摘要:

ABSTRACTStratigraphic analysis of mixed siliciclastic‐carbonate lithofacies within the Middle Cambrian Bonanza King Formation of the southern Great Basin reveals three distinct facies associations that record a range of depositional environments from semi‐arid tidal flats to deeper subtidal, restricted lagoons. Stratigraphic trends, cross‐platform facies variations and correlation of individual surfaces across 250 km of the study area suggest that these mixed lithofacies were deposited in three temporally distinct phases. (1) Extensive progradation of mixed peritidal environments culminated in a prolonged episode of subaerial exposure marked by an areally extensive intraclast breccia (0·5–1·2 m thick) that we interpret to be a major Type 1 sequence‐bounding disconformity. (2) Abrupt flooding of the exposed platform resulted in the deposition of mixed deeper subtidal lithofacies, including a condensed interval of fissile, fossiliferous shale. (3) Progressive shallowing and aggradational accumulation was accompanied by a decrease in siliciclastics and a shift to pure carbonate deposition. Deep‐water siliciclastics and megabreccias record deposition along the base‐of‐slope off the Middle Cambrian shelf‐edge, and are interpreted to represent lowstand deposits emplaced during the prolonged episode of subaerial exposure of the shallow shelf.The presence of fine siliciclastics in both peritidal facies and sharply overlying deeper subtidal facies of the study interval within the Bonanza King suggests a variable, but relatively continuous, influx of terrigenous material throughout an extended period of accommodation change, apparently asynchronous with respect to the predictive model of reciprocal sedimentation. We suggest that the primary siliciclastic source changed with relative sea‐level position. During lowered sea level, aeolian processes acting upon the unvegetated Cambrian craton transported fine siliciclastics onto peritidal and shallow‐subtidal environments. During higher sea level, coastal siliciclastic reservoirs supplied sediment that was transported for long distances by geostrophic currents flowing along the submerged platform.As opposed to many Cambro‐Ordovician grand cycles that are commonly interpreted to consist of a transgressive shaly half‐cycle grading upward into a regressive carbonate half‐cycle, the sequence boundary within this Middle Cambrian succession occurs within siliciclastic‐rich, mixed lithofacies rather than in adjoining purer carbonates, implying that some ‘grand cycles’ should not be considered synonymous with ‘sequences’. Interbasinal correlations of the Type 1 sequence boundary within the mixed unit are speculative, primarily because of the inherent imprecision of available trilobite biostratigraphy. However, there is evidence that an extended episode of subaerial exposure may have been continent‐wi

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-13.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTTwo sediment wave fields have been identified in the northern Rockall Trough; one from the north‐eastern part of the trough, developed on the flank of an elongate sediment drift, and the other at the distal edge of the Barra Fan. Wavelengths in both areas vary from 1 to 2 km and wave heights from 5 to 20 m. The seismic character of both the wave fields is similar with a lower package of well‐layered, medium‐ to high‐intensity reflectors migrating upslope, overlain by a dominantly acoustically transparent unit containing irregular, semicontinuous reflectors. Eight cores have been recovered from the two wave fields, seven from the crest‐trough areas of the distal Barra Fan wave field, and a single core from the crest of the sediment‐drift waves. Lithologically, the cores show that different processes have been active across the two wave fields. Cores from the Barra Fan field contain thin turbidites with thicker, draped hemipelagites and hemiturbidites, corresponding to the well‐layered, reflective seismic units and transparent seismic unit, respectively. These waves have been maintained by turbidity currents, perhaps over older, originally bottom‐current‐formed waves. The single core from the small sediment‐drift wave field recovered hemipelagites and glaciomarine sediments grading upwards into muddy‐silty contourite deposits, topped by a sandy contourite. These waves were constructed by contour currents. Dating of cores from these two small wave fields revealed that the sequences of thin turbidite and hemipelagite sediments from the Barra Fan correspond to the Late Glacial‐Allerød/Bölling Interstadial with the overlying hemipelagite of Younger Dryas‐Holocene age. The contourite deposits from the north‐east Rockall Trough wave field have been dated as early Holocene, reflecting increasing bottom‐current activity at the changeover from a gl

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-1.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThree categories of fibrous calcite from early to middle Caradoc platform‐marginal buildups in east Tennessee can be delineated using cathodoluminescent microscopy, minor element chemistry and stable C‐O isotopic composition. Bright luminescent fibrous cement has elevated Mn (>1000 p.p.m.), negative δ13C and intermediate δ18O values relative to other types of fibrous calcite. This cement reflects fibrous calcite that interacted with reducing Mn‐rich fluids. Dully luminescent fibrous cement has elevated Fe (>400 p.p.m.), positive δ13C and negative δ18O values relative to other fibrous cements. This cement was stabilized by burial fluids. Nonluminescent fibrous cement has low Mn and Fe (generally below 400 p.p.m.) and positive δ13C and δ18O values relative to other types of fibrous calcite. The latter cement is interpreted to be the best material for determining the isotopic composition of calcite precipitated in equilibrium with early to middle Caradoc seawater, which is δ13C=1% PDB and δ18O=−4 to −5‰ PDB. Results from this study and Ashgillian brachiopods indicate that the average δ18O composition of the Ordovician ocean, during nonglacial periods, was probably never more negative than −3‰ SMOW. Assuming an Ordovician seawater δ18O value of −1‰ SMOW, Holston Formation fibrous cements would have precipitated at temperatures between 27 and 36 °C, which is near the upper temperature limit for metazoans. A seawater δ18O value of −2‰ SMOW yields temperatures ranging from 23 to 31 °C, while a −3‰ SM

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-2.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTTrapezoidal‐type fan deltas, lacking bottomset deposits, were studied in two different tectonic settings: extensional (rifted), and compressional (piggy‐back) basins. In both cases studied fan deltas were characterized by: (i) an absence of bottomsets; (ii) development in protected or narrow basins and sub‐basins confined by intrabasinal basement highs or by topographic highs, respectively; (iii) coarse‐grained sediment fluxes, dominated by mass‐flows forming fan deltas that prograded from steep nearshore slopes basinwards; and (iv) a high‐energy environment, with powerful underflows that probably bypassed the basins and transported fine‐grained sediments outside the basins. The location of channels cut by such underflows is influenced by local tectonic style. When the supplied sediments in the rifted basins overstepped the intrabasinal basement highs, trapezoidal fan deltas were replaced upwards by Gilbert‐type deltas,

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-3.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTUppermost Tortonian to lower Messinian temperate carbonates crop out in the Agua Amarga Basin (SE Spain). They consist of four units. The lower three units can be tentatively assigned to the lowstand systems tract of a fourth‐order sequence, constituting in turn the lowstand (‘megatrough unit’), transgressive (‘breccia unit’) and highstand (‘bedded unit’) stages of a higher‐order cycle. All these materials were deposited in a small pull‐apart basin related to the sinistral Carboneras strike‐slip fault system. The best represented is the bedded unit (up to 25 m thick), which consists of bioclastic, bryozoan/bivalve‐dominated calcarenites/calcirudites with abundant fragments of echinoids, barnacles, benthic foraminifers, coralline algae, brachiopods and solitary corals. Facies trends within this unit are roughly arranged in an E‐W direction, with the coastline to the north of the basin. The depositional model is that of a gentle ramp with prograding beaches and shoals in its higher parts. Seaward of the shoals was the ‘factory area’, where most organisms lived and maximum carbonate production took place. From this area some of the skeletons were washed landwards by waves and/or currents during storms and incorporated into the shoals and beaches, and others moved downslope along the ramp as mass‐flows, accumulating to form the ‘fan‐bedded zone’.The factory‐area and fan‐bedded sediments intercalate five well‐defined, thick beds of calcarenites/fine‐grained calcirudites. They show bar morphologies (single or amalgamated), or make up sand‐waves with very consistent tabular cross‐bedding pointing landwards. These beds formed in a very shallow, wave/current‐influenced, coastal environment. The bars and sand waves in the fan‐bedded zone developed during lowstands, while those located higher up in the ramp interbedded with the factory facies are related to transgressive stages. Prograding beaches, shoals, factory facies and fan‐bedded layers developed during the highstands. Net skeletal production occurred mainly during the highstands.Sediment‐accretion values of these sediments are similar to those of present and ancient shallow‐marine, temperate carbonates considering that the whole bedded unit was deposited in a 100 000‐year interval (equivalent to the short eccentricity cycle). The five cycles inside the bedded unit would therefore correspond to

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-4.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe effects of liquefaction in saturated sand bodies under a variety of driving forces are described from shaking table experiments, and structures from the geological record are presented which are analogous to the experimental structures. The collapse of sloping heaps of cross‐bedded sand under a gravitational body force generates low‐angle, essentially uncontorted stratification. A basal zone of shearing may be present, with steepened and folded foresets. Stretching of foresets may be accommodated on normal faults, and bottomsets may be contorted into inclined folds. In natural systems the substrate may also liquefy, causing deformation driven by an unevenly distributed confining load. Stratification in the surface bedform is flattened, and stratification in the substratum contorted. Experiments failed to produce relative displacement at the interface between stacked sand bodies. Liquefaction of gravitationally unstable systems in sands generates load structures comparable to those from sand‐mud systems. Recumbent‐folded deformed cross‐bedding is formed by current shear over a liquefied bed, as has been inferred from field and theoretical analyses. Shear of nonliquefied sand forms angular folds. Other deformation mechanisms, such as fluidization or seepage, may generate structures similar to all of these. Local water‐escape structures driven by fluidization occur in the upper parts of some liquefied sand bodies. They include cusps, sand volcanoes and clastic dykes. Transient cavities formed in some experiments and seemed to be preserved as breached cusps. Although the experiments tried to isolate individual driving forces, driving forces may operate together, and there may be a continuum between deformation driven by water escape and deformation driven by loading. Different structures from those described here may form where liquefaction develops in a buried layer as opposed to at the sedim

