Lateral Changes In Thickness Research Articles

Structural controls on gold-bearing quartz mineralization in a duplex thrust system, Hyde-Macraes shear zone, Otago Schist, New Zealand

The Hyde-Macraes shear zone in east Otago, New Zealand, is a low-angle north- to northeast-dipping thrust system in which gold-bearing quartz veins have been deposited. Mineralization accompanied deformation in the shear zone which is preferentially developed in graphitic pelitic schist. The shear zone is a duplex system dominated by steep hinterland-dipping thrust faults between well-defined shallow-dipping upper and lower bounding thrusts. Imbrication and stacking have occurred in the shear zone, especially where intrashear schist is psammitic and relatively brittle. This stacking contributes to lateral thickness changes from 2 up to 125 m. Mineralized veins formed mainly in dilational jogs or localized extensional zones on the relatively shallow-dipping parts of the intrashear thrust faults. Late-stage shallow south-dipping extensional veins crosscut the duplex structures but are related to the same overall deformation. From analysis of the shear zone structure, it is possible to make predictions about the geometry of mineralized schist within unexposed parts of the shear zone. This is a potentially useful tool for further prospecting and mining in hydrothermally mineralized duplex systems.

Significance of Great Lateral Extent of Thin Units in Newark Supergroup (Lower Mesozoic, Eastern North America): ABSTRACT

The lacustrine sediments of the Newark Supergroup accumulated in rift valleys developed during the early phases of the fragmentation of Pangea. Analogy with modern rift valleys has led us to expect abrupt facies transitions and syndepositional structural complexity. However, in contrast to their supposed modern analogs, many thin (< 6 m) organic-rich lacustrine units within the Newark can be traced over large areas (+2,000 km2) with only gradual lateral changes in thickness and lithology. Lateral correlation is afforded by: (1) detailed ecostratigraphic correlation of fossils in lacustrine cycles, (2) key marker beds such as earthquake-induced fluidized beds, (3) detailed paleomagnetic reversal stratigraphy (by others), and (4) matching of microlaminae and turbidites. These observations suggest that the lakes which produced the organic-rich units were very large (+2,000 km2) and deep (+100 m) while at their maximum extent. The continuity of thin beds across the major intrabasin faults and the gradual lateral change in thickness over the basins show that these faults were not active during the deposition of the units and that the size of the subsiding blocks of these Mesozoic basins was much larger than is currently the case in the east African rifts. Simple analogy between Newark and African rift systems in structure and facies is not justified and obscures their real and important differences. End_of_Article - Last_Page 1444------------

Visco-elastic modelling of lithospheric stresses arising from density contrasts

Summary. A method has been developed for application in conjunction with finite element analysis which can be used to model the stress distributions arising from lateral changes in thickness and density of lithospheric layers. It is especially applicable to those situations where there are large lateral variations in density and rock materials are comparatively weak such as the upper crust of active and passive continental margins. The full effect of gravitational body forces is incorporated in a visco-elastic finite element model which is solved within an iterative scheme that preserves the observed or designed shape of the model. If so desired, further deformations can be studied in the ‘pre-stressed’ model. To compare the method with others used previously, it was applied to a passive continental margin. A model in which the lower crust had a viscosity a factor of 10 smaller than that of the upper crust provided a stress distribution consistent with observational evidence of structure and tectonics. The average shear stress, of 60 MPa, predicted in the continental crust was close to that derived by Artyushkov, using an analytical method.

Facies in Upper Part of Madison Group, Sawtooth Range, Northwestern Montana: ABSTRACT

