摘要:

ABSTRACTMixed carbonate‐siliciclastic sediment gravity flow deposits of Late Pennsylvanian to Early Permian age are exposed in the Death Valley ‐ Owens Valley region of east‐central California. The Mexican Spring unit constitutes the upper part of the Keeler Canyon Formation and is characterized by turbidites, debris flow deposits and megabreccias, all of mixed carbonate‐siliciclastic composition. The mixed composition of the Keeler Canyon Formation provides an opportunity to link facies architecture to controls on depositional system development. Depositional relationships indicate that the deposits represent a non‐channellized base of slope carbonate apron system with inner, outer and basinal facies associations. These gravity flow deposits are characterized by repeated stacked, small scale (100 m) coarsening and thickening upward cycles.Contemporaneous outer shelf and upper slope deposits of the Tippipah Limestone are exposed at Syncline Ridge on the Nevada Test Site. The deposits consist of carbonate buildups directly overlain by cross bedded, quartz‐rich sandstone and conglomerate which filled channels that traversed across the previously existing carbonate shelf. Detritus was transported to the west, down the upper slope by gully systems that fed the temporally persistent base of slope apron of the upper part of the Keeler Canyon Formation. This style of deposition differs from point‐sourced siliciclastic submarine fan depositional systems. However, the Keeler Canyon system has lithofacies similar to some sandy siliciclastic turbidite systems, such as the delta‐fed submarine ramp facies model, which is a line‐sourced, shelf‐fed system that is not supply limited.The mixed clastic apron systems of the Keeler Canyon Formation differ from classical carbonate aprons in that the former is characterized by an abundance of sedimentary cycles. Controls on the development of these cycles and of the facies distribution may have resulted from changes in type and rate of sediment supply, relative sea level changes and/or tectonic events. Interpretation of the data is focused on relative changes in sea level as the most significant control on development of the depositional system. Relative sea level changes serve two important functions: (1) they provide a mechanism for bringing coarse siliciclastic and bioclastic grains together on the outer shelf, and (2) shelf margin collapse may be initiated during relative lowstands allowing for transport of the sediment to the deep basin and development of deep basinal cycles. Therefore, an abundance of mixed clastic gravity flow deposits such as these in the rock record may be an indicator of periods of high frequency changes in relative sea level, which is a characteristic of Late Palaeozoi

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01389.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTAlmost 90% of 39 m of core material recovered from Scoresby Sund and the adjacent East Greenland shelf is massive diamicton, interpreted to be formed predominantly by the release of iceberg rafted debris and reworking by iceberg scouring. There is also likely to be a contribution from suspension settling of fines derived from glaciofluvial sources. Model calculations suggest that the14C derived Holocene sedimentation rate of 0.1‐0.3 m 1000 yr−1in Scoresby Sund can be accounted for mainly by iceberg rafting of debris. A further 4% of core material is of gravel or coarse sand lenses, interpreted to reflect iceberg dumping of debris. Intensive iceberg scouring, which reworks sea floor sediments, is observed on acoustic records from over 30 000 km2of the Scoresby Sund fiord system and the adjacent East Greenland shelf (69‐72°N and 75°N). The rate of iceberg production from Greenland Ice Sheet outlet glaciers, and iceberg drift tracks on the shelf, suggests that iceberg rafting and scouring may be important over a significant proportion of the 500 000 km2area above the shelf break. The relatively extensive modern occurrence of massive diamicton, formed by iceberg rafting and scouring, together with suspension settling of fines, suggests that it may also be a significant facies in the glacier‐influenced geological record. The recognition in the geological record of the massive diamicton facies described above may also indicate the former presence of fast flowing ice sheet outlet

