ARTHUR V. CHADWICK AND M. ELAINE KENNEDY
Geoscience Research Institute, Loma Linda University, Loma Linda, California 92350
Copyright © 2001 Art Chadwick
The Tapeats Sandstone, outcropping in the Grand Canyon and elsewhere in Central Arizona and forming the basal Cambrian deposit of the Tonto Platform, is generally interpreted as a shallow marine deposit. In the Grand Canyon region the sandstone was deposited on a Precambrian surface of low relief broken by a series of isolated remnant cliffs of Shinumo Quartzite and by occasional granitic hills. Associated with the Precambrian Shinumo Cliffs are Cambrian breccias containing large quartzite clasts from the cliffs, deposited along Precambrian surface topography extending from the cliff face basinward. Subsequent to deposition the breccias and associated cliffs were partly buried by continuing Tapeats influx and finally covered by the overlying Bright Angel Shale. These cliffs and the associated breccias provide an opportunity to investigate the influence of Precambrian surface relief on Cambrian sedimentary processes and to test the concept of an encroaching marine shoreline.
Paleoslope measurements and sedimentological features were recorded from all outcrops in the Grand Canyon exhibiting significant Precambrian topographic relief (63 locations). Samples were collected from the Hakatai Shale, Vishnu Schist, Tapeats Sandstone and Bright Angel Shale along 91 Mile Canyon for trace and major element analyses.
Most localities exhibited no evidence of post-depositional erosion or reworking of breccias deposited along topographic reliefs of up to 140 meters or more. In some localities portions of the breccia were preserved uncovered during deposition of the entire thickness of Tapeats Sandstone and during deposition of a significant part of the overlying Bright Angel Shale. The widespread preservation of the breccia along topographic relief would require that it was not only covered by water, but that the unit existed below wave base. This would require water depths of 200 meters or more during the deposition of the lower and middle Cambrian sediments. Other features are consistent with this conclusion.
The debris flows were initiated by some catastrophic event that simultaneously broke up and transported Shinumo clasts in a matrix of Tapeats Sandstone. These submarine flows deposited on a surface with over 140 meters of vertical relief. Their preservation across this relief would seem to require a marine environment considerably deeper than that visualized previously.
The Tapeats Sandstone, outcropping in the Grand Canyon and elsewhere in Central Arizona, forms the basal Cambrian deposit of the Tonto Platform (Beus and Morales 1990). Various authors have postulated shallow marine as well as subaerial origins for the Tapeats Sandstone and other basal Cambrian deposits of the Western United States, based on a variety of sedimentary structures that have been interpreted as indicative of shallow marine conditions. Specifically, McKee and Resser (1945) interpreted the Tapeats deposition as shallow marine restricted to less than 60 feet. Hereford (1977) considered Tapeats deposition to be tidal flat.
The earliest definitive work describing the environment of deposition of the Tapeats Sandstone in Grand Canyon, Arizona, identified the formation as a subtidal, transgressive marine sequence (McKee and Resser 1945). This view was reinterpreted by Wanless (1973) who, on the basis of petrologic data from the overlying Muav Limestone, concluded that deposition of the Cambrian sediments occurred on a very shallow tidal to subaerial platform. Working in central Arizona, Hereford (1977) concluded the Tapeats was deposited as intertidal sand bars, beach, tidal flat and tidal channel sediments based on a variety of sedimentological features. This interpretation of the Tapeats Sandstone has been applied to the Tapeats sediments in the Grand Canyon area (Beus and Morales, 1990).
Grand Canyon Paleogeography
At the onset of Paleozoic deposition, the Precambrian paleogeographic surface exhibited low relief broken by a series of isolated remnant cliffs or monadnocks and by occasional granitic hills. These resistant cliffs of Shinumo Quarzite, some rising 250 meters above the associated Precambrian landscape, occur as a linear series of strike ridges trending to the northwest (Sharp 1940a).
A persistent feature associated with the Precambrian Shinumo Cliffs in the Grand Canyon are Cambrian breccias. These breccias or debris flows, first described by Sharp (1940b), were identifed by him as subaqueous "slides". The debris flows, containing large quartzite clasts from the Shinumo Cliffs, are deposited along a topographic gradient near the base of the Tapeats.
The Shinumo cliffs provide an opportunity to investigate the influence of the Precambrian surface relief on Cambrian sedimentary processes and to test the concept of an encroaching marine shoreline. Aspects of the breccias will be used to identify the conditions present during the deposition of the Tapeats.
Paleoslope measurements and sedimentological features were recorded from all outcrops in the Grand Canyon exhibiting significant Precambrian topographic relief. These outcrops and data collection localities() include Red(4), Mineral(1), Asbestos(4), Vishnu(4), Clear(3), Bright Angel(6), Phantom(3), 91 Mile(6), Trinity(2), Dragon(2), Crystal(2), Shinumo(1), Galloway(2), Stone(1), 33 Mile(3), and Tapeats Canyons(5), as well as the outcrop along the Kaibab Trail(1), the region above Phantom Ranch(7), and Monadnock(3) and Hotauta(3) Amphitheaters (Fig. 1-map).
Paleoslope measurements were generally made using a portable transit and rod, and occasionally using a Brunton compass when conditions prohibited use of the transit. The paleotopographic detail required in a crucial region of 91 Mile Canyon necessitated plane table mapping (Fig. 2-map).
Seventy-four samples were collected from the Hakatai Shale, Vishnu Schist, Tapeats Sandstone and Bright Angel Shale along 91 Mile Canyon (Fig. 2) for trace and major element analyses. X-ray fluorescence, X-ray diffraction and Inductively Coupled Plasma Mass Spectroscopy data were compiled by a commercial testing laboratory (Appendix_A).
At outcrops in Grand Canyon where Shinumo Quartzite cliffs are exposed, locally derived breccias have been identified (Fig. 3-photo). These breccias are associated with the Lower to Middle Cambrian Tapeats Sandstone and Bright Angel Shale deposits of the Grand Canyon but do not form a basal unit in the Tapeats Sandstone. Displaying complex sedimentological relationships with the overlying and underlying deposits, the breccias locally blanket Precambrian topography including the cliff faces (some as high as 180 meters), as can be seen in localities where this relationship has not been destroyed by modern erosion. In those areas where the exhumed Shinumo cliffs extend above the Tapeats and Bright Angel deposits, the breccias continuously underlie both Tapeats Sandstone and Bright Angel Shale. In 91 Mile Canyon, the breccia underlies most of the Tapeats Sandstone as well as 112' of Bright Angel Shale along the Precambrian slope. Breccias extend for vertical distances of up to 140 meters and horizontally up to 2 kilometers from the Shinumo outcrops (Table 1). These deposits, walked out along outcrop in all possible localities, exhibit no evidence of post-depositional erosion or reworking prior to burial. The breccias persist up-section, along the Precambrian contact, in some cases underlying the entire thickness of the Tapeats Sandstone and portions of the Bright Angel Shale.
Clasts in the breccia flow range from pebble size to house size and larger. One clast, 10 meters in diameter in Red Canyon, contains a mine shaft driven by an ill-fated mining operation. Larger clasts up to 75 meters in length are found occasionally in close proximity to the Shinumo cliffs.
In many areas of the Canyon the basal Tapeats consists alternating of light and dark layers of well-sorted graded sandstone beds. The debris flows generally lie immediately above these Tapeats beds. Within the debris flows, the Tapeats sands form much of the matrix. In the wall of 91 Mile Canyon, portions of this basal Tapeats form striking rip-ups in the breccia (Fig. 4-photo). In some areas near the cliffs, isolated Shinumo clasts are incorporated into the Tapeats Sandstone at various horizons (Fig. 5-photo). In a number of localities paleoslope was determined for individual beds of Tapeats Sandstone (Table 1). The vertical relief over which these beds were deposited ranged up to 50 meters.
Similar mineralogy for the Hakatai Shale, Tapeats Sandstone, Bright Angel Shale and the breccias was determined by XRD (Appendix A) with quartz dominating all of the units and glauconite visible in hand samples. The thorium/uranium ratio for the deposits in 91 Mile Canyon is less than 5 based on calculations from Appendix A. Trace element signatures for the Tapeats Sandstone, Bright Angel Shale and the breccia matrix do not differ appreciably based on a comparison of the means (Appendix A). However, the fine-grained fraction of the Tapeats Sandstone can be differentiated from the Bright Angel Shale on the basis of the chromium content.
Brecciated debris flows containing large clasts require high energy, fluidized depositional processes (Cook et al. 1972). Large clast sizes, synchronous deposition and ubiquitous occurrence of the breccias associated with the Shinumo Quartzite cliffs require a high energy source during Lower Cambrian deposition. The inclusion of large clasts and rip-ups in the Tapeats Sandstone as well as large clasts in the Bright Angel Shale suggest that these formations also were deposited under high energy conditions. It seems likely that deposition of all of these units took place within a high energy marine system associated with the Shinumo cliffs.
The alternating layers of light and dark sandstone commonly seen in the basal portion of the Tapeats Sandstone were documented previously by Sharp (1940a) and McKee and Resser (1945). Subsequently these beds, consisting of well-sorted, normal graded sands were described by Burgert (1972) as "turbidites". If correct, this interpretation would be inconsistent with deposition in a tidal marine environment.
Beds of Tapeats Sandstone blanketing the Shinumo cliffs often exhibit original dip in response to the Precambrian highs (McKee and Resser 1945). Numerous examples of such beds display vertical relief of 50 meters or more. Such deposits imply water depths in excess of 50 meters (maximum tidal range, ref?).
In many places in the Canyon, debris flows, associated with the cliff face and occasionally in direct contact with the face, can be traced laterally into the basin over vertical distances exceeding depths typically attributed to nearshore marine environments. At Monadnock Amphitheater and other localities where exhumation of the Shinumo cliff is occurring, the relationship between the breccia and the original cliff face is preserved. The lack of post-depositional erosion or reworking of the breccia precludes a Cambrian shore line in this region. In 91 Mile Canyon portions of the breccia were unprotected during the entire deposition of Tapeats Sandstone and during deposition of the overlying Bright Angel Shale. The breccia was not covered entirely until Bright Angel Shale buried the associated Shinumo cliff. The widespread preservation of the breccia would require that it was not only covered by water, but that the unit existed below wave base. For this to be true, water depths up to 200 meters would be required during the deposition of the lower and middle Cambrian sediments.
The occurrence of abundant glauconite in both Tapeats Sandstone and Bright Angel Shale formations suggests that the geochemical marine environment remained relatively constant during the Tapeats Sandstone and Bright Angel Shale deposition. The reworking and transport of glauconite grains into deep water environments has been suggested by Weaver (1989) and this scenario may be applicable to the environment of deposition for the Tapeats Sandstone and Bright Angel Shale. Th/U ratios less than 5.0 indicate a reducing environment consistent with deep water, low oxygen conditions.
The debris flows were initiated by some catastrophic event that simultaneously broke up and transported Shinumo clasts in a matrix of Tapeats Sandstone. These flows deposited on a surface with over 140 meters of vertical relief. The flows appear undisturbed by any event subsequent to subaqueous emplacement. The deep water scenario proposed here offers an explanation for the preservation of the Shinumo cliffs through a second erosive cycle.
Continuity of the subaqueous breccia deposits across paleoslope relief in excess of 140 meters in the vicinity of the Shinumo Quartzite monadnocks provides an opportunity to explore new deep water hypotheses for deposition of the Tapeats Sandstone and the Bright Angel Shale in the Grand Canyon region. Since many of the shallow marine sedimentary indicators found by other workers in the Tapeats are present in the Grand Canyon region near the Shinumo cliffs; perhaps caution should be used when interpreting this data in light of the deep water evidence for the deposition occurring in the same region.
RERERENCES Beus, S.S., and Morales, M., 1990, Grand Canyon Geology: New York, Oxford University Press, Inc., p. 83-85.
Burgert, B.L., 1972, Petrology of the Cambrian Tapeats Sandstone, Grand Canyon, Arizona [unpublished M.S. thesis]: Northern Arizona University, 156 p.
Cook, H.E., McDaniel, P.N., Mountjoy, E.W., and Pray, E., 1972, Allochthonous carbonate debris flows at Devonian bank (`reef') margins, Alberta, Canada: Bulletin of Canadian Petroleum Geology v. 20, p. 439-497.
Hereford, R., 1977, Deposition of the Tapeats Sandstone (Cambrian) in Central Arizona: Geological Society of America Bulletin, v. 88, p. 199-211.
McKee, E.D., and Resser, 1945, Cambrian history of the Grand Canyon region: Washington D.C., Carnegie Institute, Publication 563, p. 3-168.
Sharp, R.P., 1940a, EP-Archean and EP-Algonkian erosion surfaces, Grand Canyon, Arizona: Geological Society of America Bulletion v. 51, p. 1235-1270.
Sharp, R.P., 1940b, A Cambrian slide breccia, Grand Canyon, Arizona: American Journal of Science, v. 238, p. 668-672.
Wanless, H.R., 1973, Cambrian of the Grand Canyon; a re-evaluation of the depositional environment: [Ph.D. dissertation], Ann Arbor, University Microfilms International, 113 p.
Weaver, C.E., 1989, Clays, muds, and shales: New York, Elsierver, Developments in sedimentology 44, p 396.
Observable Relief Red Canyon:
East Wall 140'
West Wall 283' - 365'?
Asbestos Canyon 61'
East Wall 273'
"Friday" Canyon: 187'
Clear Creek Canyon:
Bright Angel Canyon:
East Wall 100'
West Wall 236'
91 Mile Canyon 325'-450'
Crystal Canyon 391'
Shinumo Canyon: 247' - 297'?
Galloway Canyon 73'
Stone Canyon 100'
33 Mile Canyon
East Fork >344'
West Fork 213' & 257'