Jack Edward Harrison

Precambrian Belt Basin of Northwestern United States: Its Geometry, Sedimentation, and Copper Occurrences

Geometry and provenances of the Belt basin at various stages of its 600-m.y. time span (between about 1,450 to 850 m.y. ago) are decipherable for the middle and upper parts of the Ravalli Group, the Helena-Wallace Formations, and the lower part of the Missoula Group. The Revett and overlying St. Regis Formations (middle and upper parts of the Ravalli Group) had a cratonic source in the south and southwest and were deposited in troughs that reflect the west-northwest trend of the Osburn fault zone (or the Lewis and Clark line whose west-central part includes the Osburn fault zone). The Spokane Formation, correlative of St. Regis, had a source in the Canadian Shield and was deposited in a north-to northwest-trending trough in northwestern Montana, west of, but approximately parallel to, the north-northwest trend of the much younger Montana disturbed belt. The Helena and Wallace Formations were deposited in a broad simple northwest-trending basin between the previous troughs, which received clastic sediments from both the east and southwest, and in which extensive carbonate deposits formed on the eastern shelf. Tectonic adjustment in early Missoula time resulted in a long low dome in the Idaho Panhandle area and in the rejuvenation of the north-northwest-trending trough in the east. Clastic sediments containing abundant hematite were deposited on Helena-Wallace carbonate rocks, and Purcell Lava was poured out in the northeast. Anomalously high amounts of copper (100 or more ppm) are scattered throughout thousands of square miles of Belt terrane. The copper occurs in almost all formations, but it is most common in green strata. This mode of occurrence suggests a syngenetic or diagenetic origin. Stratabound copper ores, however, are known only as disseminations, discrete blebs, and veinlets in white quartzites and siltites of the Revett Formation in a “copper sulfide belt” along the northwestern Montana border. This belt is perpendicular to the Revett trough but parallel to the dome formed in early Missoula time, suggesting post-Revett epigenetic reconcentration of copper. Geochronologic data as interpreted by several authors are in apparent conflict with geologic history of the basin as read from the sedimentation record. Many of the conflicts can be resolved by reinterpretation of existing geochronologic and geologic data. It is obvious, however, that the dating of many events, including those that may have caused migration of copper, is in need of refinement.

Phanerozoic thrusting in Proterozoic belt rocks, northwestern United States

Newly identified listric thrust faults show eastward translation north of the Lewis and Clark line across all 800 km of Belt terrane from Spokane, Washington, on the west to Glacier National Park on the east. Right-lateral slippage on the line was accompanied by clockwise rotation on the thrusts. These movements were probably in response to the complex plate interactions that began about 200 m.y. ago along the western continental margin. Gravity and magnetic data suggest that basement rock is involved in the thrusting. Reconcentration of strata-bound copper sulfides along bedding-plane shears may help form ore near one thrust. West of the Rocky Mountain Trench, Phanerozoic strata that elsewhere have oil and gas potential may have been stepped up by the thrusting, and eroded, rather than extending beneath most of the thrust belt.

Correlations and Problems in Belt Series Stratigraphy, Northern Idaho and Western Montana

A continuous strip of geologic maps has recently been completed along the Idaho-Montana state line between Clark Fork, Idaho, and Superior, Montana. New stratigraphic and petrographic information provides the basis for stratigraphic correlations and for the interpretation of facies changes in this part of the basin of deposition of the Precambrian Belt Series. Identification of facies changes is aided by the recognition of siltite (low-grade metamorphosed siltstone) as a valid rock type, in addition to quartzite and argillite, to classify most of these rocks and to establish mappable units. The older Belt rocks (Prichard through Wallace Formations) were deposited in a trough whose axis trended northwestward, perhaps approximately through Libby, Montana. Subtle facies changes suggest an ancient shore line southwest of the Coeur d9Alene district, Idaho, perhaps near the present exposed edge of the Idaho batholith. The younger Belt formations of the Missoula Group, however, thicken markedly southeastward toward Superior. Thus, the younger Belt rocks were deposited in a trough whose main axis was about at right angles to that of the older trough and perhaps was near Missoula, Montana. Deposition in the Clark Fork area was scant and often interrupted in Missoula time. The young major cross-warp in the old Belt geosyncline undoubtedly has contributed complexities to Belt correlation farther north. Detailed petrographic studies indicate that the Belt rocks from the Pend Oreille area, Idaho, are remarkably uniform in the mineralogic composition of similar rock types throughout 40,000 feet of strata. Further mineralogic studies are required to determine whether this uniformity is local or widespread.

Audio-frequency magnetotelluric and gravity traverse across the crest of the Purcell anticlinorium, northwestern Montana

A series of audio-frequency magnetotelluric (AMT) and gravity measurements were made across the Purcell anticlinoriurn of the Belt basin in Montana. The purpose of the study was to gain information about the electrical properties of the Belt Supergroup strata before using techniques that probe deeper but are more expensive. The Bouguer gravity data indicate an anomaly of about 20 mgal that can be modelled as a block uplifted 11 km above a flat basement surface that lies at a depth of 17 km. The AMT measurements did not penetrate to the crystalline basement, but they did delineate several intermediate structural features, including a deep layer of unusually high conductivity. Both the uplifted block and the conductive unit are positioned beneath the crest of the anticlinoriurn, approximately beneath the outcrop of the Prichard Formation.

Precambrian Folding in the Idaho Springs-Central City Area, Front Range, Colorado

Metasedimentary gneisses in the Idaho Springs-Central City area of the Front Range have been deformed twice in Precambrian time. The older and major deformation was plastic folding; it was accompanied by intrusion of a series of plutons and sheets, recrystallization of the meta-sedimentary rocks, and metamorphism of the earlier members of the igneous series. It produced a major fold system consisting mainly of open, but disharmonic asymmetric and upright anticlines and synclines whose axes trend sinuously north-northeast and are spaced 1-2 miles apart. Over most of the area the fold axes are nearly horizontal or plunge at low angles, but in the southern part the axes plunge steeply northeastward. Small folds and a well-developed mineral alignment characteristically parallel the major fold axes ( B o ); small-scale folds, boudinage and sparse mineral alignment are present in the A o direction. The younger deformation was dominantly cataclastic and restricted chiefly to a 2-mile-wide zone in the southeast part of the area. Within this zone small folds were developed locally in the relatively incompetent rock masses, and intense granulation was developed locally in the more competent units. Cataclastic products are pervasively distributed, however, through all the rocks in the zone. The younger folds are mainly terrace, monoclinal, and chevron types; the largest has a breadth of about 400 feet. These folds trend N. 55° E., are remarkably straight, and plunge at various angles, largely depending upon their position on the older, larger folds. They consistently are strongly asymmetric and show their northwest limbs raised structurally. Associated with these folds are two lineations, one (B y ) parallel to the fold axes and one (A y ) oriented at about 80° to the fold axes. The younger deformation is a manifestation of extensive Precambrian shearing defined by a zone of intense cataclasis that extends both northeast and southwest of this region.

Libby Thrust Belt and Adjacent Structures--New Factors to Consider in Thrust Tectonics of Northwestern Montana: ABSTRACT

About 40 mi (65 km) west of the Rocky Mountain trench and at least 9 mi (15 km) above the sole detachment of the Rocky Mountain thrust belt is a zone of Cretaceous-Tertiary thrust faults up to 25 mi (40 km) wide in middle Proterozoic and Cambrian rocks. This zone (the Libby thrust belt) extends northward from the Lewis and Clark line to the northwest corner of Montana. Within the Libby thrust belt is a series of complex ramps, horsts, splays, and folds that accommodate a tectonic shortening End_Page 850------------------------------ of about 6.2 mi (10 km). Backsliding has occurred on some listric thrust faults, and middle Tertiary(?) extensional horst-and-graben faults offset or join most thrust faults. On the east, the lead thrust ramps up onto the broad open Purcell anticlinorium. On the west, the Libby thrust belt is overridden in the north by the lead thrust of the Yaak plate (whose central part is the broad, open Sylvanite anticline), and in the south, it is overridden by the Moyie thrust (which trends northwest and also overrides the west edge of the Yaak plate). An essentially continuous section, 46,000 ft (14,021 m) thick, of Belt rocks is displayed on the south-plunging Sylvanite anticline. The base is not exposed, and the top is eroded. A section of similar thickness exists on the west flank of the Purcell anticlinorium, where the Belt Supergroup is overlain by about 3,000 ft (914 m) of Cambrian rock. The Cambrian occurs in the broad synclinal Libby trough that is paired with the Purcell anticlinorium, and these Cambrian strata are also caught up in the Libby thrust belt. Geologic cross sections suggest that the Belt rocks have overridden the Cambrian at shallow depths only and that Cambrian and younger Phanerozoic strata probably do not occur at greater depths beneath and west of the Purcell anticlinorium. This interpretation differs significantly from interpretations that suggest intercalation of major wedges of Paleozoic and Belt rocks at depth in this same area. End_of_Article - Last_Page 851------------