Sedimentary facies analysis of the subsurface triassic and hydrocarbon potential in the United Arab Emirates

Summary The Triassic sediments in the subsurface of the United Arab Emirates has been divided into three formations (from bottom to top): Sudair, Jilh (Gulailah) and Minjur. The Sudair Formation consists of four lithofacies units composed mainly of limestones and minor dolomites interbedded with terrigenous shaley mudstones and anhydritic dolomitic limestones. These were deposited in shallow marine supratidal to subtidal settings. The Jilh (Gulailah) Formation has five lithofacies units dominated by anhydritic dolomitic limestone, fine terrigenoclastic sediments and bioclastic and intraclastic limestones. The formation was laid down under lagoonal to supratidal sabkha conditions with little normal marine influence. The Minjur Formation is composed of three lithofacies units characterized by argillaceous quartzitic sandstones, shales, mudstones, dolomitic and ferruginous limestones with thin coal seams. These facies represent deposition in prograding delta lobes, reflecting humid continental to marginal-marine conditions. Diagenesis plays a major role in the reservoir development in the Triassic sediments, the pores are occluded by dolomite and anhydrite. The grains are compacted, leached or cemented by marine cements. Porosity generally ranges from fair to poor with values from 6% to 9% in the carbonates and from 6% to 15% in the clastics. Interparticle and vuggy porosities are the main pore types. The porosity was controlled by diagenesis, depth of burial and lithology. No oil has been discovered so far in the Triassic sediments of the United Arab Emirates but pronounced gas shows have been reported from offshore fields. Western offshore United Arab Emirates is a promising area for potential hydrocarbon accumulations. The Triassic sediments have low to moderate source rock potential; the organic matter is mainly sapropelic kerogen, and the degree of thermal alteration ranges between mature to highly mature stages.     

Microfacies of Cambrian Limestones in Jordan

Summary The limestones of the Wadi Nasb Formation of the uppermost Lower Cambrian of Jordan are under- and overlain by massive sandstones of a near-shore facies. Facies analysis is based on samples from an outcrop at the northeastern shore of the Dead Sea and two oil test wells in the Wadi Sirhan Depression in eastern Jordan. Limestones were deposited in the shallow sea and within the coastal tidal area. Cyanobacteria, algae, echinoderms, trilobites and hyoliths have contributed the bulk of the carbonate and phosphatic material composing the Wadi Nasb limestone. Fine-grained facies types are composed of peloidal carbonate muds with laminar and nodular algal and cyanobacterial mats. They formed within a quiet tidallagoonal environment. The coarse grained facies types consist of carbonate sands with layers of sheell debris deposited in crossbeds in an environment with a rich endobenthic fauna. Here most particles were coated by cyanobacterial crusts. Ooids, oncoids and various coated grains are present. Consolidated sediments were commonly eroded within or near to this environment and their remains were integrated within the sands. Diagenesis is reconstructed step by step with deposition, first cementation, aragonite dissolution, compaction, pore filling, formation of pressure solution, growth of dolomite and anhydrite within the calcitic limestone and final fissure formation and filling.     

Facies architecture of the Stuttgart Formation (Schilfsandstein, Upper Triassic), central Germany, and its comparison with modern Ganga system, India

