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Home , Allochthon, Foreland basin, Orogeny

Antler and : A model

R. C. SPEED Department of Geological Sciences, Northwestern University, Evanston, Illinois 60201 N. H. SLEEP Department of Geophysics, Stanford University, Stanford, California 94305

ABSTRACT

The Antler orogeny occurred in Missis- sippian time at a of the sialic North American in Nevada and probably in Idaho. Its principal events were the emplacement of an extensive of oceanic rocks on the continental shelf, subsidence of an elongate foreland basin, and maintenance of highlands in the inte- rior region of the allochthon during and after subsidence of the foreland basin. The orogeny was apparently not accompanied by thermal phenomena or large crustal shortening. We follow earlier models by assuming that the orogeny was initiated by collision of an arc system with . Our proposal holds that a large accretionary of the probably far-traveled arc sys- tem was underthrust by the continental slope and outer shelf and became the Roberts allochthon. The Antler magmatic arc, now unexposed, thermally contracted after collision and was largely or entirely subducted in a later () arc- continent collision. The Antler foreland basin was created by vertical loading and downflexing of the continental shelf by the allochthon, and from subaerial regions of the allochthon ultimately filled and broadened the foreland basin. Quanti- tative models using the theory of a loaded thin elastic plate above a dense fluid provide reasonable agreement between calculated widths, depths, and asymmetry of plate deflections and the Antler foreland basin. Models predict that a flexural bulge of low amplitude migrated continentward ahead of the foreland basin during arc-continent clo- sure. Certain stratigraphic phenomena in pre-foreland basin strata of central Nevada may record the passage of the bulge.

INTRODUCTION Figure 1. Map showing outcrop region of rocks related to the Antler orogeny: lower oceanic rocks that are now (black) or were originally (lined) in the Roberts The Antler orogeny (Roberts, 1951) oc- Mountains allochthon and strata deposited in the Antler foreland basin. XX' is trace of curred in time at a passive section in Figure 2. Edge of Precambrian sialic North America taken from contour of initial margin of the sialic North American conti- 87Sr/86Sr = 0.706.

Geological Society of America Bulletin, v. 93, p. 815-828, 7 figs., 1 table, September 1982.

815

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nent in Nevada and probably in Idaho (Fig. Our objective is to present a general Antler-related rocks probably differ sub- 1). Principal events of the orogeny were (1) model, quantified where feasible, of the stantially because of post-Antler tectonism. displacement of oceanic strata in the Rob- Antler orogeny that gives geologically and One region in central Nevada, however, has erts Mountains allochthon as much as mechanically acceptable solutions to the apparently been little affected by such about 140 km continentward of the margin problems above. The basic ingredients of deformation (Fig. 1), and outcrops of lower above the early Paleozoic shelf (Roberts the model (Speed and Sleep, 1980) are as Paleozoic oceanic rocks within it (black, and others, 1958; Stewart and Poole, 1974; follows: the Antler orogeny was initiated by Fig. 1) may be the only remnants of the Smith and Ketner, 1977); (2) subsidence of a local collision of the passive margin with allochthon in its ap- an elongate foreland basin (foredeep, exo- an arc system as first proposed by Moores proximately original configuration. ) during Mississippian time on (1970). The arc was probably migrating Post-Antler tectonism occurred in at least the continental shelf at the toe of the alloch- with large obliquity with respect to North four episodes, as follows. thon (Poole, 1974); and (3) maintenance of America. The Roberts Mountains alloch- 1. Late Paleozoic deformation, uplift, and highlands in the interior region of the thon was derived from a thick accretionary removal of the Roberts Mountains alloch- allochthon during and after foreland basin prism of the that was underrun by thon occurred locally north of a line subsidence. The orogeny was apparently the slope and outer shelf of North America between Winnemucca and Elko (Fig. 1) not accompanied by , penetra- before convergence ceased due to the bouy- (Ketner, 1977). Such events may have been tive regional deformation or ancy of partially subducted continental related to right slip in an east-striking dis- of the , or significant crustal . The beached accretionary prism placement zone that caused a major deflec- shortening. provided a Mississippian belt tion of tectonic units and facies in northern The Antler may represent a class of orog- along the continental outboard and created Nevada. Moreover, a of rocks corre- enies in which allochthonous oceanic or at a parallel trough some 200 km wide along lated with the Golconda allochthon that is least deep basinal strata are emplaced above its continentward edge by vertically loading well east of the main allochthon about 100 the sialic shelf without associated thermal and downflexing the outer shelf. km northeast of Tonopah (Kleinhampl and phenomena inboard of the margin. Other Ziony, 1972; Laule and others, 1981) lies in examples may be the. Taconic FEATURES OF THE ANTLER OROGENY part directly on the autochthon to the orogeny (Rodgers, 1970; Williams, 1978) Roberts Mountains thrust. If the klippe is and the Pennsylvanian Regional Relationships based by the. Golconda thrust, this suggests (King, 1975; Viele, 1979). Like the Antler, that late Paleozoic • or tectonism both these events generated foreland basins The cryptic modern perimeter of the sialic may have stripped the Roberts Mountains on the continental shelf (Stevens, 1970; Precambrian of the North Ameri- allochthon in that area before Early Triassic Mclver, 1970; Galley, 1971; Nicolas and can continent (Fig. 1) may be approximated time. Rozendal, 1975). Similarly, oceanic strata by the contour of initial 87 Sr/86 Sr ~ 0.706 2. The Golconda allochthon was em- were emplaced in the Golconda allochthon for and Cenozoic magmatic rocks placed above the Roberts Mountains al- in Early Triassic time above the Nevada (Kistler and Peterman, 1978; Armstrong lochthon (Figs. 1, 2) and its partial cover of continental margin and the earlier Roberts and others, 1978; R. W. Kistler, 1979, writ- late Paleozoic shallow marine strata in Mountains allochthon (Speed, 1977) in the ten commun.). The contour follows a belt in Early Triassic time. Such thrusting was so-called , indicating that which seismic and gravity data indicate or accompanied by. little or no deformation of such tectonic processes may recur at the suggest a general eastward thickening of the sub-Golconda rocks. same site. The Sonoma event differs, how- (Prohdehl, 1978; Cogbill, 1979). The 3. and deformation ever, in that it generated either no foreland near coincidence of the 0.706 contour and of the Cordilleran thrust and belt basin or a basin so inconspicuous as to be the westernmost outcrops of autochthonous apparently spanned much of Nevada (Speed, unrecognized. Antler-type are Paleozoic rocks of outer-shelf facies (Kay 1982) and caused tectonic intercalation of distinguished from the thrust belt-foreland and Crawford, 1964; Matti and McKee, Antler-related rocks with lower Paleozoic basin associations exemplified by the Cana- 1977; Rowell and others, 1979) indicates shelf strata and with post-Antler cover as dian Rockies (Bally and others, 1966; Price that the sialic boundary in central Nevada is young as Triassic (Kerr, 1962; Silberling and Mountjoy, 1970) because the latter are probably an ancient passive margin, not a and Roberts, 1962; Larson and Riva, 1963; intracontinental and are apparently related later continental truncation such as proba- Hotz and Willden, 1965; Gilluly and Gates, to thermal phenomena within the sialic bly exist in Idaho (Hamilton, 1976) and 1965; Gilluly, 1967; Winterer, 1968; Riva, continent. California (Hamilton and Myers, 1966). 1970; Coats and Gordon, 1972; Silberling, The basic problems of the Antler orogeny The passive margin is thought to have 1973; Nolan and others, 1974; Ketner and and others of its kind are (1) the formed during late Precambrian rifting Smith, 1974; Skipp and Sandberg, 1975; that, initiated it, (2) the mechanisms that (Stewart, 1972). Sandberg and others, 1975; Smith and permitted oceanic strata to • be thrust up Figure 1 indicates the distribution of Ketner, 1977, 1978; Whitebread, 1978; Speed, some 4 km and laterally many tens of rocks related to the Antler orogeny: lower 1978; Dover, 1979; Evans, 1980). In the kilometres in the light of limits on Paleozoic oceanic rocks that occur in or are region of Jurassic and Cretaceous thrusting, length imposed by rock strength (Hubbert correlated with those of the Roberts Moun- a regional autochthon is unexposed, and and Rubey, 1959), and (3) the cause of fore- tains allochthon and Mississippian foreland only a few of the myriad thrusts that cut land basin subsidence and apparently basin strata. In general, the present and Paleozoic rocks are demonstrably of pre- coupled uplift of a hinterland to the west. Mississippian distributions and extents of Jurassic age. Dover (1979) contended that

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Figure 2. Diagrammatic section of pre-Tertiary structural relationships along XX' of Figure 1; Mz = Mesozoic, PM = Permian to Mississippian strata that overlap the Roberts Mountains allochthon, M = Mississippian foreland basin deposits.

