bluuebunny/crossref_metadata_2025_split huggingface dataset:
Depositional mechanics and sequences of base surges, Songaksan tuff ring, Cheju Island, Korea
Authors: CHOUGH S. K. , SOHN Y. K. 
Published: June 2006

ABSTRACTThe Songaksan mount in the southwestern part of Cheju Island, Korea, is a Taalian tuff ring produced by phreatomagmatic explosions at an aquifer. A detailed analysis of proximal‐to‐distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies; one lateral facies sequence (LFS) and three vertical facies sequences (VFS).The VFS 1 and 2 are representative of facies relationships in horizontal near‐vent deposits. The VFS 1 comprises scour‐fill bedded tuff, inversely graded tuff, massive tuff and laminated tuff from base to top. The VFS 2 is a variant of the VFS 1, replaced by an inversely graded lapilli tuff unit at the base. The sequences suggest traction carpet, suspension and minor traction sedimentation from a high‐concentration near‐vent base surge.The LFS 1 and the VFS 3 are distilled from outward‐dipping flank deposits. Both sequences begin with disorganized lapilli tuff, followed successively by stratified (lapilli) tuff, dune‐bedded (lapilli) tuff, very thinbedded tuff and accretionary lapilli. They are suggestive of waning base surge which decreases in particle concentration, suspended‐load fall‐out rate and flow regimes with an increase in traction and sorting.These facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space. A near‐vent base surge is turbulent, uniformly mixed and highly concentrated and produces scour‐fill bedded tuff. As capacity decreases, the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part (gravity transformation), depositing inversely graded, massive and normally graded (lapilli) tuff. Ensuing loss of sediment load and mixing of ambient air result in flow dilution (surface transformation). Stratified and dune‐bedded units are produced by tractional processes of turbulent and low‐concentration surge. Further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension.
A comparison of an autosuspension criterion to field observations of five turbidity currents
Authors: STACEY MICHAEL W. , BOWEN ANTHONY J. 
Published: June 2006

ABSTRACTAn autosuspension criterion that has been developed directly from the fluid dynamical equations, by taking into account the vertical structure of turbidity currents, is compared to field observations of five turbidity currents. It is found that the criterion is consistent with the motion of all five currents, which suggests that the criterion may, at least under certain circumstances, be a reasonable guide in estimating the conditions necessary for a turbidity current to be self‐sustaining.
Tidally flooded back‐barrier dunefield, Guerrero Negro area, Baja California, Mexico
Authors: FRYBERGER STEVEN G. , KRYSTINIK LEE F. , SCHENK CHRISTOPHER J. 
Published: June 2006

ABSTRACTA 45 km long barrier island exists west of the town of Guerrero Negro, Mexico, along the western coast of the Baja California peninsula, about 720 km south of San Diego, California. This barrier has developed in a mesotidal, arid‐climate regime characterized by steady, strong, onshore winds from the NW. The barrier island W of Guerrero Negro has prograded seaward about 1·6 km in the last 1800 years while an aeolian dunefield fed by sand blown from beaches has advanced inland up to 13 km. Landward progradation of the dune system from the barrier has occurred during relative rise in sea‐level; thus, aeolian sediments exist at or below the water table over a wide area. The progradation of dunes across marshes, tidal flats, and tidal channels, as well as the repeated submergence of interdune areas by tidal waters, has created a complex suite of mixed aeolian and subaqueous sediments in the back barrier. The complexity of the suites of aeolian sedimentary structures, together with the inclusion of subaqueously formed structures such as current and oscillation ripples, would make recognition of the aeolian origin of much of the sediments difficult in ancient rocks.In addition to the scientific importance of recognizing the aeolian deposits, the sedimentation model represented by the Guerrero Negro barrier has applications in petroleum exploration and development. Currently, most preservational models for barrier islands attach little volumetric importance to aeolian deposits. This modern example suggests that volumetrically significant aeolian deposits can be preserved behind a barrier, particularly in an arid‐climate regime. If preserved and charged with oil, the resulting productive sandstone could have an extremely irregular landward edge comprised in part of onshore‐prograded aeolian dune sandstone with excellent reservoir characteristics. As with current barrier models, the reservoir would be sealed landward and above by lagoonal mudstone and silt, evaporites, or evaporitic, sandy sabkha deposits. High organic productivity occurs in lagoons immediately adjacent to the dunefields of Guerrero Negro, suggesting that organic‐rich source rock may exist near aeolian sandstone in ancient settings similar to Guerrero Negro.
Ribbon sandstones in the Pennsylvanian Waddens Cove Formation, Sydney Basin, Atlantic Canada: the influence of siliceous duricrusts on channel‐body geometry
Authors: GIBLING MARTIN R. , RUST BRIAN R. 
Published: June 2006

ABSTRACTChannelized bodies up to 9 m thick in the Pennsylvanian Waddens Cove Formation show low width: thickness ratios (8:1 to 14:1), up to four vertically stacked storeys, and deep incision into associated floodplain strata. The presence of lateral accretion sets up to 6 m thick indicates that the migration of bank‐attached bars was a major channel‐filling process. The progressive rise in elevation of the basal part of the accretion surfaces where the sets adjoin the steeply inclined sides of the channel bodies shows that the channel floor aggraded progressively as the bars approached the channel sides. In planform, the river system probably showed a low‐sinuosity, incised valley within which the channel followed a more sinuous and partially confined course.The alluvium surrounding the channel bodies includes sandstone sheets attributed to crevasse splay and levee accumulation. The upper strata of many sheet's and some channel bodies contain siliceous duricrusts (ganisters) which formed by pedogenic lithification of sandy material commencing shortly after the landforms became inactive. The channel bodies contain duricrust‐bearing slumpblocks and show stepped margins over duricrusts. The stepped nature of the banks suggests that duricrusts were an important factor in confining the reach, and we suggest that channel‐body geometry was influenced, although not necessarily controlled, by the presence of the lithified layers.Alternation in the Waddens Cove Formation of grey, coal‐bearing zones with red, duricrust‐rich zones is attributed to water‐table fluctuation in response to base‐level changes. The low width: thickness ratios and aggradation mode'of the channel bodies may in part reflect channel incision and subsequent aggradation resulting from these allocyclic events.
Aeolian landscapes in central Australia: gypsiferous and quartz dune environments from Lake Amadeus
Authors: CHEN XIANG YANG , BOWLER JAMES M. , MAGEE JOHN W. 
Published: June 2006

ABSTRACTThere are two different dune systems in central Australia; regional quartz dunefields and transverse gypsiferous dunes associated with playa lakes. These two systems, especially gypsiferous dunes at Lake Amadeus, the largest playa in central Australia, provide a sedimentary, geomorphological and environmental history of the region during the late Quaternary. The gypsifierous dunes consist of a surficial gypcrete overlying an aeolian sediment sequence below, a mixture of gypsum sand and quartz sand. No clay pellets have been found in the dune sequence, in significant contrast to the gypsiferous clay dunes in other parts of Australia. Three possible models of the environmental controls of gypsiferous dune formation are discussed. The most plausible one suggests simultaneous gypsum precipitation and deflation. Sandsized gypsum was precipitated in a groundwater‐seepage zone around the playa margin during seasonally high water‐tables and these crystals were deflated onto land during dry intervals, forming the marginal gypsiferous dunes. These processes require conditions of high regional water‐table, strong climatic seasonality and probably a windier and overall wetter climate. At least two separate gypsiferous‐duneforming episodes can be recognized. The age of formation of the younger one has been dated by thermoluminescence at 44–54 ka. The gypcrete crust capping the dunes is characterized by intergrown microcrystalline gypsum crystals, showing evidence of leaching, dissolution and recrystallization. It is interpreted as a pedogenic product formed during a stable period after accumulation of the gypsiferous dune. After the construction of the younger gypsiferous dune, there was a major episode of activation of regional quartz dunefields which formed thick quartz sand mantles overlying gypsiferous dunes on both playa margins and the dune islands within the playa. An equivalent aeolian sand layer was deposited within the playa. Soil structures in this unit indicate that the sand sheet over the playa was later colonized by vegetation. Activation of the regional dunefields suggests a major period of dry climate, which, although not dated, may correlate with the last glacial maximum identified as a period of maximum aridity from 25 to 18 ka at other sites in Australia.
Grain‐size distributions of pyroclastic deposits
Authors: LIRER LUCIO , VINCI ANNA 
Published: June 2006