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-5.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe Late Permian Bainmedart Coal Measures form part of the Permo‐Triassic Amery Group, which crops out in the Beaver Lake area of the Northern Prince Charles Mountains, MacRobertson Land, Antarctica. The exposed strata are believed to have formed in graben or half‐graben sub‐basins on the western edge of the Lambert Graben, a major failed rift system. Sedimentological analysis has revealed that these rocks formed in alluvial environments in which swiftly flowing rivers of low sinuosity (represented by Facies A1 and A2) flowed northward down the axis of the basin, and were associated with waterlogged floodbasin and peat‐forming wetlands (Facies B1‐B4). A third Facies Association (comprising Facies C1‐C3), interpreted as the deposits of lake floor and delta environments, is exclusively developed within a distinctive, fine‐grained interval here named the Dragon's Teeth Member. The proportion of Association B facies within the succession increases markedly above the level of the Dragon's Teeth Member (at about 300 m above the base of the formation).Flat, low‐angle and undulatory bedding structures preserved within channel deposits are suggestive of sediment deposition in flow conditions which were often critical or supercritical. Presence of massive and chaotic intervals of sandstone further implies some deposition from high‐concentration aqueous flows. Alluvial channel bodies show evidence of incision into underlying substrates, both during initiation and at later stages in channel belt construction. The lack of interfingering between channel deposits and coals suggests that thick peats formed only in areas and at times of minimal clastic sediment supply.Analysis of well‐developed cyclicity within the coal measures suggests that the dominant control on sequence architecture was climatic, related to precessional Milankovitch fluctuations ofc.19‐kyr periodicity. Cycles began abruptly with the deposition of coarse‐grained material in high‐energy alluvial channels, which contracted with time in response to changes in water supply (rainfall). Upper parts of cycles are dominated by finer‐grained sediments and then coal, indicative of progressively reduced coarse sediment input. Tectonic processes overprinted this pattern at least once during the period of sediment accumulation, to for

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-6.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTPseudohexagonal aragonite crystals are common components in some hot‐spring travertines at Chemurkeu on the western shore of Lake Bogoria, Kenya. Beds, lenses and pods of aragonite crystals are intercalated with beds of white non‐crystallographic calcite dendrites. The pseudohexagonal aragonite crystals, which are up to 4 cm long and 4 mm wide, are formed of nested skeletal crystals. Each skeletal crystal is formed of cyclical twinned crystals that are constructed of stacked subcrystals. The latter are inclined at a consistent angle of 40° to the long axis of the pseudohexagonal aragonite crystal. Intense competition for space during growth modified the crystal morphology with the result that many of the pseudohexagonal crystals are distorted. Intercrystalline and intracrystalline pores are filled or partly filled by epitaxial aragonite overgrowths and/or reticulate microbial coatings that have a high concentration of Si and Mg. In places, this extracellular mucus induced etching of the underlying aragonite crystal.Today the hot (T>95 °C) Na‐HCO3‐Cl spring waters at Chemurkeu have a salinity of 5–6 g L−1TDS, a pH of 8·1–9·1, Ca2+concentrations of<2 mg L−1and Mg2+concentrations of<0·7 mg L−1, The springs of the Lake Bogoria Geothermal Field are fed by a shallow aquifer (T∼100 °C) and a deeper aquifer (T∼170 °C). Springs at Chemurkeu derive from meteoric groundwater, lake water and condensed steam, and are fed mainly from the shallow thermal aquifer. Much of the aragonite may have formed when the spring waters contained more dissolved Ca2+than today, possibly under more humid co

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-7.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTComminution in the glaciers that debouch into Guys Bight Basin, followed by selective sorting in the fluvial system, has had little effect on the bulk composition, or on the mineralogy, of the basin sands and muds. Most striking are the feldspar contents, and the feldspar‐quartz ratios in sands and muds, both of which remain similar to those of average bedrock. The feldspar contents of sands and muds range from 48 to 52% feldspar whereas average bedrock contains 51·7% feldspar. Feldspar‐quartz ratios average 1·58:1 in bedrock and 1·54:1, 1·66:1 and 1·69:1 in the medium to coarse sands, fine sands and muds, respectively, indicating minimal feldspar enrichment in the fine‐grained sediments. In the absence of appreciable chemical weathering, extreme abrasion followed by hydraulic sorting has not produced mature sediments such as quartz arenites and clay‐mineral‐rich muds. There is, however, some chemical differentiation. Preferential accumulation of mafic minerals in fine sands and muds is reflected in bulk compositions by higher abundances of MgO, FeO and TiO2, and in the mineralogy by enrichment of biotite in the fine grades. Bulk compositional studies focused solely on muds and mudstones will result in an overestimate of the mafic contribution from source rocks. This work shows that bulk compositional studies of sediments and sedimentary rocks should include all available granul

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-12.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTHolocene aeolian silts deposited on the Cape Verde Islands provide information about the origin of African palaeodusts that have fallen on the north‐eastern Atlantic ocean over the last 10 000 years. Sedimentological composition indicates that most of these aeolian silts are unquestionably of continental origin. Their Sr and Nd isotopic composition identifies a Saharan origin‐suggesting transport by Harmattan winds. We estimate that Saharan dust comprises 75–95% of material in these Holocene silts, the rest coming from the weathering of local basaltic be

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-8.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY