Shell Dissolution Research Articles

The concretions of the Bearreraig Sandstone Formation: geometry and geochemistry

ABSTRACTThe sandbodies of the Bearreraig Sandstone Formation (Inner Hebrides, UK) are cemented by two generations of calcite. The first generation, an inhomogeneous ferroan calcite (0.05−3.28 mol% FeCo3) formed during sulphate reduction (δ13C =−24 to −32%o PDB) in marine porewaters (δ18O of cement from −1 to −4%o PDB) at very shallow burial depths (a few centimetres). These cements are rare but form millimetre‐scale clusters of crystals which acted as nuclei to the later, concretionary cements.The second generation of cements are more homogeneous ferroan calcites (mean 1−58% mol% FeCo3) which evolve to progressively higher Fe/Mg ratios. They are sourced by shell dissolution (δ13C of cement from +1 to −3%o PDB) into meteoric (δ18O of cement from −6 to −10%o PDB) or mixed marine meteoric waters (δ18O of cement from −4 to −6%o SMOW). These were introduced into the formation either during Bathonian times as a freshwater lens, or, subsequent to partial inversion, by confined aquifer flow. Corroded feldspars within the concretions suggest that an interval of at least 8 Ma separated the deposition of the sediments from the onset of concretion growth.Abundant concretions are preferentially developed at certain horizons within the sandbodies, where the early generation of ferroan calcite cements provided nuclei. The latter formed close to the sediment‐water interface, the concentration of cement within the sediment being related to sedimentation rate. The relatively high concentrations of the first generation of cement, upon which the concretionary horizons are nucleated, formed during periods of minimal sedimentation.

Field relationships and stable isotope geochemistry of concretions from James Ross Island, Antarctica

Early diagenetic (precompactional) concretions are abundant throughout the Cretaceous-Tertiary Marambio Group Larsen Basin, Antarctica. Four distinct concretion types are recognised: (1) spherical-subspherical concretions: (2) sheet concretions; (3) fossil-nucleated concretions; and (4) concretionary burrow networks. All concretion types have a micritic to microsparry variably non-ferroan to ferroan calcite cement. Stable isotope analyses show a wide spread in both δ 18O and δ 13C. δ 13C values are typically negative, ranging between –3.38 and –39.15%o (PDB) (usually –16 to 230%). δ 18O ranges between –1.28 and –13.81% (PDB) with most of the values between –5 and –10%. The δ 13C signature is interpreted to represent carbon sourced from sulphate reduction and/or methane oxidation, with minor input from shell dissolution, and is consistent with a shallow burial, early diagenetic origin. A single mudstone hosted concretion has a δ 18O composition indicative of precipitation of carbonate from seawater. The low δ 18O signatures in the sandstone- and siltstone-hosted concretions are possibly due to early diagenetic modification of the pore water composition through volcaniclastic mineral dissolution/reprecipitation reactions and perhaps through input of meteoric water. Concretion distribution is related to (a) changes in sedimentation rate and (b) the dominance of diffusion on concretion cementation.

The effects of acidic seawater on three species of lamellibranch mollusc

Three species of commercial bivalve mollusc, Ostrea edulis, Crassostrea gigas and Mytilus edulis, were maintained in seawater over a pH range of 5.4–8.2 (control), normally for 30 days. Significant mortalities occurred in C. gigas at pH <6, in M. edulis at pH <6.6, in O. edulis young at pH ⩽6.9 (after 60 days) and spat at pH <7 after 18 days. Survival at a given pH level reduced with time of exposure and with increased temperature, and increased with size of animal. Growth suppression, tissue weight loss, reduced shell size, shell dissolution and suppressed feeding activity occurred at pH ⩽ 7. Abnormal behaviour analogous to narcosis (excessive shell gaping, torpor) occurred at pH ⩽6.5, possibly attributable to CO 2 excess. These and other collated results confirm that seawater at pH <7 is intolerable to bivalve molluscs.

Taphonomic Processes in the Waldron Shale, Silurian, Southern Indiana

The Waldron Shale (Silurian) in southern Indiana contains four lithofacies, ranging from very fossiliferous to nearly unfossiliferous, each with a distinct taphonomic history. Preburial taphonomic processes include primartily disarticulation of multielement skeletons, fragmentation, and limited dissolution. Transport distances were negligible even during storms. Storms resulted in shell-gravel lags in shallower water and wellpreserved smothered-bottom assemblages in deeper water. Taphonomic analysis of the fossils and stable isotope data (C, S) were used to identify a series of post-burial taphonomic events. From earliest to latest those events are: 1) pyrite shell replacement in reducing microenvironments, 2) pyrite mold formation and shell dissolution, 3) dolomitization, 4) precipitation of euhedral pyrite as overgrowths on fossils, and 5) precipitation of sparry calcite in the voids offossils partly replacing internal morphology. The effect of each event varied among taxa such that in any given sample the percentage of pyritized or dolomitized shells may range from 0 to 100% depending on the taxa. The severity of each event also varied among facies. Models based on the taphonomy of Recent shelled invertebrate communities (excluding sedimentological processes) may not be applicable to Paleozoic assemblages because of secular variations in shell mineralogy of the dominant shelled organisms, burrowing rates, and chemical and physical conditions of shallow marine environments. A high proportion of fossils, even juveniles, may have been preserved providing that favorable conditions existed in Paleozoic muddy seafloors.

Relative rates of shell dissolution and net sediment accumulation ‐ a commentary: can shell beds form by the gradual accumulation of biogenic debris on the sea floor?

Rates of shell production rarely exceed 500 g CaCO3.m-2yr-1 in clastic sediments. Loss of shell carbonate by dissolution greatly exceeds loss by bioerosion and abrasion in most habitats. Rates of shell dissolution in modern sediments, estimated from rates of organic carbon degradation or measured directly, usually exceed 1000 g CaCo3.M-2yr-1. This taphonomic loss is concentrated at or just below the sediment-water interface in the taphonomically-active zone (TAZ). Consequently, except where rates of shell production are very high or rates of organic carbon degradation very low, shells cannot permanently accumulate on the sea floor. Preservation requires rapid burial, usually by physical ‘event’ processes, to slow down taphonomic loss. Only near the base of the TAZ does the long-term sedimentation rate become an effective mediator of shell preservation as sediment accumulation gradually removes buried shell material from the taphonomically-active zone.

The influence of surface state and saturation state on the dissolution kinetics of biogenic aragonite in seawater

Laboratory observations of aragonite dissolution in seawater, under temperature and pressure conditions that approximate the natural oceanic environment, indicate that R = k{prime}((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})){sup n} is an appropriate expression describing the dependence of dissolution rate on seawater saturation, state, where R is aragonite dissolution rate in percent per day, (CO{sup 2{minus}}{sub 3}){sub s} is the carbonate ion concentration at saturation, and (CO{sup 2{minus}}{sub 3}) is the in-situ carbonate ion concentration. Under conditions in which the influence of surface alteration is minimized, plots of dissolution rate, R, versus ((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})) approach linearity. Consequently, in the absence of surface alteration effects, our results suggest that the reaction order, n, in the above expression should be equal to one. Use of single pteropod shells in extended experimental sequences indicates that progressive roughening of the shell surface by dissolution can substantially enhance shell dissolution rates. Surface alteration leads to variable values of the rate constant, k{prime}, in the expression above. For rate measurements obtained at increasing degrees of both undersaturation and shell roughness, the multiplicative factors k{prime} and ((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})) give rise to curvature in plotsmore » of rate versus ((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})). For models in which variations in k{prime} are not explicitly acknowledged, dissolution rates are generally described successfully with reaction orders (n) greater than one. Our experiments, performed at variable pressure, were modeled using several realistic partial molar volume changes ({delta} V) for aragonite dissolution in seawater.« less

Burial-dominated cementation in non-tropical carbonates of the Oligocene Te Kuiti Group, New Zealand

Cementation of bryozoan-echinoid-benthic foraminiferal temperate shelf carbonates of the Oligocene Te Kuiti Group, North Island, New Zealand, occurred mainly during subsurface burial. The calcite cements in the limestones are dominated by equant and syntaxial rim spar which typically becomes ferroan (given an iron supply) and, compared to the skeletal material with normal marine δ 18O values from +2 to −1‰ , more depleted in 18O with depth of burial, the δ 18O composition of bulk cement samples ranging from −1 to −7‰ . These trends reflect the establishment in pore waters during sediment burial of reducing conditions and gradually increasing temperatures (20–50°C), respectively. The δ 13C values ( 0 to +3‰ ) of the cements remain the same as the host marine shells, suggesting the source of carbon in the cements was simply redistributed marine carbonate derived from shell dissolution. Two gradational burial diagenetic environments influenced by marine-derived porewaters are arbitrarily distinguished: shallow burial phase and moderate burial phase. During the shallow burial phase, down to 500–600 m sub-bottom depth, the carbonates lost at least 25% of their original porosity by mechanical compaction and were selectively cemented by non-ferroan or usually ferroan, variably luminescent, slightly 18O-depleted sparry calcite cement (δ 18O −2 to −4‰ ), mainly as syntaxial rims about echinoid grains. These shallow-burial cements form less than about 10% of total cement in the majority of the limestones and their source was probably mainly mild intergranular dissolution of calcitic skeletal fragments accompanying the onset of chemical compaction. During the moderate burial phase, between about 600 and 1100 m sub-bottom depth, porosity loss continued (typically to about 70% of its original value) as a result of pressure-solution of calcitic bioclasts associated with more advanced stages of chemical compaction. This involved development of a wide variety of non-sutured and microstylolitic solution seams, including both single and composite, wispy or continuous, bedding-parallel types and non-parallel reticulate forms. The released carbonate was precipitated as ferroan (or non-ferroan where iron supply was negligible), dull luminescent, strongly 18O-depleted (δ 18O −4 to −7‰ ), mainly equant calcite spar cement, occluding available pore space in the limestones.

A comparison of bivalve (Calyptogena magnifica) growth at two deep-sea hydrothermal vents in the eastern Pacific

Abstract Analyses of specimens of a common deep-sea hydrothermal vent bivalve, Calyptogena magnifica, from two hydrothermal vent sites in the eastern Pacific, have been utilized to develop a mathematical model of the growth of this species based on accurate measurements of: (1) in situ rates of dissolution of the aragonitic outer granular shell layer; and (2) the thickness of the outer shell layer remaining at any given point in time at various distances from the umbo. The model permits the variances of each independently measured variable to be combined into a final confidence limit on age at a given size, making possible statistical comparisons of the calculated parameters of the von Bertalanffy growth equation. This model, in turn, provides a powerful tool for quantifying temporal and spatial variability in rates of biological processes both within and between deep-sea hydrothermal vent communities. Ontogenetic growth curves for C. magnifica specimens at both hydrothermal sites suggest that growth rates of this species are several orders of magnitude greater than those reported for the one bivalve (Tindaria callistiformis) analysed to date from a deep-sea, non-vent habitat. These data provide additional evidence that biological processes at submarine hydrothermal vents along the mid-oceanic ridge system proceed at rates that are extremely rapid for a deep-sea environment and are comparable with those from some shallow water temperate environments. Rates of shell dissolution decreased markedly with increasing distance from vent fluids. In a 20 cm distance spanning 10 cm into a bivalve assemblage atop an active vent at Rose Garden (Galapagos Rift) to 10 cm outside the assemblage, dissolution rates of the outer granular layer of C. magnifica declined 100-fold (from 355.4 to 3.5 μm y−1, respectively) in in situ exposures of approximately 210 days. At distances ranging from 1 to 6 m away from active vent sites, no measurable thickness (

Growth increments in the shell of the living brachiopod Terebratalia transversa

Scanning electron micrographs revealed growth increments in the primary shell layer of the extant terebratulid brachiopod Terebratalia transversa collected from Anacortes, Washington, USA, during the summers of 1982–1984. The increments extend into the secondary shell layer, but only as poorly-defined continuations. Growth increments narrow and widen cyclically, producing patterns similar to tidally-induced, fortnightly growth patterns in bivalve molluscs. “Asymmetric” increments or “doublets” that consist of paired wide and narrow bands present additional evidence that the growth pattern is produced by the interference of tidal (24 h 50 min) and solar (24 h) daily environmental influences on growth, as is the case in bivalves. Growth increments are relatively well-defined in specimens from tidal habitats and are poorly-defined in subtidal specimens. Specimens grown in laboratory aquaria and subjected to simulated tidal emersions and submersions had well-defined increments, whereas those that were continuously submerged in aquaria had poorly-defined increments, paralleling the distinctness in growth increments produced at various depths in the natural habitat. Production of growth increments in T. transversa may be related to valve movement and changes in mantle-cavity pH. When the valves are closed, pH drops and, when they are open, pH rises. Highperformance liquid chromatography (HPLC) of mantle tissue and fluid failed to detect any single organic acid that could have been responsible for the pH changes, confirming that intermediate metabolism in T. transversa is very low compared to that of bivalve molluscs. Calculations indicate that cycles in intermediate metabolism may nevertheless be responsible for cycles in growth-increment production and shell dissolution.

Sedimentology and reservoir potential of a ‘marginal facies’ limestone from the Cretaceous of the North Sea

The lower Cretaceous section of UK North Sea well 3/16-1, on the eastern flank of the Shetland Platform, drilled by Sovereign Oil & Gas plc, is dominated by marine ‘shelf-type’ limestones with a very low argillaceous content. In this respect it differs both from the typical open-sea coccolith-rich Chalk and from the argillaceous Cretaceous of the Viking Graben (the Cromer Knoll and Shetland Groups). Diagenesis has resulted in crystal overgrowths, corrosion, and development of very fine to coarsely crystalline void filling calcite. Cavities, of stromatactis type and the result of shell dissolution, are filled with radiaxial fibrous calcite. Porosity is mostly intercrystal with possible contributions from fractures and vugs. Hydrocarbon staining is confined to patches of void-filling calcite and partially cemented fractures. No oil staining was observed in the carbonate matrix. It therefore appears that oil migrated through the rock via fractures but was unable to penetrate the main fabric of the rock. The lithological and petrophysical characteristics of the interval are similar to the Chalk Group of Norwegian well 1/3-1. The thickness is considerably less in 3/16-1, probably as a result of deposition on a topographic high such as existed along the margins of the Viking Graben and the East Shetland Platform during the Cretaceous.

The effects of acidic sea water on young carpet-shell clams Venerupis decussata (L.) (Mollusca: Veneracea)

Young carpet-shell clams Venerupis decussata (L.) were maintained in sea water over a pH range of 3.5 to 8.2 (control), for periods from 8 to 30 days. Shell dissolution occurred at pH ⩽ 7.55. At pH < 7.0 feeding was inhibited, and both tissue and shell growth were significantly reduced. There were ⩾ 50% mortalities over the time scale of the experiment at pH ⩽ 6.5, with smaller (3–4 mm) clams more sensitive than larger (7–9 mm) clams. The animals showed reduced activity at pH<6.0, with abnormal feeding behaviour, exhibited by hyper-extension of the siphons; greater time spent with the valves closed at these pH levels may have marginally reduced the deleterious effects. It is concluded that pH levels below those found naturally in the sea are intolerable to young V. decussata.

Quaternary paleo-oceanography, pteropod preservation and stable-isotope record of the Red Sea

An abundance of pteropod species, their mode of preservation, and stable isotope ratios of carbon and oxygen in benthic and planktic foraminifera have been determined in core MD76-140 from the northern Red Sea. Isotopic data and benthic foraminiferal species indicate changes in O 2 concentration in the bottom sediments. The glacial stages are characterized by an almost anoxic bottom, while the interglacial bottom was generally well ventilated. In general very well preserved pteropod shells occur in glacial oxygen-depleted intervals, and also on recent well ventilated bottoms. In the interglacial stages the preservation deteriorates, and in the most extreme cases there is a complete dissolution of the aragonitic shells. In these cases most of the assemblage is lost and the only traces of the original shells are their internal molds. These are cemented with high magnesium calcite (HMC) which is a more stable carbonate phase in bottom sediments of the interglacial. Dissolution of aragonite discriminates between large and small shells, the latter tend to be removed completely, while the larger ones may be preserved as internal HMC molds. Limacina inflata is more readily molded than L. trochiformis, and thus its relative abundance increases in poorly preserved assemblages. The state of aragonitic shell preservation depends on the degree of organic matter combustion in the sediments. Well preserved shells occur in sediments where the supply of organic matter is limited and O 2 levels are high. Preservation deteriorates with increasing supply of organic carbon, O 2 consumption and increasing production of CO 2. In this case the dissolved carbonate is reprecipitated as HMC cement. With further increase in organic matter supply and reduced ventilation, nearly all oxygen is consumed and NO 3 − and subsequently SO 4 2− become the oxidizing agents. This is followed by an increasing carbonate alkalinity and excellent aragonitic shell preservation.

Effects of animal behavior on sediment trap collections: implications for the calculation of aragonite fluxes

Collection of living organisms can seriously bias the results of sediment trap collections, and the behavior of planktonic animals may cause certain species to be collected preferentially. We studied the effects of the escape response of one genus of pteropod on the rates of its collection by sediment traps. This model system was chosen because a great deal is known about the field behavior of pteropods, and because a recent paper ( BeTZER et al., 1984, Science, 226, 1074–1076) has proposed that the dissolution of sinking pteropod shells could almost balance the alkalinity budget of the Pacific Ocean. In three sediment trap deployments off the coast of Greenland, we found that large numbers of living Limacina spp., in contrast to copepods and larvaceans, were rapidly collected in a few hours. Since this artifact leads to a gross overestimate of aragonite flux, we conclude that the assertion of BETZER et al. (1984) is not proved. There are likely to be a number of other planktonic organisms whose behavior can bias sediment trap collections. Therefore, the results of short-term flux measurements made with shallow sediment traps should not be interpreted without proper controls and more knowledge of the behavior of planktonic animals.

Shell dissolution and mortality in the freshwater snailViviparus ater

Populations of the prosobranch snailViviparus ater (Christ. and Jan) in Lake Zurich and Lake Maggiore (Switzerland) were compared in order to evaluate how soft water in Lake Maggiore affects shell morphology and population dynamics. A low calcium concentration of the water (3.5 mg CaCO3l−1) was the main reason for shell dissolution in Lake Maggiore. At some places shell damage was augmented by a stony ground and wave action. In Lake Zurich (11.5 mg CaCO3l−1) the shells were not damaged. The snails in Lake Maggiore were smaller, suffered higher mortality and had a shorter life span than those in Lake Zurich.

Taphonomy and ichnology of cephalopod shells in a Maastrichtian chalk from Western Australia

The post-mortem history of a prolific Maastrichtian ammonite and nautiloid fauna preserved as phosphatic steinkerns in chalk of the upper Miria Formation of Western Australia is described. Sediment infilling of phragmocones, required for their fossilisation, was accomplished by means of perforations in the shell wall induced by the activity of abundant endoliths. These include borings ascribed to clionid sponges (Entobia), thallophytes, polychaete worms (including Caulostrepis and probable Maeandropolydora), phoronids (Talpina and Gnathichnus) and others of conjectural origin. Sediment infilling by this mechanism is considered to be more applicable to the taphonomy of phragmocones in general than sediment entry through the siphuncle and to be indicative of low sedimentation rates for the hosting strata. Nonheteromorph ammonites, and the nautiloid Cimomia, are preserved almost exclusively as phragmocones in the upper Miria Formation, and are numerically subordinate to the heteromorph Eubaculites which, together with Glyptoxoceras, is preserved predominantly as body chambers. This phragmocone/body chamber preservational contrast is attributed to the influence of shell shape on pre-burial mechanical abrasion. The apparent dominance of Eubaculites is considered to be largely a preservational artifact and ascribed to the ease with which body chambers were infilled, and thereby favoured for steinkern formation, relative to phragmocones. Phragmocones, or parts thereof. not filled with sediment were eliminated from the fossil record by diagenetic aragonite dissolution. Many ammonites with open umbilici have the early whorls missing. This we attribute to the trapping of sediment in the umbilicus prior to burial, preventing endolith attack whereby the early whorls avoided a sediment infilling. However, the protected inner whorls of involute ammonites and Cimomia, which have closed umbilici, and the inner whorls of evolute ammonites where a cemented umbilical plug supported the mouldic cavity left by shell dissolution, were preserved and were commonly infilled with calcite spar later in diagenesis. The dearth of cephalopod fossils in chalk underlying the upper Miria Formation is ascribed to diagenesis in which aragonite dissolution was not preceded by cementation.

Biodegradation of temperate-water skeletal carbonates by boring sponges on the Scott shelf, British Columbia, Canada

Boring sponges are a major agent of shell destruction and, in the process, an important contributor of fine-grained carbonate detritus in temperate skeletal carbonate deposits on Scott shelf, northwestern Vancouver Island. Intensive boring is encouraged by the generally low rates of carbonate production and sedimentation. Skeletal grains become considerably weakened by sponge boring and break preferentially along the directions of sponge-excavated tunnels and galleries within the shell. Boring also facilitates shell dissolution by significantly increasing the surface area—weight ratio of shells and by breaking down the organic matrix of shells. The large aragonitic bivalves Glycymeris and Humilaria appear especially susceptible to sponge biodegradation and have become thoroughly bored and corroded within about 1000 years in ambient shelf waters; thus, their preservation potential is low. Selective loss of aragonitic biota from ancient cool-water shelf skeletal carbonate deposits is anticipated and is evident in many temperate-latitude occurrences of New Zealand Cenozoic limestones.

Variations in the Rate of Anaerobic Succinate Accumulation within the Central and Marginal Regions of an Euryoxic Bivalve Mantle

. Anaerobic metabolism in the central and marginal portions of the mantle of Mercenaria mercenaria was compared. Anaerobic succinate accumulation was more rapid in the central region. This difference may be due to higher phosphoenolpyruvate carboxykinase activity in the central region. Thus, the central region is more specialized for anaerobic metabolism and the marginal region more for net shell growth. The original rate of succinate accumulation in the mantle is similar in isolated mantles and intact clams, suggesting that mantle succinate production does not require translocation of precursors from other tissues. However, in intact clams, the rate of succinate accumulation in the central region of the mantle slows after four hours. The reduced rate is probably caused by reducing the metabolic rate. Succinate accumulation and shell dissolution are slower in freshly collected clams than in clams that had been stored anaerobically. The difference may be due to induction of PEPCK synthesis during storage. Shell derived calcium did not accumulate in the mantle and, therefore did not alter the intracellular calcium concentration in the mantle.

Deposition and Diagenesis of Horquilla Carbonates, Big Hatchet Peak Section, Pedregosa Basin: ABSTRACT

