Precambrian Sediments Research Articles

Organismes du Schisto-calcaire, protérozoïque supérieur, de la République Populaire du Congo (Afrique Centrale)

In the formation within the Niari synclinorium that is generally regarded as Upper Proterozoic in age, the schisto-calcareous deposits provide a favourable environment for the study of trace fossils. Stromatolites appear at the top of the SCI and the SCII stage correspond to an increase in organic activity with the presence of stromatolitic biohermes ( Asterosphaeroides manangae, Alvarez and Vicat, 1987, Morulaspharea morulasphaera, Alvarez and Vicat n.sp., filaments and spherules, Asterosphaeroides sp, microbial “balls” and various microphytolites). When these organisms are compared with those described in West Africa and in Siberia, an Upper Riphean age may be proposed for the SCII formation. The SCII formation contains numerous undetermin-able organisms which are exposed within a thin lenticular level made up of biogenic limestone. Within the same formation, brecciated limestone composed of carbonaceous clastics with fossilretrs was observed. This peculiar facies correspond to important concentrations of biogenic material. In the Niari synclinorium life was highly developed as early as the Upper Proterozoic. The schistocalcareous series therefore represent a remarkable sedimentary sequence in Central Africa where Precambrian sediments are usually known to be poorly fossiliferous.

Botryococcus—A planktonic green alga, the source of petroleum through the ages: Transmission electron microscopical studies of oil shales and petroleum source rocks

The hydrocarbon secreting alga Botryococcus has been identified in organic remains of sediments ranging from Precambrian to Recent, and is believed to have been a major source material for petroleum generation throughout the geological time. In some petroleum source rocks of Lower Palaeozoic and Precambrian age, identification of the alga is only possible by electron microscopy. Transmission electron microscopy (TEM) has been used in the present study to identify microstructures of the algal remains in a range of oil shales and petroleum source rocks. It has been established that Botryococcus is the predominant alga in the Kukersite oil shale of Estonia. Similarly, the alga has been shown to be a major contributor to petroleum source rocks in Cambrian and Precambrian sedimentary basins in Australia. TEM has been applied to observations of Botryococcus in torbanites and to products from simulated maturation experiments on torbanite. A comparison with algal remains from Cambrian and Precambrian sediments ranging from undermature to overmature, enabled the distinction of organic matter in various stages of oil generation. Maturation/thermal effects on alginite have been established by reflectance and fluorescence, and compared with experimental results.

Problem of fungal contamination in Precambrian palaeobiology: A cautionary note

The paper deals with the problem of fungal contamination in palaeopalynological preparations specially from the Precambrian sediments. The vegetative and reproductive structures of extant fungal groups show a broad similarity in morphology with the morphotypes described from the Precambrian. The recent fungi when subjected to chemical treatment similar to palynological preparations (maceration) do not show major physical and morphological changes. Nine common fungal genera were selected for this study. It has been observed that they withstand hydrochloric acid and hydrofluoric acid treatment without losing much of their morphocharacters. As these are common in soil profiles, one can easily be misled when they occur amongst macerated residues. This data serves as a cautionary note to all palaeobiologists and specially dealing with Precambrian material, where every new evidence is important in adding to the meagre knowledge.

Strike-slip faulting along the Church Stretton Lineament, Old Radnor Inlier, Wales

The Old Radnor Inlier comprises Precambrian sediments unconformably overlain by Wenlock age limestones and shales. Its structure is dominated by steep strike-slip faults. Major NE-SW faults probably bound the inlier and are linked by more northerly faults to form a strike-slip duplex. Strike-slip faults within the inlier match a Riedel shear system, with observed dextral offsets on a NW striking set and implied sinistral offsets on the major inlier-parallel faults. Subordinate dip-slip faults are anomalously oriented, but may be Riedel shears reactivated during progressive sinistral transtension across the inlier. Pre-Wenlock deformation is recorded by the tilting of the Precambrian sediments and by some faults that do not cut the unconformity. On regional grounds, this deformation might be Ashgill (late Ordovician) or late Precambrian-early Cambrian in age. The strike-slip component now predominating was imposed in post-Wenlock time, probably during the regional sinistral transpression of the Acadian (Late Caledonian) event in Early Devonian time. Later Variscan and Mesozoic components cannot be ruled out.

