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JOHN H. STEWART U.S. Geological Survey, Menlo Par\, California 94025 Initial Deposits in the Cordilleran Geosyncline: Evidence of a Late Precambrian (<850 m.y.) Continental Separation ABSTRACT involved in the Kenoran (2,400 to 2,600 m.y.) and Hudsonian (1,640 to 1,860 m.y.) orogenies, Upper Precambrian and Lower Cambrian overprinted in places by the Elsonian event strata in western North America are exposed in (1,280 to 1,460 m.y.) (King, 1969b, Fig. 10, a narrow slightly sinuous belt extending from Table 3, p. 33-42). Overlying these meta- Alaska and northern Canada to northern Mexi- morphic rocks are supracrustal rocks that con- co, a distance of 2,500 mi. Within this belt, the sist of relatively unmetamorphosed sedi- strata thicken from 0 ft on the east to 15,000 to mentary and volcanic rocks belonging to two 25,000 ft in areas 100 to 300 mi to the west. major sequences, a lower sequence consisting of The basal unit of this sequence is a diamictite the Belt Supergroup and equivalent rocks (850 (conglomeratic mudstone) which is widely to 1,250 m.y.), and an upper sequence, con- distributed but discontinuous and is generally sisting of the Windermere Group and equiv- considered to be of glacial origin. Overlying alent rocks (<850 m.y.). These two great sedimentary rocks consist predominantly of groups of supracrustal rocks commonly have siltstone, shale, argillite, quartzite, and con- been considered to be closely related in origin glomerate. Tholeiitic basalt forms thick and and have been grouped together as a major widespread units near the base of the sequence tectonic unit (Bayley and Muehlberger, 1968). but is sparse higher in the section. The purpose of this article is to suggest that a The distribution pattern and lithologic change occurred in the tectonic framework of characteristics of the upper Precambrian and western North America after the deposition of Lower Cambrian sequence fit the recently the Belt Supergroup, and that this change developed concept that thick sedimentary marked the beginning of the Cordilleran sequences accumulate along stable continental geosyncline. In terms of plate tectonics theory, margins subsequent to a time of continental this change marks the time of a continental separation. The depositional pattern of the separation. sequence is unlike that of underlying rocks, a The plan of this paper is to describe the relation consistent with the idea of a con- stratigraphy of the upper Precambrian (Win- tinental separation cutting across the grain of dermere Group and equivalent rocks) along previous structures. The pattern is, on the with the overlying lithologically similar Lower other hand, similar to that of overlying lower Cambrian strata and to compare this sequence and middle Paleozoic rocks, suggesting that the of rocks with underlying and overlying rocks. diamictite and post-diamictite rocks were the The Precambrian Windermere and the Lower initial deposits in the Cordilleran geosyncline. Cambrian rocks are considered together be- Thick units of volcanic rock near the bottom cause they are closely related and cannot be of the sedimentary sequence indicate volcanic consistently separated in western North activity related to the thinning and rifting of America. the crust during the continental separation. UPPER PRECAMBRIAN AND LOWER INTRODUCTION CAMBRIAN DETRITAL ROCKS The oldest rocks exposed in western North Upper Precambrian and Lower Cambrian America are metamorphic and plutonic rocks rocks (Fig. 1) consisting of the Precambrian Geological Society of America Bulletin, v. 83, p. 1345-1360, 6 figs., May 1972 1345 Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/83/5/1345/3417741/i0016-7606-83-5-1345.pdf by University of Nevada Reno user on 29 April 2020 1346 J. H. STEWART Nahanni River Metaline area. Northwest District, Death Valley area, Territory Nelson area, Washington Pocatello, Idaho California (Gabrielse and British Columbia (Park and (Crittenden and (Hewett, 1956, others, 1965; (Little, 1960) Cannon, 1943) others, 1971) Stewart, 1970) Ziegler, 1967, FEET p. 59) EXPLANATION 30,000-i 20.000- Formation' Leola Volcanics rjnjtign, = Supergroup = Congfoiyejajf * Rocks incuded Belt Supergroup Figure 1. Precambrian and Lower Cambrian rocks in selected areas in western North America. Winderrnere Group and equivalent rocks, and Figure 1 have been indicated by Gabrielse of overlying Lower Cambrian rocks, are com- (1967) in Canada and by Crittenden and others posed predominantly of siltstone, argillite, (1971) in parts of the United States. These shale, and of fine- to medium-grained, locally correlations throughout western North Amer- conglomeratic quartzite (Fig. 1). Siltstone, ica have been described by Crittenden and argillite, and shale are generally predominant in others (1972). The similar stratigraphy and the lower half of the sequence and quartzite in probable equivalence of the diamictite and the upper half. Limestone and dolomite form associated volcanic rocks in British Columbia, conspicuous units in some areas. The