bluuebunny/crossref_metadata_2025_split huggingface dataset:
Shallowing‐upward sequences in Purbeckian peritidal carbonates (lowermost Cretaceous, Swiss and French Jura Mountains)
Authors: STRASSER ANDRÉ 
Published: June 2006

ABSTRACTPurbeckian carbonates in the Swiss and French Jura (Goldberg Formation, lower Berriasian) comprise shallow‐subtidal, intertidal, supratidal, low‐energy, high‐energy, marine, brackish, freshwater, and hypersaline facies. These facies are arranged in small (0–2–1.5 m thick) sequences which display a dominant shallowing‐upward component, and which form the fundamental units of the highly structured Purbeckian sedimentary record.Six types of small‐scale sequences can be recognized. A: intertidal to supratidal overprinting of shallow lagoonal facies; B: algal‐marsh sequence with frequent dolomitization; C: sabkha sequence, often associated with collapse breccia; D: tidal‐flat sequence with desiccation features; E: lacustrine sequence; F: terrestrial overprinting of subtidal or intertidal facies. Episodic event deposits such as tempestites are superimposed] Thin transgressive beds which rework elements of the underlying facies are frequently found at the base of the sequences. Green marls and black pebbles are common at the top and indicate long subaerial exposure. The sequences are often incomplete, as subtidal facies may be absent, or their upper part can be eroded. Lateral facies changes are common, which is due to the very shallow and partly emergent Purbeckian platform where various depositional environments were juxtaposed. However, many sequence boundaries are well developed and can be correlated over large parts of the study area.The Purbeckian shallowing‐upward sequences were generated by climatically controlled sea‐level changes. Autocyclic processes occurred locally, but were overprinted by drops of sea‐level affecting the entire platform. The small‐scale sequences are most probably related to the 20 000‐year cycle of the precession of the equinoxes. Larger sequences with usually well‐developed emersion surfaces are attributed to the 100 000 and 400 000‐year eccentricity cycles of the Earth's orbit. Identification and correlation of sequence boundaries makes it possible to set up a framework of isochronous surfaces (which often cut across facies boundaries), and thus to interpret in detail the palaeogeographic, sedimentological and diagenetic evolution of the Purbeckian peritidal carbonate environments.
Sediment transport and bed morphology at river channel confluences
Authors: BEST JAMES L. 
Published: June 2006

ABSTRACTRiver channel confluences form important morphological elements of every river system, being points at which rapid changes in flow, sediment discharge and hydraulic geometry must be accommodated. This article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence.Bed morphology at channel confluences is characterized by three distinct elements: avalanche faces at the mouth of each confluent channel, a deep central scour and a bar within the separation zone formed at the downstream junction corner. These elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels. As confluence angle and discharge ratio increase, the sediment contributions from the confluent channels are progressively segregated in their paths through the junction, with sediment being transported around rather than through the centre of the confluence. This segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence, an increase in the scour depth, a change in the orientation of the scour and an increase in the size of the separation zone bar. Field measurements closely replicate the flume simulation.A model of sediment transport and bed morphology links these features to the fluid dynamics of these sites. An understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record.
Contrasting diagenesis of two Carboniferous Oolites from South Wales: a tale of climatic influence
Authors: HIRD KEVIN , TUCKER MAURICE E. 
Published: June 2006

