Horse Spring Formation Research Articles

Middle Miocene faulting and basin evolution during central Basin and Range extension: A detailed record from the upper Horse Spring Formation and red sandstone unit, Lake Mead region, Nevada, USA

AbstractMiocene basins of the Lake Mead region (southwestern United States) contain a well-exposed record of rifting and the evolving paleogeography of the eastern central Basin and Range. The middle Miocene Horse Spring Formation and red sandstone unit allow for detailed stratigraphic, chronostratigraphic, and structural analysis for better understanding the geologic history of extension in this region. We present new data from the White Basin and Lovell Wash areas (Nevada) to interpret the evolution of faulting, basin fill, and paleogeography. We conclude that tectonics strongly influenced sedimentation and hypothesize that climate may have played a secondary but important role in creating stratigraphic variations. Deposited from 14.5 to 13.86 Ma, the microbialitic Bitter Ridge Limestone Member of the Horse Spring Formation, the stratigraphically lowest unit in this study, records a widespread shallow and uniform lake which had moderate and steady sedimentation rates, both of which were controlled by a few faults. The persistent lake was broken up by fault reorganization followed by deposition of the highly variable fluvial-lacustrine facies of the Lovell Wash Member from 13.86 to 12.7 Ma. During this time, faulting shifted from the northeast-trending, oblique normal left-lateral White Basin fault to the northwest-trending, normal Muddy Peak fault and other smaller northwest-trending faults. The lower and middle portions of the red sandstone unit, 12.7–11.4 Ma, record an increase in the sedimentation rate of basin fill near the Muddy Peak fault as well as the return to widespread lacustrine conditions. Sedimentation and faulting slowed during deposition of the uppermost red sandstone unit, but some deformation occurred post–11.4 Ma. This study records basin-fill evolution including variations in depositional environments laterally and vertically, documents changes in the location and magnitude of faulting, supports earlier work that hypothesized faulting proceeded in discrete westward steps across the Lake Mead area, and helps constrain the paleogeographic and tectonic evolution of the region.

Late Oligocene–early Miocene landscape evolution of the Lake Mead region during the transition from Sevier contraction to Basin and Range extension

Late Oligocene to Miocene sedimentary strata of the Lake Mead, Nevada, region hold evidence that can test models of central Basin and Range extension and tectonic evolution and illuminate paleodrainage systems related to the carving of the Grand Canyon. In this region, Sevier contraction and a period of relative tectonic quiescence predate Miocene extension and magmatism. The herein revised Rainbow Gardens Formation, formerly the oldest member of the Horse Spring Formation, is found throughout the Lake Mead area and spans the transition from quiescence to the start of extension. New detailed stratigraphic work within the Rainbow Gardens Formation, including paleocurrent analyses, stable isotope geochemical data, new 40 Ar/ 39 Ar dates, tephrochronologic data, and ongoing structural studies, was used to build a detailed chronostratigraphic framework and reconstruct the Rainbow Gardens basin through time. Our results show that the basin formed by a combination of inherited topography and tectonic events outside the Lake Mead region prior to ca. 25 Ma. Sedimentation rates, sedimentary facies patterns, and the lack of evidence for syndepositional faulting indicate that the basin did not form during extensional faulting, although recent work suggests that this basin was modified by uplift to the south between 18 and 20 Ma. Initially, the basin recorded fluvial sedimentation within the center of an E-NE–trending inherited valley, with two facing gravel-dominated bajadas on either side. As the valley filled, rivers from the Caliente volcanic field to the north delivered an influx of volcaniclastic material into a broad area of marshes and ephemeral lakes. In the final stages of sedimentation, just prior to the onset of extension at ca. 17 Ma, the basin filled with limestone in a series of shallow lakes. Understanding the development and evolution of this basin is crucial for testing models of extension as recorded by the younger Horse Spring Formation. In addition, the detailed sedimentology and stratigraphy place constraints on models of the formation of the Grand Canyon and suggest that a paleo–Colorado River did not deposit sediment in the Lake Mead area between 25 and 17 Ma. This work highlights the importance of detailed basin analyses, which are crucial companions to structural, tectonic, and geomorphologic studies aimed at understanding the complex evolution of lithospheric and landscape changes across a large region.

