Quartzite Beds Research Articles

Stratigraphy and Structure at the North End of the Taconic Range in West-Central Vermont

The stratigraphy of the Taconic sequence at the north end of the Taconic Range in west-central Vermont has been revised as follows: Pawlet formation: interbedded black slate and greywacke; Middle Ordovician Mount Hamilton group: undivided black, gray, green, and red argillite, with minor limestone, limestone conglomerate, and ankeritic quartzite; Upper Cambrian to Middle Ordovician West Castleton formation: black slate and phyllite, with a limestone unit near the base; Lower Cambrian Bull formation: Lower Cambrian (1) Mettawee facies. Purple and green slate, mudstone, and phyllite; this facies constitutes the bulk of the formation. (2) North Brittain conglomerate member. An intraformational limestone-pebble, slate-matrix conglomerate (3) Mudd Pond member. A thin but persistent orthoquartzite (4) Zion Hill member. A discontinuous graywacke or sub-graywacke unit ranging from a pebble conglomerate to a mudstone (5) Bomoseen member. A massive, olive-drab, medium-grained graywacke Biddie Knob formation: a chloritoid-bearing purple and green slate and phyllite with minor beds of limestone and quartzite. Lower Cambrian (?) The sense of the Lower Cambrian succession is given by load casting and graded bedding in the Zion Hill, and in rare arkoses mapped with the Bomoseen. The sense of succession in the Mount Hamilton group is given by cross-bedding, graded bedding, and by channel filling. Structurally, the Taconic sequence consists of nested thrust slices subsequently folded together. Although each slice has its own characteristic stratigraphy and structure, correlation of the units, down to the formational level, presents little difficulty. The structure is typified by the Giddings Brook recumbent bottoming fold in the area between the Taconic Range and Lake Bomoseen. The fold is at least 5 miles in amplitude, the movement was cast to west, and the axial plane is nearly horizontal. Digitations on this major structure explain the map pattern. Key units of the Taconic sequence have been recognized east of the Taconic Range; the east flank of the Range is now underlain by an inverted sequence of Lower Cambrian rocks. The Sudbury nappe at the north end of the map area is composed of the Lower Ordovician Chip-man formation. The structure does not root in the east limb of the Middlebury synclinorium as reported by Cady, but is a thrust slice, whose southeast edge tucks under the Taconic sequence. Structurally, this unit is part of the latter. The Whipple marble at the north end of Whipple Hollow is part of the Lower Ordovician Bascom formation, which in this area has been moved westward over the younger black phyllite through a recumbent fold involuted into the Taconic sequence. This structure indicates the persistence of intense deformation after the development of the Taconic structure. Topologically, the Taconic sequence is in the southward continuation of the Middlebury synclinorium. Coupled with the discovery of the inverted Lower Cambrian rocks east of the Taconic skyline, the evidence supports an allochthonous origin of the Taconic sequence. The gradational contact between the green and black phyllites east of the Taconic Range, and the presence in the black phyllite of beds characteristic of the Taconic sequence show that part of the black phyllite here is allochthonous. The thrusting is dated as Late Trenton on the basis of exotic blocks of Taconic rocks in an autochthonous black slate at Forbes Hill. Similar rocks have also been found elsewhere at the periphery of the Taconic sequence. The thrusting may have occurred as submarine gravity slides of soft rocks; slump structure is indeed abundant in the Taconic sequence. The intimate mixing of the black slates of the Taconic sequence and of the Trenton mud may make the mapping of the Taconic fault unfeasible even in principle. Continued deposition of black mud after the emplacement of the Taconic nappes could account for the reported Trenton unconformity at the margin of the Taconic sequence south of the Castleton area. The Taconic sequence probably was deposited in the area of the present Green Mountains. The upper part of the Mount Hamilton group correlates by fossils with the Moretown-Cram Hill formations of eastern Vermont. The Mount Hamilton group is free of volcanic rocks and is thin, in contrast with the Moretown-Cram Hill formations; this change is consistent with paleogeographical requirements, as the Cambro-Ordovician, typically eugeosynclinal deposits of eastern Vermont, must somehow change into the synchronous, but typically miogeosynclinal deposits of western Vermont and eastern New York.

DISCONFORMITY BETWEEN LOWER AND UPPER CRETACEOUS IN WESTERN COLORADO AND EASTERN UTAH

In a part of western Colorado and eastern Utah the contact between the Burro Canyon formation (Lower Cretaceous) and the Dakota sandstone (Upper Cretaceous) is a broadly undulating erosional surface. Angular and subangular pebbles, cobbles, and boulders of sandstone, chert, and quartzite derived from the Burro Canyon have been found in the basal part of the Dakota sandstone. These fragments and a thin quartzite bed at the top of the Burro Canyon indicate that the formation was lithified, probably under subaerial conditions, before its erosion and before deposition of the Dakota sandstone. On the basis of fossil plants R. W. Brown (1950) placed the boundary between the Lower and Upper Cretaceous at the top of the Burro Canyon. The field evidence described here supports this division and further suggests a hiatus between the deposition of Lower and Upper Cretaceous sediments in part of the Colorado Plateau.

