摘要:

ABSTRACTThe particle sizes in bottom and suspended sediments from the ocean upstream for 80 km in the Hudson River Estuary were analysed. Several modes at 4, 8 and 30 μm were prevalent throughout the estuary. The average size of particles in bottom samples varied from 150 μm near the ocean to 15–20 μm in Haverstraw Bay. The bottom sediment size appears to be controlled by two components; (1) sand from the ocean and (2) the particles in the f

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01441.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe post‐glacial succession in the Cobequid Bay — Salmon River incised valley contains two sequences, the upper one incomplete. The lower sequence contains only highstand system tracts (HST) deposits which accumulated under microtidal, glacio‐marine deltaic conditions. The upper sequence contains two, retrogradationally stacked parasequences. The lower one accumulated in a wave‐dominated estuarine environment under micro‐mesotidal conditions. It belongs to the lowstand system tract (LST) or early transgressive system tract (TST) depending on the timing and location of the lowstand shoreline, and contains a gravel barrier that has been overstepped and preserved with little modification. The upper parasequence accumulated in the modern, macrotidal estuary, and is assignable to the late TST. Recent, net progradation of the fringing marshes indicates that a new HST has begun.The sequence boundary separating the two sequences was formed by fluvial incision, and perhaps also by subtidal erosion during the relative sea level fall. Additional local erosion by waves and tidal currents occurred during the transgression. The base of the macrotidal sands is a prominent tidal ravinement surface which forms the flooding surface between the backstepping estuarine parasequences. Because fluvial deposition continued throughout the transgression, the fluvial‐estuarine contact is diachronous and cannot be used as the transgressive surface. The maximum flooding surface will be difficult to locate in the macrotidal sands, but is more easily identified in the fringing muddy sediments.These observations indicate that: (1) large incised valleys may contain a compound fill that consists of more than one sequence; (2) relative sea level changes determine the stratal stacking patterns, but local environmental factors control the nature of the facies and surfaces; (3) these surfaces may have complex origins, and commonly become amalgamated; (4) designation of the transgressive surface (and thus the LST) is particularly difficult as many of the prominent surfaces in the valley fill are diachronous facies boundaries; and (5) the transgression of complex topography may cause geologically instantaneous changes in tidal range, due to resonance under particular geographical con

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01442.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThis paper considers the magnetic properties of a range of recent and contemporary sediments from the north eastern part of the Irish Sea. Principal component ordinations of the results show a close link between magnetic property variations and particle size. The magnetic properties of a subset of samples, particles sized by a combination of sieving and pipette analysis, confirm that variations in ferrimagnetic (‘magnetite’) grain size parallel those in particle size, despite the fact that the magnetic grains in the fine grades have diameters 1–2 orders of magnitude smaller than those of the particle size fraction in which they occur. This is best explained by postulating that the fine magnetic grains occur in the clay fraction but are present in declining concentrations in the coarser grades up to 4ø as an artefact of the pipette method. Most samples have a biomodal distribution of magnetic minerals, with a coarse mode associated with heavy minerals in the sands or coarse silts, and a fine mode in the clays. Magnetic susceptibility (x) and saturation isothermal remanent magnetization (SIRM) largely pick out the coarse mode where present; anhysteretic remanent magnetization (ARM) largely picks out the fine mode. The results open up the possibility of normalizing samples from these environments for particle size and, more specifically, clay content, by means of ARM or ARM/xvalues. The most likely source for the uniform and almost exclusive stable single domain magnetite, which dominates the magnetic properties of the clays, is thought to be bacterial magnetosomes. The measurements as a whole do not appear to hold much promise for discriminating sediment source

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01443.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTNeptunian dykes and sills in Jurassic carbonate platform strata of the Betic region of Spain occupy spaces created by wholly mechanical fracturing and displacement of the host strata, and later filled by pelagic sediments from above or by precipitation of calcite from circulating solutions. In some places, joint‐bounded blocks of platform carbonates have been wholly removed, possibly by sliding down submarine slopes, leaving a staircase topography, commonly Fe‐encrusted, that was subsequently filled by pelagic sediments. Other cavities that formed during Cretaceous times were developed by dissolution and current erosion in moderately deep submarine environments, and then filled by pelagic sediments from above.None of the cavities hosted by either platform or pelagic strata contain evidence for their formation by dissolution in a subaerial environment. The Jurassic and Cretaceous history of subsidence of the Betic margins is thus simpler than in versions requiring repeated emergence to form subaerial karstic cavit

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01444.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe objective of this study was to discriminate between modern beach subenvironments based on textural characteristics obtained using the graphical (percentile) method, the moment method, and the log‐hyperbolic distribution (LHD). A total of 126 surface sedimentation units were sampled at the nodes of a 21 x 6 rectangular grid (1000 m2) on a carbonate sand beach, Oahu, Hawaii. Sampling was conducted at low energy conditions from the lower foreshore to the backshore. Non‐parametric discriminant analysis was used as an objective tool in defining distinct subenvironments. Confidence bands around the canonical variates derived from the graphic mean, sorting, skewness and kurtosis indicated four separate subenvironments (lower foreshore, mid‐foreshore, upper foreshore and backshore). Three distinct subenvironments were identified using the mean, sorting (standard deviation) and skewness measures derived by the method of moments. A similar subenvironment distinction was obtained using five statistics of the LHD (gamma, γ; nu ν; delta, δ; tau, τ; and xi, ξ). No significant difference was noted in textural characteristics between the upper foreshore and backshore zones, and these zones were grouped into one subenvironment. These results indicate that different process scenarios would be needed to explain different subenvironment partitioning based simply on the approach adopted. Discriminant analysis indicated that fewer subenvironment samples were misclassified and separation distances between subenvironments in bivariate canonical plots were greater for the standard moment measures compared with the statistics derived from fitting the computationally intensive LHD model. Examination of the mass frequency grain size distributions indicated that the LHD was generally the most appropriate model. These observations were confirmed by the hyperbolic shape triangle which indicated that the LHD rather than the more commonly used log‐normal distribution was generally optimal in describing sediments. These results support the use of the LHD statistical measures in subenvironment discrimination over the graphic‐incl

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01445.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTMiddle Triassic carbonate buildups of the Dolomites were high in relief (500–1000m) and small in size (one to a few square kilometres in area). A paradox results from the carbonate platform model that invokes the platform top, including reef rims, as the carbonate factory and flanking beds as talus deposits. Most buildups consist largely of clinoforms (inclined at 10‐50°) whereas massive reef rocks and stratified buildup interiors are poorly developed or absent. Facies and modal analysis of 323 thin sections from buildups of the Marmolada indicate that clinoforms are: (i) predominantly composed ofin situboundstones (56% of all samples); (ii) primarily made up of early cements (37 vol.%), microbial crusts (17 vol.%), micritic intraclasts (10 vol.%) andTubiphytes(8 vol.%); and (iii) contain diagnostic shallow water grains (dasyclads, coated grains) that are less abundant by 1‐2 orders of magnitude compared with buildup interior facies. These data suggest that the clinoforms themselves were the main carbonate factory of the Triassic buildups. Stratified buildup interior rocks and massive reef rocks were apparently not a prerequisite for buildup growth and clinoform progra

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01446.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTQuantitative appraisal of compaction strain is essential for the study of the burial history of shales in sedimentary basins. The results of a preliminary fabric analysis of Westphalian and Zechstein shales in the Campine Basin (Belgium) show that clay fabric analysis, using an X‐ray pole figure goniometer, is suitable for this purpose.Clay fabrics, in the range studied, are independent of depth and therefore cannot be used as depth indicators. This suggests that in the early stages of the burial history a stable clay fabric has to develop, which will basically remain unchanged during the subsequent burial history.The degree of clay particle preferred orientation not only reflects the compaction strain, but is also determined by mineralogical parameters: the presence of non‐platy particles and the relative concentrations of the different clay minerals. This degree of preferred orientation furthermore determines the degree of fissility of the shales.These mineralogical factors limit the use of clay fabrics as truly quantitative strain markers. Their use as semi‐quantitative strain markers remains advantageous, mainly because of the common occurrence of clay fabrics in the geological record. Moreover, the relative ease of measurement and the possibility of distinguishing compaction from tectonic strains favour the use of clay fabrics in the quantitative strain analysis of argillaceous

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01447.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTLarge pisoliths from the Laguna Pastos Grandes playa in the Bolivian Altiplano have a wide diversity of cortical fabrics and variable mineralogical composition. The cortical laminae are composed of radial calcite bundles, spar calcite, micrite, amorphous silica, mixed micrite‐amorphous silica, quartz, gypsum and halite. Diatoms are common in the outer parts of some radial calcite laminae and amorphous silica laminae, but cyanobacterial filaments are rare. Although the organization of the cortical laminae is highly variable, some repetitive sequences of different laminae are present. Cavities in and between pisoliths contain micrite, detrital grains, calcite bundles and peloids morphologically similar to those found in marine reefs.The pisoliths grow in shallow ephemeral pools fed by hot springs. Radial bundles of calcite precipitate rapidly by degassing and photosynthetic removal of CO2following spring snowmelt. Conditions for micrite precipitation are unclear, but there is evidence to suggest formation in partially stagnant waters, some of elevated salinity. Amorphous silica laminae precipitate mainly by evaporative concentration; quartz may precipitate from warm silica‐rich spring waters that remain below amorphous silica saturation. The evaporite minerals form during desiccation of the pools or from spray. The peloids in cavities are probably primary precipitates. Different types of laminae may form simultaneously in different pools because of the highly variable conditions across the playa.Lateral migration of spring locations through time has created a complex carbonate‐silica pavement. Large spherical pisoliths form in outflow channels near spring orifices and across discharge aprons where waters are several decimetres deep. With mineral precipitation, channels are filled and become shallow, producing discoid pisoliths and crusts. In shallow waters and on distal aprons only small pisolith gravels form. As spring pools fill with deposits, their locations shift laterally; new pisoliths form elsewhere or precipitation may recommence on older abandoned piso

