Lowermost Member Research Articles

Petrography and Geochemistry of the Lower Sylhet Sandstone Member (Therria Sandstone) of Jaintia Group Exposed Along Jowai-Badarpur Road Section, Jaintia Hills District, Meghalaya: Implication for Provenance and Palaeoclimate

ABSTRACT The Paleogene sediments of the Shillong Plateau are exposed in the southern part of Meghalaya and are designated as the Jaintia Group. The Lower Sylhet Sandstone Member (LSSM) (Therria Sandstone) is the lowermost member of the Shella Formation of Jaintia Group. To understand the depositional history of the studied sandstones, petrographic and geochemical approaches have been made. Petrographically, these sandstones are classified as quartz arenite and quartz wacke. They were derived from intermediate to upper rank metamorphic and plutonic source which were deposited in a craton interior and quartzose recycled tectonic setting. The paleoweathering indices of CIA, CIW, PIA and ICV indicate that the weathering at the provenance was intense. The tectonic discrimination diagrams suggest passive margin setting for their sedimentation and climatic condition during the deposition was humid in nature. The studied sediments were deposited in non-marine conditions as evidenced by the geochemical proxies and primary sedimentary structures existing in the LSSM.

Synopsis of Upper Tukau Deposits along the Sungai Rait Road, Miri District, Sarawak, Malaysia

The Upper Tukau Formation is exposed along the Sungai Rait Road and can be described as follows: the lowermost member is formed by soft clay and silt, the “Brick Shale”, and contains meandering sandy channels. The clay is thoroughly excavated in the area and used as raw material for bricks. The second and hilltop-forming layer is built by relatively hard amalgamated, and partly channelized sandstone layers, often lined with diagenetically precipitated iron minerals (i.e., pyrite and siderite concretions). The youngest member is characterized by intertidal channels and mud flats. Geochemical studies point to a polymictic source of already recycled sediments, deposited in predominantly oxic, partly suboxic environments, with rapid transitions observed from one milieu to the other. In broad terms, the entire sequence appears to be a shallowing-upwards cycle, leading from subtidal to intertidal realms. The surprising sharp boundaries between clay and sand packages are discussed and may be related to climate cycles.

Paragenesis of an Ediacaran carbonate-platform phosphorite: Constraints from optical petrography and texture-specific clumped isotope paleothermometry

The Salitre Formation is comprised of several hundred meters of primarily carbonate rocks, representing sedimentation in an intracratonic foreland basin during the Ediacaran Period (c. 600–550 Ma). The lowermost member of the Salitre Formation contains sedimentary phosphorite deposits, in which the dominant phosphate-bearing phase is cryptocrystalline carbonate-rich fluorapatite (CFA) cement. These CFA cements occur exclusively associated with digitate stromatolite buildups, decimeters to meters in vertical and lateral scale, which interfinger with cross-stratified grainstone. Notably, there are also non-phosphatic stromatolite buildups in close proximity to the phosphatic, in the same depositional facies. This implies that the mechanisms that control the distribution of phosphate cement development versus carbonate cement development are not driven by the location of depositional facies within the architecture of the Salitre paleobasin. Other models which could explain the distribution and style of phosphate mineralization there include: (1) differential diagenesis involving diagenetic fluids which infiltrate one lithofacies more pervasively than another to effect the replacement of primary cement mineralogies according to microtextural differences, and (2) differences in primary porewater chemistries over minute spatial scales which result in the formation of different primary cement phases. In this study, we present paired petrographic thin sections and novel measurements of ∆47, δ13Cmineral, and δ18Omineral. We provide a paragenetic framework for phosphatic and non-phosphatic lithofacies of the Salitre Formation which constrains the thermal and chemical alteration history of CFA, calcite, and dolomite cements. Structural carbonate in the CFA and calcite cements co-occurring in partially phosphatic digitate stromatolite buildups and adjacent carbonate grainstone generally yield similar δ13Cmineral values (0–5 ‰ VPDB), δ18Omineral values (−5–0 ‰ VPDB), as well as similar ∆47 temperatures (90–115 °C) and calculated δ18Owater (10–19 ‰ VSMOW), supporting a scenario in which CFA and calcite formed and then both recrystallized in equilibrium with the same diagenetic fluids under low water–rock ratio conditions at depth. Dolomite cements yielded similar δ18Omineral values to CFA and calcite, and similar to slightly higher temperatures (85–150 °C) — supporting a scenario in which dolomite cements were recrystallized in equilibrium with a different, and isotopically lighter, generation of fluids. Given the general absence of pervasive, fabric-destructive replacement is scarce, these geochemical data do not support later stage, differential diagenesis as a mechanism for controlling the distribution of phosphatic versus non-phosphatic cements. Rather, primary differences in porewater biogeochemistry seem the likeliest explanation. This is important, as it implies that the ecology of the Ediacaran seafloor controlled the style and quality of mineralization observed today, and that the presence of CFA cements in these rocks represents a unique taphonomic window for select microbial communities.

