Geology of the Spring Mountains, Nevada

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B. C. BURCHFIEL Department of Geology, Rice University, Houston, Texas 77001 R. J. FLECK U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 D. T. SECOR Department of Geology, University of South Carolina, Columbia, South Carolina 29210 R. R. VINCELETTE Trend Exploration Limited, 600 Capitol Life Center, Denver, Colorado 80203 G. A. DAVIS Department of Geology, University of Southern California, Los Angeles, California 90007

ABSTRACT II6°-0" 115®- 0"

The northwest-trending Spring Moun- tains, Nevada, contain a 45-mi-wide (75-km) cross section of the eastern part of the North American Cordilleran orogenic belt and geosyncline. This cross section is probably the most southerly exposed section which exhibits structure and stratigraphy "typical" of the eastern part of the Cordillera. Stratigraphically, the transition from Paleozoic craton to miogeosyncline is present from east to west across the Spring Mountains. The sedimentary succession through the middle Permian thickens from 8,800 ft (2,660 m) east of the Spring Moun- tains to approximately 30,000 ft (9,000 m) in the west. Thickening of individual formations accounts for 6,800 ft (2,070 m) of added section, addition of formations at unconformities accounts for 4,600 ft (1,400 m) of added section, and addition of a thick terrigenous late Precambrian sequence accounts for 9,800 ft (3,000 m) of added section. Three major thrust plates are exposed in the Spring Mountains, each structurally higher plate containing a thicker sequence Figure 1. Location map of the Spring Mountains, Nevada, showing areas of mapping responsibility. Quadrangle names are shown for the Spring Mountains area. of Paleozoic rocks. The easternmost thrust is the Keystone thrust, except where the ear- lier Red Spring thrust plate is present below within the Spring Mountains only establish easternmost part of the North American the Keystone as isolated remnants. The a much wider time bracket, post-Early Cordilleran orogenic belt. Keystone thrust appears to be a Jurassic to pre—late Cenozoic for the east- The earliest geologic work in the Spring décollement thrust, but complications at ernmost thrust faults and post—Early Per- Mountains was by G. K. Gilbert (1875), depth suggest that additional thrust slices mian to pre—late Cenozoic for the west- who served as a geologic assistant for the may be present below the thrust or several ernmost thrusts. Wheeler expeditions of 1871-1872. R. B. thousand feet of late Precambrian terrige- Rowe of the U.S. Geological Survey did ex- nous rocks may be present above the thrust. INTRODUCTION tensive work in the central part of the The structurally higher Lee Canyon thrust The Spring Mountains are located in Spring Mountains during the period plate probably contains at least 4,000 ft southeastern Nevada, 10 mi west of Las 1900-1901, but he died before his results (1,200 m) of these terrigenous rocks at its Vegas (Fig. 1). Trending northwest more could be published. His field data were in- base, and the Wheeler Pass thrust plate con- than 45 mi, they form a southern boundary corporated in the regional report of J. E. tains at least 11,000 ft (3,300 m) of these for the general north or northeast-trending Spurr (1903). Rowe clearly recognized the rocks. Pregeosynclinal basement could be ranges farther north in Nevada. Because the fault along the east side of the Spring involved in some of the higher thrust plates, northwest topographic trend is transverse Mountains now referred to as the Keystone particularly the Wheeler Pass plate, but to the north- or northeast-trending regional thrust. depths of exposure are inadequate to de- structural strike, the Spring Mountains In 1919, C. R. Longwell began system- termine its role. offer a unique opportunity to study a wide atic mapping in the region of southeastern Thrust faulting has produced a shorten- cross section of continuously exposed pre- Nevada. His work, together with that of ing of from 22 to 45 mi (36.6 to 75 km) in Tertiary rocks. Stratigraphically, the Spring Nolan (1929) and Glock (1929), led to the the géosynclinal rocks based on geometric Mountains contain Paleozoic rocks which first regional geologic map of southeastern constructions of cross sections at depth. show the transition from craton to Nevada and of the Spring Mountains This range probably represents a minimum miogeosyncline. Structurally, they contain (Bowyer and others, 1958). Longwell's figure. Some folding and thrusting occurred rocks of the undeformed craton and of work forms the basis for most of our pres- during the early Late Cretaceous, but data three major thrust plates belonging to the ent understanding of the tectonics and

Geological Society of America Bulletin, v. 85, p. 1013-1022, 6 figs., July 1974

1013

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stratigraphy in this part of the Cordilleran tains; Fig. 1) to a geosynclinal sequence in tween the Spring Mountains and Fren- thrust belt. The present report summarizes the northwest Spring Mountains. chman Mountain, but the general charac- the geology of most of the Spiing Moun- Stratigraphically, the oldest exposed teristics of the formations are maintained. tains compiled from work completed at var- parts of the most easterly and westerly sec- All the carbonate formations that have been ious times from 1961 to the present (Fig. 1). tions, and by inference the two central sec- studied suggest deposition in shallow The authors of this paper owe a great debt tions, begin with a nonmarine and shallow marine or marginal marine environments to Chester Longwell for assistance given, marine sequence of terrigenous rocks. At (Gans, 1970). ranging from selection of map areas, field Frenchman Mountain, these rocks are Early Thickening of the carbonate sequence excursions, and discussions to personal in- and early Middle Cambrian in age, approx- across the Spring Mountains takes place in spiration. imately 1,000 ft (300 m) thick, and rest un- two ways: (1) by thickening of individual conformably on Precambrian crystalline formations and (2) by addition of forma- GEOLOGY OF THE rocks. In the northwestern part of the tions at unconformities. Thickening is SPRING MOUNTAINS Spring Mountains, the terrigenous rocks marked in the Bird Spring Formation and The Spring Mountains contain geology are more than 11,000 ft (3.3 km) thick and equivalents of Late Mississippian to middle typical of the eastern part of the Cordilleran range from late Precambrian to early Mid- Perm an age which thicken from 1,600 ft orogenic belt which can be traced continu- dle Cambrian in age. South and southwest (485 m) at Frenchman Mountain (Callville ously from Canada to southern Nevada. of the Spring Mountains, the thick late Pre- Limestone and Permian red beds) to more Late Precambrian and Paleozoic rocks are cambrian terrigenous sequence rests uncon- than 6,700 ft (2,000 m) in the central characterized by a thin (approx. 8,800 ft or formably on either crystalline basement or Spring Mountains. Some formations thick- 2.6 km) cratonal sequence to the east which on an older sequence of unmetamorphosed en by 50 to 60 percent (for example, thickens northwestward (to approx. 30,000 to weakly metamorphosed late Precam- Bonanza King Formation), whereas others ft or 9.0 km) in the Spring Mountains into brian sedimentary rocks (Pahrump Group) show little if any thickening (for example, the miogeosynclinal part of the Cordilleran that rests unconformably on crystalline Monte Cristo Limestone and Sultan Lime- geosyncline (Fig. 2). Mesozoic rocks are basement. Regional correlations by Stewart stone). present only in the eastern part of the (1970) suggest that the upper part of the Addition of formations at unconformities Spring Mountains, and their relations to Tapeats Sandstone in some cratonal se- accounts for approximately 4,600 ft (1,400 Mesozoic geosynclinal development are not quences correlates with the Zabriskie m) ot thickening across the Spring Moun- clear (Stanley and others, 1971; Burchfiel Quartzite of the geosynclinal sequence (for tains. In the cratonal sequence at French- and Davis, 1972). Structures in the Spring example, northwestern Spring Mountains man Mountain, only one major unconfor- Mountains are dominated by east-directed section) and that the lower part of the mity is present in the Paleozoic carbonate thrust faults along which thicker sequences Tapeats correlates with the uppermost part sequence; the Middle Devonian Muddy of Paleozoic rocks have moved over thinner of the Wood Canyon Form ation. The transi- Peak Limestone rests unconformably on the ones. At present levels of exposure, thrust tion from the thin cratonal sequence to the Upper Cambrian Nopah Formation. In the plates carry only sedimentary reeks. These thick geosynclinal sequence must occur be- eastern part of the Spring Mountains above structural and stratigraphic characteristics neath the Spring Mountains; the thick Red Rock Canyon, two and possibly three are similar to those described from areas geosynclinal terrigenous sequence is first unconformities are present. One occurs be- farther north along the eastern oart of the exposed in the Wheeler Pass thrust plate tween Lower and Upper Ordovician rocks Cordilleran orogenic belt. where it is already fully developed. (Goodwin Limestone and Mountain Southward, however, the structural and The upper part of the terrigenous se- Sprin,s;s Formation of Gans, 1970) which stratigraphic relations change, such that the quence becomes calcareous and contains are here present below the Middle Devo- next exposed southeast cross section in the carbonate beds in lower Middle Cambrian nian unconformity. A second unconformity Mesquite and Clark Mountains, 30 mi rocks of the Bright Angel Shale, or its is present below Middle Devonian rocks farther south, contains Paleozoic sedimen- geosynclinal equivalent, the Carrara For- (Sultan Limestone) and is of regional ex- tary rocks characteristic of the craton and mation, and grades upward into a thick se- tent. A third unconformity within unfos- transitional between craton and quence of shallow-water carbonate rocks siliferous dolomites at the top of the Moun- miogeosyncline, and thrust plates that in- that range in age from Middle Cambrian to tain Springs Formation is suspected but not volve Precambrian crystalline basement Early Permian. Thin terrigenous units are proven (Gans, 1970). South along the rocks (Burchfiel and Davis, 1971, 1972). present at several stratigraphic levels, and Spring Mountains, the Ordovician and sus- Regional relations suggest a divergence of three can be followed from the cratonal pected Devonian rocks are cut out by the structural and geosynclinal units (Burchfiel into geosynclinal sequences: (1) the thin unconformities beneath Midc.le Devonian and Davis, 1972); thus, the Spring Moun- calcareous siltstone at the base of the rocks, and the Middle Devonian Sultan tains represent the southernmost example Banded Mountain Member of the Bonanza Limestone rests on the Upper Cambrian of "typical" Cordilleran tectonics. King Formation, (2) the Dunderberg Shale Nopah Formation at Mountain Springs Member of the Nopah Formation, and (3) Pass. The trend of this wedge out of Or- Stratigraphy the basal terrigenous rocks of the Bird dovician and Devonian(P) rocks must be No attempt is made here to describe all Spring Formation. Three other prominent northeast across the structural trend (that the stratigraphic units present in the Spring but thin terrigenous units are present in the is, west of Frenchman Mountain and north Mountains. Lithology, thickness, and facies geosynclinal sequence cf the northern of Mountain Springs Pass). changes are presented in Figure 2, and de- Spring Mountains, but are not present in Farther west, the Middle Ordovician tails of these sequences can be found in the eastern Spring Mountains. Those three Ninemile Formation, Antelope Valley Burchfiel, 1964; Secor, 1963; Vincelette, units are (1) Eureka Quartzite, (2) Ninemile Limes rone, and Eureka Quartzite occur be- 1964; Fleck, 1967, 1970, 1974; Gans, Formation, and (3) a sandstone unit at the low the Upper Ordovician reeks, so that 1970; Longwell and others, 1965. The four base of the Nevada Formation. in the central part of the Spring Mountains, representative stratigraphic columns pre- The carbonate formations can be fol- an unconformity is no longe:: recognized sented on Figure 2 demonstrate the change lowed through the Spring Mountains with within Ordovician strata, and the Ordovi- from a cratonal sequence at Frenchman little lithologic change: some are cut out by cian sequence is complete. Farther west, but Mountain (25 mi east of the Spring Moun- unconformities. Facies changes occur be- still in the central Spring Mcuntains, the

