Top Of Unit Research Articles

The Upper Cretaceous elasmobranch fauna from Senegal

Three successive horizons were sampled by bulk-sampling across the Campanian–Maastrichtian marine succession at the Cap de Naze cliff, South of Popenguine, Senegal. The material collected includes several hundreds of elasmobranch micro-remains as well as surface-collected specimens that are described in the present work. Twenty-six species are recorded, including three new genera and eight new batomorph species (Ptychotrygon nazeensis sp. nov., Terangabatis thiami gen. et sp. nov., ‘Rhinobatos’ popenguinensis sp. nov., Atlantobatis acrodonta gen. et sp. nov., Sowibatos minimus gen. et sp. nov., ‘Dasyatis’ reticulata sp. nov., Coupatezia casei sp. nov., Phosphatodon cretaceus sp. nov.). These new samplings complement previous records based on surface collection of the Cape de Naze section and bring the number of elasmobranch species for this fauna to 31. Increasing elasmobranch taxonomic diversity throughout the three successive assemblages indicates a transition from restricted marine to open coastal settings. This high-diversity fauna is dominated by necto-benthic batomorphs and is similar in its structure to other Tethyan and Eastern Atlantic Maastrichtian faunas. The high number of species apparently endemic to the studied area suggests high habitat differentiation in corresponding marine Maastrichtian settings. The presence of typical Maastrichtian species in the top of Unit 1 argues against a Campanian age previously proposed for this unit, while our data support a middle–late Maastrichtian age for the ferruginous sandstone bed of Unit 3.

Characterization of the inverted metamorphic gradient of the Passos Nappe (SE-Brazil) based on multiple geothermobarometers

Inverted metamorphic gradients are recognized in several Proterozoic to Phanerozoic orogens. However, the mechanisms that are responsible for the formation of these gradients are still a matter of discussion. In the southern Brasília Orogen, an inverted metamorphic gradient is recorded in the Passos Nappe, that comprises a sequence of metasedimentary rocks with minor intercalations of tholeiitic metamafic rocks, metamorphosed under greenschist facies (biotite zone) at the bottom and high-pressure granulite facies at the top. This paper presents a detailed investigation of pressure and temperature conditions along the Passos Nappe by integrating conventional geothermobarometry in metamafic rock and single-trace elements geothermometry (Zr-in-rutile and Ti-in-quartz) in quartzite. Nine lithostratigraphic units are identified in the Passos Nappe, which are informally named A to I, from the bottom to top, adding up to a minimum thickness of 4.56 km. In mafic rocks, the increase of metamorphic conditions is indicated by: (1) chemical variation of amphiboles, from actinolite/edenitic hornblende in Unit C, to pargasitic hornblende in Units E and G, and Fe-pargasite in Units H and I; (2) plagioclase composition varying from An1-13 in Unit C, An15-27 in Unit D to andesine at upper units; and (3) occurrence of clinopyroxene from the top of Unit E upwards. Additional petrographic features in quartzite also corroborate these insights, such as: the increase of grain size of both quartz and rutile, from bottom to top; the morphology of rutile crystals; the mechanism of quartz recrystallization (subgrain rotation dominates in lower units while grain boundary migration dominates in upper units) and the occurrence of rutile needles in quartz in the upper units, which suggest a continuous gradient. Combined application of Zr-in-rutile and Ti-in-quartz geothermometers in quartzite provided the following average temperature and pressure values for each unit: 673 °C and 11.7 kbar for Unit E, 690 °C and 9.5 kbar for Unit G, 747 °C and 11.6 kbar for Unit H and 758 °C and 11.3 kbar for Unit I. Our data indicate that the Zr-in-rutile and Ti-in-quartz systems do not re-equilibrate in metamorphic conditions lower than 600 °C. The extrapolation of these results to the low-grade units indicate: 496 °C and 4.8 kbar at Unit A, 504 °C and 4.8 kbar at Unit B, 502 °C and 4.4 kbar at Unit C. The results obtained with the geothermometer hornblende-plagioclase and the geobarometer garnet-rutile-ilmenite-plagioclase-quartz in metamafic rocks yielded the following average values for each unit: 486 °C and 5.0 kbar at Unit C, 568 °C and 6.0 kbar at Unit D, 622 °C and 8.1 kbar at Unit E, 650 °C and 8.4 kbar at Unit G, 743 °C and 9.4 kbar at Unit H and 723 °C and 11.2 kbar at Unit I. Since the two geothermobarometric approaches are based on independent chemical systems, the results obtained represent robust estimates for the temperature and pressure conditions for the inverted metamorphic gradient of the Passos Nappe. Comparing the inverted metamorphic gradient at the Passos Nappe with classical occurrences at the Himalaya Orogen, it is possible to assume that both origins were similar, which probably involved isotherm inversion due to subduction, channel flow process associated with shear heating and transport of higher pressure rocks over low-pressure rocks by reverse ductile shearing.