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01448.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Permian Park City Formation consists of cyclically bedded subtidal to supratidal carbonates, cherts and siltstones. Early diagenesis of Park City Formation carbonates occurred under the influence of waters ranging from evaporative brines to dilute meteoric solutions and resulted in evaporite emplacement (syndepositional nodules and cements), as well as dolomitization, silicification and leaching of carbonate grains.Major differences are seen, however, in the diagenetic patterns of subsurface and surface sections of Park City Formation rocks. Subsurface samples are characterized by extensively preserved evaporite crystals and nodules, and preserve evidence of significant silicification (chert, chalcedony and megaquartz) and minor calcitization of evaporites. In outcrop sections, the evaporites are more poorly preserved, and have been replaced by silica and calcite and also leached. The resultant mouldic porosity is filled with widespread, very coarse, blocky calcite spar.These replacements appear to be multistage phenomena. Field and petrographic evidence indicates that silicification involved direct replacement of evaporites and occurred during the early stages of burial prior to hydrocarbon migration. Siliceous sponge spicules provided a major source of silica, and the fluids involved in replacement were probably a mixture of marine and meteoric waters. A second period of replacement and minor calcitization is inferred to have occurred during deep burial (under the influence of thermochemical sulphate reduction), although the presence of hydrocarbons probably retarded most other diagenetic reactions during this time interval. The major period of evaporite diagenesis, however, occurred during late stage uplift. The late stage replacement and pore‐filling calcites have δ13C values ranging from 0·5 to ‐25·3%, and δ18O values of ‐16·1 to ‐24·30(PDB), reflecting extensive modification by meteoric water. Vigorous groundwater flow, associated with mid‐Tertiary block faulting, led to migration of meteoric fluids through the porous carbonates to depths of several kilometres. These waters reacted with thein situhydrocarbon‐rich pore fluids and evaporite minerals, and precipitated calcite cements.The Tosi Chert appears to have been an even more open system to fluid migration during its burial and has undergone a much more complex diagenetic history, as evidenced by multiple episodes of silicification, calcitization (ferroan and non‐ferroan), and hydrocarbon emplacement.The multistage replacement processes described here do not appear to be restricted to the Permian of Wyoming. Similarly complex patterns of alteration have been noted in the Permian of west Texas, New Mexico, Greenland and other areas, as well as in strata of other ages. Thus, multistage evaporite dissolution and replacement may well be the norm rather than the exception in th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01449.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTDolomites from the upper calcareous‐siliceous member of the Miocene Monterey Formation exposed west of Santa Barbara, California, were analysed for geochemical, isotopic and crystallographic variation. The data clearly document the progressive recrystallization of dolomite during burial diagenesis in marine pore fluids.Recrystallization is recognized by the following compositional and crystallographic variations. Dolomites have decreasing δ18O and δ13C compositions, decreasing Sr contents and increasing Mg contents with increasing burial depths and temperatures from east to west in the study area. δ18O values vary from 5·3‰ in the east to − 5·5‰ PDB in the west and are interpreted to reflect the greater extent and higher temperature of dolomite recrystallization in the west. δ13C values correlate with δ18O and decrease from 13·6‰ in the east to − 8·7‰ PDB in the west. Sr concentrations correlate positively with δ18O values and decrease from a mean of 750 ppm in the east to a mean of 250 ppm in the west. Mol% MgCO3values inversely correlate with δ18O values and increase from a minimum of 41·0 in the east to a maximum of 51·4 in the west.Rietveld refinements of powder X‐ray diffraction data indicate that the more recrystallized dolomites have more contracted unit cells and increased cation ordering. The fraction of the Ca sites in the dolomites that are occupied by Ca atoms increases slightly with the approach to stoichiometry. The fraction of the Mg sites occupied by Mg atoms strongly correlates with mol% MgCO3. Even in early diagenetic, non‐stoichiometric dolomites, there is little substitution of Mg in Ca sites. During recrystallization, the amount of Mg substituting for Ca in Ca sites decreases even further. Most of the disorder in the least recrystallized, non‐stoichiometric dolomites is related to subst

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01450.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY