Mountain Group Research Articles

Delaware Mountain Group, West Texas, A Case of Refound Opportunity: Part 2--Cherry Canyon Formation (E & P Notes)

Recent drilling activity in the War-Wink area of west Texas has established a new play in submarine channel and levee/overbank sandstones of the middle and lower Cherry Canyon Formation, Delaware Mountain Group (Guadalupian). Previous exploration and development, spanning the 1960s, 1970s, and early 1980s, had focused on the upper part of the formation. The new play, located a few miles west of the Central Basin platform, suggests the existence of multiple channel trends not yet explored in the Delaware basin. Entrapment in the War-Wink area is related to both structural and stratigraphic factors. Lower Cherry Canyon reservoirs have porosities ranging from 18 to 24% and permeabilities ranging from 10 to 60 md and thus are higher in quality than reservoirs of the underlying Brushy Canyon Formation. Core data indicate that permeabilities increase rapidly above a threshold porosity value of about 18%. Lower Cherry Canyon wells are completed by fracture stimulation in one or two main reservoir zones, with several secondary zones commonly behind pipe. Wells usually flow at rates of 100-200 bbl oil per day for up to 1 yr before being put on pump. Main producing zones are capable of recoveries ranging from 50,000 to 120,000 bbl. Attempts at using horizontal drilling in place of fracture stimulation have proved highly successful in certain cases. Use of this approach for Cherry Canyon development, guided by good geologic control, may provide an excellent technique applicable to many portions of the Delaware basin.

Temporal dynamics of grouse populations at the southern edge of their distribution

Studies of grouse conducted at northern latitudes have shown that tetraonids frequently exhibit cyclic fluctuations in abundance but little is known about the dynamics of grouse species at the southerly edge of their range. Hunting statistics from four species of grouse based on 30 yr of data collected from 210 hunting areas were examined from the Dolomitic Alps in the province of Trentino. These data were summed to represent 18 time series from discrete mountain groups. Analyses identified cycles of ca 5 yr in the minority of rock ptarmigan Lagopus mutus and hazel grouse Bonasa bonasia populations. These cycles only showed significant negative autocorrelation at half the cycle period and were classified as phase‐forgetting quasi‐cycles. Cycles were not found in time series of black grouse Tetrao terix or capereaillie Tetrao urogallus. Correcting time series for hunting effort or hunting restrictions tended to increase the proportion of populations that exhibited cycles but no difference in the strength of second order density dependence, A linear first order density‐dependent autoregressive model described the dynamics of most of the populations with the exception of a proportion of rock ptarmigan and black grouse populations where a non linear first order model provided the best tit. We compare the findings with studies conducted in Finland and suggest possible reasons for the reduced tendency to cycle in the populations of southern Europe.

Complex thrusting at Uniondale, eastern sector of the Cape Fold Belt, Republic of South Africa: structural evidence for the need to revise the lithostratigraphy

Three zones of thrust faulting, each zone with its own structural characteristics, have been identified in rocks of the Palæozoic Table Mountain Group, just south of Uniondale, in the Cape Fold Belt. From north to south these zones consist of (1) shallow dipping sets of thrusts spaced, on average 10 m apart, (2) closely spaced (average 4 m apart) moderate to steeply-dipping thrusts associated with mesofolds, and (3) a zone mainly of backthrusts, spaced on average 2 m apart. All three zones formed during a single compressional event which took place during the Late Palæozoic. Thrusting is interpreted to have commenced in zone (1) where thrusts developed in a break-back sequence, followed by thrusting in zones (2) and (3) where fore- and backthrusting were probably coeval. Upright fold structures in zone (2) controlled the steep orientation of thrust planes in this zone. The intensity of thrusting is so great that the ‘Cedarberg Shale Formation’, generally regarded as a regionally reliable stratigraphic marker in the Cape Fold Belt, has been eliminated in the study area. In addition, the scale of deformation seen locally in all units of the Table Mountain Group brings into sharp focus potential problems in respect of the recognition, thicknesses and correlation of lithostratigraphical units in the Cape Fold Belt.

Eurypterid trackways from the Table Mountain Group (Ordovician) of South Africa

Eurypterid trackways in paralic facies of the Graafwater and Peninsula Formations, lower Table Mountain Group (Ordovician) of the Western Cape Province, South Africa, are reviewed. The ichnotaxonomy of trackways preserved on the ‘Brandenburg slab’ from the Graafwater area is reassessed, with the recognition of two distinct forms: (1) Palmichnium capensis (Anderson, 1975), consisting of a series of four tracks, many accessory imprints and (usually) an intermittent bilobed medial impression; and (2) Petalichnus brandenburgensis isp. nov., a much narrower trackway with nine to ten tracks per series and no medial impression. Palmichnium capensis is interpreted as the locomotory trace of an onychopterellan eurypterid (Chelicerata: Eurypterida). Petalichnus brandenbergensis is provisionally ascribed to the Trilobita. The likely walking techniques employed by onychopterellan eurypterids are assessed from the revised ichnological evidence, comparisons with extant taxa and modelling procedures; subaqueous locomotion was achieved by a series of short hops across the substrate and on land these animals were slow-moving and cumbersome, employing an ungainly, undulatory gait.

Delaware Mountain Group, West Texas and Southeastern New Mexico, A Case of Refound Opportunity: Part 1--Brushy Canyon

Exploration in Permian (Guadalupian) deep-water sandstones of the Delaware Mountain Group, west Texas and southeast New Mexico, represents a success story of the 1990s derived from reevaluation of reservoirs previously deemed uneconomical. Recent discoveries have concentrated on the Brushy Canyon in New Mexico and, to a lesser extent, the Cherry Canyon in Texas. Brushy Canyon reservoirs in particular previously were overlooked due to indications of poor reservoir quality from log and well test data; however, oil shows observed on mud logs across the northern Delaware basin led to new completion efforts in the late 1980s and 1990s using gel-sand fracture stimulations. Productive reservoirs are very fine to fine-grained arkosic to subarkosic sandstones with porosities of 12-25% and permeabilities typically of 1-5 md. Better reservoir quality is concentrated in massive channel sandstones variably interpreted as deposited by turbidity or saline density currents. Significant clay content, lamination, and close interbedding between oil- and water-bearing units make log analysis and reserve estimates problematic. As a result, the mud log remains the cheapest, most practical indicator of pay. Reservoir sandstones can be divided into a series of major productive trends related to proximal/slope and more distal/basin-floor depositional settings. Well productivity is variable within each trend, but primary recovery rarely exceeds 10%. Options for enhanced recovery include pressure maintenance, waterflooding, and carbon dioxide flooding. Early indications suggest that carbon dioxide flooding may be most appropriate in these low-permeability, clay-bearing reservoirs.

Tectonostratigraphical development of the Upper Karooforeland basin: Orogenic unloading versus thermally-induced Gondwana rifting

The origins of the Karoo Basin have never been adequately explained, but twomain models have been suggested: (1) a purely foreland basin (retro-arc) model; and (2) a polyphase successor basin, beginning with extension during Upper Table Mountain Group (Silurian) times to a foreland basin in the Early Karoo (Permian) and extension in the Upper Karoo (Upper Triassic-Jurassic). Subsidence and forward modelling on the known stratigraphy of the Karoo Basin do not fit a simple foreland basin model. This is particularly true for the Upper Karoo where the stratigraphy, stacking patterns and age of the succession suggest that the foreland basin model is not viable, and that continental extension played a major role in late basin development. Evidence for extension, and when this occurred, is provided by: (1) sequence stratigraphical identification of a major Middle-Late Carnian low stand sequence boundary unconformity, related to uplift of a newly emergent source area in the southeast; and (2) the presence of volcanic detritus in Upper Karoo sediments, chemically similar to the Karoo basalts, suggesting a possible chemogenetic link between them and, by implication, with early volcanism and incipient Gondwana rifting, now well-preserved in the rift sequences of East Antarctica. This early volcanism, which began not later than the Carnian (∼230 Ma), and provenance uplift, may be related to a thermal anomaly linked to mantle plume-induced updoming, crustal extension and faulting approximately 40 Ma prior to maximum uplift preceeding Karoo Basalt eruption at 183 Ma. Thermal uplift occurred some 200 km off the present southeast coast of South Africa, just south of the Karoo Hotspot, but away from the direct influence of any known hotspot or hotspot track. This uplift, aligned parallel to the southeast coast of South Africa, at a high angle to the Cape Fold Belt, was located close to the site of later rifting and separation along the Agulhas Falkland Fracture Zone.

Infilling history of a Neoproterozoic intracratonic basin: Nd isotope provenance studies of the Uinta Mountain Group, Western United States

Neodymium isotope and selected trace element concentration data were obtained from 27 samples of shales and arkosic sandstones comprising the Neoproterozoic Uinta Mountain Group (UMG) in north-eastern Utah and northwestern Colorado to determine the provenance of siliciclastic detritus deposited in this region and to reconstruct aspects of the Proterozoic geologic history of the southern margin of the Archean Wyoming Province. In the east lobe of the Uinta Mountains, coarse-grained components of the Jesse Ewing Canyon Formation, the oldest recognized formation of the UMG, have measured ϵ Nd values of −23 to −28 and were most probably derived from Archean basement of the Wyoming Province to the north. Fine-grained sediment in the southern portions of the Jesse Ewing Canyon Formation and shales and quartz arenites from overlying undifferentiated UMG all contain at least a component of detritus derived from Proterozoic crust. In the west lobe, the oldest members of the UMG (the Red Castle Lake, Gilbert Peak and Moosehorn Lake Formations) and arkosic arenite comprising the northern portions of the overlying Mount Watson Formation were also derived exclusively from Archean sources. However, as in the east lobe, the more mature sandstones that dominate the southern sections of the Mount Watson Formation have isotopic compositions that require a significant Proterozoic-crust derived detrital component. Our data support a model in which sedimentary rocks with an Archean provenance represent the preserved remnants of alluvial fans marking the northern margin of the basin in which the UMG was deposited. A source for these sediments within the Archean Wyoming Province is consistent with the high light rare-earth element (LREE) and Th abundances of finegrained, low ϵ Nd, sedimentary rocks, given that Late Archean granitic rocks within the Wyoming province have similar trace element characteristics. Sediments with Proterozoic components in both the east and west lobes were apparently deposited from a fluvial system that transported sediment along the basin axis. The Proterozoic detrital component in these sediments originated either through direct erosion of the Colorado Province in the Neoproterozoic, or through erosion of ‘mixed’ Archean and Proterozoic crust located to the east of the UMG basin that originally formed during the Paleoproterozoic accretion of the Colorado Province to the southern margin of the Wyoming craton.

Extent and spatial distribution of Pleistocene glaciations in eastern Tibet

As a contribution to the controversial discussion regarding the nature and extent of the Pleistocene glaciation on the Tibetan Plateau, additional observations and results were obtained during several Chinese-German joint expeditions. In the investigation area of the eastern and central Tibetan Plateau, it will be shown that the maximum extent of the last glaciation was limited to the glaciation of isolated mountain groups or smaller plateau glaciations. These glaciations are characterized by glacial erosional features such as U-shaped valleys in the inner mountain massifs and cirques in the higher summit areas. Accumulation forms, such as terminal and lateral moraines (less common than basal moraines) are marked by granitic and basaltic erratics. Using two examples from the eastern margin of the Tibetan Plateau (Nianbaoyeze Shan and northern Chola Shan), the extent of Pleistocene glaciations can be mapped by the distribution of terminal and lateral moraines and erratic granite boulders on the slopes of the surrounding schists. Two to three main Pleistocene glaciations can be separated using weathering criteria, paleosols, and the overlying stratum on the moraines and associated terrace sequences. Overlying stratum consist of aeolian sandy silt or solifluction debris. The younger moraines are found to be characterized by large erratics, were therefore named “Big-Boulder-Moraine”, and can be found over the investigated area. The ice free areas are characterized by Pleistocene terraces along the main rivers, loess deposits with sand and ice wedges at higher elevations (e.g. the basin of Zoige), and paleosols in the basins below 3500 m a.s.l. (e.g. Aba basin or Garze basin). The hypothesis of an extensive plateau glaciation could not be verified for this region. A rising snowline (equilibrium-line altitudes = ELAs) is revealed in two cross-sections during the Last Glacial Maximum (LGM) at the eastern and northern margin of the Tibetan Plateau. Snowlines migrated from about 4000 m in the east (Minshan), to 4300 m in the Nianbaoyeze, and to about 4700 m in the west (Bayan Har Shan). A rise was also recorded from about 3800 m in the north (central Qilian Shan), up to a range of 4300–4950 m (different exposures) in the Kunlun Shan, to approximately 5200 m in the Tanggula Shan in the center of the Tibetan Plateau. The general increase of modern and LGM snowlines (or ELAs) towards the plateau corresponds with a decrease of precipitation from east to west and from north to south respectively. Precipitation was more pronounced at the eastern margin during glacial times than compared to present.

Stratigraphic hierarchy of organic carbon–rich siltstones in deep-water facies, Brushy Canyon Formation (Guadalupian), Delaware Basin, West Texas

The first systematic test for a predictive relationship between organic carbon content and stratigraphic hierarchy in a deep-water slope to basin-floor deposit was performed. The studied section includes the Pipeline Shale, the Brushy Canyon Formation, and the lower part of the Cherry Canyon Formation of the Delaware Mountain Group, West Texas. This interval represents one large-scale, 3rd-order genetic sequence within which 4th- and 5th-order stratigraphic cycles are recognized. Samples of fine-grained facies throughout the section were collected from outcrop and analyzed for organic carbon content and hydrogen index. Degree of pyritization was also determined for a subset of the samples. The results indicate that organic enrichment is closely correlated to the stratigraphic hierarchy at the 3rd-, 4th-, and 5th-order levels. The data suggest that quantity and quality of preserved organic matter are controlled by changes in bulk sedimentation rate (dilution vs. condensation), which affect organic matter inputs to the sediment, as well as the balance between (1) burial and preservation of organic matter and (2) its degradation on the sea floor during times of sediment starvation.

Magma batches in the Timber Mountain magmatic system, Southwestern Nevada Volcanic Field, Nevada, USA

The common occurrence of compositionally and mineralogically zoned ash flow sheets, such as those of the Timber Mountain Group, provides evidence that the source magma bodies were chemically and thermally zoned. The Rainier Mesa and Ammonia Tanks tuffs of the Timber Mountain Group are both large volume (1200 and 900 km 3, respectively) chemically zoned (57–78 wt.% SiO 2) ash flow sheets. Evidence of distinct magma batches in the Timber Mountain system are based on: (1) major- and trace-element variations of whole pumice fragments; (2) major-element variations in phenocrysts; (3) major-element variations in glass matrix; and (4) emplacement temperatures calculated from Fe-Ti oxides and feldspars. There are three distinct groups of pumice fragments in the Rainier Mesa Tuff: a low-silica group and two high-silica groups (a low-Th and a high-Th group). These groups cannot be related by crystal fractionation. The low-silica portion of the Rainier Mesa Tuff is distinct from the low-silica portion of the overlying Ammonia Tanks Tuff, even though the age difference is less than 200,000 years. Three distinct groups occur in the Ammonia Tanks Tuff: a low-silica, intermediate-silica and a high-silica group. Part of the high-silica group may be due to mixing of the two high-silica Rainier Mesa groups. The intermediate-silica group may be due to mixing of the low- and high-silica Ammonia Tanks groups. Three distinct emplacement temperatures occur in the Rainier Mesa Tuff (869, 804, 723 °C) that correspond to the low-silica, high-Th and low-Th magma batches, respectively. These temperature differences could not have been maintained for any length of time in the magma chamber (cf. Turner, J.S., Campbell, I.H., 1986. Convection and mixing in magma chambers. Earth-Sci. Rev. 23, 255–352; Martin, D., Griffiths, R.W., Campbell, I.H., 1987. Compositional and thermal convection in magma chambers. Contrib. Mineral. Petrol. 96, 465–475) and therefore eruption must have occurred soon after emplacement of the magma batches into the chamber. Emplacement temperatures of the pumice fragments from the Ammonia Tanks Tuff show a continuous gradient of temperatures with composition. This continuous temperature gradient is consistent with the model of storage of magma batches in the Ammonia Tanks group that have undergone both thermal and chemical diffusion.