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01390.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTSubaqueous tuff deposits within the lower Miocene Lospe Formation of the Santa Maria Basin, California, are up to 20 m thick and were deposited by high density turbidity flows after large volumes of ash were supplied to the basin and remobilized. Tuff units in the Lospe Formation include a lower lithofacies assemblage of planar bedded tuff that grades upward into massive tuff, which in turn is overlain by an upper lithofacies assemblage of alternating thin bedded, coarse grained tuff beds and tuffaceous mudstone. The planar bedded tuff ranges from 0.3 to 3 m thick and contains 1‐8 cm thick beds that exhibit inverse grading, and low angle and planar laminations. The overlying massive tuff ranges from 1 to 10 m thick and includes large intraclasts of pumiceous tuff and stringers of pumice grains aligned parallel to bedding. The upper lithofacies assemblage of thin bedded tuff ranges from 0.4 to 3 m thick; individual beds are 6‐30 cm thick and display planar laminae and dewatering structures. Pumice is generally concentrated in the upper halves of beds in the thin bedded tuff interval.The association of sedimentary structures combined with semi‐quantitative analysis for dispersive and hydraulic equivalence of bubble‐wall vitric shards and pumice grains reveals that particles in the planar bedded lithofacies are in dispersive, not settling, equivalence. This suggests deposition under dispersive pressures in a tractive flow. Grains in the overlying massive tuff are more closely in settling equivalence as opposed to dispersive equivalence, which suggests rapid deposition from a suspended sediment load. The set of lithofacies that comprises the lower lithofacies assemblage of each of the Lospe Formation tuff units is analogous to those of traction carpets and subsequent suspension sedimentation deposits often attributed to high density turbidity flows. Grain distributions in the upper thin bedded lithofacies do not reveal a clear relation for dispersive or settling equivalence. This information, together with the association of sedimentary features in the thin bedded lithofacies, including dewatering structures, suggests a combination of tractive and liquefied flows.Absence of evidence for elevated emplacement temperatures (e.g. eutaxitic texture or shattered crystàls) suggests emplacement of the Lospe Formation tuff deposits in a cold state closely following pyroclastic eruptions. The tuff deposits are not only a result of primary volcanic processes which supplied the detritus, but also of processes which involved remobilization of unconsolidated ash as subaqueous sediment gravity flows. These deposits provide an opportunity to study the sedimentation processes that may occur during subaqueous volcaniclastic flows and demonstrate similarities with existing models for sediment gravity flow p

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01391.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Lower Triassic succession of Barles, Alpes de Haute Provence, France, comprises an unconformable quartz arenite sand body of 90m thickness. The succession may be informally divided into (i) lower channellized cross‐bedded member overlain by (ii) an upper fining upward member. The lower member comprises vertically stacked, subtidal channel units separated into five major sand bodies by thin developments of fine grained channel margin and shoal deposits. Subtidal channel fill deposits are dominated by varying scales of cross bedding. These scales vary systematically from the base to the top of the member, with large scale planar sets dominating the lowest channel sand body (sand body 1), medium scale planar and trough cross bedding characterizing sand bodies 2‐4, the largest scale planar sets in the highest sand body (sand body 5). This upward change in cross bedding scale is concomitant with a decrease in both the relief of major channel sand body erosion surfaces, and the proportion of preserved interchannel shoal deposits. The succeeding fining upward member comprises small scale tidal channel units overlain by channel shoal and tidal flat deposits. Tidal flat sequences are characterized by parallel laminated, wave and current rippled sandstones separated by bioturbated, fine grained siltstones and mudstones.The vertical variation in facies of the Lower Triassic succession suggests two main periods of deposition. The lower member is considered to preserve successively more seaward components of a transgressive estuarine complex. The overlying upper member records the seaward progradation of tidal channel, shoal and tidal flat environments.The unconformity bounded nature of the lower member, combined with its systematic variation in facies, suggests it may represent an incised valley‐estuarine fill developed in response to an early Triassic relative sea level fall and subsequent rise. Succeeding tidal channel and tidal flat deposits forming the upper fining upward member reflect a change in sediment supply and/or rate of relative sea level rise comparable with a progradational shoreline. It is unclear whether this final depositional episode represents a period of highstand progradation or a later lowstand shoreline system developed following a further period of relative sea level fall and

