Calcitic Fossils Research Articles

The Use of Mg/Ca as a Seawater Temperature Proxy

The underlying basis for Mg/Ca paleothermometry is that the amount of magnesium in calcite precipitated from seawater is dependent on temperature. Here we review the state of the art of the Mg/Ca seawater paleotemperature proxy, summarized by the following: 1) Calcite, whether formed abiotically or biologically as foraminifera and ostracode shells, incorporates variable amounts of magnesium into the crystal structure. 2) Uptake of Mg varies positively with temperature. 3) The relationship between temperature and the amount of Mg in calcite has been quantified by experiments on synthetic calcite growth and by culture, core top, and sediment trap experiments using living organisms. 4) The most careful calibrations of the Mg/Ca paleothermometer have been done for planktic foraminifera, then benthic foraminifera; there are species-specific variations in the amount of Mg incorporated into foraminifera shells. 5) The Mg/Ca ratio of calcite from planktic foraminifera in deep-sea cores has been widely used to interpret sea surface temperatures. 6) Measurement of both Mg/Ca and δ18O in planktic foraminifera have been used to calculate δ18O in seawater, and after correction for global ice volume, salinity could be inferred. 7) Mg/Ca from benthic foraminifera have been used to reconstruct deep-sea temperatures and cooling of ~12° over the last 50 million years. 8) One problem with the Mg/Ca seawater temperature proxy is partial dissolution of foraminifer shells, which lowers the Mg/Ca, and leads to an underestimation of ocean temperature. Benthic foraminifers appear to be more resistant to partial dissolution. 9) Past changes in the Mg/Ca ratio of seawater are an important factor in determining the amount of Mg in fossil skeletal calcite, and thus add another variable to the Mg/Ca temperature proxy. All Mg/Ca paleotemperature studies on fossil calcite older than Pleistocene should take into account the Mg/Ca of the seawater from which it precipitated.

Tectonic sedimentology: sediment systems deciphering global to local tectonics

AbstractTectonic sedimentology has broad appeal because it can be applied at many time and length scales, such as sedimentary consequences of: changing rates of plate creation at mid‐ocean ridges, continent–continent collisions, plateaux uplift, active plate margins, extensional rift tectonics and the birth, growth and death of individual folds and faults. Tectonic sedimentology deals with the direct influence of neotectonics on sedimentation and basin development, and with the inverse problem of reconstructing the order and development of palaeotectonic structures. This review attempts to show how the scope and importance of the subject has developed over the last decade and is divided into five themes: (i) At the largest tectonic scale, constraints on mid‐ocean ridge activity and continent–continent collisions come from: (a) precise chemical analyses of fluid inclusions in marine halite deposits dating back to ca 550 Ma; (b) Sr‐isotopic ratios of marine Phanerozoic calcitic fossils; and (c) chemical composition of Archaean and Proterozoic banded iron formations. (ii) At regional scales, constraints on the timing and extent of continental plateau uplift/incision and cratonic tilting are given by: (a) palaeoaltimetry estimates from oxygen isotope depletion in carbonate palaeosols; (b) radiometric dating of palaeowater‐table speleothems; and (c) preferential river migration. (iii) At basin scales, catchment processes are emphasized as key controls on sediment flux from tectonic uplands to sedimentary basins. The inadequacy of sediment transport algorithms based on tractive stress and flow power to describe erosion, incision and sediment transport during the most formative catchment hydrological events is emphasized. (iv) Rates of basin‐scale fault and fold growth, displacement, death and basement uplift are provided by application of facies analysis and sequence stratigraphy to accurately‐dated sediments in sedimentary basins. (v) The effects of changing climate and varying basin base‐level in modifying tectonic influences on erosion and sedimentation are examined. In such cases, experimental and numerical modelling using multi‐variate parameters of subsidence rate, sediment supply and sea‐level change can shed light on the problem. Future developments will undoubtedly improve such models, hopefully establishing full linkage and feedback between sediment/water supply and changing climate within an overall tectonic framework that may itself be changing.

Studies of Biominerals Relevant to the Search for Life on Mars

The evidence of the water erosion on Mars is particularly interesting since present climatic conditions are such that liquid water cannot exist at the surface. But, if water was present on the planet in the past, there may have been life, too. Since the discovery of carbonates on Mars also may have very important implications on the possibility that life developed there, we are studying minerals that can have biotic or abiotic origin: calcite (CaCO(3)) and aragonite, a metastable state of calcite.We have analysed biomineral aragonite, in the form of recent sea shells, as well as crystals of mineral aragonite. Infrared spectroscopy in the 2-25 mum wavelength range reveals that, after thermal processing, the biotic samples have a different spectral behaviour from the abiotic ones. As a result, it is possible to distinguish abiotic mineral aragonite from aragonite of recent biological origin.Obviously, if life existed in the past on the Red Planet, we could expect to find "ancient" biotic carbonates, which should therefore be investigated, in order to search for a way of discriminating them from abiotic minerals. For this reason, at the beginning we have considered samples of crushed fossil shells of aragonite composition. Afterwards, in order to take into account that fossilization processes almost always produce a transformation of metastable form (aragonite) into more stable form (calcite), we also studied samples of mineral calcite and different types of fossils completely transformed into calcite. All these biotic fossil samples show the same spectral behaviour as the fresh biotic material after thermal annealing at 485 degrees C. Instead, the calcite behaves like abiotic aragonite.Furthermore, it is known that seashells and other biominerals are formed through an intimate association of inorganic materials with organic macromolecules. The macromolecules control the nucleation, structure, morphology, crystal orientation and spatial confinement of the inorganic phase: this differentiates biominerals from minerals. Analysing the aragonite or calcite fossils with a Scanning Electron Microscope, we found that the fossilization process did not modify the structure of the biominerals which maintain their microscopic characteristics. Looking at the morphology of fossil biominerals, it is evident that the crystals are arranged in complex architectures compared with the compact structure of the mineral crystals. In conclusion, the properties and structure of the biominerals are different from those of the minerals. The rapid increase of the crystalline structure developed under biotic conditions makes these minerals less resistant to thermal treatments, compared with samples of abiotic origin. This result holds both for recent shells as well as all fossil samples. The spectroscopic behaviour of all analysed calcium carbonates of biotic origin is different from that of the abiotic one. Therefore, the infrared spectroscopy is a valid technique to discern the origin of the samples and a powerful tool for analysing in-situ and "sample-return" Mars missions specimens. Also Optical and Scanning Electron Microscopy can be useful to support this type of studies.

Nanoscale effects of strontium on calcite growth: An in situ AFM study in the absence of vital effects

This experimental study presents in situ measurements of step migration rates for layer growth of calcite at various levels of superaturation and fluid Sr concentrations. Our results show that Sr has complex behavior as an impurity. At low concentrations, Sr promotes faster growth. This effect may be associated with slight shifts in calcite solubility when Sr is incorporated or may be due to as yet uncharacterized kinetic effects. At higher concentrations, Sr stops step advancement by pinning kink-sites or step edges. The threshold concentration of Sr needed to halt growth is positively correlated with supersaturation. Addition of Sr to the calcite growth system leads to significant changes in hillock morphology. Hillocks become elongate perpendicular to the projection of the c-glide plane, in contrast to the changes previously reported for Mg. Step edges also become scalloped, and the boundary between the obtuse-stepped flanks disappears and is replaced by a new step direction with edges parallel to [010]. Incorporation of Sr was measured at two supersaturation levels and identical fluid [Sr]. The results indicate a strong positive correlation between fluid supersaturation and crystal Sr content. Further, Sr is strongly fractionated between obtuse- and acute-stepped flanks by a factor of approximately two. The sensitivity of Sr uptake to supersaturation may explain apparently contradictory results in the literature regarding whether Sr uptake in the calcite produced by one-celled marine organisms is controlled by temperature. In addition, Sr contents of natural calcite samples may be good indicators of the levels of supersaturation at which the crystals grew. Results of this investigation demonstrate the importance of understanding impurity-specific interactions with calcite growth surfaces at the microscopic scale. Despite similar chemical behavior in some systems, Mg and Sr clearly have very different effects on calcite growth. If Sr and other impurities are to be used as robust indicators of growth conditions in natural calcite samples, well grounded understanding of the mechanisms of recording trace element signatures in calcite is an essential step toward correctly deciphering paleoenvironmental signals from fossil calcite compositions.

