Muddy Siltstone Research Articles

LITOFACIÁLNÍ ANALÝZA BÁZE HRADECKO-KYJOVICKÉHO SOUVRSTVÍ (NÍZKÝ JESENÍK, MORAVSKOSLEZSKÁ JEDNOTKA ČESKÉHO MASIVU)

A detailed field facies and ichnofacies analysis undertaken in the eastern part of the Nízký Jesník Mts. revealed that the basal part of Hradec-Kyjovice Formation of Upper Viséan age corresponded to coarse-grained siliciclastic turbidite system. Research was focused on detailed measurement of fifteen outcrops in the area. The formation was deposited in deep water environmental of the foreland basin by sediment gravity flows. Five facies were identified in the Hradec Member of the Hradec-Kyjovice Formation: conglomerate facies, pebble to coarse grained sandstone facies, coarse grained sandstone facies, sandstone-siltstone facies and the muddy siltstone facies. The conglomerate facies, pebble to coarse grained sandstone facies and coarse grained sandstone facies represent proximal, coarse grained channel deposits of high-density turbidite currents. The sandstone-siltstone sediments consist of a variety of turbidites deposites in lobes and interchannel environments. The muddy siltstone facies were deposited in interchannel environments by lowdensity turbidite currents. Some depositional lobes contain trace fossils of the Nereites ichnofacies. Sedimentary record of the basal parts of the Hradec-Kyjovice Formation indicates a Late Viséan a change in the development of Culm basin in Upper Viséan and beginning of new sedimentary cycle of sedimentation governed presumably by a compressional tectonic pulse.

Muddy Prodeltaic Hyperpycnites In the Lower Genesee Group of Central New York, USA: Implications For Mud Transport In Epicontinental Seas

Abstract Recent studies of marine continental shelves show that hyperpycnal flows are responsible for offshore transport of large volumes of sediment. Detailed facies analysis and petrography of the lower Genesee Group in the Northern Appalachian Basin (NAB) shows a wealth of sedimentary textures and fabrics that indicate mud deposition by lateral transport across and along the shelf under energetic conditions. Intervals of silt-rich mudstones and muddy siltstones with internal scours, diffuse stratification, soft-sediment deformation, normal and inverse lamina-set grading, and a reduced intensity and diversity of bioturbation occur in multiple facies types and “interrupt” what appears to be the overall background sedimentation. These intervals and their sedimentary features are interpreted as products of high-density fluvial discharge events, which generated turbulent flows that carried fine-grained clastics several tens of kilometers offshore from the paleoshoreline. Recognizing these sediments as products of river-flood- and storm-wave-generated offshore-directed underflows challenges previous depositional models for organic-rich mudstones in the lower Genesee succession, which call for clastic starvation and suspension settling of clay and silt in a deep stratified basin. Rapid deposition of fine-grained intervals from hyperpycnal plumes in a setting favoring preservation of organic-rich mudstones calls for a reappraisal of the depositional setting of not only the Genesee Group, but also of comparable mudstone successions in the Appalachian Basin and elsewhere.

Experiment Study on Bearing Capacity of Muddy Siltstone Foundation and its Application

Rock-socket cast-in-situ piles is a commonly used style of foundation, but theoretical analysis, experiments and test research of bearing capacity behavior about rock-socket piles in soft rock have not adapted to the requirement of engineering. The static loading test of single pile on a project in argillaceous and silt laminate, through which the skin friction and point bearing capacity were measured, the bearing capacity behavior of rock-socket cast-in-situ piles in soft rock was analyzed.

Paleomagnetic data from the Late Carboniferous-Late Permian rocks in eastern Tibet and their implications for tectonic evolution of the northern Qiangtang-Qamdo block

We report paleomagnetic results from the Late Carboniferous-Late Permian strata in eastern Tibet (China), and aim to clarify the tectonic and paleogeographic evolution of the northern Qiangtang-Qamdo block, which is the key to the study of plate boundary between the Gondwanaland and the Eurasia during the late Paleozoic. Two hundred and nineteen samples—including limestone, muddy siltstone, basalt, lava, and tuff—were collected at 24 sites in the Upper Carboniferous and Middle-Upper Permian successions. A systematic study of rock magnetism and paleomagnetism yields three reliable paleomagnetic pole positions. Both hematite and magnetite occurred in the Late Carboniferous limestone samples. The demagnetization curve shows a characteristic double-component, with the remanent magnetization (ChRM) exhibiting a positive polarity (negative inclination). In the Late Permian limestone, tuff, and basalt, magnetic information were recorded primarily in magnetite, although a small fraction of them was found in hematite in basalt. The demagnetization curve illustrates a double or single component, with the ChRM showing a negative polarity (positive inclination), which has passed the classic fold test successfully. The single polarity features of the ChRM directions of the Late Carboniferous and Middle-Late Permian rocks are respectively related to the Kiaman positive and reversed polarities under the stratigraphic coordinates. This, in turn, indicates that both ChRMs directions represent the original remanence directions. By comparison with the previously published paleomagnetic results from the late Paleozoic rocks in the northern Qiangtang Range, we suggest that: (1) Qamdo and northern Qiangtang block were independent of each other during the Late Carboniferous to the Early Permian periods. The north Lancangjiang ocean basin between the two blocks may have closed before the Middle Permian and been involved in the continent-continent collision stage in the Late Permian-Early Triassic periods. (2) The northern Qiangtang-Qamdo Block paleogeographically was situated at low to intermediate latitudes in the Southern Hemisphere in the Late Carboniferous-Late Permian periods, and began to displace northward in the Early Triassic, with an amount of more than 5000 km northward transport from its current location.

Experimental Investigation on Mechanical Properties of Soft Rock under Instantaneous Triaxial Compression

Using the RMT-150C rock mechanics test system, the instantaneous triaxial compression tests for muddy siltstone were carried out under different confining pressures, and the instantaneous mechanical properties of soft rock were obtained. The results show that the strength parameters of sample have a positive linear relation with the confining pressure, and the sensitivity of peak strength on the confining pressure is higher than that of residual strength. The elastic and deformation modulus of sample linearly increase with the confining pressure going up, and the elastic modulus is more sensitive to the confining pressure. The soft rock sample shows typical ductility failure characteristic, while the damage characteristic is mainly shear failure and the shear failure angle linearly decreases with the confining pressure going up.

Lithologic, mineralogical, and petrophysical characteristics of the Eau Claire Formation: Complexities of a carbon storage system seal

Concerns about potential climate change related to greenhouse gas emissions have spurred researchers across the world to assess the viability of geologic storage of CO2. In the Illinois Basin in the United States, the Cambrian Mount Simon Sandstone has been targeted as a reservoir for carbon capture and storage (CCS). In this CCS system, the Eau Claire Formation is expected to serve as the primary seal to prevent upward migration of the CO2 plume; however, little work has been done to specifically determine how well it will function as a seal. Although the lateral extent and thickness of the Eau Claire Formation, along with its generally low permeability, certainly make it a prime candidate to serve in this capacity, the primary depositional fabric and mineralogy, which are the fundamental controls on the petrophysical charter of this unit, remain poorly constrained. Therefore, the purpose of this study is to investigate the lithologic, mineralogical, and petrophysical properties of the Eau Claire Formation in an effort to characterize its potential as a functional seal in a CCS system. Sixty-six core-derived Eau Claire Formation samples from seven wells within the Illinois Basin are described using a combination of petrography, reflectance spectroscopy, x-ray diffraction, geochemical, and petrophysical analyses. These analyses show that the Eau Claire Formation contains five different lithofacies (sandstone, clean siltstone, muddy siltstone, silty mudstone, and shale) with fine-scale heterogeneities in fabric and mineralogy that greatly influence the petrophysical properties. Porosity, permeability, and entry-pressure data suggest that some, but not all, lithofacies within the Eau Claire Formation have the capability to serve as a suitable CCS seal. Abundant authigenic minerals and dissolution textures indicate that multiple generations of past fluid-rock interactions have occurred within the Eau Claire Formation, demonstrating that much of the formation has behaved as a fluid conduit instead of as a seal. Minerals that would be potentially reactive in a CCS system (including carbonate, glauconite, and chlorite) are common in the Eau Claire Formation. Dissolution of these and other phases in the presence of carbonic acid could potentially jeopardize the sealing integrity of the unit. Although complexities in the sealing properties exist, the dynamics of the CCS system and the potential for precipitation of new minerals should allow the Eau Claire Formation to serve as an adequate seal.

On a moderately diverse continental ichnofauna from the Permian Ikakern Formation (Argana Basin, Western High Atlas, Morocco)

The Permian Ikakern Formation of the Argana Basin yielded important tetrapod skeletal remains but is still poorly known for any other kind of fossils. Here we present a moderately diverse assemblage of invertebrate and vertebrate traces from red beds in the upper part of the formation (Tourbihine Member, T2) near Timezgadiouine. Recorded ichnotaxa include Spongeliomorpha carlsbergi (Bromley and Asgaard, 1979), Striatichnium natalisWalter, 1982, AmphisauropusHaubold, 1970, HyloidichnusGilmore, 1927, ErpetopusMoodie, 1929, and DromopusMarsh, 1894. The traces occur in laminated muddy siltstone and fine-grained sandstone that we interpret as slack-water deposits in an episodically high energy fluvial setting. Including arthropod burrows, arthropod grazing traces and tetrapod footprints, the ichnofossil assemblage corresponds to the Scoyenia ichnofacies. The described trace fossils provide evidence for various invertebrates and vertebrates hitherto unknown from the body fossil record of the study area. By extending the stratigraphic and geographic range of some of the recorded ichnotaxa, our finds strongly emphasise the importance of the Argana Basin for the reconstruction and understanding of Late Palaeozoic terrestrial ecosystems.

The significance of deep-water cryptic bioturbation in slope-channel massive sand deposits of the lower Rio Dell Formation, Eel River basin, California

Deep-water massive (featureless) sands are commonly interpreted as indicators of flow conditions during or shortly after deposition and are almost never interpreted to reflect post-depositional homogenization due to bioturbation. Deep-water deposits of the lower Rio Dell Formation (Pliocene), Eel River basin (NW California), however, reveal previously unrecognized cryptically bioturbated fabrics in massive sandstone and mudstone. This study represents the first detailed documentation of cryptically bioturbated massive-appearing sandstone in deep-water channelized environments. Although fluid escape contributed to some of the massive-appearing textures in the Rio Dell Formation, we present criteria for recognizing cryptic bioturbation fabrics: (1) tubes with lithologically distinctive concentric linings or infillings, (2) meniscate backfill in burrows, (3) lithologically distinctive spreite, (4) concentric grain orientations, and (5) grains realigned adjacent to tubular structures. Using biostratigraphic data, detailed stratigraphic descriptions, core photos, and photomicrographs, we document ichnofabrics associated with two main lithofacies and their bounding surfaces: (1) diffuse muddy siltstone lithofacies (e.g. Planolites, Phycosiphon, Schaubcylindrichnus), (2) massive sandstone lithofacies (e.g. cryptic bioturbation , Planolites), (3) lithofacies crossers, (4) burrows that prefer the top-of-sand/base-of-mud interface (e.g. Planolites, Skolithos), and (5) a Glossifungites-demarcated discontinuity at the base of the massive sand lithofacies. We also establish two main lithofacies associations with characteristic sedimentologic traits. We use relative percentages of the lithofacies associations to interpret slope-channel depositional facies: distal channel margin, proximal channel margin, and channel axis. By comparing ichnologic characteristics to depositional facies, we conclude that proximity to the channel axis appear to control the degree and type of bioturbation observed in the massive-appearing deposits. Distal channel margin deposits preserve the smallest burrows, highly developed cryptic bioturbation fabrics, and the most complete sequence of background bioturbation. Proximal channel margin and channel axis deposits record larger burrows but incomplete cryptic bioturbated fabrics and an overall smaller proportion of preserved background bioturbation.

Dinosaur eggs and dinosaur egg-bearing deposits (Upper Cretaceous) of Henan Province, China: Occurrences, palaeoenvironments, taphonomy and preservation

The Upper Cretaceous dinosaur egg-bearing deposits in Henan Province, central China are divided into three formations in ascending order: Gaogou, Majiacun and Sigou. The Gaogou Formation belongs to alluvial fan deposits containing the fossil dinosaur egg assemblage of Faveoloolithus, Dendroolithus, Dictyoolithus, Paraspheroolithus and Longiteresoolithus. The Majiacun Formation is interpreted as braided stream to meandering stream deposits with assemblage of Ovaloolithus, Paraspheroolithus, Placoolithus, Dendroolithus, Prismatoolithus, rare Youngoolithus and Nanhiungoolithus. The Sigou Formation is shallow lacustrine/palustrine to low-sinuosity river sedimentary rocks with assemblage of Macroolithus, Elongatoolithus, Ovaloolithus and Paraspheroolithus. To date, 37 oospecies, 13 oogenera and 8 oofamilies of dinosaur eggs have been distinguished. Autochthonous dinosaur eggs are preserved in the floodplain deposits, whereas allochthonous and parautochthonous dinosaur eggs are preserved in the alluvial fans. This suggests that river floodplains are the best environments for the preservation of numerous autochthonous dinosaur eggs. The preservation of most dinosaur eggs in brown to red calcic palaeosoils, muddy siltstones or mudstones suggests that the paleoclimate of the nesting area was semiarid. The presence of gray and green mudstone and coal layers, however, indicates that there existed an ephemeral subhumid climate during the middle and Late Cretaceous. It is suggested that such a climate was favorable for the development of meandering streams in a vegetated environment.

Fossil Association from the Lower Cambrian Yanjiahe Formation in the Yangtze Gorges Area, Hubei, South China

Abstract: Apart from previously reported Small Shelly Fossils (SSFs), a macroscopic fossil assemblage, comprising abundant algae, cone‐shaped tubular fossil forms, and probable impressions of a megascopic metazoan, comes from the Lower Cambrian Yanjiahe Formation in the Yangtze Gorges area of western Hubei Province, south China. The visible fossils are preserved in thinlaminated siltstone or muddy siltstone intercalated between 8–15 mm‐thick carbonate deposits, probably representing sedimentary settings of a constrained local depression in the shallow water carbonate platform during the Early Cambrian Meishucunian Stage. The macroscopic fossil association provides significant fossil evidence about the evolution of life from the late Precambrian to the ‘Cambrian explosion’ interval.

Middle Turonian dinosaur paleoenvironments in the Upper Cretaceous Kaskapau Formation, northeast British Columbia

The Kaskapau Formation is a mudstone-dominated wedge up to 950 m thick that spans late Cenomanian to middle Turonian time. The formation has a prominent wedge geometry and was deposited in the foredeep of the Western Canada Foreland Basin. In outcrop in northeast British Columbia, nearshore sandstones are locally well developed and include rare wedges of nonmarine strata. On Quality Creek, near Tumbler Ridge, 11 m of nonmarine strata contain abundant dinosaur tracks and the first in situ dinosaur bone reported from British Columbia. This site, at a paleolatitude of about 67°N, preserves a rare glimpse of Turonian terrestrial environments during global eustatic highstand. Three main dinosaur habitats are recognized: strandplain and beach-ridge; freshwater lake; and brackish lagoon. Back-beach sandstone has a relief of about 2.5 m, interpreted as beach ridges; sandy coals containing abundant dinosaur tracks represent inter-ridge slough deposits. Overlying lake deposits comprising laminated muds with freshwater molluscs grade up into rooted muddy siltstone and locally, dinoturbated sandstone. Lake deposits are capped by lagoonal mudstone with abundant brackish-water molluscs, locally including a dinosaur-trampled oyster bioherm. The upper parts of sandy lagoonal deltas are pervasively dinoturbated. Sandstone filling a tidal channel contains logs, oyster shells, and bones of dinosaurs, turtles, and crocodiles, as well as fish scales. The lagoonal succession is erosively overlain by offshore sandy mudstones. Various lines of evidence suggest that the mean annual temperature at this sea-level location was about 14 °C, with a cold-month mean no less than 5.5 °C. The high-latitude location implies a significant period of winter darkness, and correspondingly diminished plant productivity.

Differentiation of estuarine and offshore marine deposits using integrated ichnology and sedimentology: Permian Pebbley Beach Formation, Sydney Basin, Australia

Abstract This study integrates ichnology and sedimentology to refine the palaeoenvironmental and sequence stratigraphic interpretations of the Early Permian Pebbley Beach Formation, in the southern Sydney Basin, Australia. This succession has been interpreted previously to reflect entirely inner to outer shelf and slope environments of deposition. Detailed analysis of the formation reveals ichnological and sedimentological characteristics that contradict a fully marine interpretation. Instead, the interval reflects the vertical superposition and lateral juxtaposition of brackish-water and fully marine units. Marine facies comprise: (1) thoroughly bioturbated muddy siltstone (lower offshore); (2) thoroughly bioturbated sandy siltstone (upper offshore); (3) interbedded bioturbated sandy siltstone and laminated sandstone (delta-influenced offshore transition); (4) thoroughly bioturbated muddy sandstone (distal lower shoreface); (5) interbedded laminated sandstone, bioturbated muddy sandstone and dark claystone (delta-influenced lower shoreface); and (6) bioturbated, laterally variable sandstones (transgressive sand sheets). Estuarine facies comprise: (1) channelized heterolithic sandstone-mudstone (active estuarine channels); (2) sheet-like heterolithic sandstone-mudstone (active estuarine basins); and (3) laminated mudstone (abandoned estuarine channels and basins). The interpreted fully marine deposits contain ichnological suites that exhibit moderate to intense bioturbation, high diversities (31 ichnospecies belonging to 20 ichnogenera), uniform distributions of ichnogenera, and significant numbers of structures reflecting specialized feeding/grazing behaviours. In marked contrast, interpreted estuarine (brackish-water) deposits contain impoverished ichnological suites (9 ichnogenera), show variable but significantly reduced degrees of bioturbation intensity, pronounced variability in ichnogenera distributions and the predominance of a few, simple forms representing simple feeding strategies of resilient trophic generalists. The new analysis allows the recognition of a series of highly top-truncated and condensed sequences (cycles of relative sea-level fall and physical rise), which can be physically correlated over several kilometres. Sequence boundaries typically cut down through shoreface sandstones to directly overlie offshore facies, leading to an interface with little apparent lithological contrast. In the absence of laterally continuous exposure, these surfaces may be recognized by careful ichnofacies evaluation. Thus the re-evaluation presented herein has facilitated a more realistic sequence stratigraphic analysis of the Pebbley Beach Formation.

New upper Paleocene species of the bivalvePlicatulafrom southern California

Three new species of the shallow-marine, warm-water bivalvePlicatulaare reported from the upper Paleocene Santa Susana Formation of southern California.Plicatula simiensisnew species is from the middle part of the formation on the south side of Simi Valley and occurs as a displaced specimen in deep-marine turbidites.Plicatula lapidicinanew species andP. trailerensisnew species are both from coralline-algal-rich muddy siltstone just beneath a nearshore, coralline-algal limestone interval in the upper part of the formation in the Santa Ynez Canyon area, east-central Santa Monica Mountains. These three new species represent the first late Paleocene records of genusPlicatulaon the west coast of North America and the first Paleocene records of this genus in southern California.

Carbonate deposition in an ephemeral siliciclastic alluvial system: Jurassic Shuttle Meadow Formation, Newark Supergroup, Hartford Basin, USA

Nonmarine carbonate accumulation, from a sedimentologic perspective, has multiple possible origins: clastic load, springs, groundwater discharge, soil development, biochemical seasonal changes, cave speleothems, and eolian influx. Many of these carbonates contain diagnostic textures from which origin is easy to ascertain. However, differentiating between clastic and spring/groundwater carbonates, especially in depositional systems dominated by siliciclastics, is not always simple; here criteria for recognition are developed. Subsurface hydrology and tectonic origin of the depositional basin, source rocks surrounding the basin, sedimentologic packaging and extent of the carbonates in question, as well as their sedimentary structures and fossil content, and stable isotopic analyses of the carbonates can all aid in determining carbonate origin and correctly assessing sedimentary and basin evolution. The present study focuses on a nonmarine carbonate deposit found within the Jurassic Shuttle Meadow Formation of the Hartford Basin, one of many rift basins which formed as a result of the opening of the North Atlantic Ocean during the Late Triassic to Early Jurassic. The upper 50 m of the formation at Plainville, Connecticut, is interpreted as continental in origin and is composed of five facies: red ripple cross-laminated muddy siltstones to sandstones (Fr, Sr), massive red siliciclastic mudstones (Fm), buff fine- to medium-grained horizontally laminated sandstones (Sh), red to white disrupted calcareous shales (Fl 3), and rare micritic carbonates (Pm). Facies Fr, Sr, Sh, and Fm are interpreted as an alluvial plain system in an ephemeral setting with bedload sheetflooding as the dominant depositional mechanism, similar to the Channel Country drainage system of central Australia. In addition, preservation of lateral accretion sandstone sheets (accretionary benches) indicate that the Shuttle Meadow alluvial system was rapidly accreting. The anomalous existence of carbonate beds (in Facies Fl 3 and Pm) in the SMF bedload-dominated siliciclastic sequence is attributed mainly to the unique nature of the clastic sediment load in the basin; no direct evidence for a spring/groundwater input is apparent. The shallow carbonate lake margin or a pond with vegetation and periodic exposure is envisioned as the depositional environment. The intraclastic texture of the micrites with rare primary relict lamination indicates early diagenetic alteration from root penetration, desiccation cracks, and burrowing.

Shallow marine event sedimentation in a volcanic arc-related setting: the Ordovician Suri Formation, Famatina Range, northwest Argentina

The Loma del Kilómetro Member of the Lower Ordovician Suri Formation records arc-related shelf sedimentation in the Famatina Basin of northwest Argentina. Nine facies, grouped into three facies assemblages, are recognized. Facies assemblage 1 [massive and parallel-laminated mudstones (facies A) locally punctuated by normally graded or parallel-laminated silty sandstones (facies B] records deposition from suspension fall-out and episodic storm-induced turbidity currents in an outer shelf setting. Facies assemblage 2 [massive and parallel-laminated mudstones (facies A) interbedded with rippled-top very fine-grained sandstones (facies D)] is interpreted as the product of background sedimentation alternating with distal storm events in a middle shelf environment. Facies assemblage 3 [normally graded coarse to fine-grained sandstones (facies C); parallel-laminated to low angle cross-stratified sandstones (facies E); hummocky cross-stratified sandstones and siltstones (facies F); interstratified fine-grained sandstones and mudstones (facies G); massive muddy siltstones and sandstones (facies H); tuffaceous sandstones (facies 1); and interbedded thin units of massive and parallel-laminated mudstones (facies A)] is thought to represent volcaniclastic mass flow and storm deposition coupled with subordinated suspension fall-out in an inner-shelf to lower-shoreface setting. The Loma del Kilómetro Member records regress ive-transgressive sedimentation in a storm- and mass flow-dominated high-gradient shelf. Volcano-tectonic activity was the important control on shelf morphology, while relative sea-level change influenced sedimentation. The lower part of the succession is attributed to mud blanketing during high stand and volcanic quiescence. Progradation of the inner shelf to lower shoreface facies assemblage in the middle part represents an abrupt basinward shoreline migration. An erosive-based, non-volcaniclastic, turbidite unit at the base of this package suggests a sea level fall. Pyroclastic detritus, andesites, and a non-volcanic terrain were eroded and their detritus was transported basinward and redeposited by sediment gravity flows during the low stand. The local coexistence of juvenile pyroclastic detritus and fossils suggests reworking of rare ash-falls. The upper part of the Loma del Kilómetro Member records a transgression with no evidence of contemporaneous volcanism. Biostratinomic, paleoecologic, and ichnologic analyses support this paleoenvironmental interpretations and provide independent evidence for the dominance of episodic sedimentation in an arc-related shallow marine setting. Fossil concentrations were mainly formed by event processes, such as storms and volcaniclastic mass flows. High depositional rates inhibited formation of sediment-starved biogenic concentrations. Collectively, trace fossils belong to the Cruziana ichnofacies. Low diversity, scarcity, and presence of relatively simple forms indicate benthic activity under stressful conditions, most probably linked to high sedimentation rates. Contrasting sedimentary dynamics between ‘normal shelves’ and their volcaniclastic counterparts produce distinct and particular signatures in the stratigraphic record. Arc-related shelves are typified by event deposition with significant participation of sediment gravity flows, relatively high sedimentation rates, textural and mineralogical immaturity of sediments, scarcity and low diversity of trace fossils, and dominance of transported and reworked faunal assemblages genetically related to episodic processes.

Turbidite to storm transition in a migrating foreland basin: the Kendal Group (Upper Silurian), northwest England

AbstractThe uppermost Windermere Supergroup (Kendal Group) of northwest England records the passage from Wenlock and lower Ludlow deep water ‘flysch’ deposits to upper Ludlow and Přídolí shallower water ‘molasse’ deposits within an evolving foreland basin. An upwards progression is preserved from oxygen-poor basin-slope turbidite deposits through more oxygenated, bioturbated dilute density flow deposits, to storm and wave-influenced sediments. The storm-influenced sediments display hummocky cross-stratification, a Skolithos ichnofacies, shelly lags, and symmetrical wave ripple cross-lamination. Convolute lamination increases in magnitude and frequency in the upper part of the sequence, apparently nucleated above ripple crests. Tropical hurricanes probably controlled storm deposition, as suggested by late Silurian palaeogeographic reconstructions. Structures in the heterolithic muddy siltstones suggest deposition in a lower energy, wave-influenced setting. Mud-drapes, short wavelength symmetrical ripples and multi-directional ripple cross-lamination are common. The Kendal Group shows a regional palaeocurrent distribution consistent with an arcuate basin geometry, bounded to the northwest and northeast by topographic slopes. As well as a temporal trend, facies and faunal diachroneity imply a southwards migration of the foreland basin depocentre through Ludlow and Přídolí time, probably ahead of a rising mountain front to the north. The increase in pre-lithification sediment disturbance may reflect greater earthquake activity as this mountain front advanced and the basin began to invert.

Lacustrine environment during Lower Beaufort (Upper Permian) Karoo deposition in Northern Malawi

This paper provides the first detailed sedimentological facies analyses of a Lower Beaufort (Upper Permian) Gondwana sequence from Mt. Chombe are in northern Malawi. Approximately 300 m of undeformed non-marine mudstones, siltstones and sandstones are preserved in isolated grabens, exposed along the Mzuzu-Karonga Highway where it crosses the Lake Malawi escarpment. Five lithofacies are distinguished: (A) finely-laminated paper shales and muddy siltstones deposited in lakes with anoxic bottom waters, (B) massive, muddy siltstones and siltstones deposited in oxic lake waters, (C) thinly-bedded, horizontally stratified siltstones from episodic sheet flows into a lake, (D) cross-stratified and flaser-bedded siltstones and sandstones of higher energy lake environments, and (E) massive and cross-bedded, coarse-grained sandstones with small channels and intraformational pebble layers interpreted as marginal lacustrine deposits. New dates by pollen indicate an uppermost Permian interval. The whole sequence appears to have been deposited in a large shallow lake complex as none of the strata displays subaerial features such as desiccation cracks, pedogenesis, or rootlets. All facies are reworked as is also suggested by the lack of in-situ faunal or floral remains. Organic matter is mainly land-derived although some algal-dominated intervals occur. Clastic mineralogy is immature, uniform and dominated by feldspar and quartz. Clay minerals include smectite, kaolinite and illite. Carbonate contents greater than 5% and up to 40% are taken as an evidence of periods of authigenic precipitation in a freshwater lake setting. The Mt. Chombe sequence is lithologically similar, but differs in significant sedimentology aspects from equivalent sequences in the adjacent Luwanga and Ruhuhu Basins where sub-aerial deposition has been reported. Although all three basins are part of a stable, cratonic palaeoenvironment, the absence of the Lower Permian Upper Coal Measures from Karoo sequences in northern Malawi is taken as evidence of some syndepositional topographic relief in the region.

The relationship between sedimentary facies and faunal associations in the Llandovery siliciclastic Ross Brook Formation, Arisaig, Nova Scotia

The Llandovery siliciclastic Ross Brook Formation of Arisaig, Nova Scotia, comprises three broadly defined sedimentary facies. These are a mottled silty mudstone facies (facies A), a laminated shale facies (facies B), and a laminated siltstone facies (facies C). Facies A consists of variably bioturbated silty mudstones, muddy siltstones, and fissile shales. It developed in relatively shallow water conditions; mudstones were presumably deposited from suspension, and siltstone laminae and thin layers were formed by increased wave and current activity. Mottling resulted from bioturbation by epifaunal and, particularly, infaunal benthic organisms. Within facies A, two subfacies are recognized: A(i) is mudstone dominated, and A(ii) is fine-grained siltstone dominated. Facies B consists of alternating laminae of undisturbed mudstone and fine-grained siltstone probably produced as a result of deposition from suspension during a temporary upward expansion of the oxygen-minimum layer. Facies C consists of 0.5–30 cm thick fine- to coarse-grained siltstones, which occur in lenses or layers of single, composite, or amalgamated units. Internally they are extremely variable, but all are interpreted as a result of deposition from storm-generated currents.The Ross Brook Formation formed on a shallow-marine, storm-influenced, subtidal inner–mid muddy shelf and is dominated by extensive but stratigraphically variable developments of facies A and facies C. Absolute water depth per se is difficult to assess, and although fluctuations occurred, much of the sequence is believed to have accumulated at or in the immediate vicinity of fair-weather wave base in water depths estimated to have been between 30 and 60 m. Silt supply was generally low, possibly reflecting great distance from source or the presence of a mud-dominated shoreline.Five brachiopod-dominated associations, which are stratigraphically the Eocoelia hemisphaerica, the Eocoelia intermedia – Eocoelia curtisi, the Visbyella nana, the "Camarotoechia" rossonia, and the Eocoelia sulcata associations, occur through the sequence. Associations change where die sum of the facies characteristics change, suggesting that the major physical controlling factor was substrate type and related environmental parameters. The development of discrete but intergrading associations is viewed as a consequence of the long-term persistence of a set of conservative animal–sediment relationships, superimposed on which is the evolutionary pattern of immigration and extinction of individual species.

New Occurrence of Lower Eocene (Capay Stage") Strata, Lower Piru Creek, Topatopa Mountains, Southern California: ABSTRACT"

A 900-m thick siltstone unit between Canton Canyon and Piru Creek, 16 km north of the town of Piru, California, previously was unnamed and considered as undifferentiated Eocene or middle Eocene in age. The Siltstone unconformably overlies the Whitaker Peak granodiorite basement complex. At the base of the siltstone is a veneer of gruss (weathered granodiorite). The gruss is usually overlain by about a few meters of shoreface carbonaceous sandstone that grades vertically upward into transition-zone siltstone (500 m) with storm-deposit accumulations of macrofossils. Collections made at 53 localities from these lower 500 m of strata yielded numerous shallow marine gastropods and bivalves, as well as specimens of discocyclinid foraminifers, colonial corals, calcareous worm tubes, and spataganoid echinoids. This fauna is indicative of the West Coast provincial molluscan Capay Stage (lower Eocene). Common age-diagnostic species are Turritella uvasana infera, T. Andersoni, and Ostrea haleyi. Overlying and gradational with the transition-zone siltstone is 400 m of muddy siltstone with rare storm-deposit accumulations of macrofossils. This muddy siltstone thickens westward and passes into deep-sea slope and inner-fan turbidite deposits. Collections made at three localities in the muddy siltstone yielded many shallow marine gastropods and bivalves indicative of the Domengine stage (upper lowermore » through lower middle Eocene). Common age-diagnostic species are Turritella uvasana applinae and Pitar (Lamelliconcha) joaquinensis.« less

Formation and Deformation of Subglacial Intrusive Clastic Sheets in the Dwyka Formation of Northern Natal, South Africa

ABSTRACT An arenaceous diamictite rests on fissile silty mudstone and argillaceous diamictite, and is overlain by a laminated muddy siltstone east of Helpmekaar in the Dundee district of northern Natal. The argillaceous deposits originated either in the subglacial or proglacial zone of a partly grounded marine ice sheet, where sediment was supplied by debris-laden floating ice, meltwater plumes, and turbidity currents. The arenaceous diamictite is the product of melt-out debris released close to the grounding line of the ice sheet, where it was subjected to overriding by the ice and slumping and redeposition by sediment gravity flow. The laminated siltstone represents sediment that was deposited in a low-energy setting by turbidity currents and suspension settling downslope from a retreating g ounding line. Dispersed ice-rafted clasts occur in these fine-grained deposits. The upper surface of the arenaceous diamictite displays regularly spaced linear undulations that are parallel to the former iceflow direction, as are corrugations in discrete subhorizontal slip planes in its upper part. Dike-like bodies of this diamictite are found in the underlying mudstone and argillaceous diamictite. These dikes, or wedge-shaped sheets, of sandy diamictite are nearly vertical in their upper parts but at depth curl to the southeast to form sill-like bodies. These intrusive diamictites record a novel, and previously undescribed, form of subglacial bed shear by repeated hydraulic injections which shuffled the upper layers of the substrate to the south-southeast. Bed compaction and shear beneath the grounded ice is recorded by folds in steepened portions of the clastic intrusive sheets and by pinch-and-swell (and folded pinch-and-swell) of the clastic sills. Displacement of steep, folded sheet elements along subhorizontal, slicken-sided slip planes indicate that bed shear continued along spaced planes of slip during the process of compaction. The corrugated and planar slip surfaces and the formation of the listric or spoon-shaped intrusions are all shown to be different expressions of bed shear induced by traction from the overriding ice flowing to the southeast when it was temporarily grounded. The base of an ice sheet flowing from land into the sea undergoes flow acceleration and forms a ramp where it decouples from the high-friction substrate. The arenaceous diamictite mound most likely to have developed beneath this ramp is inferred to have a structural core of sediments that have been squeezed and sheared and have slipped from beneath the ice in the tidal-grounding zone.