Ca-Al silicates in low-grade metavolcanic rocks from southern Tanzawa Mountains, Central Japan.

The metavolcanics of the Miocene Tanzawa Group of the southern Tanzawa Mountains can clearly be divided into six mineral zones ; stilbite, laumontite, prehnite-pumpellyite, epidote, epidote-amphibole and amphibole zones. Four phases of mafic phyllosilicates in the five lower grade zones are identified by systematic X-ray diffraction studies; (1) smectite in the stilbite and laumontite zones, (2) randomly mixed-layer chlorite/smectite mineral which is ubiquitous in the stilbite, laumontite and prehnite-pumpellyite zones, (3) regularly mixed-layer chlorite/smectite in the stilbite zone, (4) chlorite occurs first in the laumontite zone. With increasing metamorphic grade, smectite layers progressively transform to chlorite. Chlorite is the predominant phyllosilicate in the prehnite-pumpellyite zone and is a unique phyllosilicate in the epidote and epidote-amphibole zones. Variations in Fe/(Fe+Mg) of chlorite are positively correlated with Fe/(Fe+Mg) ratios of whole rocks, but the other chemical variations are clearly unrelated to whole rock compositions. The content of Si decreases with increasing metamorphic grade.

The mineralogy of the Golden Sunlight gold-silver telluride deposit, Whitehall, Montana, U.S.A.

The Golden Sunlight gold-silver telluride deposit, hosted primarily within the Mineral Hill breccia pipe (MHBP), is spatially related to a high-level, Late Cretaceous multiple intrusive, alkaline to subalkaline porphyry system. Base metal veins and manganese (rhodochrosite) mineralization occur up to 2km from the MHBP and form part of a regional mineral zonation pattern genetically related to a low-grade porphyry molybdenum system. Proterozoic rocks of the LaHood Formation and the informally named Bull Mountain Group host the MHBP and contain stratabound sulphides/ sulphosalts (up to 50% pyrite with minor to trace amounts of chalcopyrite, tennantite, pyrrhotite, sphalerite, galena, and molybdenite). Four periods of hypogene mineralization occur in the breccia pipe. Stages I and IV constitute ,≈99% of the mineralization; native gold (4–11 wt.% Ag), calaverite, tetradymite, tellurobismuthite, Se-bearing Bi sulphosalts (aikinite, lindstromite, krupkaite, gladite, bismuthinite, and ?benjaminite), tennantite (Zn, Fe, Te, and Bi varieties), coloradoite, melonite, galena (up to 6.7 wt.% Bi and 6.4 wt.% Se), stannite, chalcocite, and the rare mineral buckhornite are included in stage Ib. Minor amounts of base metals are present in stage II. Gold-silver tellurides (krennerite, petzite, sylvanite, and possibly the rare “x-phase”) developed in stage III whereas barite, fluorite, dolomite, magnesite, trace kaolinite, and sericite formed during stage IV. Utilizing the mineral assemblages in stage Ib, calculated values of logf Te2 and logf S2 range from -10.5 to -9.7, and -12.6 to -5.5, respectively.

Depositional environments of Lower Pennsylvanian coal-bearing siliciclastics of southeastern Tennessee, northwestern Georgia, and northeastern Alabama, U.S.A.

In the Cumberland Plateaus of southeastern Tennessee, northwestern Georgia, and northeast Alabama, Pennsylvanian strata are siliciclastics containing discontinuous coal seams. Above some of these coal seams, the shale deposits contain fossils of marine or brackish fauna. The entire sequence was deposited in an asymmetric foreland basin and is thickest in the southeast. Within this general trend there are locally thick deposits of lowermost Pennsylvanian Gizzard Group sequences that mark subbasins. Conglomeratic sandstone members of the Gizzard Group are discontinuous and tend to be thicker in the subbasins. In contrast, conglomeratic to sandy units are more laterally continuous in the overlying Crab Orchard Mountains Group. The Pennsylvanian sequence overlies paleosols with subjacent freshwater-neomorphosed shallow marine carbonates, or siliciclastics of the uppermost Mississippian Pennington Formation. A provenance discrimination diagram indicates that Pennsylvanian siliciclastics were derived from an orogenic source. Profile analysis of thick quartzose sandstone sequences indicates facies, architectural elements and bounding surfaces characteristic of braided stream deposits. A dominant southwest paleoflow direction is inferred from paleocurrent indicators in sandy braided and meandering stream deposits.

Mirror of heaven: Cross‐cultural transference of the sacred geography of the Black Hills

Abstract The Black Hills are a mountain group on the northern plains of North America considered sacred by several Native American groups. Several groups shared a conceptual association between Black Hills landscape features, constellations, seasonal movements and mythology. These groups include the Lakotas, Cheyennes, Kiowas and Kiowa‐Apaches. Beliefs about the sacred geography of the area were transferred from group to group as new peoples entered the area over the course of several centuries.

Evidence for a genetic link between gold-silver telluride and porphyry molybdenum mineralization at the Golden Sunlight Deposit, Whitehall, Montana; fluid inclusion and stable isotope studies

The Golden Sunlight gold-silver telluride deposit, hosted primarily within the Mineral Hill breccia pipe, is spatially related to a high-level, Late Cretaceous multiphase, alkaline to subalkaline porphyry system (latite porphyries, quartz monzodiorite, and lamprophyres). Base metal veins and manganese (rhodochrosite) mineralization occur up to 2 km from the Mineral Hill breccia pipe and form part of a regional mineral zonation pattern genetically related to a low-grade porphyry molybdenum system. Alteration associated with this system consists of a proximal zone, pervasive in style (disseminations and veinlets), that is core to the alkaline igneous rocks and a distal zone that is vein controlled. Proximal alteration is composed of an early barren zone of quartz, hematite, pyrite, and barite as well as a pervasive zone composed of quartz, sericite, K feldspar, base metal sulfides, tellurides, molybdenite, and native gold. Distal alteration is characterized by the same mineralogy as the precious and base metal zone of the proximal zone except that it is vein controlled and does not contain molybdenite; a base metal and rhodochrosite zone occurs farthest from the center of the porphyry system. K feldspar alteration occurs as selvages to quartz-molybdenite mineralization and as a pervasive replacement of clasts in the Mineral Hill breccia pipe (early), whereas quartz-sericite alteration is dominantly associated with auriferous veins and the breccia pipe (late).Molybdenite occurs in high veinlet-density quartz veins at the margins and deeper portions of the breccia pipe, in clasts in prebreccia pipe quartz-pyrite-K feldspar veinlets, as fine-grained disseminations in breccia fragments, and as rims to fragments of xenolithic latite. Previously four periods of hypogene mineralization in the breccia pipe were identified. Stages I and IV constitute [asymp] 99 percent of the mineralization; native gold, calavertite, and Bi tellurides-sulfosalts occur in stage I whereas Au-Ag tellurides developed in stage III. Minor amounts of base metals are present in stage II whereas barite, dolomite, magnesite, trace kaolinite, and sericite develop in stage IV. Proterozoic rocks of the LaHood Formation and the informally named Bull Mountain group host the breccia pipe and contain strata-bound sulfides-sulfosalts (up to 50% pyrite with minor to trace amounts of chalcopyrite, tennantite, pyrrhotite, sphalerite, galena, and molybdenite).Fluid inclusion homogenization temperatures of primary and pseudosecondary aqueous liquid-vapor inclusions (constant liquid to vapor ratios) in quartz-pyrite-molybdenite veins range from 131.8 degrees to 398.2 degrees C. Primary and pseudosecondary inclusions with highly variable liquid-vapor ratios that coexist with rare three-phase CO 2 -H 2 O and multiphase inclusions (one or more solid phases in addition to liquid and vapor) also occur in quartz-pyrite-K feldspar-molybdenite veins and indicate periods of intermittent boiling. Homogenization temperatures for fluid inclusions in stage I auriferous pyrite veins range from 145 degrees to 345 degrees C with salinities of [asymp] 1 to 10 wt percent NaCl equiv. Values of delta 34 S for sulfides in the Mineral Hill breccia pipe, quartz-pyrite-K feldspar-molybdenite veins, Proterozoic sedimentary rocks, and distal base metal veins range from -12.2 to 3.1 per mil, -8.1 to 0.8 per mil, 0.7 to 6.1 per mil, and -8.9 to 9.5 per mil, respectively, and suggest a mixed magmatic-sedimentary sulfur source. Calculated delta 18 O values for water in equilibrium with late-stage sericite at 170 degrees C vary from 0.7 to 2.8 per mil, whereas delta D water values range from -67 to -8 per mil.Geologic, paragenetic, fluid inclusion, and stable isotope studies are consistent with an early magmatic fluid, associated with quartz-pyrite-K feldspar-molybdenite veins, that subsequently mixed with meteoric water during stages I to IV breccia pipe-hosted and auriferous pyrite vein formation. Ore-forming components (e.g., Au, Ag, Te, Cu, Bi, Mo, and much of the S) were most likely derived from the Late Cretaceous intrusive system with possible contributions from the Proterozoic host rocks.

Palaeoproterozoic, ∼ 1.88–2.0 Ga, organic matter from the Mugford/Kaumajet Mountain Group, northern Labrador

The Palaeoproterozoic, ∼ 1.88–2.0 Ga, Mugford Group of northern Labrador is a relatively undeformed cover sequence developed on Archaean crust of the Nain craton. The Cod Island Formation occurs at the base of the group and consists of mixed shale, chert, dolostone and siltstone. Thin layers of highly matured organic material similar to anthracite have been found within a shale and chert sequence. Based on the nonmigrated and sedimentological context of the layers, the material may represent some of the earliest intact, in situ, organic deposits known. δ 13C isotope ratios of the organic matter in this deposit range from −31.79 to −32.22‰ and the H/C atomic ratios from 0.05 to 0.08. The material represents the first truly biological remains known from the Nain Province of Labrador and the earliest organic material in northeastern North America.

Recurrent eruption and subsidence at the Platoro caldera complex, southeastern San Juan volcanic field, Colorado: New tales from old tuffs

Reinterpretation of a voluminous regional ash-flow sheet (Masonic Park Tuff) as two separate tuff sheets of similar phenocryst-rich dacite erupted from separate source calderas has important implications for evolution of the multicyclic Platoro caldera complex and for caldera-forming processes generally. Masonic Park Tuff in central parts of the San Juan field, including the type area, was erupted from a concealed source at 28.6 Ma, but widespread tuff previously mapped as Masonic Park Tuff in the southeastern San Juan Mountains is the product of the youngest large-volume eruption of the Platoro caldera complex at 28.4 Ma. This large unit, newly named the “Chiquito Peak Tuff,” is the last-erupted tuff of the Treasure Mountain Group, which consists of at least 20 separate ash-flow sheets of dacite to low-silica rhyolite erupted from the Platoro complex during a 1 m.y. interval (29.5−28.4 Ma). Two Treasure Mountain tuff sheets have volumes in excess of 1000 km 3 each, and five more have volumes of 50–150 km 3 . The total volume of ash-flow tuff exceeds 2500 km 3 , and caldera-related lavas of dominantly andesitic composition make up 250-500 km 3 more. A much greater volume of intermediate-composition magma must have solidified in subcaldera magma chambers. Most preserved features of the Platoro complex-including postcollapse asymmetrical trap-door resurgent uplift of the ponded intracaldera tuff and concurrent infilling by andesitic lava flows-postdate eruption of the Chiquito Peak Tuff. The numerous large-volume pre-Chiquito Peak ash-flow tuffs document multiple eruptions accompanied by recurrent subsidence; early-formed caldera walls nearly coincide with margins of the later Chiquito Peak collapse. Repeated syneruptive collapse at the Platoro complex requires cumulative subsidence of at least 10 km. The rapid regeneration of silicic magmas requires the sustained presence of an andesitic subcaldera magma reservoir, or its rapid replenishment, during the 1 m.y. life span of the Platoro complex. Either case implies large-scale stoping and assimilative recycling of the Tertiary section, including intracaldera tuffs.

Origin of compositional heterogeneities in tuffs of the Timber Mountain Group: The relationship between magma batches and magma transfer and emplacement in an extensional environment

Compositionally zoned ash flow sheets provide convincing evidence for chemically zoned magma bodies. Most workers have assumed that the high‐silica portions of these magma bodies evolved largely by differentiation processes that occurred within the magma chamber. However, chemical heterogeneities within some ash flow sheets are not consistent with these differentiation processes. The chemical variation of pumice fragments in the large volume (>1200 km3), Rainier Mesa ash flow sheet ranges from 55 to 76.3% silica. These pumice fragments occur in three distinct chemical groups. A low‐ and high‐silica group is separated by a compositional gap at about 72% silica, and within the high‐silica group there are two distinct populations based on trace element variations. There is little overlap between populations. These three magma types have been resident in same magma chamber at the same time and cannot be produced by any differentiation process of a single magma body. They must reflect discrete magma batches generated in the source area. Furthermore, the lower silica portion (<72% SiO2) of the Rainier Mesa ash flow sheet is chemically distinct from the lower silica portion of the overlying Ammonia Tanks ash flow sheet, even though they erupted within 200,000 years of each other. These ash flow sheets from the SW Nevada volcanic field are associated in time and place with Basin and Range extension, and all models for extension involve detachment surfaces that extend to great depth. A model for the relationship of these compositional heterogeneities and the regional extension involves (1) the generation of magma batches by either continuous melting of the source at different temperatures, or by melting of different sources, (2) the use of faults (shears) as conduits for transport of magma, and (3) the use of a dilatant releasing step on a detachment as storage chamber for the magma. We first address the evidence for separately generated magma batches and then propose a model for their coalescence in a single chamber prior to eruption.

A Neoproterozoic multi-phase rift sequence: the Grampian and Appin groups of the southwestern Monadhliath Mountains of Scotland

The Grampian and Appin groups of the southwestern Monadhliath Mountains form the earliest known syn-rift sequences of the Scottish central Highlands. They were likely to have formed in an intracontinental setting and represent deposition of mixed clastic and carbonate shallow and deep marine strata. The Grampian Group of the southern Monadhliath Mountains was deposited during a period of initial basin rifting (NW–SE extension) followed by a phase of thermal subsidence. Syn-rift sediments comprise a 2.5–6 km thick turbidite system. Thermal subsidence brought about the basinward progradation of shallow marine shelf sediments resulting in the infilling of pre-existing basin topography. The overlying Appin Group commenced with deposition of a shallow marine sequence alternating between nearshore tidal sand and offshore mud deposition. This formed in response to renewed rifting and concomitant subsidence. Accelerated rifting resulted in localized footwall uplift and erosion while sedimentation continued in the hanging-wall areas. Resultant subsidence, perhaps partly thermally driven, caused gradual basin widening and produced an onlapping marine sequence. There followed a period of progressive clastic deprivation when carbonates were precipitated, and at the onset of anoxic conditions, deposition of organic muds. The fundamental structural elements responsible for the formation of the Grampian and Appin group basins were also influential in the orogenic evolution of the basin-fill. Half-graben fills were deformed to produce regionally extensive folds such as the Stob Ban–Craig a’ Chail Synform.

Diagenesis, Morphology and Reservoir Effects of Authigenic Illite and Chlorite in the Cherry Canyon Formation, Delaware Mountain Group, Screwbean Field, Reeves County, Texas: ABSTRACT

Diagenesis, Morphology and Reservoir Effects of Authigenic Illite and Chlorite in the Cherry Canyon Formation, Delaware Mountain Group, Screwbean Field, Reeves County, Texas: ABSTRACT