Introduction
Hardgrounds are surfaces of synsedimentary lithification and represent a break in sedimentation (Shinn, 1969, Bromley, 1975, Kennedy and Garrison, 1975, Bathurst, 1975, Tucker, 1991, Wilson and Palmer, 1992). These surfaces may be recognized by the presence of abrasion, encrustations and borings and may cut across fossils and sedimentary structures. Some hardgrounds develop from loose sediments, through firmgrounds, to lithified layers, and indicate a progressive hardening which is characterized by a change in the fauna, particularly the burrowing organisms, and by sedimentary features (Tucker, 1991, Fürsich et al., 1981). Some hardgrounds are mineralized, with iron hydroxides (goethite-limonite), and glauconite and phosphorite impregnating the sediment, burrow walls and fossils, and occurring as discrete grains (Tucker and Wright, 1990). Both intense submarine cementation and borings and/or burrowing indicate slow sedimentation in the depositional environment (Shinn, 1969, Tucker and Wright, 1990, Macintyre, 1977, James and Ginsburg, 1979).
In the recognition of hardgrounds, geologists face two main problems. The first problem is hardground identification. The second problem is the recognition of the diagenetic environment in which lithification took place — either submarine or subaerial (Bathurst, 1975). Research on hardground recognition has been published by Shinn, 1969, Purser, 1969, Kennedy and Garrison, 1975, Garrison et al., 1987, Marshall and Ashton, 1980, Goldring and Kazmierczak, 1974, Dravis, 1979, Brett and Brookfield, 1984. This paper aims to introduce the Kilop Hardground as a Cretaceous example, and to investigate its origin.
Chronostratigraphic interpretation of the carbonate platform sequences underlying the Kilop Hardground has been made using benthic foraminifers and calcareous algae. Sequences representing the facies of the drowning platform were studied by Tasli and Özsayar (1997).
Section snippets
Kilop Cretaceous hardground
The hardground surface is well exposed in the Kilop area of Kale (Gümüshane, NE Turkey), a small town located on the road between Gümüshane and Bayburt (Fig. 1). Here the hardground caps a burrowed peloidal limestone (grainstone) of the Berdiga Formation, covering an area of 0.5km2. The Berdiga Formation is made up of platform carbonates of Late Jurassic to Early Cretaceous age (Eren, 1983, Tasli, 1991). In the field, the Kilop Hardground is overlain, apparently conformably, by the Kermutdere
Regional and tectonic settings
The Gümüshane region, in which the Kilop Harground occurs, is a part of the Eastern Pontides (Fig. 1), extending in E–W direction along the southern shore of the Eastern Black Sea. The geotectonic evolution of the Eastern Pontides is still debated. According to Sengor et al. (1984), it forms the eastern part of the Rhodope-Pontid Fragment. It is generally accepted that the Rhodope-Pontid fragment lies entirely to the north of the Neo-Tethyan oceanic sutures and therefore formed the northern
Biostratigraphy and environmental setting of the Cretaceous sequences
In the Gümüshane area, Upper Jurassic–Lower Cretaceous platform carbonates have been correlated with the Berdiga Formation first described by Pelin (1977) from the Alucra (Giresun) area. The Upper Cretaceous conglomerates, calcarenites (Fig. 7, unit E), red pelagic limestones (Fig. 7, unit F) and siliciclastic turbidites have been named the Kermutdere Formation (Tokel, 1972). Albo-Cenomanian to Turonian-Coniacian outer shelf to slope carbonates (Fig. 8, units C and D) were first described by
Hardground origin
Extensive burrows (especially vertical burrows), encrusting organisms and very rare marine cement relics are characteristics of the Kilop Hardground, and indicate slow sedimentation (Wilson, 1975, Pienkowski, 1985, Shinn, 1969). Dwelling traces of suspension feeders, such as Skolithos and Thalassinoides (?) indicate a soft substrate (Frey and Pemberton, 1984, Fürsich et al., 1981). Thalassinoides commonly forms a burrow-system below many hardgrounds (Bromley, 1967, Bromley, 1975, Goldring and
Conclusions
An ancient hardground surface is exposed in the Kilop area, characterized by extensive burrows, large encrusting organisms and scarce relics of submarine rim cement.
