Shallow-water Carbonate Research Articles

The glass ramp of Wrangellia: Late Triassic to Early Jurassic outer ramp environments of the McCarthy Formation, Alaska, U.S.A.

Abstract The marine record of the Triassic–Jurassic boundary interval has been studied extensively in shallow-marine successions deposited along the margins of Pangea, particularly its Tethyan margins. Several of these successions show a facies change from carbonate-rich to carbonate-poor strata attributed to the consequences of igneous activity in the Central Atlantic Magmatic Province (CAMP), which included a biocalcification crisis and the end-Triassic mass extinction. Evidence for a decline in calcareous and an increase in biosiliceous sedimentation across the Triassic–Jurassic boundary interval is currently limited to the continental margins of Pangea with no data from the open Panthalassan Ocean, the largest ocean basin. Here, we present a facies analysis of the McCarthy Formation (Grotto Creek, southcentral Alaska), which represents Norian to Hettangian deepwater sedimentation on Wrangellia, then an isolated oceanic plateau in the tropical eastern Panthalassan Ocean. The facies associations defined in this study represent changes in the composition and rate of biogenic sediment shedding from shallow water to the outer ramp. The uppermost Norian to lowermost Hettangian represent an ∼ 8.9-Myr-long interval of sediment starvation dominated by pelagic sedimentation. Sedimentation rates during the Rhaetian were anomalously low compared to sedimentation rates in a similar lowermost Hettangian facies. Thus, we infer the likelihood of several short hiatuses in the Rhaetian, a result of reduced input of biogenic sediment. In the Hettangian, the boundary between the lower and upper members of the McCarthy Formation represents a change in the composition of shallow-water skeletal grains shed to the outer ramp from calcareous to biosiliceous. This change also coincides with an order-of-magnitude increase in sedimentation rates and represents the transition from a siliceous carbonate-ramp to a glass ramp ∼ 400 kyr after the Triassic–Jurassic boundary. Sets of large-scale low-angle cross-stratification in the Hettangian are interpreted as a bottom current–induced sediment drift (contouritic sedimentation). The biosiliceous composition of densites (turbidites) and contourites in the Hettangian upper member reflects the Early Jurassic dominance of siliceous sponges over Late Triassic shallow-water carbonate environments. This dominance was brought about by the end-Triassic mass extinction and the collapse of the carbonate factory, as well as increased silica flux to the ocean as a response to the weathering of CAMP basalts. The presence of a glass ramp on Wrangellia supports the hypothesis that global increases in oceanic silica concentrations promoted widespread biosiliceous sedimentation on ramps across the Triassic to Jurassic transition.

Diagenetic evolution control on pore network creation in Cretaceous shallow-water carbonates (Jandaíra Formation, Potiguar Basin, Brazil): Inferences for development of secondary porosity in epigenic karst systems in carbonate reservoirs

Diagenetic evolution control on pore network creation in Cretaceous shallow-water carbonates (Jandaíra Formation, Potiguar Basin, Brazil): Inferences for development of secondary porosity in epigenic karst systems in carbonate reservoirs

Jurassic–Cretaceous intraplatform basins from NW Sicily fold and thrust belt: Implications for oblique rifting of the Southern Tethyan margin

Carbonate intraplatform basins are documented worldwide as peculiar synsedimentary tectonic features forming in rifted margins and especially in those cases where the direction of extension is oblique with respect to the rift axis. A detailed stratigraphy and facies analysis combined with subsidence history of the Upper Triassic-Cretaceous shallow-water carbonates outcropping in the Peninsula of Capo San Vito, representing the NW end of the Sicilian fold and thrust belt, has been performed. Three different sections, separated by faults and characterised by shallow-water and deep-water carbonate sedimentation, were distinguished. Their location and tectonic relationships highlight the occurrence of two structural highs (Monaco and Sparagio sections) separated by a structural low (Acci section).The Acci intraplatform basin, structured since the Pliensbachian, is characterised by mostly pelagic sedimentation with resedimentation of carbonate reef derived-elements from adjacent carbonate platform blocks. On the two structural highs, shallow-water sedimentation, since the Late Triassic, developed with progradational trends shedding out shallow-water elements towards the basin. The evolution of the narrow Acci intraplatform basin appears influenced by a transtensional regime creating a wedge-shaped basin. Backstripping analysis on the three measured sections was able to reconstruct subsidence curves, from which different evolutionary stages, marked by punctual synsedimentary tectonic activity, can be highlighted for the Late Triassic-Eocene time interval.Comparison with the adjacent basin system of Cala Rossa, located 30 km away eastward in the Palermo Mountains, allowed to recognise similar characteristics. The stratigraphic, sedimentologic and tectono-sedimentary common features, trend and kinematic of the paleofaults, and the same chronology of tectonic events, suggest that the two areas represent adjacent basins making part of a unique spindle-shaped basin system. Some variable evolutionary characteristics among the Acci and Cala Rossa basins, including the overall thicknesses, the thickness of the resedimented deposits, the occurrence of Upper Cretaceous shallow-water deposits only in the structural highs of the Acci Basin, are explained as features linked to the different location of the two basins with respect to the master fault. Many of the observed features are described from those rift systems where the direction of extension is not perpendicular to the rift axis, suggesting that this sector of the Southern Tethyan margin could be developed as an oblique rift system.

Evaluation of shallow-water corals and associated carbonate sediments as seawater Ba isotope archives in the South China Sea

Barium isotopes (δ138Ba) obtained in carbonates have a great potential for reconstructing past changes in marine productivity and associated biogeochemical cycling. Although coral skeletons and inorganic carbonates have been used as archives for seawater Ba isotopes, few studies have investigated other confounding factors that influence Ba isotope fractionation (e.g., vital effects and carbonate mineralogy), hindering their use as paleo-proxies. In this paper, we report Ba isotope data for multi-species corals, including paired sets of living and fossil coral skeletons in the inner shelf (Paipu Reef, Hainan Island) and the open-sea (Jiuzhang Reef corals) in the South China Sea (SCS). We also present Ba isotope datasets of bulk carbonate sediments from two shallow-water drill cores (A and B) in the Jiuzhang Reefs, SCS. The δ138Ba data for the Paipu Reef corals have a relatively narrow range of values (0.12‰ to 0.21‰, mean 0.17 ± 0.06‰; 2 SD, n = 8) regardless of living and fossil coral skeletons. The corals of Jiuzhang Reef show a relatively consistent δ138Ba value (0.30 ± 0.04‰; 2 SD, n = 4). The coral skeletal δ138Ba values show no significant differences across multiple species within the individual colony, which suggests that vital effects do not affect coral δ138Ba records. These findings, together with the fact that a relatively constant δ138Ba offset exists between coral skeletons and seawater, indicate that coral skeletons may be reliable archives for past seawater δ138Ba. All bulk carbonate sediment samples yield constant δ138Ba values of 0.38 ± 0.04‰ (2 SD, n = 30) with a fairly consistent deviation (Δ138Babulk_carb-sw = −0.21 ± 0.04‰) from the reported typical surface seawater of SCS. In addition, mass balance calculation suggests that Ba isotope fractionation between inorganic High-Mg calcite (HMC) and seawater is derived to be Δ138BaHMC-sw = −0.13 ± 0.12‰, exhibiting a difference less than Δ138Baaragonite-sw = −0.28 ± 0.04‰ in aragonite precipitation. Our findings suggest that shallow-water carbonates are reliable archives of past seawater δ138Ba when considering Ba isotope deviations during marine carbonate deposition.

Geochemical expression of sequence stratigraphic surfaces: A case from Upper Cretaceous shallow-water carbonates of southeastern Neo-Tethys margin, SW Iran

Providing a chronostratigraphic framework often is problematic in shallow-marine carbonates with poor biostratigraphic resolution. In such cases, an integration of geochemical, sedimentological, and paleontological data can be used to construct a reliable sequence stratigraphic framework. This study focuses on elemental concentrations and isotopic ratios of Cenomanian–Santonian (C–S) neritic carbonates in SW Iran used to construct a sequence stratigraphic framework, asses sequence ages and durations of hiatus. The diagenetic and geochemical expression of two paleoexposure surfaces, representing type-I sequence boundaries (SBs), are discussed. Increase in Fe and Rb concentrations and decrease in Ca content are recorded at and/or below distinct erosional surfaces. Mg content shows a facies-dependent response with former LMC (low magnesium calcite) or aragonitic facies showing increase while HMC (high magnesium calcite) indicates decrease in response to SBs. Mn shows considerable changes apart from sequence surfaces that are strongly facies dependent. Sr concentrations show an increase in HMC and decrease in LMC in samples which experienced semi-closed meteoric diagenesis, far below the SBs. However, in samples characterized by open system diagenesis, directly underlying the SBs, Sr tends to decrease in all components. The Sr/Rb ratio is a reliable tool in detecting erosional SBs, shown as conspicuous negative excursions. C (carbon) and O (oxygen) isotopes show distinct negative excursions at erosional SBs and a slight increase at or around the MFSs. The 87Sr/86Sr ratio shows clear positive excursions at SBs probably due to 87Rb decay in incorporated aluminosilicate detritus, which makes it as the most reliable proxy for the detection of SBs in the studied sections. Two main erosional SBs are marked by 87Sr/86Sr ratios including the CT-ES (Cenomanian–Turonian boundary exposure surface) and mT-ES (middle Turonian exposure surface) with hiatus durations of 0.53 and 2.7 Myr, respectively. These paleoexposure surfaces are correlated with other neritic sections of C–S sequences in SW Iran.

Carbonate contourite drifts in the southwest South China Sea: Sedimentary, paleoceanographic and economic implications

Contourite drifts are significant sedimentary features and provide clues for the reconstruction of paleoceanography and paleoenvironment. Although they have been increasingly identified in the world’s ocean, shallow-water contourite drifts (< 300 m depth) remain poorly understood and the examples are rare. This study documents a Middle Miocene shallow-water contourite depositional system in the southwest South China Sea by interpreting seismic reflection data and calibrating results with the previous chronological framework. The depositional system consisted of six mounded drifts and six moats. The contourite features were generated in seismic unit III (16-10.5 Ma) and distributed adjacent to carbonate reefs. They were formed on the proto-continental shelf (50-200 m depth) and shaped by the wind-driven currents. Changes in the sedimentary stacking patterns suggest three evolutionary stages of the contourite features. Stage I represents the growth of the Middle Miocene contourite depositional system between 16 and 10.5 Ma. Stage II marks the termination of carbonate drifts and the burial of the Late Miocene sedimentation during 10.5-5.3 Ma. Stage III started with the development of modern deep-water sedimentary systems since 5.3 Ma. The contourite features are compared with the examples on other South China Sea margins. Significant changes in the paleoceanography occurred at 10.5 Ma and 6.5-5.3 Ma when the dominated bottom currents shifted from the monsoonal wind-driven currents to the North Pacific waters, and then the modern circulation system. The Middle Miocene mounded drifts were likely sourced by the coarse-grained carbonate sands. Fluid flow escaped from the coarse-grained contourite layers and natural gas leakage occurs on the seafloor. Shallow-water carbonate contourite drifts can be served as a good gas reservoir and have great economic potential.

Sequence stratigraphy and paleoenvironmental significance of the Neoproterozoic Bambui Group, Central Brazil

The Bambui Group is a ca. 2,000 m-thick mixed biochemical-siliciclastic sedimentary succession that accumulated in the Sao Francisco Basin, central Brazil. Its age is contentious, but deposition during the Cryogenian-Ediacaran (ca. 635–550 Ma) is consistent with a new sequence stratigraphic model presented herein and previously published geochemical and structural data. Six stratigraphic sequences are recognized that record depositional evolution from an epeiric sea to foreland basin during the Marinoan snowball glaciation (ca. 635 Ma) to shortly after the Gaskiers (ca. 580 Ma) glaciation. Although this stratigraphic architecture differs from the classic lithostratigraphic and more recent chemostratigraphic and sequence stratigraphic interpretations of the Bambui Group, it is supported by correlation to other Cryogenian-Ediacaran successions globally. Collectively, these six sequences preserve a robust record of seawater redox structure leading up to the oxygenation threshold that triggered the evolution of multicellular animals in the Ediacaran.Sequence 1 is composed of Marinoan glacial deposits and an overlying cap carbonate with aragonite fan pseudomorphs and barite. Sequence 2 is composed of lime-mudstone that changes laterally into loess-rich peritidal phosphorites, suggesting oxygen stratification in shallow paleoenvironments. Sequence 3 is a carbonate succession with abundant stromatolites implying widespread oxygen production. The top of Sequence 3 is marked by a regional subaerial unconformity developed during uplift associated with the onset of the Brasiliano-Pan African Orogeny. Sequence 4 signals a return to loess derived sedimentation. Distal deposits contain abundant organic matter with framboidal pyrite reflecting bacterial sulfate reduction in low sulfate Ediacaran seawater. Sequence 5 is composed of well-developed aggradational parasequences composed of alternating deep subtidal organic-rich siltstones and shallow-water carbonate grainstones. Their well-defined cyclicity is interpreted to record extreme sea level fluctuations during the onset of the Gaskiers glaciation. Stromatolites suggest an oxygen-stratified water column. Sequence 6 marks the change from an epeiric sea to a foreland basin. Interbedded basinal glauconitic siltstone and shale loess-rich are correlative with shoreface sandstones. Authigenic glauconite requires suboxic bottom waters to precipitate and thus reflects oxygenation of distal settings. Most of the deposition ended by ca. 560–550 Ma when the entire Bambui Group was deformed during the assembly of West Gondwana.In sequences 1, 2, 3, 4, 5, and the lower portion of 6, the lateral continuity of lithofacies, predictable layer-cake stratigraphy, and lack of fluvial coarse siliciclastic deposits are characteristics of epeiric sea successions. The appearance of immature sandstone, poorly sorted conglomerate, braided fluvial deposits, and synsedimentary deformation in the upper part of Sequence 6 are interpreted as the onset of foreland basin development. Stacking relationships of redox sensitive, authigenic lithofacies in both the epeiric sea and foreland phases support recent research suggesting seawater redox conditions were in a state of flux during the apex of the purported Neoproterozoic Oxygenation Event. Organic matter and authigenic pyrite in Sequences 4 and 5 reflect increased primary productivity in surface waters fertilized by nutrients such as P and Fe adsorbed on loess particles. This mechanism could be expanded to a global scale and have contributed to the increase in atmospheric and oceanic oxygen levels. These findings redefine and highlight the significance of the Bambui Group for Neoproterozoic Earth history.

Spatial heterogeneity in carbonate-platform environments and carbon isotope values across the Paleocene–Eocene thermal maximum (Tethys Himalaya, South Tibet)

The Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma) is a large negative carbon isotope excursion (CIE) that testifies to a massive perturbation of the global carbon cycle and has been considered to be an ancient, deep-time analogue for present and future climate change. However, the environmental and carbon isotopic response to the PETM in shallow-water carbonate platforms has remained largely elusive. This study presents new sedimentological, biostratigraphic, and carbon isotopic data to evaluate the impact of the PETM on a shallow-water carbonate platform from more proximal southern parts of the northern Indian continental margin. Detailed biostratigraphy and carbon isotope data indicate that the PETM occurs 3 m below the boundary between nodular and thin-bedded limestones, is maintained up to the thin-bedded limestone and calcareous marl interval, and is followed by recovery at the base of thick-bedded limestone interval. Microfacies analysis testifies to a regression from open to restricted shallow-marine environments at the transition from PETM onset to PETM core, and shallowing continued through the PETM core. Restricted lagoonal deposition was renewed during PETM recovery. These environmental changes were associated with two major turnovers of shallow-water biota. We infer that the first sudden biotic change at the PETM onset may relate to intensified continental weathering, whereas the second biotic change at PETM recovery may have been caused by sea-level fall. The smaller magnitude of the CIE observed in proximal, shallower-water than in distal, deeper-water environments is ascribed to increased primary productivity due to increased nutrients' supply associated with intensified continental weathering. • A regression at the PETM onset from open to restricted shallow-marine environments, and restored to restricted lagoon at the recovery • The sudden environmental changes across PETM were associated with major changes of shallow-water biota • The smaller magnitude of the CIE observed in shallower-water than in deeper-water strata is ascribed to increased primary productivity

The origin of a complex breccia body within the Upper Cretaceous/Early Eocene succession on Pag Island (Karst Dinarides, Croatia): karstic dissolution and collapse or dilational faulting and collapse origin?

A kilometre west of Pag Town on Pag Island, Croatia, within the Upper Cretaceous shallow-water carbonate succession, there is a large breccia body that has an irregular, quasi-circular shape and a subvertical-oblique position in relation to the bedding of the host rock. The breccia clasts and fragments consist almost entirely of the Upper Cretaceous host rock with only sporadic clasts of the Lower Eocene foraminiferal (alveolinid) limestones. In the brecciated body, there are three breccia types. 1) crackle breccia, 2) mosaic breccia, and 3) chaotic breccia. Based on the textural and structural characteristics of these types of breccia such as chaotic appearance and random fabric, very poorly sorted material, angular fragments, the composition reflecting only the host rock lithology, two genetic scenarios or concepts for the origin of Pag Town breccia body were considered, with observations supporting each of them. The first concept involves host rock dissolution resulting in a widened dissolution cavity into which wall and roof rocks progressively collapsed, and the second concept involves the collapse of voids produced by dilational fault displacement. A common prerequisite to both opposing scenarios is the existence of a subsurface cavity or void where the accumulation of rock clasts and fragments occurred. It is assumed that the timing of the cavity formation is related mainly to karstification during the Late Cretaceous-Early Eocene emersion phase or is related to dilational faulting during the Palaeogene Dinarides thrusting event.

Mo isotope composition of the 0.85 Ga ocean from coupled carbonate and shale archives: Some implications for pre-Cryogenian oxygenation

This study addresses marine palaeoredox conditions of the mid-Neoproterozoic by analysing the Mo isotope, trace element, and U-Th-Pb isotope compositions of shallow water microbial carbonate, deep water pelagic carbonate, and shale from the Stone Knife Formation (SKF) in NW Canada. The U-Th-Pb isotope SKF systematics of reef microbialite carbonates, and the moderately expressed negative Ce anomalies are consistent with the presence of dissolved O2 in the surface waters. Thirteen of 14 analysed samples yield a depositional 206Pb/238U regression age of 0.850 ± 0.028 Ga. The Mo isotope data (δ98Mo) are distinct for the microbial and pelagic carbonates and the deeper water shales, with the isotopically heaviest black shales 0.4 ‰ lighter than the heaviest carbonate. The bulk digestion carbonate δ98Mo data scatter widely, ranging up to 1.64 ‰, and are not reproducible between repeat digestions. The spread in shallow-water carbonate δ98Mo cannot be attributed to a single origin (e.g., admixture of silicate-hosted Mo) and probably reflects a combination of factors, including the complex pathway of Mo into microbial carbonates. Regardless, we propose a minimum δ98Mo of 1.64 ‰ for the 0.85 Ga ocean, similar to other Neo- and Mesoproterozoic estimates from studies of proxies other than black shale. Our new black shale δ98Mo data agree with most previous results from 1.8 to 0.7 Ga shales. If interpreted as reflecting seawater, this would mean a minimum oceanic δ98Mo composition of only 1.29 ‰, implying a limited oxic reservoir compared to the modern Mo budget in agreement with previous studies. This study’s results suggest that the discrepancy could also be explained by a systematic offset between δ98Mo compositions of black shales and the overlying water columns, regardless of depositional environment, akin to the relative depth distribution of δ98Mo in modern euxinic water columns such as the Black Sea. If valid, an implied heavier seawater δ98Mo throughout the Proterozoic would indicate that the magnitude of the Mo oxic sink remained relatively stable throughout the Proterozoic, shifting the apparent expansion of oxygen towards the younger boundary of the interpreted onset of the NOE (ca. 1.0–0.54 Ga).

Cadmium isotopes in Late Ediacaran–Early Cambrian Yangtze Platform carbonates – Reconstruction of bioproductivity in ambient surface seawater

Cadmium (Cd) isotope signatures (expressed as δ114Cd values) in seawater and in suitable modern and ancient sediments have been proposed as a useful tracer for paleobioproductivity and for constraining Cd as a micronutrient. This study contributes to the calibration of Cd isotope compositions in ancient shallow-water carbonates and proposes a combination with chromium (Cr) isotopes to link bioproductivity and ocean redox condition. We analysed 64 carbonate samples from the Jiulongwan, Gaojiaxi-Yanjiahe and Xiaotan sections of the Yangtze Platform, South China, covering the Shuram negative carbon isotope excursion (SCE) and Ediacaran-Cambrian transition (ECT), for Cd concentrations [Cd] and Cd isotopes. The results of this study show δ114Cd values ranging from −0.9‰ to +0.6‰, averaging −0.20 ± 0.65‰ (2σ, n = 56) and [Cd] with substantial variations from 0.003 μg/g to 2.78 μg/g. Using the experimental fractionation factor for Cd into calcite (−0.45‰), we calculate an average ambient surface seawater δ114Cd during the entire studied period of +0.24 ± 0.65‰, largely covering today's surface and deep ocean water compositions. The δ114Cd of ambient seawater during the SCE is reconstructed to an average value of +0.48 ± 0.40‰. This elevated signature possibly reflects the result of increased nutrient upwelling and concomitant enhanced bioproductivity levels in the upper photic surface waters during this period, leading to an increased oxygenation of the surface water. The carbonates deposited during the post-Shuram-Wonoka interval reveal a strong correlation between [Cd] and [Zn] and a correlation between degree of negative Ce anomalies and positive δ114Cd excursions, which point to strongly oxidized surface waters and concomitant biotic removal of isotopically light Cd isotopes into phytoplankton. It remains unclear why surface waters during the Late Ediacaran—Early Cambrian did not exhibit more positively fractionated Cd isotope signatures as expected from elevated bioproductivity during this transitional period.

The Maldives, a key location of carbonate drifts

Wind-driven surface currents and deeper water-mass circulation shape carbonate sediment drifts in shallow- to deep-water depositional settings of the Maldives. Drift sediments are located in the platform interior, on top of drowned banks, and around the atolls. They are composed of an admixture of pelagic and shallow-water components. Drift bodies vary in dimension, architecture, and genesis. Small patch drifts and sheeted drifts are present in the platform interior and on top of drowned atolls; large patch drifts preferably form in the current-shadow of large atolls and banks. Small inter-atoll channels and large gateways are dominated by channel-related drifts. The Inner Sea, a large perched basin surrounded by atolls comprises a complex contourite depositional system of giant elongated mounded and sheeted drifts, mixed drifts, as well as delta drifts. This succession of different drift types is organized in sequences, delimited by unconformities caused by changes in the current regime. These may be triggered by a re-configuration of the inter-atoll gateways or by allo-cyclic processes such as climate variability. Effects based on to changes in the global water-mass circulation could not be proven. With exception of the delta drift, carbonate drift types in the Maldives are analogous to their siliciclastic counterparts. By contrast, however, these carbonate drifts are emplaced at a short distance to the shallow-water carbonate factory, which acts as main sediment source. Variability and abundance of drifts in the Maldives emphasize the significant role of current-controlled sediment transport and redistribution in carbonate platform development.

Tsunamites versus tempestites: Various types of redeposited stromatoporoid beds in the Devonian of the Holy Cross Mountains (Poland), a case study from the Ołowianka Quarry.

The sedimentary history of two stromatoporoid accumulations ‒ an allobiostrome and a parabiostrome-are studied in the shallow water carbonates of the Middle to Upper Devonian Kowala Formation in the Ołowianka Quarry, Holy Cross Mountains, central Poland. Sedimentological and facies observations are accompanied by morphometrical and taphonomical analyses of redeposited stromatoporoid skeletons. Stromatoporoid features, including shape profile, latilaminae arrangement, surface character, dimensions, and preservation state, are interpreted in terms of their original growth habitats and susceptibility to exhumation and transport. Sedimentary features of the studied beds are interpreted with regards to the high-energy processes that lead to their deposition. In the allobiostrome, the original stromatoporoid habitat was located below storm wave base, in a calm setting characterised by a low and stable depositional rate and clear bottom waters. The large scale onshore redeposition of stromatoporoid skeletons from such a setting was only possible due to an extraordinary event causing erosion at considerable depths: a tsunami is the most probable explanation. The sedimentary and textural features of the allobiostromal accumulation, such as clast supported textures and lack of vertical sorting, point to a single act of deposition and high flow velocities, in agreement with the tsunami interpretation. In contrast, the parabiostromal stromatoporoid accumulation does not exhibit any features that would require a non-tempestitic explanation, the default and most probable interpretation of high energy facies interbedding shallow water lagoonal sediments. This comparison has shown that studies of variously developed stromatoporoid beds, and particularly the analysis of morphometric features of stromatoporoid skeletons, can provide a unique opportunity to identify palaeotsunamites, which commonly remain undetected in the sedimentary record, leading to underestimates of their abundance.

The influence of diagenesis on carbon and oxygen isotope values in shallow water carbonates from the Atlantic and Pacific: Implications for the interpretation of the global carbon cycle

Global correlation of negative excursions in the δ13C values of shallow water carbonates have been used to interpret and identify major environmental changes through geologic time, such as the glaciations of the Neoproterozoic. As a result of their global reproducibility and similar geometry, these signals have predominantly been interpreted as being representative of the original δ13C values of dissolved inorganic carbon in the oceans, which in turn reflect major changes in the global carbon cycle. In this paper δ13C values from cores drilled in Enewetak Atoll (Pacific) and the Great Bahama Bank (Atlantic), are shown as examples of how similarly correlating variations in global δ13C values, can arise from diagenetic processes. It is further shown that while the profiles of the changes in δ13C values may appear synchronous, as a result of varying rates of deposition and subsidence, that the actual timing of the two changes can be quite different. By comparison of the geometry and global correlation of δ13C values measured in this study, it is proposed that the δ13C values of ancient sequences, may have resulted from non-synchronous diagenetic processes, rather than true changes in the global carbon system. This emphasizes the need for precise chronological control when interpreting sediment records, which is not present during many older geological time periods, in order to constrain the origin and correlation of carbon isotope variations.

Age, duration, and geochemical signatures of paleo-exposure events in Cenomanian–Santonian sequences (Sarvak and Ilam formations) in SW Iran: Insights from carbon and strontium isotopes chemostratigraphy

This study addresses the timing implications of chemical stratigraphy and chronostratigraphic correlation of unconformity-related shallow water (neritic) carbonate rock units. Cenomanian–Santonian sequences (i.e., Sarvak and Ilam formations) host considerable volumes of hydrocarbon in the Zagros region of SW Iran, which represent good examples of unconformity-related reservoirs worldwide. Geochemical traits including carbon, oxygen, and strontium isotopes of these sequences are measured in three wells from the Abadan Plain and one well from the Dezful Embayment. The altered intervals below the exposed erosional surfaces are characterized by remarkable variations in both elemental and isotopic compositions. These variations are marked by increasing in Mg, Al, Fe, Mn, and Rb elemental contents as well as decreasing in Sr and Ca elemental contents. The altered intervals show a positive shift in 87Sr/86Sr isotopic values and a negative trend in δ13Ccarb and δ18Ocarb isotopic values. The paleontological age that is available from previous studies provide a valuable basis for numerical dating of the studied intervals using the rubidium-corrected 87Sr/86Sr isotopic values. The duration of the stratigraphic hiatuses at the Cenomanian–Turonian and mid-Turonian paleo-exposures are calculated as 0.53 Myr and 2.7 Myr, respectively. The calculated durations corroborate the results of previous studies of these paleo-exposures within the Zagros region of western Iran. The concluded (carbon and Sr-isotope) chemostratigraphic correlation of the Cenomanian–Santonian sequences in the Abadan Plain and other parts of the Zagros region can provide a more reliable chronostratigraphic framework of larger scale. The latter can be used in regional correlation and interpretation of the geological-structural evolution of the Zagros region during Cenomanian–Santonian, particularly in western Iran.

238U/235U in calcite is more susceptible to carbonate diagenesis

The uranium isotopic composition (δ238U) of bulk marine calcium carbonates has been extensively explored as a promising paleoredox proxy to track the extent of global oceanic anoxia in deep time. Multiple studies have examined whether primary calcium carbonates can directly capture seawater δ238U and whether bulk measurements of recent and ancient carbonates preserve seawater U isotope signatures. Here we assess the role of diagenesis in altering δ238U signatures in carbonates sediments that have a primary calcitic mineralogy at the Paleocene-Eocene Thermal Maximum (PETM), an interval with rapid global warming and oceanic deoxygenation at ∼56 million years ago.Although primary abiotic and biogenic calcium carbonates (aragonite and calcite) can directly capture seawater δ238U with small offsets (<0.1‰) relative to modern seawater, diagenetic alteration of Bahamian shallow-water platform carbonate sediments that have a predominantly primary aragonitic mineralogy resulted in significantly larger offsets (up to 0.6‰). Since U concentration in primary aragonite is at least one order of magnitude higher than primary calcite (>1 ppm vs. <0.1 ppm), δ238U in calcite should be even more susceptible to diagenesis than that in aragonite.We find strong evidence of this effect in analysis of δ238U in PETM shallow-water carbonate sediments from Drilling Project (ODP) Hole 871C (Limalok Guyot, Pacific Ocean). Our results reveal large fluctuations in bulk carbonate δ238U from −0.69 to +0.71‰ around the PETM boundary but consistently heavier δ238U (between −0.14 and +0.47‰) than modern seawater outside of this interval. The significantly lighter δ238U values than modern seawater were interpreted to result from the operation of a Mn oxide shuttle. The heavier δ238U values are most likely caused by authigenic reductive accumulation of U(IV) in pore waters below the sediment-water interface. We found that carbonate δ238U values higher than modern seawater tend to increase with increasing U/Ca. This relationship is well-explained by an authigenic reductive accumulation model that simply assumes addition to primary calcite during diagenesis of calcitic cements containing isotopically heavier U(IV).Our work confirms expectations that δ238U in primary calcite is more susceptible to the amount of diagenetic cementation compared to primary aragonite, and that variations of δ238U in carbonate sediments with a primary calcitic mineralogy would more dominantly reflect the local redox state of depositional and early diagenetic environments. It is essential to identify the original carbonate mineralogy, the diagenetic history, and constrain the redox state of local deposition environments of sedimentary carbonate rocks when applying bulk carbonate δ238U as a global proxy for oceanic anoxia in deep time.

Multi-scale analysis of shallow-water carbonate depositional geometries from a reservoir perspective: Insights from outcrop (Middle Jurassic, Portugal) and subsurface (Lower Cretaceous, U.A.E.) contrasting case studies

Outcrop observations offer valuable insights into depositional architecture as a primary control on facies and reservoir properties variability, in contrast to subsurface cases, where observation points are limited to well locations. Two dominantly high-energy shallow-water carbonate case studies were analysed, including outcrops of a Bathonian-Callovian (Middle Jurassic) carbonate succession in the Maciço Calcário Estremenho (MCE) region, central Lusitanian Basin, Portugal and a Barremian (Lower Cretaceous) subsurface carbonate reservoir from Abu Dhabi (U.A.E.). The MCE succession comprises a range of inner ramp deposits, which consist of oobiointraclastic grainstones, biointraclastic grainstone-rudstones and coral/algal boundstones, mostly defining sandbody and coral biostrome units which represent potential reservoir facies, as suggested by their textural types and locally significant porosity, though now mostly occluded by calcite cements. The succession is exposed in recently-cut quarry faces showing minimal weathering, hence allowing for detailed observations on depositional geometries. The U.A.E. case study is a world-class reservoir characterized by mid-ramp to high-energy inner-ramp facies, ranging from biomicritic wackestones to biointraclastic packstones, bioclastic packstones, grainstones and rudstone-grainstones, interpreted using core and thin section data from four onshore wells.In the analysed outcrops, strong continuity of main major bedding surfaces is observed at larger scales (dozens to hundreds of metres), with strong spatial variability seen at smaller centimetre to metre scales and depositional geometries varying from tabular, wedge-like, lensoid bodies and coral buildups. The subsurface case also shows strong continuity of larger-scale depositional packages (at kilometre scale) and moderate to strong small-scale heterogeneities at centimetre to metre scales. Discontinuity-bounded intervals are observed in the younger section of this reservoir generally showing stronger vuggy porosity and coarser skeletal grains at the base, while the youngest set of these intervals are characterized by an abundance of rudist fragments. The interpreted depositional geometries in this reservoir consist of tabular, wedge-like and lenticular geobodies of limited lateral extension.The multi-scale heterogeneity in both these case studies is strongly influenced by depositional factors related to hydrodynamics and the balance between carbonate factory productivity and accommodation space, affecting lateral facies migration and creating complex depositional patterns. This study illustrates how the perception of heterogeneity, though reflecting an intrinsic property of carbonate rocks, is influenced by the scale of observation and, from a reservoir perspective, both cases show that addressing small-scale heterogeneities and associated geological factors in higher detail is vital in defining conceptual geological models for a better understanding of the reservoir.

The first Middle Ordovician and Gondwanan record of the cincinnaticrinid crinoidOhiocrinus byeongseonin. sp. from South Korea: biostratigraphy, paleobiogeography, and taphonomy

AbstractOhiocrinus byeongseonin. sp. from the Middle Ordovician (Darriwilian) Jigunsan Formation of South Korea in the Sino-Korean (North China) block is the oldest species ofOhiocrinusof the Cincinnaticrinidae and the first record outside Laurentia.O.byeongseoniis characterized by a loosely clockwise-coiled anal sac, isotomous branching throughout arms, long, slender xenomorphic column, and small lichenocrinid-type holdfast. The new species occurs in association with a deep-water siliciclastic environment, unlike the Laurentian species with a shallow-water carbonate environment. The monospecific crinoid assemblage is interpreted as parautochthonous, considering that the crinoids were reworked by relatively weak down-current probably caused by storm but preserved within the environment where they lived. The occurrence ofO.byeongseonipresents a considerable spatiotemporal gap and ecologic disparity in evolution ofOhiocrinusand the Cincinnaticrinidae.UUID:http://zoobank.org/240417a4-8d33-4f45-aedb-afcc07f1ecbb

The Jurassic–Cretaceous transition in deep- and shallow-water carbonate depositional settings: a case study from the easternmost Getic Carbonate Platform (Southern Carpathians, Romania)

The Jurassic–Cretaceous transition in deep- and shallow-water carbonate depositional settings: a case study from the easternmost Getic Carbonate Platform (Southern Carpathians, Romania)

A new methodology using seismic data to differentiate shallow-water carbonate deposits from similar, noncarbonate structures

A new methodology using seismic data to differentiate shallow-water carbonate deposits from similar, noncarbonate structures