Age and geochemistry of late precambrian sediments of the hammamat series from the Northeastern desert of Egypt

Late Precambrian crustal evolution in the Northeastern Desert of Egypt culminated in a major episode of strong extension, accompanied by bimodal igneous activity and deposition of clastic sediments in restricted terrigenous basins. These sediments, known as the Hammamat Series, record the uncovering of the evolving crust of the region. We have undertaken petrographic, geochemical, and Rb-Sr and K-Ar geochronologic studies of Hammamat samples collected along a single traverse. The lower half of a 400 m thick section consists of coarse detritus shed from nearby ensimatic terranes, while the upper half was derived from rapid reworking of bimodal igneous rocks similar to those of the Dokhan Volcanics and Pink Granites, units that are preserved within the region. Rb-Sr whole-rock analyses give an 8-point isochron age of 585 ± 15 Ma with an initial 87Sr/ 86Sr of 0.70323 ± 0.00013, interpreted to approximate closely the time of sedimentation. This is very similar to the K-Ar ages of the coarser clay fractions (0.5–2 μm) (567–588 Ma). This result is consistent with stratigraphic considerations and other geochronological studies indicating that Hammamat deposition occurred at ∼ 590 Ma. Rb-Sr analyses of leached and unleached size fractions indicate thermal resetting at 524 ± 17 Ma accompanied or followed by fluxing of a radiogenic Sr-rich pore fluid ( 87Sr/ 86Sr=0.7124). This is further indicated by K-Ar ages of the 0.2–0.5 μm size fraction of 532–542 Ma.

Discussion on the supposed Precambrian palaeosuture along the Dead Sea Rift

Tuvia Weissbrod & Iaakov Karcz write: The evidenc available to date is insufficient to allow a detailed correlatioi of Precambrian lithologies and tectonic events across th Arabo-Nubian massif (ANM). Hence any spatia extrapolation of local evidence requires a great deal of caution. The arguments recently presented by Abed (1985) against the supposed Precambrian ancestry of the Dead Se; Rift (e.g. Bender 1968 , 1982 ) are therefore briefly assessed here in the light of findings in Israel and Sinai. Precambrian sediments. To refute the claim that the molasse-like Sarmuj Conglomerate is preserved as relics, which escaped the intense Late Precambrian planation only along the inferred palaeosuture depression, Abed presents examples of Precambrian sediments at locations across the ANM well away from the Dead Sea Rift (DSR). None of these examples, however, is fully applicable. The Precambrian history of the ANM includes several phases of deformation, planation and deposition (e.g. Hadley & Schmidt 1980; Bentor 1985 ), so that even though preservation in structural depressions is quite common, the date and nature of the infilling, and the age of the depressions, vary considerably. Abed’s first example, the Murdama Group of Saudi Arabia (600–650 Ma), is older than the Sarmuj Conglomerate, and contrary to Abed’s assertion, pre-dates rather than postdates the Najd fault system (e.g. Delfour 1979 ; Schmidt et al. 1979 ). Since the relation, if any, of the Najd faulting to the DSR is not clear, and since no claim has ever been made that rifting was instrumental in the preservation of all Precambrian sediments

Sedimentation and tectonics in the Welsh Basin

Report of a joint meeting of the Tectonic Studies Group and the British Sedimentological Research Group of the Geological Society held at the University College of Wales, Aberystwyth on 10–12 April 1986. The organizers were Dr W. R. Fitches and Dr N. H. Woodcock. Research into the Early Palaeozoic history of Wales is like that history itself; eventful and rarely quiescent. Recent events have been the lively debate on the tectonics of Anglesey provoked by A. J. Barber and M. D. Max, the exemplary volcanological studies in North Wales by the BGS Snowdonia unit, and the definition of Silurian world stratotypes by members of the Ludlow Research Group. The stimulus for this conference was the recently increased activity in structural and sedimentological research in mainland Wales. Twenty-eight papers were given over the first 2 days, followed by a day of field trips to Snowdonia, the Aberystwyth area, and the Wenlock of eastern Mid-Wales. Precambrian events featured in three papers. D. S. Wood concentrated on the evidence for the superficial ‘soft-sediment’ origin of the Gwna melange of Anglesey and postulated a setting predating rather than accompanying the subduction system that produced the Anglesey blueschists. In the later deformation history of Anglesey, D. Carter distinguished two main deformations. A Caledonian event was preceded by one of pre-Arenig or probably pre-Arvonian age, with an unconformity present between the Monian and Lower Palaeozoic rocks. Precambrian sedimentation of the Longmyndian of Salop was described by J. Pauley in terms of a progradational basin fill.

A search for molecular fossils in the kerogen of Precambrian sedimentary rocks

Hydrous pyrolysis of the kerogen in Precambrian sedimentary rocks of a range of ages and thermal histories was used in an attempt to produce normal, iso and anteiso, isoprenoid, sterane and triterpane hydrocarbons whose structures can be related to known biochemical precursors. Gas chromatography—mass spectrometry was used in the selected ion monitoring mode to achieve a sensitivity of one part hydrocarbon in 10 9 of the rock. Small amounts of some of these hydrocarbon classes were found in certain Lower Cambrian and upper Precambrian sediments with normal alkanes predominating. The yields diminished with increasing age, even for well-preserved rocks where the minerals showed little effect of metamorphism. Samples with ages greater than 1.6 × 10 9 years yielded no detectable hydrocarbons. Spontaneous dehydrogenation of kerogen results in cross-linking, cyclization and aromatization of organic structures and extensive transformation of molecular fossils. Little information on the evolution of Precambrian life is contained in the organic matter of the rocks studied in this work. Further advances along this line will depend critically on the discovery of sediments in which the kerogen is better preserved.

Carbon and sulfur isotopes in Precambrian sediments from the Canadian Shield

Carbon and sulfur isotope ratios have been determined for more than 200 samples of Precambrian graphitic and sulfidic metasediments from the Superior and Churchill Provinces of Canada. The sediments were deposited in small sedimentary basins related to various Canadian greenstone belts. The age of the Archean samples is approximately 2.7 Ga., the Proterozoic samples about 1.8 Ga.,

Morphological and chemical record of the organic particles in precambrian sediments

Summary A review is presented of the currently available morphological and chemical evidence of life in the Precambrian, with special reference to the interrelationships between microbial activity and biomineral accumulation. Fossil organic microstructures can be detected in thin sections of the rock under the light microscope and examined in demineralized sections under the transmission electron microscope (TEM). They are chemically analyzed in microprobes and spectrophotometer microscopes. On the basis of such studies, the interaction of microorganisms with the formation of minerals in cherts, carbonates and from sediments can be traced back to early Archean times, 3800 million years (MY) ago. The stratiform gold and uranium ores of the lower Proterozoic (2700–2800 MY) indicate anoxic conditions during deposition. Certain bacteria and fungi are apparently involved in the formation of the facies. The banded iron formations occur most often in the subsequent time span 2300–1700 MY ago. The deposits are interpreted as being produced by the activity of cyanobacteria and iron bacteria. During this period, the first red beds appeared indicating that the atmosphere became increasingly oxygenated. Copper deposits can be interpreted as a marker of the younger Proterozoic 1200−650 MY ago as far as their biological significance is concerned. At this time, the first diversification of eukaryotic phytoplankton occured. It is suggested that the evolution of eukaryotic algae was accompanied by an increase in copper, coupled with a decrease in iron in the cell.

Glaciotectonic and till or tillite genesis: Examples from pleistocene glacial drift in central Sweden

A few different wasy of formation of diamictons that to some extent could be considered till are discussed. Shearing, brecciation and flow as a result of glacier action deformed and mixed waterlaid sediments to become diamictons. The structures observed illustrate the ways of formation. Similar structures found in e.g., Precambrian sediments could be an indication of glacial deformation.

Metallogenesis in the Harlech Dome, North Wales: A fluid inclusion interpretation

A regional fluid inclusion study of Cu-Au (+Zn-Pb) mineralisation in the Harlech Dome area, North Wales, gives support to the concept of two distinct metallogenic episodes. The inclusion assemblages associated with the porphyry copper mineralisation at Coed-y-Brenin are consistent with a genetic model of early potassic-propylitic alteration overprinted by later phyllic alteration. High salinity fluids, normally characteristic of potassic alteration, are confined to the host rock quartz. The meteoric/hydrothermal system is closely linked to the emplacement of late-Cambrian diorites. Integrated fluid inclusion and mineralogical studies of the Gold-belt veins suggest that the mineralising fluids were probably dehydration waters released from weakly metamorphosed Cambrian and perhaps Precambrian sediments during hydraulic fracturing in a tensional zone at the close of the Caledonian orogeny. Localisation of economic concentrations of gold in veins at the level of the Clogau Formation is ascribed to a destabilisation of metal complexes caused by a change in fluid buffering from a pyrite-magnetite assemblage in the Lower Cambrian sediments to a pyrite-pyrrhotite-graphite assemblage in the Upper Cambrian sediments. Veining associated with the Coed-y-Brenin porphyry copper deposit and related breccia pipes can be distinguished from the copper-gold veins of the coextensive Dolgellau Gold-belt by the presence in the former of inclusions notably richer in CO2. Furthermore the Gold-belt fluids have a distinctive low CO2/CH4+N2+H2 ratio.

Oil and Gas Developments in North Mid-Continent in 1984

Common to the Committee on Statistics of Drilling District 12 area are the recent exploration activities associated with the Central North American rift system or Mid-Continent geophysical anomaly (MGA), a major feature that runs from the Lake Superior area south into Kansas. For the last several years, much preliminary geologic and geophysical work has been undertaken, which usually proceeds a major play. The primary purpose is to test the Cambrian and Precambrian sediments know to have oil seeps in Wisconsin and Michigan. In 1984, Texaco USA drilled the first deep test, which was in Kansas. Although the well was apparently a dry hole, Texaco's findings have not been released. Kansas had a very active year with 7,451 completions, 45 more than those reported in 1983. The success rate of all wells drilled for oil or gas (7,307) was 57.5%, down slightly from 59.3% in 1983. Drilling for oil continued to predominate with 3,783 oil wells and 419 gas wells completed. Total footage was 22,486,535, up 4% from 1983. The average depth of a test drilled for oil or gas was 3,026 ft. In Missouri, the number of wells drilled for oil or gas declined 17% from 1983 levels. Most drilling continued to be in the western part of the state. A deep test in Vernon County penetrated 2,080 ft of Precambrian rocks. In Nebraska, 12 new discoveries were made in the western part of the state. Seven found new oil reserves, and 5 were tight holes; all were classified as new-field wildcats. The average depth was 5,465 ft in the 7 discoveries where the operator reported the total depth. In Mills County, Iowa, 4 wildcats were drilled to the Cambrian with depths from 3,000 to 3,300 ft. All were located approximately 35 mi north of the Tarkio field in northwestern Missouri. It is estimated that 2,000,000 ac are leased in Iowa along the MGA. In Minnesota, 400,000 ac were leased during 1984. The leases were concentrated mainly along the MGA from Duluth to the Iowa border. About 1,000 mi of Vibroseis was run across this feature. In Wisconsin, regional geophysical surveys along the MGA have been run. Companies are now doing more detailed seismic work. Acreage leased from October 1983 to January 1985 was estimated at 214,000 ac. A dry hole was drilled 1,000 ft into quartzite in Barron County.

Precambrian geochronology and the geological record

Summary Rb-Sr, Sm-Nd and Pb/Pb whole-rock isochron methods and the U-Pb zircon method have been widely applied to dating Precambrian igneous and meta-igneous rocks. Application of these methods in regional geochronological studies provides reliable constraints on the age and temporal evolution of rock units, even when some parent-daughter systems have suffered disturbance in metamorphic and/or metasomatic events which significantly post-date primary rock formation. Initial isotopic compositions of Sr, Nd and Pb aid geological interpretation of isochron age data, constrain crustal residence time of the protoliths and can be used to assess the relative contributions of mantle and crust in magma genesis. Precambrian sediments can be indirectly dated by reference to age data on interbedded volcanics, or by age-bracketing between dated basement rocks and dated cross-cutting intrusives. Significant progress has also been made in the direct dating of sedimentary rocks. Published isotopic age determinations on early Precambrian sediments containing biogenic remains are briefly reviewed. Provided that stratigraphical and geochronological correlations are correct, it appears that stromatohtic limestones were being deposited by c. 3400 - 3300 Ma ago, and that true microfossils occur in rocks dated at c. 3200 - 3000 Ma. Evidence relating to possible biogenic markers in sediments reliably dated at c. 3800 - 3700 Ma is not yet definitive.