A multiproxy reconstruction of the Late Pleistocene-Holocene paleoenvironment: New insights from the NW Black Sea

We provide new data on the Black Sea evolution during the preconnection and reconnection stages with the Mediterranean, based on sedimentology, geochemistry, calcareous nannoplankton and ostracods analysis, in 13 cores placed between 141 m and 1315 m water depths. Unit 1 – The Coccolith Mud, Unit 2 - The Sapropel Mud and Unit 3 – The Lacustrine Lutite, including red-brownish depositional intervals, were identified in all studied deep-water cores. These units are not present in a shallow setting, i.e., above 166.7 m water depth, being replaced by sandy levels and shell debris. The red-brownish layers are interpreted as hyperpycnites, but some of the red successions show turbidite features, especially the ones situated on canyon sides, at a water depth below 600 m. In the oldest intercepted Unit 3 of the Neoeuxinian lake, CaCO3 values are constantly low, but towards the top of this unit they show two distinct peaks, interpreted as signals of Bølling–Allerød and Preboreal interstadials. In between, a significant decrease in CaCO3 values, linked to the setting of the Younger Dryas, is coincident in the deep cores with diatom blooms and high frequency of ostracods related to cooler waters. The scarce but continuous occurrence of Emiliania huxleyi nannoplankton species from the top of Unit 3 indicates that the salinity was at least 10–11‰ prior the Sapropel Mud deposition and increase at a least 17‰ within the lower part of Unit. 2. In the same interval, the Ponto-Caspian ostracods are still present. The Mediterranean ostracods consistently occur from the top of Unit 2, when bottom water salinity reached values for allowing their survival. The first bloom of E. huxleyi is placed at the base of Unit 2, where coccolith laminae occur, while the second bloom is placed in Unit 1 that contains exclusively Mediterranean ostracods and consistent assemblages of benthonic foraminifers, similar with the ones present today.

Geochemistry of the new Permian-Triassic boundary section at Sitarička Glavica, Jadar block, Serbia

The new Permian-Triassic boundary section at Sitarička Glavica is unusual for the Alpine-Dinaride area, in that it is dominantly detrital in the early Triassic. It shows a change from late Permian limestones and shales with a diverse algal-brachiopod-echinoid-molluscan biota deposited in quieter normal marine water (unit 1), into similar sediments, with coarser calcisiltites and a reduced fauna (unit 2), into a post-extinction early Triassic calcareous micaceous siltstones and silty limestones (unit 3) with only molluscan fossils. The major element ratios to Al, reported previously, change little through the section, but with slightly higher ratios just above the extinction horizon (except for K) at the top of unit 2, and show moderate chemical weathering, and change to slightly less mature sediments above. Trace element ratios to Al show little consistent change, except for higher values of most ratios at the contact of units 1 and 2, where slight but temporary increases in TOC, δ13Corg and δ15Norg, and C/N, and decrease in δ34Spyrite occur. Low HFSE (Zr, Nb, REE, Th, U, Ta) values and ratios to other elements indicate that the resistant mineral containing these elements were somehow removed prior to deposition in residual lags by preferential wind erosion. Paleoredox indicators are inconsistent but anoxia seems present in unit 1 and 2 with mostly oxic conditions in unit 3 above. The geochemistry of this section does not show the concentrations of Ni and other elements attributed to Siberian Trap sources and evidence of volcanic input to the sediments is confined to below and above the extinction horizon. The main geochemical changes occur at the top of unit 1, corresponding with a faunal diversity decrease, but not extinction, and at the +5 metre horizon, which seems to- have no faunal changes associated with it. The carbon and other organic-related elements and isotopes indicate a terrestrial input of freshwater and organic matter just before the main negative spike in carbon isotopes, which greatly reduced the diversity of the fauna, and was shorty followed by an input of intermediate volcanic ash. The actual extinction and paleontological Permian-Triassic boundary occurred at the top of unit 2, and has little geochemical changes associated with it. These events can be recognized in other Alpine-Dinaride sections -. All Alpine-Dinaride sections also show a latest Permian ostracod-dominated horizon with a negative spike in organic carbon and sulphur isotopes that indicate an input of freshwater and land plant material, reducing the surface waters to almost brackish, followed by a return to normal salinities with the return of a reduced latest Permian fauna, accompanied by a gradual negative shift in carbonate carbon isotopes accompanied by input of intermediate volcanic material. The main extinction is accompanied by a drop in the difference between carbon and organic carbon isotopes, indicating less land plant input; but otherwise there is little geochemical change at this horizon.

Buried iceberg-keel scouring on the southern Spitsbergenbanken, NW Barents Sea

PARASOUND (3.5kHz) subbottom echosounder profiles acquired on the southern Spitsbergenbanken, NW Barents Sea, show iceberg-keel scouring features which are buried by sediment that accumulated during the post Last Glacial Maximum (LGM) sea-level rise. Four acoustic units (Units 1 to 4 in stratigraphic order) were differentiated, based on the characterization of their acoustic facies and reflection surfaces. Unit 1 shows a chaotic internal structure and is interpreted as a glacial till, whereas the laminated Units 2 to 4 accumulated by sediment settling from suspension clouds and bottom currents during the last deglaciation phase. The top of Unit 2 was frequently incised by iceberg keels, resulting in up to 12m deep ploughmarks which were later filled and buried by Unit 3 and 4 sediments. Three main paleo-evironmental changes controlled the evolution of the facies succession: (1) The major shift from till formation (Unit 1) below grounded ice to the accumulation of laminated sediments (Unit 2) which are inferred to reflect ice lifting and meltwater release; (2) Iceberg-keel scouring after sedimentation of Unit 2; (3) the probable abrupt termination of iceberg-keel scouring related to the glacio-eustatic sea-level rise. A linkage between these episodes of changes and short-lasting phases of rapid post LGM sea-level rise, such as meltwater pulses, is inferred, although further studies are needed to better understand the temporal and genetic relationships between the sedimentary events recognized in the Barents Sea and climate changes.

A late Quaternary vertebrate deposit in Kudjal Yolgah Cave, south‐western Australia: refining regional late Pleistocene extinctions

ABSTRACTWe describe the stratigraphy and chronology of Kudjal Yolgah Cave in south‐western Australia, a late Quaternary deposit pre‐ and post‐dating regional human arrival and preserving fossils of extinct and extant fauna. Single‐grain optically stimulated luminescence (OSL) dating shows that seven superposed units were deposited over the past 80 ka. Remains of 16 mammal species have been found at the site, all of them represented in Unit 7, for which seven OSL ages indicate accumulation between 80 and 41 ka. Single‐grain OSL equivalent dose distribution patterns show no evidence of reworking of older or younger sediments into Unit 7, but late Holocene charcoal has been washed into the top of it from adjacent Unit 2, deposited 1.2 ka ago. Six species that failed to survive the Pleistocene are recorded in Unit 7, but only the south‐western wombatVombatus hackettiis recorded in younger units. Two species, the large extinct kangaroosProtemnodonsp.cf.P. roechusandProcoptodon browneorum, are represented by articulated specimens near the top of Unit 7, immediately adjacent to an OSL sediment sample dated to 41 ± 2 ka. These are the youngest reliably dated records of these genera from mainland Australia, and among the youngest megafaunal remains from the continent. All species currently known from the middle Pleistocene of the south‐west persisted into the late Pleistocene, which removes a key pillar supporting the argument against a driving role for human impacts in the extinctions.

Rockmagnetic correlation between Holocene cave sediments at the mountain and loess soil deposits in Piedmont Crimea (on example of the trap cave Emine-Bair-Khosar and archaeological site Biyuk-Karasu-XIX)

Modern soils and paleosols, loess and uncemented aeolian caves sediments are carriers of a paleoclimatic signal (Evans, Heller, 2003). The saturation degree of a layer with humus material, depending on the temperature, correlates well with magnetic susceptibility and different types of magnetization of deposits (Tang et al., 2003, BosakP 3, 4 - hlb1 - the soil has features of grey or sod-calcareous soil forming, unit 3 combines horizons He and Eh, unit 4 – illuvial horizons Ihp and Pigl; 5 - pc-bg? - silty-sandy light loam. Lithofacial units are named according to “Stratigraphic scheme of Quaternary deposits of Ukraine” (Veklich et al., 1993), soil horizons are named according to Dokuchaev soil classification (Dokuchaev, 1883). Section of unconsolidated sediments in the Emine-Bair-Khosar Cave consists of the following lithofacial units: unit A - dark gray, loose, structureless, moist, light-loamy texture, contains much of organic matter. Unit B - sequence, which consists of eight packs of clay+clastic layer, reddish-brown in color, size of limestone rubbles in clastic layers increases from 1-2 mm at the top of unit, to 1 cm at the bottom. Unit C is light, fawn, homogeneous clay, with a low content of clastic material. There are two clastic units in the section, D and F, and they are formed by chaotically accumulated acute limestone debris up to 5 cm in diameter. Unit E has similar morphology as unit C. Unit G composed with the grayish-brown clay with thin clastic beds.

Hydrocarbon-Mobility Steering for Optimum Placement of a Power Water Injector

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164282, ’Hydrocarbon-Mobility Steering for Optimum Placement of a Power Water Injector Above Tar Mats - A Case Study From a Light-Oil Carbonate Reservoir in the Middle East,’ by Craig Saint and Thomas Glowig, Baker Hughes, and Ashis S.S. Swain, Nasser A. Al-Khaldi, Mohd. H. Al-Otaibi, Abdul Aziz Al Ghareeb, and Ahmad Bader Al Bader, Khafji Joint Operations, prepared for the 2013 SPE Middle East Oil and Gas Show and Exhibition, Manama, Bahrain, 10-13 March. The paper has not been peer reviewed. Tar mats are encountered in many Middle East carbonate reservoirs. In cases in which tar acts as a flow barrier between water and light oil, water injectors work most efficiently when placed horizontally immediately above the tar mat to maintain pressure support during production. As this strategy is pursued, a real-time tar-detection method is needed while drilling. The low mobility caused by tar can be detected with a formation-pressure- while-drilling (FPWD) tester, while a nuclear-magnetic-resonance (NMR) tool provides oil-viscosity estimation. Introduction The Ratawi limestone reservoir was discovered in 1963 in the offshore Neutral- Zone Concession area between Saudi Arabia and Kuwait. The production of light sour crude oil commenced in 1966. After an increase in gas/oil ratio and a sharp drop in reservoir pressure occurred, dump-water injection was started in 1975. Until recently, peripheral dump-water injection was used. Geologically, the reservoir is divided into three units, Units A, B, and C from top to base, respectively. Of these, Unit C is the main producing unit. The top of Unit C is considered as a possible sequence boundary/unconformity. Units A and B represent initial flooding over Unit C. The two units are composed of alternating porous limestone and less-porous or tight limestone, and show overall transgressive-system tract sequence. The heavy-oil zone was recognized above the oil/water contact. Additional drillstem tests and investigation of the separation between shallow and deep resistivity indicated fieldwide distribution of the heavy-oil zone. Wireline NMR and pressure data indicated presence of a tar/asphaltene zone in the heavy-oil zone. The operator’s strategy was to carry out peripheral water injection, placing the injectors above and as close as possible to the heavy-oil/tar zones, enabling better sweep and volumetric displacement of light oil toward potential producers. The reservoir characterization around the injector locations is critical for assessing carbonate heterogeneity, diagenetic uncertainty, asphaltene presence, and a transition zone of heavy-to-light oil. Assessment of risk-evaluating conventional- wirelinelog responses, seismic attributes, and nearby well dynamic information resulted in good prognosis of the area for deciding the entry point or landing point of the injectors and was crucial in well.placement. Offshore operations did not allow evaluation of the tar/asphaltene zone in each well because of higher operating costs from drilling a pilot hole. In recent wells, the placement was achieved on the basis of mobility steering as close as possible to the heavy-oil/tar zone.

A remarkable sea-level drop and relevant biotic responses across the Guadalupian–Lopingian (Permian) boundary in low-latitude mid-Panthalassa: Irreversible changes recorded in accreted paleo-atoll limestones in Akasaka and Ishiyama, Japan

The Capitanian (Upper Guadalupian) to Wuchiapingian (Lower Lopingian) shallow-marine limestones at Akasaka and Ishiyama in central Japan record unique aspects of the extinction-related Guadalupian–Lopingian boundary (G-LB) interval. The ca. 140m-thick Akasaka Limestone consists of the Capitanian black limestone (Unit B; 112m) and the Wuchiapingian light gray dolomitic limestone (Unit W; 21m), with a black/white striped limestone (Unit S; 9m) between them. The G-LB horizon is assigned at the base of Unit W, on the basis of the first occurrence of the Wuchiapingian fusulines. The Capitanian Unit B and the Wuchiapingian Unit W were deposited mostly in the subtidal zone of a lagoon, whereas the intervened Unit S and the lowermost Unit W were in the intertidal zone. A hiatus with a remarkable erosional feature was newly identified at the top of Unit S. These records indicate that the sea-level has dropped significantly around the G-LB to have exposed the top of the atoll complex above the sea-level. The Ishiyama Limestone, located ca. 10km to the north of the Akasaka limestone, retains almost the same depositional records. The extinction of large-tested fusuline (Yabeina) and large bivalves (Alatoconchidae) occurred in the upper part of Unit B, and the overlying 20m-thick limestone (the uppermost Unit B and Unit S) below the hiatus represents a unique barren interval. The upper half of the barren interval is more depleted in fossils than the lower half, and this likely represents a duration of the severest environmental stress(es) for the shallow-marine protists/animals on the mid-oceanic paleo-atoll complex. Small-tested fusulines re-appeared at the base of Unit W above the hiatus. These facts prove that the elimination of shallow-marine biota occurred during the Capitanian shallowing of Akasaka paleo-atoll before the subaerial exposure/erosion across the G-LB. The overall shallowing and the development of such a clear hiatus at the top of a mid-oceanic seamount, in accordance with the contemporary sea-level curve based on continental shelf records, indicates that a remarkable sea-level drop has occurred globally during the latest Capitanian. This further suggests that a cool climate likely has appeared even in the low-latitude domains in Panthalassa to cause the decline of the Middle Permian shallow-water protists/animals that adapted to warmer seawater. The Wuchiapingian biota first appeared immediately after this erosional episode, i.e., during the onset of warming after the G-LB.

Large-Scale Seismogenic Deformation of A Carbonate Platform Straddling the Precambrian-Cambrian Boundary, Karatau Range, Kazakhstan

Abstract The Berkuty Member of the Ediacaran Kyrshabakty Formation in the Malyi Karatau Mountains of southern Kazakhstan immediately underlies some of the world's largest sedimentary phosphate deposits of earliest Cambrian age. This 5–30 m thick unit, exposed over > 40 km strike length, is a complex carbonate unit that was formed by regional seismic deformation of semiconsolidated carbonate. Three subdivisions are recognized. The lower unit (Unit 1), ∼ 3–5 m thick, is regionally shortened by meter-scale flexural folds, and shows thrusting and imbrication of underlying siliciclastic units; local extension is accommodated by soft-sediment boudinage. The middle unit (Unit 2), ∼ 2–5 m thick, consists largely of a megabreccia of angular to subrounded dolomite clasts up to a meter across. The upper unit (Unit 3), up to 3 m thick, consists of microbially laminated dolobindstone, dolorudstone, and flat-pebble conglomerate, cut by syndepositional, low-angle normal faults. Channelization and bank-margin slumping at the top of Unit 3 was followed by karsting, prior to the formation of a high-energy, shallow-water, carbonate–phosphatic coastal environment. We interpret the Berkuty Member as an example of an incompletely and unevenly lithified carbonate platform deformed by at least one large seismic event (Mb greater than approximately 8). Deformation of the lower unit included local sliding, imbrication, and folding, whereas the middle unit was deformed by in-situ disaggregation due to extensive shaking. Regional coseismic or early postseismic uplift of soft sediment, possibly above sea level, led to widespread gravitational collapse, rapid drainage incision, and drainage fill by bank collapse and conglomeratic wedges. While each of the deformational features and stratigraphic elements observed in this unit is not diagnostic of a seismogenic origin, the remarkable lateral extent in outcrop, the wide range of structures related to soft-sediment deformation, their systematic lateral variability, and the internal process-related stratigraphy demonstrates in an exemplary way the manner in which single soft-sediment deformation events can produce a large array of features over a broad area, largely due to minor rheological differences.

Assessing tectonic and climatic causal mechanisms in foreland‐basin stratal architecture: insights from the Alborz Mountains, northern Iran

ABSTRACTThe southern foreland basin of the Alborz Mountains of northern Iran is characterized by an approximately 7.3‐km‐thick sequence of Miocene sedimentary rocks, constituting three basin‐wde coarsening‐upward units spanning a period of 106 years. We assess available magnetostratigraphy, paleoclimatic reconstructions, stratal architecture, records of depositional environments, and sediment‐provenance data to characterize the relationships between tectonically‐generated accommodation space (A) and sediment supply (S). Our analysis allows an inversion of the stratigraphy for particular forcing mechanisms, documenting causal relationships, and providing a basis to decipher the relative contributions of tectonics and climate (inferred changes in precipitation) in controlling sediment supply to the foreland basin. Specifically, A/S > 1, typical of each basal unit (17.5–16.0, 13.8–13.1 and 10.3–9.6 Ma), is associated with sharp facies retrogradation and reflects substantial tectonic subsidence. Within these time intervals, arid climatic conditions, changes in sediment provenance, and accelerated exhumation in the orogen suggest that sediment supply was most likely driven by high uplift rates. Conversely, A/S < 1 (13.8 and 13.8–11 Ma, units 1, and 2) reflects facies progradation during a sharp decline in tectonic subsidence caused by localized intra‐basinal uplift. During these time intervals, climate continued to be arid and exhumation active, suggesting that sediment supply was again controlled by tectonics. A/S < 1, at 11–10.3 Ma and 9‐6–7.6 Ma (and possibly 6.2; top of units 2 and 3), is also associated with two episodes of extensive progradation, but during wetter phases. The first episode appears to have been linked to a pulse in sediment supply driven by an increase in precipitation. The second episode reflects a balance between a climatically‐induced increase in sediment supply and a reduction of subsidence through the incorporation of the proximal foreland into the orogenic wedge. This in turn caused an expansion of the catchment and a consequent further increase in sediment supply. Copyright © 2013 John Wiley & Sons, Ltd.

Heterogeneous diagenetic patterns in the Pleistocene Ironshore Formation of Grand Cayman, British West Indies

The Ironshore Formation on Grand Cayman consists of six unconformity-bounded units (A to F) that developed in response to repeated transgressive–regressive cycles during the Middle to Late Pleistocene. Corals and matrices in limestones from the Rogers Wreck Point (RWP), offshore George Town (GT), and western onshore (WO) areas are characterized by complex diagenetic fabrics that reflect marine (bioerosion, micrite envelopes, internal sediments, fibrous high-Mg calcite, acicular aragonite, isopachous prismatic calcite cements), freshwater phreatic (circumgranular cements, even/random blocky calcite cements), and vadose (meniscus calcite cements, blocky calcite cements) diagenesis. Throughout these limestones, the matrices have undergone more meteoric diagenetic alteration than the corals. Overall, however, no systematic stratigraphic patterns exist to the distribution of these diagenetic fabrics and it is generally impossible to link the different phases of diagenesis with specific unconformities that cap each unit. These heterogenous patterns of diagenetic features can be attributed to many intrinsic and extrinsic factors, including the original compositions of the different components in the limestones, the porosity and permeability of the substrate, the nature of the diagenetic fluids, climate, and the duration of exposure during each lowstand. Integration of available data, however, indicates that maximum diagenesis took place during the lowstands denoted by the unconformities at the top of Unit C (Marine Isotope Stage 7) and Unit D (Marine Isotope Stage 5e) when long periods of exposure were accompanied by wet climates with high annual rainfalls.

Explaining the exceptional preservation of Ediacaran rangeomorphs from Spaniard's Bay, Newfoundland: A hydraulic model

Exceptional 3-D preservation of Ediacaran rangeomorph fossils is found on a single bedding plane at Spaniard's Bay, Newfoundland. This high-quality preservation has previously been explained by entrainment of organisms within the Td-e mudstone division of a distal turbidite, followed by encasement within concretions. Our sedimentological and taphonomic analysis reveals a clear association between these fossils and evidence for erosive unidirectional flows, including scours marks, tool marks, ridge-and-groove marks, parting lineations and current crescents. We suggest an alternative sequence of events that runs broadly as follows: (i) rangeomorph discs were anchored to the seafloor during deposition of planar laminated silts (our unit 2, <10mm thick; Td), now bearing pyrite framboids and pyritized organic matter; (ii) rangeomorph fronds were then felled and entrained by high velocity unidirectional currents, to lie within their own erosional scours at the top of unit 2, or to form tool marks; (iii) this topography was then draped and cast by soft-weathering sand (unit 3, Tc) associated with the growth of early diagenetic pyrite around sand grains. Pyrite grains also appear to have replaced clumps of organic matter. Fossil impressions have since been exposed by differential weathering of the ferruginous sands with respect to the silts. This new context now provides a parsimonious explanation for a range of hitherto paradoxical structures. Features previously regarded as microbial mats (‘bubble trains’) that formed in the lee of sinuous ripples on the top of unit 2 may be explained as load-casts, or by localised gas escape within areas of lowered hydraulic pressure. Rangeomorph fronds remarkably preserved in positive (rather than the more usual negative) epirelief are explained by means of sediment-casting of branches that became ruptured in the high velocity current. Paradoxical structures previously thought to be enclosing biological ‘sheaths’ around rangeomorph fronds are reinterpreted as scour marks, whereas imbricate overlaps of first order branches in Beothukis, Trepassia and Avalofractus are explained by hydraulic shear, driven by overlying currents across ruptured and deflated fronds. We find that rangeomorph bodies could be deflated, imbricated, folded over, inverted, and infilled with fine sediment. Our hydraulic model provides a null hypothesis against which future observations of rangeomorph fronds can now be tested. It removes some significant anomalies in our understanding of rangeomorph architecture, and provides a better understanding of the physical properties of their body tissues, permitting the possibility of a reasoned consideration of their puzzling biological affinities.

Textures and mineral compositions of the Xinjie layered intrusion, SW China: Implications for the origin of magnetite and fractionation process of Fe-Ti-rich basaltic magmas

The Xinjie layered intrusion in the Panxi region, SW China, hosts both Fe-Ti oxide and platinum-group element (PGE) sulfide mineralization. The intrusion can be divided, from the base upward, into Units I, II and III, in terms of mineral assemblages. Units I and II are mainly composed of wehrlite and clinopyroxenite, whereas Unit III is mainly composed of gabbro. PGE sulfide-rich layers mainly occur in Unit I, whereas thick Fe-Ti oxide-rich layers mainly occur in Unit III. An ilmenite-rich layer occurs at the top of Unit I. Fe-Ti oxides include magnetite and ilmenite. Small amounts of cumulus and intercumulus magnetite occur in Units I and II. Cumulus magnetite grains are commonly euhedral and enclosed within olivine and clinopyroxene. They have high Cr2O3 contents ranging from 6.02 to 22.5 wt.%, indicating that they are likely an early crystallized phase from magmas. Intercumulus magnetite that usually displays ilmenite exsolution occupies the interstices between cumulus olivine crystals and coexists with interstitial clinopyroxene and plagioclase. Intercumulus magnetite has Cr2O3 ranging from 1.65 to 6.18 wt.%, lower than cumulus magnetite. The intercumulus magnetite may have crystallized from the trapped liquid. Large amounts of magnetite in Unit III contains Cr2O3 (<0.28 wt.%) much lower than magnetite in Units I and II. The magnetite in Unit III is proposed to be accumulated from a Fe-Ti-rich melt. The Fe-Ti-rich melt is estimated to contain 35.9 wt.% of SiO2, 26.9 wt.% of FeOt, 8.2 wt.% of TiO2, 13.2 wt.% of CaO, 8.3 wt.% of MgO, 5.5 wt.% of Al2O3 and 1.0 wt.% of P2O5. The composition is comparable with the Fe-rich melts in the Skaergaard and Sept Iles intrusions. Paired non-reactive microstructures, granophyre pockets and ilmenite-rich intergrowths, are representative of Si-rich melt and Fe-Ti-rich melt, and are the direct evidence for the existence of an immiscible Fe-Ti-rich melt that formed from an evolved ferro-basaltic magma.

High resolution seismic study of the Holocene infill of the Elkhorn Slough, central California

The seismic analysis of the sedimentary infill of the Elkhorn Slough, central California, reveals a succession of three main seismic units: U1, U2, U3, with their correspondent discontinuities d2, d3. These units are deposited over a paleorelief representing the channel location at least 8k years ago. The location of that paleochannel has not changed with time, but the geometry of the infilling sedimentary packages has done so through the years. Discontinuities d2 and d3 show a relic island or relative high in the center of the Slough that separated the sedimentation into two main small basins at least 3k years ago. There is evidence of erosion in the last two sedimentary units showing that the present erosive pattern began decades ago at minimum. We have correlated radiocarbon data of selected cores with the high resolution chirp profiles and reconstructed the infill for the Elkhorn Slough.In the most recent unit, the occurrence of numerous lateral accretion surfaces on both ends of the main channel is discussed within their environmental setting, tidal currents and the net ebb flux of the area. We have interpreted the presence of gas in the sediments of the slough, with a gas front located at the tops of units 2 and 3, which are discontinuities that reflect an effective seal for the gas. Our data shows no obvious evidence for seepage, although the shallow presence of some of the fronts points out the fragility of the environment in the present erosive conditions.

Faunal assemblages from the upper Horseshoe Canyon Formation, an early Maastrichtian cool-climate assemblage from Alberta, with special reference to theAlbertosaurus sarcophagusbonebedThis article is one of a series of papers published in this Special Issue on the themeAlbertosaurus.

The faunal assemblage from the early Maastrichtian portion of the Horseshoe Canyon Formation is described on the basis of four new vertebrate microfossil localities and remains from the Albertosaurus bonebed. All of the localities sampled were deposited during a cool, dry climate at a palaeolatitude of ∼58°N. Thus, these assemblages provide insight into a northern cool-climate assemblage in the early Maastrichtian of western North America. This fauna is characterized by the presence of taxa with more northern affinities, such as Holostean A, champsosaurs, Troodon , and toothed birds. Warm-climate taxa, such as crocodylians, large and diverse turtles, and albanerpetontids are notable in their absence. The Albertosaurus bonebed locality at the top of unit 4 of the Horseshoe Canyon Formation was deposited during the initial stages of a trend to a warmer and wetter climate that is represented in unit 5. The bonebed shares many taxa with the underlying vertebrate microfossil localities. However, a notable difference is the presence of Atrociraptor marshalli from the Albertosaurus bonebed but not the other localities in the upper Horseshoe Canyon Formation. The presence of Atrociraptor may be attributable to this change in climate rather than local ecological conditions. Also, the assemblages are different in the paucity of fish remains in the bonebed, and the relative rarity of shed hadrosaur teeth. The low abundance of aquatic taxa and rarity of shed teeth of hadrosaurs indicate that the locality is largely autochthonous, with little material being transported into the site.

Depositional Environments and Sequence Stratigraphy of an Exhumed Permian Mudstone-Dominated Submarine Slope Succession, Karoo Basin, South Africa

Abstract The physical stratigraphy of a 470-m-thick, claystone-dominated exhumed middle to upper submarine slope succession was constrained within an area of 400 km2 in which five sand-prone units were characterized (Units D/E, E, F, G, and H). Units D/E to Unit F show an overall pattern of thickening upward and basinward stepping. This stacking pattern is reversed from the top of Unit F to the base of Unit H, above which basinward stepping is again observed. Different architectural styles of sand-prone deposits occupy predictable stratigraphic positions within the basinward-stepping section, starting with intraslope lobes through channel–levee complexes to entrenched slope valleys. Sandstone percentage is highest in the intraslope lobes and lowest in the slope valley fills, reflecting a change from depositional to bypass processes. The landward-stepping stratigraphy is dominated by claystone units with thin distal fringes of distributive deposits. The upper basinward-stepping succession (Unit H) is a distributive system possibly linked to a shelf edge delta. Across-strike complexity in the distribution of sand-prone units was controlled by cross-slope topography driven by differential compaction processes. Hemipelagic claystones separating the sand-prone units represent shutdown of the sand delivery to the whole slope and are interpreted as relative sea-level highstand deposits. Eleven depositional sequences are identified, nine of which are arranged into three composite sequences (Units E, F, and G) that together form a composite sequence set. The highly organized physical stratigraphic stacking suggests that glacioeustasy, during the Late Permian icehouse period, was the main driving process for the analyzed succession.

The Great Barrier Reef: a 700 000 year diagenetic history

AbstractA deep borehole through Ribbon Reef 5 in the Great Barrier Reef off north‐eastern Australia has identified a variety of cements, including epitaxial, radial prismatic and spherular aragonite, together with blocky, prismatic and fibrous calcite. These cements are discontinuously arranged within the sequence that consists predominantly of grainstones but locally includes clotted muddy and filamentous textures that may be of microbial origin. Calcite cements vary in morphology with groups of crystals that include acute scalenohedral, rhombohedral and flattened concordant terminations; these show varying densities of inclusions that locally define growth zones and in some terminations divide in the manner of ‘split crystals’ to form fibrous fringes. Morphological changes in calcite are inferred to reflect changes in water chemistry and crystal growth rates at the time of growth, allied to their relationship to the palaeo‐water table, and linked in turn to changes in sea‐level. Neomorphism and dissolution are widespread and variations in the severity of both imply response to the degree of undersaturation of pore waters that at times were probably balanced within very narrow limits. A total of 10 depositional units are identified. Those units at the base of the borehole reflect deposition and diagenesis within a marine environment. The influence of meteoric waters, indicated by stable isotopes, is first apparent at the top of Unit 1 and in Unit 2 (184 to 155 m below sea floor). Petrographic evidence of vadose conditions appears at the tops of Unit 3 (131 to 99 m below sea floor). Units 4 to 8, all deposited under marine conditions, provide isotopic evidence of meteoric or mixing‐zone waters and petrographic indicators of vadose conditions, typically at the top of the units. Evidence indicates that in Unit 5 the water table was mobile and Units 6a, 6b, 7 and 8, all characterized by ultraviolet fluorescent cements, are capped by sub‐aerial erosion surfaces. Unit 9 (the Holocene) reflects the recent re‐establishment of marine conditions. The extent of alteration of the entire sequence reflects the substantial and pervasive influence of meteoric waters. This effect is interpreted as a result of a greater rainfall and river flow from the mainland during early and late stages of interstadial periods. The study reflects progress in the ability to recognize the diagenetic signal generated by sea‐level change. However, whereas the isotopic results reflect the changing relationships between vadose and phreatic zones in groundwater systems beneath successive emergent surfaces, their correspondence with petrographic features is expressed only weakly and commonly lacks the systematic sequential overprinting implied by the distribution of cathodoluminescent zones of cements in many ancient limestones.

Tectonic control on sedimentary facies pattern and sediment accumulation rates in the Miocene foreland basin of the southern Alborz mountains, northern Iran

The southern Alborz mountains of northern Iran are an integral part of the Arabia/Eurasia collision zone. A magnetostratigraphic and rock magnetic investigation of the Eyvanekey stratigraphic section in the southern Alborz mountains reveals the spatiotemporal character of sedimentary facies migration in the Alborz foreland basin. The section constitutes three coarsening upward units (units 1, 2, and 3), comprising the Upper Red and Hezardarreh formations. Our data reveal that the Upper Red Formation was deposited between 17.5 and 7.5 Ma, while the depositional age of the top of the Hezardarreh Formation can be extrapolated to ∼6.2 Ma. Slow sediment accumulation rates correlate with sedimentary facies comprising prograding, coarsening‐upward units. This is likely the result of intraforeland uplift (units 1 and 2) and basin inversion, probably associated with a growth syncline located in the proximal foreland (unit 3). In contrast, fine‐grained strata at the bottom of each cycle are associated with faster sediment accumulation rates, testifying to enhanced flexural basin subsidence in the course of thrust loading. Progradation of coarse‐grained facies also occurred during relatively fast sediment accumulation (top of unit 2), suggesting that the influx of coarse‐grained sediment outpaced the storage capacity of the proximal foreland. Thus, despite an overall southward propagation of deformation into the southern Alborz foreland, the locus of active deformation must have migrated back and forth on a time scale of circa 0.7 to 2 Ma.

Highstands during Marine Isotope Stage 5: evidence from the Ironshore Formation of Grand Cayman, British West Indies

The Ironshore Formation on Grand Cayman is formed of six unconformity-bounded packages (units A–F). Units A, B, C, and D, known from the subsurface in the northeastern part of Grand Cayman, formed during Marine Isotope Stages (MIS) 11(?), 9, 7, and 5e, respectively. Unconformities at the tops of units A, B, and C are highlighted by terra rossa and/or calcrete layers. Strata in core obtained from wells drilled in George Town Harbour and exposed on the west part of Grand Cayman belong to unit D, and the newly defined units E and F. Corals from unit E yielded Th/U ages of ∼104ka whereas conch shells from unit F gave ages of ∼84ka. Unit E equates to MIS 5c whereas unit F developed during MIS 5a.Th/U dating of corals and conchs from the Ironshore Formation on the western part of Grand Cayman shows that unit D formed during the MIS 5e highstand whereas units E and F developed in association with highstands at 95–110ka (MIS 5c) and 73–87ka (MIS 5a). Unit E, ∼5m thick in the offshore cores, is poorly represented in onshore exposures. Unit F, which unconformably overlies unit D at most localities, is formed largely of fossil-poor, cross-bedded ooid grainstones. The unconformity at the top of unit D, a marine erosional surface with up to 2.5m relief, is not characterized by terra rossa or calcrete in the offshore cores or onshore exposures. Unit D formed with a highstand of +6m asl, whereas units E and F developed when sea level was +2 to +5 asl and +3 to +6m asl, respectively. Thus, the highstands associated with MIS 5e, 5c, and 5a were at similar elevations.