Grain size distributions of chalk from image analysis of electron micrographs

In the chalk of the Ekofisk formation in the Chalk Group of the North Sea, substantial depth-related variations in porosity are observed. With the aim of obtaining a textural interpretation of these porosity data, we have developed a method to assess the grain size distribution of the chalk from image analysis. The chalk is composed of a fine-grained matrix of nannofossils and predominantly calcitic fossil debris with larger microfossil grains, but the chalk may also contain significant amounts of silica and siliciclastic clay. For image analysis, we used backscatter electron images of epoxy-impregnated, polished samples from the Ekofisk Formation, Tyra field, Danish North Sea. On backscatter images the calcite phase will appear light as opposed to the dark pore space. The procedure involves the combination of data acquired at two magnifications and by this method analysis of only four images per sample is sufficient. The two applied magnifications result in images measuring 300 μm×400 μm and 30 μm×40 μm, respectively. The method does not build on image analysis alone because the siliciclastic clay is not resolved at these magnifications but will appear grey; and because the total porosity is difficult to assess from image analysis due to rim effects inherent in backscatter images at high magnification. Thus, in order to obtain a consistent interpretation, we use total (He) porosity and insoluble residue as measured in the laboratory. We find that the volume density of larger grains (cross section larger than 316 μm 2) relative to solids (one minus porosity) primarily controls the porosity of the chalk samples.

Primary and diagenetic isotopic signals in fossils and hemipelagic carbonates: the Lower Jurassic of northern Spain

Stable isotope and trace element analyses of 230 Jurassic (Pliensbachian–Toarcian) samples from northern Spain have been performed to test the use of geochemical variations in fossils (belemnites and brachiopods) and whole‐rock hemipelagic carbonates as palaeoceanographic indicators. Although the succession analysed (Reinosa area, westernmost Basque–Cantabrian Basin) has been subject to severe thermal alteration during burial diagenesis, the samples appear to be well preserved. The degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic, chemical and cathodoluminescence analyses. It is demonstrated that brachiopods and whole‐rock carbonates, although widely used for palaeoceanic studies, do not retain their primary marine geochemical composition after burial diagenesis. In contrast, there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock. Well‐preserved belemnite shells (non‐luminescent to slightly luminescent) typically show stable isotope values of +4·3‰ to –0·7‰δ13C, +0·7‰ to –3·2‰δ18O, and trace element contents of <32 μg g–1Mn, <250 μg g–1Fe, >950 μg g–1Sr and Sr/Mn ratios >80. This study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low‐Mg calcite fossil shells and hemipelagic bulk carbonates, behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated. Multiple non‐luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders, yet their δ13C and δ18O values exhibited moderate scatter. Such variability is likely to be related to the palaeoecological behaviour of belemnites and/or high‐frequency secular variations in sea‐water chemistry superimposed on the long‐term isotopic trend. A pronounced positive carbon‐isotope excursion (up to +4·3‰) is documented in the early Toarcianserpentinusbiozone, which correlates with the Toarcian δ13C maximum reported in other European and Tethyan regions.

