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

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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

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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). Here, the south-flowing Colorado planation following a post-Chuar Group of displacement "the Butte fault" because of River turns westward, crosses the Butte fault structural event and, from dating presented its proximity to the buttes. He recognized and East Kaibab monocline, and becomes in this report, represents a 200- to 250-m.y. about 120 to 1,220 m of downthrow on the

Figure 2. Stratigraphie outline of Grand Canyon Supergroup and structural relations between Proterozoic and basal Cambrian strata, eastern Grand Canyon, Arizona. Structural' offset on Butte fault is portrayed as it existed in early Paleozoic time.

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TABLE 1. SUMMARY OF MIDDLE AND formed high ranges similar to the faulted eastern North America to which the term LATE PROTEROZOIC GRAND CANYON ranges of the present Great Basin and the "orogeny" is applied. If the correlations are SUPERGROUP, NORTHERN ARIZONA desert ranges of Arizona and California. valid, block-fault mountains in the western This mountain-making event has since been cordillera of North America, located far Thickness (m) called the "Grand Canyon revolution" from the area of apparently greatest defor- (Maxson, 1961), the "Grand Canyon dis- mation, nonetheless record some discrete Cambrian turbance" (Wilson, 1962, p. 17-18), and the part of an orogenic episode. Tonto Group 180-395 "Grand Canyon orogeny" (Elston, 1979). Unconformity From details of the stratigraphy, Elston STRUCTURE Proterozoic (1979) presented evidence that the structural Grand Canyon Supergroup . 3,689-4,503 episode involving the Proterozoic strata of A Precambrian structural fabric has con- Sixtymile Formation (uncon- the Grand Canyon was marked by faulting trolled the distribution of faults that cut formities within) 60( + ) with as much as 3.2 km of structural dis- Phanerozoic strata in northern and central Chuar Group 1,896 placement, and he concluded that the dis- Arizona. The dominant faults, spaced about 594 Kwagunt Formation turbance occurred principally, if not en- 19 to 30 km apart, trend northeast and Galeros Formation 1,302 tirely, during deposition of the Sixtymile north. Northwest-trending faults are com- Unconformity Formation at the top of the Grand Canyon mon, but they characteristically are of short Nankoweap Formation . . . 113-150(?) Upper member 100( + ) Supergroup. The Sixtymile is an incom- linear extent. The Butte fault (Fig. 1) under- pletely preserved, 60-m-thick, clastic red- lies part of the north-trending East Kaibab Unconformity Ferruginous member 13( + ) bed unit that contains ancient landslide monocline, an east-facing fold of Tertiary deposits and associated breccia. At the base age (Walcott, 1890, p. 57, from studies of C. Unconformity Unkar Group 1,620-2,197 of the Sixtymile Formation (prior to land- E. Dutton, 1882). The East Kaibab mono- sliding), laminated red silty sandstone con- cline, which thus appears to reflect the trace Cardenas Lavas 224-450(300)* Dox Sandstone 884-915(900) formably overlies and is gradational with of a deep-seated Precambrian structure, can Shinumo Quartzite 324-475 (400) underlying dark gray marine shale of the be followed northward from the Grand Chuar Group. Landsliding ended deposi- Canyon for about 200 km. A group of Unconformity Hakatai Shale 131-253(200) tion of the basal laminated sandstone, and northeast-trending faults in and near the Bass Limestone 37-104 (75) the landslide and breccia came from a Grand Canyon has been called the "Bright Unconformity closely adjacent scarp that developed along Angel fault system" (Shoemaker and others, Vishnu Schist a periodically active Butte fault. Narrow 1974). This system includes the Bright stratigraphic limits for the structural epi- Angel and associated faults west of the area Note: Tonto Group from McKee and Resser sode appear to be recorded in the Sixtymile of Figure .1, and the Eminence and asso- (1945); Sixtymile Formation from Elston (1979), Formation, and the limits suggest that the ciated faults in and northeast of the area this paper; Chuar Group from Ford and Breed disturbance took place over a fairly brief shown in Figure 1. Geologic mapping (by (1972a, 1973); Nankoweap Formation and Unkar interval of time. The character of the dis- Elston in 1979) has recently revealed that Group from Elston and Scott (1976); with modi- fications from unpublished data. turbance is summarized in sections that fol- the north end of the Butte fault splays and »Nominal thicknesses for units of the Unkar low, and its age and correlation is evaluated turns abruptly to the northeast along low- Group are shown in parentheses. in light of isotopic, paleontologic, and angle faults (Fig. 1). This "bend" seen in the paleomagnetic data. Proterozoic strata, and a fold and fault in Paleozoic strata to the northeast, are align- west in Precambrian time, and he also rec- The term "disturbance" is herein applied ed with the Eminence fault. The Butte fault ognized that the Precambrian displacement to this Precambrian structural episode in thus appears to turn and join the Eminence was later mitigated by about 820 m of the Grand Canyon. Aspects Of structural fault system rather than extend northward downthrow to the east, partly during Paleo- deformation sought for by some workers to beneath the trace of the East Kaibab zoic time but mostly during the Tertiary identify an orogeny are lacking—for exam- monocline. (Walcott, 1890, p. 64). Walcott (1895, ple, there is an absence of penetrative p. 327) considered the Precambrian dis- deformation and strong concurrent igneous The history of recurring movements on placement on the Butte fault to be part of a activity. The term "disturbance," defined as the Bright Angel and associated faults in the major structural event. He noted that oro- a minor orogeny in the Glossary of Geology Grand Canyon has been investigated and graphic movement occurred following dep- (American Geological Institute, 1972), is to summarized by Maxson (1961), Sears osition of the Chuar Group, and that be preferred because high-standing block- (1973), and Huntoon and Sears (1975). In during this period of movement, rocks of fault mountains alone were formed at the Proterozoic time, strata characteristically the Vishnu Schist and the Unkar and Chuar time of the structural episode. However, as were down-faulted on the west along north- Groups were elevated and broken by faults will be seen, a combination of isotopic- and northeast-trending, high-angle reverse and for the most part tilted and gently age, paleontologic, and paleomagnetic data faults. In contrast, displacements of the folded. Noble (1913, p. 83) later described seem to reveal that formation of the com- opposite sense occurred on the same faults this event as a mountain-building move- paratively simple block-fault mountains in in Paleozoic and early Tertiary time. A ment of block faulting and tilting that broke Arizona correlates with a far-ranging dis- complex Proterozoic structural history has the 3,660 m of Unkar and Chuar strata into turbance in western North America. More- been deciphered for the Bright Angel and great crustal blocks. Additionally, Noble over, this disturbance also appears to other faults by Sears (1973), who reported speculated that the fault blocks must have correlate with part of a structural episode in that the earliest recognizable structural epi-

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sode occurred following the deposition of sandstone member of the Shinumo.. The (Fig. 1), where it unconformably overlies an the Hakatai Shale of the Unkar Group unconformity thus represents no apparently erosionally truncated section of the Carde- (Table 1). Sears also recognized several later great gap in the stratigraphic record at the nas Lavas (Fig. 2). The upper member of Precambrian offsets, some associated with Hakatai-Shinumo contact even where the the Nankoweap is composed of red sand- sills and dikes that were intruded concur- conglomeratic member is thin or absent stone and siltstone that appears to have rently with deposition of Unkar strata. Evi- (Huntoon and Sears, 1975, p. 467). Erup- accumulated in a near-shore continental dence for post-Unkar displacement is seen tion of the Cardenas Lavas signaled the end and perhaps marginal marine environment on faults near the south end of thè Butte of deposition of sedimentary strata of the of deposition. Cliff-forming sandstone at fault (Elston and Scott, 1976), where three Unkar Group. A thin flow marking the base the top of the Nankoweap grades irregularly post-Unkar episodes of Precambrian dis- of the Cardenas Lavas locally intertongues upward from red to white, appearing to placement can be recognized: (1) following with the underlying Dox Sandstone, indi-. reflect bleaching beneath an unconformity eruption of the Cardenas Lavas, (2) follow- eating no appreciable hiatus between depo- that separates red beds of the Nankoweap ing deposition of the ferruginous member of sition of the Dox and eruption of the from dark-gray marine strata of the Chuar the Nankoweap Formation, and (3) follow- Cardenas Lavas. Group. ing deposition of the upper member of the Diabase sills and mafic dikes intrude Nankoweap and the Chuar Group. The parts of the Unkar Group in and below the Chuar Group third and apparently final.episode of Pro- Dox Sandstone, which is a red bed unit that terozoic faulting, accompanied-by regional is nearly 1 km thick (Figs. 1 and 2). In the The Chuar Group (Chuar Group of WaK tilting and local folding, is recorded within, central Grand Canyon (west of the area of cott, 1883; Ford and Breed,. 1972a, 1973) and .appears to have been responsible for Fig. 1), comparatively thick (~60 m) sills was subdivided by Ford and Breed into the deposition of, the Sixtymile Formation at intrude the Bass Limestone and Hakatai . Galeros, Kwagunt,. and Sixtymile Forma- the top of the Grand Canyon Supergroup. Shale near the base of the Unkar. To the tions (Table 1). Ford and Breed also subdi- east (southwest part of the area in Fig. 1), a vided the Galeros and Kwagunt into a total GRAND CANYON SUPERGROUP thinner (~20 m) sill intrudes the Bass of seven members. The Chuar Group is here Limestone. Nearby, dikes intrude the Haka- restricted to these two lower formations, The Grand Canyon Series of Walcott tai Shale and Shinumo Quartzite at the and the Sixtymile Formation is removed (1894; formerly the Grand Canyon Group head of Hance Rapids and in adjacent Red from the Group (Table 1) because of of Powell, 1876) has been redesignated the Canyon. Still farther to the east, several marked differences in lithology and in "Grand Canyon Supergroup" (Elston and dikes intrude the Dox Sandstone, one of environment of deposition. The Galeros Scott, 1976), consisting of the Unkar which penetrates to within about 80 m of and Kwagunt total 1,900 m in thickness, Group, Nankoweap Formation, and Chuar the top of the Dox (Fig. 1). In an. area whereas an incomplete section of the overly- Group. The stratigraphic framework for the encompassing the northeasternmost expo- ing Sixtymile Formation is only 60 m thick. Grand Canyon Supergroup is further re- sures of the Dox Sandstone directly north The Galeros and Kwagunt Formations are vised in this report (Table 1) with removal of the Palisades fault (Fig. 1), sills measur- predominantly dark gray marine shale that of the Sixtymile Formation at the top of the ing in thickness from a few metres to 10 m locally contain some stromatolite-bearing Supergroup from the Chuar Group. A gen- or more intrude the lower member of the carbonate rocks and subordinate red beds. eralized description of the formations is Dox. Field relations thus indicate that In contrast, the Sixtymile Formation is a given in Appendix 1 of Elston. and Scott intrusion of the mafic dikes, and presuma- very fine- to coarse-grained, in part land- (1976). Other descriptions are given in Beus bly intrusion of the more deeply buried dia- slide- and breccia-bearing red-bed deposit and others (1974) and in Ford and Breed basic sills as well, took place principally of apparent continental origin. during the time of deposition of the Dox (1973). Three horizons of algal stromatolitic Sandstone. Paleomagnetic directions from limestone, in part biohermal, and the orga- most intrusions indicate that they were Unkar Group nism Chuaria are found within shale units emplaced at the time of deposition of the of the Chuar Group (Ford and Breed, upper middle member of the Dox Sand- The Unkar Group (Unkar terrane of 1972b, 1973). In addition, microflora con- stone, but one thin sill and one group of Walcott, 1894) comprises an approximately sisting of spheroids and filaments have been dikes appear, respectively, to have been 2-km-thick section of dominantly clastic found in a thin cherty pisolite bed in the intruded shortly before and during the time rocks. Red beds predominate, and the units Kwagunt Formation slightly more than of extrusion of the Cardenas Lavas (Elston in the succession display characteristics that 150 m beneath the Sixtymile Formation and Gromme, 1979, and unpub. data). reflect alternations between marine and (Schopf and others, 1973). More recently, near-shore continental deposition. Deposi- abundant flask-shaped chitinozoan-like mi- tion within the Unkar Group was apparent- Nankoweap Formation crofossils also have been discovered in car- ly continuous except at a disconformity that bonaceous shale in the upper part of the separates the Hakatai Shale and Shinumo The Nankoweap Formation is a thin Kwagunt Formation (Bloeser and others, Quartzite (Daneker, 1974). This unconfor- 100 m) red-bed unit. Prior to two intervals 1977). mity reflects displacement' on the Bright of pre-Chuar erosion, it once may have been Angel fault and monocline (Sears, 1973) 350 m or more in thickness (Elston and Sixtymile Formation west of the area of Figure 1. Faulting and Scott, 1976). The lower or ferruginous folding were concurrent with the initiation member, about 10 m thick, is preserved Strata assigned to the Sixtymile Forma- of deposition of the basal conglomeratic locally near the south end of the Butte fault tion were first recognized and described by

Walcott (1894, p. 503, units la and lb) from continuous across the Kwagunt-Sixtymile detached block that unconformably rests exposures at the top of Nankoweap Butte boundary, and a 10-cm-thick transition partly on a remnant of the transition zone (Fig. 1). Walcott assigned the reddish- zone records the time of marine emergence. (unit 3 of Kwagunt Formation, Fig. 3) and brown sandstone, which contains numerous On Nankoweap Butte, this transition zone partly on marine shale of the Kwagunt. This fragments of sandstone and shale, to the locally contains a 1-cm-thick layer of unre- carbonate bed is stratigraphically at least upper part of the upper division of his worked air-fall tuff at its base, which 70 m out of place, and it could only have Chuar terrane. Walcott reported a thickness records contemporaneous volcanic activity arrived in the syncline from the east. of 61 m for the sandstone on Nankoweap somewhere in the region. The transition Because its source undoubtedly was the Butte and noted that the unit becomes shaly zone is directly overlain by very fine-grained Chuar, the Chuar must have been exposed near its base and passes downward from red laminated red sandstone, the basal unit of in a scarp on the eastern, upthrown side of sandy shale into black argillaceous (Chuar) the lower member of the Sixtymile Forma- the Butte fault. The amount of displacement shale. Although the Sixtymile was subse- tion (unit 1 of lower member, Fig. 3). At on the Butte fault at that time was at least quently mistaken by some workers for the least 6 m of this laminated, non-breccia- 150 m, and from other considerations, may Tapeats Sandstone (Cambrian), Ford and bearing sandstone accumulated at the type have been much greater. Breed (1972a, p. 8; 1973, p. 1252-1255) cor- section in Sixtymile Canyon, where it is pre- One tumbled block of porous white sand- rectly recognized it as a Precambrian unit. served on the western limb of the Chuar stone is present at the base of the landslide Following Walcott's lead, they included syncline (see Fig. 4). in the axis of the syncline (Figs. 3 and 4). these strata in the Chuar Group. Although Sandstone of unit 1 of the lower member, This block appears entirely foreign to the Ford and Breed (1973, p. 1255) named this responding to an increment of folding on Chuar Group. The stratigraphically closest unit the Sixtymile Formation for outcrops the syncline, underwent folding and concur- beds of somewhat similar lithology are of breccia and sandstone in the north fork rent slumping toward the axis of a deepen- found in sandstone at the top of the Nan- of Sixtymile Canyon, their description for ing syncline. Erosion along the axis of the koweap Formation. If the exotic block the formation came principally from the syncline was followed by the deposition of a came from the Nankoweap, about 2 km of exposures on Nankoweap Butte. Ford and local breccia-bearing sandstone near the structural displacement would have been Breed (1973, Fig. 2; and Breed and Ford, axis of the fold (unit 2 in Fig. 3). At essen- required to expose the Nankoweap on the 1973, p. 12-18) considered the contact tially the same time, landslides originated eastern, upthrown side of the Butte fault. between the Kwagunt and Sixtymile For- from an escarpment to the east as apprecia- Moreover, because the block is at the base mations to be an unconformity. Because of ble to perhaps very large displacements of the landslide, such displacement would this, they assumed that an unknown thick- occurred on the nearby Butte fault. As have had to occur very early during deposi- ness of the Kwagunt had been eroded and inferred from the deposition of landslide tion of the Sixtymile Formation. that warping of the Chuar syncline and units 3 and 5 of the lower member, locally Ultimately, displacement across the Butte deposition of the Sixtymile Formation had separated by a thin bed of breccia-bearing fault appears to have been as much as 3.2 occurred in some undefined interval of late sandstone (unit 4), two strong increments of km. This is deduced from the stratigraphic Precambrian time between deposition of the movement appear to have taken place dur- level of Precambrian planation on the Kwagunt Formation and deposition of the ing deposition of the lower member of the upthrown eastern block relative to the west- Cambrian Tapeats Sandstone. This concept Sixtymile. ern block. Relations are diagrammed in has been modified in light of a detailed The landslide deposits consist of chaotic Figure 2 in which the present effects of study of the Sixtymile Formation at its type polymict breccia in which many of the Phanerozoic down-on-the-east faulting have locality (Elston, 1979). The type section and fragments of sedimentary rock appear al- been removed. West of the Butte fault, the stratigraphic relations that may be seen in tered or bleached. Nearly all of the breccia Cambrian Tapeats Sandstone rests uncon- the Chuar syncline in the north fork of Six- appears to have been derived from various formably on the Sixtymile Formation. East tymile Canyon are diagrammed here in Fig- strata of the Chuar Group. On the west side of the fault, the Tapeats rests on beds of the ures 3 and 4, and the depositional and of the Chuar syncline, the breccia locally is middle parts of the lower member of the structural history derived from the field monomict and consists of unaltered angular Dox Sandstone, well down into the Unkar relations at the type section is summarized fragments derived from closely adjacent red Group. This is seen in exposures along the below. sandstone of unit 1. The monomict breccia Colorado River, north of the Palisades fault The Chuar syncline at first developed records fragmentation of lithified sandstone (Fig. 1). If about two-thirds of the deduced gradually near the end of deposition of and its apparent local eastward transport 3 km of structural displacement occurred dark-gray marine shale of the Kwagunt toward the axis of the syncline. Large early—during the time of landsliding—the Formation. A pair of carbonate beds lying blocks of carbonate rock are present at the remainder can be inferred to have taken about 70 m below the top of the Kwagunt base of the landslide near the axis and on place at a time of deformation and disrup- are thickest in the axis of the syncline, and the east limb of the syncline. The various tion of beds of the middle member and they thin toward the east and appear to blocks in the axis are tumbled, and several lower part of the upper member of the Six- pinch out where they approach the Butte are obviously on end. However, low on the tymile Formation. fault (Fig. 4). These relations suggest that east limb, one very large carbonate body lies Events that followed deposition of the some warping in the Chuar syncline (per- nearly parallel to its stratification. It is iden- landslide deposits of the lower member of haps accompanied by minor concomitant tical in appearance and approximately the Sixtymile Formation include: (1) depo- structural displacement along the trace of equal in thickness to the upper of the doub- sition of soft, dark red, sandy siltstone at the Butte fault) occurred before the end of let carbonate beds that lie 70 m directly the top of the lower member (unit 6) record- deposition of the Kwagunt. Deposition was below (Figs. 3 and 4). The mass is clearly a ing quiet water deposition probably on an

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Dolomitic limestone

Black shale

Sandstone Conglomeratic, breccia-and chert-bearing

& A ' A'-'

Quartzite Chert- bearing

Landslide Monomict (•) and polymict (°)

Erosion surface

METERS

Figure 3, Diagram of type section of Sixtymile Formation, Sixtymile Canyon, eastern Grand Canyon (from Elston, 1979). View is to northwest.

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 Figure 4. Stratigraphie relations in Chuar syncline, north fork of Sixtymile Canyon, eastern Grand Canyon (from Elston, 1979). View is to northwest. In Walcott Member of Kwagunt Formation, note pair of ledge-forming carbonate beds that thin eastward toward the Butte fault.

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the type section. Stratigraphic details seen at Nankoweap Butte reveal that the basal, EXPLANATION non-breccia-bearing sandstone of the Six- tymile Formation conformably overlies and is gradational with the Kwagunt Forma- Sandstone Red-brown, fine- to tion. Moreover, soft-sediment deformation coarse-grained, medium caused by deposition of the lowest breccia is bedded, locally conglomeratic evident at the top of the otherwise unde- formed basal laminated sandstone (unit 1, Fig. 5). Thus, there was no hiatus between

Sandstone deposition of Chuar shale and the basal Dark red, laminated sandstone of the Sixtymile, or between to thin bedded, very fine grained and silty deposition of the basal sandstone and the to locally shaly; with overlying basal breccia. The record in suc- scattered breccia clasts ceeding strata at Nankoweap Butte, moreover, parallels the record seen at the type section. Two primary and several reworked ° " Breccia breccia layers are present in the lower and Polymict, unsorted middle members. These, and a folding and and reworked (o - chert) crenulation of interbedded laminated sandstone, appear to record structural pulses on the Butte fault and incremental deepening Shale and siltstone of the Chuar syncline predating deposition Walcott Member-black Sixtymile Formation-red of structurally undisrupted sandstone of the upper member of the Sixtymile Formation. The lower two breccia layers of the Nanko- METERS rl5 weap Butte section show little evidence for sorting or reworking, and they are presumed to correlate with the two unsorted -10 landslide deposits of the lower member of the Sixtymile in Sixtymile Canyon. Strati- -5 graphically higher layers of breccia of the middle member display the effects of trans- 0 port and sorting, and they presumably are equivalent to sparse reworked breccia in the lacustrine(?) middle member at the type section in Sixtymile Canyon. The stratigraphically highest unsorted and unreworked Figure §. Section of Sixtymile Formation on Nankoweap Butte, eastern Grand Canyon. breccia at Nankoweap Butte is found at the View is to north. Section measured by D. P. Elston and J. T. Hagstrum, October 1979, and base of the upper member. It, and a directly D. P. Elston, May 1980. underlying unconformity, correlate with the breccia and an underlying unconformity found at the base of the upper member in alluvial plain; (2) erosion and truncation of member with 1.5 m of abrupt local relief. In Sixtymile Canyon. Sandstone beds overly- this siltstone unit on the limbs of the syn- overlying strata of the upper member, brec- ing the basal breccia of the upper member cline (Fig. 4) following an increment of sub- cia fragments are reworked and are con- contain local concentrations of reworked sidence in the deepening syncline; (3) depo- tained in strata of probable fluviatile origin. clasts of breccia and chert, but no evidence sition of evenly bedded siliceous and cherty Because these highest strata show no evi- for later deformation. sandstone of the middle member in quiet, dence for postdepositional disruptions, they standing (lake?) water; (4) folding and cren- are assumed to have accumulated after the In summary, strata of the Sixtymile For- ulation of the middle member as it slumped structural disturbance. As such, they may mation are distinct from underlying marine from both the west and the east toward be the basal beds of a formerly much thicker strata of the Chuar Group, and they the axis of the syncline in response to an and extensive alluvial deposit that accumu- accumulated in an environment created by a apparently near-final structural adjustment lated adjacent to a group of fault-block structural disturbance, which we call the in the syncline and on the Butte fault; and mountains. "Grand Canyon disturbance." For this rea- (5) erosion, followed by deposition of a The stratigraphy of the Sixtymile Forma- son, the Sixtymile is herein removed from breccia-bearing sandstone that displays dis- tion at Nankoweap Butte (Fig. 5) differs the Chuar Group, but it is retained as the rupted bedding (unit 1 of upper member, somewhat in detail from that seen at the uppermost formation in the Grand Canyon Fig. 3). In the axial part of the syncline, this type section at Sixtymile Canyon. Nonethe- Supergroup. The Sixtymile Formation on highest apparently primary breccia uncon- less, it generally accords with the principal stratigraphic and lithologic grounds ap- formably overlies folded beds of the middle aspects of the history deduced from strata at pears to be analogous to strata overlying the

Little Dal Group of the Mackenzie Moun- TABLE 2. SUMMARY OF RESET K-AR AGES OF GRANITIC BASEMENT, tains of northwestern Canada (Eisbacher, AND DIABASE SILLS AND CARDENAS LAVAS OF UNKAR GROUP, 1981; Armstrong and others, 1982). The lat- GRAND CANYON, ARIZONA ter serve to separate basinal marine deposits 40 rad 40 ra<1 6 of middle Proterozoic (Y) age from glacio- Unit Material K2O Ar. Ar Age x io yr Constants genic deposits that clearly are of late Proter- dated (%) (mol/g) (%) Oldt New* ozoic (Z) age. Cardenas Lavas Whole rock 3.10 4.63 x 10"« 98 810 ±20 819 ±20 AGE AND CORRELATION Basalt Canyon . (36.11°N, 111.86° W) OF DISTURBANCE Cardenas Lavas Whole rock. 3.33 4.83 x io-9 95 790 ± 20 809 ± 20 Basalt Canyon Isotopic Ages (36.11°N, 111.86°W) Cardenas Lavas Whole rock 4.19 5.98 x IO"9 99 781 ±20 790 ± 20 Dating of the Sixtymile Formation and Tanner Canyon the record of the structural disturbance that (36.06° N, 111.83° W)

it contains is difficult because a direct iso- 9 Cardenas Lavas§ Whole rock 2.084 3.23 x io- 98 832 ± 34 843 ± 34 topic age has yet to be determined. Analysis Palisades Creek 2.094 of the volcanic ash found at the Kwagunt- (36.13°N, 111.80° W) Sixtymile boundary on Nankoweap Butte Cardenas Lavas** 845 ± 15 855 ± 15 remains to be carried out. We believe, how- (36.11°N, 111.86° W) ever, that some disturbed isotopic ages Diabase sill Pyroxene 0.303 5.19 x io-io 64.4 903 ± 40 913 + 40 obtained from rocks that were buried at Hance Rapids depths of 2 to 4 km at the onset of the dis- (36.05° N, 111.93° W) turbance reflect cooling accompanying the Diabase sill Plagioclase 1.32 2.39 x io-9 92.5 944 ±30 954 ± 30 structural disturbance. In view of distinctly Tapeats Creek greater formation ages for the rocks ana- (36.38° N, 112.48° W) lyzed, at least some of the disturbed isotopic Diabase silljt Whole rock 894 ± 100 904+100 ages appear reasonable for the time of the Tapeats Creek deformation. To anticipate, we believe that (36.38° N, 112.48°W) the new data point to a partial resetting and Diabase sill** Pyroxene 934 944 to a nearly complete resetting in two groups Basement rocks Biotite 9.36 1.7030 x io-« 98.6 948 ± 30 958 + 30 of disturbed K-Ar ages that have an age Granite-Gneiss, range of about 950 to 800 m.y. At the very Clear Creek (36.09? N,. 112.04° W) least, this age range broadly reflects a time

