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At least 1,315 records · Page 73Linked to original sources
USGS1982

Subsurface valleys and geoarcheology of the eastern Sahara revealed by shuttle radar

The shuttle imaging radar (SIR-A) carried on the space shuttle Columbia in November 1981 penetrated the extremely dry Selima Sand Sheet, dunes, and drift sand of the eastern Sahara, revealing previously unknown buried valleys, geologic structures, and possible Stone Age occupation sites. Radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand- and alluvium-filled valleys, some nearly as wide as the Nile Valley and perhaps as old as middle Tertiary. The now-vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat, hyperarid region. Underfit and incised dry wadis, many superimposed on the large valleys, represent erosion by intermittent running water, probably during Quaternary pluvials. Stone Age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation. The presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present-day oases which have been centers of episodic human habitation. Radar penetration of dry sand and soils varies with the wavelength of the incident signals (24 centimeters for the SIR-A system), incidence angle, and the electrical properties of the materials, which are largely determined by moisture content. The calculated depth of radar penetration of dry sand and granules, based on laboratory measurements of the electrical properties of samples from the Selima Sand Sheet, is at least 5 meters. Recent (September 1982) field studies in Egypt verified SIR-A signal penetration depths of at least 1 meter in the Selima Sand Sheet and in drift sand and 2 or more meters in sand dunes.

eastern Sahara↗
USGS1989

Heat flow and hydrothermal circulation in the Cascade Range, north-central Oregon

In north-central Oregon a large area of near-zero near-surface conductive heat flow occurs in young volcanic rocks of the Cascade Range. Recent advective heat flux measurements and a heat-budget analysis suggest that ground-water circulation sweeps sufficient heat out of areas where rocks younger than 6 Ma (million years ago) are exposed to account for the anomalously high advective and conductive heat discharge measured in older rocks at lower elevations. Earlier workers have proposed that an extensive midcrustal magmatic heat source is responsible for this anomalously high heat flow. Instead, high heat flow in the older rocks may be a relatively shallow phenomenon caused by regional ground-water flow. Any deeper anomaly may be relatively narrow, spatially variable, and essentially confined to the Quaternary (less than 2 Ma) arc. Magmatic intrusion at a rate of 9 to 33 cubic kilometers per kilometer of arc length per million years can account for the total heat flow anomaly. Deep drilling in the areas of high heat flow in the older rocks could indicate which model is more appropriate for the near-surface heat flow data.

