Coarse Sandstone Research Articles

Reservoir development, sequence stratigraphy and geological modelling of Westphalian fluvial reservoirs of the Caister C Field, UK southern North Sea

The earliest Westphalian B reservoir of the Caister C Field (informally named the Caister Sandstone Unit consists of erosive conglomeratic sandstones deposited in stacked low sinuosity fluvial channels. Channel types and architectural elements demonstrate an upward change to more sinuous forms. The Caister Sandstone Unit is believed to contain incised valley-fill sediments deposited as late lowstand and transgressive systems tracts when rising base level generated accommodation space. The dominant control on reservoir quality is primary depositional texture with the best permeability found in higher energy coarse sandstone and pebbly intervals associated with basal channel-fill units; crevasse channel and splay sandstones offer poor reservoir properties. The robust correlation framework, which is based upon laterally extensive coal seams, enables a sequence stratigraphic model of the field to be developed, thereby allowing a better understanding of reservoir extent and sandbody continuity. A 3D geocellular geological model is built with reservoir architecture elements controlled by the sedimentological model. This is used for volumetrics calculations, input to a reservoir simulator and in the locating and planning of in-fill wells.

Forensic Evaluation of Three Failed Cement-Treated Base Pavements

Three cement-treated base (CTB) pavements constructed around 1990 near Houston, Texas, showed severe pavement deterioration after 3 to 4 years. The unusual distress patterns included wheelpath alligator cracking and severe pumping. A forensic investigation was initiated to determine the cause and to recommend modifications to materials specification, design, and construction practices to avoid future problems. The primary cause was determined to be chemical deterioration that resulted in destruction of the cement matrix. In all cases water was trapped within the CTB layer. In two bases moisture flowed rapidly by capillary action through the CTB; these bases showed the most rapid deterioration. Water flow in one case was attributed to smectite clay contamination of the fine aggregate and in another to a highly absorptive sandstone coarse aggregate. Extensive ettringite was found in one material. In one case deterioration was caused by the growth of two phases exhibiting different morphologies: hydrated calcium aluminum silicate with a radiating needle morphology and ettringite, which showed a bladed morphology in these samples. Practical recommendations to prevent recurrence of the problem were ( a) moisture absorption as acceptance testing for coarse aggregate and ( b) elimination of designs placing different stabilized materials on top of one another. The laboratory suction test that was developed is recommended as a screening test for molded CTB specimens.

Water-holding capacity of rock fragments in rehabilitated opencast mine soils

The water-holding capacity of rock fragments sampled from spoil materials underlying rehabilitated opencast coal mine soils was determined. Four rock types, ranging from grit and coarse sandstone to carbonaceous shale, and four fractions of different size were investigated by means of ceramic plate extraction. Water retention was determined at each of four matric potentials, viz. −33, −80, −500 and −1500 kPa. Between approximately 2 and 40% (average 13.8%) water, on a volume basis, was released from saturation to −1500 kPa. In respect to smaller sized fragments (2–4 mm), 60% or more of the plant available water, taken as the fraction contained at a potential of greater than −1500 kPa, was released at high potentials (0 to -33 kPa), and in respect to larger fragments (above 4 mm), 10 to 40%. There was no relationship between water content and rock type. Rock fragment size correlated reasonably well with water held between 0 and −1500 kPa (R20.78) as well as water released between 0 and −33 kPa and between −33 and −80 kPa (R2 0.64 and 0.85 respectively). Rock fragment size correlated rather poorly with water released between −80 and −1500 kPa. Application of the results is discussed.

Spatial and mechanical controls on normal fault populations

Characteristics of normal faults have been measured from outcrops in western Sinai and northern Britain, and from the subsurface in the North Sea basin. Fault damage zone widths and the spatial distribution of minor structures around faults have been related to variation in throw along a fault surface. Secondary shear fractures, including faults and granulation seams, are most common within damage zone envelopes, in the immediate footwall and hangingwall, in the extensional field of major normal faults. Shear fractures are less common in the contractional field of normal faulted rock volumes. Fault zone thickness measurements from outcrop indicate scale invariance over several orders of magnitude within the same location and lithology. Fault zone thickness is linearly correlated to fault throw. There is a strong mechanical and lithological control on fault zone thickness. Coarse sandstones have thicker fault zones for a given throw, due to the processes of strain hardening and cataclasis, compared with argillaceous rocks which are prone to strain softening. Fault spacing in siliciclastic rocks can show scale invariance over several orders of magnitude. The influence of grain size and layer thickness on fault growth can lead to either non-fractal behaviour or changes in the power law for fault characteristics such as throw, thickness and spacing.

