Supratidal Environments Research Articles

Depositional environments of Danian/Selandian of Kurkur Formation, Kurkur Oasis, south Western Desert, Egypt

In order to document the depositional environments of the Kurkur Formation in Southwestern Desert, Egypt, four surface sections measured in this region from north to south as follows: Gebel Garra, Wadi Abu-Sayal, Kurkur Oasis, and Sinn El-Kaddab. Planktonic foraminifera indicate that the age of Kurkur Formation in the study area is Danian to Selandian. Microfacies analysis shows graduation from wackestones, floatstones, and packstones in the lower part to oolitic and bioclastic grainstones in the upper section. The lower part of Kurkur Formation characterized by calcareous foraminifera assemblages, whereas arenaceous types are concentrated toward the uppermost part. Bivalves, gastropods, and echinoids are the most abundant macrofossils. Using the microfacies analyses and fossil content, the Kurkur Formation in the studied area deposited in middle neritic zone for the lower part, which is synchronous with the beginning of the Cenozoic transgression in the study area, and changed upward to inner neritic and supratidal environments.

Intrasediment gypsum in subtidal offshore carbonates as a witness of basin-wide evaporitic precipitation. The case of the southern Variscan foreland basin (mid-Carboniferous)

Widespread calcite and quartz pseudomorphs, interpreted as originally gypsum crystals, occur within hemipelagic calci-mudstone accumulated in subtidal offshore environments in the broad marine foreland basin developed on the southern flank of the Ouachita–Alleghanian–Variscan Orogen during mid-Carboniferous times, which acted as a marine corridor connecting the Panthalassa and Palaeo-Tethys Oceans during the Mississippian, and progressively narrowed during the assembly of Pangea. In this study, 67 outcrops of radiolaria-bearing calci-mudstone deposits that contain calcite and quartz pseudomorphs located in northern Spain and southern France were studied to constrain the gypsum spatial distribution and sedimentological features. The recognised microfacies indicate intrasediment gypsum precipitation, accompanied by less abundant bottom-grown precipitates and gypsum cumulates, in extensive offshore, probably several ten to a few hundred metres deep, basinal environments. Gypsum precipitation took place during a short-lived temporal episode during the early Bashkirian time (Voznesenkian), which can be correlated, on the basis of benthic foraminifera, with the coastal (inter- to supratidal) gypsum evaporites identified in NW Africa (Tindouf and Reggan successions in Morocco and Algeria) that would represent the shallow-water counterparts. The occurrence of gypsum precipitates both in offshore hemipelagic calci-mudstones of the Variscan foreland basin and in inter- to supratidal environments of the epeiric Sahara Platform indicates that hypersaline conditions affected vast marine areas, roughly coinciding with the estimated age of closure of the Panthalassa and Palaeo-Tethys marine connection. Therefore, the studied succession represents the trace of a basin-wide evaporitic episode extending for hundreds of kilometres driven by foreland basin restriction, mid-Carboniferous sea-level fall and arid climate. This study provides new insights for the interpretation of gypsum precipitates in offshore marine environments encountered in the Phanerozoic and whose genesis is poorly understood.

Paleoenvironmental changes during the Late Pleistocene in Buenos Aires province (Argentina), using foraminifera fauna as proxy data

This paper describes the lithological and micropaleontological characteristics of a new record corresponding to the last interglacial for Buenos Aires province. Foraminifera were analyzed from an artificial outcrop 4 m thick (36°34′0.97″ S and 57°22′54.43″ W), from the quarry situated in the town of General Conesa, Buenos Aires province at an approximate elevation of 4 m a.s.l. Benthic foraminifera fauna was constituted by a predominance of calcareous forms represented mostly by the species Buccella peruviana and Ammonia parkinsoniana. The micropaleontological and lithological characteristics of all the sedimentary units involved in this locality suggest deposits associated with a transition from a low-energy shallow intertidal or subtidal zone to a higher-energy beach or supratidal environment. Although, the topographic height does not enable conclusive assignment of the age of this coastal deposit (>79.500 kyr), the environmental conditions could be correlated with the penultimate transgressive event that occurred at the beginning of the Late Pleistocene (MIS 5), which would have occurred in a shallow environment with changes in the marine influence. These findings provide new information that contributes to better understanding of the paleoenvironmental evolution of this region where two marine transgressions and two wind deposits have been recognized.

