949, Geology and ore deposits of the La Plata and Pagosa Springs quadrangles, La Plata and Archuleta district, Colorado, with sections by J. S. Williams, F. W. Counties, Colorado: U.S. Geol. Survey Oil and Gas Inv. Galbraith, and others: U.S. Geol. Survey Prof. Paper Map OM-138. 219, 179 p. [1950]. Blackwelder, Eliot, 1909, Cenozoic history of the Laramie Kelley, V. C., 1955, Regional tectonics of the Colorado region, Wyoming: Jour. Geology, v. 17, p. 429-444. Plateau and relationship to the origin and distribution Bromfield, C. S., 1961, Emplacement of the Wilson Peak of uranium; New Mexico Univ. Pubs. Geology, no. 5, stock and associated intrusive rocks, San Miguel Moun­ 120 p. tain, Colorado, in Short papers in the geologic and --- 1957, General geology and tectonics of the western hydrologic sciences: U.S. Geol. Survey Prof. Paper San Juan Mountains, Colorado, in New Mexico Geol. 424-C, p. C137-C139. Soc. Guidebook 8th Field Conf., September 1957, South­ Bromfield, C. S., and Conroy, A. R., 1963, Preliminary geo­ western San Juan Mountains, Colorado: p. 154-162. logic map of the Mount Wilson quadrangle, San Miguel Larson, E. S., Jr., and Cross, Whitman, 1956, Geology and County [and Dolores County], Colorado: U.S. Geol. petrology of the San Juan region, southwestern Colorado: Survey Mineral Inv. Field Studies Map MF-273 [1964]. U.S. Geol. Survey Prof. Paper 258, 303 p. Burbank, W. S., 1933, Vein systems of the Arrastre Basin Lee, W. T., 1922, Peneplains of the Front Range and Rocky and regional geologic structure in the Silverton and Mountain National Park, Colorado: U.S. Geol. Survey Telluride quadrangles, Colorado: Colorado Sci. Soc. Proc., Bull. 730-A, p. 1-17. v. 13,no.5,p. 135-214. Little, H. P., 1925, Erosional cycles in the Front Range of Burbank, W. S., 1940, Structural control of ore deposition Colorado and their correlation (with discussion): Geol. in the Uncompahgre district, Ouray County, Colorado, Soc. America Bull., v. 36, no. 3, p. 495-512. with suggestions for prospecting: U.S. Geol. Survey Luedke, R. G., and Burbank, W. S., 1961, Central vent ash­ Bull. 906-E, p. 189-265 [1941]. flow eruption, western San Juan Mountains, Colorado, in Burbank, W. S., and Lovering, T. S., 1933, Relation of strati­ Short papers in the geologic and hydrologic sciences: graphy, structure, and igneous activity to ore deposi- U.S. Geol. Survey Prof. Paper 424-D, p. D94-D96. CRITICAL REVIEW OF THE SAN JUAN PENEPLAIN, SOUTHWESTERN COLORADO I 19

Luedke, R. G., and Burbank, W. S., 1962, Geology of the in geology and hydrology: U.S. Geol. Survey Prof. Paper Ouray quadrangle, Colorado: U.S. Geol. Survey Geol. 475-D, p. D123-D127. Quad. Map GQ-152. Steven, T. A., Mehnert, H. H., and Obradovich, J. D., 1967, --- 1963, Tertiary volcanic stratigraphy in the western Age of volcanic activity in the San Juan Mountains, San Juan Mountains, Colorado, in Short papers in Colorado, in Geological Survey research 1967: U.S. Geol. geology and hydrology: U.S. Geol. Survey Prof. Paper Survey Prof. Paper 575-D, p. D47-D55. 475-C, p. C39-C44. Steven, T. A., and Ratte, J. C., 1960a, Geology and ore de­ Mather, K. F., 1925, Physiographic surfaces in the Front posits of the Summitville district, San Juan Mountains, Range of northern Colorado and their equivalents on the Colorado: U.S. Geol. Survey Prof. Paper 343, 70 p. Great Plains [abs.]: Geol. Soc. America Bull., v. 36, --- 1960b, Relation of mineralization to caldera sub­ no. 1, p. 134-135. sidence in the Creede district, San Juan Mountains, Colorado, in Short papers in the geological sciences: Powell, W. J., 1958, Ground-water resources of the San U.S. Geol. Survey Prof. Paper 400-B, p. B14-B17. Luis Valley, Colorado, with a section on An inflow­ --- 1964, Revised Tertiary volcanic sequence in the cen­ outflow study of the area, by P. B. Mutz: U.S. Geol. tral San Juan Mountains, Colorado, in Short papers in Survey Water-Supply Paper 1379, 284 p. geology and hydrology: U.S. Geol. Survey Prof. Paper Richmond, G. M., 1965, Quaternary stratigraphy of the Du­ 475-D, p. D54-D63. rango area, San Juan Mountains, Colorado, in Geological --- 1965, Geology and structural control of ore deposi­ Survey Research 1965: U.S. Geol. Survey Prof. Paper tion in the Creede district, San Juan Mountains, Colo­ 525-C, p. C137-C143. rado: U.S. Geol. Survey Prof. Paper 487, 90 p. Shoemaker, E. M., 1956, Structural features of the central Van Houten, F. B., 1957, Appraisal of Ridgway and Gunni­ Colorado Plateau and their relation to uranium de­ son "tillites," southwestern Colorado: Geol. Soc. America posits, in Page, L. R., Stocking, H. E., and Smith, H. B., Bull., v. 68, p. 383-388. compilers, Contributions to the geology of uranium and Van Tuyl, F. 'M., and Lovering, T. S., 1935, Physiographic thorium by the United States Geological Survey and development of the Front Range: Geol. Soc. America Atomic Energy Commission for the United Nations Bull., v. 46, p. 1291-1350. International Conference on Peaceful Uses of Atomic Wood, G. H., Jr., Kelley, V. C., and MacAlpin, A. J., 1948, Energy, Geneva, Switzerland, 1955: U.S. Geol. Survey Geology of the southern part of Archuleta County, Prof. Paper 300, p. 155-170. Colorado: U.S. Geol. Survey Oil and Gas Inv. (Prelim.) Simpson, G. G., 1948, The Eocene of the San Juan Basin, Map 81. New Mexico: Am. Jour. Sci., pt. 1, v. 246, no. 5, p. 257- Zapp, A. D., 1949, Geology and coal resources of the Durango 282; pt. 2, v. 246, no. 6, p. 363-385. area, La Plata and Montezuma Counties, Colorado: U.S. Steven, T. A., 1964, Geologic setting of the Spar City dis­ Geol. Survey Oil and Gas Inv. (Prelim.) Map 102 (2 trict, San Juan Mountains, Colorado, in Short papers sheets).

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