Precambrian Deposits Research Articles

Precambrian tectonics and mineral deposits, Lake Superior region

Precambrian tectonics and mineral deposits, Lake Superior region

Stratigraphy of upper Precambrian and lower Paleozoic deposits of the Chersk range

Stratigraphy of upper Precambrian and lower Paleozoic deposits of the Chersk range

Possible late Precambrian subduction zone in South West Africa

CONTINENTAL margin plate tectonism during the late Precambrian in southern and central South West Africa and western Botswana is strongly suggested by the geographical distribution, stratigraphy, composition and evolutionary characteristics of the pre-Damara, pre-Nama formations (1,350–900 Myr old) occurring north of the Namaqua Tectonic Province1. If the distribution of these units is considered it is apparent that a unique pattern exists (Fig. 1). The data for the generalised geological map of Fig. 1 are taken from various sources2–9 and show : first, the distribution has the form of a prominent and extensive curvilinear feature or ‘arc’. Second, the arc separates the undeformed late Precambrian deposits (Nama Group10) of the stable platform area—the Kalahari Craton or Plate—from the highly deformed and metamorphosed deposits of the late Precambrian Damara orogenic belt, suggesting that the arc occupies a marginal cratonic position, and third, the trend of the arc and arc structures (such as major faulting) parallels that of the Damaran structural trends, suggesting a possible relationship between the geotectonic development of the arc and the Damaran Orogeny (see Fig. 1).

Late Precambrian Shallow Water Deposits, Banks and Victoria Islands, Arctic Archipelago

The upper Precambrian succession of the southern tip of Banks Island comprises about 900 m of sedimentary rocks intruded by thick gabbro sills. The lowest unit exposed is a cherty stromatolitic carbonate. This is overlain by a thicker unit of sandstones and mudstones. Most of the sequence is considered to be correlative with the Glenelg Formation, which is the basal formation of the late Precambrian Shaler Group of the Minto Arch on Victoria Island. A thick shale unit at the top of the Banks Island Precambrian succession may be correlative with the basal part of the Reynolds Point Formation on Victoria Island.Paleocurrent measurements from the Banks Island rocks indicate transport essentially to the northwest. An abundantly cross-bedded sandy limestone unit in the upper part of the Reynolds Point Formation in the Minto Inlet area of Victoria Island gives a polymodal paleocurrent pattern. This unit was probably deposited in a shallow marine environment under tidal influence. Tidalites have been recognized in the lower clastic unit of the Reynolds Point Formation at several localities in the Minto Arch. Stromatolites form the lower part of the Shaler Group suggest that these rocks are early Upper Riphean (latest Helikian to early Hadrynian). The stromatolite forms appear to occur in a non-repetitive sequence that provides a powerful means of intrabasinal correlation.The depositional area is envisaged as a large embayment of the sea, comparable in scale, shape, and sedimentary fill, to the younger Michigan Basin. The northwest margin of the basin is tentatively interpreted as a breached arch, shedding terrigenous clastics to the NW in the Banks Island area, and to the east at the north end of the Minto Arch. Northeasterly elongation of stromatolites in the upper Glenelg Formation at the north end of the Minto Arch may reflect tidal currents. Circulation in the basin became progressively restricted, culminating in deposition of the evaporite-bearing sequence of the upper part of the Shaler Group.

LATE PRECAMBRIAN FORMATIONS IN THE WESTERN RICHTERSVELD, NORTHERN CAPE PROVINCE

Summary A detailed reinvestigation of rock relationships in the western Richtersveld led to a new stratigraphic classification of the Late Precambrian deposits, which were found to belong to one continuous cycle of geotectonic activity in the Orange River area and which are now grouped together in the Gariep Group. The development of a geosynclinal basin started with basement shearing and subsequent deposition of the mixed sedimentary-volcanic Stinkfontein Formation. Further subsidence led to the accumulation of mixed clastic-carbonate shoreline and shelf deposits of the Hilda Formation, followed by the clastic Numees and Helskloof formations. At the same time basic to andesitic volcanics of the Grootderm Formation were extruded in the eugeosynclinal basin, accompanied and followed by turbidite deposits of the Holgat and Oranjemund formations. Gravity deformation took place during and after deposition of the Gariep strata and was succeeded by orogenic activity which culminated in compressional eastward-di...

An Alluvial Plain and Lacustrine Model for the Precambrian Witwatersrand Deposits of South Africa

ABSTRACT A depositional model is presented for the gold-rich Precambrian deposits within the Witwatersrand basin. It is a closed basin inset in a now stable craton of the southern African Shield with an areal extent of approximately 39,000 km2 The clastic fill has a maximum thickness of 7,500 m and comprises laterally persistent conglomerates, sandstones and shales occurring within thick (up to 800 m), widespread, and continually alternating coarser and finer-grained sequences. The succession as a whole coarsens upward and is divided into a sandstone and shale dominated lower division and a sandstone and conglomerate dominated upper division. Mineralogical maturity also increases upward, the sandstones of the lower division being mainly sub-greywackes, those of the upper division be ng feldspathic quartzites. Analysis of the abundant sedimentologic data gathered during exploration and development of the gold deposits demonstrates that sedimentation within the basin fits a braided alluvial plain and lacustrine model. The origin and lateral relationships of the different rock types are explained by this depositional setting. The thick alternating coarser and finer grained sequences are explained by a shifting depositional system (sourceward and basinward) controlled by tectonism in the basin and source area. The upward increase in mineralogical maturity and grain size is explained by migration inward of the basin margins, contemporaneous with sedimentation, causing marginal reworking of the lower division sequence as well as regression of the sedimentary succession. The lower division become a distal environmental equivalent of the upper division.

Discovery of microfossils in Upper Precambrian tillites, sub-tillitic and supra-tillitic deposits of the Taoudenni syneclise (West Africa)

(1974). Discovery of microfossils in Upper Precambrian tillites, sub-tillitic and supra-tillitic deposits of the Taoudenni syneclise (West Africa) International Geology Review: Vol. 16, No. 1, pp. 80-82.

Oil-gas potential of Late Precambrian deposits

The available materials on the distribution of Precambrian nonmetamorphosed sedimentary formations and the naphthide occurrences associated with them show that oil and gas potential of the Late Proterozoic deposits can and should be regarded as an important and potentially profitable matter of great scientific and economic significance. This paper reviews distribution and geochemical properties of the Precambrian rock in Asia, Africa, Australia, Europe and the Americas. The main emphasis is on distribution in the USSR. The practical solution to the problem will expand the potential oil and gas reserves of the sedimentary mantel, both in known oil and gas basins of the USSR, and new basins located in the areas of ancient platforms.

Genesis of Precambrian banded iron deposits, Rio Arriba County, New Mexico; a reply

Comments on a paper by D. F. McLeroy (for reference, see this Bibliography Vol. 34, No. 6, 02 E70-17936)

Genesis of Precambrian banded iron deposits, Rio Arriba County, New Mexico; a reply

Genesis of Precambrian banded iron deposits, Rio Arriba County, New Mexico; a reply

Genesis of Precambrian banded iron deposits, Rio Arriba County, New Mexico

Genesis of Precambrian banded iron deposits, Rio Arriba County, New Mexico

Microfossils of Lower Cambrian and Precambrian deposits in eastern Siberia

Early Cambrian and Late Proterozoic deposits in the south of eastern Siberia (Irkutsk amphitheatre) contain a large quantity of microfossils in carbonate and terrigenous rocks. Numerous finds of fossils are particularly important for the terrigenous part of the Uschakovsk and Motsk Formations. The detected microfossils change sharply in vertical section. Fossils of the Uschakovsk and Motsk Series have wide horizontal spreading, not only within the limits of Siberia. Analogous assemblages have been found in the Russian Platform. This makes it possible to correlate the Uschakovsk Series to Upper Riphean deposits of the Serdobs Series of the Riysan—Pachelm Trough. The Middle Motsk complex is close to the Gdowsk Sandstone complex in the northwest of the Russian Platform. The Upper Motsk deposits contain Lower Cambrian microfossils. The microfossils of the same age have also been found in the Bellskaja, Bulaiskaja and Angarskaja Formations.

The Precambrian magnetite deposits of New York and New Jersey

A number of lines of evidence support the view that the gneissic host rock of the iron deposits is a product of granitization and that the iron deposits were formed by release of iron from ferromagnesian silicates during regional metamorphism. (See Buddington, ibid., v. 61, p. 484-510).

The Precambrian magnetite deposits of New York and New Jersey

Magnetite, magnetite with associated hematite, and magnetite skarns form grossly conformable tabular deposits that have the forms of deformed sedimentary deposits. The host rocks, however, include gneisses of diverse composition and amphibolite. The iron oxides now present are late minerals that have replaced their host rocks. If the iron is primarily of sedimentary origin, the original rocks must have had compositions significantly different from those of known sedimentary iron deposits, even with allowance for significant later modification by hypogene solutions. 'An origin by direct contributions and emplacement by magmatic emanations has most support.'

