Lake Hoare, Antarctica: sedimentation through a thick perennial ice cover   总被引：7，自引：0，他引：7  

STEVEN W. SQUYRES  DAVID W. ANDERSEN  SUSAN S. NEDELL  ROBERT A. WHARTON  JR 《Sedimentology》1991,38(2):363-379

Lake Hoare in the Dry Valleys of Antarctica is covered with a perennial ice cover more than 3 m thick, yet there is a complex record of sedimentation and of growth of microbial mats on the lake bottom. Rough topography on the ice covering the lake surface traps sand that is transported by the wind. In late summer, vertical conduits form by melting and fracturing, making the ice permeable to both liquid water and gases. Cross-sections of the ice cover show that sand is able to penetrate into and apparently through it by descending through these conduits. This is the primary sedimentation mechanism in the lake. Sediment traps retrieved from the lake bottom indicate that rates of deposition can vary by large amounts over lateral scales as small as 1 m. This conclusion is supported by cores taken in a 3 × 3 grid with a spacing of 1.5 m. Despite the close spacing of the cores, the poor stratigraphic correlation that is observed indicates substantial lateral variability in sedimentation rate. Apparently, sand descends into the lake from discrete, highly localized sources in the ice that may in some cases deposit a large amount of sand into the lake in a very short time. In some locations on the lake bottom, distinctive sand mounds have been formed by this process. They are primary sedimentary structures and appear unique to the perennially ice-covered lacustrine environment. In some locations they are tens of centimetres high and gently rounded with stable slopes; in others they reach ～ 1 m in height and have a conical shape with slopes at angle of repose. A simple formation model suggests that these differences can be explained by local variations in water depth and sedimentation rate. Rapid colonization and stabilization of fresh sand surfaces by microbial mats composed of cyanobacteria, eukaryotic algae, and heterotrophic bacteria produces a complex intercalation of organic and sandy layers that are a distinctive form of modern stromatolites.  相似文献   

Depositional framework and controls on mixed carbonate-siliciclastic gravity flows: Pennsylvanian-Permian shelf to basin transect, south-western Great Basin, USA   总被引：1，自引：0，他引：1  

ROBERT P. MILLER  PAUL L. HELLER 《Sedimentology》1994,41(1):1-20

Mixed 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 (<15 m) coarsening and thickening upward cycles with superimposed medium 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 Palaeozoic sea level history.  相似文献   

Origin and diagenesis of Quaternary travertine shrub fabrics, Rapolano Terme, central Italy   总被引：3，自引：0，他引：3  

LI GUO  ROBERT RIDING 《Sedimentology》1994,41(3):499-520

Millimetre to centimetre sized arborescent shrub-like calcite precipitates are common constituents of hot water travertine shallow pool deposits of Quaternary age at Rapolano Terme, Tuscany, Italy. In the presently forming travertines, the shrubs consist of apparently random associations of (i) micrite aggregates and (ii) subhedral to euhedral rhombic spar crystal aggregates. In thin section, the micrite aggregates appear dark and the spar-rhomb aggregates light, giving the shrubs a mottled appearance. Travertines are basically produced by CaCO₃ precipitation due to degassing and evaporation of the spring waters, although biological influence may also stimulate precipitation. The formation of masses of erect shrubs, rather than dense crystal crusts that form on slopes, is probably due to limited water flow in the pool environments. Microbes, including bacteria and diatoms, are important influences on shrub microfabric and external shape. The micrite aggregates are associated with bacteriform bodies, seen as tiny rods and spheres. The micrite precipitates around these bodies and in adjacent biofilm. Spar-rhomb precipitation appears to be external to the biofilm, and may be related to the presence of diatoms which are locally closely associated with the spar-rhombs, although an essentially inorganic origin, particularly for the more euhedral rhombs, cannot be ruled out. In the older Quaternary travertines, the original microfabric of the shrubs has been diagenetically altered. The original mottled appearance of the shrubs has become uniformly dark and micritic, and the evidence for the dual micritic and spar-rhomb origin of the shrubs is obscured or destroyed. Spar-micritization of the shrubs is probably due to abiotic, and locally biotic, dissolution. Previous studies did not recognize the diagenetic micritization and attributed shrub formation entirely to bacterial activity.  相似文献   

