Trench-slope channels from the New Zealand Jurassic: the Otekura Formation, Sandy Bay, South Otago

The Otekura Formation (Early Jurassic, Pseudaucella zone) at Sandy Bay comprises part of a 10+ km thick, regressive, forearc shelf and slope sequence, the Hokonui facies belt of the Rangitata Geosyncline. The Otekura Formation is dominantly fine grained, being mostly mudstone, silty mudstone and siltstone. The sediments are volcanogenic throughout. The upper 150 m of the formation contains two 20 m thick, channelized bodies of medium-thick bedded sandy flysch, each associated with thin bedded muddy flysch interpreted as overbank turbidites. Directional indicators within the channel sequence indicate emplacement from the south-southwest. In contrast, rare turbidites that occur below the channel sequence, within the background mudstone sediment, were emplaced from the east, i.e. at right angles to the channelized flows. The immediately overlying Omaru Formation contains more abundant macrofossils, intraclastic conglomerates, and appreciable amounts of traction-emplaced cross-bedded sand. Bioturbated calcareous siltstones with an in situ molluscan fauna follow (Boatlanding Formation), and are of shelf origin. The Omaru Formation is therefore interpreted as a shelf-slope break deposit, and the Otekura Formation as an upper slope facies. Reconnaissance studies indicate that the Otekura Formation is underlain by several kilometres of dominantly fine grained, deep water slope sediments, containing occasional sand and conglomerate filled channels similar to those here described in detail from the Otekura Formation. Such channels are inferred to form when a mass-transported sand, derived from failure higher on the slope, ploughs erosively into the sea floor. After their incision, the channels served for a short time as conduits for downslope transport of sediment, the redeposited deposits of which are found filling each channel. Both channel fills at Sandy Bay are capped by thin-bedded turbidites inferred to have overspilled from similar channels nearby on the slope.     

Frasnian–Famennian crisis in the Malaguide Complex (Betic Cordillera,Spain)

Devonian sediments of the Malaguide Complex potentially could include the Frasnian–Famennian boundary, one of the five greatest Phanerozoic biotic crises. Conodont biofacies and microfacies of carbonate clasts from a pebbly mudstone underlying Tournaisian radiolarites allows identification, for the first time in the Malaguide Complex, of Devonian shallow marine environments laterally grading to deeper realms. The clasts yielded Frasnian conodont associations of the falsiovalis to rhenana biozones, with six biofacies that reveal different environmental conditions in their source areas. Source sediments were dismantled and redeposited within the pebbly mudstone, whose origin is tentatively related to one of the events that are associated worldwide with the Frasnian–Famennian crisis. The latter is recorded, in two equivalent Malaguide pelagic successions, by stratigraphic discontinuities, and it was, probably, tectonically and/or eustatically controlled, as in other Alpine‐Mediterranean Paleotethyan margins.     

Facies characteristics of Middle Pleistocene (Saalian) ice-margin subaqueous fan and delta deposits,glacial Lake Leine,NW Germany

The blocking of major river valleys in the Leinebergland area by the Early Saalian Scandinavian ice sheet led to the formation of a large glacial lake, referred to as “glacial Lake Leine”, where most of the sediment was deposited by meltwater. At the initial stage, the level of glacial Lake Leine was approx. 110 m a.s.l. The lake level then rose by as much as 100 m to a highstand of approx. 200 m a.s.l.Two genetically distinct ice-margin depositional systems are described that formed on the northern margin of glacial Lake Leine in front of the retreating Scandinavian ice sheet. The Bornhausen delta is up to 15 m thick and characterized by a large-scale tangential geometry with dip angles from 10°–28°, reflecting high-angle foreset deposition on a steep delta slope. Foreset beds consist of massive clast-supported gravel and pebbly sand, alternating with planar-parallel stratified pebbly sand, deposited from cohesionless debris flows, sandy debris flows and high-density turbidity flows. The finer-grained sandy material moved further downslope where it was deposited from low-density turbidity currents to form massive or ripple-cross-laminated sand in the toeset area.The Freden ice-margin depositional system shows a more complex architecture, characterized by two laterally stacked sediment bodies. The lower part of the section records deposition on a subaqueous ice-contact fan. The upper part of the Freden section is interpreted to represent delta-slope deposits. Beds display low- to high-angle bedding (3°–30°) and consist of planar and trough cross-stratified pebbly sand and climbing-ripple cross-laminated sand. The supply of meltwater-transported sediment to the delta slope was from steady seasonal flows. During higher energy conditions, 2-D and 3-D dunes formed, migrating downslope and passing into ripples. During lower-energy flow conditions thick climbing-ripple cross-laminated sand beds accumulated also on higher parts of the delta slope.     

Triassic carbonate submarine fans along the Arabian platform margin, Sumeini Group, Oman

ABSTRACT The Sumeini Group formed along the passive continental margin slope that bounded the northeastern edge of the Arabian carbonate platform. With the initial development of this passive continental margin in Oman during Early to Middle Triassic time (possibly Permian), small carbonate submarine fans of the C Member of the Maqam Formation developed along a distally steepened slope. The fan deposits occur as several discrete lenticular sequences of genetically related beds of coarsegrained redeposited carbonate (calciclastic) sediment within a thick interval of basinal lime mudstone and shale. Repeated pulses of calciclastic sediment were derived from ooid shoals on an adjacent carbonate platform and contain coarser intraclasts eroded from the surrounding slope deposits. Sediment gravity flows, primarily turbidites with lesser debris flows and grain flows, transported the coarse sediments to the relatively deep submarine fans. Channel erosion was a major source of intraformational calcirudite. Two small submarine fan systems were each recurrently supplied with calciclastic sediment derived from point sources, submarine canyons. The northern fan system retrogrades and dies out upsection. The southern fan system was apparently longer-lived; calciclastic sediments in it are more prevalent and occur throughout the section. The proximal portions of this fan system are dominated by channelized beds of calcirudite which represent inner- to mid-fan channel complexes. The distal portions include mostly lenticular, unchannelized beds of calcarenite, apparently mid- to outer-fan lobes. Carbonate submarine fans appear to be rare in the geological record in comparison with more laterally continuous slope aprons of coarse redeposited sediment. The carbonate submarine fans of the C Member apparently formed by the funnelling of coarse calciclastic sediment into small submarine canyons which may have developed due to rift and/or transform tectonics. The alternation of discrete sequences of calciclastic sediment with thick intervals of ‘background’ sediment resulted from either sea-level fluctuations or pulses of tectonic activity.     

Tide-dominated offshore sedimentation, Lower Cambrian, north Norway

The Duolbasgaissa Formation, Lower Cambrian, of northern Norway consists of 550 m of mineralogically and texturally mature sandstones with subordinate siltstones, mudstones and conglomerates. Four facies are defined on the basis of grain size, bed thickness and sedimentary structures. Facies 1–3 consist of a variety of erosively-based, cross-stratified and parallel-stratified sandstones interbedded with siltstone and mudstone. Many of these sandstones show evidence of deposition from waning currents. Facies 4 consists of trough cross-bedded sandstones with sets up to 4 m thick. Symmetrical ripples and bioturbation are ubiquitous. Bipolar palaeocurrent distributions are common to all facies and one mode is usually strongly dominant. Lateral facies variations and sedimentary structures suggest that deposition took place in a tide-dominated, offshore, shallow marine environment in which maximum sediment transport probably occurred when storm generated waves enhanced tidal currents. The four facies are thought to represent the deposits of various parts of tidal sediment transport paths such as exist in modern seas around Great Britain. Small scale coarsening upward sequences may represent the superposition of facies independently of changing water depth. Lack of information prevents a detailed palaeogeographic reconstruction. It is suggested that sand body shape is not accurately predictable.     

Tectonic and eustatic control on deposition and preservation of Upper Cretaceous ooidal ironstone and associated facies: Peace River Arch area, NW Alberta, Canada

The Late Coniacian, shallow-marine Bad Heart Formation of the Western Canada foreland basin is very unusual in that it contains economically significant ooidal ironstone. Deposition of shallow-water and iron-rich facies appears to have been localized over the crest and flanks of a subtle intrabasinal arch, in part interpreted as a forebulge and partly attributed to reactivation of the long-lived Peace River Arch. The formation comprises two upward-shoaling allomembers, typically 5–10 m thick, that are bounded by regionally mappable ravinement surfaces. The lower unit, allomember 1, grades up from laminated mudstone to bioturbated silty sandstone, which is abruptly overlain by bioturbated ooidal silty sandstone grading into an almost clastic-free ooidal ironstone up to 7 m thick. Ooidal ironstone was concentrated into NW- to SE-trending ridges, kilometres wide and tens of kilometres long. Ironstone formation appears to have been promoted by: (a) drowning of the arch, which progressively curtailed sediment supply; and (b) enhanced reworking over the shallowly submerged arch and over a fault-bounded block that underwent episodic vertical movement of 10–20 m during Bad Heart deposition. Allomember 2 also shoals upwards from mudstone to bioturbated and laminated silty sandstone but lacks ooids, apparently reflecting a rejuvenated supply of detrital sediment from the arch. The marine ravinement surface above allomember 2 is a Skolithos firmground, above which is developed a regional blanket of ooidal sediment. In the east, ooids are dispersed in a bioturbated silty sandstone with abundant evidence of repeated reworking and early siderite and phosphate cements. Westwards, this facies grades, over about 40 km, into almost clastic-free ooidal ironstone about 5 m thick; the lateral facies change may reflect progressive clastic starvation distal to a low-relief source area. The two allomembers are interpreted to reflect eustatic oscillations of about 10 m, superimposed on episodic tectonic warping and block-faulting events. The development of ooidal ironstone immediately above initial marine flooding surfaces indicates a close relationship to marine transgression, reflecting sediment-starved conditions. Ironstone does not appear to be related to either sequence boundaries or maximum flooding surfaces. The Bad Heart Formation is blanketed by marine mudstone deposited in response to major flexural subsidence and rejuvenation of clastic sources in the Cordillera to the SW.     

Evolution of Miocene fluvial environments, eastern Potwar plateau, northern Pakistan

The Miocene-Pliocene Siwalik Group records changing fluvial environments in the Himalayan foreland basin. The Nagri and Dhok Pathan Formations of this Group in the eastern Potwar Plateau, northern Pakistan, comprise relatively thick (tens of metres) sandstone bodies and mudstones that contain thinner sandstone bodies (metres thick) and palaeosols. Thick sandstone bodies extend for kilometres normal to palaeoflow, and are composed of large-scale stratasets (storeys) stacked laterally and vertically adjacent to each other. Sandstone bodies represent single or superimposed braided-channel belts, and large-scale stratasets represent channel bars and fills. Channel belts had widths of km, bankfull discharges on the order of 10³ cumecs and braiding parameter up to about 3. Individual channel segments had bankfull widths, maximum depths, and slopes on the order of 10² m, 10¹ m and 10^?4 respectively, and sinuosities around 1-1. These rivers are comparable to many of those flowing over the megafans of the modern Indo-Gangetic basin, and a similar depositional setting is likely. Thin sandstone bodies within mudstone sequences extend laterally for on the order of 10² m and have lobe, wedge, sheet and channel-form geometries: they represent crevasse splays, levees and floodplain channels. Mudstones are relatively bioturbated/disrupted and represent mainly floodbasin and lacustrine deposition. Mudstones and sandstones are extremely disrupted in places, showing evidence of prolonged pedogenesis. These ‘mature’ palaeosols are m thick and extend laterally for km. Lateral and vertical variations in the nature of their horizons apparently depend mainly on deposition rate. The 500 m-thick Nagri Formation has a greater proportion and thicker sandstone bodies than the overlying 700 m-thick Dhok Pathan Formation. The thick sandstone bodies and their large-scale stratasets thicken and coarsen through the Nagri Formation, then thin and fine at the base of the Dhok Pathan Formation. Compacted deposition rates increase with sandstone proportion (0-53 mm/year for Nagri, 0-24 mm/year for Dhok Pathan), and palaeosols are not as well developed where deposition rates are high. Within both formations there are 100 m-scale variations (representing on the order of 10⁵ years) in the proportion and thickness of thick sandstone bodies, and tens-of-m-scale alternations of thick sandstone bodies and mudstone-sandstone strata that represent on the order of 10⁴ years. Formation-scale stratal variations extend across the Potwar Plateau for at least 100 km, although they may be diachronous: however, 100-m and smaller scale variations can only be traced laterally for up to tens of km. Alluvial architecture models indicate that increases in the proportion and thickness of thick sandstone bodies can be explained by increasing channel-belt sizes (mainly), average deposition rate and avulsion frequency on a megafan comparable in size to modern examples. 100-m-scale variations in thick sandstone-body proportion and thickness could result from ‘regional’ shifts in the position of major channels, possibly associated with ‘fan lobes’on a single megafan or with separate megafans. However, such variations could also be related to local changes in subsidence rate or changes in sediment supply to the megafan system. Formation-scale and 100-m-scale stratal variations are probably associated with interelated changes in tectonic uplift, sediment supply and basin subsidence. Increased rates of hinterland uplift, sediment supply and basin subsidence, recorded by the Nagri Formation, may have resulted in diversion of a relatively large river to the area. Alternatively, changing river sizes and sediment supply rates may be related to climate changes affecting the hinterland (possibly linked to tectonic uplift). Climate during deposition of the Siwalik Group was monsoonal. Although the deposits contain no direct evidence for climate change, independent evidence indicates global cooling throughout the Miocene, and the possibility of glacial periods (e.g. around 10-8 Ma, corresponding to base of Nagri Formation). If the higher Himalayas were periodically glaciated, a mechanism would exist for varying sediment supply to megafans on time scales of 10⁴-10⁵ years. Although eustatic sea-level changes are related to global climatic change, they are not directly related to Siwalik stratigraphic changes, because the shoreline was many 100 km away during the Miocene.     

THE CENOZOIC GEORECORDS IN THE NORTHWEST OF YUNNAN AND THE EVOLUTION OF QING—ZANG PLATEAU

THE CENOZOIC GEORECORDS IN THE NORTHWEST OF YUNNAN AND THE EVOLUTION OF QING—ZANG PLATEAU     

The Devonian–Lower Carboniferous succession in Northwest Peninsular Malaysia

A new stratigraphic nomenclature is proposed for the approximately 600 m thick, mainly clastic transitional sequence between the underlying Mempelam Limestone and overlying Kubang Pasu/Singa Formation in northwest Peninsular Malaysia. This sequence represents shallow marine deposits of the continental margin of the Sibumasu Terrane during the Middle Palaeozoic (Devonian–Carboniferous). It is separated into several formations. The Timah Tasoh Formation is an approximately 76 m sequence consisting of 40 m of laminated tentaculitid shales at the base, containing Monograptus yukonensis Jackson and Lenz and Nowakia (Turkestanella) acuaria Alberti, giving an Early Devonian (Pragian–Emsian) age, and about 36 m of rhythmically interbedded, light coloured argillo-arenites. The Chepor Formation is about 90 m thick and consists mainly of thick red mudstone interbedded with sandstone beds, of Middle to Late Devonian age. A new limestone unit is recognized and named the Sanai Limestone, which contains conodonts of Famennian age. The Binjal Formation consists of red and white mudstone interbedded with sandstone beds showing Bouma sequences. The Telaga Jatoh Formation is 9 m thick and consists mainly of radiolarian chert. The Wang Kelian Formation is composed of thick red mudstone beds interbedded with silty sandstone, and contain fossils indicative of an Early Carboniferous (Visean) age. The succession was deposited on the outer shelf, with depositional environments vertically fluctuating from prodelta to basinal marine. The Devonian–Carboniferous boundary is exposed at Hutan Aji and Kampung Guar Jentik, and indicates a major regressive event during the latest Devonian.     

Sedimentology of an ancient erg margin: the Lower Jurassic Aztec Sandstone, southern Nevada and southern California

The Lower Jurassic Aztec Sandstone is an aeolian-deposited quartzose sandstone that represents the western margin of the southerly-migrating Navajo-Nugget sand sea (or erg). Vertical and lateral facies relations suggest that the erg margin encroached upon volcanic highlands, alluvial fan, wadi and sabkha environments. In southern Nevada, 700 m thick facies successions record the arrival of the Aztec sand sea. Initial erg sedimentation in the Valley of Fire consists of lenticular or tongue-shaped aeolian sand bodies interstratified with fluvially-deposited coarse sandstone and mudstone. Above, evaporite-rich fine sandstone and mudstone are overlain by thick, cross-stratified aeolian sandstone that shows an upsection increase in set thickness. The lithofacies succession represents aeolian sand sheets and small dunes that migrated over a siliciclastic sabkha traversed by ephemeral wadis. These deposits were ultimately buried by large dunes and draas of the erg. In the Spring Mountains, a similar facies succession also contains thin, lenticular volcaniclastic conglomerate and sandstone. These sediments represent the distal margin of an alluvial fan complex sourced from the west. Thin aeolian sequences are interbedded with volcanic flow rocks, ash-flow tuffs, debris flows, and fluvial deposits in the Mojave Desert of southern California. These aeolian strata represent erg migration up the eastern flanks of a magmatic arc. The westward diminution of aeolian-deposited units may reflect incomplete erg migration, thin accumulation of aeolian sediment succeptible to erosion, and stratigraphic dilution by arc-derived sediment. A two-part division of the Aztec erg is suggested by lithofacies associations, the size and geometry of aeolian cross-strata, and sediment dispersal data. The leading or downwind margin of the erg, here termed the fore-erg, is represented by a 10–100 m thick succession of isolated pods, lenses, and tongues of aeolian-deposited sediment encased in fluvial and sabkha deposits. Continued sand-sea migration brought large dunes and draas of the erg interior into the study area; these 150–500 m thick central-erg sediments buried the fore-erg deposits. The trailing, upwind margin of the erg is represented by back-erg deposits in northern Utah and Wyoming.     

ALLUVIAL STYLES AND ARCHITECTURE AS GUIDES TO CENOZOIC TECTONIC AND ENVIRONMENTAL EVENTS AT THE NORTHERN MARGINS OF THE QINGHAI—XIZANG PLATEAU

ALLUVIAL STYLES AND ARCHITECTURE AS GUIDES TO CENOZOIC TECTONIC AND ENVIRONMENTAL EVENTS AT THE NORTHERN MARGINS OF THE QINGHAI—XIZANG PLATEAU     

黑龙江七星河含煤盆地古近纪-新近纪沉积相及层序地层特征

黑龙江省东部七星河盆地是一新生代聚煤盆地,其含煤地层为古近系宝泉岭组、新近系富锦组。宝泉岭组由各级砂岩、泥岩、炭质泥岩以及褐煤组成,发育滨浅湖相、深-半深湖相、三角洲平原相,属于湖泊沉积体系、三角洲沉积体系。富锦组主要由泥岩、粉砂岩、中砂岩、含砾粗砂岩及煤层、炭质泥岩、硅藻岩组成,发育滨浅湖相、扇三角洲平原相,分别属于湖泊沉积体系和扇三角洲沉积体系。层序SI相当于宝泉岭组,发育低位体系域、湖侵体系域和高位体系域,煤层主要发育高位体系域中后期,成煤环境以滨浅湖淤积沼泽为主。层序SII相当于富锦组,主要发育湖侵体系域、高位体系域,局部地区发育低位体系域,煤层亦主要发育高位体系域中后期,成煤环境以扇三角洲淤积沼泽和滨浅湖淤积沼泽为主。层序SI、SII的高位体系域中后期,盆地基底沉降速率和物源供给处于相对平衡状态,主要发育了扇三角洲淤积沼泽、滨浅湖和滨浅湖淤积沼泽环境,发育可采煤层。     

Tectonic control on third‐order sequences in a siliciclastic ramp‐style basin: An example from the Roper Superbasin (Mesoproterozoic), northern Australia

