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At least 811 records · Page 45Linked to original sources
USGS2005

Contrasting Proterozoic basement complexes near the truncated margin of Laurentia, northwestern Sonora–Arizona international border region

We utilize new geological mapping, conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses, and whole-rock radiogenic isotope characteristics to distinguish two contrasting Proterozoic basement complexes in the international border region southeast of Yuma, Arizona. Strategically located near the truncated southwest margin of Laurentia, these Proterozoic exposures are separated by a northwest-striking Late Cretaceous batholith. Although both complexes contain strongly deformed Paleoproterozoic granitoids (augen gneisses) intruded into fine-grained host rocks, our work demonstrates marked differences in age, host rock composition, and structure between the two areas. The Western Complex reveals a >5-km-thick tilted section of finely banded felsic, intermediate, and mafic orthogneiss interspersed with tabular intrusive bodies of medium-grained leucocratic biotite granite (1696 ± 11 Ma; deepest level), medium-grained hornblende-biotite granodiorite (1722 ± 12 Ma), and coarse-grained porphyritic biotite granite (1725 ± 19 Ma; shallowest level). Penetrative ductile deformation has converted the granites to augen gneisses and caused isoclinal folding and transposition of primary contacts. Exposed in a belt of northwest-trending folds, these rocks preserve southwest-vergent shear fabric annealed during amphibolite facies metamorphism, when crystalloblastic textures developed. Deformation and regional metamorphism occurred before emplacement of 1.1 Ga(?) mafic dikes. Throughout the Eastern Complex, meta-arkose, quartzite, biotite schist, and possible felsic metavolcanic rocks comprise the country rocks of strongly foliated medium- and coarse-grained biotite granite augen gneisses that yield mean 207 Pb/ 206 Pb ages of 1646 ± 10 Ma, 1642 ± 19 Ma, and 1639 ± 15 Ma. Detrital zircons from four samples of host sandstone are isotopically disturbed; nevertheless, the data indicate a restricted provenance (ca. 1665 Ma to 1650 Ma), with two older grains (1697 and 1681 Ma). The pervasively recrystallized Paleoproterozoic map units strike parallel to foliation and are repeated in south-trending folds that are locally refolded about easterly hinges. Southeasterly lineation developed in augen gneiss and host strata becomes penetrative in local domains of L-tectonite. Regional metamorphism associated with this tectonism persisted until ca. 1590 Ma, as recorded by metamorphic growths within some zircon grains. Mesoproterozoic intrusions that crosscut the Paleoproterozoic metasediments and augen gneisses include coarsely porphyritic biotite granite (1432 ± 6 Ma) and diabase dikes (1.1 Ga?). Emplacement of the granite was accompanied by secondary high-U overgrowths, dated at 1433 ± 8 Ma, on some of the Paleoproterozoic detrital zircons, and apparently was also responsible for resetting the whole-rock Pb isotopic systematics (1441 ± 39 Ma) within these Eastern Complex augen gneisses. Younger plutons emplaced into both Proterozoic basement complexes include medium-grained quartz diorite (73.4 ± 3.3 Ma and 72.8 ± 1.7 Ma), Late Cretaceous hornblende-biotite granodiorite, and Paleogene leucocratic biotite granite. Neogene sedimentary and volcanic strata overlie basement along unconformities that are tilted to the northeast, southeast, or southwest. A brittle normal fault, dipping gently northeast, juxtaposes Tertiary andesite with Paleoproterozoic metasandstone. These relationships suggest that the area shares a common history of mid-Tertiary extension with southwestern Arizona. Later influence of the southern San Andreas fault system is implied by multiple dextral offsets of pre-Tertiary units across northwest-trending valleys. Our structural, geochronologic, and isotopic data provide new information to constrain pre–750 Ma Rodinia reconstructions involving southwestern Laurentia. Whole-rock U-Th-Pb and Rb-Sr isotopic systematics in both Paleoproterozoic gneiss complexes are disturbed, however, well-behaved Sm-Nd analyses preserve depleted initial ε Nd values (+2 to +4) that are distinct from the Mojave crustal province, but overlapping with the Yavapai and Mazatzal Provinces of Arizona. The Eastern Complex has the appropriate age and Nd isotopic signature to be part of the Mazatzal Province, but records major tectonism and metamorphism at ca. 1.6 Ga that postdates the Mazatzal orogeny. Deformed granitoids of the Western Complex have “Yavapai-type” ages and ε Nd but display structures discordant to the southwesterly Yavapai trend in central Arizona. The Western Complex lies along-strike with similar-age rocks (1.77 Ga to 1.69 Ga) of the “Caborca block” that have only been studied in detail near Quitovac and south of Caborca. Collectively, these rocks form a northwest-trending strip of basement situated at the truncated edge of Laurentia. The present-day basement geography may reflect an original oroclinal bend in the Yavapai orogenic belt. Alternatively, the western Proterozoic belt of Sonora may represent displaced fragments of basement juxtaposed against the Yavapai-Mazatzal Provinces along a younger sinistral transform fault (e.g., the Late Jurassic Mojave-Sonora megashear or the Permian Coahuila transform). Crustal blocks with these specific petrologic, geochronologic, and isotopic characteristics can be found in south-central and northeastern portions of the Australian Proterozoic basement, further supporting a connection between the two continents prior to breakup of the Rodinian supercontinent.

Arizona, California, Nevada, Sonora↗
USGS1947

Stratigraphy of the upper Cambrian, Llano Uplift, Texas

The two formations and eight members that constitute the Upper Cambrian in the Llano uplift of central Texas are described or redefined, and their lithic characters in 19 measured sections are graphically summarized. Standard reference to them is thus furnished. The Riley formation comprises the basal Paleozoic strata of the Llano uplift. Its initial sediments were deposited on a submerged pre-Cambrian terrane having a known topographic relief as great as 800 feet. Its thickness normally averages about 680 feet but ranges from probably less than 200 to about 800 feet. At most places it is subequally divisible between the Hickory sandstone member below and the Cap Mountain limestone member above, with the thin but widespread, glauconitic Lion Mountain sandstone member capping and completing the sequence. The Wilberns formation includes five named members between the Riley formation and rocks of the Lower Ordovician Ellenburger group. It normally averages about 580 feet thick and ranges from 540 to 610 feet thick, but in the southeastern corner of the Llano uplift truncation of the upper beds has reduced it to 360 feet. The thin but widespread, nonglauconitic Welge sandstone member introduces the sequence. Above it is the Morgan Creek limestone member, grading to the succeeding argillaceous beds of the Point Peak shale. At the top of the sequence are the San Saba limestone and Pedernales dolomite members. These two are essentially equivalent and gradational facies, with the Pedernales normally overlying the San Saba.

Texas↗
USGS1947

Flysch and molasse

By definition European geologists consider a sequence of limestones, sandstones, and shales, the beds of which are thin, regular, and alternating, and which are deposited in a geosyncline or foredeep shortly before a major orogeny, as the flysch . The waste products that accumulate as a deposit flanking mountains and built in part of the deformed flysch make up the molasse . In field practice the groups of sediments called flysch and molasse, or facies of them, are formations in the American sense. However, the Europeans would not recognize all groups of beds deposited in a geosyncline just before an orogeny as flysch; the beds must possess the proper lithologic and bedding characteristics. Europeans have tacitly tied the variable of lithology and stratification to the variable of orogeny, with attendant difficulties. Originally only the first was denoted, but later the second was emphasized in definition if not in field use. Americans have considered the terms chiefly in their orogenic sense and thereby have called certain sequences flysch that do not fully meet the requirements of European usage. The writers are of the opinion that little is gained by the use of the words; by their application no new fact is told or discovery made. They simply elaborate a conclusion by way of analogy. On the other hand, a possibility of confusion is introduced by the use of the terms, and in America it is best to avoid them.

GSA Bulletin↗
USGS1951

Metamorphic and igneous rocks of the merrimac area, Plumas National Forest, California

The pre-granitic rocks of an area in the northern Sierra Nevada consist of metamorphosed sedimentary and volcanic series ranging in age from Carboniferous to Jurassic. Synkinematic ultrabasic intrusives, now serpentines, cut these rocks concordantly and discordantly. Magmatic series ranging from basalt to dacite and soda-rhyolite occur together with the normal basalt-rhyolite series among the meta-volcanics. The younger intrusives (Sierra Nevada series), ranging from gabbros to granodiorites and granites, show great chemical similarity to the meta-volcanic series. Furthermore, soda-rich members are common among the pre-granitic intrusives and younger dike rocks. The pre-granitic rocks were folded and metamorphosed to green schist and epidote-amphibolite facies prior to emplacement of granodiorite and granite batholiths. The later contact metamorphism affected the areas next to the contacts of the batholitic intrusions, causing crystallization of such minerals as garnet, diopside, epidote, and andalusite. The plutonic rocks obtained the space needed partly by pushing the country rocks aside and partly by stoping and assimilation.

California↗
USGS1953

Petrology of granophyre in diabase near Dillsburg, Pennsylvania

Small bodies of granophyre occur in the upper part of diabase bodies of Triassic age in southeastern Pennsylvania. One near Harrisburg was penetrated by a diamond-drill. Drill core specimens show a gradation from diabase to granophyre. New data include 10 chemical analyses, spectrographic determinations of trace elements, and the results of petrographic study of specimens from the drill core. The sequence, from diabase to granophyre, includes a chilled zone that represents an original magma of tholeiitic composition, normal diabase, pegmatitic facies of diabase, and granophyric diabase that is intermediate in composition and petrographic characteristics between diabase and granophyre, and finally granophyre. Alkalies and silica increase progressively from diabase to granophyre; iron increases to a maximum in transitional granophyric diabase, then decreases in the granophyre. It is concluded that crystal fractionation in a large sheetlike body of tholeiitic magma yielded a small amount of granophyre. Prior to complete solidification, a residual liquid rich in iron, alkalies, and silica accumulated locally in the upper part of the diabase sheet. In places volatile-rich iron-bearing solutions escaped into the overlying sedimentary rocks and deposited magnetite; the remaining liquid crystallized t o fine-grained granophyre.

Pennsylvania↗
USGS1959

Granitization, migmatization, and fusion in the northern Entiat Mountains, Washington

A tabular quartz diorite complex extends along the Entiat Mountains , which form a southeast-trending spur of the Northern Cascade Mountains . The country rocks of the complex are biotite gneiss, probably derived from arkosic sedimentary rocks, and hornblende schist, probably derived from basic volcanic rocks. Their present mineral composition is typical of the upper epidote amphibolite and amphibolite facies of regional metamorphism. During regional metamorphism the biotite gneiss was granitized to biotite-quartz diorite gneiss and the hornblende schist to hornblende-quartz diorite gneiss. The metamorphic origin of these quartz diorite gneisses involving relatively little replacement is shown by their chemical and mineralogical similarity to the original biotite gneiss and hornblende schist, by their enclosing long, thin, undisturbed layers of country rocks, and by their association with migmatites formed by metamorphic processes. Here and there in the quartz diorite gneisses massive rocks with hypidiomorphic granular "igneous" textures have formed principally by recrystallization, and these features are associated with small-scale swirling of the foliation and some intrusive features, which show that the gneiss was rendered plastic and mobile during granitization . Migmatites containing replacement bodies and secretions of leucocratic quartz diorite occur in the biotite gneiss and in most of the rocks of the complex - particularly in the biotite-quartz diorite gneiss. During the formation of the complex, the felsic material contained in the leucocratic rocks, principally sodium, potassium, and silicon, was generated by metamorphic differentiation. During the granitization , anatectic magmas were formed by fusion of hornblende-quartz diorite gneiss, and some similar magma intruded from below. Melting without metamorphic differentiation is indicated by the chemical similarity between the anatectic quartz diorite and the parent hornblende-quartz diorite gneiss. Smearing out of inclusions and crystals and the tabular, sill-like nature of the bodies indicate that fusion may have been triggered by differential movement and shearing. The anatectic rocks contain inclusions of hornblende schist that survived both the granitization to hornblende-quartz diorite and the fusion . Crystallization differentiation of the anatectic magmas yielded potassium-rich pegmatites and local granodioritic masses. Regional metamorphism continued after the anatectic magmas in the sills had solidified; felsic metamorphic differentiates occur in leucocratic quartz diorite that partly replaces the borders of the sills. The leucocratic material was plastic and facilitated differential movement between the sill and its walls.

