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Cambrian and Ordovician Rocks of Southeastern Arizona and Southwestern New Mexico Philip T

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Cambrian and Ordovician Rocks of Southeastern Arizona and Southwestern New Mexico Philip T New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/29 Cambrian and Ordovician rocks of southeastern Arizona and southwestern New Mexico Philip T. Hayes, 1978, pp. 165-173 in: Land of Cochise (Southeastern Arizona), Callender, J. F.; Wilt, J.; Clemons, R. E.; James, H. L.; [eds.], New Mexico Geological Society 29th Annual Fall Field Conference Guidebook, 348 p. This is one of many related papers that were included in the 1978 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, 29th Field Conf., Land of Cochise, 1978 165 CAMBRIAN AND ORDOVICIAN, ROCKS OF SOUTHEASTERN ARIZONA AND SOUTHWESTERN NEW MEXICO PHILIP T. HAYES U.S. Geological Survey Denver, Colorado GEOLOGIC HISTORY AND SETTING which subsequently covered the region, left their deposits on The Cambrian and Lower Ordovician rocks of southeastern Middle Cambrian strata in the western part of southeastern Arizona were deposited near the eastern shore of a shallow Arizona (fig. 2), on Upper Cambrian strata in most of south- shelf sea that encroached from the west or southwest across an eastern Arizona that are progressively younger eastward, on area of generally low relief carved in diverse metamorphic, Lower Ordovician strata in most of easternmost Arizona and igneous and sedimentary rocks of Precambrian age. The sea, part of westernmost New Mexico, on Middle and Upper Ordo- which apparently had a very broad intertidal zone, entered the vician strata in a very small part of the same area, and on region in late Early Cambrian or early Middle Cambrian time Silurian strata farther east in southwestern New Mexico. and extended eastward to about the position of the present The Cambrian and Ordovician rocks of southeastern Ari- Arizona-New Mexico boundary by about the end of Middle zona have been affected by many other tectonic and erosional Cambrian time (fig. 1). The position of the eastern shoreline of events in post-Devonian time. In some areas these rocks were the sea fluctuated during Late Cambrian time, but before the faulted, intruded, locally mineralized and locally eroded away end of the Cambrian began a more rapid eastward advance during Triassic and Jurassic time. They were removed from across western New Mexico. The region was covered by this large areas in the northern part of southeastern Arizona and in sea until latest Early Ordovician time. part of southwestern New Mexico by erosion during Cre - The region was probably slightly emergent during most of taceous time (fig. 3). They were locally thrust-faulted, folded, Middle Ordovician time and was subjected to some erosion, intruded, thermally and hydrothermal!) , altered and possibly more to the north than to the south. A sea again encroached, wrench-faulted during Late Cretaceous and early Tertiary this time from the southeast, near the end of Middle Ordo - (Laramide) time. They were very locally intruded, meta - vician time and reached at least as far as the eastern edge of morphosed and apparently faulted again in mid-Tertiary time. southern Arizona and possibly extended across much of it. No Finally, along with other rocks of the region, they were in - sedimentary rocks and thus no record of events exists for the volved in Basin and Range faulting and subsequent local ero- Silurian in Arizona, but a Silurian sea did reach into south- sion in late Tertiary and Quaternary time. western New Mexico; southeastern Arizona may have been What is now known of the Cambrian and Ordovician rocks inundated at that time or may have been slightly emergent. of the region has been gleaned from study of the variously The entire region was emergent and tilted slightly southeast- faulted and altered rocks in mountain-range outcrops that for- ward during Early and much of Middle Devonian time, when tuitously make up about 0.5 percent of the present land sur- Cambrian and Ordovician rocks were eroded to progressively face; very few drill holes exist to provide information on these lower stratigraphic levels towards the west and north. Con- rocks under the great thicknesses of younger rocks in the inter- sequently, late Middle Devonian and Late Devonian seas, montane basins. STRATIGRAPHY Bliss Sandstone; it rests on Nrecambrian rocks and is overlain Throughout the southwestern part of southeastern Arizona, in succession by the Ordovician El Paso and Montoya Groups. Cambrian rocks are subdivided into two formations: the Bolsa These latter-named rocks in New Mexico are described only Quartzite and overlying Abrigo Formation. In the southeastern briefly here in order to show their interpreted relations to the part of this area, in the Swisshelm and Pedregosa mountains, rocks in Arizona. these formations are overlain by Lower Ordovician rocks as- Bolsa Quartzite signed to the El Paso Limestone (figs. 3, 4), but elsewhere The Bolsa Quartzite was named by Ransome (1904) for Ordovician rocks are missing. In the eastern part of southeast- exposures in the Mule Mountains near Mount Martin (figs. 3, ern Arizona and at the westernmost edge of New Mexico, the 4) and has since been recognized by many workers over a Cambrian is represented by the Coronado Sandstone and the large part of southeastern Arizona. In the southern part of the lower member of the overlying El Paso Limestone. In most region, where local relief on the underlying Precambrian is places the Cambrian is overlain by the upper member of the El generally fairly low, the Bolsa is generally 100 to 200 m thick Paso Limestone of Early Ordovician age and, near Morenci, the but, locally, it may be slightly thicker or thinner. In the north- El Paso is overlain by the Second Value Dolomite of late Mid - ern part of the region, where local relief on the Precambrian is dle or early Late Ordovician age. Farther east in New Mexico, commonly more pronounced, it is absent in places over Pre- uppermost Cambrian sedimentary rocks are included in the cambrian hills. 168 HAYES The formation consists almost entirely of resistant beds of in beds a few centimeters to 2 m thick, are commonly lami - rather dark-weathering silica-cemented orthoquartzite, but nated, and are generally only weakly resistant. local pebble or cobble conglomerate occurs at the base. Grain The carbonate rocks in the lower member include intra- sizes in the orthoquartzite decrease upward from dominantly formational flat-pebble conglomerates, mudcracks and occa- coarse- and medium-grained sand in the lower part to domi- sional burrows. Tracks, trails and burrows are common in the nantly medium- and fine-grained sand in the upper part. The sandstone and siltstone facies, as are small- and medium-scale lower part is commonly slightly feldspathic, the upper part cross-laminations; slump structures are locally found. The non-feldspathic. Bedding thicknesses are mostly between 15 lower member of the Abrigo Formation is interpreted as repre- cm and 1.5 m, with most thicker beds near the base and thin- senting deposition in an intertidal environment. The sandy and ner beds near the top. I nterbeds of siltstone or shale are com- silty northern lithofacies indicates proximity to a northern mon in the upper part of the formation at most localities. source of sediment or to a river mouth. Small- and medium-scale planar cross-laminations are abundant Fossils, chiefly trilobites, have been found in different parts in the formation, particularly in the lower part. Animal tracks, of the southern lithofacies of the lower member at several trails and burrows are locally common, especially in the upper localities. On the basis of these fossils, the member is assigned part. The Bolsa Quartzite is interpreted to represent deposition a Middle Cambrian and mostly late Middle Cambrian age. The either in an offshore bar or a beach environment. sandy facies has yielded no diagnostic fossils but is assumed to No fossils closely diagnostic of age have been found in the be equivalent in age. Bolsa Quartzite. However, because of its conformable relations with overlying beds of late Middle Cambrian age in the Abrigo Middle member Formation, the Bolsa is believed to be of Middle Cambrian age.
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