The Origin of the Upper Cambrian Basin-Scale Massive Dolostones of the Xixiangchi Formation, Sichuan Basin, China

The thick Upper Cambrian Xixiangchi dolostones, developed in the Sichuan Basin, are an important deep exploration target, but their genesis is still controversial, which hinders predicting the porous dolomite distribution and related potential hydrocarbon play. Herein, based on the observation and sampling of field outcrops, combined with a microscopic thin section analysis, cathodoluminescence analysis, and geochemical study, their characteristics and genesis were investigated. The results showed that there are mainly three types of dolomite that can be distinguished: (1) fine crystalline dolomite with a low crystallinity (Type 1); (2) granular dolomite with coarse grains, maintaining the original particle structure (Type 2); and (3) grain-texture relict dolomite with a higher degree of crystal form and obvious recrystallization (Type 3). The Type 1 dolomite with a common lamina structure and the highest 87Sr/86Sr ratio implies the most continental-influenced seawater in a supratidal environment around paleouplift, where there is an evaporation pump effect in its formation. The Type 2 dolomite recorded a slightly higher diagenesis temperature and slightly lower brine salinity, which would be formed in a seepage-reflux model in the beach environment of the platform. The lowest REY content and higher dolomite temperature with structural residuals indicate that the Type 3 dolomite is the result of further burial dolomitization during the diagenetic process.

Biotic vs abiotic origin of unusual features from Mesoproterozoic of Vindhyan Supergroup, India

Biogenic signatures in Precambrian rocks are often difficult to confirm and debatable. We present some unusual features associated with microbial-mat related structures (MRS) from the freshly-exposed rippled bed surface of 1.6 Ga old Chorhat Sandstone, Vindhyan Supergroup, India. The features discussed here, are present within intertidal to supratidal environments often affected by storms. One of the features includes ridge-groove couplets that run across the ripple crests. Locally, the ridge-groove couplet forms a braid-like pattern. Along the ripple troughs, the ridges are considerably long and maintain a uniform width on the mm scale. Another feature shows meandering grooves bordered by ridges. The grooves swerve, form loops, cut across older grooves, and branch up. None of them have comparable equivalents in the Precambrian record described thus far. The invariably uniform width of the ridges for both the two features cannot be compared with undersurface gas bubble migration, and the swerving and reversing nature of the grooves denies passive movement of any inorganic/organic masses under the influence of an external force. They seem to have been created by movement through a microbiota-rich surficial sediment. Such unusual features raise questions about the biosphere and biotic structures during the Boring Billion (1.8–0.8 Ga).

Microfacies Analysis and Depositional Environment of the Middle Eocene Avanah Formation in Haibat Sultan Area, Iraqi Kurdistan Region

The Avanah Formation (Middle Eocene) was studied in the Haibat sultan section at Bina Bawi Anticline, Northeast of Erbil city within the Iraqi Kurdistan region. The field observation indicated that the formation occurs as Carbonate interbeds within the lower and middle parts of the Gercus Formation, which consists mainly of light yellowish to creamy color, well-bedded, marly, and dolomitic limestone with some intercalations of sandstones and marls reaching about 32 m thick characterized by lateral variations in facies and thicknesses. A petrographic study of 16 thin slices of carbonate rocks indicated that most of the matrix is composed of micrite, with a few microspars. Foraminifers, dasycladacean green alge, and Ostracods make up the skeleton grains. Non-skeletal grains mainly include peloids, intraclasts, and extraclasts. Depending on detailed microfacies analysis in the current study, five main microfacies and fourteen submicrofacies were discovered. The total of all petrographic, facies and textural investigations indicates that the Avanah Formation in the Haibat Sultan area was deposited in the shallow marine inner platform within subtidal, intertidal, and supratidal environments

Chemostratigraphic contributions to paleoenvironmental assessment of microbialites from neoproterozoic Capiru Formation, Southern Ribeira Belt – Brazil

The Capiru Formation records supracrustal metassedimentary rocks within the Southern Ribeira Belt. These rocks underwent heterogeneous deformation and metamorphism, resulting in an incomplete greenschist facies paragenesis. They are tectonically interbedded with strata that exhibit well-preserved primary structures. In the Morro Grande synform region, there is a continuous 150 m outcrop of metadolomites with preserved sedimentary records and was described in a stratigraphic profile (1:500) subsequently detailed (1:50). Dolomite, minor amounts of quartz, traces of ilite, graphite and zircon constitute the mineral assembly, defined by XRD, petrography and SEM-EDS. Distinct preserved facies are recognized by morphostructures: (1) lamina (parallel, discontinuous, crenulated); (2) stromatolites (homogeneous columnar, club-shaped, conical conophyton-like, pseudocolumnar, parallel-branching, divergent-branching, delicate-branching); (3) thrombolite (rugous) and (4) flat pebble conglomerate. Single-isotope data revials different microbial patterns, particularly in stromatolites (−1.57 to −0.40‰ δ13C; −8.21 to - 3.94‰ δ18O) and lamina (−1.89 to 1.29‰ δ13C; −7.32 to −3.55‰ δ18O). Facies associations suggest a shallow sea with a regressive trend. Tectonofacies are characterized by massive, venulated or brecciated carbonates. Chemostratigraphic profile is divided into two major intervals. At the bottom, the first does not exhibit a specific isotopic trend and can be subdivided in heterogeneous post-deformation venulas (Unit I, −1.75 to 0.40 δ13C; −2.34 to −8.16 δ18O), thrombolythic signals (Unit II, −1.43 to 0.40‰ δ13C; −6.35 to −4.11‰ δ18O), and facies variation, mainly supratidal (Unit III, 1.89 to 1.29‰ δ13C, −7.32 to −3.31‰ δ18O). Upwads in the profile, the second interval consists of microbial facies, displaying a more uniform signature with depleted values of δ13C and δ18O along with a slight enrichment marking the transition from intratidal to supratidal environments (Unit IV; −1.32 to −0.81‰ δ13C; - 8.17 to −6.25‰ δ18O) and supratidal lagoons (Unit V; −1.35 to −0.41‰ δ13C; −8.20 to −5.49‰ δ18O). Preserved EPS, graphite, and ilite confirm low-grade metamorphism, with clumped isotope thermometry temperatures ranging from 206.07 to 307.58 °C. Calcite is secondary, filling porosity and present in recrystallized facies. Isotopic signature of recrystallized facies differs from microbial ones, with the most depleted values (−2.16‰ δ13C; −14.02‰ δ18O) and lower formation temperature (122.29 ± 7.07 °C), indicating a late restricted event. This dolomitic succestion within Capiru Formation provides evidence of a shallow marine paleoenvironment with diverse microbial activity during its deposition.

PALYNOSTRATIGRAPHIC STATUS AND DEPOSITIONAL ENVIRONMENT OF GIRUJAN CLAY AND DUPITILA FORMATION EXPOSED ALONG SHARI RIVER (DIGRAIL) AND DUPIGAON, SYLHET, BANGLADESH

The Girujan Clay Formation is demarcated as Bangladeshopollenites barishalensis Zone, which is equivalent to middle-upper Siwalik sediments of Punjab Basin. The mangrove and coastal palynotaxa present in this formation indicate that these sediments have been deposited under the delta condition in a supratidal environment. The Girujan Clay Formation is considered to be of Mio-Pliocene in age. The DupiTila Formation is recognized here as Corrugatisporites sp. Zone. This formation is equivalent to Namsang bed of Assam-Meghalaya region. The palynological assemblage of this formation is suggestive of a high degree of precipitation and the presence of rainfed river system prevailing in the basin of deposition. It is of Plio-Pleistocene in age.

Long-term shoreline evolution (Late-Holocene – 2018) of the Channel 15 coast (Samborombón bay, NE – Buenos Aires province, Argentina)

Here we present an investigation with the aim of reconstructing the shoreline evolution and geomorphological changes in the central zone of Samborombón bay (Channel 15, Buenos Aires province, Argentina) during the period 2700 cal yr BP to recent times. Palynological, geomorphological and historical imagery and proxy-data analyses were combined to reconstruct the shoreline evolution and paleo-environmental changes. Palynological material (pollen and non-pollen palynomorphs -NPP-) was recovered from a continuous 117 cm long sedimentary profile extracted from an outcrop at the northern margin of Channel 15. Relict landforms, including paleo- shorelines, were reconstructed by applying enhancing filters to a Spot 6 image from 2015. Historical shorelines were identified in high-definition images (aerial and satellite imagery) between 1968 and 2018, but also by using the marsh vegetation edge as a shoreline proxy. Shoreline change was quantified using the Net Shoreline Movement and Weighted Linear Regression algorithms in ArcMap 10.1. Results based on the palynological analysis for the period 2700–2740 BP, show the passage from subtidal-intertidal to supratidal environment with influence of brackish to fresh-waters, where paleo-shorelines imprint was recognized developing over a tidal plain that was grading to marshes. Results based on aerial and satellite imagery for the period 1968–2018 show shoreline retrocession only immediately at the north of the channel mouth, while the rest of the coast prograded toward the estuary. These processes indicate that new marshes – and, therefore, wetlands – are constantly developing at the edge of Samborombón bay.

Sedimentology and New Paleo-geographic Data of the Upper Cretaceous-Lower Paleocene Haria Formation in the Gafsa Basin (Tunisia) Southern Tethyan Margin

Abstract Facies associations and environmental evolution of the late Cretaceous–lower Paleocene shallow platform of the Haria Formation (upper Maastrichtian–Danian–Selandian) have been studied from the Gafsa basin, located in southern Tunisia. Four main lithostratigraphic units have been identified. Unit 0, deposited on a restricted platform, is organized into aggradational and shallowing-upward sequences. Unit 1 is composed of shallowingand deepening-upward sequences that are deposited in externally restricted platform environments. Unit 2 is a peritidal shallowing-upward sequences deposited in shallow restricted environments ranging from the intertidal to supratidal environments. Unit 3,is composed of shallowing-upward sequences corresponding to the “muddy” type in the lower part, deposited in the proximal part of the shallow restricted external platform, and the “grainy” type in the upper part, representing wave-dominated bioclastic shoals and channel deposits. The proximal-distal transition of depositional environments gradually ranges from lagoon to offshore environments. Lateral variation of thickness and facies is generated by the reactivation of NW-SE extensive and/or transtensive faults, acting since the late Cretaceous during the intermediate closing phase of the Neotethys. These tectonic features were also expressed by changes in facies. The Haria Formation’s mixed carbonate-marls successions are classified into seven major facies associations, with carbonates considered a new facies represented by channel deposits and lumachellic limestones. This study establishes for the first time a clear and detailed regional stratigraphic correlation and paleogeographic maps based on the new litho- and biostratigraphic data.

Durability and Improvement of Cement-Based Revetment Materials Serving in Subtidal, Intertidal, and Supratidal Environments.

To improve the durability of cement-based revetment materials serving in different positions relative to the water level, slag powder and polypropylene fibers were added into cement to prepare paste, mortar, and concrete. Based on three simulated experiments of high-humidity air, dry–wet cycles-coupled chloride erosion, and complete immersion-coupled chloride erosion, the half-year durability of cement-based revetment materials was investigated. An abundant amount of Ettringite containing chloride was formed in the pores of the cement, and its formation was accelerated by dry–wet cycles. Replacing 30% of cement by slag powder and adding 0.1 vol.% of polypropylene fibers helped concrete in the intertidal zone to obtain a compressive strength of 47.58 MPa after erosion, equal to 159% of the reference. Slag powder was found to induce cement to form Friedel’s salt and C-S-H with a more amorphous structure, increasing its chemical binding ability and physical adsorption ability to chloride ions, and reduce the chloride ions’ penetration depth of concrete from 22.5 to 12.6 mm. Polypropylene fibers controlled the direction of surface cracks to be perpendicular to the specimen’s sides. These findings lay a foundation for the design of high-durability cement-based revetment materials serving in costal environments.

Origin of hematite in the Middle Cretaceous red chalks on the eastern coast of England

Chalks are generally believed to have formed in shallow marine environments. However, the consensus on sources of Fe and origin of hematite in red chalks remains elusive. To better understand the origin of hematite in the Middle Cretaceous red chalks on the eastern coast of England, the evidence has been gathered through diffuse reflection, trace element and iron isotope analysis and combining with the lithologic and stratigraphic characteristics. Rare earth element (REE) patterns exhibit upper-continental-crust-like signatures such as enrichment in light REEs and depletion in heavy REEs. The Fe isotopic composition of the red chalk is typically heavier than that of igneous rocks (except for one sample) and shows negative Eu anomalies (Eu/Eu* = 0.58–0.73), which indicates that the hematite in the red chalk is not of microbial or hydrothermal, but of sedimentary origin. At the same time, the analysis of trace elements suggests that the felsic rocks and sedimentary derivatives are the source of non-carbonate components in the red chalks. The pedogenic features such as mudcracks, secondary white chalks, and calcareous nodules appear in the red chalks, as documented through the field stratigraphic observations. Based on the above evidence and the sedimentary environment of the overlying and underlying layers, it is inferred that the red chalk was deposited in the supratidal environment that had undergone some degree of desiccation and pedogenic modification.

Youngest Cretaceous dinosaur tracksite from the Middle East (Maastrichtian, Farrokhi Formation, Central Iran)

A late early Maastrichtian dinosaur trampling site is reported from the Farrokhi Formation of the Khur area, Central Iran. The largely indeterminate footprints, some of which may represent undertracks, can be classified as natural moulds (i.e. concave epireliefs) bordered by a raised rim of displaced sediment. They reach diameters of up to 0.5 m and were impressed under very shallow to subaerial conditions in an inter- to supratidal environment. Two generations of traces have been imprinted, initially into a soft, fine-grained carbonate sand and afterwards into a superficially hardened substrate that was still plastic underneath; the change in substrate consistency is supported by a conspicuous cracking pattern around the footprints. As a result, hardly any details of the foot morphology of the trackmakers are recorded. Nevertheless, the occurrence improves our knowledge about dinoturbation and its preservation in different kinds of substrates. Furthermore, it is the youngest record (ca. 70 Ma) of dinosaur locomotion traces from Iran and, in all probability, the entire Middle East.

Metamorphic origin of anastomosing and wavy laminas overprinting putative microbial deposits from the 3.22 Ga Moodies Group (Barberton Greenstone Belt)

Anastomosing branching and wavy laminae in quartz-rich sandstones of the 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB), South Africa, have been extensively described as fossilized microbial mats developed in terrestrial to marine transitional environments. Petrological and geochemical characteristics of a ~ 350 m thick Moodies succession in the Saddleback Syncline in the central BGB were analyzed to reconstruct the post-depositional history of the sediments and to better constrain the origin of the laminae. The studied sandstones are composed mainly of quartz, potassium feldspar, and chert clastic grains with various proportions of microquartz and sericite cements. In coastal floodplain and inter- to supra-tidal environments, quiescent periods with low current velocity allowed the repeated deposition of thin (<5 mm thick) and fine-grained laminae with clay matrix and potentially organic matter. In contrast, subtidal settings under consistently high-energy conditions led to the deposition of coarse- to medium-sized sands without fine-grained lamina. Intergrain areas in those coarse sediments were infilled with a microquartz cement during burial diagenesis. Due to the presence of clay matrix, the fine-grained laminae had a reduced porosity that prevented the microquartz cementation within the laminae. This original clay matrix was later replaced with sericite cements during metamorphism, forming the anastomosing and wavy laminae observed at the hand-specimen scale in the studied sandstones. Microscopic dark areas within the laminae are composed mostly of opaque minerals with scarce carbonaceous material. The scarcity of carbonaceous material and the fluctuating energy settings, potentially inauspicious for the continuous development and preservation of biomats, are not in favor of a flourishing Paleoarchean microbial life on the Moodies sediments.

Underestimated diversity: Cryptic species and phylogenetic relationships in the subgenus Cobalius (Coleoptera: Hydraenidae) from marine rockpools

Marine rockpools are isolated patches of habitat in the supratidal environment (the so-called splash zone), at the transition between sea and land, found along the rocky shores worldwide and characterized by harsh conditions for life. Nonetheless, few specialized invertebrates successfully colonized this peculiar environment. Among them several members of the water beetles Ochthebius Leach, 1815, subgenus Cobalius Rey (1886), which are found almost exclusively in supratidal and upper-most intertidal marine rockpools from the eastern Atlantic Ocean (Cape Verde, Canary Islands and Morocco, Madeira, Azores) throughout the whole Mediterranean basin. The subgenus Cobalius before 2020 was considered to include ten valid species, based on morphological differences. In late 2020, four additional new species were described. However, recent molecular phylogenetic studies have uncovered further cryptic diversity suggesting the presence of multiple undetected species within this group, highlighting that the species boundaries remain unclear and systematics and taxonomy are in need of revision. In this study we provide a molecular phylogeny based on DNA sequence data from mitochondrial and nuclear genes obtained from ten described species belonging to the subgenus Cobalius, and extensive taxon sampling, in order to better understand the phylogenetic relationships within this genus and to infer the biogeographic processes behind its diversification. We also used a molecular clock to define a time window for diversification of distinct clades within the subgenus, and explore aspects of its evolutionary history. Finally, we used three species delimitation methods (PTP, GMYC and ABGD) to clarify taxonomy and validate species boundaries. Our phylogenetic and biogeographic results identified sixteen independent lineages grouped in four main clades and the possible origin of Cobalius was estimated to be in the Early Miocene (~22 Mya) in W Mediterranean area. Moreover, species delimitation methods suggest there are between 16 and 24 putative species, most of them diverged during the Late Miocene, Pliocene and Pleistocene (6.0–0.11 Mya).

Cambrian (Series 2 to Miaolingian) platform facies from central Sonora, Mexico and the regional correlation

This study provides new insights about depositional paleoenvironments through siliciclastic microfacies, carbonate microfacies, and biofacies analysis from sedimentary formations of the lower and middle Cambrian (Stage 4–Wuliuan), exposed in central Sonora, northern Mexico. Results of the petrographic analysis of 48 samples revealed the following lithologies: quartzarenite, oncolytic rudstone, grainstone-packstone, wackestone, mudstone, and to a lesser extent sandy limestone. Two siliciclastic microfacies were identified: (A) quartzarenite with cross-bedded and horizontal stratification deposited in an intertidal and supratidal environment; and (B) massive quartzarenite with Skolithos ichnofacies deposited in subtidal and intertidal environments. Four carbonate microfacies were identified: microfacies 1 is a sandy limestone with trilobite fragments; microfacies 2 is a grainstone with intraclasts, salterellids, hyolithids, trilobites, and echinoderms plates; microfacies 3 is an oncolytic rudstone consisting of microbes and abundant echinoderms plates; and microfacies 4 is a packstone-grainstone with abundant ooids, trilobite fragments, and echinoderm plates. Two biofacies were identified: Agnostid-polymeroid biofacies with predominance of the trilobites Pentagnostus, Bathyuriscus, Oryctocephalites, and Elrathina; and Pagetia biofacies with abundant trilobites of the genera Pentagnostus, Pagetia and Elrathina. It is concluded by the sedimentation model that changes in sea level is the most important parameter in determining the siliciclastic microfacies, carbonate microfacies and biofacies; as well as the depositional environments that vary from the coastline (subtidal to supratidal) to shallow-water open circulation marine platform with low and high energy waters. The Cambrian deposits of northern Mexico are correlated with the deposits of California and Nevada (USA), as well as to the Precordillera (Argentina), where the species in common show a strong affinity.

Oxfordian Benthic Foraminifera and Ostracods from the Hanifa Formation (Hawtah Member), Central Saudi Arabia

The Hanifa Formation in Saudi Arabia consists of mostly carbonate units of Late Jurassic age and is well exposed along the Tuwaiq Mountain escarpment. Micropaleontological analyses of samples from three outcrops have revealed the presence of benthic foraminifera and ostracods. The study of fossil faunas from the Hawtah Member, has led to the identification of six foraminiferal species: Alveosepta (Redmondellina) powersi, Oolina globosa, Nautiloculina oolithica, Kurnubia palastiniensis, Pseudomarssonella cf. maxima, Lenticulina sp. and two ostracod species: Hutsonia asiatica and Cytherella cf. umbilica. The survey of species covered in this research suggests that they indicate a predominantly euryhaline to polyhaline marine setting, possibly with a slightly brackish influence and deposited in a supratidal environment, suggesting a progressively shallowing environment on the inner shelf with intermittent shoal complexes.

Palaeontological and sedimentological characteristics of the Changhsingian (Late Permian) platform−margin sponge reef and intraplatform skeletal mound in NE Sichuan Basin, South China: their palaeoenvironmental implications

Well-preserved fossil reef/mound-building communities and shallow microfacies have been recovered from Changhsingian platform−margin sponge reef at the Panlongdong Section and intraplatform sponge skeletal mound at the Yanggudong Section in NE Sichuan Basin, South China. The present contribution focuses on the palaeoenvironmental implication of the differentiated development processes on the platform−margin reef and intraplatform skeletal mound based on the palaeontology and sedimentology. Precise field assessments and macroscopic/microscopic qualitative observations led to the identification of five reef/mound-building community and 14 reef/mound-related microfacies in the 2 sections of Changxing Formation. In the Panlongdong platform−margin sponge reef, reef-building communities are recognized into Amblysiphonella Community, Preperonidella−Amblysiphonella Community, and Preperonidella–Sollasia Community in ascending order. The reef-related microfacies are mainly classified into calcisphere−pelletoid wackestone to packstone, crinoidal wackestone to packstone of shallow water bank environment, hypercalcified sponge bafflestone, calcareous algae−hypercalcified sponge binding framestone, hypercalcifed sponge−calcareous algae bindstone, and calcareous algae−hypercalcifed sponge rudstone of reef environment, and benthic foraminifera−calcareous algae wackestone to packstone of shallower water lagoon environment. Two mound-building communities can only be found in the Yanggudong intraplatform skeletal mound, including Preperonidella−Intrasporeocoelia Community of lower part and Preperonidella−Sollasia Community of upper part. The mound-related microfacies include calcisphere−calcareous algae wackestone to packstone, crinoidal wackestone to packstone and crinoidal packstone to grainstone of shallow water bank environment, hypercalcified sponge bafflestone and cyanobacteria−hypercalcified sponge binding bafflestone of skeletal mound environment, benthic foraminifera−calcareous algae packstone grainstone of shallower water bank environment and benthic foraminifera−calcareous algae wackestone to packstone of shallower water lagoon environment. These palaeontological and sedimentological analysis results have facilitated the classification of sponge reef and skeletal mound sections corresponding to a solitary shallow benthic zone into multiple microfacies attributed to variable environmental depositional conditions. The differentiated evolutional process of multiple reef/mound-building stages at the Panlongdong and Yanggudong sections clearly demonstrate a general shallowing-upward trend from an inner shelf setting to a supratidal and low energy lagoon environment.

Sedimentology and Palaeoenvironmental Reconstruction of the Early Cambrian Kussak Formation, Salt Range, Pakistan

Comprehensive sedimentological and sequence stratigraphic analyses of the Kussak Formation were carried out at the Nilawahan Gorge, the Karuli Section and the Khewra Gorge, Pakistan. The studied Kussak Formation has been grouped into: (1) bioturbated mudstone, (2) interbedded sandstone/shale, (3) sandy dolomite, (4) glauconitic sandstone and (5) laminated sandstone and siltstone. Hard to friable sandstone is present with condensed neobolus beds in the mud at the top. Bioturbation and glauconite are in middle parts, while a few beds of sandy dolomite are also present in the upper and middle parts of the Kussak Formation. These five depositional sequences had developed during the phase of transgression. Mineralogically, the average modal composition classifies it as quartz arenite. Rock fragments belong to the igneous origin, and feldspar grains most likely indicate an igneous and metamorphic source. The field and petrographic study indicate that the source area belongs to arid and semi-arid environments. Presence of wavy and flaser bedding indicates tidal flat deposition. Burrows, mud cracks, and synaeresis cracks indicate supra-tidal environment. Subsurface data show the finning upward sequence that confirms the deposition during transgression. In summary, the environment of deposition is subtidal, intertidal to supra-tidal environment. Palaeogeographic setting shows that the Kussak Formation source belongs to Aravalli and Malani ranges.

Porosity distribution in cyclic dolomites of the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, China

ABSTRACT Increasing interests in hydrocarbon resources at depths have drawn greater attentions to the deeply-buried carbonate reservoirs in the Tarim Basin in China. In this study, the cyclic dolomite rocks of Upper Cambrian Lower Qiulitag Group from four outcrop sections in northwestern Tarim Basin were selected to investigate and evaluate the petrophysical properties in relation to depositional facies and cyclicity. The Lower Qiulitag Group includes ten lithofacies, which were deposited in intermediate to shallow subtidal, restricted shallow subtidal, intertidal, and supratidal environments on a carbonate ramp system. These lithofacies are vertically stacked into repeated shallowing-upward, meter-scale cycles which are further grouped into six third-order depositional sequences (Sq1 to Sq6). There are variable types of pore spaces in the Lower Qiulitag Group dolomite rocks, including interparticle, intraparticle, and fenestral pores of primary origin, inter crystal, and vuggy pores of late diagenetic modification. The porosity in the dolomites is generally facies-selective as that the microbially-originated thrombolites and stromatolites generally yield a relatively high porosity. In contrast, the high-energy ooidal grainstones generally have very low porosity. In this case, the microbialite-based peritidal cycles and peritidal cycle-dominated highstand (or regressive) successions have relatively high volumes of pore spaces, although highly fluctuating (or vertical inhomogeneous). Accordingly, the grainstone-based subtidal cycles and subtidal cycle-dominated transgressive successions generally yield extremely low porosity. This scenario indicates that porosity development and preservation in the thick dolomite successions are primarily controlled by depositional facies which were influenced by sea-level fluctuations of different orders and later diagenetic overprinting.