Washington↗
USGS1961

Patterns and origin of radial dike swarms associated with West Spanish Peak and Dike Mountain, south-central Colorado

West Spanish Peak and Dike Mountain in south-central Colorado are stocks which cut Tertiary sedimentary rocks near the axis of the La Veta syncline, the structural trough of the Raton basin. Associated with these stocks are radial dike swarms . The outline of the West Spanish Peak dike swarm is elliptical. The Dike Mountain swarm is more radial , and its outline is oval. Both systems are elongated normal to the axis of the La Veta syncline. The dikes of the West Spanish Peak swarm are of diverse rock types and represent several separate magmatic phases. Those of the Dike Mountain swarm are facies of syenodiorite and probably represent a single phase of magmatic invasion. The dikes occupy vertical joints that have been generally attributed to radial fissuring during doming of the sedimentary rocks by the emplacement of the stocks. Structural studies, however, indicate that several systems of shear and tension joints resulted from intermittent orogenic stresses of varying direction and magnitude during folding of the syncline before invasion of the magmas, and the writer suggests that selective intrusion into this joint complex accounts for the dike patterns .

Colorado↗
USGS1963

Gibson peak pluton: A discordant composite intrusion in the southeastern Trinity Alps, northern California

Gibson Peak pluton is the most discordant of several dominantly granitic intrusions in the Trinity Alps of northern California . It formed during Nevadan (Late Jurassic) deformation by emplacement of at least five discrete rock units that define a successively more silicic series, ranging from hypersthene gabbro to trondhjemitic tonalite. Contact features suggest that several units were incompletely crystalline when intruded by succeeding phases. Deformation of wall rocks, mainly partly serpentinized peridotite, indicates forceful intrusion , despite remarkable discordance of the pluton to regional structures. The discordance probably was controlled by regional extension fracturing during late stages of Nevadan deformation. Chemical compositions, computed from average modes of the intrusive units, are characterized by high Fe 2 O 3 -FeO and Na 2 O-K 2 O ratios. Plots of normative feldspar define a trend of trondhjemitic differentiation that diverges markedly from typical calc-alkaline trends. Contact metamorphism to mineral assemblages of pyroxene hornfels facies has been largely obscured by later low-grade hydration reactions, resulting in a net increase in serpentinization of most country-rock peridotite within the contact aureole.

California↗
USGS1965

Eclogites and eclogites: Their differences and similarities

Eclogites are divisible into three groups based on mode of occurrence: Group A, inclusions in kimberlites, basalts, or layers in ultramafic rocks; Group B, bands or lenses within migmatite gneissic terrains; Group C, bands or lenses within alpine-type metamorphic rocks. The compositions range from olivine basalt for Group A to tholeiitic basalts for Group C. New analytical data on six eclogites from glaucophane schist terrains in California and New Caledonia now permit comparisons among the three eclogite types. The pyrope content of the garnets is distinctive for each group as follows: Group A, greater than 55 per cent py; Group B, 30–55 per cent py; Group C, less than 30 percent py. Pyroxenes coexisting with these garnets also reflect a compositional change related to their occurrence. The jadeite content progressively increases from Group A through Group B, whereas the diopside content decreases. A comparison of eclogites from different geologic occurrences but with similar bulk compositions demonstrates variation in Ca-Mg partition between coexisting garnet and pyroxene. The Ca/Mg ratio increases in garnet and decreases in pyroxene from Group A through Group B eclogites. This obvious difference in the Ca-Mg partition between coexisting garnet-pyroxene in eclogites of the same bulk composition indicates a broad range of pressure-temperature conditions obtained during crystallization. Experimental synthesis of eclogite-like material at high pressures and temperatures demonstrates that some eclogites may form in the earth's mantle, but naturally occurring Group C eclogites have coexisting garnet-pyroxene with distinct Ca/Mg ratios when compared to Group A or B eclogites of similar bulk composition. This difference in the Ca/Mg ratio must reflect the pressure-temperature conditions characterizing the glaucophane schist facies.

GSA Bulletin↗
USGS1967

Belt of sigmoidal bending and right-lateral faulting in the western great basin

Betweeen the northwest-trending Sierra Nevada and the north-northeast-trending ranges that characterize most of the Great Basin section of the Basin and Range province is a belt of confused and divergent topographic forms, which is approximately 50 miles wide and 300 miles long. Along the eastern part of this belt is a topographically low lineament called the Walker Lane. A number of the ranges along the belt have an arcuate form which is controlled by the structure of rocks that are mainly of pre-Cretaceous age; locally, in the southern part of the belt, rocks as young as early Miocene show the arcuate form. The strike of tilted beds, fold axes, and in places elongate plutons, parallels the trend of the arcuate ranges, herein called “oroflexes”—defined as mountain ranges with arcuate trends that result from tectonic bending of the crust. The gross structural pattern of pre-middle Miocene rocks along the belt is a series of sigmoidal bends that suggest gigantic dextral drag. The sigmoidal bends define the oroflexes, facies boundaries, and structural trend lines. These features are cut in places by at least three, and probably five, major northwest-trending faults having right-lateral separations of many miles. Along the strike, the two largest faults seem to die out and the movement is believed to be taken up by bending. The indicated total horizontal displacement by bending and faulting along the belt is 80–120 miles, the Great Basin having moved southeastward relative to the Sierra Nevada. This movement is most likely restricted to the crust, which at some depth is thought to have moved freely over deeper material —either a deeper segment of the crust or the mantle. Available evidence permits the interpretation that the deformation began possibly as early as late Early Jurassic and that the same movement pattern has prevailed ever since. The bending and major strike-slip faulting must have been completed by early or middle Miocene time but much of it may have taken place as long ago as the Jurassic. More recent movements have been restricted to faulting.

California, Nevada↗
USGS1968

Summary of regional evidence for right-lateral displacement in the western Great Basin

Right-lateral displacement of 80 to 120 miles across the western Great Basin is indicated by the consistent disruption of sedimentary facies and thickness trends of formations ranging in age from late Precambrian to Mesozoic. Some of this displacement occurs as fault slip and some as a more pervasive large-scale drag (oroflexural bending).

California, Nevada↗
USGS1968

Gneissic amphibolite at Las Palmas, Puerto Rico, and its significance in the early history of the greater antilles island arc

The basal complex of Puerto Rico consists principally of serpentinite, minor amounts of chert and spilite, and locally small blocks of amphibolite . A detailed structural and metamorphic study of a relatively large block of gneissic amphibolite at Las Palmas reveals that the rock has undergone repeated deformation and regional metamorphism prior to contact metamorphism by intrusive serpentinite. The first event recorded by the amphibolite is amphibolite facies regional metamorphism. Foliation and some hornblende lineation were formed at this time. An increase in the intensity of regional metamorphism then produced quartzo-feldspathic segregations from the host rock, and was accompanied by small-scale folding and development of several types of lineation oblique (o the early hornblende lineation. The metamorphic intensity then decreased slightly and tectonic movement radically decreased, as nonfoliated but metamorphosed mafic dikes cut the gneissic amphibolite . As the metamorphic grade decreased further, late-stage kink-bands were developed at high angles to the earlier small-scale folding. Later, the amphibolite was intruded by serpen-tinite, causing incipient low-grade contact metamorphism and some calcium metasomatism. H. H. Hess has maintained that Puerto Rico and other islands in the Greater Antilles rest directly on oceanic crust, and that the intrusive serpentinite is derived from the bottom layer of the crust. In the Las Palmas area, the amphibolite formed prior to intrusion of serpentinite, indicating a complex preserpentinite history . Three interpretations of the gneissic amphibolite are considered. It may represent: (1) metamorphosed gabbroic rocks genetically related to the serpentinite, (2) regionally metamorphosed mafic rocks that are part of the oceanic crust, or (3) older mafic rocks regionally metamorphosed as part of an extensive pre-Late Cretaceous orogeny. The evidence from Las Palmas is not conclusive, but when considered in the regional framework, either of the last two interpretations seems reasonable.

Puerto Rico↗
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

Premetamorphic down-to-basin faulting, folding, and tectonic dewatering, Rangeley area, western Maine

The Rangeley area of western Maine is underlain by a thick sequence of dominantly eugeosynclinal metasedimentary rocks of Ordovician, Silurian, and Devonian age. The dominant structural pattern of these rocks is defined by tight, upright, northeast-trending passive flow folds and by three major normal faults along which younger rocks on the southeast are down-faulted against older rocks on the northwest. Each normal fault, together with a major syncline and a complementary anticline farther southeast, defines a geometrically related fault-fold unit. In best-exposed units, displacement along the faults increases in the direction of plunge of the synclines and of increasing structural relief in the syncline-anticline pairs. A genetic relation between normal faulting and folding is inferred. The dominant fault-fold pattern represents the oldest recognized deformation in the area. Slaty or phyllitic cleavage of this deformation is typically subparallel to the axial surfaces of folds, but locally crosses the faults and the axial surfaces of tight folds at low angles. Metamorphosed clastic dikes along the cleavage suggest that cleavage formation was in part a diagenetic dewatering process. This process probably graded, however, into low-grade metamorphism at depth. It was quickly followed by emplacement of large plutons, local superposed passive slip and flexural slip folding, and by two recognized events of greenschist and amphibolite facies metamorphism. Porphyroblasts of these events have grown across slip cleavages as well as older phyllitic cleavage, and metamorphic zones cross the dominant fault-fold pattern. Deformation, as well as sedimentation, is considered to have been controlled by the ancestral Merrimack synclinorium—a strongly linear two-sided trough that persisted at least from Late Ordovician through Early Devonian time. The fault-fold pattern is inferred to have evolved over a long period of time, as follows: (1) Rapid deposition of 15,000 to 20,000 ft of nearly-impermeable clastic sediments in Late Ordovician and Early Silurian time on the southeast-dipping slope of the sedimentary trough; mass weakened in depth by excess fluid pressure. (2) Continuing sedimentation, down-to-basin creep with associated slump faulting and folding, probably beginning in Middle Silurian time; faults flattened basinward in depth along lower boundary of zone of excess fluid pressure. (3) Horizontal compression developed parallel to slide direction as mass piled against material in the trough; incipient slaty cleavage developed normal to compression, improving vertical permeability. (4) Pore fluids expelled vertically, permitting the slumping mass to compact horizontally, and fold with at least 25 percent shortening. The process culminated in Early Devonian time, during and after deposition of the youngest exposed rocks in the area.

Maine↗
USGS1970

Thermal infrared investigations, Arbuckle Mountains, Oklahoma

Thermal-infrared images obtained on flights over the Tishomingo anticline and South Flank areas near Mill Creek in the Arbuckle Mountains, Oklahoma, were used to study the possibility of identifying some common rock types from their diagnostic reflection and emission characteristics, and to evaluate the usefulness of infrared images in structural geologic investigations. The areas flown are underlain by folded and faulted Paleozoic dolomite, limestone, sandstone, shale, and Precambrian granite. Images were obtained at 6:00 a.m., 11:00 a.m., and 2:00 p.m. The predawn (6:00 a.m.) image is the most useful in distinguishing rock types. Of particular interest is a thermal contrast of dolomite (warm) and limestone (cool), sufficient to distinguish those rock types and to reveal facies changes between them. Theoretical considerations indicate that this thermal contrast arises from a combination of albedo and thermal-inertia characteristics distinctive of dolomites and limestones in many areas. The daytime images display much stratigraphic and structural detail. Small-scale bedding detail is enhanced in the morning images of low-relief areas, and contrasts of alternating formations that form hogbacks and valleys are enhanced in the afternoon images of higher relief areas. The difference in features displayed in morning and afternoon images appears to be a function of the insolation on sunward and shadowed slopes of differing scale. Fault or fracture zones are best displayed in the predawn image; they appear cooler than surrounding ground, because of greater water content and concomitant evaporation. The abundance and throughgoing nature of lineaments (which coincide for the most part with joint systems) are more obvious in the infrared images than in aerial photographs. Lineaments striking northwest are preferentially enhanced in the morning images, and lineaments striking northeast are preferentially shown in the afternoon images. This enhancement cannot be ascribed to the effects of topography, insolation, or wind; it may relate to a combination of ground-water and vegetation effects.

