Lauren A. Wright

Late Cenozoic fault patterns and stress fields in the Great Basin and westward displacement of the Sierra Nevada block

The patterns of late Cenozoic faulting in the Great Basin apparently delimit two deformational fields, each extensional but contrasting in magnitude and style of extension. The field of smaller magnitude, which shows about 10 percent extension, occupies the northern and most of the central part of the Great Basin. It is characterized by steeply dipping normal faults and gently tilted blocks, with a preferred north to northeast trend. Evidence of greater extension occurs in the other deformational field, which lies between Walker Lane and the Sierra Nevada and extends across the narrow southern end of the Great Basin. This field contains most of the complementary strike-slip faults (northwest-striking right-lateral and northeast-striking left-lateral faults), long recognized as major components of the structural framework. It also contains abundant normal faults, most of which strike north to northeast. In certain areas, extension of 50 percent or more is indicated in the association of strike-slip and normal faults and in the palinspastic restoration of fault blocks that have been steeply tilted along gently dipping normal faults. These contrasts in structural pattern and apparent percentage of extension may be related to westward movement of the Sierra Nevada block and southward narrowing of the Great Basin. The faults along which strike-slip displacement occurred in late Cenozoic time appear to have functioned as conjugate shears, the shears and associated normal faults being first-order extensional features. The fault pattern also invites a simplistic interpretation that is based on the orientation of three mutually perpendicular directions of stress. The major normal faults, which strike north-northeast in most parts of the Great Basin, suggest a pervasive horizontal minimum compressive (maximum tensional) stress that is oriented west-northwest. Maximum compressive stress would be oriented perpendicularly where the horst and graben structure predominates and horizontally to the east-northeast where the strike-slip faults are abundant.

Giant Stromatolites and Associated Vertical Tubes from the Upper Proterozoic Noonday Dolomite, Death Valley Region, Eastern California

Stromatolitic domes having apparent growth reliefs up to 100 m or more characterize the lower member of the Noonday Dolomite, late Proterozoic, Death Valley region, eastern California. Long to short, subparallel, initially subvertical tubes penetrate the bedding of these structures at angles that range from 90° where stratification was initially horizontal to downslope angles up to 110° (and acute upslope angles) as initial dips increase. Tubes generally are missing where initial dips exceed 20°. They are common in the larger buildups and most abundant in the upper and central parts of these. They may be open or filled with coarsely sparry secondary dolomite, medium-gray ultra microgranular dolomite, silty brown dolomite, or drusy quartz. They pinch and swell irregularly along their length from a mean diameter of 1.5 cm and are subcircular to irregular in horizontal cross section. Secondary crystalline growths similar to those in many tubes are common along lamination surfaces and as irregular vug fillings. Weathering along intersecting fracture sets locally produces a subvertical lineation in the same orientation as associated tubes. Possible origins considered for the tubes and found improbable are: (1) metazoan burrows, (2) interspaces of columnar stromatolites, (3) columnar stromatolites, (4) solution pipes, and (5) root casts. Instead the spaces now represented by filled vugs, sparry lamination surfaces, and tubes are interpreted as having, in general, a common origin caused by the upward movement of fluids through the stromatolite mounds.

Death Valley bright spot: A midcrustal magma body in the southern Great Basin, California?

A previously unrecognized midcrustal magma body may have been detected by COCORP deep seismic reflection profiles in the Death Valley region of the southern Great Basin. High-amplitude, relatively broad-band reflections at 6 s (15 km) are attributed to partially molten material within a subhorizontal intrusion. This “bright spot” extends laterally at least 15 km beneath central Death Valley. A moderately dipping normal fault can be traced from the inferred magma chamber upward to a 690 000-yr-old basaltic cinder cone. The fault zone is inferred to have been a magma conduit during the formation of the cinder cone. Vertical variations in crustal reflection character suggest that the Death Valley magma body may have been emplaced along a zone of decoupling that separates a faulted brittle upper crust from a more ductile and/or intruded lower crust. The Death Valley bright spot is similar to reflections recorded by COCORP in 1977 in the Rio Grande rift, where both geophysical and geodetic evidence support the inference of a tabular magma chamber at 20-km depth.

Turtleback Surfaces of Death Valley Viewed as Phenomena of Extensional Tectonics

The controversial turtleback surfaces of Death Valley may be colossal fault mullions resulting from severe crustal extension which were localized along undulating and northwest-plunging zones of weakness that were in existence prior to this deformation. Supporting evidence includes (1) a coincidence between the surfaces and carbonate layers in folded metamorphic rocks beneath the surfaces, and (2) striations, slickensides, and extensional fractures with orientations compatible with northwest extension.

