Geology and mechanics of the Blackhawk landslide, Lucerne Valley, California
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Blackhawk Mountain, a resistant mass of marble thrust northward over uncemented sandstone and weathered gneiss, rises above southeastern Lucerne Valley at the eastern end of the rugged 4000-foot escarpment that separates the San Bernardino Mountains on the south from the Mojave Desert on the north. Spread out on the alluvial apron at the foot of the mountain is the Blackhawk rockslide, a lobe of nearly monolithologic marble breccia 30 to 100 feet thick, 2 miles wide, and nearly 5 miles long. At least two earlier similar but smaller rockslides have occurred in the area.
The rocks of the area comprise late Tertiary and Quaternary fanglomerates and breccias derived mainly from the gneiss, quartzite, Carboniferous marble, and Cretaceous quartz-monzonite of the San Bernardino Mountains. Uplift of Blackhawk Mountain occurred in two stages after deposition of the older fanglomerates and breccias: the first by over-thrusting from the south, and the second by monoclinal folding along a northwest-trending axis.
Geological evidence in the area shows that the Blackhawk rockslide traversed the gently inclined alluvial slope as a nearly nondeforming sheet of breccia moving more than 50 miles per hour. The hypothesis that compressed air, rather than water or mud, constituted the lubricating layer on which the breccia sheet slid qualitatively explains all of the principal physical features of the slide lobe. Theoretical analysis of the flow in the lubricating air layer indicates the quantitative feasibility of the air-lubrication hypothesis for the Blackhawk slide.Keywords:
Rockslide
Breccia
Escarpment
In 1950 Wheeler and McNair reported low-angle thrust faulting involving late Tertiary Humboldt lacustrine sediments in two areas in eastern Elko County, Nevada. Since this concept has an important bearing on petroleum exploration, an examination was made of the areas. Spruce Mountain area: Along U. S. Highway 93 in T. 31 N., R. 62 E., about 40 miles south of Wells, a low escarpment paralleling the highway on the east is capped by a 50-75-foot limestone breccia which was apparently believed the basal part of a thrust plate. Study of the lithologic and structural features of the breccia and of its distribution and regional topographic relations shows that it is an old alluvial fan deposit related in origin to the present fan at the mouth of a large canyon draining the western slope of Spruce Mountain. This breccia rests on the eroded surface of gently tilted Humboldt beds, with complete absence of structural features indicative of low-angle thrust faulting. A dissected piedmont fault scarp about 2 miles east of the highway probably accounts for the slight eastern tilt of the lower part of the fan and the exposure by erosion of what is undoubtedly its basal breccia. About 10 miles south, a thrust was reported at the western base of Phalen Mountain in T. 29 N., R. 63 E. Excellent exposures of the contact between the Humboldt beds and the Paleozoic rocks occur along the southwest side of the mountain. Here the basal maroon conglomeratic silt unit of the Humboldt is in depositional contact with the Paleozoic. Minor post-Humboldt normal faulting has occurred. Again, there is complete absence of structural features indicative of major low-angle faulting. Thousand Springs Valley area: No specific localities are given by Wheeler and McNair for this area but the observations made throw grave doubt on the possibility of low-angle thrusts which involve the Humboldt beds. East of Thousand Springs Creek along the western base of Nine Mile Ridge, in Secs. 27 and 34, T. 42 N., R. 66 E., the Paleozoic-Humboldt contact is well exposed. For approximately 3/4 mile along the range front, steeply west-dipping Humboldt beds rest in depositional contact on the Paleozoic. The basal depositional breccia and conglomerate show no evidence of low-angle deformation. West of Thousand Springs Creek along the western edge of T. 42 and 43 N., R. 66 E., ridges of Paleozoic rocks stand above the gravel-covered pediment which cuts across gently to steeply dipping Humboldt beds. The Paleozoic-Humboldt contact was studied at two localities. In Sec. 29, T. 43 N., R. 66 E., Paleozoic limestone occurs as an upfaulted wedge between two intersecting normal faults, one of which continues into the Humboldt sediments. In Sec. 6, T. 42 N., R. 66 E., a high-standing area of Paleozoic rocks has Humboldt beds in normal depositional contact dipping away from it on at least three sides. The Paleozoic is anticlinally folded, and relations indicate that these rocks and the Humboldt beds were folded at the same time. Directly south of the anticlinal area, the lake beds ar faulted against the older rocks. No features indicative of low-angle faulting were found.
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Research Article| May 01, 1966 Table Mountain Serpentinite Extrusion in California Coast Ranges WILLIAM R DICKINSON WILLIAM R DICKINSON Geology Dept., Stanford University, Stanford, California Search for other works by this author on: GSW Google Scholar Author and Article Information WILLIAM R DICKINSON Geology Dept., Stanford University, Stanford, California Publisher: Geological Society of America Received: 26 Oct 1964 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1966, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1966) 77 (5): 451–472. https://doi.org/10.1130/0016-7606(1966)77[451:TMSEIC]2.0.CO;2 Article history Received: 26 Oct 1964 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation WILLIAM R DICKINSON; Table Mountain Serpentinite Extrusion in California Coast Ranges. GSA Bulletin 1966;; 77 (5): 451–472. doi: https://doi.org/10.1130/0016-7606(1966)77[451:TMSEIC]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Near Parkfield in the southern Diablo Range, the smooth summit of Table Mountain, a narrow ridge 12 miles long, is capped by a subhorizontal sheet of weakly foliate serpentinite breccia composed of massive serpentinized-peridotite blocks set in a slickensided matrix of crushed serpentine. The breccia was extruded by plastic flowage from longitudinal fault fissures now occupied by steeply dipping serpentinite breccia dikes that lie near the crest of a complex anticline whose core is Jurassic-Cretaceous(?) Franciscan tectonic breccia made of exotic blocks set in a cataclasite matrix. The fissure feeders probably tapped a long-dormant mass of Franciscan serpentinite that was mobilized by fault disruption and high lateral stresses during the Pliocene-Pleistocene orogenesis of the Diablo Range. Upon reaching the surface, the breccia buried an uneven ridge crest of hills and saddles beneath a gently arched summit carapace. The breccia then moved downslope in earthflow fashion to form narrow, sloping ramps that filled draws between resistant spurs on the flanks of the ridge. On the lower slopes, the extrusion again coalesced to form an evenly sloping piedmont plate through which rejuvenated streams have cut steep gorges. Extensive landslides with prominent headwall scarps have scarred eroded edges of the extrusive sheet, and a degradational sequence of Quaternary pediment gravel surfaces at the base of Table Mountain also postdates the extrusion. The piedmont plate rests discordantly across a steeply dipping anticline limb of marine Cretaceous Panoche Group and a largely marine Tertiary sequence that includes Pliocene-Pleistocene(?) continental gravels. The Tertiary beds rest unconformably on the Panoche Group, but the Panoche Group lies upon the underlying Franciscan tectonic breccia along a folded thrust. The thrust emplacement followed widespread tectonic brecciation of the Franciscan, but preceded deposition of Eocene sandstones containing Franciscan detritus. The folded thrust is cut locally by younger, steep reverse and strike-slip faults that offset Tertiary strata. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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In 1950, Wheeler and McNair (Bull. Geol. Soc. America, Vol. 61, No. 12, Pt. 2 (December, 1950), p. 1513) reported low-angle thrust faulting involving late Tertiary Humboldt lacustrine sediments in two areas in eastern Elko County, Nevada. Since this concept has an important bearing on petroleum exploration, an examination of the areas mentioned was made. SPRUCE MOUNTAIN AREA (a) Along U. S. Highway 93 in T. 31 N., R. 62 E., about 40 miles south of Wells, a low escarpment paralleling the highway on the east is capped by a 50-75-foot limestone breccia which was apparently End_Page 2627------------------------------ believed the basal part of a thrust plate. Study of the lithologic and structural features of the breccia and of its distribution and regional topographic relations shows that it is an old alluvial fan deposit related in origin to the present fan at the mouth of a large canyon draining the western slope of Spruce Mountain. This breccia rests upon the eroded surface of gently tilted Humboldt beds, with complete absence of structural features indicative of low-angle thrust faulting. A dissected piedmont fault scarp about two miles east of the highway probably accounts for the slight eastern tilt of the lower portion of the fan and the exposure by erosion of what is undoubtedly its basal breccia. (b) About 10 miles to the south, a thrust was reported at the western base of Phalen Mountain in T. 29 N., R. 63 E. Excellent exposures of the contact between the Humboldt beds and the Paleozoic rocks occur along the southwestern side of the mountain. Here the basal maroon conglomeratic silt unit of the Humboldt is in depositional contact with the Paleozoic. Minor post-Humboldt normal faulting has occurred. Again there is complete absence of structural features indicative of major low-angle faulting. End_Page 2628------------------------------ End_Page 2629------------------------------ THOUSAND SPRINGS VALLEY AREA No specific localities are given by Wheeler and McNair for this area but the observations made throw grave doubt on the possibility of low-angle thrusts which involve the Humboldt beds. (a) East of Thousand Springs Creek along the western base of Nine Mile Ridge, in Secs. 27 and 34, T. 42 N., R. 66 E., the Paleozoic-Humboldt contact is well exposed. For approximately 3/4 mile along the range front, steeply west-dipping Humboldt beds rest in depositional contact on the Paleozoic. The basal depositional breccia and conglomerate show no evidence of low-angle deformation. (b) West of Thousand Spring Creek along the western edge of T. 42 and 43 N., R. 66 E., ridges of Paleozoic rocks stand above the gravel-covered pediment which cuts across gently to steeply dipping Humboldt beds. The Paleozoic-Humboldt contact was studied at two localities. In Sec. 29, T. 43 N., R. 66 E., Paleozoic limestone occurs as an up-faulted wedge between two intersecting normal faults, one of which continues into the Humboldt sediments. In Sec. 6, T. 42 N., R. 66 E., a high-standing area of Paleozoic rocks has Humboldt beds in normal depositional contact dipping away from it on at least three sides. The Paleozoic is anticlinally folded, and relations indicate that these rocks and the Humboldt beds were folded at the same time. Immediately south of the anticlinal area, the lake eds are faulted against the older rocks. No features indicative of low-angle faulting were found. End_of_Article - Last_Page 2630------------
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Research Article| April 01, 2006 The Poverty Hills, Owens Valley, California—Transpressional Uplift or Ancient Landslide Deposit? KIM M. BISHOP; KIM M. BISHOP 1Department of Geology, California State University, Los Angeles, 5151 State University Drive, Los Angeles, CA 90032 Search for other works by this author on: GSW Google Scholar STEVE CLEMENTS STEVE CLEMENTS 2SCS Engineers, 6601 Koll Center Parkway, Suite 140, Pleasanton, CA 64566 Search for other works by this author on: GSW Google Scholar Author and Article Information KIM M. BISHOP 1Department of Geology, California State University, Los Angeles, 5151 State University Drive, Los Angeles, CA 90032 STEVE CLEMENTS 2SCS Engineers, 6601 Koll Center Parkway, Suite 140, Pleasanton, CA 64566 Publisher: Association of Environmental & Engineering Geologists First Online: 09 Mar 2017 Online ISSN: 1558-9161 Print ISSN: 1078-7275 © 2006 Association of Engineering Geologists Environmental & Engineering Geoscience (2006) 12 (4): 301–314. https://doi.org/10.2113/gseegeosci.12.4.301 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation KIM M. BISHOP, STEVE CLEMENTS; The Poverty Hills, Owens Valley, California—Transpressional Uplift or Ancient Landslide Deposit?. Environmental & Engineering Geoscience 2006;; 12 (4): 301–314. doi: https://doi.org/10.2113/gseegeosci.12.4.301 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract The Poverty Hills are a 5 km2, 300-m-high mass of plutonic and meta-sedimentary rocks exposed along the axis of the alluviated Owens Valley graben in eastern California. Two models have been proposed for the origin of the hills. One model suggests the hills represent a transpressional uplift along a 3-km left-step in the right-lateral Owens Valley fault zone, and the other proposes that the hills are a long-runout rock-avalanche deposit. This article argues that the rock-avalanche hypothesis best fits the geology. The transpressional model is problematic because the fault segment to which right-lateral displacement is purportedly transferred has been shown to have pure dip-slip displacement. The rock-avalanche model is based primarily on lithologic evidence. Throughout the hills are scattered outcrops displaying mosaic, jigsaw, and crackle breccia textures, all of which are characteristic of rock-avalanche deposits. Also, well-exposed roadcut outcrops display preserved source-rock bedding within the matrix-poor breccia framework. Though preserved, the bedding has been contorted and distorted by cataclastic flow of the breccia, another common characteristic of rock-avalanche deposits created during emplacement. The landslide is younger than 3 Ma, the age of inception of the Owens Valley graben, and older than 640 ± 50 ka, the age of a basalt flow that post-dates the landslide. An area in the Inyo Mountains southeast of the hills appears to be the most likely source. Interpretation of the Poverty Hills as a landslide mass suggests an alternative model for the Owens Valley fault-zone kinematics. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Abstract Snake Valley is a 95-mile-long (150 km) valley in Utah and Nevada that holds great interest for residents of both states because of its ground-water resources. The geologic framework of Snake Valley and adjacent Hamlin Valley, Tule Valley, Pine Valley, Fish Springs Flat, and Wah Wah Valley and their surrounding ranges was analyzed as part of an effort to characterize ground-water conditions and regional ground-water flow. The study area lies within the boundary of the Great Salt Lake Desert regional ground-water flow system, in which the valleys are hydraulically interconnected and ground-water movement is northward to the Great Salt Lake Desert. Snake Valley is bounded on the west by high ranges: the Deep Creek Range of Utah and the Kern Mountains and Snake Range of Nevada. The ranges consist mostly of Proterozoic and Cambrian quartzite intruded by Jurassic, Cretaceous, and Tertiary plutonic rocks. Uplift occurred during late Cenozoic time along mostly high-angle basin-range normal faults, at which time the upper part of the Snake Range failed along the Snake Range decollement, a low-angle normal denudation fault. The east side of Snake Valley is bounded by the low Confusion Range and adjacent ranges of mostly middle to upper Paleozoic carbonate rocks that were folded and locally thrusted during the late Mesozoic to earliest Tertiary Sevier compressive deformational event. Hamlin Valley, south of and tributary to Snake Valley, is bounded on the northwest by the low Limestone Hills, made up of faulted Devonian carbonates, through which some ground water may flow from Spring Valley to Hamlin Valley. Southern Hamlin Valley drains the large Oligocene Indian Peak caldera complex. East of Snake Valley is Tule Valley, on the east side of which are the narrow, sharp Fish Springs and House Ranges, which consist mostly of Cambrian quartzite and carbonate rocks. Pine Valley to the south is bounded on its east side by the Wah Wah Mountains, made up of locally thrusted, east-dipping Cambrian quartzite and carbonates overlain by Oligocene ash-flow tuffs. Fish Springs Flat lies east of Tule Valley, between the Fish Springs Range on the west and the Dugway and Thomas Ranges on the east. The Dugway and Thomas Ranges consist of Eocene ash-flow tuffs and Miocene rhyolite flows that overlie Paleozoic carbonates. Wah Wah Valley to the south is bounded on the east by Proterozoic quartzite, an Oligocene stock, and Oligocene tuffs and flows in the San Francisco Mountains and low hills farther south. As in most places elsewhere in the Great Basin, the six valleys are north-trending grabens, and the ranges are north-trending horsts, formed during the late Cenozoic basin-range extensional deformational event. This deformation, resulting in the present topography, was formed by large northerly trending, high-angle, basin-range faults. Snake Valley was downfaulted thousands of feet relative to adjacent ranges, followed by erosion of the ranges and deposition of basin-fill sediments in the valley. In most places in the interior of Snake Valley, the sediments are as much as 5000 feet (1500 m) thick, but the sediments are thicker in some subbasins. The other five valleys have comparable to lesser thicknesses of sediments. We apply the concept of fracture flow, developed from extensive mapping experience in the Basin and Range Province, to explain ground-water movement. Under this concept, ground-water flow is enhanced by the north-trending faults and parallel fault-related fractures in basin-fill deposits and underlying bedrock units. The faults act as conduits to northerly flow and as partial barriers to east or west flow. Aquitards in many of the ranges include the Proterozoic and Cambrian quartzite, plutonic rocks of several ages, and the Chainman Shale and Thaynes Formation, which further restrict or prevent east or west ground-water flow.
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Research Article| May 01, 1959 GEOLOGY OF THE BUFFALO MOUNTAIN-CHEROKEE MOUNTAIN AREA, NORTHEASTERN TENNESSEE RICHARD J ORDWAY RICHARD J ORDWAY STATE TEACHER'S COLLEGE, NEW PALTZ, N. Y. Search for other works by this author on: GSW Google Scholar Author and Article Information RICHARD J ORDWAY STATE TEACHER'S COLLEGE, NEW PALTZ, N. Y. Publisher: Geological Society of America Received: 18 Jun 1957 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1959, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1959) 70 (5): 619–636. https://doi.org/10.1130/0016-7606(1959)70[619:GOTBMM]2.0.CO;2 Article history Received: 18 Jun 1957 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation RICHARD J ORDWAY; GEOLOGY OF THE BUFFALO MOUNTAIN-CHEROKEE MOUNTAIN AREA, NORTHEASTERN TENNESSEE. GSA Bulletin 1959;; 70 (5): 619–636. doi: https://doi.org/10.1130/0016-7606(1959)70[619:GOTBMM]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Buffalo Mountain-Cherokee Mountain area in northeastern Tennessee includes about 45 square miles and is located along the southeast border of the Appalachian Valley and Ridge geomorphic province. The mountainous part of the area is underlain almost entirely by the Buffalo Mountain thrust sheet, which has been separated by two minor thrust faults into three imbricate thrust blocks.Cambrian and Precambrian (?) rocks in the Buffalo Mountain thrust sheet consist of the Unicoi, Hampton, and Erwin formations (Chilhowee group) and the Shady dolomite. Younger, Cambrian-Ordovician rocks beneath the thrust sheet include the Honaker limestone, Nolichucky shale, Knox dolomite, and Athens shale.During or following the thrusting, all the rocks in the area were folded into a synclinorium trending northeast-southwest. Some folding apparently preceded the thrusting. Several “shear faults” mapped by Keith in this area do not appear to exist. An interesting feature of the structure is the number of slices that have been broken off and dragged along the thrust surfaces. Slices of younger rocks have been found between older rocks, and slices of older rocks between younger. Cleavage and a low-rank metamorphism are present. Deformation probably occurred in late Paleozoic time during the Appalachian orogeny. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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In San Gorgonio Pass, 70 miles east of Los Angeles, a complex network of faults separates two of the highest mountain ranges of Southern California. The San Andreas fault, which forms a part of the network, exhibits several unusual features in this area. Among these are the absence of rift topography, absence of lateral stream offsets, an abrupt change in trend of the fault trace, seismic evidence for the predominance of thrusting over strike-slip movements, and a lack of great earthquakes in historic time.
Crystalline rocks of Mesozoic and earlier age crop out over most of the map area. North of the pass, the San Gorgonio igneous-metamorphic complex comprises an old metamorphic terrane of intermediate to basic composition and probable igneous parentage, and Mesozoic(?) plutonic rocks of quartz monzonitic composition. These plutonic rocks have intimately intruded and in large part reconstituted the older metamorphic rocks. The resulting migmatitic gneiss is the most widespread rock of the area, and includes flaser gneiss, green-schist, and piedmontite-bearing gneiss as distinctive varietal types. Rocks of the San Jacinto Mountains south of San Gorgonio Pass are distinctly different from those to the north, and comprise texturally uniform granodioritic and tonalitic rocks that contain sparse inclusions and septa of metasedimentary rocks.
