Late Proterozoic Time Research Articles

Precambrian events at Sabaloka, near Khartoum, and their significance in the chronology of the basement complex of North-East Africa

A large inlier of basement complex rocks surrounded by Cretaceous sediments north of Khartoum comprises a grey gneiss assemblage intruded by late-tectonic adamellitic batholiths and a shallow-level granitic ring complex. The gneiss assemblage contains evidence that an early granulite facies metamorphism was followed by retrogression into the amphibolite facies and widespread migmatization. There are close affinities with basement rocks of Archaean age in northern Uganda. Amphibolite facies metasediments which overlie the grey gneiss to the north and northeast of the Sabaloka inlier probably equate with the Early and Middle Proterozoic cover sequences which in the east African sector of the Mozambique belt are strongly interfolded with thoroughly reworked Archaean basement. The adamellite batholiths are in places strongly discordant to their country rocks and may be coeval with the ‘batholithic granites’ of the Red Sea Hills, emplaced in late Proterozoic times, during the last stages of deformation in the Red Sea belt. The whole area was later affected by reheating and limited tectonism during the Pan-African event (550 ± 100 Ma) with consequent resetting of K-Ar apparent ages. Following a period of deep erosion, the Sabaloka ring complex and associated volcanic rocks were emplaced in mid-Palaeozoic times. It is suggested that the concepts embraced by the terms ‘Pan-African event’, ‘Red Sea belt cycle’ and ‘Mozambique belt cycle’ are best kept distinct from each other since their affects appear to differ in spatial and temporal distribution. One new analysis of a retrogressed and metasomatized amphibolite is presented.

Biostratigraphy and depositional environment of algal stromatolites from the Mescal Limestone (Proterozoic) of central Arizona

The Apache Group of central Arizona is subdivided into, from base upward, the Pioneer Formation, the Dripping Spring Quartzite and the Mescal Limestone. Radiometric age determinations by Silver, and Livingston and Damon indicate an age of 1.2–1.4 billion years. Within the Mescal Limestone, algal stromatolites form a conspicuous biostrome, commonly 20–25 m thick. The basal 1–5 m of the biostrome consists of up to three zones of digitate stromatolites, which often form discrete bush-like bioherms. These forms are interpreted as Baicalia baicalica, Parmites sp. and Tungussia sp.; the latter form previously reported by Cloud and Semikhatov (1969). The form Parmites is interpreted as a modification of digitate stromatolites probably by decrease in current velocity within the shallow marine environment, which allowed discrete heads to coalesce. Basal digitate forms are replaced upward in the biostrome by domal and undulatory laminated (stratiform) stromatolites, interpreted to represent gradual upward shoaling, with lower intertidal and subtidal forms (digitate morphology) being replaced by upper intertidal and possibly supratidal forms (stratiform types). The digitate form B. baicalica is suggested by Soviet workers to be indicative of Middle Riphean time (1350-950 m.y.). While many empirical data suggest the possibility of gross subdivision of Late Proterozoic time on the basis of algal stromatolite “zones”, the intercontinental applicability and the ultimate validity of this concept in unresolved.

