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At least 91 records · Page 5Linked to original sources
USGS1971

Tertiary igneous chronology of the Great Basin of western United States — Implications for tectonic models

The chronology of igneous activity in the Great Basin of western United States is used as a time framework for a simple plate model. This chronology suggests that a plate (Farallon plate) became underthrust to sufficient depth by the middle Tertiary to trigger the eruption of volcanic rocks of andesitic to rhyolitic composition in the central part of the Great Basin, 40 m.y. ago. This plate continued to be underthrust until about 19 m.y. ago, at which time it was completely consumed and volcanic activity ceased. When the oceanic ridge reached a certain point under the Great Basin about 16 m.y. ago, this resulted in the widespread eruption of olivine basalt and the main initial phase of Basin and Range faulting.

Arizona, California, Idaho, Nevada, Oregon, Utah↗
USGS1989

Diapiric transfer of melt in Kilauea Iki lava lake, Hawaii: A quick, efficient process of igneous differentiation

Kilauea Iki lava lake, formed in 1959, is a large pond of picritic basalt (average MgO content = 15.34% by weight), which has cooled and crystallized as a small, self-roofed magma chamber. Repeated drilling of the upper crust of the lake, down to its molten core, and more recent (1981) drilling, through the thermal maximum and part way through the lower crust, have made it possible to monitor the differentiation processes in the lake in detail. Differentiation processes recognized as active in the lake include rather inefficient settling of the larger (2-10 mm) olivine phenocrysts, formation of segregation veins, and formation of diapir-like vertical olivine-rich bodies, all processes which occur in one or more of the other Kilauean lava lakes as well. In addition, most of the central part of Kilauea Iki has been affected by diapiric melt transfer. In this process, relatively low-density liquid, present at 1145-1160 °C, rose from within the loose crystal mush at the base of the lens of melt and intruded the equivalent thermal horizon at the top of the lens of melt, passing through the hotter but denser melt in the core of the lake without mixing. The source volume from which the low-density liquid was extracted is depleted in TiO 2 and other incompatible elements and enriched in FeO and CaO. The upper part of the lake shows the opposite effects. The crystalline assemblage present was olivine + augite + minor plagioclase. The crystallinity of both the source and receiving layers was low enough that no obvious textural imprint was left by the transfer process; the principal evidence for its occurrence is the chemical zonation of the lake seen in core from depths of 13 to 80 m. Diapiric melt transfer was active from 1960 to 1971 and has affected most of the central part of the lake from 13 m to at least 80 m. The process ran simultaneously with the other three main differentia tion processes but started and stopped independently of the others. Calculations suggest that between 21 and 42 wt % liquid has been extracted from the depleted zone at 56-78 m in the center of the lake, making this a very efficient process of chemical differentiation.

Hawaii↗
USGS1963

Matureland of northern Chile and its relationship to ore deposits

The old surface that extends over a long north-striking belt in northern Chile probably reached the mature stage of erosion by middle Tertiary time. Low areas are mantled by the piedmont deposits of the pampa, and the higher parts exhibit rock decay, oxidation, leaching, and super-gene enrichment of sulfide mineral deposits as a result of deep weathering. The surface and its deposits are deeply eroded south of the main pampa by the Río Copiapó and its tributaries. Numerous ore deposits in the thick secondary zones have not been eroded, but ore deposits are few in the youthful valleys where the primary zones extend to the surface.

northern Chile↗
USGS1991

Anomalous folds associated with the east-central part of the Garlock Fault, southeast California

The east-central part of the left-lateral Garlock fault, in southeast California, is associated with three broad folds that trend and plunge northeast. The folds, which lie north, south, and astride the fault, postdate deposition of alluvial gravels that at one time formed a continuous northwest-sloping fan; the gravels rest conformably on lake beds that contain the Lava Creek B ash bed (0.62 Ma). The anticline, named after Christmas Canyon, has vertical relief of as much as 100 m. Topographic profiles suggest that the gravels exposed along the synclinal troughs to its southeast (Pilot Knob Valley syncline) and northwest (Teagle Wash syncline) lie at about the same elevations as they did when part of the originally active fan, implying that the synclines are passive results of the anticline development. If so, the original slope of the middle Pleistocene alluvial sediment sheet was about 0.6° northwest. The age and dimensional controls allow calculation of the rates of tilting and horizontal shortening. Corrected tilting rates vary from about 2.5° to 4.7°/m.y.; the total shortening of about 7.5 m developed at a minimum rate of 12.5 m/m.y.

California↗
USGS1969

A geophysical study of North Park and the surrounding ranges, Colorado

A geophysical study in the North Park basin and surrounding mountains, Colorado illustrates the structural relationship of various sedimentary, metamorphic, and igneous rock units. Bouguer anomalies from 1330 gravity stations range from −210 mgal over Precambrian metamorphic rocks in the mountains to −260 mgal in the Walden syncline and —280 mgal in the North Park syncline. Steep gradients delineate a fault which strikes west-northwest along the north flank of the North Park syncline. Two models fitted to the gravity data show 1 to 2 km relief on this steeply dipping fault. Density contrasts between Precambrian metamorphic and igneous rocks produce anomalies of as much as 25-mgal amplitudes in the Park and Medicine Bow Ranges. A 30-km-long seismic refraction profile, parallel to the most negative Bouguer anomaly values in the North Park basin, shows velocities increasing from 2.5 to 3.4 km/sec within Tertiary rocks at depths ranging from 1.2 to 2.0 km. Mesozoic sedimentary rocks have a velocity of 4.0 to 4.5 km/sec, a very high velocity in view of the predominance of Upper Cretaceous rocks. Precambrian basement with a velocity of 6.25 km/sec underlies the profile at depths ranging from 3.5 to 4.5 km. Strong second arrivals across the profile, observed at distances of more than 14 km from the shotpoints and interpreted as SP reflections, verified the refraction model. An aeromagnetic survey shows numerous anomalies ranging from 100 to 200γ in the Park and Rabbit Ears Ranges and in the Never Summer Mountains, to 400γ in the Front Range, and to 1200γ over the Medicine Bow Range. Positive anomalies in the Park, Medicine Bow, and Front ranges overlie metamorphic rocks. Magnetic and gravity data suggest that the Never Summer Mountains are separated from the Front Range by a north-trending, steeply east-dipping reverse fault, extending beneath the Front Range along the Colorado River valley. The magnetic data indicate that this fault may connect with a possible fault that is parallel to the Laramie River valley. In the Rabbit Ears Range, a series of magnetic anomalies show that igneous rocks are present in the eastern part of the range. A northeast-trending positive magnetic anomaly, which is parallel to foliation trends reported in Precambrian rocks, extends from the Park Range across the North Park basin to the Medicine Bow Range. On the basis of this anomaly, the high seismic velocity of the Precambrian basement, and computed profiles fitted to the gravity and magnetic data, we infer that much of the basin is underlain by high-density metamorphic rock. As shown by gravity data, the deepest part of the basin is 2.7 km below sea level, resulting in a maximum relief of 6.7 km on the basement, relative to the Medicine Bow Range. A 25-mgal negative gravity anomaly and a zone of negative magnetic anomalies outline a large granitic intrusion in the Park Range, which probably extends northeast beneath the North Park basin and connects with granitic rocks in the Medicine Bow Range.

Colorado↗
USGS1991

The neotectonic setting of Puerto Rico

The island of Puerto Rico, in the northeast Caribbean, lies within a broad deformation zone between the Caribbean and North American plates. The simplest model for the tectonic setting of Puerto Rico has major strike-slip movement on nearly east-west lines in the vicinity of the Puerto Rico Trench coupled to a small counterclockwise rotation of a Puerto Rico block within the broader plate boundary zone. This simple model is attractive because it predicts the tectonic regime south of Puerto Rico, and provides an explanation for a possible component of extension across the Puerto Rico Trench west of 65.5°W. GLORIA long-range sidescan sonar data and seismic reflection profiles have been used to test this model by mapping the major tectonic features across the plate boundary north and south of Puerto Rico. To the north, the new data help to resolve between conflicting models, of underthrusting or strike-slip motion at the Puerto Rico Trench. No direct evidence of compression is seen, although evidence for normal and strike-slip movement is abundant. This, combined with regional considerations, leads us to conclude that the main east-west-trending part of the Puerto Rico Trench between 65.5°W and 68°W lies within a strike-slip regime, although oblique convergence occurs both to the east and west where the plate boundary trends east-southeast. To the south of Puerto Rico, underthrusting of the Caribbean plate beneath the island decreases from west to east, and it is ultimately replaced by extension in the Virgin Islands Basin east of 65°W.

Puerto Rico↗
USGS1964

Relation of temperature distribution to ground-water movement in carbonate rocks of central Israel

The Cenomanian-Turonian formations of central Israel constitute a highly permeable dolomite and limestone aquifer. In this area it is on the west limb of an anticlinorium that trends north-northeast, and it contains water under artesian pressure. A graph of water temperatures and well depths suggests that there is a very small vertical temperature gradient in local segments of the aquifer. The small gradient is believed to result from a large vertical component of flow that tends to equalize the vertical temperature distribution . On a regional scale the apparent horizontal temperature distribution indicates a westward increase with increasing depth of the aquifer, suggesting a manifestation of the regional geothermal gradient. The westward increase in temperature also implies that the lateral component of flow may be in the normal range for artesian carbonate - rock aquifers whose pores consist mainly of solution cavities. Locally, pumping appears to have affected the temperature distribution by modifying the natural flow pattern. In parts of the most intensively developed area, the aquifer is hydraulically connected with overlying coastal-plain deposits, and some cooler water has been induced to move into the aquifer from this source. At three other areas, pumping has resulted in an apparent horizontal shift of the isotherms on a temperature - distribution map. The data suggest that the spatial distribution of temperature may be used to determine some of the flow characteristics of carbonate - rock aquifers.

Geological Society of America Bulletin↗
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↗
USGS1971

Thin skin distension in Tertiary rocks of southeastern Nevada

Volcanic rocks of late Tertiary age, aggregating about 17,000 ft, accumulated on a surface of low relief cut on Precambrian rocks in the Basin and Range province south of Lake Mead, in Nevada and Arizona. They consist mostly of lava and flow breccia of intermediate composition with minor ash-flow tuff, bedded tuff, and lava of rhyolitic composition. The last of three main phases of volcanism was accompanied by widespread epizonal plutonism and intense faulting. All or parts of six similarly but separately fault-deformed structural units are recognized in a 92-sq-mi mapped area. The structural units are highly distended by a system of closely spaced north-to northwest-striking shingling normal faults (many of which are low angle) that displace younger over older rocks in a west to west-southwest direction. Cumulative amounts of distension approximate the breadth of the structural units and are as much as 20,000 ft, whereas cumulative vertical displacements are much less and in some places are minimal. The structural units are floored at or near the present level of exposure by complex low-angle zones of detachment or décollement into which the numerous shingling normal faults merge. Where the units abut along their strike, they are separated by complex zones of transcurrent faults that appear to merge with the detachment structures and thus mark the ultimate limits of the structural units. Displacement on the detachment structures has the same sense as, but in some places is much greater than, that of the cumulative offset on the shingling faults, thus indicating low-angle movement of the structural units as platelike or lobate masses. These relationships indicate remarkably thin-skinned, large-scale, fault-related tectonism of a type which is present in a broad belt south of Lake Mead and in numerous other areas in the Basin and Range province. The best exposed structural units exhibit a serial eastward progression from broad areas of steeply dipping strata, low-angle faults, and deep denudation to gently dipping strata, high-angle faults, and little denudation. Reverse-drag flexing, a volume-compensating mechanism for movement on concave-upward faults, is inferred to have produced the gentle to moderate dips of the strata, whereas the nearly vertical dips in the western parts of the units probably resulted from a combination of reverse-drag flexing and rotation related to uplift. Evidence of compression-related folding is absent. The extreme distension is viewed as a surficial feature of a crustal belt that was subjected to a brief episode of tensional rifting. Rifting at subjacent levels along the belt was compensated for by emplacement of plutons. The surficial rocks were stretched and thinned over the plutons.

Nevada↗
USGS1975

The vourinos ophiolite, Greece: Cyclic units of lineated cumulates overlying harzburgite tectonite

Re-examination of the Vourinos ophiolite shows it to be composed of metamorphic tectonites, cumulates, plagiogranites, dikes, and lava. The contact between the tectonites and the cumulates is exposed and sharp. Beneath the cumulate contact, the rocks have been highly deformed and complexly folded; above that contact, they simply have been tilted vertically to expose a stratiform complex 1,500 m (4,800 ft) thick. The stratiform intrusion is characterized by cyclic units, rich in olivine at the base, and rich in feldspar at the top. Some cumulus diorites are present at the top of the section which grade into quartz diorites (plagiogranites) with hypautomorphic textures. Lineate lamination characterizes the cumulates and may indicate the direction or orientation of the Mesozoic mid-oceanic ridge crest with respect to the present position of the complex. A siliceous dike swarm cuts the upper part of the stratiform complex. The section suggests that in the case of Vourinos, a large magmatic chamber formed at a mid-oceanic ridge crest and that intrusion was a much more important process than extrusion in the formation of oceanic crust in that area. The reported presence of cumulates in many other ophiolite complexes suggests that these relations may obtain generally at most or all spreading ridges. The contact between the tectonites and the cumulates of the complex would not have corresponded with seismic M. © 1975 Geological Society of America.

Geological Society of America Bulletin↗
USGS2019

Relaxing segmentation on the Wasatch Fault Zone: Impact on seismic hazard

The multisegment Wasatch fault zone is a well-studied normal fault in the western United States that has paleoseismic evidence of recurrent Holocene surface-faulting earthquakes. Along the 270-km-long central part of the fault, four primary structural complexities provide possible along-strike limits to these ruptures and form the basis for models of fault segmentation. Here, we assess the impact that the Wasatch fault segmentation model has on seismic hazard by evaluating the time-independent long-term rate of ruptures on the fault that satisfy fault slip rates and paleoseismic event rates, adapting standard inverse theory used in the Uniform California Earthquake Rupture Forecast 3 (UCERF3), and implementing a segmentation constraint where ruptures across primary structural complexities are penalized. We define three models with varying degrees of rupture penalization: (1) segmented (ruptures confined to individual segments), (2) penalized (multi-segment ruptures allowed, but penalized), and (3) unsegmented (all ruptures allowed). Seismic-hazard results show that on average, hazard is highest for the segmented model, where seismic moment is accommodated by frequent moderate (moment magnitude, M w 6.2–6.8) earthquakes. The unsegmented model yields the lowest average seismic hazard because part of the seismic moment is accommodated by large (M w 6.9–7.9), but infrequent ruptures. We compare these results to model differences derived from other inputs such as slip rate and magnitude scaling relationships and conclude that segmentation exerts a primary control on seismic hazard. This study demonstrates the need for additional geologic constraints on rupture extent and methods by which these observations can be included in hazard-modeling efforts.

Utah↗
USGS1973

Age and correlation of the Windermere Group in northeastern Washington

Greenstone of basaltic composition forms the middle part of the Windermere System in southern British Columbia and the correlative Windermere Group in northeastern Washington. The greenstone, together with the rest of the Windermere in this region, is highly sheared, altered, and metamorphosed, except for a small exposed mass of relatively unaffected rocks near the community of Chewelah, Washington. K-Ar ages on whole rocks and mineral separates from the Chewelah area indicate the greenstone was probably extruded between 827 and 918 m.y. ago.

Washington↗
USGS1953

Geology of the west-central part of the Gunnison Plateau, Utah

A detailed study of the west-central part of the Gunnison Plateau, Utah, has disclosed stratigraphic and structural relations important in the geological history of central Utah. The area mapped includes the eastern half of the Axtell No. 2 quadrangle, Manti area (U. S. Dept. of Agriculture, Soil Conservation Service). The bedrock ranges in age from the Arapien shale (Upper Jurassic) to the Green River formation (Eocene). The North Horn formation (Cretaceous-Tertiary), the Flagstaff limestone (Paleocene-Eocene), and the Green River formation (Eocene) successively overlap the Arapien shale and the Indianola group in the northern part of the area. In the southern part of the area, the North Horn formation and the Flagstaff limestone successively overlap the Price River conglomerate with angular discordance. The latter relation establishes a post-Price River and pre-North Horn orogenic movement in central Utah. A conspicuous west-dipping monocline, broken by many high-angle faults and a graben, extends along most of the western margin of the area. This structure is similar to the monocline and graben in the Wasatch Plateau to the east. At one point in the northern part of the area, the North Horn and the overlying Flagstaff oppose the Arapien shale across a high-angle fault. The Green River formation extends across the fault and overlaps the Arapien. This relation suggests faulting between Flagstaff and Green River time because the Colton formation, which normally occurs between the Flagstaff and Green River, cannot be differentiated in this immediate area. Numerous small intrusive masses of monzonite porphyry of post-Upper Jurassic age occur in the Arapien shale.

Utah↗
USGS1973

Fusion Relations in the System NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O and Generation of Granitic Magmas in the Sierra Nevada Batholith

Chemical analyses of 167 typical specimens indicate that about 95 percent of the intrusive rocks of the central Sierra Nevada contain more than 79 percent normative Ab + An + Or + Qz. If the composition of the lower continental crust is similar to or slightly more felsic than andesite, as seems likely, the system NaAlSi 3 O 8 -CaAl 2 Si 2 O 8 -KAlSi 3 O 8 -SiO 2 -H 2 O provides an excellent chemical model for testing various schemes of fusion of the lower crust and crystallization of the resulting magmas. From consideration of this system in conjunction with field and petrographic data, we conclude that the intrusive rocks are best explained by repeated episodes of equilibrium fusion corresponding to magmatic sequences defined by field, petrologic, chemical, and geochronologic data. Fractional crystallization of the crystal-liquid mush generated by equilibrium fusion, coupled with periodic upward or lateral movement of the less crystallized central part of the magma, would produce the characteristic mafic to felsic sequence of intrusion; each mafic to felsic sequence corresponds to a separate equilibrium fusion event. In contrast, a close approach to fractional fusion of the lower crust is inadequate for obtaining most of the plutonic rocks, because rock compositions capable of being produced by this process do not match those observed. Normal amounts of conductive heat from the mantle and from radioactive decay in the crust may have been capable of causing fusion in the deepest parts of a thickened crust under the central part of the Sierra Nevada without the aid of a transient heat source from the mantle, but would have been inadequate where the crust was thin in the western Sierra Nevada. However, upward transport of andesitic and basaltic magmas generated along a Mesozoic subduction zone dipping beneath the Sierra Nevada would have provided sufficient additional heat to make fusion of the lower crust unavoidable. This implies that a major portion of the present batholith must have been derived from the lower crust.

California↗
USGS1963

Geologic history of the teays valley in West Virginia

The segment of the abandoned pre-Pleistocene Teays Valley between Scary and Huntington, W. Va. stands 130-240 feet above the Ohio and Kanawha rivers, and its bedrock floor slopes westward at about 0.6 foot per mile. The bedrock floor is overlain by highly weathered gravel in which a soil profile developed; only resistant siliceous materials remain. As much as 100 feet of locally derived sediments overlies the basal gravel. Sand was deposited at each end of the valley but in the east-central part it grades laterally into a laminated silty clay that was deposited during a period of ponding, probably in Kansan time. These deposits are deeply eroded. Probably during Illinoian time, ponding at a lower level resulted in deposition of a younger silty clay in the western part of the valley. This silty clay is weathered to a depth of about 14 feet. During a brief ponding in Wisconsin time, a widely scattered veneer of ice-rafted unweathered pebbles of igneous and metamorphic rocks was deposited. This veneer represents the youngest Pleistocene deposits in the valley and it occurs as much as 110 feet above the present Ohio River. Depositional, weathering, erosional, and topographic evidence argues that the Teays Valley in West Virginia was abandoned in late Tertiary or early Pleistocene time by normal stream-capture processes and that prolonged weathering followed. © 1963, The Geological Society of America, Inc.

West Virginia↗
USGS1971

Boulder Creek batholith, Colorado part III: Fingerprinting discordant zircon ages in a complex intrusion

The apparent ages (32 lead/alpha and 6 Pb 206 /U 238 ) of zircon as plotted on an isochron map of the Boulder Creek batholith define the following pattern: (1) very high ages (1600 to 1900 m.y.) within the outermost border zone on the southwest, south, and southeast; (2) transitional high ages (1300 to 1600 m.y.) within an inner border zone on the east and widening to the south and west to include about one-third of the batholith; (3) transitional low ages (1000 to 1300 m.y.) throughout much of the interior of the northern half; and (4) very low ages (1000 m.y. or less) limited to a small area within the northeast corner. The area of minimum age is shown to be part of the reduced-age aureole surrounding the 77 m.y. hornblende granodiorite stock at Jamestown that intrudes the Silver Plume Granite of the Longs Peak-St. Vrain batholith in the region immediately to the north of the minimum-age area of the Boulder Creek batholith. A southeastward elongation of the area of minimum age is attributed to channelway control of the solutions responsible for the recrystallization of the zircon by those northwest-trending breccia reefs that are cut by, or strike toward, the Laramide intrusion. Statistical studies of five zircon separates used for isotopic work showed that the frequency of grains having partial, or complete, rims of colorless zircon on purple to semiopaque zircon cores increased inversely with measured Pb 206 /U 238 age along a smooth curve that, when extrapolated, connected the point representing age of emplacement (0 percent rims) and the point representing the approximate age of re-crystallization (100 percent rims). Consequently, in the Boulder Creek zircon rim frequency gives a useful estimate of the amount of lead lost relative to uranium and thorium from a given sample during its recrystallization. The microstudy indicated: (1) the surface separating core and rim is a major discontinuity; (2) the greater part of the rims appear to be true overgrowths; and (3) the highest frequency of rims is found in the most metamict zircon. However, in any one sample a significant fraction of the most metamict zircon has been sheltered from reaction, presumably by inclusion within relatively impervious minerals, and remains free of rims. These observations coupled with the map evidence of selective channelway control point to warm solutions rather than dry heat as the agent of recrystallization and lead loss.

Colorado↗
USGS1942

Correlation of the outcropping cretaceous formations of the Atlantic and Gulf Coastal Plain and trans-Pecos Texas

This is Number 9 of a series of correlation charts prepared by the Committee on Stratigraphy of the National Research Council, which will cover the post-Proterozoic sedimentary formations of North America. For general information about the project the reader is referred to the general introduction preceding this paper. The senior author, Lloyd W. Stephenson, is responsible for that part of the chart treating of the Upper Cretaceous series in the Atlantic and Gulf Coastal Plain (exceptions noted below) and the Lower Cretaceous series of the North Atlantic Coastal Plain; with respect to the Upper Cretaceous his presentation is based largely on his personal knowledge of the paleontology and stratigraphy of the area, gained from field and laboratory studies ranging from reconnaissance to detailed; all published information has been considered, and much of it utilized, but it is not practicable to include a complete bibliography. References are given to papers presenting views differing from those of the author, and to recent papers not yet listed in bibliographies. A complete bibliography of Texas geology through 1932 is given by Sellards (1933, p. 819-965). Watson H. Monroe has collaborated in the preparation of that part of the chart showing therelations of the Upper Cretaceous units in Alabama and Mississippi. The classification shown in the Upper Cretaceous part of the Delaware column is based on a recently published paper by Charles W. Carter (1937, p. 237-281). Philip B. King is responsible for the remainder of the chart covering the Lower Cretaceous (Comanche series) of central Texas (largely compiled from published sources) and all of the Cretaceous in trans-Pecos Texas (from personal knowledge and published sources). In March 1941 this part of the chart was checked by R. W. Imlay, and changes were made by King at his suggestion. References to the more important literature are given. In a correlation chart of this kind, in which age equivalencies are expressed by horizontal lining, it is obviously impossible to indicate relative thicknesses by vertical spacing. Hiatuses are indicated by vertical lining. The red-line overprint is intended to indicate certain important lithologic and faunal boundaries, and to assist the eye in recognizing the age relationships of the geologic units

Geological Society of America Bulletin↗
USGS1970

Boulder Creek batholith, Colorado part I: Allanite and its bearing upon age patterns

Allanite is abundant and commonly attains unusually large size as a late-replacement mineral in: (1) the comagnatic rocks of the Precambrian Boulder Creek batholith; (2) associated amphibolite xenoliths and related hybrid rocks; and (3) distinctly younger intrusions of Silver Plume Granite that cut the complex. Allanite porphyroblasts develop by replacement of biotite, probably in the presence of emanations from the rare earth-rich and thorium-rich Silver Plume Granite. The largest allanite crystals are made up of nearly isotropic (metamict) cores and birefringent (recrystallized) rims. Smaller crystals are made up exclusively of birefringent material. The maximum birefringence is shown to be that expectable in allanite of Late Cretaceous to early Tertiary age. As plotted on maps, the birefringence increases, and the thorium and uranium contents of the allanite decrease toward a Laramide stock. The variation in birefringence is, therefore, largely relatable to variations in the post-Laramide radiation dosage brought about by differences in the amounts of uranium and thorium lost during recrystallization. The recrystallized allanite is itself partly replaced by epidote which characteristically occurs as a border between allanite and biotite. Total rare-earth oxides for the eight samples of allanite analyzed range from 17.5 to 21.3 percent by weight. In 13 samples, thorium ranged from 0.50 to 1.14 percent by weight, and uranium from 54 to 158 parts pparts per million. Ranges in optical measurements for 20 samples using the spindle stage are: Nα = 1.719–1.759, Nβ = 1.731-1.774, N γ = 1.741–1.784, birefringence = 0.020–0.032, 2V X (calc.) 70°–84°. Ranges for unit-cell data obtained on 8 samples are: a = 8.948–8.985Å, b = 5.721-5.763Å, c = 10.184–10.240Å, β = 115°7.50′–115°25.89′ and volume = 473.02–478.43Å 3 . The average value for the ratio a:b:c = 1.561:1:1.778.

Colorado↗