Marine Siltstones Research Articles

Estimating the upper limit of Proterozoic petrographic organic carbon recycling

Atmospheric oxygen levels in the Proterozoic Eon are strongly debated, with estimates ranging from < 0.1 % – 10 % present atmospheric levels (PAL). A new approach to assess Proterozoic atmospheric oxygen levels based on the concentrations of recycled petrographic organic carbon in Proterozoic-aged strata indicates that atmospheric oxygen was potentially between 2 % and 24 % PAL. However, this model lacks some precision because the petrographic organic carbon flux in the Proterozoic is poorly constrained.This study focuses on marine siltstone and shale of the post-glacial Cryogenian Tapley Hill Formation (Adelaide Basin, south Australia) that was deposited during a eustatic transgression following the Sturtian glaciation. This strata contains concentrations of 0.041–0.754 wt% total organic carbon (TOC) and experienced burial diagenesis temperatures of 195 – 220 °C based on Raman spectroscopic characterization of disordered organic carbon. However, a portion of 12.4–58.1 % of the TOC within these rocks is of higher maturity, in part fully converted to graphite, and must therefore be recycled petrographic organic carbon of clastic origin. The measured proportion (%) of petrographic organic carbon increases with decreasing TOC concentration, resulting in petrographic organic carbon concentrations of 0.019 – 0.095 wt% (average 0.052 wt%).Because of potentially high post-glacial erosion rates, we presume that the reported petrographic organic carbon concentrations mark an upper limit on organic carbon recycling for the Proterozoic Eon. However, the diagenetic overprint of the Tapley Hill Formation in combination with petrographic similarities between graphitic and disordered organic carbon particles both indicate that the given values are a minimum estimate of petrographic organic carbon recycling and that the actual concentration of recycled organic carbon in the Tapley Hill Formation is higher. Therefore, we conclude that petrographic organic carbon recycling in the Neoproterozoic during the deposition of the Tapley Hill Formation was similar to modern marine depositional systems that display high organic carbon recycling rates. Because the weathering of petrographic organic carbon functions as a sink for atmospheric oxygen, high recycling rates during the Cryogenian period would allow for an increase in atmospheric oxygen concentrations.

Mesoproterozoic surface oxygenation accompanied major sedimentary manganese deposition at 1.4 and 1.1Ga.

Manganese (Mn) oxidation in marine environments requires oxygen (O2 ) or other reactive oxygen species in the water column, and widespread Mn oxide deposition in ancient sedimentary rocks has long been used as a proxy for oxidation. The oxygenation of Earth's atmosphere and oceans across the Archean-Proterozoic boundary are associated with massive Mn deposits, whereas the interval from 1.8-1.0Ga is generally believed to be a time of low atmospheric oxygen with an apparent hiatus in sedimentary Mn deposition. Here, we report geochemical and mineralogical analyses from 1.1Ga manganiferous marine-shelf siltstones from the Bangemall Supergroup, Western Australia, which underlie recently discovered economically significant manganese deposits. Layers bearing Mn carbonate microspheres, comparable with major global Mn deposits, reveal that intense periods of sedimentary Mn deposition occurred in the late Mesoproterozoic. Iron geochemical data suggest anoxic-ferruginous seafloor conditions at the onset of Mn deposition, followed by oxic conditions in the water column as Mn deposition persisted and eventually ceased. These data imply there was spatially widespread surface oxygenation ~1.1Ga with sufficiently oxic conditions in shelf environments to oxidize marine Mn(II). Comparable large stratiform Mn carbonate deposits also occur in ~1.4Ga marine siltstones hosted in underlying sedimentary units. These deposits are greater or at least commensurate in scale (tonnage) to those that followed the major oxygenation transitions from the Neoproterozoic. Such a period of sedimentary manganogenesis is inconsistent with a model of persistently low O2 throughout the entirety of the Mesoproterozoic and provides robust evidence for dynamic redox changes in the mid to late Mesoproterozoic.