Antler-related rocks in Idaho (Fig. 1) may the regional focus of our analysis, and we thicknesses highly suspect. Nonetheless, al- have been translated east as much as 100 km assume that within the enclave (1) Antler- most all workers agree that lithotypes of the in the Mesozoic. In contrast, a tectonic related rocks are in or insignificantly allochthon accumulated in a basinal envir- enclave in central Nevada from the ancient changed from their Mississippian positions onment, probably on the ocean floor front- passive margin east for some 150 km was relative to the subjacent Paleozoic shelf ing Paleozoic North America. Arguments relatively undeformed in Mesozoic time rocks, (2) the eastern limit of lower Paleo- for a North American source of the terri- (Fig. 1), as indicated by the existence of zoic oceanic rocks is the approximate trace genous sediment in the allochthon are nearly flat-flying autochthonous late Paleo- of the Roberts Mountains thrust, and (3) if sound (Palmer, 1971), but there is no com- zoic and Triassic strata (Ferguson and oth- a regional Mesozoic décollement extends pelling reason to assume accumulation ers, 1951a, 1951b, 1952; Muller and others, from eastern to western Nevada, it lies only near the continental margin in Paleo- 1951; Roberts, 1964; MacMillan, 1972; deeply within or below the lower Paleozoic zoic Nevada. Deformation of beds in the Nichols, 1973; Burke, 1973; Stewart and shelf succession (Speed, 1982). In particu- allochthon is generally large (Gilluly and McKee, 1976). Where exposed in this lar, Ketner and Smith's (1981) suggestion Gates, 1965; Smith and Ketner, 1977; Evans region, the contact between lower Paleozoic that the Roberts Mountains allochthon was and Theodore, 1978), but the complexity oceanic and shelf strata seems to be a single, emplaced in the Mesozoic is considered apparently varies from place to place and laterally continuous tectonic surface that is unlikely for reasons given later. Outside perhaps from nappe to nappe. the Roberts Mountains thrust (Stewart and central Nevada (Fig. I), the locus and extent McKee, 1976; McKee, 1976). Thus, al- of the original Antler orogeny are uncertain. Autochthon though the Roberts Mountains allochthon most likely retains its original configuration Roberts Mountains Allochthon The autochthon of the Roberts Moun- in the central Nevada enclave, the enclave tains thrust contains calcareous strata of itself may have been translated as a giant A generalized section of the Roberts through age that crop nappe, probably cratonward, within the Mountains allochthon is shown in Figure 2, out in windows and Cenozoic horsts across Mesozoic intracontinental thrust belt. modified from Speed and Moores (1981). nearly the full width of the allochthon Furthermore, it has been postulated that The allochthon is exposed over a distance of (Stewart, 1980, Fig. 22). Such rocks were during the late Mesozoic, a fragment of si- about 100 km and probably extends west deposited on the early Paleozoic shelf of alic North America and accreted to below the Golconda allochthon another 30 North America (Roberts and others, 1958) its outer edge may have been rafted away to 50 km. It comprises pelite, radiolarian and represent inner-shelf to shelf-edge facies from southeastern Oregon, western Idaho, and other chert, tholeiite, quartz , (Kay and Crawford, 1964; Stewart and and perhaps northernmost northwestern and of Cambrian through Devo- Poole, 1974; Matti and McKee, 1977). Nevada (Hamilton, 1976; Speed, 1982). The nian age. If such rocks accumulated in Antler-age deformation of rocks of the fragment may have included the bulk of any broadly related environments, an oceanic autochthon seems markedly less than that Antler-related rocks that originally existed realm seems most apt, probably including of the allochthon (Merriam, 1963; Gilluly at those latitudes. varied depositional sites such as fan, inter- and Gates, 1965; Nolan and others, 1974; 4. Cenozoic Basin-Range tectonism fan, and basin plain. Limited structural data McKee, 1976; Smith and Ketner, 1977). caused extension of probably 10% to 20% in suggest that allochthon strata occur at least Tight folds of Antler age appear to be a west or northwest direction in central partly in that juxtapose successions absent from the autochthon except locally Nevada. Thus, the width of the Roberts of various age ranges (Gilluly and Gates, in a thin zone immediately below the Mountains allochthon in the central Ne- 1965; McKee, 1976; Smith and Ketner, Roberts Mountains thrust. A possible excep- vada enclave (Fig. 1) was initially 15 to 30 1977; Stanley and others, 1977). Although tion to this relationship is a major recum- km less. early studies (Roberts and others, 1958) bent fold in parautochthonous shelf-margin To conclude, the main of attempted to synthesize a comprehensive strata in the Toquima Range (just east of Antler orogenic features was by Mesozoic stratigraphy among rocks of the allochthon, the passive margin at about 39.5°) figured foreland thrusting throughout the central the paucity of dating and the more recent by Kay and Crawford (1964). The lack of cordillera except in the tectonic enclave in recognition of tectonic stacking within the documentation of this structure, however, central Nevada (Fig. I). Thus, the enclave is allochthon make correlations and resultant makes its existence questionable. Antler-age

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thrusting within the shelf sequence is estab- Roberts Mountains allochthon lished by the recognition of parautoch- -0 thonous nappes of outer- over inner-shelf strata below the main allochthon in contact with the Roberts Mountains thrust (Rob- erts and others, 1958; Kay and Craw- ford, 1964; Gilluly and Gates, 1965; McKee, Figure 3. Thickness-dis- 1976; Smith and Ketner, 1977). In general, tance curves for two time however, such redistribution within the divisions of foreland basin autochthon was not accompanied by perva- strata at 40° N. Data from sive deformation, metamorphism, or trans- Poole and Sandberg (1977) port of deep-seated rocks to shallow levels. and Stewart (1980). To conclude, Antler-age deformation of the autochthon seems to have been due to transmitted by a surface thrust rather than to intracontinental orogeny.

Foreland Basin clearly of allochthon , was in because maximum preserved thicknesses of Early Mississippian time. The transition is foreland basin strata are generally at or near The Antler foreland basin is an asymmet- marked by subtidal siliciclastic and calcare- their westernmost exposures, it cannot be ric structural trough defined principally by ous beds, including (Webb, Dale assumed that remnant strata include a basin a thick clastic wedge of Mississippian sedi- Canyon, and lower Eleana Formations) axis. It turns out, however, that our quan- ment derived from the west, apparently that lie above and are westerly facies of car- titative model of the Antler foreland basin from the Roberts Mountains allochthon bonate bank deposits (Joana ) of predicts maximum basin depth at the toe of (Poole, 1974; Poole and Sandberg, 1977; Kinderhookian to early Osagean age (Poole the allochthon. Harbaughand Dickinson, 1981). Maximum and Sandberg, 1977; Gutschick and others, Figure 3 shows a twofold time division of preserved thicknesses of the clastic wedge 1980). Although resedimented faunas are thicknesses of foreland basin strata from are as great as 3.5 km and occur at or near recognized in some initial foreland basin Poole and Sandberg (1977) and Stewart their westernmost outcrops' (Fig. 3); the deposits (Hose and others, 1979; C. A. (1980). Although the two curves are proba- wedge thins eastward to less than a kilome- Sandberg, 1981, oral commun. ), the occur- bly not very precise in the western trough, tre thickness at a distance of about 200 km rence of numerous samples yielding isoch- they suggest that the basin broadened with (Poole and Sandberg, 1977; Stewart, 1980). ronous faunas (C. A. Sandberg, 1981, oral time and that subsidence rates increased Preserved strata of the foreland basin commun.) suggests deposition may have westward. Again, because beds of the west- occupy a belt some 200 km wide that gener- started as early as Kinderhookian time. ern trough are imperfectly perserved or ally borders the eastern margin of the out- Succeeding Mississippian strata of the west- dated, maximum subsidence rates cannot be crop region of Antler-related lower Paleo- ern trough record a flood of terrigenous determined. zoic rocks (Fig. 1). Within the Mesozoic debris from the allochthon, first as deep tectonic enclave (Fig. 1), there is a gap of 10 marine-fan deposits (Poole, 1974) and later Dating of Events to 25 km between outcrops of the two ter- as a fan delta (Harbaugh and Dickinson, ranes, probably due to post-Mississippian 1981). The clastic wedge prograded conti- Emplacement of the Roberts Mountains erosion. Northward, the two belts narrowly nentward across the earlier starved eastern allochthon has been widely accepted to have overlap, due at least partly to Mesozoic basin and onto bordering carbonate banks occurred between late Late Devonian and thrusting and perhaps also to deposition of during the Mississippian (Poole and Sand- mid-Early Mississippian times on the basis foreland basin strata on the oceanic rocks berg, 1977; Sandberg and Gutschick, 1980). of studies by Smith and Ketner (1968, (Ketner and Smith, 1977). Thus, initial con- By Early Pennsylvanian time, the region of 1977). They stated that initial foreland basin tact relations of the two terranes are not the basin was once again largely blanketed strata of Kinderhookian age lap unconform- definitively preserved. The present distribu- by shoal-water carbonates, but Lower ably over rocks in the Roberts Mountains tion implies, however, that the width of a Pennsylvanian thicknesses, including local allochthon as young as mid-Famennian age. Mississippian zone of gaps or overlaps was coarse siliciclastic accumulations, indicate Ketner and Smith (1981), however, have small, probably less than 25 km. that subsidence of the western trough con- questioned the evidence for an unconfor- The foreland basin evolved on the conti- tinued into Atokan time (Smith and mity and suggested that instead the contact nental shelf which had earlier accumulated Ketner, 1977; Stewart, 1980). could be a thrust. In the absence of a depo- shoal-water carbonate and siliciclastic sed- The timing of depositional events in the sitional overlap of Paleozoic rocks across iments from Cambrian to Late Devonian western trough of the foreland basin, im- the Roberts Mountains thrust, Ketner and times. The onset of foreland basin subsi- portant to time-series models of the Antler Smith proposed that a Mesozoic age of dence, taken as the maximum age of basinal orogeny, is unfortunately not yet precisely emplacement of the allochthon is possible. strata in successions whose are established. Owing to the predominance of We argue that although the earlier assertion resedimented debris and the scarcity of fos- of a depositional overlap may be faulty, sils in Mississippian rocks in that region (C. present evidence supports the earlier view of 'The isopachs for Mississippian rocks west of A. Sandberg, 1980, oral commun.), it is dif- mid-Paleozoic emplacement, as follows: the locus of maximum preserved thickness shown by Poole (1974), Poole and Sandberg (1977), and ficult to track deposition and subsidence 1. The Kinderhookian age of onset of Stewart (1980) are largely extrapolated. rates and water depths with time. Further, subsidence of the Antler foreland basin and

