Age of Jurassic volcanism and tectonism, southern Owens Valley region, east-central California GEORGE C. DUNNE Department of Geological Sciences, California State University, Northridge, Northridge, California 91330 J. DOUGLAS WALKER Department of Geology, University of Kansas, Lawrence, Kansas 66045 ABSTRACT GEOLOGIC SETTING Remnants of the eastern fringe of the volcanic and volcanogenic Southern Inyo Mountains sedimentary cover of the Mesozoic Sierra Nevada batholith are exposed in the southern Inyo Mountains and adjacent Alabama Hills of east- Arc strata in the southern Inyo Mountains are preserved in co- central California. Six new U-Pb dates on volcanic units and crosscutting linear northern and southern belts that are separated by plutons intrusions reveal that the upper parts of both the Inyo Mountains and (Figs. 1, 2). Strata in these two belts are locally folded, but they are Alabama Hills sections accumulated during Middle and Late Jurassic broadly homoclinal and typically face and dip steeply southwest. In time. During this same interval, both sections were steeply tilted and both exposure belts, the volcanic sequence rests on the Union Wash locally folded during one or more episodes of contractile deformation Formation of Triassic age and is limited upward either by a thrust fault occurring in the east Sierran thrust belt. Differences between the largely carrying Paleozoic strata or by alluvial fill of Owens Valley. The Undated lower parts of the Inyo Mountains and Alabama Hills sections Union Wash Formation, of Early and early Middle(?) Triassic age suggest that they were once located farther apart, then later brought into (Stone and others, 1991) is a marine unit, most—and perhaps all—of proximity by thrust faulting or strike-slip faulting. The Inyo Mountains which was deposited prior to the onset of arc volcanism in this region. and Alabama Hills sections are similar to partly coeval strata in the In its northern exposure, the basal contact of the overlying arc se- White Mountains, in that both contain abundant sedimentaiy strata that quence clearly is unconformable, but the amount of stratal omission were in part deposited in or near terrain of moderate topographic relief. decreases southward (Stone and others, 1991). In the southern expo- Together, these areas seem to compose a distinctive arc-marginal dep- sure belt, this basal contact commonly is faulted, and its original nature ositional province different than that represented by partly coeval strata is difficult to interpret. Oborne and others (1983) suggested that the preserved in pendants to the west. contact is gradational and records a continuous transition from shal- low-marine through tidal-flat and distal-stream environments into al- INTRODUCTION luvial-fan environments of the overlying arc sequence. In contrast, Marzolf (1991) proposed that the basal contact of the arc sequence is Mesozoic volcanic and related volcanogenic sedimentaiy strata a major unconformity equivalent to the J0 unconformity of earliest composing the eastern fringe of the Sierran igneous arc are preserved Jurassic age that is widely exposed on the Colorado Plateau. Volcanic in a northwest-trending belt of discontinuous exposures in east-centred rocks in the lower third of the arc sequence are undated; thus these California (Fig. 1). The few published studies of these pendants have contrasting interpretations remain unresolved. provided the broad outlines of the eruptive, deformational, and paleo- The arc sequence of the southern Inyo Mountains can be divided geographic history of this part of the arc (Tobisch and others, 1986; into lower, middle, and upper intervals (Garvey and others, in press). Busby-Spera, 1988; Saleeby and others, 1990). Erosion is evident at each of the contacts between intervals, and the Mesozoic volcanic and volcanogenic sedimentary rocks exposed intervals may be separated by significant unconformities. Each of the in the southern Inyo Mountains and nearby Alabama Hills (Fig. 2) are three intervals can be recognized in both northern and southern ex- little-studied exposures within this belt. A few brief reports and ab- posures, although lateral facies changes result in somewhat different stracts describing these two exposures are available (Oborne and lithosome compositions, thicknesses and vertical succession within others, 1983; Dunne, 1986, 1990; Schneidereit, 1987; Garvey and correlative intervals (Fig. 3). The lower interval, ranging in thickness others, in press), but their age is poorly known because neither ra- from 200 to 480 m, consists of a basal 5- to 25-m-thick, mostly lime- diometric dates nor reliable fossil ages have been obtained for these stone-clast conglomerate (—5% of interval) overlain by volcanogenic rocks. We have determined new U-Pb dates on zircon extracted from conglomerate and breccia (—44%), pebbly volcanogenic sandstone four volcanic units in the southern Inyo Mountains and Alabama Hills and siltstone (—43%), and basaltic lava flow (—8%) units, many of exposures and from two crosscutting intrusions. In this paper, we which are arranged in complexly interfingering, laterally variable len- briefly summarize the geology of the arc volcanic sequence, present soidal arrays that are separated by erosional contacts. Oborne (1983) our new age data, and address implications of these data for the and Oborne and others (1983) concluded that these strata were de- Mesozoic evolution of east-central California. posited on alluvial fans by fluvial and debris-flow processes. Geological Society of America Bulletin, v. 105, p. 1223-1230, 4 figs., 2 tables, September 1993. 1223 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/9/1223/3381977/i0016-7606-105-9-1223.pdf by guest on 26 September 2021 DUNNE AND WALKER changes suggest that they accumulated in a terrain of lower relief than did strata of the lower interval. Features such as accretionary lapilli, mudcrack casts, raindrop impact impressions, and festoon cross-bed- ding of probable aeolian origin are evidence of subaerial deposition. The upper interval is >2,600 m thick in the southern exposure belt. If similar fossiliferous units in the southern and northern expo- sure belts are correlative (Fig. 3), then an additional few hundred meters of strata forming the youngest part of this interval may be preserved in the northern exposure. Over most of the southern ex- posure belt, the upper interval consists of volcanogenic conglomer- ate, sandstone, siltstone, and rare calcareous units (—95% of member) and welded tuff and lava-flow units (—5%). Sedimentary units are composed predominantly of sandstone and siltstone featuring parallel and low-angle cross-bedding. Individual lithosomes are laterally ex- tensive, with some traceable for 4 km. Garvey and others (in press) have interpreted these sedimentary features as indicating deposition of most of the upper interval by fluvial processes in terrain of low relief. The upper few hundred meters of this interval consists mostly of massive, matrix- to clast-supported, pebble to boulder volcanic conglomerate and breccia that was probably deposited by debris flows that may reflect an increase in topographic relief. Previously available age limits for arc strata in the southern Inyo Mountains are sparse. These rocks are younger than the Lower and lower Middle (?) Triassic strata of the underlying Union Wash For- mation, and all three stratigraphic intervals are intruded by subver- tical, northwest-trending mafic dikes that Dunne (1986) provisionally correlated with the Late Jurassic (148 Ma) Independence dike swarm (Chen and Moore, 1979). Apparently correlative fluvial strata within the upper interval in both northern and southern exposure belts (Fig. 3) contain poorly preserved fresh-water bivalves and gastropods for which a provisional age of Early to early Late Cretaceous was postulated (J. Hanley, personal commun., in Dunne, 1986). Alabama Hills In the Alabama Hills, arc strata form two exposures separated by a pluton (Fig. 2). As in the Inyo Mountains, these strata face and dip moderately to steeply southwest. The more stratigraphically complete Figure 1. Larger outcrops of Mesozoic syn-arc strata, east-central northern exposure consists of a lower and an upper interval separated California. Letter code for localities and related literature source are: by an unconformity (Fig. 3). The lower interval, which has a minimum A, Alabama Hills (this report); Ar, Argus Range (Moore, 1976); B, Butte thickness of —2,000 m, is composed of two groups of rocks present Valley (Cole and others, 1986); G, Goddard (Saleeby and others, 1990); in subequal amounts: (1) rhyolitic crystal lithic ash-flow tuff and vol- nl and si, northern and southern outcrop belts of southern Inyo Moun- canogenic sedimentary strata and (2) two or more generations of tains (this report); M, Mount Morrison (Rinehart and Ross, 1964); O, hypabyssal intrusions that take the form of northwest-trending dikes Oak Creek (Saleeby and others, 1990); RS, Ritter Range/Saddlebag area and sill-like masses. Volcanogenic sedimentary strata consist of vol- (Tobisch and others, 1986; Schweickert and Lahren, 1987); S, Slate canic-lithic to heterolithic conglomerate and breccia and less abun- Range (Smith and others, 1968); W, White Mountains (Hanson and dant, poorly bedded sandstone and siltstone. All exposures of the others, 1987). lower interval display moderate to intense hydrothermal alteration, which together with multiply intruded, abundant hypabyssal rock may