Biogenically modified reservoir rock quality: A case from the lowermost member Paleocene Funing Formation, Gaoyou Depression, Subei Basin, China

Bioturbation can influence reservoir quality and is consequential to the producibility of a reservoir. The study of samples from the lowermost member of the Paleocene Funing Formation (E1f1), Gaoyou Depression, Subei Basin, shows how bioturbation affects reservoir quality. Techniques used to study the samples include petrography, pressure decay porosimetry, pulse decay permeametry, Field Emission Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy. Sample A is intensely burrowed by Taenidium, Scoyenia, Skolithos, Palaeophycus, and other trace fossils. Increased isotropy in sample A contributes to cleaner well-sorted burrows, relative to its surrounding matrix, and 67.18% augmented burrow porosity. Taenidium and Palaeophycus in sample B indicate 20.23% improved burrows porosity. Plant debris and/or root traces in sample C have a 3.68% reduction in porosity. In samples A and B, the arithmetic mean of permeability describes all horizontal fluid flows within burrows. In sample C, the geometric mean of permeability describes the fluid flow in all directions. Porosity is ≤ 11.2%, permeability ≤1 md in samples, and sample C log-derived porosity ≤0.33%. This study demonstrates that bioturbation together with depositional factors (sorting, grain size distribution, and mud-matrix/burrow content) and diagenetic modifications (albitization, compaction, dissolution, kaolinization, and precipitation) control the quality of the high to intensely bioturbated (Bioturbation Index 4 to 5; 61–99 vol %) sandstone and siltstone reservoir facies of the E1f1.

Origen, potential and water quality of a perennial spring in Juárez city mountains, Chihuahua, México

A Small Perched Perennial Spring has been used for Irrigation for 25 years. With an area of 23.3 ha its water potential basin varies from 90.7 to 979 m3/day so water conductivity of (k=62 cm/day producing between 0.30 to 3.17 lt/sec for lowest to highest rain return period (PR1 to PR100). Water Quantity and Hydraulic conductivity already mentioned were evaluated: Firstly, RPS structure were formed for 100 m of thick impervious lowermost member Lagrima Formation (Kli) overloading for a medium porosity (Klm, Klu) as well as high porosity of Finlay and del Norte Formations (Kf, Kdn). Secondly, Rainfall-Runoff-Recession hydrology model with Recession constant of 0.6 and ratio to pick of 0.2 in the threshold were used trough the simulation. Thus, Base (underground) and Direct (surface) runoff were separated. Finally, water quality and its interaction with host rocks were performed so; two water samples were collected and studied. Both samples were qualified as: Calcic-Bicarbonated (6f-C2) with low salinization risk (C3-S2). Therefore, water could be used for irrigation. In addition, water interaction among Cretaceous rocks of Sierra de Juárez as Lagrima (Kl), Finlay (Kf) and del Norte (Kdn) formations suggests three cases; One; If Calcium reduces Sodium increases given a sodic shale-slate system as: Ca2+HCO3+shales+Na+→Na++HCO3+shales+Ca2, this suggests that water have been confined by rock sourced Cretaceous Lagrima formation (Kli). Two, RPS overloaded recharge layers; Finlay (Kf) and (Kdn) with high karstification potential as high porosity is evident so: Three. If Na+ differ from Cl- thus, the cationic exchange has more Na+ than Cl- so more risk to karstification arises as mentioned before.

The Ra Stua section: A puzzle piece of the Early Cretaceous history of the Dolomites (Trento Plateau, N. Veneto, Italy)

A Barremian hemipelagic succession from Ra Stua in the Dolomites (Southern Alps, northern Italy) is investigated stratigraphically. A short Barremian section was studied with regard to biostratigrapy, lithology, chemostratigraphy and geophysics and subsequently compared to known isochronic palaeomagnetic data and sea levels changes. The results are linked to magnetostratigraphic datasets of the same depositional area on the Trento plateau, yielding a detailed and robust age calibration of the Ra Stua succession. The Lower Cretaceous Ra Stua Formation in the Dolomites is assigned as lower Barremian (Taveraidiscus hugii Zone) to lower upper Barremian (Toxancyloceras vandenheckii Zone). Ammonoids enable a first biostratigraphic subdivision for that puzzling section at Ra Stua. This forms the basis for analyses of the faunal distribution and the palaeoenvironmental evolution of the northerneastern Trento Plateau on the western edge of the Cretaceous Tethys. The Ra Stua section is an enigmatic jigsaw piece correlatable to the Barremian of the entirely preserved sections in the Puez area. The succession consists of marlstones to marly limestone alternations of the Lower Cretaceous. These deposits cover only restricted areas in the Dolomites, but the facies type appears isochronically over wide areas in the Tethys. Lower Cretaceous successions from the Ra Stua area are known, but this isolated block is studied in detail for the first time. The Puez Formation occurs at the Ra Stua section with the Puez-Limestone Member, known as the lowermost member at the type section in the Puez area.

Ichnology and sedimentology of the trace fossil-bearing fluvial red beds from the lowermost member of the Paleocene Funing Formation in the Jinhu Depression, Subei Basin, East China

Abstract The lowermost member of the Paleocene Funing Formation, which is composed of bioturbated fluvial red beds, is extensively distributed throughout the Jinhu Depression, Subei Basin, East China. Despite its economic value as one of the most important oil-bearing reservoirs in the Subei Basin, the sedimentology, ichnology and paleoenvironment of this formation in the Jinhu Depression remain poorly understood. In this work, we investigated the ichnofossils and sedimentary facies, analyzed the ichnofacies and potential tracemakers and reconstructed the paleoenvironment of the Jinhu Depression based on cores from the lowermost member of the Paleocene Funing Formation. This formation contains a moderately diverse ichnofauna (9 ichnotaxa of the Scoyenia ichnofacies) that includes meniscate burrows (Taenidium barretti, Taenidium satanassi and Beaconites coronus), simple horizontal cylindrical burrows (Palaeophycus tubularis, Palaeophycus heberti and Planolites isp.), simple vertical or subvertical burrows (Skolithos linearis and Skolithos isp.) and plant root traces. The other complex branched and chambered burrows are left in open nomenclature. These ichnofossils are widespread throughout the three facies associations identified in this study: (1) ephemeral fluvial channel, (2) crevasse splay and (3) distal floodplain facies associations. Multiple lines of sedimentological and ichnological evidence, namely, the predominance of massive and upper flow regime structures, the occurrence of invertebrate burrows (e.g., Skolithos and Taenidium) in the lower and middle parts of the channel deposits and the dramatic changes in the color and mottling in channel and overbank sediments, indicate that the river discharge was short lived and that the water table was highly variable. The variations in the discharge and water level are attributed to the influence of seasonal rainfall. The cyclic nature of intense rainfall events and dry periods suggests that the Subei Basin was likely located in a semiarid region with a seasonal rainfall pattern during the early Paleocene. This study also reveals that the ephemeral fluvial channel is the facies association with the highest potential to yield oil-bearing reservoirs in the Paleocene red beds of the Jinhu Depression. Additionally, the influence of bioturbation on the reservoir quality in this region should be considered in future research.

Diverse Lithologies and Alteration Events on the Rim of Noachian‐Aged Endeavour Crater, Meridiani Planum, Mars: In Situ Compositional Evidence

AbstractWe report the results of geological studies by the Opportunity Mars rover on the Endeavour Crater rim. Four major units occur in the region (oldest to youngest): the Matijevic, Shoemaker, Grasberg, and Burns formations. The Matijevic formation, consisting of fine‐grained clastic sediments, is the only pre‐Endeavour‐impact unit and might be part of the Noachian etched units of Meridiani Planum. The Shoemaker formation is a heterogeneous polymict impact breccia; its lowermost member incorporates material eroded from the underlying Matijevic formation. The Shoemaker formation is a close analog to the Bunte Breccia of the Ries Crater, although the average clast sizes are substantially larger in the latter. The Grasberg formation is a thin, fine‐grained, homogeneous sediment unconformably overlying the Shoemaker formation and likely formed as an airfall deposit of unknown areal extent. The Burns formation sandstone overlies the Grasberg, but compositions of the two units are distinct; there is no evidence that the Grasberg formation is a fine‐grained subfacies of the Burns formation. The rocks along the Endeavour Crater rim were affected by at least four episodes of alteration in the Noachian and Early Hesperian: (i) vein formation and alteration of preimpact Matijevic formation rocks, (ii) low‐water/rock alteration along the disconformity between the Matijevic and Shoemaker formations, (iii) alteration of the Shoemaker formation along fracture zones, and (iv) differential mobilization of Fe and Mn, and CaSO4‐vein formation in the Grasberg and Shoemaker formations. Episodes (ii) and (iii) possibly occurred together, but (i) and (iv) are distinct from either of these.

The occurrence of a shallow-water <i>Ammobaculoides</i> assemblage in the Middle Jurassic (Bajocian) Dhruma Formation of Central Saudi Arabia

Abstract. We report the occurrence of an Ammobaculoides-dominated assemblage in the lowermost member of the Middle Jurassic Dhruma Formation exposed west of Riyadh, Saudi Arabia. The new species Ammobaculoides dhrumaensis n.sp. is described from the green shale of the D1 unit (also known as the Balum Member) of the Dhruma Formation, which has been assigned an early Bajocian age based on ammonites. Our new finding constitutes the oldest reported worldwide occurrence of the agglutinated foraminiferal genus Ammobaculoides Plummer, 1932.

The Palaeocene-early Oligocene Zacatecas conglomerate, Mexico: sedimentology, detrital zircon U–Pb ages, and sandstone provenance

ABSTRACTThis article presents detailed mapping results and the first U–Pb zircon dating and sedimentological characterization of the Zacatecas Conglomerate, which belongs to the Palaeogene red beds of central Mexico, deposited in fault-bounded basins during the Late Cretaceous to Eocene Laramide orogeny. The conglomerate was divided into five depositional facies associations according to their clast-type abundances and interlayered volcanic rocks. The lowermost member has a maximum depositional age based on young zircon grain ages varying from ca. 63 to 81 Ma. It is unconformably overlain by a continuous sequence characterized by a conglomerate rich in granite clasts at the bottom, with an interlayered tuff dated at 37.64 ± 0.36 Ma. Near the top, another tuff was dated at 30.84 ± 0.47 Ma, and a sandstone has a maximum depositional age of ca. 31.5 Ma. Normal grading, massive textures, channels, channel-form sandstone bodies, and upward-finning successions suggest that the Zacatecas Conglomerate is of fluvial origin. Late Jurassic to Early Cretaceous ages from zircons in plutonic rocks and sandstones bracket possible source regions for the Zacatecas Conglomerate. One possible source is Late Jurassic-Early Cretaceous granite derived from the Alisitos-Guerrero arc of western Mexico. Another possible source is the Tuna Manza Diorite, now exposed 250 km southeast of the study area. The lack of pre-Jurassic grains implies that possible sources such as the Nazas arc or the Potosí fan were not cropping out at that time, or at least that these areas were not affected by the fluvial system feeding the Zacatecas Conglomerate. It is possible that during the Palaeocene-early Oligocene the fluvial systems drained from west to east and from southeast to north, according to the above-mentioned constraints.

신생대 제4기 울릉도 나리칼데라 부석층에서 산출된 탄화목의 연륜연대 및 수종식별 연구

본 연구는 울릉도 나리칼데라에서 산출된 탄화목의 수종식별, 연륜분석, 방사성탄소 연대를 통해 고식생/고환경, 분출연대 및 화산쇄설물의 정치온도에 대한 정보를 제공하기 위해 수행되었다. 탄화목은 울릉도 최후기 화산쇄설층을 대표하는 나리테프라층의 최하부(Member N-5) 부석질 퇴적층 내에서 채취되었으며, 나이테의 원형(횡단면)이 잘 보존된 나무의 가지 부분이다. 분석결과, 수종은 소나무과 중 가문비나무속(Picea spp.)으로 식별되었으며 최외각 나이테의 연륜은 <TEX>$263+{\alpha}$</TEX> 년으로 확인되었다. 나이테는 중심(pith)에서 바깥쪽을 향해 그 나이가 점차 젊어지며 <TEX>$20,260{\pm}230$</TEX>, <TEX>$19,995{\pm}245$</TEX> 및 <TEX>$19,975{\pm}265cal\;BP$</TEX> 의 보정연대를 가진다. 가장 젊은 연대인 19,710 cal BP 는 Member N-5의 분출과 관련된 연대일 가능성이 높다. 이 결과는 후기 플라이스토세 동안 울릉도 나리칼데라는 가문비나무가 번성하였으며, 한랭 습윤한 기후조건하에 있었음을 지시한다. 광택, 취성, 색 및 조흔색 등의 특성에 기초하면, 탄화를 야기한 화산쇄설물의 최소 온도는 약 <TEX>$350{\sim}500^{\circ}C$</TEX> 정도일 것으로 추정된다. The purpose of this study is to provide an inquiry into paleovegetation (paleoenvironment), eruption age and inferred emplacement temperature of pyroclastic deposit using charcoal occurred in the Quaternary Nari Caldera, Ulleung Island. In order to obtain the data, we carried out species identification and tree-ring analysis, radiocarbon dating for the charcoal. This sample was collected from pumiceous deposit in lowermost member(Member N-5) of the Nari Tephra Formation, which corresponds to the wood branch that has well preserved the tree-ring structure. Speciation and outermost tree-ring of wood reveal Picea spp. and <TEX>$263+{\alpha}$</TEX> years. The calibrated ages from the center(pith) to the outermost tree-ring are <TEX>$20,260{\pm}230$</TEX>, <TEX>$19,995{\pm}245$</TEX>, and <TEX>$19,975{\pm}265cal\;BP$</TEX>, respectively, which are accordant with the tree-ring position. The youngest age, <19,710 cal BP would have implications for the eruptive age of Member N-5. Our results indicate that Picea spp. is the paleovegetation representing that Nari Caldera was under cold and wet climate conditions during the late Pleistocene. Based on the silky luster, brittleness, color, and streak of charcoal, etc., the lowest emplacement temperatures of pyroclastic flow are interpreted to have been at least as high as <TEX>$350{\sim}500^{\circ}C$</TEX>.

Gypsum domes and diapirs: common features in the Zechstein (Upper Permian) of Germany

The Upper Permian Zechstein Group contains three gypsum horizons of variable thickness. All of them are affected by gypsum diapirism which is not related to salt diapirism. Best areas for observations and descriptions of this phenomenon are outcrops and mining districts around the Harz Mts. The lowermost gypsum member (A1) is overlain by Main Dolomite (Ca2) of the Stassfurt Formation - dolomite showing mudstone and wackestone textures. The gypsum forms cupolas or elongate ridges up to 50 m high, several km long, and may project through its roof. Above the ridges, the Ca2 roof is thinner or completely removed by slumping and sliding. The uplift of the gypsum started very early before the carbonates were completely lithified. All the ridges strike between 90 and 120°, most likely, drawn out by tectonic forces. The Basal Anhydrite (A2) and the Main Anhydrite (A3) are overlain by rock salt and behave in the same manner. Here, too, only the upper parts of the gypsum are involved in the movements. Reasons of the uplift are high water content of gypsum, additional release of water by the conversion of gypsum to anhydrite and, last not least, bouyancy of the light gypsum mud against compact rock salt or semi-lithified carbonates.

