Sheet-like Bodies Research Articles

Pb-Sr-Nd Isotope Data from 30 and 300 Ma Collision Zone Carbonatites in Northwest Pakistan

Virtually all known carbonatites have been intruded within plates by anorogenic processes, usually in rift zones, and can provide information on the chemical characteristics of the under-lying mantle(e.g. Bell et aL, 19821. The carbonatites of Pakistan are especially interesting because they cropout within the Himalayan collision zone. and include both syn-orogenic and preorogenic plutons, offering unique opportunities to study the effects of the orogenic process upon the isotope data. We present isotope data for two synorogenic Sillai Patti and koe Shilman (30 Mal and two pre-orogenic Koga and Jhambi11300 Mal carbonatites, of the Indus Suture Zone. The younger carhonatites are foliated/banded sheet-like bodies emplaced along thrust faults in the metamorphosed belts of the Higher and Lesser Himalaya, within the Indus Suture Zone. They appear to be collision-related and have no relationship to the silicate rocks in the complex. The initial isotope ratios are given in Table I and plotted in a Nd-Pb correlation diagram in Fig. I. The pattern for the 30 Ma samples is atypical given ~hat carbonatites generally, yield positive ~:N,~ (and negative Es,I. In a compilation of analyses for 128 carbonatites from around the world with ages less than 200 Ma (Harmer and Gittins. 1998), 28 plutons yield negative ENa, of which 9 are from E. Africa, and 9 others are from a single complex, the Amba Dongar carbonatite of NW India (Simonetti et al., 1995). Significantly, the Pakistan Nd, Sr, and least radiogenic Pb isotope ratios also fit the pattern of carbonatites from the East African Rift, as shown in Fig. 1. Although all Nd and Sr data fit the East African Rift pattern. Pb in some samples appears to have been contaminated as a result of the collision process. The similarities between the East African Rift and the Pakistan carbonatite isotope data suggest similar sources. We suggest that a lithospheric source for the 30 Ma Pakistan carbonatites was transported with the Indian plate during migration from East Africa to the collision with the Asian continent. Intrusion of the carbonatites into rocks of the Himalayan orogenic zone affected only the Pb isotopes in some of the plutons. Thc anomalies in both terrains are probably somehow related to the presence of the East African Rift. The 300 Ma Koga and Jhambil carbonatites exhibit a very different pattern, with positive EN,dand negative Esrl, similar to that for the majority of world carbonatites, showing that the conditions that led to the anomalous features in the 30 Ma carbonatites did not exist 300 m.y. ago in the future Pakistan terrain. The tectonic settings must have differed greatly from present conditions. The Koga

Granite ascent and emplacement during contractional deformation in convergent orogens

Based on a case study in the Central Maine Belt of west-central Maine, U.S.A., it is proposed that crustal-scale shear zone systems provide an effective focussing mechanism for transfer of granite melt through the crust in convergent orogens. During contractional deformation, flow of melt in crustal materials at depths below the brittle–plastic transition is coupled with plastic deformation of these materials. The flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses. Within the oblique-reverse Central Maine Belt shear zone system, stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain, which suggests percolative flow of melt to form the migmatite leucosomes and viscous flow of melt channelized in sheet-like bodies, possibly along fractures. Cyclic fluctuations of melt pressure may cause instantaneous changes in the effective permeability of the flow network if self-propagating melt-filled tensile and/or dilatant shear fractures are produced due to melt-enhanced embrittlement. Inhomogeneous migmatite and schlieric granite occur in zones of lower strain, which suggests migration of partially-molten material through these zones en masse by granular flow, and channelized flow of melt carrying entrained residue. Founded on the Central Maine Belt case study, we develop a model of melt extraction and ascent using the driving forces, stress conditions and crustal rheologies in convergent, especially transpressive orogens. Ascent of melt becomes inhibited with decreasing depth as the solidus is approached. For intermediate a(H 2O) muscovite-dehydration melting, the water-saturated solidus occurs between 400 and 200 MPa, near the brittle–plastic transition during high- T–low- P metamorphism, where the balance of forces favors (sub-) horizontal fracture propagation. Emplacement of melt may be accommodated by ductile flow and/or stoping of wall rock, and inflation may be accommodated by lifting of the roof at shallower crustal levels and/or sinking of the pluton floor. The resultant plutons have (sub-) horizontal tabular geometries with floors that slope down to the ascent conduits. Although these plutons may have locally discordant relations with country rock structures, when viewed at the crustal-scale, granite ascent and emplacement in convergent orogens are syn-tectonic processes.

Mid-crustal magmatic sheets in the Cascades Mountains, Washington: implications for magma ascent

Diking, diapirism, ascent along faults, and ascent during heterogeneous ductile flow have all been championed as the most important means of magma ascent in the crust. We suggest that these mechanisms are end-members in a complex spectrum of ascent processes. In an attempt to evaluate which combination of ascent processes formed sheet-like bodies in the mid-crustal (20–25 km) Entiat pluton, Washington, we examined the tip regions of these sheets. The sheets have length/width ratios ranging from ∼6 to >75, with increasing ratios strongly correlated to decreasing sheet tip radii (from 850 to 100 m) and decreasing ratios of tip diameter/sheet width (from 0.66 to 0.33). Thus, these bodies have geometries falling between those of dikes and those associated with elliptical diapirs. The sheets are not associated with faults or fracture zones extending from their tips. Instead, sheet walls are oriented parallel to the axial planes of upright, syn-emplacement folds. In sheets with high length/width ratios, magmatic foliations in sheets are folded or parallel to axial planes of host rock folds. With decreasing length/width ratios, margin parallel foliations in both sheet and host rock are increasingly common. Our studies indicate that the sheets are emplaced at high angles to σ 1, not σ 3 as proposed in elastic dike models, and are always associated with complex, viscoelastic flow of host rock. These observations rule out elastic dike and fault models, and instead favor diapiric rise of magma sheets during viscoelastic behavior of host rock.

Glacier-fed(?) sandstone sheets in the Weller Coal Measures (Permian), Allan Hills, Antarctica

The upper member of the Weller Coal Measures (Permian) at Allan Hills, Victoria Land, Antarctica, consists of repetitive coal layers separated by sheetlike bodies of predominantly sandstone and subordinate siltstone and conglomerate, all of fluvial origin. Investigation of the lowermost inter-coal sheet (WC1) reveals abrupt planar basal contacts, persistent upward fining, and a strong westward palaeocurrent trend. Three lithofacies are recognized in the WC1 sheet: (A) small-scale cross-bedded pebbly sandstone, which occupies most of the basal portions of the sheet, (B) large-scale cross-bedded medium-coarse sandstone, confined to a linear NE–SW belt overlying or locally replacing facies A, and (C) siltstone and rippled fine-grained sandstone, which drapes and overlaps facies A and B and contains Glossopteris leaves and Vertebraria root traces. Facies distribution patterns indicate the WC1 body initiated as an abrupt sheetlike incursion of coarse sediment over a low-lying mire (facies A), followed by entrenchment and confinement of flow as the sheet extended distally (facies B). Facies C represents low-energy floodplain sedimentation co-existing with and later succeeding facies B as deposition evolved toward abandonment and eventual renewal of peat deposition. Relatively rapid deposition of the sandstone sheet is implied by the lack of any significant hiatus markers. Outsized clasts up to 45 cm in diameter, one observed to display prominent striations, occur sporadically throughout the WC1, including within siltstone beds of facies C. These are inferred to be ice-rafted deposits which, together with other evidence, suggest meltwater influence, possibly glacial, on sediment transport and deposition. Preliminary examination of other sandstone sheets in the upper Weller indicates attributes similar to the WC1, including abrupt bases, upward-fining character, and predominantly westward palaeocurrent patterns. The Allan Hills exposures are believed to represent a succession of low-lying mire complexes interrupted by abrupt but infrequent incursions of coarse sediment, possibly by extreme meltwater floods.

Subcircular features and autotracking artefacts in 3D seismic interpretation; a case study from the central North Sea

Tertiary deep-water sandstones are major reservoirs in the North Sea, and display an upward gradation from sheet-like bodies to increasingly channelized units. 3D seismic horizon attribute maps at near Top Forties and Top Chalk levels within the Central North Sea portray peculiar textures, which were found to be genuine, and not the result of artefacts caused by the use of autotracking in seismic interpretation. The semi-circular fabric of the near Top Forties event is the result of pre-existing NW-SE structural trends, depositional processes and differential compaction. The pockmark pattern displayed by the Top Chalk horizon is associated with small-scale fractures related to reactivated shear zones and to episodic overpressure release. Use of attribute maps and time slices highlights elusive features, thus refining fairway delineation and understanding of fracture network properties, leading to improved reservoir description and management.

Origin of a classic cratonic sheet sandstone: Stratigraphy across the Sauk II–Sauk III boundary in the Upper Mississippi Valley

Research Article| February 01, 1998 Origin of a classic cratonic sheet sandstone: Stratigraphy across the Sauk II–Sauk III boundary in the Upper Mississippi Valley Anthony C. Runkel; Anthony C. Runkel 1Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota 55114-1057 Search for other works by this author on: GSW Google Scholar Robert M. McKay; Robert M. McKay 2Iowa Department of Natural Resources, Geological Survey Bureau, 109 Trowbridge Hall, Iowa City, Iowa 52242-1319 Search for other works by this author on: GSW Google Scholar Allison R. Palmer Allison R. Palmer 3Institute for Cambrian Studies, 445 North Cedarbrook Road, Boulder, Colorado 80304-0417 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1998) 110 (2): 188–210. https://doi.org/10.1130/0016-7606(1998)110<0188:OOACCS>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Anthony C. Runkel, Robert M. McKay, Allison R. Palmer; Origin of a classic cratonic sheet sandstone: Stratigraphy across the Sauk II–Sauk III boundary in the Upper Mississippi Valley. GSA Bulletin 1998;; 110 (2): 188–210. doi: https://doi.org/10.1130/0016-7606(1998)110<0188:OOACCS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The origin of cratonic sheet sandstones of Proterozoic and early Paleozoic age has been a long-standing problem for sedimentologists. Lower Paleozoic strata in the Upper Mississippi Valley are best known for several such sandstone bodies, the regional depositional histories of which are poorly understood. We have combined outcrop and subsurface data from six states to place the Upper Cambrian Wonewoc (Ironton and Galesville) Sandstone in a well-constrained stratigraphic framework across thousands of square kilometers. This framework makes it possible for the first time to construct a regional-scale depositional model that explains the origin of this and other cratonic sheet sandstones.The Wonewoc Sandstone, although mapped as a single contiguous sheet, is a stratigraphically complex unit that was deposited during three distinct conditions of relative sea level that span parts of four trilobite zones. During a relative highstand of sea level in Crepicephalus Zone time, quartzose sandstone lithofacies aggraded more or less vertically in nearshore-marine and terrestrial environments across much of the present-day outcrop belt around the Wisconsin arch. At the same time, finer grained, feldspathic sandstone, siltstone, and shale aggraded in deeper water immediately seaward of the quartzose sand, and shale and carbonate sediment accumulated in the most distal areas. During Aphelaspis and Dunderbergia Zones time a relative fall in sea level led to the dispersal of quartzose sand into a basinward-tapering, sheet-like body across much of the Upper Mississippi Valley. During early Elvinia Zone time a major transgression led to deposition of a second sheet sandstone that is generally similar to the underlying regressive sheet.The results of this investigation also demonstrate how subtle sequence-bounding unconformities may be recognized in mature, cratonic siliciclastics. We place the Sauk II-Sauk III subsequence boundary at the base of the coarsest bed in the Wonewoc Sandstone, a lag developed through erosion that occurred during the regional regressive-transgressive event that spanned Aphelaspis to early Elvinia Zones time. Such sequence-bounding unconformities are difficult to recognize where they are contained within coarse siliciclastics of the Upper Mississippi Valley, because they separate strata that are texturally and mineralogically similar, and because erosion occurred on a loose, sandy substrate along a low, uniform gradient, and in a nonvegetated terrestrial environment. Furthermore, the ultramature mineral composition of the exposed substrate is resistant to the development of a recognizable weathering profile.The well-known sheet geometry of the Wonewoc and other units of lower Paleozoic sandstone of this area is not dependent on atypical terrestrial depositional conditions conducive to the widespread distribution of sand, as commonly believed. Sand was spread into a sheet dominantly within the marine realm in a manner similar to that inferred for many better-known sandstone bodies deposited in the North American Cretaceous Western Interior seaway and Tertiary Gulf of Mexico. The laterally extensive, thin character of the Upper Mississippi Valley sandstone bodies compared to these other sandstone bodies simply reflects deposition of a continuously abundant supply of sand on a relatively stable, nearly flat basin of slow, uniform subsidence during changes in sea level. The dearth of shale in this and other cratonic sandstones can be indirectly attributed to the same controls, which led to an uncommonly low preservation potential for fairweather deposits on the shoreface. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cretaceous Depositional Systems in the Norwegian Sea: Heavy Mineral Constraints

Deep-water Cretaceous sandstones in the Norwegian Sea were deposited by a number of distinct sediment transport systems tapping different sediment source terrains. Three distinct sandstone types (K1, K2, and K3) have been identified, distinguished, and mapped on the basis of a combination of heavy mineral parameters. Sandstone type K1 occurs on the Trondelag Platform, Halten Terrace, and Nordland Ridge, but does not appear to be present in the deeper water Voring Basin. Sandstone type K1 was ultimately derived from the Scandinavian landmass with detritus sourced from metasediments of the Caledonian fold belt, intrusives of the Trans-Scandinavian Igneous belt, and, to a smaller extent, Svecofennian basement. K1 sandstones were deposited on an unstable slope by debris flows and slumps, with minor reworking by bottom currents. Sandstone type K3 was derived from the Western Gneiss region farther south on the Scandinavian landmass. Sandstone type K2 occurs in more basinal locations in the Norwegian Sea and was not supplied by the systems operating along the Scandinavian margin because its mineralogy contrasts with that of K1 and K3. K2 mineralogy is not consistent with a source in the Lofoten area or East Greenland; therefore, K2 is believed to represent the deposits of a separate axial transport system fed by a source that lay in northeast Greenland. K2 zircon ages indicate involvement of Early Proterozoic (approximately 2000 Ma) and Archean basement, together with metasediments of the Caledonian fold belt. Previous sedimentological models for the Cretaceous of the Norwegian Sea suggest that sand deposition occurred as slumps and debris flows along the continental slope, resulting in discontinuous and unpredictable sandstone units, unless they become amalgamated into thick reservoir sequences. The mineralogical evidence indicates that this model can be applied only to sandstone types K1 and K3. By contrast, sandstone type K2 represents sediment introduced from the conjugate margin of the basin and occurs up to 200 km from its detrital source region; therefore, K2 is likely to occur as more predictable, sheetlike bodies on the basin floor.

Facies architecture and sedimentology of a meandering fluvial system: a Palaeogene example from the Weisselster Basin, Germany

The Schleenhain open pit coal mine, located 30 km south of Leipzig, Germany, exposes Upper Eocene and Oligocene non‐marine strata representing fluvial deposition in the centre of the Weisselster Basin. Active mining and successive cuts provided the rare opportunity to obtain a three‐dimensional perspective of laterally extensive surface outcrops. These were used to construct a detailed fence diagram, which provided the basis for recognition of key architectural elements in the weakly consolidated meandering stream deposits. In addition to the eight basic architectural elements of Miall (1985), the element SL (shallow lake deposits) was newly defined and the element CH (channel) was subdivided into CHg (palaeo‐river system) and CHk (small channel). The profiles contain parts of two fining‐upward cycles, which are separated by an unconformity spanning the Early Oligocene. Deposits of the first cycle begin with transverse sand bars (downstream accretion deposits‐DA) and point bars (lateral accretion deposits‐LA). The upper part of the cycle is represented by overbank fines (OF) and the element SL, which consists of laterally discontinuous lenses of dark, plant‐bearing, kaolinite‐rich clays, that were deposited in shallow lakes adjacent to the active channel. Coal seams interlayered with palaeosols are the main constituents of element OF. Sheetlike bodies of medium to fine gravels (gravel bars and bedforms‐GB) on an erosive coal surface mark the beginning of the second cycle. Dissolution of underlying Permian salts and sulphates prior to, during, and after the deposition of the Palaeogene strata caused the development of two synclines within the outcrop. Coal seams and clay horizons which thicken and dip towards the centre of the synclines, provide evidence for their chronological development.

Long-term river response to regional doming in the Price River Formation, central Utah

A profound down-basin change in alluvial architecture is coincident in time and space with the early phase of uplift of the San Rafael Swell along the Book Cliffs of central Utah. Marked thinning in the lower part of the Price River Formation (Campanian) approaching the crest of the swell demonstrates that the swell was first active as a region of reduced subsidence rate before it developed topographic relief. A comparison of total unit thickness, percentage of sandstone, and relative abundance of sheet vs. isolated sand bodies in vertical sections within the Price River Formation and laterally equivalent Farrer Formation shows changes in these measures across the swell crest. The abundance of isolated lenticular fluvial sand bodies increases in the area of reduced aggradation at the expense of sheetlike bodies. This change in alluvial architecture over the crest of the swell is found only in units laid down when the swell was active, not in overlying or underlying units. This coincidence strongly suggests a link between early growth of the swell and alluvial response. However, the sense of change—i.e., fewer connected sand bodies in the region of reduced aggradation—is opposite to that predicted by most alluvial-architecture models. After comparison to other field and experimental studies, it seems likely that different river systems may reveal different avulsion frequencies that ultimately produce varied, but not necessarily unpredictable, alluvial architectures.

Device for modifying the sound of a drum

A device for modifying the sound of a drum or other percussion instrument comprises a sheet-like body of a soft, flexible polymer such as a gelled plastisol. The device includes a planar bottom surface which establishes a vacuum seal to a vibratable surface of the instrument.

Reservoir Compartmentalization Caused by Mass Transport Deposition, Northwest Stevens Pool, Elk Hills Naval Petroleum Reserves, California: ABSTRACT

The [open quotes]A[close quotes] sands of the Northwest Stevens Pool consist of six major subdivisions (A1-A6) and numerous sublayers. These sands are above the [open quotes]N Point[close quotes] stratigraphic marker, making them much younger than most other Stevens sands at Elk Hills. Cores show the A1-A3 sands to be possibly mass transport deposition, primarily debris flows, slumps, and sand injection bodies. The A4-A6 sands are characterized by normally graded sheet-like sand bodies Hospital of traditional outer fan turbidite lithofacies. Most current production from the A1-A2 interval comes from well 373A-7R, are completed waterflood wells that came on line in 1992 at 1400 BOPD. Well 373A-7R is an anomaly in the A1-A2 zone, where average production from the other ten wells is 200 BOPD. Other evidence for compartmentalization in the A1-A2 interval includes sporadic oil-water contacts and drawdown pressures, difficult log correlations, and rapid thickness changes. In 1973, well 362-7R penetrated 220 ft of wet Al sand. The well was redrilled updip and successfully completed in the A1, where the oil-water contact is more than 130 ft lower than the original hole and faulting is not apparent. In 1992, horizontal well 323H-7R unexpectedly encountered an entirely wet Al wedge zone. Reevaluation more » of the A1-A3 and other sands as mass transport origin is important for modeling initialization and production/development strategies. « less

Palaeomagnetic constraints on the emplacement of the Bushveld complex

Pertinent results of all palaeomagnetic investigations on the Bushveld complex are reviewed and analysed to find the constraints the palaeomagnetic data impose on the genesis and structure of the Bushveld complex. It is concluded that the emplacement of the complex must be viewed as a series of magmatic events, ending with subsidence in the central area of the complex to produce a sheet-like body dipping towards the centre of the Bushveld complex.

Geologic and geochemical characteristics of the Yichun Ta-Nb-Li deposit, Jiangxi Province, South China

The Yichun Ta-Nb-Li mine is situated within a small, sheetlike body of topaz-lepidolite granite which is the most fractionated phase of the Yashan batholith. Volumetrically, the most important phase in the batholith is a coarse-grained, protolithionite-muscovite granite which has a finer grained, muscovite granite margin. The protolithionite-muscovite granite is intruded by small bodies of muscovite granite which form the locus of a tungsten vein system at the Xin Fang tungsten mine. Two sheetlike bodies, a lower Li mica granite and an upper topaz-lepidolite granite intrude the upper contact between the protolithionite-muscovite granite and the host metasediments. The granites are crosscut by unmineralized porphyry dikes that are exploited as a source of kaolin. The topaz-lepidolite granite is a major, low-grade Ta-Nb-Li deposit which is exploited via open-pit mining. The topaz-lepidolite granite is composed dominantly of albite, lepidolite. and quartz, with topaz. K feldspar, amblygonite, and accessory zircon, monazite, pollucite, columbite-tantalite, microlite, and Ta-rich cassiterite. Phenocrysts of quartz, topaz, and K feldspar contain abundant inclusions of albite laths and occasional lepidolite crystals along growth zones (snowball texture), indicating simultaneous crystallization of these minerals from a subsolvus, residual magma. The topaz-lepidolite granite is characterized by high F, Li, P, Al 2 O 3 , and Na 2 O contents (with Na 2 O > K 2 O), and by very low SiO 2 , TiO 2 , MgO, CaO, and K 2 O contents. It is strongly enriched in Rb, Nb, and Ta, and strongly depleted in Sr and Zr compared with the earlier granite phases. Chemical trends are consistent with an origin via fractionation involving especially the removal of plagioclase, K feldspar, quartz, Fe-Mg silicates, and zircon. Most of this fractionation appears to have taken place within a deeper level magma chamber, with successive magma batches emplaced into the present level of exposure. Mineralization at Yichun consists of columbite-tantalite, microlite, and Ta-rich cassiterite which occur throughout the topaz-lepidolite granite closely associated with, and as inclusions, in lepidolite. The ore minerals also occur as interstitial crystals between the major silicate phases. Crystallization of the Nb-Ta-Sn minerals is an integral part of the cystallization of the topaz-lepidolite granite, and the effects of postmagmatic alteration in the formation of mineralization are absent or minor.

Very large debris-avalanche deposit within the Eocene volcanic succession of the northeastern Absaroka Range, Wyoming

A very large debris-avalanche deposit found within the Eocene volcanic succession in the northeastern Absaroka Range, Wyoming, is a result of the failure and collapse of an ancient volcanic edifice in the vicinity of Sunlight Peak, a principal early middle Eocene center of volcanic-plutonic activity. The debris-avalanche deposit is a sheetlike body that consists of blocks, individually as large as several square kilometres, of well-stratified vent medial facies lava flows, breccias, and sandstones within a thin, heterogeneous, and complexly deformed matrix of boulder- to sand-sized volcaniclastic material. The avalanche was directed to the southeast along a broad paleovalley, and remnants have been found >40 km from the source area. The deposit initially covered an area of at least 450 km2 and had a minimum volume of 100 km3, making it one of the largest subaerial avalanches ever recognized.

The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna greenstone belt, Western Australia

Komatiites in the western part of the Norseman-Wiluna greenstone belt in the Yilgarn Block of Western Australia display a wide variety of volcanic facies, ranging from very thin differentiated Munro Township-type flow units through to very thick olivine-rich cumulate flow units containing high proportions of adcumulate dunite. Cumulate flow units have been mapped in detail in the Agnew-Wiluna segment of the Norseman-Wiluna Greenstone Belt. In the Yakabindie and Mt. Keith areas, thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet-like sequences of finer grained olivine orthocumulates. They show fine-scale internal layering, broad-scale cryptic layering and upper fractionated sequences containing harrisites, pyroxene bearing cumulates and in some cases gabbroic derivatives. Low grade disseminated sulphide mineralisation occurs within the dunite lenses. In the stratigraphically equivalent Kathleen East area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows. A similar relationship is also seen further south, where the Perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows. These field relationships indicate an extrusive origin for the adcumulate dunite bodies. The Walter Williams Formation, in the west-central part of the Norseman-Wiluna Belt, is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent. It consists of a central sheet-like body of adcumulate dunite, underlain and overlain by olivine orthocumulates. The adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite. At its northernmost extent, the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros, dolerites and pyroxene spinifex-textured material, interpreted as a periodically replenished lava lake sequence. Both the lenticular dunite bodies of the Agnew-Wiluna Belt and the sheet-like dunite body of the Walter Williams Formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates. The adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows. Flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates. Field evidence from Perseverance, and the general geometry of the dunite lenses, suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low-melting felsic volcanic substrates. The textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion. Dunite sheets form close to the eruption sites, and dunite lenses as a result of channellisation further away from the vent, or close to the vent in the situation where the substrate is non-refractory and thermal erosion can take place readily. Kambalda-type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic, and Munro-type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions, or close to the vent of very small ones. Waning eruption rates leads to proximal facies being overridden by distal ones, a common observation in komatiite sequences. The size of thermal erosion channels requires very rapid eruption rates comparable to those in Phanerozoic flood basalt terrains.

The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna greenstone belt, Western Australia

Komatiites in the western part of the Norseman-Wiluna greenstone belt in the Yilgarn Block of Western Australia display a wide variety of volcanic facies, ranging from very thin differentiated Munro Township-type flow units through to very thick olivine-rich cumulate flow units containing high proportions of adcumulate dunite. Cumulate flow units have been mapped in detail in the Agnew-Wiluna segment of the Norseman-Wiluna Greenstone Belt. In the Yakabindie and Mt. Keith areas, thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet-like sequences of finer grained olivine orthocumulates. They show fine-scale internal layering, broad-scale cryptic layering and upper fractionated sequences containing harrisites, pyroxene bearing cumulates and in some cases gabbroic derivatives. Low grade disseminated sulphide mineralisation occurs within the dunite lenses. In the stratigraphically equivalent Kathleen East area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows. A similar relationship is also seen further south, where the Perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows. These field relationships indicate an extrusive origin for the adcumulate dunite bodies. The Walter Williams Formation, in the west-central part of the Norseman-Wiluna Belt, is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent. It consists of a central sheet-like body of adcumulate dunite, underlain and overlain by olivine orthocumulates. The adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite. At its northernmost extent, the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros, dolerites and pyroxene spinifex-textured material, interpreted as a periodically replenished lava lake sequence. Both the lenticular dunite bodies of the Agnew-Wiluna Belt and the sheet-like dunite body of the Walter Williams Formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates. The adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows. Flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates. Field evidence from Perseverance, and the general geometry of the dunite lenses, suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low-melting felsic volcanic substrates. The textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion. Dunite sheets form close to the eruption sites, and dunite lenses as a result of channellisation further away from the vent, or close to the vent in the situation where the substrate is non-refractory and thermal erosion can take place readily. Kambalda-type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic, and Munro-type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions, or close to the vent of very small ones. Waning eruption rates leads to proximal facies being overridden by distal ones, a common observation in komatiite sequences. The size of thermal erosion channels requires very rapid eruption rates comparable to those in Phanerozoic flood basalt terrains.

Paleoshoreline patterns in the transgressive-regressive sequences of Pennsylvanian rocks in the northern Appalachian Basin, U.S.A.

Sheets of sponge spicule flint of Pennsylvanian age (Bashkirian, Moscovian, Kasimovian) that are present in the northern Appalachian Basin of Ohio and adjacent parts of Kentucky, Pennsylvania and West Virginia, are important indicators of paleoshorelines. This flint typically occurs with or occupies the position normally held by shallow-water limestone and contains a normal marine fauna. The flint was deposited above coal or underclay, representing the detritus-starved marine portion of a transgressive-regressive sequence and marking the eastern limit of transgression across a westward-spreading alluvial plain. Flint occurs at several stratigraphic positions in the upper Pottsville-lower Conemaugh interval. The most important are: Boggs, Upper Mercer and Kanawha flints of the upper Pottsville Group; Kilgore-Flint Ridge, Zaleski and Vanport flints of the lower Allegheny Group; and Brush Creek flint of the lower Conemaugh Group. Lithofacies maps of these beds were constructed to show the distribution of the flint. Limestone-hosted flint occurs in long discontinuous chains of sheetlike bodies, whereas shale-hosted flint occurs in single sheets with restricted geographic distribution. Chains of limestone-hosted flint attain maximum dimensions of a few meters in thickness, a few kilometers in width and several hundreds of kilometers in length. The Upper Mercer, Vanport and Brush Creek flints are particularly extensive, forming arcuate shoreline patterns that parallel the fronts of large delta systems. Beds of clay ironstone and/or coal above flint indicate that the lagoonal environment in which flint was deposited was followed closely by a change to stagnant waters. Cementation of flint with silica likely occurred under the lower pH conditions existing at that time and when depths of burial were shallow.

Crystallization of Hydrous Magmas: Calculation of Associated Thermal Effects, Volatile Fluxes, and Isotopic Alteration

The metamorphism and accompanying isotopic alteration of country rock in contact with an instantaneously emplaced sheet-like body of hydrous magma has been studied using a one-dimensional analytical solution and numerical modeling. The model includes a consideration of the complete crystallization and degassing history of the magma, coupled with conductive and convective heat flow and mass transfer in the porous country rock, and in the magma layer itself. The dynamics of cooling of the magma determine the velocity with which the solidus point (solidification front) moves downward, and this in turn gives (by conservation of fluid mass) the magnitude of the flux of aqueous magmatic fluid that is released to flow upward through the country rock. The fluid flux is therefore variable because it depends upon the temperature evolution of the magma, and this allows us to make several new statements about the paths of rocks undergoing contact metamorphism as a function of distance from the contact and the temperature, composition, and water content of the intruded magma. In previous studies of metamorphic fluid flow, the fluid flux has usually been assumed constant, or only the integrated effects of the fluid flux were studied. The parameterization of phase diagrams of hydrous magmas at fixed pressure is discussed in detail and a simple scheme is developed. For a given magmatic water content, we suggest a description of the variation of the melt fraction with temperature that includes only one dimensionless parameter, , which represents the fraction of melt generated close to the solidus temperature during melting. This parameterization allows us to calculate the evolution of temperature and volatile flux during magma crystallization. The results of the numerical calculations are shown to depend upon and two other dimensionless parameters: the Stefan number (Ste) and the solidus temperature () of the magma. Polynomials are given that describe the numerically calculated contact temperature, the contact temperature gradient, the timing of fluid release and the crystallization time as a function of these three parameters. We discuss the application of the results to natural situations and use them to classify the temporal evolution of metamorphic and isotopic zonation in contact metamorphic aureoles. Three types of time-temperature-fluid infiltration trajectory are recognized within the aureole, and these define a series of zones, following in a specific sequence moving away from the contact. Identification of such zones in metamorphic terranes allows assessment of the plausibility of magmas as the principal fluid sources, and documentation of their relative width provides a means to quantify various aspects of the crystallization history. We demonstrate that the maximum width of an oxygen isotope alteration zone caused by magmatic volatiles is unlikely to exceed 1 km, and most natural situations will involve much smaller alteration zones (e.g., tens to hundreds of meters). Because we are able to predict the thermal and isotopic effects exclusively due to outward directed flow of magmatic fluids, we can use our results to distinguish such situations from others which involve inward directed flow of externally derived fluids. Because systems domi-nated by magmatic fluids are likely to be more common at greater depth in the crust, our model may be particularly appropriate for metamorphism in the deep crust, adjacent to underplated magmas. Such volatile fluxes will exert an important influence on deep crustal melting processes.

Seismic reflections from within the Lake District batholith, Cumbria, northern England

Seismic reflection data from the Wasdale area, Cumbria, reveal reflector-poor and reflector-rich areas within components of the Lake District batholith. A representative panel of seismic data crossing the western boundary of the intrusion has been modelled with synthetic seismograms. The preferred model indicates that the Ennerdale Granophyre is a sheet-like body some 1100 m thick. Beneath this, the batholith comprises multiple poorly-reflective laccolithic granitic intrusions, interfingered with moderately reflective wedges of country rock, to give the steep western margin of the granite complex a 'cedar-tree' profile. Within the main body of the batholith, reflector-rich zones may arise from either areas of included country rock, basic sills or zones of mafic material within granite.

Contrasting development patterns of crevasse channel deposits in cretaceous alluvial successions, Korea

The Eumsung and Pyonghae basins (both of Cretaceous age) in Korea comprise a thick alluvial sequence of conglomerate, sandstone, and purplish siltstone. Laterally impersistent, gravelly sandstone beds can be classified into two types based on bed geometry, internal structure and occurrence: (a) vertically stacked, sheetlike bodies (facies 1), and (b) migratory, ribbon-shaped bodies (facies 2). Facies 1 is characterized by symmetrical channel geometry with a large ratio of width to thickness of sandstone beds which are mostly stratified and bounded by relatively distinct lower and upper boundaries. They are frequently found in aggradational style. Facies 2 shows asymmetric geometry with a relatively small ratio of width to thickness of sandstone beds. It comprises massive, cross-stratified units bounded by sharp basal contacts and gradational upper boundaries, which resulted from laterally shifting mode. Both types represent crevasse/distributary channels developed in floodplain environments. Facies 1 is suggestive of gradual migration of crevasse/distributary channels under the conditions of overall aggradation, whereas facies 2 represents discrete, abrupt diversion of crevasse channels. These features are due to differences in floodplain gradients, paleohydrology and trunk channel behaviour.