Application of Induced Polarization and Vertical Electrical Sounding to Locate Coal and Aquifers in Ojoma Akpa, Benue State

The study area Ojoma is located in Akpa district of Otukpo Local Government Area of Benue State, Nigeria. The area is underlain by Awgu Shale Formation that comprises shallow marine dark Shales, carbonaceous mudstones, limestones, siltstones and sandstones. The survey applied integrated geophysical techniques such as Induced Polarization and Vertical Electrical Sounding in order to locate a speculative coal deposit and prolific aquifers. ABEM Resistivity meter (Tetrameter SAS 1000) was used for the survey and Interpred 1D Software was used for processing, interpretation of field data and modeling of the subsurface. Four survey profiles (VES 1-4) were established with traverse lines between 1000 – 1300m each. No coal deposit was discovered. However, two aquiferous zones were located at depths of 9 – 12m and 25 – 50m respectively. It is recommended that drilling of boreholes may be carried out to tap water from the second hydrogeologic zone between the depths of 25-50m. The survey result also explained the major features revealed by the survey in terms of the types of likely geological bodies and structures which gave rise to the resistivity curves and models, thereby extending geological knowledge of the area. The underlying structures which gave rise to the geological models are the existence of folded and faulted sedimentary beds in the area.

Taxis behaviour of burrowing organisms recorded in an Ediacaran trace fossil from Ukraine

The trace fossil Archaeonassa cf. fossulata Fenton and Fenton is a bilobate, slightly undulating, corrugated ridge on upper bedding plains in the Ediacaran shallow marine siltstones of Ukraine which are no younger than 557 Ma. It was produced partly under microbial mats. This trace fossil shows strong orientation within a sector of 20º–40°, which is approximately perpendicular to the expected shoreline. The orientation points to ability of some Ediacaran burrowing organisms to taxis during early evolutional stages of bilaterians in response to physical or chemical stimuli, probably according to direction of tides. Undulations of A. cf. fossulata can be regarded as a record of the in-and-out burrowing in response to diurnal cycles of oxygen production within microbial mats. Probably, this is one of the oldest examples of such behaviour. In general, Archaeonassa is considered as a relatively common trace fossil in Upper Ediacaran deposits worldwide, especially in shallow marine deposits in the time interval from 560 to 550 Ma.

Geology, biostratigraphy and carbon isotope chemostratigraphy of the Palaeogene fossil-bearing Dakhla sections, southwestern Moroccan Sahara

AbstractNew Palaeogene vertebrate localities were recently reported in the southern Dakhla area (southwestern Morocco). The Eocene sediment strata crops out on cliffs along the Atlantic Ocean coast. Vertebrate remains come from five conglomeratic sandstone beds and are principally represented by isolated teeth belonging to micromammals, selachians and bony fishes, a proboscidean assigned to ?Numidotheriumsp. and many remains of archaeocete whales (Basilosauridae). From fieldwork five lithostratigraphic sections were described, essentially based on the lithological characteristic of sediments. Despite the lateral variations of facies, correlations between these five sections were possible on the basis of fossil-bearing beds (A1, B1, B2, C1 and C2) and five lithological units were identified. The lower part of the section consists of rhythmically bedded, chert-rich marine siltstones and marls with thin black phosphorite with organic matter at the base. The overlying units include coarse-grained to microconglomeratic sandstones interbedded with silts, indicating deposition in a shallow-marine environment with fluvial influence. The natural remanence magnetization of a total of 50 samples was measured; the intensity of most of the samples is too weak however, before or after the first step of demagnetization. The palaeomagnetic data from the samples are very unstable, except for eight from three similar sandstone levels which show a normal polarity. Matched with biostratigraphic data on rodents, primates, the selachian, sirenian and cetacean faunas, the new carbon isotope chemostratigraphy on organics (1) refines the age of the uppermost C2 fossil-bearing bed to earliest Oligocene time and (2) confirms the Priabonian age of the B1 to C1 levels.

Geological development of Heard Island, Central Kerguelen Plateau

ABSTRACTWe report 40Ar–39Ar laser step-heating age determinations on 15 stratigraphically controlled lava flows and intrusive rocks from Heard Island, Central Kerguelen Plateau (Indian Ocean). The island history began with uplift of pelagic limestone intruded by 22 Ma gabbro sills. Subaerial and wave erosion levelled the early island, producing an unconformity onto which pillow lavas, tuffaceous sediments and shallow-water, fossiliferous marine siltstone (Drygalski Formation) were deposited, beginning in late Miocene time. Two volcanic systems then formed in the late Quaternary. Big Ben dominates the larger southeast part of the island, while Mount Dixon occupies the northwest Laurens Peninsula. Feeder dykes and the early lava flows in both systems are 400–200 ka. Lava flows with evolved compositions (trachytes, trachyandesites) erupted 100–20 ka. Well-preserved parasitic cones exposed at low elevations are 15–10 ka and younger. Mawson Peak, near the summit of Big Ben, has erupted lava flows as recently as 2007. Heard Island, and nearby active McDonald Island, are subaerial features of a larger Neogene volcanic region of Central Kerguelen Plateau that includes several large sea knolls and recently identified submarine fields of small cones. This broadly distributed volcanic activity is linked to incubation of plume material at the base of the nearly stationary overlying Central Kerguelen Plateau.

New marine mammal faunas (Cetacea and Sirenia) and sea level change in the Samlat Formation, Upper Eocene, near Ad-Dakhla in southwestern Morocco

The Samlat Formation is well exposed in coastal sections bordering the Atlantic Ocean south of Ad-Dakhla in southwestern Morocco. Here some 22m of rhythmically-bedded, chert-rich, marine siltstones and marls are overlain by 1–1.5m of vertebrate-bearing microconglomeratic sandstone, another 4–8m of rhythmically-bedded siltstone and marl, and finally a second 3–6m unit of vertebrate-bearing muddy sandstone. The microconglomeratic and muddy sandstones represent low sea stands in what is otherwise a deeper water sequence. Cetacean skeletons are rare but cetacean vertebrae are common in the lower sandstone (bed B1), where many show the effects of reworking. The cetaceans in bed B1represent a minimum of five species, from smallest to largest: cf. Saghacetus sp., cf. Stromerius sp., Dorudon atrox, cf. Dorudon sp., and Basilosaurus isis. Bed B1 yields rib fragments that may represent sirenians, but sirenians, if present, are rare. The only identifiable cetacean found in the upper sandstone (bed B2) is Basilosaurus sp. Dugongid sirenians identified as cf. Eosiren sp. are the most common mammal in bed B2. We interpret co-occurrence of the typically Early Priabonian species Dorudon atrox and Basilosaurus isis with smaller species more like Middle Priabonian genera Saghacetus osiris and Stromerius nidensis to indicate that bed B1 was deposited during low sea stand Pr-2 between the Early and Middle Priabonian (between the early and middle Late Eocene). Bed B2 is separated from B1 by an interval of deeper water sediment accumulation. Bed B2 could represent a later phase of Pr-2 or a subsequent Priabonian low sea stand (possibly Pr-3).

Permian bivalves of the Taciba Formation, Itararé Group, Paraná Basin, and their biostratigraphic significance

A small and poorly diversified bivalve fauna from Taciba Formation, Itarare Group, Parana Basin (State of Santa Catarina, Mafra Municipality), is described in this paper for the first time, based on new findings. The fauna is recorded in a 30 cm thick interval of fine sandstone locally at the top of Taciba Formation, in the Butia quarry. The studied fossil-bearing sandstone bed is a marine intercalation recording a brief eustatic rise in sea-level, probably following glacier retreat and climate amelioration at the end of a broad glacial scenario. The fauna is mainly dominated by productid brachiopods, which are not described here, and rare mollusk shells (bivalves and gastropods). Two bivalve species were identified: Myonia argentinensis (Harrington, 1955), and Aviculopecten multiscalptus (Thomas, 1928). The presence of Myonia argentinensis is noteworthy since this species is also present in the Baitaca assemblage found in marine siltstones (Baitaca assemblage) of the Rio do Sul Formation, cropping out at the Teixeira Soares region, Parana State. This species is also recorded in the bivalve fauna from the Bonete Formation, Pillahinco Group, Sauce Grande Basin, Buenos Aires Province, in Argentina. Hence, the marine bivalves of the Taciba Formation are associated with the transgressive event that characterizes the Eurydesma fauna, indicating a Late Asselian-Sakmarian age for the bivalve fauna. Presence of the Myonia argentinensis megadesmid species reinforces the Gondwanic nature of the studied fauna.