Neptunian dykes and a syn-sedimentary fault with regular boundary indicate that the substrate was well lithified prior to the deposition of the overlying deeper marine argillaceous sediments which also infill the sedimentary dykes.
The hardground occurs at the top of the regressive sequences of the Berdiga Formation, where it
Acknowledgements
The authors are indebted to Prof. Alastair Robertson and Dr Mark Wilson for their constructive comments on an earlier version of the paper, and to Dr A.J. Barber for improvements to the English.
Distinguishing borings and burrows in intraclasts: Evidence from the Cambrian (Furongian) of North China
2023, Sedimentary Geology
Hardgrounds represent synsedimentary cemented stratigraphic beds that form at or near the seafloor. Borings represent a key line of evidence for investigations of hardground development and record the evolution of bioerosion and boring organisms. The unequivocal identification of borings is done through identification of the crosscutting relationship between the proposed boring and a hard substrate, such as lithoclasts and/or shells, with morphological criteria able to be used when dealing with a homogeneous substrate, such as micritic hardgrounds. Bioeroded hardgrounds and burrows with a micrite halo/lining are subject to fracturing and reworking, resulting in accumulations of intraclasts in flat-pebble conglomerates (FPC). The recognition of borings and broken burrows with a halo can be challenging in FPC. Using trace fossils preserved in situ and in FPC from late Cambrian carbonates of North China, we establish a set of criteria for distinguishing borings from burrows with a halo in FPC. Features such as the relative volume of burrows and borings versus the host pebble and the number of traces per pebble, the cross-cutting relationship with different colored laminae, and the presence of pyrite or glauconite encrustations can all be invoked to aid recognition of borings. Examination of the cross-cutting relationship and encrustation of trace fossils are not sufficient on their own. Our results suggest caution is necessary in defining borings in FPC, particularly as synsedimentary deformation of burrows with a halo in late Cambrian FPC can create structures that resemble borings.
Calibrating the Late Jurassic–Early Cretaceous shallow and deep marine bioevents by quantitative biostratigraphy: A synthesis from the Pontides Carbonate Platform (Turkey)
2022, Earth-Science Reviews
Citation Excerpt :
See AlsoProgressive deformation and reorientation of fold axes in a ductile mylonite zone: the woodroffe thrust - PDF Free DownloadChirurgie urologique et ambulatoireSeasonal and spatial variations in the source and transport of sinking particles in the Strait of Georgia, British Columbia, CanadaIchnofacies and sedimentary structures: A passive relationship with permeability of a sandstone reservoir from NW BorneoThe LO of Montsalevia salevensis seems to be confined to the Calpionellites Zone (Fig. 6). In addition to similar upper Valanginian LOs in previous studies (Altiner, 1991; Eren and Taslı, 2002; Krajewski and Olszewska, 2007), base Hauterivian (Septfontaine, 1980–81; Arnaud-Vanneau, 1998) and earliest Hauterivian (Bucur et al., 2004; Pleş et al., 2019) disappearances are also noted, providing a proximity marker for the V/H boundary in shallow marine deposits (Fig. 6). Similarly, the LOs of Coscinoconus elongatus, Mohlerina basiliensis and Protopeneroplis ultragranulata are within the darderi subzone (Fig. 6).
The Late Jurassic–Early Cretaceous is an interval of unstandardized stages and includes the only Mesozoic system boundary without a Global Boundary Stratotype Section and Point – the Jurassic/Cretaceous (J/K) boundary. Recent researches have been mainly focused on deep marine continuous successions from the Tethyan region and provided important progress in calibration of pelagic bioevents. Correlation of these pelagic zonations with the schemes from shallow marine deposits is still obscure. Biostratigraphical data from marginal carbonates containing fossils both from the platform and basinal facies can provide the required links between these two distinct depositional environments. This kind of Upper Jurassic–Lower Cretaceous carbonates widely crop out in the Pontides (northern Turkey) in close association with related shallow and deep marine successions. A biostratigraphical dataset including 17 stratigraphical sections from this Pontides Carbonate Platform is synthesized. The fossil data include organisms from various depositional environments (i.e., benthic and planktonic foraminifers, calpionellids, algae, microencrusters and crinoids) and provides 139 bioevent datums (stratigraphic levels). This fossil dataset is analyzed through the methods of Graphic Correlation (GC) and Unitary Associations (UA) in order to overcome facies (past depositional conditions) controlled local biohorizons and calibrate fossil datums from unrelated phylogenies. Calibration of the Pontides Composite Reference Section (CSRS) with the Geological Time Scale (2020) reveals relative positions of both shallow and deep marine bioevents with respect to the Oxfordian–Hauterivian stage boundaries. The Tithonian/Berriasian and the Berriasian/Valanginian boundaries can be easily delineated by calpionellid bioevents in pelagic successions. However, no synchronous shallow marine first/last occurrence bioevents are available for both of these levels. Increased rates of originations toward Berriasian provide clustering of bioevents around the Tithonian/Berriasian boundary and brackets for both pelagic and shallow marine deposits. Several last occurrences provide unreliable approximations for the Berriasian/Valanginian boundary in neritic deposits. The species richness declines mid-Berriasian onward in accordance with the general trend toward lower sea levels through the late Tithonian into the Valanginian that diminished shallow marine factories and paved the way for a general Valanginian–Hauterivian drowning phase for the Tethyan carbonate platforms. This also adds difficulties in finding reliable origination events in the shallow marine environments for this extinction dominated interval.
Discovery of enigmatic toroidal carbonate concretions on the Rio Grande Rise (Southwestern Atlantic Ocean)
2022, Marine Geology
We present enigmatic toroidal carbonate concretions retrieved from 700m water depth from two sites in the upper plateau of the Rio Grande Rise. The concretions have a diameter of ~15cm and a central hole of ~5cm, and were observed on top of loose bioclastic sand over an area of ~30m2 at 0.5–1.5m from one another. They consist of brown, porous, bioclastic grainstone, lacking internal structures. Grains consist of sand (< 3% coarse, 30% medium, 35% fine, 25% very fine), composed mainly by planktonic foraminiferal tests, and<10% lime mud. The observed foraminiferal species indicate initial deposition of the sand in an open ocean setting. Biostratigraphy suggests an age no older than Pleistocene. Petrographic thin sections and SEM reveal that the fossiliferous grainstone contains intraclastic micritic cement and isopachous rim cement made of bladed magnesian calcite. δ18O values range from +1.5 to +3.3‰ (V-PDB) and increase with the degree of cementation, while δ13C ranges from +0.5 to +2.3‰ irrespective of cementation. The cementation of the grainstones is likely to have taken place in the marine phreatic environment. Carbonate precipitation induced by methane oxidation or (subaerial) meteoric diagenesis are ruled out based on both cement fabric and isotopic composition. Plausible causes for the toroidal shape of these structures could be: 1) sediment excavation by organisms, or 2) cementation within biofilms around burrows, followed by selective seafloor erosion. However, unveiling the actual formation mechanisms warrants further investigation.
Barkerina dobrogiaca Neagu, 2000, a Valanginian marker taxon from the northern Neotethysian margin
2019, Cretaceous Research
Barkerina dobrogiaca Neagu (Valanginian of Romania), a medium-sized complex benthic foraminifera, is described from the Valanginian of the Pontides, NE Turkey. Barkerinadobrogiaca differs from the type-species B.barkerensis Frizzell & Schwartz (Albian of Texas) by its small size, and comparably thin septula and test wall. The present paper is a further contribution to the knowledge of inventory and faunal assemblages of Valanginian shallow-water carbonates. Compared to the classical Urgonian (Hauterivian to lower Aptian) these are still incompletely known. Together with another form reported from the Barremian of Romania, it also shows that representatives of Barkerina Frizzell & Schwartz are well present in the Neotethysian realm.
Age and geodynamic evolution of the Black Sea Basin: Tectonic evidences of rifting in Crimea
2018, Marine and Petroleum Geology
The timing and direction of opening of the Black Sea Basin are debated. However, parts of its margins were inverted during Cenozoic and can be studied onshore. The Crimean Mountains are located in the middle of the northern margin of the basin, and at the onshore prolongation of the mid-Black Sea High.
We present the first detailed mapping of large striated normal faults in Crimea. These faults define graben structures that trend parallel to the continental margin. Kinematic analysis of the faults combined with new biostratigraphic data show that the syn-rift sequence is Valanginian to Late Albian in age. It consists of siliciclastic deposits with limestone olistoliths. In contrast, the post-rift Late Cretaceous carbonaceous sequence of Crimea is devoid of normal faults or olistoliths. It unconformably overlies the graben structures.
The onset ages, and the trends of extension are quite similar in the northern (Crimea) and the southern (Turkey) inverted margins of the basin. The Early Cretaceous extension directions are normal to the mid-Black Sea High and the Black Sea margins. We conclude that rifting of Black Sea Basin occurred from the Valanginian to the Late Albian (∼39 Ma) and drifting during the Late Cretaceous.
Based on the directions of rifting, on the lack of evidence of strike slip motions near the mid-Black Sea High, and on published paleomagnetic data, we propose that the Black Sea opened with rotations accommodated by transform faults at its western and eastern margins, as a response to asymmetric slab rollbacks of the Neo-Tethys plate.
The inversion of the Crimean margin results from two successive shortening events: Early Eocene NE-SW compression, Eocene to Present SE-NW compression. Their timing support the idea that compressional stresses generated by continental collisions in Turkey were transmitted through the strong Black Sea lithosphere up to Crimea.
Age constraints on intra-formational unconformities in Upper Jurassic-Lower Cretaceous carbonates in northeast Turkey; geodynamic and hydrocarbon implications
2018, Marine and Petroleum Geology
Citation Excerpt :
Detrital zircons in the underlying Şenköy Formation (Akdoğan et al., in press) close to the Berdiga Formation type section near Alucra, more precisely constrain a latest Oxfordian or younger depositional onset age for the formation in this region. Rifting disrupted sedimentation on the Berdiga carbonate platform during Cretaceous time (Eren and Tasli, 2002; Konak et al., 2009; Taslı et al., 1999; Yılmaz, 2002; Yılmaz and Kandemir, 2006). This resulted in erosion, karstification or hardground formation on the highs, and a deepening and change in carbonate facies in subsiding regions.
Upper Jurassic-lowermost Cretaceous carbonate build-ups are imaged on seismic data in the Black Sea. They form important, untested, hydrocarbon reservoirs that are the focus of active exploration. Outcrop analogues to these build-ups around the Black Sea contain a series of subaerial exposure surfaces. The hiatuses associated with a number of these subaerial exposure surfaces have been dated in a well exposed Callovian or Upper Oxfordian to Barremian shallow-water inner platform carbonate succession (the Berdiga Formation) in the Eastern Pontides using strontium isotope stratigraphy and foraminiferal biostratigraphy. They span the latest Kimmeridgian to Tithonian or Berriasian, and the Hauterivian to Barremian. Less well constrained, but broadly contemporaneous stratigraphic gaps in multiple successions around the Black Sea provide additional insights and point to a regional driving mechanism. The timing of hiatus formation does not correspond to periods of eustatic lowstand. It does coincide, however, with Late Tithonian to Berriasian and Hauterivian to Early Aptian episodes of rifting in the Greater Caucasus Basin, located farther to the north. Thus, it is possible that subaerial exposure was caused by rift flank uplift during periods of regional extension. Uplift due to slab break off is discounted as a control because it post-dates (rather than pre-dates) locally developed Kimmeridgian magmatism. Rift-flank uplift is likely to have also affected carbonate build-ups on the intervening rift shoulders of the eastern Black Sea, the Shatskiy Ridge and the Mid Black Sea High. At outcrop, subaerial exposure is often associated with karstification and secondary porosity development. Similar processes may have occurred in the offshore helping to enhance the reservoir quality of these exploration targets.
Research article
The Kapanboğazı formation: A key unit for understanding Late Cretaceous evolution of the Pontides, N Turkey
Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 441, Part 3, 2016, pp. 565-581
The Pontides forming the south-western continental margin of the Black Sea consist of two tectonic units, the Istanbul Zone in the west, and the Sakarya Zone in the central and eastern parts. The Sinop Basin in the Sakarya Zone is filled, from base to top, by Hauterivian to Albian turbidites, Cenomanian–Turonian red pelagic sediments, Turonian–Campanian magmatic-arc and related deposits, and by the uppermost Campanian to middle Eocene post-magmatic units developed on the southern passive margin of the Black Sea. Based on nannofossil, dinoflagellate, Foraminifera and Radiolaria data we describe the Kapanboğazı Formation, a Cenomanian–Turonian unit in the Sinop Basin, represented by red calcareous/siliceous pelagic shales, limestones and cherts passing gradually from the Albian black shales. These sediments possibly represent deepest depositional conditions of the basin during the Cenomanian–Turonian interval and also reflect the transition from an anoxic to an oxic palaeoenvironmental setting. The Istanbul Zone to the west was emerged during the deposition of the Kapanboğazı Formation in the Sakarya Zone.
In the Pontides, red pelagic sediments were deposited at different times during the Cenomanian–Maastrichtian interval. Because the Kapanboğazı Formation was deposited only in the Sakarya Zone and because it is present in limited outcrops due to structural reorganization and thick overlying volcanoclastic pile, most previous authors assumed Cenomanian–Turonian hiatus. Herein we describe detailed palaeontological data from this unit and discuss their importance to the interpretation of depositional history and tectonics of the Black Sea region, as well as climatic and eustatic implications.
Research article
Mineralogy, early marine diagenesis, and the chemistry of shallow-water carbonate sediments
Geochimica et Cosmochimica Acta, Volume 220, 2018, pp. 512-534
Shallow-water carbonate sediments constitute the bulk of sedimentary carbonates in the geologic record and are widely used archives of Earth’s chemical and climatic history. One of the main limitations in interpreting the geochemistry of ancient carbonate sediments is the potential for post-depositional diagenetic alteration. In this study, we use paired measurements of calcium (44Ca/40Ca or δ44Ca) and magnesium (26Mg/24Mg or δ26Mg) isotope ratios in sedimentary carbonates and associated pore-fluids as a tool to understand the mineralogical and diagenetic history of Neogene shallow-water carbonate sediments from the Bahamas and southwest Australia. We find that the Ca and Mg isotopic composition of bulk carbonate sediments at these sites exhibits systematic stratigraphic variability that is related to both mineralogy and early marine diagenesis. The observed variability in bulk sediment Ca isotopes is best explained by changes in the extent and style of early marine diagenesis from one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the fluid (fluid-buffered) to one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the precursor sediment (sediment-buffered). Our results indicate that this process, together with variations in carbonate mineralogy (aragonite, calcite, and dolomite), plays a fundamental and underappreciated role in determining the regional and global stratigraphic expressions of geochemical tracers (δ13C, δ18O, major, minor, and trace elements) in shallow-water carbonate sediments in the geologic record. Our results also provide evidence that a large shallow-water carbonate sink that is enriched in 44Ca can explain the mismatch between the δ44/40Ca value of rivers and deep-sea carbonate sediments and call into question the hypothesis that the δ44/40Ca value of seawater depends on the mineralogy of primary carbonate precipitations (e.g. ‘aragonite seas’ and ‘calcite seas’). Finally, our results for sedimentary dolomites suggest that paired measurements of Ca and Mg isotopes may provide a unique geochemical fingerprint of mass transfer during dolomitization to better understand the paleo-environmental information preserved in these enigmatic but widespread carbonate minerals.
Research article
The nature, origin, and predictors of porosity in the Middle to Late Devonian Horn River Group of the Central Mackenzie Valley, Northwest Territories, Canada
Marine and Petroleum Geology, Volume 142, 2022, Article 105738
The characterization of porosity is an essential step in the evaluation of resource-bearing porous media. Here, we focus on the Devonian Hare Indian and Canol Formations, two potential unconventional mudstone reservoirs, in a core from the Horn River Group of the Central Mackenzie Valley, Northwest Territories, Canada. By combining bulk porosity, low-pressure N2 adsorption, and scanning electron microscopy (SEM) results with composition and lithofacies datasets, we assess the porosity in these successions to understand pore types, size, distribution, degree of connectivity, and controls and predictors of porosity. Mineral matrix pores (interparticle and intraparticle), organic matter pores, and lithofacies-dependant natural fractures are present. All pore types display limited connectivity in two dimensions. Mineralogy is the most significant control on porosity with trends in porosity present among lithofacies. No relationship is observed between porosity and total organic carbon (TOC), suggesting that mineral matrix pores, rather than organic matter pores, are dominant in this unit. We compare these results to other mudstone reservoirs in North America and show that the Bluefish Member (Hare Indian Formation) and the Canol Formation are characterized by comparable bulk porosity, lower N2 mesopore volume, and higher quartz content relative to the other units considered. In contrast, compared to the other unconventional reservoir examples, the Bell Creek Member of the Hare Indian Formation exhibits lower quartz and higher clay content, average bulk porosity, and lower N2 pore volume. The results collectively suggest that high quartz and low clay content are the best predictors of porosity in the Horn River Group. Natural fractures may serve as flow pathways to induced fractures; however, these units lack the network of interconnected organic matter pores that can be present in other successions.
Research article
Quantifying early marine diagenesis in shallow-water carbonate sediments
Geochimica et Cosmochimica Acta, Volume 236, 2018, pp. 140-159
Shallow-water carbonate sediments constitute one of the most abundant and widely used archives of Earth’s surface evolution. One of the main limitations of this archive is the susceptibility of the chemistry of carbonate sediments to post-depositional diagenesis. Here, we develop a numerical model of marine carbonate diagenesis that tracks the elemental and isotopic composition of calcium, magnesium, carbon, oxygen, and strontium, during dissolution of primary carbonates and re-precipitation of secondary carbonate minerals. The model is ground-truthed using measurements of geochemical proxies from sites on and adjacent to the Bahamas platform (Higgins et al., 2018) and authigenic carbonates in the organic-rich deep marine Monterey Formation (Blättler et al., 2015). Observations from these disparate sedimentological and diagenetic settings show broad covariation between bulk sediment calcium and magnesium isotopes that can be explained by varying the extent to which sediments undergo diagenesis in seawater-buffered or sediment-buffered conditions. Model results indicate that the covariation between calcium and magnesium isotopes can provide a semi-quantitative estimate of the extent and style (fluid-buffered vs. sediment-buffered) of early marine diagenesis. When applied to geochemical signatures in ancient carbonate rocks, the model can be used to quantify the impact of early marine diagenesis on other geochemical proxies of interest (e.g. carbon and oxygen isotopes). The increasing recognition of early marine diagenesis as an important phenomenon in shallow-water carbonate sediments makes this approach essential for developing accurate records of the chemical and climatic history of Earth from the chemical and isotopic composition of carbonate sediments.
Research article
The unconformity caused by the Huaiyuan movement and the deep natural gas exploration field in the Ordos Basin, China
Natural Gas Industry B, Volume 8, Issue 6, 2021, pp. 539-551
In order to clarify the hydrocarbon accumulation significance and exploration prospect of the unconformity caused by the Huaiyuan movement in the Ordos Basin, this paper studies the spatial distribution and structural plane characteristics of this unconformity and its relationships with hydrocarbon accumulation by observing field outcrops and cores and analyzing logging data, based on the previous research results and the interpretation results of 2D and new 3D seismic data. And the following research results are obtained. First, the unconformity was mainly formed in the Floian Age of Early Ordovician and widely occurs at the bottom of the Jiawang Formation and the top of the Sanshanzi Formation and the related tectonism lasts 30 Myr. Second, basal conglomerate and sandstone less than 1m in thickness are developed above the unconformity at the edge of the basin, while thin mudstone, argillaceous dolomite (limestone) and marl are developed above the unconformity in the central part of the basin. Third, the unconformity structurally consists of three layers, including a basal conglomerate layer, a paleosoil layer and a fully–semi weathered carbonate layer from top to bottom, among which, the last one is 20–90m in thickness with developed dissolution fractures and pores to form a quality reservoir. Fourth, the unconformity results in the development of a series of large valleys landforms, which incise the Lower Ordovician–Upper Cambrian. Fifth, the unconformity can act as a good channel for hydrocarbon migration, and it connects with the unconformity caused by the Caledonian movement within the paleo-uplift of the Ordos Basin, which is favorable for the the hydrocarbon of different sources in the west side of the basin eastwards to migrate, accumulate and form a gas reservoir. In conclusion, the deep Lower Paleozoic related to the Huaiyuan unconformity is expected to be an important natural gas exploration field in the Ordos Basin.
Research article
Evaluation of the properties of dolomitization fluids and diagenetic alterations of mg/Ca ratios in carbonate rocks in the cambrian series-2 to miaolingian strata in Central Uplift Belt, Tarim Basin: Constraints from halogens, REEs and isotope geochemistry
Marine and Petroleum Geology, Volume 144, 2022, Article 105838
The nature of the dolomitization fluids and the diagenetic alterations on carbonate Mg/Ca ratios in Cambrian series-2 to Miaolingian strata in Central Uplift Belt, Tarim Basin are interpreted on the basis of petrography, geochemistry (trace elements, halogens, REEs, O–C–Sr isotopes) and fluid inclusion microthermometry. Based on petrographic examination, two types of less stoichiometric dolomites: dolomicrites (DM), very fine-to fine-crystalline planar-e(s) dolomite (D1) formed at near-surface to shallow-burial settings, and two types of more stochiometric dolomites (fine-to medium-crystalline planar-e(s) dolomite (D2) and medium-to coarse-crystalline.
nonplanar-a dolomite (D3)) formed at deep-burial setting are identified. Medium-to coarse-crystalline, nonplanar-a saddle dolomite cement (DC), and early-stage calcite cement (ESCAL) and later-stage calcite cement (LSCAL) are identified. The DM, D1, D2, and D3 samples display positive Cerium anomalies (δCe) and negative Europium anomalies (δEu), and the DC display positive δCe and positive δEu. They show micritic-bladed and blocky calcite-like REE patterns, suggesting their dolomitizing fluids inherited the seawater or seawater-like REE signatures of their precursor carbonate materials at the time of crystal growth. No correlations exist between δ13C, δ18O, 87Sr/86Sr, Mg/Ca, and Mn/Sr which were tools used to evaluate the diagenetic history of the carbonates, coupled with the high Mg/Ca ratios (0.64–0.85), Mn/Sr ratios, and more depleted δ18O, δ13C values of D2, D3, and DC. This suggests that the dolomites were altered by high-temperatures or their isotopic difference may be the result of equilibrium isotopic fractionation. The increasing trend in Cl/(Ca+Mg), I/(Ca+Mg), and Br/(Ca+Mg) ratios in the D2, D3, and DC with dolomite content >50wt% can be explained by the presence of evaporite (salt) derived from seawater or sedimentary pore waters as fluid interaction with sedimentary materials in the studied strata. The comparable isotopic (δ18O, δ13C and 87Sr/86Sr) values between DM, D1, D2, D3, DC, and coeval Early Paleozoic seawater value, coupled with the relatively high homogenization temperature (Th) and salinity levels in D2, D3, and DC, indicates that their diagenetic fluids were derived from radiogenic 87Sr-enrich coeval seawater with hypersaline signatures in deep-burial settings. The dolomitization model shows that the precipitation of dolomicrites was by evaporative seepage-reflux dolomitization at near-surface burial settings. Evaporative seawater dolomitization supported by 87Sr/86Sr, δ13C values and calculated water oxygen isotope ratio was probably responsible for the D1 during shallow-burial, and D2 and D3 with irregular overgrowth rims were formed during deep-burial dolomitization. The average range of the Th values of DC is 148.2–193.4°C; mostly overlapping with the estimated ambient temperature (65°C–180°C, at a depth >4000m) for the studied strata, indicates that the growth of DC in fractures may have resulted from upward and laterally squeezed higher-temperature saline-enriched basinal fluids conveyed by geothermal convective-advective fluid flows or squeegee fluid flows at depth resulting in geothermal dolomitization.
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