Sedimentology and sequence stratigraphy of Neoproterozoic and Cambrian units across a craton-margin hinge zone, southeastern California, and implications for the early evolution of the Cordilleran margin

Sedimentary Geology 141±142 (2001) 501±522

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Sedimentology and sequence stratigraphy of Neoproterozoic and Cambrian units across a craton-margin hinge zone, southeastern California, and implications for the early evolution of the Cordilleran margin Christopher M. Fedo a,*, John D. Cooper b a

Department of Earth and Environmental Sciences, George Washington University, Bell Hall, Washington, DC 20052, USA b Department of Geological Sciences, California State University, Fullerton, CA 92834, USA Accepted 19 January 2001

Abstract Neoproterozoic±basal Cambrian strata exposed in eastern California represent the deposits of a craton-margin hinge zone that formed in response to the fragmentation of the Neoproterozoic supercontinent, Rodinia. One unresolved question regarding the late Neoproterozoic stratigraphy in the southwestern United States has been the interpretation of paleotectonic af®nity for rocks deposited above the glaciogenic Kingston Peak Formation. Of central concern has been attempts to identify the position in this stratigraphy, where rift-related sediments give way to passive-margin deposits. Subsidence analyses suggest that rifting occurred near the start of the Cambrian, perhaps coincident with the feldspathic middle member Wood Canyon Formation. It rests on a disconformity that represents the base of the Sauk Supersequence and would likely represent the break-up unconformity in subsidence analysis models. Middle member Wood Canyon and overlying Lower±Middle Cambrian strata extend far into the craton. Detailed sedimentary facies and sequence stratigraphic analysis of post-Kingston Peak, pre-middle Wood Canyon rocks indicate that depositional facies in this succession (distal alluvial through shallow subtidal) are similar to units deposited on the stable craton. Further, these pre-Sauk units, which can be divided into multiple, principally eustatically driven depositional sequences are correlated across long distances and even lap onto the edge of the stable craton. We suggest the Kingston Peak Formation is the principal rift-generated deposit and the overlying thick, predominantly siliciclastic, section represents the basal deposits of the Cordilleran passive margin. Kingston Peak strata consist of thick, coarse diamictites interbedded with turbidites; such facies are unique in the overall stratigraphy but are similar to other known rift deposits. Although the age of the Kingston Peak Formation is not well constrained and may be as young as ,590 Ma, it may correlate with the Rapitan±Sturtian glaciation (,720 Ma), which would place middle Wood Canyon strata some 150 Ma younger than the start of rifting. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Neoproterozoic; Rodinia; Sauk Sequence; Rift; Passive margin

1. Introduction * Corresponding author. Address: Department of Geology, George Washington University, Bell Hall, Washington, DC 20052, USA. Fax: 11-202-994-0450. E-mail addresses: [email protected] (C.M. Fedo), [email protected] (J.D. Cooper).

The Neoproterozoic through early Paleozoic craton margin in eastern California (Fig. 1) exposes a transitional paleotectonic setting between the slowly subsiding Laurentian craton and the more rapidly

0037-0738/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0037-073 8(01)00088-4

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Fig. 1. Location of stratigraphic control sections that constrain hinge-zone con®guration. Dashed line shows stratigraphic panel orientation for subsequent ®gures. SAFZ, San Andreas fault zone.

subsiding miogeocline. This transitional region developed as a result of the stretching of the lithosphere associated with the rifting of Rodinia during the late Neoproterozoic. Preserved within the succession is the rift-to-drift transition, although exactly where in the

stratigraphy, remains controversial (e.g. Prave, 1999). Additionally, the location of the craton margin has been debated (e.g. Cooper and Fedo, 1993a,b), largely owing to complications resulting from Mesozoic and Cenozoic structural dismemberment. It is best recognized

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on the presence or absence and thickness variations of speci®c Neoproterozoic±Lower Cambrian deposits, including, in ascending order, Johnnie Formation, Stirling Quartzite, Wood Canyon Formation, and Zabriskie Quartzite. Although the edge of the craton clearly has in¯uenced the stratigraphic organization of these units, it represents a critical bridge that links classical cratonic sequences (Sloss, 1963) with thick deposits of the continental margin basin (miogeocline). Cratonic and miogeoclinal strata are typically studied independently because hinge-zone regions are areally restricted and prone to subsequent tectonic reworking (e.g. Wehr and Glover, 1985). This paper examines the depositional systems and sequence stratigraphic framework of the Neoproterozoic±basal Cambrian succession that represents the cratonal to miogeoclinal transition in eastern California. In this light, we will also address the potential impact of paleotectonics on the succession, especially as it pertains to the complex fragmentation of Rodinia during the Neoproterozoic (e.g. Young, 1995; Prave, 1999). 2. Craton-margin hinge zone de®ned Fundamental to understanding the architecture and evolution of the nascent continental margin is delineation of the Cordilleran cratonal±miogeoclinal hinge zone (CMH). The CMH is a paleotectonic element that separates a crustal domain of relatively greater subsidence (continental margin basin or miogeocline sensu lato) from one of signi®cantly less subsidence (Laurentian craton) along the early edge of the continent. Hinge zones represent regions where there is a marked increase in depth to crystalline basement rocks concomitant with an expanded sedimentary cover section, and they typically denote the landward termination of highly attenuated continental crust during rifting (Wehr and Glover, 1985). As such, they represent the manifestation of the complexly faulted underlying rift topography and structure. Picha and Gibson (1985) recognized that the Cordilleran hinge is best located by identifying the faults, or their reactivated expressions, that accommodated the differential subsidence. In southeastern California, however, such structures have been strongly overprinted by Mesozoic thrusting and

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Cenozoic normal and strike-slip faulting. As a result, few well-documented Neoproterozoic and Cambrian faults are recognized in the region and offer little help for reconstructing the original hinge con®guration. In the absence of such relic rift-related structures, sedimentology and stratigraphy is the most sensitive indicator of continental margin tectonic activity and paleotectonic setting. Following conventional de®nitions, the CMH is interpreted to represent the original rifted edge of the continent. Therefore, we argue that the most sensitive proxy for the CMH is the initial, basal Neoproterozoic±Lower Cambrian succession. Furthermore, it was during the early rift-to-drift history of the nascent continental margin that the hinge exerted its most profound in¯uence as an active tectonic and physiographic feature. During the later Paleozoic, this Neoproterozoic±Early Cambrian hinge was buried by many kilometers of sediments and was less pronounced. In southeastern California, the presence of stratigraphic units and their thickness identi®es the paleotectonic setting. Cratonal sections begin with middle member Wood Canyon Formation (or Tapeats Sandstone lithologic equivalent) resting unconformably on Proterozoic crystalline rocks (or tilted strata in the case of the Grand Canyon). Miogeoclinal (continental margin basin) sections, in contrast, are characterized by a signi®cantly thicker, more fully developed Wood Canyon Formation that includes siltstone-dominated lower and upper members, and a kilometers-thick, Neoproterozoic, sub-Wood Canyon section (^Pahrump Group; Noonday Dolomite, Johnnie Formation and Stirling Quartzite). Transitional between these two settings is the unique craton-margin hinge zone, where stratigraphic sections begin with a comparatively thin, incompletely developed sub-Wood Canyon section that consists of Johnnie Formation (resting on gneiss) overlain by Stirling Quartzite. This sub-Wood Canyon section contains several signi®cant disconformities/sequence boundaries, which are sensitive indicators of relative base-level changes in this transitional setting. Fig. 1 shows the stratigraphic sections that constrain the hinge reconstruction. This palinspastically unrestored arrangement de®nes a broad, northeast-trending CMH in the eastern Mojave Desert that converges northward to a more northerly trend ,30 km east of Baker. North of Interstate Highway

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Fig. 2. Panel showing regional sequence boundary correlations and unit representation and thickness for miogeoclinal (Nopah Range), craton margin (Providence Mountains) and cratonal (Marble Mountains) sections. Note the thinning of units and truncation of sequence boundaries at the craton margin. Stratigraphic units: ND, Noonday Dolomite; lmJF, lower member Johnnie Formation; umJF, upper member Johnnie Formation; lmSQ, lower member Stirling Quartzite; mmSQ, middle member Stirling Quartzite; umSQ, upper member Stirling Quartzite; lmWCF, lower member Wood Canyon Formation; mmWCF, middle member Wood Canyon Formation.

15 (I-15; Fig. 1), the hinge zone is condensed to a more linear feature, where it coincides with Mesozoic thrust faults in this region. Depositional dip oriented stratigraphic transects in the Death Valley region (e.g. from the Winters Pass Hills (locality 6, Fig. 1) to the Mesquite Pass Hills (locality 7, Fig. 1)) abruptly go from inner miogeoclinal to cratonal sections. Hence the progressive loss of lower member Wood Canyon Formation and upper and middle members Stirling Quartzite in a cratonward direction and the true truncational character of disconformities can be observed only in the transect through the eastern Mojave Desert (Fig. 2). Although there have been

post-Paleozoic tectonic modi®cations of this hinge con®guration (Picha and Gibson, 1985; Burch®el and Davis, 1988), we believe it, nonetheless, re¯ects along-trend differences in the tectonic and physiographic character of the original continental margin: an apparently abrupt edge or narrow zone north of I-15 and a more transitional, downstepped margin south of I-15. It is possible that a craton-margin section has been obscured by overthrusting; however, Burch®el and Davis (1988) suggest that contractile shortening between localities 6 and 7 (Fig. 1) might not exceed 5 km, thus leaving little room for a robust craton-margin stratigraphic development. The

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original more abrupt edge of the continent may have served as a guide for later thrusting (Burch®el and Davis, 1988). Our delineated CMH in southeastern California (Cooper and Fedo, 1993a,b) differs from that proposed by Martin and Walker (1992, 1993), who invoked the presence (miogeoclinal) or absence (cratonal) of Ordovician±Silurian section and the presence of Proterozoic basement outcrop to de®ne a CMH well to the west of ours (see their Fig. 2, Martin and Walker, 1992). We caution that the presence or absence of an Ordovician±Silurian section is not an accurate indicator of the continental margin basin±cratonal transition, or hinge zone because: (1) the Silurian section (Hidden Valley Dolomite) pinches out within the Nopah Range (Fig. 1; Burch®el et al., 1983), which occupies a miogeoclinal position (southern Nopah Range also contains exposures of Proterozoic basement) and (2) presence of 75 1 m of Lower Ordovician section at Mohawk Hill, southern Clark Mountains (Cooper and Edwards, 1991; Cooper and Keller, 1995), which is in a parautochthonous tectonic block in a cratonal setting. Furthermore, absence of Ordovician±Silurian section is meaningful only where Devonian (or younger) strata rest unconformably on Cambrian rocks, a relationship that is not unequivocally documented in the highly deformed rocks of the western Mojave Desert (Martin and Walker, 1991). Well before the Ordovician, the developing continental margin sedimentary prism had evolved from a siliciclastic sedimentary wedge to a mature, thermally subsiding passive margin (Levy and Christie-Blick, 1991) dominated by carbonate deposition.

3. Depositional environments 3.1. Johnnie Formation In the craton-margin section exposed in the Providence Mountains, the basal stratigraphic unit is the Neoproterozoic Johnnie Formation, which is 35± 40 m thick and is developed as three depositional packages (Fig. 3A; Traub and Cooper, 1993). The lower package, ,5 m thick, consists of a ®ning upward succession of olive brown silty to pebbly, ®ne to coarse quartzite and greenish gray ripple-

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laminated siltstone. This thin facies succession is provisionally interpreted as distal ¯uvial or shallow marine. The middle package, which is the thickest (16±18 m), is recrystallized carbonate arranged in bundles of silici®ed microsparite, microbial laminite, cross-bedded calcarenite and intraclast breccia. The breccias (Fig. 4) show considerable thickness variation along strike and contain common imbricated intraclasts up to 10 cm long. This carbonate package is interpreted as peritidal and storm deposits, with some of the coarser breccias possibly representing hurricane ridges, similar to those described by Koerschner and Read (1989) in Cambrian carbonates in Virginia. Some imbrication fabrics are similar to those in the Lower Cambrian Sellick Hill Formation, South Australia, ascribed by Mount and Kidder (1993) to a combined ¯ow mechanism. The upper package, 13±15 m thick, consists of banded dark green±gray siltstone and very ®ne-grained quartzite, with some hummocky cross-strati®cation (HCS) and interbedded carbonate. These ®ner grained rocks are interpreted as subtidal shelf deposits above storm wave base. The sections in the Providence and Kelso Mountains are very similar, with the latter section being a bit thicker and overall ®ner grained (Fig. 3B). The intraclast breccias are not present there and the basal deposits consist of laminated ®ne-grained quartzite. Conspicuously absent in the craton-margin sections are the `Johnnie oolite' marker bed and overlying `Rainstorm Member' of the Johnnie Formation (Stewart, 1970), which are widespread in miogeoclinal rocks of the Death Valley region (Summa, 1993); the Johnnie oolite is present in the Old Dad Mountains, 25 km northwest, where it rests disconformably upon karsti®ed microbial doloboundstone. The Old Dad Mountains section provides an anchor for correlation between the craton-margin sections and those of the Death Valley region. The basal siliciclastic and middle carbonate packages correlate with strata of the lower member Johnnie Formation in the Death Valley region (Stewart, 1970), while the upper siltstone package correlates with the lower part of the upper member Johnnie Formation. Loss of the Johnnie oolite and Rainstorm Member in the hinge-zone sections results from erosional cutout beneath the base of the overlying lower member Stirling Quartzite (Fig. 2).

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Fig. 3. (A) Measured log, interpretation and sequence stratigraphy of the succession developed in the Providence Mountains. (B) Measured log, interpretation and sequence stratigraphy of the succession developed in the Kelso Mountains. Stratigraphic units: lJ, lower Johnnie Formation; uJ, upper Johnnie Formation; lSQ, lower Stirling Quartzite; mSQ, middle Stirling Quartzite; uSQ, upper Stirling Quartzite; lWCF, lower Wood Canyon Formation; mWCF, middle Wood Canyon Formation; uWCF, upper Wood Canyon Formation; ZQ, Zabriskie Quartzite. Sequence stratigraphy: TST, transgressive systems tract; HST, highstand systems tract; mfs, max ¯ooding surface; 1±4, disconformities discussed in text.

3.2. Stirling Quartzite Sharply overlying the argillites of the upper Johnnie Formation is the Neoproterozoic Stirling Quartzite (Fig. 3A and B). In its type area in the Death Valley region, the Stirling Quartzite is characterized by a lower (A and B members of Stewart (1970)) and upper (E member of Stewart (1970)) member both composed of cross-bedded, medium to coarse pebbly quartzite of dominantly braided ¯uvial origin (Fedo and Cooper, 1999), separated by a middle member (C and D members of Stewart (1970)) of shallow-marine green and red mudstone, ®ne quartzite and carbonate (Stewart, 1970). In the Death Valley area, the three members have gradational contacts, although there is

a disconformity developed in the upper Stirling Quartzite that emplaces very coarse alluvial deposits over more distal deposits (Fig. 2). In the Providence and Kelso Mountains, the lower member Stirling Quartzite is ,55 m thick and is developed as two ®ning upward facies successions, the ®rst of which begins with thick-bedded ®ne to medium quartz and feldspar pebbly, medium to coarse quartzite (Fig. 5A) overlain by cross-strati®ed and abundant horizontally strati®ed, ®ne to medium quartzite. The upper facies succession in the lower member consists of trough cross-strati®ed, coarse to medium quartzite, with dispersed paleocurrent cross-bed dips; a number of beds near the top of this unit display mm- to cm-thick very ®ne quartz granule

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Fig. 4. (A) Photograph of rip-up clast breccias in the Johnnie Formation, Providence Mountains. Scale in centimeters. (B) Photograph of Johnnie oolite marker unit from the Old Dad Mountains. Scale in centimeters.

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Fig. 5. (A) Photograph of pebble conglomerate from the base of the lower member Stirling Quartzite, Kelso Mountains. Rock hammer for scale. (B) Photograph of mudcracks as sole marks in the middle member Stirling Quartzite, Salt Spring Hills. Scale about 1 m across long edge of photograph. (C) Photograph of cross-bedded arenites from the upper member Stirling Quartzite, Nopah Range. Scale in centimeters. (D) Photograph of conglomeratic upper part of the upper member Stirling Quartzite from the Mesquite Pass Hills. Pen for scale.

conglomerate on bedding top surfaces. These strata are interpreted primarily as distal braid plain in origin, with the abundance of horizontally strati®ed sand perhaps related to aeolian sand sheet deposition (cf. Clemmenson and Dam, 1993) or interdune and de¯ationary deposits (cf. Simpson and Eriksson, 1993). Lithologies and environments are similar to

Neoproterozoic sandstones described by ChristieBlick and Levy (1989) from Utah and Idaho. Unlike the Death Valley region where there is a gradation in environments between the lower and middle members, the ®ne-grained middle member abruptly overlies lower member quartzites in the craton margin. Here, middle Stirling Quartzite strata

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Fig. 5. (continued)

consist of dark green and red platy siltstone with desiccation cracks, rip-up chips, and slightly asymmetrical ripple marks with bimodal and poly-modal paleocurrent azimuths (Crangle and Fedo, 1999; Fig. 5B) in the Kelso Mountains, and quartzitic siltstone with ¯aser, lenticular and wavy lamination in the Providence Mountains. These strata are interpreted as predominantly mixed tidal ¯at in origin (Crangle and Fedo, 1999), and exhibit many of the classic features ascribed to tidal ¯ats by Dalrymple (1992).

The thickness of the middle member Stirling Quartzite changes dramatically from 45 1 m in the Kelso Mountains to ,30 m in the Providence Mountains, a change of .30% (Fig. 4). This is signi®cantly greater than the slight changes in thickness of the Johnnie Formation and the lower member Stirling Quartzite from the Kelso to the Providence Mountains. From a facies perspective, the upper member Stirling Quartzite resembles the lower member in that it consists primarily of plane- and cross-strati®ed

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Fig. 6. (A) White arrow points at Taphrhelminthopsis trail developed in the lower member Wood Canyon Formation in the Kelso Mountains. Scale in centimeters. (B) Trough cross-strati®ed feldspathic arenite from the middle member Wood Canyon Formation, Marble Mountains. Scale in centimeters. (C) Slabbed sample of the quartz- and chert-pebble conglomerate from the Kelso Mountains. Scale in centimeters. (D) Skolithos piperock at top of middle member Wood Canyon Formation, Kelso Mountains. Pen for scale. (E) Photograph of HCS from the upper member Wood Canyon Formation, Providence Mountains. Lens cap for scale.

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Fig. 6. (continued)

slightly feldspathic arenites with very little intercalated mud rocks (Fig. 5C). Paleocurrents from trough cross-strati®cation in the Death Valley area are oriented dominantly west to southwest (also see Stewart, 1970), which is similar to the overall trend for transport direction of other Neoproterozoic and Cambrian arenites in the region (Fedo and Prave, 1991). Upper member arenites are organized into sharply bounded beds ,10±50 cm thick. The arenites are poorly sorted, despite being well rounded. The upper ,80 m of the upper member on the Old Dad

Mountains (Fig. 2) is noticeably coarser (granule to cobble conglomerates interbedded with coarse arentites) than the lower part (Fig. 5D), although other characteristics are quite similar. In the cratonmargin sections, this coarse interval is manifested as a 1±1.5 m thick ®ne quartz pebble conglomerate that is overlain by ,1 m of quartz sandstone. In the southern Kelso and Providence Mountains, this thin conglomerate represents the entire upper member. The quartz pebbles are tightly packed, rounded to well rounded, and consist mainly of vein quartz. The physical

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Fig. 6. (continued)

sedimentology is consistent with a braided alluvial origin for the upper member; the very coarse upper part is interpreted to be more proximal relative to the lower part of the member. 3.3. Lower member Wood Canyon Formation In miogeoclinal sections, there is a sharp boundary where shelf-deposited shale and siltstone of the lower member Wood Canyon Formation overlies pebbly beds of the upper Stirling Quartzite (Diehl, 1979). The lower member is .100 m thick and contains several sandy dolomite beds. The unit dramatically thins and loses carbonate interbeds as it crosses the craton-margin hinge zone (Fig. 2). In craton-margin sections, the lower member is poorly developed, only a few meters thick, and much coarser than in more basinward localities (Fig. 3). It begins with a thin siltstone interval containing burrows assigned to the basal Cambrian ichnotaxon Treptichnus pedum and Taphrhelminthopsis (Hagadorn et al., 2000; Fig. 6A), which represents the lowest occurrence of metazoans in the craton-margin section. Very poorly sorted, pebbly, feldspathic arenite beds are intercalated with the trace fossil-bearing siltstones. We interpret the lower member in the craton margin to represent the distal edge of a braid-delta environment where tongues of marine-deposited muds are intimately

associated with ¯uvial delivery of coarse sand (e.g. McPherson et al., 1987, see their Fig. 3). In this light, lower member strata in the craton margin represent a more proximal setting relative to miogeoclinal sections, which contain, in addition to thin-bedded siltstone and mud shale, abundant hummocky crossstrati®ed ®ne sandstone and m-thick carbonate interbeds of shelf origin. 3.4. Middle member Wood Canyon Formation Sharply overlying the lower member Wood Canyon Formation in craton-margin sections is the middle member Wood Canyon Formation, which consists of an ,140 m thinning and ®ning upward succession of extensively trough cross-bedded, pebbly feldspathic quartzite, with interbedded siltstone (Fig. 3). In the Death Valley region, the Wood Canyon Formation contains lower and upper members (the lower contains the Precambrian±Cambrian boundary, Corsetti and Hagadorn, 2000) of burrowed, mixed siliciclastic±carbonate rocks interpreted as shallow marine. These are separated by a thick middle member of pebbly medium to coarse, trough cross-bedded feldspathic quartzite (Fig. 6B) of dominantly braided alluvial origin (Diehl, 1979; Prave et al., 1991). In the Providence and Kelso Mountains, a distinctive 1±2 m thick quartz- and chert-pebble conglomerate

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low in the middle member (Figs. 3 and 6C) separates coarse, thick-bedded, pebbly trough cross-strati®ed deposits below from thinner bedded, ®ner grained trough and horizontally laminated deposits above. This conglomerate maintains a remarkably constant thickness and fabric from the Marble Mountains to the Providence and Kelso Mountains (Flaugher, 1993) and serves as a key bed. Above this conglomerate, thin, dark siltstone/mudstone-based cycles are capped by texturally mature thicker bedded subarkoses or submature trough cross-bedded arkoses. We consider these as braid-delta deposits (sensu McPherson et al., 1987), whereby the basal siltstones represent paralic muds periodically overrun by ¯uvially introduced sands. These lower strata of the middle member represent more proximal parts of the braid-delta terminus expressed by the lower member. We interpret the more quartz-rich, texturally mature beds to re¯ect periodic beach-face reworking of the sand by wave activity. The trough cross-bedded, coarser arkoses that compose most of the middle member from craton to miogeocline (Fig. 6A) are characteristic of alluvial braid-plain deposits (e.g. Diehl, 1979; Fedo and Cooper, 1990; Fedo and Prave, 1991). The top of the middle member is marked by a ¯ooding surface characterized by a thin, but densely burrowed, Skolithos piperock interval (Figs. 3 and 6D), which is interpreted to re¯ect slow sedimentation rates in shallow water (Droser and Bottjer, 1989). Thus, the middle member section is interpreted to record transitions from braid delta to braided ¯uvial, and then to shallow marine environments, and in total, represents the distal part of a vast Early Cambrian braid plain described by Fedo and Cooper (1990, 1991) and Fedo and Prave (1991). 3.5. Upper member Wood Canyon Formation and Zabriskie Quartzite In the Providence Mountains, the abruptly overlying ®ner grained upper member Wood Canyon Formation consists of ,45 m of olive brown mudstone, quartzitic siltstone and interbedded ®ne quartzite with trace fossils and HCS (Fig. 6E), overlain by rhythmically interbedded siltstone and very ®ne sandstone with runzelmarken and interference ripple marks (Fig. 3; Cederstrand, 1995). These strata suggest a storm-in¯uenced marine shelf, shallowing

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up to a tidal ¯at paleoenvironment, similar to deposits described by Walker and Plint (1992). The upper member is considerably thicker and more mud rich in the Kelso Mountains where it contains several meter-thick carbonate interbeds in its upper part. The thick-bedded, burrowed and cross-strati®ed, ®ne to medium quartzite of the gradationally overlying Zabriskie Quartzite is interpreted as prograding marine shoreface (Prave et al., 1991; Prave, 1992). The contact with the shallow-marine siltstones and shales of the overlying Lower to Middle Cambrian Carrara Formation is sharp.

4. Sequence stratigraphic framework 4.1. Introduction In the craton-margin sections of the eastern Mojave Desert, the Neoproterozoic±basal Cambrian succession can be divided into at least ®ve major depositional sequences, each bounded by signi®cant disconformities (Fig. 2; Cooper et al., 1994). A sequence stratigraphic approach has revealed some interesting stratigraphic architecture that has previously escaped attention and appreciation using more conventional lithostratigraphic correlation. Of signi®cance are depositional shingles, de®ned as repetitions of facies or facies assemblages on either side of a sequence boundary (Fedo and Cooper, 1994). The shingled repetitions re¯ect a resonating effect of sediment response to an ensuing transgression, involving the interplay between relative rate of sealevel rise, local to regional tectonics, provenance character and sediment ¯ux. Three styles of shingling are recognized: (1) stacked connected, whereby similar facies are in direct contact in the same section, but separated by a sequence boundary; (2) stacked disconnected, whereby similar facies are repeated in the same section, but separated by other facies as well as a sequence boundary; (3) separated disconnected, whereby facies are repeated in different sections, do not overlap, and are separated by a sequence boundary. This exempli®es the essence of sequence stratigraphy: detailing and visualizing the architecture of sedimentary successions in three dimensions at a great range of scales (ChristieBlick, 1994).

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Fig. 7. Detailed sequence stratigraphic panel of the craton-margin region, hung on disconformity 4, from Old Dad Mountains southward to Marble Mountains. Disconformity 4 represents the base of the middle member Wood Canyon Formation and the base of the Sauk Supersequence. Shaded area in the upper Stirling Quartzite highlights the motif of erosional truncation beneath sequence boundaries typical of the hinge-zone region. MM, Marble Mountains; PM, Providence Mountains; KMs, Kelso Mountains (south); KMn, Kelso Mountains (north); ODM, Old Dad Mountains. TST, transgressive systems tract; HST, highstand systems tract; mfs, maximum ¯ooding surface.

We recognize ®ve depositional sequences (A±E) in the Neoproterozoic through Cambrian succession that mantles basement rocks and straddles the craton± miogeoclinal hinge zone (Fig. 7). This number contrasts sharply with miogeoclinal sections in which there are signi®cantly more; for example, in the Nopah Range, Summa (1993) divided the Johnnie Formation alone into six sequences. The loss of section, and sequences, in a cratonward direction results from two processes, namely (1) on-lap coupled with less accommodation space linked to variable subsidence rates across differentially stretched crust and (2) top downward truncation by erosive facies (braid-plain deposits) tracking falling base level resulting in the merging of disconformities. These

factors are the hallmark of the hinge zone, which, in turn, in¯uenced sequence architecture. 4.2. Sequence architecture In the craton margin, the Johnnie Formation is interpreted as two depositional sequences (Sequences A and B, Fig. 7) with the rocks in the Old Dad Mountains (the most proximal miogeoclinal section) providing an anchor point for our correlations. The lower and middle units comprise the transgressive systems tract of the lower sequence, whose lower boundary is a nonconformity and whose upper boundary is a regional transgressive surface and sequence boundary (disconformity 1, Fig. 7). The upper unit represents a

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transgressive systems tract of the overlying sequence, a signature of widespread, rapid drowning. The most cratonward extent of the Johnnie oolite occurs in the Old Dad Mountains, where it disconformably overlies doloboundstone; this sub-Johnnie oolite disconformity merges cratonward with the basal lower member Stirling Quartzite disconformity (disconformity 2, Figs. 2, 7). Consequently, rocks comprising the Rainstorm Member of the Johnnie Formation (Stewart, 1970) are absent from the craton margin. Rather, the upper Johnnie Formation siltstones in the craton margin represent siltstones below the oolite in miogeoclinal sections. The contact with the overlying Stirling Quartzite (disconformity 2, Fig. 7) is irregular and erosive and emplaces alluvial braidplain sediments on top of marine shelf mud rocks, which expresses a signi®cant base-level drop with concomitant basinward shift of facies. This disconformity can be correlated across the Great Basin as the base of the Prospect Mountain Quartzite in Nevada and as the base of the Mutual Formation in Idaho and Utah (Christie-Blick and Levy, 1989). In this scenario, the incised valley ®lls that occur below the lower Stirling Quartzite (Summa, 1993; Charlton et al., 1997) would form a laterally discontinuous sequence that is genetically part of the Johnnie Formation. An interesting problem with determining the sequence architecture across the craton margin has been trying recognize what happens to lithologic units as they thin by on-lap, and get beveled from above by erosion. Facies of the Stirling Quartzite exemplify this dif®culty. Everywhere in the study area, lower member strata possess about the same physical sedimentologic features, which best ®t a braid-plain interpretation. In miogeoclinal sections, Stewart (1970) divided the Stirling Quartzite into ®ve members (A±E), that we combine into lower (A and B members), middle (C and D members) and upper (E member) members. Stewart's (1970) B member consists of intercalated mudstone and crossbedded sandstone that we provisionally interpret as a braid-delta deposit; it gradationally overlies alluvial arenites and is in turn gradationally overlain by tidal ¯at mudstones. B member rocks thin and disappear in a cratonward direction, such that `C member' strata rest directly on `A member' rocks in the hinge-zone region. This thinning is almost certainly associated

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with on-lap associated with transgressive and highstand marine rocks of the middle member as well as decreased accommodation space related to differential subsidence. In the hinge-zone region, the quartz pebble conglomerate that tops the lower member Stirling Quartzite is interpreted as a transgressive lag on a ravinement surface cut by retreating, wave reworked, deltaic deposits. The `D member' is also absent in the craton margin, but this probably results from erosive down cutting associated with emplacement of overlying braid-plain deposits of the `E member', which we interpret as the products of a highstand and regressive systems tracts (Fig. 2). Consequently, the Stirling Quartzite represents a nearly complete depositional sequence (Sequence C) in the miogeocline, but one that is fragmented by disconformities in the craton margin (Fig. 7). The ®ne to medium quartz pebble conglomerate and pebbly arenites that form the top of the upper member Stirling Quartzite rest on ,80 m of the lower part of the upper member Stirling Quartzite in the Old Dad Mountains and ,150 m in the Nopah Range (Figs. 1 and 2). In the northern Kelso Mountains, quartz pebble conglomerate rests on ,12 m of upper member Stirling Quartzite that is very similar to the dark-streaky cross-strati®ed arenites in the basal part of the upper member in the Old Dad Mountains. Two kilometers south, the conglomerate overlies dark gray siltstones of the middle member Stirling Quartzite, and further south in the Providence Mountains, it occurs only 30 m above the base of the middle member. Hence the conglomerate progressively truncates the upper and middle members Stirling, and its base is interpreted as a regional disconformity and type-1 sequence boundary (disconformity 3, Figs. 2 and 7). The resulting juxtaposition of lithologies in most of the study area then places alluvial deposits disconformably above alluvial deposits creating what we refer to as a stacked connected shingle. Consequently, the Stirling Quartzite represents a nearly complete depositional sequence in the miogeocline, but one that is punctuated by disconformities and periods of non-deposition in the craton margin (Fig. 7). Upper member rocks of the Stirling Quartzite that lie above disconformity 3 form the transgressive base of the following sequence (Sequence D). The pebbly arenites are abruptly overlain by a signi®cant ¯ooding surface that drowns alluvial sediments with the lower

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member Wood Canyon Formation, which represents marine shelfal and paralic marine deposits in miogeoclinal and craton-margin settings, respectively. Prave et al. (1991) interpreted the few distinct dolomites as carbonate capped parasequences, which we suggest belong to transgressive and highstand systems tracts of depositional sequence D. The succeeding middle and upper members Wood Canyon Formation, consisting of distal braided ¯uvial, braid-delta and marine-shelf facies, comprise a thick transgressive systems tract of the uppermost sequence (sequence E). This sequence rests on a regional disconformity that de®nes the base of the middle member Wood Canyon Formation (disconformity 4, Fig. 7), and the base of the Sauk Supersequence (Cooper and Fedo, 1995; Runnegar et al., 1995). The disconformity progressively truncates lower member Wood Canyon Formation in a cratonward direction (Fig. 2), such that in most miogeoclinal sections, middle member Wood Canyon ¯uvial rocks are separated from upper member Stirling Quartzite ¯uvial rocks by the marine lower member Wood Canyon Formation; this expresses a stacked disconnected shingle relationship. In the Mesquite Pass Hills (Fig. 1), the base middle member Wood Canyon Formation completely truncates lower Wood Canyon strata, so that the middle member sits unconformably on upper Stirling rocks along a contact that is very dif®cult to recognize in the ®eld, leading to a stacked connected shingle (¯uvial on ¯uvial separated by an unconformity). The unconformity clearly represents a signi®cant base-level drop with the progradation of alluvial deposits over, and completely through, a marine shelf facies. Such a relationship would be dif®cult to recognize in a single section. The middle member most likely represents accumulation of a transgressive systems tract above a regional disconformity cut during the previous lowstand. Inter¯uve paleosols (e.g. McCarthy and Plint, 1998) have not been recognized, although middle member Wood Canyon deposits rest nonconformably on a thick paleosol developed on basement rocks in cratonic settings (e.g. Marble Mountains, CA and Frenchman Mountain, NV). The Skolithos piperock developed at the top of the middle member of the Wood Canyon Formation represents the ®rst widespread marine unit and is interpreted to be a major marine ¯ooding surface

that signals a signi®cant relative rise in sea level. The upper part of the upper member Wood Canyon Formation and Zabriskie Quartzite represent the progradational highstand systems tract of this sequence. The ¯ooding surface at the top of the piperock mimics the upper Stirling Quartzite±lower Wood Canyon contact and attests to the similarity in process and product throughout much of the late Neoproterozoic±Lower Cambrian succession in the southern Great Basin. The top of the Zabriskie Quartzite is separated from the transgressive subtidal mudrocks of the overlying Carrara Formation by a regional sequence boundary (Prave, 1992). Thus, in the craton margin of the eastern Mojave Desert, the basal, Neoproterozoic±Lower Cambrian section is punctuated by at least four signi®cant, but not readily apparent (at individual locales), disconformities delineated by regional mapping and sequence stratigraphic analysis that has emphasized facies stacking patterns and lateral shifts in paleoenvironments. Furthermore, shingling of facies on either side of sequence boundaries has produced the misleading illusion of laterally continuous sheets. These regional sequence boundaries have important implications for the early paleotectonic evolution of the continental margin. Cratonward thinning of these Neoproterozoic±Cambrian sedimentary units, previously considered to re¯ect mostly stratigraphic on-lap during transgression is, in addition, the result of major erosional removal of units from the top downward (stratigraphic truncation and resulting toplap), and limited accommodation space associated with differential subsidence across the hinge zone. 5. Early evolution of the Cordilleran margin The complex fragmentation history of the late Neoproterozoic supercontinent Rodinia has commanded much attention in recent literature. Contained within the duration of the fragmentation is the postulation of one or more Snowball Earth episodes (Hoffman et al., 1998) and the rapid evolution of large metazoans (e.g. Grotzinger et al., 1995). Thermo-tectonic subsidence analyses (e.g. Bond et al., 1984, 1989; Levy and Christie-Blick, 1991) for the early Cordilleran margin suggest a signi®cant rifting event that formed the new continental margin occurred between about 600 and

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550 Ma, with the onset of thermally driven subsidence near the beginning of the Cambrian. Globally there is evidence for two Neoproterozoic rifting events linked with Rapitan±Sturtian (,720 Ma) and Varanger± Marinoan (,590 Ma) glacial deposits (e.g. Prave, 1999; Condon and Prave, 2000), with abundant stratigraphic evidence for rifting in along the Cordilleran margin at the earlier event (e.g. Young, 1995). If the subsidence analysis approach for determining the timing of rifting is correct, it would place the Neoproterozoic and basal Cambrian stratigraphy discussed here well within the rift to perhaps earliest drift phase of margin evolution. However, as is discussed the stratigraphy and sedimentology of the units is not consistent with rifting during this time frame. Hence the timing of rifting and onset of thermal subsidence still remains a problem: the elegant modeling suggests a younger rifting event, while the rocks themselves imply a much earlier event (e.g. Stewart, 1972, 1991; Ross, 1991). As we see it, the main problem seems to be the thickness, lithofacies, and remarkable along-strike uniformity of the post-Pahrump Group, upper Neoproterozoic succession. It appears to record a transitional, perhaps as much as 150 Ma duration, .1 km thick interval, between synrift deposits (Christie-Blick and Levy, 1989; Ross, 1991) composed of laterally variable debris ¯ow and glaciogenic diamictites of the upper Kingston Peak Formation (Miller, 1985; Walker et al., 1986; Prave, 1999), and unequivocal passive-margin deposits, exempli®ed by the widespread Lower Cambrian Zabriskie Quartzite (Prave and Wright, 1986). As Stewart (1991) pointed out, evidence for Early Cambrian rifting is dif®cult to reconcile with the clear evidence of widespread shelf deposition in the thick siliciclastic succession and the absence of major extensional rift structures during its deposition. Prave (1999) offered a possible resolution to the problem in an isotope study of carbonates intercalated with Kingston Peak diamictites. He postulated that the two levels of diamictite represent the Rapitan and Varanger glaciations separated by a thin interval of dominantly mudrock; each diamictite has a cap carbonate (sensu Kennedy, 1996). In this scenario, there is still as much as approximately 50 million years of time between the Kingston Peak Formation and the middle member Wood Canyon Formation, which has

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been cited as a potential rift unit based on its arkosic composition (e.g. Levy and Christie-Blick, 1991). Alternatively, if the age of rifting and continental separation is accepted to be about 720 Ma, coeval with the Rapitan glaciation in other parts of the Cordillera (Young, 1995; Dalrymple and Narbonne, 1996), then it becomes untenable to accept the continuation of rifting (e.g. Lickorish and Simony, 1995; Veevers et al., 1997) and onset of thermal subsidence as some 150 million years later. Perhaps, this transitional succession represents a composite transitional paleotectonic style related more to `jerky subsidence' (sensu Rankey et al., 1994), with periodic lithospheric ¯exuring by sediment loading on variably attenuated continental crust, and eustasy acting in concert. Such combined activity might explain the apparent evidence for rifting during Johnnie Formation deposition reported by Summa (1993). We suggest that the main sequence boundary architecture in the craton-margin hinge-zone region is driven by eustasy that is modi®ed by differential subsidence. Examination of the sequence correlations as well as facies distributions and types demonstrates that late Neoproterozoic units share a remarkable similarity with Cambrian and even later Paleozoic stratigraphy (Cooper and Keller, 1995) that is undoubtedly of passive-margin character (e.g. Sloss sequences such as the Sauk). For example, there is a consistent array of depositional facies that occur in both Sauk and sub-Sauk rocks, including alluvial braid plain, coastal and shallow marine. Striking stratigraphic similarities include: (1) the pinching of sequences by merging of sequence boundaries in the hinge-zone region, (2) abrupt drowning surfaces at the top of thick braid-plain sections (upper member Stirling Quartzite±lower member Wood Canyon Formation, and middle member Wood Canyon Formation±upper member Wood Canyon Formation contacts) and (3) the progradation of braid-plain deposits over shallow-marine shelf rocks (lower member Stirling Quartzite±upper member Johnnie Formation and middle member Wood Canyon Formation±lower member Wood Canyon Formation contacts). Further, the sub-Sauk deposits (sub-middle member Wood Canyon Formation) can be correlated across great distances along and across strike (hundreds of kilometers or more, although across strike distances do not re¯ect pre-deformational

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Fig. 8. Cartoon depicting our interpretation of tectono-eustatic evolution of southwestern Cordilleran craton margin and development of Supersequence package stacking. Phase I represents rifting and rift basin ®ll; Phase II represents early development of the Cordilleran passive margin; Phase III represents mature passive margin that blankets the entire craton. Phases I and II partially ®ll the hiatus of the base Sauk Supersequence in the cratonic interior of North America.