Palaeogeographic reconstruction of the 1.55 Ma synchronous isolation of the Ryukyu Islands, Japan, and Taiwan and inflow of the Kuroshio warm current

In the Quaternary, the Ryukyu Islands evolved from a continental margin arc to an island arc by backarc spreading of the Okinawa Trough, accompanied by subsidence and isolation of the islands, a process that has continued to the present. Trough-parallel half grabens were filled with marine siltstone. Similar sediments filling orthogonal fault-controlled and west-draining non-tectonic valleys record island separation. New Quaternary nannofossil biostratigraphic data date the deposition of the marine siltstone at 1.552 ± 0.154 Ma. At that time, the entire 1000 km-long island chain comprising the Ryukyu Islands separated from the Asian continent by rifting extending from the Okinawa Trough to the Tsushima Strait. The Tokara, Kerama, and Yonaguni gaps, branched or transverse rifts of the Okinawa Trough, separate the island chain into subgroups of the Osumi, Amami, Okinawa, and Yaeyama islands, and Taiwan. The shallow Taiwan Strait separated Taiwan from the Chinese mainland. The Kuroshio warm current that previously ran offshore of the continental margin arc began to enter the opening backarc basin through the Yonaguni gap and to exit through the Tokara gap, flowing along the axis of the Okinawa Trough. Under influence of the warm current and because of entrapment of continentally sourced detrital sediments by the Okinawa Trough, coral reefs formed around each island. These reefs make up a unit called the Ryukyu limestone. Subsidence continued through the deposition of this limestone, resulting in further isolation of each island. Some islands did not separate from the mainland but emerged above sea level later as a result of volcanic edifice construction or forearc uplift. Following initial isolation, the Japanese islands and Taiwan may have been connected to the mainland by land bridges during some sea level low stands related to glacial periods, whereas the other islands remained isolated. Based on ages of isolation of each island, a Quaternary palaeogeographic map and ‘phylogenetic tree’ of the islands can be drawn showing the separation time of each island from the mainland and from each other. This information should be useful for phylogenetic molecular biologists studying evolution of Ryukyu endemic species and vicariant speciation and could facilitate analysis of the DNA substitution rate.

Стратиграфия Маастрихтских отложений бассейна р. Каканаут (восточная часть Корякского нагорья)

Recent discovery of rich dinosaur fauna at high latitudes in northeastern Russia required the additional investigation of the Kakanaut locality for more precise definition of its age and stratigraphic position. This locality is situated in the southern part of the Koryak Upland, in the Kakanaut River basin near Pekulneyskoe Lake (Chukotka Autonomous Region). In the result of field work 2007-2009, five particular sections were described and joint geological column of the Maastrichtian deposits was composed. In this column three parts were distinguished: the marine siltstone strata, the Kakanaut Formation and the Kokuy strata. The Maastrichtian deposits are covered by effusive-pyroclastic deposits (possibly, early Paleogene in age). The marine siltstone strata contains inocerams and ammonites of the lower and lower part of upper Maastrichtian (Inoceramus (Cataceramus) pilvoensis Sok., Schachmaticeramus shikotanensis (Nagao et Mat.) Schachmaticeramus kusiroensis (Nagao et Mat.). Hypophylloceras marshalli (Shimizu), Diplomoceras sp., Pachydiscus subcompressus Mat.). The Kokuy strata contains remains of the late Maastrichtian-early Palaeocene foraminifers (Rzehakina epigone zone) and of undetermined fragments of ammonites. The Kakanaut Formation are represented by nonmarine volcanogenous-terrigenous sediments. It consists of tuffaceous sandstone and siltstones, tuffs, tuffites and andesite-basaltic rocks and represents lacustrine and fluviodeltaic deposition on near-sea lowland. These deposits contain joint association of fossil plants and dinosaur bones, teeth and eggshell fragments. The dinosaur assemblage represented by basal ornithopods, hadrosaurids, ankylosaurians, neoceratopsians, troodontids, dromaeosaurids and tyrannosaurids. Dinosaur eggshell fragments, belonging to hadrosaurids and non-avian theropods, indicate that at least several dinosaur taxa could reproduce in polar region. The Kakanaut flora includes about 50 taxa. The cycadophytes (Nilssonia, Encephalartopsis) and Ginkgo are very abundant in certain layers. The conifers are represented by mixture of the typical Late Cretaceous elements with an admixture of younger Paleocene elements belonging to families Taxodiaceae, Cupressaceae and Pinaceae. Angiosperms include about 30 species. Families that can be confidently recognized are Platanaceae (Platanus), Cercidiphyllaceae (Trochodendroides), Betulaceae (Corylus), Fagaceae (Fagopsiphyllum), Rosaceae (Peculnea, Arctoterum). Other species appear to belong to ancient groups, without clear phylogenetic links to modern taxa. The age of the fossiliferous beds can be estimated as the beginning of late Maastrichtian. The correlation of the Maastrichtian deposits of different areas at eastern part of Koryak Upland was carried out.