High energy coralgal-stromatolite frameworks from Holocene reefs (Tahiti, French Polynesia)   总被引：6，自引：0，他引：6  

GILBERT F. CAMOIN  LUCIEN F. MONTAGGIONI† 《Sedimentology》1994,41(4):655-676

Drill cores from Holocene reefs on Tahiti (French Polynesia) reveal a framework composed of massive branching acroporids encrusted by coralline algae associated with sessile vermetid gastropods and arborescent foraminifers. Laminated micritic crusts form coatings over coral branches or, more commonly, over related encrusting organisms throughout the cored reef sections; these crusts appear as a major structural and volumetric component of the reef framework. The microbial nature of these micritic crusts is inferred from their typical organic growth forms and geometry, the occurrence of microbial remains and stable isotope measurements. The reef communities accumulated at depths less than 5 m below mean sea level in a high energy environment throughout vertical growth from 7140 ± 170 yr bp to the present. The nature of the involved benthic communities, stable isotope data and high calcification rates of microbially encrusted corals strongly suggest that local environmental conditions have been optimal for reef development for the last 7000 years. The causes of the predominance of microbial communities over actual encrusters (red algae, foraminifers) remain problematic and could be related to short term fluctuations in ecological parameters. Microbial micritic crusts seemingly played a prominent role in protecting the coralgal colonies from bioeroders and grazers and, possibly, in strengthening the framework, due to rapid lithification. The record of similar microbial crusts in other Quaternary reef tracts suggests that microbial communities may have played a more prominent role in Quaternary reefs than presently recognized.  相似文献   

Re-evaluation of coquinoid sandstone depositional model, Upper Jurassic of central Wyoming and south-central Montana     

ROBERT L. BRENNER  DONALD J. P. SWIFT†  G. C. GAYNOR‡ 《Sedimentology》1985,32(3):363-372

The most extensive Jurassic marine transgression in North America reached its maximum limits during the Oxfordian Age. At this time, siliciclastic sediments were being brought into the North American seaway from an uplifted zone to the west. Within this setting, complexes of sand ridges and coquinoid sands layers were deposited. Coquinoid sandstones appear to fill erosional scours and were interpreted as channel fills. Re-evaluation of these features in the light of recently discovered attributes of modern shelf sediments and processes has produced a revised model of coquinoid sand deposition in this setting. Coquinoid sandstones which fill ‘channel-like’ scours in the Oxfordian (Upper Jurassic) rocks of central Wyoming and south-central Montana, appear to have formed through the migration of sand waves across the crests of inner shelf sand ridges during periods of storm and tidal flow. Erosion in the zone of flow reattachment in the troughs between sand waves resulted in the development of shell lags. Migration of these scour zones as the sand waves advanced resulted in the deposition of sheet-like coquinoid sandstone bodies. Sand waves crossing the ridge crest tended to migrate more slowly and to be overstepped by later sand waves. Sand wave troughs thus buried have channel-like geometries with apparent epsilon bedding.  相似文献   

Element mobility during alteration of silicic ash to kaolinite—a study of tonstein     

ROBERT A. ZIELINSKI 《Sedimentology》1985,32(4):567-579

A laterally persistent kaolinitic mudstone parting (tonstein) occurring in a Wyoming coal bed of Eocene age was sampled to estimate the compositional contrast with a probable silicic ash precursor, and to determine the compositional influence of leached ash on immediately adjacent coal. With the exception of Al, and possibly Ti, the tonstein is highly to moderately leached of major elements, relative to a range of compositions estimated for silicic ash of the region. In agreement with the behaviour of geochemically similar major elements, alkali trace elements (Rb, Cs) are highly leached, transition-series metals moderately leached, and Ga is residual. Additional immobile trace elements are Zr and Hf but some other trace elements that are considered relatively immobile during low-temperature alteration (Th, Ta, Nb, REE, Y) were apparently leached by the low-Eh, low-pH, organic-rich pore fluids of the coal-forming swamp. The migrational range of many of the leached elements is highly limited by the intimate association of tonstein with enclosing organic matter. Mixtures of coal + minor tonstein that occur within 20 mm of contacts are consistently enriched in some elements relative to either tonstein or nearby coal (U, Cu), or relative to calculated mixtures of tonstein and coal in their measured proportions (Th, Y, REE, Pb, Ba, V, Ti). Direct observations by fission-track radiography and electron microprobe indicate a preference of U and Fe for the organic component of mixtures. Metal fixation is probably by adsorption on insoluble organic matter (humic acids), or by precipitation as minor sulphides in these low-sulphur coals.  相似文献   