Nearly all of the sedimentary rocks in the pass area are of alluvial-fan or flood-plain origin, and they reflect a Quaternary and late Tertiary history of recurrent deformation and deposition. The upper Miocene(?) Coachella fanglomerate is the oldest exposed sedimentary rock, and is overlain with marked angular unconformity by all younger units. Lower Pliocene(?) incursion of tropical marine waters into the Salton trough is represented by a thin stringer of Imperial formation which is conformably underlain and overlain by continental strata of the Hathaway and Painted Hill formations, respectively. All of these rocks are overlain with marked angular unconformity by Quaternary Cabezon fanglomerate, which probably is correlative with upper beds of the Pliocene-Pleistocene San Timoteo(?) formation in the western part of the map area. Other Quaternary deposits, each showing complex structural relationships to adjacent rocks, are the deformed gravels of Whitewater River, Heights fanglomerate, and Burnt Canyon breccia. Recent alluvium covers the floor of the pass. Flows and dikes of olivine basalt occur in the Coachella fanglomerate and Painted Hill formation. Lithology of clasts in the sedimentary rocks indicates derivation predominantly from rocks of the San Gorgonio igneous-metamorphic complex to the north.
Quaternary alluvial fans of Heights and Cabezon fanglomerate, which once buried a former rugged topography, are now being dissected along the north side of the pass. Surfaces of low relief and associated stream terraces resulting from this dissection are the Beaumont plain, Banning Bench, and Pine Bench surfaces. Upstream divergence of the Banning Bench surface from present stream levels is attributed in part to tilting. Farther east, the older Cabezon surface shows many effects of warping. This surface probably is correlative with an area of low relief at altitudes of 6500 to 8000 feet near Raywood Flat, and suggests Quaternary arching of the mountain range along a north-south axis.
Within San Gorgonio Pass, alluvial fans derived from areas to the north dominate those derived from the steeper San Jacinto scarp to the south. This unequal development of fans is attributed to greater flood-producing rainfall and larger drainage area on the north, together with more easily erodable rock in this area. Most of the faults that show Recent movement are well delineated by springs and vegetative contrasts. Other springs are caused by exposure of unconformities, and by superposition of streams onto the rugged pre-Cabezon topography.
The San Andreas fault is a continuous linear feature for a distance of more than 400 miles northwest from San Gorgonio Pass, but within the pass it curves abruptly southward and butts into the east-trending Banning fault at an angle of 45 [degrees] . Recent strike-slip movement on this part of the fault probably amounts to less than one mile, and post-Mesozoic displacement probably has not exceeded a few tens of miles.
The Banning fault, a major break that delineates the north side of the pass, extends for a distance of more than 50 miles eastward from a point near Redlands through the pass into the Coachella Valley. Within the pass, it is a steeply north-dipping reverse fault except for a zone of low-angle thrusting between Millard and Whitewater Canyons. At least 5000 feet of vertical displacement has taken place on this fault since San Timoteo time, and a right lateral offset of 5 miles is suggested. Recent displacement is limited to the segment of the fault east of Millard Canyon. Pre-Pliocene lateral displacement may have been great, but is not demanded by evidence in this area.
The Mission Creek fault branches from the San Andreas fault north of Banning, and is a major north-dipping fracture that is continuous for at least 40 miles to the southeast. The Pinto Mountain fault diverges from the Mission Creek fault at a low angle, and probably is continuous for more than 50 miles to the east; in this interval it forms the southern boundary of the northwest-trending fault system of the Mojave Desert. The Mill Creek fault branches from the San Andreas fault north of San Bernardino, and has guided erosion along deep linear valleys in the high mountains; this fault apparently dies out eastward.
Within the San Bernardino Mountains all of the faults north of the Banning fault separate crystalline rocks of the same family; these rocks are similar in their migmatitic structural features, remnants of amphibolite, intrusion by quartz monzonite, and high content of titanium minerals. Thus post-Mesozoic lateral displacements of hundreds of miles along these faults seem to be precluded. Although lateral displacements of a few tens of miles are possible, no observed evidence appears to demand such movements. Late Tertiary and Quaternary vertical movements are suggested by the physiography of the mountains. No Recent movements have occurred on parts of the Mission Creek and Mill Creek faults.
Recent movements on both the Banning and San Andreas faults probably were caused by a stress system involving a generally north-south maximum principal stress, with an east-west least principal stress of only slightly lesser magnitude than the vertical stress. In the vicinity of San Gorgonio Pass, an older east-west line of weakness causes the east-west stress effectively to become the intermediate stress, so that thrust faulting predominates over strike-slip faulting in this one local area.
San Gorgonio Pass is bounded by a reverse or thrust fault on the north, and indirect evidence suggests a similar fault on the south. Quaternary and late Tertiary displacement on these faults, rather than erosion, is primarily responsible for the present physiography of the pass. Local conversion of San Andreas-type lateral strain into vertical displacements on the bounding faults is a reasonable explanation of both the pass itself and the unusually high peaks adjacent to it.
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The area discussed in this report, consisting of sixteen square miles, embraces a portion of the San Jose Hills approximately three miles south of the town of Covina, Los Angeles County, California. The area was mapped on a base map prepared from U. S. Geological Survey topographic maps, and the Brunton compass-pacing method of mapping was used.
The region is part of an upland that rises above the Los Angeles and San Gabriel basins, and consists of a group of rolling hills trending in an approximate east-west direction. The relief throughout the region is moderate, and as a result of the semi-arid climate vegetation in the area is sparse. Bedrock is well exposed, excepting in some of the areas underlain by shale, where slumping has distorted the rocks and soils and a dense grass growth further hinder exposure of the underlying formations.
All of the rocks exposed in the area are of sedimentary origin, and, with the exception of Recent and Pleistocene alluvium, are part of the Puente formation of upper Miocene age. The Puente formation is divided into three members--a lower member of shale, a middle member of sandstone and conglomerate, and an upper member of shale, sandstone, and conglomerate. The subsurface rocks, knowledge of which has been derived from wells drilled in the area, consist of the Topanga formation, the Glendora volcanics, the Mountain Meadows dacite porphyry, and the basement complex in that order with increasing depth.
There appear to have been two major periods of deformation in the San Jose Hills area during Tertiary time--one at the close of the Pliocene and one during and after the deposition of the Miocene Puente formation. The Puente deformation seems to have involved only gentle folding, but the post-Pliocene deformation was more severe, involving steep folding and some faulting. The general structural trend of the region is approximately N 60 E, and is parallel to the boundaries of the higher hills. Folding in the area has created a series of parallel anticlines and synclines, with the San Jose anticline in the northern half of the area being the major structure. The one fault of any magnitude in the area is the San Jose fault, which enters from the east and apparently dies out in the central part of the area. It is a vertical or steeply dipping reverse fault with considerable downthrow on the south side. There are other smaller faults in the southern half of the area.
The geologic history of the region largely involves erosion after the intrusion of the basement complex in Mesozoic time until the Miocene period, when the area became one of deposition. The Glendora volcanics were deposited in early middle Miocene time, followed by submergence of the area and deposition of the Topanga, Puente, and Pliocene formations. After the close of the Pliocene the region emerged from beneath the sea and again became an area of erosion.
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The San Francisco Mountains are part of a fault-block range in the Basin and Range physiographic province in west-central Utah. The rock succession in this range was mapped as a normal stratigraphic sequence by B. S. Butler. Recent investigation, however, indicates that older rocks rest on younger rocks along an extensive overthrust, as first suggested by Nolan and Hintze. The Morehouse quartzite, presumed by Butler to be of Ordovician-Silurian(?) age, is recognized to represent the Precambrian(?) to Lower Cambrian Prospect Mountain Quartzite. The Grampian assigned by Butler to the Cambro(?)-Ordovician, is identified as including Middle(?) and Upper Cambrian carbonate rocks, Ordovician Pogonip Group, Lower Mississippian(?) limestone, and Pennsylvanian Ely L mestone. The Prospect Mountain Quartzite has overridden the Paleozoic carbonate rocks. The thrust plate underlies the crest of the San Francisco Mountains north of a large quartz monzonite stock, plunges northward, and forms the bulk of the Cricket Mountains. The Prospect Mountain Quartzite normally is overlain by younger Cambrian strata in the Cricket Mountains. Two small klippen of the quartzite remain south of the intrusive body. Boulder conglomerates of probable early Tertiary age unconformably overlie the thrust plate; therefore, the age of the thrusting is post-Early Pennsylvanian and pre-early Tertiary. The direction of movement at the thrust is not evident from the local record, but regional relationships indicate an easterly to southeasterly direction. The displacement at the thrust is greater than the 4-mile width of the range and probably is of large magnitude, as the thrust has no root within the range. Misch postulates that both the Wah Wah thrust described by Miller and the San Francisco thrust of this report resulted from the frontal breakthrough of the Snake Range decollement. If so, the displacement has been several tens of miles. The Paleozoic strata underlying the San Francisco Mountains were folded and faulted prior to large-scale overthrusting. Fluviatile boulder conglomerates, probably formed as a result of the Laramide orogeny, covered the deformed and eroded Paleozoic beds. A large outpouring of volcanic flows and pyroclastic rocks followed in middle Tertiary time. Cenozoic deformation has been superposed on the earlier structures. This period of tectonism included intrusion of a quartz monzonite stock and Basin-and-Range faulting which outlined the present range. In late Pleistocene, Lake Bonneville flooded the adjacent intermontane basins and during retreat left numerous shoreline deposits along the western range flank.
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Investigation of the Spread Eagle Peak area in the north-central Sangre de Cristo Range, Colorado, was undertaken to complete a series of geologic mapping studies in this region. The bulk of the range in th~s area is composed of a thick s equence of complexly folded and faulted Pennsylvanian and Permian s trata which may have an aggregate thrckness of as much as 14,000 feet. An area of approximately 70 square miles was mapped using aerial photographs with particular emphasis on the structural f eatures of the range. Precambrian rocks comprised of granite and metasediments occur in zones as much as 1-112 miles wide on both flanks of the range. Imbricate segments of lower P aleozoic strata c rop out in a fault zone between Precambrian,rock s and Permo-Pennsylvan ian clastics on the west flank of the range. These lower P aleozoic strata are probably remnants of Ordovician, Devonian and Missis- sippian rocks, chefly dolomites, limestones and sandstones. Strata of Perm-o-Pennsylva nian age may be divided into two general units: the lower unit, the Minturn Formation, is comprised of conglomerate, sandstone, and shale with thin l imestones interbedded near the top; the upper unit, the Sangre de Cristo Formation, is comprised of coarse arkosic sandstones and conglomerate. This thick sequence of sediments w as derived predominantly by erosion of Precambrian crystalline rocks of the San Luis-Uncompahgre highland to the west, and was deposited in a rapidly subsiding narrow trough, a zeugogeosyncline. No trace was found oi a large T ertiary stock which was pre- viously mapped along the Saguache-Custer .County line. Tertiary volcanic rocks covlering about 2-112 square miles occur in the northeastern part of the area. Dikes and sills also of Tertiary age occur separately and in clusters throughout the area. Pleistoccne valley glaciers were active throughout the hlgher parts of the range. In the area mappkd, two distinct stages of Laramide thrusting of opposing direction arc inferred from field data. In adjacent areas to the north and south most of the evidence reflects only a single stage of thrusting. A klippe, previously unreported in the Sangre de Cristo Range, was mapped near the crest of the range. . The structural f ramework of the Wet Mountain Valley is reinterpreted. Evidence is presented to show that this valley is probably genetically related to basin-and-range type structure rather than an intermontane synclinal depression. It is also proposed t hat the Wet Mountain massif be extended westward to include the West Mountain Valley and the Precambrian rocks which form the east flank of the range in the report area. A normal fault is inferred along the base of the range facing the Wet Mountain Valley. Displacement on this fault ie thought to be relatively small, but its genesis is probably similar to that of the high-angle fault which bounds the range on the west.
Conglomerate
Devonian
Stratigraphic unit
Trough (economics)
Outcrop
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