摘要:

ABSTRACTSmall digitate stromatolites with diameters in the range of 0·2–20 mm (ministromatolites) are common in Early and Middle Proterozoic carbonate sequences, and extend stratigraphically from the Archaean to the Holocene. An occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial‐fibrous fabric and microscopically crinkled (microcrenate) lamination is described from the ˜ 1·9 Gyr old Belcher Supergroup (McLeary Formation) in southeastern Hudson Bay, Canada. The structures, which can be considered to be a variety of tufa, are unusually well preserved because of early diagenetic silicification. Columnar types are referable toPseudogymnosolen (Asperia), and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern Canada, where the fabric is normally preserved by a secondary mosaic of dolomite. The textural evidence of angulate cross‐sections and rectilinear divergent patterns indicates that the radial‐fibrous fabric represents primary or very early diagenetic precipitation, and that pseudogymnosolenids with mosaic dolomite originally also had radial‐fibrous structure. The precipitation may have been within, or on, mi

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00586.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTThe dominantly shallow‐marine Vendian succession of NE Spitsbergen contains distinctive types of carbonate rock. Limestones deposited before Vendian glaciation resemble those described from other Upper Proterozoic successions, being high in Sr and inferred to have been originally aragonitic, including the distinctive 5–10 Jim equant polygonal calcite of cemented shrinkage cracks. In contrast, manganoan stromatolitic limestones within marginal‐marine glacial‐outwash deposits, and consisting of micrite, microspar and fascicular‐optic calcite are interpreted as originally calcitic. The restriction of primary marine calcite tocoldseawater is comparable with Recent and Permian carbonates, although the Precambrian example formed in a sea diluted with meltwater.There is good textural preservation of relatively18O‐rich oolitic dolostones which were cemented in a supratidal environment by artesian fluids. Nevertheless, early diagenetic replacement is inferred, immediately prior to a glacial episode. Post‐glacial dolostones are either replacive marine, or evaporative lacustrine, but share rather more negative δ18O values, closer to the mean of Late Precambrian dolostones.The heaviest oxygen isotope values constrain seawater δ18O to no more negative than — 2 to — 4SMOW. The main reason for the pronounced oxygen isotopic depletion of most Late Precambrian carbonates is their initial metastable mineralogy. The possibility of determining palaeolatitudes of the enigmatic widespread Late Proterozoic glaciations by isotopic analysis of freshwater periglacial calcareous precipitates is raised. Significant carbon isotope variations reflect changes in depositional water chemistry: some of these could

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00587.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTTwo caliche profiles from a Pleistocene carbonate dune on San Salvador Island, Bahamas, were examined by petrographic and geochemical analysis. Profile A is an immature buried caliche profile characterized by caliche pisolites, a friable crust and abundant Cerion. Profile B is a more well‐developed caliche profile at the top of the dune which contains abundant pisolites, rhizomorphs, laminated calcrete, a breccia and abundantCerion.Geochemical changes in caliche profiles relative to the host rock are an increase in Al2O3, Fe2O3and total organic carbon, a decrease in Mg and Sr, and a decrease in δ13Ccarb, δ18Ocarband δ13Corg. The magnitude of these changes is probably a function of the duration of subaerial exposure and resultant colonization by dune plants and associated microflora.Abundance of calcified filaments and needle‐fibre crystals in profile A attests to the importance of microbial processes in the early development of caliche profiles. Biogenic structures are largely destroyed in profile B due to recrystallization, but indirect evidence of biological activity is retained in the form of carbon isotope

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00588.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTSmall quantities of a rare Ca‐deficient dolomite have been found coexisting in thin layers and alternating with the normal Ca‐rich variety in the Holocene sabkhas of southern Kuwait. The Ca‐poor dolomite has a molar composition of Ca46–49, whereas the Ca‐rich type is Ca51–56. The former type has been found only in two localities of the supratidal zone that are not subjected to tidal flooding today, and mostly within the fluctuating ground‐water table in these zones. The thin layers are either carbonate mud‐rich or pellet‐rich, and the amount of Ca‐poor dolomite is never more than 4 to 6 per cent by weight of the total carbonate fraction. The dolomites vary in size between 2 μm and 5 μm and exhibit characteristic rhombic

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00589.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTCarbonate hardgrounds often occur at the surface of shallow subtidal to supratidal, lacustrine, and subaerial carbonate shelf sediments. These are commonly disrupted and brecciated when the surface area of these crusts increases. In the subtidal environment, megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand. Similar megapolygons form in the supratidal, lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and/or cement also causes the area of the hardgrounds to expand. The arched up antiform margins of expansion megapolygons are known as tepees.The types of tepees found in the geological record include:(1)Submarine tepeeswhich form in shallow carbonate‐saturated waters where fractured and bedded marine grainstones are bound by isopachous marine‐phreatic acicular and micritic cements. The surfaces of these brecciated crusts have undergone diagenesis and are bored. Unlike tepees listed below they contain no vadose pisolites or gravity cements;(2)Peritidal and lacustrinetepees are formed of crusts characterized by fenestral. pisolitic and laminar algal fabrics. This similarity in fabric makes these tepees of different origins difficult to separate.Peritidaltepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical. They are associated with fractured and bedded tidal flat carbonates. Their fracture fills contain geopetal asymmetric travertines of marine‐vadose origin and/or marine phreatic travertines and/or Terra rossa sediments. The senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material.Lacustrinetepees form in the margins of shallow salinas where periodic groundwater resurgence is common. They include groundwater tepees which form over evaporitic ‘boxwork’ carbonates, and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas, but the dominant sediment type is carbonate mud. These latter tepee crusts are coated and crosscut by laminated micrite; the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust.Both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions, in time intervals of days to years. Cement morphologies reflect this and the crusts often contain gravitational, meniscus vadose cements as well as phreatic isopachous cement rinds.(3)Caliche tepeeswhich are developed within soil profiles in a continental setting. They are formed by laminar crusts which contain pisolites, and fractures filled by micritic laminae, microspar, spar and Terra rossa. Most of the cements are gravitational and/or meniscoid.In ancient carbonates, when their cementation and diagenetic fabric can be interpreted, tepee structures can be used as environmental indicators. They can also be used to trace the evolution of the depositional and hydrologica

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00590.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTThe interpretation of the depositional environments in which bedded halite is formed is a frontier of evaporite sedimentology. Facies studies, supplemented by geochemical data, define an evaporite shelf depositional environment for the halite of the San Andres Formation of the Palo Duro Basin, Texas Panhandle, that is significantly different from modern and previously described ancient halite‐precipitating environments such as playas, salinas, sabkhas, and barred basins.A coastal palaeoenvironmental setting for the San Andres halite is defined by its facies position between normal marine‐shelf carbonates and aeolian deposits. The San Andres brine pool extended over more than 10 000 km2but was very shallow. Perennial brine‐pool conditions (thick sequences of halite) alternated with ephemeral brine‐pool conditions (halite, haloturbated mudstone‐halite, mudstone interbeds, and microkarst). The depositional sequence in the perennial brine pool includes (1) flooding and minor dissolution of previously deposited halite, (2) precipitation of a thin lamina of gypsum, and (3) evaporation and precipitation of halite. Bottom‐growth forms of halite, including chevrons and vertically elongated crystals, are well preserved. Cumulates of foundered, floating crystals and rafts were probably abundant but have been almost completely recrystallized.Mudstone interbeds in halite and microkarst features cut into bedded halite record the episodic interruption of brine‐pool deposition and the establishment of subaerial conditions. Vertically elongated karst pits originate at the palaeosurfaces, cross‐cut primary brine‐pool fabrics, and are filled with geopetal sediment and coarse halite cement. Mudstone interbeds and mixtures of mudstone and halite have been haloturbated by the alternating growth and dissolution of halite reflecting conditions of fluctuating salinity. The geochemistry of halite from both ephemeral and perennial brine‐pool environments reflects the frequent influx of marine brine, indicating that communication with the marine environments to the south was ‘good’ considering that restriction was sufficien

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00591.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTIn the diagenetic history of calcareous sandstones, silicacementation and silicification may be followed by carbonatecementation and replacement andvice versa, and the change‐over from one to the other may occur more than once. This is well illustrated by calcareous and siliciclastic turbidites of the Gault Formation (Aptian to Albian) of the Eastern Alps which have been interpreted as deep‐sea trench plain and deep‐sea fan deposits.In these turbidites silicification selectively affects ooids and a few other biogenic carbonate fragments rich in organic matter (algae and bryozoans) which form a small fraction of the bulk sediment. The type and sequence of diagenetic changes are largely controlled by host‐rock composition and may vary vertically within individual beds as a result of compositional grading. In the carbonate turbidites, silicification follows widespread calcite cementation. The process is slow, resulting in relatively coarsely crystalline replacement quartz. In ooids with quartz nuclei, rim‐quartz forms mostly monocrystalline ‘overgrowths’ by outward replacement of the concentrically laminated carbonate cortex. This type of silicification is often incomplete leaving parts of the ooid cortices unaffected. In quartz arenites and sublitharenites silicification precedes calcite cementation. There the process is rapid, forming microcrystalline quartz. Even if the ooid nucleus consists of quartz, a syntaxial ‘overgrowth’ does not normally form. The replacement quartz is almost always polycrystalline. Late‐stage diagenetic calcite and dolomite which develop euhedral crystal shapes and cut across grain boundaries may replace the earlier secondary rim‐quartz of the ooids as well as other minerals.Possible sources of the silica are pressure‐solution of quartz, dissolution of opaline silica of radiolarian tests and of sponge spicules, and feldspar in the host bed. In a number of examples an increase in the degree of silicification can be observed towards the lower bedding planes of individual turbidites requiring an additional external source of silica which seems to necessitate cross‐formational flow of pore solutions. Silicification in both the carbonate and the siliciclastic turbidites probably took place at about the same time; in the carbonate turbidites it was preceded, however, by calcite cementation, which significantly reduced porosity and permeability before silicification took place.The greater degree of alteration experienced by the Gault turbidites of the Falknis and Tasna Nappes, which are more internal structural units of the Alps (compared to the Flysch Zone of the Eastern Alps), is reflected by the growth of quartz ‘beards’ and spikes from the ooids in the direction perpendicular to maximum stress. This is the only case observed where the rim quartz of the ooids grows beyond