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01392.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Lower Werra Anhydrite (Zechstein, Upper Permian) deposits of the teba area originated in a deep basin setting, in shallow to deep water conditions. Facies changes occur within small distances and suggest fluctuating boundaries between well defined basins and platforms. This pattern of local platforms and adjacent basins developed during deposition. In basinal areas, the sequence is clearly transgressive, whereas on platforms accumulation kept pace with subsidence after an initial transgression.Nodular anhydrite represents a polygenetic deposit which formed at different times with respect to deposition. Massive anhydrite with pseudomorphs after upright‐growth gypsum crystals suggest rapid precipitation in a subaqueous environment and/or fluctuating, but generally high, salinity conditions. Massive clastic sulphate originated due to periodic high energy events and resedimentation, or due to brecciation possibly connected with salinity fluctuations and the dissolution of halite. Massive, textureless anhydrite is locally porous and passes upward into breccia, indicating a strongly saline environment. Bedded anhydrite is considered to form in shallow water environments and laminated anhydrite in deep water. Bedded anhydrites contain portions which are graded. Intercalations of sulphate turbidites and upright‐growth gypsum suggest fluctuating water depths, with comparatively deep water during turbidite deposition, but shallower conditions during upright‐growth gypsum deposition.The sequence observed in slope zones at platform‐basin margins, detrital (parautochthonous) sulphate sand to graded beds to basinal laminites, indicates that redistribution processes were important. At the onset of the Lower Werra Anhydrite deposition bathymetric relief existed between the central part of the basin and its margins, where carbonate platforms remained subaerially exposed. Formation of local platforms and adjacent basins required a relatively high subsidence rate, as pre‐existing relief cannot account for the total accumulated thickness of the Lower Werra Anhydrite deposits. One implication of this is that the main argument against ‘the shallow water ‐ shallow basin’ evaporite basin model, i.e.,a very fast rate of subsidence, may not be valid for the Łeba Lower Werra

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01393.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTStudies, spanning a 3 year period, of Westphalian C strata exposed in an active quarry have enabled three dimensional reconstruction of a lacustrine delta complex. The sequence exhibits a complex history of lake infilling by sediment introduced by intermittent high energy, low sinuosity distributary channel flows.Deposition in the small 0.2 km2lake prior to delta formation was dominated by organic matter and typified rheotrophic swamp conditions. Large lycopods colonized the swamp floor. The lake was filled by a prograding delta which comprised six horizontally and vertically stacked delta lobes. The presence of lycopods aided sediment accumulation.Fluctuations in river discharge and consequent lake level rise and fall exerted a fundamental control on delta progradation and aggradation. Flooding during low lake levels first produced erosion on the existing lobe followed by a lake level rise which created accommodation for aggradation of a new delta lobe. Further lobe erosion and low stand lobe formation occurred during lake level fall.The area's proximity to alluvial fans resulted in hyperconcentrated flood flow within the distributary channels. The occurrence upon in‐channel surfaces of plant colonization, including lycopods, testifies to the ephemeral nature of the flow. The lacustrine delta complex formed at the front of a terminal alluvial fan. Northward progradation of the alluvial fan was achieved by the capture and infilling of lakes by northerly flowing distributary channel

