Marginal Facies Research Articles

Marginal Erg Facies: A Trial Approach toward a Descriptive Classification

During the late 1970s and early 1980s, sedimentologists began recognizing the margins of eolian sand seas as separate, components which differed from interior sand seas in geometry, extent, and facies. Stratigraphers have now observed these differences in eolian rocks. Erg margins may be grouped in five ways: (1) by associations with extradunal environments-coastal plain, lacustrine, periglacial, marine (tidal flat, coastal sabkha, beach, and lagoon), and arid alluvial (alluvial fan, fluvial, playa, inland sabkha); (2) by allocyclic controls-eustasy, plate tectonism, and climate; (3) by autocyclic controls-local tectonism, topography, vegetation, hydrology, structure, sediment source and supply, and wind regime; (4) by geographic position-upwind, downwind, and along-wind margins; and (5) by sedimentary facies-texture and architecture. In contrast with erg interiors, erg margins are characterized by smaller, less complex dune-forms related to thinner sand accumulation; elementary dune architecture; more vegetation and bioturbation; high occurrence of sand sheet, zibar, and serir facies; expansive, low-relief interdunes with widely distributed dunes; and a greater proportion of interbedded extradunal deposits. Some of the published studies on ancient eolian systems have identified erg margin facies that have been influences by marine and arid alluvial processes. Few reports have described lacustrine-eolian and periglacial-eolian interactions. This study is an attempt tomore » organize known features of modern and ancient erg margins into a scheme based on erg margin controls.« less

Probable Reservoir Facies of the Wapanucka Limestone (Morrowan), Frontal Ouachita Mountains, Southeastern Oklahoma

Considerable recent exploration and significant discoveries in the frontal Ouachita Mountains can be attributed to structural and stratigraphic relationships within the overthrust belt. Although, most of the production has been from the early Atokan Spiro Formation, the late Morrowan Wapanucka Formation has locally proven to be a gas reservoir. Surface imbrications allow outcrop study of the Wapanucka lithofacies and form a basis for predicting the character of rocks in the subthrust region. Repetitive sequences of platform, platform margin, and basinal facies characterize the Wapanucka Formation in outcrop. These sequences record several southward progradations of the ramp-like shelf margin. Oolitic and bioclastic calcarenites accumulated on the shallow shelf, while algal micrites developed near the shelf edge. Spiculitic limestones characterize the platform margin, and slope and basinal depositional environments are inferred for the noncalcareous spiculites and shale. Within the Wapanucka, several facies are potentially hydrocarbon reservoir rocks. These include oolitic grainstone and spiculitic packstone. The former lithology might preserve primary porosity, while the latter facies could develop significant fracture porosity in certain structural configurations. Thus, mapping their surface extent is significant because well control and seismic data do not provide this kind of information. However, detailed surface work in association with themore » subsurface geology can aid in locating potential hydrocarbon reservoirs.« less

Prediction of Reservoir Geometries in the Morrow of Southeastern New Mexico

The Morrow Formation of southeastern New Mexico is comprised of facies that were deposited in a complex of mixed siliciclastic and carbonate depositional environments. Reservoir geometries in gas-bearing sandstones are highly variable and identify a number of different depositional environments. In particular, environments identified within marginal marine facies tracts in the Morrow indicate that the shoreline was constantly shifting in response to relative sea level changes. Depositional processes were sensitive to varying sediment discharge rates as well sheltering by offshore carbonate mounds. Sandstone/carbonate sequences also indicate periodic reworking or drowning of marginal marine sediment throughout the Morrowan, which was especially prevalent in middle Morrowan time. The southern shelf of Belize, Central America, offers an excellent analog to depositional process that were operative during the Morrowan in southeastern New Mexico. Modern facies tracts along the Belizean shoreline respond to varying discharge rates of sediment from the Maya Mountains source area to the west and to sheltering from the Maya Mountains source area to the west and to sheltering from the offshore barrier reed and other carbonate mounds. Carbonates are actively developing upon drowned beach ridges and channel mouth bars, and there is evidence of drowned fluvial channels several miles from present-daymore » shoreline. Depositional patterns in Belize are identical in scale to those believed to the operative in the Morrow and offer analogs that are useful as predictive tools for exploration and development.« less