of cooling and Ar retention. Such cooling 10 1 40 4 •Using new constants: = 0.581 x 10" yr ; = 4.96 x 10^'° yr-';. K/Ktota| = 1.167 x 10" gm/gm. and Ar retention seems most simply attribu- 10 10 1 4 fUsing old constants: = 0.585 x lO" yr'; = 4.72 x io yr ; ""K/K^, = 1.19 x lO^ gm/gm. table to uplift accompanying the Grand §Analysis and age determination by M. L. Silberman, 1980. Canyon disturbance. The rock units investi- •»Ford and others, 1972, p. 225. gated include the Cardenas Lavas at the top tT40Ar-39Ar incremental heating, see Table 4 and Figure 5. of the Unkar Group, and diabase sills and crystalline basement near the base of and ing following an episode of heating about tention in the flows began sometime in the beneath the Unkar Group. 800 m.y. ago. McKee and Noble (1974) also inverval between 855 and 790 m.y. ago (see The Cardenas Lavas lie about 1.9 km proposed that these younger K-Ar ages Fig. 6), as the strata cooled as a conse- below the top of the Chuar Group (Table 1). might represent the time of the Grand quence of regional uplift and faulting. The Samples from six lava flows collected in and Canyon disturbance. Elston (1979) applied differences in age presumably reflect varia- near Basalt Canyon (Fig. 1) have yielded a the young K-Ar ages from the Cardenas tions in cooling patterns and' argon whole-rock Rb-Sr isochron of 1,070 ±70 Lavas to the structural disturbance he retention. 87 m.y. (recalculated using the new Rb decay found recorded in the Sixtymile Formation, More deeply buried rocks in the Proter- constants) (McKee and Noble, 1976). This noting their general similarity to ages ozoic sequence and in thè basement also age probably represents the minimum age reported from some late Proterozoic distur- yield disturbed isotopic ages. Diabase sills for the time of lava extrusion, in view of the bances elsewhere in North America. Three intrude strata in the lower part of the Unkar fact that Rb-Sr ages for Proterozoic rocks (recalculated) reset K-Ar ages from the Group, and these have been dated by Rb-Sr commonly seem to be somewhat younger Cardenas Lavas reported by McKee and and K-Ar analyses with contrasting results. than U-Pb ages on zircon determined from Noble (1976) are 819 ± 20, 809 ± 20, and Rb-Sr geochronology yields an age that is coeval rocks (see, for example, Bickford 790 ± 20 m.y. (Table 2); decreasing age cor- significantly older than the age obtained by and others, 1981, p. 338). Significantly responds to increasing stratigraphic depth the K-Ar method. A five-point Rb-Sr iso- younger ages were obtained from the K-Ar in the Cardenas. Two other somewhat older chron from a diabase sill in the central analysis of three of the Cardenas flows by K-Ar ages have been determined by other Grand Canyon (near Shinumo Creek near McKee and Noble (1976), who suggested workers (Table 2). These five ages, if taken the Colorado River, 36.25°N, 112.35°W) that these younger ages might reflect cool- at face value, would indicate that argon re- gives an age of 1.07 ±0.03 b.y. (1.09 b.y.

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AGE pie analyzed in an eight-step incremental m.y. 40Ar/39 Ar experiment to construct an isochron (Table 4; Fig. 8). All ages are the 700 -i E same within their analytical reproducibility tno> a and are significantly younger than the 1.07 m Figure 6. K-Ar ages of basaltic b.y. ± 0.03 b.y. Rb-Sr isochron age (Fig. 7). 40 39 lava flows, diabase sills, and gneis- The scatter on the Ar/ Ar isochron (Fig. 800 - sic basement rock from the Grand 8) and the pattern of the incremental heat- Canyon. (See Table 2 for source of ing ages of the whole-rock sample (Fig. 9) are data.) Other radiometric schemes, characteristic of rocks that have disturbed thermal histories leading to partially reset 900 Rb-Sr or U-Pb, yield ages that are 7 7—7 7—7 7 significantly older and that are ages (see, for example, Fleck and others, eset ;QQ nd basement; 930^ assumed to approximate the age 1977). „ x , 125 m.y. of emplacement of the various The crystalline basement beneath the rocks. The K-Ar ages are consid- stratified units of the Unkar Group has been 1000 ered to be reset ages. dated by the U-Pb method on congenetic zircon at 1,725 ±15 m.y. (Pasteels and Silver, 1965). Other studies of the U-Pb systems in zircon from the basement complex 1100 in the bottom of the Grand Canyon yield radiometric ages of about 1,700 m.y., cor- responding closely to the Mazatzal Revolu- 1200 tion of central Arizona (Silver, 1965). These ages, slightly greater than 1,700 m.y. for Younger ages for the diabase sills of emplacement, or about 1,700 m.y. for a Jy Table 2 major heating event, are significantly older 2/ Table 4 the Grand Canyon are given by K-Ar than K-Ar age determinations on the same 31 McKee and_Noble (1976), recalculated analysis. Potassium-argon dates, from con- with new 87Rb constants. ventional isotope dilution techniques, from rocks. K-Ar analyses on minerals from the n Figure 7 40Ar/39Ar methods, and from incremental basement most commonly give ages of 40Ar/39Ar heating analysis, yield ages be- about 1,400 m.y. (E. H. McKee, unpub. tween about 950 and 900 m.y. Five samples data). The youngest K-Ar age determina- using the old 87 Rb decay constants; see were analyzed; two pyroxene and one pla- tion has come from a biotite sample from 38 Fig. 7). The samples consist of dark diabase gioclase by isotope dilution using an Ar the Zoroaster Granite of Maxson (1961, 40 39 from 65 m and 40 m below the top of the sill tracer, one pyroxene by Ar/ Ar total 1967) at Clear Creek, which has given an (samples ba 16 and ba 18, respectively), fusion, and one crystalline whole-rock sam- age of 947 ± 20 m.y. (Table 2, Fig. 6). This light-gray diabase from 7 m below the top 0.75 (sample ba 8), and two samples of pink 1 1 / granophyric diabase 7 m and 4 m below the top (samples ba 7 and ba 8). Results of the Rb-Sr studies are given in Table 3. The iso- /bo-7 /ba-6 — chron age, which is the best-fit line through Figure 7. Rb-Sr whole- 0.74 87 86 87 86 the Rb/ Sr versus Sr/ Sr data using a rock isochron plotted on a modification of the procedure described by strontium evolution dia- York (1969), is assumed to be close to the gram. The five points are original cooling age of the sill. Within ana- whole-rock samples from 0.73

much as 50 m.y. earlier.) 87 6 Rb/^ Rb

TABLE 3. Rb-Sr ANALYTICAL DATA FOR SAMPLES OF DIABASE SILL nor as severely disturbed structurally since NEAR SHINUMO CREEK, CENTRAL GRAND CANYON, ARIZONA Precambrian time, the burial of Unkar Group rocks and the basement by 2 to 4 km Rb (ppm) Sr (ppm) Rb/Sr 87Rb/86Sr 87Sr/86Sr (2 a) of Proterozoic strata, followed by uplift and cooling at the end of deposition of the ba 6 76.8 93.3 0.823 2.392 0:7403 ± 0.0005 Chuar Group, would seem to be the ba 7 80.7 92.7 .871 2.531 .7419 ±0.0003 sequence of events responsible for the ba 8 24.5 168.4 .146 .422 ,7122±0.0002 observed reset ages. Because the K-Ar ages .063 .182 .7061 ±0.0004 ba 13 31.5 502.7 from minerals in the sills and basement are ba 16 29.1 480.7 .061 .175 .7062 ± 0.0004 disturbed but probably are not completely reset, the group of younger whole-rock ages Note: Location of sill is 36.245°N, 112.347° W. Constants used: 87RbX^ = 1.42 x 10"" yr; 87Rb = 0.283 g/g Rb. Sr isotopic data are normalized to a from the Cardenas Lavas appear to more value of 0.1194 for 86Sr/88Sr. The mass spectrometer yields a mean 87Sr/86Sr value of 0.7080 for the sensitively and accurately record the time of Eimer and Amend standard SrC03. Rb and Sr concentrations were determined by X-ray fluorescence resetting. using an Mo tube operated at 75 KV. Paleontologic Correlations and. Reported Ages sample came from the same granite-gneiss and basement rocks. Nonetheless, the dis- body as the one that yielded the 1,725 ± 15 turbed ages are all broadly the same, within Algal megafossils and microfossils, and m.y. U-Pb age. The very young K-Ar age, about 10%. This consistency, and the obser- microfossils that may represent early proto- which is essentially the same as the dis- vation that the amounts of potassium in the zoan or metazoan life, are reported from the turbed age from the diabase sill (Fig. 6), various mineral phases and whole rock Galeros and Kwagunt Formations of the