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Silurian Laketown Dolomite occurs be- from west to east truncates progressively The latter are commonly associated with neath the Middle Devonian Sultan Lime- older formations, the pre-Moenkopi uncon- thrust faults, which appear to postdate the stone and thickens westward. FinaLy in the formity truncates older formations from folds (Fleck, 1970; Vincelette, 1964; northwest Spring Mountains, Lower Devo- east to west. A basal detrital and con- Nolan, 1929). High-angle faults cut and are nian dolomite beds of the Nevada Forma- glomeratic unit overlies the unconformity cut by thrust faults, suggesting at least two tion are present beneath Middle Devonian and is overlain in turn by shallow-water periods of high-angle faulting. The high- rocks and thicken rapidly; farther west the marine limestone correlative with the Vir- angle faults most commonly trend north- unconformity within Devonian rocks is no gin Limestone member of the Moenkopi west, although north-trending faults are longer recognized. Thus, in the northwest Formation. These rocks represent the last numerous locally. Spring Mountains, all formations seem con- identified marine strata in the Spring The structural grain of the northeastern formable and range from late Precambrian Mountains area. A thin sequence of red half of the Spring Mountains has been ro- to Mississippian and probably Late Per- beds forms the uppermost beds of the tated by late Tertiary movement on the Las mian in age. Moenkopi; they are the youngest rocks Vegas Valley shear zone (Longwell, 1960; Pre-Late Permian strata (that is, pre- present in the allochthonous terrane of the Fleck, 1967), which lies in Las Vegas Valley Coconino Sandstone) on the craton are Spring Mountains and are present only in immediately northeast of the Spring Moun- 7,700 ft (2,300 m) thick. Assuming that a the Keystone thrust plate. Rocks younger tains. Thrust faults, folds, and bedding that thick section of Bird Spring Formation once than Bird Spring Formation are absent strike north in the southwestern part of the overlay the rocks exposed in the northwest above the Lee Canyon thrust fault. Detrital range arc northeast adjacent to Las Vegas Spring Mountains, the total stratigraphie rocks of the Moenkopi in the cratonal se- Valley. This bending was interpreted by thickness of pre-Late Permian géosynclinal quence on Frenchman Mountain and below Longwell (1960) to be right-lateral drag rocks may have been approximately 30,000 the Keystone thrust in the eastern Spring along the south side of the shear zone. ft (9.0 km). Estimates of the contributions Mountain continue unbroken into the Structure of the Autochthon. Autoch- to westward thickening of the section are Lower Jurassic Aztec Sandstone. The up- thonous rocks crop out only on the eastern the following: (1) thickening of individual permost part of the Moenkopi Formation slope of the Spring Mountains where they formations that carry through to the cra- and the Aztec Sandstone are not present form a homocline of Mesozoic strata that ton: 6,800 ft (2,070 m) or 32 percent; (2) west of the Keystone thrust fault. dips gently westward. In Red Rock Can- addition of formations at unconformities: Locally, channels filled by conglomerate yon, the Mesozoic rocks are overturned 4,600 ft (1,400 m) or 22 percent; and (3) with red sandstone matrix are present at the eastward below the Keystone thrust. This addition of thick terrigenous late Precam- top of the Aztec and immediately below the fold plunges below the thrust and is not ex- brian sequence: 9,800 ft (3,000 m) or 46 Keystone and Red Spring Thrusts. Secor posed north or south of the Red Rock Can- percent. The Bird Spring Formation alone (1963) and Davis (1973) have interpreted yon area. East of Mountain Springs Sum- accounts for one-half of the total effect of these channel deposits of post-Aztec age mit, near the southern boundary of Figure formational thickening. Thus, excluding and suggest that they may be as young as 3, a northwest-striking, high-angle fault, this formation, the main cause of géosyn- Cretaceous. In any case, these conglomerate the Cottonwood fault (Hewett, 1931), jux- clinal thickening is the addition of the late units are the youngest rocks in the area and taposes Devonian and Mississippian rocks Precambrian terrigenous wedge at the base. indicate that the Keystone and Red Spring on the south and Jurassic Aztec Sandstone Formations that overlie the Paleozoic thrust plates may have moved across ero- on the north. Rocks south of the fault are carbonate sequence are largely terrigenous sional surfaces (see below). part of the Contact thrust plate (Hewett, with marine carbonate rocks present only 1931; Davis, 1973) which overrides Aztec in Late Permian and Early Triassic units. Structure Sandstone south of the area shown on Fig- These terrigenous formations range in age Structure of the Spring Mountains north ure 3. Thus, rocks south of the Cottonwood from Early Permian to Cretaceous(i) and of Mountain Springs Summit is dominated fault are displaced downward several are exposed only in the cratonal sequence at by three thrust faults which trend north to thousand feet relative to rocks north of the Frenchman Mountain and in the eastern northeast through the northwest-trending fault. The fault displaces the thrust verti- part of the Spring Mountains (Figs. 2 and range (Fig. 4). Passing through the eastern cally by only 200 ft (65 m), with the south 3). The oldest of these terrigenous units is part of the range is the Keystone thrust side down, suggesting that most displace- the lower Permian red beds which lie below which forms the easternmost thrust of the ment on this fault was pre-Keystone in age. the Kaibab and Toroweap Formations of Cordilleran orogen throughout most of the Other faults that cut both the autochthon Leonardian age. In the eastern Spring Spring Mountains as well as farther south. and the Red Spring thrust farther north Mountains, they conformably overlie „ower The Lee Canyon thrust cuts through the suggest similar age relations (see below). Permian rocks of the Bird Spring Forma- central part of the mountains, and the Red Spring Thrust Plate. Longwell tion. Because the Bird Spring Formation in Wheeler Pass thrust cuts through the west- (1926) described the Red Spring thrust as a the central Spring Mountains contains ern part. Four thrusts of probably smaller low-angle fault in the eastern Spring Moun- fusulinids of Leonardian age, part or all of magnitude are also present: (1) the Red tains which carried Cambrian dolomite the red beds in the east may be equivalent to Spring thrust which lies below the Keystone eastward over Jurassic Aztec Sandstone. marine limestone units farther west. The thrust northeast of Red Rock Canyon, (2) The thrust was subsequently broken into at red beds grade upward into the marine the Kyle Canyon thrust which crops out least four east-tilted fault blocks by north- limestone and evaporite beds of the To- over a small area in the central Spring or northwest-trending high-angle faults roweap and Kaibab Formations. The Mountains, (3) the Deer Creek thrust, and (Fig. 4). Later, probably following consid- Kaibab and Toroweap Formations wedge (4) the Macks Canyon thrust. The latter erable erosion, the blocks were overridden out to the northwest and are not present in two faults crop out only in the north half of by the Keystone thrust plate. Longwell later the central Spring Mountains where they the range (Fig. 4). (1960) reinterpreted the Red Spring thrust are cut out by pre-Moenkopi erosion. Folds occur either as minor intrafolial or plate as frontal parts of the Keystone thrust The Moenkopi Formation of Early Trias- intraformational structures of limited lat- plate which were faulted, rotated, and over- sic age was deposited unconformably on eral extent or as very large amplitude, long ridden by the Keystone thrust plate. older formations. Unlike the unconformity wave-length structures, involving most or Mechanically, Longwell visualized a con- at the base of the Sultan Limestone, which all of the stratigraphic section in the area. tinuous eastward advance of the Keystone

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30,000 9'°00 feet meters

25,000

20,000 ê.,000

15,000

10,000 3,000

LEE CANYON 10,000' - WHEELER PASS THRUST KEYSTONE 10,000'

2.0 km- 5,000' - 1.0-

0 £ 5,000

1.0-

5,000' 2.0-

10,000' 10,000'

7 miles

HORIZONTAL AND VERTICAL SCALE Figure 2. Stratigraphie Sections from Frenchman Mountain to Northwest Spring Mountains. Figure 5. Geologic Sections, Spring Mountains, Nevada.

BURCHFIEL AND OTHERS, FIGURES 2 AND 5 Geological Society of America Bulletin, v. 85, no. 7

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Figure 4. Generalized tectonic map, Spring Mountains, Nevada.

plate modified by synchronous strike-slip tive with the Red Spring thrust plate. He tion; Davis (1973) believe:; that the con- movement on the Las Vegas shear zone. further interprets the Cottonwood fault as glomerates represent channel deposits cut Later work by Fleck (1967; 1970) and An- similar to faults that cut the Red Spring into the Aztec Sandstone, indicating that derson and others (1972) demonstrated a thrust. Thus he suggests, that the Red Spring the frontal parts of the Red Spring thrust late Tertiary age for the Las Vegas shear thrust plate once covered most of the Juras- moved across an erosion surface. The con- zone. Recent work by Davis (1973) and sic Aztec Sandstone shown on Figure 3, but glomerate beds contain rounded pebbles Longwell (1973) have returned to it was uplifted on a horst between the La and cobbles of late Precambrian terrigenous Longwell's original interpretation of the Madre and Cottonwood faults and was rocks, Paleozoic carbonate rocks, and Red Spring thrust. Only the westernmost subsequently eroded prior to the thrusting Mesozoic terrigenous rocks set in a matrix fault blocks containing the Red Spring of the Keystone thrust plate. of red sand reworked from the Aztec Sand- Thrust have been remapped and are shown Longwell (1926) described discontinuous stone. Longwell (1973), however, has on Figure 3. conglomerate lenses beneath the Red Spring abandoned his original interpretation of the Davis (1973) believes that rocks below thrust which he interpreted as surficial de- restricted surficial nature of the conglomer- the Keystone thrust and south of the Cot- posits overridden by the thrust plate. A re- ate beds. He now interprets them as basal tonwood fault, which belong to the Contact study of these conglomerates by Davis remnants of a thicker, mote extensive clas- thrust plate of Hewett (1931), are correla- (1973) reconfirms Longwell's interpreta- tic sequence which has been overridden and

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largely obliterated by movement of the Red and by the Griffith fault on the north; the thrust slices that are comparable to the Spring thrust plate. Because of their impor- central block is bounded by the Griffith and overturned anticline of the southern block. tance, locations of the conglomerate beds La Madre faults, and the northern block is The most important of these thrust faults is are shown by small circles on Figure 3 (sees. bounded by the La Madre fault on the the Deer Creek thrust, which carried Cam- 23, 24, and 36, T. 20 S., R. 58 E.). south and alluvium on the north. At the brian dolomites over Bird Spring Forma- Along the crest of the ridge on which the eastern end of each block, the Paleozoic tion. Shortening southwest of the Griffith Red Spring thrust crops out and is centered rocks dip homoclinally west or northwest fault is accommodated by folding. Both the on sec. 31, T. 20 S., R. 59 S., there is a long at the base of the Keystone plate; within a upper and lower plates of the Deer Creek narrow outcrop of breccia composed of few miles west of the thrust, however, each thrust are imbricated north of Kyle Can- carbonate rocks which were derived from block develops a different structural style. yon, repeating formations in several slices. Cambrian to Mississippian formations. The southern block contains a complex The lowermost thrust of the lower plate Originally mapped as a thrust klippe syncline and anticline, both of which gener- which repeats the Bird Spring and locally (Longwell and others, 1965), the breccia is ally plunge south (Fig. 5, cross section). The the Monte Cristo Limestone has been interpreted by Davis (1973) as a landslide syncline is outlined by a core of Permian red named the Kyle Canyon thrust. Several of breccia. Small outcrops can be followed beds and Moenkopi Formation, which ex- the small thrusts that repeat upper north where they seem to project below the hibit varying degrees of small-scale folding. Paleozoic rocks above the Kyle Canyon north-dipping Red Spring thrust plate. Al- The anticline, which has a core of Bird thrust die out eastward into overturned though the relation is obscured by al- Spring Formation or, locally, Monte Cristo folds, just as they do to the west. Thrusts in luvium, stream channels carrying debris Limestone, is asymmetric with its eastern this area appear to die out upward as well eroded from advancing thrust plates and limb overturned to the east. Both the major as laterally; thus, lateral continuity of landslide material shed from these pktes syncline and locally the major anticline are thrusts may be related in part to depth of may have been buried beneath the Red cut by several northwest-trending, high- exposure. Spring thrust plate as it advanced. angle faults that may have acted as tear or The northwestern part of the central Keystone Thrust Plate. The Keystone adjustment faults. Secor (1963) has shown block is cut by numerous high-angle faults, thrust fault can be followed along the entire that many of the smaller folds are truncated some of which are confined to rocks of the eastern face of the Spring Mountains, al- by these faults and do not continue in the thrust plates. Other high-angle faults ter- though its continuation east of sec. 14, T. adjacent fault blocks. The form and minate small thrusts (such as west of Deer 20 S., R. 58 E. has not been mapped since number of folds change from block to Creek campground) or fail to displace other the work of Longwell and others (1965). block. Three blocks contain thrust faults, thrusts. These relations suggest that some Along the fault, the Cambrian Bonanza each confined to a single block; one thrust of these faults are tear (or adjustment) King Formation was thrust eastward over is east directed, another west directed, and faults that date from the time of thrusting. Jurassic Aztec Sandstone and remnants of a third contains a truncated klippe. Because At the northwest end of the central block, the Red Spring and Contact plates. Along of these changes in style, some high-angle Cambrian rocks of the Lee Canyon thrust its trace, the thrust dips west approximately faults were probably present during fold- plate rest on Deer Creek thrust plate. parallel to bedding in the Bonanza King ing. Northeast of the La Madre fault, the Formation above and the Aztec Sandstone West of the axial traces of folds shown structural style of the northern block of the below. In Red Rock Canyon, the thrust is on Figure 3, there is a broad area underlain Keystone thrust plate is somewhat different exposed at a deeper level and dips approx- by unfolded Bird Spring Formation that from that of the central block. A large imately 30° west, still approximately paral- dips 5° to 25° southwest. Westward, the northeast-plunging syncline with an over- lel to bedding in the Bonanza King Forma- dips steepen to 30° to 45° before passing turned western limb trends northeastward tion of the upper plate but cutting across below the Bonanza King Formation of the through the central part of the block. The bedding in overturned Mesozoic forma- Lee Canyon thrust plate. Below the Lee core of the fold is outlined by Permian red tions of the lower plate. Farther east, the Canyon thrust, there are two slices of upper beds and Triassic Moenkopi Formation and thrust cuts across rocks of the Red Spring Paleozoic carbonate rocks which were is locally folded on a small scale. A north- thrust plate which are locally thrown into sheared from the Keystone plate. South of trending high-angle fault displaces the syn- an overturned syncline (sees. 22 and 23, T. Trout Canyon, the southern slice contains cline, causing a right-handed separation. 20 S., R. 58 E.); dips along the thrust con- mostly right-side-up Bird Spring Forma- The Deer Creek thrust is displaced by tact vary from 16° north to near vertical. tion. The northern slice in the vicinity of approximately 5,000 ft (150 m) of dip- At Red Rock Canyon, the thrust lies at Charleston Peak, however, contains over- slip movement on the La Madre fault. It the base of the Banded Mountain Member turned Bird Spring and Monte Cristo For- crops out 1 mi southeast of the Hilltop of the Bonanza King Formation, but mations which probably represent the over- campground in section 16, T. 19 S., R. 57 southward, rocks at the base of the plate turned limb of a syncline below the Lee E. High-angle faults with large displace- are folded, faulted, and brecciated on a Canyon thrust, which was sheared off ment truncate this short segment of the small scale; how closely the thrust follows along its axial surface. Limestone beds in Deer Creek thrust near Angel Peak, and the stratigraphic horizons in the Bonanza King this slice are foliated and recrystallized thrust does not crop out for a distance of 3 is obscure. On a regional basis, the thrust mylonite near the Lee Canyon Thrust. mi to the northeast. The thrust reappears, fault is located near the boundary between Structures and stratigraphic units can be however, in Lucky Strike Canyon. It can be the two members of the Bonanza King followed directly from the southern block followed from there to Las Vegas Valley. Formation (Fig. 2) for a distance of more into the central block at the ends of the No structures related to the Deer Creek than 35 mi (60 km), strongly suggesting Griffith fault; but structurally, the main thrust are found between these exposures, stratigraphic control of the thrust. part of the central block is very different although higher thrust plates, while dis- Structure within the Keystone thrust from that of the southern block. The only placed, crop out continuously across the plate consists of folds, high-angle faults, fold is a broad, open syncline in the south- block. Fleck (1974) concludes that evidence and thrust faults, which are variably de- eastern part of the block, and no rocks from these exposures and from others on veloped in three blocks bounded by two younger than the Bird Spring Formation are the Lee Canyon thrust indicates that thrust northwest-striking faults. The southern present in its core. Northwest of this syn- displacement decreases upward on at least block is bounded by alluvium on the south cline, there are a series of east-directed the smaller thrust faults and that deeper ex-

