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1.
Extensive (ca. 50,000 km2) shallow-marine platforms (< 250 m) off northern (34°S) and southern (48°S) New Zealand, and more local areas of shelf between, are blanketed by skeletal carbonate sediments > 70% CaCO3), despite proximity to a tectonically active plate margin. In these regions the terrigenous sediment supply is presently low, and growth of epibenthos is fostered by firm substrates (rock, gravels, shells, seaweeds) and the generally energetic nature and high nutrient levels of open-shelf waters. Rapid transition into adjacent terrigenous-dominated facies is characteristic. Irrespective of water depth, the carbonates are coarse-grained and fragmental; carbonate mud is rare. Calcite dominates over aragonite. High-Mg calcite, widespread off northern New Zealand, is rare in the south. Skeletal material is dominated by bryozoans and bivalve molluscs, with significant local contributions from foraminifers, barnacles, calcareous red algae and echinoderms. The name bryomol is suggested for this distinctive temperate-region skeletal carbonate facies, which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component, known to be habitat-related. Bioerosion is an important mechanism of skeletal fragmentation and degradation. Many grains, especially aragontic bivalves, are infested by endolithic borers and have low preservation potential. Ages of skeletal material in the surficial deposits range from more than 20,000 years B.P. to modern, which is consistent with both low rates of carbonate production and sediment accumulation, and the wide range in preservation state of grains. Some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons, sand waves and sand sheets driven by storm-assisted tidal flows. Tracts of modern, palimpsest and relict carbonates can occur in juxtaposition.
The facies characteristics of the New Zealand shelf carbonate deposits contrast significantly with those of the classical Bahaman-type carbonate model. However, they are similar to those reported from many other mid- to high-latitude carbonate shelves, and afford good analogues for most onland occurrences of New Zealand Cenozoic limestones. 相似文献
2.
The southern continental margin of Australia is a cool-water carbonate sedimentary province located in a high-energy, swell-dominated oceanographic setting. A vibrocore transect of 14C-dated sediments across the centre of the Eucla Shelf is the first record of Holocene shelf deposition in the Great Australian Bight. Much of the seafloor shallower than 70 m water depth, the base of wave abrasion, is bare Cenozoic limestone, in some places encrusted by (?) Late Pleistocene, coral-rich, limestone that is cemented by high-magnesium calcite (12 mole% MgCO3). The areally extensive, 100 km-wide, hard, bored substrate supports an epibiota of coralline algae, minor bryozoans and soft algae or is covered by patches of Holocene sediment up to 1.5 m thick; generally a basal bivalve lag (< 3 ka) overlain by quartzose-bioclastic palimpsest sand. This pattern of active carbonate production but little accretion on the wave-swept mid- to inner-shelf is similar to that on other parts of the southern Australian continental margin. The term shaved shelf is proposed for this style of carbonate platform, formed by alternating periods of sediment accretion, cementation and erosion.
The palimpsest sand is typically rich in bivalves, coralline algae and locally, detrital dolomite. Outer shelf Holocene sediment, below the base of wave abrasion but inboard of the shelf edge, is a metre-thick unit of fine, microbioclastic muddy sand with minor delicate bryozoans overlying a 9–13 ka rhodolith gravel. Some of this outer shelf sediment appears to have been resedimented. The shelf edge is a sandy and rocky seafloor with active bryozoan growth and sediment production.
The Holocene sediments are enriched in coralline algal particles and conspicuous large foraminifers (cf. Marginopora) and depleted in bryozoans, as compared to coeval deposits on the Lacepede and Otway shelves off southeastern Australia. These differences are interpreted to reflect warmer waters of the Leeuwin Current and prevalent downwelling in this area as opposed to the general upwelling and colder waters in the east. 相似文献
3.
长期以来,海相碳酸盐沉积物被广泛认为是温暖浅海沉积环境中的产物,然而近年来国外研究表明,在冷水陆架环境中,也可以形成规模的碳酸盐沉积,即冷水碳酸盐岩。通过对大量文献的调研,综述了冷水碳酸盐岩的概念、地质特征、沉积模式等方面的研究进展,并展望了该研究领域未来的发展方向。研究表明,冷水碳酸盐沉积是指在(古)纬度约30°~35°以上的温带及寒带地区、温度约20℃以下的沉积水体中,或是在富营养的寒冷上升流水体中形成的碳酸盐沉积物或岩石,其沉积特征与暖水碳酸盐岩不同。冷水碳酸盐沉积物中的生物颗粒组合以底栖有孔虫、软体动物、苔藓虫等异养生物和钙质红藻为主,缺乏造礁珊瑚和钙质绿藻以及鲕粒、集合颗粒等非骨架颗粒,而且沉积物中灰泥基质含量较少,矿物成分以方解石为主,氧同位素较重,胶结作用弱,以破坏性成岩作用为主。冷水碳酸盐沉积形态以缓坡为主,波浪磨蚀和再沉积作用较强。古代冷水碳酸盐岩的沉积特征、识别标志及其作为储层的油气资源潜力尚处于探索阶段,仍需进一步深入研究。 相似文献
4.
