Dolomite Layers Research Articles

Impacts of glacial/interglacial cycles on continental rock weathering inferred using Sr/Ca and 87Sr/86Sr ratios in Michigan watersheds

Michigan soils have developed on thick glacial-drift deposits that include different proportions of ground granite and gneiss from the Canadian Shield region (with radiogenic Sr) and carbonate sedimentary rocks from within the Michigan Basin (with non-radiogenic Sr). This study of the Cheboygan, Huron and Kalamazoo watersheds shows how Sr-isotope and Sr/Ca ratios in soil waters, ground waters, and soils reflect relative weathering intensities of the dominant minerals in Michigan soils, including carbonates from the Michigan Basin, and amphibole, plagioclase and K-feldspar derived from the Canadian Shield.Soil water 87Sr/86Sr ratios evolve quickly to the carbonate weathering end-member (0.709–0.711) once a calcite and dolomite layer is reached at depth (~100–200cm) in the Huron and the Kalamazoo watersheds. Dissolution of plagioclase and amphibole controls shallow soil water 87Sr/86Sr ratios (0.711–0.713), with minor contributions from K-feldspar weathering. In contrast, soils in the previously studied Cheboygan watershed are completely depleted in carbonate minerals and contain little plagioclase and amphibole in the top 300cm of the profile. As a result, soil waters in this watershed are ionically dilute with high 87Sr/86Sr ratios (0.72 and 0.74), dominantly contributed by K-feldspar dissolution. Subsequent dissolution of plagioclase and amphibole at greater depths sharply increases soil water and ground water Mg2+, Ca2+, and Sr2+ concentrations, and lowers the Sr-isotopic ratios to ~0.709 for the Cheboygan watershed.Similarly, along hydrologic flow paths, soil water Sr/Ca ratios move from the silicate end-member (defined by amphibole and plagioclase) towards the carbonate end-member. The Sr-isotopic compositions and Sr/Ca ratios of soil waters thus reveal the types, directions and extent of chemical weathering processes in Michigan soils, augmenting information from previous soil water chemistry and soil mineralogy studies.This work also highlights the two-fold impacts of glacial/interglacial cycles on the riverine and oceanic Sr isotopes: due to the great extent of continental glaciation, Paleozoic carbonate minerals from the Michigan Basin were redistributed widely within the interior of the North American continent, leading to elevated Sr fluxes with lower Sr-isotopic ratios in natural waters after glacial retreat. The glacial ice also ground up the ancient Precambrian Canadian Shield, accelerating mineral weathering rates and releasing highly radiogenic Sr from K-feldspar.

Major Ion Chemistry in a Coastal Karstic Groundwater Resource Located in Western Ireland

The extent of seawater intrusion in a coastal karstic aquifer system along the southern shore of Galway Bay, western Ireland, was investigated using spatial and temporal variations in major ion chemistry and nutrient levels. The background water was Ca- and bicarbonate-rich with variable concentrations of Mg. In general, higher Mg concentrations were detected in the Gort lowlands region compared with sites in the Burren, likely due to contact with dolomite layers. Nitrate concentrations ranged from 1.3mg/L to 78.5mg/L as NO3, and dissolved organic carbon concentrations ranged from 0.6mg/L to 4.9mg/L, suggesting that anthropogenic contamination due to surface activities occurs at some locations in the study area. Based on salinity values, on Na and Cl concentrations and on Cl/Br mass ratios, five of the 24 operational wells sampled were in a saltwater-influenced zone. Three of these wells contained <0.5% seawater, and the extent of saltwater influence was dependent on seasonal conditions. Water levels were monitored in six unused wells, and at four of these locations groundwater levels responded to the tidal variations in Galway Bay. During the study period, seawater intrusion and/or tidal influence on groundwater levels was found to occur within 5km of the coastline. The inland extent of seawater intrusion on the karst system is dependent on the hydrologic conditions at the time of measurement. The results provide a baseline dataset from which the influence of climatic and environmental changes on the aquifer system can be assessed in the future.

In situ nanoscale observations of the dissolution of [formula omitted] dolomite cleavage surfaces

Knowledge of the kinetics and mechanisms of carbonate dissolution is essential, for instance, to determine the contribution of carbonate-fluid reactions to the global carbon cycle and CO2 sequestration strategies, as well as to design new methods that mitigate the effects of weathering processes on carbonate stones. There is a significant lack of understanding of the molecular-scale reaction mechanisms of dolomite (CaMg(CO3)2), particularly in comparison to other common carbonates such as calcite (CaCO3). Here we present a systematic in situ Atomic Force Microscopy (AFM) study of dolomite dissolution in the pH range 3–10 aimed at improving our understanding of the nanoscale processes governing dolomite–fluid interactions. The results of this study indicate that the overall dolomite dissolution rate is controlled by the removal of dolomite layers by spreading and coalescence of shallow etch pits, nucleated at point defects and/or in defect-free areas. Our results also suggest that at all pH (and particularly at pH <5) and under conditions of relatively slow solution flow, dolomite dissolves via a dissolution–precipitation reaction with the formation of an Mg-rich surface precipitate. As a consequence, the effluent solution shows Ca/Mg ratios >1. This explains why in the past the dolomite dissolution was considered to be “incongruent”. In situ, direct observations of the reacting mineral surfaces are important to unambiguously ascertain the kinetics and mechanism of mineral dissolution. In fact, direct observations allow quantification of the kinetics of the process from the measurement of etch pit spreading rates, which are unaffected by the formation of a secondary precipitate whose existence has been neglected in the past.

Calcium and magnesium‐limited dolomite precipitation at Deep Springs Lake, California

AbstractDolomite [Ca,Mg(CO3)2] precipitation from supersaturated ionic solutions at Earth surface temperatures is considered kinetically inhibited because of the difficulties experienced in experimentally reproducing such a process. Nevertheless, recent dolomite is observed to form in hypersaline and alkaline environments. Such recent dolomite precipitation is commonly attributed to microbial mediation because dolomite has been demonstrated to form in vitro in microbial cultures. The mechanism of microbially mediated dolomite precipitation is, however, poorly understood and it remains unclear what role microbial mediation plays in natural environments. In the study presented here, simple geochemical methods were used to assess the limitations and controls of dolomite formation in Deep Springs Lake, a highly alkaline playa lake in eastern California showing ongoing dolomite authigenesis. The sediments of Deep Springs Lake consist of unlithified, clay‐fraction dolomite ooze. Based on δ18O equilibria and textural observations, dolomite precipitates from oxygenated and agitated surface brine. The Na‐SO4‐dominated brine contains up to 500 mm dissolved inorganic carbon whereas Mg2+ and Ca2+ concentrations are ca 1 and 0·3 mm, respectively. Precipitation in the subsurface probably is not significant because of the lack of Ca2+ (below 0·01 mm). Under such highly alkaline conditions, the effect of microbial metabolism on supersaturation by pH and alkalinity increase is negligible. A putative microbial effect could, however, support dolomite nucleation or support crystal growth by overcoming a kinetic barrier. An essential limitation on crystal growth rates imposed by the low Ca2+ and Mg2+ concentrations could favour the thermodynamically more stable carbonate phase (which is dolomite) to precipitate. This mode of unlithified dolomite ooze formation showing δ13C values near to equilibrium with atmospheric CO2 (ca 3‰) contrasts the formation of isotopically light (organically derived), hard‐lithified dolomite layers in the subsurface of some less alkaline environments. Inferred physicochemical controls on dolomite formation under highly alkaline conditions observed in Deep Springs Lake may shed light on conditions that favoured extensive dolomite formation in alkaline Precambrian oceans, as opposed to modern oceans where dolomites only form diagenetically in organic C‐rich sediments.

Investigation of the weathering layer using seismic refraction and high-resolution seismic reflection methods, NE of Riyadh city

Five seismic refraction and five high-resolution seismic reflection (HRSR) profiles were carried out in northeastern part of Riyadh city to investigate depth of the weathering layer. Results obtained from seismic refraction survey reveal the depths of weathering layer at 12, 25, 17, 12, and 16 m, respectively. On the other hand, HRSR stack sections illustrate the depths of weathering layer at 14, 28, 20, 13, and 18 m, respectively. The weathering layer is composed of alluvial sediments and gravel, which is underlain by a sequence of limestone and dolomite layer. Seismic results from site no. 2 have been found to be in good agreement with lithological information reported from the adjacent water well. The HRSR data generally reveal better signal-to-noise ratio and enhanced resolution compared to the refraction data. Although, the HRSR data failed in achieving high-quality common midpoint (CMP) stacking profile at site no. 3, it provide an improved image of the subsurface features than the refraction data, recognizing it as a potential seismic technique.