Early Paleozoic Evolution of Northern Parts of Canadian Arctic Islands: ABSTRACT

End_Page 2509------------------------------A geosyncline occupied northern parts of the Arctic Islands in Late Proterozoic time. It received sediments from the continent and deepened in a northerly direction. A northwestern belt, which included northernmost Ellesmere Island and the present shelf off Ellesmere and Axel Heiberg Islands, underwent an orogeny in latest Proterozoic or Cambrian time. The orogen behaved as an intermittently rising geanticline, and remained a site of volcanism, plutonism, and metamorphism, from Cambrian to Devonian time. Sediments derived from the geanticline accumulated in a clastic basin on its southeast side. The basin was flanked on the southeast by a subsiding carbonate shelf, in turn grading southward to stable carbonate platforms. Three phases of sedimentation are recognized in the clastic basin in northeastern Ellesmere Island: (1) Middle to Upper Cambrian (?) post-tectonic deltaic deposition; (2) Early to Middle Ordovician deep-water deposition of starved-basin type (radiolarian chert, graptolitic shale, etc.); (3) late Middle Ordovician to Middle Silurian deep-water deposition of flysch-type (graywacke, shale, etc.). The trough must have formed by subsidence of the continental crust rather than by sea-floor spreading, because the deep-water sediments lie on shallow-water sediments and not on volcanics. The trough, which was separated from subaerial parts of the geanticline by a shelf on which carbonates, clastics, and volcanics were deposited, expanded until about mid-Silurian time, then migrated southeast, ahead of the southeast-migrating geanticline. The southeast flank of the trough, characterized by graptolitic shales and limestones, has been traced from northwestern Greenland to northwestern Melville Island. There, starved-basin conditions persisted from Early Ordovician to Early Devonian time. A north-trending belt in the central islands, extending from the stable platform to the geanticline, was elevated in the Early Devonian. The uplift was basement controlled and reflects Precambrian basement trends unrelated to the early Paleozoic basin configuration. An orogeny of the entire northern regions, locally accompanied by intrusion of quartz diorite, occurred in Middle Devonian to Mississippian time. Deformation and uplift proceeded from northwest to southeast. End_of_Article - Last_Page 2510------------

Canada Basin and Lomonosov Ridge: Inferences Based on Precambrian and Lower Paleozoic Geology of Canadian Arctic Islands: ABSTRACT

Canada basin truncates and post-dates a belt of north-trending structures in the central part of the Arctic Islands. The trends, though exhibited by Phanerozoic strata, are interpreted as rejuvenated Precambrian basement features because they cut across Phanerozoic basin axes and parallel exposed Precambrian structures on the south. The maximum possible age of the Canada basin, therefore, is the age of these trends--about 1.7 b.y. A geanticline, characterized by volcanism and plutonism, rose out of a late Proterozoic and occupied the northern rim of the Arctic Islands and adjacent present offshore region, from Cambrian to Devonian time. What lay beyond the geanticline is unknown, and speculations about that region depend on the tectonic model used. If present concepts of plate tectonics are applied, it appears that Canada basin (or some predecessor) opened in middle to late Proterozoic time with a developing on the newly formed continental margin. The floor of the basin was thrust beneath the from latest Proterozoic or Cambrian to Devonian time, and produced a geanticline above and southeast of the postulated Benioff zone. Ridge: A belt of north-trending structures in northernmost Ellesmere Island lines up with the southern extremity of Ridge. The structures, formed in sialic metasedimentary and metavolcanic rocks, are partly pre-late Middle Ordovician. The belt aligned with Ridge was elevated relative to terrane on the east during 2 Paleozoic orogenies, but not during the Tertiary orogeny. A short distance inland, the north-trending belt terminates against west-trending structures subparallel with the axis of the Franklinian geosyncline. The zone of intersection, marked by ultrabasic intrusions, was a site of repeated crustal extension on a scale of miles. If the Ridge has oceanic crust the apparent alignment would be coincidental. If it has continental crust, two interpretations are possible: (1) the north-trending belt represents an early Paleozoic Lomonosov geosyncline that joined the Franklinian; (2) both Ridge and the north-trending belt in Ellesmere Island are controlled by meridional Precambrian basement trends, but these trends were rejuvenated in a different manner, and perhaps at different times, on Ellesmere Island and in the present Arctic Ocean. This hypothesis is favored. End_of_Article - Last_Page 2509------------

Continental Shelf Positions During Geologic Time: ABSTRACT

Continental Shelf Positions During Geologic Time: ABSTRACT

The Baykal-Yenisey rift (general description)

The author's personal studies in the field lead to a conclusive identification of the Baykal-Yenisey rift (ca. 800 km long, up to 60 km wide in the west, narrowing down to 15 km in the east), presumably a deep-seated sub-crustal fracture originated in late Proterozoic time, in about the same age as the Altay-Sayan geosyncline. Surficial expressions of the rift include downwarps, tectonic seams, blocks of the ancient crystalline base, and other disturbances, together with evidence of a tectonic re-activation in the Quaternary and the associated effusions of young basalts. Among the demonstrable rift-geosyncline relationships, in the course of their geologically intricate evolution, the rift zone functioned, in Sinian time, as a kind of a boundary between the preponderantly platformal sediments in the northeast and the geosynclinal sediments in the southwest. – IGR Staff.

General Geology and Hydrocarbons of the Northern Amadeus Basin, Australia: ABSTRACT

The Amadeus basin is a structural depression extending easterly from the Canning basin in Western Australia to the Great Artesian basin in Queensland. It covers about 80,000 square miles and contains up to 30,000 feet of late Proterozoic and early Paleozoic marine and continental sediments. The marine cycle of deposition started in late Proterozoic time and terminated in late Ordovician. No unconformity is present at the base of the Paleozoic sequence and both Proterozoic and Paleozoic beds are unaltered except along the northern margin of the basin where low grade metamorphic facies were developed in End_Page 349------------------------------ the sediments during the Pertnjara orogeny in Devonian time. The marine sequence consists of clastics, carbonates, and evaporites. They were deposited in shallow water, shelf environments as is shown by algal growths, biohermal and biostromal carbonates, abundant ripple marks and abundant cross laminations in sandstones, and by widespread coquinoid facies in Ordovician shales. Silled or barred basins with restricted circulation of marine waters existed from time to time during the marine cycle as is shown by salt deposits in the upper Proterozoic and Cambrian sections and by thick accumulations of dark shales with abundant pyrite in late Proterozoic and Ordovician sediments. The marine cycle of deposition was terminated by the Pertnjara orogeny. This orogenic episode created a welt north of the Amadeus basin and a bordering foredeep whose depoaxis follows the present northern margin of the basin. Marine sediments were stripped from the rising welt, transported southward, and dumped into the subsiding foredeep where they now form a thick apron of poorly sorted, coarse clastic deposits. Salt tectonics has played an important role in the growth of structures in the northern Amadeus basin. Thick salt deposits in the Bitter Springs formation of late Proterozoic age constituted a semi-plastic layer near the base of the Proterozoic sequence. Sedimentary loading on this layer produced flowage and initiated salt anticlines and salt domes. These structures grew during late Proterozoic and early Paleozoic deposition as is shown by crestal stratigraphic convergence and local unconformities confined to one structure. The evaporite layer also provided a lubricated zone along which slippage was localized during the Pertnjara orogeny, and it may be responsible in part for the large nappes and overthrusts along the northern margin of the basin. The anticlines and salt domes initiated by salt flowage were formed early in the history of deposition and thus constituted potential traps for hydrocarbons long before the Pertnjara orogeny. However, folding during the Pertnjara orogeny greatly increased structural relief on the anticlines and thereby created traps having large volumetric capacities. Two of these large structures have been tested with encouraging results. The Exoil-Magellan-United Canso groups have discovered a large wet-gas accumulation, possibly with an appreciable oil leg, in Ordovician reservoirs on the Mereenie anticline in the western part of the Amadeus basin. They also discovered a non-commercial gas accumulation in Proterozoic sediments on the Ooraminna anticline in the eastern part of the Amadeus basin. A third test of a small structure near the Ooraminna anticline encountered non-commercial oil shows in Cambrian sediments. In addition, the Australian Bureau of Mineral Resources discovered oil-saturated sands in Ordovician sediments penetrated by a well being drilled as a test for phosphates. With the exception of shallow water bores, no other wells have been drilled to date in this basin. End_of_Article - Last_Page 350------------