Two-Dimensional Forward Modeling of Lower Ordovician Platform Carbonate Sequences (Beekmantown Group, Central Appalachians): The Search for High-Frequency Autocycles

Platform carbonates of the Beekmantown Group are organized into several onlap-offlap facies tongues that record four third-order depositional sequences (each about 150 m thick; 3 myr duration). A dip-oriented, 2-D facies mosaic (600 m thick; 275 km wide) displays a lateral facies zonation that, along a given time line, consists of (1) tidal flat facies composed of peritidal fifth-order cycles (average 3 m thick; 50-100 kyr duration); (2) inner shelf facies composed of shelf-lagoon grainstones-packstones; (3) outer shelf facies composed of Renalcis-Girvanella thrombolites and grainstones recording a shelfal patch reef setting; and (4) shelf margin facies dominated by Epiphyton framestone mounds and grainstones recording a shelf-edge reef-shoal complex. Sequence boundaries are stratigraphically conformable with no evidence for erosion or downward shift in facies. Third-order accommodation changes, as revealed by systematic stacking patterns of thickening and thinning fifth-order cycles, are correlated across strike using megafossil zonation. Time series analyses detected no peritidal cycle rhythms suggestive of Milankovitch forcing. This, coupled with the progradational nature of individual cycles, suggests a Ginsburg autocyclic mechanism linked to onshore sediment transport and seaward progradation of tidal flats. The 2-D internal facies architecture of a sequence and cycle stacking patterns have been simulated via computer utilizingmore » an autocyclic mechanism for fifth-order cycle development triggered by third-order accommodation changes.« less

Tectonic Implications of Facies Patterns, Lower Permian Dry Mountain Trough, East-Central Nevada

Paleozoic tectonism is indicated by a study of a west-east facies analysis transect across the northern portion of the Lower Permian Dry Mountain trough (DMT). In an attempt to characterize the Early Permian basin-filling sequences, three broadly recognizable facies packages have been identified across the DMT: the western margin facies and the central basin facies of the DMT and an eastern shelf facies. In the western margin facies of the basin, pulses of tectonic activity are recorded at McCloud Spring in the Sulphur Springs Range. Here, shallow open-marine carbonate overlies eroded Vinini Formation and, in turn, is unconformably overlain by basinal marine carbonate. An unconformity also marks the contact with the overriding prograding coarse clastic facies. These abrupt transitions suggest the sediments were deposited in a tectonically active area where they preservation of Waltherian sequences is unlikely to occur. Similarly abrupt transitions are evident in the western part of the central basin facies. At Portuguese Springs n the Diamond Range, a thin basal marine conglomerate delineates Lower Permian sedimentation over the Pennsylvanian Ely Formation. Coarsening-upward basinal carbonate strata of pelagic, hemipelagic, and turbidite components overlie the basal conglomerate. this progression of sediments is unconformably overlain by a subaerial sequence ofmore » coarse clastic deposits. Within the eastern part of the central basin facies in the Maverick Spring Range, the Lower Permian sediments are open-marine siltstone, wackestone, packstone, and grainstone. The sediments are assigned to a gradually sloping ramp, indicating the effects of tectonism on this margin of the basin were subdued.« less

Significance of a late Caledonian igneous complex revealed by clasts in Lower Old Red Sandstone conglomerates, central Scotland

Boulder-bearing conglomerates from the Lower Old Red Sandstone (ORS) Crawton Group in east-central Scotland contain large granite clasts (0.4-1.0 m in diameter) which were derived from an area of the Midland Valley now concealed by younger rocks. These clasts provide new information concerning early Paleozoic magmatism along the margin of Laurentia. Rb/Sr mineral-whole-rock age dating for a series of calcalkaline tonalite, granodiorite, and granite boulders produces a cluster of late Wenlock-early Ludlow ages (ca. 420 Ma) which are interpreted as recording the original emplacement of hinterland plutons. The clast ages extend back to the late Ashgill (443 Ma), and the magmatism identified thus bridges a gap between an older Ordovician detrital record of coeval magmatism in the Midland Valley, and in situ ORS (Late Silurian/Early Devonian) volcanic rocks in this zone. Evidence from hornfels clasts and xenoliths within igneous boulders suggests that some of the granitoid intrusions were emplaced into an unseen, Lower Ordovician flysch sequence from which graywacke and limestone clasts were also derived. Textural and other evidence indicates that the source plutons rose to a high structural level in the crust. Both foliated and unfoliated granites have been distinguished, but these have no straight-forward relationship with Rb/Sr age. The foliated tonalite clasts are unlikely to have been derived from an autodeformed marginal facies, and the association of synchronous deformed and undeformed granitoids resembles the pattern of emplacement in areas undergoing transcurrent shear; this is consistent with evidence suggesting that juxtaposition of the northern Midland Valley and Grampian terranes was largely achieved during this interval. The cryptic igneous rocks share some features with in situ late Caledonian granites with which they overlap in age (for example, high Na, initial low 87 Sr/ 86 Sr), but differ in that tonalitic compositions and uniform positive ϵ Nd values were developed. The isotopic distinction between Silurian Midland Valley and Grampian Highlands magmas probably reflects the disparate lithospheric structure of the terranes now separated by the Highland Boundary fault and is a robust feature of subsequent magmatism, persisting for at least 70 m.y. Temporal changes in magmatism associated with the Midland Valley during the Ordovician to mid-Silurian may have been influenced by thinning of previously thickened arc crust.

The molluscan fauna of the middle pleistocene interglacial deposits at Little Oakley, Essex, and its environmental and stratigraphical implications

The molluscan fauna of the interglacial channel deposits at Little Oakley, Essex, U.K., is described. Detailed faunal analyses are presented from three boreholes located at different sites across the palaeochannel. Large-bulk samples [ ca. 1.5 t), obtained from mechanically excavated pits at one of these sites, have yielded a few additional species and enabled the resolution of several taxonomic problems that result from the fragmentary nature of some of the borehole specimens. The fauna as a whole is fully temperate throughout and indicates the presence of a large, welloxygenated river, thought to be the ancestral Thames. Mollusca from fringing marsh habitats are also present, together with xerophilous species from dry calcareous grassland. Woodland species are virtually absent, suggesting that the river here had a wide, open floodplain. The fauna indicates a Cromerian age and includes the extinct prosobranch Tanousia (= Nematurella auctt.), which is unknown anywhere in northwest Europe after this interglacial stage. Other noteworthy species include Sphaerium solidum (only British Pleistocene record), Belgrandia marginata , Helicella itala and cf. Cernuella virgata (earliest British Pleistocene records), Trochoidea geyeri and Truncatellina cylindrica (only British Cromerian records). There are significant differences between the faunas at each of the sampling sites, which are thought to reflect mid-channel and marginal facies. This interpretation is in complete accord with the sedimentology and gives strong support for the local palaeogeographical reconstructions. There are also some biostratigraphical changes: Tanousia declines upwards and is totally absent in the upper levels of one borehole that is thought to extend later into the interglacial. The significance of this disappearance as caused by either regional (climatic) or local factors is unclear.

The Moel‐y‐Golfa Andesite: An Ordovician (Caradoc) intrusion into unconsolidated conglomeratic sediments, Breidden Hills Inlier, Welsh borderland

AbstractThe Moel‐y‐Golfa Andesite is a discordant intrusive body of porphyritic pyroxene andesite, 1 km across and at least 200 m thick. Textural variation defines a central facies and a marginal facies, the latter displaying well‐formed lobes, tubes, and pillows. The vertically stacked lobes, brecciated tubes, and elongate pillows of andesite intrude and locally enclose enclaves of altered host sediment and are cut by steeply dipping slide scars. The host sediments are locally strongly silicified and cut by numerous, anastomosed vertical and subvertical fissures. Fissures are filled by disrupted host sediment and fragments of brecciated andesite. The formation and filling of each fissure system was probably concomitant with the brecciation of an intrusive magma lobe. The complex relationship of igneous and sedimentary material indicates that the Moel‐y‐Golfa Andesite was emplaced into the host sediments whilst they were unconsolidated or only partly lithified.

