Prolonged Subaerial Exposure Research Articles

Carbonate-associated sulfate as an archive of sulfur and oxygen stable isotope composition of seawater sulfate: Evidence from reef carbonate rocks of the southern South China Sea

Carbonate-associated sulfate (CAS) is a useful proxy for the reconstruction of the isotopic composition of ancient seawater sulfate, archiving information on the global sulfur cycle, Earth's surface redox evolution, and biological activity. The CAS proxy is possibly compromised by early diagenetic alteration, which may cause secondary inhomogeneities in the isotopic composition of pore water sulfate achieved in the form of CAS. However, the effects of early diagenesis on the retention of primary δ34S signals of CAS in reef carbonates are not well understood, and it is unknown whether reef carbonate deposits can faithfully record the coeval isotopic composition of marine sulfate. To provide new constraints on the resilience of CAS isotopic signatures of reef carbonate, we have measured and analyzed CAS content and CAS isotopic composition of reef carbonates from the Meiji Atoll (well NK-1) of the South China Sea, representing carbonates affected by meteoric and marine diagenesis including dolomitization. CAS contents were found to decrease rapidly during meteoric and marine diagenesis, with minimum CAS contents caused by prolonged subaerial exposure. Despite low contents, the corresponding δ34SCAS values were found to vary within a narrow range from 21.5 to 24.1‰, compared to values of 20.9 to 22.7‰ of coeval barite, suggesting that the original δ34SCAS values are preserved and largely unaffected by recrystallization. However, δ18OCAS values were found to vary widely, ranging from 7.1‰ to 15.0‰, only recording the same oxygen isotope signal as coeval marine barite for reef carbonates younger than 5 Ma. Our data reveal that reef carbonates can be used as an archive of paleoceanic change, faithfully tracking δ34S changes in coeval seawater regardless of whether reef carbonates underwent recrystallization, including dolomitization. In contrast to sulfur isotopes, the oxygen isotope composition of CAS is more susceptible to alteration during diagenesis. Overall, this study provides new insight for the future reconstruction of the sulfate sulfur and oxygen isotope record of ancient seawater.

Stratigraphy and facies of the Apennine Carbonate Platform (southern Italy): the record of Mesozoic OAEs and Miocene transgression

The Apennine Carbonate Platform of southern Italy witnesses nearly 150 Myr (Late Triassic to Late Cretaceous) of shallow-water carbonate sedimentation in the subtropical central Tethys. During this field trip, you will have a look at the stratigraphy and facies across some key intervals of global palaeoenvironmental perturbation and at some important stratigraphic discontinuities in the history of the platform.During the first day, you will visit three localities of the Matese Mts. At San Lorenzello you will look at Milankovitch cyclicity expressed in Lower Cretaceous (Valanginian-Hauterivian) peritidal carbonates. At Pietraroja you will have a look at the foramol facies of the Cusano formation, marking the Early Miocene transgression on top of the eroded Cretaceous substrate. Moving north, at the Regiapiana you will see the abandoned mines that exploited the mid-Cretaceous karst bauxites, which mark a prolonged subaerial exposure. Moreover, you will walk through the Middle Miocene synorogenic history of the platform, from its exposure and erosion in the forebulge to its incipient flexural subsidence, highlighted by the Lower Miocene carbonates of the Cusano formation, to its drowning below the photic zone, marked by a phosphatic hardground overlain by the Orbulina marls of the Longano formation.During the second day, you will head south toward Salerno. In a quarry at Mercato San Severino, you will look at the record of the early Toarcian oceanic anoxic event, marked by the abrupt demise of lithiotid bivalves and dasycladalean algae, the major carbonate producers of the Lower Jurassic carbonate platforms. At the base of Mt Tobenna, you will look at the Aptian Orbitolina level: a marker bed whose palaeoenvironmental meaning has been long debated. From Mt Tobenna you will move south toward Monteforte Cilento where you will look at the record of the Cenomanian-Turonian OAE2 in the Apennine Carbonate Platform.

Sedimentology and neoichnology of a muddy translating point bar in the fluvio-tidal transition, Serpentine River, BC, Canada

Point bars are common in the fluvio-tidal transition of rivers, yet detailed characterization of their sedimentology and neoichnology is rarely reported. Herein, we present sedimentological and ichnological descriptions for a seaward translating point bar located in the fluvio-tidal transition of the Serpentine River, BC, Canada. This bar accumulates in mainly polyhaline water, and channel flow is tide-dominated. The landward bar has steep channel banks and grain size fines from medium sand in the thalweg to mud in the upper intertidal zone. The seaward side of the bar forms a bench-like morphology that is steeply inclined in the intertidal zone, has a wide, gently dipping platform in the lower intertidal/uppermost subtidal zone, and a steep lower subtidal channel bank. Sediment sizes fine from coarse- to medium-grained sand in the thalweg, to mud in the lower intertidal zone. Medium-grained sand is deposited on the middle- and upper-intertidal zone of the seaward-side of the bar as a result of flow reversal and impingement of the flood tidal current against the seaward side of the translating point bar.The neoichnological assemblage of the intertidal part of the translating point bar is characteristic of brackish-water environments, and comprises low diversities of diminutive structures that occur locally in high densities (Bioturbation Indices 3–4). The neoichnological distribution correlates closely with the macrobenthos distribution, which in turn indicates marine-nutrient distribution. Marine nutrient supply is likely controlled by the bar's morphology. Biogenic structures attributable to Arenicolites, Skolithos, Polykladichnus and Siphonichnus mark the middle and upper intertidal zones, as well as the lower intertidal zone along the landward side of the bar. The trace makers that produce this trace assemblage are resilient to prolonged subaerial exposure and employ diverse feeding strategies. The lower intertidal zone along the seaward side of the bar is affected by decreasing current velocities across the wide bench-like morphology, which permits regular deposition of marine nutrients. Here, population counts are high and trace makers specialize in surface deposit- and filter-feeding. Traces include Gyrolithes-like structures and Siphonichnus. The Gyrolithes and Gyrolithes-like burrows that are produced by S. kowalevskii at the study site are hypothesized to potentially reflect the trace maker's anatomical predisposition, and not represent some spatial or anchoring advantage.This work represents the first published study on the neoichnology of a tide-dominated, translating point bar in the fluvio-tidal transition. The model we present is intended as an analog to show the variability in fluvio-tidal point bars and how they might be manifested in the sedimentary record.

Cherts, spiculites, and collapse breccias – Porosity generation in upper Permian reservoir rocks, Gohta discovery, Loppa High, south-western Barents Sea

Spiculitic cherts are uncommon reservoir rocks and their porosity evolution is poorly understood compared to sandstones and carbonates. In the Gohta oil and gas discovery on the Loppa High in the south-western Barents Sea, the reservoir is in brecciated, silicified, and dolomitized Permian spiculites below the Permian/Triassic unconformity. It represents the infill of several collapsed cave systems with spiculite clasts in a micritic matrix, separated by intact cave roofs of shale and spiculite facies. The cave collapse was related to dissolution of less resistant (?limestone and ?spiculitic) beds, due to percolation of freshwater during latest Permian – earliest Triassic emersion. During later transformation of opal-A to opal-CT, associated growth of silica concretions left the margins of spiculite clasts depleted in SiO2 and thus highly porous. Subsequent transformation of opal-CT to quartz resulted in precipitation of texture-preserving quartz and chalcedony cements. The latter show a systematic decrease of δ18O from the first to last precipitated phase, implying crystallization under increasing temperatures during renewed Middle–Late Triassic burial. Later diagenesis includes in situ brecciation and fracturing, dolomitization affecting mainly the micritic matrix of cave-collapse facies, chemical compaction, and calcite cementation. The best reservoir properties are in cave-collapse facies (commonly 10–25% and 0.03–19 mD) where the pore system is dominated by (1) uncemented interspicule pores and central parts of spicule molds within clast margins, and (2) intercrystalline pores between dolomite crystals in the breccia matrix. The primary depositional facies have much lower porosity and permeability (rarely exceeding 10% and 1 mD, respectively). This study shows that porosity in the cave fill most likely formed by local redistribution of silica to form concretions and dissolution of the carbonate matrix to source the growth of dolomite crystals, while prolonged subaerial exposure only played an indirect role by isolating spiculite clasts and preventing their complete silicification during burial.

Superfici di interesse culturale geo-paleontologico con orme di dinosauro del Cretaceo (Albiano superiore): l'esempio di Lama Balice nella Città Metropolitana di Bari

Along the eastern flank of the Murge, a karst plateu located in Apulia (southern Italy), the “Parco Naturale Regionale Lama Balice” (“Lama Balice” Regional Natural Park), which falls into the Metropolitan City of Bari, was established; in the Park, on a bottom surface of an abandoned quarry, several dinosaur footprints were found. The quarry develops in the Cretaceous limestones of the Calcare di Bari Fm, and shows an about 20 m thick upper Albian succession made up of carbonates and dolomites. A Research Convention stipulated between the Park authority, the University of Bari and the “Soprintendenza Archeologia, Belle Arti e Paesaggio per la Citta Metropolitana di Bari”, allowed the access to the quarry and the study of both the stratigraphic succession and the trumpled surface.In order to investigate the outcropping succession, a stratigraphic log was realised, and 54 samples were collected. Thin sections were produced and a facies analysis was carried out by means of macroscopic feature observations and microscopic ones. Several facies were recognised allowing to attribute the studied succession to a tidal flat frequently subject to prolonged subaerial exposure. The study of the surface consisted both in an ichnological analysis of individual footprints and in the identification of potential trackways. In order to highlight the main morphological features of footprints, an ichnological survey was realised by means of interpretative drawings carried out directly in situ. Then, data were processed digitally through photos and photomosaics. The best-preserved footprints allowed us to recognize the presence of theropods, ankylosaurs, and probably sauropods. All the recognized taxa belong to small- to medium-sized dinosaurs.The presence of dinosaur footprints represents an important contribute to the knowledge of Cretaceous paleoenvironments of this sector of the Apulia Carbonate Platform corresponding to the Murge region.Interestingly, the occurrence of dinosaur footprints within a park area, located close to a large city and an airport, represents a powerful educational and cultural tool.

