Dolostone Sequence Research Articles

Delineating aquitard characteristics within a Silurian dolostone aquifer using high-density hydraulic head and fracture datasets

Fractured aquifers are heterogeneous due to the variable frequency, orientation, and intersections of rock discontinuities. A ~100-m-thick Silurian dolostone sequence provides a bedrock aquifer supplying the city of Guelph, Canada. Here, fracture network characteristics and associated influences on hydraulic head were examined using several data types obtained from 24 cored holes in a study that is novel for the quantity and quality of data. High (50–90°) angle joint orientations, heights, and terminations relative to bedding features were determined from acoustic televiewer logs and outcrop scanlines. These data were compared to high-resolution hydraulic head profiles showing head loss over depth-discrete intervals identifying zones with lower vertical hydraulic conductivity. This study reveals that the marl-rich Vinemount Member, traditionally considered the principal aquitard, corresponds to head loss in only 62% of the 24 boreholes. The vertical position of head loss varies across the 90-km2 study area and occurs in any of the lithostratigraphic units of the Lockport Group. Within this sedimentary sequence, aquitards are laterally discontinuous or “patchy” at variable depths and relate to: (1) the frequency of the high-angle joints; (2) shorter joint height; and (3) the type of joint terminations. The head loss occurs in thin (2–2.5 m) intervals where the frequency of the high-angle joints is low. Where a large proportion of small joints cross-cut marl bedding planes, head loss is negligible, suggesting that the vertical hydraulic conductivity is not reduced. Overall, these findings are potentially applicable to assessing aquitard and cap rock integrity in carbonate sedimentary sequences worldwide.

The covariation between Mg[sbnd]C isotopes and trace elements in dolostones: Implications for reconstructing the seawater chemistry

Magnesium isotopes of marine dolomites have the potential to reconstruct the Mg isotopic signals of coeval seawater. However, a massive dolostone sequence can form via the coupling effects of multiple dolomitization events, which can potentially reset the Mg isotopic compositions of dolomites during each migration of the dolomitization fluid. How to distinguish changes of seawater chemistry and early diagenetic effects which have been recorded in dolomites is crucial for the reconstruction of seawater Mg isotopic signals. Here we studied the Mg-Sr-C-O isotope systematics and trace elements of a dolostone sequence from the Qiangtang Basin in the hinterland of Tibet. Multiple lines of evidence consistently indicate the lack of deep-burial diagenesis and high-temperature alteration in the carbonates. The stratigraphic variations of isotopic and elemental compositions of the dolostone profile are generally characterized by the co-variation between MgC isotopes and trace element contents. Our modeling results indicated that a single downward dolomitization can cause the large Mg isotopic variability of dolomites, ca. 1 ‰, within a dolomitization front. However, when the limestones in the profile were completely dolomitized, the coupling effects of multiple downward dolomitizations will homogenize the δ26Mg values of dolostones. Therefore, we attribute the observed co-variation in isotopic and elemental signals in the dolostone sequence to the changes in the chemistry of evolved seawater within the platform. The reconstructed seawater δ26Mg values from the Carnian to Early Jurassic are lower than previous model-based estimations. This result implies an overestimation of the production of shallow-water carbonate in literature during Late Triassic to Early Jurassic. Our study highlights the importance of utilizing multiple geochemical proxies to identify the controlling factors driving Mg isotope variability in dolomites.

The Cenozoic Seawater Conundrum: New constraints from Mg isotopes in island dolostones

The Mg/Ca ratio of seawater increased from ∼1.5 to its present value of 5.4 over the Neogene, indicating a fundamental imbalance in the Mg–Ca cycle during the late Cenozoic. The imbalance in Mg–Ca cycles since the late Cenozoic, however, is contrasted by an arguable constancy in the Mg isotope compositions of seawater (δ26Mgsw) reconstructed from several low-resolution carbonate records. Addressing such Cenozoic Seawater Conundrum requires reliable seawater Mg isotopic records. The Xisha Islands in the South China Sea host large Cenozoic dolostone sequences that record seawater chemistry over the past 23 Myr. This study reports the Mg isotopic and trace elemental compositions of island dolostones in the well-studied Xike-1 drill core from the Xisha Islands. Petrographic, C–O–Sr isotopic, and paleomagnetic data from the drill core samples collectively support a “fluid-buffered”, syn-depositional origin for the dolostones. The dolostones from the Xike-1 core have a narrow range of δ26Mg (−2.83‰±0.12‰), confirming that δ26Mgsw have been stable around the modern value (−0.83‰) since the late Cenozoic. This record provides a new opportunity to better understand the Mg–Ca cycle during the late Cenozoic. We ran numerical models of global seawater Mg–Ca contents and δ26Mgsw with the most recently published flux and isotope fractionation data. The results show that the contrast between the constancy of δ26Mgsw values and the dramatic increase in seawater Mg/Ca ratios cannot be explained by a single mechanism (i.e., a decrease in the rate of phyllosilicate formation on the seafloor alone). Instead, processes involving different Mg fluxes need to be coupled to explain the observed trends in Mg content and isotopic compositions of Cenozoic seawater. The solution to the Cenozoic Seawater Conundrum includes simultaneous decreases in the rates of both clay formation and dolomitization on the seafloor, or an increase in the rate of both continental weathering together with decreased seafloor clay formation. Our study underlines the sensitivity of the Mg cycle to the interactions between climate, continental weathering, and seafloor processes during the late Cenozoic.

Integrated petrophysical and microfacies analyses for a reservoir quality assessment of the Asmari Dolostone sequence in the Khesht Field, SW Iran

The Asmari Formation is one of the most important prolific reservoirs along both sides of the Persian Gulf. The present study aims to characterize the fractured dolomite of the Asmari reservoir sequence in SW Iran. Such characterization can be achieved through a petrophysical reservoir characterization using core data integrated with a detailed microfacies analysis and a detailed study on the dominant diagenetic features. The detailed microfacies analysis indicates that the Asmari reservoir can be discriminated into seven non-clastic microfacies (laminated to massive anhydrite, sandy dolomudstone, peloidal bioclastic dolowackestone, peloidal foraminiferal dolowackestone/dolopackstone, peloidal bioclastic dolopackstone/dolograinstone, framestone, and dolograinstone microfacies). These microfacies have been primarily deposited in tidal flat, lagoonal, and carbonate shoal depositional environments. Petrophysically, some X-Y plots were established to present the available petrophysical core data (porosity ∅He, horizontal and vertical permeabilities, kH and kV, respectively) to enable a mathematical reservoir modeling. Besides, the reservoir quality has been defined using the flow zone indicator (FZI), reservoir quality index (RQI), and the effective pore radius (R35). Based on these petrophysical core data and quality parameters, the studied samples can be subdivided into five reservoir rock types (RRTs). Also, reservoir zonation on the borehole scale using the improved stratigraphic modified Lorenz technique (ISML) indicated that the characterized reservoir can be subdivided into four hydraulic flow units (HFU-1, HFU-2, HFU-3, and HFU-4). The HFU-2 (dolograinstone, dolopackstone, and framestone samples) is characterized by the best reservoir quality (average ∅He = 7.8%, kH = 95.2 md, RQI = 0.84 µm, FZI = 15.9 µm, and R35 = 10.5 µm); it is composed of RRT1 and RRT2 samples. On the other side, the lowest reservoir quality is assigned to the HFU-3 which is composed of the RRT5 (dolomudstone and evaporates, average ∅He = 11.5%, kH = 0.24 md, RQI = 0.04 µm, FZI = 0.22 µm, and average R35 = 0.21 µm). The prevailed reservoir characteristics of the Asmari dolostone sequence in SW Iran are mostly controlled by replacive and pervasive dolomitization, fracturing, and cementation by anhydrite.

Microfabric features of microbial carbonates: experimental and natural evidence of mold holes and crusts

Results of our study based on examination of induced precipitation of carbonate by a cyanobacterium, Lyngbya in the laboratory, and the analyses of microphotographs of both modern and ancient microbial carbonates, demonstrated the importance of recognition of mold holes and carbonate crusts in understanding microbial carbonates. In the experiment, only cyanobacteria Lyngbya can induce precipitation of carbonate, forming scattered grains on the surface of Lyngbya filaments. Carbonate crusts enclosing the old parts of the filaments were formed through aggregation of these scatter grains while mold holes were formed after decay of the filaments. Mainly based on the experiment, six different ways of microbial carbonate formation were recognized: (1) trapping without mold holes, (2) trapping with mold holes, (3) particle-forming induced-precipitation of carbonate, (4) discrete crust-forming induced-precipitation of carbonate, (5) induced precipitation, forming tangled crusts that build a porous construction, and (6) induced precipitation, forming a dense construction. And mold holes and crusts can form in ways (4), (5), and (6). Examination of both modern microbial carbonates from the Shark Bay of Australia, Highborne Cay of Bahamas and the atoll of Kiritimati and the microbialites from the Cambrian dolostone sequence in Tarim, Xinjiang, China all demonstrated the limitation of recognizing only mesofabric features and importance of examining microfabric features for understanding of the genesis of the microbial carbonates and their proper classification. The shape, size and arrangement of the mold holes, crusts, and the features of the minerals filling in pores between the crusts, which are referred as the microfabric features here, are keys to better understand the formation and environments of both modern and ancient microbial carbonates.

Towards understanding the origin of massive dolostones

The origin of ancient massive dolostones, i.e. continuous dolostone sequence with a thickness >100 m and a platform-wide distribution, is the key issue of the ‘Dolomite Problem’ that cannot be clearly demonstrated by any existing dolomitization model individually or sequentially. It has been proposed that the massive dolostone could be generated by the stacking of multistage dolomitization events linked to the sea-level fluctuation, which results in repeatedly occurring of limestone precipitation-dolomitization cycles. However, the sequence of dolomitization events cannot be differentiated by any sedimentological or traditional geochemical techniques. Here we report Mg isotopic compositions of the massive dolostone (δ26Mgdol) from the middle Cambrian Qinjiamiao Formation (QJM) in the Yangtze Platform, South China, which consists of cyclic depositions of shoaling upward sequences. The stratigraphic variation of δ26Mgdol is coincident with the depositional cycles, suggesting the dolomitization might be periodic and be coupled with the sea-level oscillation. As dolomitization fluids experience changes in δ26Mg values during dolomitization processes, the intra-cycle stratigraphic δ26Mgdol profile reflects the processes of dolomitization. Our study indicates that the massive dolostone could be generated by the temporal and spatial stacking of multiple dolomitization events that are associated with sea-level fluctuation. If this model can be verified by other massive dolostone successions, the origin of massive dolostone may be resolved.

Acoustic and strength characterization of Upper Cretaceous dolostones from the Bahariya Oasis, Western Desert, Egypt: The impact of porosity and diagenesis

The Western Desert of Egypt has huge reserves of good-quality natural stones such as limestones, travertines and dolostones. In this paper we assess the dolostones of El Heiz and Ain Giffara formations exposed at the Bahariya Oasis by investigating their lithological, textural, acoustic, and strength properties, and accordingly their economic significance and potential contribution to the national economy of Egypt. Representative dolostone rocks were collected at carefully-selected field sites. The microfacies analysis revealed that three main facies are recognized in the collected rocks: vuggy dolostone, marly dolostone, and sandy dolostone. The measured density and porosity of the studied rocks have been largely affected by the sediment composition, depositional setting, and the diagenetic alterations which, in turn, influenced the acoustic properties and strength of the rocks. Results of the uniaxial unconfined compressive strength (UCS), tensile strength (σt), point load strength (IS50), and the Schmidt Hammer rebound (SHR) number of the studied samples indicate generally high-strength rocks. Various interrelationships between the acoustic and strength properties were obtained using the regression analysis and have been interpreted in light of the rock microfacies and the observed diagenetic processes. Significant relationships have been obtained particularly between the UCS versus the acoustic velocities and the SHR. The resulting empirical relationships with the moderate/high coefficients of correlation can be used to compute important rock parameters from those that are measured on a routine basis; hence enabling a much quicker and efficient evaluation of the rocks. Results of the present study are also useful for studying the strength and acoustic properties of similar surface and subsurface dolostone sequences in the Western Desert of Egypt, as well as other areas of similar geological setting and depositional facies.

Late Paleoproterozoic geodynamics of the North China Craton: Geochemical and zircon U–Pb–Hf records from a volcanic suite in the Yanliao rift

The Yanliao rift zone along the northern margin of the North China Craton exposes a suite of volcanic rocks comprising trachybasalts and trachytes interlayered with dolostones in the Tuanshanzi Formation of the Changcheng Group. The overlying Dahongyu Formation is composed of olivine basalts, trachybasalts, trachyandesites, trachytes, and minor sub-alkaline basalts, intercalated with a lower sandstone and an upper dolostone sequence. LA-ICP-MS zircon U–Pb age data from the volcanics reveal that the Tuanshanzi Formation was deposited at ca. 1670Ma, and the Dahongyu Formation at ca. 1625–1664Ma. Combined with previous studies, these new data indicate that the lowest boundary age of the Changcheng Group, marking the timing of initiation of the Yanliao rift, is ca. 1680Ma. Except for the sub-alkaline basalts, the other alkaline volcanic rocks of the two formations are characterized by high Nb/Y ratios (1–3), and possess SiO2 contents of 43.3–60.5wt.%, K2O of 0.72–15.69wt.%, MgO of 0.10–8.56wt.%, and with Mg# of 2–64. These rocks also display strongly fractionated REE patterns and generally positive Nb and Ta anomalies, with high (Nb/La)PM and (Nb/Th)PM values of 0.85–1.39 and 0.96–2.05, respectively. They show positive zircon εHf(t) values (+0.1 to +1.8 for the trachybasalt sample 11PG31-2) and whole-rock εNd(t) values (+0.83 to +2.25). These geochemical features, together with trace element modeling, suggest that the alkaline volcanic rocks were generated by fractional crystallization of magma derived from low-degree partial melting of an OIB-like depleted asthenospheric mantle source, with only minor involvement of melts generated from the overlying enriched lithospheric mantle source. In contrast, the sub-alkaline basalts are characterized by low Nb/Y ratios of 0.09–0.19, and show fractionated REE patterns and pronounced negative Nb–Ta anomalies with low (Nb/La)PM and (Nb/Th)PM values of 0.07–0.21 and 0.10–0.33, respectively. In combination with the positive zircon εHf(t) values (+0.8–+4.3), we infer that these rocks were derived from the partial melting of a depleted lithospheric mantle source previously metasomatized by fluids from subducted slab. Combined with previous studies on the 1680–1780Ma magmatic rocks along the northern margin of the NCC, our study identifies a late Paleoproterozoic asthenospheric upwelling in this region. The generation of the Yanliao rift and the eruption of these volcanic rocks were possibly triggered by the delamination of the continental lithosphere in a post-orogenic setting following the final amalgamation of the North China Craton within the Columbia supercontinent.

Sedimentary constraints on the duration of the Marinoan Oxygen-17 Depletion (MOSD) event

The ~635 Ma Marinoan glaciation is marked by dramatic Earth system perturbations. Deposition of nonmass-dependently (17)O-depleted sulfate (SO4(2-)) in worldwide postglacial sediments is, thus far, unique to this glaciation. It is proposed that an extremely high-pCO2 atmosphere can result in highly (17)O-depleted atmospheric O2, or the Marinoan Oxygen-17 Depletion (MOSD) event. This anomalous (17)O signal was imparted to sulfate of oxidative weathering origin. However, (17)O-depleted sulfate occurs in limited sedimentary intervals, suggesting that Earth surface conditions conducive to the MOSD had a finite duration. An MOSD duration can, therefore, provide much needed constraint on modeling Earth system responses at that time. Unfortunately, the sulfate (17)O record is often sparse or lacks radiometric dates. Here, we report 11 barite layers from a post-Marinoan dolostone sequence at Wushanhu in the South China Block. The (17)O depletion fluctuates in magnitude in lower layers but is persistently absent up section, providing the most confident first and last sedimentary appearance of the anomaly. δ(13)C chemostratigraphy is used to correlate the Wushanhu section to two proximal sections on the same shallow platform that lack barite layers but have published U-Pb dates that occur in dolostone and shale. Assuming a similar pattern and rate for carbonate and shale deposition among the different sections, we estimate the MOSD duration at 0-0.99 My. This number can be further constrained by new radiometric dates from equivalent sequences worldwide, thus underpinning models on the nonsteady-state Earth system response in the immediate aftermath of the Marinoan meltdown.

Dechlorinating Microorganisms in a Sedimentary Rock Matrix Contaminated with a Mixture of VOCs

Microbiological characterizations of contaminant biodegradation in fractured sedimentary rock have primarily focused on the biomass suspended in groundwater samples and disregarded the biomass attached to fractures and in matrix pores. In fractured sedimentary rock, diffusion causes nearly all contaminant mass to reside in porous, low-permeability matrix. Microorganisms capable of contaminant degradation can grow in the matrix pores if the pores and pore throats are sufficiently large. In this study, the presence of dechlorinating microorganisms in rock matrices was investigated at a site where a fractured, flat-lying, sandstone-dolostone sequence has been contaminated with a mixture of chlorinated and aromatic hydrocarbons for over 40 years. The profile of organic contaminants as well as the distribution and characterization of the microbial community spatial variability was obtained through depth-discrete, high-frequency sampling along a 98-m continuous rock core. Dechlorinating microorganisms, such as Dehalococcoides and Dehalobacter, were detected in the rock matrices away from fracture surfaces, indicating that biodegradation within the rock matrix blocks should be considered as an important component of the system when evaluating the potential for natural attenuation or remediation at similar sedimentary rock sites.

Miocene south directed low‐angle normal fault evolution on Kea Island (West Cycladic Detachment System, Greece)

New structural, petrologic, and thermochronologic data from Kea, West Cyclades, define a crustal‐scale ductile shear zone and ductile/brittle low‐angle normal fault (LANF) system. Both the greenschist‐facies shear zone forming the footwall and the overlying LANF zone formed during constrictional strains, with a consistent top‐to‐SW‐S shear sense, with increasing finite strains toward higher structural levels but decreasing temperatures from footwall to hanging wall. The tectonostratigraphy comprises a ∼450 m thick footwall of shallowly dipping schists and calcite marbles representing the Intermediate Unit of the Attic‐Cycladic Crystalline (ACC). Above the footwall is a ∼60 m thick highly strained LANF zone, consisting of phyllonites, cataclastic schists, ultramylonitic calcite marbles, (proto) mylonitic calcite marbles, and cohesive cataclasites that arch over the whole island. These fault rocks exhibit multistage LANFs, evolving from ductile to brittle conditions. An up to ∼50 m thick brecciated limestone and dolostone sequence forms the unmetamorphosed hanging wall which is most probably part of the Upper Unit of the ACC. Multiequilibrium P‐T estimates on chlorite‐white mica pairs in the footwall yield 7–5.5 kbar/360°C–450°C for inclusions in albite and epidote, 5.5–3 kbar/400°C–350°C for the main foliation, and 3–2 kbar/350°C–280°C in localized shear bands (C and C′ foliations). The40Ar/39Ar white mica footwall cooling ages demonstrate that greenschist‐facies retrogression occurred between ∼21 and 17 Ma. Localized, late decimeter thick shear zones were active and dynamically recrystallized before ∼15 to 13 Ma. The LANF on Kea, together with similar structures in South Attica and the West Cyclades define the West Cycladic Detachment System, characterized by ductile to brittle top‐SW‐S shear sense.

Composition and origin of Early Cambrian Tiantaishan phosphorite–Mn carbonate ores, Shaanxi Province, China

The Tiantaishan phosphorite–Mn carbonate ores occur in the Early Cambrian Tananpo Formation in complexly folded and faulted rocks located in southern Shaanxi Province. About 65×10 6 tonnes of 17% P 2O 5 ore reserves exist and Mn-ore reserves are about 8.3×10 6 tonnes of +18% Mn. The stratigraphic sequence in ascending order consists of black phyllite, black to gray phosphorite ore, black phyllite, rhodochrostone ore, Mn mixed-carbonates, and dolostone. Data are presented from microprobe mineral chemistry, whole-rock chemistry, stable isotopes of carbonates, X-ray mineralogy, petrographic and SEM observations, and statistical analysis of chemical data. The dominant ore-forming minerals are hydroxy- and carbonate fluorapatite and Ca rhodochrosite, with Mg kutnahorite and dolomite comprising the Mn mixed-carbonate section. Pyrite occurs in all rock types and alabandite (MnS) occurs throughout the rhodochrostone section. The mean P 2O 5 content of phosphorite is 31% and argillaceous phosphorite is 16%, while the mean MnO content of rhodochrostone ore is 37%. Phosphorite ores are massive, spheroidal, laminated, and banded, while rhodochrostone ores have oolitic, spheroidal, and granular fabrics. The most distinguishing characteristics of the ores are high total organic carbon (TOC) contents (mean 8.4%) in the phosphorite and high P 2O 5 contents (mean 2.7%) in the rhodochrostone ore. The atypically high TOC contents in the Tiantaishan phosphorite probably result from very strong productivity leading to high sedimentation rates accompanied by weak reworking of sediments; poor utilization of the organic matter by bacteria; and/or partial replacement of bacterial or algal mats by the apatite. The depositional setting of the ores was the margin of an epicontinental seaway created as a direct consequence of global processes that included break-up of a supercontinent, formation of narrow seaways, creation of extensive continental shelves, overturn of stagnant, metal-rich deep-ocean waters, and marine transgression. Water depth increased from deposition of the black phyllite sequence through deposition of the Mn mixed-carbonate sequence, then shallowed again during deposition of the overlying dolostone sequence. Bottom waters were mostly dysoxic to suboxic, but fluctuated from oxic to anoxic. Productivity was high during deposition of the black phyllite sequence, increased during precipitation of phosphorite, and then decreased to moderate levels during precipitation of rhodochrostone ores. Biosilica contributions occur in each lithology, but are greatest in rhodochrostone. Changes in sedimentation were determined by changes in water depth, productivity, upwelling, sea-level change, and ventilation of the depositional basin. The source of the phosphorus was organic matter produced in great quantities during deposition of the black phyllite and phosphorite sequences in a zone of coastal upwelling. Organic matter accumulation was rapid. Globally, Mn was supplied by overturn of stagnant, metal-rich deep-ocean waters, which were redistributed to areas of coastal upwelling and seaways; that process may have been initiated by latest Proterozoic glaciations which would have promoted density stratification and accumulation and storage of metals. Regionally, Mn was supplied by terrigenous input into the shallow seaway and hydrothermal input into the deeper water parts of that seaway. Locally, Mn sources included leaching and transport of metals from the sediment column. Manganese was stored locally in low-oxygen (not anoxic) seawater prior to Mn-ore formation. The source of the carbon in the Mn carbonates and dolostones was predominantly seawater bicarbonate and secondarily CO 2 derived from the oxidation of organic matter in the bacterially mediated diagenetic zone of sulfate reduction.

An Organogenic Origin for Widespread Dolomite in the Cambrian Eilean Dubh Formation, Northwestern Scotland

ABSTRACT Dolomite precipitation from seawater is normally inhibited by the presence of sulfate, the low activity of the carbonate ion, and the high enthalpy of hydration of the magnesium ion. However, in modern microbial sediments, anoxic degradation of cyanobacterial material by sulfate-reducing bacteria can significantly alter ambient water chemistry: sulfate is removed, and enzymatic breakdown of proteins releases ammonia, thereby increasing pH and carbonate alkalinity to levels necessary for dolomite formation. Magnesium may be supplied from complexed ion pairs in solution, and from the breakdown of cyanobacterial sheaths and mucilage. In the ancient past, where benthic microbial communities dominated the depositional environment, it is postulated that large-scale, shallow subsurface modif cation of ambient waters by sulfate-reducing bacteria created conditions favorable to widespread dolomite formation. Early diagenetic cherts from the Cambrian Eilean Dubh Formation of northwestern Scotland preserve successive stages of the taphonomic history of cyanobacterial biofabrics in sediments of a continuously aggrading shelf. Depth-related sequential degradation of cyanobacterial mat is accompanied by the appearance and increase in abundance of authigenic dolomite, culminating in a crystalline dolomitic fabric with only residual organic clots. Silicified oolitic grainstone fabrics preserve various ooid morphologies. These form a diagenetically modified sequence in which progressive degradation of component microbial material, together with leaching of the ooid shell, is accompanied by the appearance and increase in abundance of intragranular authigenic dolomite. Total dissolution of ooids re ulted in the release of a package of crystals as sediment; similar sequences are observed in peloids and intraclasts. Ensuing dolomitic rock fabrics bear no clue to their origin, except where grains are preserved in early-silicified windows. These relationships can be explained by biochemical modification of porewaters during organic diagenesis, and provide evidence for widespread, authigenic dolomite formation in the shelf sediments. This organic degradational model may serve as a basis for understanding the origin of thick platformal dolostone sequences of the Proterozoic.