documents at least local, pronounced Ar differ widely (0.3% to 9.26% K20, Table 2), Chuar Group (see above). They serve as a loss in biotite in the basement rocks. is evidence against appreciable slow leakage biota to which similar forms recognized Disturbed K-Ar ages shown in Table 2 of argon over geologic time. Slow leakage elsewhere in the world are referred. There- range from 958 to 790 m.y., and they fall would have had to occur in the exact pro- fore, ages assigned to other strata that con- into two groups (Fig. 6). The group of portions in both the minerals and the whole tain a Chuar-like biota, and to which a younger ages, which averages 823 ± 26 m.y. rock to produce the similar ages—an un- systematic position and age significance (one standard deviation), is from five likely situation. Rather, the two groups of have been attached, are briefly reviewed whole-rock determinations of cryptocrystal- ages would seem to be best explained if they here. line basaltic andesite from the Cardenas occurred as a consequence of argon-loss A varied biota has been reported from the Lavas. The group of older ages, which aver- during a single episode of heating and cool- Little Dal Group in the Mackenzie Moun- age 930 ± 25 m.y., is from five analyses of ing, an episode that resulted in partial to tains of northwest Canada. It is a biota that mineral separates and the whole-rock perhaps complete resetting of the Ar clock. includes Chuaria circularis and a probable 40 39 Ar/ Ar analysis of a coarse-grained Because strata of the Grand Canyon Super- metazoan trace fossil (Hofmann and Ait- holocrystalline sample. The whole-rock K- group have neither been as deeply buried ken, 1979). Hofmann and Aitken have esti- Ar ages obtained from the lavas are distinctly younger than the mineral ages obtained from the diabase and gneiss, perhaps because argon diffused out and was lost more easily from the cryptocrystalline rock matrix of the flows than from the crystal lattice structure of phenocrysts of the sills

Figure 8. 40Ar/39Ar isochron from seven-step incremental heating of a holocrystalline whole-rock sample of diabase from g the sill at Tapeats Creek (36.38° N, 112.48° W) central Grand Canyon. Temperatures of the heating steps are shown on the diagram. The age is the best line through the points determined by a least-squares fit using a modification of the procedure described by York (1969). The ± value is la.

300 400 500

39Ar/36Ar

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TABLE 4. ANALYTICAL DATA FOR 40Ar/39Ar TOTAL. FUSION AND INCREMENTAL-HEATING EXPERIMENT ON MINERAL AND WHOLE-ROCK SAMPLES OF DIABASE SILL AT TAPEATS CREEK AND ON MINERAL AND WHOLE-ROCK SAMPLES OF SILL NEAR SHINUMO CREEK, CENTRAL GRAND CANYON, ARIZONA

40 * 36 40 39 37 39 36 39 39 Material Ar Arca Ar/ Ar Ar/ Ar Ar/ Ar J Temperature ArK Apparent °C (percent of age (m.y.)t total)

(Total fusion) Pyroxene - - 52.3 22.9 101.0 178.2 0.212 0.01023 — 100. 907 ± 35 (Incremental-heating) Whole rock - - 73.2 0.8 31.5 .88 .029 .01023 500 3.8 410 ± 9 Do. 97.6 0.15 67.8 2.41 .006 .01023 700 .14.8 985 ± 12 Do. 97.5 5.5 64.4 1.16 .005 .01023 780 17.2 945 ± 12 Do. 98.5 6.3 58.9 .75 .003 .01023 860 23.8 899 ± 11 Do. 98.7 7.5 .54.4 .69 .003 .00123 940 20.5 836 ± 11 Do. 95.9 8.3 •44.7 2.09 .007 .01023 1020 7.7 695 ±9 Do. 95.3 17.5 67.5 8.37 .013 .01023 1100 5.3 966 ± 12 Do. 89.2 10.7 62.1 9.96 .025 .01023 1200 7.0 860 ± 12 Isochron age, 904 ± 100 (m.y.)

Note: location of sill at Tapeats Creek is 36.38° N, 112.48° W; location of sill near Shinumo Creek is 36.245°N, 112.347°W. 10 1 10 1 fAge using new constants: Kt = 0.581 x 10 yr ; Kß = 4.96 x 10 yr ; "°K/Ktotal = 1.167 x lO^gm/gm.

mated a late Neohelikian to early Hadry- into the time of deposition of the lower part chron (Armstrong and others, 1982). Geo- nian age (about 1,100 to 800 m.y.) for the of the Windermere Supergroup (Eisbacher, logic relations in the Mackenzie Mountains Little Dal assemblage. The Little Dal 1981). Strata of the Rapitan Group of the thus indicate that deposition of Chuaria- Group is in the upper part of the Macken- Windermere Supergroup (upper Protero- bearing marine strata of the Little Dal zie Mountains . Supergroup (Jefferson, •zoic) overlie the Coppercap Formation Group ended as a consequence of a struc- 1978). Lava flows at the top of the Little . conformably or unconformably. Sills and tural-disturbance, as was the case for the Dal Group are overlain by the Redstone dikes intrude the Little Dal Group and Chuar Group in the Grand Canyon. On River and Coppercap Formations, which underlying strata. Emplacement either pre- paleontologic grounds, the Little Dal ap- are informally referred to as a "copper dates or was contemporaneous with extru- pears to correlate with the Chuar Group, cycle" sequence by some Canadian workers. sion of the lava flows at the top of (or just •and the overlying Redstone River Forma- These copper cycle strata were deposited at above) the Little Dal Group; in any case, tion of the Mackenzie Mountains thus . a time of block faulting, and they form the the sills are not much younger than the time would seem to correlate with the Sixtymile upper part of the Mackenzie Mountains of deposition of the basal units of the Win- Formation. The isotopic age for the intru- Supergroup (Eisbacher, 1977, Figs. 4, 6, 8, dermere Supergroup. The age of two of the sions provides the approximate time for the arid 1981; Helmstaedt and others, 1979). sills that intrude the Tsezotene Formation onset of the disturbance in northwest Can- Faulting, with relative vertical displace- beneath the Little Dal Group is reported as ada (770 m.y.), and the reset K-Ar age from ments locally greater than 800 m, occurred 770 ± 20 m.y. from an internal Rb-Sr iso- the Cardenas Lavas of the Grand Canyon (823 m.y.) appears to record the time of the disturbance in the Grand Canyon. The paleontologic correlations and generally similar ages thus suggest a disturbance of

10000-1 continental proportions in western North 8000" America, an inference that also is supported 6000- 4000- (O K < 2000- >LU 1000- (A 800- O 600- E 400- o IO 4 39 e> o> Figure 9. ®Ar/ Ar age spectra for a holocrystalline < 200- whole-rock sample of diabase from the sill at Tapeats W Creek. Individual ages calculated for the argon released 100- IO 80- at different temperature'steps are shown. This release t. 60" < curve is similar to curves produced from rocks with 40- known disturbed thermal histories (Lanphere and Dal- . 20- rymple, 1971).

10- —I— -1— —I— —I— 50 70 90 20 30 40 50 60 70 80 90 100 39Ar RELEASED, CUMULATIVE PERCENT

from paleomagnetic correlations. Arm- with those found in beds of the middle and Pringle, 1973, recalculated in Sturt and oth- strong and others (1982) consider emplace- lower-upper Visingso Group, as well as in ers, 1975, p. 711). ment of the diabase at about 770 m.y. ago to the Vadso Group of northern Norway have occurred close to the boundary be- (Vidal, 1981). The upper Visingso Group is Paleomagnetic Correlations tween the Mackenzie Mountains and dated at 707 ± 37 m.y. (Rb/Sr whole-rock Windermere Supergroups, and to reflect a age on shale cited in Vidal, 1979b, p. 10), A stratigraphically controlled apparent rifting event that initiated the "younger than and the Vadso Group is dated at 807 ± 19 polar wandering path is being developed 850 m.y." continental separation (Stewart, m.y. (Rb/Sr whole-rock age on shale of from middle Proterozoic strata of Arizona, 1972), which also is considered to have been responsible for the origin of the west-facing Cordilleran miogeocline. The stratigraphic distribution and age range for the problematic fossil Chuaria and associated structures have been summarized by Hofmann (1977, Table 1), who reported a maximum age range of 1,150 to 570 m.y. Some ages are only broad esti- mates, such as the 1,000- to 600-m.y. age range that Hofmann estimated for the Chuar Group. An age of 950 to 925 m.y. is reported for the Chuaria-btaxmg Red Pine Shale, the upper formation of the Uinta Mountain Group of Utah and Colorado, determined from whole-rock Rb-Sr analysis of the shale (Crittenden and Peterman, 1975; Chaudhuri and Hansen, 1980). No other kind of isotopie age has been reported for the Uinta Mountain Group. Bloeser and others (1977) discovered flask-shaped chitinozoan-like microfossils in the Walcott Member of the Kwagunt Formation, and they suggested that these fossils may be early representatives of pro- tozoan or metazoan life. Similar if not identical microfossils have been found in Prot- erozoic rocks in Saudi Arabia in either the Haliban Group (750 to 650 m.y.) or the Murdama Group (630 to 600 m.y.) (Binda and Bokhari, 1980). In addition, vase- shaped, chitinozoan-like microfossils regarded as comparable to those in the Kwagunt Formation, as well as Chuaria circularis, also have been recovered (1) from Figure 10. Apparent polar wandering path for Belt and Grand Canyon Supergroups the upper part of the Eleonore Bay Group (generalized from Elston and Bressler, 1980 and unpub. data; Elston and Gromme, 1974, of East Greenland, underlying tillite beds of 1979, and unpub. data). B-B', Belt Supergroup, Montana and Idaho. Grand Canyon Vendian (Hadrynian) age (Vidal, 1979a); (2) Supergroup, northern Arizona: U-U', Unkar Group; N-N', Nankoweap Formation; C-C', in beds of the upper Visingso Group of Chuar Group; S, Sixtymile Formation. Uinta Mountain Group, Utah and Colorado southern Sweden (Knoll and Vidal, 1980); (Bressler, 1981); preliminary pole 6 near top of Uinta Mountain Group shows westward and (3) in the Vadso Group of northern shift similar to shift seen in Sixtymile Formation. Little Dal Group, Mackenzie Mountains, Norway (Vidal, 1981). Paleontological sim- northwest Canada (Park, 1980,1981); pole "a" is from detrital(?) magnetite and a hematite ilarities have led Vidal (1979a, p. 35) to component, and pole "b" is considered due to a red hematite pigment; ~ 770-m.y.-old sills in correlate the upper part of the Eleonore Bay Tsezotene Formation beneath Little Dal Group (J. K. Park, in W. A. Morris and J. K. Group of Greenland and beds of the Vi- Park, 1980, written commun.); LD-L and LD-M, poles from lower (unaltered) and upper singso Group of Sweden (Vidal, 1974) with (altered, mixed polarity) parts of lava flows at the top of the Little Dal Group (W. A. the Kwagunt Formation of the Chuar Morris and J. K. Park, 1980, written commun.). Thermochron poles for Grenville Prov- Group. The paleontologie correlation be- ince rocks, Gra, Grb, Grc (McWilliams and Dunlop, 1977, Table 1) have been assigned tween the Arizona, Swedish, and Norwe- inferred ages of >1,000,1,000 to 9S0, and 950 to 900 m.y., respectively. A pole Grd (= Gra) gian sections has been further improved. has been assigned an age of < 900 m.y. A pole residing in a low temperature (-<250 °C) Vidal (1981, written commun.) has observed component of magnetization in the Haliburton intrusions of the Grenville Province, identi- acritarchs from a number of samples from cal with the Chuar poles, has been assigned an age of ~<820 m.y. from an isotopically the Chuar Group, and he reported that an derived cooling curve, whereas a pole residing in a high temperature component of magnet- assemblage from the Kwagunt Formation ization (650-520 ° C), which plots at the southern end of the apparent Grenville path, is is, species by species, directly comparable assigned an age of 980 m.y. (Berger and others, 1979). See text for discussion.

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Montana and Idaho, and Utah (Elston and Group and Sixtymile Formation of the part of the Belt Supergroup correlates with Gromme, 1974, 1979, and unpub. data; Grand Canyon Supergroup, and results strata of the Grand Canyon Supergroup. Elston and Bressler, 1980 and unpub. data; from an extensive study of the Belt Super- An age of about 1,100 m.y. has been Bressler, 1981). A composite polar path has group of Montana and Idaho; in this report, reported from the Rb-Sr analysis of sedi- emerged, the generalized trace of which is we propose an over-all correlation of mid- mentary rocks of the Belt that underlie and shown in Figure 10. The path has allowed dle Proterozoic strata of North America that include much of the Missoula Group, correlations to be made with a number of that is based on polarities as well as on and a younger provisional age of about 900 other paleomagnetically studied sections poles. An important goal of the stratigraph- m.y. has been reported from strata of the and rocks of North America. These include ically oriented paleomagnetic work in the upper Missoula Group (Obradovich and Proterozoic strata and intrusions of the Proterozoic rocks of the western United Peterman, 1968). The 1,100-m.y. Rb-Sr age western cordillera of the United States and States is to closely tie a well-documented for part of the Missoula Group and underly- Canada, Keweenawan strata and intrusions polar path and polarity zonation to the ing strata is the same as an ~ 1,100-m.y. K- of the Lake Superior region, and some depositional record and to the structural Ar whole-rock age reported from the Pur- rocks of the Grenville Province of eastern history that can be deduced from the geo- cell Lava of the Missoula Group (Hunt, North America. The correlations permit the logic record. The interrelated data are help- 1962). However, the polar path from the extrapolation of U-Pb and Rb-Sr ages, ing to clarify the Proterozoic tectonic Belt rocks clearly does not contain the pro- which serve to improve age assignments for history of the North American craton and nounced north-trending loop seen in the otherwise undated strata whose poles plot also are providing a temporal framework Proterozoic rocks of Arizona and Lake on discrete parts of the composite, strati- for assessing the history of evolution of Superior. This loop is well dated from the graphically controlled polar path. The most plant and animal life during the Protero- U-Pb analysis on zircon of rocks of Lake reliable, nonconflicting ages derive from the zoic. Superior. If the Belt basin were an integral several U-Pb ages determined from zircon In this report, we are concerned with the part of the North American plate (and no in igneous rocks reported from Lake Super- age and correlation of a structural episode suggestion to the contrary exists), the polar ior, Arizona, and Montana; these are aug- that appears to coincide with, and to over- path shown on Figure 10 would seem to mented by some closely supporting, non- lap, the boundary between the deposition of indicate that deposition of Belt Supergroup conflicting Rb-Sr isochron ages on igneous strata of middle and late Proterozoic age. A rocks largely predated deposition of rocks rocks. Some Rb-Sr ages, however, appear correlation on paleomagnetic, paleontolo- of the Grand Canyon and Keweenawan slightly to distinctly too young, as do K-Ar gic, and isotopic ages exists for strata of the Supergroups. From the lack of paleomag- ages in general that have been reported for western cordillera of North America. A less netic correlation (on polarity as well as these ancient rocks. For this reason, the certain correlation exists with respect to poles), strata of the Belt appear to be no composite polar path is controlled by only a some of the rocks, and to an inferred struc- younger than ~ 1,250 m.y. relatively few of the many isotopic ages that tural event, in the Grenville Province. Grand Canyon (Lower) and Keweenawan have been reported for the Proterozoic Because of this, we first review the status of Supergroups. A key magnetostratigraphic rocks of North America. As a corollary, the paleomagnetic correlations and ages for section has been obtained from the Unkar construction of polar paths from poles rocks of the western and central craton. The Group of the Grand Canyon Supergroup whose ages are ordered principally by a mix stratigraphic units, and correlations, are (Fig. 10). Poles from the upper Apache of isotopic ages leads to paths that differ treated in groupings of decreasing age. Group and from ~ l,150-m.y.-old sills (U- markedly from our composite stratigraphi- Pb zircon age of Silver, 1963, 1972) from cally controlled path (for example, see Belt Supergroup. The polar path for pre- central Arizona plot very near the top of the Irving and others, 1974; Irving and Mc- Grand Canyon Supergroup rocks is con- pronounced loop in the Unkar polar path. Glynn, 1976; Henry and others, 1977; Roy trolled by poles from the Belt Supergroup The Unkar polar path is drawn only on the and Robertson, 1978). of Montana and Idaho (Fig. 10). On the basis of poles and polarities, the Belt Super- basis of normal polarity poles. Essentially Details of the stratigraphically controlled group (except for the upper part of the identical normal polarity poles from the polar path developed from paleomagnetic Pilcher Quartzite at the top) appears to middle parts of the Dox Sandstone define studies in the western United States are entirely predate strata of the Grand Canyon the top of the loop, and strata containing given in two reports that presently are Supergroup (Elston and Bressler, 1980, Fig. these poles bound an ~ 200-m-thick interval undergoing review within the U.S. Geologi- 7; and supported by considerable additional containing mixed polarity directions. Some cal Survey. One report (by Elston and S. unpub. data). The Belt polar path lies in of the reversed polarity directions are Gromme) describes the polar path that has the ~ l,400-m.y.-old part of the North broadly antiparallel to the normal polarity been obtained from the Unkar Group and American polar path (Spall, 1971). Sills directions. However, some reversed polarity Nankoweap Formation of the Grand Can- intrude the lower part of the Belt Super- directions are scattered and tend to be offset yon Supergroup, supplemented by some group in the western Belt basin, and a U- from the enclosing (and from some in- data from the Proterozoic rocks of central Th-Pb age on zircon of 1,433 ± 10 m.y. has cluded) normal polarity directions. The Arizona. In it, we propose a detailed corre- been determined for one of these (Zartman sense of offset is similar to that seen in re- lation between Proterozoic rocks of Ari- and others, 1982). However, other types of versed polarity directions reported from zona and those of the Keweenawan Super- isotopic age studies in the middle to upper middle Keweenawan rocks of the Lake group of Lake Superior. The other report parts of the Belt Supergroup have given Superior region (Palmer, 1970; Robertson, (by Elston and S. L. Bressler) summarizes much younger ages, and these ages have led 1973; Massey, 1979; Pesonen, 1979). The paleomagnetic results from the Chuar some workers to believe that a considerable reversed polarity directions in the Keween-