Low-latitude carbonate muds often are composed either of entire units of skeletons (e.g., algal muds) or of precipitates, whereas high-latitude carbonate muds are bioerosional or result from maceration. Bioerosion at high latitudes is most intense in the photic zone, particularly down to 25 m depth. Shelly substrata may be crushed, bitten, drilled, bored or scraped. Clionid sponges, endolithic algae, acmaeid gastropods and regular echinoids are the most significant agents. Clionids produce distinctive facetted carbonate silt chips when boring, which have been described from both high- and low-latitudes. Faecal pellets break down to yield mud-sized carbonate particles that are more irregular than those produced by maceration. Exhumed infaunal bivalves are often preferred to epifaunal organisms as substrata. Bioerosion occurs very rapidly; shells may be totally infested with boring algae in three months. A “moth-eaten” appearance therefore does not denote a relict grain. Reliable rates of fine sediment production are not yet available.
The mud fraction of northwest European shelf sediment generally contains 10–20% CaCO3, though an inshore and offshore belt with higher values may be identified. Some Holocene supratidal mud-flats exceed 50% CaCO3. Much of the shelf represents a modern-day equivalent of the “calcareous shale” facies common in the geological record. Instances of synsedimentary cementation are not uncommon, particularly in association with heavily burrowed muds. 相似文献
5.
Terence P. Scoffin 《Sedimentary Geology》1988,60(1-4):107-124
The dominant calcareous organisms and sediment characteristics are described for eight different physical settings on the shelf west of Scotland, each having a different depth, substrate and degree of hydrodynamic exposure.
The principal sites of carbonate production are on shallow rocky substrates where barnacles, molluscs, echinoderms and serpulids are the dominant calcareous organisms. In sheltered shallow sandy zones, molluscs, echinoderms and benthic foraminiferans are the active producers, though the sediments are commonly barnacle-rich. Where tidal currents are enhanced between islands and the waves are suppressed, calcareous red algae (Phymatolithon calcareum) and mussel shells build localised banks. In deep, open-shelf water molluscs are the major skeletal contributor to the sediment, though on rocky sea beds bryozoans, serpulids and echinoderms are important.
The major sites of deposition are where persistent hydro- (and aero-) dynamic conditions sweep together grains from active production sites (e.g., sand ribbons or beaches and dunes adjacent to shallow rocky platforms) or in sinks where the physiographic configuration favours the deposition and retention of locally produced sediment or sediment derived from suspension. The well-sorted, cross-bedded, beach and dune sands commonly contain > 75% CaCO3. In sheltered depressions, bioturbated muds accumulate with up to 30% calcite silt, which is probably the breakdown product of barnacles and benthic foraminiferans. 相似文献
6.
The southeastern coastal plain of South Australia contains a spectacular and world-renowned suite of Quaternary calcareous eolianites. This study is focused on the provenance of components in the Holocene, actively forming sector, of these carbonate eolian deposits. Research was carried out along seven transects across a lateral distance of 120 km from ～30 m water depth offshore across the beach and into the dunes. Offshore sediments were acquired via grab sampling and SCUBA. Results indicate that dunes of the southern Lacepede and Bonney coasts are composed of siliciclastic particles (mainly quartz), relict allochems, Cenozoic and limestone pieces, but dominated by Holocene invertebrate and calcareous algal biofragments. The most numerous grains are from molluscs > benthic foraminifera ≥ coralline algae, > echinoids and > bryozoans. Most of these particles originate in carbonate factories such as macroalgal forests, rocky reefs, seagrass meadows and low-relief sea-floor rockgrounds. Incorporation of Holocene carbonate skeletons into coastal dunes, however, depends on a combination of: (1) the addition of infauna from intertidal and nearshore environments; (2) the physical characteristics of different allochems and their ability to withstand bioerosion, fragmentation and abrasion; (3) the character of the wave and swell climate; and (4) the nature of eolian transport. Most eolian dune sediment is derived from nearshore and intertidal carbonate factories. This is well illustrated by the abundance of robust infaunal bivalves that inhabit the nearshore sands and virtual absence of bryozoans that are common as sediment particles in offshore water depths >15 m. Importantly, the calcareous eolianites in this cool-water, open-platform carbonate setting are not simply an allochthonous reflection of the offshore marine shelf factories, but more a product of autothonous shallow nearshore–intertidal skeletal production and modification. These findings explain the preponderance of mollusc fragments and lack of bryozoans in similar older Pleistocene calcareous eolianites up to ca 1 million years old across ～2000 km of southern Australia with implications for the older rock record. 相似文献
7.
S. Anne Reeckmann 《Sedimentary Geology》1988,60(1-4):209-219
Temperate shelf carbonates form in cool marine waters and have skeletal and mineralogical compositions which are different from their tropical counterparts. They commonly lack non-skeletal grains and are often composed of low- and high-magnesium calcite with subordinate aragonite. Many of the aragonitic components found in tropical carbonates, such as corals, ooids, blue-green algae and lime mud, are absent.
Temperate shelf carbonates undergo diagenesis in marine waters with lower carbonate saturation than do tropical carbonates, and are exposed to cool climates with moderate to low rainfall. Marine cementation is rare because of low carbonate saturations in the surrounding waters. However, aragonite and high-magnesium calcite cements have been reported forming under specialized conditions associated with biogenic precipitation, submarine methane and sulphate-reducing bacteria, and more commonly in the intertidal environment where evaporation has increased carbonate concentrations.
In Pleistocene and Tertiary temperate shelf carbonates from southeastern Australia, evidence of marine diagenesis is rare to absent. Diagenetic stabilization of aragonite and high-magnesium calcite has taken from 80,000 y to 1 My, or longer, during subaerial exposure. This is slower than rates reported from tropical climates. A general lack of aragonite in some facies within these temperate carbonates leads to a lack of secondary porosity and only sparse low-magnesium calcite cement, even after prolonged fresh-water diagenesis. However, with lengthy exposure and under the right climatic conditions, karstic solution and calcrete precipitation can occur.
In sequences containing siliciclastic clays, pyrite and glauconite, abundant iron is present in interstitial waters leading to the precipitation of ferroan calcite cements in the phreatic and shallow burial environments, and to the substitution of iron for magnesium in stabilizing high-magnesium calcite skeletal material.
A unique void-filling, micritic internal sediment occurs in discrete layers in many of the Tertiary temperate shelf carbonate sequences in southeastern Australia. This internal sediment is localized as a pore-filling material above permeability barriers such as fine-grained sediments or volcanics, and above paleo-water tables which formed during periods of subaerial exposure. It is a feature of the vadose zone and lithifies to form a dense micritic low-magnesium calcite cement with characteristic pink/brown coloration, often associated with erosion surfaces and nodule beds.
Dolomite is uncommon in the southeastern Australian temperate shelf carbonates. It forms associated with preferential fluid pathways or mixing zones. Ferroan dolomite forms in siliciclastic clay-rich carbonates in the shallow burial environment. The ubiquitous fine, evaporite-related dolomite so common in tropical carbonates is absent. 相似文献
8.
Uppermost sands of the Red Crag at Walton-on-the-Naze (Essex) and elsewhere in East Anglia have been decalcified to iron-stained quartz sands. In contrast, lower sands are only minimally altered and contain aragonitic and calcitic shells. Aragonitic shells are slightly dissolved (chalkified), but calcitic shells are unaffected. Cementation is limited to an addition of iron oxides, now mainly haematite, which also coat carbonate grains. Abundant iron-oxide fines in the upper decalcified sands were liberated from the coatings of shells; shells that have since dissolved. The diagenetic nature of the contact between decalcified upper and unaffected lower sands is evident where it transects cross-bedding. The contact is knife-sharp, even smoothly truncating large shells, and is usually planar and subhorizontal. Shelly sands immediately beneath the boundary contain similar amounts of aragonitic material, as do sands further below. Locally the decalcification boundary has been contorted by cryoturbation, implying that carbonate dissolution was a Pleistocene event. Decalcification probably occurred when the area was affected by permafrost. Lower sands were cemented by ice and protected from dissolution. Upper sands were above the ice table and subject to chemically aggressive waters during summer thaws. Decalcification is believed to have taken place during an episode of climate amelioration when downward retreat of the ice table accompanied replacement of tundra by boreal forest. Highly acidic and podzolic soils developed, beneath which shell-carbonate dissolved. Sharp based decalcified zones in Lowestoft Till and Devensian deposits in other parts of England can also be attributed to dissolution associated with permafrost. 相似文献
9.
Lindsay B. Collins 《Sedimentary Geology》1988,60(1-4):15-49
The Rottnest Shelf is a narrow, wave-dominated open shelf on the passive continental margin of southwest Australia, adjacent to a hinterland of low relief and sluggish drainage. High physical energy, low nutrients in cool subtropical waters, and rapid postglacial transgression have limited carbonate productivity, restricted grain types, and reworked the transgressed surface to form only a thin ( < 1 m) blanket of carbonate and relict sediment, with little terrigenous influx. Subaerial weathering of the shelf during Late Pleistocene emergence was followed by postglacial drowning, erosional shoreface retreat, and generation of a transgressive lag deposit. Establishment of the warm temperate biota, dominated by bryozoans and calcareous red algae, resulted in bioerosion of the shelf disconformity surface and generation of hardground veneers and thin skeletal carbonate sheets. Linear topographic ridges of Pleistocene limestone partition the shelf into systems with varying physical energy, biota and sediment supply. The Holocene sediments are a shallowing-upward coastal sequence; wave-ripple cross-stratified grainstone (Inner Shelf); and bioturbated bryozoan grainstone to skeletal wackestone (Outer Shelf to Upper Continental Slope), characterised by seaward fining and increasing percentages of planktic carbonate sediment.
Given sufficient time, the Rottnest Shelf could recover from drowning, and form blanket-like skeletal carbonates. Thin ( < 1 m) lags overlying disconformities, which underlie shallowing-upward coastal and shelf sediments a few metres thick, will be generated by glacio-eustatic cycles of sedimentation (105 y duration). Thick (several tens of metres) sediment bodies, composed of wave-rippled to bioturbated skeletal carbonate sediment with a temperate biota, will be formed during longer term (1–10 My) sedimentation cycles. Such cycles have characterised passive margins during the Cenozoic. The Rottnest Shelf thus provides a facies model for temperate shelf sedimentation along passive continental margins. 相似文献
10.
The 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. 相似文献
11.
Selective dissolution of aragonitic grains is emerging as a volumetrically significant process that affects a broad range of modern carbonate settings. This study explores mechanisms and implications of aragonite loss in Challenger Mound, a giant cold‐water coral (Lophelia pertusa) mound of Pleistocene age, which lies on the continental slope off south‐west Ireland. A comprehensive sampling scheme allowed the integration of petrographic data with geochemical analyses of sediment and pore water. The mound remains virtually unlithified and consists of stacked, fining‐upward cycles of silty coral floatstone–rudstone and bafflestone grading into wackestone. Whereas calcitic grains appear unaltered, aragonitic grains are corroded and fragmented. Aragonite dissolution is attributed to organic matter oxidation at/near the sediment–water interface and, at greater depths, to the initial stages of bacterially mediated sulphate reduction, when alkalinity production is outpaced by the generation of H+. Pore water profiles indicate that undersaturated waters are diffusing towards the mound interior from two centres of sulphate reduction: one located in the upper 10 m of the sediment column and a second that lies below an erosional unconformity which marks the base of the mound. Continued aragonite dissolution is expected to gradually lower the diagenetic potential of the Challenger Mound and delay lithification until deep burial, when solution‐compaction processes come into play. Despite a fundamental role in predestining the final taphonomic and textural characteristics of Challenger Mound, the processes described here are expected to leave little trace in the geological record due to a lack of cementation and calcitization. Assuming that similar processes have been active throughout the Phanerozoic, results imply that the understanding of diagenetic processes in carbonate systems may be incomplete. 相似文献
12.