Seal turns into reservoir: Sudair equivalents in outcrops, Al Jabal al-Akhdar, Sultanate of Oman

ABSTRACT The Lower Triassic Sudair Formation represents the deposits of an epeiric carbonate-evaporite-siliciclastic platform. It is widely known as top seal for the underlying Khuff reservoir in some of the largest hydrocarbon accumulations in the world (e.g. the North Dome and the Ghawar field). Towards the seaward edge of the platform however, in northeast Oman for example, the sealing anhydrites and shales pinch out. Dolomite layers turn increasingly grainy, forming up to 8 m-thick individual grainstone bodies. Thus, on a regional scale, a top seal turns laterally into a potential reservoir. This paper outlines facies, depositional environment and stratigraphic architecture of the Middle Mahil Member, the outcrop equivalent of the Sudair Formation in northeast Oman. The Middle Mahil, some 260 m thick, is made up of just a few facies types: argillaceous, muddy, microbially-laminated, graded, and cross-bedded dolomites. These represent tidal flat, backshoal and shoal environments. Facies are arranged in regular sequences; the smallest units being 2–5 m thick cycles. A clear hierarchical organization of sequences is apparent with bundling of 4–8 cycles into cycle sets, and 2–3 cycle sets forming depositional sequences. The lower third of the Middle Mahil is dominated by rather muddy textures, the middle part is grain-dominated and the upper part again is muddier with a higher percentage of microbial laminites. The maximum grainstone thickness is observed in the middle of the Middle Mahil, at the interpreted zone of maximum flooding. Overall the Middle Mahil shows “layer-cake” type architecture over distances of 4–8 km. Grainstone layers are laterally continuous at this scale. Pinching and swelling geometries of grainstones are widespread. Grainstones gradually increase in thickness towards the northeast, the direction of the seaward platform edge. Mapping of Middle Mahil grainstones suggest excellent reservoir potential in such platform margin settings. The observations from Oman provide a calibration point to explorationists targeting the Lower Triassic carbonate sequence in the Middle East.

The exponential flow law applied to necking and folding of a ductile layer

SUMMARY The uniaxial exponential law, , has been applied to experimental results for steady-state creep, including that of wet and dry olivine, pyroxenite and carbonates, at a deviatoric stress greater than ∼100 MPa. Such stress levels likely occur in the upper-mantle lithosphere and middle crust. In a layered rock, in layer-parallel extension or shortening, high deviatoric stress can occur in the stiffest layers, for example, a dolomite layer in fine-grained marble. Under assumptions of isotropy and incompressibility, the uniaxial law yields where J2=sijsij/2 is an invariant of the deviatoric stress tensor, sij. Linearization about a homogeneous basic-state of flow yields equations identical in form to those obtained for the familiar power law. This establishes expressions for an effective viscosity, and stress exponent, , where is an invariant of the basic-state deformation rate, . These results allow application of existing analytical folding and necking solutions for a rock layer of power-law fluid to a rock layer with an exponential flow law. The effective stress exponent for the exponential flow law increases with decreasing temperature, through the dependence of C on the latter and, weakly, with increasing deformation rate. For dry olivine, effective stress exponents are between 10 and 30 for temperatures between 400 and 600°C with little dependence on deformation rate. Finite element simulations employing full non-linear forms of the flow laws show that large strain necking is nearly identical for power law and exponential flow laws. The results suggest that the instability in necking and folding in ductile rock layers can be considerably stronger than inferred from results based on flow laws representing diffusion and dislocation creep. The large values of the effective stress exponent, ne > ∼15, that may be attained for exponential flow laws can account for observed outcrop-scale ductile necking.

The long runout of the Heart Mountain landslide: Heating, pressurization, and carbonate decomposition

The Heart Mountain landslide of northwestern Wyoming is the largest subaerial landslide known. This Eocene age slide slid ∼50 km along a shallow 2° slope, posing a long‐standing enigma regarding its emplacement mechanism. We suggest here a mechanism for the catastrophic emplacement of the Heart Mountain landslide that is independent of slide triggering. The mechanism is a feedback between shear heating, thermal pressurization, and thermal decomposition of carbonates at the slide shear zone. Such a feedback arises when a porous, fluid‐filled shear zone heats up because of frictional sliding. If the shear zone is confined, the generated heat leads to pore pressure rise, which in turn reduces frictional resistance to sliding, leading to acceleration. Temperatures at the shear zone quickly reach the decomposition temperature of carbonates. Since the shear zone of the Heart Mountain slide is located within a dolomite layer, it is expected that thermal decomposition of dolomite occurred within the Heart Mountain shear zone. This prediction is supported by ample field evidence for carbonate decomposition during the emplacement. Simulation of the sliding dynamics of the Heart Mountain block, accounting for feedback between shear heating, thermal pressurization, and thermal decomposition of carbonates, successfully reproduce the travel distance of the Heart Mountain block. The simulation results also predict that the maximum sliding velocity ranged between tens of meters per second to more than 100 m s−1 (depending on model assumptions) and that the duration of sliding was of the order of a few tens of minutes, in agreement with previous assessments.

