Ripple Cross-laminated Sandstones Research Articles

Sedimentological and Geochemical Characterization of the Early Cambrian Eastern Yunnan, Southwestern China

The accurate reconstruction of the early Cambrian paleoclimate and paleoceanographic conditions on the Yangtze Plate is crucial for understanding the ancient environment during the Cambrian Explosion. It is also a key factor in understanding the ecological habits of organisms during the Cambrian Explosion. The study utilized field outcrops, thin section analysis, and major and trace elements to investigate the sedimentary environment, provenance, paleoweathering, and paleoclimate of the Lower Cambrian Hongjingshao (HJS) Formation (Cambrian Stage 3, ~515 Ma) in the Yangtze Basin, eastern Yunnan, SW China. The HJS sandstones are composed of 10 lithofacies, including massive and weakly bedded gravel supported by coarse sandstone (Gm), trough cross-bedded sandstone (St), planar cross-bedded sandstone (Sp), ripple cross-laminated sandstone (Sr), horizontal bedded sandstone (Sh), scour-fill sandstone (Se), massive sandstone (Sm), fine to medium sandstone with thin bed muddy siltstone (Fl), muddy siltstone (Fsc), and mudstone (Fm). On the basis of these lithofacies, channel fill and over-bank deposits in delta and shallow shelf depositional environments are suggested for HJS Formation. The major elements-based provenance discriminant function and mineral composition indicate that felsic rocks from the recycled orogen and continental block are the main sediment source terrane for the HJS sandstones of the study area. CIA, PIA, and CIW values range from 71.29 to 93.72, indicating an intermediate to intense chemical weathering and semiarid to humid climate conditions in Cambrian Stage 3. The research findings have clarified the paleoclimate and paleoceanographic environment of the Early Cambrian in the Yangtze region, which is of significant importance for understanding the early biological and ecological marine environment in the study area.

Sedimentological study of Bima sandstone in Dumne and Environs, Yola Sub-basin, Upper Benue Trough, Northeastern Nigeria.

Field mapping was carried out on Bima sandstone from Dumne and its environs with the aim to understand the geology and determine the paleoenvironment of deposition. The integrated (outcrop study with granulometric and pebble morphometric) studies were used to achieve this aim. Based on outcrop studies, nine (9) lithofacies were identified from the proximal alluvial fan facies to the mid-fan braided river: massive, matrix-supported, gravel-dominated facies (Gmm); matrix-supported gravel (Gmg); clast-supported gravel stratified (Gh); gravel stratified trough cross-bedded sandstone (Gt); trough cross-bedded sandstone (St); planar cross-bedded sandstone (Sp); ripple cross-laminated sandstone (Sr); massive sandstone (Sm) and silt and mud (Fsm). The outcrop sampled materials were subjected to granulometric and morphometric analyses. The result of the granulometric shows that the Bima sandstones of the study area have an average graphic mean (Mz) of 0.82 (coarse grains), an inclusive graphic standard deviation (δi) of 1.33 (poorly sorted), a graphic kurtosis (K) of 1.11 (leptokurtic), and an inclusive graphic skewness (Ski) of 0.17 (positive skewed). The result of the pebble morphometrics shows that the grains have an average elongation ratio (ER) of 0.80, coefficient of flatness (FI) of 60.6%, oblate–prolate index (OPI) of 0.35, maximum projection sphericity index (MPSI) of 0.77 and roundness (ρ) of 0.43. The roundness versus elongation ratio plot indicates a fluvial to transitional environment. Several bivariate and ternary plots that include the granulometric and pebble morphometric data suggest a river environment for the Bima sandstone in the studied area.

The primary depositional structures of the Upper Bima Member from Fufore and environs, Yola Arm of the Upper Benue Trough, Northeastern Nigeria: implications for paleoenvironment

Primary depositional structures are important parameters that can provide the means of unraveling the processes and process responses in sedimentary depositional settings. This paper is aimed at determining the paleoenvironment of the Upper Bima Member in Fufore and environs using the primary depositional structures. This research was carried out in two phases which involved fieldwork that dealt with logging the outcrops sections and desk work whereby the logged sections and other field data are digitized, remodeled and analyzed using relevant computer programs. The result of the study shows that the Upper Bima Member is composed of five lithofacies succession; trough cross-bedded sandstone facies (St), planar cross-bedded sandstone facies (Sp), horizontal bedded sandstone facies (Sh), ripple cross-laminated sandstone facies (Sr), and mudstone facies (Fm). These lithofacies gave rise to two facies associations; sand dominated and fine-grained facies dominated facies association. The cross-stratified facies show an overall paleocurrent trend in the NW direction. These facies succession represent both shallow perennial and ephemeral sand-bed in a braided river depositional environment.

Sedimentary and diagenetic characteristics of the Z21 field in the Huizhou depression, Pearl River Mouth basin, South China Sea

The Huizhou Depression in the Pearl River Mouth basin has prospective hydrocarbon potential, with Miocene sandstones as its main oil and gas-bearing reservoir. The sandstones in Miocene formation of the Z21 offshore oil–gas field composed of medium-grained, moderately sorted subarkose and lithic arkose. In this study, a total of six depositional lithofacies, namely Massive fine- to medium-grained sandstone (Sm), ripple cross-laminated fine-grained sandstone (Sr), parallel-laminated siltstone and claystone (Fl), lenticular siltstone (Sl), parallel-bedded fine-grained sandstone (Sp), wavy laminated siltstone (Sw), and two depositional systems, namely nearshore sand bar (SB) and sand sheet (SS) were identified based on core observations and seismic study. Distributions of the porosity (13.9%) and permeability (35.8 mD) reveal that the Miocene sandstones have characteristics of low porosity and low permeability, with high heterogeneity. The sedimentary system, primary texture and diagenesis jointly control the reservoir quality. Sandstones with sand bars as well coarse-grained tend to exhibit a higher quality. Mechanical compaction and calcite (average 6.81%) cementation are the major determinants to reductions in porosity and permeability. The total clay minerals (average 5.27%) generally lead to reduction of porosity, whereas chlorite coatings and illite within a certain content range may enhance the preservation of porosity in eodiagenesis. Dissolution of feldspar and debris contribute significantly to improving the reservoir quality.

Sedimentology, petrography, and deposition of the Upper Cretaceous Codell Sandstone in the Denver Basin

We integrate new and previous stratigraphic and petrographic data for the mid-Turonian Codell Sandstone to interpret its provenance, depositional characteristics, and environments. Our focus is on sedimentologic, X-ray diffraction, and X-ray fluorescence analyses of cores and thin sections spread throughout the Denver Basin, augmented by interpretation and correlation of well logs, isopach maps, outcrops, and provenance data. Although we treat the Codell as a single mappable unit, it actually consists of two geographically disjunct sandstone packages separated by a southwest-northeast-trending gap, the NoCoZo, short for No Codell Zone. The Codell is everywhere capped by a significant unconformity and across much of the northern Denver Basin rests unconformably on the underlying shales of the Carlile Shale. In the southern Denver Basin, the Codell commonly contains two parasequences, each of which becomes less muddy upward. Biostratigraphic and geochonologic data suggest that the unit represents deposition over a relatively brief time, spanning ~0.4 Ma from ~91.7 to ~91.3 Ma. The Codell is predominantly a thin (<50 ft) sheet-like package of pervasively bioturbated coarse siltstone and very fine-grained sandstone dominated by quartz and chert grains 50 to 100 μm in diameter. The unit is more phosphatic than the underlying members of the Carlile Shale, and its grain size coarsens to medium-grained in the northern part of the basin. An unusual aspect of the Codell across our study area is the generally excellent grain sorting despite the presence of an intermixed clay matrix. This duality of well sorted grains in a detrital clay matrix is due to the bioturbation that dominates the unit. Such burrowing created a textural inversion that obscures most of the unit’s primary sedimentary structures, except for thin intervals dominated by interlaminated silty shale and very fine sandstone. A relatively widespread and unburrowed example of this bedded facies is preserved in a thin (<10 ft) interval that extends across most of the northern Denver Basin where it is informally called the middle Codell bedded to laminated lithofacies. Sparse beds with hummocky or swaley cross-stratified and ripple cross-laminated fine-grained sandstone are present locally in this bedded facies. We hypothesize that Codell sediments were derived from a major deltaic source extending into the Western Interior Seaway from northwestern Wyoming, and that the Codell was deposited and reworked southward on the relatively flat floor of the Seaway by waxing and waning shelf currents as well as storms and waves. Codell sediments were spread across an area of more than 100,000 mi2 in this epeiric shelf system that spans eastern Colorado, southeastern Wyoming, western Kansas, parts of Nebraska and beyond.

Bioturbation, heavy mineral concentration, and high gamma-ray activity in the Lower Cretaceous McMurray Formation, Canada

In the Lower Cretaceous McMurray Formation (Alberta, Canada), many intervals of intensely bioturbated (Bioturbation Index = 5–6) fine-grained sediments are characterized by high gamma-ray (GR) readings. Several methods, including sedimentary facies analysis, thin-section petrography, handheld spectral gamma-ray, portable X-ray fluorescence, X-ray diffraction, inductively coupled plasma-mass spectrometry, microprobe of K-feldspar, energy dispersive spectroscopy, and detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry, were used to investigate the interval of interest in core samples. The mineralogical analysis shows that these intervals are enriched in heavy mineral grains, and particularly in zircons. The content of radioactive elements is variable. Thorium is commonly elevated up to three times, uranium nil to two times, and potassium content usually remains normal. The studied intervals consist of interbedded, bitumen-saturated cross-bedded and/or ripple cross-laminated sandstone (high-energy deposits) and light-gray bioturbated mudstone (low-energy deposits), commonly addressed as inclined heterolithic strata (IHS). IHS represent tidally influenced, brackish-water, upper point-bar deposits. The zircon grains become concentrated while hydraulic processes interact with bioturbation: the burrowing animals cause significant sediment mixing that allows the lightest sediment particles to go back into the suspension. Additionally, bioturbation increases the surface roughness along the sediment-water interface and, causes more turbulent flow, allowing for quartz and other light grains to be removed by traction and/or saltation, while dispersed heavier zircon grains become trapped and concentrated in open burrows.So far, this study is the first to demonstrate the importance of bioturbation in the enrichment of zircon grains in IHS. The interaction of bioturbation and hydraulic processes explains the apparently counter-intuitive enrichment of heavy minerals in a low-energy depositional setting. This scenario likely applies to numerous intervals characterized by similar GR and/or zirconium spikes across the McMurray Formation. Furthermore, it can be expected that in other sedimentary basins and stratigraphic units, similar studies will demonstrate that the proposed mechanism is universal.

Architectural analysis of an Early Cambrian braided-river system on the north Gondwana margin: The lower sandstone of the Lalun Formation in the Shirgesht area, central Iran

The Lalun Formation (Early Cimmerian), up to 867 m thick, was studied in the Shirgesht area, Central Iran. It was laid down across the northern edge of Gondwana, where fluvial sandstones were covered by marine shales, carbonates and sandstones. The Lower Sandstone unit of the formation is dominated by sand-rich braided river deposits (97%) with a general scarcity of fine-grained deposits (3%), and the unit consists nine lithofacies (Se, Spb, Sm, Sp, St, Sh, Sr, Fl and Fm). Common sedimentary structures in the succession include planar and trough cross-bedded, plane-laminated and ripple cross-laminated sandstone, and mudcracks in laminated and structureless mudstone. Architectural elements are defined as channel forms and fills (CHs and CHm), sheet sandstone bodies (SB, DA, and LS) and mudstone bodies (FF). The Early Cambrian fluvial system was operating in a non-vegetated setting, and it is probable that the major flooding of the system took place on a seasonal basis. Six orders of surfaces are identified in the outcrops. The highest order surface (sixth-order) includes the lower contact of the Lalun Formation (Lower Sandstone), which was laid down on the Zaigun Formation, and the upper contact of the Lower Sandstone with estuarine deposits of the Middle Shale, denoting a marine transgression. The paleoflow direction is consistent with the reconstructed position of the Lalun Formation during the Cambrian and indicates that sediment was transported eastward to northeastward in the northern part of the Gondwana margin. The petrographic analysis suggests craton interior and transitional continental sources under seasonal humid conditions, consistent with the reconstructed position of Iran along the northeast margin of the Arabian Shield.

The facies analysis of a fan delta by integrating multiple discipline data — A case study of the KL-A oilfield, Bohai Bay Basin, China

The third member of the Shahejie Formation (Es3) is the main hydrocarbon exploration target in the KL-A oilfield, Bohai Bay Basin, East China. The facies classification is an essential step in mapping the depositional system of Es3 within the Bohai Bay Basin. The facies model is important for identifying the “sweet” lithology within the reservoirs. We characterize the facies of Es3 by integrating cores, well logs, and seismic data. The analysis indicates that the Es3 consists of fan delta deposits. The analysis using well logs and cores indicates that there are five different lithofacies for the Es3 Formation: massive sandstone, horizontally bedded sandstone, trough cross-bedded sandstone, ripple cross-laminated sandstone, and massive mud. The microfacies of Es3 include distributary channel, distributary mouth bar, interdistributary bay, and lacustrine mudstone deposits. The sweet lithologies for hydrocarbon accumulation within the KL-A oilfield include the coarse- and fine-grained sand developed within the channels and distributary mouth bar. We use seismic rms amplitude strata slices to determine the horizontal distribution of facies of the fan delta. The correlation between rms amplitude and well logs indicates that relatively low rms amplitude anomalies on the strata slices can be used as an indicator for sandstone within the KL-A oilfield.

Pedogenic Mud Aggregates Preserved In A Fine-Grained Meandering Channel In the Lower Permian Clear Fork Formation, North-Central Texas, U.S.A.

Abstract: The Lower Permian Clear Fork Formation of north-central Texas is among few examples from the rock record where mud aggregates are well-preserved as the main sediment component of a meandering channel. This rare occurrence provides an excellent opportunity to evaluate the origin and depositional setting of aggregates, as well as the conditions responsible for their preservation. At the study site, lateral-accretion deposits define an exhumed point bar composed of thick mudstone layers rich in aggregates, interbedded with thin layers of ripple cross-laminated sandstone. Petrographic analysis indicates that red-brown and gray aggregates constitute > 68% of the mudstones, with minor quartz, feldspar, ferruginous grains, and dolomite rhombs. When size estimates are corrected for 2D sections and for ∼ 17% flattening during compaction, the aggregates are of fine-sand size and more than twice the size of associated quartz grains, which were probably size-limited on the distal plain. Despite the difference in grain size and density, the aggregates and quartz populations were both entrained at very low velocities ranging from 1.2 to 1.5 cms –1 . Based on their relatively uniform size, variation in iron-oxide content, and association with small ferruginous concretions, the aggregates formed by reworking of associated vertic paleosols. In the absence of discrete smectite, their formation probably reflects a minor swelling-clay component in illite, Fe-rich chlorite, and mixed-layer clays, as indicated by glycolation results. Deficiency in smectite is probably a result of parent-rock composition, rather than burial illitization in this shallowly buried formation. In modern floodplains, aggregates typically lose their pelleted texture during shallow burial. Their excellent preservation at the study site is attributed mainly to the lack of swelling clays in the mud aggregates and their rapid incorporation into a migrating point bar, set within a “compartment” of indurated paleosols that limited compactional effects. Some early cementation by dolomite, gypsum, and barite of the aggregate-bearing and associated beds would have enhanced rigidity.

Shoreface to estuarine sedimentation in the late Paleocene Matanomadh Formation, Kachchh, western India

Late Paleocene sedimentation in the pericratonic Kachchh Basin marks the initial marine transgression during the Cenozoic era. A 17m thick sandstone-dominated succession, known as the clastic member (CM) of the Matanomadh Formation (MF), is exposed sporadically in the basin. Three facies associations are reconstructed in the succession in three different sections. Facies association-1 contains matrix-supported pebbly conglomerate facies, horizontally-laminated sandstone-mudstone alternation facies, hummocky- and swaley cross-bedded sandstone facies, wave-rippled sandstone facies and climbing ripple cross-laminated sandstone facies. This facies association developed between shoreface and foreshore zone under the influence of storms on a barrier ridge. Facies association-2 contains sigmoidal cross-bedded sandstone facies, sandstone-mudstone alternation facies, flaser-bedded sandstone facies, herringbone cross-bedded sandstone facies and tangential cross-bedded sandstone facies. This facies association possessing tidal bundles and herringbone cross-beds developed on a tidal flat with strong tidal influence. Facies association-3 comprises pebbly sandstone facies, horizontally-bedded sandstone facies, tangential cross-bedded sandstone facies exhibiting reactivation surfaces and tabular cross-bedded sandstone facies. This facies association represents sedimentation in a river-dominated estuary and reactivation surfaces and herringbone cross-beds indicating tidal influence. The bipolar paleocurrent pattern changes to unipolar up-section because of the change in the depositional currents from tidal to fluvial. The sedimentation took place in an open coast similar to the Korean coast. The presence of neap-spring tidal rhythmites further suggests that a semidiurnal system similar to the modern day Indian Ocean was responsible for the sedimentation. Here, the overall sequence developed during the transgressive phase where barrier ridge succession is succeeded by the tidal flat succession and the latter, in turn, is succeeded by the estuarine succession. This study resolves the most debated issue of initial marine transgression in the Kachchh Basin during the Cenozoic.

Forced regressive tidal flats: response to falling sea level in tide-dominated settings

Abstract During forced regression, the coastline advances seaward irrespective of sediment supply and relocates to a topographically lower position. Forced-regressive deposits are a component of the falling-stage systems tract. They are recognized by a basal unconformity which records erosion during the seaward facies shift, and are in turn capped by another unconformity due to subaerial exposure or wave ravinement during subsequent transgression. In wave-dominated settings, the basal “regressive surface of marine erosion” is a scoured surface cut by wave action, because erosion is needed to maintain the seaward-sloping bathymetric profile in equilibrium with the wave energy. The response to falling sea level in tide-dominated settings has not been as thoroughly discussed. The stratal architecture of the Lower Cambrian Gog Group of the southern Canadian Rocky Mountains reveals a new mechanism for the formation of this surface landward of the lever point of balance between sedimentation and erosion in the subtidal environment. Subtidal parasequences of the Lake O’Hara Member are typically composed of thin-bedded mudstone and ripple cross-laminated sandstone, followed by Skolithos pipe rock and thin- to medium-bedded cross-stratified sandstone overlain by thick-bedded cross-stratified sandstone. However, at the top of the Lake O’Hara Member, an erosional surface truncates a pipe-rock interval and the expected capping cross-stratified sandstones are absent. Overlying this surface, the Lake Oesa Member comprises tidal-flat deposits composed mainly of thinly interbedded mudstone and sandstone. This jump in facies at the erosional surface can be explained as a response to a fall in sea level. As the shoreline is forced to regress, the laterally continuous tidal flats advance and the pre-existing shallow-subtidal compound dunes are scoured by strong tidal currents, along with waves, that gradually carve a new equilibrium profile. We argue that the accretion of intertidal flats on top of subtidal sands is an overlooked yet predictable component of falling-stage systems tracts in tide-dominated settings.

Glaciation and deglaciation of the Libyan Desert: The Late Ordovician record

Detailed outcrop studies at the flanks of Al Kufrah Basin, Libya, reveal the nature of glacially-related sedimentation and post-depositional deformation styles produced in association with the Late Ordovician glaciation, during which ice sheets expanded northward over North Africa to deposit the Mamuniyat Formation. At the SE basin flank (Jabal Azbah), the Mamuniyat Formation is sand-dominated, and incises interfingering braidplain and shallow marine deposits of the Hawaz Formation. The glacially-related sediments include intercalations of mud-chip bearing tabular sandstones and intraformational conglomerates, which are interpreted as turbidite and debrite facies respectively. These record aggradation of an extensive sediment wedge in front of a stable former ice margin. An increase in mudstone content northward is accompanied by the occurrence of more evolved turbidites. A widespread surface, bearing streamlined NW–SE striking ridges and grooves, punctuates this succession. The structures on the surface are interpreted to have formed during a regional north-westward ice advance. Above, siltstones bearing Arthrophycus burrows, and Orthocone-bearing sandstones beneath tidal bars testify to glaciomarine conditions for deposition of the underflow deposits beneath. By contrast, the northern basin margin (Jabal az-Zalmah) is appreciably different in recording shallower water/paralic sedimentation styles and major glaciotectonic deformation features, although facies analysis also reveals northward deepening. Here, a siltstone wedging from 8 to 50 m toward the north was deposited (lower delta plain), succeeded by climbing ripple cross-laminated sandstones up to 60 m in thickness (distal through proximal delta mouth bar deposits) with occasional diamictite interbeds. These rocks are deformed by thrusts and > 50 m amplitude fault-propagation folds, the deformation locally sealed by a diamictite then overlain by conglomeratic lag during ultimate deglaciation. Integrating observations from both basin margins, a model of fluvial-dominated delta systems feeding a pulsed debrite and turbidite fan system in a shallow proglacial shelf is proposed.

Trace fossils of the arthropod Camptophyllia from the Westphalian (Carboniferous) rocks of Lancashire, UK and their palaeoenvironmental context

Two arthropod trace fossils are described and analysed from the Carboniferous Lower Westphalian ( C. communis and basal A. modiolaris chronozones) coal-bearing strata of Lancashire. The biserial trackway Diplichnites triassicus consists of five overlapping en echelon sets of 7–9 tracks preserved as epichnia and hypichnia in lacustrine siltstones. The trackway suggests subaqueous in-phase walking by a multi-segmented producer with a body length of 35–40 mm, width 17–22 mm, and 7–9 appendages. Curved, clustered, or laterally repeated, hypichnial lobes with transverse striations on the base of ripple cross-laminated sandstone are identified as Rusophycus versans. This trace fossil is interpreted as shallow resting or furrowing burrows of a homopodous arthropod, 30–60 mm long, 15–30 mm wide, and probably the same kind of arthropod as produced D. triassicus. A review of contemporary arthropod body fossils from Lagerstätten in Lancashire favours the onisciform, or Arthropleura like arthropod Camptophyllia as a potential producer of both of these trace fossils in a lacustrine palaeoenvironment. This study integrates the analysis of sediments, trace fossils and body fossils for reconstructing the arthropod biota and ecology in Westphalian lacustrine and crevasse splay fluvial palaeoenvironments.

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.

Advances in Arabian stratigraphy: Origin and stratigraphic architecture of glaciogenic sediments in Permian-Carboniferous lower Unayzah sandstones, eastern central Saudi Arabia

ABSTRACT In the subsurface of eastern central Saudi Arabia, deposition of lower Unayzah sediments commenced in late Carboniferous (late Moscovian = late Westphalian-Stephanian) times during the earliest stages of the late Paleozoic Gondwanan glaciation. The Unayzah C member is the lowermost unit, and comprises quartzose sandstones laid down on an extensive glacio-fluvial outwash braidplain upon the pre-Unayzah unconformity (so-called “Hercynian unconformity”). They were overridden during (probably several) glacial re-advances, when the sediments were thrust over each other during the construction of push moraines ahead of the advancing ice sheets. The upper surface of the Unayzah C member represents the final sub-glacial contact of this longlived series of glacial advances and retreats, and demonstrably represents a significant post-Unayzah C unconformity. Subsequent (early Permian) deposits represent the final glacial retreat phase. They include the Unayzah B member and a new, un-named middle Unayzah member that underlies the Unayzah A member. The Unayzah B member is dominated by glaciogenic sediments, including highly deformed material attributed to seasonal push moraines (and hence representing minor glacial re-advances); ice-proximal, subaqueous outwash fans (high-energy lacustrine turbidites with associated dropstones, and massive diamictites); and ice-distal, glaciolacustrine deposits including laminites, stratified diamictites, ripple cross-laminated sandstones and mudrocks with associated dropstones. In contrast, the un-named middle Unayzah member is characterized by widespread, fine-grained red-beds, representing low-lying alluvial floodbasin deposits. Embedded within these occur aeolian and fluvial sandstones. A significant drainage event is thus implied between the Unayzah B member and the un-named middle Unayzah member. It is marked by a sharp contact between the two members that represents a regionally correlatable disconformity. The end of lower Unayzah deposition is represented in places by paleosol development, indicating a further disconformable contact, separating the un-named middle Unayzah member from the overlying Unayzah A member. This study significantly extends the record of the advance and retreat of the Gondwanan ice sheets in southern Saudi Arabia, and provides sufficient confidence to propose a degree of stratigraphic correlation with coeval deposits in Oman.

Life in an active volcanic arc: Petrology and sedimentology of dinosaur track beds in the Jindong Formation (Cretaceous), Gyeongsang Basin, South Korea

The Jindong Formation was deposited as a thick succession of fine-grained volcaniclastic sediments in a rapidly subsiding intra-arc basin. These sediments contain abundant dinosaur and bird tracks that occur on many bedding planes throughout the approximately 110 m of stratigraphic section measured for this study. The main lithologies in the Jindong Formation are ash-rich mudstone, siltstone, and very fine sandstone. Minor lithologies are coarser sandstone, conglomerate, carbonates, and ash tuff. The sandstones are feldspathic and lithic arenites and wackes. They contain plutonic feldspars, volcanic rock fragments, volcanic ash, and a variety of quartz types. All lithologies show evidence of extensive post-depositional alteration, including devitrification and replacement of detrital grains by authigenic minerals. The alterations are typical of burial metamorphism with high heat flow. Sediments of the Jindong Formation can be grouped into three facies associations. Association 1 (proximal) comprises 16% of the measured sections. It consists of various types of bedded, cross-bedded, laminated, and ripple cross-laminated sandstone and minor conglomerate. These are interpreted as fluvial or distributary channel and proximal lake deposits. Association 2 (medial) comprises 53% of the measured sections. It consists mainly of heterolithic couplets of very fine sandstone and mudstone that are mm to dm thick. They contain graded bedding, laminations, current ripples, wave ripples, and mud cracks, and are interpreted as lacustrine hyperpycnal flow deposits. Association 3 (distal) comprises 31% of the measured sections. It consists mainly of bedded shale, mudstone, and siltstone. These are interpreted as distal lake deposits. Associations 2 and 3 contain minor carbonates, which are interpreted as lacustrine and/or diagenetic in origin, and minor tuffs, which are interpreted as tephra. Small wave ripples, dinosaur tracks, invertebrate burrows, and mud cracks are common throughout the measured sections; thus the Jindong lakes are inferred to have been shallow and ephemeral in nature. Wave ripple-crest orientations are consistently WNW-ESE, indicating a prevailing wind direction from the NNE or SSW. Because the sediments are mostly fine-grained and the ash beds thin, the study area is inferred to have been in a distal lowland setting, at least tens of km away from erupting volcanoes. Paleoclimate reconstructions suggest that the study area had a seasonal climate with wet summers and dry winters. Cooler, wetter conditions are thought to have prevailed to the north, and warmer, drier conditions to the south. Modern volcanic arc environments with seasonal and/or humid climates are characterized by rapid aggradation of ashy sediment that fills drainages and causes widespread flooding. This style of sedimentation accounts for the facies found in the Jindong Formation, as well as the many bedding-plane exposures of tracks. The unusual occurrence of both sauropod and ornithopod tracks in the Jindong sections may be the result of a climate boundary that existed in the area at the time. Facies analysis of track types indicates that various track makers had the same environmental preferences, and facies association 2 contains the greatest number of tracks of all types. It is inferred that the tendency for only one track type to occur on a given bedding plane is the result of different types of animals visiting the study area at different times, rather than differing environmental preferences. The Jindong Formation contains an abundance of subaqueously deposited facies, an abundance of tracks, and many track levels that were wet when animals were walking on them. These features are not consistent with previous interpretations of aridity during Jindong time.

Fluvial facies architecture in small-scale river systems in the Upper Dupi Tila Formation, northeast Bengal Basin, Bangladesh

The late stage basin-fill history of the fluvial Dupi Tila Group (Plio-Pleistocene) is described. These rocks have been deposited in the Sylhet trough, a sub-basin of the Bengal Basin, in a foreland basin setting. This outcrop study, carried out in Sylhet, Bangladesh, presents the first detailed facies analysis of the Upper Dupi Tila Formation. Four facies have been identified: trough cross-bedded sandstone (St), ripple cross-laminated sandstone (Sr), finely laminated mud with ripples (Fl), and massive mud with rootlets (Fm). Facies analysis supplemented with embedded Markov chain analysis, reveals small-scale fining-upward cycles (average 4.5 m thick). Facies architectural elements include channel (CH), lateral accretion (LA), sandy bedforms (SB), and overbank fines (OF) with limited vertical and lateral connectivity of the sand bodies. The average channel depth and width is 5 and 30 m, respectively. Sand body geometry ranges from tabular, to sheet, to shoestring with a 0.45 net to gross ratio. This study shows that the Upper Dupi Tila Formation is composed of small-scale, mudstone-reach meandering river deposits. In Bangladesh, the Dupi Tila Formation is the main aquifer presently being utilized. Understanding of facies architecture and sand body geometry of this Formation is crucial in examining the issue of arsenic and other contaminations of ground water in Bangladesh.

ABSTRACT: Scales of Facies Architecture in Top-truncated Lowstand Delta Lobes, Upper-Cretaceous Wall Creek Member, Frontier Formation, Powder River Basin, Wyoming, U.S.A

Abstract The Turonian Wall Creek Member (Wall Creek) has been interpreted as being deposited as an offshore bar, as storm deposited prodelta sand-sheets, and as a wave-dominated delta. An outcrop investigation was initiated in order to evaluate these varying hypotheses. This paper outlines each hypothesis' strengths and weaknesses then compares them with recent fieldwork completed during the 2000 summer field season. Fifty-nine vertical measured sections, including outcrop gamma ray scintillometry, photographs, and video were collected over a 900 square kilometer (347 square mile) area. The Wall Creek shows a mixture of different upward coarsening facies successions, associated with distinctly different overlapping sandstone bodies separated by mudstones. These are interpreted as different delta lobes. The south lobe grades upward from burrowed, to current rippled sandstones and mudstones, into structureless to flat stratified and ripple cross-laminated sandstones interpreted as delta front turbidites. These beds dip southeastward, the same direction as paleocurrents, suggesting they are delta front clinoforms. Dune-scale cross-stratified sandstones cap upward coarsening facies successions to the north. The presence of double mud drapes suggests tidal influence. In the east, the Wall Creek shows sharp-based mud-free cross-bedded sandstones, which suggest a wave-influenced shore face. Pebbles occurring at the sharp base may indicate a forced regression. Pebbles occur at the tops of sandstone bodies, including the inclined beds, all of which are truncated. Therefore, we interpret the Wall Creek as representing top-truncated lowstand deltaic deposits. We identify several scales of facies variability within the Wall Creek. Variability of different sand bodies with length scales of several kilometers to tens of kilometers represents large-scale reservoir compartments. These compartments show varying internal reservoir heterogeneity, depending on the proportion of river-, tide-, and wave-influence within lobes.

Post glacial lacustrine event sedimentation in an ancient mountain setting: Carboniferous Lake Malanz�n (Western Argentina)

Lacustrine deposits of the Malanzan Formation record sedimentation in a small and narrow mountain paleovalley. Lake Malanzan was one of several water bodies formed in the Paganzo Basin during the Late Carboniferous deglaciation. Five sedimentary facies have been recognized. Facies A (Dropstones-bearing laminated mudstones) records deposition from suspension fall-out and probably underflow currents coupled with ice-rafting processes in a basin lake setting. Facies B (Ripple cross-laminated sandstones and siltstones) was deposited from low density turbidity currents in a lobe fringe environment. Facies C (Massive or graded sandstones) is thought to represent sedimentation from high and low density turbidity currents in sand lobes. Facies D (Folded sandstones and siltstones) was formed from slumping in proximal lobe environments. Facies E (Wave-rippled sandstones) records wave reworking of sands supplied by turbidity currents above wave base level.

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.