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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 55 (1971)

Issue: 3. (March)

First Page: 454

Last Page: 477

Title: Tectonic and Sedimentologic History of Lower Jurassic Sunrise and Dunlap Formations, West-Central Nevada

Author(s): K. O. Stanley (2)

Abstract:

The Sunrise and Dunlap Formations (Lower Jurassic) are exposed sporadically in structurally complex mountains of the Basin Range province in west-central Nevada. Sunrise strata are generally older but appear to be in part age-equivalent of Dunlap rocks. The Sunrise Formation consists of 1,200-2,400 ft of marine carbonate rocks and mudstone. Dunlap clastic sedimentary rocks, more than 4,000 ft thick, are a heterogeneous mixture of four types: (1) easterly derived pure quartz sandstone, texturally and compositionally like the Navajo and Aztec Sandstones; (2) shallow marine and intertidal carbonates; (3) locally derived "orogenic" breccias, conglomerates, and sandstone; and (4) volcaniclastic sediments derived from local Lower Jurassic and Triassic eruptive volcanics, princi ally andesitic flows and coarse pyroclastics.

Shallow-marine Early Jurassic deposition in west-central Nevada was superseded by alluvial fan deposition during the late Early Jurassic. Stratigraphic relations suggest that fan sediments (Dunlap Formation), in part red, extended to the marine environment with the development in the transitional area of intertidal carbonate and red mud (Dunlap Formation).

The deposition of Dunlap Formation "orogenic" sediments was accompanied by late Early Jurassic warping and volcanism, but probably not by thrusting as suggested by Muller and Ferguson. This diastrophism appears to have been a local event, much like earlier structural disturbances that developed in the Cordilleran mobile belt during the Late Triassic and Early Jurassic Epochs in other areas related to a volcanic island arc system on the west in California. All the thrusting appears to be younger. Apparently, sedimentary conditions typical of the eastern Cordillera (miogeosyncline) extended uninterrupted across Nevada during the Early Jurassic. Thus, contrary to previous opinion, there was no persistent Mesocordilleran geanticline in central Nevada during the Early Jurassic.

Text:

INTRODUCTION

The Sunrise and Dunlap Formations are on the eastern margin of the Cordilleran eugeosyncline (Fraser Belt of Kay, 1947), and adjacent to the western edge of the so-called Mesocordilleran geanticline, a feature incorporated into many Early Jurassic paleogeographic maps of the United States (Fig. 1). Similarly situated Lower or Middle Jurassic rocks in the United States are known only in northwestern Nevada (Speed and Jones, 1969) and in southeastern California (Grose, 1959). Most Sunrise and Dunlap rocks are exposed in the Hawthorne and Tonopah one-degree quadrangles, particularly in the Gabbs Valley Range, Garfield Hills, and Pilot Mountains (Fig. 2). Isolated exposures also are present in surrounding mountain ranges and in the Westgate area on the north (Fig. 1). Rocks of less certai affinity, but included in the Dunlap Formation by Ferguson and Muller (1949) and Ross (1961), are in the Toiyabe Range, Gillis Range, and Southern Cedar Mountains (Fig. 1).

Because of the dearth of Lower Jurassic outcrops in large parts of the Cordilleran mobile belt (Fig. 1), Sunrise and Dunlap rocks are critical data-points that bear on any interpretation of Early Jurassic sediment dispersal or paleogeography for the Nevada part of the mobile belt as proposed by Stanley et al. (1971). Moreover, the onset of Jurassic diastrophism in west-central Nevada coincided with the beginning of Dunlap deposition (Muller and Ferguson, 1939). The extent of this diastrophism has not been defined heretofore for Early Jurassic time. The principal objective of this paper is to describe the petrology of Sunrise and Dunlap rocks, and particularly the tectonic and sedimentologic processes that influenced their deposition.

The classic works of Muller and Ferguson (1939) and Ferguson and Muller (1949) are the primary sources of present knowledge of Sunrise and Dunlap rocks. Exposures in the Union District (Silberling, 1959), Cedar Mountains (Mottern, 1962), and Pilot Mountains (Nielsen, 1964) have been remapped and the

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stratigraphy defined. Additional Sunrise rocks have been described in the Westgate area (Corvalan, 1962), and Sunrise-like lithology with Lower Jurassic fossils is reported in the Stillwater Range (Page, 1965) and Humboldt Range (Silberling and Wallace, 1969). These prior investigations have provided stratigraphic and structural bases for the present study. Local detailed mapping was undertaken to provide structural control for stratigraphic sections and the rotation of paleocurrent data.

Detailed maps were made only at areas in the Gabbs Valley Range, Garfield Hills, and Pilot Mountains, where marker beds, sedimentary structures, and "reasonable" exposure permitted the definition of local structure and the

Fig. 1. Index map of Nevada and adjacent states showing generalized locations of Lower Jurassic exposures (in black); Hawthorne and Tonopah 1° quadrangles (area I); boundary between Fraser and Millard belts (line II); western margin of hypothesized Lower Jurassic Mesocordilleran geanticline (line III); eastern margin of Mesocordilleran geanticline (line IV); and Nielsen's (1964) Luning embayment (line V). Lettered areas are: (B) Bridgeport, (C) Cedar Mountains, (E) Excelsior Mountains west of Huntoon Valley, (G) Gillis Range, (h) Humboldt Range, (P) Pine Nut Range, (s) Stillwater Range, (T) Toiyabe Range, (U) Union District,and (W) Westgate area. Small box is location of Figure 2.

Fig. 2. Geologic map of part of Hawthorne quadrangle modified from Ferguson and Muller (1949) and Nielsen (1964), showing principal Sunrise and Dunlap exposures and pre-Tertiary geology. Location shown on Figure 1.

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stratigraphic sequence. Aerial photographs (scale 1:24,000 and enlarged to 1:12,000) and new United States Geological Survey preliminary topographic maps of the Hawthorne quadrangle (1:24,000 and enlarged to 1:12,000) were used to plot data. Stratigraphic sections were taped only where detailed maps were made.

Generally, Dunlap rocks strike nearly east-west in the Toiyabe Range, Cedar Mountains (Mottern, 1962), Pilot Mountains (Nielsen, 1964), Gabbs Valley Range, and in the eastern Garfield Hills (Ferguson and Muller, 1949). In the western Garfield Hills the strike bends southward, and Lower Jurassic rocks disappear beneath Tertiary volcanic rocks (Ferguson et al., 1954). Imposed on this general pattern are folds trending nearly north-south and east-west (Ferguson and Muller, 1949; Nielsen, 1964).

STRATIGRAPHIC SETTING

Stratigraphic successions of late Paleozoic and early Mesozoic rocks in west-central Nevada have been studied in detail by Muller and Ferguson (1936; 1939) and Silberling and Roberts (1962), who recognized two Lower Jurassic formations in the Hawthorne and Tonopah quadrangles--the Sunrise and the Dunlap (Fig. 3). More recently, Nielsen (1964) has proposed a third formation, the Gold Range (Fig. 3).

Gold Range Formation

Rocks included in the Gold Range Formation by Nielsen (1964) are volcanic flows, breccias, and pyroclastic rocks intercalated with volcanic-derived sedimentary rocks that previously were included in either the Excelsior, Luning, or Dunlap Formations. In this paper the name Gold Range is applied to those rocks included in the formation by Nielsen (1964) and, in addition, lahars, tuff breccias, and tuffs intercalated with volcanic-derived sediments that unconformably overlie Excelsior strata in the southwestern Garfield Hills (Fig. 2, areas IV, V). These rocks originally were placed in the Dunlap Formation by Ferguson and Muller (1949), but are lithologically more akin to the clastic member of the Luning Formation exposed in the western Garfield Hills and the Gold Range Formation expose in the Excelsior Mountains, both of which unconformably overlie upturned Excelsior Formation beds (Fig. 3).