Sedimentology of a lacustrine fan‐delta system, Miocene Horse Camp Formation, Nevada, USA

ABSTRACTA 1600‐m‐thick succession of the Miocene Horse Camp Formation (Member 2) exposed in east‐central Nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan‐delta environments and nearshore and offshore lacustrine environments. These four depositional environments are distinguished by particular associations of individual facies (14 defined facies). Subaerial and subaqueous fan‐delta facies associations include: ungraded, matrix‐and clast‐supported conglomerate; normally graded, matrix‐ and clast‐supported conglomerate; ungraded and normally graded sandstone; and massive to poorly laminated mudstone. Subaqueous fan‐delta deposits typically have dewatering structures, distorted bedding and interbedded mudstone. The subaerial fan‐delta environment was characterized by debris flows, hyperconcentrated flows and minor sheetfloods; the subaqueous fan‐delta environment by debris flows, high‐ and low‐density turbidity currents, and suspension fallout.The nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments. High‐energy oscillatory wave currents, probably related to a large fetch, reworked grains as large as 2 cm into horizontally stratified sand and gravel. Offshore‐directed currents produced uncommonly large (typically 1–2 m thick) trough cross‐stratified sandstone. In addition, stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high‐energy conditions alternating with periods of carbonate precipitation under reduced energy conditions. Massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout.A depositional model constructed for the Horse Camp Formation (Member 2) precludes the existence of all four depositional environments at any particular time. Rather, phases characterized by deposition in subaerial fan, nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan‐delta, subaqueous fan‐delta and offshore lacustrine deposition. This model suggests that high‐energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge, thus preventing development of a subaqueous fan‐delta environment and promoting development of a well‐defined nearshore lacustrine environment. Low‐energy nearshore currents induced by shallow water along the

High-resolution Stratigraphy and Sedimentary Evolution of Coarse-Grained Canyon-Filling Turbidites From the Upper Cretaceous Transgressive Megasequence, Campos Basin, Offshore Brazil

ABSTRACT Coniacian to Lower Maastrichtian coarse-grained turbidites fill intra-slope, fault-controlled canyons in the Campos Basin, off-shore Brazil. They form part of an Upper Albian to Lower Paleocene transgressive succession characterized by onlapping, deepening-upward sedimentation throughout the eastern Brazilian margin. In the Carapeba and Pargo (CRP-PG) oil fields the turbidites consist mainly (> 95%) of graded beds without any other sedimentary structures. Individual beds are up to 12 m thick and are composed of small pebble(< 2 cm) to granule-rich sandstones, and medium to very coarse sandstones. The finer-grained portions of these graded beds show better sorting and higher porosity, and these characteristics can be recognized in density logs. Use of 92 density logs calibrated w th 13 cores permitted the mapping of 198 coarse-grained turbidites in the CRP-PG turbidite system. The turbidites form eight thinning- and fining-upward facies successions, some bounded by regional unconformities or local erosion surfaces. The successions are 27-140 m thick and contain 7-58 turbidites. Their durations are estimated to be between 0.4-0.9 m.y. They form 1-12 km wide, tabular or linguoid sandstone bodies in which the younger or more distal turbidites become finer-grained, thinner-bedded, and more discontinuous upsection and downcanyon. The successions are stacked in an overall retrogradatioual pattern for at least 20 km, recording the backfilling of the CRP-PG canyon. Turbidites in the CRP-PG area were probably deposited during falls of relative sea level that punctuated the overall transgressive setting of the late Cretaceous and early Tertiary. There is a mismatch between the number of successions (eight) and the number of time-equivalent third-order fluctuations of relative sea level (four and part of a fifth; Haq et al 1988), suggesting that eustatic sea level fluctuation was not the dominant control on most of the successions. Cross sections and isopach maps of the successions do not suggest internal channeling, and a channel-fill origin for the fining- and thinning-upward successions seems unlikely. We therefore suggest that episodic tectonic reactivation in the uplifted Precambrian source area, and faulted basin margin, led to relatively abr pt increases in the volume of sediment supplied to the head of the canyon across a narrow shelf. Steady denudation and decreasing supply of sediment led to formation of the fining- and thinning-upward successions.