TThe Roper Group is a cyclic, predominantly marine, siliciclastic succession of Calymmian (Early Mesoproterozoic) age. It has a distribution of at least 145 000 km² and a maximum known thickness of ～5000 m. In the Roper River district the middle part of the Roper Group (～1300 m thick) is characterised by the cyclical alternation of mudstone and sandstone units, and can be divided into six third‐order depositional sequences. A typical sequence is broadly progradational in aspect, and comprises a lower, mudstone‐rich, storm‐dominated shelf succession (up to 330 m thick), and a sequence‐capping unit dominated by tidal‐platform cross‐bedded sandstone (up to 80 m thick); both are interpreted as highstand systems tracts. Transgressive strata are poorly represented but where present are characterised by paralic to fluvial redbed assemblages that include ooidal ironstone. Roper Group sequences lack a distinct condensed section and sequence boundaries are mostly conformable. Erosional contacts separate mud‐rich shelf facies from sequence‐capping sandstones. We infer that these erosion surfaces were generated by episodic flexural tectonism, which also generated the accommodation and sediment supply for Roper sequences.     

Eocene tidal sedimentation in the Anambra basin,Southern Nigeria

The Eocene Nanka Formation of the Anambra basin in southern Nigeria consists, in its type area, of four sand subunits each 50–90 m thick, separated by three gypsiferous glauconitic shales each about 2.5 m thick. The sand subunits are unconsolidated, uncemented quartzarenites, planar and trough cross-stratified, flaser and lenticular bedded, and burrowed (Ophiomorpha and Skolithos). Texturally the sands are medium to coarse, moderately sorted, positively skewed and leptokurtic. Two parallel belts, each with a distinctive paleocurrent pattern, are identified: one lying along the present eastern and northern margin of the formation, characterised by a shore-normal paleocurrent pattern; and the other lying on the west, distinguished by a shore-parallel orientation.An integrated analysis of stratigraphic, petrographic, textural, and structural data of the formation enabled reconstruction of an environmental model for the sand body. The model depicts sedimentation in a tidally influenced marine shoreline environment in which an intertidal and a subtidal zone can be delineated. Each zone is characterised by an assemblage of several sedimentary features. Interlaminations of shale and thin sand lenses, gypsiferous and glauconitic shale beds, flaser and lenticular bedding, herringbone structures, and shore-normal paleocurrent pattern are among the features that delineate the intertidal facies. The abundantly cross-stratified, coarse elastic sand-bars with rapid fining-upward sequences, burrowed pebbly horizons overlain by fine sand, and shoreparallel paleocurrent are the distinguishing features of the subtidal facies. The sandflat facies of the intertidal zone and the sandbar facies of the subtidal zone are, however, inseparable in most cases.     

Multilayer folds initiated in bulk plane strain,with the axis of no change perpendicular to the layering

Buckle folds in internal multilayer systems will initiate and grow in a bulk plane strain condition, in which the principal axis of no change, Y (X >Y >Z), is perpendicular to the layering and to the fold axial direction, providing that the multilayer is confined both above and below. The bulk extension direction, X, is then parallel to the fold axial direction.     

Provenance of the Bosnian Flysch

Sandwiched between the Adriatic Carbonate Platform and the Dinaride Ophiolite Zone, the Bosnian Flysch forms a c. 3000 m thick, intensely folded stack of Upper Jurassic to Cretaceous mixed carbonate and siliciclastic sediments in the Dinarides. New petrographic, heavy mineral, zircon U/Pb and fission-track data as well as biostratigraphic evidence allow us to reconstruct the palaeogeology of the source areas of the Bosnian Flysch basin in late Mesozoic times. Middle Jurassic intraoceanic subduction of the Neotethys was shortly followed by exhumation of the overriding oceanic plate. Trench sedimentation was controlled by a dual sediment supply from the sub-ophiolitic high-grade metamorphic soles and from the distal continental margin of the Adriatic plate. Following obduction onto Adria, from the Jurassic–Cretaceous transition onwards a vast clastic wedge (Vranduk Formation) was developed in front of the leading edge, fed by continental basement units of Adria that experienced Early Cretaceous synsedimentary cooling, by the overlying ophiolitic thrust sheets and by redeposited elements of coeval Urgonian facies reefs grown on the thrust wedge complex. Following mid-Cretaceous deformation and thermal overprint of the Vranduk Formation, the depozone migrated further towards SW and received increasing amounts of redeposited carbonate detritus released from the Adriatic Carbonate Platform margin (Ugar Formation). Subordinate siliciclastic source components indicate changing source rocks on the upper plate, with ophiolites becoming subordinate. The zone of the continental basement previously affected by the Late Jurassic–Early Cretaceous thermal imprint has been removed; instead, the basement mostly supplied detritus with a wide range of pre-Jurassic cooling ages. However, a c. 80 Ma, largely synsedimentary cooling event is also recorded by the Ugar Formation, that contrasts the predominantly Early Cretaceous cooling of the Adriatic basement and suggests, at least locally, a fast exhumation.     

Reassignment of the absorption spectra of purple yoderite

Polarized spectra E∥X, E∥Y, and E∥Z of purple yoderite, taken at 295 and 100 K, result in a revised interpretation for the mineral. Major bands at around 16,900 (X>Y?Z), 18,600 (X?Y), and 20,600 cm^?1 (X≈Z>Y) may be attributed to spin-allowed transitions of Mn³⁺ in trigonal bipyramids (A2 or A3). Minor features may be assigned to single ion Fe³⁺. However, charge transfer possibilities for bands at 18,600 and 25,500 cm^?1 in yoderite cannot be ruled out and are discussed. The extremely high intensity of Mn³⁺ spin-allowed bands (?, 216 to 1,900 [1·g-atom^?1·cm^?1]) is attributed to fivefold coordination of the A2 and A3 position and to the covalency of the Mn³⁺-O bonds.     

Sedimentary processes and facies of the waterlaid Anvil Spring Canyon alluvial fan, Death Valley, California

The Anvil Spring Canyon fan of the Panamint Range piedmont in central Death Valley was built entirely by water-flow processes, as revealed by an analysis of widespread 2- to 12-m-high stratigraphic cuts spanning the 9·7 km radial length of this 2·5–5·0° sloping fan. Two facies deposited from fan sheetfloods dominate the fan from apex to toe. The main one (60–95% of cuts) consists of sandy, granular, fine to medium pebble gravel that regularly and sharply alternates with cobbly coarse to very coarse pebble gravel in planar couplets 5–25 cm thick oriented parallel to the fan surface. The other facies (0–25% of cuts) comprises 10- to 60-cm-thick, wedge-planar and wedge-trough beds of pebbly sand and sandy pebble gravel in backsets sloping 3–28°. Both facies are interpreted as resulting from rare, sediment-charged flash floods from the catchment, and were deposited by supercritical standing waves of expanding sheetfloods on the fan. Standing waves were repeatedly initiated, enlarged, migrated, and then terminated either by gradually rejoining the flood or by more violent breakage and washout. The frequent autocyclic growth and destruction of standing waves during an individual sheetflood resulted in the deposition of multiple coarse and fine couplet and backset sequences 50–250 cm thick across the active depositional lobe of the fan. Erosional intensity during washout of the standing wave determined whether early-phase backset-bed deposits or washout-phase sheetflood couplet deposits were selectively preserved in a given cycle. Two minor facies are also found in the Anvil fan. Pebble–cobble gravel lags (0–20% of cuts) are present above erosional scours into the sheetflood couplet and backset deposits. They consist of coarse gravel concentrated through fine-fraction winnowing of the host sheetflood facies by sediment-deficient water flows. This reworking occurred during recessional flood stage or from non-catastrophic discharge during the long intervals between major flash floods. This facies is common at the surface, giving rise to a ‘braided-stream’ appearance. However, it is stratigraphically limited, present as thin, continuous to discontinuous beds or lenses that bound 50- to 250-cm-thick sheetflood sequences. The other minor facies of the Anvil fan consists of clast-supported and imbricated, thickly stratified, pebbly, cobbly, boulder gravel present in narrow, radially aligned ribbons nested within sheetflood deposits. This facies is interpreted as representing deposition in the incised channel of the fan, a subenvironment characterized by greater flow competence resulting from maintained depth from channel-wall confinement, and by more frequent water flows and winnowing events caused by its direct connection with the catchment feeder channel.     

