The proterozoic history of southern Britain
25
Citation
32
Reference
10
Related Paper
Keywords:
Basement
Cite
Citations (0)
An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.
Cite
Citations (0)
A small assemblage of primitive microscopic fossils is here reported for the first time from the lower part of the Belcher Group in Hudson Bay. The microbiota includes chains and clumps of bacteria, and filamentous and spheroidal structures of probable algal or fungal affinities. In addition, a variety of isolated and clustered spheroids and other structures of probable biologic origin is present. The structures are more than 1600 m.y. old, and occur in a black chert associated with thick dolomite beds considered to be of Aphebian (Early Proterozoic) age. Morphologically comparable Precambrian microfossils occur in the Gunflint Formation (Early Proterozoic, Ontario) and the Bitter Springs Formation (Late Proterozoic, central Australia).
Acritarch
Cite
Citations (38)
Summary A new stratigraphic scale of the USSR Precambrian officially adopted by the All-Union Precambrian Meeting in Ufa (1977) and approved by the National Stratigraphic Committee (1978) is discussed. Archean and Proterozoic have been distinguished in the Precambrian, the boundary between being 2600 Ma. Proterozoic is subdivided into the Lower- and Upper-Proterozoic with the boundary between them 1650 Ma. Riphean and Vendian are distinguished in the Upper Proterozoic. More detailed subdivisions of the Upper Proterozoic are Burzyanian, Yurmatian, Karatavian, Kudashian and Vendian. Lithological and palaeontological characteristics of these subdivisions and their radiometric age are given.
Riphean
Radiometric dating
Cite
Citations (18)
Cite
Citations (0)
Abstract. Precambrian glaciations appear to be confined to two periods, one in the early Proterozoic between 2.5 and 2 Gyears BP (Before Present) and the other in the late Proterozoic between 1 and 0.57 Gyear BP. Possible reasons for these broad features of the Precambrian climate have been investigated using a simple model for the mean surface temperature of the Earth that partially compensates for the evolution of the Sun by variations in the atmospheric CO2 content caused by outgassing, the formation of continents and the weathering of the Earth's land surface. It is shown that the model can explain the main changes in the Precambrian climate if the early Proterozoic glaciations were caused by a major episode of continental land building commencing about 3 Gyears BP while the late Proterozoic glaciations resulted from biologicallyenhanced weathering of the land surface due to the proliferation of life forms in the transition from the Proterozoic to the Phanerozoic that began about 1 Gyear BP.
Early Earth
Cite
Citations (17)
Skaly prekambryjskie tworzą podloze calego kraju, lecz zostaly szczegolniej wyroznione i zbadane w trzech regionach: na prekambryjskiej platformie wschodnioeuropejskiej i w Karpatach, gdzie tworzą fundament krystaliczny ukryty pod skalami pieter osadowych, oraz na Dolnym Śląsku, gdzie są wlączone w system faldow waryscyjskich, wypietrzone i odsloniete. Skaly prekambryjskie roznią sie nie tylko polozeniem, ale takze budową tektoniczną i strukturalną, metamorfizmem regionalnym, wiekiem i stylem przebudowy. Odmiennie ksztaltowaly sie dzieje ich badan i niejednolity jest stopien rozpoznania. THE PRESENT STATE AND DIRECTIONS OF STUDIES ON THE PRECAMBRIAN ROCKS OF POLAND Although the whole basement of Poland is built of Precambrian rocks, these rocks were recorded and recognized in three regions only. They form crystalline basement overlain by rocks of sedimentary stages in the Precambrian Platform and the Carpathians, being incorporated in the Variscan fold system, uplifted and exposed in the Lower Silesia. Precambrian rocks of these three regions differ in location as well as in tectonic and structural units, regional metamorphism, age and style of reactivation. The history of studies of these rocks is also different, similarly as the degree of knowledge. In Poland, these rocks built crystalline basement on which rocks of Paleozoic and Meso-Cenozoic sedimentary stages were originating and undergoing structural rebuilding. The development of the latter was determined by movements of crystalline basement blocks, responsible for regional and tectonic differentiation of the sedimentary covers. The studies of crystalline basement of the East-European Platform were initiated in 1954 by making three drillings preceded by geophysical surveys. In the next years, there took place intense development in there studies, determined by search for metal ore deposits and bitumens in the sedimentary cover of the Platform. The geological studies on the crystalline basement are concentrated in the Geological Institute which makes possible appropriate planning of the works and synchronization of geophysical, drilling and mapping programs. On the basis of results of petrographic, mineralogical, geochemical, geochronological and deposit studies of core material from almost 200 boreholes, and rich geophysical data (including those from magnetic and gravimetric semi-detailed and detailed mapping), it was possible to prepare petrological map of the top of crystalline basement in the scale 1:500 000 and, subsequently, the map in the scale 1:200 000 in 34 sheets. The tectonic model and stratigraphic scheme of rocks of the crystalline basement were also prepared. The studies have been crowned with discovery of vanadium-bearing titanomagnetite deposit in anorthosites in the Suwalki area and demonstration of its resources. Moreover, chemogenic hematite taconites were found at Łochow near Warsaw at 2400 m depth, and concentrations of rare earth elements – in nepheline syenites and carbonatites in the areas of Elk and Tajno. The history of studies on the Precambrian basement of the Carpathians and southern Poland began when crystalline rocks were found by a drilling at Rzeszotary in 1909. However, at that time it was not certain whether the drilling entered an exotic or autochtoneous crystalline massif. This question has not been solved before the end of the sixties, when deep drillings penetrated the Carpathian Flysch and entered crystalline basement. The knowledge of crystalline basement in this part of Poland is unsatisfactory as petrological maps, tectonic model and stratigraphic scheme are still missing. The available data show that the basement is strongly eroded ill the west, where the Carpathian Flysch rests on amphibolites fades metamorphic rocks, whereas in the east the Flysch rests on strongly diagenesed or weakly metamorphic rocks of the uppermost Precambrian. After the world war II, the Geological Institute began studies on crystalline rocks of the Lower Silesia, with detailed geological mapping of areas for which maps were still missing. Up to the present, 100 sheets of geological map in the scale 1: 25000 are completed and the remaining ones are in preparation. To carry out programs of stratigraphic, lithological, petrographic and tectonic studies and search for deposits, there has been called into existence the Lower Silesian Branch of the Geological Institute in Wroclaw. Along with time, the Lower Silesian Branch of the Geological Institute started introducing modern research techniques, as e.g. mesoscopic analysis in studies on tectonics, petrotectonic studies, geochemical analysis, analysis of radiogenic age, physical studies of rocks, geophysical studies, and biostratigraphic studies of “silent rock series”. The developments in the studies were accompanied by widening their range by deep drillings, especially in the forefield of the Sudety Mts. The results of these studies markedly changed views on stratigraphy, lithological development and metamophic alterations of the Precambrian.
Basement
Cite
Citations (0)