Sedimentary Condensation Research Articles

Unraveling marine phosphogenesis along the Miocene coast of Peru: Origin and sedimentological significance of the Pisco Formation phosphorites

Phosphorite deposits form through phosphogenesis, a mechanism of diffusion and concentration of interstitial phosphate in sediments leading to Ca-phosphate mineral precipitation in a favorable sedimentary regime at the seafloor. Stretching along the Peruvian coast, the East Pisco Basin is known to host phosphorite beds of Miocene age. In this study, we investigate the petrographic, mineralogical and chemical composition of the Miocene phosphorite layers of the East Pisco Basin, and discuss the phosphogenetic mechanism(s) and sedimentary dynamics that led to the genesis and accumulation of P-rich deposits. Phosphorite samples were collected from layers overlaying stratigraphic unconformities within the Pisco Formation and subsequently analyzed through multiple methods: optical microscopy, XRD, XRF, SEM-EDS, EPMA, ICP-MS and LA-ICP-MS. Our results indicate that these layers consist of phosphatic clasts and nodules along with dolomite clasts, basement boulders, shark teeth, mollusk molds and vertebrate bone fragments. The P-rich clasts are mainly composed of phosphatic intraclasts and small phosphatic nodules. They occur associated with minor amounts of dolomite clasts coated by Fe-oxyhydroxides and laminites. Clasts and nodules are cemented by both early and late diagenetic cements, such as dolomite, calcite, silica, gypsum/anhydrite and baryte. Our observations suggest that the Pisco phosphorite deposits are transgressive lags laying on ravinement surfaces. They formed just below the sediment-water boundary during the early transgression phases, when the porewaters were enriched in phosphorus due to the abundance of organic-rich sediments and the microbial activity, as typical of upwelling settings. These deposits formed in shallow marine conditions, characterized by high biological productivity and low net sedimentation rates, with suboxic conditions at and beneath the sediment-water interface. The concentration of phosphate-coated clasts was then favored by a sedimentary condensation mechanism of dynamic bypassing. Overall, this study provides new insights into the genesis of phosphorite layers in the East Pisco Basin and highlights the importance of understanding the complex interplay between authigenic and sedimentological processes in the formation of these economically and geologically relevant sedimentary rocks.

Hyperthermal events recorded in the Palaeogene carbonate sequence of southern Gulf of Mexico—Santa Elena borehole, Yucatan Peninsula

Based on geochemical and magnetic susceptibility analyses, maximum warming events (hyperthermal) in the Palaeogene are recognized in the carbonate rocks of the Santa Elena borehole (SEB) in the Yucatan Peninsula, the Palaeocene‐Eocene Thermal Maximum (PETM) and the Eocene Thermal Maximum 2 (ETM‐2). The site records the continental shelf marine response during these global events. Major and trace element records (Al, Ba, Ca, Fe, K, Si, and Ti), Ca/Fe, Si/Al ratios, and magnetic susceptibility are used as proxies of terrigenous input, and Ba/Al ratio as a proxy of palaeoproductivity. The hyperthermal events are characterized by the dilution and/or dissolution of biogenic carbonates. The high input of terrigenous materials is linked to extreme precipitation, common during these warming events. Our records suggest a decrease in palaeoproductivity associated with a nutrients gradient in a shallow ecosystem, with deeper thermocline and stratified column water. The PETM is characterized by high eustatic sea‐level conditions, with a high contribution of detrital material, indicating sedimentary condensation and marked increase in precipitation, calcite dilution and/or dissolution, and low productivity. The ETM‐2 event is less extreme than the PETM, with high precipitation, although evaporation could also play an important role, as evidenced by the presence of evaporites in this interval. These changes might affect the higher trophic levels of the shelf sea ecosystem, declining productivity. The study contributes to our understanding of the global and regional effects of these past warming events and the future climate change.

Phosphorites, glass ramps and carbonate factories: The evolution of an epicontinental sea and a late Palaeozoic upwelling system (Phosphoria Rock Complex)