New records of Ediacaran Acraman ejecta in drillholes from the Stuart Shelf and Officer Basin, South Australia

Abstract— New occurrences of the Acraman impact ejecta layer were recently discovered in two South Australian drillholes, SCYW‐79 1a (Stuart Shelf) and Munta 1 (Officer Basin) using lithostratigraphy, acritarch biostratigraphy, carbon isotope stratigraphy, and biomarker anomalies to predict the stratigraphic position. The ejecta layer is conspicuous because it consists of pink, sandsized, angular fragments of volcanic rock distributed along the bedding plane surface of green marine siltstone. In SCYW‐79 1a it forms a layer 5 mm thick; in Munta 1 the ejecta layer is thin and discontinuous because of its distance (˜550 km) from the impact structure. Palynological, biomarker, and carbon isotope anomalies can now be shown to coincide with the ejecta layer in SCYW‐79 1a and Munta 1 suggesting the Acraman impact event may have had far reaching influences on the rapidly evolving Ediacaran biological and geochemical cycles.

FOSSIL ARGONAUTS (MOLLUSCA: CEPHALOPODA: OCTOPODIDA) FROM LATE MIOCENE SILTSTONES OF THE LOS ANGELES BASIN, CALIFORNIA

Fossilized argonaut egg cases have been recovered from marine siltstones of the late Miocene exposed around margins of the Los Angeles Basin, California. Low radial ribs on the thin, keelless, planispirally coiled egg cases suggest referring them to Mizuhobaris lepta new species. Occurrence of these egg cases in fine-grained Monterey Formation sediments with mesopelagic fish fossils, nannofossils, and Foraminifera indicate deposition in middle-to-upper bathyal depths. Argonaut egg cases have been described from Tertiary strata in Japan, New Zealand, Sumatra, and Europe, but this is the first report of fossilized argonaut egg cases from the Western Hemisphere.Secretion of egg cases by argonauts probably developed during the Paleogene as a solution to problems of spawning encountered by octopi as they acquired an epipelagic, open-ocean habitat. Shape and sculpture of the egg cases represent responses to hydrodynamics rather than an inheritance from or copying of ectocochliate cephalopods. The cases may provide the eggs with protection from ultraviolet radiation present in the argonauts' near-surface habitat.

The role of tectonism in sequence development and facies distribution of Upper Oligocene cool‐water carbonates: Coromandel Peninsula, New Zealand

AbstractThe Torehina Formation is part of a cool‐water carbonate succession of Oligocene age in New Zealand that crops out on the Coromandel Peninsula, North Island. It contains two major transgressive sequences that record successive onlap of a once emergent landmass. The first sequence records marine flooding of non‐marine to marginal marine fan delta/estuarine facies, followed by deepening upward and formation of a low‐energy, deep (100+ m) muddy carbonate ramp. The capping sequence boundary is characterized by differential uplift and varies considerably in its character over a small (9 km2) area, varying from a burrowed glauconitic firmground to an erosional hardground to an undulatory marine contact to a palaeokarst with &lt; 25 m relief. Sequence 2 sediments, which overlie the palaeokarst with minor (&lt; 10°) angular unconformity, are clayey, marine (offshore) siltstones, whereas open‐marine limestones of equivalent age overlie the other boundary types with no apparent angular discordance. The siliciclastics could either represent lowstand channel deposits or may define interbank deposits contemporary with adjacent carbonates. Palaeogeographic restriction of palaeokarst and sequence 2 siliciclastics identifies a structural corridor oriented strike‐parallel to the adjacent Harauki Graben, which began to develop by this time. Palaeogeographical differences in the character of the basal limestone facies of sequence 2 also occur. These differences identify variation in accommodation during initial stages of deposition imposed by previous differential movement of fault blocks. As a result, relatively warm‐water (20 °C) Amphistegina‐bearing limestones in one area contrast with co‐existing deeper water, silty foraminiferal (benthic &gt; planktic), echinoderm and bivalve limestones in another. This variation disappears upsection, which suggests that initial bathymetric differences were eliminated with renewed rise in sea level, yielding deeper water inner‐shelf sediment facies followed by the accumulation of still deeper, but higher energy, outer‐shelf bivalve and bryozoan facies. The sequence architecture of the Torehina Formation is controlled by tectonism, both long‐term subsidence and short‐term differential uplift. This arose as a result of increasing tectonic activity throughout proto‐New Zealand during the Late Oligocene. In such a system, local and regional variation in tectonism among adjacent basins can impose subtle to marked differences in the timing of sequence boundaries and the character of basin‐fill patterns.