basal unit in northern Washington, and near Pocatello, of these rocks is a widely distributed, but dis- Idaho, have been suggested by Crittenden and continuous, diamictite1 (also called conglom- others (1971) and in part by Yates (1968). The eratic mudstone, conglomeratic subgraywacke, correlation of the diamictite from California to tillite, or tilloid). Volcanic rocks form thick British Columbia was also shown in an illustra- units within or directly above the diamictite tion by Stewart (1970, Fig. 36), in which the but are sparse higher in the upper Precambrian top of the diamictite was used as a datum for and Lower Cambrian sequence. an isopach map of upper Precambrian and The general correlations of upper Pre- Lower Cambrian strata in the western United cambrian and Lower Cambrian rocks shown in States and parts of British Columbia. The 1 Diamictite: A nonsorted sedimentary rock consisting possible equivalence of the diamictite has also of sand and/or larger particles in a muddy matrix been discussed by Cloud (1971). (Crittenden and others, 1971, modified from Flint and The key unit in most of these correlations is others, 1960). the diamictite, which is commonly considered Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/83/5/1345/3417741/i0016-7606-83-5-1345.pdf by University of Nevada Reno user on 29 April 2020 EVIDENCE OF A LATE PRECAMBRIAN CONTINENTAL SEPARATION 1347 to have a glacial origin (Hazzard, 1937; John- northern Canada to northern Mexico, a son, 1957; Blackwelder, 1910; Hintze, 1913; distance of 2,500 mi. Within this belt, the Ludlum, 1942; Aalto, 1971; Little, 1960; strata thicken from 0 ft in eastern areas to Ziegler, 1959). The unit is lithologically unique 15,000 to 25,000 ft in areas 100 to 300 mi to the in the Precambrian and Paleozoic sequence in west. The western thickening of these rocks western North America. It consists of rounded seems well established in California and south- or subangular pebbles to boulders (some 5 to 8 ern Nevada (Stewart, 1970), western Utah ft across) of diverse rock types in a sandy or (Crittenden and others, 1971), and southern argillaceous matrix. In places, some of the British Columbia (Okulitch, 1956, Fig. 4). diamictite or associated layers are graded Farther north in British Columbia and in the (Condie, 1966, 1969; Troxel, 1967) and can be Yukon Territory, thickness trends are less well considered to be turbidites; elsewhere, isolated known and changes in the generalized isopachs coarse fragments occur within thick, massive, given in Figure 2 will doubtless be made as argillaceous layers. Striated clasts of possible more information becomes available. The glacial origin have been noted in Utah (Crit- isopach map is not shown on a palinspastic base tenden and others, 1971). Troxel (1967), and thus does not represent the exact original Crittenden and others (1971), and Aalto (1971) thickness distribution. Thrust faulting (Crit- suggest that the diamictite unit may have had a tenden and others, 1971), strike-slip faulting complex origin; some parts may have been and associated drag features (Albers, 1967; deposited directly from glaciers along the Stewart, 1967; Stewart and others, 1968), and margins of a marine basin, other parts may have large tectonic discontinuities (Yates, 1968) been dropped from ice rafts, and still other have affected various parts of the outcrop belt. parts may be glacial material redeposited by In spite of these complications, the gross thick- turbidity currents. ness trends shown on the isopach map seem The diamictite units are scattered through- well established. out western North America (Fig. 2 and Table The upper Precambrian and Lower Cam- 1). They may be virtually synchronous units brian strata are considered to be predominantly if they represent a time of widespread glacia- marine deposits (Okulitch, 1956, p. 704-706; tion. Even if they are not glacial in origin, they Stewart, 1970; Crittenden and others, 1971). may be correlative and perhaps even syn- The primary source area lay to the east as chronous, judging from their uniqueness in the indicated by studies of current directions and stratigraphic sequence. facies changes (Ressor, 1957, p. 160-162; The upper Precambrian and Lower Cam- Mountjoy and Aitken, 1963; Seeland, 1968; brian rocks above the basal diamictite are Stewart, 1967 and 1970), although a western divided into many local or semiregional map source has been suggested in parts of Canada units. Some units can be traced for several (Gabrielse, 1967, p. 275). The strata were hundred miles, but none is widespread enough deposited predominantly in shallow water to be useful in establishing correlations through- (Stewart, 1970). Tidal currents probably out the entire 2,500-mi length of the upper spread the sands that form the widespread Precambrian and Lower Cambrian belt. Un- quartzite units in the upper part of the conformities occur at various horizons within sequence (Merifield and Lamar, 1968; Seeland, the upper Precambrian and Lower Cambrian 1968; Stewart, 1970).

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