Bedding In Sedimentary Rocks Research Articles

Contrasting constraints on the temporal and spatial extents of normal faults from the Hilltop and Lewis mining districts, northern Shoshone Range, Nevada, USA

Mapping of rock types, structural geology, and hydrothermal alteration, supported by geochronology and thermochronology, sheds light on the original spatial relationships of hydrothermal systems to intrusions in the northern Shoshone Range in north-central Nevada. Rocks in the Hilltop district are tilted ~35–40°E, as indicated by orientations of flattened pumice fiamme and bedding in sedimentary rocks along a single set of presently low-angle normal faults that initiated at 60–70°W dips. New U-Pb zircon geochronology from two sets of dikes in the Lewis district could suggest late Eocene–early Oligocene extension, but definitive crosscutting relations are lacking to demonstrably support this potential earlier period of normal faulting. Reinterpretation of previously reported apatite fission-track cooling ages with a new palinspastic restoration in the Lewis mining district concurs with middle Miocene extension as documented to the south at the Caetano caldera; however, the depth of burial of the Lewis district—and thus the significance of the apatite fission-track cooling ages—is uncertain. The comparable orientations and tilting history, supported by fault scaling relations, suggest that the temporally coincident extension in the Caetano caldera to the south represents the along-strike continuation of the same system of normal faults as in the Hilltop and Lewis districts, with changes in observed offset, percent extension, and fault spacing attributed to the gradual tipping out of the fault system northward.

Hummocky cross-stratification is not produced purely under progressive gravity waves

Hummocky cross-stratification is a type of bedding in sedimentary rocks caused by the draping of sand around low-steepness three-dimensional bedforms. This distinctive form of stratification has been thought to be diagnostic of the former action of storm waves and hence its occurrence has been used to reconstruct ancient palaeogeographies and marine processes. Here I examine theoretical and empirical aspects of hummocky cross-stratification pertaining to its origin and argue that the notion that it was produced purely under progressive storm waves is incompatible with observed hummock spacings and possible values of the ratio a/λ under waves (a is the amplitude of oscillation near the bed and λ is the bedform spacing). My view is supported by recent observations on naturally-occurring hummocky megaripples from the sea floor of the inner Atlantic Shelf of North America.

Geochemical exploration models for sedimentary uranium deposits

The geochemical detection of uranium deposits in sedimentary rocks, especially sandstone-type deposits, depends on the geochemical behavior of U and pathfinder elements. Uranium is extensively dispersed under oxidizing conditions but is immobile under reducing conditions. Adsorption on freshly precipitated Fe-oxides and certain types of organic matter also limits dispersion unless high concentrations of CO 3 2− or other complexers are present. Thorium accompanies U in most plutonic processes, but the two elements are separated under oxidizing conditions. Background values for U in water are increased by evaporation and transpiration in arid regions, and are low because of extensive leaching in humid regions. Possible pathfinder elements associated with U in sandstone-type deposits include S, V, Mo, Se, As and at some deposits Cu, Ag, Cr, Pb, Zn, Ni, Co, Re, Be, P, Mn and rare earths, plus He, Rn and other radioactive decay products. Exploration models must take into account channelling of dispersion and possible concealment of deposits by bedding in sedimentary rocks. Most uranium deposits in sedimentary rocks are associated with geochemical provinces enriched in U and Th or with U-rich intrusives or volcanic rocks. However, the deposits may be separated from these U-rich source rocks by tens of kilometers or more. Weak regional U and Th anomalies in sediments containing U deposits are found in the Catskill Fm. of eastern Pennsylvania and may be present around other districts. Zircons from the Catskill sediments are also enriched in U. Halos of anomalous U and small concentrations of U are present around many U deposits but have not been documented in detail. The paleoaquifer leading to a Wyoming-type roll deposit in Pennsylvania contains distinctly anomalous U. Anomalies in U, Se, Mo, V, As, He, Rn, and other pathfinder elements in rock and in ground and surface waters can furnish geochemical guides to ore, as can thermoluminescence, S and C isotopes, and textures of Fe and Ti oxides.

Layering in rapakivi granite, SW Finland

Layering in rapakivi granite occurs locally in the islet Soderoren, Aland, SW Finland. The layering consists of 3—5 cm thick, rhythmically repeated layers with high concentrations of biotite, hornblende, allanite and olivine. Almost all layers are asymmetric with sharp lower contacts but gradually pass-ng into the overlying light layers. Many layers show cross-cutting structures resembling current bedding in sedimentary rocks. It is concluded that the layering is a gravity stratification probably due to convective magmatic currents in the roof zone of a granitic magma.

Lead Isotope Evidence on the Genesis of the Silver-Arsenide Vein Deposits of the Cobalt and Great Bear Lake Areas, Canada

Silver veins of Proterozoic age in the Cobalt area, Ontario, are associated with the thick Nipissing diabase sheet, and those at Great Bear Lake, N.W.T., are also, in part, associated with a diabase sheet. Host rocks in the Cobalt area are flat-lying Aphebian sedimentary rocks, steeply dipping Archean volcanic and associated sedimentary rocks, and diabase, while host rocks in the Great Bear Lake area are intermediate and felsic volcanic rocks and tuffs of the Echo Bay group.Model-lead ages for ordinary galenas are about 1,630 and 2,280 m.y. for the Great Bear Lake and Cobalt veins, respectively, whereas the pitchblende U-Pb age for the Great Bear Lake veins is about 1,445 m.y., and the apparent Rb-Sr age for the Cobalt veins is about 2,160 m.y. An Rb-Sr isochron age of 1,425 + or - 48 m.y. has been obtained for the diabase sill in the Port Radium area, Great Bear Lake.In the Cobalt area essentially identical lead isotope compositions were obtained for interflow base metal mineralization in Archean volcanic rocks, galena along bedding in sedimentary rocks of the Cobalt group, and some late-stage sulfide veins. One interflow sample with a more primitive composition has a model age of at least 2,805 m.y. Mineralization, including deposition of most of the lead in Archean interflow beds, is interpreted to have taken place at about 2,160 m.y. and to be genetically related to the Nipissing diabase. In addition, two anomalous lead lines are defined, one of which suggests an age of 3,260 + or - 100 m.y. for Archean volcanic rocks. A shallow line, represented in only late-stage sulfide veins, is difficult to interpret but could be due to the late addition of a radiogenic component during Paleozoic or younger events not normally considered as causing mineralization, or even from ground water.Leads from the Great Bear Lake area could represent a single anomalous lead line with a slope of 0.1085 + or - 0.0070, and thus have a maximum possible age of 1,130 m.y. Geological events younger than this are known in the Bear Province, but not in the vicinity of the veins, and the same problems of interpretation exist as for the Cobalt area.The source of lead is not defined in either area. However, a source with homogeneous lead is required and the lack of extensive wall-rock alteration suggests that the lead was not leached from the country rocks. The diabase is tentatively favored as the source, but a connate brine or other source is possible.

Clay Colloids as a Cause of Bedding in Sedimentary Rocks

Colloidal clay of the montmorillonite-beidellite group extracted from the Putnam soil of Missouri was studied under the petrographic microscope while undergoing flocculation. The floccules were relatively coarse aggregates so well oriented as to resemble crystals. They were platy in shape, resembling in that respect the mineral in a bedded clay shale. It is suggested that bedding in shale may originate in part in the colloids contributed to it, that clay may be carried far seaward as a colloid and be flocculated and deposited in coarser floccules, and that clay partings and films in limestone may originate from flocculated clay colloids. The texture of fire clays is contrasted with that of shale.