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Age and Correlation of the Late Proterozoic Grand Canyon Disturbance, Northern Arizona

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Age and Correlation of the Late Proterozoic Grand Canyon Disturbance, Northern Arizona Age and correlation of the late Proterozoic Grand Canyon disturbance, northern Arizona DONALD P. ELSTON U.S. Geological Survey, Flagstaff, Arizona 86001 EDWIN H. McJCEE U.S. Geological Survey, Menlo Park, California, 94025 ABSTRACT ing arid Ar retention accompanying the •to 770 m.y. for a structural disturbance in Grand Canyon disturbance. western North America that separates the A structural disturbance that ended de- Strata of .the upper Grand Canyon Su- deposition of middle Proterozoic from late position of Chuaria-bearing marine shale pergroup correlate paleontologically and Proterozoic strata. This range brackets the of middle Proterozoic age in-northern Ari- paleomagnetically with sedimentary rocks nominal 800 m.y.- age assigned to the boun- zona is recorded by strata within the and isotopically dated intrusive rocks of the dary between Proterozoic Y and Z rocks on Sixtymile Formation at the top of .the Little Dal Group of the Mackenzie Moun- a geologic time scale adopted by the U.S. Grand Canyon Supergroup. The distur- tains Supergroup (northwest Canada), and Geological- Survey in 1980. Deposition of bance involved marine emergence and up- with sedimentary rocks of the Uinta Moun- the Sixtymile Formation in the Grand lift, accompanied by block faulting with as tain Group (Utah and Colorado). An Canyon, which accompanied and which much as 3.2 km of structural displacement. apparent correlation also exists with some also appears to have postdated the distur- It was the most severe episode of structural poles reported from crystalline rocks of the bance, is assigned to the late Proterozoic. deformation to affect either the Proterozoic Grenville Province of eastern North Amer- The structural disturbancein the western or Phanerozoic strata of the Grand Canyon, ica, and this correlation accords with the United States, here called the "Grand but it was markedly less severe than an ear- broad range of disturbed K-Ar ages from Canyon-Mackenzie Mountains distur- lier deformation which resulted in meta- the Grand Canyon (950 to 800 m.y.). Intru- bance," was marked by a westward shift in morphism and intrusion of the underlying sions emplaced after deposition of the the apparent polar wandering path. A l,700-m.y.-old crystalline basement. Two CVwana-bearing Little Dal Group, very somewhat similar westerly shift is seen in groups of disturbed K-Ar ages, obtained near the top of the Mackenzie Mountains some of the reset poles reported from rocks from the Proterozoic rocks, 930 ± 25 m.y. Supergroup of Canada, are geologically of the Grenville Province, but a strong . and 823 ± 26 m.y., appear to broadly reflect well controlled and have an internal Rb-Sr southerly shift also is seen, which has given the time of uplift and faulting of the Grand isochron age of 770 ± 20 m.y. Intrusion in rise to an apparent southerly track (and a Canyon disturbance. The older age was the Mackenzie Mountains preceded or postulated loop) in the Grenville polar path. obtained from mineral analyses of ~ 1,150- accompanied deposition of basaltic lava The southerly shift is not seen in the strati- m.y.-old sills and l,700-m.y.-old crystalline flows and sedimentary strata that accumu- graphically controlled polar path from the basement buried at a depth of about 4 km at lated during a time of block faulting, and western cordillera, which temporally over- the time, of disturbance. The younger age these strata-are in turn overlain by glacio- laps the age range assigned to the Grenville . was obtained from whole-rock analyses genic deposits assigned to the late Protero- poles. Although the apparent southerly shift of ~ l,100-m.y.-old lava flows buried only zoic Windermere Supergroup. On isotopic- unquestionably is present in the Grenville half as deeply at the time of.the disturbance. age, paleontologic, and paleomagnetic paleomagnetic data, and also is seen in A high consistency of ages (values within grounds, the structural disturbance in the poles from Grenville-age rocks from Fen- about 10%) was obtained from both mineral Grand Canyon thus appears to correlate noscandia, we suggest that the shift may be and whole-rock-samples having widely vary- . with post-Little Dal to early -Windermere an artifact recording the uncorrected effects ing K2O contents, suggesting, that little dif- faulting in northwest Canada, a faulting of- structure and structural rotation. The ferential loss of Ar has occurred since a time that in turn has been correlated with the Grenville terrane very likely was subjected of general resetting. Moreover, scatter on disturbance called the "East Kootenay orog- to at least some structural deformation at 40 39 an Ar/ Ar isochron and a disturbed eny". in the southern