In the Big Hatchet Peak section the Pennsylvanian-Permian (Morrowan to Wolfcampian) Horquilla Formation is 3,230 ft (985 m) thick and consists of shallow shelf carbonates which accumulated along the outer shelf margin of the Alamo Hueco basin. The lower Horquilla (Morrowan to Desmoinesian) is 1,363 End_Page 762------------------------------ ft (415 m) thick and consists largely of micrite and fossiliferous limestones with abundant chert in many beds. Depositional cycles, consisting of grainstones, packstones, and wackestones, occur throughout the interval. The upper Horquilla (Desmoinesian to Wolfcampian) is 1,867 ft (569 m) thick and consists of limestones and dolostones; chert is less abundant than in the lower Horquilla. Phylloid algal biostromes are present in some upper Horquilla limestone intervals. Six laterally extensive dolostone units range in thickness from 48 to 154 ft (15 to 47 m). In both upper and lower Horquilla limestones many grainstones were subjected to intertidal and subtidal cementation within marine environments. Micritization of ooids and skeletal grains, which also developed in the marine environment, is abundant in most Horquilla limestones. Nearly all Horquilla limestone intervals bear two or more of the following evidences of having stabilized within fresh-water diagenetic environments: (1) selective dissolution of aragonitic shells and formation of hollow micrite envelopes; (2) precipitation of meteoric vadose and phreatic calcite cements; (3) recrystallization of original lime mud matrix; (4) replacement by silica which first nucleated within ooids, shells, and peloids and then invaded the surrounding matrix to form nodules or discontinuous layers. At one stage most Horquilla limestones contained large volumes of primary and/or secondary porosity, all or virtually all of which has been occluded by subsequent precipitation of marine or meteoric cements. All upper Horquilla dolostones were formed by neomorphic dolomitization of originally unstable carbonate sediments. At one stage considerable secondary intercrystalline and moldic porosity existed in most dolostone intervals; much of this was subsequently obliterated by epitaxial dolomite cements on original rhombs and by coarse recrystallization. Gravitational calcite cements partly filled many of the larger voids, and outer tips of many of these microstalactites subsequently became paramorphically dolomitized. In the upper Horquilla, anhydrite porphyroblasts and nodules were emplaced in many limestone and dolostone intervals by hypersaline groundwaters to form tertiary (third order) stairstep molds, many of which are still open in dolostones. The only preserved porosity is in dolostones and these may represent potential hydrocarbon reservoirs in the subsurface. End_of_Article - Last_Page 763------------

The Sedimentology, Stratigraphy, and Preservation of Intertidal Deposits in the Minas Basin System, Bay of Fundy

ABSTRACT Intertidal sediments of the Minas Basin, a macrotidal estuary at the head of the Bay of Fundy, can be subdivided into six major facies: tidal marsh, beach, mudflat, sandflat, channel lag, and sandbar. Each is characterized by its grain size distributions, sedimentary structures, and biological communities. These sedimentary facies occur in three types of high-to-low tide zonation, depending on the local sediment source and wave and current energy. The Holocene stratigraphy presently preserved in the upper intertidal zone of Minas Basin consists of a relatively thin (on the average less than 1-2 m) wedge of sediment overlying bedrock. Only in a few sheltered locations, where deposits accumulate to several meters, is the marine transgressive stratigraphy well represented. The hypothetical progradational or marine regressive stratigraphy of upper intertidal sediments would probably resemble, from top to bottom, the high to low tide sequence of facies occurring today on sheltered tidal flats. Lebensspuren would provide the most reliable representation of the ecology of intertidal sediments in Minas Basin. They are useful in delineating communities as well as being sensitive indicators of sediment deposition. Microfauna are too easily transported or reworked. Field data from relict intertidal deposits (dated 2300-3400 yr. B.P.) indicate calcareous organism hard parts would probably provide a biased fossilized record. Indirect evidence is discussed, which suggest in situ dissolution of shells in certain intertidal sediments.

Role of a Unionid Clam Population in the Calcium Budget of a Small Arctic Lake

Growth rate and morphometric parameters were estimated for the Anodonta grandis (Pelecypoda, Unionidae) population in Shell Lake, N.W.T. (68°N), based on sampling during the summer of 1972. Variation in density and size of Anodonta within the lake as a function of varying depth and slope of bottom was estimated by SCUBA during the summer of 1973. Predictive models based on these data estimate total calcium in shells of the Anodonta population to be 10.1 Mg, which is approximately half the amount of calcium dissolved in Shell Lake water. The youngest four or five year-classes are missing from the estimated age distribution, and recent pollution may be a factor. Independent estimates of calcium used for shell growth, and of calcium released by death of individuals and subsequent shell dissolution, approximate 50% (of the calcium in the Anodonta population) per year. This implies a mean residence time in the Anodonta population of approximately 2 yr.Key words: calcium, clam, unionid, lake, arctic