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Lower Permian sediment-gravity-flow sequence, eastern California
Sedimentary Geology, 64 (1989) 1-12 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
1
Lower Permian sediment-gravity-flow sequence, eastern California C A L V I N H. STEVENS 1, M I C H A E L S. L I C O 2 and P A U L S T O N E 3 Department of Geology, San Jose State University, San Jose, CA 95192 (U.S.A.) 2 U.S. Geological Survey, 705 N. Plaza St., Carson City, NV89701 (U.S.A.) 3 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 (U.S.A.) Received October 5, 1988; revised and accepted April 20, 1989
Abstract Stevens, C.H., Lico, M.S. and Stone, P., 1989. Lower Permian sediment-gravity-flow sequence, eastern California. Sediment. Geol., 64: 1-12. The Lower Permian (middle Wolfcampian) Zinc Hill sequence, a 65- to ll0-m-thick series of beds in the Owens Valley Group in east-central California, comprises sediment-gravity-flow deposits consisting of carbonate sediment that originated on, and siliciclastic sediment that may have been generally ponded behind, a carbonate shelf to the east and northeast. Thickness patterns and paleocurrent indicators show that the sediment forming this sequence was transported primarily southeastward and deposited in a southeast-trending, lobe-shaped body. Evidently, the sediment was carried from the shelf by sediment-gravity flows that travelled westward down the slope and then turned southeastward upon reaching a southeast-trending basin at the base of the slope. Data derived from the study of this basin, which paralleled the shelf edge and is thought to have formed parallel to a southeast-oriented segment of the Early Permian continental margin, constitute one of the most important arguments favoring a Pennsylvanian to Early Permian age of truncation of the western North American continental margin.
Introduction
The Lower Permian section in a large area of east-central California (Fig. 1) is composed of a thick sequence of basinal carbonate and finegrained siliciclastic rocks that in many places overlie older carbonate shelf rocks. This superposition of facies formed in such topographically and sedimentologically different environments suggests that a major readjustment of tectonic features occurred prior to the deposition of the Permian rocks. The event responsible for this reorganization was postulated by Stone and Stevens (1988) to have been associated with the truncation of the southwestern part of the North American continent, an idea previously postulated to explain the apparent termination of Paleozoic facies belts 0037-0738/89/$03.50
© 1989 Elsevier Science Publishers B.V.
in central California (Hamilton and Meyers, 1966), and assumed to have occurred in the early Mesozoic (Davis et al., 1978). Analysis of the Permian basinal rocks in east-central California, which were deposited near the truncated margin, is critical to the understanding and interpretation of this important tectonic event. In Darwin Canyon and the northern Argus Range, the area of this study, the Lower Permian section is composed of about 2000 m of calcareous siltstone, limestone conglomerate, bioclastic limestone, argillaceous limestone, and calcareous mudstone, much of which was deposited by sedimentgravity flows (Stone et al., 1987). Although the Lower Permian section in the study area appears monotonous, some series of beds are so distinctive they can be correlated throughout the entire area
Fig. 1. Index map of east-central California showing distribution of Permian rocks of the Owens Valley Group. B C = Bendire Canyon: C M = Conglomerate Mesa; DC = Darwin Canyon; D H = Darwin Hills; K C = Knight Canyon; M C = Marble Canyon; OC = Osborne Canyon; P S = Panamint Springs. Area in rectangular box is enlarged for Fig. 2.
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Fig. 2. Index map of the Darwin Canyon-northern Argus Range area showing locations of measured sections.
covered (Fig. 2). These are here referred to as localities A through G.
(Lico, 1983; Stone, 1984). Within this Lower Permian section we here describe and interpret the paleogeographic significance of a series of especially distinctive beds, approximately 65-110 m thick, which will be referred to as the Zinc Hill sequence. This sequence of rocks is named for a local geographic feature in the northern Argus Range. We have selected these beds for detailed study because they can be recognized throughout a wide area and because they typify the Lower Permian section in this region. The lateral extent of the Zinc Hill sequence was ascertained through detailed study of Permian rocks throughout the area shown in Fig. 1 (Stone, 1984; Stone and Stevens, 1984). This study showed that the Zinc Hill sequence is recognizable throughout an area of about 75 k m 2 including all of Darwin Canyon and the northern Argus Range to south of Osborne Canyon (Fig. 1). Good, structurally uncomplicated exposures of this sequence, however, are rare; only seven sections reasonably suitable for detailed study were dis-
Stratigraphy The Lower Permian rocks in Darwin Canyon and the northern Argus Range are assigned to the Owens Valley Group (Stone and Stevens, 1987), which, in this area, has been divided into the Osborne Canyon Formation and the overlying Darwin Canyon Formation (Stone et at., 1987). The latter formation has been subdivided into the lower Millers Spring and upper Panamint Springs Members (Stone et al., 1987; Fig. 3). The Zinc Hill sequence, which is entirely middle Wolfcampian in age (Magginetti et at., 1988), consists of rocks within the lower part of the Millers Spring Member, specifically unit 2 and the lower part of unit 3 (Fig. 3) as defined by Stone et al. (1987). In this report, the Zinc Hill sequence is divided into nine subunits which consist of five predominantly limestone beds containing transported, shallowwater fossils and four intervening zones composed
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Fig. 3. Stratigraphy of the Owens Valley Group and the Zinc Hill sequence in the Darwin C a n y o n - n o r t h e r n Argus Range area.
largely of very fine-grained siliciclastic sandstone and coarse siltstone. The limestones, in ascending order, are referred to as subunits 1L-SL: the intervening, dominantly siltstone and sandstone subunits are numbered 1S-4S (Fig. 3). The major characteristics of these subunits are as follows: Subunit 1: Medium-dark-gray- to mediumlight-gray-weathering, fossiliferous, clast-supported limestone conglomerate with clasts up to 5 cm near the base grading upward through a horizontally laminated division into argillaceous, micritic limestone near the top. The thickness generally is 4 or 5 m, but it ranges up to 14 m. Subunit IS: Grayish-orange-to moderate-yellowish-brown-weathering, calcareous, siliciclastic siltstone and sandstone in beds 0.2-0.7 m thick with internal horizontal and convolute laminations. Platy, pale-red, siliciclastic mudstone is present between some of the siltstone and sandstone beds. This subunit ranges in thickness from less than 2 m to 10 m. Subunit 2L: Medium-dark-gray- to brownishgray-weathering, fossiliferous, clast-supported limestone conglomerate with clasts up to 2 cm near the base grading upward through a horizontally laminated division into micrite at the top. The thickness range from about 3 to 22 m with a mean of about 7 m. Subunit 2S: Grayish-orange- to moderate-yellowish-brown-weathering, calcareous, siliciclastic siltstone and sandstone in beds 0.2-2.0 m thick with internal horizontal and convolute lamination. Rare calcareous concretions and interbeds of moderate-red and dark-gray mudstone occur in this subunit. A fine-grained, medium-gray, structureless limestone represents this interval in section E, and similar limestone occupies more than half of the interval at section F. This unit ranges in thickness from about 1 to over 13 m, and generally is about 10 m thick. Subunit 3L: Grayish-orange- to light-brownweathering, fossiliferous, matrix-supported limestone conglomerate with contorted clasts and slabs of calcareous siltstone up to 3 m long. The thickness ranges from 10 to about 47 m. Subunit 3S: Grayish-orange- to moderate-yellowish-brown-weathering, calcareous, siliciclastic siltstone and sandstone in beds 0.2-2.0 m thick
with horizontal, cross- and convolute lamination. At section C the upper part of this interval ~s occupied by medium-light-gray, fine-grained limestone with rare limestone clasts near the base. This interval ranges in thickness from about 1 to more than 12 m. Subunit 4L: Pale-red- to light-brown-weathering, fossiliferous, matrix-supported limestone conglomerate with clasts up to about 8 cm long. This bed ranges from 1 to 8 m in thickness. Subunit 4S: Grayish-orange- to moderatebrown-weathering, calcareous, siliciclastic siltstone and sandstone in beds 0.2-0.6 m thick, with internal horizontal, cross- and convolute lamination and some graded beds. This interval ranges from about 6 to 16 m in thickness. Subunit 5L: Medium-dark-gray- to mediumlight-gray-weathering, fusulinid-rich, clast-supported, small-pebble limestone conglomerate showing a sequence of graded bedding, horizontal lamination and cross-lamination from base to top. The thickness ranges from about 0.3 to 1.0 m.