Oregon↗
USGS1997

Synthesis of the paleoclimatic record from Owens Lake core OL-92

During much of the late Quaternary, Owens Lake overflowed into one or more of four successively lower-elevation basins. Most of the water came from the high, eastern slopes of the southern Sierra Nevada, and changes in the volumes of that water reflect a dominant climatic cycle of ~100 k.y. Variations in the inflow to, and outflow from, Owens Lake since ca. 800 ka left biological, chemical, mineralogical, and geophysical evidence in the sediments of those changes. Biological evidence includes fossil ostracodes, diatoms, fish, and mollusks (and δ 18 O data from their shells) which indicate fresh or brackish lake water on the basis of their modern habitats. Fossil pollens indicate ~20 regional vegetation cycles during the same period. Chemical evidence of high inflow and, commonly, outflow volumes is provided by the low inorganic- and organic-C content of some sediments, reflecting short lake-water residence times; long residence times produced higher and more variable quantities of these components. Mineralogical variations in illite/smectite ratios indicate changes in weathering processes and glacial comminution. High magnetic susceptibility correlates with other criteria that indicate high runoff. Between 810 ka and 645 ka, Owens Lake was fresh, several meters deep, and depositing silt with a few beds of sand; it supported a flora and fauna now found in fresh, sometimes very cool, waters. (Note that most geologic ages describing the OL-92 chronology have been rounded to the nearest 5 or 10 ka.) A shallow-but-freshwater lake may have been the result of accelerated sedimentation during an earlier (>800 ka) glaciation in the Sierra Nevada, choking the basin with sediment nearly to its spillway level. Between 645 ka and 450 ka, the lake was probably even shallower, depositing beds of coarse to fine sand, but overflowing periodically allowing its water to remain fresh. Between 450 ka and 5 ka, Owens Lake was mostly deep, alternating between spilling and being closed part of the time. It deposited silt and clay on its floor, yet underwent detectable variations in salinity caused by climate changes; this part of the record is the most easily interpreted and constitutes the main basis for comparing this paleoclimatic record with other long records. From 5 ka to A.D. 1913, when the Owens River was diverted into an aqueduct, Owens Lake was shallow (~2 m to ~15 m), moderately saline (~5% to <15% salts), and depositing oolites. After 1913, the lake desiccated. Comparison of the Owens Lake water-depth record with that of Searles Lake, two-basins downstream during much of late Pleistocene time, shows that they underwent similar responses to climate, but sedimentation changes documenting those responses commenced thousands of years apart, apparently because changes in precipitation volumes occurred gradually. Owens Lake, at the base of high mountains, was the first to reflect increasing amounts of regional precipitation; Searles, in a more arid environment, was the first to reflect decreasing amounts of precipitation. Devils Hole, 150 km east of Owens Lake, has a well dated isotopic-temperature record that resembles the Owens Lake-depth record. Marine records of Pleistocene glacial fluctuations, which measure high-latitude ice-sheet volumes and thus both precipitation and temperature at those latitudes, also resemble the Owens Lake history. There are, however, differences between the ages of the maxima and minima of climatic events as reconstructed from the Owens Lake core and similar-appearing inflections in the other two records; the differences range from 0 to 33 k.y. and average ~15 k.y. The question arises whether the differences between those ages are results of errors in the time-scale used for the Owens Lake record, or were there significant differences in the times when atmospheric climate change began to affect its different elements. The three records compared here are measurements of different elements and combinations of elements in two latitude belts: the deep-sea marine records measure combinations of temperature and precipitation that determined global ice volumes (at mostly high latitudes), the Devils Hole record measures atmospheric temperatures (in its mid-latitude region), and the Owens Lake record measures effective precipitation (in the same mid-latitude region).

California↗
USGS1947

Geology of the Borah Peak quadrangle, Idaho

This report is on result of a long program of geologic investigation in south-central Idaho, undertaken as an aid in the development of the mineral resources of the region. This quadrangle was examined because of the exceptional opportunities for the study of stratigraphy and structure afforded by the Lost River Range, which is the highest in Idaho and contains comparatively few intrusive igneous rocks. The Borah Peak quadrangle differs from the region to the west and northwest in that it contains parts of two sharply defined, narrow ranges elongated in a north-westerly direction, instead of broad, irregular mountain masses, in which the trends of most local topographic units are ill-defined. These ranges, the Lost River and the Lemhi, are flanked by intermontane valleys sparsely populated by stock ranchers. The Lemhi and Swauger quartzites, regarded as of Belt (pre-Cambrian) age, are named. The two Cambrian (?) formations of the Bayhorse region are not recognized in this quadrangle. The Ramshorn slate (Lower Ordovician), widespread and thick there, does not appear to be exposed anywhere in the Borah Peak quadrangle with the possible exception of small areas east of the Elkhorn Ranch, where relations are obscure. The higher Paleozoic formations, named in order of decreasing age, are the Kinnikinic quartzite, Saturday Mountain formation, Laketown dolomite, Jefferson dolomite, Grand View dolomite, Three Forks limestone, Milligen formation, Brazer limestone, and Wood River formation. The Three Forks limestone, nowhere much over 250 feet thick, is the only one of these not recognized farther west. It provides substantial aid in the interpretation of upper Paleozoic stratigraphy. Equivalent beds to the west are presumably grouped with the Milligen. The grit that introduced uncertainties into the correlation of the Wood River and Milligen formations in the southeastern part of the Bayhorse quadrangle is absent here, and both formations have close lithologic resemblances to those in the type localities in the Wood River region. Wood River strata (Pennsylvanian) crop out only in the southwest corner of the quadrangle, and the Brazer may have been the last Paleozoic unit to be deposited over most of the area. Certainly it can have had comparatively little cover over it throughout the Mesozoic. The Brazer is regarded as Mississippian, but some of the fossils in it have Pennsylvania affinities. The carbonate rocks in the Three Forks and later formations are dominantly calcareous, while those in earlier units are dolomitic. Carbonaceous matter is present in most of the formations but is abundant only in the Milligen. The Challis volcanics (Oligocene or Miocene) constitute the first stratified unit to be laid down after Paleozoic deposition ceased, although there was a little intrusive activity late in the Mesozoic. The volcanics were once widely distributed on the flanks of the mountains and are still plentiful in the northwestern corner of the quadrangle, near the head of the valley of the Pahsimeroi, and in smaller areas. They are neither so abundant nor so diversified as they are farther west. They are locally succeeded by an alluvial formation, here termed the Donkey fanglomerate, of possible Pliocene age. Abundant Quaternary glacial and alluvial deposits are present in the larger valleys. The Lemhi and Swauger quartzites were broadly folded before Paleozoic sedimentation began. Apparently the later deformation affected them only enough to render the more impure beds somewhat schistose. The Paleozoic rocks have been folded into closely spaced, asymmetric anticlines, locally broken by thrusts at and near their crests. These folds approximately parallel the trends of the present range, whereas those in the old rocks strike more nearly north. Later deformation twisted some of the folds and produced thrusts of lower dip and greater extent. The folded rocks in the Lost River Range have been arched into an anticlinorium. The original, tight folds may have preceded the emplacement of the Idaho batholith farther west, while the anticlinorium and the late twists and low thrusts result from deformation during the long period in which the batholith came to place and adjusted itself. Local intricately contorted beds in the Brazer limestone are interpreted as the effects of flowage in calcareous material subjected to tangential pressure under comparatively light supercumbent load. The quadrangle contains numerous normal faults of diverse trends. Most of those that can be conclusively demonstrated strike transverse to the trends of the ranges, but especially along parts of the southwest front of the Lemhi Range normal faults along the range fronts may have helped locally to guide erosion of the mountains. Some of the faults have displacements of thousands of feet and are thought to have originated in connection with the low-angle thrusts. Most of these, as well as minor breaks of diverse trends, affect also the Challis volcanics, which shows that renewed movement occurred in Tertiary time. The present mountain masses and broad intermontane valleys are in about the positions occupied by similar features in the early Tertiary. Several incomplete erosion cycles since then have greatly modified the topography but have not obliterated the influence of these ancient land forms. The results of early episodes in the development of the topography are much obscured by the rugged forms that result from active Pleistocene glaciation and later vigorous stream erosion, but modified remnants of the post-Challis and Donkey Hills surfaces can be clearly distinguished. Several less widespread remnants mark intermediate steps in the process. Exceptionally abundant and permeable, coarse alluvial and glacial deposits floor intermontane valleys and choke the larger mountain gorges to such an extent as to interfere with normal erosion and make the surface-water supplies even scantier than might be expected from the climate of the region, which is moderately humid in the mountains and semiarid in the valleys. Active erosion in the high mountains and comparatively static conditions on fans at the range borders result in striking contrasts.

Idaho↗
USGS1957

Notes on the structural geology of Puerto Rico

Two major structural and stratigraphic rock units occur in Puerto Rico: the older complex, ranging in known age from Late Cretaceous to late Paleocene or early Eocene and the middle Tertiary sequence, ranging from late Oligocene possibly to late Miocene. The former rocks are eugeosynclinal in character and are very badly faulted but for the most part apparently only moderately folded. With the exception of a large, partly low-angle thrust, the writer has not recognized evidence of strong tangential stresses. Intra-formational folding in the older complex is interpreted as caused by submarine sliding (slump structure). Except in the vicinity of the larger plutonic intrusions and in the northeastern and southwestern corners of the island, the main strike alignment of the older complex is northwestward. The plutons are roughtly concordant with the structure of the country rock and show varying degrees of differentiation. Their average composition seems to be more acidic than that of the volcanic and volcanogenetic rocks into which they are intruded. The middle Tertiary sequence is nonvolcanic, made up dominantly of calcareous marine sediments. It crops out on the north and south sides of the island and in structural troughs on the west coast. On the north coast the beds dip gently to the north, and, except for slight terracings and a flexure at the northwestern corner of the island, are not folded. The middle Tertiary sequence on the south side of the island is somewhat folded. Seismic-reflection studies of the north coast indicate, however, a pronounced northward thickening, possibly some folding, and unconformities at depth. Unconformities which may be local have also been noted at several places on the surface. Several large faults in the middle Tertiary sequence have been recognized in both the north- and south-coast belts. The pattern of master joints that is inferred in the north-coast middle Tertiary belt from topographic alignments seems to indicate (1) control by fades contacts, (2) possible downwarping associated with the sinking of an arm of the Puerto Rican Trench, and (3) tension during upwarping along the island axis. The dates of the major diastrophic events that are decipherable from the Puerto Rican data are: (1) early Tertiary (possibly late Paleocene, but more probably Eocene), when the older complex was deformed; (2) Miocene, when the middle Tertiary sequence was deformed; (3) late Pliocene and possibly early Pleistocene, when block faulting on a large scale produced the present topographic relief. Remnants of the fault scarps resulting from the latter deformation occur widely. Quaternary marine deposits and marine terraces suggest that Puerto Rico has been relatively unaffected by crustal movement at least since the late Pleistocene.