Oklahoma↗
USGS1970

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↗
USGS1973

Variations in lead-isotopic compositions in Mesozoic granitic rocks of California: A preliminary investigation

Six alkali feldspar and two whole-rock samples of granitic rocks from the Sierra Nevada batholith and adjacent Klamath Mountains were analyzed for their lead-isotope compositions. The samples represented each of three 87 Sr/ 86 Sr groupings (< 0.704, 0.704 to 0.706, and > 0.706) for granitic rocks north of the Garlock fault in California. The isotopic compositions of lead in the samples from the Sierra Nevada batholith range from 18.73 to 19.37 for 206 Pb/ 204 Pb, 15.61 to 15.71 for 207 Pb/ 204 Pb, and 38.44 to 39.10 for 208 Pb/ 204 Pb. A crude parallel correspondence was found between lead and strontium isotopes, in that the specimens with the most radiogenic strontium also tend to have the most radiogenic lead similar to the previously studied Boulder batholith of Montana. A parallel correspondence is thought to imply characteristics of the source rocks for the plutons rather than consequences of partial melting or natural contamination. Lead-isotopic compositions for the Sierra Nevada batholith and the Boulder batholith differ, average values of 206 Pb/ 204 Pb being at least 18.8 for the Sierra Nevada batholith and about 18 for the Boulder batholith. In the Late Cretaceous part of the Sierra Nevada batholith, the secondary isochron “age” for the lead data in these rocks is about 2,900 m.y., far older than known Precambrian in California. Sources are proposed for these plutons from the lower continental crust and upper continental mantle or dominantly recycled continental materials, probably of intermediate composition and possibly carried down to the zone of melting by subduction. This source material may have been formed in Pre-cambrian times but did not undergo a Precambrian metamorphism greater than upper amphibolite facies which would have reduced the values of 238 U/ 204 Pb in the source rocks and resulted in Mesozoic leads like those found in the Boulder batholith and elsewhere in the Rocky Mountain region. A trondhjemite from the Klamath Mountains has a lead-isotope composition ( 206 Pb/ 204 Pb, 18.57; 207 Pb/ 204 Pb, 15.50; 208 Pb/ 204 Pb, 38.08) similar to that of oceanic volcanic rocks, particularly like those of island volcanics on oceanic ridges. Derivation of this trondhjemite from an oceanic mantle or recycled mantle material is indicated by this observation and supports the conclusion of Kistler and Peterman (1973) based on its alkali abundances and 87 Sr/ 86 Sr value.

California↗
USGS1983

Evidence for two pulses of glaciation during the late Proterozoic in northern Utah and southeastern Idaho

A record of glaciation during late Proterozoic time is preserved in a number of localities extending from the Sheeprock Mountains, Utah, to Pocatello, Idaho, and from the Park City area 40 km east of Salt Lake City to the Deep Creek Range along the Utah-Nevada line. Over much of this area, the glacial deposits and associated rocks thicken westward and form the basal part of a miogeoclinal wedge that accumulated near the late Proterozoic and early Paleozoic continental margin. In the east, such deposits are thin and rest on Archean basement or rocks of Proterozoic Y age; in the west, they are part of thicker sequences in which deposition apparently continued without significant interruption from late Proterozoic into Cambrian time. In many places, the original continuity between the western and eastern parts of the depositional wedge has been obscured by thrusting of Cretaceous and early Tertiary age that carried the thick basinal sequences eastward over those deposited on the continental platform. Recent mapping of Fremont Island in Great Salt Lake, the Wasatch Range between Ogden and Brigham City, and the Sheeprock Mountains shows that glacial episodes represented either by diamictite or by dropstones enclosed in finegrained laminated beds are separated by as much as 1,000 m of non-glacial deposits, including black slate, alternating graywacke and siltstone, quartzite, and conglomerate. Using reasonable sedimentation rates for such deposits and by comparison with modern analogues, we infer that two episodes of glaciation, each probably consisting of multiple advances and retreats, were separated by a non-glacial interval of a few hundred thousand to a few million years' duration. Correlation of the allochthonous, miogeoclinal glacial deposits with the single glacial unit present in autochthonous and parautochthonous platform sites is uncertain, but our interpretation of sedimentary facies and paleogeography suggests that only the younger of the two episodes recorded in the allochthon is represented by the diamictites of the autochthon.

Idaho, Utah↗