Hydrocarbon Accumulation Conditions of Ordovician Carbonate in Tarim Basin

Abstract:Based on comprehensive analysis of reservoir‐forming conditions, the diversity of reservoir and the difference of multistage hydrocarbon charge are the key factors for the carbonate hydrocarbon accumulation of the Ordovician in the Tarim Basin. Undergone four major deposition‐tectonic cycles, the Ordovician carbonate formed a stable structural framework with huge uplifts, in which are developed reservoirs of the reef‐bank type and unconformity type, and resulted in multistage hydrocarbon charge and accumulation during the Caledonian, Late Hercynian and Late Himalayan. With low matrix porosity and permeability of the Ordovician carbonate, the secondary solution pores and caverns serve as the main reservoir space. The polyphase tectonic movements formed unconformity reservoirs widely distributed around the paleo‐uplifts; and the reef‐bank reservoir is controlled by two kinds of sedimentary facies belts, namely the steep slope and gentle slope. The unconventional carbonate pool is characterized by extensive distribution, no obvious edge water or bottom water, complicated oil/gas/water relations and severe heterogeneity controlled by reservoirs. The low porosity and low permeability reservoir together with multi‐period hydrocarbon accumulation resulted in the difference and complex of the distribution and production of oil/gas/water. The distribution of hydrocarbon is controlled by the temporal‐spatial relation between revolution of source rocks and paleo‐uplifts. The heterogenetic carbonate reservoir and late‐stage gas charge are the main factors making the oil/ gas phase complicated. The slope areas of the paleo‐uplifts formed in the Paleozoic are the main carbonate exploration directions based on comprehensive evaluation. The Ordovician of the northern slope of the Tazhong uplift, Lunnan and its periphery areas are practical exploration fields. The Yengimahalla‐Hanikatam and Markit slopes are the important replacement targets for carbonate exploration. Gucheng, Tadong, the deep layers of Cambrian dolomite in the Lunnan and Tazhong‐Bachu areas are favorable directions for research and risk exploration.

Pedogenesis of a catena of the Farmdale–Sangamon Geosol complex in the north central United States

The Farmdale–Sangamon Geosol pedocomplex consists of the Sangamon Geosol and the overlying Farmdale Geosol, which form the most extensive terrestrial record of the last interglacial to glacial transition in the Midwest United States. The geosol complex formed for upwards of 100 ka, extending from the end of MIS 6 through 4 for the Sangamon Geosol, then the Farmdale Geosol for during a brief episode at the end of MIS 3 following slow accumulation and pedogenic modification of eolian silt deposited on top of the Sangamon Geosol. Our study site consists of a buried paleo-hillslope transect that forms a catena, enabling evaluation of slope effects on interglacial-scale soil formation. The Sangamon Geosol is formed in calcareous and illitic glaciogenic sediment. Along the catena the Sangamon Geosol profiles display some morphological changes, namely in terms of colors that we interpret as indicators of differences in drainage. Most thickness and horizonation characteristics are similar all along the transect, with intact upper sola horizons (AE and E horizons) that overlie clay-enriched Bt horizons. The Bt horizons contain abundant clay that exists as illuvial clay coatings, matrix infills, and as mosaic-speckled domains. The clay originated both by in situ weathering and through illuviation from the clay depleted upper sola. Slope does not appear to affect Bt characteristics beyond redder hues of the matrix and clay coatings in the upper slope position. With depth, effects of carbonate leaching and infilling of clay in the matrix decrease and clay coatings are restricted to walls of voids adjacent to aggregates. Clay mineralogy shows illite depletion, but no interstratified kaolinite-expandable minerals, indicating the degree of weathering is not as great as is typical of Sangamon Geosol profiles formed in loess or in glaciogenic sediment of the central Illinois type area. Clay mineralogy is also stratified with depth, coincident with particle size, which probably indicates sorting of layers of illitic dolomite and shale. Variation of horizon and profile characteristics appears to largely be a function of particle size variability and stratification than topographic position in the catena. The influence of hillslope position on soil redistribution during formation of the Sangamon Geosol appears negligible given the uniformity of upper solum horizon thickness and sandy particle size characteristics, so we conclude that a bioturbation and rainwash origin of the upper solum and the texture contrast in these profiles is not the best process model explanation. We suggest that the base-rich nature of these soils led to ecosystem characteristics that discouraged erosion and encouraged infiltration and a lessivage-type origin of the texture contrast. No convincing evidence of MIS 6 through MIS 4 loess occurs at this site. The Farmdale Geosol formed in the Robein Silt, which is Roxana Silt (MIS 3 loess) that was redistributed downslope. The Robein Silt is thicker and finer in the topographic low and indicates the cooler and forested environmental conditions during MIS 3 were conducive to downslope movement of soil and also produced greater differences in drainage-induced soil morphological changes in the Farmdale Geosol.

