Planar Cross-stratified Sandstone Research Articles

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.

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.

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.

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.

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.

Analysis of sedimentary facies and depositional environments of the Permian Gondwana sequence in borehole GDH-45, Khalaspir Basin, Bangladesh

Lithofacies analysis of the Permian Gondwana sequence in borehole GDH-45 of the Khalaspir Basin was performed with a view to deduce the nature of depositional environments. On the basis of dominant lithofacies association, the sequence is divided into six lithostratigraphic units (units A to F). Five lithofacies (conglomerate, sandstone, siltstone, mudstone/shale and coal) are identified within these units. Several sub-lithofacies, such as massive, crudely stratified, cross-stratified, ripple and parallel laminated sandstones are also identified within these lithofacies. The sequence forms a fining-upward trend with a rare coarsening-upward unit. The generalised Gondwana sequence is characterised mainly by channel lags, pebbly massive to crudely cross-stratified sandstone, trough and planar cross-stratified sandstone, ripple laminated sandstone/siltstone, massive to parallel laminated siltstone, mudstone/shale and coal in ascending order. The facies associations represent several repeated fining-upward units and cycles, indicating various sub-environments (channel, floodplain, flood basin/backswamp) in fluvial regime. The conglomerates might have been deposited as debris flow or channel lag deposits. The sandstones were deposited mainly as multistoried channel and lateral bars in moderately braided and sinuous streams. The siltstone and mudstone lithofacies indicate bar top, natural levee or floodplain to flood basin environments. The coal lithofacies suggests deposition in low-lying, short to long persistent, moderately to well drained and sparse to densely vegetated backswamps in fluvial channel-flood-plain complex. The overall succession of the Gondwana borehole sediments suggests that the depositional basin became, with time, gentler in slope gradient, resulting in a more sinuous stream setting.

Lacustrine fan-deltaic sedimentation in a Proterozoic rift basin: the Sopa-Brumadinho Tectonosequence, southeastern Brazil

The Sopa-Brumadinho Tectonosequence (100 to 200 m thick) is the fourth unit in ascending order of the up-to-2000 m thick Mesoproterozoic Espinhaço Supergroup, representing the depositional products of the third and main phase of the rift stage of the Espinhaço basin. The Sopa-Brumadinho Tectonosequence in the study area consists of diamond-bearing lacustrine fan-delta deposits, bounded by prominent angular and erosional unconformities. Detailed lithofacies studies were carried out in the central part of the southern Serra do Espinhatço, southeastern Brazil, to characterize the paleogeography and basin-fill architecture of the Sopa-Brumadinho Tectonosequence. Thirteen lithofacies are present: massive clast-supported conglomerates (lithofacies UGCS), inversely graded clastsupported conglomerates (lithofacies IGCS), normally graded clast-supported conglomerates (lithofacies NGCS), massive to crudely stratified clast-supported conglomerates (lithofacies CSCS), sandy matrix-supported conglomerates (lithofacies S-MS), muddy matrix-supported conglomerates (lithofacies M-MS), talus-breccia clast-supported conglomerates (lithofacies TB-CS), massive sandstones (lithofacies Sm), horizontal stratified sandstones (lithofacies Sh), trough cross-stratified sandstones (lithofacies St), planar cross-stratified sandstones (lithofacies Sp), graded-stratified sandstones (lithofacies Sg), and pelites (lithofacies P). They represent the products of several types of subaerial and subaqueous sediment gravity flows (ranging from turbidity currents to cohesive debris flows) and ephemeral braided streams. The deposits are usually arranged in coarsening- to fining-upward tectono-depositional intervals. Each tectono-depositional interval represents a progradational lobe, which starts at the base with lacustrine pelites and evolves with time from distal (subaqueous) to proximal (subaerial) characteristics. The fining-upward part represents the abandonment phase of the depositional lobes. The origin, characteristics, distribution and spatial arrangement of the various lithofacies, the interbedding of gravityflow and stream-flow deposits, the predominance of debris-flow conglomerates over aqueous current-formed ones, the semi-radial, fan-like dispersion pattern of the paleocurrents, and the recognition of associated subaerial and subaqueous depositional settings are all indicative of a lacustrine alluvial-fan/fan-delta environment, or a fan-delta system. An interplay between several intrabasinal and extrabasinal controls probably determined the fan evolution. The sedimentation of the lacustrine fan-delta systems of the Sopa-Brumadinho Tectonosequence probably took place in a N-S elongated continental rift basin, compartmentalized by N-S trending normal faults and W-E trending transfer faults, under a tropical semi-arid climate. The basin-fill architecture was probably determined by episodic subsidence, clockwise block tilting and asymmetric-graben evolution, as well as by inherited characteristics of the source area. Additionally, a new descriptive scheme for classification of conglomerate lithofacies is here proposed.

Braidplain sedimentation in a Proterozoic rift basin: the São João da Chapada Formation, southeastern Brazil

The São João da Chapada Formation (100–200 m thick) is the basal unit of the Middle Proterozoic Espinhaço Supergroup. This up to 3000 m thick and slightly metamorphosed sedimentary succession is exposed in the central part of the southern Serra do Espinhaço, southeastern Brazil. Detailed lithofacies studies were carried out to characterize the sedimentation and paleogeography of the São João da Chapada Formation. Six lithofacies are present: horizontal to low-angle cross-stratified sandstones (lithofacies Sh/Sl), trough cross-stratified sandstones (lithofacies St), planar cross-stratified sandstones (lithofacies Sp), ripple cross-laminated sandstones (lithofacies Sr), massive to horizontally stratified conglomerates (lithofacies Gm), and fine-grained rocks (lithofacies F). These are usually arranged in fining- and thinning-upward successions, characterizing waning-flood cycles, which are always bounded by erosional surfaces. No complete cycles have been found, however, and the cycles are normally composed only of lithofacies Sh/Sl and St. The other lithofacies are rare. The observed sedimentary structures and their arrangement, the geometry of the bodies (mainly sheet-like to slightly lenticular), the low-relief morphology of the erosion surfaces bounding the waning-flood cycles, the coarse and poorly sorted texture of the sediments, and the measured paleocurrent pattern (unidirectional with little dispersion throughout the study area) point to deposition on a wide, ephemeral braidplain, characterized throughout by relative uniformity of facies and paleocurrent directions. Water depths on this subsiding braidplain were maintained by high-discharge streams debouching onto it from the upland catchment areas. As these rivers spread onto the depositional plain, the changes in gradient and substratum transformed them into channels of remarkable width and very large width/ depth ratios. These rivers ran in braided tracts, and floods were temporarily confined within banks in these tracts. Channel-switching events associated with tilting and subsidence of the basin floor resulted in the accumulation of extensive sandstone sheets, interfingering laterally with one another and succeeding one another vertically, which built up a braidplain succession extending up to 35 km transverse to the paleoslope. The São João da Chapada braidplain flanked the west margin of a NS elongated rift basin.

Coalescence megafan: multistorey sandstone complex of the late-orogenic (Mio-Pliocene) sub-Himalayan belt, Dehra Dun, India

In the Himalayan foreland basin, a prominent multistorey sandstone complex, in the Middle Siwalik Subgroup, is exposed along the southern flank of the Dehra Dun valley in the Mohand anticline. Extending laterally for ∼ 40 km, the Middle Siwalik Subgroup consists of sandstone-mudstone couplets (300–450 m thick) in the lower part and the multistorey sandstone complex (900–1200 m thick) in the upper which forms a gross coarsening upward sequence. The major lithofacies are trough cross-stratified sandstone, planar cross-stratified sandstone, ripple drift laminated sandstone, horizontal laminated sandstone and massive sandstone with subordinate amounts (<20%) of mudstone and conglomerate. The multistorey sandstone complex is punctuated by several erosional surfaces over which abundant mud clasts are present. The trough cross-stratification exhibits a dominantly southward palaeoflow with much variation in the southeast and southwest quadrants. However, at outcrop level the palaeoflow data reveal internal consistency. This spatial and temporal variability in palaeoflow direction indicates the existence of two major drainage systems of coalescing alluvial megafan type. The estimated palaeochannel sinuosity is of the order of 1.04 to 1.23 with a return period of 4 to 8 × 103 years.Palaeoflow variability, facies distribution, frequent avulsion and the coarsening upward sequence collectively suggest the progradation of coalescing alluvial fans towards the south and southwest. This was principally controlled by thrust movements and differential basin subsidence at the margin of the Himalayan orogen.

Longshore-Drift Dispersed, Storm-Generated Cross-Stratified Sandstone from Some Cretaceous Shallow Marine Strata, Rocky Mountain Region: ABSTRACT

Most Cretaceous shallow marine strata of the Rocky Mountain region are characterized by asymmetrical upward-coarsening and upward-thickening sequences. The strata typically contain similar lithofacies (i.e., normally graded planar parallel laminated claystone, siltstone, and sandstone; hummocky cross-stratified sandstone; symmetrical and asymmetrical ripple cross-lamination; and trough and planar tublar cross-stratified sandstone) and display an upward increase in the thickness and frequency of sharp-based sandstone beds that grade into amalgamated cross-stratified sandstone. Most workers agree that sharp-based sandstone beds and hummocky cross-stratified sandstone are storm generated. However, the origin of trough and planar tabular cross-stratified sandstone is controversial. Most workers interpret these sedimentary structures as deposited from either storm-generated traction currents or combined flow currents on the shelf, tide-generated traction currents, or tidally dispersed, storm-generated suspension clouds. Detailed analysis of three-dimensional outcrops has revealed several significant features of these sedimentary structures that indicate they may have been deposited by longshore drift dispersed, storm-generated suspension clouds. Sets of trough and planar cross-stratified sandstone form medium-scale discontinuous, irregularly shaped sand bodies, bound by erosional surfaces and composed of unidirectional dip-oriented cross strata. Individual cross stratum commonly have a sigmoidal shape, are bound by either reactivation surfaces or mudstone drapes, and contain normally graded concordant laminae.more » Top-set laminae, are truncated by the upper set boundary, whereas bottom-set laminae, become asymptotic to the lower set boundary and commonly are reworked and overlain by wave generated, ripple cross-lamination or mudstone drapes.« less

Braidplain Evolution in the Pennsylvanian South Bar Formation, Sydney Basin, Nova Scotia, Canada

ABSTRACT The South Bar Formation is a 1 km-thick alluvial succession in the Pennsylvanian Morien Group of the Sydney Basin, Nova Scotia that fines upward from conglomerate to pebbly sandstone to sandstone facies assemblages. The conglomerate assemblage comprises mainly massive to horizontally stratified conglomerate and planar cross-stratified sandstone, attributed to deposition in proximal braided rivers confined between bedrock ridges. The sandstone assemblage shows uniformity of facies and paleocurrents across the basin, and is interpreted as a distal braidplain deposit; the pebbly sandstone assemblage is transitional between the other two. Both sandstone assemblages are dominated by trough cross-stratified sandstone formed by dune migration in channel tracts about 4 m deep. This succession differs markedly from Platte River and Icelandic sandy braidplain deposits, which are mainly planar cross-stratified sands formed by broad, shallow flow. The South Bar sandstones are more like South Saskatchewan River deposits, but differ in 1) greater abundance of trough sets and 2) rarity of planar sets and other components of shallow water channel fill. These features are ascribed to 1) inheritance of relatively deep channels from confined upstream reaches, 2) maintenance of deep flow in active tracts by local confinement between temporary banks, and 3) rapid avulsive abandonment of active tracts, which inhibited development of channel-fill sequences. Evidence for local temporary confinement is preserved as in situ and slumped channel-margin mudrock units. Lateral and vertical transitions from the upper South Bar Formation to the mud-rich Waddens Cove and Sydney Mines Formations are ascribed to changes in channel pattern from braided to meandering. The former is attributed to decreased slope, the latter to eustatic rise in sea level.

Sedimentology of the lower huronian supergroup (early proterozoic), Elliot lake area, Ontario, Canada

The Early Proterozoic Huronian Supergroup is a slightly metamorphosed sedimentary succession locally > 10 km thick, exposed to the north of Lake Huron, Ontario, Canada. The lower part of the supergroup, comprising the Matinenda, McKim, Ramsay Lake and Pecors formations, was studied in the Quirke Syncline area, Elliot Lake, and regions immediately to the west and south. Thirteen lithofacies are present: trough and planar cross-stratified sandstone, quartz pebble conglomerate, clay-rich sandstone, lenticular-bedded sandstone-siltstone-mudstone, laminated sandstone-siltstone-mudstone, massive sandstone- massive pebbly sandstone, massive diamictite, stratified matrix-supported diamictite, stratified clast-supported diamictite, graded diamictite, and massive siltstone. The lithofacies may be grouped into associations which correspond approximately to the mapped formations. The crossbedded sandstones and conglomerates comprise a fluvial association (Matinenda Formation), which was formed mainly in shallow braided channels. Clay-rich sandstone and the lenticular and laminated lithofacies, constituting portions of the McKim and Pecors Formations, were deposited on the strand and offshore areas of braid deltas, interfingering laterally and vertically with the Matinenda fluvial deposits. Four glacigenic subassociations are present in the Ramsay Lake Formation. An ice-proximal subassociation (I) consists of massive pebbly sandstone and diamictite which progressively truncates the Matinenda and McKim Formations to the north. The massive sandstone may be an ice-override deposit formed by erosion and reworking of the underlying Matinenda Formation, or a sub-ice shelf rain-out accumulation of locally eroded glacial debris. Subassociations II and III consist mainly of subaqueous sediment gravity flow and ice rain-out deposits. Subassociation IV is interpreted as an ice-proximal fluvial outwash deposit containing resedimented blocks of moraine material. Stratigraphic and paleocurrent data indicate a south-dipping paleoslope that tilted southeast during the deposition of the fluvial and deltaic units. A trough of relatively deep water extended NW-SE across the region, transverse to the E-W divergent continental margin hinge line that is assumed to have existed to the south of the project area. Analogies are suggested to Mesozoic-Recent basins on the Atlantic margin of Canada, where transverse paleogeographic trends reflect either landward extension of active oceanic fracture zones, or reactivation of earlier structural lineaments.

Walls Formation, Western Shetland: distal alluvial plain deposits within a tectonically active Devonian basin

Synopsis The Walls Formation of western Shetland is a severely deformed, structurally isolated sequence of sandstones and siltstones of probable middle Devonian age. The sequence is probably extremely thick, and can be subdivided into a number of distinct sedimentary lithofacies. These include: trough and planar cross-stratified sandstones; very low angle to horizontally stratified sandstones; small scale trough cross-laminated sandstones; ripple cross-laminated silty sandstones; alternating siltstones and sandstones; and siltstones. The lithofacies occur in three distinct lithofacies associations, which represent deposition within high sinuosity (meandering) streams, sheltered floodplains and low sinuosity (braided) streams respectively. The braided stream sequences are the most common, and include sheet flood deposits which may indicate that the streams were ephemeral in nature. The sandstones are generally finegrained, and arkosic to subarkosic in composition; in places they contain high proportions of a varied suite of heavy minerals. Syndepositional seismicity is reflected by overturned and convolute lamination which is abundant throughout the succession. The Walls Formation represents first-cycle erosion of the emerging Caledonian mountains, and deposition upon a distal alluvial braidplain within a rapidly subsiding basin. Basin development may have been controlled by strike-slip tectonics, related to movement on the transcurrent Melby and Walls Boundary Faults.

Development of a braided-fluvial facies model for the Devonian Battery Point Sandstone, Québec

Eight distinct facies have been defined in a 110 m thick section of the Lower Devonian Battery Point Sandstone near Gaspé, Québec. The first is a scoured surface overlain by massive sandstone with mudstone intraclasts. Facies A and B are trough cross-bedded sandstones, with poorly- and well-defined stratification, respectively. Facies C and D consist of large isolated, and smaller multiple, sets of planar cross-stratified sandstones, respectively. Facies E comprises large sandstone-filled scours, facies F comprises ripple cross stratified fine sandstones with interbedded mudstones, and facies G comprises sets of very low angle cross-stratified sandstones.The overall context of the Battery Point Sandstone, the presence of rootlets, and the abundance of trough and planar-tabular cross bedding, all suggest a generally fluvial environment of deposition. Analysis of the facies sequence and interpretation of the primary sedimentary structures suggest that channel development began by scouring, and deposition of an intraclast lag. Above this, the two trough cross bedded facies indicate unidirectional dune migration downchannel (vector mean direction 291°). The large planar tabular sets are associated with the trough cross bedded facies, but always show a large (almost 90°) paleoflow divergence, suggesting lateral movement of in-channel transverse bars. The smaller planar tabular sets occur higher topographically in the fluvial system, and the rippled silts and muds indicate vertical accretion. Because of the very high ratio of in-channel sandy facies to fine-grained vertical accretion facies, and because of the evidence of lateral migration of large in-channel bars, the Battery Point River appears to resemble modern braided systems more than meandering ones.