Tidal Flat Settings Research Articles

Ypresian–Lutetian platform evolution in the Indus Basin, Southwestern Pakistan: An interplay between local and regional tectonic changes

The present investigation aims to analyze the depositional environment, sequence stratigraphy, and geochemistry of the Early–Middle Eocene succession in the Laki Range of the Southern Indus Basin, Pakistan. The sequence is subsequently correlated with other sections within the Southern Indus Basin, Central Indus Basin, Upper Indus Basin, and Hazara Basin in Pakistan. For this study, the Lakhra and Laki formations were sampled from four sections in the Laki Range. Based on detailed outcrop observations, eight lithofacies were identified including four clastic lithofacies (CLF-1 to 4) and four limestone lithofacies (LLF-1 to 4). Similarly, based on detailed petrographic examination, seven microfacies were identified in limestone units. Considering the biotic paleoecology, facies texture, and chemical composition, depositional environments were assigned to the microfacies and lithofacies. All the microfacies, and the shale and sandstone lithofacies (CLF-1 and CLF-4) represent shallow inner to deeper outer-shelf settings whereas the other two lithofacies (CLF-2 and CLF-3) represent deltaic and tidal flat settings, respectively. All the microfacies and lithofacies information was used to establish sequence stratigraphy for the studied strata. The top of the Lakhra Formation and the whole of the Laki Formation in the Bara Nala Section (BNS) represent two complete, and two partial, third-order sequences, further divided into fourth order and small-scale cycles. The sequence's development is primarily influenced by regional and local tectonics. The regional correlation of the Indus Basin and Hazara Basin indicates that the regional basin's bathymetry and resultant depositional sequence were significantly influenced by the Himalayan Orogeny. The regional depositional pattern indicates that subsequent to India–Asia collision, the closure of eastern Tethys took place in a temporal succession from northwestern to the southwestern parts of the Indus Basin. In the Central Indus Basin, marine environments persisted until the Priabonian Stage (SBZ20) compared to those of the Upper (SBZ11–12) and Southern (SBZ13–14) Indus basins. This study offers valuable insights into both the local and regional depositional frameworks and the influence of local and regional tectonics on a carbonate platform evolution.

Depositional setting and trace fossil suites of the early Cambrian (series 2, stage 4) Khussak Formation, east-central salt range, north-west sub-Himalayas, Pakistan

This is the first integrated ichnologic-sedimentologic study of the Khussak Formation at the Salt Range, Pakistan. This integrated approach led to identifying thirteen lithofacies and six recurring facies associations. Facies association 1 (FA-1) is made up of massive to parallel laminated mudstone with rare interbeds of sandstone. FA-1 is moderately bioturbated with trace fossils belonging to the Cruziana ichnofacies (Cruziana, Planolites, Rusophycus). FA-1 is interpreted to record deposition in the upper intertidal mud flat setting. Facies association 2 (FA-2) consists of thinly interbedded sandstone (wavy and current rippled) and mudstone (massive to parallel-laminated). The heterolithic wavy bedding is the dominant bedding type. FA-2 is variably bioturbated (BI = 0–5). Trace fossils of moderate diversity of the Cruziana ichnofacies are present. FA-2 represents deposition in the middle intertidal mixed flat setting. Facies association 3 (FA-3) comprises gradational-based, laterally extensive, crudely bedded to massive, medium to lower fine-grained sandstone with thin (cm-scale) mudstone beds. FA-3 is intensely bioturbated (BI = 3–5). Robust and diverse trace fossils belonging to the Skolithos ichnofacies are present (Arenicolites, Cylindrichnus, Diplocraterion, Phycodes, Planolites, Siphonichnus, Skolithos, and Treptichnus). FA-3 records deposition in the lower intertidal sand flat setting. Facies association 4 (FA-4) consists of muddy to silty lower-upper fine-grained sandstone with rare mudstone drapes. FA-4 displays inclined heterolithic stratification (IHS). Tidal couplets (mud and mica drapes) and mud clasts are common. Bioturbation is absent to sparse (BI = 0–2) with low-diversity trace fossils of Skolithos ichnofacies (Cylindrichnus, Diplocraterion, Palaeophycus, Planolites, Skolithos, and rare fugichnia). FA-4 is interpreted to represent deposition in an intertidal creek point bar. Facies association 5 (FA-5) consists of well sorted sandstone (upper medium to fine-grained). The sandstone is erosionally based, and laterally extensive. Planar parallel lamination, trough cross-stratification, reactivation surfaces, and mud drapes are present only locally. The degree of biogenic reworking is low to moderate (BI = 0–4), and traces are sporadically distributed. Trace fossils present are of moderate diversity and abundance, including Arenicolites, Cylindrichnus, Diplocraterion, Monocraterion, Palaeophycus, Planolites, Siphonichnus, and Skolithos. FA-5 is interpreted as subtidal bar deposits. Facies association 6 (FA-6) is characterized by bioturbated muddy sandstone, heterolithic sandstone, and massive mudstone. Physical bedforms are commonly obliterated by bioturbation. Planar horizontal laminae, wavy laminae, small ripples, and synaeresis cracks may occur locally. FA-6 is moderate to intensely bioturbated (BI = 3–6). The distribution of traces (within beds and across the FA) is heterogeneous (sporadically) to homogeneous (locally). Robust Bergaueria, Cylindrichnus, Planolites, Rhizocorallium, Siphonichnus, Teichichnus and Zoophycos are present. FA-6 records deposition in the subtidal bay region. The tidal flat deposits of the Khussak Formation show a distinctive ichnofacies trend. The Cruziana ichnofacies characterizes the lower energy proximal deposits (intertidal mixed and mud flat). In contrast, the high-energy distal deposits (lower intertidal sand flat and shallow subtidal) are characterized by Skolithos ichnofacies. The ichnologic variation between the mud flat, mixed flat, sand flat, and subtidal deposits are gradational, but they can be used as facies boundary indicators. The bioturbation intensity and diversity of trace fossils within the lower Cambrian (Series 2, Stage 4) tidal flat deposits of the Khussak Formation suggest elevated levels of bioturbation intensity during stable environmental conditions in the Cambrian. Furthermore, it implies that the Agronomic Revolution was already emplaced in lower Cambrian (Series 2, Stage 4) tidal flat settings.

Conodont biostratigraphy, carbonate microfacies and climate changes during the Late Devonain (Frasnian–Famennian) in the Preslavtsi 2 well (Moesian Terrane, NE Bulgaria)

Five conodont zones (falsiovalis, rhenana, triangularis, marginifera, and praesulcata) were recognized within the Upper Devonian carbonate sequence in the Preslavtsi 2 well from the Moesian Terrane in NE Bulgaria. Microfacies study indicates a climate change (from the Subtropical Arid to the Warm Temperate Climatic Zones) recorded in the Frasnian–Famennian succession. Thus, during the Givetian and early Frasnian, sedimentation took place in an arid tidal flat setting, where microbial mats and lagoon carbonates have been formed. In the late Frasnian, a transgression, followed by a rapid regression was recorded that are presumably related to the Late Devonian Kellwasser event. At the beginning of the Famennian, a climate change has occurred and various shallow- and open-marine carbonates have been formed under warm temperate climate conditions.

Characteristics of Early Neoproterozoic Stromatolites from Southern Liaoning, North China: Insights into the Formation of Stromatolites

Stromatolites, among the earliest fossils in Earth’s history, are widely distributed on the margins of the North China Precambrian carbonate platform. The formation processes of stromatolites reveal the biomineralization and evolution of early life in the Precambrian. The well-preserved stromatolitic dolostones recorded in the Ganjingzi Formation are developed around Yuanjiagou village, in southern Liaoning Province. The morphology of the Ganjingzi stromatolites manifests in stratiform, columnar, and domal forms. A tripartite lamina structure including light laminae and two types of dark laminae is observed in thin sections. The origins of dark laminae were related to microbial metabolism, while the light laminae were the result of the recrystallization of synsedimentary marine cement. Hardground substrate and carbonate fragments were suitable for microbes to colonize, suggesting that microbes can adapt to various current energy settings. A comparison of the growth environment, morphology, and laminae features between the Ganjingzi stromatolites and modern carbonate stromatolites from Hamelin Pool and Lagoa Vermelha suggest that the Ganjingzi stromatolites may have been formed in a restricted tidal-flat setting with high salinity and evaporation. The role of microbes that form modern stromatolites in inducing precipitation of carbonate or binding sediments, might contribute to the formation of the Ganjingzi stromatolites. The formation process of the Ganjingzi stromatolites indicates that the microbial communities, favorable substrate, and synsedimentary marine cement were the key factors in promoting the development of the Neoproterozoic stromatolites on the northeastern margin of the North China Craton.

Geological understanding of permeability upscaling in mixed carbonate siliciclastic reservoirs: An outcrop approach, Miocene Dam formation, eastern Saudi Arabia

In reservoir modeling and simulation, upscaling a fine-grid model (scale of millions of cells), which includes numerous geological details to a coarser grid model (scale of thousands of cells), is indispensable to reduce unachievably high computational time and costs. The upscaling process usually results in losing essential details of the fine-scale model and may introduce overestimating or underestimating reservoir properties. Although numerous mathematical approaches have been developed to reduce such limitations, linking upscaling process to the conceptual understanding of sedimentary geology is less well addressed. Such linkage is critical to establishing a predictive conceptual model of factors that affect the upscaling process, such as heterogeneity and connectivity of reservoir rocks. This study generated several upscaling scenarios and performed multiple runs of fluid flow simulation for three-dimensional porosity and permeability models of carbonate strata deposited in a tidal flat setting (the Miocene Dam Formation, eastern Saudi Arabia). The objective was to understand porosity and permeability upscaling in tidal flat carbonate reservoirs based on lithological distribution and mainly answer questions related to the impact of increasing cell size on the distribution of petrophysical properties and the effect of averaging methods (harmonic, geometric, and arithmetic) on permeability upscaling of tidal flat carbonate. The results showed that porosity values in the tidal flat carbonate are sensitive to upscaling procedures. The mean of the porosity values increases substantially with increasing the cell size of the upscaled models and results in an overestimation of the volumetric parameters of the fine-grid model. A possible reason for such overestimation is the diminishing of the percentage of the cells representing the mud-dominated facies, which in the field occur as thin and interbedded layers. Upscaled permeability showed variation based on averaging methods, and the results from fluid flow simulation provide means to select the appropriate upscaled permeability based on lithofacies distribution. Collectively, these results suggested that a geological understanding of the lithofacies distribution is necessary for the upscaling process. Such knowledge helps to select the appropriate cell size and method for permeability averaging.

Non-avian theropod phalanges from the marine Fox Hills Formation (Maastrichtian), western South Dakota, USA.

We report here the first dinosaur skeletal material described from the marine Fox Hills Formation (Maastrichtian) of western South Dakota. The find consists of two theropod pedal phalanges: one recovered from the middle part of the Fairpoint Member in Meade County, South Dakota; and the other from the Iron Lightning Member in Ziebach County, South Dakota. Comparison with pedal phalanges of other theropods suggests strongly that the Fairpoint specimen is a right pedal phalanx, possibly III-2, from a large ornithomimid. The Iron Lightning specimen we cautiously identify as an ornithomimid left pedal phalanx II-2. The Fairpoint bone comes from thinly bedded and cross-bedded marine sandstones containing large hematitic concretions and concretionary horizons. Associated fossils include osteichthyan teeth, fin spines and otoliths, and abundant teeth of common Cretaceous nearshore and pelagic chondrichthyans. Leaf impressions and other plant debris, blocks of fossilized wood, and Ophiomorpha burrows are also common. The Iron Lightning bone comes from a channel deposit composed of fine to coarse sandstone beds, some of which contain bivalves, and a disseminated assemblage of mammal teeth, chondrichthyan teeth, and fragmentary dinosaur teeth and claws. We interpret the depositional environment of the two specimens as marginal marine. The Fairpoint bone derives from a nearshore foreset setting, above wave base subject to tidal flux and storm activity. The Iron Lightning specimen comes from a topset channel infill probably related to deposition on a tidal flat or associated coastal setting. The taphonomic history and ages of the two bones differ. Orthogonal cracks in the cortical bone of the Fairpoint specimen suggest post-mortem desiccation in a dryland coastal setting prior to transport and preservation in the nearby nearshore setting described above. The pristine surface of the Iron Lightning specimen indicates little transport before incorporation into the channel deposit in which it was found. The Fairpoint bone bed most probably lies within the Hoploscaphites nicolletii Ammonite Zone of the early late Maastrichtian, and would therefore have an approximate age of 69 Ma. The Iron Lightning bone is from the overlying H. nebrascensis Ammonite Zone, and is thus about one million years younger.

Ediacaran-Cambrian microbialites of the Southern Amazon Craton: relation with the metazoan rise, sea-level changes, and global tectonics

Microbialites are the most abundant life evidence in Precambrian sedimentary rocks. They are produced by microbial interaction activity and sedimentary processes reflecting paleoenvironmental conditions. The Ediacaran-Cambrian carbonate and siliciclastic successions in the Southern Amazon Craton in Central Brazil, provide a key opportunity to understand how the metazoan life coexisted with the microbial communities. The spatial and temporal distribution of microbialites as well as morphological and paleoenvironmental changes have been assessed, reinterpreting previous works and including new data from the Araras-Alto Paraguai and Corumbá basins. The deposition was controlled by subsidence and sea-level changes that affected these basins, considered extensions of epicontinental seas during the Gondwana assembly. The stromatolites are restricted to coastal deposits and experienced thriving flourishment intervals after the Marinoan Glaciation (635 Ma). Post-glacial transgression was marked by microbial colonization in shallow platforms represented by stratiform and giant domical stromatolites in the Araras-Alto Paraguai Basin. The continuity of the transgression generated a moderately deep aragonite sea at about 622 Ma. A progressive sea-level fall caused the implantation of coastal environments under greenhouse conditions with tidal flat and sabkha settings colonized by centimetric-scale stromatolites. The sea retreat was accompanied by progressive uplift, causing a moderate inversion of the basin and erosion of the succession until ~560 Ma with the deposition of the last preserved tidal flat deposits with the occurrence of thrombolites. The subsiding Corumbá Basin was the site of microbially-induced deposition of carbonates in a shallow platform connected to an offshore setting with the proliferation of metazoan straddling the Ediacaran-Cambrian boundary. Microbial communities were restricted to lagoon deposits during the Lower Cambrian transgression in the Araras-Alto Paraguai Basin and the last phase refers to the sea retreat towards southeast, developing a fluvial system connected with the arid and evaporitic tidal flats colonized by microbialites that lasted until the upper Cambrian. Except for the post-glacial stromatolites, the columnar and domal microbialite indicate that the coastal settings dominated the Ediacaran-Cambrian transition. The preservation of microbialites in the post-glacial intervals can be associated with the Mg-Ca-CO3 oversaturation in dolomitic platforms. The rapid calcification and ability to resist the dissolution and replacement have increased the stromatolites’ preservation potential reported here, where its well-preserved occurrence in tidal flats and sabkha occurs due to intense early diagenetic silicification. The change from carbonate accumulation to siliciclastic-rich environments contributed to the demise of microbially-induced strata. In general, the scarce coexistence between coastal stromatolite and metazoan-bearing marine deposits makes it challenging to establish a competitive relationship between these organisms, as previously postulated.

Microbially induced sedimentary structures from the Xinji Formation (Cambrian Series 2), Western Henan, North China

Abundant well‐preserved microbially induced sedimentary structures (MISS) are exposed in the Xinji Formation, the earliest Cambrian sedimentary unit in North China. The MISS herein are categorized into microbial mat growth structure (wrinkle structures and mat biolaminites) and microbial mat destruction structure (sand cracks and ‘Manchuriophycus’ structures). The MISS developed on the surface of thick quartz sandstones, as well as the lithologic interface between thin sandstone and mudstone. Most of the sand cracks lack the direct evidence of microbial activities, whereas the mat biolaminites that consist of filamentous mica, clay minerals, and quartz grains may record the in situ microbial mats. The Xinji Formation deposited along with the Cambrian explosion when biodiversity and abundance of metazoan stayed relatively low. Besides, the intermittently exposed tidal flats are not the ideal place for early life, which exacerbated the ecological vacuum therein. The massive occurrence of the MISS in the siliciclastic rocks of the Xinji Formation indicated that the tidal flat setting during this period was still beneficial for accumulations of microbial mat because of the deficient grazing metazoans and weak bioturbation.

Modeling the Depositional Environment of the Sandstone Reservoir in the Middle Miocene Sidri Member, Badri Field, Gulf of Suez Basin, Egypt: Integration of Gamma-Ray Log Patterns and Petrographic Characteristics of Lithology

Understanding the depositional setting of siliciclastics reservoir is fundamental process to exploration and development of hydrocarbon reservoirs and to the multi-phase cycle of the oil and gas industry. Typically, core samples from existing or potential reservoirs can be used for interpretation of depositional environment. However, the lack of core samples in certain reservoirs represents a challenge for reservoir development plans and further exploration. To overcome the absence of core samples in the middle Miocene Sidri Member in the Badri field, Gulf of Suez, this study aimed to reconstruct its depositional settings by coupling well logging patterns and petrographic characterization of ditch cuttings. Consequently, 30 thin section samples representing the reservoir section of four wells were described in detail and standard petrographic characteristics were determined. Then, gamma-ray (GR) log patterns of the studied reservoir wells were used for interpretation of depositional environment. Petrographic analysis indicates that the sandstone reservoir is fine- to medium-grained arkose with dominant siliceous cement and composed mainly of quartz, feldspars, and lithic fragments. Pores reflecting primary and secondary porosity as well as inter-granular pores are dominant. The facies analysis indicates that the studied member has a heterogeneous nature and rapid facies change; its main lithofacies are blocky sandstones, intercalated sandstones and shales, and shales. Three electrofacies were interpreted in the studied section: (1) electrofacies-A (blocky sandstones), which is characterized by a cylindrical-shaped (right boxcar) GR trend; (2) electrofacies-B (intercalated sandstones and shales), which is characterized by an irregular log trend with serrated shape; and (3) electrofacies-C (shales), which is characterized by irregular GR trend and has no character. The interpreted results indicate a tidal channel depositional setting for electrofacies-A, mixed tidal flat depositional setting for electrofacies-B, and low relief offshore mudstone depositional setting for electrofacies-C. Finally, the results illustrate how the coupling of GR patterns with the analysis of petrographic characteristics can be used to understand the depositional setting of a hydrocarbon reservoir that lacks core samples. This work could be useful for assessment of reservoir distribution and quality, for reduction of uncertainty during field development, as well as for providing useful insight to similar hydrocarbon reservoirs elsewhere.

Holocene development and human use of mangroves and limestone forest at an ancient hong lagoon in the Tràng An karst, Ninh Binh, Vietnam

How past environments and communities responded to episodes of coastal inundation can inform preparations for future resilience to predicted rises in sea level. Southeast Asia’s extensive coastlines and expanding coastal populations mean vast natural and human capital is at risk from future sea level rise. Regional mangroves provide many ecosystem services that can help mitigate such risks, but deforestation has left them threatened and compromised. The present study examines the Holocene development and human use of mangrove forest in northern Vietnam, where existing palaeo-records derive from sedimentary archives in tidal flat, estuarine and deltaic settings. Here, we expand that coverage by describing conditions at an enclosed doline within the Tràng An limestone karst in Ninh Binh province that would have been sheltered from deltaic and marine processes. We present a multi-proxy assessment incorporating pollen analysis of the 8125-year-old discontinuous sediment core obtained from the doline floor, combined with inferences from erosional tidal notches in the enclosing limestone, and analyses of phytolith, vertebrate and mollusc assemblages from an adjacent archaeological cave site with deposits of comparable age. The results provide a detailed example of how enclosed coastal environments and communities responded to Middle Holocene marine inundation. High percentages of pollen from mangroves (17–57%) suggest their colonisation of the doline from ∼8100 cal. BP and persistence until ∼250 cal. BP, well after the intertidal zone regressed seaward beyond the massif. Archaeological assemblages dating to ∼5500 cal. BP and containing palm and woody eudicot phytoliths and sponge spicules, neurocrania of the fish genus Pomadasys (‘grunts’, ‘grunters’ or ‘javelins’) and brackish-water molluscs Sermyla riqueti and Neripteron violaceum support the persistence of mangrove environments through the Middle Holocene high-stand, a period of hiatus within the core, and indicate human foraging and fishing activities took place in mangrove and lagoonal habitats alongside hunting in the surrounding limestone forest. Subsequent structural opening of this latter forest formation from ∼1075 cal. BP (875 CE), evident in the pollen record, coincides with the Medieval Climate Anomaly and with the adjacent development of the ancient capital at Hoa Lu. We propose that given the long-term persistence of mangrove habitats and associated resources documented in this study, regional initiatives aimed at rehabilitating mangroves (with the positive consequences that this holds for biodiversity and socio-economic conditions) may wish to consider selective restorative measures within Tràng An and similar sheltered sub-coastal karst settings.

Porosity evolution and diagenetic history of the Upper Jurassic Mozduran Formation, eastern Kopet-Dagh Basin, NE Iran

The Upper Jurassic carbonates of the eastern part of the Kopet-Dagh Basin, with thickness of 470 m, are the major gas-bearing reservoir in NE Iran. The objectives of this study are recognition of diagenetic history and estimation of porosity related to dolomitization. Based on field and laboratory study, four carbonate facies associations have been identified at the Mazdavand outcrop. Most of the carbonate rocks were deposited in an open marine, tidal channel and barrier, lagoon and tidal flat setting. Using petrographic and CL analyses, the main diagenetic processes that affected these rocks are micritization, cementation, compaction, fracturing, dissolution, neomorphism, silicification, and dolomitization. Dissolution of grains and calcite cement generated secondary porosity, whereas compaction and cementation are the primary cause of porosity reduction. The average porosity of core plugs is 15.78% and match well with two-dimensional estimates from thin sections. The more porous samples are either highly fractured or contain interparticle and intercrystalline porosity, within the lower strata that contain more dolostone.

Tidal Flat Depositional Response to Neotectonic Cyclic Uplift and Subsidence (1–2 m) as Superimposed on Latest-Holocene Net Sea Level Rise (1.0 m/ka) in a Large Shallow Mesotidal Wave-Dominated Estuary, Willapa Bay, Washington, USA

The late-Holocene record of tidal flat deposition in the large shallow Willapa Bay estuary (43 km in length), located in the Columbia River Littoral Cell (CRLC) system (160 km length), was investigated with new vibracores (n=30) and gouge cores (n=8), reaching 2–5 m depth subsurface. Reversing up-core trends of muddy sand to peaty mud deposits in marginal tidal flat settings demonstrate episodic submergence events resulting from cyclic tectonic uplift and subsidence (1–2 m) in the Cascadia subduction zone. These short-term reversals are superimposed on longer-term trends of overall sediment coarsening-up, which represent the transgression of higher-energy sandy tidal flats over pre-existing lower-energy tidal flat mud and peaty mud deposits in late-Holocene time. Fining-up trends associated with channel lateral migration and accretionary bank deposition occurred only infrequently in the broad intertidal flats of Willapa Bay. Vibracores and gouge cores were dated by 14C (n=16) and paleo-subsidence event contacts (n=17). Vibracore median probability 14C ages ranged from 0 to 6,992 yr BP and averaged 2,174 yr BP. Dated sample ages and corresponding depths of tidal flat deposits yield net sedimentation rates of 0.9–1.2 m ka-1, depending on the averaging methods used. Net sedimentation rates in the intertidal flat settings (~1.0 m ka-1) are comparable to the rate of net sea level rise (~1.0 m ka-1), as based on dated paleo-tidal marsh deposits in Willapa Bay. Reported modern inputs of river sand (total=1.77x104 m3 yr-1), from the three small rivers that flow into Willapa Bay, fall well short of the estimated increasing accommodation space (1.9x105 m3 yr-1) in the intertidal (MLLW-MHHW) setting (1.9x108 m2 surface area) during the last 3 ka, or 3.0 m of sea level rise. The under-supply of tributary sand permitted the influx of littoral sand (1.1x105 m3 yr-1) into Willapa Bay, as based on the net sedimentation rate (~1.0 m ka-1) and textural composition (average 60 % littoral sand) in analyzed core sections (n=179). The long-term littoral sand sink in Willapa Bay’s intertidal setting (55 % of total estuary area) is estimated to be about 5 % of the Columbia River supply of sand to the CRLC system, and about 30% relative to the littoral sand accumulated in barrier spits and beach plains during late-Holocene time. A 2.0 m rise in future sea level could yield a littoral sand sink of 2.2x108 m3 in the Willapa Bay intertidal setting, resulting in an equivalent shoreline retreat of 600 m along a 50 km distance of the barrier spit and beach plains that are located adjacent to the Willapa Bay tidal inlet. Willapa Bay serves as proxy for potential littoral sand sinks in other shallow mesotidal estuary-barrier-beach systems around the world following future global sea level rise.

Sedimentology and sequence stratigraphy of evaporites in the Middle Jurassic Buqu Formation of the Qiangtang Basin, Tibet, China

Qiangtang Basin is a large Mesozoic marine sedimentary basin located on the northern Qinghai-Tibet plateau. Multiple sets of evaporites were observed in the Middle Jurassic Buqu Formation through field outcrop description and thin sections analysis, so as to form the sets of evaporite–carbonate sedimentary sequences. The evaporites in the Buqu Formation are represented by gypsum and anhydrite rocks that contain minor secondary diagenetic features such as massive, rosette, chicken-wire, and satin spar gypsum. Two lithofacies association types were identified in the Buqu Formation, including sparry oolitic limestone–dolomite–gypsum/anhydrite and micrite–carbonaceous mudstone–gypsum/anhydrite. The former was deposited in an intertidal–supratidal (tidal flat) setting and the latter was deposited in a subtidal lagoon setting. Two depositional sequences were distinguished, including Buqu sequence 1 and Buqu sequence 2. Gypsum of the intertidal setting is mainly developed in a lowstand system tract in both Buqu sequences 1 and 2, while gypsum of the subtidal setting is mainly developed in a highstand system tract in Buqu sequence 2. Combining the measured evaporites thickness in the field with sedimentary facies analysis, the distribution of the Middle Jurassic Buqu Formation evaporites is clear in the Qiangtang Basin, which may serve as a local seal for oil and gas plays in this region.

Transitional environments of the lower Maastrichtian South-Pyrenean Basin (Catalonia, Spain): The Fumanya Member tidal flat

Sedimentological, palaeontological and geochemical data provide detailed evidence of a marine-to-lagoon environmental succession around the Campanian-Maastrichtian boundary in the Ibero-Armorican domain. This regression is recorded by the succession of several environments of the south-central Pyrenees basin: open marine mixed shelf, marine restricted mixed shelf (both rudist-rich), tidal flat, lagoon and fluvial dominated. The tidal flat setting belongs to the Fumanya Member here described, which is the base of the Posa and Massana formations (Tremp Group). The Fumanya Member is 5 m thick, is built up of marly limestones and was an elongated tidal mudflat (100 km long, 25 km wide) developed in a foreland trough and was separated from the Atlantic Ocean by an island-barrier system. In the lagoon environment, marginal marine waters and continental fresh waters alternated as documented by geochemistry and fossil molluscs. The Fumanya Member is a dinosaur megatracksite, reporting the roaming activity only of sauropods in tidal flats, a likely secure area against predators. Feeding activity of these herbivores took place in the lagoonal-lacustrine environments of the Posa Formation above the Fumanya Member.

Chemo-stratigraphy as a tool for sequence stratigraphy of the Early Triassic Kangan Formation, North of the Persian Gulf

The Lower Triassic Kangan Formation, which is equivalent to the Upper Khuff, is the main reservoir for natural gas in SW Iran and the northern Persian Gulf. Investigations of this formation in Salman Field indicate that it is composed of 14 facies deposited in tidal flat, lagoon and oolitic barrier settings in the inner part of a carbonate ramp. Vertical variations of microfacies and gamma-ray log profiles show that this formation consists of three depositional sequences (KG3–KG1), each including of transgressive and high stand systems tract and each bounded above and below by type-two unconformities. Carbon and oxygen stable isotope profiles from a well-dated carbonate section were compared with the sedimentological data and results from the sequence stratigraphic study of the Kangan Formation, showing negative and positive peaks coinciding with sequence boundaries and maximum flooding surfaces Boundaries and Maximum Flooding Surfaces, respectively.

A trace element and Pb isotopic investigation into the provenance and deposition of stromatolitic carbonates, ironstones and associated shales of the ∼3.0 Ga Pongola Supergroup, Kaapvaal Craton

Major and trace element, and Pb isotopic data for chemical and clastic sedimentary rocks of the Mesoarchaean Pongola Supergroup are employed to infer aspects of the provenance and depositional environment, including ambient seawater composition. Stromatolitic carbonates of the Nsuze Group were formed in a tidal-flat setting, whereas ironstones of the Mozaan Group were deposited in an outer-shelf setting during marine transgression. Geochemical criteria, employed to test for crustal contamination and diagenetic/metamorphic overprinting, demonstrate that carbonates and ironstones preserved their primary chemical signature. In comparison to other documented Precambrian stromatolites, shale-normalised REE+Y patterns for Nsuze carbonates show pronounced enrichment in middle REE, but lack strong elemental anomalies (La, Gd, Y) that are diagnostic for derivation from open marine waters. In contrast, normalised REE+Y for ironstones exhibit distinct positive La, Gd and Y anomalies. Both rock types are devoid of normalised Ce anomalies and show only minor enrichment in Eu, suggesting deposition in anoxic environments (with respect to the Ce3+/Ce4+ redox couple) accompanied by minor high-temperature hydrothermal input. Trace element geochemical data are most consistent with deposition of Nsuze carbonates in a shallow-water epicontinental basin with restricted but variable exchange to the open-ocean and dominant fluvial input, whereas ironstone precipitated in a deeper-water, epicontinental sea. Estuarine fractionation and organic complexation due to microbial activity is possibly indicated by MREE enrichment of the carbonates, also consistent with a restricted environment.Shales belonging to the Mozaan Group are characterised by high concentrations of Al and K relative to Ca, Na and Sr, indicative of pronounced in-situ weathering, coupled with K-metasomatism. The provenance is mixed, comprising (ultra)mafic and granitic source rocks.Pb isotope regression for Nsuze carbonates documents a widespread, tectono-thermal or fluid percolation event at around 2.4Ga. Two-stage modelling of Pb isotope data, in association with published Sr and Nd isotope data, requires a source for Nzuse carbonates that was derived from evolved continental crust with an elevated U/Pb ratio (μ-value) and an approximate crustal residence time of ∼100–600Ma.

Sedimentary phosphate and associated fossil bacteria in a Paleoproterozoic tidal flat in the 1.85 Ga Michigamme Formation, Michigan, USA

Phosphorus is a nutrient fundamental to life and when it precipitates in modern environments bacteria are intimately involved in its release, concentration, and mineralization. Preserved fossil bacteria in phosphate crusts and grains from the ca. 1850 million-year-old Bijiki Iron Formation Member of the Michigamme Formation, Michigan provide insight into the longevity and nature of this relationship. The Michigamme Formation accumulated near the end of the Earth's initial phosphogenic episode (ca. 2.2 and 1.8Ga) to produce one of the first granular phosphorites. Phosphatic lithofacies consist of fine- to medium-sand-sized francolite peloids concentrated on bedding surfaces in peritidal facies. Granular beds are up to 2cm thick and peloids are often partially to completely replaced by dolomite and chert. The grains contain organic matter and pyrite framboids that suggest bacterial breakdown of organic matter and bacterial sulfate reduction.The peritidal nature of phosphorite in the Michigamme Formation is in sharp contrast to Phanerozoic phosphogenic environments in deeper coastal upwelling settings. Peritidal settings were well suited for phosphogenesis under the very low oxygen and low dissolved sulfate levels of the Paleoproterozoic as cyanobacteria produced oxygen in shallow water and evaporation led to increased sulfate concentrations. Such concomitant processes helped establish focused redox interfaces in the sediment that chemosynthetic bacterial communities (sulfur oxidizers, reducers, forms that concentrate P, and possibly iron oxidizers) could exploit. Phosphate released from organic matter by heterotrophic bacteria and Fe-redox pumping was further concentrated by these chemotrophs; a process that forms late Neoproterozoic to Phanerozoic phosphorites but on a much larger scale.This early example of a granular phosphorite demonstrates that, like their Phanerozoic counterparts, Paleoproterozoic phosphorites are the concentrated indirectly biomineralized products of bacterial communities. But unlike younger analogs, which accumulated across subtidal shelves and shelf margins, these ancient deposits formed only in tidal flat settings where phosphogenic redox processes could be established in the sediment. From this early beginning, the zone of phosphogenesis likely migrated into deeper water settings as oxygen and sulfate levels rose, expanding the zone of chemosynthetic bacterial and associated phosphogenesis across the shelf.

Palaeo-earthquake events during the late Early Palaeozoic in the central Tarim Basin (NW China): evidence from deep drilling cores

Abstract Various millimetre-, centimetre- and metre-scale soft-sediment deformation structures (SSDS) have been identified in the Upper Ordovician and Lower-Middle Silurian from deep drilling cores in the Tarim Basin (NW China). These structures include liquefied-sand veins, liquefaction-induced breccias, boudinage-like structures, load and diapir- or flame-like structures, dish and mixed-layer structures, hydroplastic convolutions and seismic unconformities. The deformed layers are intercalated by undeformed layers of varying thicknesses that are petrologically and sedimentologically similar to the deformed layers. The SSDS developed in a shelf environment during the early Late Ordovician and formed initially under shear tensile stress conditions, as indicated by boudinage-like structures; during the latest Ordovician, SSDS formed under a com-pressional regime. The SSDS in the Lower-Middle Silurian consist mainly of mixed layers and sand veins; they formed in shoreline and tidal-flat settings with liquefaction features indicating an origin under a compressional stress regime. By Silurian times, the centre of tectonic activity had shifted to the south-eastern part of the basin. The SSDS occur at different depths in wells that are close to the syn-sedimentary Tazhong 1 Fault (TZ1F) and associated reversed-thrust secondary faults. Based on their characteristics, the inferred formation mechanism and the spatial association with faults, the SSDS are interpreted as seismites. The Tazhong 1 fault was a seismogenic fault during the later Ordovician, whereas the reversed-direction secondary faults became active in the Early-Middle Silurian. Multiple palaeo-earthquake records reflect pulses and cyclicity, which supports secondary tectonic activity within the main tectonic movement. The range of SSDS structures reflects different developments of tectonic activity with time for the various tectonic units of the centralbasin. The effects of the strong palaeo-earthquake activity coincide with uplift, fault activity and syn-tectonic sedimentation in the study area during the Late Ordovician to Middle Silurian.

Horizontal gene transfer and the rock record: comparative genomics of phylogenetically distant bacteria that induce wrinkle structure formation in modern sediments

Wrinkle structures are sedimentary features that are produced primarily through the trapping and binding of siliciclastic sediments by mat-forming micro-organisms. Wrinkle structures and related sedimentary structures in the rock record are commonly interpreted to represent the stabilizing influence of cyanobacteria on sediments because cyanobacteria are known to produce similar textures and structures in modern tidal flat settings. However, other extant bacteria such as filamentous representatives of the family Beggiatoaceae can also interact with sediments to produce sedimentary features that morphologically resemble many of those associated with cyanobacteria-dominated mats. While Beggiatoa spp. and cyanobacteria are metabolically and phylogenetically distant, genomic analyses show that the two groups share hundreds of homologous genes, likely as the result of horizontal gene transfer. The comparative genomics results described here suggest that some horizontally transferred genes may code for phenotypic traits such as filament formation, chemotaxis, and the production of extracellular polymeric substances that potentially underlie the similar biostabilizing influences of these organisms on sediments. We suggest that the ecological utility of certain basic life modes such as the construction of mats and biofilms, coupled with the lateral mobility of genes in the microbial world, introduces an element of uncertainty into the inference of specific phylogenetic origins from gross morphological features preserved in the ancient rock record.

Depositional environments and sequence stratigraphy of the Late Carboniferous−Early Permian coal-bearing successions (Shandong Province, China): Sequence development in an epicontinental basin

This study focuses on the Late Carboniferous−Early Permian coal-bearing successions in Shandong Province, North China in order to understand the depositional processes and sequence-stratigraphic framework in an epicontinental basin. Based on detailed analysis of eleven facies, five facies assemblages (FAs) were recognized in the studied succession. FA1–3 are present mainly in the Benxi and Taiyuan formations, and consist of mixed siliciclastic and carbonate lithofacies, representing eluvial-lagoon, barrier-lagoon, and tidal-flat environments. FA4 occurs in the Shanxi formation and consists mainly of interbedded medium to fine sandstone, siltstone, mudstone, and coal lithofacies, representing river-dominated deltaic environments. FA5 is characterized by interbeds of trough cross-stratified coarse sandstone, and silty mudstone, mainly in the Lower Shihezi Formation, which was deposited in meandering river channel and floodplain. Three third-order sequences were established based on the vertical arrangement of facies assemblages and identification of physical surfaces (i.e., subaerial unconformity, transgressive surface, and regressive surface). Each sequence comprises a transgressive systems tract (TST) and a highstand systems tract (HST). TST of sequence 1 is composed of eluvial lagoonal deposits (FA1), whereas HST formed in lagoon-barrier and tidal-flat settings (FA2 and FA3). TST of sequence 2 formed in a barrier-lagoon system (FA2), whereas HST is characterized by repetitive accumulation of interbedded limestone, sandstone, mudstone, and coal, deposited under lagoonal and tidal-flat settings (FA2 and FA3). TST of sequence 3 comprises FA2, and HST mainly FA4, deposited in a river-dominated shallow-water delta system. Sequence 3 is overlain by a fluvial sequence (FA5). The three third-order sequences in the Shandong region are generally correlated with those in the Taebaeksan Basin (South Korea), the eastern part of the North China Block. The relative sea-level curves established in the two regions show a generally similar long-term rising trend.