Rock magnetism, geologic history and aeromagnetic anomalies: A case study of the Ulupa Siltstone, Adelaide Geosyncline

Marine siltstones and fine sandstones of the Brachina Formation and olive-green siltstones of its more distal equivalent, the Ulupa Siltstone, were deposited during the Marinoan in the Adelaide Geosyncline. These sediments have since been deformed and metamorphosed during the Ordovician Delamerian Orogeny. In the Flinders Ranges, the relatively unmetamorphosed sediments, though iron-rich, are only weakly magnetic. In the Mount Lofty Ranges, where peak metamorphic grades were reached, the Brachina Formation and its equivalent, the Ulupa Siltstone, are extremely magnetic. Magnetic markers within these formations have been used to trace the macrostructure of the Southern Adelaide Foldbelt and have been especially useful where outcrop is limited. While the magnetic markers are consistent with respect to their continuity and presence, the amplitude and character of the anomaly changes significantly along strike. This is basically a function of metamorphic grade as anomalies at lower grades can be successfully interpreted (since both depth and dip are often known) assuming induced magnetisation only. At the highest grade, where the rocks are andalusite schists, the magnetic anomalies are inconsistent with the direction of the present magnetic field, and remanence must represent the greater component of the anomaly. The results of the rock magnetism study show a close correlation between metamorphic grade and the variation in magnetic mineralogy and natural remanent magnetisation. The rocks display multiple components of NRM including a weak primary detrital component acquired during deposition and a much stronger thermal or chemical component acquired during the Delamerian Orogeny. The secondary component has been related to the NRM direction indicated by the nearby Black Hill Norite which is believed to have intruded during the waning phases of the Delamerian Orogeny. The results of the rock magnetism study have been interpreted in terms of the geologic history of the Ulupa Siltstone and have been used in the interpretation of the aeromagnetic anomalies it causes.

Geochemistry of Upper Pliocene silty and sandy sediments from the well Mt-7, Moravci Spa, North-Eastern Slovenia

Bulk chemical composition of Upper Pliocene silts and sands, cored in the well Mt- 7, is strongly controlled by mineralogy - the abundance of quartz, carbonates and phyllosilicates in the sediments. Trace elements are in general more abundant in silts than in sands (Li, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Sb, Cs, REEs, Th and U), except for Y, Zr, Hf and Ba. The majority of yttrium probably originates from xenotime, Zr and Ht from zircone and Ba from alkali feldspars and the traces of barite. YREE patterns, normalised to PAAS show very similar shapes, although the abundance in sands are lower. Positive europium anomalies can be ascribed to the presence of plagioclase feldspars, and positive cerium anomalies to the presence of monazite. Sm/Nd ratios are relatively close and do not differ very much from Kiscellian marine siltstones termed »sivica« from Kozjansko, indicating that the source rocks for sediments probably did not change significantly.

The Permian–Triassic boundary in central East Greenland: past and present views

The latest Permian, Changhsingian fauna in East Greenland is a typical northern cool-water fauna, dominated by a low diversity assemblage of boreal brachiopods and lacking fusulinids, small foraminifers and fish. Continuous sedimentation across the Permian–Triassic boundary is recorded at the Fiskegrav locality, southwestern Jameson Land but the precise location of the boundary is disputable within an interval covering the uppermost few metres of the Schuchert Dal Formation and the lowermost 23.5 m of the Wordie Creek Formation. Based on palynological data the boundary is best placed 50 cm below the top of the Schuchert Dal Formation at Fiskegrav, in a monotonous interval of bioturbated, grey offshore marine siltstone characterised by a marked drop in the ä13C of organic carbon. ä13C of carbonate from brachiopods shows a temporary drop of 2‰, 4–5 m below the top of the Schuchert Dal Formation, while the ä13C of whole rock carbonate reaches a minimum in the lowermost part of the Wordie Creek Formation. Comparisons to the ä13C carbonate curves from the boundary stratotype section at Meishan, China are therefore highly subjective and allow the boundary to be placed within a 24–27 m thick interval in the topmost Schuchert Dal Formation and the lowermost Wordie Creek Formation, – here termed as the Permian–Triassic boundary interval. The lower part of this interval, below the palynological boundary includes a low diversity Permian-type fauna of two brachiopod and four agglutinated foraminifer genera. Above the palynological boundary, in the uppermost 50 cm of the Schuchert Dal Formation remains of the Triassic-type fish Bobasatrania occur together with agglutinated foraminifers, and at the base of the Wordie Creek Formation a more diverse, Triassic-type fish fauna with five genera is present. In the stratotype section at Meishan the first Triassic-type faunal elements appear above the negative ä13C anomaly in what is regarded as a topmost Permian mixed fauna interval. A similar situation may possibly exist in East Greenland so the old East Greenland issue of faunal mixing with Permian-type fauna in the Triassic may turn out to be a matter of having Triassic faunal elements in the Permian.

RESPONSE OF FORAMINIFERAL FACIES TO TRANSGRESSIVE-REGRESSIVE CYCLES IN THE CALLOVIAN OF NORTHEAST SCOTLAND

At Brora on the northwestern margin of the Inner Moray Firth Basin, an Upper Bathonian to Middle Oxfordian (Jurassic) sedimentary succession of marine shales, siltstones and sandstones is subdivided into four 4th order transgressive–regressive (T-R) cycles. The middle two of these cycles, B2 and B3, comprise the Callovian Brora Argillaceous Formation which is the subject of this study, integrating foraminiferal, sedimentological and stratigraphical data. The principal factor acting on the foraminiferal distribution pattern both in cycle B2 and B3 is oxygen supply to the benthic environment. The oxygenation is controlled mainly by the changing water depth during transgressive and regressive episodes. In the studied faunal succession these changes are reflected by two main biofacies trends: 1) Diversity decrease, accompanied by increasing dominance and decreasing proportion of calcareous taxa. This faunal trend reflects increasing stagnation during transgression. 2) Diversity increase, coupled with reduction in dominance and expansion in proportion of calcareous taxa. In typical cases, this development reflects improving oxygenation during regression. The intervals which include the maximum flooding surface, in both cycle B2 and B3, are typified by significantly increased organic carbon content and strongly reduced faunal diversity, developed in dark shales representing severe oxygen depletion. The transgressive phase of cycle B2 shows a faunal diversity decrease, while that of cycle B3 reveals a diversity expansion in its lower part. The regressive phase of both cycles is characterized by a diversity increase. In addition to these major faunal trends, the distribution of species and genera appears useful for “fingerprinting” individual components of T-R developments.

The Lower and Middle Devonian of the south-eastern Baltic Sea

In the Baltic region, Devonian sedimentary bedrock is exposed in a wide area extending from north-eastern Estonia through Latvia to the south-eastern Baltic Sea. Westwards in the Baltic Sea, the outcrop terminates against a presumably Permian fault, which extends southwards from Gotland. In the southernmost part of the Baltic Sea, the Devonian sedimentary bedrock was eroded during the Permian-Carboniferous uplift event. The Devonian sedimentary bedrock of the investigated area southeast of Gotland, consists of near-shore marine deposits, namely sandstone, siltstone, clay, dolomite, dolomitic marl, and limestone. Gypsum and anhydrite deposits occur frequently in the lower parts. The Silurian-Devonian boundary is marked by a stratigraphic break across the presently investigated offshore area. All regional stages of the Lower and Middle Devonian are present in exposures in the area. In the north-eastern part, offshore Latvia, the basal Devonian Gargždai deposits follow ontop of the Silurian sedimentary bedrock. Further to the south-west, the Gargždai beds are missing, and deposits of the next younger Kemeri Stage rest directly on the Silurian. In the Middle Devonian, the Pärnu Stage contains widespread sandbars, which are locally deeply truncated. This may indicate a shallow nearshore environment. Within the Old Red Sandstone facies of the late Middle Devonian Aruküla-Burtnieki complex, several discontinuity surfaces are present. The reflectors exhibit a mound-like pattern, which again may indicate a shallow nearshore environment with rapid deposition, erosion, and redeposition.