ABSTRACTA multivariate statistical technique (principal component analysis) has been used to investigate the granulometric features of 414 samples of pyroclastic rocks: 216 samples were collected from areas of recent and active volcanism in Central and Southern Italy; data for the remaining 198 samples were obtained from the literature and relate to products from Somma‐Vesuvio, Mount St Helens and the Japanese volcanic arc. The analysis offers an improved means of discriminating pyroclastic deposits using their grain‐size characteristics.
Detrital carbonate grains as provenance indicators in the Upper Cretaceous Pietraforte Formation (northern Apennines)
Authors: FONTANA DANIELA 
Published: June 2006

ABSTRACTThe Upper Cretaceous Pietraforte Formation, an allochthonous unit of the Ligurian domain in the northern Apennines, provides a case study of the importance of detrital carbonate grains for provenance determination in sandstones.The Pietraforte Formation is composed of turbidite sandstones with subordinate conglomerate, deposited in an external sector of the Ligurian ocean, close to the Adriatic margin. The sandstones have a lithic composition, characterized by abundant sedimentary and metasedimentary rock fragments (35–56% of the terrigenous framework), little feldspar (<7%) that is almost exclusively plagioclase, and a high ratio of fine‐ to coarse‐grained polycrystalline quartzose grains to total quartzose grains (average Qp/Qt=0.37).Carbonate rock fragments dominate the lithic association of both sandstones and conglomerates and provide the most detailed information for provenance determination. They are composed primarily of dolostones and a wide variety of limestones containing identifiable age‐diagnostic microfossils. Fossils and rock textures of carbonate clasts document the erosion of Upper Triassic to Lower Cretaceous shelf and pelagic carbonate units which can be matched with Mesozoic rock types present in the Tuscan domain of the northern Apennines. Compositional results constrain the source of the Pietraforte Formation sandstones to the western margin of the Adriatic plate, from uplifted sedimentary and metasedimentary rocks of the Tuscan domain and its low‐grade metamorphic basement.Coeval intrabasinal sources provided additional supplies to the depositional basin of the Pietraforte Formation; this intrabasinal supply consists of shelf carbonate allochems, planktonic foraminifera and argillaceous rip‐up clasts. The presence of carbonate grains from shallow‐water environments may indicate the existence during deposition of marginal shelf areas favourable for carbonate allochem production.
Pleistocene, multiple‐growth, lacustrine oncoids from the Poacher's Point Formation, Etosha Pan, northern Namibia
Authors: SMITH A. M. , MASON T. R. 
Published: June 2006

ABSTRACTThe Poacher's Point Formation is located within the Etosha Pan, northern Namibia. It contains Pleistocene stromatolites, ooids and silica nodules. The stromatolites were constructed by cyanobacteria in very lowenergy lacustrine conditions. The internal structure of the oncoids is complex and subdivided into shells by unconformities. Each unconformity‐bounded shell probably represents stromatolite growth during a distinct lacustrine event. Shallower conditions are represented by colloform growth and deeper water by smooth oncoid mat morphology. The unconformities represent hiatuses of unknown duration. At least six stromatolite growth shells are recognized. Silica concretions which may have developed during shallowing lake stages are also present. The lack of clastic material in the carbonate lithologies suggests that the lake waters may have accumulated from collecting rainwater or have been spring fed. The lowest bed of the succession is a saline clay which represents a seasonally flooded salt pan deposited in an arid to semi‐arid climate similar to that of today. The overlying carbonates represent sedimentation during a series of transient lacustrine events, representing intermittent wetter periods. Finally, due to progressive aridity, the lacustrine succession was terminated by calcrete development.
Amalgamated accumulations resulting from climatic and eustatic changes, Akchar Erg, Mauritania
Authors: KOCUREK GARY , HAVHOLM KAREN G. , DEYNOUX MAX , BLAKEY RONALD C. 
Published: June 2006

ABSTRACTThe Akchar Erg of the Sahara of western Mauritania shows a morphology and stratigraphy that can be recognized as the amalgamation of late Pleistocene and Holocene deposits that reflect eustatic and climatic events. Mapping, trenching, and dating by 14C methods and artefacts show that the prominent complex linear dunes (draas) of the Akchar Erg are actually composite features showing at least three constructional and two destructional phases. The constructional phases are represented by three convex‐up layers: (i) a modern veneer moulded into superimposed crescentic dunes, which partially mantle the larger linear bedforms; (ii) a middle, partly root‐turbated sand deposited sometime during the last 4000 years; and (iii) a core of linear dune sand formed during the last glacial period (13 000–20 000 yr BP), which today shows relict relief, intense root‐turbation, and pedogenesis. These constructional phases are separated by super bounding surfaces that coincide with erg destructional phases. Surface 2 bounds the middle aeolian sand, and is marked by a lag surface of small granules. Surface 1 is a very prominent surface with an abundance of Neolithic artefacts, and represents stabilization of the linear dunes during the humid, interglacial period (4000–11 000 yr BP). Interdraa deposits originated during the interglacial period, and consist of continental lacustrine limestones and sandstones, humic sands deposited in marshes, and sabkhas on the coast. The sabkhas originated during interglacial highstand of sea‐level when interdraa areas were marine embayments, and subsequently dried during regression. The draa and interdraa sequences, therefore, in spite of being adjacent facies, actually represent different events and were not formed simultaneously. The upwind sand‐sheet margin of the Akchar Erg shows exposures of the middle and core aeolian sands (which were previously protected from deflation by vegetation) being progressively cannibilized in the current phase of erg construction, and revealing a crystalline basement rock. In this proximal area, conditions are not favourable for the incorporation of these aeolian accumulations into the stratigraphic record.
The anatomy of an early Dinantian terraced floodplain: palaeo‐environment and early diagenesis
Authors: ANDREWS JULIAN E. , TURNER MICHAEL S. , NABI GHULAM , SPIRO BARUCH 
Published: June 2006

ABSTRACTThe lowermost Carboniferous rocks in the Cockburnspath area of east Berwickshire (southern Scotland) are interpreted as coastal floodplain sediments. A lower mudstone‐dominated unit is composed of silty mudstones and shales with subordinate sandstones and argillaceous ferroan dolomites (cementstones). These are interpreted as distal floodplain sediments with periodic crevasse‐splay deposition. The dark grey colour of the mudrocks suggests deposition in reducing conditions, probably in floodplain lakes. Most of the cementstones are concretionary, some with septarian cracks, suggesting an early diagenetic origin. An immature palaeosol suggests periodic pedogenesis under improved drainage. A synsedimentary erosion surface indicates incision of a valley into the floodplain, presumably in response to base‐level lowering.An upper sandstone‐dominated unit starts with fine‐grained rippled sandstones, cut by small channel sandstones. These are interpreted as floodplain lake deposits fed by crevasse channels. A distinctive conglomerate with cementstone clasts, wood fragments and fish remains is interpreted as a major overbank deposit, dumped into a pre‐existing floodplain lake. A bivalve fauna was established in the overlying mudstones, followed by a thin limestone with a restricted marine fossil assemblage, showing that seawater flooding of the lakes occurred at times.Mudrocks throughout the sequence contain no pyrite, except for the marine band which has an organic‐carbon/sulphur ratio and degree of pyritization value typical of marine sediments. The concretionary cementstones have δ13C values around —4 to —6%0/00 PDB which are interpreted as indicative of anaerobic oxidation of organic matter. The combined geochemical data suggest a significant involvement of iron reduction in cementstone formation, although the δ13C values are ambiguous in assessing the relative involvement of methanogenesis and methane oxidation. Limited seawater inundation of the floodplains might have supplied magnesium and calcium ions for dolomite formation assuming that any H2S derived as a result of sulphate reduction was oxidized by iron reduction. Alternatively a weathering source for solutes might have been involved.
Sedimentology of a Proterozoic erg: the Venkatpur Sandstone, Pranhita‐Godavari Valley, south India
Authors: CHAKRABORTY TAPAN 
Published: June 2006

ABSTRACTReappraisal of the Late Proterozoic Venkatpur Sandstone indicates that the bulk of the sandstone is aeolian in origin. Aeolian stratification types, namely (i) inverse graded translatent strata, (ii) adhesion laminae, (iii) grainflow strata and (iv) grainfall strata, are present throughout the outcrop belt. Nine facies have been identified that represent both aeolian and related aqueous environments within a well‐developed erg. Cosets of large cross‐beds at the Bellampalli section in the NW of the study area record dune fields in the interior of the sand sea. To the SE, at the Godavari River and Ramgundam sections, a progressive increase in the relative proportion of the flat‐bedded to cross‐bedded facies and intercalated non‐aeolian facies delineates the transition from the dune‐field to sand‐sheet environment. An alternating sequence of aeolian and marine sediments at Laknavaram, in the extreme SE, marks the termination of the sand sea. Palaeocurrent data suggest that the NW‐SE trend of the sections represents a transect across the sand sea in a direction normal to the resultant primary palaeowind direction.Abundant horizontally stratified units in the Vankatpur Sandstone do not always represent the interdune sediments. On the basis of the thickness and geometry of the units, nature of bounding surfaces and associated facies sequence, the facies is variously interpreted to represent interdune, inland sabkha, sand sheet and coastal sand flat deposits.
Origin of a cool‐water, Oligo‐Miocene deep shelf limestone, Eucla Platform, southern Australia
Authors: JAMES NOEL P. , BONE YVONNE 
Published: June 2006

ABSTRACTThe Abrakurrie Limestone is an areally extensive, bryozoan‐rich unit within the Eucla Platform, a Tertiary carbonate shelf which caps the central part of the southern Australian continental margin. The onshore portion, the topic of this study, has been exposed since middle Miocene time and lies beneath the Nullarbor Plain. The rocks are fine‐sand‐ to granule‐sized calcarenites, composed of bryozoans, bivalves, benthic foraminifera and echinoids with lesser numbers of brachiopods, solitary corals and serpulids. They conspicuously lack significant numbers of planktonic foraminifera and coralline algae. Most bryozoan remains are from delicate branching cyclostomes although delicate branching, robust branching, foliose, fenestrate, multilaminar encrusting and free‐living cheilostomes are variably abundant in specific units. The poorly lithified sequence is punctuated by well‐cemented layers with erosional tops, which are interpreted as hardgrounds.The limestone is interpreted as a cool‐water, deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the Eucla Platform. A model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation. The shelf is envisaged as partitioned by the depth of the zone of wave abrasion. Sediments were produced throughout, but accumulated only below this depth. When the seafloor was above this depth it was an environment of cementation and erosion.The vertical sequence correlates in a general way with the global sea‐level model for the mid‐Cenozoic. While accumulation rates for southern Australian carbonates are similar to rates of cool‐water carbonate deposition elsewhere (c. 2.5 cm kyr‐1), the rate of Abrakurrie accumulation is much less, suggesting that significant time periods are represented by the hardgrounds.