Stable isotopic evidence for Neogene surface downdrop in the central Basin and Range Province

Authigenic calcite and smectite stable isotopic records were determined for five Neogene terrestrial stratigraphic sections from the central Basin and Range Province and surrounding regions. We recognized a general pattern of increasing authigenic mineral δ 18 O values with decreasing age since the middle Miocene throughout the region. We suggest this pattern results from regional elevation decrease and associated changes in atmospheric circulation patterns. An anomalous pattern of decreasing δ 18 O values with decreasing age is observed in the Miocene Horse Spring Formation exposed in the Lake Mead Basin, which is contemporaneous with local magmatic activity. We suggest this pattern results from surficial uplift of local highlands feeding water into the Horse Spring depositional system. The topographic histories suggested here provide insight into the response of the land surface to crustal processes. Neogene extensional deformation in the central Basin and Range Province resulted in a net regional elevation decrease.

New 40Ar/39Ar isotopic dates from Miocene volcanic rocks in the Lake Mead area and southern Las Vegas Range, Nevada

New 40Ar/39Ar dates on volcanic rocks interlayered with synextensional Miocene sedimentary rocks in the western Lake Mead area and southern end of the Las Vegas Range provide tight constraints on magmatism, basin formation, and extensional deformation in the Basin and Range province of southern Nevada. Vertical axis rotations associated with movement along the Las Vegas Valley shear zone occurred after 15.67 ± 0.10 Ma (2 sigma ), based on a 40Ar/39Ar date from a tuff in the Gass Peak formation in the southern Las Vegas Range. Basaltic magmatism in the western Lake Mead area began as early as 13.28 ± 0.09 Ma, based on a date from a basalt flow in the Lovell Wash Member of the Horse Spring Formation. Isotopic dating of a basalt from the volcanic rocks of Callville Mesa indicates that these rocks are as old as 11.41 ± 0.14 Ma, suggesting that volcanic activity began shortly after formation of the Boulder basin, the extensional basin in which the informally named red sandstone unit was deposited. The red sandstone unit is at least as old as 11.70 ± 0.08 Ma and contains megabreccia deposits younger than 12.93 ± 0.10 Ma. This result shows that formation of the Boulder basin was associated with development of topographic relief that was probably generated by movement along the Saddle Island low-angle normal fault. Stratal tilting associated with extension occurred both prior to and after 11.5 Ma.

Kinematics and timing of Tertiary extension in the western Lake Mead region, Nevada

Explanation of the origin of the complex array of structures in some extensional terranes (including folds and normal, strike-slip, and reverse faults) includes many models that implicitly assume kinematic compatibility between and contemporaneous operation of these structures. We present new stratigraphic and age data from the highly extended western Lake Mead region, Nevada, together with an analysis of fault kinematics (technique of Marrett and Allmendinger, 1990) to test the assumptions of kinematic compatibility and contemporaneity of structures in an area of excellent exposure and superb stratigraphic control. Our analysis indicates an overlapping but clearly distinct chronology of deformation. Early regional extension (>18?-13.5 Ma) is marked by development of a basin into which the middle Miocene lower Horse Spring Formation was deposited. In the western Lake Mead region, this basin was disrupted by more areally restricted, post-13 Ma normal and kinematically coupled right-slip faulting along the Las Vegas Valley shear zone. Kinematic analysis of faults indicates an average regional extension direction of nearly due west for the middle and late Miocene. Extension and right-slip faulting was followed by development of dominantly southvergent contractional structures including tight, east-plunging folds and east-striking reverse faults. These structures deform the post 8.5 Ma Muddy Creek Formation; the Muddy Creek Formation is not cut by the Las Vegas Valley shear zone. Older faults, including the eastern Las Vegas Valley shear zone, reactivated as south-vergent reverse faults. Northeast-striking left-slip faults cut folds and reverse faults. These observations show that north-south shortening and left-slip faulting postdate the major phase of extension and right-slip faulting in the western Lake Mead area. Dynamic models that invoke either a single stress field or rotating stress fields to explain development of structures in the western Lake Mead area are inconsistent with the kinematic and age data. Similarly, kinematic models that view all structures in the context of a single strain field are precluded by systematic cross-cutting relationships that demonstrate at least partial diachroneity of deformational styles. Large-magnitude extension south of the Las Vegas Valley and Lake Mead fault zones appears to have been followed by north-south contraction that was highly localized near the region of greatest extension. We suggest that lateral pressure gradients arising from differential crustal thinning at the northern end of the Colorado River extensional corridor may have provided the driving mechanism for localized contractional deformation.

Seismic stratigraphy and tectonic development of Virgin River depression and associated basins, southeastern Nevada and northwestern Arizona

Virgin River depression is a Neogene basin, with a surface area that exceeds 1,500 km 2 in the Basin and Range structural province of south-eastern Nevada and northwestern Arizona. The depression formed within the foreland of the Sevier orogenic zone, a region that was characterized in Paleogene time by a flat-lying section of Cambrian to Cretaceous platform strata about 5 km thick. Well data from Mobil Virgin 1A on Mormon Mesa reveal 2,000+ m of Neogene basin fill that consists mostly of the Muddy Creek Formation (4-10 Ma), the red sandstone unit (10-12 Ma) of Bohannon (1984), and the Lovell Wash Member of the Horse Spring Formation (12-13 Ma). Seismic reflection data from six primacord and two vibroseis lines show that the Muddy Creek Formation uniformly fills Virgin River depression to a depth of 1-2 km. Two older and less-extensive basins, the Mormon and Mesquite, lie beneath the Muddy Creek and are separated from one another by a complex buried ridge. The basins are mostly filled with rocks of the red sandstone unit and Lovell Wash Member to depths locally exceeding 6 km. Two older members of the Horse Spring Formation, the Rainbow Gardens and Thumb, also occur in Mormon basin (the western one), where they rest disconformably on the pre-Tertiary strata. The basins are east-tilted half grabens that are bounded on the east and southeast by large listric normal fault systems. The faults that bound Mormon basin are buried by the Muddy Creek Formation, but the Piedmont fault, on the east side of Mesquite basin, cuts Quaternary alluvium. The Virgin River depression formed in three stages. The period from 24-13 Ma is characterized by slow subsidence in Mormon basin and little noticeable deformation of the basin substrate. The Mormon and Mesquite basins became fully differentiated during the period from 13-10 Ma. This stage is associated with large displacements on the normal faults bounding both basins and the buried ridge. Proterozoic crystalline rocks were exposed locally, providing a source for part of the red sandstone unit deposited in the basins. Tectonic denudation during the 13-10 Ma stage locally removed large amounts of the pre-basin section. By 10 Ma, most of the fault activity had ceased, the ridge between the basins was overlapped, and Virgin River depression began to subside uniformly over a wide area. This stage lasted until the commencement of the modern period of dissection associated with the Colorado River. Our structural analysis suggests that upper crustal extension within the basin, mostly during the 13-10 Ma stage, might have exceeded 60%. The basin subsidence was partly due to extension in the upper crust and partly due to viscous flow in the deeper crust beneath the basin. It is not clear to us what caused the uplifts that flank the depression, but isostatic rebound due to tectonic denudation remains aviable possibility.

Petrography and Petrology of a Miocene (?) Tuff, Las Vegas Range, Clark County, Nevada

Within the Horse Springs Formation, east of Wamp Spring, several ash flow tuffbeds are interbedded with Tertiary sediments. The predominantly fine-grained crystal vitric tuff beds probably originated from a nearby undescribed source area, and it is presumed that these volcanic rocks of Miocene(?) age were laid down before or during local Basin and Range faulting (Guth, 1981). The stratigraphic and structural relationships of the Horse Spring Formation do not preclude hydrocarbon occurrences.

Tertiary extension north of the Las Vegas Valley shear zone, Sheep and Desert Ranges, Clark County, Nevada

Detailed mapping reveals the presence of high-angle extensional faults and low-angle gravity slides on the west side of the Sheep Range. Three major high-angle faulting events each account for 20° of eastward rotation and accommodate extension between the Las Vegas Range and the Desert Range. Low-angle faults represent surficial slides in response to topography produced by extension on the high-angle faults. Faulting took place during the Miocene, synchronously with deposition of the Horse Spring Formation and with displacement on the Las Vegas Valley shear zone. The extension in the Sheep Range took place without volcanism, intrusion, or metamorphism of the Paleozoic sedimentary rocks. Offset thrust faults suggest that the area west of the Sheep Range extended almost 100% during the Miocene, while the corresponding area south of the Las Vegas Valley shear zone did not extend significantly. The shear zone bounded the extending terrane on the south, acting as a transform fault. This extension west of the Sheep Range may in part balance that mapped by Anderson (1971) in the Eldorado Mountains. The Las Vegas Valley shear zone and the Lake Mead fault system may have acted together to compensate for areas of localized extension between the Colorado Plateau and the vicinity of the Specter Range.

Lithium-bearing rocks of the Horse Spring Formation, Clark County, Nevada

The Horse Spring Formation of Miocene age in Clark County, Nevada, contains as much as 0.5% Li in individual samples. Rock sequences which average 0.1% Li range from 3 m thick near Gold Butte (south of Mesquite, Nev.) to as much as 40 m thick near Lava Butte (east of Las Vegas, Nev.) about 75 km to the west. The lithium-bearing beds are light colored to white and contain hectorite in a dolomite, magnesite, or calcite matrix. Varied amounts of gypsum, halite, celestite, clinoptilolite, quartz, feldspar, biolite and colemanite are also present locally. Hectorite is the only lithium mineral recognized to date.The lithium-rich rocks contain low concentrations of most other minor elements except that boron and strontium are enriched. Rarely, barium, arsenic, and zinc are present in anomalously large amounts.The lithium-enriched part of the Horse Spring Formation was formed from a series of volcanic ashes which were deposited in a playa. Relict volcanic ash is recognizable in thin sections as remnant glass shards and vitroclastic textures. Most of the original glass has been altered to clay minerals, carbonate minerals, or zeolites, presumably through interaction with highly saline pore waters. Abundant evidence of spring activity suggests that thermal waters played a part in releasing the lithium from volcanic materials.

Radiometric Ages and Stratigraphic Sequence of Volcanic and Plutonic Rocks, Southern Nye and Western Lincoln Counties, Nevada

The geochronology of Tertiary igneous events at the Nevada Test Site and adjacent area is outlined by 36 recently determined K-Ar ages, together with other published K-Ar ages. The first evidence of Tertiary igneous activity is the ash-fall bedded tuffs in the Horse Spring Formation. One such tuff has been dated as 29 m.y. old (late Oligocene). Other ash-flow tuffs and lavas formed during the Miocene and Pliocene, according to radiometric age determinations. The youngest ash-flow tuff in this area is about 6 m.y. old. Great volumes of ash and lava were spewed forth 13 to 11 m.y. ago to form the Paintbrush and Timber Mountain Tuffs. Sixteen replicate age determinations on minerals from four densely welded ash-flow tuffs from these formations gave a pooled standard deviation of about ± 2 percent error, provided anomalous ages were rejected on the basis of rock alteration or analytical difficulties. In the Air Force Gunnery Range, just north of the test site, K-Ar ages suggest that the oldest ash flows, the Monotony Tuff, were emplaced 27.6 m.y. ago (late Oligocene) and were followed by outpourings of lava and ash throughout most of the Miocene. Youngest dated lava is about 13 m.y. old. In the southern Egan and northern Seaman Ranges of central Nevada, the Needles Range (?) Formation has an averaged K-Ar age of about 30 m.y., which compares closely with 29.2 m.y., the average of four earlier K-Ar ages determined by other investigators on known Needles Range Formation in eastern Nevada and western Utah. K-Ar ages given by micas from two exposed plutons in the Nevada Test Site suggest emplacement of these plutons at about 93 m.y. ago (early Late Cretaceous), although earlier emplacement in the Mesozoic would be more consistent with Pb-α ages.