Palaeoflow Direction Research Articles

Current‐aligned dewatering sheets and ‘enhanced’ primary current lineation in turbidite sandstones of the Marnoso‐arenacea Formation

AbstractTurbidite sandstones of the Miocene Marnoso‐arenacea Formation (northern Apennines, Italy) display centimetre to decimetre long, straight to gently curved, 0·5 to 2·0 cm regularly spaced lineations on depositional (stratification) planes. Sometimes these lineations are the planform expression of sheet structures seen as millimetre to centimetre long vertical ‘pillars’ in profile. Both occur in the middle and upper parts of medium‐grained and fine‐grained sandstone beds composed of crude to well‐defined stratified facies (including corrugated, hummocky‐like, convolute, dish‐structured and dune stratification) and are aligned sub‐parallel to palaeoflow direction as determined from sole marks often in the same beds. Outcrops lack a tectonic‐related fabric and therefore these structures may be confidently interpreted to be sedimentary in origin. Lineations resemble primary current lineations formed by the action of turbulence during bedload transport under upper stage plane bed conditions. However, they typically display a larger spacing and micro‐topography compared to classic primary current lineations and are not associated with planar‐parallel, finely laminated sandstones. This type of ‘enhanced lineation’ is interpreted to develop by the same process as primary current lineations, but under relatively high near‐bed sediment concentrations and suspended load fallout rates, as supported by laboratory experiments and host facies characteristics. Sheets are interpreted to be dewatering structures and their alignment to palaeoflow (only noted in several other outcrops previously) inferred to be a function of vertical water‐escape following the primary depositional grain fabric. For the Marnoso‐arenacea beds, sheet orientation may be linked genetically to the enhanced primary current lineation structures. Current‐aligned lineation and sheet structures can be used as palaeoflow indicators, although the directional significance of sheets needs to be independently confirmed. These indicators also aid the interpretation of dewatered sandstones, suggesting sedimentation under a traction‐dominated depositional flow – with a discrete interface between the aggrading deposit and the flow – as opposed to under higher concentration grain or hindered‐settling dominated regimes.

Controls on bedrock bedform development beneath the Uummannaq Ice Stream onset zone, West Greenland

This paper investigates the controls on the formation of subglacially eroded bedrock bedforms beneath the topographically confined region upstream of the Uummannaq Ice Stream (UIS). During the last glacial cycle, palaeoglaciological conditions are believed to have been similar for all sites in the study, characterised by thick, fast-flowing ice moving over a rigid bedrock bed. Classic bedrock bedforms indicative of glacially eroded terrain were mapped, including p-forms, roches moutonnées, and whalebacks. Bedform long axes and plucked face orientations display close correlation (parallel and perpendicular) to palaeo-ice flow directions inferred from striae measurements. Across all sites, elongation ratios (length to width) varied by an order of magnitude between 0.8:1 and 8.4:1. Bedform properties (length, height, width, and long axis orientation) from four subsample areas, form morphometrically distinct populations, despite their close proximity and hypothesised similarity in palaeoglaciological conditions.Variations in lithology and geological structures (e.g., joint frequency; joint dip; joint orientation; bedding plane thickness; and bedding plane dip) provide lines of geological weakness, which focus the glacial erosion, in turn controlling bedform geometries. Determining the relationship (s) between bedding plane dip relative to palaeo-ice flow and bedform shape, relative length, amplitude, and wavelength has important ramifications for understanding subglacial bed roughness, cavity formation, and likely erosion style (quarrying and/or abrasion) at the ice-bed interface. This paper demonstrates a direct link between bedrock bedform geometries and geological structure and emphasises the need to understand bedrock bedform characteristics when reconstructing palaeoglaciological conditions.

Understanding the impact of open-framework conglomerates on water–oil displacements: the Victor interval of the Ivishak Reservoir, Prudhoe Bay Field, Alaska

The Victor Unit of the Ivishak Formation in the Prudhoe Bay Oilfield is characterized by high net-to-gross fluvial sandstones and conglomerates. The highest permeability is found within sets of cross-strata of open-framework conglomerate (OFC). These cross-strata are preserved within unit-bar deposits and assemblages of unit-bar deposits within compound (braid)-bar deposits, and may form thief zones limiting enhanced oil recovery. We incorporate recent research that has quantified important attributes of preserved sedimentary architecture into high-resolution models. Waterflooding experiments using these models demonstrate the control that such architecture has on oil production rate, water breakthrough time, and spatial and temporal distribution of residual oil saturation. We found that when the pressure gradient is orientated perpendicular to the palaeoflow direction, the total oil production and the water breakthrough time are larger, and the remaining oil saturation is smaller, than when it is orientated parallel to palaeoflow. The pressure difference between production and injection wells does not affect sweep efficiency, although the spatial distribution of oil remaining in the reservoir critically depends on this value. Oil sweep efficiency decreases slightly with increase in the proportion of OFC cross-strata. Whether or not clusters of connected OFC span the domain does not visibly affect sweep efficiency.

Distinct styles of fluvial deposition in a Cambrian rift basin

AbstractProcess‐based and facies models to account for the origin of pre‐vegetation (i.e. pre‐Silurian) preserved fluvial sedimentary architectures remain poorly defined in terms of their ability to account for the nature of the fluvial conditions required to accumulate and preserve architectural elements in the absence of the stabilizing influence of vegetation. In pre‐vegetation fluvial successions, the repeated reworking of bars and minor channels that resulted in the generation and preservation of broad, tabular, stacked sandstone‐sheets has been previously regarded as the dominant sedimentary mechanism. This situation is closely analogous to modern‐day poorly vegetated systems developed in arid climatic settings. However, this study demonstrates the widespread presence of substantially more complex stratigraphic architectures. The Guarda Velha Formation of Southern Brazil is a >500 m‐thick synrift fluvial succession of Cambrian age that records the deposits and sedimentary architecture of three distinct fluvial successions: (i) an early rift‐stage system characterized by coarse‐grained channel elements indicative of a distributive pattern with flow transverse to the basin axis; and two coeval systems from the early‐ to climax‐rift stages that represent (ii) an axially directed, trunk fluvial system characterized by large‐scale amalgamated sandy braid‐bar elements, and (iii) a distributive fluvial system characterized by multi‐storey, sandy braided‐channel elements that flowed transverse to the basin axis. Integration of facies and architectural‐element analysis with regional stratigraphic basin analysis, palaeocurrent and pebble‐provenance analysis demonstrates the mechanisms responsible for preserving the varied range of fluvial architectures present in this pre‐vegetation, rift‐basin setting. Identified major controls that influenced pre‐vegetation fluvial sedimentary style include: (i) spatial and temporal variation in discharge regime; (ii) the varying sedimentological characteristics of distinct catchment areas; (iii) the role of tectonic basin configuration and its direct role in influencing palaeoflow direction and fluvial style, whereby both the axial and transverse fluvial systems undertook a distinctive response to syn‐depositional movement on basin‐bounding faults. Detailed architectural analyses of these deposits reveal significant variations in geometry, with characteristics considerably more complex than that of simple, laterally extensive, stacked sandstone‐sheets predicted by most existing depositional models for pre‐vegetation fluvial systems. These results suggest that the sheet‐braided style actually encompasses a varied number of different pre‐vegetation fluvial styles. Moreover, this study demonstrates that contemporaneous axial and transverse fluvial systems with distinctive architectural expressions can be preserved in the same overall tectonic and climatic setting.

Regional tectonic control on large size explosive eruptions: Insights into the Green Tuff ignimbrite unit of Pantelleria

Original geological and structural data, which derive from the analysis of the rheomorphic Green Tuff ignimbrite unit of Pantelleria, have offered the opportunity to define its modes of emplacement and the location of the eruptive sources in terms of distribution and geometry. The Green Tuff displays a wide range of rheomorphic structures consisting of preserved penetrative foliations, lineations and folds which, developed at distinct times, have been assigned to three major (D1–D3) deformation events accompanying and following the ignimbrite unit emplacement. The first D1 event produced distinct sets of structures developed along ductile shear zones generated during the emplacement of pyroclastic density currents along current-deposit boundaries. Palaeoflow directions of this event are completely independent from topography and are directly related to high-energy currents generated from the eruption. D2 event is characterized by folding due to down-slope post-emplacement flows related to gravity sliding processes whereas the D3 event was dominated by semi-brittle to brittle structures developed after the complete emplacement of the flow units and their subsequent cooling and compaction. The statistical analysis of these structural data has led to the hypothesis that the Green Tuff eruption developed from fissural sources that are largely superimposed on the NNE-trending dip-slip normal fault zones of the island (the Zinedi and the Montagna Grande faults). Our model also implies that the Green Tuff ignimbrite deposit can be the result of several events within a single eruptive cycle. The orientation of the fissural eruptive systems is evidence that the feeding structures for this large-size explosive event were strongly controlled by the E-W to ESE-WNW directed extension structures that affect the island of Pantelleria and, as a whole, the entire region of the Sicily Channel.

Palaeomagnetism and magnetic fabrics of the Late Palaeozoic volcanism in the Castejón-Laspaúles basin (Central Pyrenees). Implications for palaeoflow directions and basin configuration

AbstractThe Castejón-Laspaúles basin is one of the South Pyrenean basins of Late Variscan age that were strongly inverted during the Alpine compression (Late Cretaceous–Tertiary). It is mainly composed by Stephanian pyroclastic and volcanic deposits that reach a maximum thickness of ~ 500 m, and are overlain by Permian and Triassic sedimentary units. A palaeomagnetic and magnetic fabrics (AMS) study was carried out in the Stephanian units, where the general absence of flow markers at the outcrop scale and the Alpine inversional structure prevent the straightforward reconstruction of the original volcanic and basinal configuration. Magnetic fabric data are not overprinted by Alpine internal deformation and can be interpreted in terms of primary volcanic and pyroclastic fabrics. The obtained directions coincide in the different sampled units, suggesting a constant source area during the development of the basin, and show the dominance of N–S-trending K1axes that are interpreted to be parallel to flow directions. Palaeomagnetic data indicate the presence of a pre-folding palaeomagnetic component that is rotated clockwise by an average of +37° (±32°) with regards to the Stephanian reference. This rotation probably took place during Alpine thrusting since it is also registered by the overlying Triassic deposits. The whole dataset is interpreted in terms of basin development under sinistral transtension with two main fault sets: deep-rooted E–W-striking faults, probably responsible for magmatic emissions, and shallow-rooted, listric faults of N–S orientation.

Mud dispersal across a Cretaceous prodelta: Storm‐generated, wave‐enhanced sediment gravity flows inferred from mudstone microtexture and microfacies

AbstractDetermining sediment transport direction in ancient mudrocks is difficult. In order to determine both process and direction of mud transport, a portion of a well‐mapped Cretaceous delta system was studied. Oriented samples from outcrop represent prodelta environments from ca 10 to 120 km offshore. Oriented thin sections of mudstone, cut in three planes, allowed bed microstructure and palaeoflow directions to be determined. Clay mineral platelets are packaged in equant, face‐face aggregates 2 to 5 μm in diameter that have a random orientation; these aggregates may have formed through flocculation in fluid mud. Cohesive mud was eroded by storms to make intraclastic aggregates 5 to 20 μm in diameter. Mudstone beds are millimetre‐scale, and four microfacies are recognized: Well‐sorted siltstone forms millimetre‐scale combined‐flow ripples overlying scoured surfaces; deposition was from turbulent combined flow. Silt‐streaked claystone comprises parallel, sub‐millimetre laminae of siliceous silt and clay aggregates sorted by shear in the boundary layer beneath a wave‐supported gravity flow of fluid mud. Silty claystone comprises fine siliceous silt grains floating in a matrix of clay and was deposited by vertical settling as fluid mud gelled under minimal current shear. Homogeneous clay‐rich mudstone has little silt and may represent late‐stage settling of fluid mud, or settling from wave‐dissipated fluid mud. It is difficult or impossible to correlate millimetre‐scale beds between thin sections from the same sample, spaced only ca 20 mm apart, due to lateral facies change and localized scour and fill. Combined‐flow ripples in siltstone show strong preferred migration directly down the regional prodelta slope, estimated at ca 1 : 1000. Ripple migration was effected by drag exerted by an overlying layer of downslope‐flowing, wave‐supported fluid mud. In the upper part of the studied section, centimetre‐scale interbeds of very fine to fine‐grained sandstone show wave ripple crests trending shore normal, whereas combined‐flow ripples migrated obliquely alongshore and offshore. Storm winds blowing from the north‐east drove shore‐oblique geostrophic sand transport whereas simultaneously, wave‐supported flows of fluid mud travelled downslope under the influence of gravity. Effective wave base for sand, estimated at ca 40 m, intersected the prodelta surface ca 80 km offshore whereas wave base for mud was at ca 70 m and lay ca 120 km offshore. Small‐scale bioturbation of mud beds co‐occurs with interbedded sandstone but stratigraphically lower, sand‐free mudstone has few or no signs of benthic fauna. It is likely that a combination of soupground substrate, frequent storm emplacement of fluid mud, low nutrient availability and possibly reduced bottom‐water oxygen content collectively inhibited benthic fauna in the distal prodelta.

Principal component analysis of the geochemistry of soil developed on till in Northern Ireland

This paper presents results from the first regional statistical analysis of soils developed on till in Northern Ireland, using the Geological Survey of Northern Ireland's Tellus geochemical database. Till geochemistry is largely determined by its parent bedrock and soils developed on tills are known to inherit this geochemical signature. Soil geochemistry from areas of till can therefore be used to establish sediment provenance which in turn provides information on palaeo ice flow directions and ice sheet history. In this study, we use Principal Component Analysis to establish geochemical groupings that can be related back to likely bedrock parent material for the tills in the region. The maps, presented here at 1:445,000 scale, show the results of the first regional investigation of soil geochemistry of part of the northern sector of the Irish Ice Sheet. The results indicate that the upper surface of the majority of tills in the study area have a close relationship to local bedrock with rapid geochemical changes observed at lithological boundaries. This suggests that tills in this sector of the Irish Ice Sheet are primarily local in origin, indicating rapid entrainment of bedrock and low rates of evacuation of debris to the ice margin.

Fluvial terraces of the northwest Iberian lower Miño River

A new fluvial terrace map with a tectonic framework for the northwest Iberian lower Miño River is presented. It is the first integrated map to cover the entire lower, 67-km reach of the Miño River, and to cover both the Spanish and Portuguese side of the river. The map is presented at a scale of 1:200,000, although its features were mapped at a scale of 1:5000. Various map layers can be viewed, such as a digital elevation model (DEM), fluvial sediment thickness layers, a palaeoflow direction layer, a lineament and fault layer, and two terrace and tectonic basin layers, showing up to 10 fluvial terraces and a floodplain level. Interpretation of the map shows that next to regional tectonic uplift and glacioeustacy, local basin subsidence and small-scale block movement are very important for the fluvial network, localised fluvial terrace formation, and preservation.

Local sediment sources and palaeoflow directions in Upper Miocene turbidites of the Pannonian Basin System (Croatian part), based on mapping of reservoir properties

The source of the clastic sediments in the Croatian part of the Pannonian Basin System (CPBS) during the Late Pannonian and Early Pontian was the Eastern Alps. Clastic sediments were redeposited several times before they reached the Sava Depression. The depositional environment and sediment transport mechanisms have been subject to detailed analysis described in many publications, and this study builds on previous research. We have carried out geostatistical mapping of selected Upper Pannonian and Lower Pontian reservoir variables of the Klostar Field, located to the west of the Moslavacka gora Mt. (Croatia). This has shown that the Moslavacka gora Mt. was a secondary, local source of sediment, in contrast to the previous interpretation of a single, distant clastic source (Eastern Alps) for the CPBS during the Late Miocene. As the mineralogical composition of the Moslavacka gora Mt. and the Eastern Alps is very similar, the dominant direction of turbidity currents obtained by sequential indicator simulations are used to suggest that a modest amount of detritus was eroded from the Moslavacka gora Mt. and mixed with detritus sourced from the Eastern Alps.

Taphonomy of the vertebrate bone beds from the Klūnas fossil site, Upper Devonian Tērvete Formation of Latvia

Combined sedimentological and taphonomical study of the siliciclastic sequence of the Tērvete Formation in the stratotypical area was aimed at revealing the formation of the three oryctocoenoses discovered and related structural and textural features of the deposits, as well as at detailed observation of the taphonomical peculiarities of the obtained palaeontological material. The fossil vertebrate assemblage is represented by 14 taxa comprising placoderms, acanthodians, sarcopterygians and actinopterygians. The three oryctocoenoses, first recognized in 2010, differ in the proportions of repeatedly buried material, in the number and degree of preservation of small and fragile skeletal elements, as well as in the evaluated current velocity and the transportation distance. Sedimentary concentrations of marine vertebrate remains, dominated by the antiarchs Bothriolepis ornata and B. jani, have been formed under the influence of fluvial and tidal processes in the shallow-water environment, deltaic or estuarine settings. Elongated placoderm and sarcopterygian bones are probably better indicators of the palaeoflow direction than acanthodian spines or sarcopterygian teeth.

Baddeleyite geochronology and geochemistry of mafic cobbles from the Dwyka diamictite: New insights into the sub-Kalahari basement, South Africa

U–Pb baddeleyite geochronology and geochemistry of mafic cobbles from the ~ 300 Ma Dwyka diamictite on the western boundary of the Kalahari Basin provide new insights into the sub-Kalahari basement. Palaeo-ice flow directions indicate that the origin of the Dwyka diamictite deposits on the border between South Africa and Namibia is from beneath the Kalahari sands to the east or north-east. This work shows that the mafic cobbles from three localities near Rietfontein come from intrusions related to the Umkondo Large Igneous Province (LIP), located in the Kheis or Rehoboth Provinces. The lack of Banded Iron Formation (BIF) cobbles in these glacial diamictites indicates that their source region lies west or north of the BIF-dominated Griqualand West Basin of the Kaapvaal Craton. The four dated mafic cobbles give U–Pb ages of 1111–1123 Ma, coeval with dolerites of the Umkondo LIP. The cobbles experienced greenschist facies metamorphism before the Dwyka glaciation, but are unfoliated. The geochemistry of the cobbles indicates that they are genetically similar and have a within-plate setting. They show many similarities to the Umkondo LIP dolerites and the coeval Rouxville Formation, an upper unit of the Koras Group. The most likely sources of the cobbles are the mafic intrusives and lavas along the Kalahari Line, southern equivalents of the ~ 1.11 Ga Tshane and Xade complexes or intrusive and extrusive mafic units related to the Umkondo LIP, now concealed beneath the Kalahari sands.

A new ‘microstructural mapping’ methodology for the identification, analysis and interpretation of polyphase deformation within subglacial sediments

Although integrated clast macro and microfabric studies have been used to provide evidence of palaeo-ice flow directions, to discriminate between diamictons deposited in differing sedimentary environments, and to model the response of tills to subglacial deformation, such studies have typically only examined the ‘bulk’ fabric developed within the diamicton, largely ignoring the potential for several phases of fabric development within these typically polydeformed deposits. We present a new graphical method for the identification, description and analysis of clast microfabrics in polydeformed tills. This method utilises high resolution digital scans of orientated thin sections imported into a standard computer graphics package to measure the orientation of long axes of detrital (skeleton) grains included within the till. These data are then used to identify the main clast microfabrics present within the till, allowing the delineation of clast microfabric domains in which elongate grains share a similar preferred orientation. This process leads to the identification, characterisation and, most importantly, the interrelationships between the various generations of microfabrics. The relationships between these clast microfabrics and other microstructures (e.g. plasmic fabrics, turbate structures, shears, folds, faults…etc) can also be examined, providing a detailed microfabric-microstructural map of the thin section. Clast long axis orientation data collected for different parts of a thin section are plotted on a series of rose diagrams allowing the variation in clast microfabric orientation to be displayed graphically. A new set of terminology for the description of clast microfabrics in tills, which is based upon the system used by metamorphic petrologists, is also presented. Three case studies of the fabrics present within subglacial sediments of Pleistocene age as well as the present are used as examples of how to apply this methodology.

Pleistocene calcareous aeolian–alluvial deposition in a steep relief karstic coastal belt (island of Hvar, eastern Adriatic, Croatia)

Pleistocene aeolian and alluvial deposits occur on the island of Hvar, belonging to the eastern Adriatic karstic coastal belt along the north-eastern Mediterranean margin. The depositional mechanism of the aeolian, alluvial and talus sediments are interpreted based on facies analysis and mineralogical composition. Aeolian deposits are represented by cross-bedded, cross-laminated, and sub-horizontally laminated fine- to medium-grained calcareous sands. The cross-bedded units form tabular sets stacked into cosets. Sets and cosets are separated by distinct low-angle bounding surfaces which mostly dip towards the east interpreted as the upwind direction. Cross-laminated units form isolated sets within the cross-bedded and sub-horizontally laminated units. Cross-bedded and cross-laminated units represent dunes dominated by grain fall and wind ripple deposition, respectively. Dunes produced by sand flow also occur, but are less common. The cross-bedding is truncated by reactivation surfaces. Transverse dunes and some small dome-shaped dunes were also present. Sub-horizontally laminated sandy units represent aeolian sand sheets developed by wind ripple migration in the interdune area. A few wind-rippled dune apron deposits also occur. Trace fossils are locally very abundant within the aeolian deposits, mostly produced by plants. Soft-sediment deformation, such as contorted cross-bedding and pocket structures occur scattered, and rare reddish horizons show pedogenesis. Unlikely most Quaternary coastal aeolian sands, these sands do not include marine bioclasts in composition. The sands are composed of extraclasts dominated by limestone with subordinate chert, quartz-sericite schist, quartzite and quartz. Amphiboles, pyroxene and epidote are the most abundant translucent heavy minerals. The mineralogical composition and palaeoflow directions indicate that the Dinarides were the main provenance of the sand transported by primary easterly and northerly continental winds causing migration of dunes towards the palaeo-Adriatic Sea. Alluvial deposits are intercalated with the aeolian sands. They are composed of breccia derived from a nearby steep hill-ridge, and by resedimented aeolian sand deposited by traction currents during flash floods. Talus fragments reached the aeolian sands only sporadically. Climate strongly affected aeolian, alluvial and talus depositions. During arid conditions dunes migrated forming a dune field, whilst more humid climate triggered heavy rains and generated erosion of aeolian deposits, alluvial sedimentation, colonisation of plants and pedogenesis. Deposition was in a topographic basin, and was controlled by capacity of source area and wind. However, local orography characterised by developed and steep karstic relief strongly affected wind directions, and in that way had specific controls on the dune field evolution. Dominating winds caused seaward dune migration differing from most Mediterranean Quaternary coastal aeolian dune localities characterised by landward migration.

CHARACTERIZING THE SEDIMENTARY HISTORY OF CAVE DEPOSITS, USING ARCHAEOMAGNETISM AND ROCK MAGNETISM, ATAPUERCA (NORTHERN SPAIN)

We use a combination of rock magnetism (anisotropy of magnetic susceptibility, AMS) and magnetic polarity to characterize cave deposits and as a proxy for sedimentary fabric. In three localities at the Atapuerca archaeological site (Galeria, Gran Dolina and Sala de los Cíclopes), magnetic foliation (Kmax/Kint) is always greater than lineation (Kint/Kmin), consistent with a primary, depositional, sedimentary fabric. Our results, although preliminary, reveal a higher degree of anisotropy in autochtonous deposits compared to allochthonous deposits, possibly indicative of a higher hydrodynamic regime in the former. At two localities the magnetic lineation (Kmax) defines a cluster, which is thought to be antipodal to the palaeocurrent direction. Hence we are able to retrieve palaeoflow directions in deposits that otherwise lack any other sedimentary structure. We conclude that AMS is a powerful tool for determining the hydrodynamic character of depositional environments in cave sediments at the Atapuerca archaeological site. A better understanding of the depositional environment and how sedimentation occurred allows reconstruction of the karst evolution and ultimately a better definition of human interaction with the environment.

Sedimentological and sequence stratigraphic analysis of Carboniferous deposits in western Libya: Recording the sedimentary response of the northern Gondwana margin to climate and sea-level changes

Detailed sedimentological and sequence stratigraphic analysis of Carboniferous (Tournaisian to Moscovian) strata exposed in the north-western Murzuq Basin and southern Ghadames Basin, western Libya, provides new insights into the sedimentary response of the northern Gondwana margin to climate and sea-level change. The Lower Carboniferous Marar and Assedjefar Formations can be divided into five depositional sequences of 3rd order. In total 27 facies types are defined, grouped into four facies associations: offshore shales, shallow marine clastics, fluvial sandstones and marine carbonates. The bulk of the Lower Carboniferous interval is dominated by an alternation of offshore shales and shallow marine clastics, which were deposited during the transgressive and highstand systems tracts. The clastic deposits mostly consist of laterally persistent coarsening and thickening upward cycles with a common succession from basal hummocky cross-stratified sandstones to ripple-laminated sandstones, capped by multidirectional cross-stratified sandstones. Within the lowstand systems tracts, lenticular sandbodies have been identified, which vary in thickness from 1.5 m (ca. 40 m wide) to 50 m (ca. 1.5 km wide). These are interpreted to be fluvial channel complexes based on their geometry, erosive base, and presence of thick stacked sandstones with unidirectional planar and trough cross-bedding, the absence of bioturbation and occurrence of land plant fragments. These channel complexes mostly cut down into offshore shales, and are interpreted to be bound at the base by sequence boundaries. Palaeogeographic maps generated for each lowstand system show the location and palaeoflow direction of these fluvial channel complexes. They are interpreted to represent large incised valleys filled with thick fluvial sandstones. Their identification and distribution indicates repeated exposure of large areas of western Libya, most-likely controlled by major eustatic sea-level changes. The Assedjefar Formation exibits a gradual decrease in coarse clastic sediment supply throughout the Serpukhovian and by the Bashkirian and Moscovian during the deposition of the Dembaba Formation a carbonate depositional system was established. Limestones are dominantly made up of a heterozoan fauna (brachiopods, crinoids, bryozoans, gastropods) and are mostly preserved as shallow marine storm and coastal deposits. It is hypothesised that a local increase in aridity and/or the gradual erosion and decreasing topography of the hinterland mountains, with the resulting reduction in discharge, controlled this shift from clastic to carbonate deposition. Available data indicate that the Murzuq Basin was interconnected with the Ghadames Basin at this time and is a postdepositional basin with respect to the Carboniferous interval. The sedimentological and sequence stratigraphic data from the present study offer new insights into the depositional setting and facies distribution in the Carboniferous, and the recognition of major incised fluvial systems has significant implications in the search for potential Carboniferous hydrocarbon reservoirs in the Murzuq, Ghadames, and Illizi Basins.

Contrasting sources for glacial and interglacial shelf sediments used to interpret changing ice flow directions in the Larsen Basin, Northern Antarctic Peninsula

Marine sediment cores from the former Larsen A Ice Shelf area reveal three lithological units deposited during ice sheet advance and retreat from the continental shelf. The uppermost Unit 1 consists of a diatom-bearing silty clay with sparse ice-rafted debris (IRD). Clasts include rock types which can all be matched to onshore outcrops along the eastern side of the Antarctic Peninsula. Most clasts are very angular, angular or subangular. The clasts in this unit are interpreted as supraglacial or englacial debris deposited below an ice shelf distal from the grounding line and/or by icebergs in an open marine setting during the Holocene. In the underlying Units 2 and 3 clast lithologies are quite different and mainly comprise Cretaceous and Jurassic sedimentary rocks. These lithologies were derived locally from the continental shelf (the Larsen Basin) rather than the limited onshore outcrops. Of the 2750 clasts studied in Units 2 and 3, most are subrounded or subangular, and some are striated. Unit 2 is interpreted as being deposited beneath an ice shelf proximal to the grounding line. Unit 3 is interpreted as subglacial diamict deposited by grounded ice flowing across the shelf during the last glacial period. The availability of sedimentary rocks at the seabed may have facilitated the development of a deforming till layer with low shear strength (i.e. soft or deformation till) within Unit 3. There are no consistent differences in clast composition or roundness between the deformation till, which was probably deposited at the base of an ice stream, and an underlying till with high shear strength (i.e. stiff till). On the inner shelf where the bedrock lies very close to the seabed, clast composition in the subglacial diamicts affords a way of identifying the presence of Mesozoic stratigraphic units. The contrasting provenance of the coarse fraction between Unit 1 and Units 2 and 3 is used to interpret ice sheet basal thermal regime and produce a conceptual model of changing palaeo-flow directions between glacial and interglacial periods. Grain roundness and petrology are shown to be a potent method for interpreting different glacial/interglacial processes and environments.

Sedimentary record of a Late Cretaceous volcanic arc in central Patagonia: petrography, geochemistry and provenance of fluvial volcaniclastic deposits of the Bajo Barreal Formation, San Jorge Basin, Argentina

The Upper Cretaceous Bajo Barreal Formation (San Jorge Basin, Argentina) is a fluvial succession mostly composed of channel sandstones interbedded with thicker floodplain deposits dominated by tuffaceous strata. The goal of this contribution is to infer the provenance of the unit through combined petrographical and geochemical data of channel sandstones, primary tuffs and tuffo-psammites (reworked, no-mixed tuffs). Channel sandstones are dominantly litharenites and feldspathic litharenites with abundant participation of volcanic lithic fragments with different textures (porphyritic, eutaxitic, felsitic, pilotaxitic, trachytic and vitric-vitrophyric), pumice and plagioclase. K-feldspar, quartz and sedimentary rock fragments are scarce. QFLu diagram and several provenance indicators including dominance and textural types of rock fragments, K-feldspar/plagioclase ratio and inclusion-free quartz grains indicate a provenance from felsic to intermediate, pyroclastic-rich, arc-related volcanic rocks. This interpretation agrees with the values of various elemental ratios of trace elements such as La/Sc, Th/Sc, Cr/Th, Eu/Eu ∗ and (La/Lu) N and discriminant functions based on major elements. Pyroclastic floodplain strata are commonly pumice-rich, occasionally glass shard-rich, vitric tuffs suggesting an origin from plinian-like eruptions. Subordinate components include volcanic lithic fragments, quartz, orthoclase and plagioclase. Zr/Ti versus Nb/Y and SiO 2 versus K 2O diagrams, as well as a multi-element diagram of selected trace elements shows that the pyroclastic rocks were originated from medium-K content, intermediate, arc related magmatic source. Considering the palaeoflow direction towards the east and south-east and presence of subduction processes along the western margin of Patagonia during the Late Cretaceous, it is interpreted that Bajo Barreal Formation constitutes the sedimentary record (distal facies of volcano-flanking fan or apron) of this coeval volcanic arc. Channel sandstones probably were mostly derived from the Divisadero Formation volcanic rocks because of similar petrographical features and REE pattern. In contrast, pyroclastic tuffaceous floodplain deposits would be derived from vents located over the actual outcrops of the Patagonian Batholith.

Carbonate aeolianites of western Saurashtra, India: experimental decipherment of the depositional mechanisms

Granular carbonate deposits of Late Pleistocene to Early Holocene age, commonly referred to as ‘miliolite limestone’, occur in a linear belt, parallel to the southern coast of Saurashtra, India. In the present study area these carbonate deposits are found in select valleys between ridges and mounds of pyroclastic material present in the Deccan trap plateau. Two different depositional histories have been proposed for these sediments. The presence of marine bioclasts led to the postulation of a marine origin for these deposits. The second school of thought propounded redeposition of the coastal sediments by aeolian processes. Although a few features could not be explained by the proposed aeolian model, critical comparison of these two views favoured the aeolian origin. The mode of occurrence, lithological and structural attributes, and microscopic evidence presented here, also support a possible aeolian origin for these deposits. Experimental observation indicates that these carbonate aeolianites represent backflow deposits, which accumulated because of the flow separation caused by the presence of topographic highs. The conspicuous concave-up geometry of the deposit conformed to the shape of the separation bulb. In view of the inferred depositional mechanism, the disposition of the deposits and the signature of the palaeoflow direction suggest that the carbonate particles were derived from the north-western coast of Saurashtra by strong south-easterly winds. Massive granular carbonates with outsized basement clasts appear to be the product of avalanching of granular material from the higher contours because of oversteepening of the primary deposit.

Deposition and palaeogeography of a glacigenic Neoproterozoic succession in the east Kimberley, Australia

Facies, provenance and palaeocurrent analysis of the predominantly siliciclastic Neoproterozoic succession of the east Kimberley district of northwestern Australia indicates southwesterly-directed regional palaeoice-flow directions associated with continental glaciation. Facies relationships at three key study sites in the east Kimberley record glacial advance, retreat and marine transgression and the likely effects of ice dynamics on local basin depositional history. Ice-scour linked with provenance data indicate ice advance from the northeast. Additional ice-flow data from the west Kimberley and the Daly River area of Northern Territory are consistent and record a regional palaeoice-flow direction towards the southwest. Palaeocurrent data and facies relationships in the east Kimberley, however, suggest that sediment transport directions within the east Kimberley basin were influenced by a combination of ice-induced lithospheric flexure, pre-existing topography and modification of basin topography and sediment dispersal paths by ice-induced redistribution of debris. A basin evolution model incorporating these concepts within the framework of ice advance, retreat and marine inundation is proposed. Identification of regional ice-flow trends of this scale should ultimately prove a useful tool in testing possible palaeogeographic reconstructions of continent configurations in a post-Rodinian world.