Integration of static and dynamic data and high-resolution sequence stratigraphy to define reservoir architecture and flow units within a ‘super giant’ gas condensate and oil field, Kazakhstan

Abstract The Upper Serpukhovian (Mississippian) interval of the Karachaganak reservoir comprises stratified platform interior carbonates, which are characterised by relatively high porosity and matrix permeability (maximum 23.8% and 2198 mD, respectively), and comprise high frequency (fourth-order) glacio-eustatic cycles. Based on static (geological) and dynamic (well test and production) data, this cyclicity exerts a strong control on gas migration pathways, and reservoir quality is predictable within a third- and fourth-order sequence stratigraphic framework that defines two discrete flow units. The contribution to flow made by fourth-order cycles within flow units is dependent on their position within third-order transgressive or highstand systems tracts. An upper flow unit comprises thick (up to 40 m) fourth-order cycles within a third-order transgressive systems tract. These fourth-order cycles contain large scale, clinoformal bedforms, can be correlated for up to 6 km, and are the main pathways of injected gas through the Upper Serpukhovian reservoir. A lower flow unit comprises fourth-order cycles (within a third-order highstand systems tract) that are thinner, and less laterally continuous, than those in the upper flow unit, and contribute far less to horizontal gas flow. Pressure data indicate that vertical permeability baffles are developed at the third-order sequence boundary that separates the two flow units (linked to diagenetic alteration during prolonged subaerial exposure), and also within the low porosity, mud-rich, transgressive bases of thick fourth-order cycles of the upper flow unit, that formed during periods of increased accommodation space across the platform top during third-order transgression. The scale at which this data set is observed is critical to understanding the dynamic behaviour of the reservoir. Relatively small differences in the composition of microfacies of cycles within both flow units do not explain the variations in dynamic data. This variation is better explained within the broader context of the sequence stratigraphic framework as defined from analysis of cycle stacking patterns. Thicker high frequency cycles deposited in increased accommodation space on the platform top during third-order transgressive intervals are key to developing thick intervals of net pay.

Temporal dynamics of coral and algal cover and their potential drivers on a coral reef of Gorgona Island, Colombia (Eastern Tropical Pacific)

Los arrecifes coralinos son ecosistemas altamente dinámicos, a menudo afectados por diversas perturbaciones naturales. Sin embargo, descensos dramáticos en la cobertura coralina durante décadas recientes llevan a preguntar si tales descensos han sido causados por crecientes impactos antropogénicos. Con este fin, la dinámica de la cobertura de coral y algas se estudió a partir de datos recolectados entre 1998-2014 de 20 transectos fijos de 10 m a dos profundidades en dos sitios de La Azufrada, un arrecife coralino libre de impactos humanos locales, situado en Isla Gorgona, Colombia. La cobertura coralina disminuyó de 66,9 % en 1998 a 39,4 % en 2008, pero luego aumentó a 50,7 % en 2014. La cobertura de algas carnosas y de césped fue baja entre 1998-2004, aumentó a 49,5 % en 2007 y luego disminuyó a < 33,0 %. Las algas coralinas incrustantes fueron las más escasas hasta 2009, cuando aumentaron al disminuir las algas carnosas y de césped. La variación temporal fue diferente entre profundidades con las áreas someras exhibiendo los mayores descensos en la cobertura coralina hasta 2008, pero una recuperación significativa desde entonces. En contraste, la cobertura coralina en áreas profundas disminuyó sólo ligeramente. La exposición subaérea prolongada de los corales durante mareas bajas extremas parece impulsar un ciclo de perturbación y recuperación de los corales. El crecimiento de los corales hace que el arrecife sea más propenso a la exposición subaérea, después de lo cual los corales se blanquean, mueren y son colonizados por algas filamentosas y carnosas. Los erizos de mar y los peces herbívoros aumentan su abundancia en respuesta al aumento de la cobertura de algas y controlan el crecimiento de algas, dejando el sustrato cubierto por algas coralinas incrustantes que lo adecúan para el asentamiento de larvas de coral producidas sexualmente. La fragmentación por agentes físicos y biológicos (como los coralívoros) incrementa el reclutamiento de coral y el arrecife entra en una fase de recuperación coralina. Aunque este arrecife es resiliente a la exposición subaérea porque está protegido de perturbaciones antropogénicas, las crecientes amenazas del cambio climático global pueden comprometer su futura resiliencia. © 2017. Acad. Colomb. Cienc. Ex. Fis. Nat.

Unravelling the origin of a Paleogene unconformity in the Latium-Abruzzi carbonate succession: A shaved platform

The Latium-Abruzzi carbonate platform constitutes the sedimentary record from the Triassic to the late Miocene of the southern Tethyan margin outcropping in the Apennines. This sedimentary succession is punctuated by an important hiatus between the end of the Cretaceous and the early Miocene. This Paleogene hiatus has long been interpreted as the result of prolonged subaerial exposure. However, no evidence of such exposure has ever been documented on the Latium-Abruzzi platform. This is contrary to the coeval and adjacent Apulia and Lessini carbonate platforms where a broadly-developed paleokarst system formed between the Eocene and the early Miocene. The assumption that the Miocene marine transgression deeply eroded a karstified Cretaceous substrate on the Latium-Abruzzi platform is not supported because this surface appears in the field as a perfectly flat, bioeroded paraconformable surface. In this work, a marine origin is proposed for this unconformity. From late Paleocene to early Miocene, the Latium-Abruzzi carbonate platform was a shaved isolated platform exposed to wave action in the middle of the proto-Mediterranean area. Bioclastic sediment accumulated during transgressive and sea-level highstand phases, whereas in the following falling and lowstand stages, sediment was eroded as the seafloor came into the zone of wave abrasion. The sediment eroded from the platform was shed into the basin where it produced coarse bioclastic intercalations in the hemipelagites of the Scaglia Formation. At the end of the Oligocene, when the adjacent basin was filled, a small ramp was established in the transitional zone between the platform and the basin. There, cross-bedded carbonates accumulated in middle to outer ramp environments. The inner ramp remained within the zone of wave abrasion such that sediment generated there was washed out into the middle and outer ramp environments. Finally, in the early Burdigalian, the Mediterranean progressively evolved into closed sea during the initial stages of the closure of Indo-Pacific connection. The reduction of the wave-base depth, typical of an enclosed sea, created increased accommodation space allowing sediments to accumulate and form the complete inner to outer ramp facies belts of the Miocene Latium-Abruzzi ramp.

Diagenesis of Miocene, incised valley-filling limestones; Provence, Southern France

The Cenozoic of southeastern France is characterized by a series of incised valleys that were filled by a succession of marine carbonate and then siliciclastic sediments culminating in the modern Rhone River depositional system. The earliest of these paleovalleys (Miocene — Burdigalian) is located in the Pernes Hills between the towns of Saumane and Venasque. It was filled by a succession of marine carbonates in the form of two third-order stratigraphic sequences (S1 and S2) and three fourth-order subsequences (S1a, S1b and S1c). The deposits are heterozoan throughout, composed of echinoids, bryozoans, coralline algae, mollusks, and benthic foraminifers. They comprise a succession of spectacular cross-bedded calcarenites that accumulated in the seaward part of a tide-dominated estuary. Diagenesis is interpreted to have taken place in four stages: 1) minor synsedimentary precipitation of inclusion-rich carbonate cements, 2) shallow burial physical and chemical compaction, 3) subaerial exposure and widespread precipitation of clear, zoned, epitaxial and isopachous, followed by subsequent clear, unzoned, calcite cements and, 4) prolonged subaerial exposure (middle Miocene to Holocene), that involved dissolution, karstification, and precipitation of minor clear and locally pendant calcite cement. The rocks were essentially uncemented during shallow burial only to be well lithified during the early phases of subsequent telogenesis. The main controls on such lithification are interpreted to have been: 1) the dissolution of minor aragonite biofragments and precipitation of some LMC cement, 2) the abundance of echinoid particle nuclei for epitaxial cement nucleation, and 3) increasing rainfall together with regional tectonic uplift to the east that resulted in increased subsurface water flow. This study not only emphasizes the variable paragenesis of calcite-rich, heterozoan carbonates but also highlights the utility of these Cenozoic limestones with extant components as models for the diagenesis of older Paleozoic limestones that formed in shallow calcite seas.

Sedimentary facies analysis of a high-frequency, small-scale, peritidal carbonate sequence in the Lower Jurassic of the Tripolis carbonate unit (central western Crete, Greece): Long-lasting emergence and fossil laminar dolocretes horizons

The study examines a Lower Jurassic (late Liassic) cyclic lagoonal–peritidal stratigraphic unit outcropping in central western Crete (Tripolis unit), which corresponds to the eastern (internal) part of the mainland Gavrovo-Tripolis platform, the most significant external platform of the Hellenides. The studied Tripolis carbonate sequence consists of meter-scale, shallowing-upward successions of restricted inner-carbonate platform facies, including cyclically repeated subtidal, intertidal and supratidal facies, that are separated by erosion surfaces (elementary cycles). Each cycle starts with relatively open-marine facies, which are overlain by shallower, more restricted facies (tidal flat progradation). The lithofacies association includes dolomitic intraclastic–peloidal–bioclastic wackestones–packstones/floatstones and grainstones/rudstones dominated by a restricted shallow-marine fauna (bivalves, gastropods, ostracods and seldom benthic foraminifers), representing a shallow subtidal to intertidal, moderately high-energy environment within an inner-platform setting (peritidal environment to restricted lagoon). This lithofacies association has been intermittently subaerially exposed and has undergone diagenetic processes in an inter- or supratidal environment, exhibiting features of vadose diagenesis and pedogenesis due to long-lasting exposure along certain horizons. The peritidal facies are capped by dolocretes controlled by root-activities (laminar dolocretes, peloidal–pisoid dolocretes and massive dolocretes), marking the end of each depositional cycle, and, thus, distinguishing the successive episodes of a prolonged subaerial exposure period and birth of paleosol horizons. Dolocretes consist a diagenetic facies, characterized by several vadose and pedogenic fabrics, including fenestral cavities with geopetal structures, “flower spar” to blocky sparry cement in primary pores, micritic coatings, crudely pelleted walls, alveolar-septal texture, infills of rootlet moulds, inter-granular micritic bridges, meniscus cement, sinuous desiccation cracks, circum-granular cracking and in-situ brecciation. The stratigraphic distribution of the cyclothems, suggests relative sea-level control on the peritidal cyclicity controlled by uniform tectonic subsidence and eustasy (allocyclic processes), in an inner- ramp-to-shelf carbonate setting with tidal flat and restricted lagoon depositional environment. However, a combination of allocyclic with autocyclic processes controlling accommodation space and sediment accumulation, should be also taken in consideration. The studied high-frequency cycles are interpreted to have been formed during the falling stage and correspond to lowstand systems tract (LST) sediments, which consists part of a regressive peak of the Lower/Middle Jurassic Transgressive–Regressive Facies Low Frequency Cycle (2nd order). Due to the absence of massive evaporites, dolomitization has been attributed to reflux of slightly increased-salinity marine fluids (i.e. penesaline), that are driven into the underlying lagoonal–peritidal carbonate sediments during periods of long term sea-level fall in a marginal marine setting.

Peritidal carbonate cycles induced by carbonate productivity variations: A conceptual model for an isolated Early Triassic greenhouse platform in South China

Eustasy has commonly been invoked to explain peritidal carbonate cyclicity, but is difficult to explain cycles formed in a greenhouse climate when eustasy is minimal. We propose that peritidal cycles on an Early Triassic isolated carbonate platform in Guizhou, South China, were formed by hierarchical carbonate productivity variations. Most of the 149 shallowing-upward cycles are typically terminated by flooding over intertidal facies and contain rare supratidal facies and no prolonged subaerial exposure. Low-diversity benthos in the platform interior during the post-end-Permian biotic recovery were sensitive to environmental perturbations, which caused variations in benthic sediment productivity in the subtidal carbonate factory. The perturbations may be driven by changes in salinity and degree of eutrophication, or repeated platform mini-drowning by anoxic and/or CO2-charged deep water upwelled onto the banktop. They were modulated by Milankovitch orbitally-driven climatic and oceanographic factors as suggested by the hierarchical stacking pattern and spectral signals of these cycles. A one-dimensional conceptual model shows that hierarchical productivity variations alone may generate hierarchical peritidal carbonate cycles under conditions of constant subsidence and no sea-level fluctuation.

On the peritidal cycles and their diagenetic evolution in the Lower Jurassic carbonates of the Calcare Massiccio Formation (Central Apennines)

Abstract This paper shows the environmental changes and high-frequency cyclicity recorded by Lower Jurassic shallow-water carbonates known as the Calcare Massiccio Formation which crop out in the central Apennines of Italy. Three types of sedimentary cycle bounded by subaerial erosion have been recognized: Type I consists of a shallowing upward cycle with oncoidal floatstones to rudstones passing gradationally up into peloidal packstone alternating with cryptoalgal laminites and often bounded by desiccation cracks and pisolitic-peloidal wackestones indicating a period of subaerial exposure. Type II shows a symmetrical trend in terms of facies arrangement with peloidal packstones and cryptoalgal laminites present both at the base and in the upper portion of the cycle, separated by oncoidal floatstones to rudstones. Type III displays a shallowing upward trend with an initial erosion surface overlain by oncoidal floatstones to rudstones that, in turn, are capped by pisolitic-peloidal wackestones and desiccation sheet cracks. Sheet cracks at the top of cycles formed during the initial phase of subaerial exposure were successively enlarged by dissolution during prolonged subaerial exposure. The following sea-level fall produced dissolution cavities in subtidal facies, while the successive sea-level rise resulted in the precipitation of marine cements in dissolution cavities. Spectral analysis revealed six peaks, five of which are consistent with orbital cycles. While a tectonic control cannot be disregarded, the main signal recorded by the sedimentary succession points toward a main control related to orbital forcing. High frequency sea-level fluctuations also controlled diagenetic processes.

Subaerial speleothems and deep karst in central Sweden linked to Hirnantian glaciations

The limestones of the upper Katian Boda mud mounds (Ordovician) of the Siljan district in central Sweden are deeply fractured. The fissures were partly synsedimentary and are often lined with stromatolite-like crusts. These crusts thus far are the only known subaerial Ordovician speleothems. They reach depths of up to 30 m below the former mound top. Macroscopically the crusts form decimetre-sized, cone-shaped domal aggregates, stalactites and stalagmites. Microfabric and morphology identify them as microbially mediated speleothems in a dark environment. Combined Sr and C isotope values indicate a formation of the speleothems from meteoric waters without influence of a significant soil horizon. For the first time the age of the speleothems can be precisely constrained by δ 13 C whole-rock and brachiopod shell isotope data to the mid-Hirnantian. Repeated and/or prolonged subaerial exposure of the Boda mud mounds during the Hirnantian is evident from karst surfaces and early cements in the mound capping carbonates. The speleothems and the karst surfaces record an estimated sea-level fall in the range of 80–130 m within the time window of the Hirnantian Isotopic Carbon Excursion. This massive regression coincides with maximum ice sheet extent inferred from sections in West Gondwana. Supplementary material: 87 Sr/ 86 Sr isotope ratio of selected brachiopod shells and results of Energy-dispersive-X-ray spectroscopy are available from http://www.geolsoc.org.uk/SUP18809

Late Palaeozoic lithostratigraphy of the Andean Precordillera revisited (San Juan Province, Argentina)

New data on some neopalaeozoic lithostratigraphic units of the central sector of the Argentinean Precordillera are presented. The western Precordillera displays occasional outcrops of siliciclastic rocks of the Late Palaeozoic. These rocks are separated by two main unconformi­ties related to the development of the Chanic (Upper Devonian-early Carboniferous) and Gondwanan (late Carboniferous-early Permian) orogens. A major unconformity marks the beginning of the Andean cycle in middle Permian times. The Del Raton Formation (800 m) in the studied area is made up of conglomerates and can be divided into two units. The basal part (Lower Member) was deposited in a fjord environment. The Upper Member was generated by an alluvial system that was mainly dominated by upper flow-regime plane beds, whereas its uppermost part accumulated in a fan delta setting. The clast provenance and the main palaeocurrents suggest the existence of a significant topographic high to the W and NW of the present outcrops. The El Planchon Formation (1400 m) is mainly composed of shales and sandstones as a result of sedimentation in a deep-sea fan environment. These lutite-dominated materials graded laterally north­wards into conglomerates in a glacial setting. The Del Raton and El Planchon formations of the Lower Carboniferous correspond to the synorogenic Chanic sedimentation (Chanic foreland basin). Thereafter, the stratigraphic succession underwent deformation which led to its prolonged subaerial exposure and rubefaction. In Permian times, a marine transgression gave rise to the Del Salto Formation (600 m), which is constituted by beach deposits with some aeolian reworking and which lies unconformably over the Lower Carboniferous rocks. Conglomerates (Quebrada del Alumbre Formation, 100 m) produced by an alluvial system prograded episodically into coastal areas. The Escombrera Formation (350 m) is composed of beach deposits prograding seawards. This formation probably represents the end of the Palaeozoic sedimentary record. Thereafter, a playa-lake and an alluvial fan system were generated (Quebrada de la Arena Formation). This unit (750 m) was the last infill of the Gondwanan retroarc foreland, the total thickness of which was about 4000 m.

Sedimentary and foraminiferal evidence of the 2011 Tōhoku-oki tsunami on the Sendai coastal plain, Japan

The 2011 Tōhoku-oki megathrust earthquake (Mw 9.0) generated a tsunami that reached the Sendai coastal plain with flow heights of ~2 to 11m above TP (Tokyo Peil). We examined the tsunami deposit exposed in 14 shallow trenches along a ~4.5‐km transect perpendicular to the coast. We primarily document the stratigraphical, sedimentological, foraminiferal and geochemical characteristics of the Tōhoku-oki tsunami deposit and perform a preliminary comparison with sediments deposited by the Jōgan tsunami of A.D. 869.In the coastal forest and rice fields inundated by the Tōhoku-oki tsunami, a poorly sorted, dark brown soil is buried by a poorly sorted, brown, medium-grained sand deposit. In some trenches located more than 1.2km inland, the sand is capped by a thin muddy-sand layer. The tsunami deposit, although highly variable in thickness, is generally thickest (25cm) near the coastal dune and thins to less than 5mm at ~4.5km inland. The tsunami deposit was discriminated from the underlying soil by the appearance of recent and fossil foraminifera and a pronounced increase in grain size that fined upward and landward. The recent foraminifera preserved in the sandy facies of the deposit are rare and showed evidence of prolonged subaerial exposure (e.g. pitting, corrosion, fragmentation). Recent foraminifera likely originated from coastal dune and beach sediments that were breached by the tsunami. Calcified and sediment in-filled, fossil foraminifera are abundant and were eroded from sedimentary units and transported by fluvial or wave activity to Sendai Bay. Trends associated with test size (e.g. decreasing concentration of large test sizes with distance inland) are in agreement with grain size data. At two locations a decrease in total organic carbon and an increase in δ13C were found in the tsunami sand compared with the underlying soil, supporting a beach to intertidal origin for the upper unit.

Taphonomy of an Upper Triassic vertebrate bonebed: A new rhynchosaur (Reptilia; Archosauromorpha) accumulation from India

The Upper Triassic Tiki Formation of India is a rich storehouse of varied vertebrate fossil assemblages. So far, there is no information on taphonomic signatures of the Tiki vertebrate assemblages in comparison to that of other fossiliferous horizons of India. We report a new, low diversity, mono-dominant, multitaxic vertebrate accumulation where rhynchosaurs constitute the dominant component. The formation of the rhynchosaur bonebed is attributed to biological aggregation with a hydraulic overprint resulting in a mixed-origin concentration. Other taxa include rauisuchid, phytosaur, small indeterminate archosaur and cynodonts. Taphonomic study based on 617 skeletal specimens of rhynchosaurs collected from nine sites within an area of about 250×217m2 shows that most of the specimens are disarticulated and disassociated but in close spatial proximity to one another; some are associated specimens and few are articulated. About 13 to 20 individuals of rhynchosaur at different ontogenetic stages are estimated from the specimens collected, suggesting gregarious behavior, possibly herding. These specimens show varying degree of weathering, breakage, encrustation, abrasion and deformation. The bonebed is preserved within the Tiki red mudstone unit and is found in association with paleosol profiles, suggesting prolonged subaerial exposure. Spatial distribution and relative bone frequencies show differential susceptibility of the skeletal specimens to fluvial transport. 55.4% of the collected skeletal specimens belonged to Voorhies Group I, whereas 12.4% and 24% constituted Voorhies Groups II and III respectively, and 8.2% of the collected specimens belonged to the intermediate Groups I and II. It appears that the animals concentrated in the vicinity of the water sources during prolonged period of aridity and died possibly during high seasonal rainfall that resulted in a major flood event. Subsequently, the soft tissues decomposed, and the skeletons suffered prolonged subaerial exposure when the water receded leading to disarticulation and fragmentation followed by minor dispersion by low velocity water currents. This resulted in segregation of skeletal specimens, which were gradually covered by mud deposited during later flooding events. Based on the known flora and fauna, the Tiki Upper Triassic ecosystem is reconstructed for the first time. In the aquatic ecosystem, the metoposaurid labyrinthodonts occupied the top of the food pyramid together with the semi-aquatic parasuchids, which occupied an ecological niche similar to that of the present day crocodilians. The abundance of herbivorous rhynchosaurs at the base with few large and carnivorous rauisuchids and parasuchids at the top suggest a trophic structure similar to that of a modern day terrestrial ecosystem.

Investigating Variation in the Prevalence of Weathering in Faunal Assemblages in the UK: A Multivariate Statistical Approach

AbstractThis article presents an exploratory multivariate statistical approach to gaining a more comprehensive understanding of variation in sub‐aerial bone weathering in a British context. Weathering is one of the most common taphonomic modifications and provides a crucial line of evidence for reconstructing the taphonomic trajectories of faunal assemblages and archaeological deposits. It provides clear evidence for prolonged subaerial exposure, either prior to deposition in a context or as a result of later disturbance. In combination with other taphonomic indices such as gnawing, trampling, abrasion and fracture patterns, weathering can be used to reconstruct depositional histories and to investigate the structured treatment of different body parts or taxa. As a broad range of factors affect the prevalence and severity of weathering, weathering patterns can rarely be interpreted at face value indeed many variables such as predepositional microenvironment cannot be traced archaeologically. Other contributory factors pertaining to the structural properties of elements and taxa can be discerned and must be taken into account in interpreting weathering signatures although disagreement exists regarding which variables are most important in mediating weathering. In addition, for zooarchaeologists to interpret modification patterns, it is necessary for elements and taxa that are most likely to be affected by weathering to be defined. Deposits that are dominated by susceptible classes of remains are likely to exhibit greater modification than deposits that are not, even if depositional histories were similar. Through a combination of classification tree and ordinal regression analysis, this article identifies which archaeologically recoverable variables explain the greatest variance in weathering and which anatomical elements and taxa are most likely to be affected in archaeological deposits in the UK. Copyright © 2011 John Wiley & Sons, Ltd.

Carbon and oxygen isotope evidence for high-frequency (10 4–10 5 yr) and My-scale glacio-eustasy in Middle Pennsylvanian cyclic carbonates (Gray Mesa Formation), central New Mexico

We combine cyclo- and sequence stratigraphy along with whole rock δ 13C and conodont apatite δ 18O analysis to document high-frequency (10 4–10 5 yr) and My-scale sea-level changes for the Middle Pennsylvanian (Desmoinesian or Moscovian) Gray Mesa Formation of central New Mexico. Approximately 75 subtidal cycles (1–8 m) are grouped into 4 1/2 My-scale depositional sequences (40–80 m). About 50% of the cycles show evidence of prolonged subaerial exposure at cycle tops with the development of calcretes, diagenetic mottling, and regolith intraclasts. High-resolution δ 13C analysis of whole rock limestones across nine of the cycles indicates that the cycle tops were diagenetically altered by isotopically light, meteoric fluids during sea-level fall and lowstand. These δ 13C trends support the interpretation that high-frequency sea-level changes were responsible for cycle development. Conodont apatite δ 18O values from sampled cycles indicate that the high-frequency sea-level changes were driven by glacio-eustasy combined with changes in surface seawater temperature (SST). δ 18O values from conodont apatite, spanning parts of three depositional sequences indicate that My-scale glacio-eustasy and/or SST changes controlled sequence development. δ 18O shifts indicate that the magnitudes of 10 4–10 5 yr glacio-eustasy were between ∼ 55 and 170 + m combined with tropical SST changes of ∼ 1.5°–6 °C. Calculated My-scale glacio-eustatic oscillations were between ∼ 60 and 140 m with SST changes of < 3.5 °C. The most plausible driver for the My-scale paleoclimate changes is long-period obliquity (∼ 1.2 My) variations. These calculated high-frequency, glacio-eustatic values are similar or greater than Pleistocene values, and lie within the range estimated for other Middle Pennsylvanian successions using a variety of independent eustatic proxies. The similarity in range of magnitudes between high-frequency and My-scale sea-level changes combined with the large differences in magnitudes between individual high-frequency sea-level oscillations helps explain the lack of systematic cycle-stacking patterns within these Pennsylvanian icehouse sequences.

Effects of the Permian–Triassic boundary on reservoir characteristics of the South Pars gas field, Persian Gulf

AbstractThe Permian–Triassic boundary (PTB) is a world‐wide event characterized by the most extensive mass extinction in the history of life. In the Persian Gulf, the rock record of this time interval host one of the most important hydrocarbon reserves in the world: the South Pars Gas Field and its southern extension, the North Dome (or North Field). These carbonate and evaporite successions were sampled in eight wells for petrographic, geochemical and porosity–permeability studies. An important characteristic of the Dalan and Kangan formations is the centimetre‐scale lithological heterogeneities caused by facies changes and diagenetic imprints that led to the compartmentalization of these reservoirs. These Permian–Triassic (P‐T) sediments were deposited in a shallow marine homoclinal ramp. The PTB in this hydrocarbon field is represented by a reworked coarse‐grained intraclastic/bioclastic grainstone facies deposited during a marine transgression. Prolonged subaerial exposure in the P‐T transition caused hypersaline and meteoric diagenesis, including extensive cementation, dolomitization and some dissolution, influencing reservoir characteristics of bordering units. Both δ18O and δ13C values in this succession mirror worldwide excursions typical of other P‐T sections, with some variations due to diagenetic alterations. A pronounced decline in 87Sr/86Sr values, reflective of global seawater geochemistry for most of the Permian is evident in our data. Reservoir quality declines through the late Permian, as a result of facies change and diagenesis. The Late Permian is succeeded by a Triassic transgressive facies and decline in reservoir quality. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Sequence stratigraphic controls on early-diagenetic carbonate cementation of shallow marine clastic sediments (the Devonian Zakeen Formation, southern Zagros, Iran)

Distribution pattern of carbonate cements of the Middle to Upper Devonian Zakeen Formation of southern Zagros, suggests a close relationship between relative sea-level changes and diagenetic processes. Ferroan dolomite cementation is closely linked to genetically related facies (parasequences, systems tracts, sequences) and/or major stratal surfaces (transgressive and maximum flooding surfaces and sequence boundaries). The cements are systematically distributed within the lowstand, transgressive and highstand systems tracts. Eogenetic ferroan dolomite cement (δ18O from 1.06 to −6.00‰ VPDB and δ13C from −1.23 to −4.94‰ VPDB) was developed in the form of: 1) ferroan dolomite cemented conglomerates in LST estuarine environments, 2) laterally extensive ferroan dolomite cemented sandstones in LST estuarine and TST foreshore-shoreface environments and along maximum flooding surfaces and 3) ferroan dolomite cemented nodules in LST, TST and HST foreshore-shoreface environments. Prolonged subaerial exposure of the strata, resulted from epeirogenetic movements of the Hercynian Orogeny, is considered to be responsible for cement and grains leaching and porosity development in sandstones underlying the major sequence boundary at the top of the Zakeen Formation.