Microbial alteration of Bahamian deep-sea carbonates 总被引：4，自引：0，他引：4
An analysis of microborings within sediment and hardground samples collected from the Northwest Providence Channel and the western margin of the Little Bahama Bank was conducted to characterize the endolithic assemblage present, to examine the role of microboring organisms in the alteration of deep-sea carbonates, and to evaluate the palaeoecologic potential of the aphotic microboring assemblage found. Samples examined in this study were collected at depths ranging from 210 to 1450 m. The microboring assemblage was found to contain: (a) filamentous fungal borings of five distinct types, (b) a 1.0 × 2.5 μm vermicular form of fungal or bacterial origin, (c) an 8-12 μm tubular, branching form of probably fungal origin, (d) a subapically branched form considered to be fungal, (e) a spinate form of uncertain affinity and (f) sponge borings. Three of these forms are known only from the deep-marine environment; the remainder are also known from shallow-marine sediments found well within the photic zone. Both carbonate sediments and lithified hardgrounds are highly altered through the activity of endolithic organisms. Infestation of individual skeletal fragments by microborers may be so extensive as to produce heavily bored envelopes resembling those previously reported to occur only under shallow-marine conditions. Although the geological ranges of these microborings remain to be established, the presence of such an aphotic assemblage, coupled with the absence of photosynthetic algal borings, could provide a valuable tool in palaeoecological studies. Other potential applications include the determination of turbidite sediment sources and the establishment of relative water depths for the formation of hardground surfaces. 相似文献
13.
CAMPBELL S. NELSON 《Sedimentology》1978,25(6):737-771
Shelf limestones are widely distributed in New Zealand Cenozoic sequences and are especially well developed in the Oligocene. Detailed field and laboratory work on several Oligocene occurrences, and reconnaissance field-work at most other sections have elucidated the major characteristics of the environment, texture, composition and diagenesis of these sediments. Several generalizations emerge which contrast with the commonly accepted characteristics of shallow marine carbonate sedimentation established from studies of tropical and subtropical deposits. The limestones are either calcarenites or, less commonly, calcilutites and, in general, these two lithologies are mutually exclusive, both in time and space. The allochems and interparticle carbonate mud (where developed) in calcarenitic limestones consist almost exclusively of fragmented skeletal material derived primarily from bryozoan, echinodermal, benthic foraminiferal, barnacle, brachiopod, bivalve and coralline red algal tests. The calcilutitic limestones consist mainly of whole and disintegrated tests of pelagic foraminifers and coccolithophorids. Non-skeletal carbonate components such as ooids, pellets and aggregates are conspicuously absent from both lithologies. Reefal structures are also absent or rare and are mainly oyster reefs. The limestones commonly contain a significant content of terrigenous material and/or glauconite and at the stratigraphic level the limestones are intimately associated with terrigenous formations. The distribution of the carbonate sediments has been governed mainly by rate of supply of river-derived terrigenous material, by subsequent dispersal patterns of this material over the shelf, and by current sorting. As a consequence of selective grain transport, bedding in the limestones is often defined by the cyclic alternation on a wide range of scales of carbonate units that are relatively enriched and relatively impoverished in terrigenous material. The primary (carbonate) mineralogy of the carbonate sediments was completely dominated by magnesium calcite and/or calcite with only small amounts of aragonite and no dolomite or associated evaporite minerals. The metastable magnesium calcite and aragonite grains were probably altered on, or close below, the shallow sea-floor. Among other factors, transformation was encouraged by the absorption of magnesium in pore waters by montmorillonitic clays and by the complete oxidation of all organic matter in the bottom sediments. Magnesium calcite grains were stabilized by texturally non-destructive incongruent dissolution, but aragonite was often dissolved without trace from the sediment, especially in grainstones. Thus submarine diagenesis has been characterized by selective dissolution phenomena. Cementation by granular and syntaxial rim orthosparite of calcite and/or ferroan calcite composition occurred mainly during shallow subsurface burial and was associated with the intergranular solution of calcitic skeletal fragments, especially at those levels in the sediment relatively enriched in terrigenous material. This lithification process has worked to accentuate and modify original litho-logic differences and sedimentary structures in the primary sediments and has produced a kind of rhythmic vertical alternation of less well cemented, microstylolitized, impure limestone beds (‘cement-donor’ beds) and well cemented, more open textured, purer limestone beds (‘cement-receptor’ beds). The New Zealand limestones formed between latitudes 60° S and 35° S under generally cool temperate to warm temperate climate conditions. Oxygen isotopes suggest that surface waters were mainly significantly cooler than 20°C, so that shelf waters may have experienced extended periods of undersaturation with respect to calcium carbonate. Generally open circulation patterns maintained near normal salinity values over the entire shelf platform. Calculated sedimentation rates for the New Zealand carbonate sediments are generally very low (< 5 cm/1000 years). Periods of more active deposition commonly alternated with longer periods of non-deposition and by-passing or erosion. It is concluded that many characteristics of the New Zealand shelf limestone occurrences are explained best by a temperate latitude model of shallow marine carbonate sedimentation. 相似文献
14.