Portions of the Mississippian Madison Group are gas reservoirs in the plains adjacent to the Sawtooth Range, northwestern Montana, and are equivalent to Mississippian carbonates that are major gas producers in the Canadian Foothills. In the Sawtooth Range, three facies are recognized in the upper 125 m (410 ft) of the Madison Group; they comprise a carbonate shelf sequence that is several hundred meters thick, shoals upward, and is unconformably overlain by Jurassic strata. Economically significant porosity may occur in the upper part of the Madison Group, controlled by the eogenetic secondary dolomitization of a conspicuous crinoidal grainstone unit within it. This dolomitized crinoidal grainstone unit (termed facies C) is the lowest of three facies in the upper part of the Madison Group, and it abruptly overlies lagoonal limestone that forms the major part of the group. Facies C is massively bedded and exhibits large-scale planar cross-stratification suggestive of its origin as a subaqueous dune field. Measured porosity in surface samples of the dolomitized grainstone of facies C is a maximum of 18% and consists of vuggy, intergranular, and intercrystalline pores. The upward transition from limestone to secondary dolomite commonly occurs in the lower part of facies C. The thickness of facies C ranges from 35 to 75 m (115 to 250 ft) and is inversely proportional to the thickness of the intertonguing and overlying facies B. The uppermost two facies, termed B and A, reflect the upward transition from and open platform to a restricted platform environment. Facies B ranges in thickness from 25 to 75 m (80 to 250 ft) and is a nonporous, dolomitized mudstone and wackestone sequence generally containing some 1 m (3 ft) interbeds of porous dolomitized grainstone. This sequence is capped by < 10 m (30 ft) of intertidal rocks of facies A, which are thin-bedded, partly algally laminated, dense dolomites. Locally, facies A and parts of B have been removed as a result of pre-Jurassic folding and erosion in the Sawtooth Range. However, all lateral thickness changes in facies C reflect its intertonguing with B. Although original facies patterns are greatly telescoped by thrusting, the porous grainstones of facies C are best developed in the vicinity of Blackleaf Canyon, Montana, the site of a recently developed commercial gas field in dolomite of the Madison Group. End_of_Article - Last_Page 1350------------

The Caballos Novaculite Revisited: Part II: Chert and Shale Members and Synthesis

ABSTRACT The chert and shale members consist largely of rhythmically intercalated beds of green or gray chert 1 to 15 cm thick separated by shale partings 1 to 5 mm thick. Chert beds are composed of equant grains of microcrystalline quartz, minor amounts of illite and Radiolaria, and trace amounts of pyrite, carbonate and other minerals and organic matter. Chert beds formed by the diagenetic alteration of sediment composed chiefly of biogenic silica from Radiolaria. Fractures and crackle breccias developed in gray chert following lithification; green siliceous sediment, whose lithification was impeded by clay, filled these pre-orogenic fractures. Beds of red shale, chert pebble and cobble conglomerate, sandstone, limestone, dolomite, and lumpy manganiferous and jasperitic chert make up only 3% of the chert and shale members, but are of controversial origin and environmental significance. The chert conglomerate beds, for example, are interpreted as tidal-channel deposits by RLF and as mass-flow deposits by EFM. Jasper beds are bizarre: they are lumpy uneven beds 0.2 to 2 m thick composed of cherry-red chert with local geopetal cavities, contorted laminae. manganiferous zones, cauliflowerlike quartz-filled cavities, zebraic chalcedony, lutecite, quartzine, pseudocubic quartz crystals, and filamentous structures resembling algae. These beds are interpreted by RLF as the product of diagenetic alteration of sabkha evaporite nodules and siliceous ooze during and following soil development and subaerial exposure; and by EFM as the product of diagenetic alteration of evaporite beds deposited in deep water and sandwiched between radiolarian ooze. Synthesis of evidence on the origin of both the novaculite and chert and shale members leads to contrasting interpretations of water depth during deposition. Evidence presented by RLF that favors shallow water include: mounds of possible algal origin, birdseye and Stromatactis-like structures, geopetal fabric, karst features, founder breccias, intraformational unconformities, tidal-channel conglomerate, biosparite beds, petrified logs, and jasper beds that contain evaporite pseudomorphs, paleosoil features, and fresh water algae. Evidence against deep-water deposition includes lateral changes in thickness of novaculite members; absence in novaculite of clay, Radiolaria, cosmic spherules, P and Mn; and narrowness of the Marathon trough during Caballos deposition. Evidence presented by EFM that favors deep water include: similarity of Caballos chert, red shale, and manganese concretions with deep-sea siliceous ooze, red clay, and manganese nodules; spumellarian Radioluria similar to living bathyal to abyssal forms; Helminthoida-like trace fossil; presence of slump structures and mass-flow deposits: Caballos is under and overlaid by deep-water formations; vertical sequences in Caballos of novaculite-radiolarian chert-red shale resembles the vertical sequence of oceanic deposits off spreading ridges (calcareous ooze--siliceous ooze-red clay); some plate tectonic models show the presence of oceanic crust in the greater Ouachita trough during mid-Paleozoic time. Evidence against shallow-water deposition includes absence of shallow-water mari e fossils except in one 3-m thick lithosome, absence of shallow-water trace fossils, and absence of current-formed stratification.