Cenozoic Sedimentary Rocks Research Articles

Correlation of aromatic hydrocarbons of oils and organic matter from Mesozoic-Cenozoic sedimentary rocks of Ciscaucasia

The same polycyclic aromatic hydrocarbons (PAH) are encountered in oils and chloroform-soluble bitumens of Mesozoic--Cenozoic sedimentary rocks. Our analysis of PAH indicates a genetic relationship between oils and organic matter (OM) and we suggest that the major bulk of oils is generated by the sedimentary rocks where they accumulate.

Preliminary analysis of the spectral content of P and S waves from local earthquakes in the Garm, Tadjikistan region

abstract This paper reports preliminary results of an analysis of the spectral content of seismic waves from over 1,000 local earthquakes in the Garm, Tadjikistan region. Very low values of Q (∼100) were obtained for the Mesozoic and Cenozoic sedimentary rocks of the Peter I Range compared with those in the crystalline rock of the South Tien Shan (Q ≳ 500). For events with the same low frequency spectra, earthquakes in the South Tien Shan are recorded with more energy at higher frequencies than earthquakes in the Peter I Range, at least in part, because of the greater attenuation of the sedimentary rock in the latter region. There is a wide variety of spectra radiated by earthquakes throughout the Garm region, but a marked difference between spectra radiated by earthquakes from these two regions is not apparent. Nevertheless although calculated stress drops of earthquakes appear to vary considerably throughout the region, there appears to be a dependence on the type of rock in which the earthquakes occurred. The variation in calculated stress drops, however, is greater within either region than between the two. For most regions and for most of the range of seismic moments studied the shape of the spectrum is relatively independent of seismic moment. Consequently, calculated stress drops increase with seismic moment, with the functional dependence varying from region to region. For the largest events, the spectrum shifts to lower frequencies with increasing moment and stress drops become essentially independent of moment. Preliminary results suggest that the stress drops are not detectably different for earthquakes with thrust or normal faulting fault-plane solutions. A limited amount of data are consistent with a change in spectral content and stress drop of earthquakes in the vicinity of and before stronger earthquakes but a clear, consistent pattern is not yet evident. Toward high frequencies, S-wave spectra both begin to decrease at lower frequencies and seem to decrease more rapidly than P-wave spectra, in contrast with predictions of published dislocation models of earthquakes.

An array of strong-motion accelerographs in Bear Valley, California

Fifteen strong-motion accelerographs, each with the capability of writing the WWVB absolute time code on the recorded accelerogram, have been deployed in an elliptical array, at a station spacing of several kilometers, along the San Andreas Fault in the Bear Valley region of central California. Ten accelerograms were obtained for the June 22, 1973, earthquake (M = 3.9), located near the center of the array. Preliminary analyses of these accelerograms support previous suggestions that the crystalline rocks of the Gabilan Range possess higher material velocities and lower intrinsic absorption than do the Cretaceous and Cenozoic sedimentary rocks northeast of the fault zone. These accelerograms clearly indicate that a strong-motion accelerograph array of this sort can provide the basic data for source mechanism, wave propagation, and local ground-motion studies for earthquakes with magnitudes as small as 3.5-4.0.

Crustal Structure of the Texas Gulf Coastal Plain

To obtain new information about deep crustal structure in the Texas Gulf Coast region, tripartite arrays of long-period vertical seismographs were deployed to observe Rayleigh wave dispersion. Additional gravity measurements were also made and combined with those of a previous study to produce four detailed gravity profiles crossing the study area. Interpretation of the Rayleigh wave dispersion data and the gravity data together suggests a crustal structure consisting of a Paleozoic subduction zone and orogen buried beneath thick Mesozoic and Cenozoic sedimentary rocks.

The Economics of Developing Canadian Arctic Gas

In the remote Canadian Arctic, exploration for oil and gas is costing $300 to $400 million a year. Thus far, the most significant discoveries have been of natural gas. Examined here are the costs and potential results of exploring in the area and the wellhead prices required to justify the efforts. The wellhead prices are translated into prices in the markets that would logically receive such gas. Introduction The existence of petroleum in Canada's North has been known since 1798, when Alexander Mackenzie recorded the presence of oil seeps along the banks of the great river that bears his name, near the present site of Norman Wells. About the turn of the century, other explorers recorded the discovery of oil and gas seepages along the north shore of Great Slave Lake, and near Fort Good Hope, down-river from Fort Norman. In 1920, a subsidiary of Imperial Oil Ltd. drilled the discovery well of the Norman Wells oil field and Canada's North became, in fact, an oil-producing region. Except for a flurry of development and production in the early 1940's, brought about by the unique demands of World War II, exploration activity in the North remained at a low ebb until recent years. The harsh climate, remoteness, lack of communications, high costs, and attractions of risk capital to more amenable regions resulted in a long period during which only limited exploration activity was conducted. Forecasts of supply and demand for Canadian oil and gas demonstrate that substantial new reserves must be developed during the next 10 to 20 years to satisfy the needs of our expanding domestic markets and the requirements for exports to the U. S. As a result, exploration is expanding into the frontier regions and a large exploratory play is now in progress in Canada's Arctic region. Major gas discoveries have been made in two widely separated Arctic areas (Fig. 1). The more southerly region is in the Mackenzie Delta on the Arctic mainland, some 325 miles east of the Alaska North Slope discoveries at Prudhoe Bay. The other Arctic discoveries have been made in the Sverdrup basin in the central part of the Canadian Arctic Islands. Since the principal discoveries to date have been of natural gas, the following analysis examines the cost and potential results of this exploration in terms of wellhead prices required to justify the efforts being made. Mackenzie Delta - Geology and Potential The Mackenzie Delta portion of the Beaufort basin (Fig. 2) is a geographical and structural entity delineated by the Richardson Mountains and the Aklavik Range on the west and the Tuktoyaktuk Peninsula - Caribou Hills escarpment to the east, Peninsula - Caribou Hills escarpment to the east, both lineaments converging to an apex in the Point Separation area. The Mackenzie Delta is comparable in many respects with the U. S. Gulf Coast and the West African Niger Basins, and consists of a thick succession of Mesozoic and Cenozoic sedimentary rocks developed upon the northern continental margin of North America. The sequence overlies older Paleozoic sedimentary rocks that within the basin are Paleozoic sedimentary rocks that within the basin are deeply buried but at the southern margin are exposed within a complex structural element, the Aklavik Arch (Fig. 3). JPT P. 1199

Geology of Georges Bank Basin

The Georges Bank basin is a basement structural depression beneath the continental shelf southeast of New England. The deepest part of the basin is an arcuate trough approximately 120 mi long, containing 26,000 ft or more of Mesozoic and Cenozoic sedimentary rock. This estimate is based on interpretation of geophysical data utilizing new velocity information from wells drilled on the western Scotian shelf. Overall stratigraphic similarity to rocks in the western Scotian shelf is likely. Sedimentary rocks form a southward-thickening wedge beneath the continental shelf. Geophysical data indicate the existence of more than 14,000 ft of Lower Cretaceous and Jurassic carbonate rock, marine shale, evaporite, and consolidated sandstone, and as much as 12,000 ft of Upper Cretaceous and Tertiary sandstone and shale in the Georges Bank basin. Jurassic units in the basin are probably thicker than their western Scotian shelf equivalents. Salt diapirs similar to those in the Scotian shelf have not been detected. Structural deformation consists of high angle normal faulting in basement rocks. Estimates of recoverable oil and gas from sedimentary rocks beneath the continental shelf and continental slope have been made by petroleum-industry groups and federal agencies. The 30,000 cu mi of Lower Cretaceous and Jurassic sedimentary rocks in the Georges Bank basin may contain a significant share of these hydrocarbon reserves, thus assuring the basin a favorable place among eastern North America's frontier exploration areas.

Geology of Labrador Shelf, Canada: ABSTRACT

The area includes the shelf and continental slope of eastern Canada between north latitudes 50° and 67°. Water-depths average 450-800 ft. Some parts of the slope covered with as much as 6,000 ft of water are included. The area encompasses 200,000 sq mi (130 million acres). Mainland Labrador and southern Baffin Island are Precambrian in age. The land surface is an uplifted Cenozoic peneplane that is tilted away from the sea. The coastal area is fiord indented and rugged. In contrast, the shelf is underlain by soft Mesozoic and Cenozoic sedimentary rocks. The shelf is smooth and quite unlike the mainland. The boundary line between the young sediments and the Precambrian is mainly coincident with a submarine marginal channel near the shoreline, and parallel with it along the coast. Until the Triassic, Greenland and Canada were joined and some of the early Paleozoic seas covered the entire area. Perhaps part of the area was covered by late Paleozoic seas. The first manifestations of rifting probably were in the Triassic. Block faulting in the Early Jurassic made basins which started to receive marine sediments. It is not clear when new ocean floor formed in the Labrador Sea but certainly it was widespread in the middle Cretaceous and may have been present as early as Middle Jurassic. Sediments which crop out on the eastern part of the marginal channel and on the continental slope range in age from Jurassic to late Cenozoic. Climate during this time span was mostly subtropical to tropical. Jurassic and Cretaceous clastic sediments came from adjacent areas. Cenozoic sediments, which now cover the floor of the Labrador Sea, ware transported from central Canada by a river system which entered at Hudson Strait. The Mesozoic rifting controlled the early sedimentation patterns. First drilling targets will be associated with the horsts. Theoretically, the warmer water deposits of clastic rocks over and around fault blocks is ideal for the generation and accumulation of hydrocarbons. End_of_Article - Last_Page 2148------------

Tectonic Framework of Cordilleran Foldbelt in Southwestern New Mexico

The Cordilleran foldbelt trends west-northwest through the southwestern corner of New Mexico and is characterized mainly by flat thrust faults and subordinate, closely compressed, overturned folds of Laramide age. Postorogenic Cenozoic volcanic and sedimentary rocks cover much of the region, and only scattered areas of preorogenic and synorogenic rocks are exposed; this cover, in addition to superimposed Basin-Range deformation, makes regional structural synthesis difficult. Apparently, however, the contact of the foldbelt with the platform on the north is marked by thrust faults exposed in the Florida, Victorio, and central Peloncillo Mountains. Yielding on the thrust is northward toward the foreland, with displacement as much as several miles. Anomalous minor thrusts in the Big Hatchet Mountains that appear to have yielded toward the southwest are of unknown origin. The regional distribution of thrusts suggests that the entire foldbelt in this region is underlain by thrusts, even though considerable parts of the preorogenic sedimentary section may have escaped deformation in some localities. In southwestern New Mexico, the zone separating the area of the foldbelt from the less deformed foreland on the north appears to mark the Texas lineament.

Sub-La Jolla Unconformity in Vicinity of San Diego, California

Rock units of the San Diego area include two basic divisions separated by a high-relief unconformity: (1) an intensely deformed late Mesozoic basement complex and (2) an essentially flat-lying succession of latest Cretaceous and Cenozoic sedimentary rocks. The lower part of the latter division in the northern part of the San Diego area consists of several Upper Cretaceous rock units and the Eocene La Jolla Formation. Upper Cretaceous (Campanian-Maestrichtian) marine sandstone and shale of the Rosario formation crop out along the coastal area at Point Loma, La Jolla, and near Carlsbad. The Rosario is overlain unconformably by the La Jolla Formation. Another Upper Cretaceous unit crops out in the vicinity of Rancho Santa Fe east of the Rosario outcrop belt; in marked contrast to the Rosario, it is nonmarine and is dominantly a massive reddish boulder conglomerate. This unnamed unit unconformably overlies the basement complex and is separated from the La Jolla Formation by a high-relief (up to 450 ft) disconformity. The unnamed boulder conglomerate at Rancho Santa Fe apparently correlates with a sub-Rosario conglomeratic redbed unit reported from several wells in the San Diego area, and a conglomerate-sandstone unit beneath the Rosario near Carlsbad. If this correlation is correct, and if the contact between the Rosario and the unnamed unit is essentially planar, these relations indicate a post-Rosario uplift and slight westward tilting, an episode of erosion exposing both the Rosario and the sub-Rosario conglomerate, and subsequent La Jolla deposition on either of the Cretaceous formations, or on the basement complex, but without total destruction of the pre-La Jolla topography.

Geology of Central Grant Range, Nevada

Approximately 26,000 ft (7,930 m) of Paleozoic carbonate and subordinate clastic sedimentary rocks is exposed in the central Grant Range. Cambrian quartzite, shale, and minor limestone are the oldest exposed rocks in the area. The overlying 11,200 ft (3,416 m) consists of limestone, dolomite, shale, and minor quartzite and sandstone; this sequence begins with the Ordovician Pogonip Group and ends with the Mississippian Joana Limestone. The overlying strata are Upper Mississippian and Pennsylvanian shale, and minor sandstone and limestone. The Paleozoic sequence is overlain unconformably by Cenozoic sedimentary and volcanic rocks. Three major low-angle faults that separate younger rocks from older are exposed in the area. They are, in ascending order, the Grant Canyon, Heath Canyon, and Beaty Canyon faults. The Grant Canyon fault emplaces gently dipping Middle Cambrian limestone and shale over steeply dipping Lower Cambrian Prospect Mountain Quartzite. Rocks considered to be Lower Ordovician are separated from overlying Cambrian units by the Heath Canyon fault. The Beaty Canyon fault generally carries concordant Silurian through Pennsylvanian strata over Lower Ordovician rocks. Relative eastward movement of the upper plate rocks is demonstrated by folds in the lower plates, which are overturned to the east. In addition, five subordinate low-angle faults are recognized. Synorogenic regional metamorphism, predating the faulting, converted part of the Cambrian and Lower Ordovician strata into tectonically banded marbles, and chlorite-muscovite and biotite phyllites. Postmetamorphic low-angle faulting resulted in superposed local mylonitization. Medium- to high-grade calc-silicate rocks are associated with a granitic intrusion. Folding within the range is locally intense, and high-angle faulting, with the exception of the range front fault or faults, is mostly of small displacement. A Mesozoic age of metamorphism and thrusting is inferred from regional evidence.

International Correlation of the Lower Pleistocene of Japan

Combined paleomagnetic and biostratigraphic works have been carried out on the Cenozoic sedimentary rocks of Japan, chiefly in the Boso Peninsula (Nakagawa et al., 1969) and Choshi Peninsula (Sakai et al., 1970), Chiba Prefecture, Kinki District (Ishida et al., 1969), Shiga, Kyoto, Osaka, and Hyogo Prefectures, Uonuma area (Nitobe and Yamanoi, 1970), Niigata Prefecture, and Aizu Basin (Manabe et al., 1969), Fukushima Prefecture. Many reports have been published on the geomagnetic and biostratigraphic frameworks made on the deep-sea sediment cores (for example, Hays et al., 1969). On the other hand, a preliminary paleomagnetic and biostratigraphic investigation was made on the rocks from the proposed strasotype of the lower limit of the Pleistocene in Southern Italy by Nakagawa et al. (1970). The results of these investigations made it possible to correlate the Lower Pleistocene of Japan with those in Southern Italy and with the deep-sea sediment cores of the equatorial Pacific. A tentative correlation of the Lower Pleistocene is shown in Fig. 1.

New ideas on geologic history of Eastern Zeagli-Darvaza uplift in connection with Cenozoic hydrovolcanism

Cenozoic hydrovolcanic activity was inferred in a small area in the Kara-KumDesert by lithologic, mineralogic and morphologic studies. Several petuliar Cenozoic sedimentary rocks were interpreted as being deposited in vertical channels by hydrovolcanic waters. Associated with these channels are sulfur-capped hillocks with up to 60 m of relief. These hillocks are considered hydrovolcanic depositional features. – A. J. Woloshin.

Mantled Gneiss Domes in the Albion Range, Southern Idaho

The Albion Range, in southern Idaho east of the Antler orogenic belt and west of the Sevier orogenic belt, exposes a northeast-trending chain of four mantled gneiss domes. The Green Creek Complex (Precambrian [2.5 b.y.] gneiss and metasediments), which forms the cores of the domes, is unconformably overlain by the Paleozoic Dove Creek Group, consisting of sparsely fossiliferous metasediments approximately 22,000 feet thick. The basal quartzite and overlying schists, marbles, and thin quartzites are of probable Cambrian age. A massive quartzite and overlying schist and limestone may be Cambrian or Ordovician in age. Ordovician quartzite more than 7000 feet thick forms the middle part of the metasedimentary section, and this is overlain by massive pure dolomite, calcareous black schist, and quartzite of Ordovician through Mississippian(?) age. Devonian strata are thin or absent. At the top of the metasedimentary section are limestone, schist, and quartzite of the Pennsylvanian Oquirrh(?) Formation. Younger Paleozoic sandstone, limestone, and chert, and Triassic(?) shale occur in fault contact overlying the metasediments. Cenozoic sedimentary and volcanic rocks unconformably overlie the deformed older rocks. Metamorphic grade (greenschist and amphibolite facies) increases northwestward and downward in the area. Staurolite and kyanite occur in Cambrian rocks in the northern half of the range; cordierite, andalusite, and sillimanite occur in Precambrian rocks in the southern half of the range, but these may be due to Tertiary contact metamorphism. Deformation during Mesozoic metamorphism produced two fabric systems; locally a Precambrian metamorphic fabric survived the younger metamorphism. Early bedding foliation and northeast-trending, northwest-vergent folds and lineations predated rise of the domes. Northwest-trending folds and lineations with northeast and southwest vergence were produced during and after doming; over much of the western half of the range these structures destroyed all older fabrics. During the Oligocene (30 m.y.) a postkinematic adamellite stock—the Almo Pluton—and related granitic dikes were emplaced. Following emplacement of the pluton, uplift of the range accelerated to an average rate of .5 mm per year, bringing the gneiss domes to the surface from a depth of at least 10 km. The Albion Range is part of a belt of rocks along the western part of the Cordilleran miogeosyncline which was affected by high-grade regional metamorphism during the middle of the Mesozoic. The metamorphic rocks are part of the Cordilleran infrastructure, which can be traced from California into southern British Columbia and beyond.

On the Amount of Bottom Sediments in Reservoirs

This paper presents the results of the study on the amount of bottom sediments in reservoirs.The study concerns the geology, topography, annual precipitation in the watershed, discharge of rivers flowing into the reservoir, and amount of bottom sediments in the reservoir. The characteristic features of erosion and transportation in the watershed could be found from this study.1. As regards the 52 reservoirs studied, the amount of the sediments (the mean annual amount per unit area of the watershed) has little or no relation directly with any one factor of geology, relief and annual precipitation during heavy rains in the watershed. Each of the reservoirs based upon the geological characteristics (rock distribution) of their watersheds, can be classified into the following three groups, namely, A: Reservoirs which have the watershed consisting mainly of Paleozoic and Mesozoic rocks.B: Reservoirs which have the watershed consisting mainly of acidic plutonic, hypabyssal and their metamorphic rocks (Granitic rocks).C: Reservoirs which have the watershed consisting mainlyf Cenozoic sedimentary rocks, effusive rocks or crystalline schists, or sedimentary, plutonic and effusive rocks of nearly equal distribution.2. For the reservoirs of each group included in the three groups by the characteristics of rock distribution in the watershed, the larger amount of bottom sediments is found for the reservoir with the watershed which has larger numerical value of product of those representing the relief and the annual precipitation during heavy rains.3. Moreover, from comparison of the amount of bottom sediments in the reservoirs of these three groups with each other, it is clear that the finer the watershed products, the more the sediments are deposited. The same relation is also found for the reservoirs.of the three subgroups (B1, B2 and B3) of B group which were classified based upon the climatic condition of the watershed.4. Based on the aforementioned relations between the amount of bottom sediments in the reservoirs and the several factors of the watersheds, it is concluded that for each reservoirs of A, B and C groups, the larger the numerical value of the product of those representing the relief and the annual precipitation during heavy rains, the more the clastics are eroded in the watershed and transported into the reservoirs. Furthermore, for the reservoirs with the watershed of which combined factors of relief and annual precipitation during heavy rains is nearly equal, the finer the sizes of the clastics, the more clastics are eroded in the watershed and transported into the reservoir. 5. For each of the several main reservoirs selected from the aforementioned 52, the annual variation of the amount of bottom sediments is little relatad with that of the annual discharge of inflow, whereas it is controlled by that of the yearly maximum flood discharge. Therefore, it is considered that for all the reservoirs which have their own rock distribution and relief, the amount of the clastic sediments transported by the rivers has a close relation with the flood discharge.

Tectonic Development of Idaho-Wyoming Thrust Belt

Three stages are evident in the tectonic development of southeastern Idaho and western Wyoming. First are the changing patterns of tectonic elements during deposition; second, development of northward-trending folds and thrust faults; and third, development of block faults that produced horst ranges and graben valleys. During Paleozoic time about 50,000 feet of marine sediments, mostly limestone and dolomite, were deposited in a miogeosyncline and about 6,000 feet of mixed marine sediments were deposited on the shelf to the east. Detritus came from both east and west from Cambrian time on. Starting in Mississippian time, the belt between shelf and miogeosyncline, where thicknesses increase markedly, shifted progressively eastward. During Mesozoic time about 35,000 feet of marine and continental sediments were deposited in the western part of the region and about 15,000 feet in the eastern part, with terrestrial deposits becoming increasingly dominant. Western positive areas became the chief source of detritus. The belt of maximum thickening and the site of maximum deposition were relocated progressively eastward; maximum thicknesses of succeeding geologic systems are not superposed. In Late Triassic a belt on the west rose and the miogeosyncline started to break up. As Mesozoic time progressed the western high spread eastward, until by the end of the Jurassic the miogeosyncline gave way to intracratonic geosynclinal basins that received thick deposits, particularly in Cretaceous time. Cenozoic sedimentary rocks are products of orogeny in the region. The second stage, which overlapped the first, produced folds overturned toward the east and thrust faults dipping gently west in a zone, convex to the east, 200 miles long and 60 miles wide. Stratigraphic throw on many larger faults is about 20,000 feet; horizontal displacement is at least 10 to 15 miles. Lack of metamorphism and mylonite along the faults is striking. From west to east, the thrust faults cut progressively younger beds, have progressively younger rocks in their upper plates, and are estimated to be successively younger. Thrusting started in the west in latest Jurassic and ended in the east perhaps as late as early Eocene time; detritus shed from emergent upper plates is preserved in coarse terrestrial strata of corresponding ages. West of the thrust belt is a northwestward-trending area underlain mostly by lower Paleozoic rocks and flanked on east and west by upper Paleozoic and Mesozoic rocks. Scattered pieces of eastward-dipping thrust faults have been reported west of the older rocks. This central area of old rocks has been interpreted as: (1) part of a large continuous thrust sheet moved scores of miles from the west; or (2) an uplifted segment of the earth's crust from which thrust sheets on the east and west were derived. Both interpretations have defects; relative thrust ages are difficult to explain under the first; a large positive gravity anomaly, expectable under the second, is apparently absent. Block faulting, the third stage of tectonic development, started in Eocene time. Faulting has continued to the Recent, as indicated by broken alluvial fans, displaced basalt flows less than 27,000 years old, and earthquakes. North-trending and east-trending fault sets are recognized. Old east-trending steep faults in the Bear River Range may be tear faults genetically related to thrusting. Movement along many faults has been recurrent. Patches of coarse Tertiary gravel on the flanks and crests of ranges, for which there is no provenance with present topography, may record reversed vertical movement along some north-trending faults. Present topographic relief of basins and ranges is tectonic.

Tectonic Development of Idaho-Wyoming Thrust Belt: ABSTRACT

Three stages are evident in the tectonic development of southeastern Idaho and western Wyoming. First, the changing patterns of tectonic elements during deposition; second, development of northward-trending folds and thrust faults; and third, development of block faults that produced horst ranges and graben valleys. During Paleozoic time about 50,000 feet of marine sediments, mostly limestone and dolomite, were deposited in a miogeosyncline and about 6,000 feet of mixed marine sediments were deposited on the shelf to the east. Detritus came from both east and west recurrently from Cambrian time on. Starting in Mississippian time, the belt between shelf and miogeosyncline, where thicknesses increase markedly, shifted progressively eastward. During Mesozoic time about 35,000 feet of marine and continental sediments were deposited in the western part of the region and about 15,000 in the eastern, terrestrial deposits becoming increasingly dominant. Western positive areas became the chief source of detritus. The belt of maximum thickening and the site of maximum deposition shifted progressively eastward; maximum thicknesses of succeeding geologic systems are not superposed. In Late Triassic a belt to the west rose and the miogeosyncline started to break up. As Mesozoic time progressed the western high spread eastward, until by the end of the Jurassic the miogeosyncline gave way to intracratonic geosynclinal basins that received thick deposits, particularly in Cretaceous time. Cenozoic sedimentary rocks are products of oroge y in the region. The second stage which overlapped the first, produced folds overturned to the east and thrust faults dipping gently west in a zone, convex to the east, 200 miles long and 60 miles wide. Stratigraphic throw on many larger faults is about 20,000 feet; horizontal displacement is at least 10 to 15 miles. Lack of metamorphism and mylonite along the faults is striking. From west to east, the thrust faults cut progressively younger beds, have progressively younger rocks in their upper plates, and are estimated to be successively younger. Thrusting started in the west in latest Jurassic and ended in the east perhaps as late as Early Eocene time; detritus shed from emergent upper plates is preserved in coarse terrestrial strata of corresponding ages. West of the thrust belt is a northwestward-trending area underlain mostly by lower Paleozoic rocks and flanked on east and west by upper Paleozoic and Mesozoic rocks. Scattered pieces of eastward-dipping thrust faults have been reported west of the older rocks. This central area of old rocks has been interpreted as (1) part of a large continuous thrust sheet moved scores of miles from the west, or (2) an uplifted segment of the earth's crust from which thrust sheets to the east and west were derived. Both interpretations have defects: relative thrust ages are difficult to explain under the first; a large positive gravity anomaly, expectable under the second is apparently absent. Block faulting, the third stage of tectonic development, started in Eocene time. Faulting has continued to the Recent, as indicated by broken alluvial fans, displaced basalt flows less than 27,000 years old, and earthquakes. North-trending and east-trending fault sets are recognized. Old east-trending steep faults in the Bear River Range may be tear faults genetically related to thrusting. Movement along many faults has been recurrent. Patches of coarse Tertiary gravel on the flanks and crests of ranges, for which there is no provenance with present topography, may record reversed vertical movement along some north-trending faults. Present topographic relief of basins and ranges is tectonic. End_of_Article - Last_Page 1878------------

Seismic-refraction measurements of crustal structure between Santa Monica Bay and Lake Mead

A reversed seismic-refraction profile was recorded between Santa Monica Bay, California, and Lake Mead, Nevada, during November 1961. Depth to the M discontinuity was determined to be approximately 29 km at Santa Monica Bay, 36 km under the Trans-verse Ranges, 26 km under the Mojave Desert, and 30 km at Lake Mead. Prominent events on the seismograms in the distance range 30 to 150 km are interpreted as reflections from the M discontinuity and from a crustal layer of intermediate velocity. These reflected events are used to make a detailed interpretation of crustal structure. The velocity of compressional waves in the mantle immediately below the M discontinuity was determined to be 7.8 km/sec. The velocity of compressional waves in the intermediate layer is near 7.0 km/sec. The apparent velocity of the direct arrival in the crustal rocks near the surface is 6.3 km/sec northeast of Santa Monica Bay and 6.1 km/sec southwest of Lake Mead. The higher apparent velocity for the direct arrival from Santa Monica Bay seems to be the result of thinning toward the east of low-velocity rocks near the surface. These low-velocity near-surface rocks are Cenozoic sedimentary rocks and fractured and weathered granitic and metamorphic rocks. The velocity of Sg was determined to be 3.4 km/sec near Lake Mead. A prominent phase of apparent velocity 6.3 to 6.4 km/sec was recorded at distances beyond 200 km. This phase is identified as P and is interpreted as a reflection from the intermediate layer. Amplitude measurements support the conclusion that the P phase is a reflected arrival.

DRILLING SUPER-DEEP HOLES IN THE CENTRAL PART OF THE NEAR-CASPIAN DEPRESSION

The Near-Caspian Depression is a deeply subsided southeastern block of the Russian platform consisting of Paleozoic, Mesozoic, and Cenozoic sedimentary rocks as thick as 15, 000 m. Most geologists hold, under the salt complex, there are pre-Kungurian Paleozoic sediments of platform type composing upwarpings and downwarpings (anteclises and syneclises). The'outlook for, oil and gas in the Near-Caspian is very good; it is especially true of the complex overlying the salt and the Emba district where oil was found; the Paleozoic deposits below the salt are also promising. Prospecting for oil and gas here is possible only with the use of super-deep drilling, and of special importance is drilling of two test super-deep boreholes (to 7, 000 m). First of all it is necessary to select such areas within which deposits below the salt could be struck at a depth not more than 5 or 6 km and where a Paleozoic section from 1, 000 to 2, 000 meters thick can be found. It is necessary as well to carry out seismic surveys wi...

Origin of the Gulf of California

The probable cumulative Late Cretaceous and Cenozoic right-lateral strike-slip displacement along the San Andreas fault in central California is 350 miles. The San Andreas and the allied faults into which it branches southward trend longitudinally into the Gulf of California, and the seismicity of the region indicates that the fault system follows the length of the Gulf and enters the Pacific basin south of Baja California. Crustal structure of most of the Gulf is of oceanic type, so that an origin by structural depression of continental rocks is not possible. Tectonic styles north and south of Los Angeles differ greatly. To the north, the Coast Ranges expose thick Upper Cretaceous and Cenozoic sedimentary rocks that were deposited in local basins and deformed tightly and repeatedly. To the south, in the Peninsular Ranges and Baja California, correlative rocks are thin and show little compressive deformation. The California batholith of mid-Cretaceous age and allied crystalline rocks form the basement of Baja California, southwestern Arizona, and northwestern Sonora and probably extend along the coast of mainland Mexico; the Gulf apparently bisects the crystalline belt longitudinally. These features suggest that Baja California initially lay 300 miles to the southeast, against the continental-margin bulge of Jalisco. The Gulf of California may be a pull-apart feature caused by strike-slip displacement plus up to 100 miles of cross-strike separation of the continental plate, subcontinental materials having welled up into the rift gap. The strike-slip motion has a tensional component across the continental margin south of Los Angeles but a compressional component to the north.

STRUCTURAL SIGNIFICANCE OF GRAVITY SURVEYS IN THE VIRGINIA CITY—MOUNT ROSE AREA, NEVADA AND CALIFORNIA

Gravity surveys were made in the Mount Rose and Virginia City quadrangles to investigate the concealed Cenozoic structure of the alluviated basins and to explore for significant gravity differences between basins and ranges. The gravity data have been computed as complete Bouguer anomalies, which incorporate corrections for all the accurately assessible factors affecting gravity values, including topographic irregularities but not isostatic compensation. Gravity variations in the basins are closely related to changes in thickness of Cenozoic sedimentary rocks and alluvium in the basins. The gravity data indicate that light Cenozoic sedimentary rocks in Washoe Valley are at least 1800 feet thick; that the sediments in the northern part of Steamboat Valley are only about 600 feet thick and thin gradually eastward but thin abruptly to the west near Steamboat Hills; that east of Reno, between Sparks and the Virginia Range, sediments are roughly 2800 feet thick and thin abruptly about half a mile west of the range; and that in the basin west of Reno sediments are locally 1800 feet or more thick. The variations in thickness reflect important structural deformation in the basins. The late Cenozoic structures that separate the basins from the mountains are a complex of contemporaneous faults and folds; folding is predominant in some areas and normal faulting in others. From the eastern boundary of the area surveyed, the gravity values decrease westward about 1 mgal per mile, a total of 30 mgals, to a minimum at the east side of the Carson Range of the Sierra Nevada and then increase toward the main Sierra block. There is little or no gravity expression of the Virginia Range or the Carson Range relative to sediment-free parts of the basins adjacent to the ranges. Gravity measurements of the Coast and Geodetic Survey combined with the present measurements demonstrate that the Bouguer anomaly from San Francisco eastward shows a general inverse relationship to elevation but is a minimum along the east side of the Carson Range rather than at the main summit of the Sierra. This gravity profile may be the result of a thickening of the earth9s crust eastward beneath the higher part of the continent, combined with a smaller effect from greater-than-average amounts of granitic rock in the crust of the Sierran region.