The Miocene Gachsaran Formation evaporite cap rock, Zeloi oilfield, SW Iran

Petroleum at the Zeloi oilfield (SW Iran) is produced from the Oligocene/Miocene Asmari Formation. Evaporites in the overlying lowermost member of the Gachsaran Formation form an efficient cap rock seal. In the present study, this cap rock was studied and interpreted on the basis of gamma-ray and sonic logs data from eleven boreholes. Geochemical data obtained from X-ray fluorescence analyses of the anhydrite samples and scanning electron microscope analyses of the halite samples have been interpreted in terms of depositional environment. Chemostratigraphy based on the use of Na2O/Al2O3, Na2O/Fe2O3, Na2O/TiO2, and Na2O/K2O ratios allowed division of the cap rock into three marker units: (1) a unit of weathering-resistant elements; (2) an index key unit; and (3) a unit of highly soluble elements. The stratigraphy of the cap rock suggested the occurrence of cyclic regression and transgression. Geochemical data in combination with textural analyses in scanning electron microscopy indicate arid depositional conditions for this part of the Gachsaran Formation. Cycles of marine transgression–regression taking place in a barred coastal sabkha-lagoon basin, must have been responsible for the formation of the evaporite sequences of the Gachsaran Formation.

Carnian (Triassic) aridization on the Levant margin: evidence from the M1 member, Mohilla Formation, Makhtesh Ramon, south Israel

The Carnian Pluvial Event was followed by extreme aridity around the Mediterranean region, but the relationship between these climatic modes and the evaporite deposits of the north Arabian margin has not been established. The Mohilla Formation in Israel was deposited on the rifted Levant margin of the Neotethyan seaway during the Carnian (Late Triassic). The lowermost member, M1 (carbonate, shale, minor evaporite), records sea-level and climate change of the transition from open-marine deposits to the thick evaporite of the overlying M2 member. Field, petrographic and cathodoluminescence investigation of the M1 member exposed at Makhtesh Ramon, southern Israel, enabled tracking of changing paleoenvironments. The M1 member can be subdivided into six sea-level controlled sedimentary cycles, each terminating in subaerial exposure. Open-marine, lagoonal, and supratidal belts are represented, but environments become increasingly restricted upwards. Three of the exposure horizons are marked by well-developed early diagenetic features, including dissolution porosity and meteoric cements. These changing facies belts and the superposed diagenetic modes formed under an oscillating climate regime of three relatively humid episodes dominated by deposition of carbonate, alternating with evaporitic sediments indicating aridity. Deposition of the M1 member can be correlated with the transition in the western Tethys from the Carnian pluvial episode to aridity. This transition in the Levant region is characterized by an oscillatory climate that culminated in a thick evaporite unit from the extreme arid phase. The oscillations may be attributed to shifting global climate belts, overprinted by monsoonal strengthening and weakening, in the equatorial belt.

Correlation of Middle Pleistocene crystal-rich tephra layers from Daisen Volcano, southwest Japan, based on the chemical composition and refractive index of mafic minerals

Analyses of chemical compositions and refractive indices of mafic minerals have been effective correlation tools, especially for crystal-rich tephra layers containing isolated volcanic glass shards. The analyses are applied to the correlation of the Middle Pleistocene tephra layers in the Kinki and Chubu districts, central Japan, with those erupted from Daisen Volcano in the Chugoku Mountains, west of the districts. On the basis of chemical compositions of hornblende phenocrysts and refractive indices of hornblende, orthopyroxene and cummingtonite phenocrysts, four groups of tephra layers in the Kinki and Chubu districts are successfully correlated to bvs2, cpm, gpm and hpm1, or DOP tephra layers of the Lowermost Member of the Daisen Tephra Formation. The correlation reveals that two tephra layers, bvs2 and cpm, with estimated eruption ages of 410 ka and 370–400 ka, respectively, are present in the Kinki district. The hpm1 tephra, about 210 ka, is also widespread and recognized at the eastern foot of Yatsugatake Volcanoes about 400 km east of the source. These tephra layers can contribute to establishing the regional correlation and chronology of the Middle Pleistocene sequences in central Japan, especially for the time from MIS 7 to MIS 11.

The Middle Cambrian Mount Roosevelt Formation (new) of northeastern British Columbia: evidence for rifting and development of the Kechika Graben System

The eastern margin of the Kechika Graben in the vicinity of Muncho and Moose lakes, northeastern British Columbia, contains Middle Cambrian rift-related strata herein named the Mount Roosevelt Formation. The formation conformably overlies a quartzite package equivalent to the uppermost clastic unit of the informal late Early Cambrian Gataga group ( Bonnia – Olenellus zone) and is conformably overlain by a thick, unnamed carbonate sequence of Middle Cambrian age ( Plagiura – Poliella zone). The Mount Roosevelt Formation is subdivided into three members. The lowermost member is characterized by oöid-bearing siltstone and sandstone, interbedded with dolostone, limestone, and hematitic conglomerate. Conformably above this, the middle member is a thick sequence of polymict cobbly pebble conglomerate. The upper member includes karstified dolostone, calcareous-cemented conglomerate and sandstone, and limestone. Collectively the Mount Roosevelt Formation reflects alluvial fan delta progradation into a transgressive marine environment. Deposition occurred in an active fault-controlled basin, located on the eastern margin of the Kechika Graben adjacent to the Muskwa High. Basin initiation in the late Early Cambrian coincided with the reactivation of pre-existing regional faults. High rates of subsidence during the initial phase of extension were accommodated on these faults, which provided a locus for fan delta deposition. Continuing high rates of subsidence limited basinward fan delta development. Deposition of the formation ended with base-level transgression in the early Middle Cambrian that drowned the fan deltas and the adjacent Muskwa High and allowed development of the Kechika Trough above the older graben system.

Ignimbrites of the Pliocene Bagaces Formation near Canas (Guanacaste, Costa Rica)

By this study, four units within the so far mostly undivided Bagaces Formation - a major ignimbrite unit of the Plio- to Pleistocene magmatic succession in northern Costa Rica - were identified and their ignimbrite sequences stratigraphically distinguished. The basal welded Montenegro ignimbrite of mid-Pliocene age forms the lowermost stratigraphic member. It is followed by three non-welded ignimbrite units of Upper Pliocene age, Sandillal (4.1-4.15 ± 0.02 Ma) and Canas (2.06 ± 0.02 Ma) and the locally exposed Rio Tenorio ignimbrite (estimated age 2 Ma). The ignimbrite units of the Bagaces Formation show decreasing thicknesses from NE to SW. For the non-welded Sandillal and Canas units, distal lobes could be defined. Lateral pumice and lithics size distribution, as well as isopach data indicate a paleotopographical control for the deposition of the ignimbrites and allowed defining the source area of the ignimbrites. Ar-Ar-ages and compositions analysis of the pyroclastic units refer to the eruptive evolution of the Neogene volcanic arc.

Metamorphism of the Permo-Triassic Cape Fold Belt and its basement, South Africa

The Permo-Triassic Cape Fold Belt around the southern tip of Africa consists of a thick sequence of Palaezoic siliciclastic sedimentary and pre-Cape basement rocks believed to be of Pan-African age. Both the basement rocks and the supracrustal rocks of the Cape Supergroup display only low metamorphic grades. Application of chlorite, chlorite-chloritoid Fe-Mg exchange, and calcite-graphite carbon isotope geothermometry to rocks from the unconformable contact between pre-Cape basement and the Cape Supergroup made it possible to distinguish pre-Cape and syn-Cape metamorphic overprints. During Pan-African metamorphism temperatures of up to middle greenschist facies conditions (around 400 °C) were reached, whereas lowermost greenschist facies conditions (around 300 °C) were not exceeded during the 220–290 Ma Cape orogeny. In the past, most if not all of the pre-Cape basement rocks, which form the Pan-African Saldania Belt, were considered to be of Neoproterozoic age. A hiatus of about 100 °C observed between two adjacent limestone horizons that previously had been grouped together into a single formation at the bottom of the allegedly Neoproterozoic Kango Group indicates that almost all of this group is syn- to post-orogenic with respect to the Pan-African orogeny. A revision of the stratigraphy of the Kango Group is therefore suggested. Only its lowermost member is truly Pan-African and probably related to about 620–740 Ma post-Sturtian cap carbonates in other Pan-African belts of southern Africa. The remainder of the Kango Group reflects the successive development of two stages of orogen-related intra-continental basins: The older stage led to a typical syn-orogenic foreland basin related to tectonic loading in the Gariep and Damara orogenic belts further north(west) between 570 and 540 Ma; the younger is believed to have formed either a further foreland basin or an intra-orogen pull-apart basin caused by later tectonic loading in the Ross orogenic belt and its continuation into the southern Saldania Belt between 510 and 480 Ma.

The marine transgression in the Middle Carboniferous of Brøggerhalvøya (Svalbard)

The change from continental to marine conditions in the Middle Carboniferous on Broggerhalvoya started at the end of the Bashkirian with short-term transgressive events at the top of the Broggertinden Formation. Local basin subsidence was responsible for the pulsatory nature of the transgression. The establishment of a shallow marine carbonate-dominated environment is represented by the Moscovian Scheteligfjellet Member which overlies the post-Caledonian red beds of the Broggertinden Formation. The Scheteligfjellet Member is the lowermost member of the Nordenskioldbreen Formation and shows distinct lateral facies variations. Three facies associations can be distinguished: lagoonal facies, shoal facies and open marine facies. The succeeding two members were deposited in subtidal areas of the carbonate platform. A basin subsidence event at the Carboniferous/Permian boundary was responsible for a short shift into deeper depositional environments during a time of worldwide regression. After this a continuous regression led to supratidal conditions at the top of the Nordenskioldbreen Formation.

Appalachian Stress Study: 1. A detailed description of in situ stress variations in Devonian shales of the Appalachian Plateau

We describe an experiment to measure variations in the state of stress within a horizontally bedded Devonian shale/sandstone/limestone sequence in western New York. A total of 75 stress measurements were made in three wells a kilometer or so apart using a wireline‐supported hydraulic‐fracturing system. The stress profiles indicate that a major drop in horizontal stress level occurs in the generally massive shales. This drop occurs principally across the lowermost member of a group of sand beds and corresponds to an offset in Sh and SH of 3.5 and 9 MPa, respectively. Above the sands, “thrust” regime conditions prevail, although the amount by which Sh exceeds Sv is undetermined since instantaneous shut‐in pressures (ISIPs) were clipped at the level of Sv due to fracture rotation. Below the sands, the regime is strike slip with both horizontal stresses showing lateral uniformity despite substantial variations in topography. The magnitude of Sh in the sand beds themselves and a lower limestone remains at least as great as Sv despite the decline in shale stress. Hence stress contrasts between these beds and neighboring shales become pronounced with depth. The contrast in Sh and SH between the lowermost sand and the immediately underlying shale is at least 6 and 14.5 MPa, respectively. SH levels in the lower strike‐slip regime are about 1.75 times greater than Sh and are less than the value required to initiate slippage on favourably oriented frictional interfaces. For the upper thrust regime and the sand and limestone beds, however, the inferred lower bound on SH is close to the slippage threshold for a Coulomb friction coefficient of 0.6. The orientation of SH is ENE with a standard deviation of 20°. Fracture traces were usually splayed, occasionally spanning 30° of well bore. No systematic correlation between mean orientation and lithology is evident. Significantly different orientations were obtained for adjacent tests in which almost identical ISlPs were observed, suggesting that the fractures quickly reorient themselves to propagate normal to the least principal stress direction. Similarly, vertical traces were observed in those tests where ISIPs apparently reflect Sv, suggesting that rotation to horizontal was rapid.