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posures in the Angel Peak area would reveal Canyon thrust is small and the fault is here Exposures near Wheeler Pass show that the the Deer Creek thrust. This, interpretation considered a subsidiary thrust in the Lee dip of the thrust is steeper than bedding, requires that crustal shortening related to Canyon thrust plate. cutting older strata at deeper levels. Rocks the thrust faulting be accommodated by In upper Clark Canyon, the Macks Can- n:;ar the thrust are brecciated and con- some other means, probab'y folding. The yon thrust is cut by a curving high-angle torted, but mylonitization was not detected. lateral transition of folds into thrust faults fault that has west-sid:-down displacement. One small imbricate slice with an eastward in this area (Fleck, 1970) supports this This fault is one oi several intersecting overturned anticline above it is present near interpretation. high-angle faults of similar displacement the base of the plate northeast of Wheeler The Lee Canyon thrust plate has overrid- that can be followed from 4 mi north of Well (sec. 20, T. 18 S., R. 55 E.). den the Deer Creek thrust plate; an over- Wheeler Well (sec. 31, T. 17 S., R. 55 E.) Within the Wheeler Pass thrust plate, turned slice of Bird Spring Formation is southeast across Clark Canyon, then south structure is relatively simple. In its eastern present beneath it and represents the over- across Wallace Canyon, and beneath a part, the plate contains a north-plunging turned limb of a syncline that was sheared broad expanse of alluvium to the north side syncline (the Wheeler syncline); in its along the axial surface. This relation is very of the ridge north of Trout Canyon (sec. 26, northwest part (north and west of the similar to that present in the southern block T. 20 S., R. 55 E.). The down-dropped northwest corner of the Mount Stirling of the Keystone thrust plate. blocks probably contain the upper plate of quadrangle), the plate contains a large Total shortening in the northern block of the Macks Canyon thrust at depth, but be- northeast-plunging overturned syncline and the Keystone thrust plate is probably larger cause displacement on the Macks Canyon locally an overturned anticline farther than in the other two blocks to the south- thrust just before intersecting the high- north. Rocks in the reminder of the plate west. The northern block contains the Deer angle fault at Clark Canyon is less than a dip at moderate angles to the north and Creek thrust of the central block, a large few hundred feet, it: is possible that the northeast and are repeated by northwest- or overturned syncline and anticline similar to thrust never extended much farther west. north-striking, high-angle faults. Most of those in the south block, and several smal- North and west of the Macks Canyon the northeastward or eastward dip of the ler folds and thrust slices. We suggest that thrust, progressively younger rocks are beds and northeast plunge on the folds that the total shortening increases northeast- present in the Lee Canyon thrust plate be- occur in the western pari: of the plate were ward in the Keystone thrust plate, perhaps cause of a general northwestward-to- probably caused by east-northeast tilting of in part compensating for a decrease in dis- westward dip. Folding of these rocks is var- this part of the Spring Mountains during placement on the Lee Canyon thrust (see iable. In the southwestern part of the Lee Cenozoic time. below). Canyon plate (T. 20 S., west part of R. 54 Small-scale folding within the Wheeler Lee Canyon Thrust Plate. The Lee Can- E., east part of R. 55 E.), numerous small Pass thrust plate is mostly confined to the yon thrust fault is the middle of three major folds are present; many trend north, but well-stratified, thinly bedded terrigenous thrust faults that can be followed through others are more dome-shaped and have no formations (that is, the Carrara and John- the Spring Mountains. It can be traced from obvious trend. In the western part of the nie Formations). The Carrara contains a small outcrop south of Trout Canyon plate, there is a syncline-anticline pair (sees. many small-scale folds near the southwest (sec. 18, T. 21 S., R. 56 E.) to the west side 17, 18, 19, T. 19 S., R. 55 E.), and the anti- end of the Wheeler syncline. These folds are of Charleston Peak, through Lee Canyon cline is adjacent to the Wheeler Pass thrust. overturned eastward in the west flank of the northeastward on the ridges south of the Small but unmapped thrusts probably syncline which is inconsistent with an Lee Canyon road to Las Vegas Valley. Un- complicate this atypical relation between origin as parasitic folds for the Wheeler like the Keystone thrust, the Lee Canyon folds and the Wheeler Pass thrust. Farther syncline. Axial trends of these small folds thrust cuts across significant thicknesses of north, 2 mi southv/est of Willow Spring have been rotated by the Wheeler syncline strata. At Trout Canyon, the upper (sec. 2, T. 18 S., R. 55 E.), a large over- and are regarded as folds formed by in- Bonanza Formation is exposed immediately turned syncline is present beneath the traformational slip during thrusting, but above the thrust. Farther northeast, the Wheeler Pass thrust. To the northeast, prior to the development of the Wheeler thrust cuts through Ordovician, Silurian, across several miles of alluvium, an over- syncline. Devonian, and Mississippian formations turned slice of Bird Spring Formation is Small folds are common in the middle until (just before reaching Las Vegas Val- present below the projected but covered and lower parts of the Johnnie Formation ley) it dies out into overturned folds in the trace of the Wheeler Pass thrust. This slice in the northwest part of the Spring Moun- Bird Spring Formation, demonstrating a probably represents the overturned limb of tains. Folds generally increase in intensity decrease in displacement northeastward. a syncline which was thrust along or near downward and in proximity to the large Folding of the upper-plate rocks is common the axial surface of the fold, a relation sug- overturned folds in the northwest part of in the southern and northeastern parts of gested for similar slices below the Lee Can- the map area. Axial plane cleavage is better the Lee Canyon thrust plate; whereas in Lee yon and Deer Creek thrust faults. developed downward and to the northwest Canyon, imbricate thrusts are more com- Wheeler Pass Thrust Plate. The Wheeler in the Johnnie Formation. mon. Pass thrust trends northeast across the The projection of the Wheeler Pass thrust The Macks Canyon thrust, here consid- west-central part of the Spring Mountains at depth in the northwest Spring Mountains ered part of the Lee Canyon plate, can be and carries a thick sequence of late Precam- is equivocal. Nolan (1929) believed the traced from Clark Canyon through Macks brian terrigenous rocks eastward over the Wheeler Pass thrust flattened at depth and Canyon (sees. 27 and 33, T. 18 S., R. 56 E.) Bird Spring Format: on. The eastern trace of reappeared along the contact between the and northeastward to the Las Vegas Valley the thrust is covered by alluvium, but the Johnnie Formation and the Stirling Quartz- along the ridge north of the Lee Canyon fault undoubtedly passes between an over- ite as the Johnnie thrust. Subsequently, the Road. At its west end in Clark Canyon, turned slice of Bird Spring Formation and Johnnie thrust played a large part in the in- Bonanza King Formation is thrust over it- rocks of the Cambrian Carrara Formation terpretation of thrusting in southern self or over Nopah Formation. In its middle on Indian Ridge at the north end of the Nevada (Longwell, 1945) and as recently as part, Cambrian and Ordovician rocks are Mount Charleston quadrangle. 1960, King described it as a classical exam- imbricated; in its northeastern part, Missis- Beds of the Wood Canyon Formation or ple of a thrust fault which flattens at depth sippian and Pennsylvanian rocks are re- Stirling Quartzite lie immediately above the and follows stratigraphic units. Remapping peated. Thus, displacement on the Macks thrust and dip 25° to 45° west or northwest. of the area of the Johnnie thrust by several

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workers (Burchfiel, 1965; Hamill, 1966; others are nearly vertical. Relations tenta- struct their three-dimensional geometry en- Livingston, 1964; Vincelette, 1964) has not tively suggest that the north block has counters several difficulties and ambiguous supported the existence of the Johnnie moved down along the gently dipping nor- relations that cannot now be resolved. thrust. mal faults that were parts of a single fault However, some limits as to the range of Nolan's observations supporting the ex- which was subsequently cut by later high- possible interpretations can be presented. istence of the Johnnie thrust were (1) a angle faults. The faulted contact between All interpretations presented here involve a white quartzite at the base of the Stirling Cambrian dolomite and late Precambrian geometry in which the thrust faults flatten Quartzite that he believed was a tectonic terrigenous rock probably represents the with depth. This is a common interpreta- microbreccia formed along the thrust plane, northwest boundary of the main part of the tion for the eastern part of the Cordilleran and (2) that beds in the Stirling Quartzite Spring Mountains block elevated during thrust belt; it is supported by evidence from above and beds in the Johnnie Formation Cenozoic time. the Clark Mountain area 30 mi to the south below were truncated by this microbreccia. Landslide Deposits. Within the Spring (Burchfiel and Davis, 1971). The Clark Vincelette (1964) demonstrated that the Mountains, there are large and small areas Mountain thrust complex is the continua- white quartzite is locally fractured but underlain by rocks which have been de- tion of at least part of the thrust belt rep- otherwise undeformed. The white quartzite tached and moved downslope under the resented in Spring Mountains but exposed is a basal member of the Stirling; at many influence of gravity. Many of the smaller at a deeper level. The thrusts flatten at localities, it makes a transitional sedimen- rock masses are thoroughly brecciated depth whether or not they are stratigraphi- tary contact, locally unconformable with fragments, chaotically mixed. Most of the cally controlled (Burchfiel and Davis, the Johnnie Formation. Furthermore, the material in these masses was derived from 1971). Thus, this geometry is used as a intensity of deformation generally increases one or more Paleozoic formations. These basis for interpretation in the Spring downward away from the Stirling-Johnnie masses were probably em placed as rock-fall Mountains. contact, not toward it, as might be expected avalanches or rock slides. The Red Spring thrust fault cannot be if this contact were a thrust fault. Locally, In most of the larger landslides, however, projected to depth, because it is truncated the contact shows evidence of displace- breccia is present only in the basal 50 to along its west side by the later Keystone ment, but we regard this as differential 100 ft of the deposit; the remainder of the thrust fault. This relation indicates that movement between strongly contrasting deposit is unbrecciated and retains its originally more westerly parts of the Red lithologie units. Similar movement has been coherence such that stratigraphic units can Spring thrust plate, not removed by pre- detected along the upper and lower con- be traced through the deposits. Some of Keystone erosion, were incorporated into tacts of the Eureka Quartzite. these landslide deposits, such as the com- the frontal parts (that is, easterly parts) of If the Wheeler Pass thrust does flatten at posite landslide north of the mouth of the Keystone plate and transported at least depth, it must flatten below exposed parts Trout Canyon, reach large size. Here the far enough to the east so that post-Keystone of the Johnnie Formation. Even if it does landslide is made of several individual land- erosion could remove evidence of their exis- flatten below the Johnnie, a simple slide blocks composed of Devonian through tence. Some problems in projecting the décollement geometry cannot apply, be- Permian formations bounded by brecciated Keystone thrust fault to depth (see below) cause a reasonable projection of the thrust rock at their basal contacts. These blocks might be resolved, however, by inserting places the base of the Johnnie Formation in are probably the erosional remnants of a remnants of the Red Spring thrust plate the Wheeler Pass thrust plate approxi- sheet of landslide debris which was once below the fault. mately 10,000 ft higher than corresponding more extensive and may have covered more The magnitude of displacement on the rocks in the overridden Lee Canyon plate than 20 sq mi. From their coherent char- Red Spring thrust cannot be calculated be- below. This relation suggests complications acter, we infer that these masses moved cause its geometry has been disrupted by of thrust geometry at depth which is com- downslope rather slowly. pre-Keystone high-angle faults. A minimum mon for all thrust faults in the Spring In all cases, the source of the landslide estimate can be obtained, assuming the Mountains and will be treated separately debris can be found upslope from its pres- thrust originally dipped 30° and cut below. ent position. None of the landslide masses through a stratigraphic section with a The Wheeler syncline in the eastern part are cut by either thrust faults or high-angle minimum thickness of 10,000 ft (3,000 m). of the Wheeler Pass thrust plate does not faults. Some masses, such as those at Trout Using these assumptions, an estimate of 4 appear to be the result of ramping produced Canyon, are being removed by the present mi can be used as a minimum figure. above a riser on a décollement thrust, an in- drainage. Most of the deposits are found in On a regional basis, the Keystone thrust terpretation which at first appears reason- topographically low areas of present basins fault appears stratigraphically controlled able. The axial trace of the syncline con- or at the foot of the mountains, suggesting and can be reasonably projected as a fault verges with the trace of the Wheeler Pass that they are related to topography not that flattens at depth probably within or thrust too rapidly to fit the geometry for a much different than that existing today, al- near the base of the Bonanza King Forma- ramp origin (Fig. 3). though most of the blocks are being buried tion. Beneath the eastern part of both the Large overturned folds in the Johnnie by recent alluvium. With one exception, all southern and northern blocks of the Key- Formation in the northwestern part of the evidence suggests a late Cenozoic age of stone plate, the thrust fault is projected to a Wheeler Pass plate cannot now be related emplacement for these masses. Only the depth of approximately 7,000 ft below sea to any regional features. An overturned an- landslide deposit east of the Red Spring level, which is close to the projected depth ticline is present in sec. 15, T. 16 S., R. 53 thrust in the eastern Spring Mountains may of the same stratigraphic level in the au- E., and is truncated obliquely by a high- be older. Davis (1973) has suggested that it tochthonous rocks, making décollement angle fault that dips steeply west, so that may have been shed from the front of the geometry appear reasonable (Fig. 5). West only its normal west limb is exposed over Red Spring thrust plate and was later over- of the large overturned syncline in these much of the area west of this fault. Near the ridden by the thrust plate — such a relation two blocks, the depth to the base of the northwest corner of the map area, Cam- would suggest a Mesozoic age for this Bonanza King rises to less than 4,000 ft brian dolomite is faulted against the late landslide. below sea level beneath a broad anticline. Precambrian terrigenous sequence along Geometry of Thrust Faults at Depth. This suggests that the fault is folded, cuts several intersecting faults. Some of these Projecting the thrust faults of the Spring through older rock units in the plate, or faults are low angle, dipping 30° north; Mountains to depth in an attempt to con- contains several splay faults which die out

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upward below the anticline. If the thrust graphic level in the terrigenous sequence high-angle faults that cut the Red Spring fault is warped, such as are thrusts in the westward. thrust might also cause complications in the Canadian Cordillera (Bally and others, Configuration of the thrust below the geometry of the Keystone thrust at depth; 1966), other thin slices of Paleozoic rocks central block presents additional problems, however, the problem area is confined to may be present above the décollement but because the projected depth to the thrust, if the central block, suggesting a relation to below the Keystone thrust (Fig. 6a). Slices it is at the base of the Bonanza King Forma- the two bounding faults, the Griffith and La of this type are known from the Clark tion, is nowhere more than 3,000 to 4,000 Madre faults. An alternate interpretation is Mountain area 30 mi to the south ft (900 to 1,200 m) below sea level (Fig. 5, that the central block lias been elevated, (Burchfiel and Davis, 1971). An alternate A-A'). The corresponding stratigraphie relative to the blocks on either side, be- interpretation is to change the geometry of level in the autochthon is at 7,000 ft (2,100 tween the La Madre and Griffith faults. the plate such that added stratigraphie units m), thus producing a disparity of nearly Displacements on the two faults increase are present in the Keystone plate and above 3,000 to 4,000 ft (900 to 1,200 m) between westward, causing greater elevation in the a décollement surface that dips gently the two positions. Possible solutions to this central part of the block and resolving the westward (Fig. 6b). Older formations space problem are again the same as those space problem. added at the base of the plate could include presented for the north and south blocks of The Deer Creek thrusi: of the central and 3,000 ft (900 m) of the late Precambrian the Keystone plate, however, the eastern northern blocks can easily be projected into and Cambrian terrigenous sequence, and limit of the added slices or stratigraphie the lower part of the carbonate sequence the eastern limb of the overturned anticline units would be several miles farther east in because the same stratigraphic levels of the would represent a step in the upper plate the central block, suggesting the La Madre upper and lower plates are comparable at a where the thrust cuts across the base of the and Griffith faults may extend to the base point 3 mi west of the surface trace of the Bonanza King Formation to a lower strati- of the allochthonous units. Pre-Keystone Deer Creek thrust (Fig. 5, sections A-A'). The Kyle Canyon thrus.t appears to be a high-level thrust slice below the Deer Creek thrust (Fig. 5, A-A'). If the model involving thrust slices below the Keystone thrust is accepted, nearly the entire width of the Keystone plate would be allochthonous and the magnitude of eastward displacement would be approximately 14 mi (23 km) (Fig. 6a). If the model involving the addition of the terrigenous sequence at the base of the Keystone plate is accepted, the magnitude of eastward displacement is approximately 7 mi (11.6 km). Movement on the Deer Creek thrust adds approximately 3 mi (5 km) for the western part of the central and northern blocks. The Lee Canyon thrust fault dips 30° to 40° west and cuts less steeply dipping bedding in the upper plate at an angle of 10° to 15°. At a depth of 2,000 to 4,000 ft (600 to 1,200 m) below the surface, the thrust must either change dip, if it follows the base of the Bonanza King Formation, or continue to dip westward and cut into older rocks such as the late Precambrian terrigenous sequence (Fig. 6). This latter case would require that 2,000 to 4,000 ft (600 to 1,200 m) of the terrigenous sequence be added to the Lee Canyon plate. In either case, folds in the Lee Canyon plate can be interpreted as originating from the overrid- ing of ramp segments in the lower plate by the Lee Canyon plate (Fig. 6). From the geometry projected at dspth, merger of the Lee Canyon and Keystone thrusts at depth can be inferred to occur 7 to 8 mi (11.6 to 13.3 km) west of the surface trace of the Lee Canyon thrust where stratigraphic levels in the two plates are at comparable elevations.

Figure 6. Interpretative cross sections from Figure 5. 6A. Space problem is solved by adding a thrust slice. 6B. With either model proposed for the Lee Space problem is solved by adding late Precambrian terrigenous rocks to plates. 6C. Space problem is solved by Canyon thrust, the eastward displacement adding Precambrian crystalline basement rocks to plates. Inclusion of crystalline basement in the Lee Canyon and is approximately 4.5 mi (7.5 km) (Fleck, Keystone thrust plates is to emphasize the non-décollement geometry which is possible. Note: high-angle faults have 1970). In addition, evidence presented been removed, and topography is hypothetical. Dot pattern is late Precambrian terrigenous sequence, vertical line pattern is added thrust slice, gray pattern is autochthonous terrigenous sequence, and curved lines are Precambrian above suggests that the Lee Canyon thrust crystalline rocks. is losing displacement eastward, and this

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may be the reason for the cross-cutting rela- thrust or as far west as the Wheeler Pass liest occurrence of detrital material from tions between the thrust and stratigraphic thrust. these rocks is in the Baseline Sandstone units in the upper plate (that is, as it loses Work in the Clark Mountains thrust (Longwell, 1952; Armstrong, 1968). The displacement, the thrust trace more clearly complex has demonstrated that thrusts, younger Overton Fanglomerate appears to expresses the ramp or cross-cutting charac- even though they flatten at depth and in- be syn- or post-tectonic, but its age is un- ter toward the point at which the thrust volve only sedimentary rocks, are not certain; it is Cretaceous(P) or Tertiary(P). finally dies out). stratigraphically controlled in detail Postassium-argon ages of approximately 23 The Macks Canyon thrust probably does (Burchfiel and Davis, 1971). Thrusts in m.y. from ash beds lying conformably not merge at depth with the Lee Canyon some places cut subparallel to stratigraphic above the Overton were determined by thrust in the central Spring Mountains. At units but at other places cut across folds Armstrong (1963); thus the upper limit of its westernmost exposure, displacement on and formation contacts both downdip and deformation in this area can only be dated the thrust is small and it appears to be dying parallel to strike. Thus, these thrusts are not as Miocene. out within the upper part of the Bonanza décollement faults in a strict sense. Proba- In the Clark Mountain area to the south, King Formation. Farther northeast where it bly similar relations are present below the thrust faults, including the continuation of has larger displacement, the thrust may Spring Mountains, particularly in the Lee the Keystone thrust, are intruded by plu- merge at depth with the Lee Canyon thrust. Canyon and Wheeler Pass thrust plates, and tons that yield radiometric dates between A great disparity in the structural eleva- the geometry presented in Figure 6 must be 84 to 94 m.y. (Adams and others, 1968). tions of stratigraphic units on either side of viewed as an over-simplified construction. These relations are consistent with regional the Wheeler Pass thrust is present. A differ- Estimates of the magnitude of eastward data presented by Armstrong (1968) that ence of 10,000 ft (3,000 m) in elevation is displacement vary considerably, depending thrusting in the eastern part of the Cordille- present between the base of the Bonanza upon which model is used for the geometry ran orogen from central Utah to southern King Formation in the upper and lower of the thrust faults at depth. In the models Nevada was concluded before the end of plates (Fig. 5). If the thrust flattens into the presented here, the minimum figures range the Cretaceous. late Precambrian terrigenous sequence at from 22 to 45 mi (36.6 to 75 km). The Recent work by Burchfiel and Davis depth, it must flatten below the exposed lower figure presented here is within the (1971) has demonstrated that in the Clark parts of the Johnnie Formation. Because the range (18 to 32 mi; 30 to 50 km) presented Mountains, thrusting began as early as Late Johnnie is near the base of the conformable by Fleck (1970) based on arguments other Triassic or Early Jurassic time in some of geosynclinal sequence, only the Noonday than those presented here. The 45-mi the thrust plates above the Keystone plate. Dolomite would be older and yet conform- (75-km) value is within the range (40 to 60 Structures from this older episode (or able (Stewart, 1970); the thrust might mi; 67 to 100 km) presented by Armstrong episodes) project northward and perhaps flatten into the basal part of the Johnnie. If (1963) as average for the eastern thrust belt continue into the central part of the western that is the case, the thrust would flatten at a of the Cordilleran orogen. The figure could Spring Mountains. The time of inception of depth comparable to the base of the be even larger, however, as no estimate was deformation in the central and western Bonanza King Formation in the lower plate, made of additional displacement required Spring Mountains thus remains uncertain. suggestng that at least 5,000 ft (1,500 m) because of (1) the geometry of eroded parts and probably 7,000 to 8,000 ft (2,100 to of the plates, (2) the western extent of the SUMMARY 2,400 m) of strata of the lower plate extend Wheeler Pass thrust plate, (3) a more exten- The Spring Mountains, Nevada, contain beneath the entire exposed width of the sive displacement of the Red Spring thrust probably the most southern example of Wheeler Pass thrust plate in the Spring plate than estimated above, and (4) the pos- "typical" eastern Cordillera structure. Mountains. This geometry would require sible presence of additional thrust slices at Three major east-directed thrust plates an eastward displacement of at least 20 mi depth that cannot be predicted from surface move geosynclinal Paleozoic and late Pre- (33 km) (Fig. 6b). geology. cambrian sedimentary rocks eastward over An alternative hypothesis is that Precam- Age of Deformation. The age or ages of cratonal equivalents. Too much uncertainty brian basement rocks are involved in the deformation in the Spring Mountains is not exists in the projection of thrust faults to Wheeler Pass thrust (Fig. 6c). Work in the closely bracketed by stratigraphic units. depth to determine the involvement of older Clark Mountains has demonstrated that The Keystone and Red Spring thrusts can basement rocks, although the reconstruc- over distances of as much as 16 mi (26 km), be placed only in the interval Early Jurassic tions suggest that these rocks may be pres- some thrusts flatten at depth into older (post-Aztec) to pre-late Cenozoic (alluvial ent in the Wheeler Pass thrust plate. Thrust- basement rocks (Burchfiel and Davis, deposits). The age of the channel-fill de- ing in the Spring Mountains produced a 1971). If this is the case for the Wheeler posits below the Keystone and Red Spring minimum shortening estimated from the Pass thrust, its geometry at depth is more thrusts unfortunately is unknown. décollement model to be between 22 and 45 difficult to determine, and the amount of Closer dating of the deformation comes mi (36.6 to 75 km). Some deformation oc- displacement could be as small as 7 or 8 mi from the Muddy Mountains 40 mi (67 km) curred during Late Cretaceous time, but re- (11.6 to 13.3 km) or as large as 20 mi to the northeast and from the Clark Moun- cent work suggests that part of the defor- (33.3 km). tains 30 mi (50 km) to the south. In the mation could be early or middle Mesozoic Assuming that thrusts in the Spring Muddy Mountains, the Willow Tank For- in age. Mountains flatten at depth, the geometry of mation and Baseline Sandstone overlie the Stratigraphy across the Spring Moun- the thrust belt appears similar in style to Aztec Sandstone with slight angular uncon- tains shows a transition from a Paleozoic that of other parts of the Cordilleran formity. Fossils from the upper part of the cratonal sequence in the east to a geosyncli- orogen (Armstrong, 1968; Bally and others, Willow Tank yield early Late Cretaceous nal sequence in the west. Addition of a 1966). In general, a detached miogeosyn- ages (Longwell, 1949), and ash beds in the thick section of late Precambrian terrige- clinal sequence seems to have moved east- lower part of the Willow Tank have yielded nous rocks at the base of the geosynclinal ward and imbricated as thrust faults west- K/Ar ages of 98.4 and 96.4 m.y. or early sequence accounts for nearly one-half of the ward into older stratigraphic units. It is un- Late Cretaceous (Fleck, 1970). Regional thickening in the geosynclinal sequence. certain where the thrust faults first cut into studies show that pre-Silurian strata were Where this section of terrigenous rocks first the late Precambrian terrigenous sequence; not exposed in southeastern Nevada until occurs beneath the Spring Mountains is unit may occur as far east as the Keystone early Late Cretaceous time, since the ear- known, but geometry of the thrust plates at

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depth suggests that they may occur in or 76, p. 175-192. 258-A, p. 115-125. below the Keystone thrust. Burchfiel, B. C., and Davis, G. A., 1971, Clark Livingston, J. L., 1964, Stra'igraphic and struc- Mountain thrust complex in the Cordillera tural relations in a portion of the northwest ACKNOWLEDGMENTS of southeastern California: Geologic sum- Spring Mountains, Nevada [M.S. thesis]: mary and field trip guide: California Univ., Houston, Texas, Rice Univ., 35 p. Financial support for the work presented Riverside, Campus Mus. Contr., no. 1, p. Longwell, C. R., 1926, Structural studies in in this report has been provided from 1-28. southern Nevada and western Arizona: numerous sources and is acknowledged as 1972, Structural framework and evolution Geol. Soc. America Bull., v. 37, p. 551-584. follows: Burchfiel, National Science Foun- of the southern part of the Cordilleran 1945, The mechanics of orogeny: Am. Jour. dation Grants GP-1154 and GA-21375; Orogen, western United States: Am. Jour. Sci., v. 243-A, p. 417—445. Davis, National Science Foundation Grant Sci.,v. 272, p. 97-118. 1949, Structure of the northern Muddy GA-1562; Fleck, National Science Founda- Davis, G. A., 1973, Relations between the Key- Mountain area, Nevada: Geol. Soc. tion and Department of Geology and Geo- stone and Red Spri ig thrust faults, eastern America Bull., v. 60, p. 923-967. physics, University of California, Berkeley; Spring Mountains, Nevada: Geol. Soc. 1952, Structure of the Muddy Mountains, America Bull., v. 84, p. 3709-3716. Nevada: Utah Geol. and Mineralog. Survey Secor, D. F. Hewett Fund from Stanford Fleck, R. J., 1967, The magnitude, sequence, and Guidebook no. 7, p. 109-114. 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