bluuebunny/crossref_metadata_2025_split huggingface dataset:
Soft‐sediment deformation (fluid escape) features in a coarse‐grained pyroclastic‐surge deposit, north‐central New Mexico
Authors: Nocita Bruce W. 
Published: June 2006

ABSTRACT The Puye Formation in north‐central New Mexico is a very coarse‐grained fanglomerate which was deposited on the eastern flank of the Jemez caldera. Pyroclastic deposits occur within the Puye in the form of airfall pumice beds and the remnant of at least one pyroclastic‐surge deposit. This pyroclastic‐surge deposit shows the effects of fluidization and soft‐sediment deformation in the form of: (1) intrusive sedimentary plumes; (2) upwardly injected gravelly pipes; (3) ‘pocket structures’ similar to those of Postma (1983), and; (4) oversteepened and deformed cross‐stratification.Fluidization and soft‐sediment deformation resulted from a combination of the mechanical instability and high, possibly pressurized, fluid content of the deposit. This metastable condition was a consequence of the nature of the flow which deposited the sediment: a rapidly depositing, high‐velocity sediment gravity flow. The fluids in pyroclastic surges may be either gas or liquid. However, because of the coarse grain‐size of the fluidized sediment, it is suggested that liquids were responsible for the features described in this paper. Evidence also suggests that locally fluidization, liquefaction, and soft‐sediment deformation took place penecontemporaneously with deposition.
Late Pleistocene mixing zone dolomitization, southeastern Barbados, West Indies
Authors: HUMPHREY JOHN D. 
Published: June 2006

ABSTRACT Field, geochemical, and petrographic data for late Pleistocene dolomites from southeastern Barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters. The dolomite is localized in packstones and wackestones from the algalAmphistegina fore‐reef calcarenite facies.Stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization. Carbon isotopes in pure dolomite phases average about ‐15%0 PDB. This light carbon is attributed to the influence of soil gas CO2, and precludes substantial mixing with seawater. A narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint. Dolomitization likely occurred with as little as a five per cent admixture of seawater.Strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy. The dolomite is characterized by low sodium values. Low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization.A sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric. Early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix. Limpid, euhedral dolomite cements precipitated in primary intra‐ and interparticle porosity subsequent to replacement dolomitization. As waters became progressively less saline, dolomite cements alternated with thin bands of syntaxial calcite cement. The final diagenetic phase precipitated was a blocky calcite spar cement, representing diagenesis in a fresh‐water lens. This sequence of diagenetic features arose as the result of a single fall in eustatic sea‐level following deposition.A stratigraphic‐eustatic‐diagenetic model constrains both the timing and rate of dolomitization in southeastern Barbados. Dolomitization initiated as sea‐level began to fall immediately following the oxygen isotope stage 7–3 high stand, some 216 000 yr bp. Due to the rapidity of late Pleistocene glacio‐eustasy, dolomitization (locally complete) is constrained to have occurred within about 5000 yr.
The diagenetic history of some septarian concretions from the Kimmeridge Clay, England
Authors: ASTIN T. R. , SCOTCHMAN I. C. 
Published: June 2006

ABSTRACT Large septarian concretions from the Kimmeridge Clay, up to 1.2 m in diameter, have centres comprising anhedral calcite microspar passing into margins of radiating fibrous calcite microspar, with a pyrite‐rich zone at the transition. Septarian veins formed and were lined with brown calcite synchronously with fibrous matrix growth, with white calcite precipitated in septarian cavities after concretion growth ceased. Septarian veins, filled only with white calcite, formed later, at the same time as the outermost calcite microspar crystals were enlarged.The concretions were buried in the Late Jurassic to about 130 m, and in the Late Cretaceous to about 550 m, with uplift between. Oxygen isotopes show that the concretion grew throughout the first burial, with septarian veins forming from about 30 m depth onwards. Later septarian veins formed between about 200 and 500 m during the second burial.Carbon isotopes show that the compact inner matrix grew in the sulphate reduction zone, the end of which is marked by the pyrite‐enriched zone. Dissolving shells, and possibly minor methanogenic carbonate, slowly diluted sulphate reduction‐zone carbonate during deeper burial. During early concretion growth, Mg and Sr were depleted in the pore water. During later stages of the first burial, Mg, Sr, Mn and Fe all increased, especially after concretion growth ceased. During the second burial, Fe, Mn and Mg decreased as calcite precipitated, implying relatively closed systems for these elements.Synchronous formation of septarian fractures and fibrous calcite matrix shows that the Kimmeridge Clay became overpressured during the later stages of both burials.
Textural and geochemical alternations in Late Cenozoic Bahamian dolomites
Authors: DAWANS JOHN M. , SWART PETER K. 
Published: June 2006

ABSTRACTDolomitized intervals of a core from San Salvador Island, Bahamas, exhibit variations of two texturally and geochemically distinct end‐members. In the Pliocene section of the dolomitized interval, the two end‐members alternate in a pattern that may reflect originally and/or diagenetically modified depositional facies. Formerly mud‐free intervals, locally capped by exposure surfaces are massive crystalline, mimetic dolomites (CM). Muddier sediments are replaced by friable microsucrosic dolomites (MS). CM and MS dolomites also differ in porosity (< 10% vs > 30%), permeability (< 10 md vs > 100 md), mol% MgCO3 (44–9 vs 47–7) mol%), oxygen isotopic composition (1–7 vs 2–7‰) and strontium content (241 vs 106 ppm).These data indicate that depositional and diagenetic fabric are the principal controls governing the distribution of dolomite types. Differences in texture and geochemistry are suggested as arising through differential rates of crystallization produced as a result of variations in permeability and reactivity of the precursor sediments and rocks.
The genesis of nodular limestones in the Ordovician and Silurian of the Oslo Region (Norway)
Authors: MÖLLER NICOLA KERSTIN , KVINGAN KNUT 
Published: June 2006

ABSTRACTNodular limestones have been studied from the Lower Chasmops Shale (Middle Ordovician) and the Rytteráker Formation (Lower Silurian). Observations on nodule‐host‐rock relations and variations of ferroan/non‐ferroan calcite cements help explain the role of precipitation, dissolution and redistribution of carbonate.Distribution and frequency of nodules depends on environmental parameters such as carbonate/clay ratio, grain size distribution and bioturbation, though final shapes are the result of pressure‐dissolution and cleavage.
Evidence for surface reaction‐controlled growth of carbonate concretions in shales
Authors: RAISWELL R. 
Published: June 2006

ABSTRACTEstimates for the rate of concretionary growth in shales are based on models which assume that growth is diffusion‐controlled. However, laboratory and field studies of CaCO3 precipitation in organic carbon‐rich sediments indicate that surface reactions control growth, due to inhibition by various dissolved species. The spatial distribution of carbonate concretions in the Jet Rock (Lower Jurassic, England) is also inconsistent with diffusion‐controlled precipitation of CaCO3 into concretions, and growth must have been at least partly surface reaction‐controlled.
Sur submarine slide, Monterey Fan, central California
Authors: NORMARK WILLIAM R. , GUTMACHER CHRISTINA E. 
Published: June 2006

ABSTRACTThe Sur debris slide and associated debris flow cover more than 1000 km2 of the eastern part of Monterey submarine fan and extend from the base of the continental slope near the apex of the fan to the Monterey fan valley. The flow is generally confined between the continental slope and remnants of an older channel (Monterey East fan valley). The hummocky surface of the debris slide and diffuse echo returns from the surface of the nearly acoustically transparent debris flow, as seen in 3.5 kHz profiles, are common to large submarine slides described from passive‐margin continental‐slope and rise deposits. Piston cores from the Sur slide recovered contorted and sheared fan turbidite units as well as clast‐ and matrix‐supported mudlump debris flows. The cores show that the slide debris is overlain by 0.23–0.87 m of mud and turbidite sand that limit the age of the slide to latest Holocene, although more than one stage of emplacement is possible.
Magnesian calcite cements and their diagenesis: dissolution and dolomitization, Mururoa Atoll
Authors: AISSAOUI DJAFAR M. 
Published: June 2006

ABSTRACTThe most ubiquitous syn‐sedimentary cements affecting Mururoa atoll are composed of magnesian calcite. Three main types are distinguished: fibrous, bladed and sparitic on the basis of petrography, morphology and MgCO3 concentration of the constituting crystals, while peloid infills, a particular form of HMC chemical precipitation, also exist. Petrographic evidence and isotopic signatures are compatible with marine precipitation.Mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of Mg‐calcite cements. These alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology. The first stage of the partial dissolution of Mg‐calcite induces the development of chalky, white friable zones within the initially crystalline, hard cement layers. At ultrascale, this is due to the creation of micro‐voids along the elongate cement fibres. Advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and/or between superposed cement layers. Mg‐calcite dissolution is related to meteoric diagenesis during periods of Quaternary exposure. The creation of voids within Mg‐calcite layers is due to the mechanical removal of previously altered calcium carbonate, a process suggesting marine or non‐marine water flow, probably in the vadose environment.Selective dolomitization of Mururoa cements involves alternations of calcite and dolomite which form successive cement‐like rinds within primary cavities. At Mururoa, these alternations are the result of selective dolomitization of the pre‐existing Mg‐calcite cements rather than successive precipitation of calcite and dolomite. Selective dolomitization of Mg‐calcite cements at Mururoa indicates that a given cement succession is not necessarily a simple chronological sequence. Oxygen isotope values of dolomites are enriched in δ186 by about 3‰ PDB within calcite‐dolomite pseudo‐alternations. The dolomitizing fluid at Mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization.
Cathodoluminescent bimineralic ooids from the Pleistocene of the Florida continental shelf
Authors: MAJOR R. P. , HALLEY R. B. , LUKAS K. J. 
Published: June 2006

ABSTRACTA bored and encrusted late Pleistocene ooid grainstone was recovered from the seafloor at a depth of approximately 40 m on the outer continental shelf of eastern Florida. Ooid cortices are dominantly bimineralic, generally consisting of inner layers of radial magnesian calcite and outer layers of tangential aragonite. Ooid nuclei are dominantly rounded cryptocrystalline grains, although quartz grains and a variety of skeletal grains also occur as nuclei. Ooids are partially cemented by blocky calcite, and interparticle porosity is partially filled by micrite.Radial cortex layers are composed of brightly cathodoluminescent magnesian calcite having a composition of approximately 12 mol% MgCO3 and 1000 ppm strontium. The iron and manganese concentrations in radial cortex layers are generally in the range of 500–1000 ppm and 100–250 ppm, respectively. Tangential cortex layers are composed of noncathodoluminescent aragonite containing approximately 11 500 ppm strontium and less than 0.5 mol% MgCO3. Iron concentrations in tangential cortex layers are generally in the range of 150–400 ppm, and manganese concentrations are generally below the detection limit of 100 ppm. Echinoderm skeletal fragments, which are present as accessory grains, are composed of brightly cathodoluminescent magnesian calcite. Some ooid nuclei and the thin outer edges of some blocky calcite cement are cathodoluminescent; micrite matrix and the bulk of blocky calcite cement are noncathodoluminescent. Ooids do not exhibit textural evidence of recrystallization.The ooid grainstone underwent an episode of meteoric diagenesis. but ooid cortices were not affected by the event. We propose a previously unrecognized process by which the magnesian calcite cortex layers underwent diagenetic alteration in oxygen‐depleted seawater. During this diagenesis, magnesium was lost and manganese was incorporated without apparent textural alteration and without mineralogical stabilization. Thus, we Suggest that cathodoluminescence may result from diagenetic alteration on the sea‐floor.
Sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt: Ellensburg Formation (late Miocene), central Washington
Authors: SMITH GARY A. 
Published: June 2006

ABSTRACTLate Miocene volcaniclastics of the Ellensburg Formation (Washington, northwestern USA) are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a Cascade Range source area. Two facies associations are recognized. The first is composed of laterally restricted, well‐sorted, polymictic conglomerates representing a gravel‐bedload regime during inter‐eruption periods. The second comprises laterally extensive sheets, bounded by deeply‐developed paleosols, composed of monomictic pebbly, pumiceous dacitic sandstones with intercalated debris‐flow and hyperconcentrated flood‐flow deposits. These sheets aggraded in response to eruptive events that are sometimes recorded by air‐fall tephras at the bases of sequences. Debris‐flow and hyperconcentrated flood‐flow deposits occur as far as 120 km from source, but are uncommon beyond 50 km. Hyperconcentrated flood‐flow deposits initially increase in abundance away from source as debris‐flow deposits diminish, suggesting formation of hyperconcentrated flood flows by dilution of debris flows. Sandy facies form broad sheets dominated by scour‐fill bedding, in proximal and medial settings, and grade to narrower, trough cross‐bedded sheets in distal settings, suggesting moderation of flood discharges with distance. Base‐level changes associated with episodic sediment influx caused incision‐aggradation cycles in mainstream settings, and episodic impoundment of tributaries to form lakes or rapidly aggrading, poorly drained floodplains. Although volcanism was the primary control on depositional style, concurrent development of the Yakima fold belt produced a structure‐consequent drainage pattern that determined sediment dispersal, and basin subsidence permitted preservation of both syn‐eruption and inter‐eruption facies. Detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics, but dominates deposits of small tributary streams flowing toward, rather than from, the Cascades.
Depositional processes on an ancient and modern muddy shelf, northern California
Authors: LEITHOLD ELANA L. 
Published: June 2006

ABSTRACTThe modern Eel River shelf and analogous Pleistocene Rio Dell Formation in northern California provide an ideal opportunity to combine the advantages of studying a modern environment with those of studying an ancient sequence, and thereby enables further understanding of muddy‐shelf processes. The modern shelf is the site of accumulation of a thick deposit of Holocene mud. Both large‐scale sediment distribution patterns and small‐scale stratigraphy on the shelf indicate that river floods play an important role in sediment accumulation, even on a high‐energy, ‘storm‐dominated’ coast. The major factors in preservation of this flood ‘signature’ are the cohesive behaviour of fine‐grained sediments and episodically rapid rates of sediment input.The Rio Dell Formation includes approximately 400 m of mostly fine‐grained shelf deposits that accumulated offshore from a palaeo‐Eel River mouth. The shelf sediments comprise four depositional sequences. Sequence 1 records progradation from outer to inner shelf depths. Facies trends closely resemble across‐shelf trends on the modern shelf, suggesting that processes were similar. Detailed examination of these deposits provides insight into the nature and role of various processes on both the ancient and modern shelf.Muddy facies of the Rio Dell sequence are characterized by bioturbated, clayey silts, interbedded with event layers of several types. Clay‐rich silt layers are interpreted as flood deposits and physically stratified, coarse‐silt layers are interpreted to record transport and deposition of coarse silt on the midshelf during storms. Sediment‐transport calculations and consideration of grain‐size distributions of bioturbated sediments, which form the bulk of the Rio Dell sequence, suggest that these sediments are the result of biological homogenization of the fine‐grained flood deposits and of the coarser‐grained storm deposits. The results of this study in general indicate that fine‐grained shelf deposits do preserve a distinguishable, if subtle, record of depositional processes.
Bedload transport of mud as pedogenic aggregates in modern and ancient rivers
Authors: RUST BRIAN R. , NANSON GERALD C. 
Published: June 2006