awan rocks result in easterly offset reversed Superior determinations, and from a dis- seen between the Kwagunt and Sixtymile polarity poles, and these poles led to the cussion of the reliability of Rb-Sr ages with Formations. Data recording the shift in the construction of a northeasterly trending respect to U-Pb ages on zircon (Bickford upper Uinta Mountain Group rocks are "Great Logan Loop" (Robertson and Fah- and others, 1981, p. 338), we consider that considered preliminary, pending acquisition rig, 1971). This loop has been the subject of the 1,070 m.y. Rb-Sr date on the Cardenas of additional data. Nonetheless, the close no little discussion (see, for example, Pe- Lavas to be a minimum age. The maximum apparent paleomagnetic correlation appears sonen and Halls, 1979). The coincidence of age for the Cardenas probably is no greater reasonable in light of a paléontologie corre- normal polarity directions that enclose and than 1,100 m.y., to allow at least 15 m.y. for lation and a broad concordance of isotopic that also are within the interval of mixed development of much of the descending leg ages. polarity in the Dox Sandstone, and the gen- of the loop. Mackenzie Mountains Supergroup. Poles erally scattered nature of the reversed polar- Grand Canyon Supergroup (Middle and from strata of the Little Dal Group, lying ity directions, have cast serious doubt that Upper) and Uinta Mountain Group. The very near the top of the Mackenzie Moun- poles calculated from the reversed polarity polar path for the Grand Canyon Super- tains Supergroup, and a pole from 770-m.y.- directions can be employed in a polar path group is incompletely controlled for an old sills intruded in the underlying Tsezo- meant to reflect motion of the North Amer- interval that includes the Nankoweap For- tene Formation following deposition of the ican plate. mation. For this reason, the polar path is Little Dal, are identical with the pole for the Normal polarity poles from the lower, shown as a dashed line (Fig. 10). Three Sixtymile Formation (ovals overlap at the middle, and upper parts of the Keweenawan unconformities are recognized between the 95% confidence level) (Fig. 10). Preliminary Supergroup (for example, see DuBois, Unkar and Chuar Groups (Table I; Fig. 2). poles from lava flows that directly overlie 1962; Books, 1968, 1972; Robertson and The middle unconformity, on both geologic strata of the Little Dal Group plot to the Fahrig, 1971; and Pesonen and Halls, 1979) and paleomagnetic grounds, appears to north, but it must be noted that the timing correlate with the Unkar normal polarity mark a significant hiatus. No age control of magnetization for the Little Dal lavas has path from a stratigraphic position begin- exists for the time of deposition of the Nan- not yet been established and the directions ning at about the level of the Shinumo koweap and the lower Chuar Group. We may not accurately reflect apparent polar Quartzite. If early Keweenawan deposits suggest a provisional age of 1,050 m.y. for wander. Poles are needed from strata under- accumulated about 1,200 m.y. ago at about the beginning of deposition of the upper lying the Little Dal Group to obtain infor- the beginning of the Grenville orogeny, then member of the Nankoweap, above the mation bearing on a westerly polar shift deposition of the earlier parts of the Unkar major unconformity. This would allow similar to that seen in the Chuar Group and Group presumably began sometime earlier, ample time for erosion on the ~ 1,100-m.y.- Sixtymile Formation and in the Uinta presumably in the interval 1,250 to 1,200 old Cardenas Lavas, followed by deposition Mountain Group. Poles also are needed m.y. ago. Extrusive and intrusive rocks of and erosion of the lower (ferruginous) from the "copper cycle" strata at the top of middle Keweenawan age in the Lake Super- member of the Nankoweap. As pointed out the Mackenzie Mountains Supergroup to ior region are well dated by the U-Pb above, deposition of marine shale of the develop control from rocks that accumu- method on zircon as having accumulated in Chuar Group has been estimated from sev- lated shortly before deposition of the glaci- the interval 1,145 to 1,115 ± 15 m.y. ago eral considerations to have begun by about ogenic late Proterozoic Windermere Super- (Silver and Green, 1963, 1972). Poles from 1,000 m.y. ago. group. Although such poles remain to be determined, a general correlation of strata these rocks control the top of and a small The Grand Canyon polar path appears to of the upper Mackenzie Mountains Super- part of the descending leg of the loop. Poles have been stable during the time of deposi- group with strata of the upper Grand from intrusions of middle Keweenawan age tion of the upper member of the Nanko- Canyon Supergroup derive not only from of Lake Superior plot near the top of the weap Formation and during deposition of paléontologie and isotopic age data, but Unkar loop, as do poles from the major much of the Chuar Group. The paleomag- also from paleomagnetic data. intrusions of central and northern Arizona. netic record from the Chuar Group has Contemporaneity is indicated both from come from stratigraphically rather widely Grenville Province. Some poles reported generally identical U-Pb ages and from the spaced red beds. Sometime before deposi- from the Grenville Province appear to coin- poles. tion of the Sixtymile Formation, during cide with poles from the Chuar Group and The age for the base of the descending leg deposition of the upper part of the Kwagunt Sixtymile Formation, but a number of of the Unkar loop is probably no less than Formation, a pronounced westerly shift in Grenville poles plot far to the south (Fig. 1,070 m.y., the Rb-Sr isochron age for the the pole took place. This shift, and presum- 10). No such poles are known from any stra- Cardenas Lavas reported here. However, ably its termination, seem related to onset tigraphic succession in North America. The this age for the flows (and for the Grand of the Grand Canyon disturbance. southerly poles have been employed to Canyon sills, as well) is identical to a A preliminary apparent polar wandering anchor an inferred looping Grenville polar number of Rb-Sr ages reported for middle path has been developed from the Uinta path, which begins and ends in the area of Keweenawan intrusive and extrusive rocks Mountain Group of Utah and Colorado the Chuar poles. The proposed Grenville of the Lake Superior region (Van Schmus, (Bressler, 1981), and it appears to correlate loop has been diagrammed as trending in a 1971; Chaudhuri, 1974), ages that are dis- well with the polar path from the Chuar clockwise sense (for example, see Berger tinctly younger than the U-Pb ages on zir- Group and Sixtymile Formation (Fig. 10). and others, 1979) and in a counterclockwise con. Additionally, some even younger ages Rocks near the top of the Uinta Mountain sense (for example, see Piper, 1980), reflect- also have been reported from the middle Group record an apparent westerly shift of ing uncertainties as to the temporal order of Keweenawan rocks. In view of the Lake the pole, seemingly identical to the shift the magnetizations. The paleomagnetic di-

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rections have been derived from multicom- chrons calculated across the Grenville Prov- cooling of the Grenville terrane. In contrast, ponent thermoremanent (TRM) magneti- ince (Baer, 1976) show an irregular north- low-temperature components of magnetiza- zations that reside variously in magnetite, west to southeast decrease in age from tion might not survive a post-cooling dis- titanomagnetite, and hematite, and the about 1,000 to 900 m.y. These thermo- turbance, but might very well record a magnetizations are presumed to have been chrons were employed to estimate the ages paleomagnetic direction acquired during acquired during cooling of the Grenville ter- of groups of reset paleomagnetic directions final cooling at the end of the disturbance. rane following metamorphism and intru- (McWilliams and Dunlop, 1977, Table 1), The two poles that anchor the south-to- sion that accompanied the Grenville oro- which are summarized in Figure 10 to dis- north-trending leg of the Grenville loop of geny. The southerly polar shift is recorded play the nature of the discrepancy between Berger and others (1979) thus might as read- in rocks of Grenville age in Fennoscandia, the Grenville poles and poles from the west- ily reflect a structural rotation as plate

and a similar shift also has been reported ern cordillera. Poles Gra and Gn>, to which motion. Not presently known are poten- from Grenville-age rocks in Africa. An thermochron ages of > 1,000 m.y. and 1,000 tially important structural relations between apparent congruency of the three polar to 950 m.y., respectively, were assigned, sites and between the multicomponent paths led Piper (1980, Fig. 3b) to suggest a appear to correlate with poles for the Chuar magnetizations in the Haliburton intrusions plate reconstruction for the interval 1,050 to Group and Sixtymile Formation (however, that might be used to investigate the true 800 m.y. that differs markedly from other pole Grb displays a southerly as well as a nature and cause of the apparent "Grenville

middle Proterozoic plate reconstructions. westerly shift). Pole Grc, with a thermo- loop." The southerly trending Grenville shift in chron age of 950 to 900 m.y., plots far to the A southerly loop is lacking in the strati- poles, although not stratigraphically con- south. A still younger (< 900 m.y.) fourth graphically controlled, isotopically and trolled, appears to be real and to reflect group of poles (Grj, which is identical in paleontologically dated polar path from the some sort of event in the Grenville rocks. position with pole Gra) plots back in the western cordillera, which leads to the sug- However, several uncertainties remain, and area of the poles for the Chuar Group. gestion that the apparent shift in some of some critical assumptions have been made An intrusion that plots paleomagnetically the Grenville poles may be an artifact. We for the construction of the inferred looping at pole G^ has been dated at 980 m.y. do not believe that such a shift could have Grenville polar path. In particular, poles are (hornblende) and 945 m.y. (biotite) from been missed in the western sections, nor do 40 39 reported without the benefit of any structur- Ar/ Ar release ages, and rocks contain- we believe that the "Grenville loop" was al corrections. If the rocks containing the ing two poles that plot at Gr

CONCLUSIONS AND DISCUSSION regional metamorphism and minor folding disturbance in the west appears to have had that followed deposition of the Pureell Ser- its counterpart in eastern North America Paleomagnetic data, supported by isotop- ies (of Scofield, 1912) about 800 m.y. ago with the development of the Appalachian ic-age and paleontologic data-, indicate (Gabrielse, 1972). Because paleomagnetic -salients and recesses, it seems unlikely that that the Chuar Group and the Sixtymile correlations suggest an age greater than the adjacent Grenville Province could have Formation correlate with the Uinta Moun- ~ 1,250 m.y. for most strata of the Belt escaped deformation. Rather, we suggest tain Group of Utah and Colorado, and with (Pureell)-Supergroup,-a major hiatus ap- that a deformational event of continental the Little Dal Group of the Mackenzie pears to exist at the unconformity underly- proportions, beginning about 820 m.y. ago, Mountains of northwest Canada. An ap- ing Windermere Supergroup strata in the generated the structural framework that has proximate age range of about 820 to 770 southern cordillera of Canada. The "event" controlled depositional patterns on and m.y. is indicated for a wide-ranging struc- called the "East Kootenay orogeny" is char- adjacent to the craton to this day. We tural disturbance that ended deposition of acterized by regional and local unconformi- further suggest that this structural distur- Chuaria-bearing marine strata of middle ties that reflect a time of crustal instability, bance was the terminal event of the Gren- Proterozoic age in western North America, and the structural disturbance .along the ville orogeny. Unconformities.that developed as a conse- Canadian cordillera (Eisbacher, 1981) is quence of this disturbance serve to mark the marked by the transition from deposition of ACKNOWLEDGMENTS boundary between middle Proterozoic (Hel- carbonate-quartzite strata of middle Prot- ikian and Riphean) and late Proterozoic erozoic age to deposition of clastic strata of Permission given by the National Park (Hadrynian-and Vendian) strata as they are late Proterozoic age (Windermere Super- Service to work and sample in the Grand otherwise commonly recognized on the group). The structural disturbance in the Canyon National Park is gratefully ac- basis of paleontologic and climatic indica- Mackenzie Mountains thus is now consi- knowledged, as is the kind cooperation of tors in the sedimentary record. dered by some Canadian workers to corre- David C. Ochsner and Robert Yearout of This structural disturbance led to a late.with'the "East Kootenay orogeny," and the Park Service. Most samples of the Car- marked change in -the tectonic setting of • together, this disturbance in Canada in turn denas Lavas, diabase sills, and crystalline North America (Stewart, 1976, p. 14), when correlates with the Grand Canyon distur- basement used for the determination of iso- deposition in scattered locally deep epicra- bance. We propose that this event be called topic ages were collected during joint field tonic troughs gave way to deposition on a the "Grand Canyon-Mackenzie Mountains studies by the authors. We thank Ivo Luc- continental shelf. On the west, the shelf is disturbance" for its geographic end mem- chitta, John H. Stewart, Jack E. Harrison, called the "Cordilleran miogeocline" (Ste- bers, and that the term "East Kootenay Chester A. Wrucke, and Andrew F. Shride wart and Poole, 1974). Stewart considered orogeny" be abandoned, for their reviews of an early version of this the shelf to be marginal to and possibly Reset paleomagnetic poles from the manuscript. We.thank Robert J. Fleck and extend almost entirely around North Amer- Grenville Province assigned to the Grenville Max D. Crittenden for their helpful reviews ica. Rankin (1976) has proposed that on the orogeny (originally defined as occurring in of the present manuscript, and Richard L. east coast of North America, Appalachian the interval 1,000 to 880 m.y. ago but now Armstrong and Henry Halls, reviewers for salients and recesses are inherited from the extended to -<820 m.y. ago) in part corre- the journal, for important comments and initial breakup of a continental mass about late with poles from the Chuar and Uinta suggestions. We also thank Gen H. Eis- 820 m.y. ago. Additionally, cooling of the Mountain Groups. The correlations exist bacher for providing us with information on Grenville terrane to below 250 °C also is for Grenville rocks to which both old the progress of mapping and correlations in reported to have occurred -<820 m.y. ago (>1,000 m.y.) and younger (< 900 to 820 Proterozoic rocks of the Canadian cordil- (Berger and others, 1979). The structural m.y.) ages have been assigned. Some Gren- lera, for friendly and helpful discussions, • disturbance recorded in the upper part of ville poles display a westward polar shift, and for his comments on this manuscript. the Grand Canyon Supergroup (Elston, similar to the-shift seen in rocks of the Our thanks also go to Gonzalo Vidal for 1979), and that recorded in the upper part Grand Canyon to which an ~823 m.y. age is providing important information bearing of the Mackenzie Mountains Supergroup now . assigned. However, .the strong south- on the paleontologic and isotopic age corre- (Armstrong and others, 1982), appear re- erly shift seen in a number of Grenville lations of several C/iwaWa-bearing groups of strata. Continuing research in Proterozoic lated to this continent-wide reorganization poles, giving rise to an apparent loop, is not paleomagnetism is being supported by the of depositional patterns. Its onset in both seen in the stratigraphically controlled polar Geologic Framework Program of the U.S. eastern and western North America appears path from the western cordillera of North Geological Survey. to have taken place about 820 m.y. ago. America. We propose that the southerly Strata equivalent to the Redstone River shift is an artifact reflecting uncorrected and Coppercap Formations of,the Macken- rotations in declination of stable high REFERENCES CITED zie Mountains do not appear to be present • temperature components, of magnetization in the southern cordillera of Canada, where arising from a structural deformation that Armstrong, R. L., Eisbacher, G. H., and Evans, glaciogenic deposits of the Windermere marked the end of a long period of cooling. P: D., 1982, Age and stratigraphic-tectonic significance of Proterozoic diabase sheets, Supergroup (Young and others, 1973) di- Cooling of the Grenville terrane ended • Mackenzie Mountains, northwestern Can- 40 39 rectly overlie strata of the Belt (Pureell) about ~<820 m.y. ago, from Ar/ Ar ada: Canadian Journal of Earth Sciences, Supergroup. 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R„ and Roberts, D„ lar wandering path from Canadian shield 1975, Caledonian nappe sequence of Finn- MANUSCRIPT RECEIVED BY THE SOCIETY rocks during the Neohelikian Era: Canadian mark, northern Norway, and the timing of FEBRUARY 13, 1981 Journal of Earth Sciences, v. 8, p. 1355. orogenic deformation and metamorphism: REVISED MANUSCRIPT RECEIVED Roy, J. L„ and Robertson, W. A., 1978, Paleo- Geological Society of America Bulletin, AUGUST 24, 1981 magnetism of the Jacobsville Formation and v. 86, p. 710-718. MANUSCRIPT ACCEPTED SEPTEMBER 2, 1981 Primed in U.S.A.

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