The importance of maintaining horizontal sieve screens when using a Ro-Tap     

ROBERT METZ 《Sedimentology》1985,32(4):613-614

Sieve size analysis, using a Ro-Tap shaking machine, for horizontal and non-horizontal sieve screen nests are compared. The non-horizontal sieve condition results in more sediment being retained on the topmost sieve, with less sediment reaching certain sieves within the nest. It is thus essential to maintain the nest of sieves perfectly horizontal.  相似文献   

Geomorphic evolution and stratigraphy of Price and Capers Inlets, South Carolina     

ROBERT S. TYE 《Sedimentology》1984,31(5):655-674

A three-dimensional model for a tidal inlet-barrier island depositional system was constructed through examination of 37 vibracores and 10 auger drill holes on Capers and Dewees Islands, South Carolina. Two cycles of southerly inlet migration and subsequent abandonment resulted in beach ridge truncation on the northern ends of both barriers. Historical evidence indicates that these tidal inlets migrated 1.5 km to the south owing to a dominant north-south longshore transport direction. The hydraulic inefficiency of these over-extended inlet channels caused shorter, more northerly-oriented channels to breach through the ebbtidal deltas. After inlet reorientation, large wave-formed swash bars migrated landward closing former inlet channels. Weakened tidal currents through the abandoned channels permitted clay plugs to form thick impermeable seals over active channel-fill sand and shell. Price and Capers Inlets formed during the onset of the Holocene transgression following submergence of the ancestral Plio-Pleistocene Santee River drainage system. Coarse, poorly sorted inlet-deposited sand disconformably overlies Pleistocene estuarine clay and is capped by a dense clay plug. Shoreline reorientation and landward retreat of a primary barrier island chain occurred between the first and second cycles of inlet-channel migration and abandonment. Beach ridges prograded seaward over the first inlet sequence. A second cycle of inlet migration truncated the northernmost portion of these beach ridges and scoured into the clay plug of the earlier inlet deposit. Abandonment of this channel resulted in deposition of a second abandoned inlet-channel clay plug. Abandoned tidal inlet channels exhibit U-shaped strike and crescentic- to wedge-shaped dip geometries. Basal, poorly sorted inlet sands are sealed beneath impermeable, abandoned-channel silt and clay, washover deposits, and salt marsh. Multiple episodes of inlet migration and abandonment during a rising sea-level deposited stacked inlet-fill sequences within the barrier islands. The resultant stratigraphy consists of interlayered, fining-upward, active inlet-fill sand overlain by thicker abandoned inlet-fill clay plugs. These clay plugs form impermeable zones between adjacent barrier island sand bodies. Shoreline transgression would remove the uppermost barrier island deposits, sealing the inlet-fill sequences between Pleistocene estuarine clay and shoreface to shelf silt and clay.  相似文献   

Evolution and stratigraphy of a regressive barrier/backbarrier complex: Kiawah Island, South Carolina     