The Stuttgart Formation (Schilfsandstein) is approximately 50 m thick in Thuringia, representing deposition during the “Mid-Carnian Wet Intermezzo”. Stratigraphically it occurs between the Grabfeld and Weser formations, which formed under arid conditions. It comprises NNE-SSW-trending elongate, anastomosing channelized sand-rich bodies with erosional bases (channel belts) that are several kilometres wide and pass laterally into predominantly mudstones deposited in interfluve areas. The source area of these clastics was the uplifted Norwegian Caledonides. Muddy interfluve facies is dominant in exposures in Thuringia, Central Germany.The Lower Stuttgart Formation has an unconformable base that is locally overlain by metre-thick “Basal Beds”. These consist of grey mudstones and thin sandstones deposited under humid conditions in predominantly shallow brackish water environments after a marine ingression via the Eastern Carpathian/Upper Silesian Gate. The following 30-40 m-grey, fine-grained sandstones, siltstones and mudstones were deposited in fluvial environments in channel belts and interfluve areas under humid conditions. These are followed by predominantly reddish mudstones and sandstones of mainly fluvial origin, deposited under somewhat drier conditions with seasonal droughts. The Upper Stuttgart Formation may be more than 16 m thick; it comprises reddish and grey sandstones and mudstones that were mostly deposited in lake delta settings by recurring flash floods. During the deposition of this unit climate was weakly humid with less prominent seasonal draughts.The modern Ganga Plain of India is an analogue for the depositional setting of the Stuttgart Formation. Climatic conditions in Ganga Plain are humid monsoonal with seasonal droughts and roughly comparable with those interpreted for Mid-Carnian times in Germany. The sandy deposits of incised channel belts and channels and muddy deposits of interfluve areas in the Ganga Plain are comparable with the sandstone-dominated channelized facies and mudstone dominated interfluve facies of the Stuttgart Formation, respectively.     

Upper Ladinian to Lower Carnian sedimentary evolution in the Idrija-Cerkno Region,Western Sovenia

Summary An Upper Ladinian to Lower Carnian succession in the Idrija-Cerkno region (W Slovenia) is described and correlated with similar successions in the Dolomites. Structurally, the area belongs to the Rodne unit (Trnovo nappe, NW Dinarides). The succession was reconstructed from three stratigraphically superimposed sections. The Orehovska Grapa section is characterised by finegrained turbidites composed of sandy mudstones with intercalations of lenses and beds of trachy-andesite tuff and resedimented tuffs. Beds of hemipelagic light grey wackestone are rarely interstratified. These rocks are correlative with the Upper Ladinian Wengen Group. The Police1 section is composed of black shaly marls and mudstones, hemipelagic wackestone, tuffaceous sand-stones, and in the upper part, of calciturbidites overlain by black laminated shales. The section is correlated with the lower part of the San Cassiano Formation. The Police 2 section consists mainly of wavy bedded peloidal and bioclastic limestone, alternating with thin interbeds of shaly mudstones and marls. The limestone and mudstones are interpreted as tempestites and gradually pass into bedded and massive dolomite of Early Carnian age. This succession is similar to the transition from the San Cassiano Formation to the Cassian Dolomite. The studied succession represents a shallowing upward basinal sequence capped by carbonate platform deposits. Palaeogeographically it is a Late Ladinian transition from the carbonate platform in the south to the typical basinal area in the north.     

Lithostratigraphy and facies development of upper cretaceous carbonates in east central Sinai,Egypt

Summary The Upper Cretaceous exposures in east central Sinai are represented by carbonate-dominated successions interbedding few sandstone, chert, shale and marl horizons. The recognised rock units are correlated with their counterparts commonly used in the Gulf of Suez region and central Sinai including from base to top: the Raha Formation, Abu Qada Formation, Wata Formation, Matulla Formation and the Sudr Chalk. Twelve limestone microfacies are encountered and are categorised as mudstones (pelmicrite and ostracod micrite), wackestones (pelagic biomicrite and foraminiferal biomicrite), grainstones (foraminiferal biopelsparite and oosparite), boundstones (bindstone and framestone), floatstones (coated-grained biomicrudite, rudist biomicrudite and shelly biomicrudite) and rudstones (shelly biosparudite). The dolostone microfacies include fine-medium crystalline ostracod dolostones and shelly dolostones. These microfacies have been compared with the Standard Microfacies Types and their depositional environments are discussed. The encountered litho- and biofacies suggest that the Cenomanian shallow transgressive sea had covered east central Sinai as far south as the Dahab region. By the advent of the Turonian, open marine subtidal conditions prevailed. This was followed by transitional conditions with shoals and tidal bars in the Late Turonian pointing to a regressive phase more pronounced at the southern localities. The rocks of the Matulla Formation were deposited in an oscillating environment of shallow subtidal to intertidal conditions during Coniacian-Santonian. In the Late Santonian and during most of the Campanian-Maastrichtian, sedimentation was influenced by open marine conditions with low sedimentation rates; local shallow subtidal regressive events occurred.     

Foraminiferal biostratigraphy of the lower Miocene Hamzian and Arashtanab sections (NW Iran), northern margin of the Tethyan Seaway

Lower Miocene strata exposed in the Khoy-Bostan Abad area (NW Iran) document marine platform environments. Paleontological and lithological characteristics of these deposits were investigated to develop detailed bio- and lithostratigraphic frameworks. The shallow marine units, composed of bioclastic carbonates in Bostan Abad and mixed siliciclastic carbonates in Khoy area, were deposited under different local tectonic regimes. Relatively diverse assemblages of benthic foraminifera from these shallow marine units were studied to establish a high resolution biostratigraphy in the context of the standard shallow benthic zonation of Western Tethys. The co-occurrence of Miogypsina intermedia Drooger and Borelis melo curdica (Reichel) characterizes Zone SBZ25, indicating a Burdigalian age as also indicated by planktonic foraminifera.     

Late Cretaceous (Cenomanian to Campanian) paleoenvironmental history of the Eastern Canadian margin of the Western Interior Seaway: bonebeds and anoxic events

Upper Cretaceous strata in the Pasquia Hills of the northern Manitoba Escarpment, eastern Saskatchewan, Canada provide a detailed paleoenvironmental and sea-level record of the eastern margin of the Western Interior Seaway. Sediments deposited during the Cenomanian/Turonian Greenhorn marine cycle are dominantly black mudstones deposited in a stratified water column, with bottom-water anoxia recurrently reaching into the photic zone. A middle Cenomanian sea-level lowstand event followed by transgression left a series of bonebeds within the Belle Fourche Member of the Ashville Formation, indicating a sedimentary environment starved of coarse siliciclastics. Maximum sea level resulted in the formation of limestone beds within the Favel Formation, further favoured by reduced terrigenous sediment input compared to the western margin. Limestone sedimentation was followed by a phase of increased freshwater input under lower sea level conditions, and reducing zoo- and phytoplankton diversities. During final Greenhorn regression, eastern Saskatchewan probably turned into a restricted basin severely limiting marine circulation. Poor or absent benthic foraminiferal assemblages and biomarker analysis suggest prevailing watermass stratification throughout the Cenomanian/Turonian transgressive/regressive cycle. This was caused either by a freshwater lid, stratification of Boreal and Tethyan-derived watermasses, or both, to various intensities affected by changing sea level. Basin oxygenation during Niobrara time varies between localities along the eastern margin as documented by presence/absence of benthic and planktic foraminifera.     

Foraminiferal response to Holocene environmental changes of a tidal estuary in Victoria, southeastern Australia

The Holocene strata in the Anderson Inlet area in Victoria can be stratigraphically divided into four units, Unit I, Unit II, Unit III, and Unit IV. Unit I and Unit IV lack fossils and were deposited in non-marine, probably fluvial environments. Unit II and Unit III contain abundant foraminifera with molluscs, ostracods and bryozoans. Foraminiferal analysis suggests that Unit III was formed in a partially sheltered marine environment, while the high plankton content and relatively high diversity of benthic species in Unit II indicate that this unit was deposited in an open bay at water depths possibly less than 5 m. The foraminiferal data are integrated with radiocarbon dates to arrive at the following Holocene palaeoenvironmental history in this area: (1) low alluvial plain stage (10,000–7000 yr B.P.); (2) open bay environment stage (7000–5500 yr B.P.); (3) partially sheltered marine environment stage (5500–4500 yr B.P.); (4) alluvial plain and coastal lagoon environment stage (since about 4500 yr B.P.). The foraminiferal fauna show a clear response to these palaeoenvironmental changes. Globigerina bulloides can be used as an indicator for cold water marine environments. The high concentration of this species in these middle Holocene sediments shows a strong cold water influence on the coastal environments which reduced the effect of regional warm currents during this period. The Holocene palaeoenvironmental changes in the area were controlled by the Holocene sea-level fluctuations associated with the deglaciation history during this period. Similar integrated studies of shallow to marginal marine strata in southern Africa, America and New Zealand will lead to a better understanding of Holocene relative sea-level change and the interplay between Holocene cold and warm water regimes in the Southern Hemisphere.     