Puerto Rico↗
USGS1957

Thermal waters of volcanic origin

Waters of widely differing chemical compositions have been considered at least in part volcanic in origin, and are commonly associated with each other in the same area. Do any or all of these types contain volcanic components, and if so, how are the different types derived? To determine the probable characteristics of volcanic waters, the writer has selected hot-spring groups that are particularly high in temperature and associated heat flow, are associated with late Tertiary or Quaternary volcanism, and are therefore most likely to contain some water and chemical components of direct volcanic origin. Of the different types of water that occur in these groups, one of the most common is characterized chemically by a dominance of sodium chloride. Isotopic evidence indicates that the contribution of water of direct volcanic origin is not large and is probably no more than 5 per cent in typical sodium-chloride springs. The compositions of volcanic waters are believed to be determined by: [1] type of magma and stage of crystallization; [2] temperature and pressure of the emanation at different stages during and after departure from the magma; [3] chemical composition, relative quantity, and depth of penetration of mixing meteoric water and water of other origin; and [4] reactions with wall rocks. Although the type of magma and its stage of crystallization are of major interest and have been emphasized in the past, the outstanding characteristics of volcanic emanations at and near the surface of the earth seem to be controlled for the most part by the other factors. Nonvolatile compounds are slightly to highly soluble in steam at high pressure, and high-density steam has solvent properties similar to those of liquid water. In the volcanic sodium-chloride waters, the high ratio of lithium to sodium and potassium is shown to indicate that alkalies were transported as alkali halides dissolved in a dense vapor. This in turn demands a deep circulation of meteoric water for steam to condense at high pressure and for the halides to remain in solution. The depth of circulation of meteoric water in the sodium-chloride spring systems is believed to be in the order of 2 miles. Where circulation of meteoric water is shallow, the vapors rise and expand at low pressure, which does not permit transport of substances of low volatility; some type of water other than the sodium-chloride type is formed. The common volcanic sodium-chloride waters are therefore concluded to be the diluted product of high-density emanations, modified by reactions with wall rocks and by precipitation of the less soluble components. Emanations at high temperature and relatively low pressure consist almost entirely of steam and volatile components. Their compositions are therefore relatively simple, and their ability to transport matter of low volatility is very limited. The sodium-chloride type is probably gradational into acid-sulfate-chloride waters. There is some evidence that, under conditions not well understood, sulfur may be emitted as SO 2 , SO 3 , or other sulfur species of intermediate valence, rather than as H 2 S or S. Other major types of volcanic waters are called sodium bicarbonate, acid sulfate, and calcium bicarbonate; the first two tend to be distinct, but the calcium-bicarbonate type clearly grades into the sodium-chloride type. The writer concludes that, in general, all these are derived from the sodium-chloride waters as a result of physical environment or of reactions with wall rocks.

GSA Bulletin↗
USGS1959

History of Imuruk Lake, Seward Peninsula, Alaska

A study of Imuruk Lake, a large, shallow lake in north-central Seward Peninsula, Alaska, illuminates the climatic history of northwestern Alaska and the tectonic history of central Seward Peninsula during Pleistocene and Recent time. Special interest attaches to the older lake sediments, because they contain evidence concerning the climate, fauna, and flora that existed in the vicinity of Bering Strait at a time when the Bering land bridge was open and when animal and plant populations were being exchanged between the eastern and western hemispheres. The lake is 8 miles long and less than 10 feet deep; bottom sediments consisting of reworked wind-blown silt bury a rolling bedrock topography of much greater relief. Analysis of the hydrologic regime indicates that much of the water draining into the lake is lost by evaporation; smaller quantities are lost by discharge through the outlet, the Kugruk River, and by leakage into the lava flows along the lake shore. Changes in the duration and temperature of the summer ice-free season would result in changes in the amount of water lost by evaporation and thus in appreciable changes in lake level. Imuruk Lake occupies an initial low area on basaltic lava flows of Quaternary age, but the initial low area has been modified by faulting and now lies in a poorly defined graben. Topographic evidence confirmed by study of lacustrine terraces indicates that until recently Imuruk Lake drained westward into the Noxapaga River instead of eastward into the Kugruk River. A history of repeated warping of the lake basin, on which is superimposed a history of oscillating lake level which is due to changes in climate, is recorded by three systems of abandoned shore-line features found along the shores: a warped shore cliff of probable Illinoian age, a double set of warped terraces of probable Wisconsin age, and a low, horizontal terrace of Recent age. Bones of bison, horse, and mammoth were found in peaty sediments containing many twigs but no large wood; their presence indicates that these mammals, at least, were capable of surviving in a tundra environment during cold stages of the Pleistocene epoch and at a time when the Bering land bridge was in existence nearby. The sediments filling the deeper parts of the bedrock basin of Imuruk Lake probably contain an uninterrupted pollen record that reflects vegetation changes in central Seward Peninsula beginning in middle Illinoian time and terminating a few thousand years ago. Core drilling and pollen analysis of these sediments would greatly amplify our understanding of late Pleistocene events in the vicinity of the Bering land bridge.

Alaska↗
USGS1961

Origin and development of the Three Forks Basin, Montana

The Three Forks Basin sprawls where the intricately deformed sedimentary and volcanic rocks of the Disturbed Belt along the Rocky Mountain front are faulted against the Precambrian metamorphic rocks that make the core of the Tobacco Root, Madison, Gallatin, and Beartooth ranges. Its eastern edge is linear, controlled by steep faults at the west front of the Bridger Range. All other boundaries are sinuous and show little sign of structural control. Tertiary deposits in the basin , rich in contemporaneous rhyolitic and latitic ash, are about equally of lake, bolson, and stream origin . The western part of the basin is dominated by moderately folded Eocene and lower Oligocene rocks, more than 2000 feet thick. They dip eastward beneath apparently unfolded upper Miocene and Pliocene rocks, more than 1300 feet thick, that also dip gently eastward to the basin edge. Thin but extensive Quaternary deposits lying unconformably on the Tertiary and pre-Tertiary rocks are mainly of rounded terrace and flood-plain gravel, angular fan gravel, and wind-blown silt. The basin began as part of an east-flowing stream system that developed in Late Cretaceous and Paleocene time, concurrently with Laramide folding and thrusting; the faulted contact between metamorphic and sedimentary rocks was especially erodible and became a main drainage way. Recurrent uplift to the west throughout the Tertiary provided gradient and load to the streams; additional load was provided by showers of ash from unknown vents. Relative uplifts of the Bridger Range in Eocene and early Oligocene time, and again in late Miocene and Pliocene time, impeded flow from the basin and led to deposits in channels, flood plains, and lakes. During most of Oligocene and Miocene time, however, the basin was being eroded. By the end of the Tertiary the basin was deeply filled and became part of a regional surface of low relief. Regional northwestward tilting stimulated headward erosion of the Missouri River which then captured the formerly east-draining or closed basin . The Tertiary deposits have been deeply eroded, and the rugged pre- basin surface partly exhumed.

Montana↗
USGS1968

A fossil assemblage from the wicomico formation in Berkeley County, South Carolina

Both megafossils and fossil Foraminifera have been obtained from unconsolidated sediments of the Wicomico Formation , which underlies the Penholoway terrace, a coastal feature that formed when the shoreline of the Atlantic Ocean stood about 70 feet higher than at present. Some of the fossils are Tertiary species and are assumed to have been derived from rocks of that age; the others are species commonly found in near-shore deposits of Quaternary age as well as in present-day south -Atlantic coastal waters. The Penholoway terrace in this area is shown to have been formed during a pause in marine regression from the Wicomico stand, when the sea stood near 100 feet. The relation of the Wicomico and Penholoway terraces to other coastal terraces suggests a Yarmouth age for the fossil -yielding beds.

South Carolina↗
USGS1969

Cretaceous, Tertiary, and early Pleistocene rocks from the continental margin in the Bering Sea

Rocks dredged from the continental margin in eastern Bering Sea in and near the Pribilof Canyon indicate that the acoustic basement represents the upper surface of thoroughly lithified turbidite beds of graywacke and siltstone of Late Cretaceous age. The stratified sequence covering the acoustic basement is gently deformed and includes marine clastic and diatomaceous sediments ranging in age from middle or late Miocene through early Pleistocene. Dense argillite, siltstone, and calcareous sandstone of early Tertiary age in and near the Zhemchug Canyon probably represent an older part of this sequence. The main layered sequence accumulated above the acoustic basement in shallow water, and, because the older beds now lie as much as 1000 m below sea level, the continental margin must have undergone considerable subsidence during late Tertiary and Quaternary time. A rich pollen flora indicates that the shoreline lay only a few tens of kilometers away from the site of the Pribilof Canyon during late Miocene time.

Alaska↗
USGS1969

Active metamorphism of upper Cenozoic sediments in the Salton Sea geothermal field and the Salton Trough, southeastern California

The Salton Sea geothermal system is entirely within Pliocene and Quaternary sediments of the Colorado River delta at the north end of the Gulf of California. At the time of deposition, these sediments consisted of sands, silts, and clays of uniform original mineralogic composition, but under the elevated temperatures and pressures of the geothermal system they are being transformed to low-grade metamorphic rocks of the greenschist facies. We have studied these transformations by X-ray, petrographic, and chemical analyses of cuttings and core from deep wells that penetrate the sedimentary section. Temperatures within the explored geothermal system range up to 360° C at 7100 feet. The wells produce a brine containing over 250,000 ppm dissolved solids, primarily Cl, Na, Ca, K, and Fe, plus a host of minor constituents.

California↗
USGS1970

Sea-floor spreading at the junction between Gorda Rise and Mendocino Ridge

A Quaternary deformation pattern revealed by new sub-bottom acoustic profiles supplements previous knowledge derived chiefly from magnetic and seismic evidence on the contemporary tectonics off northern California. An inferred age for the sedimentary cover along the axis of the southern part of Gorda Rise suggests that no volcanism has occurred along that part of the rise for more than 100,000 years. Sea-floor spreading at the rise crest has been accommodated by sinking of a keystone block that forms the floor of Escanaba Trough, the median valley of the rise. Differential movement between Pacific and American tectonic plates could have caused the deformation pattern, and during this movement, the displacement offshore beyond the northwest end of the San Andreas fault is inferred to have divided at Cape Mendocino between a fault along Mendocino Ridge and a fault segment that connects with the Blanco fault farther northwest.

California↗
USGS1971

Sedimentary and gravity-slide emplacement of serpentinite

Large deposits of serpentinite in alpine-type orogenic areas have been formed by sedimentary processes ranging from the detrital accumulation of bedded serpentinite sandstone and shale to the emplacement of chaotic breccias (olistostromes) and gigantic slide blocks. Known occurrences of sedimentary serpentinite are listed, and eight deposits from the circum-Pacific, Caribbean, and Mediterranean areas are described in detail. Sedimentary serpentinites range in age from early Paleozoic to Quaternary, although most are Cretaceous or Tertiary. Most were deposited in eugeosynclinal environments, early in the geosynclinal cycle. Individual deposits range in thickness from a few centimeters to nearly 3 km, and several extend laterally for tens of kilometers. Graded bedding is common, and many deposits contain marine fossils. Serpentinite is the dominant rock constituent, and clasts foreign to the alpine ultramafic assemblage are rare. Chemical analyses often detrital serpentinites show that these rocks contain slightly more silica and alumina than do nondetrital serpentinites, due to contamination by aluminosilicate minerals and quartz during deposition. This and nine other criteria are potentially useful in the recognition of sedimentary serpentinites. Several features suggest that most sedimentary serpentinites were deposited very rapidly by submarine landslides, mudflows, or turbidity currents. The sources of this serpentinite debris are postulated to be upward-migrating serpentinite protrusions which penetrate the seafloor or Earth's surface upslope from eventual depositional sites. Sedimentary serpentinites are much more abundant in alpine-type orogenic areas than is commonly thought, and many ultramafic masses presently regarded as igneous intrusions or tectonic protrusions may in fact be coeval with, instead of younger than, their enclosing sedimentary or metasedimentary rocks. In eugeosynclinal sequences such as the Franciscan Formation, some elongate bodies now regarded as serpentinite sills may be beds of ultramafic detritus whose sedimentary features have been masked by post-depositional shearing; isolated masses may be exotic slide blocks. A sedimentary origin can explain some of the most persistent and perplexing characteristics of many alpine serpentinites: their conformity with enclosing sedimentary rocks, their grossly planar shapes, and the absence of metamorphism along their contacts.

California↗
USGS1972

Hawaiian-emperor chain and its relation to cenozoic circumpacific tectonics

The Hawaiian Ridge and Emperor Seamounts appear to form a single chain of tholeiitic shield volcanoes that erupted sequentially on the sea floor of the central Pacific Ocean during Tertiary and Quaternary time. The chain cuts obliquely across the older Cretaceous structural patterns of that sea floor. While the pattern of the chain as a whole is linear, the individual volcanoes lie on short, sigmoidal, en echelon loci that are subparallel with respect to each other and that may represent extensional features in the crust and upper mantle. In general, the order of eurption progressed from northwest to southeaśt along the chain, but the rate of progression of volcanism along individual loci is nonlinear where best studied in the southeastern part of the chain. Furthermore, simultaneous eruptions appear to have occurred within a distance along the chain of about 200 to 400 km. The available data are consistent with a genesis related to the motion of the Pacific crust over a melting spot in the mantle. This melting spot, which may be due to either excess heat or pressure release, appears to have a diameter of about 300 km and is presently centered slightly north of the island of Hawaii. We concur with the idea that the bend in the Hawaiian-Emperor chain probably reflects a significant change in the motion of the Pacific plate. Our best estimate of the age of the Hawaiian-Emperor bend, based on the existing radiometric data, is 24.6 ± 2.5 m.y., which correlates with a time of increased tectonic activity in the western Pacific island arcs and along the northern and eastern boundaries of the Pacific plate. The vector change in the motion of the Pacific plate (with respect to the melting spot) that is required to produce the bend is about 12 cm/yr in a west-southwest-ward direction. © 1972, The Geological Society of America, Inc.

Hawaii↗
USGS1973

Reconnaissance study of the strontium isotopic composition of Cenozoic volcanic rocks in the northwestern Great Basin

Sixteen mafic and intermediate lava flows of Eocene to Pleistocene age from the northwestern Great Basin have initial Sr 87 /Sr 86 ratios of from 0.7029 to 0.7047. Seven upper Miocene mafic and intermediate lava flows have initial ratios of from 0.7037 to 0.7041, suggesting a common source for the Steens Basalt and contemporaneous rocks of the northwestern Great Basin. Values of 0.7047 and 0.7033 obtained on olivine basalts of early Miocene and Quaternary ages, respectively, suggest that these lavas were derived from different source materials than were the late Miocene rocks. Unusually low values of 0.7029 obtained on two specimens of andesite from the Eocene Cedarville Series of Russell (1928) suggest that these lava flows were derived from still another source material. Ten silicic volcanic rocks, most from widespread and voluminous ash-flow sheets and lava complexes, have initial ratios of approximately 0.7023 to 0.7057. Some of the more radiogenic values, obtained on very highly differentiated and strontium-poor rock units, provide only an upper limit for the initial Sr 87 /Sr 86 ratios of the parent magmas. The close similiarity of the strontium isotopic composition of the silicic rocks to those of spatially and temporally associated intermediate lavas supports the concept that the widespread Miocene silicic volcanic rocks of the northwestern Great Basin were produced by the fractional crystallization of enormous volumes of mantle-derived mafic magma. The data also provide indirect support for the interpretation that the relatively radiogenic character of many salic volcanic rocks from other parts of the Great Basin largely reflects the presence of an unusually radiogenic mantle rather than the involvement of crustal material.