ABSTRACTTwo oolites in the Dinantian (Mississippian/Lower Carboniferous) of Glamorgan, SW Britain, were deposited in similar depositional environments but have contrasting diagenetic histories. The Brofiscin and Gully Oolites occur in the upper parts of shallowing‐upward sequences, formed through strandplain progradation and sand shoal and barrier growth upon a southward‐dipping carbonate ramp. The Brofiscin Oolite is characterized by a first‐generation cement of equant calcite spar, preferentially located at grain‐contacts and forming non‐isopachous fringes around grains, interpreted as meteoric vadose and phreatic in origin. Isopachous fibrous calcite fringes of marine origin are rather rare and occur only at a few horizons. Burial compaction was not important and porosity was occluded by poikilotopic calcite spar. Fitted grain‐grain contacts locally occur and could be the result of near‐surface vadose dissolution‐compaction. Syntaxial overgrowths on echinoderm debris are common. Pre‐compaction overgrowths are cloudy (inclusion‐rich) and probably of meteoric origin, and post‐compaction overgrowths are inclusion‐free. By contrast, the Gully Oolite has little first‐generation cement. However, marine fibrous calcite is common in oolitic intraclasts, as isopachous fringes of acicular calcite crystals closely associated with peloidal internal sediment; and early equant, drusy calcite spar occurs in the uppermost part of the Gully, beneath a prominent palaeokarst where pedogenic cements also occur. The major feature of Gully diagenesis is burial compaction, resulting in extensive grain‐grain dissolution and microstylolitic grain contacts, and post‐compaction poikilotopic spar occluded remaining porosity.The Brofiscin Oolite is pervasively dolomitized up‐dip but the Gully Oolite for the most part only contains scattered pre‐compaction dolomite rhombs and late veins of baroque dolomite, with less pervasive dolomitization.The difference in diagenetic style of the two Dinantian oolites is attributed to prevailing climate. The paucity of early meteoric cements in the Gully is a result of an arid climate, and this is supported by the nature of the capping palaeokarst. The abundant meteoric cements in the Brofiscin reflect a more humid climate, and effective meteoric recharge also resulted in up‐dip pervasive mixing‐zone dolomitization. The style of early diagenesis in these two oolites exerted a major control on the later burial diagenesis: in the Brofiscin, the early cements inhibited grain‐grain dissolution and pressure solution, while these processes operated extensively in the Gully Oolite. Thus, prevailing climate can influence a limestone's diagenetic history from near‐surface through into deep burial.
Architecture of the Castissent fluvial sheet sandstones, Eocene, South Pyrenees, Spain
Authors: MARZO MARIANO , NIJMAN WOUTER , PUIGDEFABREGAS CAI 
Published: June 2006

ABSTRACTThe Castissent Formation represents a phase of strong fluvial progradation within the fluvio‐deltaic Montanyana Group, probably enhanced by the late Ypresian sea‐level fall. The structural setting is characterized by the emplacement of the South Pyrenean nappes. The major architectural feature of the Formation is the superposition of three multilateral and multistorey sheet sandstone complexes (A, B and C), interbedded in finer floodplain deposits and dark‐grey mudstones of brackish transitional origin. The structure of the sheet sandstone complexes is exemplified by the detailed analysis of the lower sheet sandstone complex (A), 25 m thick, 6 km wide and exposed over 25 km of its length. Three types of channel‐fill sandstones serve as components: (1) lenticular‐bedded bodies, interpreted as braided stream deposits, (2) lateral accretion‐bedded bodies, intepreted as meandering stream deposits, and (3) ribbons, originating from laterally stable and short‐lived offshoot or crevasse channels. By coalescence and/or stacking of these channel‐fill types, higher‐order architectural units (A1, A2, A3) have been formed that together build the sheet sandstone complex. On the basis of downstream variation in proportion and geometrical relation between the channel‐fill components, three depositional zones have been differentiated within the fluvial system. Each zone is characterized by specific architectural processes: (1) alternation of minor aggradational‐degradational cycles in the proximal valley, (2) generation of new alluvial belts in the intermediate nodal avulsion zone, and (3) successive random avulsion processes with development of semipermanent crevasse channels or minor distributaries in the distal distributive zone. The sedimentation of the Castissent Formation was structurally controlled by an interplay of vertical basement movement due to thrust stacking in the hinterland and surficial thrust displacement to the foreland resulting in alternating southward and northward shift of the fluvial system.
Suspended‐load fallout rate as an independent variable in the analysis of current structures
Authors: LOWE DONALD R. 
Published: June 2006

ABSTRACTThe dynamic interpretation of most current‐structure sequences derives directly from experiments on the succession of bedforms produced by flows in flumes. The results of these and related studies have been used to construct stability field diagrams in which the fields of individual bedforms are usually expressed as a function of flow intensity (power, velocity, bed shear stress, etc.) and grain size.The data underlying existing stability‐field diagrams were collected largely from the study of flows carrying coarse‐grained sediment entrained through particle‐by‐particle bed erosion. Many flows, however, do not entrain sediment through simple bed erosion. Most turbidity currents originate by the development of turbulence in slumps, slides, and other slope failures. Such flows generally form with highly concentrated suspended loads and their bed‐load layers derive sediment from the collapsing suspended‐sediment clouds. Because the collapse properties of such clouds may be related as much to suspended particle concentration, size distribution, particle interactions, and other factors as to flow intensity, the stability fields of bedforms developed beneath such flows may differ in flow intensity‐grain‐size relationships from those beneath flows deriving sediment from bed erosion alone.Useful stability‐field diagrams for turbidity currents must include suspended‐load fallout rate as a third variable, independent of flow intensity and mean grain size. A preliminary stability‐field diagram of this type indicates that Bouma Tabc sequences may theoretically form with essentially no velocity variation of the attendant flow. This type of analysis may have considerable relevance to the interpretation not only of turbidites but also of other deposits formed where bed‐load layers are fed from above rather than below. These include shallow‐shelf storm units deposited from highly concentrated flows and volcaniclastic layers formed where pyroclastic debris falls directly into moving water.
Paleosols of Late Pennsylvanian cyclic strata, New Mexico*
Authors: GOLDSTEIN ROBERTH. 
Published: June 2006

ABSTRACTCyclically deposited siliciclastic and carbonate rocks of the Late Pennsylvanian (Virgilian, Holder Formation) of south‐central New Mexico were deposited on a narrow shelf between the Pedernal uplift and the Orogrande basin. In the Dry Canyon area, Sacramento Mountains, shelf‐to‐basin palaeotopography is indicated by onlapping beds and primary dips. Calcareous alteration zones are developed on the tops of major carbonate units. These detailed studies confirm earlier interpretations that alteration zones represent subaerial exposure events and provide new evidence for and further documentation of a paleosol environment. The paleosols contain: (1) rhizoliths; (2) tangential needle fibres of calcite; (3) five types of alveolar texture; (4) upward oriented ‘ribbon spar’; (5) brownish and clear isopachous spar; (6) blackened grains; (7) random needle fibres of calcite; (8) coated grains; (9) micritized grains; (10) glaebules; (11) desiccation cracks; (12) possible Microcodium; and (13) well‐developed laminated crusts. Isopachous cements, upward‐oriented cements (which mark the former tops of capillary fringes), dark colour and shallow depth of penetration of rhizoliths (less than 1 m) indicate significant paleosol formation in waterlogged (hydromorphic) and drier conditions. Pennsylvanian palaeohydrogeology of the system is reflected in the type of paleosol alteration.Paleosols developed on subtidal strata and paleosols which drape tens of metres of palaeotopography indicate cyclic sedimentation was not controlled by delta lobe switching. Local tectonic up and down movement would be an unlikely mechanism to explain the repeated tens of metres of relative sea‐level fluctuation. Eustatic sea‐level fluctuations, induced by Late Pennsylvanian Gondwana glaciation, probably were of the correct magnitude and frequency to have caused the Holder cycles.
The dynamics of star dunes: an example from the Gran Desierto, Mexico
Authors: LANCASTER N. 
Published: June 2006