Algal and cryptalgal structures and platform environments of the late pre-Phanerozoic Noonday Dolomite, eastern California

A late pre-Phanerozoic platform, almost continuously blanketed by algal mats, is recorded in the distributional pattern, composition, and textures of the Noonday Dolomite, Death Valley region, California. The platform, apparently L-shaped, was at least 160 km long. It lay west and south of the Nopah upland, established much earlier, and north of a fault-bounded east-trending depression containing units of the older Pahrump Group. A lower member of the Noonday consists of very finely crystalline, laminated, and relatively pure dolomite. The laminations, although commonly deposited horizontally, also outline mounds having a synoptic relief of a few metres to nearly 200 m. Algal or cryptalgal fabrics are common. Especially large mounds in the Nopah Range lie on the down-dropped south side of a contemporaneous fault, indicating tectonic control of size and location. The laminated mounds are characterized by large-scale vertical stacking, best explained as produced by differential growth of algal mats. The configuration of the laminations indicate that the mats grew to depths of at least 100 m but probably did not extend more than 150 m below the mound crests. The absence in the lower member of features attributable to moving water and the paucity of clearly detrital material indicate quiet water and very low relief in the bordering land. An upper member consists of (1) discontinuous bodies of thinly and evenly bedded siltstone and silty dolomite that ordinarily occupy the deeper of the intermound lows; (2) a laterally continuous unit of silty dolomite with wavy laminations and small, rippled to subhemispherical, laterally linked stromatolites; and (3) an upper unit of massive to laminated silty dolomite, containing large domal and bosslike stromatolites and associated with bodies of strongly cross-bedded quartz-dolomite sandstone. These features evidence the entry onto the platform of detritus supplied by increasingly stronger uplift of the Nopah Upland and the sweeping of the platform by increasingly stronger currents. We interpret the carbonate of the lower member and much of the carbonate of the upper member as having been precipitated from ambient waters by removal of CO2 during algal photosynthesis.

Limitations on Right-Lateral, Strike-Slip Displacement, Death Valley and Furnace Creek Fault Zones, California

Sedimentary units of late Precambrian and Cambrian age in the southern Death Valley–Kingston Range region of eastern California contain several through going linear features with configurations that apparently place a several-mile limit on the total right-lateral displacement that may be assigned to the Death Valley–Furnace Creek fault system. Much greater magnitudes have been suggested previously. Precambrian paleogeologic contacts, recorded in the northeastward truncation of successively older units of the Pahrump Group by the unconformity at the base of the Noonday Dolomite, can be traced discontinuously as nearly straight features from the Kingston Range west-northwestward to the Panamint Range, about 75 miles. A belt of talc mineralization, which also trends west-northwestward, is marked by a crudely linear arrangement of deposits reflecting the distribution of a dolomitic facies of a carbonate unit in the lower part of the Pahrump Group. An “algal” dolomite unit, which forms the lower part of the Noonday Dolomite, thins to the southwest and terminates along a line that can be traced northwestward for about 25 miles along the east margin of Death Valley to Virgin Spring Wash and probably thence westward to the southern part of the Panamint Range. The plots of these linear features, all of which apparently cross either the Death Valley fault zone or Furnace Creek fault zone, or both, indicate that no more than 5 and 2 miles, respectively, of right-lateral, strike-slip movement has occurred on these zones. Indeed, the position and orientation of the lines require no lateral displacement at all for their explanation. Similar limitations on movement along Death Valley fault zone seem to be imposed by the presence, on the east side of Death Valley, of a wedge of shale and graywacke within the Noonday Dolomite. The wedge thins and becomes progressively finer-grained northeastward. It also contains a basal conglomerate with clasts of the “algal” dolomite and the Kingston Peak Formation. These features suggest a source area west of southern Death Valley. Right-lateral, strike-slip movement of 10 miles or more on the Death Valley fault zone, if restored, would anomalously juxtapose the clastic wedge with a terrane in the Panamint Range where the “algal” dolomite is preserved. The possibility that large-scale, right-lateral displacement in the region of central and southern Death Valley has been accomplished by crustal flexing, as advocated by some geologists, remains unsupported by the available structural data and is difficult to reconcile with the configurations of the geologic lines.

Isopach pattern of the Lower Cambrian Zabriskie Quartzite, Death Valley region, California-Nevada: How useful in tectonic reconstructions?

New thickness measurements of the Lower Cambrian Zabriskie Quartzite indicate an original isopach pattern much less regular than once supposed, and thus less useful as an indicator of the magnitude of lateral slip on the Death Valley fault system. But within a wide range of possible restorations, the isopachs conform with the locations of Proterozoic highlands and basins. The Zabriskie Quartzite, being mature, laterally persistent, and, unlike the underlying formations, devoid of evidence of rifting, apparently records the beginning of the passive stage of the developing Cordilleran miogeocline.

Stratigraphy and tectonic implications of the latest Oligocene and early Miocene sedimentary succession, southernmost Funeral Mountains, Death Valley region, California

A multistage, pre–basin and range history is recorded in latest Oligocene and early Miocene sedimentary rocks at the southern end of the Funeral Mountains. These deposits, as much as 1,400 m in exposed thickness, overlie Paleozoic forma- tions in two fault blocks that border the Furnace Creek fault on the northwest. The blocks originally formed the upper and lower plates of a northeast-striking Meso- zoic thrust fault. They were later eroded to a low-lying surface on which the Tertiary sediments were deposited. The blocks are now separated by normal displacement on a fault that, in part, coincides with the earlier thrust fault. The Tertiary cover dis- plays marked stratigraphic and stuctural differences from one block to the other. But the underlying crust apparently remained structurally coherent, undisturbed by major faulting, during deposition of the cover and initiation of movement on the bor- dering Furnace Creek fault. The Tertiary strata record deposition, in alternating alluvial and lacustrine environments, along the northern margin of a paleotopographic high that appar- ently existed through much or all of the area now occupied by southern and central Death Valley. The lower part of the succession, herein named the Amargosa Valley Formation, forms a southward thinning sedimentary wedge, as much as 800 m thick. It contains tuff beds with K/Ar biotite ages from ca. 20 to 25 Ma. We divide the formation into four parts: a basal unit, termed the conglomerate member and consisting of locally derived fanglomerate, records the uplift that initiated the topo- graphic high; and three overlying units, which consist, in upward order, of a lime- stone member, composed mostly of lacustrine limestone and tuff, a fluvial red sandstone member composed of sandstone, conglomerate, and mudstone, and an upper member, composed of interfingering lacustrine limestone and clastic fluvial deposits with subordinate tuff. We interpret the three as recording a southto southeastward progradation of a fluvial facies over a lacustrine facies and thence an opposite retrogradation. We assign the fluvial facies to a period of active subsidence of the area to the northeast of the topographic high, and probably within the 22to 20-Ma interval. The lower Cemen, I., Wright, L. A., and Prave, A. R., 1999, Stratigraphy and tectonic implications of the latest Oligocene and early Miocene sedimentary succession, southernmost Funeral Mountains, Death Valley region, California, in Wright, L. A., and Troxel, B. W., eds., Cenozoic Basins of the Death Valley Region: Boulder, Colorado, Geological Society of America Special Paper 333.

Strata of Late Precambrian-Cambrian Age, Death Valley Region, California-Nevada

The Noonday Dolomite, Johnnie Formation, Stirling Quartzite, Wood Canyon Formation, and Zabriskie Quartzite, which seem best assigned to the late Precambrian-Cambrian interval, are a conformable and predominantly detrital part of the stratigraphic record of the Death Valley region. They can be traced from type localities in the Spring Mountains and in the well-known section in the Nopah Range southwestward to the Salt Spring Hills and Silurian Hills in southern Death Valley and southeastward to the Kingston Range-Winters Pass area. Satisfactory correlation of these widespread formations is facilitated by (1) regional persistence of most of the members as originally recognized in the Nopah Range, (2) the distinctive lithologic character of the Noonday Dolomite, and (3) the presence of two marker units--an oolite and a volcanic ash(?)--high in the Johnnie Formation. Isopachous contouring of the stratigraphic interval from the top of the Zibriskie Quartzite to the base of the Noonday Dolomite appears to outline a late Precambrian-Cambrian marine trough. The major axis of this trough strikes north-northwest and is approximately coincident with the present Amargosa Valley and southern Death Valley. Along this axis, the combined thickness of these strata or their apparent equivalents decreases progressively from nearly 12,000 feet in the Funeral Mountains to only a few hundred feet in the Marble Mountains 160 miles toward the south. The isopachous lines drawn near the southeastward bend in the Garlock fault zone are nearly parallel with this major break, and the projected zero contour is about 10 miles southwest of it. This pattern strongly suggests that the structural element south and southwest of the Garlock fault, generally referred to as the Mojave block, existed as a topographic high in late Precambrian-Cambrian time.

Tertiary extensional features, Death Valley region, eastern California

The southeastern part of the Death Valley region (Fig. 1) displays two remarkable structural features: turtlebacks (Curry, 1938) and the Amargosa chaos (Noble, 1941). The changing ideas during the past half-century about the origin of these features reflect the growth of understanding of the major aspects of Basin and Range tectonics. Although these features were initially believed to be related to thrust faulting, a consensus now exists that they are different aspects of widespread Tertiary extension associated with the development of the Basin and Range province. The evidence upon which this historical debate is based is discussed in the site descriptions presented herein.