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Stratigraphy and Depositional Setting of the Upper Precambrian Dox Formation in Grand Canyon

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Stratigraphy and Depositional Setting of the Upper Precambrian Dox Formation in Grand Canyon Stratigraphy and depositional setting of the upper Precambrian Dox Formation in Grand Canyon GENE M. STEVENSON Tierra Petroleum Corporation, 1625 Broadway, Denver, Colorado 80202 STANLEY S. BEUS Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 ABSTRACT thick. The Solomon Temple Member con- deposition in shallow-marine or transitional sists of red mudstone and sandstone 280 m environments ranging from lagoonal to The Dox Formation is part of the Unkar thick. The Comanche Point Member occu- possible deltaic-plain, tidal-flat, and fluvial. Group, which forms the lowermost 2,000 m pies more than half the Dox outcrop area in of upper Precambrian strata in the Grand eastern Grand Canyon and is a slope- INTRODUCTION Canyon. At the original type section near forming succession of orange-red mudstone Dox Castle, the Dox is incomplete, but it is 188 m thick. It contains five distinct white The Dox Formation is the uppermost fully exposed in the easternmost Grand bleached marker zones, one of which over- sedimentary unit in the Unkar Group of late Canyon where it is 950 m thick and is lies a stromatolite bed. The uppermost Precambrian age and is exposed only in divided into four members. The lowermost Ochoa Point Member is 91 m of slope- central and eastern Grand Canyon, Ariz- Escalante Creek Member is tan to brown, forming, reddish-brown mudstone that only ona. Precambrian sedimentary rocks in lithic and arkosic sandstone interbedded rarely exhibits bedding. Lithology and sed- Grand Canyon were first reported by Powell with green to brown mudstone, and is 390 m imentary structures in the Dox indicate (1874, 1875, 1876), who considered them to Geological Society of America Bulletin, v. 93, p. 163-173, 9 figs., 1 table, February 1982. 163 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/2/163/3434430/i0016-7606-93-2-163.pdf by guest on 30 September 2021 Figure 2. Geologic map and composite stratigraphie section of the Dox Formation outcrops in eastern Grand Canyon. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/2/163/3434430/i0016-7606-93-2-163.pdf by guest on 30 September 2021 PRECAMBRIAN DOX FORMATION, GRAND CANYON 165 be Silurian in age. Walcott (1894, 1895) uppermost and thickest (950 m) of the four into four members in ascending order: Esca- named and described the Grand Canyon sedimentary formations recognized in the lante Creek Member, Solomon Temple Series in detail and divided the sedimentary Unkar Group. It conformably overlies the Member, Comanche Point Member, and succession into two terranes: the Unkar Shinumo Quartzite and is overlain confor- Ochoa Point Member. The type sections for (below) and the Chuar (above). He reported mably by the Cardenas Lavas. all four members are in the Unkar Valley a thickness of about 3,660 m for the entire The Dox Formation is completely ex- area of eastern Grand Canyon, the location Grand Canyon Series, of which the lower posed in the Vishnu Temple Quadrangle of the original type section of the "Unkar 2,073 m were assigned to the Unkar. from Hance Rapids (mile 76.5) upstream to Terrane" of Walcott (1894, 1895). These Noble (1914) refined Walcott's descrip- Tanner Canyon (mile 65.4) in the "Big sections constitute a composite alternate tions, referred to the Unkar and Chuar as Bend" area of eastern Grand Canyon (Fig. type section for the Dox Formation (Fig. 2). groups, and recognized five formations 1). Additional smaller outcrops of incom- Table 1 summarizes thickness data for the within the Unkar: (1) the Hotauta Conglo- plete Dox sections occur (1) near the mouth Dox in eastern Grand Canyon. merate, (2) Bass Limestone, (3) Hakatai of Muav Canyon and adjacent smaller Shale, (4) Shinumo Quartzite, and (5) Dox canyons on both sides of. the Colorado ESCALANTE CREEK MEMBER Sandstone. The Unkar Group was subse- River within the northern Havasu Point quently studied by Van Gundy (1934, 1946, and southern Powell Plateau quadrangles— Stratigraphy 1951), who assigned the topmost part of this area is the Shinumo quadrangle of Walcott's Unkar succession to the Chuar Noble (1914) in central Grand Canyon, This lowermost member of the Dox and separated out the next unit below where all the type sections for the sedimen- Formation is 390 m thick in the type section (Unkar division 1 b-e; Walcott, 1899, tary units of the Unkar are located; (2) along Escalante Creek. Complete sections p. 216) as the Nankoweap Group, because it along Bright Angel Creek and Clear Creek are also exposed along the North Kaibab is bounded by unconformities above and in eastern Grand Canyon; (3) in lower Nan- Trail in