Half-graben lacustrine sedimentary rocks of the lower Carboniferous Strathlorne Formation, Horton Group, Cape Breton Island, Nova Scotia, Canada

The Strathlorne Formation is the middle formation of a three-part Horton Group stratigraphy present throughout the post-Acadian Orogeny Maritimes Basin in Atlantic Canada. It is up to 600 m in thickness and is of Tournaisian age. The formation was deposited in a complex lacustrine system during the period of maximum fault-bounded extensional subsidence within two asymmetric half-graben sub-basin segments of a large rift. This rift was located at a palaeolatitude of 10–15°S. Four facies assemblages are identified and interpreted: (1) dark grey mudstone (open lacustrine), (2) grey, very fine to fine-grained sandstone (nearshore/shoreline), (3) grey, medium-grained sandstone to conglomerate (fan delta) and (4) red siltstone to fine-grained sandstone (interdeltaic mudflat). Interpreted structural asymmetry of the fault-bounded sub-basins is evidenced by asymmetry of sediment input, facies distribution and palaeoflow in the lacustrine sedimentary fill. These indicators suggest that the sub-basins, which were linked end-to-end, had opposed polarity of structural asymmetry during deposition of the Strathlorne Formation. Open lacustrine sediments are typified by stacked shallowing-upward sequences, each representing deepening due to sub-basin-wide subsidence events followed by gradual infilling to shallow water depths. Sub-basin asymmetry is also reflected in the contrast of thick sequences and grouped thinner sequences at marginal and axial positions, respectively. The lakes which occupied the sub-basins were large (up to 100 × 50 km), tens to hundreds of metres deep and periodically stratified (presence of an anoxic hypolimnion, at least near sub-basin axes).     

The Cambro-Ordovician Cap Enragé Formation, Queébec, Canada: conglomeratic deposits of a braided submarine channel with terraces

The Cambro-Ordovician Cap Enragé Formation is interpreted as a deep submarine channel complex of conglomerates, pebbly sandstones and massive sandstones. The formation is up to 270 m thick, and crops out in a coastal belt 50 km long. In general terms, it has previously been interpreted as a deep sea channel deposit, with the channel about 300 m deep, at least 10 km wide and trending south-westward, parallel to the coastal outcrops. Eight facies have been defined in this study and they have been grouped into three major facies associations. In the Coarse Channelled Association, conglomerates with carbonate boulders up to about 4 m are associated with graded-stratified finer grained conglomerates. Facies of this association make up about 25% of all the beds in the formation. The association is also characterized by abundant major channels 1–10 m deep and up to 250 m wide. Excellent outcrop allows the reconstruction of topographic highs (bars) within the channels and the association is interpreted as a braided channel and bar system. The second association, Multiple-Scoured Coarse Sandstones, contains some graded-stratified fine conglomerates, along with massive to structureless coarse and pebbly sandstones, and rare cross-bedded pebbly sandstones. Deep channels are absent, but multiple channelling on the scale of 0.5–1 m is characteristic. In the absence of the very coarse conglomerates and deeper channelling, this association is interpreted as being deposited on topographically higher terrace areas adjacent to the main braid plain. The third facies association, Unchannelled Sandstones, is characterized by massive sandstones with abundant fluid-escape structures, classical turbidites and thin shales. In the absence of any scouring deeper than a few tens of centimetres, this association is interpreted as being deposited on an even higher and smoother terrace, farther from the braid plain. Palaeoflow directions for conglomerate facies indicate fairly consistent south-westward transport, apparently parallel to the base of the Cambro-Ordovician continental slope. Flow directions in the finer-grained facies are rather variable, suggesting complex bar development and overbank spills. Thinning-and fining-upward sequences are present on two scales. The smaller, 1–10 m sequence, is related to channel filling and abandonment. Thicker sequences (10–100 m), with facies of the Multiple Scoured, and Unchannelled Sandstone Associations, may indicate switching of a main channel away from the area and its subsequent burial by marginal terrace and higher terrace deposits.