P. T. Harris 《Australian Journal of Earth Sciences》2013,60(3):241-254
Surficial deposits of the tidally influenced Australian shelf seas exhibit a variation in fades related to energy gradient. These deposits comprise a high energy gravelly facies, a mobile sand sheet facies and a low energy muddy sand facies. Such a facies distribution conforms generally with the existing model of continental shelf tidal sedimentation, derived for the west European tidal seas. However, the carbonate rich and mainly warm water deposits of the Australian shelf differ from the mainly quartzose and temperate cold‐water deposits of the European type case in terms of: (i) the role of seagrasses in trapping fine‐grained sediment; and (ii) the relative importance of the production of carbonate mud by mechanical erosion of carbonate grains. Seagrasses in Spencer Gulf, Gulf of St Vincent and Torres Strait are located in regions of strong tidal currents, associated with bedforms and gravel lag deposits. Thus, in the case of tropical carbonate shelves, seagrass deposits containing fine‐grained and poorly sorted sediments are located in close proximity to high energy gravel and mobile sand facies. In contrast, the European model (for temperate, siliciclastic shelves) places facies in a regional gradient with a wide separation (in the order of 50–100 km). Of the locations reviewed, the Gulf of St Vincent, Bass Strait, southern Great Barrier Reef, Torres Strait and Gulf of Carpentaria exhibit zones of carbonate mud accumulation. The production and winnowing of carbonate mud from the mobile sand facies is a factor that must be taken into account in the assessment of a sediment budget for this facies, and which is of relatively greater importance for carbonate shelves. Insufficient data are presently available from the macrotidal North West Shelf to test the applicability of the model to this region. 相似文献
15.
Abstract The north-east Australian margin is the largest modern example of a tropical mixed siliciclastic/carbonate depositional system, with an outer shelf hosting the Great Barrier Reef (GBR) and an inner shelf dominated by fluvially sourced siliciclastic sediment wedges. The long-term interplay between these sediment components and sea level is recorded in the Queensland Trough, a 1–2 km deep N–S elongate basin situated between the GBR platform and the Queensland Plateau. In this paper, 154 samples from 45 surface grabs and six well-dated piston cores were analysed for total carbonate content, carbonate mineralogy and Sr concentration to establish spatial and temporal patterns of carbonate accumulation in the Queensland Trough over the last 300 kyr. Surface carbonate contents are lowest on the inner-shelf (<5%) and in the trough axis (<60%) because of siliciclastic dilution. Carbonate on the shelf is mostly Sr-rich aragonite and high-Mg calcite (HMC), whereas that in the basin is mostly low-Mg calcite. Once normalized to remove the effects of siliciclastic dilution, surface Sr-rich aragonite and HMC abundances decrease linearly to background levels ≈ 100 km seaward of the shelf edge. Core samples show that, over time, normalized aragonite and Sr abundances are greatest during periods of shelf flooding and lowest when sea level drops below the shelf edge. This is consistent with changes in the production of coral and calcareous algae, and the shedding of their debris from the shelf. Interestingly, normalized HMC concentrations on the slope peak during periods of major transgression, perhaps because of maximum off-shelf transport from inter-reef areas or intermediate water dissolution. After accounting for siliciclastic dilution, there are strong similarities in both spatial and temporal patterns of carbonate minerals between slopes and basins of the north-east Australian margin and those of pure carbonate margins such as the Bahamas. A limited set of basic processes, including the formation and breakdown of carbonate on the shelf, the transport of carbonate off the shelf and eustatic sea level, probably controls carbonate accumulation in slope and basin settings of tropical environments, irrespective of proximal siliciclastic sediment sources. 相似文献
16.
Diurnal variation in rates of calcification and carbonate sediment dissolution in Florida Bay 总被引：2，自引：0，他引：2
Water quality and criculation in Florida Bay (a shallow, subtropical estuary in south Florida) are highly dependent upon the
development and evolution of carbonate mud banks distributed throughout the Bay. Predicting the effect of natural and anthropogenic
perturbations on carbonate sedimentation requires an understanding of annual, seasonal, and daily variations in the biogenic
and inorganic processes affecting carbonate sediment precipitation and dissolution. In this study, net calcification rates
were measured over diurnal cycles on 27 d during summer and winter from 1999 to 2003 on mud banks and four representative
substrate types located within basins between mud banks. Substrate types that were measured in basins include seagrass beds
of sparse and intermediate densityThalassia sp., mud bottom, and hard bottom communities. Changes in total alkalinity were used as a proxy for calcification and dissolution.
On 22 d (81%), diurnal variation in rates of net calcification was observed. The highest rates of net carbonate sediment production
(or lowest rates of net dissolution) generally occurred during daylight hours and ranged from 2.900 to −0.410 g CaCO3 m−2d−1. The lowest rates of carbonate sediment production (or net sediment dissolution) occurred at night and ranged from 0.210
to −1.900 g CaCO3 m−2 night−1. During typical diurnal cycles, dissolution during the night consumed an average of 29% of sediment produced during the day
on banks and 68% of sediment produced during the day in basins. Net sediment dissolution also occurred during daylight, but
only when there was total cloud cover, high turbidity, or hypersalinity. Diurnal variation in calcification and dissolution
in surface waters and surface sediments of Florida Bay is linked to cycling of carbon dioxide through photosynthesis and respiration.
Estimation of long-term sediment accumulation rates from diurnal rates of carbonate sediment production measured in this study
indicates an overall average accumulation rate for Florida Bay of 8.7 cm 1000 yr−1 and suggests that sediment dissolution plays a more important role than sediment transport in loss of sediment from Florida
Bay. 相似文献
17.
SARAH ANNE REECKMANN 《Sedimentology》1981,28(5):643-652
A 3 km long lateral facies transition from Oligo-Miocene skeletal grainstones into packstones and mudstones displays systematic variation in carbonate skeletal fragment alteration and secondary carbonate mineral composition. Within the skeletal grainstone end-member all aragonitic grains have dissolved, all Mg-calcite skeletal fragments have altered to calcite, and calcite has been precipitated as both void-filling spar and as syntaxial overgrowths on echinoid fragments. Within the transitional grainstone-packstone lithologies some skeletal fragments show preservation of their original mineralogy, while calcite, ferroan calcite spar and dolomite have precipitated in void spaces. Within the packstone-mudstone end-member of the transition, aragonitic grains have been preserved and Mg-calcite skeletal fragments have retained most of their magnesium. Thin, originally porous grainstone layers within the pack-stone-mudstones contain sideroplesite and ferroan calcite spar in void spaces, and iron has been incorporated in skeletal fragments which were originally Mg-calcite. In these grains iron has been incorporated at the expense of magnesium until the grains approach the composition of the surrounding ferroan calcite cement. The lateral variation in diagenetic alteration across this facies transition is a function of differing groundwater movement and composition, controlled ultimately by the composition, porosity and permeability of the host lithologies. 相似文献
18.
M.E. Brookfield D.P. Hemmings P. Van Straaten 《Journal of South American Earth Sciences》2009,28(2):180-192
The Napo phosphorites were deposited at the edge of a stable marine shelf during the Upper Cretaceous (Coniacian) oceanic anoxic event (OAE 3) at the transition from bioclastic limestone to organic-rich shale facies. Phosphogenesis was triggered in the shelf margin environment by a number of factors including strong upwelling currents, high biological activity, plankton blooms, and large amounts of organic matter production and subsequent accumulation. Dissolved phosphate levels increased in the sediment from a combination of anoxic conditions and microbial activity. Once dissolved phosphate concentrations were high enough, apatite began to form around nucleic sites including mineral grains, shells, wood fragments, and foraminifera tests forming peloidal fluorine rich carbonate fluoroapatite (francolite). As the peloids formed, sedimentation continued and dissolved phosphate concentrations diminished. A period of minor winnowing ensued, and as dissolved phosphate concentrations remained low, shale layers were deposited separating the various phosphate layers. 相似文献
19.