California, Idaho, Nevada, Oregon↗
USGS1973

Structure and development of the continental margin of British Honduras

The continental margin of British Honduras is characterized by elongate ridges that are sub-parallel with the coast and that diverge slightly northward. Each ridge apparently is formed by aligned fault blocks composed of continental basement rock that rotated and subsided during rifting of the margin. The western part of the Cayman Ridge may be underlain by similar rock that extends southwestward into the margin. Little sediment has been trapped in the basins between the ridges, presumably as a result of long-term, low rates of sedimentation. A canyon system that is controlled by both fault-block ridges and cross faults transects the margin and opens into the Yucatan Basin. Deep-sea sediments are characterized by a succession of pelagic deposits above acoustic basement rocks; these deposits are covered by turbidites that have formed deep-sea fan deposits in some places. The change from pelagic to turbidite deposition may have occurred at the end of Cretaceous time when regional uplift provided a source for clastic materials. The principal channel for transport of sediment to the deep sea is believed to be a graben that extends northeastward from the southwest corner of the Gulf of Honduras. Development of the margin may have begun in Late Jurassic time when sphenochasmic rifting created the ancestral Gulf of Honduras. Near the end of Cretaceous time, a new set of motions began, with the Caribbean plate moving relatively eastward and slightly away from the North American plate. This new motion resulted in continued movement of basement blocks through Tertiary and Quaternary time. Although this margin is not young, it has a geomorphically youthful aspect in that the sedimentary sequences do not obscure the rift structures.

British Honduras, Caribbean Sea, Gulf of Honduras↗
USGS1980

Leg 67: the Deep Sea Drilling Project Mid-America Trench transect off Guatemala.

Drilling on the Cocos plate recovered a basal chalk sequence deposited during early and mid-Miocene time, a short interval of abyssal red clay, and an upper sequence of late Miocene and younger sediment deposited within an area influenced by a terrigenous source. In the trench, a mud and sand fill less than 400,000 yr old overlies the oceanic sequence. The entire section shows no evidence of compressive deformation. In contrast, the section cored on the trench's landward slope 3 km from the trench axis is affected by tectonism. The section contains a Cretaceous to Pliocene claystone sequence capped by Pliocene to Quaternary hemipelagic slope deposits.- from Authors

Geological Society of America Bulletin↗
USGS1981

Sedimentary framework of the Potomac River estuary, Maryland

Analyses of seismic-reflection profiles, sediment cores, grab samples, and side-scan sonar records, along with previously collected borehole data, reveal the characteristics, distribution, and geologic history of the shallow strata beneath the Potomac River estuary. The lowermost strata are sediments of the Chesapeake Group (lower Miocene to lower Pleistocene) that crop out on land near the shore but are buried as much as 40 m below the floor of the estuary. The top of these sediments is an erosional unconformity that outlines the Wisconsinan valley of the Potomac River. This valley has a sinuous trend, a flat bottom, a relief of 15 to 34 m, and axial depths of 34 to 54 m below present sea level. During the Holocene transgression of sea level, the ancestral valley was filled with as much as 40 m of sandy and silty, fluvial-to-shallow estuarine sediments. The fill became the substrate for oyster bars in the upper reach and now forms most marginal slopes of the estuary. Since sea level approached its present position (2,000 to 3,000 yr ago), the main channel has become the locus of deposition for watery, gray to black clay or silty clay, and waves and currents have eroded the heterogeneous Quaternary sediments along the margins, leaving winnowed brown sand on shallow shoreline flats. Pb-210 analyses indicate that modern mud is accumulating at rates ranging from 0.16 to 1.80 cm/yr, being lowest near the mouth and increasing toward the head of the estuary. This trend reflects an increased accumulation of fine-grained fluvial sediments near the turbidity maximum, similar to that found in nearby Chesapeake Bay. The present annual accumulation of mud is about 1.54 million metric tons; the cumulative mass is 406 million metric tons.

Maryl↗