AbstractThe Permian Phosphoria Rock Complex of the western USA contains an enigmatic assemblage of bioelemental rocks (i.e. phosphorites and cherts) that accumulated in a depositional system with no modern analogue. This study utilizes detailed sedimentological, stratigraphic and petrographic examination to evaluate the genetic relations of phosphorites, spiculitic chert and carbonates of the Ervay cycle (depositional sequence) and propose a unified oceanographic model for their deposition. The Ervay cycle contains three marine and one terrestrial facies association, each of which composes the bulk of a single lithostratigraphic unit. The marine facies associations include: (i) granular phosphorites (Retort Member); (ii) spiculitic cherty dolostones (Tosi Member); and (iii) marine to peritidal carbonates (Ervay Member). Red beds and intercalated gypsum (Goose Egg Formation) accumulated in the vast desert adjacent to the sea. The three marine members are chronostratigraphically distinct, successive and conformably stacked. They are not coeval facies belts. They reflect the progressive evolution of the epicontinental sea from the location of: (i) authigenic phosphogenesis (lowstand to transgression); to (ii) a glass ramp with biosiliceous (sponge) deposition (transgression); to (iii) a carbonate ramp (regression). This succession of switching biochemical sediment factories records the evolution of sea‐level, nutrient supply, upwelling, oxygenation and dissolved Si. Intense upwelling, potentially coupled with aeolian input, led to sedimentary condensation and phosphogenesis. Decreased upwelling intensity during transgression increased oxygenation sufficiently for a siliceous sponge benthos. Sponges were favoured over biocalcifiers due to elevated dissolved silica and a low carbonate saturation state. The cessation of sponge dominance and transition to a carbonate ramp occurred due to decreasing upwelling intensity, Si drawdown and an increased carbonate saturation state. These results provide insight into the role of Si loading in faunal turnover on glass ramps and highlight how differences in dissolved Si utilizers in pre‐Cretaceous versus post‐Cretaceous upwelling systems influence the resultant deposits.

Turbidite‐induced re‐oxygenation episodes of the sediment‐water interface in a diverticulum of the Tethys Ocean during the Oceanic Anoxic Event 1a: The French Vocontian Basin

AbstractWidespread anoxic events affected the Tethys Ocean during the Mesozoic. The Ocean Anoxic Event 1a (Early Aptian), expressed as the Selli Level or Goguel Level (GL) in European basins. The GL was deposited in the French Vocontian Basin, a semi‐enclosed basin connected to the Tethys Ocean. This study presents an integrated approach (Rock Eval, clay minerals, grain‐size, inorganic geochemistry and molecular biomarkers), applied to four sections in the basin distributed along a proximal‐distal transect. This study shows that the GL was perturbed by turbidites. In particular, the results demonstrate: (a) a homogeneous redox status of the basin that experienced oxic to suboxic conditions, according to trace element distributions and (b) low organic matter contents (total organic carbon ca 1 wt%) in the three sections where turbiditic deposits are observed. The distal, condensed section exhibits the highest organic matter contents (total organic carbon >3 wt%). In addition, the presence of gammacerane and isorenieratene derivatives in the distal sections suggests that the water‐column was intermittently stratified, with hypoxia developing in the photic zone. This stratification did not result from strong surface productivity but more likely from: (a) limited renewal of deep water in the basin; (b) reputedly high surface‐water palaeotemperatures during the Early Aptian; and (c) the influx of waters, possibly depleted in free oxygen and in some dissolved trace elements, into the basin. The turbiditic inputs, in addition to organic matter dilution in the sediments and a brief rupture of the water‐column stratification in the proximal areas of the basin, ventilated the sea floor and more specifically re‐oxidized the sediment‐water interface as well as underlying sediments. Such episodes of benthic re‐oxygenation could have altered the long‐term palaeoredox record, even in the distal sections where reducing conditions prevailed during deposition. In the area deprived of turbiditic input, sedimentary condensation, coupled with low oxygen conditions, furthered organic matter preservation and concentration.

Contourite facies model: Improving contourite characterization based on the ichnological analysis

The calcareous contourites from the Late Oligocene and Early Miocene in the Petra Tou Romiou type section (southern Cyprus) have been studied in detail, being regarded as outstanding examples of fossil bioclastic contourites and representative of the standard facies model of contourite bigradational sequence. These bigradational sequences of sand-dominated contourites consists of whitish calcarenite beds with wavy layering in the middle portion of the sequence. Gradual alternations of whitish and greenish calcilutites appear above and below these calcarenites. The internal wavy layering is composed by compacted interlayers and non-compacted interstratified layers. In the present study, an in-depth ichnological analysis of the bigradational sequence has been conducted, focusing on ichnological features such as length, shape, diameter, and orientation of individual burrow segments, the configuration of burrow systems, as well as external features, fill material, and finally taphonomy. Trace-fossil assemblages differ, containing Planolites isp., Chondrites isp., and ?Thalassinoides isp. at the compacted interlayers, and non-compacted layers bearing only Planolites structures. The compacted interlayer structures are flattened and elliptical, containing similar fill material as the calcilutite host sediment, though lighter in color. In the non-compacted layers, Planolites are near un-deformed, being cylindrical and tubular, with circular to sub-circular cross-sections. These are located mainly at the base or occupy the entire layer, locally showing longitudinal striae (Planolites reinecki Książiewicz), and filled with calcarenite material. The different trace-fossil assemblages in the compacted layers and non-compacted interlayers, varying in fill material, evidence an original primary differentiation, presumably indicative of fluctuating paleoenvironmental conditions. In consecutive non-compacted layers, the distribution of Planolites is separated by compacted interlayers. This indicates that the middle portion of the contourite type facies, with internal wavy layering, underwent a multiphase deposition. In the non-compacted layers, the almost undeformed Planolites, marked by striae, is consistent with a higher substrate consistency, which is interpreted as being associated with sedimentary condensation or omission, immediately before the next compacted interlayers were deposited. This evidence clearly supports the contention that sedimentary processes governing contourite deposition were intermittent rather than continuous, as traditionally proposed. These findings raise basic questions regarding the bottom current processes, and consequent bedform development in these deep-marine environments.