Origin of channel systems in the Upper Cretaceous Chalk Group of the Paris Basin

The Upper Cretaceous Chalk Group of the Anglo-Paris Basin is known to show wedging beds and channel-like features which disrupt the quietly deposited pelagic chalk that covered most of NW Europe in the Late Cretaceous. Two-D reflection seismic data from the Brie region, SE of Paris, show the presence of at least two distinct intra-chalk discordant reflections: a Top Santonian and a Mid-Campanian reflection. These reflections are in places associated with up to 120-m-deep channel-like structures trending preferentially N–S and NW–SE. The Mid-Campanian reflection is also sporadically associated with a massive secondary dolomite layer, the thicknesses of which may reach 110 m. Diagenesis does not seem to account for the formation of the discordant reflections, as there is neither a one-to-one relationship between the dolomite and discordant reflections, nor are there signs of systematic collapse of the Cenozoic succession over the channel-like features as a result of intra-chalk dissolution. Both reflections correlate with indurated chalk layers and hardgrounds, and represent real unconformities. The Mid-Campanian reflection is furthermore associated with a stratigraphic hiatus. A submarine origin is suggested due to the uninterrupted deep-marine chalk facies below and above both unconformities, and the unrealistically large sea-level drop of more than 200 m, which would be necessary for subaerial exposure of the central Paris Basin during the Campanian. The channels are oriented parallel to the margins of the basin, and important bathymetric elements which could induce erosion by slope failure are not observed. The channels are thus interpreted as having formed by strong, mainly slope-parallel bottom currents. Major channeling events are common in the Chalk Group throughout NW Europe and represent palaeoceanographic re-organization of bottom currents, probably driven by changes in sea level and water temperature.

The Kuzha barite-base-metal deposit

The Kuzha barite-base-metal deposit is located on the western slope of the southern Urals and is hosted in strongly deformed Riphean rocks. The Main barite zone is related to a tectonic suture accompanied by alkaline and basic dikes. The stratiform base-metal ore mineralization is confined to a dolomite layer conjugated with the suture zone. The ore mineralization was formed as a result of interaction between heated mineralized water ascending along the suture zone and descending sulfate-bearing vadose water. Biogenic sulfate reduction played the main role in sulfide precipitation. The heating of the ore-forming medium up to 120°C terminated sulfate reduction and facilitated deposition of barite. The Kuzha deposit has much in common with other base-metal and barite-base-metal deposits in Russia, Kazakhstan, and the United States.

Textural and compositional evidence for the evolution of pedogenic calcite and dolomite in a weathering profile on the Kohala Peninsula, Hawai’i

Pedogenic dolomite and high magnesium calcite (HMC) occur as micritic matrix and void filling microspar in soils developed on semi-arid 350 Ka basalt surfaces on Hawai’i Island. The void filling carbonate presents an unusual appearance of alternating concentric < 5 μm to ≈ 30 μm layers of HMC and dolomite at shallow (100 cm) depths and similar layers that are exclusively dolomite between depths of 170 cm and 350 cm. Elevated Mg/Ca ratios in soil water, caused by rapid basalt weathering, likely provided geochemical conditions that permitted evaporative precipitation of magnesian carbonates. Variations in the amount of soil water, soil water chemistry, and/or crystallization rate probably controlled the precipitation of HMC vs. dolomite at 100 cm, whereas increased residence time of soil water and more stable soil water chemistry below the 170 cm depth led to the exclusive formation of dolomite.

Sealevel changes control diagenetic dolomite formation in hemipelagic sediments of the Peru Margin

The first experimentally determined temperature dependent oxygen-18 fractionation factor between dolomite and water at low temperatures [Vasconcelos, C., McKenzie, J.A., Warthmann, R., Bernasconi, S., 2005. Calibration of the δ 18O paleo-thermometer with dolomite formed in microbial cultures and natural environments. Geology 33, 317–320] allows now the precise calculation of temperatures during early diagenetic dolomite precipitation. We use δ 18O values of early diagenetic dolomite beds sampled during ODP Legs 112 and 201 on the Peru continental margin (Sites 1227, 1228 and 1229) [Meister, P., McKenzie, J.A., Vasconcelos, C., Bernasconi, S., Frank, M., Gutjahr, M., Schrag, D.P., 2007. Dolomite formation in the dynamic deep biosphere, results from the Peru Margin, OPD Leg 201. Sedimentology 54, 1007–1032] to calculate paleo-porewater temperatures at the time of dolomite precipitation. We assumed unaltered seawater δ 18O values in the porewater, which is supported by δ 18O values of the modern porewater presented in this study. The dolomite layers in the Pleistocene part of the sedimentary columns showed oxygen isotope temperatures up to 5 °C lower than today. Since Sites 1228 and 1229 are located at 150 and 250 m below sealevel, respectively, their paleo-porewater temperatures would be influenced by considerably colder surface water during glacial sealevel lowstands. Thus, Pleistocene dolomite layers in the Peru Continental margin probably formed during glacial times. This finding is consistent with a model for dolomite precipitation in the Peru Margin recently discussed by Meister et al. [Meister, P., McKenzie, J.A., Vasconcelos, C., Bernasconi, S., Frank, M., Gutjahr, M., Schrag, D.P., 2007. Dolomite formation in the dynamic deep biosphere, results from the Peru Margin, OPD Leg 201. Sedimentology 54, 1007–1032], where dolomite forms episodically at the sulphate methane interface. It was shown that the sulphate methane interface migrates upwards and downwards within the sedimentary column, but dolomite layers may only form when the sulphate–methane interface stays at a fixed depth for a sufficient amount of time. We hypothesize that the sulphate–methane interface persists within TOC-rich interglacial sediments, while this zone is buried by TOC-poor sedimentation during glacial times. Thus, the presented oxygen isotope data provide additional information on the timing of early diagenetic dolomite formation and a possible link between episodicity in dolomite formation and sealevel variations. A similar link between early diagenesis and oceanography may also explain spacing of dolomite layers in a Milankovitch type pattern observed in the geological record, such as in the Miocene Monterey Formation.

Joint nucleation from cavity‐shaped flaws: Field observations, probability of occurrence, and 3‐D mechanical analysis

We present field observations, probability of occurrence, and three‐dimensional mechanical analysis of joint nucleation from cavity‐shaped flaws. A unique set of cavity‐shaped flaws and their associated joints were documented in Cretaceous dolomite layers that crop out in central Israel. In this set, cavities that initiate joints (“initiator cavities”) are commonly larger and located farther away from the bottom of the layer than non‐initiator cavities. They also have significantly less neighboring cavities. The probability of occurrence of initiator cavities is calculated by a logistic regression and compared to a mechanical analysis. Stress distribution around a cavity is calculated analytically using the solution for the elastic stress distribution around a spherical cavity in an infinite medium under triaxial loading and numerically using three‐dimensional simulations. The analytical solution and numerical modeling show that a single cavity transforms the remote compression into local tension around its periphery. The numerical simulations enable us to relate the magnitude of the tensile stress to the cavity size, location within the bed, and the relative orientation of the neighbor cavity. Our observations and analyses indicate the existence of effects arising from both the size of cavities and their distribution in the rock mass. It is shown that the reduction of rock strength by increasing porosity cannot simply apply to heterogeneous media such as carbonatic rocks. These results demonstrate the role played by large, isolated, and preferably located cavities in initiating joints in heterogeneous media.

Orbital control on Upper Triassic Playa cycles of the Steinmergel-Keuper (Norian): A new concept for ancient playa cycles

The Norian Steinmergel‑Keuper (SMK) represents a low-latitude cyclically-bedded playa system of the Mid-German Basin. We investigated a drilling site (core Morsleben) and sections from marginal positions. Dolomite/red mudstone beds form rhythmic alternations that were associated with varying monsoon activity. Hence, low K/Al ratios of dolomite beds suggest increased chemical weathering of the crystalline hinterland and therefore increased monsoonal rainfall. High K/Al ratios in red mudstone beds reflect increased physical weathering of the hinterlands during dryer periods. Dolomite layers reflect the lake stage (maximum monsoon) while red mudstones indicate the dry phase (minimum monsoon) of the playa cycle. We distinguished five major types of cyclic facies alternations, representing specific facies zones in the playa system. We have implemented spectrophotometry as a tool for high-resolution cyclostratigraphy. The dense sampling increment (up to 1 cm) allows for the recognition of all orbital frequencies. Sediment colour profiles reveal striking hierarchical cycles from semi-precession (SP, 99 kyr) over precession (P, 19.8 kyr) and obliquity (O, 36 kyr) to eccentricity (E1-2 109 kyr; E3, 413 kyr). A significant ∼ 2 Myr-signal is attributed to the longer-term eccentricity E4. One monsoonal (precession) cycle includes two carbonate precipitation events. We propose that stratified mudstone and red mudstone are associated with maximum and minimum monsoon during the transition of the solstices in perihelion and aphelion, respectively. The two carbonate precipitation events were most likely created when equinoxes were in perihelion and aphelion, respectively. A sedimentary semi-precession response cycle is a novel finding for the Norian strata. The obliquity signal is attributed to incoming atmospheric moisture from the northeast of the SMK basin. The E4 cycle controls lake-level changes over long times. Apparently, E4 is responsible whether or not a threshold value is crossed. Bundles of 109 kyr and 413 kyr in red mudstones suggest a dry system with reduced monsoonal activity. In contrast, humid periods reveal thick layers of dolomite beds, indicating that during those intervals the monsoonal activity was strong enough to prevent the playa system from drying out completely.

Dolomite formation in the dynamic deep biosphere: results from the Peru Margin

AbstractEarly diagenetic dolomite beds were sampled during the Ocean Drilling Programme (ODP) Leg 201 at four reoccupied ODP Leg 112 sites on the Peru continental margin (Sites 1227/684, 1228/680, 1229/681 and 1230/685) and analysed for petrography, mineralogy, δ13C, δ18O and 87Sr/86Sr values. The results are compared with the chemistry, and δ13C and 87Sr/86Sr values of the associated porewater. Petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate. Dolomite precipitation often pre‐dates the formation of framboidal pyrite. Most dolomite layers show 87Sr/86Sr‐ratios similar to the composition of Quaternary seawater and do not indicate a contribution from the hypersaline brine, which is present at a greater burial depth. Also, the δ13C values of the dolomite are not in equilibrium with the δ13C values of the dissolved inorganic carbon in the associated modern porewater. Both petrography and 87Sr/86Sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment (&lt;30 m below the seafloor). A significant depletion in the dissolved Mg and Ca in the porewater constrains the present site of dolomite precipitation, which co‐occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate–methane interface. It has been hypothesized that microbial ‘hot‐spots’, such as the sulphate–methane interface, may act as focused sites of dolomite precipitation. Varying δ13C values from −15‰ to +15‰ for the dolomite are consistent with precipitation at a dynamic sulphate–methane interface, where δ13C of the dissolved inorganic carbon would likewise be variable. A dynamic deep biosphere with upward and downward migration of the sulphate–methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter. Thus, the study of dolomite layers in ancient organic carbon‐rich sedimentary sequences can provide a useful window into the palaeo‐dynamics of the deep biosphere.

Contribution of geophysical surveys to the study of fine grained lacustrine sediments. Application to the Sarliève marsh (Massif Central, France)

Abstract This paper presents the results of a geophysical survey which was performed in the Sarliève marsh, located in the north of the Massif Central (France). The studied area corresponds to an ancient lake filled with 6 m of Lateglacial to historic sediments. The numerous cores and excavations previously performed in basin areas show the presence of thick fine grained and carbonated lake deposits. The mineralogical study of these deposits allows us to distinguish two main types of formations, which differ in their composition and origin. In the lower part of the sedimentary filling, the SDLF formation is generally characterized by abundant chalky and dolomitic layers, probably formed by chemical precipitation in an endoreic environment. It was found only in the distal (or depocenter) parts of the ancient lake. The upper part of the filling corresponds to calcitic clayey silty sediments (CF), mainly of detrital origin. They can be found both in the distal and proximal parts of the ancient lake. In order to precise the geometry of the marsh sedimentary filling and to discriminate between these two fine grained sediment types (SDLF and CF), an electromagnetic survey was carried out using an EM31 conductivity meter. It was completed by 16 electrical soundings performed within the marsh. The electromagnetic survey enables us to draw a large-scale conductivity map, which highlights the presence of a highly conductive zone (HCZ) in the central part of the marsh, more pronounced in the south. The HCZ is bordered by more resistive and heterogeneous fields, where conductive structures are visible. We show that there is no clear correlation between the measured conductivity and the total lacustrine sediment thickness. However, the HCZ is related to the distal parts of the ancient lake, locally identified from borehole data. In the corresponding sedimentary filling, the SDLF formation appears as a very low resistive layer (about 1.2Ω.m) on the electrical soundings. It explains the high conductivity of the distal lacustrine sediments. We finally combine the cores and the geophysical data to estimate the spatial extent and the volume of the SDLF formation of the whole marsh. This study shows that geophysics can contribute to distinguish, in a globally homogeneous fine grained sedimentary filling (carbonated clayey silts), lacustrine sediments formed in very different contexts, one as chemical precipitation, the other of detrital origin.