upper Bright Angel Canyon (221 m) below. Maxson (1961, 1967) mapped the koweap Valley in eastern Grand Canyon; and along Shinumo Creek (293 m). Precambrian units in eastern Grand Canyon and (4) at Tapeats Creek in central Grand In the type section four distinct lithologic and referred to the Nankoweap as a forma- Canyon (Fig. 1). units are recognized. Unit 1, the lowest, is 73 tion, but included it in the Unkar Group. In the Grand Canyon, exposures of the m thick and is predominantly white to We consider the Nankoweap to be a forma- Unkar Group strata generally plot along a grayish-tan, friable, medium-grained quartz tion between the Unkar and Chuar Groups. N55° W line, which is roughly perpendicular sandstone. The basal 12 m of the unit is The Chuar Group has recently been studied to the dip direction of the beds. This predominantly dark green to black, fissile, and divided into formations by Ford and imposes limitations on the accurate deter- slope-forming shale, which in outcrop Breed (1973). mination of paleoslope and paleocurrent makes a distinct notch between the resistant Between the Unkar sedimentary units and directions. cliff-forming Shinumo Formation below the Nankoweap Formation, there is a suc- and the overlying resistant sandstone more cession of basaltic lavas nearly 305 m thick. STRATIGRAPHIC NOMENCLATURE characteristic of unit 1 above. Thin sand- These rest conformably on the Unkar sedi- stone interbeds also occur in the lower mentary strata and are designated the Car- Walcott (1894, p. 11) measured about 985 shale. The most distinctive structures in the denas Lavas by Keyes (1938, p. 110), Ford m of Dox strata in his description of the lower part of the quartzitic sandstone are and others (1972), and McKee and Noble "Unkar Terrane" of the Grand Canyon contorted beds (slurry slumps of Elliot, (1976), but were mapped as the Rama For- Group. The Dox Sandstone was named by 1965) indicative of soft sediment deforma- mation by Maxson (1961, 1967). The Car- Noble (1914) from exposures near Dox Cas- tion (Fig. 3a). In the upper part of unit 1, denas Lavas are considered the uppermost tle in central Grand Canyon. He reported a tabular-planar cross-beds dipping 25-30 unit of the Unkar Group. thickness of 700 m in the type section. This degrees to the northeast are the predomi- value seems excessive inasmuch as only the nant structures. GENERAL STRATIGRAPHY lower half of the Dox is present there. Pos- Unit 2 is an 80-m succession of green to sibly some of the Shinumo Quartzite was greenish-brown shale, mudstone, and sand- The Unkar Group consists of about 1,525 included in Noble's original section. stone, with micaceous shale and mudstone m of mainly fine-grained, red-brown sand- Because of the varying lithology of the constituting 70% of the unit. Subordinant stone, siltstone, and shale, with minor Dox, only half of which is sandstone, the sandstone interbeds are from 0.3 to 4 m dolomite and conglomerate, and is capped name is herein changed to Dox Formation. thick and average 0.9 m. Cyclic sequences with 305 m of basaltic lava. The Dox is the The Dox Formation is herein subdivided composed of a basal sandstone grading TABLE 1. THICKNESSES OF THE DOX FORMATION MEMBERS Shinumo Escalante Cardenas Tanner Espejo Creek Creek-Unkar Creek Canyon Creek Creek -42 km- Composite -1.6 km- -3.2 km- -3.2 km- Ochoa Point Member 92 m 92 m 53 m Comanche Point Member 188 m 155 m 130 m Solomon Temple Member Present but 280 m 58+ m 51+ m 198+ m incomplete (incomplete) (incomplete) (incomplete) Escalante Creek Member 293 m 390 m Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/2/163/3434430/i0016-7606-93-2-163.pdf by guest on 30 September 2021 fmmm Figure 3. Sedimentary structures in the Dox Formation. A. Slurry-slump bedding in the Escalante Creek Member; B. Tabular-planar cross-beds in channel-fill deposits, upper Solomon Temple Member (cm scale); C. Flute casts, upper Solomon Temple Member (cm scale); D. Mud-crack casts, Comanche Point Member (2-cm white bar); E. Salt crystal casts, Comanche Point Member (5-cm white bar); F. Stromatolite bed, Comanche Point Member (cm scale); G. Slightly asymmetrical anastomosing ripples, Comanche Point Member (cm scale); H. Interference ripple marks, Comanche Point Member. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/2/163/3434430/i0016-7606-93-2-163.pdf by guest on 30 September 2021 PRECAMBRIAN DOX FORMATION, GRAND CANYON 167 upward into siltstone are common. The duced by overloading of rapidly deposited, unit 4 are similar to those described by Col- bases of the graded sandstone beds are loose, water-saturated sediment. linson (1969, p. 208) in the lower Grind- commonly scoured, exhibit load casts, and The upper 12 m of unit 1 are highly cross- stone Shale (upper Carboniferous), which contain shale clasts 2-20 m in diameter.
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