Clast shape and textural associations in peperite as a guide to hydromagmatic interactions: Upper Permian basaltic and basaltic andesite examples from Kiama, Australia

Upper Permian shallow marine siltstone and sandstone units of the Broughton Formation are intercalated with basaltic and basaltic andesite sheets at Kiama, New South Wales. Parts of the two sheets examined in this study display peperite texture. The lower example (Blow Hole Latite Member) can be divided into two units with peperitic contacts suggesting their intrusion into wet unconsolidated sediments of the overlying Kiama Sandstone Member. The Bumbo Latite Member overlies the Kiama Sandstone Member and has been interpreted by previous workers as a lava. Well‐developed columnar joints cut the interior of the sheets. Along contacts with sedimentary facies and peperitic dykes which penetrate the sheets, columnar joints merge into a several metre‐wide zone of blocky jointing, pseudo‐pillows and hyaloclastite. In peperitic facies, sandstone or siltstone fills joints and fractures that define pseudo‐pillows, polyhedral joint blocks and columns (closely packed fabric) or sediment matrix‐rich breccia contains fragments and apophyses of basalt and basaltic andesite (dispersed fabric). Along some contacts, peperite with dispersed fabric passes through a zone of closely packed peperite into coherent facies. Alternatively, closely packed peperite passes directly into coherent facies. Examples of peperite with more than one clast type (globular, blocky, platy), and involving sedimentary matrix of constant grain‐size, are common. In some examples, globular surfaces formed during an early, low‐viscosity phase of magma emplacement into wet sediment. Planar and curviplanar fractures cut some globular surfaces suggesting that these formed slightly later as the magma became more viscous (cooler) and/or vapour films at the magma‐sediment interface broke down. However, the complexities of peperite, in respect to clast types, abundances and distribution, as well as grainsize and structures in the sedimentary component, suggest that a spectrum of fragmentation and mixing processes were involved in fragmenting the sheets. Many peperitic domains include poorly and strongly vesicular parts, resulting in apparent polymictic breccias. Vesiculation of the sheets is interpreted to have occurred in two phases: an early degassing of primary magmatic volatiles and a later, scoria‐forming event, both of which progressed as the magma mixed with unconsolidated sediment. During the later phase, magma incorporated limited amounts of steam from the wet sediment and a vesicular front propagated out into the magmatic component. Confining pressures were insufficient to prevent vesiculation of the magma or to suppress fluidisation of the host sediment along magma‐sediment contacts, but large enough to inhibit large‐scale steam explosivity. Displacement of sediment along contacts may have reduced confining pressures sufficiently to promote vaporisation of pore water, and induce local vesiculation of the magma.

Detached mud prism origin of highstand systems tracts from mid‐Pleistocene sequences, Wanganui Basin, New Zealand

Marine siltstone successions, 1–20 m thick, form the upper part of sequences in the mid‐Pleistocene Castlecliff section (≈ 0·98–0·35 Ma). The siltstones were deposited within a broad shelf embayment at and about glacioeustatic highstands and are interpreted as highstand systems tracts (HST). Shell‐rich to relatively shell‐poor contacts at the base of Castlecliff HST are interpreted as downlap surfaces, which mark the quenching of transgressive in situ biogenic accumulation (backlap shellbed). Nonetheless, the basal parts of Castlecliff HST successions are enriched in fossil content in the context of the highstand successions as a whole and represent downlap shellbeds. Castlecliff HST are truncated above by sequence‐bounding ravinement surfaces, such that complete sandier‐upward successions and subaerial exposure surfaces associated with sequence boundaries sensu stricto are never preserved. Modern highstand sedimentation in the Taranaki Bight offshore from Castlecliff is characterized by a mid‐shelf mud depocentre and a coastal shore‐connected sand prism, both of which are encroaching upon intervening shell‐rich relict and palimpsest transgressive deposits. The mud depocentre is up to 9 m thick, and deposition is influenced by a gyre caused by bathymetric steering of storm‐driven currents along the embayed coastline. Modern highstand deposition in the Taranaki Bight, in which the mud depocentre is in part detached from the contemporary shore‐connected sand prism, is regarded as an analogue for the deposition of the preserved lower parts of Castlecliff HST. The inferred architecture of Castlecliff HST therefore need not refer to the shore‐connected, progradational geometry predicted by traditional sequence models. The model proposed herein may have application to other shelf palaeo‐embayments in which mid‐shelf focusing of fine‐grained sediment has resulted from coastal steering of currents.