Shallow-water Carbonate Rocks Research Articles

Laser In Situ U–Pb Isotope Dating of Carbonate Rocks in Weijia Guyot in the Western Pacific Ocean and Its Geological Significance

Shallow-water carbonate rocks constitute a crucial component of large guyots, arising in distinct environments and harboring valuable insights into the evolutionary stages of seamount islands as well as the tectonic conditions of the underlying oceanic plate. Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) was used to conduct in situ U–Pb isotope dating of carbonate minerals with low uranium content collected from Weijia Guyot. This dating approach yielded crucial evidence for the vertical development of the seamount. Our study indicates that shallow-water carbonate rocks in Weijia Guyot had a temporal range between 91 My and 137 My. The carbonate rocks underwent two growth phases, Hauterivian to Barremian and Cenomanian to Turonian, with a hiatus of approximately 20 My. Since the Hauterivian age, the shield volcano of Weijia Guyot is essentially complete, with its seamount top exposed at or near sea level and receiving its first stage of shallow-water carbonate sedimentation. Based on the dating of both shallow-water carbonate rocks and hawaiite within the Weijia Guyot, it is inferred that approximately 10 My elapsed from shield-building volcanism to late alkalic volcanism. During the Turonian age, the main reason for the second phase of shallow-water carbonate rocks in the seamounts was the regional tectonic uplift triggered by the drift of the Weijia Guyot along with the Pacific Plate toward the Society hotspot.

Pore network characteristics as a function of diagenesis: Implications for epigenic karstification in shallow-water carbonates

Karst processes may be critical for developing secondary porosity and permeability within carbonate reservoirs and aquifers. Karstification can significantly influence reservoir storage capacity and subsurface fluid flow. This study investigates the interplay between fluid-flow pathways and diagenesis in an epigenic karst setting in Cretaceous shallow-water carbonate rocks, Potiguar Basin, Brazil. The results indicate that dissolution occurred at all diagenetic stages, but the last stage is the most important because it is related to the most recent subaerial exposure. Dissolution produced during the last karst stage occurred in the vadose and phreatic environments, leading to the precipitation of isopachous, blocky and syntaxial cements. Dissolution affected the original pore network within the matrix and localized on preexisting discontinuities such as stylolites, fractures, and bedding surfaces. The resulting karst cavities are classified as vugs, channels and caverns, varying from a few millimeters to hundreds of meters in size. In epigenic karst systems, high porosity percentages are commonly due to moldic, intragrain/intrafossil and intercrystal porosities. The results of our study show how such porosities can be connected to fractures, veins and stylolites at the microscale. Karst dissolution can further increase porosity and the overall rock permeability due to aragonite and calcite cement dissolution during eodiagenesis, microfractures and stylolites dissolution during mesodiagenesis and all cements, fractures and stylolites during telodiagenesis.

Effects of early marine diagenesis and site-specific depositional controls on carbonate-associated sulfate: Insights from paired S and O isotopic analyses

Carbon, sulfur and oxygen isotope profiles in Silurian strata of the Baltoscandian Basin (Estonia), coincident with the Ireviken Bioevent, provide insights into basin-scale and platform-specific depositional processes. Paired carbon isotope records preserve a positive isotope excursion during the early Wenlock, coincident with faunal turnover, yet δ13C variability of this excursion compared to other locations within the paleobasin reflects local depositional influences superimposed on a global signal. In comparison, sulfur isotope records do not preserve a systematic isotopic excursion over the same interval. Instead, sulfur isotope records have high sample-to-sample stratigraphic variability, particularly in shallow-water carbonate rocks (scatter up to ~10‰ for δ34SCAS and ~ 25‰ for δ34Spyr). This pattern of isotopic variability is also found between sites from the same carbonate platform, where the magnitude and isotopic variability in δ34SCAS and δ34Spyr differ depending on relative local sea level (and therefore facies). Such facies-dependent variability reflects more closed- versus more open-system diagenetic conditions where pulses of increased sedimentation rate in the shallow water environments generates greater isotopic variability in both δ34SCAS and δ34Spyr. Increased reworking and proximity to the shoreline results in local sulfide oxidation, seen as a decrease in δ34SCAS in the most proximal settings. Platform-scale evolution of isotopically distilled pore-fluids associated with dolomitization results in increased δ34SCAS in deep water settings. Correlations in paired δ34SCAS-δ18OCAS data support these conclusions, demonstrating the local alteration of CAS during deposition and early marine diagenesis. We present a framework to assess the sequence of diagenetic and depositional environmental processes that have altered δ34SCAS and find that δ34S of ~27–28‰ approximates Silurian seawater sulfate. Our findings provide a mechanism to understand the elevated variability in many deep-time δ34SCAS records that cannot otherwise be reconciled with behavior of the marine sulfate reservoir.

Tracing seawater- and terrestrial-sourced REE signatures in detritally contaminated, diagenetically altered carbonate rocks

As two typical non-skeletal carbonates, ooids and calcified microbial structures have a potential for use in paleo-seawater rare earth element (REE) and yttrium (REY) reconstruction, but strong diagenetic alteration and detrital contamination may cover/overprint their primary carbonate REY signatures and significantly undermine their reliability and usability in recovering original aqueous information from carbonate rocks. In this study, we analyzed a series of Cambrian (Stage 3) oolites and microbialites with minor terrigenous admixtures in the Hannan-Micangshan area, South China using various fractions (bulk rock, acid soluble and insoluble, individual siliciclastic and heavy minerals), and solution- and laser ablation (LA)-based analytical techniques. The acetic acid digested samples exhibit enriched middle REEs (MREEs) and light REEs (LREEs) when normalized to shales, which may indicate the effects of carbonate and detrital diagenetic alteration. The LA-based analysis of the ooid cortices and calcified microbes revealed varying REE patterns from flat to heavy REE (HREE)-enriched features despite their average REY compositions being similar to those of the acetic acid digested samples. In contrast, the early isopachous cements exhibit seawater-like REY patterns with the least diagenetic and contamination characteristics, which potentially indicate seawater-sourced porewater fluids approaching the composition of the overlying surface ocean in the shore settings. In addition, we identified HREE-enriched detrital fractions (mostly distributed in the clay-sized particles) in all three study sections, which are likely sourced from the neighboring mafic igneous rocks in the Hannan-Micangshan area. This HREE-enriched signature would significantly mislead our judgement of the coexisting carbonate REY characteristics (e.g., Ce anomalies and Y/Ho ratios) if the contamination fraction could not be rigorously excluded. Although large quantities of contaminated (to varying degrees) LA-based data cannot be used for paleoceanographic reconstruction, their unique values have been demonstrated to indicate their terrigenous mineralogical sources based on multiple groups of contamination fingerprint elements, which conforms well to the elemental compositions of individual siliciclastic and heavy minerals. For detritally contaminated, diagenetically altered shallow-water carbonate rocks, in general, the LA technique can be applied not only to paleoceanographic reconstruction based on the most pure and least altered components (i.e., early marine cements) but also to provenance analysis based on fine-grained particles admixed in the ooid cortices and calcified microbes.

Maastrichtian-Paleocene larger Foraminifera biostratigraphy and facies of the Şahinkaya Member (NE Sakarya Zone, Turkey): Insights into the Eastern Pontides arc sedimentary cover

Maastrichtian to Paleocene shallow-water carbonate rocks are common in many parts of Turkey. The occurrence of the Şahinkaya Member limestones in the Çayırbağı-Çalköy (Düzköy/Trabzon) district of the Sakarya Zone represents the neritic expression in marginal areas of a more extensive back-arc deposition, dominated by the hemipelagic sediments of the Tonya Formation. In this contribution, the larger Foraminifera biostratigraphy of the Şahinkaya Member is revisited in order to define the presence of hiatuses associated with the Eastern Pontides arc-continent collision. The study presented here allows for the definition of an upper Maastrichtian and an overlying Paleocene sequence. They are separated from each other by an unconformity of regional meaning extending between the uppermost Maastrichtian to the lower Danian. Shallow Benthic Zones 1–3 (upper Danian to lower Thanetian) have been recognized. The stratigraphy of the Şahinkaya Member and the paleoenvironmental distribution of some larger Foraminifera suggest that, unlike other areas of the Eastern Pontides, the Paleocene deposition was ongoing but still influenced by regional tectonism.

Structurally-controlled hydrothermal fluid flow in an extensional tectonic regime: A case study of Cretaceous Qamchuqa Formation, Zagros Basin, Kurdistan Iraq

Shallow water carbonate rocks of the Lower Cretaceous Qamchuqa Formation host a significant reserve of oil in the NNE Arabian plate-Southwestern Zagros Sutured Belt in the Kurdistan Region, Iraq. Such carbonates consist of several dolostone bodies with unique and complex arrays of saddle and zebra dolomite textures especially in fractured intervals. Integration of field, core description, microstructural analysis, petrographic, stable C, O and Sr isotopes, and fluid inclusion analyses reveals two main phases of multiple fluxes of hydrothermal fluid circulation during the Zagros Orogeny at the end of Cretaceous and Cenozoic times. An early regional non-focused hydrothermal dolomitization and a subsequent fault- and fracture- controlled hydrothermal fluid flow, which led to replacement and cementation by a range of saddle dolomite and zebra textures. Several generations of matrix dolomite, saddle dolomite and calcite cement were identified. The host dolostone has been affected by shortening, folding, fracturing, and thrust faulting. Saddle dolomite pipes, in particular, were found to be associated with en-échelon folding. The morphology and areal extent of the zebra dolomite are controlled by the pore geometry of the host dolostone in relationship to fracturing and faulting.Both matrix and saddle dolomite cements have comparable, overlapping stable isotopic values ranging from 0.39‰ to 3.57‰ and −0.12 to 3.11‰ VPDB, respectively for δ13C and δ18O values range from −10.53‰ to −4.47‰ and −12.87 to −7.79‰ VPDB, respectively. 87Sr/86Sr ratios of pervasive matrix dolomite range from 0.707702 to 0.707839, fall close to late Cretaceous seawater values. Slightly more radiogenic values of saddle dolomites ranging from 0.707759 to 0.708024 resulted from mixing of basinal brines with connate waters. Isotopic signatures of later calcite cements show more radiogenic Sr ratios from 0.707755 to 0.708198 and negative δ13C values 1.76 to −7.45‰, possibly related to late diagenetic processes due to incursion of meteoric waters during subaerial exposure.Fluid inclusion data of pervasive matrix dolomite and saddle dolomites show homogenization temperatures (Th) with a wide range from 71.3 to 169.7 °C and 82.3 to 188.2 °C, and salinity between 14.4 to 25.4 and 15.6 to 27.9 wt% NaCl eq., respectively. These values demonstrate the effect of hydrothermal fluid flow of the alteration of host dolomite and formation of saddle dolomite in fractures and faults.

Chemostratigraphy of Late Sinemurian – Early Pliensbachian shallow-to deep-water deposits of the Central High Atlas Basin: Paleoenvironmental implications

Shallow water carbonate rocks are especially prone to diagenetic alteration. As such they are sometimes problematic archives of past carbon cycle perturbations, casting doubt on the reliability of shallow-water carbonates carbon isotopes analyses for chemostratigraphic purposes. In this paper, bulk organic carbon isotopes (δ13Corg) is used as a robust replacement of the more sensitive carbonate carbon isotope systems for the establishment of a refined chronostatigraphic framework for the evolution of Sinemurian-Pliensbachian neritic carbonate systems in the Central High Atlas Basin of Morocco. These data show very similar patterns and characteristics in the organic carbon isotope curves of basinal and platform sections and thus illustrate the reliability of these archives. Simultaneously, we explore the expression of the global carbon isotope excursion at the Sinemurian-Pliensbachian boundary (SP event) in the study area in Morocco. This event has been previously described as a negative carbon isotope excursion of ∼2‰ associated with a large transgression in numerous basins. In Morocco and other neritic carbonate records, the SP event is better described as a return to previous values after a positive carbon isotope excursion coinciding with a major latest Sinemurian regression recognised on a global scale. Similar patterns are also found in other sections, although sometimes poorly expressed. However, it shows, that the SP event has ambiguous characteristics and questions the conventional hypothesis of it being linked to the massive injection of 13C-depleted carbon into the atmosphere-ocean system.

Composition and history of giant carbonate seep mounds, Mesoproterozoic Borden Basin, Arctic Canada

The Mesoproterozoic Ikpiarjuk Formation (Borden Basin, Nunavut) consists of a series of very large (>200m thick; kms diameter) dolostone mounds that accumulated on the floor of a restricted basin. The mounds are isolated from contemporaneous shallow-water carbonate rocks, accumulated at the same time as black shale in deep-water regions of the basin, and are geographically limited to the vicinity of basin-scale fault zones. The mounds contain extensive deposits of benthic thrombolites, a microbial texture that is not common in Mesoproterozoic carbonate rocks.Field relationships of individual mounds indicate that mound growth was initiated during regional deposition of organic-rich black shale, mounds developed significant relief above the basin floor, and some mound tops were eventually exposed and karsted during differential uplift. The mounds accumulated in a deep-water, sub-photic-zone setting as a result of fluid venting along basin-scale faults. Interpreted basin depths indicate that the thrombolitic mound texture is probably not related to cyanobacteria. Chemosynthetic microbes exploited the chemical gradient in the vicinity of the vents and built a mound framework of thrombolites. Framework voids contain both carbonate mud and isopachous linings of fibrous cement, indicating that the mixing of vent fluid with basin water favoured production of abiogenic carbonate minerals in addition to providing a chemical gradient for microbes to exploit.Proterozoic deep-water carbonates and thrombolites are rare, and the Ikpiarjuk Formation mounds represent microbial buildups that are fundamentally different from other examples in the Precambrian rock record.

The Late Triassic Sequence-Stratigraphic Framework of the Upper Yangtze Region, South China

In the transitional period between the Middle and the Late Triassic, the Indochina orogeny caused two tectonic events in South China: (1) the formation and uplift of the Qinling-Dabie orogenic belt along the northern margin of the South China Plate, due to its collision with the North China Plate; and 2) the development of a 1300-km-wide intra-continental orogen in the southeastern part of the South China Plate, which led to a northwestward movement of the foreland thrust-fold zone. These tectonic events resulted in the ending of the Yangtze Platform, and were a stable paleogeographic factor from the Eidacaran to the end of the Middle Triassic. This platform was characterized by the widespread development of shallow-water carbonates. After the end of the Yangtze Platform, the upper Yangtze foreland basin (or Sichuan foreland basin) was formed during the Late Triassic and became a accumulation site of fluvial deposits that are composed of related strata of the Xujiahe Formation. In western Sichuan Province, the Xujiahe Formation overlies the Maantang Formation shallow-water carbonate rocks of the Xiaotangzi Formation siliciclastic rocks (from shelf shales to littoral facies). The sequence-stratigraphic framework of the Upper Triassic in the upper Yangtze foreland basin indicates a particular alluvial architecture, characterized by sequences composed of (1) successions of low-energy fluvial deposits of high-accommodation phases, including coal seams, and (2) high-energy fluvial deposits of low-accommodation phases, including amalgamated river-channel sandstones. The spatial distribution of these fluvial deposits belonging to the Xujiahe Formation and its relative strata is characterized by gradual thinning-out, overlapping, and pinching-out toward both the east and south. This sedimentary record therefore expresses a particular sequence-stratigraphic succession of fluvial deposits within the filling succession of the foreland basin. The sequence-stratigraphic framework for the Upper Triassic in the Upper Yangtze region provides a record of the end of the Yangtze Platform and the formation of the upper Yangtze foreland basin.

Large-sized Early Permian “caninioid” corals from the Karavanke Mountains, Slovenia

AbstractA new monospecific “caninioid” genus,Preisingerellan. gen., from the lower Permian of the Karavanke Mountains (Southern Alps, Slovenia) is erected. The type species isPreisingerella stegovnikensisn. sp. The new taxon can be differentiated from otherCaninia-type genera by its specific ontogeny and features of its dissepimentarium. Corals with such morphology had a wide distribution during the Carboniferous and early Permian, occurring in mostly shallow-water carbonate rocks. The phylogenetic relationships within this group are mostly unclear due to similarities in the adult stages. The earlier stages reveal the main distinguishing features that are decisive for a generic assignment, but these have rarely been well preserved and properly considered. The new taxon is compared with related genera of the Cyathopsidae and species ofCaninellaGorskiy, 1938 characterized by lateral dissepiments. Large numbers of specimens of the new species, representing a monospecific assemblage, have been collected from the Born Formation at Mt. Stegovnik. Sedimentological and microfacies characteristics, as well as macro- and microfossil assemblages, underline this correlation. The fusulinoidean assemblage of the Born Formation, withSphaeroschwagerina carniolica(Kahler and Kahler, 1937), as the predominant species, corresponds to the time span between theSphaeroschwagerina moelleri-Schwagerina fecundaandPseudofusulina moellerizones, indicating a late Asselian to early Sakmarian age in the Southern Urals.

Lithofacies palaeogeography of the Late Permian Wujiaping Age in the Middle and Upper Yangtze Region, China

The lithofacies palaeogeography of the Late Permian Wujiaping Age in Middle and Upper Yangtze Region was studied based on petrography and the “single factor analysis and multifactor comprehensive mapping” method. The Upper Permian Wujiaping Stage in the Middle and Upper Yangtze Region is mainly composed of carbonate rocks and clastic rocks, with lesser amounts of siliceous rocks, pyroclastic rocks, volcanic rocks and coal. The rocks can be divided into three types, including clastic rock, clastic rock–limestone and limestone–siliceous rock, and four fundamental ecological types and four fossil assemblages are recognized in the Wujiaping Stage. Based on a petrological and palaeoecological study, six single factors were selected, namely, thickness (m), content (%) of marine rocks, content (%) of shallow water carbonate rocks, content (%) of biograins with limemud, content (%) of thin-bedded siliceous rocks and content (%) of deep water sedimentary rocks. Six single factors maps of the Wujiaping Stage and one lithofacies palaeogeography map of the Wujiaping Age were composed. Palaeogeographic units from west to east include an eroded area, an alluvial plain, a clastic rock platform, a carbonate rock platform where biocrowds developed, a slope and a basin. In addition, a clastic rock platform exists in the southeast of the study area. Hydrocarbon source rock and reservoir conditions were preliminarily analyzed based on lithofacies palaeogeography. Sedimentary environments have obvious controls over the development of the resource rocks. With regard to the abundance of organic matter, the hydrocarbon potential of the coastal swamp environment is the best, followed by the basin environment and the carbonate rock platform. The gas reservoir types of the Wujiaping Stage can be classified as conventional and unconventional gas reservoirs, like coal bed gas and shale gas; all of them have well exploration prospects.

Lower Triassic anachronistic facies capping the Qinghai-Tibet Plateau seamount: Implications for the extension of extraordinary oceanic conditions deep into the interior Tethys Ocean

The Bayan Har area, situated in the northeast of the Qinghai-Tibetan Plateau, has long been regarded as a deep-water turbidite basin during Early Triassic. However, a sequence of Lower Triassic shallow-water carbonate rocks has been documented from the region, indicating the existence of isolated seamount carbonate platforms in the basin. The seamount carbonate platform sediments differ from synchronous deep basin flysch sediments, and consist of shallow-water oncoids, ooids and cortoids. Microfacies studies reveal that the oncoids have a thick cortex comprised of irregular, non-concentric and partially overlapping micritic laminae and contain abundant well-preserved foraminifera, indicating a relatively low-energy environment in subtidal settings. Ooids exhibit thinner and more regular concentric laminations, which are interpreted to have formed by frequent overgrowth while rolling in a relatively high-energy setting. Cortoids are predominantly composed of tiny fossil fragments and intraclasts with a micritic envelope, indicating a shallow-marine warm water environment located in the intertidal or supertidal zone. Ooids in the Bayan Har area differ from those found on stable platforms, in that they sometimes nucleated onto volcanic quartz grains, reflecting an unstable tectonic setting that was frequently affected by volcanic activity. Widespread ooids and other microbial carbonates formed in the Early Triassic have been regarded anachronistic facies that are indicative of harsh marine conditions. Most Lower Triassic anachronistic facies are distributed along the shallow margin of the Tethys Ocean and the western margin of Pangea; however, Bayan Har was located in the center of the Palaeo-Tethys. The discovery of oncoids and oolites capping a seamount in this area provides strong evidence that extraordinary marine conditions spread far into the interior of the Palaeo-Tethys Ocean.

Carbonate formation of the Lower Carboniferous in Central Part of Volga-Ural Basin

Carbonate rocks of the Lower Carboniferous (Tournaisian stage) of the central Volga-Ural basin (the eastern portion of the East European platform) are of practical scientific interest to geologists, particularly because they contain large reserves of oil. Although such layers have been studied, various questions pertaining to development of sedimentation schemes for the rocks have not been answered. We have attempted to resolve these by studying a wealth of drill core materials. The study involved structural and genetic analysis of rocks and facies reconstructions. The rocks are mainly represented by different types of shallowwater limestone. The thickness of coeval layers and their lithological structures changes from well to well within an oilfield, primarily due to the different environments of sedimentation during the Tournaisian stage. Therefore, to identify the characteristics of carbonate sedimentation, we have studied the sequences of different types of limestone and analysed their thickness. As a result, we have developed principle schemes of sedimentation for shallow-water carbonate rocks of the Tournaisian stage. This may help in the predictive search for reservoir rocks in the region of study.

Sea-level and environmental changes around the Devonian–Carboniferous boundary in the Namur–Dinant Basin (S Belgium, NE France): A multi-proxy stratigraphic analysis of carbonate ramp archives and its use in regional and interregional correlations

The paper focuses on high-resolution multidisciplinary research on three Devonian–Carboniferous boundary sections in shallow-water carbonate rocks in the Namur–Dinant Basin (Belgium, France). The aim of the study is to provide palaeo-environmental reconstructions and correlations supported by several independent quantitative proxies. We describe several correlative horizons and provide their sequence-stratigraphic interpretation based on facies analysis, spectral gamma-ray data, element concentrations (XRF) and δ13Ccarb, with foraminifer-biostratigraphy age control. The most prominent surface is a basal surface of forced regression, which is indicated by a sharp basinwards facies shift and a drop in clay-gamma-ray values and Al concentrations at the base of the Hastière and Avesnelles formations in more distal settings. In proximal settings, this surface merges with a hiatus at the Devonian–Carboniferous boundary inferred from foraminifer biostratigraphy. This hiatus can be correlated with the global Hangenberg sandstone event, which indicates a glacioeustatic sea-level fall. Increasing values of Zr/Al, K/Al, Sr/Al and Mn/Al coincide with the proximal facies of the falling stage system tract and lowstand system tract in the Hastière and Avesnelles formations as a consequence of the enhanced input of siliciclastics and nutrients during low sea levels. The top of the middle Hastière member is interpreted as the maximum regression surface, which is overlain by transgressive system tract of the upper Hastière member. The patterns of gamma-ray, δ13Ccarb, Th/K, Al and Zr/Al curves are well correlated between the studied sections. The δ13Ccarb excursions are correlated with the unnamed excursion in the Upper expansa conodont zone (Carnic Alps) and with the global Hangenberg event s.l. excursion in the kockeli conodont zone. This sequence-stratigraphic framework is used for correlations with deltaic successions from the Tafilalt Basin, Morocco.The basal surface of the forced regression equivalent to the Hangenberg sandstone event, which is typical for deeper-water settings, is easily recognisable and correlatable with gaps in more-shallow water settings. We suggest that it should be taken into account as a possible candidate for the “natural solution” of the Devonian–Carboniferous boundary in discussions concerning its redefinition.

Upper Maastrichtian to Lutetian nannofossil biostratigraphy, United Arab Emirates, west of the Northern Oman Mountains

The latest Cretaceous (Maastrichtian) to Middle Eocene (Lutetian) interval of shallow water carbonate rocks in the NE part of the Arabian Pensinsula (Oman Mountains) includes important regional oil reservoir units. These carbonates are richly fossiliferous in foraminifera, which have been useful in correlating sequences and formations. Previous foraminiferal studies have indicated the existence of several hiatuses or lacunae related to sea level changes or due to erosion. Subsequent studies of the abundant calcareous nannofossils have permitted definition of these hiatuses via high resolution biostratigraphy. However, these previous studies were of limited extent. In this study a total of 103 nannofossil species were identified from the upper Maastrichtian–Lutetian successions at Jabal Qarn El Barr, El Faiyah Range Mountains (Jabal Thanays and western side of Jabal Buhays), United Arab Emirates, as well as Jabal El Rawdah, west of the Northern Oman Mountains, Oman.

Organic carbon isotope gradient and ocean stratification across the late Ediacaran-Early Cambrian Yangtze Platform

Organic carbon isotope (δ 13Corg) data from two well-preserved sections across a shallow-to-deep water transect of the late Ediacaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δ 13Corg values of the late Ediacaran Dengying Formation range from −29‰ to −24‰. In the deep-water Longbizui section, δ 13Corg values from time-equivalent strata of the Dengying Formation are mostly between −35 and −32‰. These new data, in combination with δ 13Corg data reported from other sections in South China, reveal a 6‰-8 shallow-to-deep water δ 13Corg gradient. High δ 13Corg values (>−30‰) occur mostly in shallow-water carbonate rocks, whereas low δ 13Corg values (<−32‰) dominate the deep-water black shale and chert. The large temporal and spatial δ 13Corg variations imply limited buffering effect from a large dissolved organic carbon (DOC) reservoir that was inferred to have existed in Ediacaran-Early Cambrian oceans. Instead, δ 13Corg variations between platform and basin sections are more likely caused by differential microbial biomass contribution to total organic matter. High δ 13Corg values (>−30‰) documented from shallow-water carbonates are within the range of typical Phanerozoic δ 13Corg data and may record the isotope signature of organic matter from primary (photosynthetic) production. In contrast, low δ 13Corg values (<−32‰) from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δ 13Corg data provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform.

Carboniferous and Lower Permian sedimentological cycles and biotic events of South China

AbstractThe sedimentary successions and four fossil groups, including rugose corals, brachiopods, fusulinaceans and conodonts, from the Carboniferous and Lower Permian of South China have been studied in order to reveal the sedimentary characteristics and evolutionary pattern of main biological groups in the East Tethyan region during the Late Palaeozoic Ice Age. The Lower Carboniferous lithology of South China is diverse, ranging from basinal and shelf carbonate rocks to coal measures and continental clastics, while the Upper Carboniferous and Lower Permian are composed mostly of shallow-marine carbonates. From uppermost Devonian to Lower Carboniferous, five major regression events are recognized at the topmost Devonian, middle and upper Tournaisian boundary, Tournasian–Viséan boundary, uppermost Viséan and the Mid-Carboniferous boundary in South China, separately. The Upper Carboniferous and Lower Permian shallow-water carbonate rocks consist of remarkable, high-frequency cyclothems. Moreover, another major sea-level fall is recognized and characterized by an extensive sedimentary hiatus at the Sakmarian–Artinskian boundary throughout South China. All of the sedimentary basins of South China were formed in extensional tectonic settings during this time; thus, multiple regressive events that occurred throughout South China should be primarily induced by glacio-eustatic drawdown. In addition, two biotic events characterized by a remarkable decline in the diversity of benthic biota and a turnover in the composition of fossil assemblages occurred, respectively, at the Mid-Carboniferous and Sakmarian–Artinskian boundaries, consistent with two major regressions, and were probably caused by the glaciations in Gondwana.

Facies evolution and sequence chronostratigraphy of a “mid”-Cretaceous shallow-water carbonate succession of the Apulia Carbonate Platform from the northern Murge area (Apulia, southern Italy)

The “mid”-Cretaceous carbonate succession of the Apulia Carbonate Platform cropping out in northern Murge area (Apulia, southern Italy) is composed of shallow-water carbonate rocks and is over 400 m in thickness. This paper focuses on the lithofacies analysis of this carbonate succession, its paleoenvironmental interpretation, and its sequence-chronostratigraphic architecture. Lithofacies analysis permitted to identify deposits which can be grouped into the following three facies belts: (1) terrestrial facies belt formed by: intraclast-supported paleosoils; solution-collapse breccias; (2) restricted facies belt made up of lithofacies deposited in protected peritidal environments; (3) normal-marine facies belt made up of lithofacies formed in moderate- to high-energy subtidal environments. The detailed study both in outcrops and in thin-sections revealed that, at the bed scale, lithofacies are cyclically arranged and form shallowing-upward small-scale depositional sequences comparable to parasequences and/or simple sequences. The following three small-scale sequence types have been distinguished: (1) subtidal sequences mostly made up of lithofacies formed in the normal-marine open subtidal domain; (2) peritidal sequences made up of lithofacies formed in the restricted peritidal domain; (3) peritidal sequences showing a cap formed by paleosoils. Small-scale sequences are not randomly arranged in the compiled succession but form discrete packages, or sets, that alternate in the sedimentary record. The repetition of such small-scale sequence packages in the succession has been the key to recognize large-scale sequences comparable to third-order depositional sequences. Although sedimentological data are often fragmentary due to late dolomitization, four large-scale sequences have been distinguished. The data support a generalized landward-backstepping of facies belts during transgression, which implies a gradual gain of accommodation culminating with the deposition of a package of small-scale sequences formed by normal-marine subtidal deposits. These mark periods of maximum accommodation space and form the maximum-flooding zones of large-scale sequences. A gradual seaward progradation of facies belts is recorded during highstand conditions, which implies a gradual loss of accommodation culminating with the deposition of a package of peritidal small-scale sequences capped by paleosoils or by solution-collapse breccias. The occurrence of terrestrial deposits marks periods of minimum accommodation on the platform and determines the sequence boundary of large-scale sequences. The large-scale sequences identified in this study fit with the main transgressive/regressive cycles published in the sequence-chronostratigraphic chart of European basins. As a consequence, it is interpreted that changes of the sea level recorded at the scale of European basins played an important role in determining the sequence-stratigraphic architecture of the studied succession. In spite of this, the occurrence of solution-collapse breccias, which implies a significant gap in carbonate sedimentation in between Early and Middle Cenomanian times, may also have an alternative interpretation. In particular, this deposit may represent the local fingerprint of the well-known tectonic phase which, during Late Albian-Early/Middle Cenomanian times, determined the subaerial exposure of large parts of Periadriatic carbonate platforms producing a marked regional unconformity.

Geochemical characteristics of the oceanic island-type volcanic rocks in the Chiang Mai zone, northern Thailand

The oceanic island volcanic rocks in the Chiang Mai zone, northern Thailand, are usually covered by Lower Carboniferous and Upper Permian shallow-water carbonate rocks, with the Hawaii rocks and potash trachybasalt being the main rock types. The alkaline series is dominant with sub-alkaline series occurring in few cases. The geochemical characteristics are described as follows: the major chemical compositions are characterized by high TiO2, high P2O5 and medium K2O; the rare-earth elements are characterized by right-inclined strong LREE-enrichment patterns; the trace element patterns are of the upward-bulging K-Ti enrichment type; multi-component plots falling within the fields of oceanic island basalts and alkali basalts, belonging to the oceanic island-type volcanic rocks, which are similar to the equivalents in Deqin and Gengma (the Changning-Menglian zone) of Yunnan Province, China.

How fracture systems affect permeability development in shallow-water carbonate rocks: An example from the Gargano Peninsula, Italy

Fracture networks control the permeability of many reservoirs. Since the fracture patterns of petroleum reservoirs in situ are difficult to study in detail, field analogues are very important for understanding their fracture-related permeability. Here we present the results of a study of the fracture system of carbonate rocks of Lower Cretaceous age in a quarry associated with the damage zone and fault core of a major fault zone on the Gargano Peninsula in South Italy. We measured the attitude of 1541 fractures and faults along several vertical and horizontal scan lines. There are two main fracture sets: one strikes between E–W and ESE–WNW, the other NNE–SSW. A total of 675 fracture-spacing measurements indicate log-normal spacing distributions, with an arithmetic mean fracture spacing of 0.29 m and a median of 0.15 m. The data, plotted on a log–log plot, suggest three main spacing subpopulations, each of which follows approximately a power law with different fractal dimensions. Subpopulation 1, where the spacing ranges from 1 to 10 cm and the straight-line slope D (“fractal dimension”) is 0.20, represents fractures confined to laminated carbonate mudstones (multilayers) that form the microbial mat deposits of a peritidal cycle. Subpopulation 2, where the spacing ranges from 11 to 55 cm and D is 0.77, represents fractures confined to thicker layers, forming a part of a peritidal cycle, the contacts of which are marked by stylolites. Subpopulation 3, where the spacing ranges from 56 to 243 cm and D is 2.81, represents fractures that dissect comparatively thick units of an entire peritidal cycle. For the spacing, the minimum coefficient of variation, C v, defined as standard deviation divided by the mean, is 1.00 (essentially randomly spaced fractures) while its maximum C v is 1.62, suggesting that some fractures form clusters, some clusters being denser than others. The clusters, composed of fractures with varying attitudes and therefore commonly intersecting, are likely to contribute significantly to the overall permeability of the carbonate rock. Fracture-aperture (opening) data ( N = 324) also show a log-normal size distribution, with a mean opening of 1.01 cm and median of 0.29 cm. Log–log plots indicate that a part of this data groups into two subpopulations, I and II, each of which follows approximately a power law. The straight-line slope D (“the fractal dimension”) of subpopulation I is 0.46, whereas that of subpopulation II is 1.49. We present boundary-element models showing that laminated carbonate mudstones and their contacts modify the local stress fields so as to encourage fracture offset and, commonly, arrest. Our results also show that when a fluid-driven subpopulation 2 fracture approaches subpopulation 1 fractures, the induced tensile stresses may result in the opening up of many of the subpopulation 1 fractures directly above the tip of the subpopulation 2 fractures. If, in addition, the contacts between the multilayers are weak, they also tend to open up, thus generating a large interconnected cluster of vertical fractures and horizontal contacts. The results suggest that the tensile stresses induced by a comparatively large fluid-driven subpopulation 2 fracture may contribute to the formation of an interconnected cluster of subpopulation 1 fractures and associated contacts, thereby significantly increasing the permeability of the carbonate rock.