Thermoluminescence dating of the Ryukyu Limestone

Thermoluminescence [TL] dating of calcite is useful for corals in the Pleistocene Ryukyu Limestone in Okinawa Island, in which no aragonite is found. Fossil calcite corals in the Ryukyu Limestone have a variety of induced TL glow curves with peaks at about 150, 210, 250 and 310°C. The TL growth of a group of them exhibits supralinear behavior, while that of another group exhibits saturation behavior. Almost all of the TL ages of the corals which show the supralinear behavior are roughly consistent with ages estimated from geology and uranium-series and ESR ages.

The δ44Ca‐temperature calibration on fossil and cultured Globigerinoides sacculifer: New tool for reconstruction of past sea surface temperatures

We report direct δ44Ca‐temperature calibration on cultured and fossil calcite foraminifera, showing that Ca isotopes are potentially a new proxy for past sea surface temperatures (SST). Samples have been analyzed using a 43Ca‐48Ca double spike and thermal ionization mass spectrometry (TIMS). In order to avoid species‐dependent isotope fractionation we focused our investigations on a single foraminifera species (Globigerinoides sacculifer), which is known to inhabit shallow euphotic waters in tropical and subtropical oceans. Ca isotope measurements were performed on cultured G. sacculifer that grew in seawater kept at temperatures of 19.5°, 26.5°, and 29.5°C. A δ44Ca change of 0.24 ± 0.02 per 1°C is defined by the weighted linear regression through reproduced δ44Ca data of the three temperatures (95% confidence level). Application of this new method to fossil G. sacculifer of an Equatorial East Atlantic sediment core (GeoB1112; 5°46.7′S, 10°45.0′W, 3125 m) indicates that the δ44Ca difference between marine isotope stage 1 (MIS‐1) and MIS‐2 is 0.71 ± 0.24. According to the current δ44Ca‐temperature calibration this value corresponds to a temperature difference between MIS‐1 and MIS‐2 of ∼3.0 ± 1.0°C.

Paleoecology and Taphonomy of Faunal Assemblages in Gray "Core" (Offshore) Shales in Midcontinent Pennsylvanian Cyclothems

Faunal composition and diversity of macroinvertebrate assemblages in the gray facies of nineteen core shales of Mid-continent Pennsylvanian cyclothems confirm the offshore marine origin previously established for those shales. Diverse assemblages of only calcitic fossils that show grain corrosion at northern Midcontinent localities (Nebraska/Iowa) indicate deposition under oxic to slightly dysoxic conditions within the calcite lysocline but below the aragonite compensation depth for this particular sea. Southward (Kansas), sparser calcitic faunas indicate that core shales there were deposited farther from the northern shoreline in water of greater depth and less bottom oxygen, closer to or locally below the calcite compensation depth. Farther southward near the Ouachita detrital source (Oklahoma), greater influx of sediment permitted rapid burial of shelled organisms to overprint the effects of lysoclines and compensation depths. Both the originally aragonitic molluscs as well as calcitic organisms are preserved in those shales. Fine detrital grain size, radiolarian-bearing nonskeletal phosphate, and pelagic fauna (ammonoids and conodonts) further substantiate an offshore marine origin for the black core shale facies that is surrounded by the gray facies, and which was deposited in a probably deeper, more continually anoxic environment where most benthos was excluded.

Diagenetic alteration of calcitic fossil shells: Proton microprobe (PIXE) as a trace element tool

Trace element contents (Mn, Fe, Sr) of a set of Lower Carboniferous brachiopod shell fragments have been measured using the Bochum proton microprobe. The resulting data are in good agreement with ICP measurements of the same shells and serve as a quantitative criterion to constrain the preservation state of the sample material for oxygen isotope analyses. They thus help to interpret correctly the isotope data. Linescans and elemental maps yield information on the lateral distribution of trace elements.

Structure of recent and fossil mysid statoliths (Crustacea, Mysidacea).

Statoliths of 61 Recent species representing all subfamilies of Mysidae were studied with special emphasis on internal structure. In addition 5 samples of fossil statoliths from Miocene deposits were examined. Species of Boreomysinae and Rhopalophthalminae show simple roughly spherical organic statoliths, with setae originating from the sensory cushion and anchored in the statolith with distal branches extending shortly below the surface. All other subfamilies possess mineralized statoliths of greater structural complexity, with differentiation in core and mantle, where each part may consist of up to three layers. Habitus is hemispherical to discoidal. External gross structures are dorsal tegmen, ventral fundus, and the ambitus forming the outer toroidal to semi-toroidal circumference. Setae penetrate the mantle through mineralic canals and insert on the surface of the core. As suggested by congeneric species of Schistomysis, there is no principal structural difference between statoliths mineralized with fluorite compared to vaterite. However, vaterite statoliths tend to be more often of moruloid appearance and are exceptional by showing a central conical hole (the hilum) or a central cavity in certain forms. These structures are typical of fossil calcite statoliths. In vaterite and fluorite statoliths, the mantle shows radially arranged (= spherulitic) crystal aggregates. Such arrangements are badly preserved in fossil calcite statoliths. In large extant statoliths, concentric structures, mainly in the form of superficial striation and/or concentric microstrata, are visible in coexistence with radial aggregates. Stratification is possibly due to stratified deposition of the nonmineralized gland product, while the spherulitic structure is indicative of subsequent radial growth of crystal aggregates. The structure of accessory fluorite statoliths in the statocyst of Mesopodopsis slabberi leads to the hypothesis that mantle material is formed by secretions of the caudal statocyst gland. After demineralization of fluorite, vaterite and calcite statoliths, an organic template remains showing most essential morphological features of the statolith. From this we conclude that the structure of the statolith is (almost) entirely matrix mediated. © 1993 Wiley-Liss, Inc.

Dissolution of high-Mg calcite fossils and the formation of biomolds during mineralogical stabilization

Biomolds formed by the dissolution of high-Mg calcite (HMC) porcellaneous foraminifera occur in Holocene limestones from the Schooner Cays, Bahamas, and in Pleistocene limestones from the Belmont Formation of Bermuda and the Lucayan Formation of the Bahamas. In all of these cases, the HMC biomolds are associated with either a water table or a subaerial exposure surface. A single foraminiferal test may exhibit both a moldic fabric and a low-Mg calcite (LMC) recrystallization fabric that preserves original texture. Other types of HMC bioclasts, such as red algae and echinoids, do not exhibit biomolds but are mineralogically stabilized. The fact that foraminifera are more susceptible to dissolution and biomold formation than other types of HMC bioclasts indicates that an intrinsic factor, such as skeletal microstructure, is partially responsible. However, because both mold formation and texture-preserving recrystallization may occur in the same grain, intrinsic properties of the bioclasts alone do not determine the alteration pathway. An extrinsic factor is also implicated. It is proposed that this factor is CO2 flux across diagenetic interfaces during mineralogical stabilization. Abrupt and large influxes of CO2 across a subaerial exposure surface or water table will reduce saturation states in bulk pore waters and within skeletal grains. Recrystallization by spatially and temporally synchronous dissolution and precipitation, which is the normal HMC alteration process, will cease and only dissolution of HMC will occur. Bioclasts with high reactive surface areas, such as foraminifera, will be most susceptible to this dissolution. Once the CO2 influx has been buffered, HMC-to-LMC recrystallization will resume. In this manner, an originally HMC bioclast can exhibit the products of both dissolution and recrystallization pathways. This would be particularly true if recharge, and thus CO2 flux, is a seasonal phenomenon, as is the case in most carbonate regimes.

The carbon and oxygen isotopic records of fossils from the Lower Oxford Clay

The excellent preservation of calcareous invertebrates and phosphatic vertebrates in the Lower Oxford Clay provides a good opportunity for paleooceanographic reconstruction based on stable isotopic abundances. We present here our initial results and interpretations on carbon and oxygen isotopic analyses on fossils of different depth habitats. Benthic fossils include epifaunal oysters and infaunal nuculacean bivalves. We also analyzed “pendant” bivalves whose depth habitat is uncertain. Fossil nekton are represented by ammonites and belemnites. Organisms that inhabited the uppermost part of the water column are represented by marine reptiles, such as icthyosaurs and plesiosaurs, and probable pelagic fish.The oxygen isotopic compositions of calcareous benthos and nekton overlap substantially (δ180 = −2 to +1 permil vs. PDB). The wide scatter in δ180 values probably reflects physiological (non-equilibrium) effects in calcification rather than paleoenvironmental variations. Mean δ180 values for oysters, pendant bivalves, and belemnites (all calcitic) and nuculacean bivalves (aragonitic) correspond to precipitation at isotopic equilibrium with non-glacial seawater at temperatures of 15°-18°. The mean isotopic paleotemperature for ammonites (aragonitic) is slightly higher (20°) but is probably not significantly different from those for other calcareous macro-invertebrates. Preliminary oxygen isotopic results on phosphate extracted from bones, teeth, and gill rays correspond to paleotemperatures of 20°–25°.Carbon isotopic results are limited to data from calcareous benthos and nekton. δ 13C values for individual taxa are quite variable (+2 to +5 permil for aragonitic fossils, 0 to +3 permil for calcitic fossils), suggesting physiological isotope effects. Nonetheless, mean δ 13C values are consistent with calcification in seawater having a carbon isotopic composition similar to that of modern average seawater. The presumably high flux of 13C-depleted CO2 into bottom waters from the diagenesis of sedimentary organic matter is not recorded in the carbon isotopic composition of benthic fossils.Thermal stratification implied by the oxygen isotopic record suggests the penetration of cool, nutrient-rich waters into the Lower Oxford Clay sea. Upward advection of deep waters together with runoff from adjacent landmasses must have provided sufficient nutrients to maintain the inferred high productivity of surface waters. The influence of productivity on the carbon isotopic composition of surface waters will be tested by the analysis of calcareous phytoplankton.

Thermoluminescence dating of calcite shells in the pectinidae family

Previously we investigated the thermoluminescence (TL) of a calcite shell, Pectinidae Pecten (Notovola) albicans (Schröter) (abbreviated to albicans), and we found that TL dating was possible for fossil calcite shells of albicans from 5 × 10 5 years ago to the present. In the present work, we investigate the TL emission spectra and the first glow-growth of 5 other species in the Pectinidae family, and it is found that the TL characteristics of these species are the same as those of the albicans. This means that the application of TL dating can be extended to these species. Furthermore, we tried to date fossil calcite shells older than 5 × 10 5 years ago, and we found that the upper limit for TL dating of fossil calcite shells is about 6 × 10 5 years.

Factors Controlling the Distribution of Reservoir Quality in the Pennsylvanian Bend Conglomerate Fan-Delta System

Chert conglomerates, and associated sediments, of the Bend Conglomerate in Taylor Draw field, Upton Co., Texas, were deposited in a series of fan deltas that prograded into an open-marine system. The distribution of reservoir quality in these deposits is facies controlled. Permeabilities range from near zero to 30,000 md. Low-permeability conglomerates contain either a mud matrix or abundant calcite fossil fragments. The calcite grains serve as nucleation sites for calcite cementation and, through dissolution, as a source of calcite cement. High-permeability, sandy (chert and quartz) conglomerates have no mud matrix or calcite grains. Some of them are cemented with a thin rim of chalcedony, while others have no cement and show strong pressure solution at grain contacts. They are commonly friable and are recovered as loose gravels in the core barrel. The mineralogy of matrix gains is related to the distribution of the depositional facies within the fan delta. The low-permeability conglomerates were deposited in the subaerial to subaqueous distal fan-delta complex where calcareous marine organisms or mud were mixed with the conglomerate. The high-permeability conglomerates were deposited as thick channel-gravels in the subaerial proximal and mid-fan sections of the fan deltas. Depositional facies within the fan-delta complex controlled themore » distribution of calcite grains in the matrix and hence reservoir quality. The fossiliferous conglomerates became tightly cemented with calcite during burial, whereas the nonfossiliferous conglomerates remained permeable. Facies maps of the fan-delta complex therefore can be used to delineate high-permeability conglomeratic reservoirs.« less