A re-investigation of the Steall area, Upper Glen Nevis

The outcrop of the Glencoe Quartzite near Steall is extremely sinuous, and published sections across the area show the Quartzite to be recumbently folded. An examination of the attitude of the current-bedding within the Quartzite has shown that a hypothesis based solely on recumbent folding is untenable. The folds of the Glencoe Quartzite are now thought to be accompanied by slides, and an excellent exposure is described in which one such slide can be seen to truncate successive beds of Quartzite.

Ordovician Stratigraphy in Marathon Folded Belt, West Texas

Stratigraphic study of little known Ordovician exposures south and east of the Marathon uplift, West Texas, shows that a facies change exists from predominantly shaly beds of the Marathon region to more arenaceous beds in the Solitario and Dove Mountain quadrangle areas. Discovery of graptolites in a few incomplete measured sections indicates that probably Dagger Flat type of sandstone rises in the stratigraphic sequence to replace, in part, the lower Marathon limestone and shale. The upper Marathon and Alsate shale formations of the Marathon anticlinorium are represented, in the exposures considered, by sandy laminated limestones and possibly by overlying beds of massive quartzite. Typical Fort Pena and Woods Hollow formations (Middle Ordovician) of the Marathon uplift a e also present in the Solitario and Dove Mountain areas. The Maravillas is present but thin, and is overlain by about 25-40 feet of shale, probably of Late Ordovician age; the latter is here named the Persimmon Gap formation. Regional correlation shows close similarity between the sections studied and the McCurtain County, Oklahoma, exposures which also contain Early Ordovician sandy beds at approximately the same stratigraphic position. The Ordovician section of the interior (southeasternmost) folds of the Ouachita-Marathon system suggests that orogenic activity occurred in the system in Ordovician time and that this long enduring mobile belt may be considered an early and middle Paleozoic geosyncline despite its comparatively thin stratigraphic sequence.

GEOLOGY OF THE BROKEN HILL ORE DEPOSIT, BROKEN HILL, N.S.W., AUSTRALIA

The lead-silver-zinc ore deposit of Broken Hill, New South Wales, is among the great ore deposits of the world because of its size, richness, and continuity. Up to the end of 1946 approximately £50,000,000 in dividends had been won from recoverable metals worth £210,500,000 gross contained in 63,800,000 tons of ore. The deposit is a hypothermal deposit of Precambrian age resulting from the selective replacement of two closely adjacent, tightly and complexly folded stratigraphic rock layers. The original sedimentary rocks of the area now consist of tightly folded sillimanite-garnet gneisses with subordinate thin quartzite beds. These contain numerous folded sills of augen gneiss (granite), amphibolite (gabbro), and pegmatite. Post-folding peridotite, granite, and pegmatite occur. Probably following considerable uplift and erosion, thin dikes of diabase (dolerite) were intruded, followed by pegmatite dikes and silicifying solutions, and finally by ore-bearing solutions. The folds of the region are tight, steeply inclined, and extremely complex structures resulting from plastic deformation. Individual minor folds were studied in great detail by a method of axial-plane and axial-line analysis. An angular relationship exists between minor and major folds due to strain under torsional stresses. The regional pitch is flatly south. However, sudden reversals of pitch and divergences of pitch in adjacent folds are common. “Second-order” folds or folded folds exist. Cutting and offsetting the folds are “buckles” with vertical axes and crush zones of schisted rocks resulting from post-folding but pre-ore faulting movements. The lode occurs in a “belt of attenuation” between a wide arch on the west and wide basin on the east. It consists of massive lead-zine-sulphide replacement ore bodies forming (before erosion) a long continuous, irregularly shaped, flat, curving pencil of ore, 2000–3000 feet high and 300 feet thick. In longitudinal section it describes a flat arc pitching downward at each end. The “lead lodes” resulted from the selective replacement of two closely adjacent highly folded favorable beds. Each lode is distinguishable by its gangue mineralogy and metal ratios. No. 3 Lens (the lower) has fluoritic-rhodonitic gangue and comparatively high Zn:Pb and Ag:Pb ratios. No. 2 Lens, the upper, has calcitic gangue and comparatively low Zn:Pb and Ag:Pb ratios. At least three “zinc lodes”, with similar mineralogy, occur at higher stratigraphic horizons at the south end of the field. There is little observable zoning. Ore solutions are believed to have migrated up the regional pitch from the south. Intra-mineralization fracturing helped to localize ore shoots within favorable formations.

A copper deposit of the Ducktown type near the Coeur d'Alene district, Idaho

A copper deposit about 40 miles southwest of the Coeur d9Alene district, Idaho, shows a marked structural and mineralogical resemblance to the copper deposits of the Ducktown type. It occurs as a replacement along bedding plane fractures and fissures in quartzitic beds of the Belt series and its ore consists chiefly of pyrrhotite, chalcopyrite, and lesser cubanite in a silicate and carbonate gangue. Fracturing repeated several times during mineralization caused deposition in three distinct mineral stages, shown by the partial replacement of one group of minerals by later ones. Minerals deposited during the first stage include the silicates (diopside, tremolite, actinolite, biotite, epidote, tourmaline, microcline, and muscovite), and a little quartz; during the second stage, the carbonate (ferriferous dolomite); and during the third stage, the sulphides (pyrrhotite, chalcopyrite, arsenopyrite, and cubanite), and a little quartz and calcite. Except for its mode of occurrence, the deposit possesses most of the characteristic features of a pyrometasomatic deposit but is, nevertheless, a hypothermal deposit formed probably under exceptionally intense temperature conditions.

Sienna ("ocher") deposits of the Cartersville District, Georgia

Sienna (ocher) deposits in the Cartersville district, Georgia, occur in the contact zone between dolomitic limestone and underlying quartzite of Lower Cambrian age. These rocks, together with interbedded schists, were folded and recrystallized probably near the close of Paleozoic time. Simple fissures and transverse and oblique faults, formed during the late stages of folding, admitted hydrothermal solutions, which deposited concentrated masses of hematite and pyrite, and other ore minerals, in the dolomites and calcareous schists. There is some evidence that large bodies of iron carbonate also were deposited.Subsequent weathering has removed the dolomite near the surface, and its residuum in many places rests upon the more resistant quartzite. Iron derived from the hematite and pyrite, and possibly from iron carbonate, was transferred by meteoric waters and deposited as hydrous oxides by partial replacement of (1) irregular, structureless bodies of the residuum, and (2) irregular areas in schists both at the base of the dolomite and beneath the uppermost quartzite bed. A relatively thin superficial portion of the uppermost quartzite bed also was replaced. Transfer of the iron has taken place over short distances only, and the present positions of the sienna deposits reflect essentially the positions of the deposits of source minerals.

THE GEOLOGY OF CEYLON

The Island of Ceylon is composed almost entirely of crystalline rocks of Archean or pre-Cambrian age. In the extreme north these are overlain by limestones of Miocene or later age. At one point on the northwest coast a very small area of Jurassic rocks lies on the surface of the coastal plain.The ancient crystalline rocks resemble in many respects the Grenville series of the Canadian Shield. They are largely biotite gneisses, interstratified with which are at least two great beds of white crystalline limestone, often dolomitic, with associated beds of quartzite and of sillimanite-bearing rocks of undoubted sedimentary origin. Charnockite and allied rocks are present also in large amount, especially in the higher parts of the island. The petrography of these rocks is described, and is illustrated by a series of chemical analyses, the first that have been made of the rocks of this island. In structure Ceylon is a portion of a great syncline, deeply eroded, closed on the south, open to the north, where it plunges beneath the Miocene cover.Three distinct plains of denudation can be clearly recognized in Ceylon. The coastal plain, about 100 ft. above sea-level, a second plain at about 1,600 ft., and a third at 6,000 ft. above sea-level.The paper presents the first account which has been given of the geological structure of the island, and the accompanying geological map of the island is the first which has been prepared.

On Certain Bodies, apparently of Organic Origin, from a Quartzite Bed near Inveraray

The magnificent sections of the Archæan and Palæozoic rocks which are presented on the north-west coast of Scotland, have long been known to all British geologists. They occur principally in the counties of Sutherland and Wester Ross, from Cape Wrath on the north to Loch Kishorn on the south. They are not less striking in an artistic, than they are instructive in a scientific point of view. The height of the mountains, their abrupt and precipitous forms, and the thinness, or almost total absence of, superficial covering, are characteristics which combine with great variety and richness of colouring, to produce scenery which stands altogether by itself

On some Points in the Geology of South Africa

Abstract T he author had observed in Namaqualand the occurrence of horizontal siliceous beds, covering other siliceous inclined beds, the silicification of the latter being apparently due to the infiltration of silica from the upper quartzose beds into the inclined beds below. In this communication Dr. Rubidge details the evidences that he observed of such a process having taken place, and points out how the observations on some of the Namaqualand rocks by Mr. Bain, Mr. Bell, and Dr. Atherstone, respectively, tend to support his views on this point. The inclined beds of this district are gneissic, and, in the instances referred to, very quartzose. The horizontal sandstones of this district he correlates with the Table Mountain sandstones*; he has found in them a few obscure traces of fucoidal or other plants†. The author then passes on to the Cape district; and, first offering his testimony to the industry and general exactitude of Mr. Bain as a geologist, he proceeds to compare Mr. Bain’s section of Mitchell’s Pass‡ with a section which he lately made for himself on two hasty journeys. Mr. Bain figures the indurated sandstone or quartzite of Mitchell’s Pass as being at first horizontal, and then suddenly dipping at a strong angle northward, so as to underlie the Devonian schists of the Bokkeveld, at Ceres, and to divide them from the slates of the Cape district. Dr. Rubidge points out the apparent difficulty of explaining such an inclination of the quartzite, the slates underlying both the inclined and horizontal portions not presenting any evidence of a difference in dip; and he suggests that the inclined beds of quartzite have nothing to do with the horizontal quartzite, except that, being immediately beneath the horizontal siliceous beds, they have by some means been silicified from above, and thus rendered in appearance identical.