ABSTRACTAbundant sand‐sized mud aggregates in the Cooper and Diamantina Rivers, Lake Eyre Basin, Australia are attributed to bedload transport of aggregates formed in deeply‐cracked floodplain soils. The conditions required for formation of pedogenic mud aggregates are: (i) abundant clay containing at least minor swelling clay, and (ii) a climate with at least seasonally hot dry periods. The worldwide distribution of these soils (Vertisols) suggests that a significant amount of mud is transported as pedogenic aggregates by modern rivers.Ancient analogues in which mud aggregates and Vertisol profiles have been recognized are the Jurassic East Berlin Formation (Connecticut, USA) and the Carboniferous Maringouin Formation (New Brunswick and Nova Scotia, Canada). The dominant red mudstones of these formations are interpreted as mainly bedload sediments deposited by sheet floods in semi‐arid palaeoclimates. The Triassic Hawkesbury Sandstone (NSW, Australia) also contains sand‐sized mudstone aggregates, thought to be pedogenic, but its paleosol and other facies point to formation in a wetter palaeoclimate. The indications are that bedload transport of mud as pedogenic aggregates was as significant a process in ancient rivers as it is at present.
Early Cambrian reefs, reef complexes, and associated lithofacies of the Shackleton Limestone, Transantarctic Mountains
Authors: REES MARGARET N. , PRATT BRIAN R. , ROWELL A. J. 
Published: June 2006

ABSTRACTThe Shackleton Limestone formed a carbonate platform that bordered part of the Greater Antarctic craton in middle and late Early Cambrian time. In the Holyoake Range of the central Transantarctic Mountains, this unit records deposition on a stable shelf on which flourished ecological reefs composed of microorganisms and archaeocyathans. Burrow‐mottled lime mudstone, wackestone and packstone with patch reefs represent accumulation in shelf areas of relatively low to moderate energy. Thick ooidal grainstone units reflect deposition in higher energy shoals and as sand sheets that were associated with extensive reef complexes.The framework of these reefs was principally the product of micro‐organisms, by inference mostly cyanobacteria. Archaeocyathans constitute as much as 30% of some reefs, but commonly they form less than 10% and are absent from some. On the basis of microbial composition, three reef types are recognized. The first type is a Renalcis boundstone that lacks archaeocyathans. Within these, abundant upward‐directed thalii of Renalcis formed a framework that trapped fine‐grained sediment. The second type, which forms the core of some larger reefs, is composed of stromatactis‐bearing, microbial boundstone. The third, yet most common, reef type is variable in composition. It is characterized by the presence of abundant Epiphyton, but may include archaeocyathans, and the microbial microfossils Girvanella and Renalcis as well as cryptomicrobial clotted micrite. In this type of reef, frame‐building organisms typically constructed highly porous structures that had small interparticle and fenestral pores and large growth‐framework cavities, as well as rare metre‐sized caverns. Within these spaces, Epiphyton and, less commonly Renalcis, encrusted framework elements, fine‐grained sediments accumulated, and pervasive sea‐floor cements were precipitated.Boundstone fabrics in the Shackleton Limestone are highly complex, with fabrics analogous to younger, more metazoan‐rich reefs, as well as deep‐water stromatactis‐bearing mud‐mounds. The Epiphyton‐Girvanella‐archaeocyathan frameworks and stromatactis‐bearing boundstones, both of which seemingly first appeared in the middle Early Cambrian, are regarded as the precursors, in structure, composition, and preferred hydrologic setting, of the more extensive reefs and complex framework styles of later Phanerozoic time.
Environmental control on diverse stromatolite morphologies in the 3000 Myr Pongola Supergroup, South Africa
Authors: BEUKES NICOLAS J. , LOWE DONALD R. 
Published: June 2006

ABSTRACTA unique outcrop of partly silicified dolomite in the White Umfolozi section of the Pongola Supergroup, South Africa indicates that stromatolites were diverse and adapted to a range of shallow, tidal depositional settings 3000 Myr ago. Composite columnar stromatolitic bioherms 0.7‐1.6m high and 0.4‐1.0m in diameter formed along the margins of a tidal channel. They were flanked, away from the channel, by flat stratiform and small domical stromatolites growing in low energy tidal flat environments. Conical stromatolites, 0.05‐0.30m high and 0.03‐0.10m in diameter, accreted in high‐energy coarse‐grained carbonate sand along the bottom of the tidal channel. The stromatolites probably formed through the activities of filamentous, oxygen‐producing, photoautotrophic cyanobacteria.