Trough Cross-stratified Sandstone Research Articles

Heterolytic tide-dominated sedimentary facies and depositional environment: An Example from the Boka Bil formation, Sitapahar anticline, Bangladesh

The Baraichari Shale Formation, which is equivalent to the Boka Bil Formation, is well exposed within the Neogene sequence of the Sitapahar Anticline in the Chittagong-Tripura Fold Belt, Bengal Basin, Bangladesh. Regardless of its importance in hydrocarbon accumulation, a comprehensive facies analysis is essential to understand and explain the heterogeneity within different depositional architectural elements. The purpose of this work is to delineate the lithofacies characteristics, depositional environment heterogeneities and propose a generic depositional model for the Middle to Late Miocene shallow marine sediments of the formation. The sedimentary facies were investigated at three outcrops along the Kaptai–Chandraghona road cut sections of the Sitapahar anticline. The formation is composed of dark to bluish-gray shale, silty shale, yellowish brown siltstone, and gray to yellowish brown fine-to medium-grained sandstone. Eight distinct sedimentary facies have been identified and were categorized into three facies associations: subtidal or estuarine, mixed flats with tidal creeks, and tidal mud flats. The subtidal facies, predominant in the middle member, consists of fine to medium-grained ripple cross-stratified sandstone-siltstone, herringbone cross-stratified sandstone, and trough cross-stratified sandstone, indicating sub-tidal channel deposition. Mixed flats with tidal creeks exhibit heterolithic facies, suggesting energy level changes in intertidal to sub-tidal zones influenced by tidal currents. Tidal mud flats are predominantly laminated shale/mudstone and lenticular laminated sandstone-siltstone, reflecting deposition during tide transitions in the intertidal zone. Fining upward cycles within the formation indicate progressive progradation or changes in inner tidal flats conditions during regressive periods. These depositional settings serve as analogs for equivalent subsurface reservoirs and contribute to the construction of reservoir models with greater accuracy.

Re-examining models of shallow-water deltas: Insights from tank experiments and field examples

Shallow-water deltas remain enigmatic in terms of placing the observed facies within a coherent process-based depositional model. Here we report tank experiments on mouth-bar formation from shallow water pure and stratified jets that, combined with recent flume experiments on bedforms, suggest new interpretations of field observations from shallow-water delta outcrops.Our experiments imply that the height, geometry and bedforms of the mouth bars depend on the jet properties and grain size of the supplied sediment. Pure jets with very coarse-grained sediment formed a high and steep mouth bar that is characterised by steep angle-of-repose cross bedding with related avalanche processes (grain flows) on the lee side. The experiments with stratified jets imply that mouth-bar deposition and growth are dominated by supercritical density flows that evolve from the initial jets on the lee side of the growing mouth bar. In stratified jets with very coarse-grained sediment, deposition on the mouth-bar lee side was both from grain-flow avalanches and density flows. While deposition on the upper lee slope was dominated by grain flows, a concentric field of low relief, asymmetric, downflow-migrating bedforms evolved on the lower slope and beyond the mouth bar. In the stratified jet with medium-grained sediment a very low relief mouth bar formed within a concentric field of low, asymmetric, downflow-migrating bedforms covering the entire lee slope and the area beyond.Many previous field studies show that mouth bars deposited from dense stratified jets (hyperpycnal flows) are characterised by a distinct facies assemblage of coarse-grained cross-stratified or low-angle cross-stratified sandstone passing downslope into finer-grained plane-parallel, or “quasi-parallel” laminated sand and into climbing-ripple cross-laminated sandstone. Comparison to flume and tank experiments suggests that the proximal coarse-grained planar and trough cross-stratified sandstones could represent deposition by supercritical dunes that pass downslope into antidunes, characterised by sinusoidal stratification and/or low-angle cross stratification. The repeated vertical transition between antidune deposits and climbing-ripple cross-laminated sandstone may indicate the superposition of ripples onto antidunes in finer-grained sediments, indicating ripple formation under supercritical flow conditions. Similar bedforms/sedimentary structures have previously been interpreted as hummocky cross-stratification or swaley cross-stratification and attributed to combined flows in storm-dominated settings, which probably in some cases must be revised.

Plio-Pleistocene to Recent tectonostratigraphic evolution of the Lalmai anticline in the western Indo-Burman Range (Bangladesh): insights from lithofacies analysis and structural synthesis

The Lalmai anticline is one of the westernmost structures within the Eastern Fold Belt of the Bengal Basin that has surficial expression. The tectonic evolution of the Eastern Fold Belt is directly linked with the development of the Indo-Burman Range in response to the late Cenozoic convergence between the Indian and Burmese plates. However, very little is known about the Plio-Pleistocene to Recent tectonic evolution of the Lalmai anticline in the context of regional tectonics. Therefore, this research is aimed at lucid understand the Plio-Pleistocene to Recent tectonostratigraphic evolution of the Lalmai anticline based on detailed lithofacies and structural observations. Results show that Plio-Pleistocene Dupitila and Pleistocene Madhupur Clay formations are exposed in the Lalmai area. Based on sedimentary texture and structures, a total of six prominent lithofacies types have been identified. These are trough cross-stratified sandstone (St), planar cross-stratified sandstone (Sp), parallel laminated sandstone (Sl), massive sandstone (Sm), laminated shale (Fl), and mudstone (Fm) facies. The facies characteristics indicate a possible fluvial depositional environment for both the formations. Structural mapping shows that the bedding surfaces are nearly horizontal. Based on the missing of strata (both the Dupitila and Madhupur Clay formations) and presence of a fault escarpment, a N-S trending east-dipping reverse fault (namely, the Mainamati Fault) has been identified on the western margin of the Lalmai anticline. The tectonostratigraphic evolutionary models show that the last major structural activation along this Mainamati Fault can be linked to late Pleistocene to Recent time. This structural activation was possibly controlled by the latest ongoing E-W crustal shortening in response to the Indo-Burman subduction processes to the far east.

Sedimentary facies, paleoenvironments and reservoir potential of the Afikpo Sandstone on Macgregor Hill area in the Afikpo Sub-basin, southeastern Nigeria

Sedimentary facies, petrographic and textural analyses of sections of the Afikpo Sandstone on Macgregor Hill area in the Afikpo Sub-basin, southeastern Nigeria, were carried out to evaluate the paleoenvironments and reservoir potential. Result of the sedimentary facies analysis revealed eight lithofacies grouped into estuarine and subtidal sand ridge/foreshore lithofacies associations. The facies include planar cross-stratified sandstone, bioturbated sandstone, trough cross-stratified sandstone, parallel laminated sandstone, bioturbated cross-stratified sandstone, sandy bioturbated heterolith, conglomerate and laminated mudstone. The estuarine lithofacies associations show tripartite subdivision into a mudflat deposits, fluvio-estuarine sandstones in bayhead delta environments and central estuarine dominated by intertidal bayfill and lagoonal muddy deposits. Skolithos–Cruziana ichnogenera of high-energy shallow water were recorded in the bioturbated lithofacies. They showed moderate to high intensity of bioturbations, dominantly and intermittently distributed feeding structures. Petrography of the sandstones shows that samples from different lithofacies differ in compositions and textures. Generally, the sandstone consists of mainly quartz and subordinate weathered feldspar and metamorphic rock fragments as framework elements. The textural characteristics of the Afikpo Sandstone using univariate parameters of mean, standard deviation, skewness and kurtosis and the multivariate discriminate functions suggest the sediments were deposited in high-energy fluvio-marine setting. The sandstones showed good porosity range from 0.485 to 0.499 with an average value of 0.489 and the empirical permeability of 26.03–42.64 md with an average value of 39.86 md, which reveal the sandstones as good reservoirs indicating the Afikpo Sandstone as potential for hydrocarbon reservoir for subsurface analogue.

Alluvial fan to shallow marine sedimentation record in the ~3.0 Ga Keonjhar Quartzite, Singhbhum Craton, India: An example of Phanerozoic style passive margin sedimentation from the Mesoarchean

The arenite-dominant Mesoarchean Keonjhar Quartzite (>700 m) succession unconformably overlies the granitoid-greenstone core of the Singhbhum craton in the north and west of the Keonjhar town in eastern India. The Keonjhar Quartzite is characterized by eleven lithofacies. The lower part of the succession represents proximal subaerial fan association dominated by cohesive and cohesionless mass-flow conglomerates deposited in a falling stage to lowstand systems tracts (FSST-LST). The channelized conglomerates are likely to be incised valley fills (IVF) from the FSST. The differentiated mass-flow deposits interbedded with tractional deposits such as trough cross-stratified sandstone with ubiquitous evidence of wave-reworking represent subaqueous delta front setting. The upper 3/4th of the succession is dominated by mature wavy bedded arenites and is arranged in aggradational stacking pattern representing TST to early stage of HST of a tectonically driven sea level rise cycle. The Keonjhar Quartzite is a rare example of well preserved record of stable siliclastic shelf development during the Mesoarchean time. The arenite-dominated Keonjhar Quartzite is comparable to only a few well studied examples of such Mesoarchean stable siliciclastic shelf sedimentation, for example, the arenite-dominated units of the Mozaan Group of the Pongola Supergroup and its correlatable Westrand and Centralrand Groups of the Witwatersrand Supergroup in the Kaapvaal craton. The presence of thick mature arenite and wave-tide reworking implies existence of Mesoarchean stable shelf depositional setting similar to the modern day passive continental margins at least in the Singhbhum and in the Kaapvaal cratons.

Lithofacies characteristics and depositional environment interpretations of the Middle Jurassic Fort Member rocks, Jaisalmer Formation, Western Rajasthan, India

The present study is focused on the identification of lithofacies characteristics and depositional environments for the Middle Jurassic sediments of Fort Member, Jaisalmer Formation, Western Rajasthan (NW India). Three facies associations are documented as tidally influenced fluvial, tidal/inter-tidal flat and wave-dominated shoreface facies deposits. Facies association-1 contains matrix-supported clasts, planar and flaser cross-stratified sandstone, interbedded sandstone and horizontal laminated sandstones; indicative of tidally influenced fluvial deposits. Facies association-2 characterized by herringbone cross-stratified sandstones, horizontal laminated sandstones, planar and trough cross-stratified sandstones, the alternate band of sandstone and shale, mud-crack sandstone developed on a tidal/inter tidally influence deposits. Facies association-3 consists of wave-dominated shoreface facies association consisting of wavy and lenticular beds, interference ripples, horizontal laminated sandstone, small scale planar cross-stratified sandstone, trough, and hummocky cross-stratified sandstone as well as fossiliferous limestone. Palaeoflow patterns from unipolar to bipolar indicate a change from tidal influence fluvial to marine depositional environments. The overall lateral and vertical distribution of lithofacies shows that the depositional environment was tidal-dominated fluvial channel to the marine realm.

Tropical cyclone deposits in the Pliocene Taiwan Strait: Processes, examples, and conceptual model

The characteristics of tropical cyclone (TC) deposits are highly variable, and in the shallow waters of straits their character has not been established. Multiple coarse-grained beds in lower Pliocene strata in the Western Foreland Basin of Taiwan are interpreted as TC deposits, and the sedimentology of these beds reflects evolving TC processes and storm stages. By comparing sedimentological data from these beds with previous research on TCs, and with analysis of TCs that cross Taiwan from east to west, a conceptual model is built for understanding deposition in the Taiwan Strait during tropical cyclones.Taiwan is impacted by more than three TCs per year, including more than two super-typhoons every 10 years. These storms can deliver immense volumes of rain over very short time periods and are associated with intense winds. During TCs, and in shallow water in the Taiwan Strait (10–35 m water depth), hyperpycnal discharge, strong bottom currents, and oscillatory wave motion all impact deposition. These processes are manifested in the resulting deposits, and we propose that the best sedimentological criteria for recognizing TC deposits, include: 1) a basal scour surface; 2) an absence of within-bed bioturbation; 3) trough cross-stratification that preserves multi-directional, mainly shore-parallel flow with a 180° change in flow direction; 4) preservation of oscillatory structures towards the top of the bed; and, 5) organic lamina and clasts throughout the bed. Mudstone intercalated with trough cross-stratification is also a useful indicator of TC deposition as it probably reflects wave- and current-redistribution of sediment delivered to the strait through hyperpycnal and hypopycnal discharge from rivers. Contemporaneous mud deposition within TC beds is manifested as mudstone drapes on foresets and toesets, and discontinuous mudstone beds between trough cross-stratified sandstone beds. While not all TC beds exhibit all sedimentological characteristics, we hypothesize that high-probability TC beds should show three or more of them.

Development of a streamflow-dominated alluvial-fan system in the southwestern margin of Gyeongsang Basin (Lower Cretaceous): implications for initial basin-fill history

This paper focuses on the initial basin-fill processes in an extensional nonmarine back-arc basin. Sedimentary facies analysis in the southwestern part of the Gyeongsang Basin (Jinju Subbasin), Hapcheon area reveals that the succession consists of four facies assemblages. The assemblage I mainly consists of disorganized conglomerate deposited by flash floods in a streamflow-dominated alluvial fan. The assemblage II is dominated by stratified conglomerates and massive, stratified, and cross-stratified sandstones, showing architecture of stacked channels and bars. The assemblage III largely comprises massive, stratified to laminated, and trough cross-stratified sandstones, which are organized into channel-fill and bar-accretion structures. It most likely formed in sandy sinuous rivers. The assemblage IV comprises gray mudstone interlayered with fine sandstone, representing water-logged floodplains. Conglomerate and sandstone bodies formed in the fluvial systems appear to be randomly distributed within floodplain fines, displaying a great lateral and vertical lithologic variation. This study helps understand the complexity of basin-fill history, i.e., switching of fluvial networks in time and space, in contrast to the simple notion of margin-parallel stacking of lithostratigraphic units.

Stratigraphic Controls on Petrophysical Attributes and Fluid-Flow Pathways in an Exhumed Fluvial Reservoir