cordillera of Canada. •the time of a wide-ranging disturbance pattern of incremental heating ages indicate The disturbance in the Grand Canyon is beginning about 820 m.y. ago. It is a distur- that the Ar clock in minerals from the dia- assigned a nominal age of 823 m.y. from an bance that is reported to have affected adja- base sills was not completely reset. This average of the reset K-Ar dates for-the Car- cent rocks in -the Appalachian- region of implies that the Ar clock in the-lava flows denas Lavas. This age would seem to. imply eastern North America. Along the western was more nearly if not completely reset, that the onset of structural activity oc- margin of the craton, the. disturbance gave presumably because of a lower Ar retentiv- curred somewhat earlier in Arizona than in rise to block-fault mountains and to the ity of the cryptocrystalline matrix of the northwest Canada! If such is the case, data Cordilleran miogeocline. We suggest that flows. The 823 m.y. age from the lavas thus from the Grand Canyon and northwest this mountain-making event was the termi- is believed to generally reflect a time of cool- Canada provide an age range of- about 820 nal event of the Grenville orogeny. Geological'Society of America.Bulletin, v. 93, p.. 1-699, 10 figs., 4 tables, August 1982. 681 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/681/3419292/i0016-7606-93-8-681.pdf by guest on 30 September 2021 EXPLANATION Pzu Sedimentary rocks, undivided ,OQC t o t Tapeats Sandstone. As PALEOZOIC mapped locally includes Bright Angel Shale ui I- \ Sixtymile Formation < Gl 3 O o> © - Bc_~ Chuar Group Q. 3 CO c / //// < En// Nankoweap Formation o > / LU >. 7///Z _J o o >PROTEROZOIC Unkar Group TJ C Cardenas Lavas O Lower part of' Unkar Group CD l/Eul^ ( Mafic intrusion A ¿-A ^ EV> Vishnu Schist /V V J Contact Fault, high angle — Bar and ball on downthrown side; circle indicates Proterozoic displacementj dot indicates post-Proterozoic displacement; dashed where approximately located; dotted where concealed Axial trace of Chuar syncline- Dashed where approximately located ; dotted where concealed Axial trace of anticline; arrow indicates direction of plunge Monocline— Long arrow indicates approximate lateral extent of fold; short arrow indicates locally abrupt base of fold Mesa, or rim of canyon — Hachures downslope Figure 1. Generalized geologic map of eastern Grand Canyon, northern Arizona. Modified from Maxson (1967) and Huntoon and others (1976) with additions from Elston and Scott (1976), Elston (1979), and unpublished data of D. P. Elston. ELSTON AND MCKEE, FIGURE 1 Geological Society of America Bulletin, v. 93, no. 8 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/681/3419292/i0016-7606-93-8-681.pdf by guest on 30 September 2021 682 ELSTON AND McK.EE INTRODUCTION deeply entrenched in a high plateau formed gap in the geologic record. The term by the East Kaibab uplift. Key exposures are "great unconformity" also was applied by We have studied an episode of structural found west of the Colorado River and north Walcott (1895, p. 317) to the boundary that deformation in the Grand Canyon of north- of its big bend in a narrow, north-trending separates the unmetamorphosed Protero- ern Arizona. It is an episode that led to the Proterozoic syncline that closely parallels zoic strata from the underlying crystalline end of deposition of marine strata of middle the Proterozoic and Phanerozoic Butte basement (Fig. 2). This stratigraphically Proterozoic age very, near the top of the fault. The stratigraphie framework for the lower unconformity, which represents a Grand Canyon Supergroup. Following up- Proterozoic section in the Grand Canyon is larger (~< 500 m.y.) gap in the geologic lift and block faulting, a long period of con- shown schematically in Figure 2, and sum- record, is not dealt with in this report. tinental erosion and deposition ensued, the marized in Table 1. The position of the After the explorations of John Wesley record for which was subsequently nearly "great unconformity" beneath the Cam- Powell (1876), the pioneer work of C. D. completely removed by erosion. Erosion brian Tonto Group (Powell, 1876; Walcott, Walcott in 1882-1883 in the eastern Grand ultimately resulted in planation, and this 1883, p. 440) also is shown in Figure 2. It is Canyon (Yochelson, 1979) led to the identi- was followed by a marine transgression and the youngest of seven episodes of erosion fication of a line of Precambrian displace- the deposition of strata of Early and Middle currently recognized in Proterozoic strata ment west of six high-standing buttes, Cambrian age. that overlie the crystalline basement and which lie between the Colorado River and The area of study is in the eastern Grand underlie the Cambrian Tonto Group. This the Precambrian Chuar terrane (Walcott, Canyon of northern Arizona (Fig. 1, fold- unconformity resulted from erosion and 1890, 1894). Walcott (1890) named the line out).
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