Lateral distribution The Zinc Hill sequence is recognizable throughout Darwin Canyon and the northern Argus Range as far south as Osborne Canyon (Fig. 1). In sections A, B, C, and F (Fig. 2), all nine subunits of the sequence are present (Lico, 1983). In section D, subunits 4 L - 5 L are exposed; in section E, subunits 2 L - 5 L are exposed; and in section G, subunits 1 L - 4 L are exposed (Fig. 4). Elsewhere in the region, individual beds that could be the same as those of the Zinc Hill sequence are present, although the entire sequence is not represented. For example, a clast-supported, graded limestone conglomerate bed is present in the stratigraphic position of subunit 1L in the Darwin Hills west of the study area (Fig. 1), but no other recognizable part of the Zinc Hill sequence is present. Similarly, one thick matrix-supported conglomerate bed, lithologically similar to subunit 3L of the Zinc Hill sequence and in about the same stratigraphic position, is present both in Marble Canyon to the northeast and in Knight Canyon to the south (Fig. 1), but neither is associated with other recognizable subunits of the Zinc
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Hill sequence. These other conglomerates may or may not have been deposited during one of the events that formed the limestone subunits of the Zinc Hill sequence, but regardless, the sequence is not recognizable. Thus, the Zinc Hill sequence presently crops out within an area of about 75 km 2 (Fig. 2). It must terminate to the southwest under alluvium because it is not present in the Darwin Hills where rocks of the appropriate age are exposed. Neither does the Zinc Hill sequence extend into the Conglomerate Mesa area farther northwest (Fig. 1), where the depositional sequence is quite different from that in Darwin Canyon. It also must terminate to the south between outcrops in Osborne Canyon and Knight Canyon, where it is not present. Thus, the termination of the Zinc Hill sequence to the northwest, west, and south is relatively well constrained.
Major rock types and modes of deposition Five major rock types are represented in the Zinc Hill sequence: (1) siltstone and very-fine-
grained sandstone, (2) matrix-supported limestone conglomerate, (3) clast-supported limestone conglomerate, (4) mudstone, and (5) fine-grained limestone. All exposures of each rock type share many characteristics, so a general description can be written for each type.
Siltstone and very-fine-grained sandstone These rocks form moderately well exposed outcrops in subunit 4S and most sections of subunits 1S, 2S, and 3S. They are medium to dark gray on fresh surfaces and grayish orange to moderate yellowish brown on weathered surfaces, and occur in beds 0.1-2 m thick. Individual beds are laterally extensive and have planar contacts above and below. Horizontal lamination (Fig. 5A), which commonly characterizes entire beds, is replaced in the upper parts of some beds by disturbed bedding, convolute lamination, or medium-scale trough cross-lamination (Fig. 5B). The crosslamination commonly occurs in multiple sets, each up to 10 cm thick. Many beds appear massive, but thin sections show that at least some of them are
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Fig. 5. A. Typical siltstone bed showing horizontal lamination. Pen is about 15 cm long. Unit 4S at section A. B. Cross-lamination in the upper part of a very fine-grained sandstone. Scale is 15 cm long. Unit 4S near section F. C. Matrix-supported limestone conglomerate. Hammer is 31 cm long. Unit 3L at section A. D. Relatively thin, clast-supported limestone conglomerate. The coarse, basal division (at the bottom of photograph) is overlain by well developed plane beds (of which the upper ones apparendy are inversely graded and represent Lowe's (1982) $2 unit) and an upper convoluted division. Scale is 15 cm long. Unit 5L at section C. E. Lower part of a clast-supported limestone conglomerate. The erosional base is on the left. The lower, coarse, crudely graded division is overlain by plane bedded, alternating fine and coarse beds, some of which are inversely graded. This division grades into the fine calcarenite shown in F. Unit 2L at section A. The clipboard is 23 cm wide. F. Lower part of the uppermost division of the bed shown in E showing parallel laminae below hammer handle overlain by highly disturbed sediment. The hammer is 31 cm long.
very-fine-grained sand. Detrital muscovite occurs in relatively minor quantities. The T b division of the Bouma sequence dominates, but some beds lack horizontal lamination and show subtle grading suggesting the Bouma Ta division. Mediumscale cross laminae, similar to those referred to by Lowe (1982) as a Tt division and interpreted by him as deposits of a residual current following sedimentation of a high-density load, also are present. Most of these beds seem to best fit Lowe's (1982) models 9 and 10 which probably are deposited by moderate-density turbidity currents.
Matrix-supported limestone conglomerate Matrix-supported conglomerates (Fig. 5C) form subunits 3L and 4L. They are medium gray on fresh surfaces and grayish orange to brown on weathered surfaces. These conglomerates occur in single beds up to 40 m thick that typically form ridges or cliffs. The beds are completely matrixsupported and without organization except for rare hints of clast imbrication. Contacts with underlying beds are sharp and planar to undulatory without signs of scour. Contacts with overlying beds also are sharp and undulatory. The clasts, which range in size from less than 1 cm to 3 m, consist of medium-dark-gray, micritic limestone; dark-gray, fossiliferous limestone; grayish-orange to moderate-yellowish-brown siltstone; pale-red mudstone; and fossils, mostly fusulinids, crinoid debris and colonial coral fragments. These clasts are mainly angular to subrounded, but the larger ones are tabular with large length to width ratios. Some large, originally tabular clasts have been distorted into "S"- or "rolled"-shapes. The matrix is composed of silty micrite, generally with less than 5% quartz silt. All of the above features typify Lowe's (1982) model 1 deposits of debris flows.