Modern carbonate and terrigenous clastic sediments on a cool water, high energy, mid-latitude shelf: Lacepede, southern Australia 总被引：1，自引：0，他引：1
NOEL P. JAMES YVONNE BONE† CHRISTOPHER C. VON DER BORCH‡ VICTOR A. GOSTIN† 《Sedimentology》1992,39(5):877-903
The wide Lacepede Shelf and narrow Bonney Shelf are contiguous parts of the south-eastern passive continental margin of Australia. The shelves are open, generally deeper than 40 m, covered by waters cooler than 18°C and swept by oceanic swells that move sediments to depths of 140 m. The Lacepede Shelf is proximal to the ‘delta’of the River Murray and the Coorong Lagoon. Shelf and upper slope sediments are a variable mixture of Holocene and late Pleistocene quartzose terrigenous clastic and bryozoa-dominated carbonate particles. Bryozoa grow in abundance to depths of 250 m and are conspicuous to depths of 350 m. They can be grouped into four depth-related assemblages. Coralline algae, the only calcareous phototrophs, are important sediment producers to depths of 70 m. Active benthic carbonate sediment production occurs to depths of 350 m, but carbonate sediment accumulation is reduced on the open shelf by continuous high energy conditions. The shelf is separated into five zones. The strandline is typified by accretionary sequences of steep shoreface, beach and dune carbonate/siliciclastic sediments. Similar shoreline facies of relict bivalve/limestone cobble ridges are stranded on the open shelf. The shallow shelf, c.40–70 m deep, is a wide, extremely flat plain with only subtle local relief. It is a mosaic of grainy, quartzose, palimpsest facies which reflect the complex interaction of modern bioclastic sediment production (dominated by bryozoa and molluscs), numerous highstands of sea level over the last 80 000 years, modern mixing of sediments from relatively recent highstands and local introduction of quartz-rich sediments during lowstands. The middle shelf, c.70–140 m deep, is a gentle incline with subtle relief where Holocene carbonates veneer seaward-dipping bedrock clinoforms and local lowstand beach complexes. Carbonates are mostly modern, uniform, clean, coarse grained sands dominated by a diverse suite of robust to delicate bryozoa particles produced primarily in situ but swept into subaqueous dunes. The deep shelf edge, c. 140–250 m deep, is a site of diverse and active bryozoa growth. Resulting accumulations are characteristically muddy and distinguished by large numbers of delicate, branching bryozoa. The upper slope, between 250 and 350 m depth, contains the deepest platform-related sediments, which are very muddy and contain a low diversity suite of delicate, branching cyclostome bryozoa. This study provides fundamental environmental information critical for the interpretation of Cenozoic cool water carbonates and the region is a good model for older mixed carbonate-terrigenous clastic successions which were deposited on unrimmed shelves. 相似文献
20.
New interpretations of Great Salt Lake ooids and of ancient non-skeletal carbonate mineralogy 总被引：2，自引：0，他引：2
PHILIP A. SANDBERG 《Sedimentology》1975,22(4):497-537
Earlier interpretations of textural alteration affecting Great Salt Lake ooids have greatly influenced concepts of ooid diagenesis. Scanning electron microscope study shows, however, that the coarse radial aragonite rays are depositional, that no recrystallization of pellet cores has occurred, and that Great Salt Lake ooids have not suffered noticeable diagenesis. As suggested by Kahle (1974), radial texture in ancient calcitic ooids is probably mainly original, not diagenetic. Retention of such fine textures has been attributed to organic matter (since found to be equivalent in modern skeletal and non-skeletal grains) or to paramorphic replacement (proposed for non-skeletal grains whose original aragonite mineralogy was only inferred from modern analogs). Pleistocene ooids known to have been aragonite alter like aragonite shells to coarse neomorphic calcite, often with aragonite relics. The striking uniformity of that coarse texture in neomorphic calcite replacing known skeletal aragonites throughout the geologic record has been noted for over 100 years. In contrast, Mississippian ooids retain fine texture as do calcite layers of coexisting gastropods, but unlike the strongly altered aragonite layers of these same gastropods. Therefore, inferences of original aragonitic mineralogy of ancient non-skeletal carbonate grains (including muds) which are now calcite but retain fine texture appear unwarranted, as do assumptions of differential diagenetic behaviour of ancient aragonitic skeletal and non-skeletal grains. Accordingly, modern depositional environments of marine ooids and carbonate muds must be rejected as chemically unrepresentative of comparable ancient environments. It is inferred that ancient non-skeletal carbonates were originally predominantly or exclusively calcite because of an earlier lower oceanic Mg/Ca ratio (<2/1) which altered progressively to values favouring aragonite (modern Mg/Ca value = 5/1). Major influencing factors are: selective removal of calcium by planktonic foraminifers and coccolithophorids since Jurassic-Cretaceous time and by abundant younger, Mg-poor aragonite skeletons and an erratic trend toward decreasing dolomite formation (decreasing removal of oceanic Mg). The change to aragonite dominance over calcite for non-skeletal carbonates was probably during early to middle Cenozoic time. 相似文献