Paleoceanographic modeling of temperature‐salinity profiles from stable isotopic data

Temperature‐salinity (T‐S) profiles in ancient seawater can be estimated from oxygen isotopic compositions of well‐preserved calcitic fossils sampled from known paleodepth gradients. The proposed model uses isotopic compositions of fossils to determine possible combinations of temperature and salinity during deposition (given estimates of average salinity, isotopic composition, and net global hydrologic fractionation for ancient oceans). Increasing density with depth constrains those combinations to narrow ranges. This graphic method superposes curves for density and isotopic composition in T‐S space, to reveal possible ancient T‐S profiles. Application of this method to oxygen isotope data from Ordovician brachiopods deposited on the eastern continental slope of North America demonstrates its usefulness and yields a stable seawater column in which brackish intermediate waters overlie warmer, more saline deep waters.

Mechanism and Controls of Silicification of Fossils in Limestones

Silicification of calcitic fossils occurs along thin solution films at which calcite dissolves and silica precipitates. The mechanism is replacement controlled by force of crystallization, whereby the growth of the silica phase exerts pressure across the silica-carbonate contact, increasing the Gibbs free energy and hence solubility of shell calcite. Physical evidence for force of crystallization-controlled replacement includes restriction of carbonate dissolution to the silica-carbonate contact, and euhedral quartz terminations at some quartz-calcite contacts. Correlations among some replacement quartz types and fossil taxa indicate that shell microstructure and bulk pore water silica concentration control replacement quartz type.

Diagenesis of Upper Devonian Platform Deposits in Emanuel Range, Canning Basin, Western Australia: ABSTRACT

The Emanuel Range consists of 600 m of peritidal and subtidal carbonate platform deposits of the Pillara Limestone (Givetian-lower Frasnian). Diagenesis of the lower 75 m contrasts markedly with the upper 520 m, principally in the occurrence of extensive dolomite. Formation of extensive secondary porosity by tectonic fracturing, in-situ brecciation, and dissolution of calcitic fossils was enhanced in previously dolomitized strata, exhibiting only minor occurrences in nondolomitized rocks. Formation of secondary porosity occurred over an extended period, marked by a succession of calcite cements, generally following a sequence from slightly ferroan, luminescently zoned cements to nonferroan, nonluminescent cement. Beckite chalcedony and hematite/goethite postdate but are associated with the final calcite cement. The post-dolomite diagenetic sequence suggests initial cementation under reducing burial conditions to final cementation under strongly oxidizing conditions. Fluids moving through the extensive Late carboniferous karst system in the platform may be the diagenetic agent for the later calcites, chalcedony, and iron oxides.

Thermoluminescence measurements of a calcite shell for dating

For thermoluminescence (TL)-dating of fossil calcite shells of Pecten, a new procedure is introduced. A supralinear function fitted to the ‘first-glow growth’ of the fossil shell makes it possible to evaluate the natural dose. From the natural doses obtained, we can estimate TL ages of fossil shells from about 5 × 10 5 years ago to more recently.

Thermoluminescence Dating of Fossil Calcite Shells

Thermoluminescence [TL] and electron spin resonance [ESR] of fossil calcite shells were investigated. The TL emission spectrum consists of a wide spectral band at 600 nm. The first- and second-glow growths observed at 240°C exhibit a common supralinear behavior, while the first- and second-ESR growths also exhibit a common saturation behavior. The total doses estimated from TL were consistent with that from ESR. Ages estimated from TL are compared with ones estimated by other methods. It is revealed that TL dating is useful for fossil calcite shells from about 5×105 years ago to recent.