Sedimentology and stratigraphic affinities of Neoproterozoic coarse clastic successions, Glenshirra Group, Inverness-shire, Scotland
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Synopsis Records of ancient environments and past basin histories can be preserved in metasedimentary successions, despite their subsequent deformation and metamorphism. In the Central Scottish Highlands SE of Loch Ness, the Garva Bridge Psammite and the Glen Buck Pebbly Psammite Formations (hitherto included within the Glenshirra Subgroup at the base of the Neoproterozoic Grampian Group) represent a continuum of alluvial fan to shallow water sediments, deposited in a SE thinning fan-delta clastic wedge. These sediments, derived from an uplifted granitoid hinterland to the west, contrast with the overlying marine sedimentary rocks of the Corrieyairack Subgroup, which were deposited by sediment gravity flows within a submarine slope setting. The Glen Buck Pebbly Psammite/Garva Bridge Psammite Formations and the Corrieyairack Subgroup represent two genetic stratigraphic sequences divided by a sharp sequence boundary that records a major reorganization in basin architecture. Hence, we propose that the Garva Bridge Psammite and Glen Buck Pebbly Psammite Formations be included within a separate Glenshirra Group, genetically unrelated to either the marine deposits of the immediately overlying Grampian Group or the earlier, locally migmatized (Moinian?) basement to the Central Highlands. The Glenshirra Group thus represents the earliest phase of post-Knoydartian extension, predating the main Dalradian basin development.Keywords:
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Recently,the Proterozoic rapakivi granites(dike) with the paleo-weathered mantle-clastic rocks covered directly by the sandstones of the Changzhougou Formation of Changchengian System are discovered in the Miyun County,Beijing.The detrital components of the rocks embrace the in-situ weathered mantle saprolites and coarse-grained clastic rocks from rapakivi granites.Through SHRIMP and LA-ICP-MS dating methods,we have obtained the U-Pb dating ages of 1682±20 Ma and 1708±6 Ma from the detrital zircons sampled in the rocks;those age data are the same with the nearby Miyun rapakivi granite.The existence of this suite of weathered saprolites and clastic rocks,as well as the analysis of the dating results,have shown that the age of the base of the Changzhougou Formation should be younger than 1682 Ma.
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Synopsis Records of ancient environments and past basin histories can be preserved in metasedimentary successions, despite their subsequent deformation and metamorphism. In the Central Scottish Highlands SE of Loch Ness, the Garva Bridge Psammite and the Glen Buck Pebbly Psammite Formations (hitherto included within the Glenshirra Subgroup at the base of the Neoproterozoic Grampian Group) represent a continuum of alluvial fan to shallow water sediments, deposited in a SE thinning fan-delta clastic wedge. These sediments, derived from an uplifted granitoid hinterland to the west, contrast with the overlying marine sedimentary rocks of the Corrieyairack Subgroup, which were deposited by sediment gravity flows within a submarine slope setting. The Glen Buck Pebbly Psammite/Garva Bridge Psammite Formations and the Corrieyairack Subgroup represent two genetic stratigraphic sequences divided by a sharp sequence boundary that records a major reorganization in basin architecture. Hence, we propose that the Garva Bridge Psammite and Glen Buck Pebbly Psammite Formations be included within a separate Glenshirra Group, genetically unrelated to either the marine deposits of the immediately overlying Grampian Group or the earlier, locally migmatized (Moinian?) basement to the Central Highlands. The Glenshirra Group thus represents the earliest phase of post-Knoydartian extension, predating the main Dalradian basin development.
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Based on the information of sedimentary structure,fossils and metamorphic-deformation,the authors hold that the Shaohuotonggou Formation in the lower part of Hanmushan Group,which has high-grade metamorphic conglomerate,fine-grained clastic rock and carbonate assemblages,was formed in Sinian.Whereas the Caodaban Formation in the upper Hanmushan Group,which is characterized by clastic rock and carbonate assemblage of relatively low-grade metamorphism,was formed in Cambrian.The Caodaban Formation is comparable with the Lower and Middle Cambrian strata in the western margin of Ordos basin.
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A thin-section examination of sixteen clastic rock samples returned by the Apollo 14 mission from the Fra Mauro region of the moon suggests the presence of at least two distinctly different lithologies. Five of the samples (group I) are characterized by an abundant glassy matrix and glass particles and lesser amounts of plagioclase and pyroxene grains, and lithic clasts. The other eleven samples (group II) are relatively fine grained, very poorly sorted, and consist largely of pyroxene, plagioclase, and lithic clasts set in an abundant mineralic matrix. Group I and II lithologies were probably both deposited from impact generated base surges. The differences between them stem not as much from the basic sedimentary processes as from the differences in the magnitude of the events generating the base surges and the resultant difference in available detrital materials.
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ABSTRACT Detailed investigation of continuous cores from 44 wells and borehole geophysical logs from 200 wells extends sedimentological interpretations and sequence stratigraphic correlations of Lower Silurian siliciclastic rocks across a major part of the Appalachian foreland basin. Stratal surfaces in the Medina Group and Tuscarora Sandstone are correlated regionally across depositional strike, made possible by the large subsurface database. Above a sequence-bounding unconformity at the base of the Medina Group are an incised-valley fill and a transgressive systems tract represented by the Whirlpool Sandstone and the overlying Cabot Head Shale. The thickest sandstones, and most important hydrocarbon reservoirs, are in the Grimsby Sandstone, which is interpreted as a highstand systems tract with basinward-prograding deposits formed during a normal regression. Three major depositional systems are recognized for the Grimsby Sandstone: (1) wave-dominated shelf, (2) wave- and tide-influenced inner shelf, and (3) tide-dominated shoreline. In the landward direction, the Grimsby Sandstone shoreline deposits are correlative with coarse-grained fluvial and estuarine deposits of the Tuscarora Sandstone. Marine-flooding surfaces in the Medina Group are correlated regionally with flooding surfaces interpreted in the Tuscarora Sandstone. A marine-flooding surface in the upper part of the Grimsby Sandstone, as identified in cores from Ohio, Ontario, and western Pennsylvania, is correlated with the contact between fluvial/estuarine facies of the Tuscarora Sandstone and the overlying, much finer-grained Castanea Member in central Pennsylvania. This flooding event marks the first landward shift in facies of the Lower Silurian highstand accumulation, and is therefore recognized as an important, regionally correlative surface. A second regionally extensive marine-flooding surface occurs between uppermost Lower Silurian argillaceous sandstones and overlying carbonate rock and shale of the Clinton Group.
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