MARINE REPTILES FROM THE LOWER CRETACEOUS OF SOUTH AUSTRALIA: ELEMENTS OF A HIGH-LATITUDE COLD-WATER ASSEMBLAGE

Abstract: The Lower Cretaceous rocks of South Australia have yielded a diverse marine reptile assemblage of up to five families of plesiosaur (including a new cryptoclidid or cimoliasaurid, indeterminate elasmosaurids, a possible polycotylid, rhomaleosaurids, and pliosaurid) and one family of ichthyosaur (ophthalmosaurid). Other common associated vertebrates include chimaerids and osteichthyans. Sharks, dipnoans and dinosaurs are uncommon and marine turtles are notably absent. The main fossil‐producing strata belong to the Lower Aptian–Lower Albian Bulldog Shale although the Upper Albian Oodnadatta Formation has produced isolated elements. Both these units comprise finely laminated shaly mudstones and claystones deposited in a transgressive shallow coastal, epicontinental marine environment. Estimates of palaeolatitude place South Australia between 60° and 70°S, in the late Early Cretaceous. Sedimentary structures (including lonestone boulders and glendonites), fossils, isotope data and climatic modelling also indicate that seasonally cool–cold conditions (possibly with winter freezing) prevailed during deposition of the Bulldog Shale. This contrasts markedly with climate regimes typically tolerated by modern aquatic reptiles but suggests that some of the South Australian Mesozoic taxa may have possessed adaptations (including elevated metabolic levels and/or annual migration) to cope with low temperatures. A high proportion of juvenile plesiosaur remains in the Bulldog Shale might also indicate that nutrient‐rich cold‐water coastal habitats functioned as both ‘safe calving grounds’ and refuges for young animals prior to their entering the open sea as adults. The occurrence of plesiosaurs and ichthyosaurs in the high‐latitude Lower Cretaceous of southern Australia, along with plesiosaurs and mosasaurs in the Upper Cretaceous of South America, Antarctica, New Zealand and the Chatham Islands, demonstrates that Mesozoic marine reptiles utilized southern high‐latitude environments over a considerable period of time, and that these records do not represent casual occupation by isolated taxa.     

Palaeoecology of the Aptian Santana Formation (Northeastern Brazil)

In the interior of northeastern Brazil there occurs the Santana Formation of Aptian age, composed lithologically of three members: Crato — an alternation of thin limestones and shales; Ipubi — gypsum; and Romualdo — almost pure limestone. The formation is very fossiliferous; pollen, plant remains, ostracodes, conchostracans, mollusks, echinoids, fishes and a few reptiles. The fishes and reptiles occur in limestone concretions. All data on sediment character and on fossils have been considered together for the interpretation of the palaeoenvironment of the formation.It was concluded that at the time of the Crato Member the deposition occurred in shallow lakes and swamps. Later (Ipubi Member), a marine invasion took place during a time of dry climate, causing anhydrite precipitation. Gradually the connection with the sea became obstructed so that the environment at the end of the depositional period became once more one of fresh water (Romualdo Member). Faunal assemblages and sediments point to a fairly great supply of river water during the whole time of basin deposition, under warm and dry climatic conditions. The connection with the sea persisted for a rather short period.     