Paleoclimatic and tectonic control on the accumulation of North American cratonic sediment

Variation in accumulation rate as a function of depositional latitude has been determined for North American limestone, dolostone, sand-stone, gray-black shale, red shale, and evaporite rocks from tabulations by Ronov and co-workers, from data in lexicons of United States and Canadian stratigraphy, and from isopach and lithofacies maps published in 1975 by Cook and Bally. Phanerozoic sediment is similar to that on other continents, consisting of 63% terrigenous elastic facies (23% sandstone; 40% shale), 36% carbonate, and 1% chert, evaporite, and other minor rock types. Latitudinal variation in accumulation rate is influenced primarily by past positions of foreland and intracratonic basins. Additional variation related to paleoclimate is recorded as strong dependence of limestone, dolostone, red shale, and evaporite deposition on latitudes of accumulation, a relation reflecting the importance of ambient temperature and meteoric precipitation on the formation of various authigenic phases. Poleward increase in dolostone/total carbonate ratio in both hemispheres implicates the importance of syndepositional diagenesis in the formation of most dolostone sequences. The ratio of sandstone/total terrigenous clastic sediment exhibits little variation with latitude, a constancy indicating that most sand and mud is derived from older sediment rather than from the weathering of crystalline rock. As a result, terrigenous lithofacies record little climatic influence in the proportional delivery of mechanically versus chemically derived weathering products to basins of accumulation.

Dolomitization, time boundaries and unconformities: examples from the dolostone of the Taurus mesozoic sequence, south-central Turkey

A Mesozoic carbonate unit in the Taurus platform consists of thick dolostone sequences with both massive, thick beds and thinner beds intercalated with limestone. Dolomite preferentially replaced micrite in the original carbonate. In an initial phase of dolomitization, dolomicrite formed, which later as through dissolution and recrystallization resulted in clear dolomite crystals and dolosparites. An additional type of dolomite is dolosparite filling voids. These dolomite crystals represent either replacement of precursor calcite cement or growth into void space. Dissolution of late, coarsely crystalline dolosparite, precipitation of saddle crystals, local dedolomitization, and some late calcite cement in the voids, all indicate short-term atmospheric exposure of the sequence during or after the main dolomitization event. Dolomitization does not appear to be restricted to a specific environment. There is no evidence that the dolomite formation occurred in an evaporite, supratidal, or high intertidal setting. Our present state of knowledge does not allow an unequivocal explanation of the dolomitization event of massive platform dolomites. However, the association of dolomitization with sea-level fluctuations such as repetitive and minor transgressive and regressive periods might be a clue to the mechanism for the formation of thick dolomite beds in the Taurus Mesozoic platform, as noted by the increasing of the dolostone/limestone ratio near the time boundaries (Triassic/Jurassic and Jurassic/Cretaceous). Stable isotope data support mostly marine-derived solutions as the dolomitizing fluid. Marine solutions can be the only source of the large quantities of Mg 2+ ions needed for dolomitization of massive platform dolomite as was suggested by Land (1985) and Given and Wilkinson (1987).

The Genadendal Zn-Pb-Mn mineralization, South Africa; a possible early Proterozoic alkaline hydrothermal system

A complex hydrothermal system is described in a 2.3-Ga sedimentary succession. It comprises an epigenetic part developed just below an uncomformity, characterized by intense desilicification of cherty dolostone and by mineralization forming the matrix of a breccia, probably representing a collapsed paleocave horizon. Two facies can be identified in this desilicification zone: a blue facies that is characterized by replacement of chert by coarsely crystallized Mn-Fe-rich dolomite and a breccia matrix filled in by the same dolomite plus some galena and sphalerite, and a red facies, that is characterized by chert replacement by a similar dolomite, which is only Mn-rich, rubefaction of dolostone, and deposition of primary Mn,Fe oxides (+Zn, Pb). The syngenetic part immediately overlies the uncomformity and forms Zn shale interpreted as former metalliferous mud deposited in a paleosinkhole flooded by hydrothermal solutions.Theoretical calculations suggest that the desilicification of the dolostone can be best explained by mixing of warm, relatively dilute hydrothermal solutions with cold alkaline surficial brines. It is proposed that the latter resulted from surface evaporation of deep bicarbonated solutions, which then descended into the substratum where mixing occurred.The Genadendal occurrence is part of a local group of Pb-Zn deposits in a cherty dolostone sequence. These deposits have not been controlled by igneous intrusions and may be related to the Mississippi Valley type. Comparisons are made with other mineral deposits such as Kombat (Namibia) and Franklin Furnace (New Jersey) which also may have formed under alkaline conditions.

Lingulid brachiopods and Lingulichnus from a Silurian shelf–slope carbonate sequence, Delorme Group, Mackenzie Mountains, Northwest Territories

Structures assigned to the ichnogenus Lingulichnus, commonly containing the burrow-forming brachiopod, were collected from an argillaceous unit in a Silurian shelf dolostone sequence (Delorme Group). The sediments, benthic fauna, and inclined orientation of all lingulid burrows indicate the presence of a bottom current and restricted conditions on the margin of the Selwyn Basin. High-abundance–low-diversity faunas of lingulids and Lingulichnus are indicative of limiting conditions at the substrate surface. Under low-energy conditions, they seem to be indicative of the Zoophycos ichnofacies.

The carbon isotope record of dolostones as a stratigraphic tool: A case study from the upper cretaceous shelf sequence, Israel

Accumulation of evidence on past global changes in carbon isotopes in marine carbonate rocks points to the usefulness of the carbon isotope record as a stratigraphic tool. This method will be important in sections with a poor representation of indicative fauna, such as dolostones. Isotope analyses of dolostone sequences from the Upper Cenomanian-Turonian sequence of Israel suggest that on the average a depletion of 1.1% in δ 13C ratio can be detected across the boundary. The Israeli trend agrees with the δ 13C record across this boundary reported from sections with detailed biostratigraphic records.

The Evolution of a Middle Precambrian Dolostone Sequence--A Spectrum of Dolomitization Regimes

ABSTRACT Most Precambrian dolostones are considered here to represent replacement of original limestone or calcareous sediment, and hence such dolostone sequences can be interpreted in much the same way as Phanerozoic dolostone sequences. The sequential development of dolomite is recorded in a middle Precambrian carbonate-clastic platform succession in the Belcher islands. The timing of dolomite formation ranges from penecontemporaneous (eogenetic) to late burial (mesogenetic). The platform sequence is divided into three distinctive megacycles: 1) a siliciclastic-dominated phase, wherein pisolitic tufas formed in the upper reaches of a supratidal flat--incipient penecontemporaneous dolomite formed in a setting somewhat analagous to the modern Coorong alkaline lakes; II) a mixed clastic-carbonate and stromatolitic carbonate phase, where penecontemporaneous dolomite formed under conditions of high evaporation rates and high salinity, and is comparable to recent dolomite in sabkha environments; and III) a stromatolite bui dup--slope-basin phase where incipient dolomitization may have taken place within a mixed seawater-freshwater aquifer system. However, pervasive dolomitization of the platform sequence did not occur until a later stage of burial diagenesis, and postdated the main stage of silica diagenesis. Recrystallization was a late mesogenetic event that resulted in only minor crystal enlargement; stylolitization also produced local crystal growth but was not responsible for pervasive dolomitization. Fracture-filling dolomite accumulated as a result of deformation and uplift during the Hudsonian Orogeny (telogenesis).