Clast-supported limestone conglomerate This lithology forms subunits 1L, 2L, and 5L. It is typically medium dark gray on fresh surfaces and medium light gray on weathered surfaces. The total bed thickness ranges from 0.3 to 20 m. The thick beds generally crop out as well exposed
resistant ridges, whereas the thin beds commonly are covered. All beds are graded, although large pieces of massive corals belonging to the Thysanophyllum coral belt of Stevens (1982) and generally the largest clasts present, occur in the middle at many localities. The thick beds are variable in their interval organization, but they generally show three divisions: (1) a lower, clastsupported, graded division, commonly about 2 m thick; (2) a middle calcarenite division, commonly 2 m thick, with some reverse-graded horizontal laminae; and (3) an upper, silty to micritic division, commonly about 3 m thick, with local horizontal lamination a n d / o r contorted and disrupted bedding (Fig. 5D-F). Locally, a matrix-supported conglomerate occurs at the base of these beds and at one locality, one of the thick conglomerates is broken by two shale partings indicating two sedimentological breaks. Imbricated clasts of mudstone, siltstone and limestone are present at a few localities. Sole marks, including flute casts, groove casts, and lineations, occur on the base of some of the thin beds. The basal contacts are sharp and undulatory, and some how evidence of channeling. The upper boundaries grade into massive calcareous mudstone, or are sharp and planar. The maximum clast size in the thick beds ranges from 2 cm to 1.5 m and, in the thin beds, from about 0.2 to 3 cm. Clasts consist of medium-darkgray micritic limestone; medium-dark-gray limestone containing a variety of invertebrate fossils, and rarely, dasycladacean algae and ooliths; quartz-rich calcarenite; grayish-orange- to moderate-yellowish-brown-weathering, horizontal and cross-laminated siltstone; pale-red mudstone; crinoid, coral, and bryozoan fragments; and whole fusulinids. The matrix consists of micritic limestone. The lower parts of the thicker limestone beds (those several meters in thickness) are very similar to Lowe's (1982) model 8 in which a R3 division (coarse grained and normally graded) is overlain by a S1 division (medium grained with horizontal and cross laminae) a n d / o r a Bouma Ta division. These beds may be capped by Bouma T b and Tc divisions. The lower parts of these beds apparently were deposited by high density turbidity currents; the uppermost parts presumably were deposited
by low density parts of the same flows. Some of the thin limestones show Bouma T~_ d divisions corresponding to model 10 of Lowe (1982). These beds probably were deposited by turbidity currents of relatively low density.
Mudstone This lithology occurs in subunit 1S (section C) and subunit 2S (sections A, B, C, and F). It includes calcareous, pale-red and dark-gray, siliciclastic mudstone in beds 2-30 cm thick, commonly separating siliciclastic siltstone a n d / o r sandstone beds. The mudstone is laminated or structureless and unfossiliferous except for rare ammonoids. Although radiolarians might be expected in these rocks, none were observed in thin section. These rocks are inferred to represent the background hemipelagic sedimentation.
Fine-grained limestone This lithology occurs in subunit 2S (sections E and F) and in subunit 3S (section C). It is composed of moderately to well exposed, mediumdark- to dark-gray limestone occurring in beds 0.5-8 m thick. No internal structure has been noted, but in places limestone lithoclasts, crinoid and coral fragments, and fusulinids appear to "float" in the fine-grained, extensively recrystallized limestone. Quartz grains occur sparingly. The depositional mode of this rock type is uncertain. The floating allochems, however, suggest that it was formed by some type of sedimentgravity flow, perhaps a fine-grained debris flow.
Basin analysis Data bearing on the nature of the basin in which the Zinc Hill sequence was deposited are: (1) the nature of the sediment and the sedimentary structures, (previously discussed), (2) paleocurrent indicators, and (3) thickness patterns. For plotting the paleocurrent data (Fig. 6) and the thickness data (Figs. 7, 8), we have removed the interpreted lateral displacement on three major faults in the area: the Darwin, Osborne Canyon, and West Argus Range faults (Fig. 2). Because the
Darwin and Osborne Canyon faults show similar displacement and terminate against the West Argus Range fault from opposite directions, they are considered to have been originally two segments of the same fault that were offset 1.6 km in a right-lateral sense on the West Argus Range fault. Matching of rocks and structures on each side of the other faults indicates 1.2 km of leftlateral displacement on the Darwin fault, and about 1 km of left-lateral offset on the Osborne Canyon fault.
Directional features Several different indicators of transport direction are present in the rocks of the Zinc Hill sequence, although completely convincing features are relatively rare. The most common indicators, in decreasing order, are: trough cross-lamination, generally in the upper parts of siltstone beds; pebble imbrication; flute casts; lineations; and groove casts. Trough cross-lamination and flute casts are considered to be most useful. Pebble imbrication was used in only one instance where it is clearly primary and there are lineations on the base of the bed from which a more precise direction of transport could be ascertained. Elsewhere, especially in some thick limestone conglomerates, imbrication may be pronounced, but it is parallel with a tectonic foliation and may have resulted from rotation or stretching of clasts during deformation. The overall transport direction to the southeast (Fig. 6) is consistent with previous studies of paleocurrents in the Owens Valley Group in the Darwin Canyon area by Stevens and Stanton (1980), who reported a mean transport direction of S30°E; by Lico (1983), who computed mean current directions ranging from S 2 0 ° E to $73 ° E; and by Stone (1984) and Stone and Stevens (1984), who reported a modal transport direction of about S15°E.
Thickness trends Isopach maps were constructed on the theory that the shapes of the sediment bodies would give some indication of the shape a n d / o r orientation
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Fig. 6. Summary of paleocurrent data (including data of Lico, 1983). Letters refer to locations of measured sections after restoration of offset on the Darwin, West Argus Range, and Osborne Canyon faults (see Fig. 2). Each arrow indicates the vector resultant for measurements taken from one bed except in section A where A' indicates measurements from a second bed. Numbers indicate the number of measurements made.
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"(o) Fig. 7. Isopach map of the subunit 2L-5L interval (in meters). Letters refer to locations of measured sections after restoration of offset on the Darwin, West Argus Range, and Osborne Canyon faults (see Fig. 2). Thickness for location D is a reconstructed estimate.
Fig. 8. Isopach map of subunit 3L (in meters). Letters refer to locations of measured sections after restoration of offset on the Darwin, West Argus Range, and Osborne Canyon faults (see Fig. 2).
of the basin in which they were deposited. Thicknesses of some subunits in several areas could not be accurately ascertained because of unresolved structural problems in this complexly deformed area. so the isopach maps must be considered approximations of the true thickness patterns. Lico (1983) showed some different thicknesses than those shown here because of differences in interpretation of the stratigraphy, but the resulting isopach maps are very similar. Because data on the interval from the base of subunit 2L to the top of subunit 5L and on subunit 3L are the most complete and believed to be the most accurate, isopach maps of these intervals are presented here (Figs. 7, 8). They suggest that the sediment bodies are tongue-shaped, oriented approximately $20 ° E, and do not extend south as far as Knight Canyon.
Paleogeography and sources of sediment
Origin of sediment The major paleogeographic elements in eastern California during the Early Permian were a series of basins in which sediment of the Owens Valley Group was deposited west of a large carbonate shelf upon which the Bird Spring Formation and equivalent rocks were deposited. This carbonate shelf extended throughout southern and eastern Nevada and the eastern Mojave desert (Stevens, 1977; Stone, 1984; Fig. 9). During most of the Early Permian (middle Wolfcampian and later time), this carbonate shelf, which lay east of the present Panamint Range, had a southeast-trending margin (Stone, 1984). The carbonate sediment in the basinal Owens Valley Group is interpreted to have been derived from this shelf because of (1) the large volume of this sediment, which suggests derivation from an extensive carbonate shelf or bank, (2) the occurrence of ooliths, dasyctadacean algae, and large fragments of colonial corals, which indicates a source in very shallow water, and (3) the abundance of colonial coral fragments representing genera from the Thysanophyllum coral belt, which are known only from that shelf (Stevens, 1982). Collapse of the outer portions of the shelf is
Interpretations Although the paleocurrent and thickness data are not conclusive, they suggest that the southeastflowing density currents deposited southeasttrending sediment bodies that thin southeastward. This in turn suggests that the portion of the basin in which the Zinc Hill sequence was deposited sloped southeastward.
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11 inferred to have given rise to the thick carbonate sediment-gravity-flow deposits now exposed in Darwin Canyon and the northern Argus Range. Siliciclastic debris, which composes a large part of the Lower Permian section in southeastern California, rarely is thoroughly mixed with the limestone debris suggesting that the two types of sediment originated in somewhat different areas. The siliciclastic material almost certainly represents a southwestward extension of the enormous body of craton-derived quartz sand and silt deposited during the Early Permian throughout the western Rocky Mountains and the eastern Great Basin (Sheriff, 1975). This sediment probably was carried into the shallow portions of the miogeosynclinal sea by wind and/or water, and perhaps, as proposed by Stone (1984), was ponded on the landward side of the carbonate shelf. From there the siliciclastic debris may have been evacuated periodically in sediment-gravity flows through channels cut into the carbonate shelf and deposited in the deep basins to the west. No such channels, however, have been recognized. Basin characteristics
Some aspects of the basin in which the Lower Permian rocks of the Owens Valley Group in Darwin Canyon and the northern Argus Range were deposited can be inferred from data on various sediment bodies. First, unit 1 of the Millers Spring Member (Fig. 3) of the Darwin Canyon Formation occurs widely in eastern California including Darwin Canyon, the Darwin Hills, the eastem Conglomerate Mesa area, Marble Canyon, and the northern Argus Range to south of Knight Canyon (Fig. 1). This siliciclastic unit, which is 120 m thick, lies immediately beneath the Zinc Hill sequence in Darwin Canyon and the northern Argus Range. It is unbroken by significant limestone sediment-gravity-flow deposits, suggesting that the region was experiencing tectonic quiescence during deposition of this unit and that the limestone shelf was stable. The entire area covered by this sandstone and siltstone unit may have constituted a single depositional basin. The appearance of coarse-grained limestone sedimentgravity-flow deposits, including the limestone sub-
units of the Zinc Hill sequence, throughout this basin probably represents the sudden collapse of parts of the carbonate shelf, perhaps caused by earthquakes. Such earthquakes may have been produced by movement on faults that fragmented the original large basin into smaller components such as the one in which the Zinc Hill sequence was deposited. Internal directional features and the shape of the Zinc Hill sediment body show a southeast orientation, indicating that this basin had a southeast trend and that it was relatively narrow. The area of outcrops shows that it was at least 9 km wide, and the isopachous maps (Figs. 7, 8) suggest that it probably was about 15 km wide. The length of the basin is unknown, but the sediment deposited within it presently is exposed over a longitudinal distance of 10 km. Although the Zinc Hills sediment body evidently lies parallel to the shelf margin from which the carbonate material was derived, directional features in the clast-supported conglomerates indicate that it is not a carbonate slope apron or base-of-slope apron such as those described by Mullins and Cook (1986) and Colacicchi and Baldanza (1986). Instead, it appears that the sediment-gravity flows feeding the basin carried the basin carried sediment off the carbonate shelf, down the westward-facing slope, and then down the southeastward-dipping axis of the basin at the base of the slope (Fig. 9).
Regional tectonic implications The basinal Lower Permian rocks east and southeast of the Inyo Mountains, including those discussed in this report, accumulated in basins that formed by middle Wolfcampian time in an area close to the continental margin (Stone and Stevens, 1984, 1988). These rocks contrast strongly with Mississippian and older, primarily shallowshelf deposits that underlie them, and represent a period of major subsidence. A very important aspect of the paleocurrent data and basin reconstruction is that they indicate that the orientation of these Early Permian basins (southeast) was at a relatively high angle (45-60 °) to that of earlier basins and shelf edges in the region (Stevens, 1986), and that in the Early Permian,
sediment was carried toward the continent as it was constituted in the Early and Middle Paleozoic. The appearance of these new, southeast-oriented basins in the Early Permian constitutes one of the major lines of evidence that the continental margin was fragmented at a profound angle to its earlier trend. We interpret this to mean that the apparent truncation of Early and Middle Paleozoic facies belts in central California as postulated by Hamilton and Meyers (1966) is real, and that the truncational event occurred in the Late Paleozoic rather than in the early Mesozoic as suggested by Davis et al. (1978).
Acknowledgements We are grateful to David Andersen who constructively criticized an early version of the manuscript, and to him and Monty Hampton who carefully reviewed later versions. Additional reviews by Mario Coniglio and Paul Enos also were very helpful.
References Colacicchi, R. and Baldanza, A., 1986. Carbonate turbidites in a pelagic basin: Scaglia Formation, Appennines--comparison with siliciclastic depositional models. Sediment. Geol., 48: 81-105. Davis, G.A., Monger, J.W.H. and Burchfiel, B.C., 1978. Mesozoic construction of the Cordilleran "collage", central British Columbia to California. In: D.G. Howell and K.A. McDougall (Editors), Mesozoic Paleogeography of the Western United States. Pacific Coast Paleogeography Symposium 2. Pac. Coast Sect. Soc. Econ. Paleontot. Mineral., pp. 1-32. Hamilton, W. and Meyers, W.B., 1966. Cenozoic tectonics of the western United States. Rev. Geophys., 4: 509-549. Lico, M.S., 1983. Lower Permian submarine sediment gravity flows in the Owens Valley Formation, southeastern California. M.S. Thesis, San Jose State University, San Jose State University, San Jose, Calif., 80 pp. Lowe, D.R., 1982. Sediment gravity flows, II. Depositional models with special reference to the deposits of high-density turbidity currents. J. Sediment. Petrol., 52: 279-297.
Magginetti, R.T., Stevens, C.H. and Stone. P,. 1988. t,,arl~ Permian fusulinids from the Owens Valley Group, e~Js~central California. Geol. Soc. Am. Spec. Pap., 217:61 pp. Mullins, ft.T. and Cook, H.E., 1986. Carbonate apron models: alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration, Sediment. Geol., 48: 37- 79. Sheriff, A.. 1975. Origin and distribution of terrigenous detritus in the late Early Permian of the central Cordilleran Miogeosyncline. M.S. Thesis, San Jose State University, San Jose, Calif., 69 pp. Stevens, C.H., 1977. Permian depositional provinces and tectonics, western United States. In: J.H. Stewart, C.H. Stevens and A.E. Fritsche (Editors), Paleozoic Paleogeography of the Western United States. Pacific Coast Paleogeography Symposium 1. Pac. Sect. Soc. Econ. Paleontol, Mineral., pp. 113-135. Stevens, C.H., 1982. The Early Permian Thysanophyllum coral belt: another clue to Permian plate-tectonic reconstructions. Geol. Soc. Am. Bull., 93: 798-803. Stevens, C.H., 1986. Evolution of the Ordovician through Middle Pennsylvanian carbonate shelf in east-central California. Geol. Soc. Am. Bull., 97: 11-25. Stevens, C.H. and Stanton, W.R., 1980. Stylastraea-bearing gravity flow sequence of Early Permian age in eastern California. Geol. Soc. Am. Abstr. Progr., 12 (3): 154. Stevens, C.H. and Stone, P., 1988. Early Permian thrust faults in east-central California. Geol. Soc. Am. Bull., 100: 552-562. Stone, P., 1984. Stratigraphy, depositional history, and paleogeographic significance of Pennsylvanian and Permian rocks in the Owens Valley-Death Valley region, California, Ph.D. Thesis, Stanford University, Stanford, Calif., 399 pp. Stone, P. and Stevens, C.H., 1984. Stratigraphy and depositional history of Pennsylvanian and Permian rocks in the Owens Valley-Death Valley region, eastern California. In: J. Lintz, Jr. (Editor), Western Geological Excursions, 4. Geological Society of America and Mackay School of Mines, University of Nevada, Reno, Nev., pp. 94-119. Stone, P. and Stevens, C.H., 1987. Stratigraphy of the Owens Valley Group (Permian), southern Inyo Mountains. California. U.S. Geol. Surv. Bull., 1692:19 pp. Stone, P. and Stevens, C.H., 1988. Pennsylvanian and Early Permian paleogeography of east-central California: implications for the shape of the continental margin and the timing of continental truncation. Geology, 16: 330-333. Stone, P., Stevens, C.H. and Magginetti, R.T., 1987. Pennsylvanian and Permian stratigraphy of the northern Argus Range-Darwin Canyon area, California. U.S. Geol. Surv. Bull., 1691 : 30 pp.