ABSTRACTObservations of patterns of erosion and deposition and surface wind velocity and direction on a 40 m high star dune in the Gran Desierto sand sea indicate that interactions between dune form and airflow as winds change direction seasonally play a major role in the formation of this dune type. Such interactions lead to deposition of sand in the central parts of the dune, giving rise to its pyramidal shape, as well as to some extension of the linear arms.The major arms of the dune studied are oriented NE‐SW, or transverse to summer SSE and winter NNW winds. An avalanche face up to 10 m high develops during the course of each season. Flow separation at the main crestline gives rise to a wide zone of lee side secondary flow which moves sand along the base of the avalanche face towards the central part of the dune, where it is deposited as wind ripples migrate into zones of locally reduced flow velocity. Reattachment of the separated flow occurs on the lower part of the N or S arms, parallel to the flow. Spring westerly winds move sand obliquely up the S and N arms of the dune and outwards on the E arm. Large scale flow separation and diversion are replaced by the development of strong helical eddies in the immediate lee of the main crestline which move sand along avalanche faces and into zones of lower flow velocity at the end of dune arms.Formation of star dunes in the Gran Desierto follows a sequence in which crescentic dunes migrating into areas of opposed winds first develop a reversing crestal ridge. Convergent leeside secondary flows are developed, which result in the formation of linear elements parallel to each major wind direction and the concentration of sand in the central part of the dune. Examples of star dunes at different stages of their development can be documented.
Glacial advance and retreat sequences in a Permo‐Carboniferous section, central Transantarctic Mountains
Authors: MILLER JULIA M. G. 
Published: June 2006

ABSTRACTEpisodes of glacial advance and retreat can be recognized through analysis of vertical facies sequences in the Permo‐Carboniferous Pagoda Formation of the Beardmore Glacier area, Antarctica. The formation includes a remarkably complete record of continental sedimentation near the terminus of a temperate glacier. Facies sequence is pre‐eminent for inferring glacial advance and retreat. Other important criteria are abundance and geometry of sandstone interbedded with diamictite, diamictite character and nature of bed contacts. Using these characteristics advance and retreat sequences 5–60 m thick are recognized. A sharp contact, with a striated surface and erosional relief, overlain by structureless diamictite (lodgement till) is typical of grounded ice advance. Grounded ice retreat is characterized by structureless diamictite (lodgement till), overlain by crudely stratified diamictite (melt‐out till) and then by diamictite interbedded with sandstone and conglomerate (flow till and glacio‐fluvial or glacio‐lacustrine deposits). Gradational contacts between shale overlain by diamictite and diamictite overlain by shale characterize advance and retreat, respectively, in subaqueous settings.Pauses in sediment accumulation, minor(?) fluctuations of the ice margin, and/or changes in subglacial dynamics are indicated by specific features within diamictite units such as probable frost‐wedge casts, single layer boulder beds, sharp sedimentary contacts and changes in diamictite character. These minor(?) events are superimposed upon the main advance‐retreat cycles. Study of both the overall facies sequence and of individual diagnostic structures, albeit in an incomplete stratigraphic record, permits a distinction between major and minor advance‐retreat events. As many as six major advance‐retreat cycles exist in some Pagoda sections, but the number of cycles present varies in different sections.
Glaciomarine facies within subglacial tunnel valleys: the sedimentary record of glacioisostatic downwarping in the Irish Sea Basin
Authors: EYLES N. , MCCABE A. M. 
Published: June 2006

ABSTRACTCoastal exposures of Late Pleistocene sediments deposited after 19 000 yr BP near Dublin, Ireland, provide a window into the infill of a subglacially‐cut tunnel valley. Exposures close to the steeply dipping bedrock wall of the valley show boulder gravels within multi‐storey U‐shaped channels cut and filled by subglacial meltwaters driven by a high hydrostatic head. Gravels are truncated by poorly sorted ice‐proximal glaciomarine sediments that record the pumping of large volumes of subglacial debris along the tunnel valley to a tidewater ice sheet margin. The sedimentary succession is dominated by sediment gravity flow facies comprising interbedded diamict and massive, poorly sorted gravel facies interpreted as subaqueous debris flow deposits. Gravel beds show local inverse and normal coarse‐tail graded facies recording the restricted development of turbulent flow. Sediment gravity flow deposits fill broad (<2 km) shallow (10 m) and overlapping channels. Penetrative deformation structures (e.g. dykes) are common at the base of channels.The same subglacially‐eroded topography and glaciomarine infill stratigraphy can be identified on high resolution seismic profiles across nearly 600 km2 of the western Irish Sea. Tunnel valleys are argued to have been exposed to glaciomarine processes by the rapid retreat of a calving tidewater ice sheet margin in response to marine flooding caused by glacio‐isostatic downwarping below the last British Ice Sheet. The facies associations described in this paper comprise an event stratigraphy that may be found on other glaciated continental shelves.
Erg deposits in the Lower Jurassic Wingate Sandstone, northeastern Arizona: oblique dune sedimentation
Authors: CLEMMENSEN LARS B. , BLAKEY RONALD C. 
Published: June 2006

ABSTRACTThe Lower Jurassic erg (aeolian sand sea) deposits of the Wingate Sandstone on the Colorado Plateau are beautifully exposed near Many Farms, Arizona. These 3‐D outcrops allow a detailed study of structures and sequenses in the erg body.The erg sequence comprises chiefly oblique dune deposits. The dune facies are in most cases characterized by a well‐developed tripartite upbuilding. Each dune coset contains unusually thick and intricate bottomsets, medial low‐angle dipping toesets, and upper steeply dipping foresets.The foresets reveal significant across‐crest transport of sand and dip within a narrow range of directions towards the ESE. The bottomset beds are composed of compound cross‐bedding that documents strong along‐crest transport towards the SSW, whereas the toeset beds reveal upslope, downslope, and along‐crest transport of sand.The ancient dunes apparently formed in a directionally varying wind flow with prevailing winds (early summer) from the NW and periodic strong winds (late summer) from the SW. The dunes were oblique not only to seasonal transport directions, but also to the resultant annual transport direction and dune migration direction. This was caused by the interaction of the dune system with the primary winds which resulted in secondary airflow and significant along‐crest transport of sand.The erg deposits at Many Farms are separated by a number of super bounding surfaces suggesting several episodes of erg formation and destruction. The initial erg system was dominated by transverse dunes, but overlying ergs only contained oblique dunes. All erg systems were bounded to the SW by wide regions of erg margin environments in which aeolian sand sheet, fluvial, and lacustrine facies were deposited. Even though fluvial deposits are absent from the main part of the sequence at the study area, the effects of this system are reflected within the erg deposits at Many Farms.
Franciscan Complex olistostrome at Crescent City, northern California
Authors: AALTO K. R. 
Published: June 2006

ABSTRACTAn olistostrome and bounding turbidites are exposed within the late Mesozoic Franciscan Complex along the Crescent City (California) coastline. Facies grade in character from Mutti & Ricci Lucchi (1978) mixed facies B, C and D, to F (the olistostrome), to mixed A, B and E, progressing upwards within the Franciscan stratigraphic section. The facies F unit outcrop is up to 600 m thick and extends 12 km along strike. It consists of oblate to tabular blocks, up to 200 m in maximum dimension, of greenstone, tonalite, radiolarian chert, limestone, phyllite and greywacke dispersed in a scaly argillite matrix. The olistostromal origin of the unit is indicated by depositional contacts with bounding turbidites, by the presence of abundant recycled sedimentary clasts within the unit, and by the presence of sedimentary breccias and associated dismembered, slump‐folded turbidites both within the olistostrome and among subjacent turbidites. Sandstones are chiefly feldspathic litharenites that were very likely derived from the partially dissected, late Mesozoic Sierran‐Klamath magmatic are.Franciscan rocks record an early pervasive, layer‐parallel flattening strain in such features as extensional faults, necking and pinch‐and‐swell structures. Several scales of extensional faulting account for the juxtaposition of turbidites of different facies and/or with varying degrees of stratal disruption, the formation of sandstone lozenges, and the formation of scaly foliation in the olistostrome matrix. The latter resulted from the juxtaposition of lenticles with varying concentrations of silt and clay. These were ultimately derived from the finer divisions of turbidite beds that were incorporated into the olistostrome. The presence of gradational contacts between some sandstone olistoliths and the olistostrome matrix, and of sandstone dykes that intrude fractures and associated drag‐folded turbidite beds indicate that Franciscan sediments were not lithified during their early deformation. These sediments were deposited in either a trench or trench slope basin, and were first deformed either by gravity collapse of the trench slope cover or, less likely, by vertical loading beneath the toe of the accretionary wedge. They later were folded during internal shortening of the growing Franciscan accretionary prism.