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provenance of the bulk of foreland basin nance was deposited in the Pilot Shale or not accompanied by significant horizontal sediments from a proximal western high- Joana Limestone, it is difficult to be certain strains in the rocks below it. land lithologically like the Roberts Moun- of an Antler affiliation of the aforemen- tains allochthon. tioned events. However, our model of the INITIATION OF THE 2. The wide area of overlap of the Antler orogeny, given later, predicts that a ANTLER OROGENY Roberts Mountains allochthon above lower laterally propagating upwarp preceded the Paleozoic shelf strata and the near coinci- emplacement of the allochthon on the con- The initiation of the Antler orogeny was dence of boundaries of present outcrop tinental shelf. The upwarp could in fact almost certainly related to some form of areas between the allochthon and Antler account for the vertical motions recorded continental margin tectonism as indicated foreland basin rocks; two correlative points by the three events given above. by the translation of an allochthon of oce- are: (a) our model for the origin of the fore- Strata of the foreland basin indicate that anic deposits across the then passive conti- land basin by vertical loading of the outer the Roberts Mountains allochthon was a nental margin. Among various conceptual shelf by the allochthon predicts that thickest subaerial sediment source from Early Mis- processes for such tectonism reviewed by basin strata should occur at the toe of the sissippian to Early Pennsylvanian times. By Nilsen and Stewart (1980), we consider a allochthon, as observed (Fig. 1), and (b) the end of the Early Pennsylvanian, how- collision of the passive margin with an there is no reason for the near coincidence ever, the differential motion of the foreland island-arc system (Moores, 1970; Dickin- of the allochthon toe and western margin of basin-western highland couple seems to son, 1977) in a form presented by Speed foreland basin strata if the allochthon were have ceased. The succeeding Paleozoic (1977, 1979) as the best candidate. As dis- post-Mississippian and unrelated to the behavior of central Nevada was one of cussed below, no other process satisfies as Antler foreland basin. regional uplift and erosion, local shallow- many of the observational conditions out- 3. The existence of nearly undeformed marine deposition on the former highland, lined above. Triassic and late Paleozoic strata in central and general easterly transport of terrigen- Our conceptual model of the collision of Nevada above the Roberts Mountains al- ous debris into carbonate basins farther an Antler arc system and the western edge lochthon; such rocks would seem more shelfward (Ketner, 1977). of sialic North America held fixed, and likely to be considerably deformed if they There seems to be no evidence for wide- some succeeding Paleozoic events is illus- had been transported to their present sites spread regional shortening of the shelf of trated in Figure 4. The intraoceanic arc sys- on top of the allochthon. Paleozoic North America in central Nevada tem before collision (Fig. 4A) is assumed to If our argument for Early Mississippian during or after the Antler, orogeny, except have had a large forearc region (Seely, 1979) emplacement of the allochthon is correct, for the questionable recumbent fold figured with dimensions like those of the southern latest Devonian and Early Mississippian by Kay and Crawford (1964). Phenomena Lesser Antilles forearc (Westbrook, 1975). sediment in its path was obducted onto the described above indicate heterogenous ver- The Antler accretionary prism was a pile allochthon and either resedimented concom- tical motion. The only feature implying lat- of slices chiefly of subwavebase depos- itantly or after emplacement. Moreover, eral tectonic transport is the Roberts Moun- its of pelagic and terrigenous origin scraped foreland basin strata probably lapped west tains allochthon, and its emplacement was off the descending slab of oceanic litho- across the probably steep lower slope of the allochthon as inferred by Poole (1974). GOLCONOA ACCRETIONARY Aside from the evident tectonic relation- ship between the emplacement of the alloch- thon and Antler foreland basin, it has also been proposed that certain events preceding the Mississippian foreland basin on the con- tinental shelf were caused by early pulses of the Antler orogeny (Poole, 1974; Poole and Sandberg, 1977; Sandbergand Poole, 1977; Sandberg and others, 1980; Gutschick and others, 1980; Sandberg and Gutschick, 1980). Such events include: (1) development of slopes leading to deposition of sediment gravity flows in the Pilot Shale beginning in early Late Devonian (late Frasnian) time, (2) uplift and development of an erosional in latest Devonian (late Fa- mennian) time within the Pilot Shale, and (3) deposition of largely shallow-water de- posits including carbonate banks (Joana Limestone) of Kinderhookian age above the Pilot Shale. The upper early Osagean part of the Joana Limestone was deposited in deeper water (Gutschick and others, 1980) and was in turn overlain by initial deposits Figure 4. Time series of conceptual events that initiated the Antler orogeny (4A and 4B) of the foreland basin. Without convincing and followed it (4C and 4D). W = water, O = , AP = accretionary prism; see evidence that sediment of allochthon prove- text for discussion.

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sphere of the Devonian North American respect to the Roberts Mountains alloch- derland caused isostatic uplift which may be plate. Like some modern accretionary thon, of how to thrust a thin, layered suc- recorded in the Pennsylvanian and Permian prisms, the Antler prism may have been as cession of length many times that allowed and lithotypes of the region thick as 20 km at its structural high. It may by the strength of rocks. In the model, the of the Antler , as discussed have extended with diminishing thickness allochthon is prepackaged as a tectonic pile above. By Permian time (Fig. 4D), cooling 100 to 200 km to its oceanward toe, over- of great thickness in dynamic equilibrium of the Antler arc was probably nearly com- riding the slab with aseismic décollement. with a basal décollement before emplace- plete, and the increased density of the arc As inferred below, the relative motion ment on the continental shelf. Although lithosphere probably permitted it to be between the two plates was probably highly there may have been small adjustments to largely if not completely subductible. So- oblique to the continental margin. the prism configuration when it ramped up nomia (Speed, 1979), an alien microplate As closure between arc and continent the continental slope, these were probably fronted by an active arc system, migrated continued in the Devonian, the accretionary insignificant compared to the inherent struc- toward North America in the Permian, con- prism encountered the base of the continen- tural complexity of the prism. suming first the oceanic lithosphere and/or tal slope and was subsequently underthrust 2. The model explains the strong contrast back-arc basin that lay outboard of the con- by the continent. Plate closure continued in deformation of the allochthon and tracted Antler arc, and then the arc litho- until of continental lithosphere autochthon and the apparent lack of short- sphere itself. The ultimate collision of below the Antler arc was arrested in Early ening in the autochthon. The latter point Sonomia emplaced the Golconda accre- Mississippian time by buoyancy or some stands to reason because the whole conti- tionary prism over the outer reaches of the other force (Fig. 4B). Thereupon, conver- nental lithosphere was involved in the colli- Early Triassic continental shelf and Roberts gence was taken up at an unknown bound- sion, not simply the Paleozoic shelf strata. Mountains allochthon. If Sonomia stopped ary oceanward of the Antler arc, the arc If the thin zone of moderate deformation short of completely consuming the Antler system became sutured to the continent, seen at the top of the autochthon in a few arc lithosphere, the deep structure of west- and the oceanic slab initially attached to exposures is representative, it can be sug- central Nevada may include lower Paleo- North America may have broken off and gested that the prism was generally de- zoic igneous rocks. sunk (Fig. 4B). If the prism was 180 km coupled or that large strain occurred only A maximum rate of subsidence from wide between structural high and toe at within the allochthon during emplacement. thermal contraction of the lithosphere be- the time of suturing, continental underrid- 3. The postulated collision configuration neath the Antler magmatic arc can be ing would have translated the prism com- eliminates need for magmatism and for obtained from depth-age relationships of plex at least 130 km across the continental thermal or deformation-related metamor- ocean ridges (Parsons and Sclater, 1976, shelf. In Figure 4B, the beached accretion- phism in the continental part of the orogen. equations 11 and 21). The subsidence his- ary prism is the Roberts Mountains alloch- 4. The model of Figure 4A has a potential tory of the top surface of the arc lithosphere thon, and its basal décollement is the modern analog in that accretionary prisms may be approximated by cooling of a half- Roberts Mountains thrust. with dimensions as great as those of the space for which subsidence is proportional to If the Roberts Mountains allochthon is in Roberts Mountains allochthon currently the square root of cooling time: fact a relict accretionary prism, it contains exist (Westbrook, 1975). If a continent were no recognized systematic changes of litho- on the subducting plate below one of these types with height in the tectonic stack of the prisms, it is easy to envision that the colli- type suggestive of normal convergence and sion product would be like the Roberts parallelism of facies in ocean-floor strata Mountains allochthon. where d is the depth to basement relative to and a continental margin. Thus, we postu- The hypothesis of an arc-continent colli- the initial elevation, t is the time after the

late that the Antler arc migrated with large sion suffers only from the absence of a rec- heating event, pa is the density of the obliquity to the margin and that it accreted ognized Antler magmatic arc or relics of it. , pf is the density of the fill strata from a long stretch of ocean border- However, the lack of exposure of the Antler above the contracting lithosphere, and A is ing western North America. Assuming the arc can be explained by its deep subsidence a constant which depends on thermal and near-margin deposits consisted of laterally due to thermal contraction upon suturing to geometric parameters. Analysis of North sporadic fan and channel build-ups of terri- North America and isolation from subduc- Pacific and North Atlantic data by Parsons genous sediment interspersed with zones tion-related heating (Fig. 4B). The con- and Sclater (1976) indicates that A is 244 rich in pelagic and hemipelagic lithotypes, tracted arc lithosphere was then partially or (m.y.)1 for times less than 70 m.y. In the the conceived process would have produced totally consumed in a later arc-continent present case, the actual subsidence rate, a lithologically heterogenous accretionary collision, as described below. especially for small times, was somewhat prism. If the idea of oblique convergence is Figures 4C and 4D show conceptually the less than this, because the upper regions of correct, it follows that structurally higher post-collision behavior of the sutured Ant- the lithosphere of an arc are not as hot as strata were transported great distances from ler arc. By mid-Pennsylvanian time (Fig. those of a mid-ocean ridge. The principal their sites of deposition. Unfortunately, dat- 4C), about 50 m.y. after collision, the uncertainties are the initial thermal state of ing in the allochthon is too sparse to allow lithosphere below the Antler arc had cooled the arc and the effect of unknown amounts comparison of age ranges of tectonic pack- significantly and contracted to the extent of of radioactive heating on the final thermal ets with position. reinstituting a deep ocean basin imme- state of the arc. The arc-continent collision model shown diately west of North America. The new To estimate subsidence history, we as- in Figures 4A and 4B is virtuous in several basin invited westward flow of sediment sume that contraction started at the end of

ways: from the continental borderland and its cap the Kinderhookian (340 m.y. B.P.), that pa 1. It eliminates the classical mechanical of rocks of the Roberts Mountains alloch- was 3,300 kg/m3, and that the basin was problem (Hubbert and Rubey, 1959), with thon. The consequent erosion of the bor- filled with water, pf = 1,000 kg/m3. From

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D

C

A

0 50 100 150 200

km Figure 5. Time series of conceptual events in the evolution of the Antler foreland basin; magmatic arc arbitrarily held fixed; W = water; F = forearc basin; see text for discussion.

equation l the arc subsided 1.5 km by the Paleozoic strata of the Triassic Golconda the allochthon. As noted above, the basin- end of Mississippian time (320 m.y. B.P.), allochthon (Fig. 2), which are presumably highland phase of the orogeny seems not 2.2 km by mid-Pennsylvanian time (300 derived in part from the region above to have been associated with regional m.y. B.P.), and 2.9 km by mid-Permian the. subsided Antler arc, are subwavebase shortening or extension, magmatism, or time (270 m.y. B.P.). If the basin above the deposits. metamorphism. We infer, therefore, that Antler arc was covered by sediment (pf . basin-highland evolution was not a product = 2,300 kg/m3) rather than water, the sub- ANTLER FORELAND BASIN of plate boundary tectonics but was simply sidence would be.a factor of 2.3 greater than AND HIGHLANDS a consequence of loading of the continental we computed, or 5 km by mid-Pennsylvan- shelf by the Roberts Mountains allochthon. ian time and 6.7 km in mid-Permian time. Following the emplacement of the Rob- Our concept of the process is illustrated in The actual amount of subsidence was erts Mountains allochthon, the Antler Figure 5, and Figures 6 and 7 present quan- probably intermediate between these esti- orogeny continued through Mississippian titative models of the foreland basin and mates. On one hand, the paleogeographic time with subsidence of the Antler foreland related continentward arch in time series. position of the Antler arc precluded a basin and generation of the Antler high- The story begins (Fig. 5A) with the starved basin (Fig. 4C). On the other, upper lands outboard of the basin in the region of encroachment of the Antler accretionary

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prism on the base of the slope of the North The emergent Antler accretionary prism caused the basin to become shallow water American continent. The prism is assumed of stage C (Fig. 5C) formed the Antler high- or subaerial if sediment had not already to be about 130 km wide and to thicken lands. Assuming pointwise isostatic bal- filled it (Fig. 5D). At this stage, concentra- arcward, rapidly at first, then more gradu- ance, the elevation of the highlands would tion of sediment in the foreland basin pro- ally to a thickness of 15 to 20 km at the have been about one-quarter of the local gressively diminished, and the region soon outer rise. Choice of prism shape is by mod- thickness of the prism, thus 3 to 5 km at the regained a more shelf-like configuration. ern analog, as discussed later. outer rise. In fact, the distributed elastic To quantify the above ideas, we used the The prism-arc system loads the litho- deflection of the subjacent lithosphere theory of flexure and an elastic lithosphere sphere that underrides it. An elastic response would cause the western 50 km or so of the floating on a dense substratum, the asthe- of the lithosphere would yield a regional prism to be at least a factor of 2 higher than nosphere. This formulation is sufficient to downwarp (equations 2-4) of maximum the value given by (see discussion of illustrate the general features of the process. displacement equal to that created by. isos- equations 3 below). More complex calculations are not war- tasy well under the prism. The downward The basin that fronted the beached accre- ranted because the geometry of the accre- displacement of the lithosphere would be tionary prism at the time of closure (Fig. tionary prism and the Antler foreland basin equal to or somewhat less than one-half the 5C) was the Antler foreland basin. Its is too poorly known. Our purpose is not to isostatic value at the toe of the prism and westward-facing slope was created by the determine the detailed mechanical proper- would diminish to zero at a point 100 to 200 elastic deflection of the continental litho- ties of the lithosphere but km continentward of the toe. A low- sphere, and its eastward facing slope was rather to use modern analogs to appraise amplitude elastic arch of some 200-km formed by the flank of the prism. At the our proposed mechanism for the Antler width would occur continentward of the outset (Fig. 5C), the basin would have been foreland basin. downwarp. The actual dimensions of the deepest at the toe of the prism. Assuming The deflection of a thin elastic plate is elastic deflection depend, of course, on the passage of sediment was blocked from con- described by characteristics of the prism and the litho- tinent to basin, terrigenous debris from the sphere, as discussed below. highlands would have prograded continent- d4w N + kw = p(x) (2) As North America and the arc system ward in a clastic wedge across earlier pelagic closed, the elastic deflection propagated and hemipelagic deposits on the submerged inboard, as suggested in Figures 5A to 5C prism toe and the foreland basin floor. where N is the flexural rigidity of the plate, (where the arc is held fixed). If cessation of During the Mississippian, w is the vertical displacement, x is horizon-

closure occurred at first contact between probably caused significant changes in high- tal distance, k = (pa - pf)g is the difference continental and arc (Fig. 5C) land-foreland basin geography. Early in the in specific weight between the astheno-

and if the average rate of underthrusting of period, sediment from the Antler highlands sphere (of density pa) and the material that the prism was about 1.0 cm/yr, the stage was presumably transported east to the fills the basin (of density pf), g is gravity, represented by Figure 5 A was about 13 m.y. foreland basin and west to a postulated and p(x) is the driving load per unit area. earlier. However, if the continental litho- trough, a relict forearc basin (Seely, 1979), Analytic solutions which follow (equations sphere had subducted more, the part out- that lay between the highlands and the 3-5) have been obtained for three models of board of that shown in Figure 5C pre- magmatic arc. By mid-Mississippian time the accretionary prism which probably span sumably broke off and sank with the slab, and thereafter, however, contraction of the the range of physically relevant configura- and there would have been a greater time magmatic arc was sufficient to have allowed tions. The first solution is for the load of a differential between stages A and C of Fig- deposition of highland sediment on the arc semi-infinite sheet between x = and ure 5. surface and far to the west on the deep- x = 0: In stage A (Fig. 5A), the accretionary ocean floor (Figs. 4C and 5D). The isostatic prism was almost entirely submerged, and a response to the sediment transfer was to p deep starved basin existed between the uplift the highlands and depress further the structural high of the prism and the shelf- foreland basin and the inactive magmatic w(x) = cos (x/a) exp (-x/a); x > 0 (3a) slope break. As the continental slope under- arc. A second response was the continent- or thrust the prism (Fig. 5B), the basin shal- ward elastic broadening of the foreland w(x) = r [1 - >A cos (x/a) exp (x/a)] ; lowed, and carbonate may have accumu- basin because sedimentation effectively x^0 (3b> lated on the prism flank and outer conti- caused the prism load to migrate eastward. nental shelf at this time. Between stages B As the highlands continued to rise during where the flexural parameter a = (4N/k)'/4 and C (Figs. 5B and 5C), the accretionary the Mississippian, early foreland basin de- has dimensions of length. From (3a) the prism emerged above sea level and became posits that lapped on the lower prism slope deflection of the lithosphere is a depression widely subaerial by the time of final closure were uplifted and resedimented eastward of width Tva/2 between the toe of the accre- (Fig. 5C). Transport of debris from the toward the foreland basin axis. tionary prism and a more continentward exposed outer part of the prism to the sub- By the end of Mississippian time, the elastic upbulge of slight amplitude. At the merged toe and beyond to the continental highlands were much reduced and the toe of the prism (x = 0), w = 0.5 p/k and shelf presumably began before closure mechanical load initially supplied to the one-half the deflection given by pointwise stopped. The sediment and possibly some outer continental shelf by the prism de- isostasy. Equation 3b gives the deflection thickness of subjacent layered rock on the creased by transport of sediment west to the below the prism and shows that the full elas- shelf either accreted to the encroaching oceanic basin (Fig. 4C). The load reduction tic deflection is first attained at x = -ira ¡2. prism or was obducted above it and then would have resulted in regional uplift of The second solution for a very narrow resedimented. both highland and foreland basin and accretionary prism of some width X along x