1
Lower Permian sediment-gravity-flow sequence, eastern California C A L V I N H. STEVENS 1, M I C H A E L S. L I C O 2 and P A U L S T O N E 3 Department of Geology, San Jose State University, San Jose, CA 95192 (U.S.A.) 2 U.S. Geological Survey, 705 N. Plaza St., Carson City, NV89701 (U.S.A.) 3 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 (U.S.A.) Received October 5, 1988; revised and accepted April 20, 1989
Abstract Stevens, C.H., Lico, M.S. and Stone, P., 1989. Lower Permian sediment-gravity-flow sequence, eastern California. Sediment. Geol., 64: 1-12. The Lower Permian (middle Wolfcampian) Zinc Hill sequence, a 65- to ll0-m-thick series of beds in the Owens Valley Group in east-central California, comprises sediment-gravity-flow deposits consisting of carbonate sediment that originated on, and siliciclastic sediment that may have been generally ponded behind, a carbonate shelf to the east and northeast. Thickness patterns and paleocurrent indicators show that the sediment forming this sequence was transported primarily southeastward and deposited in a southeast-trending, lobe-shaped body. Evidently, the sediment was carried from the shelf by sediment-gravity flows that travelled westward down the slope and then turned southeastward upon reaching a southeast-trending basin at the base of the slope. Data derived from the study of this basin, which paralleled the shelf edge and is thought to have formed parallel to a southeast-oriented segment of the Early Permian continental margin, constitute one of the most important arguments favoring a Pennsylvanian to Early Permian age of truncation of the western North American continental margin.
Introduction
The Lower Permian section in a large area of east-central California (Fig. 1) is composed of a thick sequence of basinal carbonate and finegrained siliciclastic rocks that in many places overlie older carbonate shelf rocks. This superposition of facies formed in such topographically and sedimentologically different environments suggests that a major readjustment of tectonic features occurred prior to the deposition of the Permian rocks. The event responsible for this reorganization was postulated by Stone and Stevens (1988) to have been associated with the truncation of the southwestern part of the North American continent, an idea previously postulated to explain the apparent termination of Paleozoic facies belts 0037-0738/89/$03.50
© 1989 Elsevier Science Publishers B.V.
in central California (Hamilton and Meyers, 1966), and assumed to have occurred in the early Mesozoic (Davis et al., 1978). Analysis of the Permian basinal rocks in east-central California, which were deposited near the truncated margin, is critical to the understanding and interpretation of this important tectonic event. In Darwin Canyon and the northern Argus Range, the area of this study, the Lower Permian section is composed of about 2000 m of calcareous siltstone, limestone conglomerate, bioclastic limestone, argillaceous limestone, and calcareous mudstone, much of which was deposited by sedimentgravity flows (Stone et al., 1987). Although the Lower Permian section in the study area appears monotonous, some series of beds are so distinctive they can be correlated throughout the entire area
Fig. 1. Index map of east-central California showing distribution of Permian rocks of the Owens Valley Group. B C = Bendire Canyon: C M = Conglomerate Mesa; DC = Darwin Canyon; D H = Darwin Hills; K C = Knight Canyon; M C = Marble Canyon; OC = Osborne Canyon; P S = Panamint Springs. Area in rectangular box is enlarged for Fig. 2.
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covered (Fig. 2). These are here referred to as localities A through G.
(Lico, 1983; Stone, 1984). Within this Lower Permian section we here describe and interpret the paleogeographic significance of a series of especially distinctive beds, approximately 65-110 m thick, which will be referred to as the Zinc Hill sequence. This sequence of rocks is named for a local geographic feature in the northern Argus Range. We have selected these beds for detailed study because they can be recognized throughout a wide area and because they typify the Lower Permian section in this region. The lateral extent of the Zinc Hill sequence was ascertained through detailed study of Permian rocks throughout the area shown in Fig. 1 (Stone, 1984; Stone and Stevens, 1984). This study showed that the Zinc Hill sequence is recognizable throughout an area of about 75 k m 2 including all of Darwin Canyon and the northern Argus Range to south of Osborne Canyon (Fig. 1). Good, structurally uncomplicated exposures of this sequence, however, are rare; only seven sections reasonably suitable for detailed study were dis-
Stratigraphy The Lower Permian rocks in Darwin Canyon and the northern Argus Range are assigned to the Owens Valley Group (Stone and Stevens, 1987), which, in this area, has been divided into the Osborne Canyon Formation and the overlying Darwin Canyon Formation (Stone et at., 1987). The latter formation has been subdivided into the lower Millers Spring and upper Panamint Springs Members (Stone et al., 1987; Fig. 3). The Zinc Hill sequence, which is entirely middle Wolfcampian in age (Magginetti et at., 1988), consists of rocks within the lower part of the Millers Spring Member, specifically unit 2 and the lower part of unit 3 (Fig. 3) as defined by Stone et al. (1987). In this report, the Zinc Hill sequence is divided into nine subunits which consist of five predominantly limestone beds containing transported, shallowwater fossils and four intervening zones composed
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Fig. 3. Stratigraphy of the Owens Valley Group and the Zinc Hill sequence in the Darwin C a n y o n - n o r t h e r n Argus Range area.
largely of very fine-grained siliciclastic sandstone and coarse siltstone. The limestones, in ascending order, are referred to as subunits 1L-SL: the intervening, dominantly siltstone and sandstone subunits are numbered 1S-4S (Fig. 3). The major characteristics of these subunits are as follows: Subunit 1: Medium-dark-gray- to mediumlight-gray-weathering, fossiliferous, clast-supported limestone conglomerate with clasts up to 5 cm near the base grading upward through a horizontally laminated division into argillaceous, micritic limestone near the top. The thickness generally is 4 or 5 m, but it ranges up to 14 m. Subunit IS: Grayish-orange-to moderate-yellowish-brown-weathering, calcareous, siliciclastic siltstone and sandstone in beds 0.2-0.7 m thick with internal horizontal and convolute laminations. Platy, pale-red, siliciclastic mudstone is present between some of the siltstone and sandstone beds. This subunit ranges in thickness from less than 2 m to 10 m. Subunit 2L: Medium-dark-gray- to brownishgray-weathering, fossiliferous, clast-supported limestone conglomerate with clasts up to 2 cm near the base grading upward through a horizontally laminated division into micrite at the top. The thickness range from about 3 to 22 m with a mean of about 7 m. Subunit 2S: Grayish-orange- to moderate-yellowish-brown-weathering, calcareous, siliciclastic siltstone and sandstone in beds 0.2-2.0 m thick with internal horizontal and convolute lamination. Rare calcareous concretions and interbeds of moderate-red and dark-gray mudstone occur in this subunit. A fine-grained, medium-gray, structureless limestone represents this interval in section E, and similar limestone occupies more than half of the interval at section F. This unit ranges in thickness from about 1 to over 13 m, and generally is about 10 m thick. Subunit 3L: Grayish-orange- to light-brownweathering, fossiliferous, matrix-supported limestone conglomerate with contorted clasts and slabs of calcareous siltstone up to 3 m long. The thickness ranges from 10 to about 47 m. Subunit 3S: Grayish-orange- to moderate-yellowish-brown-weathering, calcareous, siliciclastic siltstone and sandstone in beds 0.2-2.0 m thick
with horizontal, cross- and convolute lamination. At section C the upper part of this interval ~s occupied by medium-light-gray, fine-grained limestone with rare limestone clasts near the base. This interval ranges in thickness from about 1 to more than 12 m. Subunit 4L: Pale-red- to light-brown-weathering, fossiliferous, matrix-supported limestone conglomerate with clasts up to about 8 cm long. This bed ranges from 1 to 8 m in thickness. Subunit 4S: Grayish-orange- to moderatebrown-weathering, calcareous, siliciclastic siltstone and sandstone in beds 0.2-0.6 m thick, with internal horizontal, cross- and convolute lamination and some graded beds. This interval ranges from about 6 to 16 m in thickness. Subunit 5L: Medium-dark-gray- to mediumlight-gray-weathering, fusulinid-rich, clast-supported, small-pebble limestone conglomerate showing a sequence of graded bedding, horizontal lamination and cross-lamination from base to top. The thickness ranges from about 0.3 to 1.0 m.
Lateral distribution The Zinc Hill sequence is recognizable throughout Darwin Canyon and the northern Argus Range as far south as Osborne Canyon (Fig. 1). In sections A, B, C, and F (Fig. 2), all nine subunits of the sequence are present (Lico, 1983). In section D, subunits 4 L - 5 L are exposed; in section E, subunits 2 L - 5 L are exposed; and in section G, subunits 1 L - 4 L are exposed (Fig. 4). Elsewhere in the region, individual beds that could be the same as those of the Zinc Hill sequence are present, although the entire sequence is not represented. For example, a clast-supported, graded limestone conglomerate bed is present in the stratigraphic position of subunit 1L in the Darwin Hills west of the study area (Fig. 1), but no other recognizable part of the Zinc Hill sequence is present. Similarly, one thick matrix-supported conglomerate bed, lithologically similar to subunit 3L of the Zinc Hill sequence and in about the same stratigraphic position, is present both in Marble Canyon to the northeast and in Knight Canyon to the south (Fig. 1), but neither is associated with other recognizable subunits of the Zinc
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Hill sequence. These other conglomerates may or may not have been deposited during one of the events that formed the limestone subunits of the Zinc Hill sequence, but regardless, the sequence is not recognizable. Thus, the Zinc Hill sequence presently crops out within an area of about 75 km 2 (Fig. 2). It must terminate to the southwest under alluvium because it is not present in the Darwin Hills where rocks of the appropriate age are exposed. Neither does the Zinc Hill sequence extend into the Conglomerate Mesa area farther northwest (Fig. 1), where the depositional sequence is quite different from that in Darwin Canyon. It also must terminate to the south between outcrops in Osborne Canyon and Knight Canyon, where it is not present. Thus, the termination of the Zinc Hill sequence to the northwest, west, and south is relatively well constrained.
Major rock types and modes of deposition Five major rock types are represented in the Zinc Hill sequence: (1) siltstone and very-fine-
grained sandstone, (2) matrix-supported limestone conglomerate, (3) clast-supported limestone conglomerate, (4) mudstone, and (5) fine-grained limestone. All exposures of each rock type share many characteristics, so a general description can be written for each type.
Siltstone and very-fine-grained sandstone These rocks form moderately well exposed outcrops in subunit 4S and most sections of subunits 1S, 2S, and 3S. They are medium to dark gray on fresh surfaces and grayish orange to moderate yellowish brown on weathered surfaces, and occur in beds 0.1-2 m thick. Individual beds are laterally extensive and have planar contacts above and below. Horizontal lamination (Fig. 5A), which commonly characterizes entire beds, is replaced in the upper parts of some beds by disturbed bedding, convolute lamination, or medium-scale trough cross-lamination (Fig. 5B). The crosslamination commonly occurs in multiple sets, each up to 10 cm thick. Many beds appear massive, but thin sections show that at least some of them are
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Fig. 5. A. Typical siltstone bed showing horizontal lamination. Pen is about 15 cm long. Unit 4S at section A. B. Cross-lamination in the upper part of a very fine-grained sandstone. Scale is 15 cm long. Unit 4S near section F. C. Matrix-supported limestone conglomerate. Hammer is 31 cm long. Unit 3L at section A. D. Relatively thin, clast-supported limestone conglomerate. The coarse, basal division (at the bottom of photograph) is overlain by well developed plane beds (of which the upper ones apparendy are inversely graded and represent Lowe's (1982) $2 unit) and an upper convoluted division. Scale is 15 cm long. Unit 5L at section C. E. Lower part of a clast-supported limestone conglomerate. The erosional base is on the left. The lower, coarse, crudely graded division is overlain by plane bedded, alternating fine and coarse beds, some of which are inversely graded. This division grades into the fine calcarenite shown in F. Unit 2L at section A. The clipboard is 23 cm wide. F. Lower part of the uppermost division of the bed shown in E showing parallel laminae below hammer handle overlain by highly disturbed sediment. The hammer is 31 cm long.
very-fine-grained sand. Detrital muscovite occurs in relatively minor quantities. The T b division of the Bouma sequence dominates, but some beds lack horizontal lamination and show subtle grading suggesting the Bouma Ta division. Mediumscale cross laminae, similar to those referred to by Lowe (1982) as a Tt division and interpreted by him as deposits of a residual current following sedimentation of a high-density load, also are present. Most of these beds seem to best fit Lowe's (1982) models 9 and 10 which probably are deposited by moderate-density turbidity currents.
Matrix-supported limestone conglomerate Matrix-supported conglomerates (Fig. 5C) form subunits 3L and 4L. They are medium gray on fresh surfaces and grayish orange to brown on weathered surfaces. These conglomerates occur in single beds up to 40 m thick that typically form ridges or cliffs. The beds are completely matrixsupported and without organization except for rare hints of clast imbrication. Contacts with underlying beds are sharp and planar to undulatory without signs of scour. Contacts with overlying beds also are sharp and undulatory. The clasts, which range in size from less than 1 cm to 3 m, consist of medium-dark-gray, micritic limestone; dark-gray, fossiliferous limestone; grayish-orange to moderate-yellowish-brown siltstone; pale-red mudstone; and fossils, mostly fusulinids, crinoid debris and colonial coral fragments. These clasts are mainly angular to subrounded, but the larger ones are tabular with large length to width ratios. Some large, originally tabular clasts have been distorted into "S"- or "rolled"-shapes. The matrix is composed of silty micrite, generally with less than 5% quartz silt. All of the above features typify Lowe's (1982) model 1 deposits of debris flows.
Clast-supported limestone conglomerate This lithology forms subunits 1L, 2L, and 5L. It is typically medium dark gray on fresh surfaces and medium light gray on weathered surfaces. The total bed thickness ranges from 0.3 to 20 m. The thick beds generally crop out as well exposed
resistant ridges, whereas the thin beds commonly are covered. All beds are graded, although large pieces of massive corals belonging to the Thysanophyllum coral belt of Stevens (1982) and generally the largest clasts present, occur in the middle at many localities. The thick beds are variable in their interval organization, but they generally show three divisions: (1) a lower, clastsupported, graded division, commonly about 2 m thick; (2) a middle calcarenite division, commonly 2 m thick, with some reverse-graded horizontal laminae; and (3) an upper, silty to micritic division, commonly about 3 m thick, with local horizontal lamination a n d / o r contorted and disrupted bedding (Fig. 5D-F). Locally, a matrix-supported conglomerate occurs at the base of these beds and at one locality, one of the thick conglomerates is broken by two shale partings indicating two sedimentological breaks. Imbricated clasts of mudstone, siltstone and limestone are present at a few localities. Sole marks, including flute casts, groove casts, and lineations, occur on the base of some of the thin beds. The basal contacts are sharp and undulatory, and some how evidence of channeling. The upper boundaries grade into massive calcareous mudstone, or are sharp and planar. The maximum clast size in the thick beds ranges from 2 cm to 1.5 m and, in the thin beds, from about 0.2 to 3 cm. Clasts consist of medium-darkgray micritic limestone; medium-dark-gray limestone containing a variety of invertebrate fossils, and rarely, dasycladacean algae and ooliths; quartz-rich calcarenite; grayish-orange- to moderate-yellowish-brown-weathering, horizontal and cross-laminated siltstone; pale-red mudstone; crinoid, coral, and bryozoan fragments; and whole fusulinids. The matrix consists of micritic limestone. The lower parts of the thicker limestone beds (those several meters in thickness) are very similar to Lowe's (1982) model 8 in which a R3 division (coarse grained and normally graded) is overlain by a S1 division (medium grained with horizontal and cross laminae) a n d / o r a Bouma Ta division. These beds may be capped by Bouma T b and Tc divisions. The lower parts of these beds apparently were deposited by high density turbidity currents; the uppermost parts presumably were deposited
by low density parts of the same flows. Some of the thin limestones show Bouma T~_ d divisions corresponding to model 10 of Lowe (1982). These beds probably were deposited by turbidity currents of relatively low density.
Mudstone This lithology occurs in subunit 1S (section C) and subunit 2S (sections A, B, C, and F). It includes calcareous, pale-red and dark-gray, siliciclastic mudstone in beds 2-30 cm thick, commonly separating siliciclastic siltstone a n d / o r sandstone beds. The mudstone is laminated or structureless and unfossiliferous except for rare ammonoids. Although radiolarians might be expected in these rocks, none were observed in thin section. These rocks are inferred to represent the background hemipelagic sedimentation.
Fine-grained limestone This lithology occurs in subunit 2S (sections E and F) and in subunit 3S (section C). It is composed of moderately to well exposed, mediumdark- to dark-gray limestone occurring in beds 0.5-8 m thick. No internal structure has been noted, but in places limestone lithoclasts, crinoid and coral fragments, and fusulinids appear to "float" in the fine-grained, extensively recrystallized limestone. Quartz grains occur sparingly. The depositional mode of this rock type is uncertain. The floating allochems, however, suggest that it was formed by some type of sedimentgravity flow, perhaps a fine-grained debris flow.
Basin analysis Data bearing on the nature of the basin in which the Zinc Hill sequence was deposited are: (1) the nature of the sediment and the sedimentary structures, (previously discussed), (2) paleocurrent indicators, and (3) thickness patterns. For plotting the paleocurrent data (Fig. 6) and the thickness data (Figs. 7, 8), we have removed the interpreted lateral displacement on three major faults in the area: the Darwin, Osborne Canyon, and West Argus Range faults (Fig. 2). Because the
Darwin and Osborne Canyon faults show similar displacement and terminate against the West Argus Range fault from opposite directions, they are considered to have been originally two segments of the same fault that were offset 1.6 km in a right-lateral sense on the West Argus Range fault. Matching of rocks and structures on each side of the other faults indicates 1.2 km of leftlateral displacement on the Darwin fault, and about 1 km of left-lateral offset on the Osborne Canyon fault.
Directional features Several different indicators of transport direction are present in the rocks of the Zinc Hill sequence, although completely convincing features are relatively rare. The most common indicators, in decreasing order, are: trough cross-lamination, generally in the upper parts of siltstone beds; pebble imbrication; flute casts; lineations; and groove casts. Trough cross-lamination and flute casts are considered to be most useful. Pebble imbrication was used in only one instance where it is clearly primary and there are lineations on the base of the bed from which a more precise direction of transport could be ascertained. Elsewhere, especially in some thick limestone conglomerates, imbrication may be pronounced, but it is parallel with a tectonic foliation and may have resulted from rotation or stretching of clasts during deformation. The overall transport direction to the southeast (Fig. 6) is consistent with previous studies of paleocurrents in the Owens Valley Group in the Darwin Canyon area by Stevens and Stanton (1980), who reported a mean transport direction of S30°E; by Lico (1983), who computed mean current directions ranging from S 2 0 ° E to $73 ° E; and by Stone (1984) and Stone and Stevens (1984), who reported a modal transport direction of about S15°E.
Thickness trends Isopach maps were constructed on the theory that the shapes of the sediment bodies would give some indication of the shape a n d / o r orientation
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Fig. 8. Isopach map of subunit 3L (in meters). Letters refer to locations of measured sections after restoration of offset on the Darwin, West Argus Range, and Osborne Canyon faults (see Fig. 2).
of the basin in which they were deposited. Thicknesses of some subunits in several areas could not be accurately ascertained because of unresolved structural problems in this complexly deformed area. so the isopach maps must be considered approximations of the true thickness patterns. Lico (1983) showed some different thicknesses than those shown here because of differences in interpretation of the stratigraphy, but the resulting isopach maps are very similar. Because data on the interval from the base of subunit 2L to the top of subunit 5L and on subunit 3L are the most complete and believed to be the most accurate, isopach maps of these intervals are presented here (Figs. 7, 8). They suggest that the sediment bodies are tongue-shaped, oriented approximately $20 ° E, and do not extend south as far as Knight Canyon.
Paleogeography and sources of sediment
Origin of sediment The major paleogeographic elements in eastern California during the Early Permian were a series of basins in which sediment of the Owens Valley Group was deposited west of a large carbonate shelf upon which the Bird Spring Formation and equivalent rocks were deposited. This carbonate shelf extended throughout southern and eastern Nevada and the eastern Mojave desert (Stevens, 1977; Stone, 1984; Fig. 9). During most of the Early Permian (middle Wolfcampian and later time), this carbonate shelf, which lay east of the present Panamint Range, had a southeast-trending margin (Stone, 1984). The carbonate sediment in the basinal Owens Valley Group is interpreted to have been derived from this shelf because of (1) the large volume of this sediment, which suggests derivation from an extensive carbonate shelf or bank, (2) the occurrence of ooliths, dasyctadacean algae, and large fragments of colonial corals, which indicates a source in very shallow water, and (3) the abundance of colonial coral fragments representing genera from the Thysanophyllum coral belt, which are known only from that shelf (Stevens, 1982). Collapse of the outer portions of the shelf is
Interpretations Although the paleocurrent and thickness data are not conclusive, they suggest that the southeastflowing density currents deposited southeasttrending sediment bodies that thin southeastward. This in turn suggests that the portion of the basin in which the Zinc Hill sequence was deposited sloped southeastward.