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01394.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTUpper Visean limestones in the Campine Basin of northern Belgium are intensively fractured. The largest and most common fractures are cemented by non‐ferroan, dull brown‐orange luminescent blocky calcite.First melting temperatures of fluid inclusions in these calcites are around ‐57°C, suggesting that precipitation of the cements occurred from NaCl‐CaCl2‐MgCl2fluids. The final melting temperatures (Tmice) are between ‐5 and ‐33°C. The broad range in theTmicedata can be explained by the mixing of high salinity fluids with meteoric waters, but other hypotheses may also be valid. Homogenization temperatures from blocky calcite cements in the shelf limestones are interpreted to have formed between 45 and 75°C. In carbonates which were deposited close to and at the shelf margin, precipitation temperatures were possibly in the range 70‐85°C and 72‐93°C, respectively.On the shelf, the calcites have a δ18O around ‐9.3‰ PDB and they are interpreted to have grown in a fluid with a δ18O between −3.5 and +1.0‰ SMOW. At the shelf margin, blocky calcites (δ18O∼ ‐ 13.5‰ PDB) could have precipitated from a fluid with a δ18O betweenn ‐4.0 and ‐1.1‰ SMOW. The highest oxygen isotopic compositions are comparable to those of Late Carboniferous marine fluids (δ18O= ‐ 1‰ SMOW). The lowest values are more positive than a previously reported composition for Carboniferous meteoric waters (δ18O= ‐7‰ SMOW). Precipitation is likely to have occurred in marine‐derived fluids, which mixed with meteoric waters sourced from near the Brabant Massif. Fluids with a similar negative oxygen isotopic composition and high salinity are actually present in Palaeozoic formations. The higher temperature range in the limestones near the shelf margin is explained by the upward migration of fluids from th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01395.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTSediments and diatoms from the mudflats of the Bay of Bourgneuf in western France were examined in an electron microscope study of biofilms and microbial mats. The sediments were kept in an aquarium for study and a diatom culture was made of the benthic diatoms. The sediment biofilm was composed of exopolymeric substances (EPS), incorporated clay particles and, rarely, bacteria. This film coated all particles at the sediment‐water interface. Its surface morphology reflected its composition and internal structure. Thin films were smooth, whilst a lumpy structure or incorporated fibrils produced either a mammillated or ropy surface, and clays in the structure gave rise to a flaky morphology. At shallow depths in the sediment column (0.5 cm) the biofilm was already degraded. The biofilm coating degraded diatom frustules in the benthic diatom culture consisted of EPS and bacteria and presented a ragged appearance.Microbial mats occurred on the surface of the fresh littoral sediments as well as those in the aquarium, and on the wall of the aquarium. The mat on the surface of the aquarium sediments had an open structure with webs of fibrils and bacteria in the pore space. It formed in a relatively quiet environment. Pore space was more limited in the mat from the surface of the fresh littoral sediments, in which direct contact between biofilm coated particles was common. In the exposed environment of the aquarium wall there was a thick, resistant coating of EPS.In addition to binding particles together, the presence of mats and biofilms in sediments affects sediment physical properties such as porosity and permeability, the flux of dissolved substances in pore waters and the dissolution of particles and can, therefore, influence early diagenesis. Mats and biofilms seem to be more readily preserved in the geological record than the micro‐organisms, such as bacteria, which produce them. Their identification in the sedimentary record would greatly aid interpretation of sediment genesis and evaluation of the microbial role in sediment format

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01396.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTHigh resolution seismic data, multichannel seismic data and sediment cores were used to examine the Songwe Sequence, the uppermost of four depositional sequences identifiable on multichannel seismic data from Lake Malawi (Nyasa). The sequence has a maximum thickness of about 115 m in two areas of the lake, but is typically less than 70 m thick over most of the basin. The sequence is distributed along the entire length of the 560 km long lake, and is concentrated in three main depocentres.14C age dates from sediment piston cores are extrapolated to provide an age estimate of about 78 000 yrbpfor the oldest sediments within the Songwe Sequence. In the North and Central bathymetric basins of the lake, high resolution seismic data indicate a dynamic depositional environment, dominated by turbidity and mass flow deposits. Seismic data from the southern basin show acoustically transparent sediments with relatively low amplitude internal reflections, indicative of pelagic and hemipelagic sedimentation. In many areas the Songwe Sequence is underlain by a pronounced angular unconformity, suggestive of a significant, prolonged, low lake stage prior to deposition of the sequence. Seismic reflectors within the Songwe Sequence can be correlated to younger low lake stages identified from sediment core data. Major late Quaternary low lake level stages in Lake Malawi, interpreted from features identified in the seismic data and sediment core analyses, are tentatively interpreted at 6000 to 10 000 yrbp, 28 000 to>40 000 yrbp, and prior to 78 000 yrbp. Budget calculations indicate mean sediment concentrations from catchment runoff during the period of deposition of the Songwe Sequence to be about 190 mg 1−1, comparable to estimates of modern rainy season discharges from the major river systems. Erosion rates within the drainage basin are estimated to be higher than the African average by a factor of three or more, probably due to the high relief within the Lake Malawi catchmen

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01397.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Sedimentary Basins of North Greenland, Ed. by John S. Peel and Martin SøderholmGlacigenic Sediments, by K. Brokzikowski and A. J. van Loo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01398.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY