Remagnetization and northward translation of Mesozoic red chert from Cedros Island and the San Benito Islands, Baja California, Mexico: Reply
No abstract available.
Baja California↗
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Distribution of the Toquima-Table Head (Middle Ordovician Whiterock) Faunal Realm in the Northern Hemisphere
Discovery of a Whiterock trilobite assemblage in the Albany Mudstone, Girvan District, southwestern Scotland, led to an assessment of the distribution of Middle Ordovician brachiopod and trilobite faunas previously assigned to the White-rock Stage of Cooper (1956). These faunas lie within a belt designated as the Toquima-Table Head Faunal Realm. This realm is closely related to the position of the transition from miogeosynclmal to eugeosynclinal facies, presumed to indicate the position of Ordovician continental margins. In Middle Ordovician time North America, parts of Ireland, Scotland, Norway, Sweden, and northeastern Asia may have constituted a single continental mass.
Ayrshire County↗
Sierra Nevada plutonic cycle: Part I, origin of composite granitic batholiths
Intrusion of Mesozoic batholiths in California and the western North America Cordillera began in the Late Triassic 210 m.y. ago and ended in the Late Cretaceous 80 m.y. ago. Emplacement of granitic rocks was apparently not continuous but was accomplished during five major epochs of intrusion at approximately 30 m.y. intervals, each epoch taking 10 to 20 m.y. to complete. A progressive transgression of epicontinental seas onto the midcontinent occurred during the same interval of time as the batholithic emplacement to the west. A penecontemporaneous deformation near the loci of granitic emplacement and a temporary regression during the major progressive transgression of seas onto the midcontinent are correlated with each intrusive epoch. The locus of Mesozoic granitic rocks was a source of sediments during most of the period of time required to emplace the batholiths; the origin of the batholithic magmas cannot be related only to localized down-warping of geosynclines. The source of the major proportion of the mobile granodioritic magmas of the Sierra Nevada was within the mantle, as is indicated by Sr isotope data. All plutons now exposed in the Sierra Nevada, whether of Cretaceous age or older, were emplaced at depths of a very few kilometers, the shallowest having been emplaced at depths of 4 km or less. The spatial relationships among these synchronous geologic phenomena and the geochemical and geophysical data from the same region are accounted for by a northwestward drift of North America in the region of the western Cordillera of the United States onto and across a Mesozoic feature that had characteristics like present-day oceanic rises.
California↗
Age of the Morton and Montevideo gneisses and related rocks, southwestern Minnesota
Granitic gneisses in the vicinities of Morton and Montevideo in the Minnesota River Valley are dated at 3550 m.y. ago and are the oldest rocks so far found in North America. The gneisses were altered in varying degree by younger events of which two have been dated at 2650 m.y. and 1850 m.y. old. The event which occurred 2650 m.y. ago was a high-grade metamorphism accompanied by the intrusion of a large volume of granitic magma. Only the U-Pb zircon and the Rb-Sr whole-rock ages survived this event, and both types are discordant. A two-stage model that explains the U-Pb discordant ages combines a primary discordance produced during the metamorphism of 2650 m.y. ago with a secondary discordance developed approximately 100 m.y. ago when uplift and erosion brought the rocks close to the surface. This secondary discordance is also shown by the zircon from granite near Sacred Heart (2650 m.y. old) and from a younger granitic pluton (1850 m.y. old) near Granite Falls. The discordance in the Rb-Sr whole-rock ages is attributed primarily to the loss of radiogenic Sr 87 that probably occurred largely during the metamorphism of 2650 m.y. ago. Some later loss, however, is indicated in the younger ages of biotite and K-feldspar. Granitic material introduced or mobilized during the metamorphism is also a complicating factor. The 1850-m.y.-ago event was a low-grade metamorphism that reset the K-Ar and Rb-Sr ages of biotite in the rocks between Granite Falls and Ortonville. A number of small plutons, ranging in composition from gabbro to granite, and basaltic dikes were emplaced in the gneisses at this time, but only the granitic pluton near Granite Falls has been dated by both U-Pb and Rb-Sr methods. The mineral ages show variations that are difficult to explain, and the low apparent ages of the biotite may be in some way related to epeirogeny and the stabilizing of the K-Ar and Rb-Sr systems. The southeastern part of the valley, underlain by the Morton Gneiss and the granite at Sacred Heart, was stabilized 2400 to 2600 m.y. ago, but the northwestern part, underlain by gneiss in the Granite Falls-Montevideo area and by granite in the Ortonville area, was not stabilized until 1700 to 1850 m.y. ago. The Morton Gneiss was formed by synkine-matic intrusions of trondhjemitic and granitic magmas, and the structure dates back to the time of the intrusions, 3550 m.y. ago. A similar origin as a synkinematic intrusion of granite is favored to explain the gneiss at Montevideo. The country rock appears to have been a layered series of basaltic lavas, sedimentary rocks, and possibly some sill-like masses of diabase or gabbro. The structure of the region probably was considerably modified during the high-grade metamorphism 2650 m.y. ago. The rock types that were involved in the Mortonian event 3550 m.y. ago are similar to more recent crustal rocks and do not represent a protocrust.
Minnesota↗
Mineral paragenesis of precambrian rocks in the Tenmile Range, Colorado
A Precambrian complex of granulite, gneiss, and migmatite, intruded by numerous plutons of granitic rocks correlated with the Silver Plume granite, is exposed in a long narrow belt along the crest and upper slopes of the Tenmile Range , Colorado . The metamorphic rocks are predominantly felsic; bands, lenses, and irregular bodies of mafic rocks rich in biotite, hornblende, and locally in sillimanite and garnet, are interlayered with the felsic rocks . The major lithologic variations in the metamorphic rock complex are believed to be due chiefly to variations in the original sedimentary rocks , which probably were interbedded sandstone, shale, and limestone. The metamorphic rocks and the Silver Plume granite reveal the age relations of quartz and the feldspars, and these relations afford considerable information on the origin and progressive transformation of the rocks . Quartz is the earliest mineral in the metamorphic rocks and is probably a relict mineral of a sandstone. It has been partially replaced by feldspar. It occurs chiefly in irregular clusters, some of which show sutured grains, enclosed in a ramifying network of feldspar. Irregular small apophyses, barbs, and prongs of feldspar penetrate the quartz clusters along grain boundaries and healed fractures in the quartz. In some of the least feldspathized quartzose metamorphic rocks the feldspar is clearly interstitial to the quartz. Quartz also occurs in feldspar as small spherical inclusions. The relations of the quartz to the feldspars show clearly that a quartzose host rock was replaced by feldspar along quartz grain boundaries, pre-existing healed fractures, and margins of shadowy areas in strained quartz grains. The textural relations of the other principal minerals in the metamorphic rocks show that plagioclase formed earlier than the microcline and that the micas were the last of the principal minerals to form. Identical paragenetic relations are found in the Silver Plume granite, and the writer concludes that the Silver Plume granite was derived by partial fusion of quartzose metamorphic rocks .
Colorado↗
Glaciation of the east slope of Rocky Mountain National Park, Colorado
The eastern slope of Rocky Mountain National Park , Colorado , has been subjected to at least three separate Pleistocene glaciations, which from oldest to youngest are correlated with the Buffalo, Bull Lake, and Pinedale glaciations of Blackwelder in the Wind River Mountains of Wyoming. In this area, deposits of the oldest glaciation are known from only one locality. Deposits of the Bull Lake glaciation comprise two sets of moraines indicative of two advances of ice separated by a significant recession; those of the Pinedale glaciation comprise three sets of moraines indicative of a maximum advance of the ice and two recessional halts or minor readvances. Moraines of two minor advances of the ice, correlated with the Temple Lake and historic stades of Neoglaciation in the Wind River Mountains, occur in the cirque heads.
Colorado↗
Transcurrent faulting and volcanism in Owens Valley, California
In the Owens Valley region of California , volcanic activity of Cenozoic age was confined mainly to three areas near the ends of important faults. The volcanic eruptions seemingly took place in regions of relative tension, if the horizontal movement along these faults was left lateral. The deep depression of Owens Valley may have resulted from compression associated with left-lateral horizontal fault movement. The transfer of molten rock from beneath this deep depression laterally into the regions of tension and thence to the surface seems to account for the relief of abnormal stresses and the volume of the volcanic rocks.
California↗
Thickness and consolidation of deep-sea sediments: A discussion
Hamilton (1959) concluded that in most sediments excess pore-water pressure is equal to zero - that is, the hydrostatic pressure is at atmospheric pressure. This note points out that in terrestrial environments the occurrence of artesian water (excess pore-water pressure) is commonplace and widespread and that such excess pressure is the source of energy for flowing water wells and for many flowing oil wells, "gushers," or "blowouts".
Bulletin of the Geological Society of America↗
Geology of the Mayagüez area, Puerto Rico
The Mayagüez area forms the southwestern corner of Puerto Rico , west of 67° W. and south of 18° 15' N. One-third of the 640 square kms is covered by thick alluvium. Unconformities separate a basal complex, two sequences of highly folded igneous and sedimentary rocks, and a younger sequence of gently dipping sedimentary rock. The basal Bermeja complex contains serpentinite, silicified porphyritic volcanic rock with some sedimentary rock, and minor spilite, amphibolitized spilite, and amphibolite. It is exposed chiefly in some anticlinal cores in southwestern Puerto Rico . Limestone, mudstone, andesite, and basalt form the older folded sequence. The Río Loco formation, bronzite andesite porphyry in part with pillow structures, was extruded perhaps in the Cenomanian. The Mayagüez group includes most of the rocks in southwestern Puerto Rico : the Yauco mudstone, Parguera limestone, Brujo limestone, Melones limestone, Maricao basalt, Sabana Grande andesite, and El Rayo volcanic rocks. The maximum possible age range is Turonian to Maestrichtian. The group ranges in thickness from about 800 m in the south to 3800 m in the north, and it varies in lithology from limestone in the south to mudstone and volcanic rock in the north, indicating a volcanic center to the north during that time. The second folded sequence contains andesitic volcanic rock, bedded tuff, and massive limestone. The San Germán formation (Maestrichtian) includes andesite, the Cabo Rojo agglomerate member, and the Cotui limestone member. The Jicara formation, massive limestone and bedded tuff, is Paleocene; there is one exposure of an unnamed ? Eocene marl. Post-Eocene limestone and conglomerate are also exposed in the area . The structure of the basement complex is obscured by its massiveness and by the cover of younger rocks. Two major deformations have affected the rocks of southwestern Puerto Rico since Cenomanian to Santonian time. In the Maestrichtian, the first of these formed folds with a N. 60° W. trend, asymmetric or overturned to the south. Near the south coast the folding of thin Mayagüez group rocks was probably influenced by trends in the Bermeja complex which caused deviations in the regional trends and also some overturning to the north. The San Germán formation, deposited unconformably on the eroded surface of the folded Mayagüez group, contains large allochthonous blocks of older and contemporaneous rocks. These blocks, up to 2 km by 1 km in exposure, were deposited by slumping or sliding due to gravity within and at the base and top of the San Germán formation near Lajas and San Germán. Most rocks in the blocks are extremely contorted and contain deformed Foraminifera. The San Germán and Jicara formations and perhaps the ?Eocene marls were deformed into gentle open folds trending east in the area covered by this report. Oligocene, Miocene, and younger sedimentary rocks have been tilted and uplifted. Large east-west left-lateral transcurrent faults cross the area , offsetting and offset by two sets of transverse faults (N. 45° E., N. 20° W.): most faults are probably Maestrichtian to Oligocene, although minor faulting has continued to the present. Dikes and sills of quartz diorite porphyry and mica-quartz dacite porphyry intrude the ?Maestrichtian San Germán formation and older units. A diorite plug cuts the Bermeja complex, and a granodiorite plug intrudes the Mayagüez group.
Puerto Rico↗
Geophysical investigation of Mono Basin, California
Gravity and seismic studies in Mono Basin , Mono County, California , completed during the summer of 1957 revealed a large, roughly triangular block that had subsided about 18,000 ± 5000 feet and received an accumulation of about 300 ± 100 cubic miles of light clastic sediments and volcanic material of Cenozoic age. The seemingly near-vertical faults that bound this great block are displaced toward the center of the basin from the surrounding mountain masses, but in general they are parallel to well-defined Basin and Range trends. The gravity minimum anomaly associated with the Mono Basin structure has a residual gravity relief of about 50 mgals, and the lowest gravity readings (on Paoha Island) yield a complete Bouguer gravity value of about - 260 mgals with respect to the International Ellipsoid. The computed depth of subsidence is based on a density of 2.3 gms/ cm 3 for the basin fill and 2.7 gms/cm 3 for the basement rocks. Seismic-refraction profiles at several places in the basin demonstrate that the Cenozoic deposits are thick where the gravity is low and relatively thin where the gravity is higher. Along common seismic and gravity profiles steep seismic dips coincide with steep gravity gradients. Numerous seismic reflections are present within the basin fill. Anomalies on four aeromagnetic profiles are related in part to volcanic material within the Cenozoic section. It is concluded that Mono Basin may be a volcano-tectonic depression caused by subsidence along faults, following extrusion of magma from a magma chamber at depth. Volcanic rocks of Pliocene(?) and Pleistocene ages are exceptionally abundant in this area.
California↗
Foothills fault system, western Sierra Nevada, California
A large fault system , here named the Foothills fault system , is the dominant structural feature of the western Sierra Nevada . The steeply dipping to vertical component faults trend northwestward through an area about 200 miles long and 30 miles wide north of 37°30' north latitude. The faulted Paleozoic and Mesozoic rocks are overlapped by unfaulted younger rocks, and the total extent of the fault system is not known. It is probably not limited to the western Sierra Nevada . Faults are marked by belts as much as 4 miles wide of cataclastically deformed and recrystallized rocks and by truncated folds. Along one fault , Upper Jurassic rocks are juxtaposed against Paleozoic rocks for at least 100 miles. The direction of fault movement has not been determined. Net displacement on some of the component faults exceeds 3000 feet and may be measurable in miles. Major faults cut beds of Late Jurassic age and are in turn cut by plutonic rocks of probable Late Jurassic and Middle Cretaceous age. Faults that controlled deposition of quartz veins and gold ore bodies of the Mother Lode belt are apparently younger and structurally less important features superimposed on one of the fault zones of the large system .
California, Nevada↗
Stereoscopic-pair projection of aerial photographs in map compilation
No abstract available.
Bulletin of the Geological Society of America↗
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↗
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.
Gulf of California↗
Origin of a salt-water lens in permafrost at Kotzebue, Alaska
Frozen sediments were found to a depth of 238 feet in the drilling of a 325-foot well at Kotzebue , Alaska . Between 79 and 86 feet, however, highly saline water was found in a gravel lens . The writer suggests that the salt water originated by fractionation by freezing. Analyses of this water and of slightly saline water from below the permafrost are given.
Alaska↗
Subaerially carved Arctic seavalley under a modern epicontinental sea
A shallow seavalley , averaging 6 feet in relief, extends from the mouth of Ogotoruk Creek, northwest Alaska, for 15 miles across the floor of the Chukchi Sea to a depth of 135 feet. The seavalley is considered to be a drowned subaerial valley of Pleistocene age, which was excavated on an eustatically emerged epicontinental shelf during periods of glacially depressed sea level.
Bulletin of the Geological Society of America↗
Granitic formations in the east-central Sierra Nevada near Bishop, California
This report establishes lithologic units among the granitic rocks of the east-central Sierra Nevada near Bishop , California . In this area the Sierra Nevada batholith is composed chiefly of quartz-bearing plutonic rocks ranging in composition from quartz diorite to alaskite but includes scattered small masses of darker and older plutonic rocks and remnants of metamorphosed sedimentary and volcanic rocks. The granitic rocks are in discrete plutons, either in sharp contact with one another or separated by thin septa of metamorphic or mafic igneous rock or by late aplitic dikes. The granitic rocks are grouped into lithologic units on the basis of composition, texture, and intrusive relations. The units include six new formations , three informal units made up of the rocks in several plutons, and four informal units that include the rocks in single plutons. The new formations are the Inconsolable Granodiorite, Tinemaha Granodiorite, Wheeler Crest Quartz Monzonite, Round Valley Peak Granodiorite, Lamarck Granodiorite, and Tungsten Hills Quartz Monzonite.
California↗