Distal Settings Research Articles

Chronostratigraphy of the Upper Cretaceous high productivity sequence of the southern Tethys, Israel

The Levantine high productivity sequence is a product of an extensive upwelling system that operated in the Late Cretaceous along the SE Tethyan margin. This system resulted in the deposition of a unique sequence of carbonate, chert, porcellanite, phosphorite and organic-rich (oil shale) sediments in a series of basins located proximally and marginally to the upwelling center. This study presents a detailed and updated chronostratigraphic framework for the high productivity sequence in Israel based on eight sections covering a N–S cross section of ∼90 km. The Shefela Basin (central Israel) represents the thickest and the most complete penetrated stratigraphic interval of the oil shale deposits in Israel. The newly drilled Aderet borehole in the Shefela Basin provided a continuous core record coupled with high quality geophysical well logs and was used in this study as a ‘type-section’ for the detailed chronostratigraphic scheme of the high productivity sequence. A total of 23 datum levels were recognized using planktic and benthic foraminiferal biostratigraphy, lithostratigraphy and gamma ray well log markers. The varying lithostratigraphic units (containing chert, phosphate, porcellanite and organic-rich carbonates) of the more proximal basins of southern Israel (Negev) were individually correlated to specific horizons within the monotonous organic-rich carbonates of the distal setting of the Shefela Basin.The first occurrence of the Late Cretaceous organic-rich carbonates in Israel is documented in the Negev during the late Coniacian, within the upper Dicarinella concavata Zone, and corresponds chronologically to the upper part of the lower Menuha Formation. The regional unconformity around the Santonian/Campanian boundary was found to be less substantial at the distal localities in comparison to the proximal ones. The distinct appearance of the ‘Mishash Tongue’ chert in the Shefela was correlated to the massive Chert Member (Mishash Formation) in the Negev, and assigned to the middle Campanian. Deposition of the overlying phosphate series spans from the lower Contusotruncana plummerae to the base of the Pseudoguembelina palpebra Zones (78.3–71.7 Ma) and co-occurs in both proximal and deeper distal areas, although in a much lesser magnitude in the latter. The top of the phosphatic unit is marked by a regional unconformity. The oil shale deposits in southern Israel coincide with the 100 m richest TOC interval (average of 15.2 wt.% TOC) in the Shefela. The diminishing phase of organic-rich deposition occurred in a diachronous step-wise manner across Israel, from the top of the P. palpebra Zone to the upper part of the Abathomphalus mayaroensis Zone in southern and central Israel, respectively. This indicates that the full duration of the high productivity sequence in Israel spans approximately 19 myr.Arabian Plate-wide maximum flooding surfaces (MFS) recorded during the Tectonic Mega-Sequence (TMS) AP9 timeframe (92–63 Ma) are identified here in the local sedimentary system of the Levant: K160 signifying the onset of organic-rich carbonates; K170 at the upper part of the massive and brecciated Chert Member; and K180 marking the diminishing phase of the high productivity sequence in Israel.

Middle Ordovician cephalopod biofacies and palaeoenvironments of Baltoscandia

During the Middle Ordovician cephalopods became an important part of the macrofauna of the Baltoscandian carbonate platform. The earliest cephalopod abundance peak was reached during the early Darriwilian, within the Kunda Stage Yangtzeplacognathus crassus and Lenodus pseudoplanus conodont zones. In sediments of this time interval large orthoconic cephalopods often occur in masses with more than one specimen per square-meter on bedding surfaces. The assemblages are characterized by the strong dominance of often large endocerids. In proximal depositional settings coiled tarphycerids and other cephalopod groups are an important additional component. In the most distal settings orthocerids are the most important secondary component. Correspondence Analysis of assemblages throughout Baltoscandia revealed three distinct biofacies, which here are termed Orthocerid, Proterovaginoceras and Anthoceras Biofacies, respectively. The biofacies reflect differences in depth and proximity to the shoreline and are consistent with the Baltoscandian Confacies Belts. Spatial changes in absolute abundance and taxonomic composition indicate increased original cephalopod population densities and habitat expansion within the Y. crassus and L. pseudoplanus conodont zones. A nearly coeval abundance peak in a similar facies in South China indicates supraregional causes of the mass occurrence, probably reflecting a globally increased nutrient availability in the water column during the Darriwilian.

Insight into the development of a carbonate platform through a multi-disciplinary approach: a case study from the Upper Devonian slope deposits of Mount Freikofel (Carnic Alps, Austria/Italy)

The development and behavior of million year-scaled depositional sequences recorded within Palaeozoic carbonate platform has remained poorly examined. Therefore, the understanding of palaeoenvironmental changes that occur in geological past is still limited. We herein undertake a multi-disciplinary approach (sedimentology, conodont biostratigraphy, magnetic susceptibility (MS), and geochemistry) of a long-term succession in the Carnic Alps, which offers new insights into the peculiar evolution of one of the best example of Palaeozoic carbonate platform in Europe. The Freikofel section, located in the central part of the Carnic Alps, represents an outstanding succession in a fore-reef setting, extending from the Latest Givetian (indet. falsiovalis conodont zones) to the Early Famennian (Lower crepida conodont zone). Sedimentological analysis allowed to propose a sedimentary model dominated by distal slope and fore-reef-slope deposits. The most distal setting is characterized by an autochthonous pelagic sedimentation showing local occurrence of thin-bedded turbiditic deposits. In the fore-reef slope, in a more proximal setting, there is an accumulation of various autochthonous and allochthonous fine- to coarse-grained sediments originated from the interplay of gravity-flow currents derived from the shallow-water and deepwater area. The temporal evolution of microfacies in the Freikofel section evolves in two main steps corresponding to the Freikofel (Unit 1) and the Pal (Unit 2) limestones. Distal slope to fore-reef lithologies and associate changes are from base to top of the section: (U1) thick bedded litho- and bioclastic breccia beds with local fining upward sequence and fine-grained mudstone intercalations corresponding, in the fore-reef setting, to the dismantlement of the Eifelian–Frasnian carbonate platform during the Early to Late Frasnian time (falsiovalis to rhenana superzones) with one of the causes being the Late Givetian major rift pulse; (U2) occurrence of thin-bedded red nodular and cephalopod-bearing limestones with local lithoclastic grainstone intercalations corresponding to a significant deepening of the area and the progressive withdrawal of sedimentary influxes toward the basin, in relation with Late Frasnian sea-level rise. MS and geochemical analyses were also performed along the Freikofel section and demonstrate the inherent parallel link existing between variation in MS values and proxy for terrestrial input. Interpretation of MS in terms of palaeoenvironmental processes reflects that even though distality remains the major parameter influencing MS values, carbonate production and water agitation also play an important role.

Sequence stratigraphy of the Upper Cambrian (Furongian; Jiangshanian and Sunwaptan) Tunnel City Group, Upper Mississippi Valley: Transgressing assumptions of cratonic flooding

New data from detailed measured sections permit comprehensive analysis of the sequence framework of the Furongian (Upper Cambrian; Jiangshanian and Sunwaptan stages) Tunnel City Group (Lone Rock Formation and Mazomanie Formation) of Wisconsin and Minnesota. The sequence-stratigraphic architecture of the lower part of the Sunwaptan Stage at the base of the Tunnel City Group, at the contact between the Wonewoc Formation and Lone Rock Formation, records the first part of complex polyphase flooding (Sauk III) of the Laurentian craton, at a scale smaller than most events recorded by global sea-level curves. Flat-pebble conglomerate and glauconite document transgressive ravinement and development of a condensed section when creation of accommodation exceeded its consumption by sedimentation. Thinly-bedded, fossiliferous sandstone represents the most distal setting during earliest highstand. Subsequent deposition of sandstone characterized by hummocky or trough cross-stratification records progradational pulses of shallower, storm- and wave-dominated environments across the craton before final flooding of Sauk III commenced with carbonate deposition during the middle part of the Sunwaptan Stage. Comparison of early Sunwaptan flooding of the inner Laurentian craton to published interpretations from other parts of North America suggests that Sauk III was not a single, long-term accommodation event as previously proposed.

Depositional features of co-genetic turbidite–debrite beds and possible mechanisms for their formation in distal lobated bodies beyond the base-of-slope, Ulleung Basin, East Sea (Japan Sea)

Abstract A detailed analysis of the MR1 (11–12 kHz) sonar images, chirp (2–7 kHz) profiles, core sediments and 14 C ages from the latest Quaternary lobated bodies (LB) on the deep (> 2100 m water depth) basin plain of the western Ulleung Basin reveals depositional features and context of co-genetic turbidite–debrite beds in the LB on the distal setting and a plausible mechanism for generating these beds. Eight LB are present in the basin plain, ca. 30–60 km beyond the base-of-slope, and were generally deposited retrogressively. Older and more distal LB 1–4 have large dimensions (> 27 km long, 15–25 km wide). In contrast, younger and less distal LB 5–8 are small (8.8–31.5 km long, 1.2–12 km). The muddier, larger LB 1 and 2 were most likely originated from the relatively large-scale sediment failures on the muddy upper slopes (> 350–400 m water depth) between 18.5 and 20.0 cal. ka B.P. On other hand, the sandier, smaller LB 6 and 7 were deposited between 17.0 and 17.5 cal. ka B.P., probably by the relatively small-scale sediment failures on the sandy uppermost slope, shallower than 350–400 m water depth. In LB 1, a lower sandy-mud turbidite is transitional upward into an upper mud-matrix debrite having small, rounded mud clasts. In contrast, LB 6 exhibits a gradual upward change from a lower clay-poor, sandy turbidite to an upper clay-rich, sand-matrix debrite with large, interlocking mud clasts. Each mixed bed in LB 1 and 6 can represent a co-genetic (or linked) turbidite–debrite bed formed by the down-dip flow transformation from turbidity current to debris flow at a point during the same event. The abundant large-scale (up to 20–25 m deep, 3–5 km long) erosions of fine-grained substrates near the base-of-slope area suggest that the down-dip transformation was driven by incorporation of mud via erosional bulking by turbidity currents reaching the base-of-slope area. The different clay/sand content and size/shape of mud clasts in the co-genetic turbidite–debrite beds between LB 1 and 6 imply that the composition (sandy or muddy) and dimension of initial turbidity current approaching the base-of-slope, the volume of eroded masses from the fine-grained substrates, and the transport distance of sediment gravity flows from the erosional areas of muddy substrates can be all important factors controlling the depositional styles of co-genetic turbidite–debrite beds.

Mechanisms of dust emission from Pleistocene loess deposits, Nebraska, USA

AbstractSaltation bombardment is commonly believed to be the primary mechanism by which large quantities of dust can be generated. Direct entrainment of silt, on the other hand, is thought to generate minimal quantities of dust. If so, loess landscapes should rarely be major sources of dust unless disturbed by human activity. To test the mechanisms by which loess can be eroded by the wind, we used the Portable In‐Situ Wind Erosion Lab (PI‐SWERL). PI‐SWERL tests were conducted on benches that were carved into loess at field locations. Coarse to fine loess was also collected and tested in a controlled setting using (1) dry, unconsolidated loess and (2) crusted loess. Results indicate that in most cases, the threshold friction velocity for silt is less than that for sand and that most dust generation occurs by direct entrainment of dust. Saltation, even in coarse loess, was nonexistent to intermittent. In a few cases, saltation of sand or soil aggregates preceded dust generation or enhanced emissions after direct entrainment began. Emission fluxes of loess are potentially high in both proximal and distal settings, suggesting that loess is easily entrainable if vegetation density is low. We hypothesize that an arid and windy climate of the late Pleistocene, paired with lower vegetation density, facilitated large‐scale erosion of Peoria Loess in Nebraska to generate wind‐aligned ridges and troughs. More broadly, our results indicate that loess landscapes have the potential to be major sources as well as sinks of dust.

Sequence stratigraphic hierarchy of the Upper Devonian Foreknobs Formation, central Appalachian Basin, USA: Evidence for transitional greenhouse to icehouse conditions

The Foreknobs Formation (Upper Devonian; Upper Frasnian to basal Famennian) comprises the uppermost marine strata of the progradational “Catskill clastic wedge” of the south-central Appalachian Mountains (Virginia-West Virginia; USA). The Foreknobs Formation consists of 14 lithofacies arranged in four facies associations which record the following depositional settings: 1) storm-dominated distal to proximal offshore to shoreface (facies association A); 2) sharp-based conglomeratic shoreface (facies association B); 3) fluvial redbed (facies association C); and 4) incised-valley fill (IVF; facies association D). Vertical juxtaposition and stacking patterns of lithofacies and facies associations permit recognition of a hierarchy of three scales of cyclicity. Up to 70 short-term 5th-order cycles, each averaging ~65kyr, consist of coarsening-upward parasequences of storm-dominated offshore marine facies in the distal setting which correspond to high frequency (unconformity bound) sequences (HFS) of fluvial redbed strata overlain by offshore marine strata in the proximal setting. These facies relationships are a consequence of 10–15m of sea-level fluctuations. Up to 12 intermediate-term 4th-order cycles, each averaging ~375kyr, consist of stacked 5th-order cycles. The 4th-order cycles are bounded by regressive surfaces of marine erosion (RSME) at the base of sharp-based conglomeratic shoreface sandstones in the distal setting that correspond with paleosols in the proximal setting. In some cases, the 5th-order cycles within each 4th-order cycle exhibit stacking patterns indicative of increasing or decreasing accommodation space. These facies relationships are a consequence of 25–35m of sea-level fluctuations. Three complete and portions of two additional 3rd-order cycles, each averaging ~1.12Myr, consist of stacked 4th-order cycles. The 3rd-order sea-level trends reflected in the Foreknobs Formation are nearly identical to published eustatic sea-level curves. Incised-valley fills are present at one of the 3rd-order cycle boundaries and are a consequence of a 35–45m sea-level fluctuation. The amplitudes of the inferred sea-level fluctuations are comparable to the expansion and contraction of ice volumes within the current Greenland and Antarctic ice sheets which suggests glacioeustasy was the primary control on sea-level fluctuations and cyclicity within the Foreknobs Formation. Such an interpretation is consistent with knowledge of Devonian climate, transitioning from Middle Devonian greenhouse to Late Devonian icehouse, as indicated by evidence of glaciation during parts of the Late Devonian in South America and the Appalachians.

Composite Trace Fossil Assemblage in a Distal Carbonate Setting from the Tethys (Middle Jurassic, Betic Cordillera, Southern Spain)

Ichnological analysis of a Middle Jurassic carbonate surface from the Betic Cordillera (southern Spain) reveals a complex trace fossil assemblage, including softground Ophiomorpha, firmground Arenicolites, Thalassinoides and Gastrochaenolites, and hardground Trypanites as well as possible Gastrochaenolites. The ecological replacement in the macrobenthic community is interpreted according to successive suites that are controlled mainly by substrate consistency. Variations in composition and abundance of trace fossils between suites can be ecologically and/or taphonomically determined.

Isotopic (Sr–Nd) and major element fingerprinting of distal tephras: an application to the Middle-Late Pleistocene markers from the Colli Albani volcano, central Italy

We describe the diagnostic lithological and chemical features of distal tephras from major Middle-Late Pleistocene (560–36 ka) explosive eruptions of the Colli Albani volcanic district, central Italy. In particular, we explore the time-dependent variability of the Sr and Nd isotope compositions as a tool for recognising and pinpointing individual Colli Albani tephra in distal settings. The distal tephras investigated are in lacustrine and fluvial sediments of central Apennine intermountain basins located 70 km–100 km east of Colli Albani. The recognition of the Colli Albani tephras is essentially based on the K-foiditic composition of their glass, which, within the Italian volcanological framework, is a distinctive character of the Colli Albani pyroclasts. In detail, these tephras are attributed to the following eruptive units: Tufo Pisolitico di Trigoria (561 ± 2 ka); Tufo del Palatino–Tufo di Bagni Albule (530 ± 2/527 ± 2 ka), Tufo di Bagni Albule–Pozzolane Rosse air-fall sequence (517 ± 1 to 500 ± 3 ka), Pozzolane Rosse (457 ± 4 ka), Villa Senni (365 ± 4 ka), and Albano 5–7 (41 ± 7 to 36 ± 1 ka). These correlations are supported by 40Ar/39Ar dating of the distal tephras correlated to the Pozzolane Rosse (457.4 ± 1.7 ka), Villa Senni (365 ± 2 ka) and Albano 5–7 (41 ± 9 ka) and by 87Sr/86Sr measured on clinopyroxene crystals and fresh glassy scoria from distal Colli Albani tephras that range from ∼0.711 to ∼0.709. These ratios are similar to those that characterise the individual proximal correlative units, and show the same decreasing trend over time. In contrast, the 143Nd/144Nd ratios for proximal and distal bulk samples and clinopyroxene increase from ∼0.51212 to ∼0.51215 from the oldest to the youngest tephra deposit. In summary, the study of Sr and Nd isotope compositions that is here applied on products from the Colli Albani volcanic district is a powerful, complementary tool to the more traditional tephrostratigraphic methods (e.g., componentry and electron microprobe analysis) for fingerprinting of distal tephras over a large region of the central Mediterranean, and over a large time interval, such as from 560 ka to 36 ka.

The Composition and Depositional Environments of Mesoarchean Iron Formations of the West Rand Group of the Witwatersrand Supergroup, South Africa

This paper documents the sedimentological setting, mineralogy, and geochemistry of several iron formation units interbedded with siliciclastic strata of the Mesoarchean Witwatersrand Supergroup, well known for its world-class conglomerate-hosted Au-U deposits. Four major iron formation beds, with associated magnetic mudstones, are present in two distinctly different lithostratigraphic associations, namely shale- and diamictiteassociated iron formation. The shale association is represented by the Water Tower and Contorted Bed iron formations in the Parktown Formation of the Hospital Hill Subgroup in the lower part of the succession and the diamictite association by the Promise and Silverfield iron formations in the overlying Government Subgroup. The iron formation units have been subjected to lower greenschist facies metamorphism. Oxide (magnetite and limited hematite), carbonate, and silicate facies iron formations are recognized. The iron formations typically overlie major transgressive flooding surfaces in the succession and, in turn, form the base of progradational coarsening-upward increments of sedimentation comprising magnetic mudstone, nonmagnetic shale, and interbedded siltstone-quartzite. The upward transition from iron formation into magnetic mudstone is accompanied by a change in mineralogical composition from hematite-magnetite iron formation at the base in the most distal setting through magnetite-siderite- and siderite-facies iron formation in the transition zone to magnetic mudstone. The siderite with associated ankerite displays highly depleted δ13C values, suggesting crystallization via iron respiration in presence of organic carbon. The iron formations display positive postArchean Australian shale-normalized Eu and Y anomalies with depletion in light rare-earth elements relative to heavy rare-earth elements, indicating precipitation from marine water with a high-temperature hydrothermal component. Integration of sedimentological, petrographic, and geochemical results indicates that the shale-associated iron formation was deposited during the peak of transgression, when reduced iron-rich hydrothermal waters entered the Witwatersrand Basin over a limited vertical extent due to neutral buoyancy, with the top of the plume occurring below the photic zone. It is suggested that chemolithoautotrophic iron-oxidizing bacteria, which would have been able to exploit the difference in chemistry between the iron-enriched plume water and ambient ocean water to fuel metabolic activity in the presence of limited free molecular oxygen, were responsible for precipitation of initial ferric iron oxyhydroxides. The vertical facies associations in the iron formations most likely developed in response to the limited vertical extent of the hydrothermal plume, with (from distal to proximal) hematite preserved where the base of the plume was not in contact with the basin floor, magnetite where the plume water was in contact with bottom sediment, iron-rich carbonates where organic carbon input was high, iron-rich alumosilicates where siliciclastic input became significant in more proximal settings, and iron-poor sediment above the top of the plume. Diamictite-associated iron formations in the Witwatersrand are inferred to have been deposited in a fashion similar to the shale-associated iron formations, with the exception that major transgressions and hydrothermal plume invasion were preceded by glacial ice cover. The climate warming and increased volcanic activity required could have been related to increased tectonic activity inferred for the Witwatersrand Supergroup during deposition of the glacially associated iron formations.

The Upper and Lower Nisyros Pumices: Revisions to the Mediterranean tephrostratigraphic record based on micron-beam glass geochemistry

The Greek island of Nisyros is the source of tephra found in distal settings throughout the Aegean. In this contribution, we define geochemical fingerprints for the Lower and Upper Pumice eruptions of Nisyros based on micron-beam major (EMPA) and trace (LA-ICP-MS) element data. These two eruptions are compositionally distinct from each other. The high phenocryst content of proximal Lower and Upper Pumice tephra means that the differences between the two magmas are masked in whole-rock analyses. We demonstrate that all previous reports of distal marine and continental tephras correspond to the younger Upper Pumice not the older Lower Pumice. This has important implications for the marine tephrochronology of the Mediterranean and for the age of the Upper and Lower Pumice eruptions. We suggest an age of at least 47ka for the Upper Pumice and an older, unconstrained age for the Lower Pumice.

Geochemistry of the Phlegraean Fields (Italy) proximal sources for major Mediterranean tephras: Implications for the dispersal of Plinian and co-ignimbritic components of explosive eruptions

Volcanic activity at Phlegraean Fields, Italy, produced several major marker tephras over a 50ka period. The caldera forming eruptions of the Campanian Ignimbrite (CI) and Neapolitan Yellow Tuff (NYT) are of particular importance for tephrostratigraphy in Europe. Other key eruptions from this source include the Pomici Principali (PP) and the Tufi Biancastri eruptions. We combine analyses of fresh glasses from proximal locations (i.e., juvenile clasts in proximal flow and fall deposits) with data for key tephra layers from Lago Grande di Monticchio, 120km to the east. The micron-beam major (EMPA) and trace (LA-ICP-MS) element glass dataset allows us to: (a) distinguish between tephra units produced from the Phlegraean Fields before and during the CI eruption (CI-series), and before and during the NYT and PP eruptions (NYT-series/PP); (b) discriminate between the CI and the geochemically similar Pre-CI pyroclastic deposits; (c) separate the NYT from Pre-NYT tephra units, although both major and trace elements do show significant overlap. The complex compositional overlap between Pre-NYT tephras may present a problem for tephra correlations in the 14–39ka time window and may have resulted in incorrect proximal–distal and distal–distal correlations. The diagnostic chemical criteria detailed herein permits more accurate matching of distal tephras with their proximal equivalents and hence will improve chronostratigraphy of distal settings and give insight into tephra dispersal. We show that the dispersal of PP tephra was more limited than previously thought. The surge/fall (Lower Member) and subsequent pyroclastic density current (Upper Member) phases of the NYT eruption can be recognised in distal settings. Both the NYT Lower and Upper Members are found in distal localities to the east of the Phlegraean Fields, however the Lower Member is found in the absence of the Upper Member in locations to the far north of Phlegraean Fields. Chemical compositions of the Plinian and ignimbrite phases of the CI eruption overlap extensively, but can be distinguished on a plot of Zr–Th.

Multiple scale characterization of sand-rich distal lobe deposit variability: Examples from the Annot Sandstones Formation, Eocene–Oligocene, SE France

The present paper investigates the outcrop-scale architecture of submarine gravity flow deposits in both moderately to highly confined settings. A significant dataset was gathered on the sand-rich, confined, siliciclastic turbidite system of the Annot Sandstone Formation (SE France). We focused on thick sedimentary bodies formed by depletive gravity flows, in relatively distal settings (sand-rich lobes). The purpose was to investigate the architectural variability of their small scale components (beds and bedsets) and consequently to evaluate the associated heterogeneities (facies changes, grain size, sedimentary features, sediment thickness, etc.). Therefore, we provide a set of detailed descriptions on well-exposed outcrops, through very high-resolution correlation panels built thanks to multiple vertical logs. These turbidite system end-members have rarely been described in such details and have often been considered as laterally continuous and homogeneous. On the basis of the hierarchical subdivisions of turbidite lobes, results show a high complexity in internal architecture from lobe bed to lobe scale whereas large scale (scenery-seismic scale) is characterized by apparently homogeneous tabular and very extensive sandbodies (thus corresponding to the classical definition of “sheet-sands”). This complexity implies heterogeneities not only in terms of sediment thickness changes (abrupt pinch-outs and variable scale compensation features), but also in terms of facies and associated sedimentary features distributions (tractive structures, internal scours with associated by-pass, freezed mudclast-rich layers, etc.). This facies variability is controlled by the interplay between internal erosive bedforms (i.e. large scours) and constructive bedforms, such as dome-shaped or laterally stacked bedsets that are related to local topographic controls and compensation processes. Variability then appears both laterally and vertically. Besides, results also showed small scale evidences of interactions between turbidite flows and basin floor, only expressed at lobe bed to lobe element scale, especially on the basin sides where paleoslope is subject to erosion and remobilisation and where flows can undergo significant changes. Concerning both seismic and reservoir implications, although those deposits are characterized by a sheet-like geometry, this high internal variability necessarily implies heterogeneities in terms of petrophysical characteristics (porosity, permeability) that may have a significant impact on fluid circulation. These elements lead us to reconsider both sedimentary processes involved in their building and also reservoir models that can be established on field analogues.

Chemostratigraphic constraints on early Ediacaran carbonate ramp dynamics, Río de la Plata craton, Uruguay

C- and Sr-isotope data for marine carbonates can provide a detailed record of isotopic variations in seawater through time and have proven to be a valuable tool for interpreting biogeochemical events and correlating Neoproterozoic sedimentary successions worldwide. Negative carbon isotope excursion to values as low as −4.5‰ in the Ediacaran Polanco Limestones Formation, Uruguay has been interpreted as recording the aftermath of a post-Gaskiers glacial event occurred elsewhere on southwest Gondwana. The record of both deep- and shallow-water settings in the Polanco Limestone Formation provides an opportunity to examine δ13C variability across the platform and to evaluate whether the carbon isotope values reflect local or global processes. Herein we provide high-resolution δ13C-chemostratigraphy and Sr-isotope data from stratigraphic sections comprising proximal and distal settings on the carbonate platform. Carbon isotopic values are negative in deep-water facies but progressively rise towards positive values in shallow-water settings. This previously unrecognized trend suggests that deposition of Polanco carbonates occurred in a stratified marine basin where degradation of organic matter below the redox boundary led to lower δ13C values at greater depths. The negative δ13C excursion is restricted to specific horizons in shallow-water facies and is interpreted as being originated due to higher levels of reworking under storm-dominated conditions, which produced a strong local oxidation of organic matter and ultimately, the negative carbon isotope signatures. 87Sr/86Sr and δ13C chemostratigraphy coupled with new radiometric data allow us to revise the previously proposed age for the unit. This new data support an age between 590 and 560Ma for the deposition of the Polanco Limestones Formation.

Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China

This study reports a high-resolution carbon and strontium isotope profile for the Ediacaran–Cambrian Xiaotan section, situated in northeastern Yunnan, South China. Xiaotan section represents a more distal setting than the more condensed Meishucun section in eastern Yunnan, and is, biostratigraphically and chemostratigraphically, the best constrained section on the Yangtze platform covering the Ediacaran–Cambrian interval. The carbonate carbon isotopic ratios of the late Ediacaran, upper Dengying Formation to the early Cambrian, Zhujiaqing Formation, exhibit a large negative excursion (N1, −12.2‰) in the Daibu Member, just below the first appearance of small shelly fossils (SSF) at the base of the Zhongyicun Member, and a sustained pre-‘Tommotian’ positive excursion (P4, δ13C up to +7.3‰) which coincides with the occurrence of the Heraultipegma yunnanensis (=Watsonella crosbyi) Assemblage SSF in the Dahai Member. Least altered strontium isotope ratios, based on sequential acid leaching of limestones as well as dolostones and phosphorites following rigid petrographic and geochemical selection, reveal a systematic decreasing trend from 0.7085–6 in the latest Ediacaran (Daibu Member) to about 0.7082–3 by the FAD of Watsonella crosbyi in China (Dahai Member). The carbon and strontium isotope features of the Xiaotan section can be correlated with those from sections in Morocco, Mongolia and Siberia, and confirm a decrease in seawater 87Sr/86Sr during Cambrian Stage 1 accompanied by C-isotope oscillations, which together may assist global stratigraphic correlation of the early Cambrian. As previously intimated by C-isotopes and biostratigraphy, this new Sr-isotope evidence confirms a depositional gap during the pre-Tommotian in the Aldan River area of SE Siberia. The decreasing trend of 87Sr/86Sr in the Cambrian Stage 1 marks a temporary (∼16Myrs) reversal of the overall increasing trend during the Ediacaran and Cambrian periods, and potentially records a short-term decrease in continental weathering within a prolonged interval of generally increasing weathering rates. Other factors, such as increased submarine hydrothermal alteration, ocean spreading rates and/or chemical weathering of volcanic provinces and young carbonate dissolution might also have affected this decrease in seawater 87Sr/86Sr.

Key features of mixed carbonate-siliciclastic shallow-marine systems: the case of the Capo Colonna terrace (southern Italy)

The late Pleistocene Capo Colonna terrace (southern Italy) is composed of a mix of shallow-marine carbonate and siliciclastic sediments. Shoreface sandstones, dominated by a terrigenous component and by sedimentary structures related to waves and currents, pass seaward into shelf coralline algal frameworks and associated calcarenites. Siliciclastic sandstones and rigid bioconstructions are locally juxtaposed. Main features of the studied system include ( i ) a bipartition between mixed siliciclastic-bioclastic sedimentation in proximal settings and coralline algal growth in distal settings, ( ii ) an analogy between the stratal architecture of the clastic-dominated part of the system and that of siliciclastic shelf/ramp systems, and ( iii ) the control on the gradient of the shoreface-shelf by inherited physiography and transgressive processes rather than the carbonate productivity.

The flow dynamics of an extremely large volume pyroclastic flow, the 2.08-Ma Cerro Galán Ignimbrite, NW Argentina, and comparison with other flow types

The 2.08-Ma Cerro Galan Ignimbrite (CGI) represents a >630-km3 dense rock equivalent (VEI 8) eruption from the long-lived Cerro Galan magma system (∼6 Ma). It is a crystal-rich (35–60%), pumice (<10% generally) and lithic-poor (<5% generally) rhyodacitic ignimbrite, lacking a preceding plinian fallout deposit. The CGI is preserved up to 80 km from the structural margins of the caldera, but almost certainly was deposited up to 100 km from the caldera in some places. Only one emplacement unit is preserved in proximal to medial settings and in most distal settings, suggesting constant flow conditions, but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges, it interacted with valley walls, resulting in flow instabilities that generated multiple depositional units, often separated by pyroclastic surge deposits. The CGI preserves a widespread sub-horizontal fabric, defined by aligned elongate pumice and lithic clasts, and minerals (e.g. biotite). A sub-horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed. The CGI is poor in both vent-derived (‘accessory’) lithics and locally derived lithics from the ground surface (‘accidental’) lithics. Locally derived lithics are small (<20 cm) and were not transported far from source points. All data suggest that the pyroclastic flow system producing the CGI was characterised throughout by high sedimentation rates, resulting from high particle concentration and suppressed turbulence at the depositional boundary layer, despite being a low aspect ratio ignimbrite. Based on these features, we question whether high velocity and momentum are necessary to account for extensive flow mobility. It is proposed that the CGI was deposited by a pyroclastic flow system that developed a substantial, high particle concentration granular under-flow, which flowed with suppressed turbulence. High particle concentration and fine-ash content hindered gas loss and maintained flow mobility. In order to explain the contemporaneous maintenance of high particle concentration, high sedimentation rate at the depositional boundary layer and a high level of mobility, it is also proposed that the flow(s) was continuously supplied at a high mass feeding rate. It is also proposed that internal gas pressure within the flow, directed downwards onto the substrate over which the flow was passing, reduced the friction between the flow and the substrate and also enhanced its mobility. The pervasive sub-horizontal fabric of aligned pumice, lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow, consistent with the existence of a laminar, shearing, granular flow regime during the final stages of transport and deposition.

The record of the Messinian salinity crisis in the Tertiary Piedmont Basin (NW Italy): The Alba section revisited

The Alba succession (Tertiary Piedmont Basin, NW Italy) preserves the northernmost record of the Messinian salinity crisis (MSC) and was deposited on the southern margin of a wide wedge-top basin, related to the involvement of the Piedmont Basin in the Apennine compressional tectonics. Pre-MSC sediments consist of a cyclic succession of marine euxinic shales and calcareous marls, deposited under the influence of precession-modulated climate changes, and document the progressive restriction of the basin prior to the onset of the MSC. They are followed by the Primary Lower Gypsum unit (PLG), deposited during the first MSC stage (from 5.96 to 5.60 Ma). These sediments show a clear precession-related cyclic stacking pattern and record the lateral transition from a shallow water marginal setting in the SW to a deeper one in the NE. In marginal settings, six PLG cycles are recognised, truncated by an erosional unconformity placed at the base of the post-evaporitic sediments. The lowermost five cycles are composed of massive and banded selenite beds separated by thin shale intervals. A sharp change, evidenced by the appearance of the branching selenite facies, is recorded by the 6th gypsum bed that represents a distinctive marker bed, here called Sturani key-bed, that can be mapped throughout the study area. Basinward, the lower PLG cycles are transitional to decimetre-thick carbonate-rich layers interbedded to euxinic shales, that are overlain by the Sturani key-bed. Above the marker bed, other seven PLG cycles are present. The gypsum beds form thinner bodies compared to the Sturani key-bed and are characterised by a greater amount of fine-grained terrigenous fraction, suggesting an increase of continental runoff related, in turn, to humid climate conditions at the end of the first MSC stage. PLG cycles are followed by slumped mudstones and clastic gypsum beds that correspond to the resedimented and chaotic facies (Resedimented Lower Gypsum), deposited in the Mediterranean basins during the second MSC stage (from 5.60 to 5.55 Ma). They are in turn overlain by continental and brackish water facies with Lago Mare fossil assemblages, recording the final stage of the MSC. The Messinian succession of Alba provides the opportunity to reconstruct the lateral facies transition between marginal and distal settings and to shed new light on the deep water MSC sedimentary record. Moreover, the appearance of the branching selenite facies from the 6th PLG cycle upward provides a tool for properly placing the Piedmont record in the MSC chronostratigraphic framework, allowing us to investigate the influence of climate gradients on the sedimentary response to the Mediterranean salinity crisis.

Interannual (1999–2005) morphodynamic evolution of macro-tidal salt marshes in Mont-Saint-Michel Bay (France)

This paper provides a detailed study on the sedimentation patterns and the recent morphodynamic evolution affecting the macro-tidal salt marshes located west of the Mont-Saint-Michel (France). Twenty-two stations along three transects on the marshes were seasonally monitored for marsh surface level variations from 1999 to 2005, using a sediment erosion bar. The corresponding erosion/accretion rates were obtained together with data on topography, vegetation cover, and grain size of surface sediment. To examine the mechanisms contributing to the salt marsh sedimentation, the data and their evolution were treated with respect to tides, relative mean regional sea level, and wind speed/frequency variations. From 1999 to 2005, the marsh was globally accreting (from 3.45 to 38.11 mm yr −1 in the low marsh, up to 4.91 mm yr −1 in the middle marsh, and up to 1.35 mm yr −1 in the high marsh), while the study was conducted during a window of decreasing trend in mean regional sea level (−2.45 mm yr −1 according to regional-averaged time series). These sedimentation rates are one of the highest recorded worldwide; however, the sedimentation was not found to be continuous over the period in question. This pattern is illustrated by the strong extension of the marshes from 1999 to 2002, and the relative stability observed from 2003 to 2005. The imported and reworked sediments are trapped and fixed by the dense vegetation ( Puccinellia maritima, Halimione portulacoides), inducing the general seaward extension of the marshes. The processes governing sediment budget (accretion/erosion) show annual, seasonal, and spatial variability on the marsh. Spatial variations display contrasted patterns of erosion/sedimentation between the low, middle, and high marsh, and between the different transects. These patterns are a result of distance from sediment sources, strong heterogeneity in vegetation cover (human induced or not), and contrasting topographic and micro-topographic characteristics. The higher accretion rates are observed in distal settings in the low marsh, and strongly decrease toward the middle and high marsh. This evolution results from a decrease in accommodation space/water column thickness, and frequency of inundation coupled with an increase in station elevation, but also from the cumulated effects of vegetation cover and micro-topography. The vegetation cover of the low and middle marsh enhance the settling and fixing of fine sediments imported through tides or dispersed by flood and ebb currents. The seasonal evolution of the marshes is marked by contrasting effects of water storage in the sediment. The overall seasonal sediment budget is controlled by the variation of the frequency of inundation relative to tidal range and marshes topography. Autumns are influenced by the tide (equinoxes), relative mean regional sea level, and variations in wind speed/frequency. Winter wind speed and frequency in relation with tidal variations appear to be the main parameters regulating winter marsh evolution. Summers are predominantly under the influence of local variations in water storage (desiccation) while external parameters generally display a low influence. Although it is not governed by any one parameter, springtime sediment budget seems to result from strong interaction between the above-cited parameters, despite the significant frequency of inundation (equinoxes).

Chapter 35 The Tatonduk inlier, Alaska–Yukon border

Abstract Glaciogenic deposits of the Rapitan and Hay Creek Groups are exposed in the Tatonduk inlier of east-central Alaska and the western Yukon. The Rapitan Group ranges in thickness from c. 50 to 700 m with Fe-formation common in the upper 10 m. In the most distal settings, the Rapitan Group is separated from the diamictite of the Hay Creek Group by over 100 m of sandstone and siltstone; however, the Hay Creek Group contains large erosive surfaces and cannibalizing breccia, and rarely preserves strata between the two glaciogenic deposits. The diamictite of the Hay Creek Group is capped by a white- to buff-coloured dolostone with pseudo-teepee structures, bed-parallel, isopachous sheet-crack cements, and a depleted C-isotope signature. Late Neoproterozoic glacial deposits in the Tatonduk inlier were formerly assigned to the Tindir Group. To simplify the nomenclature in the northwestern Canadian Cordillera, the Tindir Group was abandoned and replaced with nomenclature consistent with that of the Windermere Supergroup in the Mackenzie Mountains. The mixed lithology and anchizone-grade metamorphism distinguish the Rapitan and Hay Creek Groups in the Tatonduk inlier as attractive future targets for integrated micropalaeontology, geochemistry, palaeomagnetism and geochronology.