Geology and Reservoir Description of a Dolomitized Upper Devonian Reef, Westerose Field, Alberta: ABSTRACT

The Westerose field produces hydrocarbons from a dolomitized Upper Devonian Leduc reef. The reef is 1.6 km wide, 5.5 km long, and up to 175 m thick. Core description, statistical analysis of whole core data, well logs, and pressure buildup data, were integrated in the development of a reservoir model. In contrast to well-known limestone reservoirs in western Canada, the organic reef margin facies and reef interior facies are not strongly differentiated. The reef interior does not contain laterally extensive barriers to vertical fluid flow. The secondary pore system dominates porosity. The large solution vugs and fossil molds are well connected through the dense matrix by a vertical fracture system. Porosity and permeability increase from the reef flank to the interior and are optimum within the lower reef interior. The reservoir model, interpreted from pore system distribution and supported by reservoir pressure response, is a disordered arrangement of permeability blocks. Cumulative probability plots indicate that the horizontal permeability data set is top truncated, with up to 35% of the highest permeability values not represented in the data set. Examination of a Leduc equivalent outcrop yielded data on pore-size frequency and porosity at scales larger as well as smaller than coremore » diameter. The sample size necessary to make representative measurements of reservoir properties can be estimated from the outcrop analysis.« less

Lower Cretaceous (Neocomian Age) Calvin Limestone Shelf-Margin Complex, Natchitoches Parish, Louisiana: ABSTRACT

ABSTRACT The Calvin Limestone is a lower Cretaceous (Neocomian age) carbonate shelf margin complex. This carbonate complex is stratigraphically located between the Winn Limestone and the Knowles Limestone (Tithonian age). Regional geologic and seismic correlations show this shelf margin to be stratigraphically equivalent with the Calvin Sandstone, Calvin Field Winn Parish, Louisiana. The potential reservoirs in the Calvin Limestone are identified as Lithocodium encrusted coral/stromatoporoid boundstone and as rubbly coral/stromatoporoid grainstones formed in areas of high wave energy associated with the reef growth along the shelf margin. These shelf margin facies are limited in an updip (shelfal) direction, as they grade into a wackestone inner shelf lithofacies. The cores from the ARCO Huffman-McNeely Natchitoches Parish, Louisiana, represent slope (grainstones and packstones), shelf-margin (boundstone and grainstone), and the inner shelf (wackestone) carbonate facies. The Calvin Limestone contains microporosity, resulting from the recrystallization of former Mg-Calcite allochems to subhedral to euhedral microrhombic calcite crystals ranging from 1 to 5u in size. The predominant recrystallized material is Lithocodium, a problematical green algae. The excellent textural preservation and minimicrite crystal size of the nonporous portions of Lithocodium grains suggest that these grains were originally Mg-Calcite. No originally aragonite allochem or equant calcite cements are now microrhomic calcite. The paragenetic relationships and minor elemental analysis of the microrhombic calcite in the Calvin Limestone point to an origin in the shallow subsurface, meteroic ground water system after the initial stabilization of the metastable carbonate sediments. The microporosity associated with the boundstone and grainstone facies is the predominant pore network present in the deeply buried Calvin Limestone (>15,000). This deep occurrence suggests that microporosity associated with microrhombic calcite may have excellent preservation potential with burial. End_of_Record - Last_Page 817-------

Geologic Model of the Middle-Upper Devonian Snipe Lake BHL 'A' Pool Platform-Reef Complex, Western Canada: ABSTRACT

The Snipe Lake BHL 'A' pool reef is one of a number of large Swan Hills atoll reef complexes of Late Devonian age in central Alberta. This study has developed a model for predicting the distribution of reservoir facies in the Snipe Lake A' pool. The reservoir description is an integral component for tailoring an appropriate depletion plan. Variations in reservoir quality are due primarily to depositional attributes. Reservoir facies include reef flat/shoal margin, reef margin/upper foreslope, middle foreslope, and allochthonous foreslope facies. Parts of the grainy lagoon may also be effective reservoir, although vertical permeability tends to be much reduced compared to other reservoir facies. Reef/shoal margin configuration will also affect reservoir quality and continuity. Muddy margin facies are mainly confined to reentrants and may behave as low-permeability barriers to structurally downdip flank water injectors. A chronostratigraphic framework using cycle correlation in the Snipe Lake platform-reef reservoir helped determine facies-controlled reservoir continuity and distribution of low-permeability facies. The platform (53 m thick) consists of five major cycles (A1-5) arranged in a backstepping stacking pattern. Reservoir facies are confined to the uppermost three platform cycles within the pool unit boundary. The Snipe Lake reef complex initiated on the A5 accretionarymore » platform high and comprises four major cycles (BO-2, C) within the unit. The BO cycle contains in excess of 60% of the Snipe Lake recoverable reserves. This lowermost reef cycle is characterized by progradational margins and expanded reservoir facies, presumably due to a combination of prolonged relative stillstand and subdued shallow bathymetry. Other platform-reef cycles are typified by aggradational margins and narrower reservoir facies belts.« less

Windward-leeward effects on Triassic carbonate bank margin facies of the Dolomites, northern Italy

Triassic carbonate bank margins of the Dolomites show, compared to modern carbonate margins, relatively low-energy, narrow facies belts. Distinctive facies depend on the geographical margin orientation and suggest windward-leeward effects. Low-energy deposits such as algal-stabilized sand flats and tepee-pisolite facies occur along margins facing deeper water in the north and east. Higher-energy deposits such as algal reefs and cross-bedded sand bars characterize margins facing southwest and west. Locally, clinoforms show asymmetric distribution being thick at the eastern bank margin and thin or absent at the western bank margin. While the overall low-energy conditions could have been caused by the small size and, in the late Anisian, the shallowness of the basins, facies differentiations suggest a net energy flux of surface waters, caused by storms and, possibly, consistent winds, from the southwest to west in the Triassic of the Dolomites.

Paleontology and Sedimentology of Upper Clastic Member of Wanakah Formation, Chama Basin, New Mexico: Lacustrine Paleoenvironmental Implications: ABSTRACT

Lacustrine strata of the upper part of the Jurassic Wanakah Formation were restricted to the Chama basin of north-central New Mexico by mid-Jurassic tectonic activity in the Brazos and Nacimiento uplifts and along the Gallina-Archuleta anticlinorium. Lateral and vertical facies of the upper Wanakah exposed around the southern margin of the Chama basin indicate that the deeper part of the lake was north of the outcrop belt. The upper 3-5 m of the Wanakah consists of thin-bedded rippled sandstone, interbedded mudstone, and limestone containing trace fossils and freshwater mollusks characteristic of marginal lacustrine facies. Taphonomic studies of mollusks in the Wanakah Formation have been combined with application of ecophenotypic variation documented in extant unionid bivalves to produce paleoenvironmental interpretations of these lacustrine rocks.

Depositional History of Upper Triassic Carbonate Platforms on Wrangellia Terrane, Western British Columbia, Canada: ABSTRACT

Upper Triassic (Karnian-Norian) limestones of the Quatsino Formation, Vancouver Island, and the Kunga Formation, Queen Charlotte Island, were deposited within a complex rifted island arc system, which was accreted to the western margin of North America as Wrangellia during the mid-Cretaceous. Carbonates sedimentation began after a transgressive event submerged the subaerially exposed Karmutsen volcanics. In both Vancouver and Queen Charlotte Islands, the lower part of the carbonate sequence records the widespread development of a shallow, subtidal platform composed mainly of two facies: open-platform deposits (bioturbated fossiliferous wackestones) and platform sand-shoal deposits (oolitic-bioclastic grainstones). This early depositional stage evolved gradually into a second stage characterized by the fragmentation of the platform and development of a carbonate bank/basin topography controlled by extensional tectonics within Wrangellia. The interior facies of these carbonate banks consist of cyclic subtidal lagoonal and tidal-flat limestones, each cycle commonly capped by a vadose diagenetic layer. The bank margin facies comprises exclusively high-energy, oolitic sandshoal deposits. The adjacent basin facies include (1) pelagic/hemipelagic limestones, (2) platform-derived and slope-derived carbonate breccia, and (3) proximal to distal carbonate turbidites. In both island systems, carbonate sedimentation ended abruptly when the isolated carbonate banks were drowned and covered by basinal clastics in responsemore » to rapid subsidence probably caused by tectonic activity within Wrangellia.« less

The Holocene non-tropical coastal and shelf carbonate province of southern Australia

Carbonate-dominant sediments are currently forming and accumulating over the extensive marine shelf of the passive margin of southern Australia. A dearth of continental detritus results from both a very low relief and a predominantly arid climate. The wide continental shelf is bathed by cold upwelling ocean waters that support luxuriant growths of bryozoans and coralline algae, together with sponges, molluscs, asteroids, benthic and some planktonic foraminifera. The open ocean coast is battered by a persistent southwest swell, resulting in erosion of calcrete-encrusted Pleistocene eolianites. Much sediment is reworked and overall shelf sedimentation rates are low. High-energy microtidal beach/dune systems occur between headlands and along the very long ocean beach in the Coorong region. The northern, more arid coastal areas also contain saline lakes that precipitate gypsum from infiltrated sea water, and display marginal facies of aragonite boxwork to fenestral carbonate crusts, with stromatolites and tepee structures. In contrast, the southern, seasonally humid Coorong region, has a predominantly continental groundwater regime where sulphate is rare, and the high summer evaporation precipitates dolomite, magnesite and aragonite muds. Fenestral crusts, breccias, tepees and some stromatolites are also present. St. Vincent and Spencer gulfs both afford some protection from ocean swell, but tidal amplitude and currents increase, and a depth and inundation-related zonation of plants and animals is established. Muddy carbonate sand accumulates on the sea floor below 30 m, where filter-feeding bryozoans, bivalves and sponges dominate. In shallower regions, seagrass meadows contain a rich fauna that results in rapid accumulation of an unsorted muddy bioclastic sand. Mangrove woodlands backed by saline marsh with cyanobacterial mats are common, and accumulate mud-rich and gastropod-bearing sediment. As tidal amplitude and desiccation increase northward into both gulfs, a supratidal zone bare of vegetation (sabkha) becomes the site for deposition of gypsum-rich and fenestral calcitic mud.

Sedimentology and reservoir potential of a ‘marginal facies’ limestone from the Cretaceous of the North Sea

The lower Cretaceous section of UK North Sea well 3/16-1, on the eastern flank of the Shetland Platform, drilled by Sovereign Oil & Gas plc, is dominated by marine ‘shelf-type’ limestones with a very low argillaceous content. In this respect it differs both from the typical open-sea coccolith-rich Chalk and from the argillaceous Cretaceous of the Viking Graben (the Cromer Knoll and Shetland Groups). Diagenesis has resulted in crystal overgrowths, corrosion, and development of very fine to coarsely crystalline void filling calcite. Cavities, of stromatactis type and the result of shell dissolution, are filled with radiaxial fibrous calcite. Porosity is mostly intercrystal with possible contributions from fractures and vugs. Hydrocarbon staining is confined to patches of void-filling calcite and partially cemented fractures. No oil staining was observed in the carbonate matrix. It therefore appears that oil migrated through the rock via fractures but was unable to penetrate the main fabric of the rock. The lithological and petrophysical characteristics of the interval are similar to the Chalk Group of Norwegian well 1/3-1. The thickness is considerably less in 3/16-1, probably as a result of deposition on a topographic high such as existed along the margins of the Viking Graben and the East Shetland Platform during the Cretaceous.

A seabed radiometric survey of Haig Fras, S. Celtic Sea, U.K.

At Haig Fras an Hercynian intrusion, probably a separate body from the S.W. England batholith, forms a seabed ridge made up of 3 main outcrops of well-jointed granite. The granite intrudes Devonian/Carboniferous metasediments in an inlier bounded by Cretaceous and younger strata. Relatively low and high radioactivity areas of granite result mainly from variations in Th content. This is ascribed to the breakdown of monazite by hydrothermal alteration. The alteration may also have mobilised U since U concentrations, interpreted as vein mineralisation, occur in slates adjacent to low activity granite. The high activity granite is depleted in U compared to fresh S.W. England granite but this probably results from surface weathering. Enhanced U concentrations near the base of the chalk are believed to reflect a uraniferous phosphatic horizon. The fine to medium grained granites from Haig Fras differ from the coarse grained megacrystic varieties prevalent in S.W. England, but this may be a sampling anomaly. Foliated granite, prominent in early dredge samples, appears to comprise only a subordinate marginal facies. Granitic permeation gneisses may represent a sliver of crystalline basement emplaced along a faulted margin of the granite, where basic dykes appear also to have been introduced.

Geology and origin of the Death Valley uranium deposit, Seward Peninsula, Alaska

A uranium deposit discovered in 1977 in western Alaska, by means of airborne radiometric data, is the largest known in Alaska on the basis of industry reserve estimates. At about latitude 65 degrees N, it is the most northerly known sandstone-type uranium deposit in the world. The deposit lies in Eocene continental sandstone near the eastern end of the Seward Peninsula, in the southern end of a graben that extends northward into the Death Valley depositional basin.The deposit is apparently of epigenetic and supergene origin. The uranium was derived from the Cretaceous granite of the Darby pluton that forms part of the western side of Death Valley. Uranium from primary mineralization is in the subsurface in a marginal facies of the Tertiary sedimentary basin where nearshore coarse clastic rocks are interbedded with coal and lacustrine clay. Primary mineralization occurred when uranium-bearing oxidizing ground water moved downdip from the pluton eastward through transmissive clastic beds or on the surface. Uranium was deposited where the coal or other carbonaceous material produced a reducing environment in arkosic host rocks. The supergene enrichment is related to a soil horizon at the present ground surface. The most common uranium mineral is meta-autunite, but cofflnite has been identified in the primary deposits. The host rocks for the primary deposits were partly covered by basalt flows that issued from nearby vents. Some of the basalt is highly altered, and some basalt float from the supergene zone has alteration rinds that are enriched in uranium.Extensive exploratory drilling took place from 1979 to 1981. The average grade of the potential ore is 0.27 percent U 3 O 8 and the average thickness is 3 m. The calculated reserves are 1,000,000 lbs U 3 O 8 ; additional drilling would probably add to this figure.

On the formation conditions of porphyry Cu-Mo polymetallic deposits in the Xiaosigou district

The host mass consists of quartz diorite-porphyry (marginal facies), granodiorite porphry (intermediate facies) and granite porphry (central facies). The petrographical facies are asymmetrically spread. The formation temperature of the rock mass ranges from 890 to 800°C, the pressure from 330 to 380 bars and the depth from about 1.5 to 2 km. Mineralization is controlled by the N-E contact zone and adjacent fault structures. Mo-mineralization occurs in the granite porphyry with strong potash metasomatism. Cu-Mo-mineralization is distributed in the inner contact zone of the rock mass, and the wallrock is K-silicified granite porphyry. Cu-pyrite-mineralization is recognized in skarn and serpentinite. Pb-Zn ore veins occur in marbleized limestones. Rock and mineral analyses, fluid inclusion studies and highT-P experiments indicate that extensive precipitation of Cu-bearing pyrite took place atT = 290–250°C, andP = 330–380 bars and that of copper atT = 400–310°C andP = 330–380 bars. Precipitation of Pb and Zn was followed by the transformation of hydrothermal solutions from alkaline to intermediate with decreasing temperature.

Vadose Pisoide und zonare Dolomitkristalle in der Keuper-Randfazies Luxemburgs

Vadose Pisoide und zonare Dolomitkristalle in der Keuper-Randfazies Luxemburgs