Chester R. Longwell

Geology of the Boulder Reservoir floor, Arizona-Nevada

INTRODUCTION The site of Boulder Dam is in Black Canyon of the Colorado River, approximately 75 miles below the mouth of the Grand Canyon (Fig. 1). At the dam, the river, when at low-water stage, is slightly less than 650 feet above sea level. As the top of the dam is 580 feet higher, the water in the reservoir can rise almost to the 1230-foot contour. The resulting lake will be 115 miles in extreme length, and will have several long, narrow arms in valleys tributary to the Colorado. The width will vary from a few hundred feet in the narrow canyons to a maximum of eight miles in the low country near the mouth of the Virgin River. As a result of field studies in neighboring areas, the writer realized that this lake would cover geologic features of great interest. Only hasty reconnaissance surveys had been made of most . . .

Significance of K-Ar Ages of Tertiary Rocks from the Lake Mead Region, Nevada-Arizona

K-Ar ages for 43 igneous and tuffaceous rocks of the Lake Mead region, Nevada-Arizona, are reasonably consistent with mapped stratigraphic and structural relations. They serve to establish age ranges for episodes of Miocene and Pliocene volcanism, plutonism, and tectonism in the region. The late Tertiary geology of the northwestern part of the region contrasts sharply with that of the southeastern part. The northwestern part was characterized by active sedimentation in local basins followed by intense disruption by normal and transcurrent faults. The southeastern part was characterized by brief episodes of overlapping volcanism, plutonism, extreme tensional rifting, followed by severe local uplift. Although diverse in style, the tectonism of the two areas appears to have been largely synchronous. The synchronous character of the brief igneous and tectonic processes suggests strong genetic ties between them. The southeastern part of the region may be a volcanic rift zone that originated as a zone of tension near the end of a major transcurrent fault—the Las Vegas shear zone.

Measure and Date of Movement on Las Vegas Valley Shear Zone, Clark County, Nevada

Large-scale offsetting, by fracturing and bending of major geologic units, indicates clearly that a fault zone with large right-lateral displacement extends northwest across a pattern of ranges bordering Las Vegas Valley. Evidence indicating the date and measure of the movement is supplied by an unusual sedimentary unit within the Frenchman Mountain block east of Las Vegas. The Thumb Formation, dated isotopically as being Miocene in age (±17 m.y.), contains large landslide masses of brecciated metamorphic and granitic rock. These masses moved southward from an area in which only a thick section of sedimentary strata is now widely exposed. The granite is recognized as part of the distinctive Gold Butte Granite, a conspicuous unit in the South Virgin Mountains near the Nevada-Arizona border. The only present exposures of that unit near the projected location of the shear-zone axis are more than 64 km east-southeast from the landslide masses within the Thumb section. The thick Muddy Creek Formation, the basal part of which is dated as being early Pliocene, was laid down after movement on the shear zone ended.

STRUCTURE OF THE NORTHERN MUDDY MOUNTAIN AREA, NEVADA

Instead of a single large thrust in the Muddy Mountain area, Nevada, as reported from earlier field study, the writer distinguishes two superposed thrusts which may represent distinct orogenic episodes separated by a considerable time interval. Both thrusts “root” to the west. The structurally lower thrust (for which the name Muddy Mountain thrust is retained) is the more extensive; as reported previously, it has brought Paleozoic carbonate formations over Jurassic sandstone, with the heave-component of slip at least 15 miles. The higher thrust (here called the Glendale thrust) has heave-displacement of at least 5 miles. Together with associated smaller thrusts, it involves formations of early Upper Cretaceous age, as well as thick piedmont deposits that may be considerably younger. “Orogenic deposits” several thousand feet thick were laid down in front of the Glendale thrust as it advanced. Accurate dating of these strata would give an exceptional record of structural events. Conglomerate at the base of the Upper Cretaceous section, containing boulders and cobbles derived from resistant units in older systems as low as the Permian, indicates earlier strong deformation not far west of the Muddy Mountain area. This earlier orogenic episode may have included development of the Muddy Mountain and other large thrusts in the region which are not known to involve formations younger than Jurassic. Therefore the earlier orogeny can now be dated merely as post-Jurassic and pre-Upper Cretaceous. Overturned and faulted folds associated with the Glendale thrust rival in complexity some structural features of the Swiss Alps. Important transverse faults with large strike-slip component pose problems of origin; the largest of these displaces the Muddy Mountain thrust plate as much as 2 miles vertically and may be genetically related to the Glendale thrust. Numerous normal faults, variously oriented, bear witness to movements ranging in date from the Glendale thrusting episode to late Cenozoic time.

Structural Studies in Southern Nevada and Western Arizona

Introduction Grand Wash, a tributary of Colorado River in northwestern Arizona, lies in a wide valley whose position is determined by a zone of profound faulting. This valley marks throughout its length a sharp and definite boundary between two major physiographic and structural provinces. East of the valley the Grand Wash Cliffs rise precipitously, as a pair of giant steps, to an elevation averaging 4,000 feet above the valley floor. These cliffs form the abrupt western limit of a great tabular mass, the Shiwits division of the Colorado Plateau, in which the stratified rocks are nearly horizontal over a wide area. West of Grand Wash Valley and reaching into southern Nevada lies the Virgin Range, a complexly broken fault block, in which the same formations exposed in the Grand Wash Cliffs are tilted at high angles. Continuing to the west, other ranges structurally similar to the Virgin Range alternate with . . .

Geologic interpretation of gravity anomalies in the southern New England-Hudson Valley region

Pendulum stations occupied by the U. S. Coast and Geodetic Survey at locations chosen for their geologic significance provide a net in southern New England, eastern New York, and northern New Jersey comprising about 120 stations. Numerous supplemental stations have been added with a modem gravimeter. Specific gravities of representative rocks near the pendulum stations have been determined. Isanomaly maps based on isostatic, Bouguer, and free-air anomalies are remarkably alike in their major features. Some groups of anomalies reflect bedrock units that have exceptional densities. When corrections are applied for these local masses, regional tendencies are clarified. Two belts of negative anomalies are separated by a belt of positive anomalies. The negative belts correspond in a general way to axes of subsidence during Paleozoic periods. The entire region experienced orogenic deformation, followed by peneplanation. Since the groups of Bouguer anomalies are little affected by the ordinary isostatic corrections, and are related only in minor degree to the visible bedrock, it appears that they either indicate considerable departures from isostatic balance or reflect differences in density that are concealed. Glennie9s hypothesis of crustal warping, in a modified form, offers an attractive explanation of the outstanding anomalies.

Problems of Pennsylvanian-Permian Boundary in Southern Nevada

Within the past twenty years, students of stratigraphy have drawn the base of the Permian system in the Grand Canyon district at progressively lower horizons. On the basis of fossil plants, first the Hermit shale and, more recently, the red beds of the Supai formation have been included in the Permian section with the Kaibab limestone. The Bird Spring formation, more than 5,000 feet thick, which lies beneath the Supai in southern Nevada, has heretofore been placed in the Pennsylvanian, except for a few hundred feet in the lower part, which Girty considers Upper Mississippian. Fusulinids collected in the Las Vegas Quadrangle indicate that the upper 2,900 feet of the Bird Spring formation is correlative with the Wolfcamp formation and part of the Leonard formation in the Glass Mountains of Texas.

Sedimentation in relation to faulting

INTRODUCTION In the literature of geology, there are numerous descriptions of sedimentary deposits that demonstrably are related to movements on faults. There are also many attempts to demonstrate, by using the circumstantial evidence in sedimentary formations, the existence of active fault blocks while the sediments were accumulating. Some conclusions from studies of this sort are not wholly convincing. Generally, it is desirable to have contributory evidence, for localized strong warping may result in a sedimentary record that is suggestive of faulting. Study of areas in which both sedimentary and structural features are well displayed may supply criteria that have general application in problems of this kind. EXAMPLES FROM THE BASIN RANGES Many parts of the Basin-and-Range province offer favorable opportunities to study the relation of sedimentation to large-scale faulting. In most parts of the region of interior drainage, however, observation in the vertical section is seriously limited by the continuous . . .

Structural Studies in Southern Nevada and Western Arizona: A Correction

In a report published in 1926, based on reconnaissance study in a wide area including Spring Mountains, Nevada, I suggested that a mass of sheared rubble now exposed at one locality represents rock fragments derived from and overridden by an advancing thrust plate. More recent study establishes clearly that the sheared objects are well-rounded pebbles and cobbles of resistant rock, representing the basal conglomerate unit of a sedimentary formation that was overridden and largely obliterated by movement of a thrust plate during the Sevier orogeny. Careful study of the sheared remnants reveals striking similarities to the basal conglomerate of a Cretaceous(?) formation now exposed in the Frenchman Mountain block, about 25 miles to the east. Remnants of the Red Spring thrust plate, correctly identified in 1926, were mistakenly interpreted in a later report (Longwell and others, 1965) as a much broken part of the Keystone thrust plate.

A New Ordovician Formation in Nevada Dated by Conodonts

An Ordovician section similar to that near Eureka is well displayed north of Las Vegas Valley as far east as the Dry Lake Range. About 10 miles farther east, in the Muddy Mountains thrust plate, a section of impure carbonate rocks has been regarded as probably of Ordovician age. An assemblage of conodonts indicates an Early Ordovician date for that unit, here described as the Monocline Valley Formation. No Ordovician rocks are known east of Virgin Valley.