Karoo Rift basins of the Luangwa Valley, Zambia

Abstract An integrated study of the Luangwa Valley Karoo Rift basins of NE Zambia has been carried out. Approximately 3000 km of seismic data were used in addition to gravity and aeromagnetic surveys and data from two exploration wells. The present day Luangwa Valley is underlain by two opposing Karoo half-graben separated by a central accommodation or transfer zone. The basins contain up to 8000m of Permo-Triassic, mainly clastic, sediments. The Lower Karoo consists of basal glacial deposits overlain by a sequence of interbedded coal, shale and siltstone, and capped by a very thick (up to 5000 m) lacustrine section of interbedded siltstone and mudstone. The Upper Karoo consists of interbedded coarse sandstones and siltstones thought to be of braided stream and flood plain origin. Rifting appears to have been initiated in the early Permian. The timing of subsequent rifting and inversion events has been related to strike-slip movements along the Mwembeshi Shear Zone, a major Precambrian lineament. Study of the seismic data has revealed a number of structural and stratigraphic plays in the basin. The best potential for hydrocarbon accumulation occurs within the lowest part of the Karoo section where source rocks and reservoirs are postulated in the Luwumbu Formation and the lowest part of the Madumabisa Mudstone. Since these intervals were not reached by either of the exploration wells drilled it is felt that the hydrocarbon potential of the Luangwa Graben has not yet been fully tested.

Sedimentology and Depositional Sequences of the Jordan Formation (Upper Cambrian), Northern Mississippi Valley

ABSTRACT The Jordan Formation, a typical cratonic quartz sandstone, has been the focus of study and contention for more than a century, especially because of its relation to the Cambrian-Ordovician boundary in the type area of Upper Cambrian strata in North America. The Jordan is approximately 30 m thick; the presence of trace fossils and scattered trilobite and brachiopod shells indicates a marine environment of deposition. The formation was deposited in an epeiric sea at least tens of meters deep (the moat) lying between the still-exposed continental nucleus, which supplied clastic sediments, and an extensive carbonate shoal, which developed on the continental margin and advanced into the craton interior during Late Cambrian time (the Great American Bank). We recognize three facies in the Jordan on the basis of sedimentary structures and grain size. The hummocky-stratified facies, typified by the (basal Jordan) Norwalk Member, is composed of very fine to fine, subangular, feldspathic quartz sandstone, with extensive planar to low-angle bedding. This facies is interpreted to represent episodic storm deposition below fair-weather wave base in an offshore setting. The trough cross-stratified facies, typified by the (overlying) Van Oser Member, is composed of fine to medium, well-rounded, pure quartz sandstone, in pervasive high-angle trough cross-sets. Cross-bed orientation shows a regional southward (modern coordinates) transport direction, with much scatter and local bi-modal patterns. This facies was deposited as subaqueou dune bed forms. It is interpreted to have formed nearer the shoreline and above fair-weather wave base, with dune movement induced largely by storm-enhanced marine currents. The large-scale cross-stratified facies, present at the top of the Van Oser Member at a few scattered outcrops, is composed of coarse sandstone in cross-sets 1-2 m thick. This facies is interpreted as the shallowest and most energetic of all, whose deposition was dominated by currents of probable tidal origin. Because we recognize evidence for both storm and tidal processes in the Jordan, we favor a comparison with modern shelves in which combined processes are active. In general, the Jordan displays a coarsening-upward facies sequence, implying a progradational origin for the formation. Locally, part or all of a second coarsening-upward sequence is present, indicating that two progradational cycles took place. The upper contact of the formation is an unconformity, which variably truncates the progradational sequences. Above the unconformity, in overstep relationship to the truncated Jordan, lies the Lower Ordovician Prairie du Chien Group, a shoal-water carbonate unit with reworked quartz sand in its lower zone. The Prairie du Chien represents an extensive marine transgression that followed the sea-level lowstand at the end of Jordan deposition. Because of the scarcity of fossils in the upper Jordan, the age of the drawdown is not yet tightly const ained, but it is very near the Cambrian-Ordovician boundary.

Lithostratigraphy and depositional environment of the Permian Nowra Sandstone in the southwestern Sydney Basin, Australia

Lithofacies in the mid‐Permian Nowra Sandstone indicate a middle/upper shoreface to foreshore environment of deposition under the influence of storm‐generated waves and north‐northeasterly directed longshore currents. Palaeogeographic reconstruction for the Nowra Sandstone portrays a sand‐dominated high energy shelf and offshore shoal forming a sequence thickening seaward away from the western shore of the Sydney Basin. The shoal‐crest at the outer edge of the shelf trends north‐northeast. It is characterized by fine‐ to medium‐grained sandstone with upper flow regime structures and a high proportion of conglomerate, whereas coarser sandstone with lower energy bedforms occurs along the seaward side of the shoal. In the deeper water to the east, the lower Nowra Sandstone becomes rapidly thinner as it passes seaward, via bioturbated storm redeposited sandstone beds, into the shelf deposits of the Wandrawandian Siltstone. This sequence accumulated during a regressive event and the base of the formation becom...

An integrated analytical approach for determining the origin of solid bitumens in the McArthur Basin, northern Australia

An integrated analytical approach using petrological, geochemical and isotopic techniques has been used to determine the origin of solid bitumens in the Middle Proterozoic Roper Group, McArthur Basin, northern Australia. Sandstone and dolerite samples, which in hand specimen contain dark material believed to be solid bitumens, were examined from six wells. Under ultraviolet illumination, non-fluorescent solid bitumens can be distinguished from two generations of crude oil, one of lower maturity (orange fluorescence), the other of higher maturity (white fluorescence). Other petrographic observations indicate that the precursor hydrocarbons to the solid bitumens and the subsequent charges of crude oil postdate all diagenetic clay and quartz cementation. Geochemical analyses indicate that many of the sandstones contain both an aliphatic-rich oil and solid bitumen, mixed in varying proportions, thus corroborating the petrological results. On the basis of n-alkane distributions, the oil appears to have two 3ubtly-different source inputs, one of which is typified by a greater proportion of high molecular weight n-alkanes which have a slight even carbon-number predominance and an isotopically lighter aliphatic hydrocarbon fraction. One sandstone sample contains solid bitumen in a fracture which is characterized by a high abundance of asphaltenes, polar compounds and sulphur and may have formed by deasphalting of a precursor oil. The solid bitumens recovered from fractures in dolerite sills were formed by thermal processes which in one sample resulted in an unusual series of ketone isomers. However most solid bitumens, particularly where present in the coarser sandstones, are related to the generation of secondary porosity. The large proportion of unresolved complex mixture, the lack of n-alkanes and the isotopically heavier aliphatic hydrocarbon fractions of samples containing predominantly solid bitumens indicate that bitumen formation occurred mainly by biodegradation of a precursor oil.

Early Cambrian alluvial fan‐deltas in the Georgina Basin, Australia

Two formations, the Mt Baldwin Formation and Andagera Formation, show a full spectrum from proximal to distal facies associated with shelf type alluvial fan‐delta sedimentation. These facies reflect prevailing tectonic and eustatic conditions and the resultant sedimentation patterns during the Early Cambrian. The synsedimentary tectonic and eustatic history is divided into a number of stages: Stage 1, continuous gradual uplift during a lowstand in sea level and the deposition of thickly bedded coarse sandstone and granule to pebble conglomerate with medium‐scale planar cross‐beds and channel‐scoured bases; Stage 2, continuous gradual uplift during a highstand in sea level marked by a minor marine transgression and deposition of laminated yellow and green siltstone and fine sandstone with occasional flaser bedding, water‐escape structures and convolute bedding; Stage 3, a fall in sea level and gradual progradation of an alluvial fan into a coastal environment with deposition of wavy laminated siltstone, co...

The Influence of Lateral Basinal Slopes on Turbidite Sedimentation in the Annot Sandstones of SE France

ABSTRACT The deep-water deposits of the lower Oligocene Annot Sandstones of SE France were deposited in topographically dissected basins. Axially derived turbidites show marked lateral onlap onto the underlying marls. The original lateral slopes were at least 250 m high, and up to 12° prior to compaction-induced steepening; they commonly show a stepped or undulatory profile. Lateral tracing of individual beds away from their position of onlap termination down the paleoslope reveals a characteristic succession of microfacies: thin-bedded, nongraded fine sandstones with upper-flow-regime planar lamination pass downslope into medium-bedded, partially graded medium sandstones with ripple and planar lamination. At the base of the depositional slope, the turbidites become thick to very thick-bed ed, fully graded (commonly stepwise) coarse sandstones to mudstones. The most likely interpretation of the lateral transition from lower-flow-regime to upper-flow-regime conditions from the axial trough to the lateral margin of the basin involves pulsatory flow conditions. Episodes of increased flow thickness and velocity deposited upper-flow-regime bed forms on the higher parts of lateral ramps, while periods of waning current deposited lower-flow-regime bed forms in the axial trough. During accumulation of the sands, the underlying marls were differentially compacted, with maximum compaction under the axis of the basin, decreasing to zero compaction at the edges of the basin. This resulted in secondary slopes on the upper surface of the sandstones overlying the marl slopes, which may have been important in controlling the accumulation of successive sand packages. Successive onlap and overstep of sand layers onto the marls resulted in a vertical profile that shows the same succession of facies as traced downslope in individual beds. The resulting vertical sections coarsen and thicken upwards over 5-10 m. The characteristics of the thin-bedded fine sandstones at the base of cycles may be used to distinguish between prograding and laterally onlapping turbidite packages. Sandstone packages in the basin axis are thick (30-60 m), sharp based, and sandstone rich. The laterally equivalent part of a package overlying a confining slope is thinner, asymmetric with a coarsening upward signature, and more mudstone rich.

Deepening-upward Sequences in Oligocene and Lower Miocene Fan-delta Deposits, Western Santa Ynez Mountains, California

ABSTRACT Coarse, matrix-supported conglomerates, an abundant invertebrate faunal assemblage, and large vertical burrows record high-energy nearshore deposition in the Vaqueros Formation of the westernmost Santa Ynez Mountains. These strata document the effect of Late Oligocene-Early Miocene eustatic sea-level rise on fan-delta deposits of the tectonically active southern California continental margin. The Vaqueros fan-delta succession can be divided into six facies deposited in three fan-delta facies assemblages: fan-delta plain, fan-delta front, and fan-delta slope. The fan-delta-plain facies assemblage consists of inversely to normally graded, matrix-supported, boulder to pebble conglomerates. These conglomerates are angular, nonfossiliferous, and laterally persistent. The fan-delta front facies assemblage is characterized by non-graded to normally graded, sandy, cobble to pebble conglomerates and conglomeratic sandstones organized in both channelized and nonchannelized bedding units. Pectens, oysters, and shallow-water gastropods are abundant in this facies, and individual beds have sharp bases and burrowed tops. Small-scale lenticular bedding and reworked lenses of fossil mater al are common throughout. Locally, the nonchannelized units show subhorizontal bedding. Fan-delta-slope strata are dominated by extensively bioturbated, locally cross-bedded, pebbly, coarse sandstone overlain by sandstone/fossil conglomerate couplets. Pebble imbrication throughout the sequence, along with channel orientations and cross-bedding in the fan-delta-front units, record paleoflows to the east and the southeast. These facies assemblages represent deposition in a deepening-upward fan-delta system on the southern flank of the then-emergent Santa Maria basin (the paleotrench-slope break). Fan-delta-plain conglomerates record braided fluvial and intertidal deposition; fan-delta-front conglomerates and sandstones record channel-mouth and bar/spit deposition; and the fan-delta-slope sandstones record deposition in shoreface to inner-shelf environments. The sequence documents local sedimentary response during a period of regional tectonic subsidence and general eustatic sea-level rise.

The influence of half-graben syn-depositional tilting on thickness variation and seam splitting in the Brunner Coal Measures, New Zealand

The Brunner Coal Measures (Buller Coalfield, New Zealand) comprise a fluviatile sequence of Eocene age that is generally 30–100 m thick. The sequence rests on basement and is dominated by coarse to granular—very coarse sandstone. The measures contain one mineable coal seam (the Buller Seam Member) that is laterally extensive and up to 20 m thick. It is characterized by simple splitting with a thin upper split, a thick lower split and a split separation of up to 30—50 m. Locally, the Buller Seam Member exhibits splitting to form numerous thin seams. Areas of shale-out comprise thin linear belts between areas of thick coal with a low ash yield. The coincidence of faults and maximum thickness of seam isopachs indicates that the thickness and splitting characteristics of the Buller Seam Member were largely controlled by syn-depositional faulting and tilting of small half-graben blocks. Thickness variation in basal sequences is primarily due to syn-depositional faulting. Syn-depositional tilting of fault blocks also influenced the migration of river channels and hence the stacking and distribution of sandstone and locally thick mudstone units overlying coal.

Drill Core Orientation using Palaeomagnetism

Palaeomagnetic orientation of drill core from the Sydney-Bowen Basin is feasible. A consistent magnetisation is observed in coal measures in the basin enabling the remanence of an unoriented sample to be utilised to orient the sample. Palaeomagnetic orientation of drill core for fracture analysis has been used successfully in the Sydney-Bowen Basin. The direction of magnetic remanence in the sediments of the southern Sydney Basin is consistent across a wide area, this direction being north and up. Similarly, the remanence direction observed in Bowen Basin coal measures is consistent from the south (Moura mine) to the north (Goonyella mine) of the basin, and this direction is also north and up. The consistency of the remanence direction is lithology dependent. Fine sandstone and siltstone/mudstone units gave the most reliable results. Coarse sandstones gave inconsistent remanence directions and should not routinely be used for drill core orientation using palaeomagnetism. A longcore magnetometer constructed by the CSIRO Rock Magnetism Group enables measurement of the remanence of HQ drill core without the need to subsample. The measurement of the remanence of HQ drill core using a longcore magnetometer shows similar results to the remanence measured using laboratory magnetometers indicating that drilling does not alter the NRM. Thus, the noninvasive orientation of drill core by measurement of remanence using a longcore magnetometer is feasible for samples from the Permian coal measures of the Sydney-Bowen Basin. As well, storage of the drill core has no or limited effect on the remanence of the drill core. A small component is sometimes present but is easily removed by AF demagnetisation. Weathering has a severe effect on the remanence of samples, inhibiting orientation by palaeomagnetism. Weathering can produce hematite/goethite which retains a strong chemical remanent magnetisation and thus dominates the NRM of the sample.

Structural and fluid evolution of a young accretionary complex: The Hoh rock assemblage of the western Olympic Peninsula, Washington

The Hoh rock assemblage of the western Olympic Peninsula, Washington, was Deposited in Eocene to Miocene time, and accreted, deformed, uplifted, and eroded by the late Miocene. Northeast-dipping fault zones within the Hoh are characterized by intensely deformed mud matrix melange, and they range from ∼200 m to >1,000 m thick; kinematic indicators within the matrix indicate thrusting to the west-southwest. Both hanging-wall and footwall sections are strongly folded and faulted. Wall-rock sections are lithostratigraphically distinct, with hanging walls composed of massive coarse sandstone and conglomerate, and footwall sections composed of thin- to medium- bedded sandstone and siltstone. Folds within the wall-rock sections are oblique to, and are truncated by, the later fault-zone melanges, indicating that the fault zones postdate folding. Evidence of fluid involvement in the Hoh rocks includes petroliferous outcrops, carbonate and clay cement, and veins of carbonate and laumontite. The progressive evolution of veins and cements (paragenetic sequence) can be interpreted in terms of crystallization under conditions of increasing consolidation, decreasing salinity, and changing fluid composition. Suppressed carbonate luminescence may reflect crystallization under reducing conditions, perhaps related to the sulfate reduction zone, the presence of hydrocarbons, or clay dehydration. Although deformational features may be associated with the earliest phase of veining, most of the localized grain breakage and matrix shear zones formed after the early micritic veins and before the last phase of sparitic carbonate (+laumontite) veins. Laumontite occurred syn-deformationally and in equilibrium with carbonate, and also post-deformationally. The paragenetic sequences observed in all samples from the Hoh rock assemblage can be correlated, suggesting that the sources and processes controlling the fluid emplacement were similar across a wide region. In addition, the paragenetic sequence in the Hoh rocks is similar to that documented in autochthonous coeval basins of southwest Washington. This similarity suggests that the precipitates, formed during the evolution of the Hoh rocks, involved fluids that originated within the matrix and did not involve exotic or far-traveled fluids.