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ran,
~ ,-, ..... k , ' ~ ' / / \
/
.'/
/
I
Fig. 9. Present position of the carbonate shelf rocks (A) and interpreted Early Permian paleogeography in east-central California and southern Nevada (B).
11 inferred to have given rise to the thick carbonate sediment-gravity-flow deposits now exposed in Darwin Canyon and the northern Argus Range. Siliciclastic debris, which composes a large part of the Lower Permian section in southeastern California, rarely is thoroughly mixed with the limestone debris suggesting that the two types of sediment originated in somewhat different areas. The siliciclastic material almost certainly represents a southwestward extension of the enormous body of craton-derived quartz sand and silt deposited during the Early Permian throughout the western Rocky Mountains and the eastern Great Basin (Sheriff, 1975). This sediment probably was carried into the shallow portions of the miogeosynclinal sea by wind and/or water, and perhaps, as proposed by Stone (1984), was ponded on the landward side of the carbonate shelf. From there the siliciclastic debris may have been evacuated periodically in sediment-gravity flows through channels cut into the carbonate shelf and deposited in the deep basins to the west. No such channels, however, have been recognized. Basin characteristics
Some aspects of the basin in which the Lower Permian rocks of the Owens Valley Group in Darwin Canyon and the northern Argus Range were deposited can be inferred from data on various sediment bodies. First, unit 1 of the Millers Spring Member (Fig. 3) of the Darwin Canyon Formation occurs widely in eastern California including Darwin Canyon, the Darwin Hills, the eastem Conglomerate Mesa area, Marble Canyon, and the northern Argus Range to south of Knight Canyon (Fig. 1). This siliciclastic unit, which is 120 m thick, lies immediately beneath the Zinc Hill sequence in Darwin Canyon and the northern Argus Range. It is unbroken by significant limestone sediment-gravity-flow deposits, suggesting that the region was experiencing tectonic quiescence during deposition of this unit and that the limestone shelf was stable. The entire area covered by this sandstone and siltstone unit may have constituted a single depositional basin. The appearance of coarse-grained limestone sedimentgravity-flow deposits, including the limestone sub-
units of the Zinc Hill sequence, throughout this basin probably represents the sudden collapse of parts of the carbonate shelf, perhaps caused by earthquakes. Such earthquakes may have been produced by movement on faults that fragmented the original large basin into smaller components such as the one in which the Zinc Hill sequence was deposited. Internal directional features and the shape of the Zinc Hill sediment body show a southeast orientation, indicating that this basin had a southeast trend and that it was relatively narrow. The area of outcrops shows that it was at least 9 km wide, and the isopachous maps (Figs. 7, 8) suggest that it probably was about 15 km wide. The length of the basin is unknown, but the sediment deposited within it presently is exposed over a longitudinal distance of 10 km. Although the Zinc Hills sediment body evidently lies parallel to the shelf margin from which the carbonate material was derived, directional features in the clast-supported conglomerates indicate that it is not a carbonate slope apron or base-of-slope apron such as those described by Mullins and Cook (1986) and Colacicchi and Baldanza (1986). Instead, it appears that the sediment-gravity flows feeding the basin carried the basin carried sediment off the carbonate shelf, down the westward-facing slope, and then down the southeastward-dipping axis of the basin at the base of the slope (Fig. 9).
Regional tectonic implications The basinal Lower Permian rocks east and southeast of the Inyo Mountains, including those discussed in this report, accumulated in basins that formed by middle Wolfcampian time in an area close to the continental margin (Stone and Stevens, 1984, 1988). These rocks contrast strongly with Mississippian and older, primarily shallowshelf deposits that underlie them, and represent a period of major subsidence. A very important aspect of the paleocurrent data and basin reconstruction is that they indicate that the orientation of these Early Permian basins (southeast) was at a relatively high angle (45-60 °) to that of earlier basins and shelf edges in the region (Stevens, 1986), and that in the Early Permian,
sediment was carried toward the continent as it was constituted in the Early and Middle Paleozoic. The appearance of these new, southeast-oriented basins in the Early Permian constitutes one of the major lines of evidence that the continental margin was fragmented at a profound angle to its earlier trend. We interpret this to mean that the apparent truncation of Early and Middle Paleozoic facies belts in central California as postulated by Hamilton and Meyers (1966) is real, and that the truncational event occurred in the Late Paleozoic rather than in the early Mesozoic as suggested by Davis et al. (1978).
Acknowledgements We are grateful to David Andersen who constructively criticized an early version of the manuscript, and to him and Monty Hampton who carefully reviewed later versions. Additional reviews by Mario Coniglio and Paul Enos also were very helpful.
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Magginetti, R.T., Stevens, C.H. and Stone. P,. 1988. t,,arl~ Permian fusulinids from the Owens Valley Group, e~Js~central California. Geol. Soc. Am. Spec. Pap., 217:61 pp. Mullins, ft.T. and Cook, H.E., 1986. Carbonate apron models: alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration, Sediment. Geol., 48: 37- 79. Sheriff, A.. 1975. Origin and distribution of terrigenous detritus in the late Early Permian of the central Cordilleran Miogeosyncline. M.S. Thesis, San Jose State University, San Jose, Calif., 69 pp. Stevens, C.H., 1977. Permian depositional provinces and tectonics, western United States. In: J.H. Stewart, C.H. Stevens and A.E. Fritsche (Editors), Paleozoic Paleogeography of the Western United States. Pacific Coast Paleogeography Symposium 1. Pac. Sect. Soc. Econ. Paleontol, Mineral., pp. 113-135. Stevens, C.H., 1982. The Early Permian Thysanophyllum coral belt: another clue to Permian plate-tectonic reconstructions. Geol. Soc. Am. Bull., 93: 798-803. Stevens, C.H., 1986. Evolution of the Ordovician through Middle Pennsylvanian carbonate shelf in east-central California. Geol. Soc. Am. Bull., 97: 11-25. Stevens, C.H. and Stanton, W.R., 1980. Stylastraea-bearing gravity flow sequence of Early Permian age in eastern California. Geol. Soc. Am. Abstr. Progr., 12 (3): 154. Stevens, C.H. and Stone, P., 1988. Early Permian thrust faults in east-central California. Geol. Soc. Am. Bull., 100: 552-562. Stone, P., 1984. Stratigraphy, depositional history, and paleogeographic significance of Pennsylvanian and Permian rocks in the Owens Valley-Death Valley region, California, Ph.D. Thesis, Stanford University, Stanford, Calif., 399 pp. Stone, P. and Stevens, C.H., 1984. Stratigraphy and depositional history of Pennsylvanian and Permian rocks in the Owens Valley-Death Valley region, eastern California. In: J. Lintz, Jr. (Editor), Western Geological Excursions, 4. Geological Society of America and Mackay School of Mines, University of Nevada, Reno, Nev., pp. 94-119. Stone, P. and Stevens, C.H., 1987. Stratigraphy of the Owens Valley Group (Permian), southern Inyo Mountains. California. U.S. Geol. Surv. Bull., 1692:19 pp. Stone, P. and Stevens, C.H., 1988. Pennsylvanian and Early Permian paleogeography of east-central California: implications for the shape of the continental margin and the timing of continental truncation. Geology, 16: 330-333. Stone, P., Stevens, C.H. and Magginetti, R.T., 1987. Pennsylvanian and Permian stratigraphy of the northern Argus Range-Darwin Canyon area, California. U.S. Geol. Surv. Bull., 1691 : 30 pp.