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is obtained by adding a negative sheet TABLE 1. CALCULATED FORELAND BASIN WIDTHS between x = -X and x = -oo to the solution in equation 3a. If X ^ not¡4, this solution k N a Accretionary prism shape (n7ra/4) differs little from that of a semi-infinite (kg-m"'1) (nt-m) (km) wedge (n = 1) sheet (n = 2) line (n = 3) sheet (equation 3). It is thus not necessary to know the details of the prism farther than 2,300 10" 65 51 102 153 that from the toe. For a prism significantly narrower than ira/4, a line load centered at 2,300 10» 115 90 180 270 x = 0 is appropriate. The solution is: 700 10» 87 69 136 207 700 I024 154 122 242 365 Xp w(x) = •tt/4) ; Note: widths are in kilometres. Quantities are defined in the text. -^j^exp (-x/a) cos (x/a- (4)

x >0 Here the width of the depression is 3ira/4. greater by a factor of (2.3/0.7)'/< or 1.35. and 7. The density of the prism is assumed A third solution (equation 5) is for a Table 1 presents calculated width of depres- to be 2,500 kg/m3, as determined for the wedge-shaped prism which thickens at a sions for various combinations of prism Lesser Antilles accretionary prism by West- constant rate and consequently exerts a shape and flexural parameter. brook (1975). For convenience, the same steadily increasing load (constant dp/dx) The initial profile shape of the Roberts density is used for the sediment fill of the between x = 0 and x = -X and is of constant Mountains allochthon is unknown because foreland basin. The prism is assumed to thickness at x < -X. At x >0, this solution it has been much eroded. Our conceptual thicken by 0.1 km/km from the toe where is: model (Fig. 5) implies that the load was under water. The rate of increase in load, extensive rather than concentrated ocean- dp/dx, is doubled where the prism is over ward of the basin edge. Modern accretion- the continental slope (-50 to 0 km on the ary prisms (Karig and Sharman, 1975; horizontal axis of Fig. 6) to give a smooth (5) Westbrook, 1975) thicken from the toe with upper surface. The rate of elevation increase ex x+X x+X . x+X , P -(—)[cos —-sin — a convex-up profile or have a comparatively of the prism above water is reduced by a steep outer face of width less than Tra / 2 and factor of 0.6 relative to its submarine profile If X is small, equation 5 is identical to that a gentle inner face. Therefore, the most to give a constant dp/dx and to provide a for a semi-infinite sheet (equation 3a). If, on likely shape of the Antler prism was slope at a realistic angle of repose. Horizon- the other hand, the wedge extends far from between a wedge and a sheet, implying a tal distances are referenced to an origin at the prism toe (x large), the depression con- value of n between 1 and 2 in Table 1. the shelf-slope break (Figs. 6 and 7). tinentward of the toe will have a width of The early Antler foreland-basin deposits At stage A (Fig. 6A), the prism toe has 7ra/4. are subwavebase and perhaps deep-water, arrived at the base of the continental slope. To conclude, the width of the depression implying that the early basin was chiefly The continental outboard is downflexed in front of the accretionary prism is proba- water-filled. Accordingly, the range of min- between distances of -50 and 80 km with an bly within the limits (naj4 and 37ra/4) and imum predicted widths for a water-filled amplitude of about 1 km at the shelf-slope depends on the profile shape of the prism. A early basin is 51 to 180 km (Table 1). Later break. Between 80 and 420 km, the conti- concentrated or line load gives the maxi- basin deposits indicate that the sedimenta- nent is upwarped with a maximum eleva- mum width, and a load that increases grad- tion rate exceeded the subsidence rate and tion of 250 m (Fig. 7, curve A). ually from the toe, the minimum. Local that the basin ultimately became sediment- At stage B (Fig. 6B), the prism toe has variations in load distribution near the filled. The corresponding range of predicted arrived at the shelf-slope break and thick- prism toe (x = 0) and gross variations at widths for the later basin is 69 to 242 km ened somewhat during transit of the conti- large distances from the toe (x<-7ra/2) (Table 1). From Figure 3, the approximate nental slope. The foreland basin is 3 km have little effect on the width of the widths of the actual Antler foreland basin deep at the toe of the prism and extends 140 depression. between the allochthon toe and eastern km continentward of the toe. The upwarp Formation of a significantly wide depres- basin margin (point of zero slope on thick- (Fig. 7, curve B) lies between 140 and 500 sion requires the lithosphere to be relatively ness curves) are 150 km at the end of early km and is over 300 m high at a distance of cold and, hence, stiff. Relevant estimates of Meramecian time and 200 km at the end of 230 km from the prism toe. the flexural rigidity (N) are 1023 nt-m for Chesterian time. The increase in actual At stage C (Fig. 6C), the prism has ceased the Atlantic coast of the United States basin width with time evidently reflects the migration after traversing 80 km of conti- (Steckler and Watts, 1978) and 1024 nt-m increased proportion of sediment in the fill nental shelf and attaining a total continental for older oceanic lithosphere seaward of with time. The agreement between observed overlap of 130 km. The foreland basin is 2.6 trenches (Chappie and Forsyth, 1979). For and predicted values in magnitude and time km deep at the prism toe and extends 110 water-filled basin, the specific weight con- trend indicates the proposed loading mech- km continentward of the toe. The upwarp trast (k) is 23 x 103 nt/m3, giving flexural anism is credible and that the Antler accre- has a maximum elevation of about 350 m at parameter estimates of 65 km and 115 km tionary prism was more sheet-like than 290 km and extends to 560 km (Fig. 7, curve when combined with the above values of N. wedge-like. C). For a sediment-filled basin, the specific Computed models of the effects of the Stage D (Fig. 6D) represents a later time, 3 3 weight contrast is 7 x 10 nt/m , and the propagation of an accretionary prism over a when the beached prism has been reduced flexural parameter is correspondingly passive margin are illustrated in Figures 6 to sea level by erosion and its foreland basin

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100 -100 0 100 200 DISTANCE. KM D DISTANCE. KM Figure 6. Calculated flexure of the lithosphere from a simplified thrust sheet is plotted for various times. The initial water depth inland from the shelf break (0) is assumed to be sea level: The initial water depth seaward of SO km is 5 km. The thrust sheet has constant thickness to the left of the drawing at times A, B, and C. At time D the entire region is eroded and sedimented to sea level. The position of the thrust edge is indicated assuming the thrust stopped at time B. The position had the thrust stopped at time C is the dashed line. See text.

has been completely filled with sediment. yields basin widths, depths, and asymmetry marine sediment that lapped, west onto the Line C shows the prism-sediment boundary that generally agree with those of the Antler Roberts Mountains allochthon was narrow, for a prism that migrated as far as stage C, foreland basin. Three particular points con- perhaps 30 to 50 km (Fig. 6C). The width of whereas line B shows the same boundary for cerning sediment onlap on the allochthon, the sediment overlap would have been even a prism that stopped migration at stage B. time-dependent effects, and the flexural narrower if the lower slope of the alloch- The curves of Figure 6D illustrate the prin- bulge migration are discussed below. thon had been steeper than that assumed. ciple that the farther inboard the prism The basin model of Figure 6 predicts that Thus, given a very steep allochthon front, propagates, the smaller the volume of fore- maximum thickness of sediment in the fore- the model provides a primary origin for the land basin sediments that would be ulti- land basin occurs at the toe of the accre- apparent meager width of overlap between mately preserved. tionary prism. It also indicates that for the the Roberts Mountains allochthon and To conclude, our highland-basin model shape of the prism, the width of the belt of Antler foreland basin deposits. The exis- tence of a steep allochthon face is not * r unreasonable because (1) debris flow and other plastic bodies typically have bulbous toes (Johnson, 1970, p. 450) and thus have more slablike than wedgelike profiles, and (2) the broad width of the Antler foreland basin also suggests the allochthon had a steeply rather than shallowly inclined front, as discussed above. Figure 7. The uplift in front of the thrust sheet is shown for the times in Figure 6. At The evolution of the foreland basin after times A, B, and C no erosion is assumed to occur. At time D a region originally at sea level complete emplacement of the accretionary is assumed to have been uplifted by flexure and then eroded back to sea level. The broad prism would depend on sedimentation and uplift may produce an observable unconformity which is time transgressive. on mechanical properties of the lithosphere.

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Sediments derived from the prism but far Roberts (1972) and Ketner (1977) sought nental lithosphere, and (3) cooling at the from its toe would provide new loads in the explanations that involve no plate-boun- base of the lithosphere along a locus that basin and would increase subsidence and dary tectonics. Moores (1970) proposed was to become the forearc basin. width of the basin. Carbonate banks and that the orogeny was caused by one of a The first idea is mechanically infeasible if far-traveled cratonic sediment would have series of collisions between passive western conceived as an elastic buckle because the the same effect on the foreland basin. How- North America and arcs that crossed or half-wavelength of the first buckle of a ever, sedimentation from the toe of the were generated in the proto-Pacific Ocean. lithosphere over a viscous substrate should allochthon to the nearby basin floor would Our collision configuration differs from be many times that of the Antler foreland produce only a minor shift of load and Moores' in two respects: (1) the rocks that basin and because the needed to cause little uplift or subsidence. Once the became the Roberts Mountains allochthon buckle the lithosphere is excessive. basin is filled, any subsequent removal of were brought to the continental slope by the We have investigated the second mecha- mass from the highlands or basin would arc as a prepackaged tectonic edifice, not as nism and found that the heat gained by the produce uplift of the system. If the loaded a complex created by collision with the con- continental lithosphere by lateral conduc- lithosphere could maintain elastic strain tinent, (2) the arc system may have migrated tion from the cooling Antler magmatic over the erosion history of the allochthon, obliquely along western North America, under optimum transfer conductions (verti- the flexture would broaden and diminish in and (3) no subcontinental subduction was cal contact) would expand the continental amplitude as the load was spread out by involved. Moreover, our model holds that outboard by no more than 100 m. Such erosion and sedimentation. If, on the other the Antler magmatic arc was removed from relief seems inadequate to have initiated an hand, creep could relax the strains related view by thermal contraction, whereas adjacent foreland basin by sedimentary to flexure, the allochthon would subside Moores (1970) provided no explanation for loading. more and the basin would rebound and nar- the lack of exposure of the postulated arc. The third scheme assumes that hot mate- row with time. The geometric effects of Burchfiel and Davis (1972), Silberling rial is created or emplaced at the base of the creep are likely to be obscured by the (1973), Churkin (1974), and Poole (1974) continental lithosphere inboard of the Paleo- broadening of the basin by sediment filling. concluded that the Roberts Mountains zoic passive margin, either by rifting or by allochthon emerged from a backarc basin The model also predicts that a broad convection following detachment and sink- that lay between the continental slope and a flexural bulge of height as great as 350 m ing of the slab upon collision of North local magmatic arc, now located in the should have propagated across the conti- America and the Antler arc (Fig. 4B). In and the Klamath Mountains, nental shelf landward of the foreland basin analogy with subsidence of mid-ocean that surmounted a subcontinental subduc- during closure between the accretionary ridges, thermal contraction of the litho- tion zone. The orogeny was thought to be prism and the North American continent sphere above the hot region may have caused by a collapse of the backarc basin, (Fig. 7). Late Devonian and Early Missis- created the Antler foreland basin, in the but the mechanisms of of the sippian stratigraphic features that preceded way inferred for Atlantic continental mar- basin sediments in these treatments are the subsidence of the Antler foreland basin, gins and interior basins by Sleep (1971). The vague. The paucity of arc-derived rocks in as recounted above, may be related to the thickness (S) of sediments in the basin ver- the Roberts Mountains allochthon argues migration of a flexural bulge. Late Frasnian sus time, assuming sedimentation keeps against a backarc basin origin of allochthon and Famennian turbidite in the Pilot Shale pace with subsidence, is: sediment. Moreover, there is little corre- may have been derived from the leading spondence between early Paleozoic volcanic edge of the bulge, and the intra-Famennian histories of the Sierra and the Klamath lacuna within the Pilot Shale below the S = So(x)[l-exp(-t/50m.y.)] (6) Mountains and between the latter and tim- western trough may represent erosion on ing of Antler events. Whereas the early passage of the bulge. The overlying deposits Paleozoic rocks of the Klamath and Sierra for a thermal contraction origin, where including the Joana Limestone may have provinces may have been long-term local S0(x) is a slowly varying function of hori- accumulated in shallow water on the trail- residents, they may also represent fragments zontal position and t is time. Depth-time ing edge of the bulge, and the deepening of of arcs of other terranes that were generated curves for sediment thickness of the Antler the Joana may track the easterly migration at distant sites along the western margin of foreland basin, interpolated from Figure 3, of the bulge. Strata above the Joana Lime- North America (Speed, 1979). Dickinson can be reasonably fit by equation 6. The stone indicate increasingly basinal condi- (1977) considered underthrusting of the correspondence suggests that thermal con- tions and are interpreted to record the continental edge beneath oceanic rocks as a traction is a feasible origin of the foreland encroachment of the Antler foreland basin. more likely origin of the Roberts Moun- basin. Unfortunately, the dating available to us of tains allochthon than other mechanisms he The sedimentation rate, however, is a key horizons is insufficiently precise to surveyed. We agree. poor indication of subsidence rate in a basin determine a closure velocity. that was chiefly water-filled in its early his- Three alternative schemes for generation tory but sediment-filled later. The declining EARLIER MODELS of the Antler highland-foreland basin cou- rates of sedimentation are more likely due ple are (1) sustained lateral loading of the to filling of the depression and With respect to initiation of the Antler continental margin by Mississippian sub- of the source than to thermal con- orogeny, Moores (1970) was first to invoke duction with a subcontinental slab (Poole, traction of the lithosphere. the collision of an arc with North America. 1974), (2) lateral heat conduction from the To conclude, the alternative mechanisms Most subsequent models, including ours, cooling Antler magmatic arc into and con- for generation of the Antler foreland basin employ some form of collision, although sequent expansion of the peripheral conti- are either inadequate or unduly complex.

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We contend that flexure of the continental prism. Moreover, it suggests that there nental margin, in Stewart, J. H., Stevens, C. shelf by vertical loading due to the Roberts would have been meager overlap of Antler H., and Fritsche, A. E., eds.. Paleozoic paleogeography of the western United Mountains allochthon, as given above, is foreland basin deposits on the Roberts States: Society of Economic Paleontologists the only tractable hypothesis yet proposed. Mountains allochthon if the toe of the and Mineralogists, Pacific Section, Pacific allochthon had a relatively steep slope dur- Coast Paleogeography Symposium 1, p. OTHER FORELAND BASINS ing emplacement. Analytic models of elastic 137-155. deflections of the continental slope and Dover, J. H., 1980, Status in Antler orogeny in central Idaho—Clarifications and constraints shelf give widths, depths, and asymmetry If our model of the Antler orogeny has fron the Pioneer Mountains, in Fouch, T. general applicability to a class of orogenies that accord well with values of the Antler D., and Magathan, E. R., eds., Paleozoic discussed above, it should account for the foreland basin. Models predict that a flex- paleogeography of the west-central United seeming variability of foreland basin devel- ural bulge as high as 350 m preceded the States: Society of Economic Paleontologists and Mineralogists, Rocky Mountain Sec- opment among examples presented. Assum- migrating foreland basin. Certain Late tion, Rocky Mountains Paleogeography Devonian and Early Mississippian features ing that the lack of association of foreland Symposium 1, p. 371-386. basin deposits with the Golconda alloch- that underlie Antler foreland basin deposits Evans, J. G., 1980, Geology of the Rodeo Creek thon is not due to erosion, lack of recogni- may record the passage of the bulge. NE and Welches Canyon quadrangles, Eu- tion, or overriding by reactivation of the reka County, Nevada: U.S. Geological Sur- vey Professional Paper 1060, 18 p. allochthon, the main variables are the char- ACKNOWLEDGMENTS Ferguson, H. G., Muller, S. W., and Roberts, R. acters of the accretionary prism and of the J., 1951a, Geology of the Winnemucca continental margin at time of collision. We are grateful to L. L. Sloss and C. quadrangle, Nevada: U.S. Geological Survey If the accretionary prism were small, an A. Sandberg for critical reviews of the Geologic Quadrangle Map GQ 11. 1951b, Geology of the Mount Moses orogeny resulting from arc-continent colli- manuscript. quadrangle, Nevada: U.S. Geological Sur- sion might differ significantly from that dis- vey Geologic Map GQ 12. cussed above. The width and thickness of REFERENCES CITED Ferguson, H. G., Roberts, R. J., and Muller, the prism might be small enough that the S. W., 1952, Geology of the Golconda toe would not migrate beyond the shelf- Armstrong, R. L., 1968, Sevier orogenic belt in quadrangle: U.S. Geological Survey Geo- logic Quadrangle Map GQ 15. slope break and the prism might not break Nevada and : Geological Society of Galley, J. E., 1971, Summary of petroleum the water surface. Further, the elastic America Bulletin, v. 79, p. 429-458. Armstrong, R. L., Taubeneck, W. H., and Hales, resources in Paleozoic rocks of Region 5— depression of the continental shelf would be P. O., 1977, Rb-Sr and K-Ar geochrometry north-central and west and eastern proportionally less. As a result, no high- of Mesozoic rocks and their Sr iso- : American Association of Pe- lands would emerge to provide coarse detri- topic composition, Oregon, Washington, troleum Geologists Memoir 15, p. 726-737. Gilluly, J., 1967, Geologic map of the Winne- tus; the basin would undergo less sediment and Idaho: Geological Society of America Bulletin, v. 88, p. 397-441. mucca quadrangle, Pershing and Humboldt loading; and the basinal deposits would be Bally, A. W., Gordy, P. L., and Stewart, G. A., Counties, Nevada: U.S. Geological Survey pelagic, hemipelagic, or carbonate turbidite 1966, Structure, seismic data and orogenic Geologic Quadrangle Map GQ 656. types, which might be difficult to recognize evolution of southern : Gilluly, J., and Gates, O., 1965, Tectonic and as foreland-basin deposits. If the continen- Canadian Petroleum Geologists Bulletin, v. igneous geology of the northern Shoshone Range, Nevada: U.S. Geological Survey tal slope were broad relative to the typical 14, p. 337-381. Burchfiel, B. C., and Davis, G. A., 1972, Structu- Professional Paper 465, 153 p. 50-km modern value, the chances increase ral framework and evolution of the southern Gutschick, R. C., Sandberg, C. A., and Sando, that a small collided prism would not have part of the Cordilleran orogen, western W. J., 1980, Mississippian shelf margin and evolved into a highland. United States: American Journal of Sci- carbonate from to Ne- ences, v. 272, p. 97-118. vada, in Fouch, T. D., and Magathan, E. R., Burke, D. B., 1973, Reinterpretation of the Tobin eds., Paleozoic paleogeography of the west- CONCLUSIONS thrust: Pretertiary geology of the southern central United States: Society of Economic Tobin Range, Nevada [Ph.D. thesis]: Stan- Paleontologists and Mineralogists, Rocky Mountain Section, Rocky Mountains Paleo- Our model of the Antler orogeny holds ford, California, Stanford University, 82 p. Chappie, W. M„ and Forsyth, D. W., 1979, geography Symposium 1, p. 111-128. that initiation of activity was by an arc- and bending of plates: Journal Hamilton, W., 1976, Tectonic history of west- continent collision and that succeeding of Geophysical Research, v. 84, p. 6729- central Idaho: Geological Society of Ameri- motions of a highland-foreland basin cou- 6741. ca Abstracts with Programs, v. 8, p. 378. ple were due simply to vertical loading of Churkin, Michael, Jr., 1974, Paleozoic marginal Hamilton, W., and Myers, W. B., 1966, Cenozoic the continental shelf by a beached accre- ocean basin systems in the tectonics of the western United States: Review of Geophysics, v. 4, p. 509-549. tionary prism. The model accounts well for Cordilleran foldbelt, in Dott, R. H. Jr., and Shaver, R. H., eds., Modern and ancient Harbaugh, D. W., and Dickinson, W. R„ 1981, regional geologic characteristics of the orog- géosynclinal sedimentation: Society of Eco- Depositional facies of Mississippian clastics, eny, answers the mechanical problem of nomic Paleontologists and Mineralogist Spe- Antler foreland basin, central how to emplace oceanic strata tens of cial Publication 19, p. 174-192. Range, Nevada: Journal of Sedimentary kilometres across the shelf-slope break, and Coats, R. R., and Gordon, M., 1972, Tectonic Petrology, v. 51, p. 1223-1234. explains the absence of associated magma- implications of the presence of the Edna Hose, R. K., Wrucke, C. T., and Armstrong, A. in northern Elko K., 1979, Mixed Devonian and Mississip- tism, metamorphism, and regional shorten- County, Nevada: U.S. Geological Survey pian conodont and foraminiferal faunas and ing. It predicts that an Antler magmatic arc Professional Paper 800C, p. C85-C94. their bearing on the Roberts Mountains subsided deeply after collision by thermal Cogbill, A. H., 1979, Relationships of crustal thrust, Nevada: Geological Society of Amer- contraction, that the initial Antler high- structure and seismicity, western Great Basin ican Abstracts with Programs, v. 11, p. 446. [Ph.D. thesis]: Evanston, Illinois, North- Hotz, P. E., and Willden, R„ 1964, Geology and lands were the subaerial part of the accre- western University, 253 p. mineral deposits of the Osgood Mountains tionary prism, and that the deepest part of Dickinson, W. R., 1977, Paleozoic quadrangle, Humboldt County, Nevada: the foreland basin was at the toe of the and the evolution of the Cordilleran conti- U.S. Geological Survey Professional Paper

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431, 128 p. tologists and Mineralogists, Pacific Section, Paleontologists and Mineralogists, Pacific Hubbert, M. K„ and Rubey, W. W„ 1959, Role Pacific Coast Paleogeography Symposium Section, Pacific Coast Paleogeography Sym- of fluid pressure in mechanics of overthrust 1, p. 181-215. posium 1, p. 39-65. faulting: Geological Society of America Bul- Mclver, N., 1970, Appalachian turbidites, in Price, R. A., and Mountjoy, E. W., 1970, Geo- letin, v. 70, p. 115-205. Fisher, G. W., and others, Studies of Appa- logic structure of the Canadian Rocky Johnson, A. M., 1970, Physical processes in lachian geology, central and southern: New Mountains between Bow and Athabasca geology: San Francisco, California, Free- York, Interscience Publishers, p. 69-81, Rivers: Geological Association of Canada man-Cooper and Co., 653 p. McKee, E. H., 1976, Geology of the northern Special Paper 6, p. 7-26. Karig, D. E., and Sharman, G. F„ 1975, Subduc- part oftheToquima Range, Lander, Eureka, Prohdehl, C., 1979, Crustal structure of the west- tion and accretion in trenches: Geological and Nye Counties, Nevada: U.S. Geological ern United States: U.S. Geological Survey Society of America Bulletin, v. 86, p. Survey Professional Paper 931, 49 p. Professional Paper 1034, 74 p. 377-389. Merriam, C. W., 1963, Paleozoic rocks of Ante- Riva, John, 1970, Thrusted Paleozoic rocks in Kay, M., and Crawford, J. P., 1964, Paleozoic lope Valley, Eureka, and Nye Counties, the northern and central HD range, nor- facies from the miogeosynclinal to the Nevada: U.S. Geological Survey Profes- theastern Nevada: Geological Society of eugeosynclinal belt in thrust slices, central sional Paper 423, 61 p. America Bulletin, v. 83, p. 1383-1396. Nevada: Geological Society of America Bul- Moores, E., 1970, Ultramafics and orogeny, with Roberts, R. J., 1951, Geology of the Antler Peak letin, v. 75, p. 425-454. models of the U.S. Cordillera and the Tethys: quadrangle, Nevada: U.S. Geological Survey Kerr, J. W., 1962, Paleozoic sequences and thrust Nature, v. 228, p. 837-842. Geologic Quadrangle Map GQ 10, scale slices of the Seetoya Mountains, Independ- Muffler, L.J.P., 1964, Geology of the Frenchie 1:62,500. ence Range, Elko County, Nevada: Geologi- Creek quadrangle, north-central Nevada: 1964, Stratigraphy and structure of the cal Society of America Bulletin, v. 73, p. U.S. Geological Survey Bulletin 1179, 99 p. Antler peak quadrangle, Humboldt and 439-460. Muller, S. W., Ferguson, H. G., and Roberts, R. Lander Counties, Nevada: U.S. Geological Ketner, K. B., 1977, Late Paleozoic orogeny and J., 1951, Geology of the Mount Tobin quad- Survey Professional Paper 459A, 91 p. sedimentation, southern California, Nevada, rangle: U.S. Geological Survey Geologic 1972, Evolution of the Cordilleran fold belt: Idaho, and Montana, in Stewart, J. H., Quadrangle Map GQ 7. Geological Society of America Bulletin, v. Stevens, C. H., and Fritsche, A. E., eds., Nicolas, R. L., and Rozendahl, R. A., 1975, 83, p. 1989-2003. Paleozoic paleogeography of the western Subsurface positive elements within Oua- Roberts, R. J., Hotz, P. E„ Gilluly, J., and Fer- United States: Society of Economic Paleon- chita foldbelt in Texas and their relation to guson, H. G., 1958, Paleozoic rocks of tologists and Mineralogists, Pacific Section, Paleozoic margin: American Association of north-central Nevada: American Associa- Pacific Coast paleogeography Symposium 1, Petroleum Geologists Bulletin, v. 59, p. tion of Petroleum Geologists Bulletin, v. 42, p. 363-369. 193-216. p. 2813-2857. Ketner, K. B., and Smith, J. F., 1974, Folds and Nichols, K. M., 1972, Triassic depositional his- Rodgers, John, 1970, The tectonics of the Appa- overthrusts of Late Jurassic or Early Cre- tory of China Mountain and vicinity, north- lachians: New York, Wiley-Interscience, teceous age: U.S. Geological Survey Journal central Nevada [Ph.D. thesis]: Stanford, 270 p. of Research, v. 2, p. 417-419. California, Stanford University, 142 p. Rowell, A. J., Rees, M. N., and Suczek, C. A., 1981, Mid-Paleozoic age of the Roberts Nilsen, T. H„ and Stewart, J. H., 1980, The 1979, margin of the North American conti- thrust unsettled by new data from northern Antler orogeny—Mid-Paleozoic tectonism nent in Nevada during late Cambrian time: Nevada: Geological Society of America in western North America: Geology, v. 8, American Journal of Science, v. 279, p. Abstracts with Programs, v. 13, p. 64. 298-302. 1-18. King, P. B., 1975, The Ouachita and Appalachian Nolan, T. B., Merriam, C. W„ and Blake, M. C., Sandberg, C. A., and Gutschick, R. C„ 1980, orogenic belts, in Nairn, A. M., and Stehli, 1974, Geologic map of the Pinto Summit Sedimentation and biostratigraphy of Osa- F. G., eds., The ocean basins and margins, quadrangle, Nevada: U.S. Geological Survey gean and Merimecian starved basin and Volume 3, The Gulf of Mexico and the Miscellaneous Geologic Investigations Map foreslope, western United States, in Fouch, Caribbean: New York, Plenum Press, p. 1-793. T. D., and Magathan, E. R., eds., Paleozoic 210-237. Palmer, A. R., 1971, The Cambrian of the Great paleogeography of the west-central United Kistler, R. W„ and Peterman, Z. E., 1978, Basin and adjoining areas, western United States: Society of Economic Paleontologists Reconstruction of crustal blocks of Califor- States, in Holland, C. H., ed., Cambrian of and Mineralogists, Rocky Mountain Sec- nia on the basis of initial Sr isotopic compo- the New World: New York, Wiley-Inter- tion, Rocky Mountains Paleogeography sitions of Mesozoic plutons: U.S. Geological science, p. 1-78. Symposium 1, p. 129-147. Survey Professional Paper 1061, 27 p. Parsons, B., and Sclater, J. G., 1977, An analysis Sandberg, C. A., and Poole, F. G., 1977, Cono- Kleinhampl, F. J., and Ziony, J. I., 1972, Possible of ocean floor bathymetry with heat flow dont biostratigraphy and depositional com- outlier of Golconda thrust plate in the Moni- and age: Journal of Geophysical Research, plexes of Upper Devonian cratonic-platform tor Range (Nevada): U.S. Geological Survey v. 82, p. 803-827. and continental-shelf rocks in the western Professional Paper 800A, p. A42. Poole, F. G., 1974, deposits of Antler fore- United States: Riverside, California, Univer- Larson, E. R., and Riva, J. F., 1963, Preliminary land basin, western United States, in Dick- sity of California at Riverside, Campus map and geologic sections of the Diamond enson, W. R., ed., Tectonics and sedimenta- Museum Contribution 4, p. 144-182. Springs quadrangle, Nevada: Nevada Bu- tion: Society of Economic Paleontologists Sandberg, C. A., Hall, W. E., Batchelder, J. N., reau of Mines Map 20. and Mineralogists Special Publication 22, p. and Axelsen, C., 1975, Stratigraphy, cono- Laule, S. W., Snyder, W., and Ormiston, A. R., 58-82. dont dating, and paleotectonic interpreta- 1981, Willow Canyon Formation, Nevada, Poole, F. G., and Sandberg, C. A., 1977, Missis- tion of type Milligan Formation (Devonian), an extension of the Golconda allochthon: sippian paleogeography and tectonics of the Wood River area, Idaho: U.S. Geological Geological Society of America Abstracts western United States, in Stewart, J. H., Survey Journal of Research, v. 3, p. with Programs, v. 13, p. 66. Stevens, C. H., and Fritsche, A. E., eds., 707-720. MacMillan, J. R., 1972, Late Paleozoic and Paleozoic paleogeography of the western Sandberg, C. A., Poole, F. G., and Gutschick, R. Mesozoic tectonic events in west-central United States: Society of Economic Paleon- C., 1980, Devonian and Mississippian strati- Nevada [Ph.D. thesis]: Evanston, Illinois, tologists and Mineralogists, Pacific Section, graphy and conodont zonation of Pilot and Northwestern University, 146 p. Pacific Coast Paleogeography Symposium Chainman Shales, Confusion Range, Utah, Matti, J. C„ and McKee, E. H„ 1977, 1, p. 181-215. in Fouch, T. D., and Magathan, E. R., eds., and Lower Devonian paleogeography of the Poole, F. G., Sandberg, C. A., and Boucot, A. J., Paleozoic paleogeography of the west-cen- outer continental shelf of the Cordilleran 1977, Silurian and Devonian paleogeog- tral United States: Society of Economic , central Nevada, in Stewart, J. raphy of the western United States, in Ste- Paleontologists and Mineralogists, Rocky H., Stevens, C. H., and Fritsche, A. E., eds., wart, J. H., Stevens, C. H., and Fritsche, A. Mountain Section, Rocky Mountains Paleo- Paleozoic paleogeography of the western E., eds., Paleozoic paleogeography of the geography Symposium 1, p. 129-147. United States: Society of Economic Paleon- western United States: Society of Economic Seeley, D. R., 1978, Evolution of structural highs

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bordering major forearc basins: American Pacific Coast Paleogeography Symposium Canada Special Paper 7, p. 165-177. Association of Petroleum Geologists Mem- 1, p. 349-362. Stewart, J. H., 1972, Initial deposits of the Cor- oir 29, p. 245-260. 1978, Paleogeographic and plate tectonic dilleran geosyncline: Evidence of a late Pre- Silberling, N. J., 1973, Geologic events during evolution of the early Mesozoic marine pro- cambrian (850 m.y.) continental separation: Permian-Triassic time along the Pacific vince of the western Great Basin, in Howell, Geological Society of America Bulletin, v. margin of the United States: Alberta Society D. G., and McDougall, K. A., eds., Meso- 83, p. 1345-1360. of Petroleum Geologists Memoir 2, p. zoic paleogeography of the western United 1980, : Nevada Bureau of 345-362. States: Society of Economic Paleontologists Mines and Geology Special Publication 4, 1975, Age relationships of the Golconda and Mineralogists, Pacific Section, Pacific 136 p. , Sonoma Range, north-central Coast Paleogeography Symposium 2, p. Stewart, J. H„ and McKee, E. H., 1977, Geology Nevada: Geological Society of America Spe- 253-270. and mineral deposits of Lander County, cial Paper 163, 28 p. 1979, Collided Paleozoic microplate in the Nevada: Nevada Bureau of Mines and Geol- Silberling, N. J., and Roberts, R. J„ 1962, Preter- western United States: Journal of Geology, ogy Bulletin 68, 106 p. tiary stratigraphy and structure of north- v. 87, p. 279-292. Stewart, J. H., and Poole, F. G., 1974, Lower western Nevada: Geological Society of 1982, Evolution of the sialic margin in the Paleozoic and uppermost Precambrian Cor- America Special Paper 72, 58 p. central-western United States: American As- dilleran miogeocline, Great Basin, western Skipp, B., and Sandberg, C. A., 1975, Silurian sociation of Petroleum Geologists Memoir, United States, in Dickinson, W. R., ed., and Devonian miogeosynclinal and transi- Hedberg Volume (in press). Tectonics and sedimentation: Society of tional rocks of the Fish Creek Reservoir Speed, R. C., and Moores, E. M., 1981, Geologic Economic Paleontologists and Mineralogists , central Idaho: U.S. Geological cross section of the Sierra Nevada and the Special Publication 22, p. 27-57. Survey Journal of Research, v. 3, p. Great Basin along 40°N lat.; northeastern Viele, G. W., 1979, Geologic map and cross sec- 691-706. California and northern Nevada: Geological tion, eastern , Arkan- Sleep, N. H., 1971, Thermal effects of the forma- Society of America Map and Chart Series sas: Geological Society of America Map and tion of Atlantic continental margins by con- MC-28L. Chart Series MC-28F. tinental breakup: Geophysical Journal, v. Speed, R. C., and Sleep, N. H., 1980, Antler oro- Westbrook, G. K., 1975, The structure of the 24, p. 325-350. geny, a model: Geological Society of Amer- crust and upper in the region of Bar- Smith, J. F., and Ketner, K.. B, 1968, Devonian ica Abstracts with Programs, v. 12, p. 527. bados and the Lesser Antilles: Geophysical and Mississippian rocks and the date of the Stanley, K. O., Chamberlain, C. K., and Stewart, Journal, v. 43, p. 1-42. Roberts Mountains thrust in the Carlin- J. H., 1977, Depositional setting of some Whitebread, D. H., 1978, Preliminary geologic Pinon Range area, Nevada: U.S. Geological eugeosynclinal Ordovician rocks and struc- map of the Dun Glen quadrangle, Pershing Survey Bulletin 1251-1, p. 11-118. turally interleaved Devonian rocks in the County, Nevada: U.S. Geological Survey 1977, Tectonic events since early Paloezoic in Cordilleran mobile belt, Nevada, in Stewart, Open-File Report 78-407. the Carlin-Pinon Range area, Nevada: U.S. J. H., Stevens, C. H., and Fritsche, A. E., Williams, H., 1979, Appalachian orogen in Can- Geological Survey Professional Paper 867c, eds., Paleozoic paleogeography of the west- ada: Canadian Journal of Sciences, v. 18 p. ern United States: Society of Economic 16, p. 792-807. 1978, Geologic map of the Carlin-Pinon Paleontologists and Mineralogists, Pacific Winterer, E. L., 1968, Tectonic erosion in the Range area, Elko and Eureka Counties, Section, Pacific Coast Paleogeography Sym- Roberts Mountains, Nevada: Journal of Nevada: U.S. Geological Survey Miscel- posium 1, p. 259-275. Geology, v. 76, p. 347-357. laneous Investigations Series Map 1-1028. Steckler, M„ and Watts, A. B., 1978, Subsidence Speed, R. C., 1977, Island-arc and other paleo- of the Atlantic-type continental margin off geographic terranes of late Paleozoic age in New York: Earth and Planetary Science Let- the western Great Basin, in Stewart, J. H., ters, v. 41, p. 1-13. Stevens, C. H., and Fritsche, A. E., eds., Stevens, R. K., 1970, Cambro-Ordovician flysch MANUSCRIPT RECEIVED BY THE SOCIETY Paleozoic paleogeography of the western sedimentation and tectonics in west New- MARCH 16, 1981 REVISED MANUSCRIPT RECEIVED United States: Society of Economic Paleon- foundland and their possible bearing on a SEPTEMBER 4, 1981 tologists and Mineralogists, Pacific Section, proto-Atlantic ocean: Geological Society of MANUSCRIPT ACCEPTED SEPTEMBER 9, 1981

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