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Results for “Precambrian Research”

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USGS1987

[Book review] The geochronology and evolution of Africa

This book was written 'to provide an up-to-date data bank from which those wishing to construct models concerned with the evolution of Africa .... can draw.' As such, it attempts a survey of 'integrated geology and geochronology' of the African continent throughout the Precambrian and into the Phanerozoic. Political and language divisions often hinder the synthesis of continent-wide data, therefore, this well-indexed inventory of selected data and synthesis of present geochronological knowledge for Africa as a whole provides an important reference for researchers and explorationists, many of whom have limited access to complete collections of the geological literature of Africa.

Precambrian Research↗

USGS2024

Layered intrusions in the Precambrian: Observations and perspectives

Layered intrusions are plutonic bodies of cumulates that form by the crystallization of mantle-derived melts. These intrusions are characterized by igneous layering distinguishable by shifts in mineralogy, texture, or composition. Layered intrusions have been fundamental to our understanding of igneous petrology; however, it is their status as important repositories of critical metals – such as platinum-group elements, chromium, and vanadium – that has predominantly driven associated research in recent decades. Many layered intrusions were emplaced during the Precambrian, predominantly at the margins of ancient cratons during intervals of supercontinent accretion and destruction. It appears that large, layered intrusions require rigid crust to ensure their preservation, and their geometry and layering is primarily controlled by the nature of melt emplacement. Layered intrusions are best investigated by integrating observations from various length-scales. At the macroscale, intrusion geometries can be discerned, and their presence understood in the context of the regional geology. At the mesoscale, the layering of an intrusion may be characterized, intrusion-host rock contact relationships studied, and the nature of stratiform mineral occurrences described. At the microscale, the mineralogy and texture of cumulate rocks and any mineralization are elucidated, particularly when novel microtextural and mineral chemical datasets are integrated. For example, here we demonstrate how mesoscale observations and microscale datasets can be combined to understand the petrogenesis of the perplexing snowball oiks outcrop located in the Upper Banded Series of the Stillwater Complex. Our data suggest that the orthopyroxene oikocrysts did not form in their present location, but rather formed in a dynamic magma chamber where crystals were transported either by convective currents or within crystal-rich slurries. Critical metals may be transported to the level of a nascent intrusion as dissolved components in the melt. Alternatively, ore minerals are entrained from elsewhere in a plumbing system, potentially facilitated by volatile-rich phases. There are many ore-forming processes propounded by researchers to occur at the level of emplacement; however, each must address the arrival of the ore mineral, its concentration of metals, and its accumulation into orebodies. In this contribution, several of these processes are described as well as our perspectives on the future of layered intrusion research.

Precambrian Research↗

USGS1976

Geology and Rb-Sr ages of reactivated Precambrian gneisses and granite in the Marenisco-Watersmeet area, northern Michigan

Rb-Sr dating of Precambrian W (lower Precambrian) gneisses and granitic rocks that form the cores of antiforms or domes mantled by metamorphosed Precambrian X (middle Precambrian) bedded rocks in the Marenisco-Watersmeet area, northern Michigan, shows that the rock systems have been disturbed to different degrees by complex tectonic and thermal events subsequent to primary crystallization. The disturbance is indicated by a lowering of the whole-rock Rb-Sr ages of many samples of gneiss from 2,600 m.y. or older to about 1,800 m.y. and by the scattering of data points, although some of the points can be fit to secondary isochrons; probably the major mechanism was redistribution of previously accumulated radiogenic Sr, which is indicated by the high initial 87 Sr/ 86 Sr of the resultant isochrons. The disturbance is interpreted as having resulted from internal cataclasis and recrystallization accompanied by metamorphic segregation and (or) partial melting during reactivation of the gneisses about 1,800 m.y. ago. Variations in the degree of reconstitution and mobility of individual bodies of basement rocks are indicated by differences in structural reconstitution of the basement rocks and in the metamorphic grade of the Precambrian X bedded rocks.

Michigan↗

USGS1977

Preliminary data on some Precambrian deposits of zinc-copper-lead sulfides and zinc spinel (granite) in Colorado

Precambrian sulfide deposits in the Southern Rocky Mountains in Colorado are being studied and re-evaluated according to geologic concepts which were developed in recent years in other parts of the world during successful research regarding economic massive sulfide deposits. These studies, initiated in 1974 in Colorado by the U.S. Geological Survey, have indicated that a new look at areas containing long dormant mines and prospects may well lead to the discovery of minable Precambrian sulfide deposits. The zinc, copper, and lead contents of ores investigated to date, supplemented by silver and gold contents, indicate that many long-forgotten deposits are minable in terms of grade. The deposits occur in Precambrian rocks metamorphosed to the lower amphibolite facies in one major region and to the upper amphibolite facies in other regions. Field studies have provided ample evidence indicating that the search for commercial tonnages can he facilitated by using newer concepts of economic geology regarding ore-host rock associations as prospecting guides and by using structural considerations aimed toward learning how metamorphism and folding have modified the shapes and distribution of the ore bodies. A newly recognized concept, originating from the current studies by the U.S. Geological Survey in Colorado, concerns the potential economic significance of gahnite, a zinc spinel. The field and laboratory data indicate that gahnite can be used by geologists as a prospector's guide to ore and can be considerd by mining engineers as a potential major ore mineral, contributing significant amounts of zinc to the sulfide ores in many of the deposits. Studies to date indicate that the Gunnison area, the Salida area, and the Guffey area are particularly favorable to the search for minable deposits, and the potential is equally present in many other areas.

Colorado↗

USGS2026

Sackung at Bald Eagle ridge, central Colorado: An updated interpretation of ridge-spreading movement, structures, and mechanisms from 50 years (1975–2025) of U.S. Geological Survey research

Slow gravitational failures of mountain peaks and ridges are poorly understood. Herein, we report on 50 years of studies at a slowly spreading castellate ridge in the Sawatch Range in central Colorado. The orientations of geomorphic-structural features indicate that the fractured Precambrian granitic rock underlying the ridge has extended and spread northwestward toward the formerly glacier-covered Busk Creek valley. Results from surveying, field-based geomorphic-structural mapping using lidar, rock mass quality measurements, a passive seismic survey, and satellite radar provide a major update to research started by U.S. Geological Survey researchers in the 1970s and 1980s. New insights include a recognition that the entire ridge has slowly moved by concurrent sliding along an inferred northwest dipping, compound basal-slip surface (or zone), and through the formation of multiple grabens by normal faulting and flexural toppling along sets of pre-existing fractures that dip moderately (∼45°) to the southeast. We were unable to distinguish the presence of a sudden and strong contrast in seismic velocity across the inferred slip surface. Movement during the 50-year study period has been episodic and gradually decreasing, in correspondence with decreasing cumulative annual precipitation and increasing mean annual air temperatures. The fastest moving area, just upslope from the glacier trimline, had an average horizontal velocity of 3 – 4 mm/yr. Evidence suggests that movement started as a paraglacial response mechanism, but because of the site’s proximity to the Rio Grande Rift, we cannot exclude earthquake shaking as a mechanism for initiation or enhancement of slope movement. An estimate of longer-term horizontal movement from the exposed basal-slip surface at the uphill side of the ridgetop graben is ∼1.1 mm/yr for the 13 – 14 ky post-glacial period. Broad implications of our work are that: (1) long-term measurements (decades or longer) of slope movement can add insights into how sackungen form and evolve through time; (2) the identification of thrust faults and toes in zones of compression near valley bottoms can be crucial for interpreting sackung failure mechanisms, and (3) the use of passive-seismic techniques to identify the depth to a slip surface may not be successful in granitic terrain dominated by planar fractures and subtle changes in rock-mass characteristics.

Colorado↗

USGS1975

Andesite sills in the Red Mountain area, Scapegoat Wilderness, Lewis and Clark County, Montana

Sills 2-200 ft (0.6-60 in) thick in the Red Mountain area of the Scapegoat Wilderness in northwest Montana consist of altered andesite, basaltic andesite, and dacite and occur over a stratigraphic range of 8,000 ft (2,400 m) in the Helena and Snowslip Formations. The approximate compositional range of the sills is 50-80 percent plagioclase, 0-25 percent orthoclase, and 5-30 percent amphibole, pyroxene, and biotite; the uppermost sill contains about 15 percent quartz in the groundmass. Accessory minerals include quartz, magnetite, ilmenite, apatite, and pyrite. The petrology and alteration of the sills distinguish them from other sills in the region. Field observations and laboratory studies consisting of chemical, isotopic and X-ray analyses, and petrographic and mineralogic investigations suggest that the sills may have been emplaced in Precambrian Y time at about the same time as the extrusion of the Purcell Lava in the vicinity of Glacier National Park. Alternatively, the sills may represent a Precambrian igneous event not previously recognized in the Belt basin.

Montana↗

USGS2012

Preliminary correlations of MAGSAT anomalies with tectonic features of Africa

An overview of the MAGSAT scalar anomaly map for Africa has suggested a correlation of MAGSAT anomalies with major crustal blocks of uplift or depression and different degrees of regional metamorphism. The strongest MAGSAT anomalies in Africa are closely correlated spatially with major tectonic features. Although a magnetic anomaly caused by a rectangular crustal block would be offset from the block's center by the effects of magnetic inclination, an anomaly caused by real crustal blocks of varying uplift, depression, and degree of regional metamorphism would be located nearer to the locus of greatest vertical movement and highest grade of metamorphism. Thus, the Bangui anomaly may be caused by a central old Precambrian shield, flanked to the north and south by two relatively young sedimentary basins.

Geophysical Research Letters↗

USGS2000

The crustal thickness of Australia

We investigate the crustal structure of the Australian continent using the temporary broadband stations of the Skippy and Kimba projects and permanent broadband stations. We isolate near-receiver information, in the form of crustal P-to-S conversions, using the receiver function technique. Stacked receiver functions are inverted for S velocity structure using a Genetic Algorithm approach to Receiver Function Inversion (GARFI). From the resulting velocity models we are able to determine the Moho depth and to classify the width of the crust-mantle transition for 65 broadband stations. Using these results and 51 independent estimates of crustal thickness from refraction and reflection profiles, we present a new, improved, map of Moho depth for the Australian continent. The thinnest crust (25 km) occurs in the Archean Yilgarn Craton in Western Australia; the thickest crust (61 km) occurs in Proterozoic central Australia. The average crustal thickness is 38.8 km (standard deviation 6.2 km). Interpolation error estimates are made using kriging and fall into the range 2.5–7.0 km. We find generally good agreement between the depth to the seismologically defined Moho and xenolith-derived estimates of crustal thickness beneath northeastern Australia. However, beneath the Lachlan Fold Belt the estimates are not in agreement, and it is possible that the two techniques are mapping differing parts of a broad Moho transition zone. The Archean cratons of Western Australia appear to have remained largely stable since cratonization, reflected in only slight variation of Moho depth. The largely Proterozoic center of Australia shows relatively thicker crust overall as well as major Moho offsets. We see evidence of the margin of the contact between the Precambrian craton and the Tasman Orogen, referred to as the Tasman Line.

Journal of Geophysical Research B: Solid Earth↗

USGS2020

Detrital zircon age spectra of middle and upper Eocene outcrop belts, U.S. Gulf Coast region

Recently reported detrital zircon (DZ) data help to associate the Paleogene strata of the Gulf of Mexico region to various provenance areas. By far, recent work has emphasised upper Paleocene-lower Eocene and upper Oligocene strata that were deposited during the two episodes of the highest sediment supply in the Paleogene. The data reveal a dynamic drainage history, including (1) initial routing of western Cordilleran drainages towards the Gulf of Mexico in the Paleocene, (2) an eastward shift of the western continental divide, from the Jura-Cretaceous cordilleran arc to the eastern edge of the Laramide province after the Paleocene and (3) a southward shift, along the eastern Laramide province, of the headwaters of river systems draining to the Mississippi and Houston embayments at some time between the early Eocene and Oligocene. However, DZ characterisation of most (~20 Myr) of the middle Eocene-lower Oligocene section remains limited. We present 60 DZ age spectra, most of which are from the middle or upper Eocene outcrop belts, with 50–200-km spacing. We define six to eight distinct groups of DZ age spectra for middle and upper Eocene strata. Data from this and other studies resolve at least six substantial temporal changes in age spectra at various positions along the continental margin. The evolving age spectra constrain the middle and upper Eocene drainage patterns of large parts of interior North America. The most well-resolved aspects of these drainage patterns include (1) persistent rivers that flowed from erosional landscapes across the Paleozoic Appalachian orogen either into the low-lying Mississippi embayment or directly into the eastern Gulf; (2) at least during marine regressions, a trunk channel that likely flowed southward along the axial part of Mississippi Embayment and integrated tributaries from the east and west; and (3) rivers that flowed to the Houston embayment in the middle Eocene that likely originated in the Laramide province in central Colorado and southern Wyoming, as Precambrian basement highs in those source areas were being unroofed.

Gulf Coast region↗

USGS1977

Geology and Rb-Sr age of Precambrian W Puritan Quartz Monzonite, northern Michigan

The Puritan Quartz Monzonite, in the western part of the Upper Peninsula of Michigan, composes a batholith at least 50 kilometres long and as much as 20 km wide on the south side of the Gogebic iron range. It is associated with subaqueous metavolcanic rocks assigned to the Precambrian W Ramsey Formation and with a complex gneissic unit of the same age. The quartz monzonite has a Rb-Sr isochron age of 2,710±140 million years and an initial 87 Sr/ 86 Sr of 0.7015±0.0017. The scatter of data points in excess of analytical error is interpreted as indicating that minor cataclasis and retrograde metamorphism produced slight open-system behavior in Precambrian X (circa 1,800 m.y.) time. The Puritan Quartz Monzonite and associated volcanic rocks are equated with similar Precambrian W greenstone-granite complexes in northern Minnesota, and clearly are a part of the Precambrian W greenstone terrane, as defined previously for the Lake Superior region.

Michigan, Wisconsin↗

USGS2008

The April 18, 2008 Illinois earthquake: an ANSS monitoring success

The largest-magnitude earthquake in the past 20 years struck near Mt. Carmel in southeastern Illinois on Friday morning, 18 April 2008 at 09:36:59 UTC (04:37 CDT). The Mw 5.2 earthquake was felt over an area that spanned Chicago and Atlanta, with about 40,000 reports submitted to the U.S. Geological Survey (USGS) “Did You Feel It?” system. There were at least six felt aftershocks greater than magnitude 3 and 20 aftershocks with magnitudes greater than 2 located by regional and national seismic networks. Portable instrumentation was deployed by researchers of the University of Memphis and Indiana University (the first portable station was installed at about 23:00 UTC on 18 April). The portable seismographs were deployed both to capture near-source, high-frequency ground motions for significant aftershocks and to better understand structure along the active fault. The previous similar-size earthquake within the Wabash Valley seismic zone (WVSZ) of southeastern Illinois and southwestern Indiana was a magnitude 5.0 in June 1987. The seismicity associated with the WVSZ is thought to occur in a complex horst and graben system of Precambrian igneous and metamorphic units at depths between 12 and 20 km. Paleoliquefaction evidence suggests several major shaking events have occurred within the past 12,000 years (Munson et al. 1997).

Illinois↗

USGS1992

Lake trout spawning habitat in the Six Fathom Bank-Yankee Reef lake trout sanctuary, Lake Huron

Attempts to reestablish self-sustaining stocks of lake trout ( Salvelinus namaycush ) in the lower four Great Lakes, where the species was extinguished in the 1950s and 1960s, have been largely unsuccessful. To avoid many of the problems believed to be contributing to this failure, the fishery management community recently established several sanctuaries in the offshore waters of the Great Lakes where the development and protection of self-sustaining stocks of lake trout would be a primary management objective. One of these, the Six Fathom Bank-Yankee Reef sanctuary, was created in the south-central portion of Lake Huron. This sanctuary covers 168,000 ha and includes the shallower portions of the Six Fathom and Ipperwash scarps, which are major bathymetric features in the southern half of the lake. Historical accounts describe Six Fathom Bank as the most important lake trout spawning ground in the lake. Here we present the results of lake bed surveys conducted in the sanctuary with side-scan sonar, underwater videocamera systems, and a small research submarine. Our observations of the lake bed are consistent with what is known of the bedrock stratigraphy, glacial history, and karst geomorphology of the Lake Huron basin. Most of the loose rock we found seemed to be derived from local carbonate bedrock formations, although non-carbonate rock probably from Precambrian sources to the north was also present in some areas. Much of the bedrock and loose rock displayed karst solution features described for the Bruce Peninsula on the Ontario shoreline. Our surveys revealed substantial areas of lake bed at water depths of 20–36 m that resembled suitable spawning and fry production habitat for the shallow-water strains of lake trout that are the focus of the rehabilitation effort. Low mid-lake nutrient levels documented recently by others and the extremely high abundance of Mysis relicta (an important item in the diet of young lake trout) that we documented on Yankee Reef also contributed to our evaluation of the sanctuary as a site with high potential to support a self-sustaining population of lake trout.

Journal of Great Lakes Research↗

USGS2012

Two-mica granites of northeastern Nevada

The field settings are described and analytical data are presented for six two-mica granites from north-eastern Nevada. High δ 18 O and 87 Sr/ 86 Sr values indicate that all are S-type granites, derived from continental crust. The major element chemistry and accessory mineral contents of these rocks also are characteristic of S-type granites. Chemical, X ray, and other data are presented for the micas recovered from these granites. The muscovites are notably high in Fe 2 O 3 , FeO, and MgO. Except for one hydrobiotite, each of the biotites has an MgO content near 6.0 weight percent. Two different types of two-mica granites are recognized in the area of this study. One type is distinguished by the presence of many biotite euhedra within muscovite phenocrysts and by an unusual suite of accessory minerals completely devoid of opaque oxides. This type probably resulted from anatexis of late Precambrian argillites under conditions of relatively low oxygen fugacity, along a line that roughly coincides with the westward disappearance of continental basement. In the other textural type of two-mica granite the micas are equigranular and there is a greater variety of accessory minerals. The magmatic evolution of this type also appears to reflect the influence of late Precambrian argillites; there may be age differences between the two types of two-mica granites.

Journal of Geophysical Research Solid Earth↗

USGS2010

Glimpses of East Antarctica: Aeromagnetic and satellite magnetic view from the central Transantarctic Mountains of East Antarctica

Aeromagnetic and satellite magnetic data provide glimpses of the crustal architecture within the Ross Sea sector of the enigmatic, ice-covered East Antarctic shield critical for understanding both global tectonic and climate history. In the central Transantarctic Mountains (CTAM), exposures of Precambrian basement, coupled with new high-resolution magnetic data, other recent aeromagnetic transects, and satellite magnetic and seismic tomography data, show that the shield in this region comprises an Archean craton modified both by Proterozoic magmatism and early Paleozoic orogenic basement reactivation. CTAM basement structures linked to the Ross Orogeny are imaged 50–100 km farther west than previously mapped, bounded by inboard upper crustal Proterozoic granites of the Nimrod igneous province. Magnetic contrasts between craton and rift margin sediments define the Neoproterozoic rift margin, likely reactivated during Ross orogenesis and Jurassic extension. Interpretation of satellite magnetic and aeromagnetic patterns suggests that the Neoproterozoic rift margin of East Antarctica is offset by transfer zones to form a stepwise series of salients tracing from the CTAM northward through the western margin of the Wilkes Subglacial Basin to the coast at Terre Adélie. Thinned Precambrian crust inferred to lie east of the rift margin cannot be imaged magnetically because of modification by Neoproterozoic and younger tectonic events.

Journal of Geophysical Research B: Solid Earth↗

USGS2017

Erosion of the Laurentide region of North America by glacial and glaciofluvial processes

Collection of seismic reflection data from continental margins and ocean basins surrounding North America makes it possible to estimate the amount of material eroded from the area formerly covered by Laurentide ice sheets since major glaciation began in North America. A minimum estimate is made of 1.62 ?? 106 km3, or an average 120 m of rock physically eroded from the Laurentide region. This figure is an order of magnitude higher than earlier estimates based on the volume of glacial drift, Cenozoic marine sediments, and modern sediment loads of rivers. Most of the sediment produced during Laurentide glaciation has already been transported to the oceans. The importance of continental glaciation as a geomorphic agency in North America may have to be reevaluated. Evidence from sedimentation rates in ocean basins surrounding Greenland and Antarctica suggests that sediment production, sediment transport, and possibly denudation by permanent ice caps may be substantially lower than by periodic ice caps, such as the Laurentide. Low rates of sediment survival from the time of the Permo-Carboniferous and Precambrian glaciations suggest that predominance of marine deposition during some glacial epochs results in shorter lived sediment because of preferential tectonism and cycling of oceanic crust versus continental crust. ?? 1985.

Quaternary Research↗

USGS2007

Differentiating pedogenesis from diagenesis in early terrestrial paleoweathering surfaces formed on granitic composition parent materials

Unconformable surfaces separating Precambrian crystalline basement and overlying Proterozoic to Cambrian sedimentary rocks provide an exceptional opportunity to examine the role of primitive soil ecosystems in weathering and resultant formation of saprolite (weathered rock retaining rock structure) and regolith (weathered rock without rock structure), but many appear to have been affected by burial diagenesis and hydrothermal fluid flow, leading some researchers to discount their suitability for such studies. We examine one modern weathering profile (Cecil series), four Cambrian paleoweathering profiles from the North American craton (Squaw Creek, Franklin Mountains, Core SQ-8, and Core 4), one Neoproterozoic profile (Sheigra), and one late Paleoproterozoic profile (Baraboo), to test the hypothesis that these paleoweathering profiles do provide evidence of primitive terrestrial weathering despite their diagenetic and hydrothermal overprinting, especially additions of potassium. We employ an integrated approach using (1) detailed thin-section investigations to identify characteristic pedogenic features associated with saprolitization and formation of well-drained regoliths, (2) electron microprobe analysis to identify specific weathered and new mineral phases, and (3) geochemical mass balance techniques to characterize volume changes during weathering and elemental gains and losses of major and minor elements relative to the inferred parent materials. There is strong pedogenic evidence of paleoweathering, such as clay illuviation, sepic-plasmic fabrics, redoximorphic features, and dissolution and alteration of feldspars and mafic minerals to kaolinite, gibbsite, and Fe oxides, as well as geochemical evidence, such as whole-rock losses of Na, Ca, Mg, Si, Sr, Fe, and Mn greater than in modern profiles. Evidence of diagenesis includes net additions of K, Ba, and Rb determined through geochemical mass balance, K-feldspar overgrowths in overlying sandstone sections, and K-feldspars with reaction rims in weathered basement. The sub-Cambrian paleoweathering profiles formed on granite are remarkably similar to modern weathering profiles formed on granite, in spite of overprinting by potassium diagenesis. ?? 2007 by The University of Chicago. All rights reserved.

Journal of Geology↗

USGS2019

Leakage and increasing fluid pressure detected in Oklahoma's wastewater disposal reservoir

The Arbuckle Group is the principal reservoir used for wastewater disposal in Oklahoma. In Osage County—a seismically quiet part of the state—continuous measurements of fluid pressure reveal that pressure in the reservoir is increasing by at least 5 kPa annually and sometimes at a much higher rate. Tidal analysis reveals that fluid level changes lead the local strain tides, with no apparent influence from transient permeability changes; this indicates a response that is inconsistent with flow in a radially extensive, confined reservoir. We investigate whether this is due to vertical flow to the water table, vertical flow within the Arbuckle, or local distortions from fractures. While none of these alternative models can fully explain both the observed tidal phases and amplitude ratios, the observed response to teleseismic waves supports a mechanism related to leakage rather than fracture effects. At this location fluid influx associated with wastewater disposal is offset by migration into surrounding layers, which include the Precambrian basement below. Thus, our findings suggest the need to monitor for changes in the induced seismicity hazard, while pore pressures increase in a leaky disposal reservoir.

Oklahoma↗

USGS1996

Earthquakes and the southeastern boundary of the intact Iapetan margin in eastern North America

Earthquakes at three localities in eastern North America have been attributed on geological and seismological grounds to compressional reactivation of some of the late Proterozoic or early Paleozoic normal faults in the northeast-trending Iapetan passive margin. Assessment of seismic hazard can be aided by identifying the boundaries of the area of Iapetan faulting. A previous paper located the northwestern boundary. This report interprets deep seismic-reflection profiles as showing that the margin comprises a seismically active northwestern part, where Precambrian crust contains some Iapetan faults but remains mostly as it was formed, and a southeastern part, where later deformations likely destroyed or modified the Precambrian crust and Iapetan faults. Accordingly, the boundary between the northwestern and southeastern parts of the margin, which coincides approximately with the Appalachian gravity gradient, can be taken as the southeastern limit of potentially seismogenic Iapetan faults.

eastern North America↗