Origin and significance of compositional layering in Late Precambrian sediments, Blue Ridge Province, North Carolina, U.S.A.

Basal arkosic sandstones of the Late Precambrian Grandfather Mountain Formation contain smallscale deformation zones, usually 1 mm thick and spaced 5 to 10 mm apart. The deformation zones are composed of green phengitic muscovite which concentrated in the deformation zones during pressure-solution removal of quartz. Assuming plane strain, the deformation zones have lost 34% of their area due to solution of quartz, and axial ratios of strain ellipses are on the order of 16:1 in the principal plane. The deformation zones and associated tight to isoclinal folds in the basal arkoses form a structural style which is transitional between ductile deformation zones in the crystalline basement and kink-style folding in the interbedded siltstones and arkoses of the upper Grandfather Mountain Formation. Since deformation zones may be more common in sedimentary rocks and low-grade metasediments than previously realized, one should exercise caution when using penetrative fabrics to unravel the structural history of multiply deformed rocks. Small-scale deformation zones can be misinterpreted as a crenulation cleavage cutting an earlier penetrative fabric.

Precambrian microfossils from the Longmyndian of Shropshire

The late Precambrian sediments of the Stretton Series of the Longmyndian Supergroup near Church Stretton, Shropshire, contain abundant organic-walled microfossils or cryptarchs. Some are big enough to be seen with the naked eye and may represent metazoa. Narrow filaments grew in dense networks on the sediment surface, while round forms probably represent planktonic microorganisms. All of the microfossils are flattened and carbonised and can only be studied in thin sections cut at a low angle to bedding. Successive very thin laminae often contain completely different microbiotas. The overall aspect of the assemblages is different between the bottom and top of the Stretton Series and they may prove to be stratigraphically useful.

Faixa de Perau: uma unidade faciológica e tectono-metamórfica do Grupo Açungui

The purpose of this work is to demonstrate that only if we integrate the sin-sedimentary tectonic, paleo-environmental and subordinated magmatic agents and processes, it will be possible to reconstruct the entire evolution of a litho-stratigraphical column of Precambrian sediments of the Acungui Group, in Southern Brazil, and to include in it the metallogenetic episodes that originated several mineralizations in Pb, Ag, Zn, Ba and Cu (Perau Mine), Ba (Agua Clara and Tigre Mines) and Pb (Seabra Mine). A geological cartography is included to give evidence of the evolution presented.

Magnetic fabric, strain and paleostress in the Canadian Rocky Mountains

The paper reports a detailed study of the anisotropy of magnetic susceptibility at two localities 100 km apart in the Canadian Rocky Mountains, both in the Miette Group of Precambrian sediments which was folded during the Laramide Orogeny. From the orientations of the susceptibility ellipsoids we infer principal strain axes and, assuming irrotational deformation, possible Laramide paleostress orientations. At Wynd Siding, near Jasper in the Main Ranges, oriented samples were taken from both limbs and the nose of a plunging syncline. Near Tête Jaune Cache, at the western boundary of the Main Ranges, a small syncline and steeply dipping beds of one limb of a large anticline were sampled. Minimum principal susceptibilities transverse to the strike of the Rockies dominate the magnetic fabric at both localities and it is clear that regional shortening of the whole rock mass has obliterated any original depositional fabric, and any small-scale, fold-related fabric, at both sites. Writing K 1 ≥ K 2 ≥ K 3 for the principal susceptibilities, the K 3 axis (and by inference the axis of principal shortening) is horizontal, with azimuth N52°E, at Wynd; and plunges 10°–20° along azimuth N26°E, at Tête Jaune Cache. K 1 is vertical at Wynd, but horizontal and along the fold axis (N61°W) at Tête. The uniformity of axes of susceptibility and inferred strain through each locality suggests irrotational deformation and encourages the further inference of possible paleostress orientations, with σ 1 aligned with K 3, σ 2 with K 2 and σ 3 with K 1. The magnetic fabrics then indicate a thrust type paleostress field at Wynd, consistent with the Laramide thrusts in the eastern Rockies, but a strike-slip type paleostress field at Tête Jaune Cache. The change of stress orientation may reflect deeper burial, in Laramide times, of the rocks sampled at Tête Jaune Cache, consistent with the much higher grade of metamorphism there.

Correlations between the chemical and the geologic origins of anthraxolite from the Gunflint Formation, Thunder Bay, Ontario

Beds and lenses of coal and veins of anthraxolite are common occurrences in relatively unmetamorphosed Precambrian sediments of the Lake Superior region. Anthraxolite is the name for a black, combustible coallike solid found in Precambrian rocks. Anthraxolite resembles anthracite coal in many ways but occurs in veins and fissures which cut across bedding. Anthraxolite was regarded as having been formed by very low grade metamorphism of liquid bitumens that were probably derived from living organisms. Anthraxolite was studied using modern methods of chemical analysis and compound identification which have been so successful in elucidating the structures and compositions of various coals. Since the major steps in the catagenesis of organic material in coal, as it passes from low to high rank, are reasonably well understood and fairly well defined chemically, it appears possible to correlate the chemical age of the organic matter in anthraxolites with ages determined by radiometric dating. 28 references.

Geology and Petroleum Potential of Flathead Region, Flathead County, Montana, and Southeastern British Columbia: ABSTRACT

Discovery of oil seeps in the Flathead region led to wildcat drilling by 1902. Exploration on the U.S. side stopped by 1910 because of the formation of Glacier Park, and the fact that the oil seeps were in Precambrian sedimentary rocks. Exploration continued through the 1930s on the Canadian side with drilling in the vicinity of major oil seeps. Exploration in Canada from the 1950s to present has included seismic work and six wildcat tests in the Flathead region. A land play has been going on in northwestern Montana for the past several years and recently released seismic data have demonstrated major structures that apparently involve Paleozoic rocks. Plans for drilling have been announced. The Glacier-Waterton and Flathead region are on the Lewis thrust plate. At the Sage Creek, British Columbia, oil seeps, drilling in 1952 penetrated the Lewis fault after drilling 4,400 ft (1,341 m) of Precambrian sedimentary rocks, and then drilled a strongly faulted sequence of upper Paleozoic carbonates and sandstones with several oil shows. The Precambrian rocks on the Lewis plate in the U.S. thin into Canada, eventually to zero where Paleozoic and Mesozoic rocks are carried by the Lewis fault. An area of hundreds of square miles of Paleozoic and Mesozoic outcrop, including the Fernie basin, is present immediately across the international boundary from areas of Montana which have been mapped as containing great thicknesses of Precambrian sediments. In the northern Whitefish Range o Montana, nearly 40 mi (64 km) from the leading edge of the Lewis thrust, is 30 mi2 (78 km2) of Paleozoic and Mesozoic outcrop containing several petroliferous units. This sequence is the only part of the extensive Paleozoic and Mesozoic outcrop of southeastern British Columbia which extends into the U.S., but it is important in understanding the involvement of Paleozoic and Mesozoic rocks in complexly faulted northwestern Montana. The Whitefish Range Paleozoic and Mesozoic sequence is cut by several minor thrust faults as well as having been overthrust by the Hefty plate, present now as klippe on the highest peaks of Paleozoic rocks. The Couldry and Tuchuck faults cut these rocks a few miles to the west. Between the Whitefish Range and the Livingstone Range of Glacier Park lies the Kishenehn basin, a graben to half-graben formed by the listric normal Flathead fault on the Lewis plate. This basin is filled with Oligocene Kishenehn formation, a fluvial End_Page 426------------------------------ to lacustrine sequence, which may be as thick as 16,000 ft (5 km) on the east side, thinning to a few thousand feet or to zero on the west where fault bounded. The Kishenehn formation contains lignite and oil shale beds which may be buried deep enough in parts of the basin to generate hydrocarbons; recent drilling discovered oil and gas shows at less than 200 ft (61 m). The presence of oil and gas shows in many formation as well as the presence of major structures containing porous host beds indicates that this area has an excellent potential for the discovery of hydrocarbons. End_of_Article - Last_Page 427------------