Deep-water stromatolites andFrutexites Maslov from the early and Middle Jurassic of S-Germany and Austria

Summary Despite extensive discussions during the last 20 years stromatolites are still used by many geologists as unequivocal indicators of very shallow-water conditions. We investigated four stratigraphic units from the Lower and Middle Jurassic of southern Germany (Posidonien-Schiefer, Amaltheen-Ton) and of the Northern Calcareous Alps (Adneter Kalk, Klauskalk), which were formerly interpreted as shallow marine sediments by some authors due to the occurrence of stromatolites. Our interpretations of the macro-, micro- and ultrafacies of these sediments are not compatible with shallow-water settings. We therefore propose a deep-marine, aphotic origin of these stromatolites. Former interpretations of the Posidonien-Schiefer as a shallow-water deposit are mainly based on the occurrence of stromatolites. We favour the model of a temporarily stagnant, deep, aphotic basin for these planktonrich sediments. Particles resembling ooids, but lying within mudstones cannot be taken as evidence for shallow agitated water. They either formed within the mud or are allochthonous. The deep-water setting of the red limestone of the Alpine Early and Middle Jurassic is indicated by a lack of platform-typical components like coated grains and phototrophic benthos and by shells of plankton and nekton forming a major part of the sediment. Stromatolites occur on the steep slope of a drowned Rhaetian reef with an estimated relief of 50–100 m and immediately below and within radiolarian limestones, deposited below the aragonite compensation depth (ACD). The aphotic stromatolites show some morphological differences to their shallow water counterparts. In all of our sections they occurred during intervals of reduced sedimentation. They form only thin horizons and probably grew very slowly. Mineralizations by Fe−Mn oxides and phosphate are very common. The presence of a microbial film is evident from binding of sedimentary particles, but the nature of the microbes is not known. Growth habits within the very distinct environments of red limestone and black shales show some common features, but also clear differences. The microproblematicumFrutexites Maslov is a very common component in deep-water stromatolites, but may also itself form small crusts or dendrolites. It occurs in two different forms. Opaque, slender forms with indistinct outlines probably grew within the weakly lithified sediment. Thicker, transparent forms with well defined outlines are found in cavities and probably also grew on the seafloor. Well preserved specimens display an internal fabric of radially arranged fibres of Fe−Mn oxides and calcite. It is suggested that calcite or aragonite were one original mineralogy ofFrutexites, which was later replaced by Fe−Mn oxides or phosphate. It is not certain whetherFrutexites is an organic, biomineralized structure or an inorganic mineralization, but the variable mineralogy and growth forms in different environments point to an organic origin. But even if organic, the occurrence in cryptic habitats and negative phototactic growth-directions make it clear thatFrutexites was not phototrophic.     

Facies characteristic and paleoenvironmental reconstruction of the Fahliyan Formation,Lower Cretaceous,in the Kuh-e Siah area,Zagros Basin,southern Iran

The Lower Cretaceous Fahliyan Formation, part of the Khami Group, unconformably overlies the Hith Formation and is conformably overlain by the Gadvan Formation in the study area in southern Iran. The Fahliyan Formation is a reservoir rock in Zagros Basin. This formation was investigated by a detailed petrographic analysis in order to clarify the depositional facies and sedimentary environment in the Kuh-e Siah Anticline in Boushehr Province. Petrographic studies led to the recognition of 25 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open-marine in mid-ramp environment. An absence of turbidite deposits, reefal facies, and gradual facies changes indicate that the Fahliyan Formation was deposited on a carbonate ramp. Calcareous algae and benthic foraminifera are abundant in the shallow marine carbonates of the Fahliyan Formation. These skeletal grains have been studied in order to increase the understanding of their distributions in time and space. A total of ten genera belonging to different groups of calcareous algae and 16 genera of benthic foraminifera are recognized from the Fahliyan Formation at Kuh-e Siah section.     

Lacustrine sedimentation in the diapir-controlled Miocene Bicorb Basin, eastern Spain

The Miocene Bicorb Basin is a small elongated basin developed by normal faulting of a thick Jurassic–Cretaceous carbonate succession and subsequent diapirism of Upper Triassic mudstones and evaporites. The basin fill comprises a sequence over 650 m thick formed by two units. The lower, alluvial unit consists of a complex alternation of conglomerates, sandstones, mudstones, and minor lacustrine limestones. The upper unit comprises marginal alluvial and lacustrine deposits in which seven lacustrine facies associations have been distinguished. The inner lacustrine deposits comprise mudstones and carbonates with minor evaporitic deposits. At the northeast and southwest basin edges, alluvial inputs led to the development of delta and fan-delta environments where sandstone and conglomerate deposits dominate. The latter stages of the lake evolution are represented by an expansive thick limestone sequence which close to the NW fault-bounded margin pass laterally to breccias. The hierarchical arrangement of the upper unit shows five different orders of sequences. The first-order sequence defines a major vertical trend of lake expansion whereas three second-order sequences are linked to major flooding-expansion episodes. All these sequences are related to the tectonic evolution of the diapir. Metre-scale, third- and fourth-order sequences and fifth-order sequences, which consist of finely laminated rhythmite deposits probably record climatically forced processes. The diapir–graben system evolution and the climatic variations have exerted major controls on the sequential arrangement and evolution of the lacustrine system. Both factors have also strongly influenced the changes in the gastropod assemblages.     

Vertebrate-rich Plattenkalk of Pietraroia (Lower Cretaceous,Southern Apennines,Italy): a new model

The Lower Cretaceous Pietraroia Plattenkalk is a fossiliferous, fine-grained cherty limestone from the Matese Mountains – Southern Apennines, Italy. The deposits are well known for the exceptional state of vertebrate fossil preservation. First considered as shallow lagoonal deposits or as intra-platform basin-fill, the Pietraroia Plattenkalk sequences are here interpreted as abandon deposits of a submarine channel (Pietraroia Channel) documenting a major transgressive event. Transgression was associated with the development of suboxic to anoxic conditions at the seafloor which favoured the preservation of fossils, as well as the deposition of coprolith-rich and bituminous layers. A peculiar paleogeographic and paleotopographic setting, which was strongly controlled by local tectonic, experienced the contiguity of wide emerged areas with a relatively deep-water channelised area where fossiliferous Plattenkalk sequences were deposited.     

Reworked marine sandstone concretions: a record of high-frequency shallow burial to exhumation cycles

Concretions, with abundant calcite-dolomite cement-replacement textures originally hosted in shallow-marine sandstones, were reworked into Lower Cretaceous fluvio-deltaic conglomerates and shoreface sandstones (External Zones, Betic Cordillera). A cycle of host sand deposition, early diagenetic concretion formation and concretion reworking is documented: (1) Well-sorted shoreface sandstone deposited. (2) Spherical to ovoid, non-ferroan calcite-cemented concretions formed below flooding surfaces at shallow-burial depths during early eodiagenesis. Non-ferroan calcite cements were precipitated from the bicarbonate derived from seawater and from dissolution of marine bioclasts, as shown by isotope analyses. (3) Concretions were reworked and exposed on the seafloor in a high-energy setting as indicated by the presence of numerous bivalve borings (Entobia ichnofacies), laminated micritic microbial crusts around the concretions, and epilithobionts (oysters, barnacles and corals) on the concretion surface. Concretions also appear as erosional remnants on the floor of channels which were incised into the shoreline sandstone when sea-level fell. (4) The fluvio–deltaic channels were filled with sediment during flooding in the late lowstand of sea-level. (5) The concretions are partly dolomitized, and the presence of siderite, pyrite and barite in the outer part of the concretions precipitated before the dolomite, suggests that the latter formed during shallow burial.     

A revised ammonite zonation of the Boreal Oxfordian and its application in northeast Greenland

A revised ammonite zonation for use in the Middle–Upper Oxfordian of the Boreal province is put forward. The zonation is used to date late Jurassic sediments in the Wollaston Forland area of northeast Greenland. The sediments broadly comprise basinal mudstones and shallow marine and shoreline sandstones, which were deposited in westerly tilted fault blocks during three transgressive pulses. On the peneplaned surfaces of wide tilted fault blocks a thick sequence of tidally and fluvially influenced marine sandstones of Middle Jurassic age was deposited. In the Upper Jurassic deposition initially took place in the centre of the basin where 80 m of shoreface sandstones accumulated (Jakobsstigen Member). In the second pulse the upthrown margin of the next fault block to the west was transgressed and deposition of basinal mudstones (Bernbjerg Formation) commenced in the basin centre on the main part of the dip-slope of the eastern block. Finally the crestal areas of the block on Kuhn Ø were inundated in Kimmeridgian times, and then subsided rapidly.     

Paleoecology of Upper Eifelian and Lower Givetian coral limestones in the northwestern Sauerland (Devonian; Rhenish Massif)

Summary The prevailing sandy/silty lower part of the Middle Devonian in the northwestern Sauerland includes two coral limestone horizons, which contain a rich fauna of corals, stromatoporoids, and calcareous algae. The Ihmert-Formation is subdvided into three parts. The older coral limestone horizon is the Grünewiese-Member of the Ihmert-Formation (uppermost Eifelian), the younger is in the Bredenbruch-Member of the Unterhonsel-Formation (lower Lower Givetian). Conclusions about the environmental constraints are drawn from the sedimentology and the fossil content of the coral limestones. Predominant biostromes are built between storm wave base and normal wave base. Only the few bioherms grew above the normal wave base. These coral limestones were deposited in a tropical or subtropical normal marine environment in the shallow euphotic zone. Among the reef-builders epoecism is very frequent, and until now this phenomenon has not been investigated in detail. Fragile rugose and tabulate corals lived as commensals with stromatoporoids. Some other aspects of paleoecology are concisely presented.     

Stratigraphy and Palaeontology of mid-Carboniferous Sequences at Phu Bo Bit in Loei Province,Thailand

The rocks from the Phu Bo Bit section consist of shale and limestone along with an abundant and diverse invertebrate fauna consisting of brachiopods, corals, bryozoans and crinoids, interpreted as having been deposited in shallow marine or shelf environments. At Phu Bo Bit, thick limestone lenses in shale contain microfossils. Based on the foraminiferan and algae assemblage, the stratigraphic sequence of Phu Bo Bit extends from the late Bashkirian to the early Moscovian. According to lithofacies and palaeontology, the Phu Bo Bit section represents the change from carbonate to siliciclastic sedimentation, which provides new evidence on the marine regression during the mid-Carboniferous.     

Palaeoenvironment and taphonomy of a Late Jurassic (Late Tithonian) Lagerstätte from central Poland

A rich assemblage of exceptionally preserved marine and terrestrial fossils occurs in fine‐grained limestones in the upper part of the Late Tithonian (Middle Volgian) shallowing upward carbonate sequence in Central Poland. The richest horizon, a deposit known locally as the Corbulomima horizon, is named after the shallow burrowing suspension feeding bivalve Corbulomima obscura, moulds of which occur in densities of up to 500 per square metre on some bedding planes. The fauna in this bed also includes organic and phosphatic remains of a wide range of other creatures including the exuviae of limulids and decapods, disarticulated fish skeletons and rare isolated pterosaur bones and teeth. There are also perfectly preserved dragonfly wings and beetle exoskeletons. The average stable carbon and oxygen isotope values for ostracod shells and fine‐grained sediment from this horizon suggest precipitation of the calcium carbonate from warm seawater of normal marine salinity. The carbonate sediments overlying the fossiliferous horizon have been interpreted as nearshore to shoreface facies. These pass abruptly into coarse reworked intraclastic sediments interpreted as possible tsunami or storm surge over‐wash deposits. The clasts in this deposit have more positive oxygen isotope values than those in the underlying limestone, which may indicate that they were lithified in a slightly more evaporative, perhaps intertidal, setting. The succession terminates with silicified fine‐grained limestones likely to have formed in extremely shallow lagoonal environments. In contrast with the Solnhofen limestones of Lower Tithonian age in south‐central Germany the Corbulomima horizon is interpreted as a transitional deposit formed in a shallow marine setting by rapid burial with elements of both Konservat‐ and Konzentrat‐Lagerstätte preservation. □Konzentrat and Konservat‐Lagerstätte, Taphonomy, Palaeoenvironment, Paleogeography, Late Jurassic, Poland.