Sedimentological Description Research Articles

Sedimentation in the Tethyan pelagic realm during the Cenomanian: Monotonous settling or active redistribution?

Sea bottom processes of the pelagic realm are still not completely understood and represent an intriguing subject. This paper focuses on the relationships between “normal” settling processes, redistribution of sediments and oceanographic parameters in a pelagic setting, during the Cenomanian. Five key Tethyian localities in the Cenomanian Umbria-Marche and Belluno Basins have been studied in order to understand the interplay among sea bottom processes that acted on the sea floor. The dataset consists of the mm-scale sedimentological description of the sections complemented by microfacies analysis on selected samples. Different sedimentological indications, such as presence of intraclasts, lined forams, pervasive plane-parallel lamination, suggest a continuous reworking under action of bottom-currents with varying intensity and direction. All the identified facies are here illustrated in detail and organized in a comprehensive schematic facies framework, the “facies matrix”, that leads to recognize two depositional facies suites: the “settling dominated” and the “traction current dominated”, under different oxygenation conditions. Our results suggest that settling of biogenic and inorganic particles represents the main source of pelagic sediments, but not the unique depositional process: under the action of sea-bottom currents of different intensity, sediments are continuously redistributed on the sea floor. All the collected evidences contribute to the proposal of a comprehensive depositional model for these reworked and redistributed fine-grained sediments, that represent true calcareous pelagic contourites. The model suggests that the identified traction-related facies can be used as a proxy for bottom current intensity and, indirectly, as an indicator of changing ventilation regimes at the sea floor through time.

Beachrock facies variability and sea level implications: a preliminary study

Kelly, C.S., Green, A.N., Cooper, J.A.G. and Wiles, E. 2014. Beachrock facies variability and sea level implications: a preliminary study. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 736–742, ISSN 0749-0208.In spite of the worldwide abundance of beachrocks and their acknowledged utility as an indicator of former sea level position, some studies have expressed doubt as to their position of cementation on paleo shorelines. These criticisms are not, however, coupled with nuanced sedimentological studies of beachrocks. Instead, few beachrock studies acknowledge any facies and therefore disregard important signatures of the depositional environment and, consequently, utility as paleo sea level indicators. This study presents detailed sedimentological descriptions and interpretations from two beachrock localities along the subtropical, microtidal, wave-dominated eastern coastline near Durban, South Africa. The outcrops record the migration of a paleo inlet and deposition in sub, inter, and supra tidal environments. Understanding the inferred depositional environment, and observed stratigraphic relationships between various beachrock facies is critical to teasing out the local evolution of shoreline and relative sea level. The outcrops studied here record multiple episodes of Holocene sea level rise and fall.

Morpho-stratigraphic characterization of a tufa mound complex in the Spanish Pyrenees using ground penetrating radar and trenching, implications for studies in Mars

The Isona tufa mound complex (ITMC), associated with artesian springs of the Areny-Montsec aquifer, Spanish Pyrenees, is a potential analog for water constructed landforms on Mars. We used Ground Penetrating Radar (GPR), trenching, sedimentological description of exposures, and radiocarbon and U-series dating methods for the geological characterization of the ITMC. Preliminary geomorphological mapping combined with sedimentological analyses permitted the recognition of the different facies and their spatial distribution. GPR surveys conducted next to an outcrop and a trench provided electromagnetic wave velocity in tufas (0.09 and 0.11 m ns−1) and determined the correspondence of the radar signatures with facies types. This was used to reconstruct the tufas internal structure and the depositional stages for two different contexts: (1) an upper unit representing the morpho-stratigraphic record of paleosprings – Tufa 1 – composed of relict tufa mounds older than 350 ka BP; and (2) a lower unit – Tufa 3 – associated with groundwater aquifer outlets (Basturs Lakes). The GPR data allowed depicting the signatures for the vent, pool, rimstone, palustrine, dam, cascade and slope facies. A relationship was inferred between the age of the tufas and the radar signature, in terms of relative amplitude and signal attenuation. Older dry tufas with advanced diagenesis and karstification are characterized by well-defined GPR reflectors and lower attenuation than younger tufas, associated with aquifer discharge and shallower water tables. U-series and radiocarbon ages obtained from the Basturs Lakes tufas indicate that these have been active since 106 ka BP during both cold and mild Marine Isotopic Stages (MIS). We hypothesize that tufas related to the deep-seated Areny-Montsec confined karst aquifer were insensitive to climate variations. Landforms reminiscent of the ITMC have been detected during the last decade on Mars. Since GPR will be part of the ExoMars Rover of the European Space Agency (ESA) mission projected for 2018, we anticipate that our results may be able to constrain the interpretation of landforms possibly related to water on Mars.

Depositional and Diagenetic controls on Reservoir Characteristics of X-well and K-well, Ogban Field, Niger Delta

Sedimentological subsurface study of X-well and K-well reservoir sand bodies located in the Niger Delta was carried out to determine the influence of depositional environment and diagenetic control on the reservoir characteristics. Lithofacies analysis reveals the presence of sandstones, siltstones, mudstones and heterolithics. Based on the detailed sedimentological description, wire line logs evaluation and trace fossils analysis, the X-well and K-well reservoirs are interpreted as a barrier complex deposit of fluvio-deltaic—shallow marine environment. Petrographic studies reveal quartz as the dominant mineral with minor amount of feldspars. Cements include silica, quartz overgrowth and siderite. Facies analyses indicate porosity loss due to compaction, and variations in the diagenetic pathway observed in the different facies are closely related to primary mineralogy as influenced by palaeodepositional environments.

Seismic facies analyses as aid in regional gas hydrate assessments. Part-I: Classification analyses

The identification of areas of natural gas hydrate occurrence, defining its concentration and regional distribution, as well as understanding the processes that control gas hydrate formation is a crucial component of regional gas hydrate assessments. In this study, we incorporate an additional element into a regional assessment strategy by including the depositional environment defined through seismic facies classes. The seismic facies classification is attempted using regional 2D seismic data and a 3D seismic volume, as well as core and log-data from two gas hydrate drilling expeditions carried out in the Ulleung Basin, East Sea, to conduct a fully integrated gas hydrate assessment. The facies classification described in this study is part of a procedure to conduct an assessment including: (1) Lithologic description of the recovered core and describe the deposition character of the sediment facies identified; (2) seismic facies classification using 2D and 3D seismic data and their derived various seismic attributes; (3) Linking seismic facies classes to core-derived sedimentological descriptions to define the host-strata most favourable for gas hydrate occurrences; (4) Defining reservoir physical properties and linking these to seismic facies classes for reservoir property description; (5) Calculating regional elements of the gas hydrate petroleum system (top of gas hydrate occurrence and base of gas hydrate stability); (6) Merging seismic facies and reservoir properties, definition of the top and base of the gas hydrate stability zone, to calculate gas hydrate saturations in each seismic facies and the total volume of gas hydrate present in the study area; (7) Defining probabilistic elements of the predicted volume of gas hydrate by combining statistical information of the individual input parameters through exhaustive Monte Carlo simulations. The seismic facies classification algorithm used in this study is based on identifying clusters (classes) that show similar properties in the seismic data and derived attribute characteristics. Our classification scheme uses seven seismic attributes (amplitude, instantaneous frequency and amplitude (also referred to as envelope), sweetness, acoustic impedance, thin-bed- and parallel-bedding indicator) and incorporates the use of principle component analysis for data reduction. The achieved classifications are overall robust and yield identical facies classes for the co-located 2D and 3D data. Linking the seismic facies classes to the recovered core and sedimentological descriptions results in a sedimentological interpretation of the seismic facies. Cross-plots of individual components as well as analysing the Eigen-vectors and -values from the principle component analysis (PCA) helps understand the physical meaning of these seismic facies classes and their seismic character. The five facies classes assigned to the 3D volume and 2D seismic line after PCA represent the mass transport deposit (MTD) units (class #1), hemipelagic mud with some sandy turbidite (class #2), hemipelagic mud with many sandy turbidites (class #3), as well as water-bearing mud with few sand layers and occurring mostly at shallow depth near the seafloor or beneath the base of the gas hydrate stability zone (classes #4 and #5). Within the study area and seismic data sets used, gas hydrate occurrences are only linked to facies classes #2 and #3.

Characterization of the Heterogeneous Flow and Pollutant Transfer in the Unsaturated Zone in the Fluvio-glacial Deposit

Infiltration basins are part of the best management practices. They are aimed at infiltrating stormwater to prevent additional collection and treatment through rainwater systems. In the suburbs of Lyon (France), many of these infiltration basins were built over fluvio-glacial deposit. These basins have been the subject of research programs on vadose zone flow and fate of pollutants. This study focuses on the impact of the heterogeneity of the fluvio-glacial deposit on both flow pattern and solute transfer. A proper geological and sedimentological description is first proposed to characterize the efficient water transfer properties of the fluvio-glacial deposit at the work scale (1ha). The local geological and sedimentological architecture of the deposit and its lithofacies were investigated locally through trenches using both particle size analysis and sedimentological approach. This information was extended to the whole work by combining several geophysical techniques, i.e. GPR, electric resistivity and seismic refraction tomography (data not shown). Then water infiltration experiments were performed on each lithofacies to derive the hydrodynamic properties through BEST algorithm (Beerkan estimation of Soil Transfer properties), leading to the corresponding hydrofacies. In addition, soil-column and batch experiments were performed to estimate hydrodispersive parameters by tracer experiments and the geochemical properties of lithofacies for a model pollutant, i.e. copper (Cu). All these data were implemented into Hydrus to model flow and solute transfer through a 2D soil profile with a precise description of the hydrofacies at the basin scale (flow domain 14x2 m2). The results are highly relevant because they emphasize different types of preferential flow due to either the presence of capillary barriers, drainage layers or pipe flow, which may be responsible for the enhancement of pollutant transfer. In particular, they show that sand lenses may play an important role whereas unconnected gravels may have insignificant effect on flow. This methodology may help in understanding the mechanisms which that are responsible for preferential solute transfer. A sensitivity analysis combining the distribution of lithofacies, the soil initial water content and unsaturated hydraulic properties allows bettering understanding the development of preferential flows across the vadose zone underneath the basin. This data is of great interest in terms of the optimization of basin infiltration monitoring.

Diachronic development of the Lake of Abusir during the third millennium BC, Cairo, Egypt

The paper introduces results of the survey of the Lake of Abusir, performed by the Czech Institute of Egyptology over the last few years. A detailed sedimentological description of four trenches located within the area of the so-called Lake of Abusir is given, together with the micromorphological results of selected strata, molluscs and archaeobotanical findings. All the presented results document the paleoenvironmental and geoarchaeological history of the study area. Five main developmental phases of the Lake of Abusir are interpreted. The area was used as a principal entryway to the cemeteries of Saqqara and Abusir during the third millennium BC. These cemeteries were built to serve the capital of ancient Egypt, the White Walls, which was established around 3000 BC. The old Nile terrace deposits constituted the background for the tiled pavement built during the Old Kingdom period. Later, this pavement was partly destroyed by heavy outwash, and the anthropogenic features were overlain by a layer of pure sand. Subsequent sedimentological development is characterized by increased desiccation interrupted by several phases of elevated moisture due to the activity of the local hydrological system. Due to the function of Wadi Fetekti, thick colluvial deposits were preserved.

Utilising borehole image logs to interpret delta to estuarine system: A case study of the subsurface Lower Jurassic Cook Formation in the Norwegian northern North Sea

The Lower Jurassic Cook Formation forms a regressive and transgressive sandstone wedge of shallow marine reservoir sandstones. It is distributed mainly in the Norwegian sector of the northern North Sea and the formation has proven to be hydrocarbon bearing. A case study of this formation from the Tampen Spur area presents a methodology for reconstructing depositional environments in areas of scarce data coverage and poor seismic quality using limited core-coverage, wire-line logs and borehole image logs. The core material and image logs are from different wells. The latter offer interesting opportunities for sedimentological descriptions and interpretations both in cored and uncored sections, particularly as the resolution of the tool (mm-scale under optimal conditions) enables identification of sedimentary structures. In order to avoid over-interpretation, a system of descriptive, simple and robust image facies was established for this study. These include: horizontal lamination, low-angle lamination, cross-stratification, as well as mottled and deformed strata. The Cook Formation is interpreted here as a regressive tidal-fluvial delta to transgressive wave-dominated estuary couplet with offshore shale above and below. The tidal-fluvial delta of the regressive part seems to be at odds with the regional context as the regressive part of the Cook Formation in the Tampen Spur area is interpreted as a wave-dominated delta system. Internally, the regressive part of the Cook Formation thickens westwards with 68% which is unusual for the otherwise tabular regressive Cook Formation in the Tampen Spur area. Both the thickness and depositional environment difference of the regressive part of the Cook Formation can be explained with temporary fault movement of a blind fault located basinward during basin infill. This could have created a fault-induced monocline that led to wave sheltering of the tidal-fluvial delta system. A spit-barrier system was probably associated with the monocline sourced by longshore drift from the otherwise wave-dominated coast of the regressive Cook Formation. After basin infill and removal of basin relief, the subsequent transgression resulted in the formation of a wave-dominated estuary.

Subglacial deforming bed conditions recorded by late Quaternary sediments exposed in Vineland Quarry, Ontario, Canada

There has been considerable interest in recent years in the development of theoretical models of subglacial transport and deposition of sediment, but relatively few studies report field documentation of the resultant sediment stratigraphies. This paper presents detailed sedimentological description and analysis of a succession of late Quaternary deposits interpreted to record subglacial overriding and deformation of previously deposited lacustrine sediments exposed in the Vineland Quarry that sits close to the crest of the Niagara Escarpment within the Lake Ontario basin. The predominately fine-grained sediments record deposition under glaciolacustrine conditions followed by deformation and deposition by overriding glacial ice. Laminated silt and clay deposits overlie the striated bedrock surface and were deposited within a lake that formed as the Ontario Lobe of the Laurentide Ice Sheet advanced during the Port Huron stadial and ponded water against the Niagara Escarpment. The laminated silt and clay facies show increasing amounts of deformation up-section, passing from planar through ductile to brittle deformation. This succession of deformed facies is overlain by a macroscopically massive clay-rich diamict that caps the section. This pattern of sediment deposition and deformation is consistent with that proposed by current models of subglacial sediment deformation with the disrupted laminated silts and clays representing ‘glacitectonites’ resulting from downward penetrating stresses imparted by an overriding ice sheet. The uppermost massive diamict unit represents full macroscopic homogenization of the overridden sediment and is classified as a subglacial ‘traction till’. The gradual transition from undisturbed laminated deposits through increasingly deformed sediment to structureless, diamict suggests that these deposits record a single episode of ice advance across the region. This ice advance was probably the short-lived advance of the Ontario Lobe of the Laurentide Ice Sheet that occurred at approximately 13,000 ybp.

Revealing the last 13,500 years of environmental history from the multiproxy record of a mountain lake (Lago Enol, northern Iberian Peninsula)

We present the Holocene sequence from Lago Enol (43°16′N, 4°59′W, 1,070 m a.s.l.), Cantabrian Mountains, northern Spain. A multiproxy analysis provided comprehensive information about regional humidity and temperature changes. The analysis included sedimentological descriptions, physical properties, organic carbon and carbonate content, mineralogy and geochemical composition together with biological proxies including diatom and ostracod assemblages. A detailed pollen study enabled reconstruction of variations in vegetation cover, which were interpreted in the context of climate changes and human impact. Four distinct stages were recognized for the last 13,500 years: (1) a cold and dry episode that includes the Younger Dryas event (13,500–11,600 cal. year BP); (2) a humid and warmer period characterizing the onset of the Holocene (11,600–8,700 cal. year BP); (3) a tendency toward a drier climate during the middle Holocene (8,700–4,650 cal. year BP); and (4) a return to humid conditions following landscape modification by human activity (pastoral activities, deforestation) in the late Holocene (4,650–2,200 cal. year BP). Superimposed on relatively stable landscape conditions (e.g. maintenance of well established forests), the typical environmental variability of the southern European region is observed at this site.

Palaeoenvironmental reconstruction of the Mufara Formation (Upper Triassic, Sicily): High resolution sedimentology, biostratigraphy and sea-level changes

This study contributes to the reconstruction of the geological and palaeoenvironmental settings of the Mufara Formation (Upper Triassic, Sicily), which consists of monotonous marly deposits intercalated with relatively thin limestones levels. Field study included description of sixteen sections corresponding to thirteen outcrops, from which ~ 500 samples were taken. Outcrops are mainly located in north-western to central Sicily (i.e., Monti di Palermo, Monti Sicani, Monti Madonie and Caltanisetta Basin). The stratigraphic contact of the Mufara Fm. with the underlying Anisian–Lower Carnian Lercara Formation, and the overlying Upper Triassic to Lower Jurassic formations (Panormide shallow carbonate platform, Fanusi slope deposits, Scillato and Calcari con selce basinal series) has not been observed. Investigations involved detailed facies and sedimentological descriptions, high resolution biostratigraphy, and genetic stratigraphy. Sedimentological data provide evidence for a new interpretation of the depositional setting of the Mufara Fm. The depositional framework extended from a shallow to deeper water offshore zone along a distally steepened shelf or ramp, on which sponge patch reefs developed. This ramp, which developed during Ladinian to Carnian, probably evolved into a rimmed carbonate platform during Late Triassic, due to changes in basin morphology and tectonic evolution. Biostratigraphy, mainly based on foraminifers and supported by conodonts and palynomorphs, indicates a Late Ladinian/Early Carnian ( Frankites regolenadus/ Trachyceras aon zones) to Late Carnian ( Tropites subbullatus/ Anatropites spinousus zones) age for the Mufara Fm. An Anisian–Early Carnian age for the underlying Lercara Formation implies that the transition between the Lercara and the Mufara formations took place during the late Middle Triassic to early Late Triassic interval, and was probably strongly diachronous.

Sedimentology and environmental significance of the Cryogenian successions of the Yangtze platform, South China block

The Neoproterozoic glaciations during Sturtian (c. 720 Ma) and Marinoan (c. 600–635 Ma) times are considered events of extreme climate change of global dimension, sometimes interpreted according to the snowball Earth hypothesis. Here we present, for the first time, a detailed sedimentological description and facies analysis of the deposits of both glacial events recorded on the Yangtze platform of the South China block. The Yangtze platform was located at c. 40° latitude during the older glaciation, and close to the equator during the younger one. The studied successions of Nanhuan–Sinian age, including both diamictite-bearing Units, vary in thickness from ∼ 90 m to ∼ 1800 m and show some remarkable differences between the deposits of Sturtian and Marinoan age. In general, both diamictite-bearing successions are composed of diamictites, debris flow deposits, turbidites, structureless or laminated sand- and siltstones, and mudstones. However, diamictites of the lower Unit were deposited from proglacial debris flows, whereas those of the upper Unit formed as lodgement deposits of the grounded zone. Laminated sand- and siltstones containing dropstones are abundant in the lower Unit and very rare in the upper one. Deposits of subaqueous mass flows are present in both Units. The lower diamictite Unit is not overlain by cap carbonates. The upper diamictite Unit grades transgressively into 3 to 8 m thick successions of thin-bedded and partly clast-rich carbonate rocks representing the basal strata of the Ediacaran Doushantuo Formation. In view of a conformable contact between the upper diamictite Unit and these carbonate layers, and their similar lithostratigraphic position to those on other continents, we consider them to represent cap carbonates. Our sedimentological data indicate different patterns of development of the two glaciations. The Sturtian event in South China is characterized by deposition of ice-rafted debris distal to ice margins thus indicating ample open ocean space available for iceberg drift. The Marinoan record in South China registers depositional processes proximal to ice-margins including lodgement deposits. In consideration of the palaeogeographic position of the Yangtze platform in intermediate (Sturtian) and low (Marinoan) latitudes during Neoproterozoic time, there were probably significant differences in intensity and extent between the glaciations, the older one being less severe and characterized by regionally limited ice centres. There is no evidence of a shutdown of the hydrological cycle during the deposition of the two diamictite Units. In conclusion, our sedimentological data neither support a Sturtian nor a Marinoan “hard“ snowball Earth glaciation.

Very high energy sedimentation (supratidal hurricane deposits) and Mid-Holocene highstand on carbonate platforms, Andros, Bahamas: An alternative view

Twenty-eight 14C dates were obtained from 90 cores drilled through the tidal flats of western Andros Island, a large area (2000 km 2), mostly covered by mangroves and stromatolitic deposits. These dates are correlated with location of the samples, sedimentological description, position of the samples at the time of deposition and their present-day sedimentological position. Measurements were made on bulk algal sediments and location of the relative paleo-sea-level is inferred from paleobathymetric reconstructions. On the basis of 14C dates, I conclude that the whole area was flooded about 5000 years BP and then was covered by the subtidal greyish mud facies, rich in Peneroplids, in the area of Deep Creek (north of the study area), Pelican Creek and Wide Opening (southeast of the study area). The level of this paleo-Great Bahama Bank Sea continued to rise until 3500 years BP and then slowly fell asymptotically toward the present-day level. Thick hurricane marks (aragonitic lime mud with Didemnids spicules, several cm thick), previously defined as hurricane trails, can be observed throughout the cores, from 2000 years BP onwards. From 2000 years BP onwards it seems that the modern physiography was already established, including tidal estuaries, channels and stromatolite-bearing swamps. Paleogeographic maps and sections have been drawn, and accumulation rates calculated in the various sedimentological zones of the tidal flats of western Andros. These vary between 0.5 to 0.04 mm/yr. The modern topography of the tidal flats was established by 2000 years BP; similarly, the modern geological features, controlled by a tropical climate strictly dependent on hurricanes, were already established by 2000 years BP and seem to have been initiated between 2500 and 2000 years BP.

Structural and stratigraphic controls on fold-related fracturing in the Zagros Mountains, Iran: implications for reservoir development

Abstract New observations of fracture corridors > 150 m tall and planes of bedding-parallel slip are integrated with sedimentological descriptions of the Asmari Formation to understand the main controls on the development of fractures in the Zagros Mountains of Iran. In the Kuh-e Pahn, fold-related fracture corridors are axis-parallel (NW-SE) and occur in the crest of the anticline. They form by neutral surface folding, but at a critical dip of the beds (c. 15°), bedding-parallel slip by flexural slip folding is the predominant mechanism. This relationship is substantiated by curvature calculations. Crestal fractures have a large vertical extent in mechanical unit B (> 150 m), primarily due to the lithological homogeneity of massive packstones within the Asmari Formation. Northerly and easterly trending fracture corridors, interpreted from satellite imagery, are spatially unrelated to the detachment folding of the cover series, but represent the distributed effect of deep-seated basement reactivation related to fault movement. These trends define high production zones in the nearby Gachsaran super giant oilfield. Observations from an adjacent eroded box fold, the Kuh-e Mish, with steeper dipping limbs (60°), revealed a contrast in the style of deformation, and we interpret these folds to represent different stages in box fold evolution.

Origin of drumlins on the floor of Lake Ontario and in upper New York State

Upper New York State, USA contains one of the largest drumlin fields (12,000 km 2) on the North America continent consisting of some 10,000 drumlins located between Lake Ontario in the north and the Finger Lakes; drumlin-like bedforms have also recently been discovered in deep water (∼ 200 m) on the nearby floor of the Rochester Basin in Lake Ontario. The New York State drumlin field was produced under the Laurentide Ice Sheet 18,000 years ago when it extended southwards to terminate against the Allegheny Plateau. Outcrops of drumlin cores exposed by erosion along the Lake Ontario shoreline provide windows into the stratigraphy and sedimentology of the northern part of the field. Sedimentological description of 10 coastal drumlin cores, combined with quantitative morphometric analysis of 1487 drumlins, shows that they are the product of subglacial erosion of till sheets separated by glaciolacustrine deposits. A two-phase subglacial model for drumlin formation is suggested consisting of initial aggradation of extensive till deposits during ice advance followed by erosion and streamlining possibly accompanying stillstand of the ice margin and building of moraines. Systematic changes in drumlin morphometry and spacing are evident, ranging from small numbers (< 15% of total bedforms) of widely separated, spindle-shaped forms (Length/Width ratios between 6 and 18) to more numerous (< 40%) broader forms (L/W ratios between 3 and 6) to most numerous (> 60%) closely spaced, ovoid drumlins (L/W ratios between 1 and 3). Drumlin shape appears to be systematically related to sediment thickness available for erosional streamlining; elongate spindle forms occur on the crests and lee sides of bedrock escarpments where sediment cover is thin. Larger ovoid drumlins occur in topographic depressions where sediment thickness is greater. Offshore streamlined forms on the floor of the deep water Rochester Basin in Lake Ontario have long axes at right angles to the trend of onshore drumlins. These are composed of rock (rock drumlins) and have an orientation structurally controlled by a newly identified inlier of Proterozoic Grenville Province metasediments.

Porosity–permeability relationships in Miocene carbonate platforms and slopes seaward of the Great Barrier Reef, Australia (ODP Leg 194, Marion Plateau)

AbstractThis paper reports a series of 700 porosity–permeability analyses and supporting petrographic and sedimentologic descriptions from Early to Late Miocene carbonate strata cored on the Marion Plateau, offshore from north‐eastern Australia, during Ocean Drilling Program (ODP) Leg 194. The samples analysed are not only mainly coarse bioclastic limestones and dolomitized equivalents from platform‐top facies, but also include 79 plugs from deeper‐water slope to hemipelagic drift facies. Outstanding characteristics of this data set are the wide ranges of porosity and permeability in both limestones and dolostones, the large degree of short‐range heterogeneity typical of these strata, and the better porosity–permeability correlation of dolostones than limestones. The platforms have experienced widely varying calcite cementation, dolomitization and dissolution but show little clear evidence of meteoric diagenesis, suggesting that subaerial exposure may have played little role in porosity–permeability evolution. Permeability‐for‐given‐porosity is controlled by grain size and calcite cement content in grainstones and by occurrence of larger shelter pores and vugs in mud‐rich samples. Dolomitization tends to reduce the variation of permeability‐for‐given‐porosity by recrystallizing mud matrix to form intercrystalline macroporosity that connects vugs and moulds to become integrated with the effective pore system. As a result, there are no differences in permeability–porosity trends for different dolostone textures, whether dominated by intercrystalline, vuggy, or preserved intergranular pore types. Two platform‐top sites separated by only 5 km display a major lateral variation in dolostone porosity–permeability characteristics within the youngest dolostone units. This difference is interpreted as reflecting a relatively ‘windward’ (current‐facing) setting of the site with the overall higher permeability‐for‐given‐porosity (Site 1199) that led to less muddy depositional facies, greater cementation, and lesser grain dissolution. Pore‐geometry parameters measured by petrographic image analysis confirm that the ‘windward’ dolostones have pores that are both larger and less intricate than dolostones comprising the more current‐protected location.

Nature and dating of karstic filling in the Hainaut Province (Belgium). Karstic, geodynamic and paleogeographic implications

The Carboniferous limestones of the Hainaut Province (Belgium) provide evidence of complex karstification. The forms and deposits recently discovered are (1) ghost-rocks with residual weathered and decayed rock, (2) fluviatile endokarsts filled with pebbles rounded by rolling, and (3) ghost-rocks taken up later by underground fluviatile erosion and containing sandy and clayey deposits. We here provide a detailed sedimentological description, a granulometric study, a palynological dating and geochronological dating (K-Ar method on grains of glauconite and ferriferous illite) of the karstic filling. This palaeokarst stratigraphy of the studied area allows us to propose the following evolution: (1) a fluviatile endokarstification in a hummocky relief in late Early Cretaceous and early Late Cretaceous, (2) the formation of ghost-rocks on a flat relief during the Cenomanian-Turonian, (3) fluviatile deposition of sands and clays filling, as a result of tectonic reactivation during the Maastrichtian-Danian. This multidisciplinary study, taking into consideration karstic, hydrodynamic, tectonic and palaeogeographic factors within a well-defined stratigraphic framework may be of use in studying of the palaeokarst successions.

Debris characteristics and ice-shelf dynamics in the ablation region of the McMurdo Ice Shelf, Antarctica

AbstractThis paper presents observations and measurements of debris characteristics and ice-shelf dynamics in the ablation region of the McMurdo Ice Shelf in the Ross Sea sector of Antarctica. Ice-shelf surface processes and dynamics are inferred from a combination of sedimentological descriptions, ground-penetrating radar investigations and through ablation, velocity and ice-thickness measurements. Field data show that in the study area the ice shelf moves relatively slowly (1.5–18.3ma–1), has high ablation rates (43–441 mm during 2003/04 summer) and is thin (6–22 m). The majority of debris on the ice shelf was originally transported into the area by a large and dynamic ice-sheet/ice-shelf system at the Last Glacial Maximum. This debris is concentrated on the ice-shelf surface and is continually redistributed by surface ablation (creating an ice-cored landscape of large debris-rich mounds), ice-shelf flow (forming medial moraines) and meltwater streams (locally reworking material and redistributing it across the ice-shelf surface). A conceptual model for supraglacial debris transport by contemporary Antarctic ice shelves is presented, which emphasizes these links between debris supply, surface ablation and ice-shelf motion. Low-velocity ice shelves such as the McMurdo Ice Shelf can maintain and sequester a debris load for thousands of years, providing a mechanism by which ice shelves can accumulate sufficient debris to contribute to sediment deposition in the oceans.

Climatic control on Quaternary fluvial sedimentology of a Fenland Basin river, England

AbstractUnderstanding and interpreting climate response from ancient fluvial deposits requires a combined approach of detailed sedimentological description and interpretation, local climate proxies, and the accurate comparison of these sequences with further independent climate records. Application of these multiple lines of evidence is particularly effective in the Quaternary Period, where higher resolution and more detailed palaeoenvironmental reconstruction are possible. This paper presents results from a Devensian (Weichselian) age sequence in northern Fenland, England, that integrates these approaches. Detailed sedimentological description has identified three fluvial facies associations overlain by a period of ice‐wedge cast formation. Fossil plant and coleopteran material within these sediments have been studied to provide palaeoenvironmental reconstructions for the local landscape, including quantitative temperature estimates using the mutual climatic range method on fossil Coleoptera. In addition, joint application of radiocarbon and optically stimulated luminescence (OSL) dating allows this sequence to be compared with the stacked marine isotope record, although dates do not agree well at the limit of the radiocarbon technique. Sedimentological interpretations, palaeoenvironments and global ice‐volume variations (a proxy for climatic severity) are all in agreement and show clearly how the River Nene responded to subtle changes in climate and vegetation cover during the Devensian (Weichselian) Stage. The level of detail possible in this reconstruction shows the importance of this approach in understanding fluvial response to past climate changes.

Paleosecular variations recorded by Holocene-Pleistocene sediments from Lake El Trébol (Patagonia, Argentina)

Paleomagnetic studies carried out on six cores from the bottom sediments from lake El Trébol, South Argentina are described. Measurements of directions declination and inclination ( D and I) and intensity of natural remanent magnetisation (NRM), magnetic susceptibility at low and high frequency (specific, X and volumetric, k), isothermal remanent magnetisation (IRM), saturation isothermal remanent magnetisation (SIRM), back field and anhysteric remanent magnetisation (ARM) were carried out. Stability of the NRM was investigated by alternating-field demagnetisation. Rock magnetic studies suggest that the main carriers of magnetisation are ferrimagnetic minerals, predominantly pseudo single domain magnetite. Records of NRM and k show a good correlation with the sedimentological description, and also with those of shorter cores previously studied. The tephra layers were identified from the lithologic profiles and also from the magnetic susceptibility logs. Due to their different chronological meaning and their rather bad behaviour as magnetic recorder, these layers were removed from the sequence and the gaps that were produced along the profiles by the removal were closed, obtaining a shortened depth. Radiocarbon age estimates from these cores and from earlier studies allow us to construct a paleosecular variation (PSV) record for the past 24,000 years. These results show good correlation with the previous one. The spectrum obtained from spectral analysis shows major peaks at periodicities of about 1150, 1500, 2490, 3630, 5240 and 11,810 years. Both clockwise and counterclockwise precession of the geomagnetic vector is evident from analysis of the Bauer plots, with a preponderance of clockwise movement.