A Middle Triassic paleomagnetic pole for North America

Two stratigraphic sequences were sampled through the early Anisian Anton Chico Member of the Moenkopi Formation in north-eastern New Mexico. Two polarities of magnetization are present: a normal-polarity interval succeeded by a reversed-polarity interval and followed by a short normal- and reversed-polarity couplet. Detailed thermal demagnetization (10 to 17 steps) was employed to separate magnetic vectors. The secondary magnetization is largely a direction similar to the present-day and/or axial-field directions. Demagnetization above 520°C reveals a near-horizontal characteristic magnetization. The lithology of the stratigraphic sequence is such that the reversed polarity is contained almost exclusively within coarse sandstone lithologies, and hence, the reversed-polarity characteristic magnetization direction is rarely completely separated from the secondary magnetization. Because of this, the paleopole was calculated from only the lower normal-polarity portion of the section. The pole, calculated from the samples of two localities, is located at 121.4°E, 43.2°N, (α 95 = 5.3). This Middle Triassic paleopole is in good agreement with published Triassic paleomagnetic poles for cratonic North America and statistically overlaps the Early Triassic paleopoles. The large-scale relative motion during the Triassic between the magnetic pole and the North American plate is constrained by this study to have begun after early Middle Triassic time; it suggests that as much as 10° of apparent-polar wander occurred between the late Anisian-Ladinian (late Middle Triassic) and the middle to late Carnian (early Late Triassic), a time interval of between 11 and 14 m.y.

Palaeo‐yardangs: wind‐scoured desert landforms at the Permo‐Triassic unconformity

ABSTRACTIn Canyonlands National Park, south‐east Utah, at least 29 partly exhumed, aligned sandstone ridges trending generally N20°W occur at the upper unconformable surface of the Lower Permian (Leonardian) White Rim Sandstone. The ridges are at least 1·5 km long, 250 m wide and have up to 14 m of vertical relief (mean of 9 m). A thin lag of coarse sandstone that contains wind‐ripple laminae and granule ripples directly overlies the ridges. Angular blocks of sandstone within the lag and sand‐filled fissures immediately below the lag, within the ridges, attest to early cementation of the ridge‐forming material. SE‐dipping aeolian cross‐strata within the White Rim Sandstone and within the lag closely parallel the ridge trend. The ridges are interpreted as wind‐sculpted desert landforms (yardangs) that developed on the lithified upper surface of the White Rim Sandstone during an extended period of hyperaridity towards the end of the Permian.

Amounts and sources of phosphate in hill country rocks, south-eastern North Island, New Zealand

The phosphate (P) present in a range of parent rocks for hill country soils in Wairarapa region, New Zealand, has been studied. In general the highest P concentrations occur in the mudstones which contain 50% more P than the fine to medium sandstones, and at least 10 times more P than the coarse sandstones. Most of the P is in the 20-2 �m and &lt;2 �m size fractions of the rocks. The P is present both in apatite and calcite mineral grains. Although the highest P concentrations occur in the heavy fraction (containing apatite), most of the total P is held in the light fraction in which calcite is abundant and where some apatite also occurs within aggregates. Since both calcite and apatite are relatively soluble in this soil environment, and since both have large reactive surfaces, they may readily release P for plant growth.

New early Triassic Bryozoa (Trepostomata) from Spitsbergen, with some remarks on the stratigraphy of the investigated horizons

AbstractNew trepostomatous bryozoan species, Paralioclema winsnesi sp. nov., Paralioclema mariaholmensis sp. nov. and Paralioclema sp. cf. mariaholmensis are described from the Lower Triassic succession of Spitsbergen. The investigated bryozoan faunas are associated with coarse sandstones, calcareous siltstones and shelly limestones. The bryozoan colonies are mostly well preserved in situ, and show limited evidence of transportation. Some zoarial fragments show bioerosion. The conodont species Neogondolella elongata, Ellisonia triassica, Neospathodus dieneri and Neospathodus svalbardensis extracted from the investigated horizons are Dienerian–Smithian in age.