Fluvial, floodplain and lake strata of the Green River Formation (Eocene) occur within an exhumed oil reservoir exposed in a quarry near Sunnyside, Utah. Strata in the quarry highwalls define three, asymmetrical, 15- to 20-m thick, base-level-rise genetic sequences arranged in a long-term base-level-rise (landward stepping) stacking pattern. Variable intensity of oil stain on rock surfaces is a qualitative measure of pore volumes, as all permeable facies are fully saturated with oil. Visual estimates of oil-stain intensity, combined with petrophysical measurements and petrographic analysis of the different facies, were used to define fluid-flow compartments and their boundaries. Strata and facies that functioned as fluid-flow conduits, retardants and barriers were mapped on photomosaics of the quarry highwall. Three separate fluid-flow compartments coincide with the three genetic sequences. Amalgamated fluvial sandstones at the base of each genetic sequence functioned as flow units of varying permeability and degree of interconnectedness. Laterally continuous floodplain and/or lacustrine mudstones, which cap each genetic sequence, entirely lack oil in matrix porosity and functioned as fluid-flow barriers and compartment boundaries. Petrophysical properties of specific sedimentary facies are sensitive to stratigraphic position at three spatial scales, even though the sedimentary facies appear nearly identical. At the long-term scale, porosity and permeability of the same facies (trough cross-stratified sandstone is the most common) in channel sandstones of the three genetic sequences decrease in stratigraphic succession. Within each genetic sequence, porosity and permeability are highest at the base and decrease quasilinearly to the top. Using oil-stain intensity as a proxy, porosity and permeability generally decrease from base to top of each scour-based channel macroform. Petrophysical variations coincide with subtle variations in grain size and trough cross-stratification set thickness within otherwise indistinguishable sedimentary facies. These results demonstrate that conventional crossplots of porosity/permeability versus sedimentary facies are unnecessarily broad and imprecise. When such petrophysical data are plotted in a stratigraphic context, porosity and permeability values have significantly reduced scatter. Porosity and permeability measurements and predictions of each sedimentary facies should be made from a stratigraphic perspective. From our observations of variations in intensity of oil stain, homogeneity of fluid flow may not be equated directly with facies homogeneity. At one extreme of an apparent continuum, fluid-flow pathways are tortuous and extremely variable within homogeneous, high permeability, amalgamated channel sandstones. Sweep efficiencies may be low in these cases. At an intermediate position in the continuum, increased diversity of sedimentary facies and stratigraphic variability usually cause sufficient stratigraphic separation of permeable and impermeable strata such that fluid-flow pathways are more confined and have a reduced tortuosity. Sweep efficiencies may be high in these cases. At the other extreme of the continuum, where diversity of sedimentary facies and stratigraphic variability is very high, stratigraphic units are discontinuous and restricted in area. In such cases, fluid-flow pathways are not laterally connected, and sweep efficiencies would be low.

Depositional environment of Himmatnagar Sandstone (Lower/Middle Cretaceous): a perspective

Himmatnagar Sandstone (lower to middle Cretaceous) is exposed in between Sabarmati River in the west to Vantada in the east around Himmatnagar Town in north Gujarat, India. The sequence is divisible in two members: The lower member is 65 m thick, mostly massive, horizontally stratified to hummocky stratified with abundant plant and trace fossils in assorted shales and sandstones. The upper member is ~ 12 m thick, cross–stratified and medium to coarse grained–gritty to cobbly in nature. Six lithofacies have been identified in the sequence, viz. 1. grey wacke (GW), 2. silty–shale (SS), 3. cross–stratified sandstone (CS), 4. horizontally stratified sandstone (HSS), and 5. planar cross–stratified sandstone (PCS) in the lower member; and 6. gritty–cobbly cross–stratified sandstone (GCCS) in the upper member.
 The lower member consists of plant fossils which are poor to moderately preserved and transported. The silty–shale lithofacies contains plant fossils (Pagiophyllum, Brachyphyllum, Gleichenia, Araucarites, circinate vernation of ferns, Williamsonia flower, twigs, petrified wood, conifer and its cone, etc.), body fossil (insect wing) and trace fossils (Skolithos, Monocraterion, Psilonichnus, Thalassinoides, Chondrites, Planolites, Palaeophycus, Calycraterion, Circulichnus, Ophiomorpha, Phoebichnus, etc.). In the cross–stratified sandstone lithofacies, body fossils (mainly fragmented bivalves, plant fossils (Weichselia reticulata, Matonidium indicum, Ptilophyllum, cycadean frond and fossil wood) and trace fossils (Monocraterion, Chondrites, Calycraterion, Thalassinoides, Psilonichnus and Skolithos) are recognized. On the other hand, in horizontally stratified sandstone lithofacies plant fossils (Sphenopteris, Pagiophyllum, Gleichenia, Elactocladus, Brachyphyllum, ferns, petrified wood, etc.) and trace fossils (Skolithos, Ophiomorpha, Psilonichnus, Monocraterion, Arenicolites, Diplocraterion, Thalassinoides, Teichichnus, Palaeophycus, Planolites, etc.) are present. While, large crustacean and vertebrate burrows, Skolithos, Thalassinoides, Ophiomorpha, etc are found in planar cross–stratified sandstone lithofacies. The trace fossils belong to Psilonichnus, Skolithos and Cruziana ichnofacies as per Seilacher (1967). The member also contains wedge shape geometry of beds similar to tidal partings as well as ridge and runnel structures, low–angle to hummocky cross–stratification, herringbone structure and parting lineation. Here, north to northeast palaeo–current direction is indicated by cross–stratification in the member. All these features lead to the depositional environment, which seems to be foreshore–tidal flat to middle shoreface for the lower member of the sequence.
 The upper member is composed of trough cross–stratified sandstones showing prominently southwest to south palaeo–current direction with angular to sub–rounded pebbles and cobbles of underlying rocks and fossil wood with lower erosional contact and channel structures at places. Based on above characteristics, depositional environment of upper member can be interpreted from estuarine to fluvial.

Virtual and classical Precipice Sandstone outcrops mapping for reservoir modelling

The Lower Jurassic Precipice Sandstone is an important hydrocarbon and water reservoir in the Surat Basin. It is the basal infill of the Surat Basin, commonly considered an intracratonic sag basin, although the triggering mechanism for subsidence remains unresolved. Its interpreted origin is a fluviatile system that formed a thick belt of sandstone that corresponds to the Mimosa Syncline structural axis. The Precipice Sandstone outcrops along the northern margin of the basin forming laterally continuous cliffs. This provides good conditions for 2D and 3D photogrammetry and classical analysis of sedimentary architectures, bedding and facies. Photogrammetry is a measurement technique that builds 3D photorealistic virtual models in which every pixel on the image corresponds to a real 3D point in georeferenced space. This was used to measure surfaces, correlate stratigraphy, and to measure bed and body geometries for export to a reservoir modelling system, providing a bridge between the subsurface drilling data and the outcrop analogue. The field survey mapped the lower Precipice, defined by the predominance of southeast-flowing planar and trough cross stratified sandstone (the braided stream facies), and upper Precipice, defined by a predominance of heterolithic, ripple and plane parallel stratification and slumps that transition upward into the Evergreen Formation mud-dominated unit. Sedimentary structures in outcrop suggest a northward flow on the eastern margin of the outcrop for the upper Precipice. The basin asymmetry, coincident with a major, meridional-trending fault system—the Goondiwindi-Moonie-Burunga system—and changes in upper and lower unit thickness suggest some syn-depositional control on the sedimentary architecture.

Sedimentology and Depositional Environment of D2 Sand in Part of Greater Ughelli Depobelt, Onshore Niger Delta, Nigeria

The sedimentology and depositional environment of D2 sand in part of the Greater Ughelli depobelt have been studied using well logs and core data. Three wells were correlated to establish the lateral continuity of the D2 sand across the field and standard gamma ray log trends was used to infer depositional environments. Cored section of well B was described to identify lithology and delineate depositional environments based on facies types and sedimentary structures. Petrophysical characteristics of the reservoir of interest was generated using Archie Equation for water saturation, Timur formula for permeability computation and porosity values was determined quantitatively from density log. The results of the analysis showed that the D2 sand cut across the field. The reservoir displayed a funnel shaped coarsening upward gamma ray motif typical of a deltaic and progradational depositional profile. Four facies associations indicating four subenvironments within the deltaic front have been identified from the cored interval (3444.5 to 3458.5 m) of the D2 reservoir. Each facies unit was identified based on lithology and sedimentary structures of the core sample, textural characteristics and gamma ray log trend. The four lithofacies associations: A, B, C and D include muddy heterolithic sandstone, trough cross stratified sandstone, sandy/silty mudstone and crossbedded sandstones. These facies correspond to Lower Shoreface, Upper Shoreface, Tidal Flat and Channel sand respectively. Petrophysical analysis revealed the trend of reservoir quality within the facies. Porosity range from 0.79 to 10.32%, whereas permeability from 0.25 to 8.8 mD. Water saturation is high (0.8) within the poor quality facies and 0.3 in the highest porosity and permeability interval. Good porosity and permeability occurred within the channel and upper shoreface facies, whereas the tidal flat and lower shoreface have poor porosity and permeability values. The Shoreface facies has the best reservoir properties (10.32% porosity and 8.8 mD permeability) due to lack of shale intercalations and good sorting resulting from the sediments being properly reworked by wave action. The Channel facies (D) deposited by high energy current also has good reservoir properties, especially towards its base. The Tidal Flat facies has the lowest reservoir quality due to high proportion of shale/Clay that creates permeability barriers and occurred between the Upper shoreface facies and Channel sand facies. Sedimentology and depositional environments of facies have significant control on the quality of sand bodies.

Various styles of deposition of Pleistocene glacigenic deposits in Essex, England

Late Ordovician siliciclastic glacial and related deposits on the North Gondwana continental shelf comprise striking incised features referred to as large-scale channel-fill structures. These are described from exposures and aerial photographs in two areas (Adrar and Hodh) in Mauritania. The channel-fill structures are up to several kilometres in length and several hundred metres wide. They are slightly sinuous narrow sandstone bodies deeply incised into the Late Ordovician glacial drift and the Cambro–Ordovician bedrock. Palaeogeographical reconstructions indicate that these structures were located in the ice-marginal zone. They are preferentially oriented parallel to palaeo-ice-flow directions. Sedimentary facies analysis reveals three vertically stacked architectural units infilling a U-shaped erosional basal unconformity. Unit 1 is thin and made up of conglomeratic sandstones of various origins comprising debris-flows, deltaic-like progradational foresets and trough cross-stratified sands. In Adrar, architectural unit 2 shows sheet-like, evenly laminated, fine- to medium-grained, well sorted sandstones characteristic of a high-energy marine environment. In contrast, in the Hodh area, unit 2 displays vertically stacked successions of coarse-grained sandstones that indicate high-discharge sediment-laden flows; these are interpreted as meltwater sediments. In both Adrar and Hodh, architectural unit 3 is made up of gravelly coarse-grained, trough cross-stratified sandstone emplaced in a braided, low sinuosity fluvial environment. The three-stage infilling history and the palaeogeographic location combined with size and shape criteria make it possible to compare the channel-fill structures with Pleistocene tunnel-valleys found in areas of low-relief in cool temperate and low Arctic zones. Such features are incised by high-pressure subglacial meltwater and later infilled by proglacial to postglacial deposits. Based on this comparison, architectural unit 1 represents subglacial or proglacial outwash, high sediment discharge in unit 2 took place in a proglacial environment in the Hodh area while an isostatically downwarped shelf was responsible for a marine incursion in the Adrar channels, and deposits in unit 3 are fluvial postglacial sediments. Identification of preserved channel-fill structures may be useful in reconstructing Late Ordovician ice-sheet dynamics.

The Campbellton Formation, New Brunswick, Canada: paleoenvironments in an important Early Devonian terrestrial locality

Strata of the Campbellton Formation, nearly 1 km-thick and known for its diverse fossil assemblage of early plants, arthropods, and fish, can be divided into six facies associations: (1) restricted lacustrine, (2) marginal lacustrine, (3) near-shore lacustrine, (4) coastal-deltaic, (5) sandy to gravelly alluvial plain, and (6) gravelly proximal alluvial environments. Lacustrine deposits with restricted circulation, due to depth or stagnation, are fine-grained with preserved organic material. The marginal lacustrine association consists of massive siltstone and very fine sandstone, interbedded with conglomerate. The latter are interpreted to have shed from older volcanic units forming the basin walls. The near-shore lacustrine association is characterized by rippled sandstone with microbialites. Alluvial strata include interbedded imbricate to nonimbricate conglomerate, trough cross-stratified sandstone, and barren to plant-bearing siltstone. Rare exposures of thickly bedded imbricate to weakly imbricated cobble–boulder conglomerate with sandy plant-bearing lenses are interpreted as products of hyperconcentrated debris flows. In the western belt, a braided-fluvial system had paleocurrents flowing WNW. Coastal-deltaic deposits west of the fluvial outcrops, containing aquatic vertebrates and invertebrates, had paleocurrents flowing ESE, suggesting a confined body of fresh or brackish water. In lower parts of the eastern belt, lacustrine facies are prevalent, representing a large open lake. Alluvial facies dominate upper parts of the formation, representing an eastward-flowing axial braided river system, with proximal alluvium shed transversely from the basin margins. Although most strata have a volcanic provenance, only one outcrop in the lacustrine beds shows evidence of active volcanism during deposition of the Campbellton Formation.

Skolithospipe rock and associated ichnofabrics from the southern Rocky Mountains, Canada: colonization trends and environmental controls in an early Cambrian sand-sheet complex

Desjardins, P.R., Mangano, M.G., Buatois, L.A. & Pratt, B.R. 2010: Skolithos pipe rock and associated ichnofabrics from the southern Rocky Mountains, Canada: colonization trends and environmental controls in an early Cambrian sand-sheet complex. Lethaia, 10.1111/j.1502-3931.2009.00214.x The Lower Cambrian Gog Group of the southern Rocky Mountains of western Canada offers an opportunity to explore animal–sediment relationships in a high-energy setting, during the early phase of Phanerozoic diversification. Its strata record a sand-sheet complex on the broad pericontinental shelf of West Laurentia. Six ichnofabrics are recognized: Skolithos IF-1 characterized by Skolithos linearis in planar cross-stratified sandstone; Skolithos IF-2 with Skolithos linearis and accessory Diplocraterion parallelum in ripple cross-laminated, and planar and trough cross-stratified sandstone; Skolithos IF-3 with S. linearis and Planolites montanus in wavy- and flaser-bedded sandstone, interfingering with bioturbated mudstone exhibiting P. montanus and Teichichnus rectus; Skolithos IF-4, characterized by abundant Skolithos linearis in planar and trough cross-stratified sandstone; Rosselia IF-1 consisting of Rosselia isp. in planar cross-stratified sandstone; and Rosselia IF-2 consisting of scattered small Rosselia isp. in hummocky, cross-stratified sandstone. The presence of contrasting ichnofabrics within a single early Cambrian sand-sheet complex illuminates how the colonization trends of suspension and detritus feeders were controlled by factors specific to the various sub-environments. □Gog Group, Ichnofabric, Lower Cambrian, Pipe Rock, Rosselia, Skolithos.

Sedimentology and ichnology of the Lower Triassic Montney Formation in the Pedigree-Ring/Border-Kahntah River area, northwestern Alberta and northeastern British Columbia

Abstract Very fine-grained sandstone beds in the lowermost Triassic (lower Induan = Griesbachian) lower Montney Formation are the primary reservoir lithology in the Pedigree, Ring-Border and Kahntah River gas fields of northwestern Alberta and northeastern British Columbia. Hydrocarbon production from Pedigree and Ring-Border fields occurs primarily from well-sorted, low-angle to hummocky and trough cross-stratified sandstone beds. Hydrocarbon production from Kahntah River field occurs primarily from highly bioturbated silty/muddy sandstone with hummocky and trough cross-stratified sandstone interbeds. The three gas fields included in this study occur within shallow marine successions in strata that partially coincide stratigraphically, but are successively younger, from east to west. Thus the cored interval in Pedigree Field overlaps with, but is overall slightly older than, the cored interval in the Ring/Border Field, which overlaps with, but is overall slightly older than, the cored interval in the Kahntah River Field. Six lithofacies have been identified in the study interval: L-I (dark gray laminated shale); L-II (interbedded very fine-grained sandstone, siltstone and silty shale); L-III (low-angle to hummocky cross-stratified, very fine-grained sandstone); L-IV (convolute-bedded shale, siltstone and sandstone); L-V (bioturbated muddy/silty sandstone); and L-VI (trough cross-stratified, very fine-grained to fine-grained sandstone). These lithofacies comprise an overall shallowing upwards proximal offshore through shoreface succession. Strata interpreted as proximal offshore contain few trace fossils, primarily isolated Planolites. Trace fossils in strata interpreted as offshore transition include Aulichnites, Conichnus, Cruziana, Diplichnites, Fustiglyphus, Halopoa, Helminthopis, Lockeia, Monomorphichnus, Palaeophycus, Planolites, Protovirgularia, Rhizocorallium, Rusophycus, Spongeliomorpha, Thalassinoides and Treptichnus. Although trace fossil diversity is very high, abundances are low and few of these taxa represent indigenous infauna. Most forms represent the dwelling and feeding traces of storm-transported, opportunistic colonizers. Strata deposited in the lower shoreface contain a diverse trace fossil assemblage consisting of Asterosoma, Cylindrichnus, Cruziana, Diplocraterion, Palaeophycus, Phycosiphon, Phycodes, Planolites, Rhizocorallium, Rosselia, Skolithos, Spongeliomorpha, Teichichnus and Thalassinoides. Upper shoreface sandstone beds are apparently barren. Several lines of evidence, including low diversity of indigenous infauna, support the hypothesis of dysoxic depositional conditions in the lower Montney offshore transition during the Griesbachian. Decreased thickness and preservation of storm beds suggest that the intensity and frequency of storms in the study area appear to have decreased through the Griesbachian, resulting in longer interstorm intervals and deeper, more pervasive bioturbation. Periodic influx of turbid fresh/brackish water by local ephemeral deltas may have also had a dampening effect on local biodiversity.

Stratigraphic framework and estuarine depositional environments of the Miocene Bear Lake Formation, Bristol Bay Basin, Alaska: Onshore equivalents to potential reservoir strata in a frontier gas-rich basin

The Miocene Bear Lake Formation is exposed along the coast and mountains of the central Alaska Peninsula and extends offshore as part of the Bristol Bay Basin. The Bear Lake Formation is up to 2360 m (7743 ft) thick in an offshore well and is considered to have the highest reservoir potential in this gas-rich frontier basin. Our new macrofossil and palynological data, collected in the context of measured stratigraphic sections, allow us to construct the first chronostratigraphic framework for this formation. Biostratigraphic age assignments for the numerous, commonly isolated, onshore exposures of the Bear Lake Formation show that deposition initiated sometime before the middle Miocene (15 Ma) and extended to possibly the earliest Pliocene. The bulk of the Bear Lake Formation, however, was deposited during the middle and late Miocene based on our new findings. We interpret the Bear Lake Formation as the product of a regional transgressive estuarine depositional system based on lithofacies analysis. The lower part of the formation is characterized by trough cross-stratified sandstone interbedded with coal and pedogenic mudstone deposited in fluvial and swamp environments of the uppermost parts of the estuarine system. The lower-middle part of the formation is dominated by nonbioturbated, wavy- and flaser-bedded sandstone and siltstone that were deposited in supratidal flat environments. The upper-middle part of the Bear Lake Formation is characterized by inclined heterolithic strata and coquinoid mussel beds that represent tidal channel environments in the middle and lower tracts of the estuarine system. The uppermost part of the formation consists of tabular, bioturbated sandstone with diverse marine invertebrate macrofossil faunas. We interpret this part of the section as representing the subtidal tract of the lower estuarine system and possibly the adjacent shallow inner shelf. A comparison of our depositional framework for the Bear Lake Formation with core and well-log data from onshore and offshore wells indicates that similar Miocene depositional systems existed throughout much of the Bristol Bay Basin. The documented changes in depositional environments within the Bear Lake Formation are also important for understanding upsection changes in the geometries of potential reservoirs.

Lithofaciology and Palaeoenvironmental Analysis of Holocene Beachrocks in Northeastern Brazil

Abstract Five lithofacies were identified in beachrock ledges distributed along the eastern and northern coasts of the state of Rio Grande do Norte, northeastern Brazil: (1) massive to weakly stratified conglomerate, (2) low-angle cross-stratified conglomerate and sandstone, (3) medium-scale tabular-planar and trough cross-stratified sandstone, (4) Skolithos-bioturbated conglomeratic sandstone, and (5) massive sandstones. Most of the described beachrocks (53% relative to total thickness) were deposited on the upper shoreface zone, represented by lithofacies 3 and 4. Lithofacies 2, which was deposited on the foreshore zone, represents 31% of the sections described. The remaining 16% are attributed to collapse of overlying material as a result of sea cliff undercutting (massive conglomerate, lithofacies 1), transport as a traction carpet deposit (weakly stratified conglomerate, lithofacies 1), and a high degree of alteration (lithofacies 5). The distribution of 14C-dated beachrock samples in a sea level env...

Alluvial and lacustrine facies in an extensional basin: The Miocene of Malatya basin, eastern Turkey

During the Neogene, both strike-slip and extensional regimes coexisted, giving rise to a number of fault-bounded basins in eastern Turkey. One of these basins, the Malatya basin, contains a thick sequence (up to 2000 m) of alluvial and lacustrine deposits. This paper focuses on the Miocene alluvial and lacustrine deposits in the northern part of the basin. Alluvial and lacustrine deposits in the study area, having a thickness over 1200 m, are divided into three units on the basis of their vertical sequential changes and lithologic characteristics. These are the lower alluvial unit, lacustrine unit, and upper alluvial unit. The lower alluvial unit is composed of stratified conglomerate (Gm), planar cross-stratified conglomerate (Gp), trough cross-stratified sandstone (St), planar cross-stratified sandstone (Sp), horizontally stratified sandstone (Sh), ripple cross-laminated sandstone (Sr), red mudstone (Fm), carbonaceous claystone (Fl1), and rarely limestone (Pf). Facies Gm, Gp, and Sh characterize alluvial fan to sheetflood deposits. Facies St, Sp, Sh, and Sr are interpreted as braided stream to sheetflood deposits. Facies Fm, Fl1 and siltstone are interpreted as deposits in an alluvial plain system with an ephemeral setting. Carbonaceous claystone (Fl1) and limestone (Pf) are interpreted as a shallow lake, situated as a distal environment in alluvial plain. The middle unit, interpreted as channel/crevasse-splay, swamp and lacustrine deposits, is composed of interbedded sandstone–siltstone–grey claystone (SF), low-angle cross-stratified sandstone (Sl), planar cross-stratified sandstone (Sp), limestone (Pf), carbonaceous claystone (Fl1), marl (Fl2), and coal (C). The upper alluvial unit is composed of Gm, Gp, St, Sp, Sh, Sr, and Fm characterizing a low sinuosity river. Most of the alluvial input into the basin was from the north, northeast, and east. Soft-sediment deformation structures (slump folds) which indicate seismically active areas were formed within the lacustrine facies. The sediment supply, lake level fluctuation, and facies architecture were mainly controlled by synsedimentary tectonism.

Late Ordovician (Hirnantian) depositional pattern and sea-level change in shallow marine to shoreface cycles in central Sweden

The Upper Ordovician Kyrkås Quartzite Formation at the Nifsåsen Quarry (Jämtland, Sweden) exhibits c. 90 m of siliciclastic sedimentary rocks deposited on a shallow shelf at the craton-attached part of the Caledonian foreland basin. Five lithologies are distinguished, including claystone, mudstone, siltstone, subarkose and sublitharenite. Based on these five lithologies, sedimentary structures and biota, three marine facies associations are defined: the Mudstone association (FA1) deposited close to storm wave base, the Sandstone/mudstone association (FA2) formed between storm and fair-weather wave bases, and the Sandstone association (FA3) accumulated above fair-weather wave base. The facies associations are arranged in two sequences, c. 50 and 40 m thick, separated by a transgressive surface, indicating repeated shoreline progradation. Both sequences commence with marine heterolithic shales and siltstones, with upwardly increasing frequency of tempestites. Continued shoaling is indicated by a dominance of hummocky and trough (locally tabular) cross-stratified sandstone beds in the upper part of each sequence. Sand beds are increasingly amalgamated up-sequence, reflecting progressively diminishing accommodation space. The depositional style and sedimentary structures indicate that the study area was storm-dominated with an abundant supply of siliciclastic material. Biostratigraphic data tie the depositional changes to the globally recognized Late Ordovician (Hirnantian) glacial interval. These data suggest that the first sequence was formed during the initial phase of regression in the earliest Hirnantian. The lowermost part of the overlying sequence contains elements of a typical Hirnantia fauna followed by beds yielding Normalograptus persculptus, suggesting a second regressive cycle in the Jämtland basin during the early N. persculptus Biozone.