Sulfur isotopic analyses from Park County, Montana

Sulfur isotope ratios from Precambrian gold-arsenopyrite ore deposits in schist have a narrow range and show slight enrichment in the heavier isotope. The ratios suggest a magmatic hydrothermal origin.

Remarks on the origin of Precambrian mineral deposits in Egypt (U. A. R.)

Remarks on the origin of Precambrian mineral deposits in Egypt (U. A. R.)

Syngenetic pyrite associated with a Precambrian iron ore deposit

The Steeprock pyritic zones are conformable, lenticular bodies in and along the hanging wall of an iron ore-bearing member of a Precambrian volcanic-sedimentary sequence in north western Ontario. The zones contain about 50 percent pyrite. About 90 percent of this pyrite consists of fragments possessing coloform and cryptocrystalline textures. This type of pyrite does not replace any minerals or rocks, but is veined and replaced by later crystalline pyrite, goethite, and hematite. A trace element study reveals a marked paucity in the number and quantity of the trace elements present. This, combined with the low cobalt content, low cobalt-nickel ratios, presence of arsenic in the pyrite, and the negligible selenium content, indicates that, with respect to the trace element content, the Steeprock pyrite is more akin to known sedimentary pyrite than it is to hydrothermal pyrite. The Steeprock pyrite, contrary to other published opinions, is regarded as a syngenetic sedimentary deposit of Precambrian age precipitated in restricted basins under anaerobic conditions. Iron is considered to have been derived through chemical weathering of a nearby ferruginuous limestone-dolomite member of the Steeprock group; sulfur is thought to have been contributed by anaerobic bacteria and volcanic activity that is clearly closely related in time and space to the pyrite.

Visings öseriens utbredning i trakten av finnerödja

Abstract The Visingsö Series in the Finnerödja Region. The work is based on more than one hundred pebble analyses carried out in the Finnerödja region in the summer 1960. The intention was to explore the occurrence of sandstone bedrock east of Lake Skagern in the northwestern part of the province of Västergötland. The affinity of the Finnerödja sandstone to the Visingsö Series is not completely proved but there are strong evidences indicating it to be contemporary with the Late Precambrian deposits in the Vättern depression. Data are presented in maps that picture the occurrence of sandstone pebbles. The areas covered by sandstone are probably rather small. In addition to isolated occurrences only the depression extending from Lake Skagern toward the community of Finnerödja seems to contain Visingsö Sandstone. The results corroborate previous assumptions concerning the occurrence of sandstone bedrock in the area investigated.

STRUCTURES IN SHALLOW‐WATER PRECAMBRIAN SEDIMENTS FROM NORTHWEST SCOTLAND

The paper deals with sedimentary structures in shallow‐water Precambrian deposits of Lower Torridonian age in the island of Skye in northwestern Scotland. The structures described include irregular lamination, ripple bedding, festoon bedding and structures produced by local erosion and reworking. Evidence of penecontemporaneous disturbance thought to be due to the movements of quicksands is briefly referred to. Variations in lithology and structure within the succession are attributed to local differences in conditions within a possible environment of sedimentation in an intertidal zone with off‐shore sandbanks.

Dolnokarbońska facja kulmowa w podłożu zapadliska przedkarpackiego

LOWER CARBONIFEROUS CULM FACIES IN SUBSTRATUM OF THE CARPATHIAN FORE-DEEP Amidst the ten bore-holes, in which the Carboniferous was encountered in the substratum of the Carpathian Fore-deep. Southern Poland, the most interesting is bore-hole Podborze 10 completed in 1959-1960. The Carboniferous deposits discovered in this bore-hole are developed mostly in the Culm facies, hitherto not known within the substratum of the unit discussed in the paper. The sediments were pierced at the depth from 2326,00 to 2718,00; they Occur beneath the Zechstein, and on the Precambrian deposits. At the bottom, they are represented by fine-grained sandstones and limestones (data only from fragmental samples), rich in ,detritus of crinoids brachiopods and ostracods, as well as by a thick series of . dark-grey; clayey, locally clayey-siliceous shales with interbeddings of siltstones .and sandstones. At its top part the series is often cherry-red in colour, lacks fauna, but contains flora and thin interbeddings of strongly squeezed claystones.