positions) and on-lap up to the edge of the stable craton. In effect, we see nothing in the Neoproterozoic stratigraphy of the craton margin that separates it from younger cratonic cover rocks in terms of paleotectonic af®nity. Such characteristics are not consistent with a rift origin, but rather, favor a passive-margin interpretation for much (all(?)) of the post Kingston Peak Formation stratigraphy. This interpretation is in keeping with stratigraphic relationships exposed in the northern part of the Cordilleran margin (e.g. Ross, 1991; Dalrymple and Narbonne, 1996), although it leaves the problem of reconciling the subsidence modeling and stratigraphic±sedimentologic relationships still unresolved (also see Christie-Blick, 1997). Fig. 8 shows our interpretation of the evolution of the southwestern Cordilleran margin, from rift-to-drift stages, within the context of the hinge zone. Stage I consists of rift sequences (upper Kingston Peak Formation, including glaciogenic diamictites; Miller, 1985; Prave, 1999) and records earliest evolution of the hinge zone. Stage II, initiated by the base Noonday Dolomite unconformity (most likely the break-up unconformity, sensu Falvey, 1974), represents early passive-margin development, but stratigraphic packaging (Noonday Dolomite through lower member Wood Canyon Formation) was con®ned to the belt basinward of the rift shoulder Ð the cratonward margin of the hinge zone. Combinations of eustasy and ¯exural loading, perhaps at times involving

reactivation of earlier rift faults, account for several major disconformities in this `transitional' succession. In Stage III, initiated by the base middle member Wood Canyon Formation disconformity, the rift shoulder has been worn down and passive-margin sedimentation associated with a ®rst-order sea-level rise and transgression has overstepped the hinge and advanced well into the craton, inaugurating the cratonal Sauk Supersequence. 6. Conclusions Stratigraphic and sedimentologic studies of the Neoproterozoic and Lower Cambrian stratigraphy in the Mojave Desert, southeastern California permit us to draw the following conclusions about the succession: ² The depositional basin that preserves the examined strata formed in response to the fragmentation of Rodinia during the Neoproterozoic. Subsidence modeling suggests that the rifting may have occurred as late as the earliest Cambrian. Consequently, the modeling predicts that the section should contain both rift and passive-margin deposits. ² Strata that compose the Neoproterozoic and Lower Cambrian stratigraphy in the ®eld area were deposited across the craton-margin hinge zone.

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This represents a paleotectonic element that bridges the stratigraphic architecture and development between the stable craton and more rapidly subsiding miogeocline. Rocks deposited in the hinge zone rest nonconformably on crystalline basement and share many of the stratigraphic units/facies of the miogeocline, but the section is comparatively thin because of erosional beveling by braid-plain deposits and lack of accommodation space because of differential subsidence. ² There is a broad spectrum of environments preserved within these strata that record widespread shallowmarine shelf and alluvial braid-plain settings. Marine deposits include shallow wave- and tide-in¯uenced shelf, tidal ¯at and shoreface. Thick quartz and feldspathic arenite blankets occur throughout the section and represent the products of a vast westward sloping alluvial plain that persisted for tens of millions of years. Lower Cambrian units that clearly formed in a passive tectonic setting on the craton share the same paleoenvironments as units that, based on age, should have formed during rifting in accordance with subsidence models. The overwhelming similarity of paleoenvironments throughout the section supports the notion that they formed in a consistent geodynamic setting. ² The section is divided into a number of disconformity bounded genetic packages that can be correlated across long distances. The main disconformities in this section are interpreted as having formed principally by eustatic processes that were possibly modi®ed by the effects of differential subsidence across the hinge zone. Sequence boundaries in the section that lie below the Precambrian± Cambrian boundary all pinch out within the craton margin, such that the basal unit of the Sauk Supersequence rests nonconformably on Proterozoic basement in cratonic sections. ² We suggest that the early evolution of the Cordilleran margin developed in three distinct phases. Phase I represents the formation and ®lling of rift basins, which is represented by the Kingston Peak Formation, a unit well known to have been deposited in a glacio-marine setting during active tectonism. Phase II represents a wedge-shaped package of sediments that were deposited in the early, immature stages of the passive margin. Although there is tremendous along-strike presence of this section,

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it is bounded by the relic rift shoulder in a stratigraphically up-dip direction. Phase III represents widespread blanketing of cratonic basement rocks in a well-established, more mature passive-margin setting. It is in Phase III that the classical cratonic sequences of Sloss emerge on Laurentia. Acknowledgements We gratefully acknowledge ®nancial support for this project from the Petroleum Research Fund of the American Chemical Society (25327-B2 to Cooper and 33251-G8 to Fedo); unrestricted grants for research in the eastern Mojave Desert from Chevron Petroleum Technology Company (to Cooper); senior faculty research grants to Cooper from Of®ce of Faculty Research, CSUF; a George Washington University, University Facilitating Fund grant to Fedo; and CSUF Departments Associations Council Grants to Cederstrand and Traub. We appreciate valuable ®eld perspectives from Tony Prave, Cathy Summa, Marge Levy, Martin Keller, Oliver Lehnert, Rob Crangle, Bill Fritz, Paul Myrow, Roland Gangloff and Bruce Runnegar. We thank referees Tony Prave and Ed Simpson for providing constructive comments which have improved the paper. References Bond, G.C., Nickerson, P.A., Kominz, M.A., 1984. Breakup of a supercontinent between 625 Ma and 555 Ma: new evidence and implications for continental histories. Earth and Planetary Science Letters 70, 325±345. Bond, G.C., Kominz, M.A., Steckler, M.S., Grotzinger, J.P., 1989. Role of thermal subsidence, ¯exure, and eustasy in the evolution of early Paleozoic passive-margin carbonate platforms. Controls on Carbonate Platform and Basin Development, Crevello, P., Wilson, J.L., Sarg, J.F., Read, J.F. (Eds.), SEPM Special Publication 44, 40±61. Burch®el, B.C., Davis, G.A., 1988. Mesozoic thrust faults and Cenozoic low-angle normal faults, eastern Spring Mountains, Nevada, and Clark Mountains thrust complex, eastern California. In: Weide, D.L., Faber, M.L. (Eds.), This Extended Land: Geological Excursions in the Southern Basin and Range. Field Trip Guidebook. Geological Society of America, pp. 87±106. Burch®el, B.C., Hamil IV, G.S., Wilhelms, D.E., 1983. Structural geology of the Montgomery Mountains and the northern half of the Nopah and Resting Spring Ranges, Nevada and California. Geological Society of America Bulletin 94, 1359±1376.

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