NEW INSIGHT INTO STRUCTURAL AND COMPOSITIONAL VARIABILITY IN SOME ANCIENT EXCESS-Ca DOLOMITE

Samples of non-stoichiometric dolomite of different age and origin have been studied by powder X-ray diffraction, scanning electron microscopy and electron-microprobe analysis. Most samples containing a high excess of Ca consist of two or even three phases, differing in the degree of excess Ca uptake and degree of structural order. On the basis of Rietveld refi nements of samples with two 3D periodic phases, only the B sites have a mixed occupancy of Ca and Mg, and the phases differ from each other only in the content of Ca at the B site. A simulation of the experimental XRD patterns of samples showing a high and a low excess of Ca with a defective layer-structure was carried out in terms of mixed-layer structures in which different proportions of nonstoichiometric dolomite layers are interstratifi ed with a small amount of calcite-like as well as stoichiometric dolomite layers. Electron-microprobe analysis and BSE images show that large crystals in the two-phase excess-Ca dolomite samples have sharply bounded zones with different levels of Ca concentrations. Each BSE contrast zone has the same composition (within error), and in each sample there are three or four non-overlapping groups of such zones differing in composition. Remarkably, fi ne-grained micritic portions of these samples contain the same discrete sets of Ca concentration zones as the large zoned euhedral crystals. This result supports the hypothesis that crystals of excess-Ca dolomite adopt only certain non-continuous compositions, and that growth and composition are structurally controlled.

Cycles in Perilacustrine Facies of Late Mesozoic Rift Basins, Southeastern Mongolia

Detailed study of 365 meters of core recovered from southeastern Mongolia reveals fluvial, marginal lacustrine, and profundal lacustrine depositional environments in an Early Cretaceous rift basin. Multiple examples of cyclic sedimentation are superposed on these large-scale facies transitions. In fluvial facies, more than 20 upward-fining successions (2–10 m thick) are present. These contain cross-stratified sandstone units, with erosional bases, that fine upward to mudstone with root marks and burrows. They are interpreted as channel-fill successions in a fluvial depositional environment, whereas finergrained, less erosional intervals with abundant soft-sediment deformation and climbing ripples may represent a transition to fluvial-deltaic environments at the lake margin. Coarse fluvial-deltaic facies form the most likely hydrocarbon reservoirs, as indicated by frequency of oil staining in the core. Physical transport processes dominate well-oxygenated fluvial and marginal lacustrine facies, whereas cycles in more distal lacustrine facies mainly reflect biogeochemical processes. Profundal lacustrine strata include kerogen-rich laminated micrite, distal turbidite beds, and dolomitic breccia reminiscent of lacustrine facies in the Green River Formation of the western United States. Evidence for anoxia at the lake floor includes framboidal (biogenic) pyrite, high TOC content, mainly light amorphous organic constituents, and paucity of trace fossils relative to the rest of the core. Approximately 25 dolomite layers are interbedded with laminated micrite and calcareous siltstone units in poorly defined meter-scale cycles. These beds typically have brecciated or clotted textures, and suggest evaporative concentration of magnesium ions at the lake floor. The profundal lacustrine sequence displays a distinctive high-resistivity, low spontaneous-potential log character that is observed in well logs from several fields in the Zuunbayan subbasin. Correlative units can be traced at least 60 km to the northeast and southwest of the Zuunbayan field, indicating that this is a widespread lacustrine depositional unit. These lacustrine strata include the most likely petroleum source facies found to date, and are likely contemporaneous with other large lake systems in China and Mongolia.