The Impact of Hydrodynamics, Authigenesis, and Basin Morphology On Sediment Accumulation In An Upwelling Environment: The Miocene Monterey Formation At Shell Beach and Mussel Rock (Pismo and Santa Maria Basins, Central California, U.S.A.)

ABSTRACT: In the sections at Shell Beach (Pismo Basin) and Mussel Rock (Santa Maria Basin)—both along the central California coast—the Miocene Monterey Formation is composed of laminated, biosiliceous, organic-rich and phosphate-rich sediments, which are intercalated with carbonates and volcanogenic deposits. The presence of small- and large-scale scours, conglomerates, gravity-flow deposits, slumps, and condensed intervals reveals the importance of sediment reworking, which is related to the presence of bottom currents, the general morphology of the basins, and recurrent high-energy events of likely seismic origin. The lithologies reflect strong upwelling, enhanced primary productivity, and the presence of an oxygen-minimum zone, which is indicated by ubiquitous lamination, a general absence of bioturbation (with the rare exception of traces related to the gravity-flow displacement of trace makers), high pyrite, Mo, and U concentrations, and molar ratios of total organic carbon and organic phosphorus (TOC/P org ) of up to 2000. Oxygen depletion was particularly severe during the early Langhian, and the late Serravallian and early Tortonian, and favored the preservation of organic matter. Episodes of deposition of organic-rich sediment were followed by longer phases of subdued sediment accumulation and early diagenetic phosphogenesis, which resulted in the formation of a low-porosity, less permeable “lid” in the near-surface sediments that assisted in preserving organic matter. Phosphogenesis occurred throughout the time interval considered (approximately 17.5–6.5 Ma), and phosphate-rich sediments formed especially during periods of sedimentary condensation in the Langhian and earliest Serravallian, and the latest Tortonian and early Messinian. A comparison of temporal trends in the type and quantity of deposited sediments between the two sections at Shell Beach and Mussel Rock, and three sections of the Santa Barbara Basin (El Capitan State Beach, Naples Beach, and Haskell Beach) confirms that towards the top of the Monterey Formation, sediments become enriched in biosilica in all three basins. The transition into this lithology occurred diachronously, during the Serravallian in the Pismo and Santa Maria basins and Tortonian in the Santa Barbara Basin, and was probably related to a diachronous shift to more intense upwelling in all three basins. A phase of important condensation and phosphogenesis terminated Monterey-“type” sedimentation during the latest Tortonian and early Messinian in both the Santa Maria and Santa Barbara basins. The uniform expression of this phase is interpreted as the result of improved connections between the two basins.

Depositional and palaeoenvironmental variation of lower Turonian nearshore facies in the Bohemian Cretaceous Basin, Czech Republic

Dark grey strata belonging to the basal horizons of the Bílá Hora Formation (lower Turonian) were exposed during quarrying at the locality of Plaňany (Bohemian Cretaceous Basin). Based mainly on quarry maps, the early Turonian rocky bottom was reconstructed in the area of about 14,800 m2. Two sedimentologic and palaeoecological settings were recognized in the area. Dark grey deposits form part of the first setting, representing a fill of large and deep depressions on the northern foot of the Plaňany elevation. The second setting with a phosphatic lag is located on the elevated part of the area. Dark grey sedimentation belongs to the UC6a and particularly to the UC6b nannoplankton zones. During the latter zone the dark sedimentation passed upwards into light siltstones. The enrichment of Corg and S, clay minerals with an important kaolinite peak, formation of framboidal pyrite and the enrichment of macrofauna and phosphatic particles are characteristic of the basal portions of the dark deposits. The sulphate reduction zone is suggested for this sedimentary environment. In the associations of phosphatic particles, shark coprolites, faecal pellets and sponge fragments prevail. No phosphatic lag is developed. On the other hand, the phosphatic lag directly overlying the Cenomanian relics is most characteristic of the second setting. This lag is a product of sedimentary condensation, characterized by a long-lasting concentration of phosphatic particles and phosphogenesis, accompanied by encrustation of closely adjacent free rock surfaces by a faunal community with Terebella. Additional biostratigraphic data presently contributed to a proposed correlation of both settings. Micropalaeontological data (foraminifera, palynomorphs, nannoplankton) indicate that the phosphatic lag and basal dark grey deposits may be approximately coeval. The stagnant depositional conditions with only very slow sea-level rise are thought to have lasted for a relatively long period that includes a significant part of the Whiteinella archaeocretacea Zone (lowermost Turonian). In elevated parts, condensation could proceed under conditions of prevailingly weak currents and strong oxidation of organic matter, while decomposition of organic matter was probably very slow and incomplete in depressions below the elevation. The sedimentary condensation in both settings is highlighted by the remarkable formation of abundant glauconite in local deposits.

Not all aragonitic molluscs are missing: taphonomy and significance of a unique shelly lagerstätte from the Jurassic of SW Britain

The Blue Lias Formation at Lyme Regis (Dorset, UK) includes an exceptional pavement of abundant large ammonites that accumulated during a period of profound sedimentary condensation. Ammonites were originally composed of aragonite, an unstable polymorph of calcium carbonate, and such fossils are typically prone to dissolution; the occurrence of a rich association of aragonitic shells in a condensed bed is highly unusual. Aragonite dissolution occurs when pore-water pH is reduced by the oxidization of hydrogen sulphide close to the sediment-water interface. Evidence suggests that, in this case, the oxygen concentrations in the overlying water column were low during deposition. This inhibited the oxidation of sulphides and the associated lowering of pH, allowing aragonite to survive long enough for the shell to be neomorphosed to calcite. The loss of aragonite impacts upon estimates of past biodiversity and carbonate accumulation rates. The preservational model presented here implies that diagenetic loss of aragonite will be greatest in those areas where dysoxic-anoxic sediment lies beneath an oxic waterbody but least where the sediment and overlying water are oxygen depleted. Unfortunately, this implies that preservational bias through aragonite loss will be greatest in those biotopes which are typically most diverse and least where biodiversity is lowest due to oxygen restriction.

Warming, euxinia and sea level rise during the Paleocene–Eocene Thermal Maximum on the Gulf Coastal Plain: implications for ocean oxygenation and nutrient cycling

Abstract. The Paleocene–Eocene Thermal Maximum (PETM, ~ 56 Ma) was a ~ 200 kyr episode of global warming, associated with massive injections of 13C-depleted carbon into the ocean–atmosphere system. Although climate change during the PETM is relatively well constrained, effects on marine oxygen concentrations and nutrient cycling remain largely unclear. We identify the PETM in a sediment core from the US margin of the Gulf of Mexico. Biomarker-based paleotemperature proxies (methylation of branched tetraether–cyclization of branched tetraether (MBT–CBT) and TEX86) indicate that continental air and sea surface temperatures warmed from 27–29 to ~ 35 °C, although variations in the relative abundances of terrestrial and marine biomarkers may have influenced these estimates. Vegetation changes, as recorded from pollen assemblages, support this warming. The PETM is bracketed by two unconformities. It overlies Paleocene silt- and mudstones and is rich in angular (thus in situ produced; autochthonous) glauconite grains, which indicate sedimentary condensation. A drop in the relative abundance of terrestrial organic matter and changes in the dinoflagellate cyst assemblages suggest that rising sea level shifted the deposition of terrigenous material landward. This is consistent with previous findings of eustatic sea level rise during the PETM. Regionally, the attribution of the glauconite-rich unit to the PETM implicates the dating of a primate fossil, argued to represent the oldest North American specimen on record. The biomarker isorenieratene within the PETM indicates that euxinic photic zone conditions developed, likely seasonally, along the Gulf Coastal Plain. A global data compilation indicates that O2 concentrations dropped in all ocean basins in response to warming, hydrological change, and carbon cycle feedbacks. This culminated in (seasonal) anoxia along many continental margins, analogous to modern trends. Seafloor deoxygenation and widespread (seasonal) anoxia likely caused phosphorus regeneration from suboxic and anoxic sediments. We argue that this fueled shelf eutrophication, as widely recorded from microfossil studies, increasing organic carbon burial along many continental margins as a negative feedback to carbon input and global warming. If properly quantified with future work, the PETM offers the opportunity to assess the biogeochemical effects of enhanced phosphorus regeneration, as well as the timescales on which this feedback operates in view of modern and future ocean deoxygenation.

Controls on basal Zechstein (Wuchiapingian) evaporite deposition in SW Poland

The development of basal Zechstein (Wuchiapingian) strata inSW Polandindicates the existence of a diversified relief inherited after the flooding of the pre-existing depression by the transgressing Zechstein sea. The deeper parts of the basin were the place of development of thin basinal Zechstein Limestone showing sedimentary condensation manifested by bored and encrusted grains and thick evaporites (mostly halite), and in shallow parts Zechstein Limestone reefs followed by thinner evaporite sequences (dominated by anhydrite) occur. The analysis of 3D seismic sections showed that instead of three conventionally recognized evaporite units of stratigraphic potential in the PZ1 cycle, five units occur (from the base to the top: Lower Anhydrite, Lower Oldest Halite, Middle Anhydrite, Upper Oldest Halite, Upper Anhydrite). In a particular place their number may vary from two (Lower Anhydrite at the base of the PZ1 cycle and Upper Anhydrite at the top of the PZ1 cycle) to five. There are two complexes of Lower Anhydrite occurring throughout the platform and basinal zones showing deepening-upward (transgressive) trend. The halite sedimentation in the deepest parts of salt basins began shortly after the deposition of the upper Lower Anhydrite complex while in the sulphate platform areas the sulphate deposition lasted still for a long time. The Lower Oldest Halite deposits occur in the depressions. Between the halite basins, anhydrite platforms occur, and the thickness of anhydrite platform deposits is smaller than it is observed in salt basins. The Upper Oldest Halite in turn is recorded above the anhydrite platform. The two halite units represent different phases of development of halite basins. The Lower Oldest Halite basins are related to the pre-Zechstein depressions, although in some cases their syndepositional subsidence was controlled by reactivation, during the deposition of basal Zechstein strata, of former faults. In turn, the Upper Oldest Halite basins used the accommodation space created due to anhydritization of the Lower Anhydrite deposits composed originally of selenitic gypsum. The 3D seismics evidences that the PZ1 evaporites inSW Polandhave been deposited in far more complex and dynamic system than it was assumed before.

Carbon isotope evidence for sedimentary discontinuities in the lower Toarcian of the Lusitanian Basin (Portugal): Sea level change at the onset of the Oceanic Anoxic Event

The Toarcian Oceanic Anoxic Event (T-OAE), Early Jurassic, was an episode of rapid warming and deep perturbation of the carbon cycle, as suggested by the large carbon and oxygen isotope excursions recorded by various carbonate and organic materials of this age. Previous studies have shown that strata deposited immediately below the T-OAE are marked by widespread discontinuities, but their duration and synchronicity remain uncertain. In this study, we use the carbon isotope profiles of twelve sections from the Lusitanian Basin, Portugal, to correlate shallower and deeper settings and identify the durations and possible causes of these discontinuities. The obtained correlation suggests the existence of three major discontinuities. The first occurs close to the Pliensbachian–Toarcian boundary and locally corresponds to an interval of condensation due to rapid transgression. The second and most marked discontinuity occurs in the mid-upper part of the Polymorphum ammonite Zone. Depending on the location in the Lusitanian Basin, this discontinuity is interpreted to reflect either marine erosion (in deeper parts of the basin) or subaerial exposure (in the shallow parts of the basin), both induced by a high-amplitude sea-level fall. In intermediate parts of the basin, this discontinuity is interpreted as a transgressive ravinement surface due to subsequent sea-level rise. This latter transgression induced the formation of a third discontinuity of sedimentary condensation (uppermost Polymorphum Zone) that corresponds to the interval of highest rate of sea-level rise just before the onset of the T-OAE. The comparison of the carbon isotope record of the Lusitanian Basin with that of other European basins indicates that these discontinuities are present on a wide paleogeographical scale within the western Tethys, and correspond to marked changes in seawater temperatures and CO2 levels, implying a control by high-amplitude, likely glacio-eustatic sea-level changes.

The influence of authigenic clay formation on the mineralogy and stable isotopic record of lacustrine carbonates

The mineralogical, compositional and stable isotopic variability of lacustrine carbonates are frequently used as proxies for ancient paleoenvironmental change in continental settings, under the assumption that precipitated carbonates reflect conditions and chemistry of ancient lake waters. In some saline and alkaline lake systems, however, authigenic clay minerals, forming at or near the sediment water interface, are a major sedimentary component. Often these clays are rich in Mg, influencing the geochemical budget of lake waters, and are therefore expected to influence the properties of contemporaneous authigenic carbonate precipitates (which may also contain Mg). This paper documents evidence for a systematic feedback between clay mineral and carbonate authigenesis through multiple precessionally driven, m-scale sedimentary cycles in lacustrine oil-shale deposits of the Eocene Green River Formation from the Uinta Basin (NE Utah). In the studied section, authigenic, Mg-rich, trioctahedral smectite content varies cyclically between 9 and 39wt.%. The highest concentrations occur in oil-shales and calcareous mudstones deposited during high lake level intervals that favored sedimentary condensation, lengthening the time available for clay diagenesis and reducing dilution by other siliciclastic phases. An inverse relation between dolomite percentage of carbonate and trioctahedral smectite abundance suggests the Mg uptake during clay authigenesis provides a first order control on carbonate mineralogy that better explains carbonate mineralogical trends than the possible alternative controls of (1) variable Mg/Ca ratios in lake water and (2) degree of microbial activity in sediments. We also observe that cyclical change in carbonate mineralogy, believed to be induced by clay authigenesis, also causes isotopic covariation between δ13CPDB and δ18OPDB of bulk sediments because of differences in the equilibrium fractionation factors of dolomite and calcite (∼2‰ and ∼2.6%, respectively). This provides an alternative mechanism for the common pattern of isotopic covariation, which is typically attributed to the effect of simultaneous changes in water balance and biological activity on the carbon and oxygen isotopic composition of lake waters. These findings may help improve paleoenvironmental reconstructions based on lacustrine carbonate records by adding to the factors known to influence the mineralogical, compositional and stable isotopic signals recorded by lacustrine carbonates.

Asymmetrical condition of the Bogor Basin (West Jawa, Indonesia) during the Middle Miocene to Pliocene based on taphonomic study of shellbed and its sequence architecture

Examples of lithology, fossils and taphonomic features of shellbeds and intervening less-fossiliferous intervals are presented from two Middle Miocene to Pliocene successions of the Bogor Basin in West Jawa, Indonesia: the Nyalindung Formation in the Sukabumi area which was deposited in the western part of the basin and the Kaliwangu Formation in the Sumedang area, which is the eastern extent of sediments in the basin. As for Miocene to Pliocene 6th order depositional sequences, the two successions contain a variety of shellbeds which are often associated with surfaces or intervals that are characterized by sedimentary condensation, omission or erosion (e.g., sequence boundaries, ravinement surfaces, downlap surfaces and condensed sections). Stratigraphic patterns in the shellbeds tend to be similar and repetitive within a basin with respect to position and age. For example, shellbeds formed in the context of toplap are common only in the Nyalindung Formation, and are characterized by thicker units of trangressive systems tracts (TST) and thin units of highstand systems tracts (HST). In contrast, sedimentary cycles from the eastern part of the basin in the Kaliwangu Formation can be recognized as formed under conditions of marine backlap and downlap, where the TST units are thin but the HST units are thicker. Such contrasting cycle architectures are interpreted to reflect an asymmetrical shelf depositional setting between the western and eastern edges of the Bogor Basin since Miocene to Pliocene time.

Ammonoid taphonomy, palaeoenvironments and sequence stratigraphy at the Bajocian/Bathonian boundary on the Bas Auran area (Subalpine Basin, south‐eastern France)

Bas Auran, in south-eastern France, is the candidate area for Global Boundary Stratotype Section and Point (GSSP) of the base of the Bathonian Stage (Middle Jurassic). In the Bas Auran area, upper Bajocian and lower Bathonian deposits are made up of limestone beds alternating with marls that correspond to the ‘Marno-calcaires a Cancellophycus’, below the ‘Terres Noires’ Formation. Taphonomic analyses of the successive ammonoid fossil assemblages provide new and complementary data for biostratigraphic completeness, palaeoenvironmental setting and sequence stratigraphy. Lithologic differentiation between limestone and marl intervals resulted from alternating episodes of carbonate input and starvation. Both lithologic phases may contain evidence for sedimentary and taphonomic reworking, associated with scours, that reflects low rates of sedimentation and stratigraphic condensation. Three successive types of elementary cycles resulted from increasing rates of stratigraphic condensation, sedimentary condensation and substrate stabilization during early Bathonian. The occurrence of reelaborated ammonoids (i.e. exhumed and displaced before their final burial) implies that tractive current flows or winnowing affected the burial of concretionary internal moulds. In the lower Bathonian strata, the dominance of homogeneous concretionary internal moulds of phragmocones, completely filled with sediment, is indicative of low rates of sedimentation and sediment accumulation, respectively associated with low degrees of stratigraphic and sedimentary condensation. However, at the Bajocian/ Bathonian transition, hemipelagic, bed-scale limestone–marl alternations show a maximum value of biostratigraphic completeness and there is no evidence for taphonomic condensation in the ammonoid fossil assemblages. Taphonomic analyses of the successive ammonoid fossil assemblages and taphofacies confirm the development of a deepening phase associated with sedimentary starvation, which characterizes the last episode within the deepening half-cycle of third and second order cycles, in the Bas Auran area of French Subalpine Basin during early Bathonian.

EARLY DEVONIAN BIOEROSION IN THE RADE DE BREST, ARMORICAN MASSIF, FRANCE

Abstract: Abundant and diverse bioerosion trace fossils in the Lower Devonian of the Rade de Brest, France, were produced by boring ctenostomate bryozoans, polychaete annelids and algae. Two unidentifiable traces of encrusting sclerobionts are also considered. Two new taxa are described: Pinaceocladichnus cristatus ichnosp. nov. and Ichnogutta erectus ichnogen. et ichnosp. nov. Both are attributed to the work of boring bryozoans. Sedimentary condensation during intervals of sea-level rise seems to have had an important influence in promoting infestation by organisms, the activities of which are now preserved as trace fossils. A zonation of bioerosion trace fossils within the offshore zone, in the Lower Devonian of the Armorican Massif, is also suggested.

Pliocene–Pleistocene sequences bounded by subaerial unconformities within foramol ramp calcarenites and mixed deposits (Salento, SE Italy)

Nine discrete, metre-scale sequences, of Early Pliocene to Middle Pleistocene age, were deposited in the small Novoli graben (Salento peninsula, Puglia, S-Italy). They consist of carbonate and mixed carbonate-siliciclastic sediments, deposited on a slowly subsiding foreland ramp. Skeletal concentrations and intervening less fossiliferous intervals have been examined to provide information on major environmental parameters and infer the dynamics of their changes. Taphonomic and palaeoecological analyses indicate that storm-induced waves and currents, reduced sediment input, and settling behaviour of components were the main factors controlling the features of the various shellbed types. The concentrations were formed below fair-weather wave base in low-stress inner-to-outer shelf environments and are often associated with surfaces or intervals that are characterized by sedimentary condensation. Vertical change in the fossil content within individual cycles indicates water depth changes that were parallel to climatic fluctuations, hence may result from glacio-eustatic sea-level changes. Most sequences are bounded by subaerial, karstic unconformities. Because of the regional setting of low subsidence rate, the record of the relative sea-level fluctuations is incomplete. Episodes of subaerial exposure and concomitant effects of vadose diagenesis are documented by: (i) diagenetic changes leading to hardening of unconformity horizons; (ii) local subvertical solution pits developed in the vadose zone below unconformity surfaces; (iii) networks of polygonal cracks below unconformities; and (iv) infilling of solution pits and polygonal cracks with vadose silt and marine sediment inwashed during transgressions following the subaerial stages. Lower sequences are characterized by tighter cementation and significant increase in moldic and vuggy porosity, due to superimposition of the diagenetic effects of repeated high-amplitude sea-level fluctuations.

Calcareous K-bentonite deposits in the Utica Shale and Trenton Group (Middle Ordovician), of the Mohawk Valley, New York State

Numerous cemented and partially cemented K-bentonite beds occur in the long, complex succession of altered ash beds with the Middle to Late Ordovician Trenton Group and Utica Shale of New York State. Petrography and trace element geochemistry suggest a generally felsic-intermediate, calc-alkaline composition of the volcanic component. Cementation by carbonate is expressed as nodular concretionary masses within K-bentonite clay, or as a complete transformation of ash layers to hard, tabular ledges. Cementation typically imparts a sparry texture to K-bentonites, which is distinct from that of normal concretionary micrite layers observed at many levels. Cemented K-bentonites display remarkably well-developed glass shard textures, suggesting that carbonate cementation sometimes occurs prior to alteration of glass to clay minerals. The presence of cross-lamination in several cemented K-bentonites, dramatically shows that post-depositional hydraulic reworking or bioturbation of tuffaceous material was an important process on the Ordovician seafloor. Preliminary observations show the potential for detailed petrographic textural characterization of certain ash layers where cementation occurred soon after burial. Given that ashes are often associated with sedimentary condensation and frequently occur above submarine discontinuities, petrographic study of cemented ash should reveal if, and to what degree, ash layers record multiple depositional events. REE patterns show enrichments in light REE and moderate to pronounced Eu anomalies consistent with a felsic-intermediate igneous signature. Individual Calcareous ash bed trace element patterns and ratios – for example, REE and Hf/Zr, Th/U, K/Rb – may persist laterally in the Mohawk Valley, allowing their use as correlation tools.

Latest Neoproterozoic to Middle Cambrian body fossil record in Spain (exclusive of trilobites and archaeocyaths) and their stratigraphic significance

The chronostratigraphical aspects of four Iberian non-trilobite and non-archaeocyathan associations and the record of two separate fossil occurrences are discussed. These fossils are Cloudina and Platysolenites. The associations have been defined as the Anabarella-Aldanella, Arthropoda-Tannuolina-Latouchella, Alanís Fauna, and Hadimopanella-Archiasterella assemblages. They comprise a time span from the late Vendian to the early Middle Cambrian. The existence of upper Vendian associations in the Rio Huso and Ibor groups outcropping through Central Iberia is indicated by the presence of Cloudina. Its biostratigraphic overlapping with vendotaenids and Sabellidites may correlate it with the Sabellidites-Vendotaenia assemblage. In the upper member of the Rio Huso group (Valdelacasa Anticline) and Upper Alcudian strata (Alcudia Valley) the Anabarella-Aldanella association has been found. It may belong to the middle-upper Tommotian in the Siberian Platform and Mongolia and also with the upper Alcudian in Iberia. In the Sierra de Córdoba area, Southern Spain, an Arthropoda-Tannuolina-Latouchella association has been found in the Pedroche Formation, which has been correlated with the late Tommotian to early Atdabanian, based on problematical taxa, archaeocyathid and trilobite stratigraphy. Based on its trilobite associations, a middle Marianian age for the Alanís Fauna and Platysolenites antiquissimus Eichwald, 1860 is established. The youngest association, the Hadimopanella-Archiasterella association, has been found in the lower part of the Láncara Formation of the Cantabrian Ranges. Owing to a diachronism, there are only some localities where the griotte member starts in the Eoparadoxides mureroensis Zone. However, green glauconitic limestones, probably formed during a sedimentary condensation, yield Hadimopanella, so it could appear at the beginning of the Eoparadoxides mureroensis Zone or at the end of the Bilbilian.

Les nodules silico-alumineux: une expression de la condensation sédimentaire sur une plate-forme distale

The Ordovician siliceous-aluminous nodules of the Armoricain Massif are made up of terrigenous constituents (quartz/phyllosilicate) and biogenic constituents (silica and apatite). The nodules were formed during the early diagenesis. Their formation could be linked to processes of sedimentary condensation governed by eustatic variations in the distal shelf.

The relationship between shellbed type and sequence architecture: examples from Japan and New Zealand

Examples of lithology, fossil content and taphonomic features of shellbeds and intervening less fossiliferous intervals are presented from four Plio–Pleistocene successions (Shimosa Group, Boso Peninsula, Omma Formation, Hokuriku area, Japan, and Okehu, Kai-iwi, and Shakespeare groups in Wanganui, and the Rangitikei Group along the Rangitikei River in New Zealand). As for pre-Pliocene 3rd- and 4th-order depositional sequences, Plio–Pleistocene 5th- to 7th-order depositional sequences contain a variety of shellbeds which are often associated with surfaces or intervals that are characterized by sedimentary condensation, omission or erosion (e.g. sequence boundaries, ravinement surfaces, downlap surfaces and condensed sections). Stratigraphic patterns of shellbed type tend to be similar and repetitive within a basin and a locality. This demonstrates that a specific palaeogeography played an important role in determining the nature of shellbeds. For example, shellbeds formed in the context of toplap are common only in the Shimosa Group, which was deposited in a moderately sheltered sea, the palaeo-Tokyo Bay. Toplap shellbeds are rare in other sequences formed in more open conditions. Despite the variability resulting from such basin characteristics, common styles of shellbeds can be recognized that formed under conditions of marine onlap, backlap, downlap and toplap. Each type of shellbed has a characteristic fossil composition and taphonomy. Onlap and toplap shellbeds contain low-diversity macrobenthic associations including Glycymeris, Mercenaria, Paphies or other bivalves having robust shells, which are often abraded or fragmented. Backlap shellbeds, which are equivalent to the condensed section formed at the maximum transgression, are characterized by dominance of epifaunal macrobenthos such as bryozoa, brachiopoda, pectinid and ostreid bivalves, preserved in a slightly cemented, glauconitic muddy matrix. In contrast to fossils in such condensed sections, the shell density and species diversity of downlap shellbed associations are rather low, and in a few examples the macrobenthic association was buried rapidly in a lower unit of the highstand systems tract (HST) stratigraphically located above the condensed sections. Variations in the stratigraphic distribution of shellbed types are reflected in symmetrical and asymmetrical sequence architectures. Symmetrical sequences have roughly the same thickness of transgressive systems tracts (TST) and highstand systems tracts (HST), and have well segregated shellbeds that were formed during marine onlap and backlap. Asymmetrical cycles have very thin TSTs and much thicker HSTs, and are characterized by the amalgamation of condensed onlap and backlap shellbeds. Such contrasting cycle architectures are interpreted to reflect inner (symmetrical) and outer (asymmetrical) shelf palaeodepositional settings. The amalgamated onlap/backlap shellbeds appear to be common in Plio–Pleistocene sequences. Owing to the short duration of glacio-eustatic sea-level changes with dominant frequencies of 20,000, 40,000 or 100,000 years, shellbeds in the Plio–Pleistocene are relatively simple and thin compared to those formed in ordinary third-order depositional sequences. Infauna-dominated benthic associations are generally more common than in third-order cycles, and epifaunal associations facilitated by taphonomic feedback on sediment-starved shell-gravel substrates occur only in the condensed section corresponding to maximum transgression in most Plio–Pleistocene sequences.