Pre-Carboniferous Paleotectonics of the San Juan Basin, New Mexico G
Total Page：16
File Type：pdf, Size：1020Kb
New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/28 Pre-carboniferous paleotectonics of the San Juan Basin, New Mexico G. M. Stevenson and D. L. Baars, 1977, pp. 99-110 in: San Juan Basin III (northwestern New Mexico), Fassett, J. F.; James, H. L.; [eds.], New Mexico Geological Society 28th Annual Fall Field Conference Guidebook, 319 p. This is one of many related papers that were included in the 1977 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. Free Downloads NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks. Copyright Information Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. New Mexico Geol. Soc. Guidebook, 28th Field Conf., San Juan Basin III, 1977 99 PRE-CARBONIFEROUS PALEOTECTONICS OF THE SAN JUAN BASIN G. M. STEVENSON Consultant Durango, Colorado and D. L. BAARS Department of Geology Fort Lewis College Durango, Colorado INTRODUCTION considered to be Late Cambrian by most workers (fig. 1). The This paper summarizes and updates the state of knowledge reader is referred to Barnes (1954), Baars and Knight (1957), concerning Cambrian-Devonian rock units in the San Juan Rhodes and Fisher (1957), Baars (1958) and Parker and Rob- Basin of northwest New Mexico and southwest Colorado. erts (1963) for discussions concerning correlation problems, Earlier workers mapped all outcropping rock units of the lithologic descriptions, evidence of age designations, faunal Cambrian and Devonian in the Four Corners region. Since this data and history of terminology. mapping took place 20 to 25 years ago, little information has The Ignacio Formation is considered to have been deposited been published about the early Paleozoic rock units. by an eastward-transgressing sea; a thin blanket of sandstone Deep wells drilled in the late 1950's and early 1960's of Late Cambrian age covered the Precambrian in the Four penetrated the sequence and provided a regional understanding Corners region, extending to the San Juan Mountains of south- of the Cambrian and Devonian environments of deposition. west Colorado where it is exposed in outcrops. The transgres- Since then, many more deep tests have been drilled and a sive nature of the basal sandstone toward the east indicates much more complex paleotectonic history is becoming that the region was slightly positive until Late Devonian time apparent. (Baars, 1958). Figure 1 is a correlation of all Paleozoic rock units in the Although the eastward-transgressing Cambrian seas un- northern and southern San Juan Basin. Baars and Stevenson doubtedly deposited thin sandstones in northwest New (elsewhere in this volume) discuss the Permian stratigraphy in Mexico, the Ignacio is locally preserved only in relatively the San Juan Basin. However, for proprietary reasons, the small, isolated blocks that appear to be fault-bounded (figs. 2 Mississippian and Pennsylvanian stratigraphic relationships and 3). Here the Ignacio is restricted to down-thrown fault cannot be disclosed at this time. All available mechanical logs blocks and is flanked by rocks of younger age that noncon- and commercial sample logs have been used in constructing the formably overlie the Precambrian. Deposition of Late Cam- maps and cross section illustrated in this paper. [Editor's note: brian sandstone was not restricted to these grabens, but tol- see papers by Armstrong on the Mississippian and Jentgen on lowing post-depositional faulting and erosion of the Cambrian the Pennsylvanian elsewhere in this volume.] sediments from the structurally positive areas, only isolated remnants were preserved. STRATIGRAPHY Devonian Precambrian Aneth Formation As shown in Figure 2, the Precambrian basement of the San The Aneth Formation was named by Knight and Cooper Juan Basin is nonconformably overlain by all representative (1955) as the oldest recognizable Devonian strata of the Colo- Paleozoic rock units of the Colorado Plateau. Sufficient out- rado Plateau. In the type locality it consists of up to 200 ft. of crop and well control is currently available in the northwest dark-colored, resinous limestone and argillaceous dolomite and southeast portions of the San Juan Basin to indicate that with intercalated black shale and dark siltstone. The Aneth the basement is somewhat more complex than previously con- conformably underlies the Elbert Formation and discon- sidered (fig. 2). It should be pointed out that all outcrop and formably overlies the Late Cambrian elsewhere in the Four well control in Figure 2 penetrates the Precambrian. Corners (Baars and Campbell, 1968). The Aneth is restricted The Precambrian varies from quartzite to schist and granite to the subsurface in extreme southeast Utah and northwest in the Four Corners region. However, the variations in Precam- New Mexico and does not crop out in any known localities. brian rock types and their relationship to the overlying Parker and Roberts (1963) considered the Aneth to be the Paleozoic sediments are beyond the scope of this paper. Com- basal euxinic facies of the Elbert Formation, deposited in local parison with relationships seen in the San Juan Mountains and sags or basins. These local sags may, in fact, be tectonically Paradox basin suggests that the variations probably resulted controlled and not entirely depositional as Parker and Roberts I from Late Precambrian tectonism. inferred. In Figure 2, notice the fault-controlled pattern of Aneth nonconformably overlying Precambrian rocks and its Cambrian relationship to the Cambrian. A depositional history similar to Ignacio Formation that of the Ignacio is inferred, as indicated by the isolated The Ignacio Formation was named by Cross, Howe and fault-bounded occurrences. Figure 4 shows that slight tectonic Ransome (1905) for exposures north of Rockwood, Colorado. movement appears to have been ongoing shortly before Aneth The unit consists of quartzite, quartzose sandstone and friable deposition, with renewed movement during or shortly after sandstone with local shale lenses. Although the age of the Ignacio has been the subject of extensive discussion, it is now 104 STEVENSON and BAARS deposition. Fault blocks that were tensional (grabens) at the upper member nonconformably overlies the Precambrian Ignacio time were compressional (horsts) at Aneth time, and in the extreme southwest corner of Colorado (figs. 5 and 7). vice versa (fig. 5). Fish fauna, identified as Late Devonian in age, have been The age of the Aneth Formation is considered to be Late found in the upper member of the Elbert in the San Juan Devonian on the basis of fish remains recovered from cores in Mountains (Baars and Campbell, 1968). the type well in southeast Utah (Knight and Cooper, 1955; Parker and Roberts, 1963, 1966; Baars and Campbell, 1968). Ouray Formation The Ouray Formation was named by Spencer (1900) for Elbert Formation 100-200 ft. of massive limestone at Ouray, Colorado. The Ouray is a dark-brown limestone and dolomite that con- The Elbert Formation was named by Cross (1904) for formably overlies the Elbert Formation throughout the Colo- exposures along Elbert Creek north of Rockwood, Colorado. The Elbert Formation conformably overlies the Aneth in the rado Plateau. Four Corners region, but disconformably overlies Late Cam- The Ouray, as here mapped, extends much farther into the brian rocks in most other areas. Knight and Cooper (1955) San Juan Basin than it was previously mapped (fig. 8). Al- redefined the Elbert and divided it into two members; the though no faults dissect the Ouray, the basement lineaments appear to have been reactivated to the extent of continuing to basal McCracken Sandstone Member and an upper dolomite control or influence sedimentation (figs. 5 and 8). and shale member. The exact age of the Ouray has been the subject of debate The McCracken Sandstone Member is