AILEEN WOJTAL DUC  ROBERT S. TYE† 《Sedimentology》1987,34(2):237-251

Analysis of 75 vibracores from the backbarrier region of Kiawah Island, South Carolina reveals a complex association of three distinct stratigraphic sequences. Beach ridge progradation and orientation-controlled backbarrier development during the evolution of Kiawah Island, and resulted in deposition of: (1) a mud-rich central backbarrier sequence consisting of low marsh overlying fine-grained, tidal flat/lagoonal mud; (2) a sandy beach-ridge swale sequence consisting of high and low marsh overlying tidal creek channel and point bar sand, and foreshore/shoreface; and (3) a regressive sequence of sandy, mixed, and muddy tidal flats capped by salt marsh that occurs on the updrift end of the island. Central backbarrier deposits formed as a result of the development of the initial beach ridge on Kiawah Island. Formation of this beach ridge created a backbarrier lagoon in which fine-grained estuarine and tidal flat mud accumulated. Washovers, oyster mounds, and tidal creek deposits form isolated sand and/or shell-rich lenses in the lagoon. Spartina alterniflora low marsh prograded into the lagoon as the tidal flats aggraded. Barrier progradation and sediment bar-bypassing at Stono Inlet created digitate beach ridges on the northeast end of Kiawah Island. Within the beach-ridge swales, tidal flats were disconformably deposited on shoreface and foreshore sand of the older beach ridges. Tidal creek drainage systems evolved to drain the swales. These rapidly migrating creeks reworked the tidal flat, foreshore, and shoreface sediments while redepositing a fining-upward sequence of channel lag and point bar deposits, which served as a substrate for salt marsh colonization. This resultant regressive sedimentary package marks the culmination of barrier island development and estuary infilling. Given enough time and sedimentation, the backbarrier sequence will ultimately prograde over the barrier island, reworking dune, beach, and foreshore sediments to form the upper sand-rich bounding surface of the barrier lithosome. Preservation of the regressive sequence is dependent upon sediment supply and the relative rate of sea-level rise, but the reworking of barrier islands by tidal inlets and migrating tidal creeks greatly alter and complicate the stratigraphic sequence.  相似文献   

Shelf construction in a foreland basin: storm beds, shelf sandbodies, and shelf-slope depositional sequences in the Upper Cretaceous Mesaverde Group, Book Cliffs, Utah     

DONALD J. P. SWIFT  PETER M. HUDELSON  ROBERT L. BRENNER  PETER THOMPSON 《Sedimentology》1987,34(3):423-457

The Upper Cretaceous (Campanian) Kenilworth Member of the Blackhawk Formation (Mesaverde Group) is part of a series of strand plain sandstones that intertongue with and overstep the shelfal shales of the western interior basin of North America. Analysis of this section at a combination of small (sedimentological) and large (stratigraphical) scales reveals the dynamics of progradation of a shelf-slope sequence into a subsiding foreland basin. Four major lithofacies are present in the upper Mancos and Kenilworth beds of the Book Cliffs. A lag sandstone and channel-fill shale lithofacies constitutes the thin, basal, transgressive sequence, which rests on a marine erosion surface. It was deposited in an outer shelf environment. Shale, interbedded sandstone and shale, and amalgamated sandstone lithofacies were deposited over the transgressive lag sandstone lithofacies as a wave-dominated delta and its flanking strand plains prograded seaward. Analysis of grain size and primary structures in Kenilworth beds indicates that there are four basic strata types which combine to build the observed lithofacies. The fine- to very fine-grained graded strata of the interbedded facies are tempestites, deposited out of suspension by alongshelf storm flows (geostrophic flows). There is no need to call on cross-shelf turbidity currents (density underflows) to explain their presence. Very fine- to fine-grained hummocky strata are likewise suspension deposits created by waning storm flows, but were deposited under conditions of more intense wave agitation on the middle shoreface. Cross-strata sets in this region are bed-load deposits that accumulated on the upper shore-face, in the surf zone. Lag strata are multi-event, bed-load deposits that are the product of prolonged storm winnowing. They occur on transgressive surfaces. While the graded beds are tempestites in the strict sense, all four classes of strata are storm deposits. The distribution of strata types and their palaeocurrent orientations suggests a model of the Kenilworth transport system driven by downwelling coastal storm flows, and probably by a northeasterly alongshore pressure gradient. The stratification patterns shift systematically from upper shoreface to lower shoreface and inner shelf lithofacies partly because of a reduction in fluid power expenditure with increasing water depth, but also because of progressive sorting, which resulted in a decrease in grain size in the sediment load delivered to successive downstream environments. The Kenilworth Member and an isolated outlier, the Hatch Mesa lentil, constitute a delta-prodelta shelf depositional system. Their rhythmically bedded, lenticular, sandstone and shale successions are a prodelta shelf facies, and may be prodelta plume deposits. Major Upper Cretaceous sandstone tongues in the Book Cliffs are underlain by erosional surfaces like that beneath the Blackhawk Formation, which extend for many tens of kilometres into the Mancos shale. These surfaces are the boundaries of Upper Cretaceous depositional sequences. The sequences are large-scale genetic stratigraphic units. They result from the arranging of facies into depositional systems; the depositional systems are in turn stacked in repeating arrays, which constitute the depositional sequences. The anatomy of these foreland basin sequences differs  相似文献