According to Nielsen (1964), the dominantly volcanic Gold Range Formation is equivalent to the Luning, Gabbs, Sunrise, and part of the Dunlap Formations--a relation I could neither verify nor refute. Parts of the Gold Range Formation are lithologically similar to the clastic member of the Luning Formation, hence correlation of the two formations seems reasonable. That Gold Range volcanics are correlative with Sunrise and Dunlap rocks is tenuous at best, being based solely on one Weyla (?) fragment found by Nielsen (1964) in a silty limestone unit exposed in the Excelsior Mountains (Fig. 2, area VII), which I believe to be in fault contact with surrounding Gold Range chert breccias. Volcanic rocks included in the Gold Range Formation, regardless of their age, are lithologically distinc from adjacent units, and therefore have been included in a separate formation to facilitate discussion of potential sources of detritus in the Dunlap Formation.

Sunrise and Dunlap Formations

Paleontology and regional correlations:
Ammonite-bearing Sunrise Formation rocks conformably overlie uppermost Triassic (Rhaetic) strata and contain faunas representative of all four standard Lower Jurassic ammonite stages (Hallam, 1965; Imlay, 1968). Similar faunas have been reported in mudstone and volcanogenic sequences in westernmost Nevada (Noble, 1962) and in eastern California along

Fig. 3. Correlation chart of Paleozoic and Mesozoic rocks in Hawthorne and Tonopah quadrangles, Nevada. Stratigraphic hiatus indicated by vertical ruling, lack of data by diagonal ruling.

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the crest of the Sierra from Ritter Range (Rinehart et al., 1959) northward to the Bridgeport quadrangle (Halsey, 1953), the Lake Tahoe area (Clark et al., 1962; Imlay, 1968; Loomis, 1961), Taylorsville (McMath, 1966), and to the Pit River (Sanborn, 1960).

In the Westgate area (Fig. 1) Sunrise shales bearing upper Toarcian fossils (Grammoceras) grade upward into quartz sandstone, which is conformably overlain by limestone with Middle Jurassic (Bajocian) fossils (Corvalan, 1962; Hallam, 1965). South in the Hawthorne and Tonopah quadrangles (Fig. 1), the youngest fossils reported in Sunrise limestone are Uptonia (lower Pliensbachian) from the Union District (Silberling, 1959) and Harpoceras (Toarcian) from the Gabbs Valley Range (Hallam, 1965). In these areas Sunrise limestone is conformably overlain by the Dunlap Formation (Fig. 4). The few diagnostic fossils found by Muller (Muller and Ferguson, 1939; Hallam, 1965) in the Dunlap Formation--specifically the reported presence of Harpoceras in the Garfield Hills a few hundred feet above it base--indicate an Early Jurassic age.

In Europe the known range of the ammonite genus Harpoceras is from upper Pliensbachian (spinatum zone) into upper Toarcian (thousarsense zone), which partly overlaps the upper Toarcian (thousarsense and levesquei zones) range of the genus Grammoceras, a relation also recognized in the western United States (Imlay, 1968, Fig. 6; Hallam, 1965). Both these genera have been reported in the Sunrise Formation in the Westgate area, in the Gardnerville Formation in the Pine Nut Range (Noble, 1962), and in the Sailor Canyon Formation (Imlay, 1968; Clark et al., 1962) west of

Fig. 4. Columnar section showing lithology of Sunrise Formation at type locality in Gabbs Valley Range (see Muller and Ferguson, 1936, 1939, for original description of type locality). Also shown are Hallam's (1965) suggested correlations (dashed lines) of principal Lower Jurassic sections in western Nevada.

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Lake Tahoe (Fig. 1). Dunlap rocks are, therefore, partly age-equivalent of all these formations. The age of the youngest strata in the formation is not known, but there is no paleontologic evidence that the Dunlap Formation extends into the Middle Jurassic (Hallam, 1965). Dunlap rocks are principally fanglomerates, and like modern alluvial fan deposits in the Basin-Range province, they probably were deposited rapidly during a short period of time. Hence, all Dunlap rocks could be early Jurassic.

Sunrise Formation lithologic succession:
Conformable uppermost Triassic (Gabbs Formation) and Lower Jurassic (Sunrise Formation) interstratified limestone and mudstone beds were first described and measured by Muller and Ferguson (1939), who separated them solely on paleontologic evidence. The type section in New York Canyon was remeasured as part of this study (Fig. 4). Stratigraphic sections in the Union District and Westgate area (Fig. 4) also were visited but not measured. No attempt was made to measure incomplete, poorly exposed, or complexly deformed Sunrise rocks in the Gillis Range, Pilot Mountains, and Garfield Hills. Instead, lithologic characteristics of rocks adjacent to known units (i.e., Dunlap Formation) or those with distinctive fossils were noted together with their relative time-stratigraphic position (e.g., H ttangian, etc.).

Mudstone with or without limestone interbeds is the dominant Sunrise lithology. However, a wide range of carbonate-mudstone ratios is present, from sparse, thin carbonate beds in mudstone to repetitive limestone-mudstone rhythms, to massive carbonate beds with shale partings (Fig. 4). Massive silty mudstone and shale characterize the mudstone units, but dark brown sandstone and white tuff units also occur (Fig. 4).

Carbonate beds are chiefly bluish-gray (5B 5/1 : 7/1) calcilutite and calcarenite wackestone, with lesser amounts of calcarenite packstone and grainstone (Dunham, 1962). Grayish orange (10 YR 7/4) dolomite and dolomitic limestone also occurs sparingly, but only near the Sunrise-Dunlap contact. Gray and brown mottling in some limestone beds, particularly those at the type area, is believed to be secondary. Such an interpretation is suggested by truncation of fossil fragments at sharp boundaries between areas of gray and brown weathering, with no discernible change in mineralogy or fabric. Generally, limestone beds are massive and devoid of internal primary bedding. Notable exceptions are laminated dolomite and dolomitic limestone beds and calcarenite grainstone beds, which locally are rossbedded and also exhibit internal erosion surfaces.

Sections of the Sunrise Formation, including those illustrated in Figure 4, were analyzed for possible vertical lithologic trends and lateral correlations, but results were largely inconclusive. No vertical or lateral trends could be recognized in carbonate-mudstone ratios or in the distribution of silty limestone, sandstone, and tuff, probably because of insufficient data and/or lack of such trends. However, a tendency was indicated for consistent regional vertical change in carbonate lithology. In the Gabbs Valley Range-Garfield Hills area and Union District, Sunrise carbonates pass upward from calcilutite and less abundant calcarenite wackestone at the base of the formation, through calcarenite wackestone and packstone, to calcarenite grainstone at the top of the formation. Laminat d dolomitic limestone and dolomite also are present locally above the grainstone and directly below Dunlap clastic rocks. These relations could not be recognized in the Westgate area.

In the Sunrise Formation, the absence of evidence for the jamesoni (lower Pliensbachian) through falcifer (lower Toarcian) zones in the Sunrise Formation led Hallam (1965) to suggest a period of nondeposition or erosion. In the Gabbs Valley Range, upper Sinemurian ammonites 15 ft below the top of unit g (Fig. 4) are within 50 ft of overlying harpocertid-bearing (Toarcian) mudstones (Hallam, 1965). Careful examination of this partly talus-covered stratigraphic interval revealed no evidence of erosion, and the exposed beds appeared to be conformable. However, limestone at the top of unit g is, in part, composed of phosphatic oolite and pellet grains (Fig. 4). Accumulation of phosphate grains in sediments, particularly high concentrations, are generally taken to indicate slow accumulatio of detrital material, both organic and inorganic, over a long period of time (Emigh, 1967). Hence, proximity of upper Sinemurian and Toarcian beds may reflect periods of very slow deposition represented only by the phosphatic limestones, and perhaps similar unrecognized beds in unit h. In addition, Hallam (1965) has suggested that unit c (Fig. 4), a 10-ft-thick oolitic bioclastic limestone, is a stratigraphically condensed deposit representing parts of two Sinemurian ammonite zones. This is supported

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by the presence of phosphatic oolite and echinoderm grains in the unit.

Dunlap Formation lithologic succession:
Vertical and lateral variations in Dunlap lithology must be inferred from small and scattered outcrops (Fig. 1). Attempts to map units or marker beds in the formation proved futile in all but a few outcrops in the Gabbs Valley Range, Garfield Hills, and Pilot Mountains (Fig. 5). At other localities, Dunlap strata are complexly deformed, lack distinct marker units to delineate structure and, locally, are poorly exposed.

Unlike subadjacent strata, Dunlap sedimentary rocks are mostly dusky-red (SR 3/4) and reddish-brown (10 R 3/4) breccia, conglomerate, sandstone, and mudstone. Also abundant are eruptive volcanic rock and gray sedimentary breccia and conglomerate (Fig. 6). Intercalated with most of these rock types are minor amounts of bluish-gray (5B 7/1) and grayish-orange (10 YR 7/4) carbonate, and white, fine-grained, pure quartz sandstone. The maximum preserved thickness of Dunlap strata is about 5,000 ft reported by Ferguson and Muller (1949) and Nielsen (1964) in the western Pilot Mountains.

Where Dunlap rocks overlie limestone of the Sunrise Formation, the contact is nearly everywhere gradational, and basal Dunlap units vary from impure quartz sandstone (Union District) to volcanic sandstone and conglomerate (Fig. 2, area II), to limestone conglomerate (area I), to sandstone composed of volcanic and limestone detritus (areas I, III). However, where Dunlap rocks overlie upturned Luning, Gold Range, or Excelsior strata, basal Dunlap units reflect the composition of underlying rocks (Fig. 6). Beds above the basal units were measured and described in detail where sedimentary structure (scour and fill or crossbedding), marker beds, and relatively "simple" structure provided some degree of confidence in local stratigraphy (Fig. 5). Stratigraphic sections in these areas show no consistent vertical lithologic succession (Fig. 6); rather, the formation is characterized by numerous local successions which are confined to a single outcrop or mountain range, and which cannot be correlated with adjacent stratigraphic sections. The lack of a consistent vertical lithologic succession in the Dunlap Formation, as well as variation in basal Dunlap lithology, seems to imply either (1) abrupt intertonguing of Dunlap units derived from complex source terranes, (2) slight variation in the age of Dunlap rocks exposed at different outcrops, (3) lack of continuous Dunlap deposition between areas of present exposure, or (4) combinations of these alternatives.

Abrupt intertonguing of Dunlap units has been documented by the mapping of Nielsen (1964) and Ferguson and Muller (1949) in the Pilot Mountains and Garfield Hills. However, reconstruction of lateral change in lithology and how abruptly it takes place is severely restricted by the paucity of continuous exposures where interfingering can be mapped, as in the Pilot Mountains and Garfield Hills, and by post-Early Jurassic deformation, particularly thrusting of slightly different stratigraphic facies into juxtaposition (Ferguson and Muller, 1949, Pls. 2, 7). Intertonguing of Dunlap and Sunrise strata, and perhaps slight variation in the age of basal Dunlap units, is suggested not only by paleontologic evidence cited heretofore, but also by stratigraphic relations. Both the presence of Dunl p-like conglomerate in the Sunrise Formation exposed in the Union District (Fig. 4) and the gradational transition from Sunrise limestone into Dunlap conglomerate exposed in the Gabbs Valley Range (Fig. 6) suggest intertonguing of Sunrise and Dunlap facies. Variation in the age of basal Dunlap units also is suggested by buttressing of Dunlap strata against cherts of the Excelsior Formation exposed in the Pilot Mountains (Nielsen, 1964, Pl. 2).

VOLCANIC ROCKS

Volcanic activity during the Early Jurassic in, or around, west-central Nevada is recorded by tuff beds and volcanic detritus in the Sunrise Formation, and volcanic flows, pyroclastics, and volcanic detritus in the Dunlap Formation. The volcanic rocks intercalated with Dunlap sedimentary rocks are dominantly andesite, although minor amounts of more silicic material, principally dacite tuff and tuff breccia, are present in the Gabbs Valley Range. Flows are, without exception, andesites containing plagioclase phenocrysts (core labradorite, rim andesine) set in a groundmass of plagioclase microlites (andesine?) and/or iron oxide. Less abundant are iron oxide pseudomorphs of hornblende and pyroxene, which in a few crystals exhibit euhedral outline and cleavage angles.

Andesitic eruptive rocks are amygdaloidal flows and flow breccias with preserved pilotaxitic and porphyritic textures; flows with relic hyalopilitic textures; and devitrified vitric, crystal-lithic,

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Fig. 5. Geologic maps of select areas in Gabbs Valley Range, Garfield Hills, and Pilot Mountains showing locations of stratigraphic columns illustrated on Figure 6, and location of "thrust conglomerate" mapped by Ferguson and Muller (1949, Pl. 7) in Pilot Mountains. Lines of measured section for columns A (left) and C (right) in Garfield Hills, and columns A and C in New York Canyon area, Gabbs Valley Range, are shown on geologic maps, but Pilot Mountains column is composite section based on relations shown on Dunlap Canyon area map. No map is shown for column B in Gabbs Valley Range nor column B in Garfield Hills, both of which were measured in areas mapped by Ferguson and Muller (1949, Fig. 5, Pl. 2) and field checked during this study. Locations of these columns are shown on index ap (areas 6, 7).

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Fig. 5. Continued. See caption on page 460.

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Fig. 6. Columnar sections of Dunlap Formation showing lithologic character and variation. Breaks in columns indicate that thickness of unit is not known because either poor exposures or local complex structure prohibits accurate measurements. Locality of each section is shown on Figure 5.

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and crystal tuffs. Most tuffs lack relic textures or have only vague ghosts of shards. Preserved vitroclastic texture is indicated by iron oxide films that outline bubble-wall shards. Hyalopilitic texture in flows is suggested by interstitial fibrous chlorite or iron oxide between microliths in groundmass or between phenocrysts. Most dacitic and more silicic rocks (i.e., rhyolite) in the Dunlap Formation are devitrified tuff and tuff breccia with quartz and plagioclase (oligoclase) crystals, as well as sanidine in more silicic rocks, set in a cryptocrystalline or microcrystalline mosaic of quartz and feldspar. Glassy materials in Dunlap volcanics are devitrified and, like microlites and phenocrysts, are generally partly or wholly replaced by calcite or, less commonly, by sericite.

Andesite lava flows with "prominent square plagioclase phenocrysts" have been described by Ferguson and Muller (1949, p. 18, Fig. 3) intercalated with Dunlap sedimentary rocks. However, these rocks exhibit discordant relations with surrounding strata, contain engulfed fragments of overlying rocks, and exhibit baked upper and lower contacts, all of which suggest that they are intrusive dike- and sill-like bodies. Hence, they have been excluded from the Dunlap Formation in this report, although they may represent feeders for later Dunlap volcanic eruptions.

CONGLOMERATE AND SANDSTONE PETROGRAPHY

The terrigenous mineralogy of Sunrise and Dunlap rocks has been determined by point counts of 250 thin sections, pebble counts, and X-ray diffraction patterns. A "typical" sample basis was used to choose specimens in the field for analysis. This collecting method was designed to select samples within individual, lithologically homogeneous units that would show variations between units within outcrops; between outcrops in isolated mountain ranges; and between ranges. It was the only feasible method in light of the distribution of outcrops, complexity of structure, and time available for study. Specimens usually were collected from the base, middle, and top of each unit, but samples collected between outcrops and mountain ranges were determined largely by the scattered and isolated natu e of Sunrise and Dunlap rocks. Samples were collected from the Sunrise and Dunlap outcrops in the Hawthorne and Tonopah quadrangles shown on Figures 1 and 2.

Rudites

Pebble counts were made in the field on rudites judged to be typical of the Dunlap Formation (Table 1). The data exemplify the components present in Dunlap "chert," limestone, and volcanic sedimentary breccia and conglomerate. The relative abundance of components shown on Table 1 is characteristic of the majority of Dunlap rudites. Locally, however, the abundance of chert, limestone, and volcanic clasts is extremely variable, resulting in limestone-chert and limestone-volcanic rudites. Volcanic clasts are amygdaloidal and porphyritic flow rocks, devitrified vitric tuffs, lithic crystal tuffs, and crystal tuffs of andesitic through rhyolitic composition. Limestone clasts are principally bluish-gray calcilutite. However, in the Pilot Mountains carbonate clasts are largely silicified lim stone and dolomite composed of aggrading neomorphic rhombs. Similarly, grayish-orange aphanitic dolomite clasts comprise the detritus in Dunlap conglomerate near the base of the formation in the Garfield Hills (Fig. 6).

Clast compositions indicate that the rudites of the Dunlap Formation were derived from a provenance composed of volcanic and sedimentary rock. Most clasts in the rudites are from a local source in the Hawthorne and Tonopah quadrangles, including reworked Dunlap beds.

Arenites

Modal analyses of Sunrise and Dunlap sandstones (Table 2) indicate three distinct suites: (1) volcanic arenite and wacke; (2) chert arenite and wacke ranging to quartz-chert arenite; and (3) quartz arenite (Williams et al., 1954, p. 298-324). Only volcanic sandstones are present in the Sunrise Formation (Fig. 7A). Modal analyses of all rocks were done with the aid of a Swift automatic point-counting apparatus. Some samples were stained (Bailey and Stevens, 1960) to aid in distinguishing among feldspars and also volcanic rock fragments. The major difficulty encountered in point counting Sunrise and Dunlap sandstones was the differentiation of devitrified volcanic rock fragments and sedimentary chert. In thin section, these two components were separated on the bases of texture, particul rly vitroclastic textures, and the presence of well-rounded quartz fragments versus quartz or plagioclase crystals. It is possible, however, that nearly all of the grains classed as chert herein are volcanic

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in origin. In any case, double counts by the same operator gave consistent results which suggests that any counting errors are systematic.

The most noteworthy features of Dunlap sandstones are the absence of plutonic and metamorphic rock fragments, and the sharp mineralogic contrast between the three sandstone suites (Table 2), particularly the quartz-poor nature of chert and volcanic sandstone in contrast with the pure quartz sandstones (Table 3). Also notable are the quartz types in the sandstone suites. Five distinct quartz types are recognized: (1) angular, monocrystalline straight-extinction grains; (2) euhedral and subhedral bipyramidal quartz crystals, usually embayed; (3) angular polycrystalline grains; (4) well-rounded, monocrystalline, straight to undulose-extinction grains with some microlites and vacuoles; and (5) well-rounded, polycrystalline, undulose grains. Quartz types 1 and 2 are characteristic of Dunla volcanic arenite (Fig. 7), as are sparse chalcedony grains. Quartz types 3 and 4 are characteristic of "chert sandstones," and types 4 and 5 of pure quartz sandstone (Fig. 7). In addition to the sandtone suites shown on Table 2, there is a relatively small amount of lithic arenite in the Dunlap Formation composed in places almost entirely of limestone fragments, and in other places of mudstone fragments. Lithic arenite composed of limestone detritus is commonly associated with limestone rudite, and may contain chert and volcanic grains.

Terrigenous sand-size material in the Sunrise Formation is restricted to thin brown sandstone beds associated with mudstone, and sand-size grains in carbonate units. These grains almost invariably are andesite fragments or plagioclase laths, and rarely chert (Fig. 7). At the type area, unit c of the Sunrise Formation (Fig. 4) contains about 2 percent andesite fragments. The clastic content of the carbonate rocks is variable. Most contain scattered silt-size monocrystalline, angular, straight-extinction quartz grains; they grade into calcareous quartz siltstones. Sunrise mudstones contain quartz silt and, in addition, chlorite and sericite (based on X-ray diffraction patterns). This mineralogy could reflect alteration of original volcanic material.

Diagenesis

Various amounts of cement, altered framework grains, and pressolved grain boundaries are present in Dunlap sedimentary rocks. The principal secondary mineral in these rocks is calcite, both as interstitial cement and as an alteration or replacement product in framework grains. Framework grains, particularly volcanic

Table 1. Average of Results of Regional Modal Analyses of Dunlap Rudites

Table 2. Averages, Ranges, and Standard Deviations of Results of Modal Analyses of Dunlap Arenite Types

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rock fragments and plagioclase, are partly replaced by calcite or, less commonly, by iron oxide, white mica, chlorite, dolomite, and chert. Where calcite cement is present, grain surfaces are etched. In addition to calcite, the following mineral cements are present in Dunlap sedimentary rocks: chlorite, hematite, chert, and quartz overgrowths. However, quartz overgrowths are uncommon and occur only in chert sandstone. Pressolved grain boundaries are most evident in quartz sandstone not cemented with calcite, and both slightly and moderately pressolved boundaries can be recognized (Thomson, 1959, p. 98-99). More intense alteration occurred only where igneous bodies or mineralized zones intrude Dunlap rocks.

PROVENANCE

Local Sources

There is little doubt as to the local origin of most terrigenous detritus in the Sunrise and Dunlap Formations. Muller and Ferguson (1939) first recognized Sunrise, Luning, and Excelsior Formation detritus in the Dunlap; their interpretation has been accepted by Ross (1961) and Nielsen (1964). Comparison of the petrography of rudite clasts and sandstones from each of the three Dunlap sandstone suites (Table 2) with pre-Dunlap rocks in the Hawthorne and Tonopah quadrangles reveals similarities with limestones of the Sunrise, Gabbs, and Luning Formations; volcanics of the Gold Range and Dunlap Formations; cherts

Click to view image in GIF format. Fig. 7. [Grey Scale] A. Photomicrograph of Sunrise Formation mudstone and volcanic wacke showing volcanic fragments (V) and plagioclase (P). Note shape of silt-size grains in mudstone, many of which appear to be rectangular plagioclase laths. X-ray diffraction patterns of mudstones indicate presence of quartz, plagioclase, chlorite, and sericite. Sample collected 40 ft above unit C in Sunrise Formation at type locality, Gabbs Valley Range.

B. Photomicrograph of Dunlap volcanic arenite showing (V) variety of volcanic fragments; (P) plagioclase; and (Q) angular monocrystalline quartz grains. Sample collected from crossbedded sandstone unit exposed in New York Canyon, Gabbs Valley Range (Fig. 5).

C. Photomicrograph of Dunlap "chert" arenite showing (Q) well-rounded monocrystalline quartz grains and (C) chert fragments. Note similarity of so-called chert fragments and devitrified matrix of volcanic fragments shown in B. Sample collected adjacent to Dunlap quartz arenite unit exposed in Pilot Mountains, a relation that emphasizes marked contrast between Dunlap pure quartz sandstone (Fig. 8) and adjacent chert sandstones.

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of the Excelsior Formation; and chert conglomerates and sandstones of the Luning Formation (Stanley, 1969). Quartz arenite and chert-quartz arenite clasts in Dunlap conglomerate are similar to clasts in the clastic member of the Luning Formation, as well as quartz sandstone intercalated with volcanics of the so-called "Excelsior Formation" exposed in northern Nye County (Ziony and Kleinhampl, 1968) and in the southern Gillis Range. However, they are compositionally and texturally unlike Dunlap quartz arenite. Reworked Dunlap carbonate, volcanic, and mudstone debris also is in Dunlap sedimentation units, particularly where underlying Dunlap units have been scoured. In fact, much of the volcanic detritus that makes up Dunlap sandstone and conglomerate could have been derived from local unlap volcanic rocks.

Terrigenous detritus in sandstone and impure limestone of the Sunrise Formation is andesite rock fragments, plagioclase laths, and silt-size quartz, probably of volcanic origin. Like bioclastic debris in the Sunrise Formation, terrigenous material indicates a local source, namely Early Jurassic volcanism.

Distant Sources

Although most of the rudite and arenite in the Dunlap Formation is composed of detritus derived from erosion of local Mesozoic sedimentary and volcanic rocks exposed in the Hawthorne and Tonopah quadrangles, Dunlap quartz arenite is compositionally and texturally distinct from these sources, and from other Dunlap arenites (Table 3, Fig. 8). Distinctive features of the quartz arenite are (1) their compositional and textural maturity; (2) their mineralogy, particularly the presence of microcline and polycrystalline, well-rounded quartz

Table 3. Mineralogic and Textural Comparison of Dunlap Quartz Arenites with Local Sources and Distant Navajo Quartz Arenite

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grains; and (3) their size distribution. Each of these features is not duplicated in adjacent Dunlap sandstones or in Ordovician through Triassic rocks exposed in the Hawthorne and Tonopah quadrangles (Table 3). However, compositional and textural similarities between Dunlap and Navajo-Nugget Sandstone are striking (Table 3). Both contain microcline and polycrystalline quartz grains in similar proportions. Also similar are heavy mineral fractions composed principally of tourmaline and zircon. Tourmaline in both Dunlap and Navajo-Aztec sandstones exhibit the same pleochronic formula. Hence, texture, as well as light and heavy mineral assemblages present in Dunlap and Navajo quartz sandstones, are in agreement with the hypothesis of a common source.

Quartz sandstone similar to quartz arenite in the Dunlap Formation has been reported in the Westgate area conformably overlying upper Lower Jurassic Sunrise strata (Corvalan, 1962); in the Humboldt Range conformably on probably Lower Jurassic rocks (Speed and Jones, 1969); and in the Stillwater Range as part of the Lower (?) or Middle Jurassic (?) Boyer Ranch Formation that overlies unconformably Upper Triassic rock (Speed and Jones, 1969). Quartz sandstones of the Boyer Ranch Formation have been interpreted by Speed and Jones as subaqueous sand reworked from Navajo coastal dunes. Navajo-like sandstone, probably of Early Jurassic age, also has been reported near Currie, Nevada (Fig. 1) by Wheeler et al. (1949) and near Baker, California, by Grose (1959). Hence, quartz sandstone, miner logically and texturally akin to the Navajo Sandstone, is present at several localities in western Nevada and southeastern California. Its widespread distribution suggests that Navajo sand was dispersed across much of what is now western Utah and eastern Nevada, and that distal Navajo deposition took place as far west as the Hawthorne and Tonopah quadrangles in Nevada, as well as in southeastern California. Farther west along the crest of the Sierra Range and in the Pine Nut Range of westernmost Nevada, no pure quartz arenite was deposited; there, rocks are entirely andesite flows, pyroclastics, and sediments derived principally from volcanic sources.

Quartz-sand transport into, and deposition in, Nevada is believed by Stanley et al. (1971) to have taken place in a widespread, relatively shallow sea, with only minor variation in bottom topography and sharp variation in water circulation and agitation. Different sediment environments (i.e., quartz sand, carbonate, and mud), oriented approximately parallel with the shoreline, would reflect equilibrium conditions between bottom topography, water circulation, and organic activity. Such belts would migrate according to changes in marine currents, supply of sand, and changes of relative sea level.

DEPOSITIONAL ENVIRONMENTS

Conditions in western Nevada during the Early Jurassic led to a pattern of sediment transport and deposition interpreted by Muller and Ferguson (1939) to represent continental alluvial fan (Dunlap Formation) and marine (Sunrise Formation) deposition. Intertonguing alluvial fan marine deposits are nowhere actually fully exposed; therefore, relations between products of these two environments must be inferred from vertical lithologic sequences

Click to view image in GIF format. Fig. 8. [Grey Scale] A. Photomicrograph of Dunlap pure quartz arenite showing grain texture and microcline (m). B. Photomicrograph of Navajo sandstone showing similarity between Navajo and Dunlap quartz sandstones both as to texture and composition.

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(Fig. 6). Such relations suggest that alluvial fan deposition extended to the marine environment with the development of local supratidal (?) dolomite and limestone, as well as intertidal red muds (Fig. 9).

Red Alluvial Fan Deposits

Nearest the source area, alluvial fan deposits are unbedded or poorly bedded sandy conglomerates. These units are of debris-flow and water-flow origin. Outward from the source area, fan deposits are principally of water-flow origin. These deposits include sheetlike, planar-bedded sandy conglomerate, sandstone with rare crossbedding, scour channel deposits, and interbedded mudstone and ripple-laminated siltstone.

Dunlap fanglomerates are characterized by their red color, which is absent from older rocks and from the partly contemporaneous Sunrise sediments. Red pigment typically appears as stain both on matrix and grains, and as hematitic cement in sandstone and siltstone beds. The red color was not inherited from local source rocks, but appears to have formed after the sediments were deposited. This conclusion is supported by the presence of hematitic cement in cross-laminated siltstone, red jasperized fossil wood, and the absence of red pigment in Dunlap pure limestone fanglomerate. The formation of red pigment in Dunlap sediments appears to be related, therefore, to post-depositional conditions. Such conditions have been outlined by Walker (1967) for recent fan deposits in Baja California. ediments that characterize the latter sequence are coalescing alluvial fan deposits, separated from the Gulf by a broad intertidal mud and salt flat with dune sands, widely spaced shell-rich beach ridges, and low tide terrace sands (Walker, 1967, p. 355-356). The red fanglomerates of the Dunlap Formation are interpreted herein as ancient analogues of the redbeds described by Walker (1967).

The association of debris-flow and sheetlike water-flow deposits, as well as red pigment and desiccation cracks, implies that the Dunlap fanglomerates, like the Baja California sediments, were deposited in a somewhat arid region.

Debris-flow deposits:
Dunlap Formation conglomerates interpreted as debris-flow deposits are most common in the Pilot Mountains, Gabbs Valley Range, and to a lesser extent, in the Garfield Hills. These deposits consist of cobbles and boulders imbedded in a matrix of finer material. They are poorly sorted, and individual units are unstratified (Fig. 10). Angular to subrounded silt- to cobble-size material, mostly of grain-support type, is dominant in these deposits. Embedded in this material are boulders and blocks up to 7 ft in diameter (Fig. 10).

Single Dunlap debris-flow beds are typically 2-10 ft thick, and do not exceed thicknesses of 15 ft. They have sharp and well-defined bases that appear to overlap the underlying deposits (Fig. 10). Many deposits have basal protrusions similar to scour channels, and they tend to have flat tops that grade abruptly into planar-bedded sandy conglomerate. However, some debris-flow deposits in the Gabbs Valley Range have sharp erosional contacts overlain by crossbedded limestone-pebble conglomerate. Modern subaerial debris-flow deposits in arid regions of California have been studied in detail (Hook, 1967; Sharp and Nobles, 1953),

Fig. 9. Idealized restored cross section of Dunlap sediments showing their relation to contemporaneous Sunrise Formation strata, and to underlying rocks of Sunrise, Luning (including Gold Range Formation of Nielsen, 1964), and Excelsior Formations. Line of section extends from southern Pilot Mountains north to Westgate area (Fig. 1).

Click to view image in GIF format. Fig. 10. [Grey Scale] Dunlap Formation debris-flow deposit, Gabbs Valley Range. Note sharp base and large blocks floating in finer material.

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and provide an excellent comparison for Dunlap deposits. Dunlap debris-flows are similar in texture, lack of stratification, and in their sharp bases, flat tops, and thickness.

Dunlap debris-flow deposits, like modern debris-flows (Bull, 1968, p. 102), probably originated in an area of high relief with only moderate vegetation, and were associated with tectonic activity. The fact that many Dunlap debris-flow beds are composed chiefly of one kind of material (either limestone, chert, or volcanic) suggests that each flow began from a limited source. Deposits composed of mixtures of these components (that is limestone-chert deposits in the Pilot Mountains) may represent more complex source areas.

Water-flow deposits:
Dunlap alluvial fan deposits are principally water-flow deposits, including both channel and sheetlike units. Scour channels 1-10 ft thick are filled with crossbedded pebble conglomerate, sandstone, or planar-bedded conglomerate; a few channel deposits also contain fossil logs up to 10 ft long. Crossbedding in channels consists of small- to medium-scale tangential tabular sets with inclinations of up to 20°. Current directions indicated by crossbeds in channels are parallel with directions indicated by similar structures in adjacent sheetlike deposits, hence channel and sheetlike sediments presumably were deposited by related processes.

Sheetlike deposits represent deposition from both the upper and lower flow regimes (Simons et al., 1965). Upper flow regime deposits are red conglomeratic sandstone and sandy conglomerate characterized by planar-bedding (Fig. 10). Sole marks are present locally on the undersurfaces of these deposits, whereas mudstone tops, where present, commonly exhibit desiccation cracks and clay galls. Within the individual sheetlike deposits the only sedimentary structures present are pebble imbrications and a few, low-angle (less than 15°), nontangential tabular crossbeds. Basal contacts of these sheets are typically gradational, and units grade vertically and laterally from sandy conglomerate into conglomeratic sandstone (Fig. 11). Lower flow regime sediments are characterized by finer grai ed strata, including medium-scale crossbedded and ripple-laminated sandstones.

These deposits are morphologically and texturally similar to recent water-flow sediments exposed on dissected alluvial fans in Nevada. The gradational contacts, planar-bedding, low-angle tabular crossbedding, and pebble imbrication in Dunlap sheetlike water-flow deposits are similar to those in modern fans. In addition, channel deposits are common, a feature also typical of modern fans (Bull, 1968; Hooke, 1967).

Tidal Flat Red Mudstones

Dunlap Formation rocks interpreted as ancient tidal flat sediments include red mudstone and interstratified, red ripple-laminated siltstone and silty mudstone. These deposits are associated with marine carbonate, quartz sandstone, and red alluvial fan deposits in the Cedar Mountains, Garfield Hills, Gabbs Valley Range, Excelsior Mountains west of Huntoon Valley, and Pilot Mountains (Fig. 6). The red mudstones contain impressions of marine fossils, usually high-ribbed rhynchonellids. Sedimentary structures are abundant, principally ripple-laminae in siltstone, very-fine sandstone, and silty mudstone. Desiccation cracks and clay galls also have been recognized. Walker (1967, p. 355, 361) found similar intertidal red mudstone and fine-grained sandstone in Baja California associated with liocene red fanglomerate and shell-rich beach ridges. Recent intertidal nonred mud there exhibits similar facies relations. Hence, Dunlap red mudstones are interpreted as an ancient analogue of those because of their lithology, facies relations, and fossils.

Intertidal Carbonate

Grayish-orange laminated dolomite and dolomitic limestone units of probable intertidal or supratidal origin, 2-30 ft thick, are present sporadically in the Dunlap Formation associated with crossbedded quartz sandstone, red mudstone, and sandy conglomerate, are present locally in the Sunrise Formation directly below

Click to view image in GIF format. Fig. 11. [Grey Scale] Dunlap Formation water-flow deposit, Garfield Hills, showing planar-bedding and lateral and vertical changes.

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clastic sediments of the Dunlap. These deposits are scarce, but outcrops are well exposed in the Gabbs Valley Range, Garfield Hills, and Union District (Fig. 1). Their most characteristic features are laminations (0.1-4.0 mm), which commonly are discontinuous and locally form laterally linked hemispheroids 6-10 mm high and 20 mm long (Fig. 12); laminae converge at the margins of the hemispheroids. The resultant structures are similar to algal mat and laterally linked stromatolite structures described by Roehl (1967) from intertidal and supratidal deposits on Andros Island, and by Ginsburg et al. (1954) from Florida Bay. Primary sedimentary structures other than the laminae are uncommon, principally desiccation cracks and less common flat-pebble breccia. Flat-pebble breccia forms beds nd channel-like deposits. The clasts are similar in size and shape to in-place desiccation polygons. These structures are like those described in supratidal deposits in the Bahamas (Roehl, 1967, p. 2006-2014).

Two types of dolomite are in laminated carbonates: (1) aggrading neomorphic dolomite rhombs (0.2 mm) in dolomitic limestone, and (2) microcrystalline dolomite. The latter is similar in grain size (less than 0.02 mm) to recent penecontemporaneous intertidal and supratidal dolomite reported from south Florida by Shinn and Ginsburg (1964), Andros Island by Shinn et al. (1965) and the Persian Gulf (Illing et al., 1965). Therefore, these microcrystalline dolomites are interpreted as ancient analogues of recent penecontemporaneous dolomite.

Subtidal Marine Deposits

The most significant variables in subtidal limestone and mudstone of the Sunrise Formation are the framework grains and their enclosing matrix. The chief grain types are skeletal fragments, particularly echinoderm, pelecypod, and gastropod debris. Less common are brachiopod, cephalopod, and very sparse coral fragments. Nonskeletal grains include pellets, composite grains, intraclasts, oolites, and less abundant

Click to view image in GIF format. Fig. 12. [Grey Scale] A. Negative print of acetate peel of Dunlap Formation laminated limestone with stromatolite-like structures, Gabbs Valley Range.

B. Photomicrograph of Sunrise Formation mixed fossil calcarenite grainstone showing micrite rim on gastropod (C) and brachiopod (B) fragments. Also present are echinoderm fragments with syntaxial overgrowths (D), intraclasts (A), and pellets. Sample collected 20-ft below Sunrise-Dunlap contact in Gabbs Valley Range.

C. Photomicrograph of Sunrise phosphatic calcarenite packstone showing phosphate pellets with coatings of black and light-brown collophane laminae to form collophane oolites (A), phosphatic oolites replaced by crystalline calcite (B), and phosphatized echinoderm fragments (C). Sample collected from unit C of Sunrise Formation at type area in Gabbs Valley Range.

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oncolites and phosphatic grains (Fig. 12). Phosphate pellets, oolites, and phosphatized echinodermic material comprise up to 30 per cent of bed c and limestone at the top of unit g in the Sunrise Formation (Fig. 4). Oncolites in both Sunrise and Dunlap limestones are concentrically stacked spheroids of the type described by Logan et al. (1964) from shallow water environments. The abundance of micrite and marine fossils, together with the absence of current structures within limestones in the lower part of the Sunrise Formation and, locally, in the Dunlap Formation, suggests deposition under normal marine conditions with no reworking by currents or waves. Between these deposits and Dunlap clastic rocks are calcarenite packstone and grainstone units (Fig. 12) of the Sunrise Formation an , locally, the laminated dolomite and dolomitic limestone units. The calcarenite grainstone-packstone lithology is characterized by well-sorted skeletal debris (echinoderm and gastropod), low carbonate mud, and locally good current stratification. These beds are interpreted as indicative of subtidal marine deposition within the zone of wave action. The dearth of terrigenous material, except for volcanic-derived detritus and some phosphatic material, indicates deposition offshore on a relatively stable platfrom away from areas of Early Jurassic tectonism and volcanism.

Ambiguous Deposits

Dunlap quartz sandstone and limestone-pebble conglomerate units, both crossbedded and well sorted, represent two lithologies for which no specific depositional environment could be defined. Both are characterized by low-angle, small- to medium-scale (less than 3 ft), tabular and trough crossbeds; and both are associated with subtidal carbonate beds and redbeds. It is believed that the quartz sandstone units are strandline deposits, as suggested by Nielsen (1964, p. 69), or perhaps submarine sand waves formed in strongly agitated water. Similarly, the limestone conglomerates could represent beach or nearshore deposits, but definite criteria to determine environments are not available.

EARLY JURASSIC DIASTROPHISM AND DUNLAP SEDIMENTATION

Knowledge of the nature of Early Jurassic diastrophism in west-central Nevada and of its relation to Dunlap sedimentation is still largely speculative. Reliable information from the region is so sparse that attempts at interpretation have been based mainly on data from limited observations in the Pilot Mountains, Garfield Hills, and Gabbs Valley Range. From their examination of these critical areas, Muller and Ferguson (1939) and Ferguson and Muller (1949) recognized seven stages of Jurassic diastrophism, two of which were presumed contemporaneous with Early Jurassic Dunlap deposition: (1) warping on the southern margin of the Luning embayment, a hypothetical local bight in the shoreline of a Late Triassic-Early Jurassic seaway in Nevada (Fig. 1), and (2) development of local structur l troughs that received detritus from upfolded Luning limestone and advancing surface thrusts. This view of Early Jurassic diastrophism and Dunlap sedimentation generally has been accepted (Gilluly, 1963, 1967), but the unexpected nature of some of the evidence collected during this study indicates that these traditional ideas of Early Jurassic structures and Dunlap sedimentation may need revision.

As noted by Ferguson and Muller (1949), deposition of Dunlap sediments marked the beginning of Early Jurassic diastrophism and followed the accumulation of conformable marine carbonates of the Luning, Gabbs, and Sunrise Formations. The present eastern limit of these carbonates has been regarded by Ferguson and Muller (1949, p. 5) as the approximate extent of a Mesozoic sea in Nevada--a premise believed supported by the southward increase in chert conglomerate in the clastic member of the Luning Formation, and cited as evidence for the Luning embayment. However, the mere absence of marine strata of late Late Triassic and Early Jurassic ages beyond their present extent is not alone sufficient evidence for postulating a shoreline in the Hawthorne and Tonopah quadrangles, the southern mar in of the Luning embayment. Furthermore, uppermost Triassic (Luning and Gabbs Formations) and Lower Jurassic (Sunrise Formation) carbonates show no lateral changes that suggest a nearby and continuous shoreline; rather, they are deposits more characteristic of shallow quiet water below the zone of current and wave action (Silberling, 1959, p. 14-18; Mottern, 1962; Nielsen, 1964, p. 35-36).

Basal Dunlap Unconformity

Whether the Luning embayment is accepted or rejected as a working hypothesis of Early Jurassic paleogeography directly influences interpretation of the unconformity beneath the Dunlap Formation. In northernmost exposures

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(Union District, Gabbs Valley Range, northwestern Garfield Hills), Dunlap rocks are conformable on limestone of the Sunrise Formation. South and southeast of these areas Dunlap sediments unconformably overlie the Luning Formation which, like Dunlap rocks farther south, overlies bedded chert of the Excelsior Formation with marked angularity (Ferguson and Muller, 1949). Generally, there is an increase in angular discordance southward between Dunlap and underlying formations. It is possible that southward increases in angularity resulted from (1) warping and erosion of Luning rocks, exposing the more intensely deformed Excelsior strata beneath the basal Luning unconformity, or (2) warping and erosion of Excelsior rocks exposed beyond the depositional limit of Luning and Sunrise strata. T e latter interpretation was favored by Ferguson and Muller (1949, p. 9), who suggested that Dunlap sedimentation roughly paralleled the southern shoreline of the Luning embayment and extended beyond the southern margin of the Luning. However, I prefer to believe the former interpretation (Fig. 9). I could find no data from Lower Jurassic rocks that indicate a nearby continuous shoreline, or the derivation of terrigenous detritus from anywhere other than local tectonic (chert conglomerate) and volcanic (submarine and subaerial volcanic rocks described by Nielsen, 1964) sources. Moreover, the individual Dunlap debris- and water-flow deposits exposed in the Pilot Mountains, Toiyabe Range, and Excelsior Mountains west of Huntoon Valley (Fig. 1) contain both chert and carbonate detritus indic ting a common source terrane. As noted by Ferguson and Muller (1949, p. 31), chert was derived from Excelsior rocks, which were present on the south. Dolomite and limestone detritus in these units was derived from the Luning Formation. Hence, although long since removed by erosion, Luning carbonates must have existed beyond the postulated southern margin of the Luning embayment. However, removal of Luning strata prior to the deposition of Dunlap sediments on folded Excelsior beds may not have been responsible solely for the apparent southward increase in angular discordance between Dunlap and underlying rocks. It is possible also that Excelsior beds beneath Dunlap units in the Pilot Mountains and southern Cedar Mountains underwent more intense Early Jurassic deformation than less deforme Luning strata exposed beneath Dunlap rocks on the north in the Garfield Hills and northern Cedar Mountains. Available data do not indicate whether or not such variations are significant.

Sediment Dispersal

According to Ferguson and Muller (1949, p. 9-10), Dunlap sedimentation was controlled initially by warping, which was followed by localized folding and thrusting of Luning carbonates and the development of local structural troughs that received detritus from eroding Luning folds. Limestone breccia and conglomerate, believed indicative of folding and trough development, presumably were derived from folds north of troughs in the northwestern Pilot Mountains, northern Garfield Hills, and Gabbs Valley Range (Ferguson and Muller, 1949, p. 18, 26, 31). Sediment dispersal patterns produced from such a complex set of hypothetical structural controls should be characterized by several transport directions, particularly southward in limestone conglomerates eroded from folded Luning rocks and de osited in troughs in the Pilot Mountains, Garfield Hills, and Gabbs Valley Range.

Current directional features:
Primary current directional features were measured to determine the general dispersal of Dunlap sediments, and to evaluate the concept of Early Jurassic diastrophism and Dunlap sedimentation. Directional features measured included trough and tabular crossbeds in pure quartz sandstone, volcanic sandstone, and limestone pebble conglomerate; sole marks on the underside of water-flow deposits; and pebble imbrication in water-flow deposits. It was hoped at the outset that extensive paleocurrent data could be obtained but, unfortunately, the paucity of these features and the problems associated with their restoration to original depositional position preclude any meaningful rigorous statistical analysis or regional multivariate analysis. Nevertheless, the data, even with their uncertainties, a e valuable criteria to indicate whether transport was northward or southward (or both).

Measurements of current directional features were taken only in small areas where sedimentary structures, marker beds, and orientation of beds indicate a local homoclinal sequence facing upward. Data from each of these areas were plotted together as a rose diagram; readings were grouped in 30° class intervals (Fig. 13). The dispersion in readings shown by each rose diagram reflects the variation in readings taken within a local homoclinal sequence. Most paleocurrent data were measured in nearly east-west striking beds. Current directional features

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were rotated about fold axes (north-south-trending and east-west-trending folds) where such data were available. In other areas paleocurrent features were rotated about bedding, generally dipping less than 40°. Errors caused by the lack of structural control for rotation of Dunlap paleocurrent data could be large; however, such errors should not be larger than 90°. Hence, they should not affect definition of regional northward or southward (or both) sediment dispersal.(FOOTNOTE 3)

The general northward orientation of current directional features shown on Figure 13 suggests dispersion of Dunlap sediments northward without development of structural troughs that received detritus by southward transport as postulated by Ferguson and Muller (1949). In the Garfield Hills, crossbedded limestone-pebble conglomerate and intercalated volcanic sandstone show the same transport direction, as do crossbedded limestone-pebble conglomerate and volcanic sandstone units exposed in the Gabbs Valley Range (Fig. 13). These relations, as well as the general northward sediment dispersion indicated by current directional features, suggest that limestone, chert, and volcanic detritus were derived from a local complex source. Dunlap sedimentation units buttress against the basal unconfo mity in the Pilot Mountains (Nielsen, 1964, Pl. 2). The Lower Jurassic terrane that supplied detritus appears, therefore, to have been buried in its own debris.

"Thrust conglomerate":
Critical relations cited as evidence for contemporaneous movement on the Mac and Spearmint thrusts (Ferguson and Muller, 1949; Muller and Ferguson, 1939) and Dunlap sedimentation are the compositional similarity of "thrust conglomerate" to Dunlap conglomerate in the Pilot Mountains, and the penetration of Dunlap strata by wedges of thrust conglomerate. The "thrust conglomerates" (Muller and Ferguson, 1939, p. 1619-1620; Ferguson and Muller, 1949, p. 26, 31-32) in the Gabbs Valley Range and Pilot Mountains are conformable with the fault plane and are discordant with underlying Luning, Sunrise, and Dunlap strata. They are composed of carbonate fragments. However, the largest exposure of

Fig. 13. Orientation of current-directional features in Dunlap strata exposed in Pilot Mountains, Garfield Hills, and Gabbs Valley Range. Azimuth of resultant vector (Curray, 1956) of all 109 coset readings is N15°W, which is in the same quadrant as sole mark and imbrication orientations. Vector magnitude of the resultant vector of all coset readings is 45 percent; probability (using the Rayleigh test) that azimuth is due merely to change is less than 10-5. Data do not show southward movement of Dunlap sediment in Garfield Hills or Gabbs Valley Range.

FOOTNOTE 3. Dunlap strata in the Hawthorne and Tonopah quadrangles strike nearly east-west and could have been rotated as much as 90° about a major fold (the "oroflex") hypothesized by Albers (1967) to encompass much of west-central Nevada.

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"thrust conglomerate" mapped just south of the mouth of Mac Canyon (Ferguson and Muller, 1949, Pl. 7) consists of limestone and andesite (5 percent) clasts set in a micrite matrix (Fig. 5). As suggested by Muller and Ferguson (1939), "thrust conglomerate" was eroded from advancing scraps of surface thrusts and overridden by them; hence the conglomerates should reflect the composition only of directly overlying rocks. "Thrust conglomerates" exposed south of Mac Canyon (Fig. 5) contain volcanic clasts that do not correspond to lithologies in the upper plate of the thrust. Neither the "thrust conglomerate" composed entirely of limestone clasts nor those south of Mac Canyon composed of limestone and volcanic clasts are similar to Dunlap limestone conglomerate, for they contain no rounded hert clasts. Such chert clasts are characteristic of all Dunlap limestone conglomerates.

"Thrust conglomerates" mapped by Ferguson and Muller (1949) generally are in fault contact with both underlying and overlying rocks. Only at one locality in the Pilot Mountains have these conglomerates been mapped lying directly on Dunlap strata (Fig. 14). Muller and Ferguson (1939, p. 1619) thought that wedges of the "thrust conglomerate" penetrated, as well as truncated, Dunlap strata at that location; however, these conglomerates clearly postdate Dunlap deposition (Fig. 14). There is, in fact, no evidence that the "thrust conglomerate" is related to Dunlap sedimentation; therefore, herein they are reinterpreted as thrust breccias of uncertain age. The hypothesis of thrusting contemporaneous with Dunlap sedimentation is not documented. Indeed, there is evidence that the Mac and Spea mint thrusts, believed contemporaneous with deposition of Dunlap limestone conglomerates, are post-Dunlap in age.

Geosynclinal Setting

Deposition of the Dunlap Formation appears to have been accompanied by warping and volcanism, but there is little compelling evidence for contemporaneous thrusting. The local origin of detritus that makes up Dunlap "orogenic" sediments, as well as Dunlap dispersal patterns (Fig. 13) and distribution (Fig. 1), suggest that late Early Jurassic Dunlap tectonism was restricted largely to the Hawthorne and Tonopah quadrangles. Hence, Dunlap sedimentation appears to reflect a local orogenic pulse on the margin of the volcanic phase of the Cordilleran mobile belt. The local nature and relative intensity of Dunlap diastrophism resemble Early Jurassic tectonic pulses in other parts of the volcanic phase of the mobile belt, particularly in eastern California (Clark et al., 1962) and Oregon (Dic inson, 1965), and earlier Triassic pulses in the Hawthorne and Tonopah quadrangles (Ferguson and Muller, 1949).

As noted by Noble (1962, p. 19), Lower Jurassic rocks in westernmost Nevada and eastern California are thicker and contain considerably less carbonate than their eastern counterparts, the Sunrise and Dunlap Formations. Moreover, calc-alkaline lavas and pyroclastic and sedimentary rocks derived from them are the principal units that make up these western Lower Jurassic sequences (Dickinson, 1962). As suggested by Dickinson (1969), the calc-alkaline volcanogenic suites probably reflect ancient island-arc assemblages within the Cordilleran eugeosyncline. Unconformities are common (Clark et al., 1962; Noble, 1962) and Early Jurassic granitic rocks also are present locally (Kistler et al., 1965). Subaerial eruptions and perhaps volcanic islands are suggested by accretionary lapilli in pyro lastic units (Moore, 1962) and

Fig. 14. "Thrust conglomerate" below Mac thrust (originally sketched by Muller and Ferguson, 1939, Fig. 4) showing discordant relation with Dunlap Formation below, as mapped by writer. Location of exposure is shown on Figure 5, no. 4, northwestern Pilot Mountains.

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large-scale (10-ft amplitude) crossbedding in volcaniclastic sandstones.

Clastic sediments of the Sunrise and Dunlap Formations in the Hawthorne and Tonopah quadrangles appear to represent a transitional zone between the volcanic suites on the west (eugeosyncline) and pure quartz sandstones (Nugget-Navajo-Aztec Sandstones) on the east (miogeosyncline). The presence of Navajo-like sandstone in western Nevada (Dunlap quartz arenite) suggests that central Nevada was not a positive area (Mesocordilleran geanticline) influencing dispersion of Early Jurassic sediments (Stanley et al., 1971). It appears, instead, that Early Jurassic sedimentary conditions typical of the eastern Cordillera (miogeosyncline) extended uninterrupted across Nevada. Near the present Nevada-California border these rocks intertongue with volcanogenic deposits and local "orogenic" sediment produced by volcanism and tectonism on the eastern margin of the calc-alkaline island arc complex eugeosyncline).

CONCLUSIONS

1. In west-central Nevada four distinct types of Early Jurassic clastic sediments were deposited: (a) easterly-derived pure quartz sandstone, (b) shallow-marine carbonates, (c) "orogenic" sediments, and (d) volcaniclastic strata. Both (a) and (b) are most typical of the miogeosyncline farther east, but here they are intertongued with locally derived subaerial "orogenic" fanglomerate and sandstone, and volcaniclastic strata. Andesitic lavas and pyroclastic rocks also are present and reflect conditions more characteristic of the eugeosyncline.

2. Condensed stratigraphic sequences in the Sunrise Formation indicate periods of very slow deposition represented by phosphatic limestone.

3. Dunlap pure quartz sandstone and Navajo Sandstone farther east appear to have had the same source. Dunlap quartz arenite is interpreted, therefore, as a distal part of Navajo Sandstone deposition, most of which was deposited in Utah.

4. Dunlap "orogenic" sediments were derived locally from the Sunrise (Jurassic), Gabbs-Luning-Gold Range (Triassic), and Excelsior (Permian ?) Formations.

5. Dunlap "orogenic" sediments (in part red) were deposited by both debris-flow and water-flow processes on subaerial alluvial fans that extended to the marine environment. Marine carbonate in the Dunlap and Sunrise Formations represents deposition in a shallow, subtidal marine environment with varying amounts of agitation. The transition from continental to marine sedimentation is represented by intertidal carbonate and red mudstone of the Dunlap Formation. These conditions are similar to those described by Walker (1967) for modern and late Cenozoic sedimentation in Baja California.

6. Apparently, Early Jurassic sedimentary conditions typical of the eastern Cordillera (miogeosyncline) extended uninterrupted across Nevada. Near the present Nevada-California border, uplift and volcanism produced local tectonic and volcanic islands. No continuous Mesocordilleran geanticline could have persisted in central Nevada during the Early Jurassic.

7. Dunlap diastrophism represented a local structural event in west-central Nevada during late Early Jurassic time, that included warping, erosion, sedimentation, and volcanism.

8. Contrary to the interpretation of Muller and Ferguson (1939) and Ferguson and Muller (1949), all thrust faults appear to postdate Dunlap sedimentation. Also, contemporaneous folding does not appear to have controlled the dispersal of Dunlap sediments as closely as they had postulated; sediments appear to have been dispersed generally toward the north across the entire study area.

References:

Albers, J. P., 1967, Belt of sigmoidal bending and right-lateral faulting in the western Great Basin: Geol. Soc. America Bull., v. 78, p. 143-156.

Bailey, E. H., and R. E. Stevens, 1960, Selective staining of K-feldspar and plagioclase on rock slabs and thin section: Am. Mineralogist, v. 45, p. 1020-1025.

Bateman, P. C., and C. Wahraftig, 1966, Geology of the Sierra Nevada, in Geology of northern California: California Div. Mines and Geology Bull. 190, p. 107-172.

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Acknowledgments:

(2) University of Nebraska.

This paper represents part of a doctoral dissertation submitted to the University of Wisconsin. The writer thanks R. H. Dott, Jr., for his guidance and helpful discussions, and N. J. Silberling, Lewis M. Cline, and Lloyd C. Pray for their critical review of the thesis manuscript. N. J. Silberling also provided valuable consultation before and during the project, which is greatly appreciated.

Financial assistance was provided by The Geological Society of America in the form of Penrose Bequest research grants for the summers of 1967 and 1968, and by a Sigma Xi research grant for the summer of 1968.

Copyright 1997 American Association of Petroleum Geologists

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