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00592.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTAlthough it is a pelagic sediment, fine‐grained calcareous ooze may be mobilized prior to general lithification and redeposited as allochthonous units. Numerous occurrences of allochthonous chalk have been reported in recent years, having been recognized by large‐scale bedding features seen in outcrop. Smaller‐scale internal features, such as contorted laminae, and larger features, such as smeared burrows and imbricated flint nodules, attest to a significant amount of soft‐sediment deformation and synsedimentary slumping in European chalk sections of Late Cretaceous age.Truly autochthonous chalks contain complex, tiered ichnofabrics and in some cases exhibit a diagenetic nodular fabric that is undisturbed by transport. In some situations, such as stagnant water conditions, autochthonous chalks may exhibit primary lamination, although this is very uncommon in European chalk sequences.Different types of redepositional processes produce an array of varied allochthonous fabrics. Glide and slump units, for example, contain internal deformational features produced during sliding. Ooze flow causes plastic deformation of chalk units, internally as well as externally. Resuspension and fluid flow of chalk sediment produces a deposit having a totally new fabric, such as a conglomerate composed of detrital chalk clasts.In this paper, typical macroscopic, sedimentary fabric types are illustrated, and the means of identifying them are discussed in terms of bioturbation features,in situdiagenetic nodules versus detrital clasts, physical deformation structures and development of

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00593.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTThe sedimentation mechanisms that occur in ice‐covered Lake Hoare, Antarctica are examined, to determine how sediment enters the lake, and how the sedimentation pattern affects blue‐green algal growth at the lake bottom. The 3 m‐thick ice cover contains pebbly sand as much as 2 m below the surface. Sediment with similar texture and mineralogy is found at the lake bottom. This evidence, together with the lack of sediment in the inflowing stream and the markedly different texture of sediment from the other terrains around the lake suggest that most of the sediment at the lake bottom comes in through the ice cover. Sand grains intermittently migrate through porous ice on the surface, water‐filled vertical gas‐channels penetrating two‐thirds of the ice cover, and possibly through cracks in the ice that act as conduits. The algae at the lake bottom are able to survive in part because sediment that comes through the ice cover does not obli

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00594.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTSmall, steep‐sided narrow channels, locally developed within the braided fluviatile Lower Carboniferous Fell Sandstone of northeast England, are attributed to mass‐flow processes initiated by channel‐bank slumping. The channels have sharp undisturbed margins and are filled with moderately well‐sorted medium to coarse sandstone identical to the sandstone which they cut. Most channel‐fills are structureless, but a few show diffuse concentric marginal laminations. Laminations also occur towards the top of channel fills, but they are less regular and show low‐amplitude trough‐like undulations. Channel margin inclinations range from 13° to 55°, and channel depths vary between 1·7 m and 2·5 m. The channels, which are only seen in profile, are orientated perpendicular to the local and regional palaeocurrent trend, and cut through structureless sandstone and planar cross‐stratified sandstone characterized by compound and compound‐complex bar bedforms. Only one channel has both margins preserved; and of the remainder (seven) only one has its northern margin preserved.Undercutting and retrogressive slumping of the sandy channel bank is thought to have initiated a subaqueous sediment gravity flow, which moved across the main channel along a transverse scour at the base of the slip face of a large composite bar bedform. Such scours have strong asymmetric profiles with the steeper slope along the base of the active slip face (northern side) and the lower slope on the downstream (southern) side of the scour. Thus, the northern channel margin was steeper, less stable and more prone to collapse than the southern margin, and is almost never preserved. Flow reconstruction indicates that on entering the channel a hydraulic jump developed at the change in slope, and the flow became more turbulent and erosive. Scoured sand incorporated into the flow increased concentration and shearing along the boundaries. Reverse shear was also exerted by the overlying current, on top of the flow, but the central part remained relatively unaffected. The presence or absence of marginal laminations is probably related to marginal shearing and flow dilution. Deposition occurred in response to deceleration and frictional free

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00595.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY