Ripple Cross-lamination Research Articles

Occurrence of the trace fossil Hillichnus in Upper Cretaceous deposits in association with dinosaur footprints: A case from the Torotoro National Park in Bolivia

We present here the occurrence of the scarce, complex trace fossil Hillichnus associated with abundant theropod tracks and locally bird tracks at the Carreras Pampa dinosaur track site in the Torotoro National Park (TTNP), Bolivia. Hillichnus is a complex trace fossil produced by tellinacean bivalves, revealing various behaviours including locomotion, feeding, respiration and defecation. The specimens occur on the surface of a grainstone bed in the Maastrichtian El Molino Formation, characterized by the presence of low-angle planar cross-stratification, ripple cross-lamination, wavy or convoluted lamination and wave ripples. The associated invertebrate trace fossil assemblage mainly consists of superficial sinuous structures of variable size, a variety of holes (vertical burrows?), and radial/rosette trace fossils. The associated vertebrate trace fossil assemblage consists of theropod footprints, many well-preserved theropod swim traces arranged in trackways and bird tracks. The record of Hillichnus, associated to the ichnological assemblage and to sedimentological data, suggests a marginal-marine setting with freshwater influence in shallow water for the Upper Cretaceous of the El Molino Formation at the Carreras Pampa area. Local stratigraphic successions suggest both marine and lacustrine/fluvial environments. The track-bearing bed shows evidence of exposure and is covered by sediments deposited in deeper water. The occurrence of Hillichnus in Bolivia, in association with dinosaur tracks on the same bed, has relevant implications for the characterization of paleoenvironmental conditions during the production of dinosaur tracks at the Carreras Pampa site in the TTNP.

Compound-clinoform model for tide-influenced deltas in the mixed siliciclastic–carbonate Cisco Group, Upper Pennsylvanian (Virgilian), Eastern Shelf of the Permian Basin, U.S.A.

ABSTRACT Deltas in the Cisco Group contain significant volumes of potential reservoir sandstones in the Upper Pennsylvanian (Virgilian stage) mixed siliciclastic–carbonate succession in the Eastern Shelf of the Permian Basin. While early studies predominantly supported a river-dominated model for these deltas, our findings establish the presence of tide-influenced deltas characterized by the following features: i) predominant heterolithic lithology, ii) diagnostic structures such as flaser bedding, bidirectional ripple cross-lamination, or double-mud drapes, iii) an abundance of interpreted fluid muds, and iv) elongated bars on the subaqueous platform. Through analysis of 290.2 m of core, we identify six facies associations related to tide-influenced deltaic systems in highstand and falling-stage intervals of the proximal Eastern Shelf: delta plain–tidal flat, distributary channel and distributary-mouth bar (delta front) deposits on the subaerial delta; subaqueous delta topset, subaqueous delta foreset, and prodelta deposits on the subaqueous delta. Moreover, two additional facies associations, transgressive shelf deposits and subtidal shallow-water carbonates, record continuous episodic transgressions in the deltaic packages. Well-log cross sections reveal repetitive stacking of deltas, typical of icehouse systems, and sandstone mapping suggests elongated tidal bars forming the interpreted subaqueous delta platforms. These features suggest a compound-clinoform morphology of the Cisco Group deltas akin to modern tide-influenced deltas that have carbonate banks such as the Gulf of Papua. This compound-clinoform morphology has implications for the elongate geometry and isolation of sandstone reservoirs, impacting fluid migration and storage. Moreover, these insights extend beyond the Cisco Group to other Pennsylvanian and Permian levels on the Eastern Shelf, as well as to other deltas in mixed siliciclastic–carbonate systems in icehouse conditions.

COMPOUND CLINOFORM MODEL FOR TIDAL-INFLUENCED DELTAS IN THE MIXED SILICICLASTIC-CARBONATE CISCO GROUP, VIRGILIAN (GZHELIAN), EASTERN SHELF OF THE PERMIAN BASIN, U.S.A.

Deltas in the Cisco Group contain significant volumes of potential reservoir sandstones in the Late Pennsylvanian (Virgilian/Gzhelian) mixed siliciclastic-carbonate succession in the Eastern Shelf of the Permian Basin. While early studies predominantly supported a river-dominated model for these deltas, our findings establish the presence of tide-influenced deltas characterized by the following features: (i) predominant heterolithic lithology; (ii) diagnostic structures such as flaser bedding, bidirectional ripple cross-lamination, or double-mud drapes; (iii) an abundance of interpreted fluid muds; and (iv) elongated bars on the subaqueous platform. Through analysis of 290.2 m of core, we identify six (6) facies associations related to tide-influenced deltaic systems in highstand and falling-stage intervals of the proximal Eastern Shelf: lower-delta-plain/embayed tidal flat, distributary channel and distributary mouth bar/delta front deposits on the subaerial delta; subaqueous delta platform, subaqueou s delta foreset, and prodelta deposits on the subaqueous delta. Moreover, two (2) additional facies associations, transgressive shelf deposits, and subtidal shallow-water carbonates, record continuous episodic transgressions within the deltaic packages. Well-log cross sections reveal repetitive stacking of deltas, typical of icehouse systems, and sandstone mapping suggests elongated tidal bars forming the interpreted subaqueous delta platforms. These features suggest a compound clinoform morphology of the Cisco Group deltas akin to modern tide-influenced deltas that have carbonate banks such as the Gulf of Papua. This compound clinoform morphology has implications for the elongate geometry and isolation of sandstone reservoirs, impacting fluid migration and storage. Moreover, these insights extend beyond the Cisco Group to other Pennsylvanian and Permian levels on the Eastern Shelf, as well as to other deltas in mixed siliciclastic-carbonate systems in icehouse conditions.

Centimeter-scale sedimentary structures in a lacustrine delta front, northern China: Ripples or Froude supercritical-flow bedforms?

Sedimentary strata are a significant record of the Earth and planetary history, and accurate recognition of sedimentary structures and their link to environmental conditions is a key component in deciphering past surface processes. Centimeter-scale sedimentary structures are commonly acknowledged as ripple cross-laminations, but here we document ambiguous centimeter-scale structures from a lacustrine delta front in northern China, that at careful look do not seem to fit with the known ripple cross-lamination criteria. The here documented sedimentary structures range from scour-and-fill, irregular lenses with structureless or low- and high-angle, up- and downstream dipping, concave and convex laminations. These centimeter-scale sedimentary structures thus considerably differ from ripple cross-laminations in their outer shape, internal organization, and morphometric parameters. Detailed comparison of these centimeter-scale structures with Froude supercritical-flow structures suggests that they were likely produced by Froude supercritical flows. Such centimeter-scale supercritical structures are not unique in the Bantanzi delta, as they have been also documented in a variety of settings ranging from rivers to deepwater turbidites. In light of this finding, we expand the Froude supercritical-flow sedimentary structures to centimeter scale, and advocate caution in interpreting centimeter-scale sedimentary structures axiomatically as ripple laminations.

Implications of Salt Diapirism in Syn-Depositional Architecture of a Carbonate Margin-to-Edge Transition: An Example from Plataria Syncline, Ionian Zone, NW Greece

The present study examines the imprint of salt tectonics on carbonate depositional patterns of the Ionian zone platform edge to slope transition. The study area is part of an overturned rim syncline adjacent to a salt diapir. The Ionian zone is made up of three distinct stratigraphic sequences (pre-, syn- and post-rift sequences) represented by evaporites and shallow water carbonates at the base that pass gradually to a sequence consisting of pelagic limestones with shale intervals. In the study area, six cross sections were constructed, mainly covering the edge-to-slope overturned succession of Early Cretaceous to Eocene carbonates (post-rift stage) in the northern limb of the syncline. In the measured sections, abrupt changes in sediment texture resulted in the formation of distinct, thick-bedded carbonate layers, identified as packstones to grainstones–floatstones, with abundant fossil fragments, indicating deposition by debrites in a platform slope or slope-toe environment. Planar and ripple cross-lamination also suggest the involvement of turbidity currents in the depositional process. In the upper levels of the Lower Cretaceous carbonates, chert bodies with irregular shapes indicate soft sediment deformation due to instability of the slope triggered by salt intrusion. Internal unconformities identified in the field and in the available seismic data combined with the vertical to overturned dipping of the strata correspond to a basal megaflap configuration. Syn-sedimentary deformation resulted in the accumulation of debritic and turbiditic layers, while the compressional regime established in the area from the Late Cretaceous to Early Eocene enhanced the fracture porosity of carbonates, which could eventually affect the reservoir properties.

Frond orientations with independent current indicators demonstrate the reclining rheotropic mode of life of several Ediacaran rangeomorph taxa

AbstractFossils from the deep-sea Ediacaran biotas of Newfoundland are among the oldest architecturally complex soft-bodied macroorganisms on Earth. Most organisms in the Mistaken Point–type biotas of Avalonia—particularly the fractal-branching frondose Rangeomorpha— have been traditionally interpreted as living erect within the water column during life. However, due to the scarcity of documented physical sedimentological proxies associated with fossiliferous beds, Ediacaran paleocurrents have been inferred in some instances from the preferential orientation of fronds. This calls into question the relationship between frond orientation and paleocurrents. In this study, we present an integrated approach from a newly described fossiliferous surface (the “Melrose Surface” in the Fermeuse Formation at Melrose, on the southern portion of the Catalina Dome in the Discovery UNESCO Global Geopark) combining: (1) physical sedimentological evidence for paleocurrent direction in the form of climbing ripple cross-lamination and (2) a series of statistical analyses based on modified polythetic and monothetic clustering techniques reflecting the circular nature of the recorded orientation of Fractofusus misrai specimens. This study demonstrates the reclining rheotropic mode of life of the Ediacaran rangeomorph taxon Fractofusus misrai and presents preliminary inferences suggesting a similar mode of life for Bradgatia sp. and Pectinifrons abyssalis based on qualitative evidence. These results advocate for the consideration of an alternative conceptual hypothesis for position of life of Ediacaran organisms in which they are interpreted as having lived reclined on the seafloor, in the position that they are preserved.

New insights into deep-lacustrine architectural elements: Examples from the upper Triassic Yanchang Formation, Ordos basin

Deep-lacustrine systems are considerably less studied than their marine counterparts, mostly due to the low number of large and continuous outcrops worldwide. This study details outcrops and cores from Chang 7, Ordos Basin, China, which are composed dominantly of fine-grained deposits interbedded with amalgamated sandstones. Thick sandstone deposits, previously described as massive sandstones and interpreted as sandy debrites, are here re-interpreted as amalgamated sandstones, deposited by high-density turbidity currents associated with deep-lacustrine slope channels. Two outcrops composed of these deposits, which form intervals ranging from 6 to 20 m thick, laterally ranging from 600 to 1200 m, preserving evidence of sediment bypass such as erosional features and abundant trough cross-bedding, have been interpreted as lacustrine slope channels. Deposits composed dominantly of interbedded thin-bedded normal-graded sandstone and mudstone, with common occurrences of ripple cross-lamination (>70% of the beds), interbedded with sporadic amalgamated medium-bedded sandstones have been interpreted as levee deposits. These deposits outcrop laterally to slope channel deposits, supporting the interpretation they are genetically and architecturally related. Similar deposits are also described in core, overlying channelised sand-prone deposits. Interbedded thin- to medium-bedded normal-graded sandstones and mudstones, which are not associated with a confining surface (non-channelised), nor an adjacent slope channel, have been interpreted as terminal splays. The re-interpretation of the sandstone-prone deposits of the Yanchang Formation as slope channel-levees to terminal splays provides a different approach to lateral and temporal distribution of architectural elements in intracratonic lacustrine basins, and therefore hydrocarbon exploration and production.

Facies analysis of the Neoproterozoic to Late Cambrian Amadeus Basin, Central Australia

The study aims to fill research gaps in sedimentology by presenting a sedimentological study of theNeoproterozoic to Late Cambrian section in the Eastern MacDonnell Ranges to the east of Alice Springs,northeastern area of the Amadeus Basin. This study used the facies analysis in order to improve thecurrent understanding of lithofacies which assists in clarifying depositional environments interpretationin the northeast of the Amadeus Basin. This study recognised six lithofacies types (massive claystone,trough cross-stratification sandstone, hummocky cross-stratification sandstone, wave ripple laminationsandstone, massive siltstone, ripple cross-lamination sandstone) from the sedimentological log section.The Arumbera Sandstone and Shannon Formation were interpreted as deltaic settings. The Todd RiverDolomite and Giles Creek Dolomite are possibly transitional marine to deltaic-peritidal setting.Sedimentological analysis in this study classified the Arumbera Sandstone and Todd River as arkoses andthe Shannon Formation as dolostone.

Organic-rich, fine-grained contourites in an epicontinental basin: The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation, Argentina

Over the last few years, recognition and characterization of sedimentary processes in fine-grained successions is receiving considerable attention in part due to the increased importance of unconventional reservoirs. Recent sedimentologic analyses of these successions have revealed abundant traction transport structures that suggest bottom current activity of different origins. The present study constitutes a detailed sedimentologic analysis of the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation (Neuquén Basin, Argentina), which comprises organic-rich, fine-grained subaqueous clinoform deposits. Locally, intervals showing traction structures such as parallel lamination, ripple cross-lamination and cross-bedding, and displaying high bioturbation, occur. These intervals can be differentiated into three facies associations, namely crinoidal mixed to calcareous mudstone, mixed to calcareous mudstone, and calcareous to mixed mudstone interbedded with calcareous mudstone. These facies associations were deposited by contour currents showing semi-permanent activity, low sediment concentration, and generating long-term oxygen supply to the seafloor. Locally, some successions display an increasing to decreasing bioturbation intensity pattern that may have a similar origin to the bi-gradational sequence of contourites. The present sedimentologic analysis supports the idea that contour currents are associated with high bioturbation levels due to increased oxygenation, which controls the degree of preservation of traction structures, and consequently the possibility to delineate these processes in fine-grained successions. These currents occurred in relatively shallow water, probably as part of a weakened estuarine or anti-estuarine, basin-wide circulation system intensified by the enhanced action of dense water cascading during arid and cool climate periods.

Systematic organization of thin-bedded turbidites in ancient deep-marine levees: Possible evidence of rhythmic pulsing in turbidity currents

ABSTRACT Deep-marine levees are areally extensive features that border submarine channel systems and act to confine throughgoing sediment gravity flows. Compared to the adjacent channel, where episodes of erosion and bypass are commonplace, levees are mostly depositional features that experience little erosion, and therefore high preservation potential of individual beds, and as a consequence are generally assumed to provide a nearly continuous depositional record of sediment transport events down deep-marine slopes. Nevertheless, despite their size, volumetric prominence, and interpretive significance in the deep-marine sedimentary record, levees have received much less research attention compared to the adjacent channels. Exceptionally well exposed levee deposits in the Neoproterozoic Windermere Supergroup are dominated by thin-bedded, upper-division turbidites that form sharply bounded bedsets typically comprising 2–10 beds with similar grain size, bed thickness, Tc and Td/e division thickness, number of sets of ripple cross-lamination per bed, and ripple height, width, and spacing; additionally, the lithological and dimensional characteristics of the component beds in a bedset change at similar rates and spatial position along depositional strike. Such similarity suggests that deposition of each bed in a bedset is not associated with separate flow events (i.e., turbidity current), but instead represent systematic and recurring pulses or surges during a single flow event with similar hydraulic and textural conditions as they overspilled the channel margin and spread over the levee. Additionally, pulses were of sufficiently low frequency to allow the accumulation of a Td/e layer at the top of each bed and at least partial consolidation of the underlying bed. Finally, studies that have monitored and evaluated the frequency of turbidity currents in modern deep-marine systems suggest that there are a smaller number of beds, and even fewer bedsets preserved in ancient levee deposits than flow events. This suggests that many flows do not travel far enough downslope, or are of insufficient height, to result in any significant deposition on the levee. The bulk of levee strata, therefore, is likely deposited by anomalously large, pulsing flows that deposit multiple beds, but occur only once every several hundred to even several thousand years.

A record of deglaciation-related shifting of the proximal zone of a sandur \u2014 a case study from the Gwda sandur, NW Poland (MIS 2)

The study analyses a 7.5-m-thick sedimentary succession deposited in the largest sandur (the Gwda sandur, a glacial outwash plain) in NW Poland, during the late Weichselian (MIS 2). Although the study site is located in the distal zone of the sandur, its sedimentological features and palaeohydrological parameters reflect the presence of an energetic, powerful environment typical of the proximal zone. Three sedimentary units were recognized in the studied sedimentary succession: (1) lower unit — fine-grained sands with ripple cross-lamination and horizontal lamination; (2) middle unit — gravelly coarse-grained sands and sandy gravels with planar cross-stratification; and, (3) upper unit — sands and gravelly sands with trough cross-stratification. Although the age of deposition of the sandur is accepted to be convergent with that of end-moraines of the same phase, the sediments in the distal zone of the Gwda sandur correlate with an earlier glaciation phase/subphase. Our findings hint at a complex problem: large sandurs such as the Gwda sandur were probably deposited over a long time, and their successions might record the textural and structural features of the proximal subenvironment, even in their distal parts due to deglaciation-related shifting of the proximal zone of a sandur. This paper presents a new approach to analysing the depositional processes in large sandurs, shows a new light on glaciofluvial water supply dynamics of distal parts of sandurs, and may solve several fundamental problems related to the sandur deposition.

Lithofacies and petrography of Miocene Murree Formation, Peshawar basin, NW Pakistan: implications for provenance and paleoclimate

The Miocene Murree Formation is an enormous sedimentary deposit and is considered as foreland basin sediments of Himalayas, extending EastWest throughout the foothills of Himalayas. Lithofacies analysis was performed to establish the depositional environment, the following lithofacies were identified: clast-supported massive gravel (Gcm), sandstone with planar cross lamination (Sp), sandstone trough cross lamination (St), sandstone with horizontal lamination (Sh), sandstone with ripple cross lamination (Sr), sandstone with low-angle cross lamination (Sl) and mud and silt massive with roots bioturbation (Fr). These lithofacies depict shallow channel depth, deposition on the distal part of point bar, natural levees and flood plain of a meandering river. The modal composition data indicates that the Murree sandstone is arkosic to lithic wacke. The mineralogical and textural data suggests that sandstone is compositionally submature to immature and texturally immature to submature. The debris of rock fragments proposes diverse sources for Murree Formation. The ternary plots of detrital quartz type of the current study designate plutonic provenance. The high percentage of unstrained monocrystalline quartz (Qnu) relative to undulatory monocrystalline quartz (Qu) and polycrystalline quartz (Qp) proposes plutonic source area for Murree Formation. The paleoclimate plot discloses semi-humid–humid environment.

Ediacaran life close to land: Coastal and shoreface habitats of the Ediacaran macrobiota, the Central Flinders Ranges, South Australia

ABSTRACTThe Rawnsley Quartzite of South Australia hosts some of the world's most diverse Ediacaran macrofossil assemblages, with many of the constituent taxa interpreted as early representatives of metazoan clades. Globally, a link has been recognized between the taxonomic composition of individual Ediacaran bedding-plane assemblages and specific sedimentary facies. Thorough characterization of fossil-bearing facies is thus of fundamental importance for reconstructing the precise environments and ecosystems in which early animals thrived and radiated, and distinguishing between environmental and evolutionary controls on taxon distribution. This study refines the paleoenvironmental interpretations of the Rawnsley Quartzite (Ediacara Member and upper Rawnsley Quartzite). Our analysis suggests that previously inferred water depths for fossil-bearing facies are overestimations. In the central regions of the outcrop belt, rather than shelf and submarine canyon environments below maximum (storm-weather) wave base, and offshore environments between effective (fair-weather) and maximum wave base, the succession is interpreted to reflect the vertical superposition and lateral juxtaposition of unfossiliferous non-marine environments with fossil-bearing coastal and shoreface settings. Facies comprise: 1, 2) amalgamated channelized and cross-bedded sandstone (major and minor tidally influenced river and estuarine channels, respectively), 3) ripple cross-laminated heterolithic sandstone (intertidal mixed-flat), 4) silty-sandstone (possible lagoon), 5) planar-stratified sandstone (lower shoreface), 6) oscillation-ripple facies (middle shoreface), 7) multi-directed trough- and planar-cross-stratified sandstone (upper shoreface), 8) ripple cross-laminated, planar-stratified rippled sandstone (foreshore), 9) adhered sandstone (backshore), and 10) planar-stratified and cross-stratified sandstone with ripple cross-lamination (distributary channels). Surface trace fossils in the foreshore facies represent the earliest known evidence of mobile organisms in intermittently emergent environments. All facies containing fossils of the Ediacaran macrobiota remain definitively marine. Our revised shoreface and coastal framework creates greater overlap between this classic “White Sea” biotic assemblage and those of younger, relatively depauperate “Nama”-type biotic assemblages located in Namibia. Such overlap lends support to the possibility that the apparent biotic turnover between these assemblages may reflect a genuine evolutionary signal, rather than the environmental exclusion of particular taxa.

Sedimentology of the modern seasonal lower Ganges River with low inter-annual peak discharge variance, Bangladesh

The Ganges River, one of the largest rivers on Earth, is a typical monsoonal and flood-controlled system but has low inter-annual peak discharge variability. The seasonal discharge can reach 70 000 m 3 s −1 during the wet season but maintains a low base flow of 500–3000 m 3 s −1 during the dry season. However, the constancy in peak discharge every year categorizes the lower Ganges River as a river with low inter-annual peak discharge variability. This paper examines the modern lower Ganges River by conducting a detailed process-oriented investigation of the main channel, channel margin and overbank deposits, supplemented by satellite image observation and comparison with other modern fluvial systems. The channel and braid bar deposits show a dominance of small-scale to medium-scale cross-sets, with a variety of accretion processes constructing braid bars. The braid bar and channel deposits are typical of facies models of rivers with low inter-annual peak discharge variance. In contrast, the channel flank deposits are dominated by planar lamination, massive sand and mud couplets, and some ripple cross-lamination, with very little cross-bedding. Characteristic channel margin deposits represent sediments that accumulated by high-speed flows, multiple-surge and rapidly depositing flows, rapid or regular waning flows and hyperconcentrated flows. The overbank deposits predominantly comprise current ripples with long, thin bedforms and soft sediment deformation structures, which record flow transformation on the muddy flat topography and the processes of an unstable river bank. Our study shows that the channel margin and floodplain deposits are entirely different from those of the braid bar and channel. The bedform distribution of the fluvial deposits here (main channel, channel margin and overbank) may be an important tool in the identification of similar seasonal rivers with low inter-annual peak discharge variance and in the interpretation of fluvial processes. Supplementary material : The sediment texture and the depositional age is available at: https://doi.org/10.6084/m9.figshare.c.5144403

Early Pennsylvanian fluvial - Aeolian interplay in the Amazonas Basin (central-western Gondwana) and its relation to marine transgression

The Monte Alegre Formation corresponds to the main reservoir rock in the Amazonas Basin petroleum system in Brazil and is correlated with the Pennsylvanian rocks of the Parnaíba and Solimões basins. Previous outcrop and well-based studies are concentrated on the southern edge of the basin and show that the Monte Alegre Formation basically represents an aeolian, deltaic and transitional marine succession. However, there is no published research that demonstrates the depositional system in the northern edge of Amazonas Basin. In this sense, outcrop-based facies and stratigraphic analysis is carried-out to understand the distal influence of marine incursion into the basin and its relationship with paleogeography in the Upper Carboniferous. The data show that the aeolian Monte Alegre system is represented by a braided fluvial system formed by five architectural elements: (i) sandy bedform, (ii) downstream-accretion macroform, (iii) gravel bars and bedforms, (iv) channel and (v) laminated sand sheets, pointing to a braided depositional system. The aeolian succession overlies the fluvial system and is represented by medium-scale trough cross-bedding and large-scale planar cross-bedded sandstones resulting from the accumulation and migration of transverse dunes toward the northwest that grade upward to dry interdunes and aeolian sand sheet deposits that are interbedded, consisting basically of translatent ripple cross-lamination and low-angle cross-stratified lamination sandstones related to water table raises. The stratigraphic record defines a wetting-upward stacking pattern of Monte Alegre Formation resulting from local sea level rise as a result the advance of the marine incursion that settled in the central-western region of Gondwana during the Late Carboniferous. Thus, in addition to corroborating the paleo wind directions predicted for the region during the Late Carboniferous, these deposits help to understand the paleoenvironmental relations between the northern and southern edges of the Amazonas Basin.

TB or not TB: banding in turbidite sandstones

ABSTRACT Recognition and interpretation of sedimentary structures is fundamental to understanding sedimentary processes. Banded sandstones are an enigmatic sedimentary facies comprising alternating mud-rich (as matrix and/or mud clasts) and cleaner sand layers. The juxtaposition of hydrodynamically different grain sizes contradicts established models of cleaner-sand bedform development. Here, outcrop, subsurface core, and petrographic data from three deep-water systems, with well-constrained paleogeographic contexts, are used to describe the range of sedimentary textures, bedform morphologies, and facies associations, and to quantify the mud content of banding. Banding can occur in any part of a bed (base, middle, or top), but it typically overlies a structureless basal sandstone or mud-clast conglomerate lag, and is overlain by clean parallel-laminated sandstone and/or ripple cross-lamination. Banding morphology ranges from sub-parallel to bedforms that comprise low-angle laminae with discontinuous lenses of mudstone, or asymmetric bedforms comprising steeply dipping foresets that transition downstream into low-amplitude bedwaves, or steeply dipping ripple-like bedforms with heterolithic foresets. This style of banding is interpreted as a range of bedforms that form progressively in the upper-stage plane-bed flow regime via tractional reworking beneath mud-laden transitional plug flows. The balance of cohesive and turbulent forces, and the rate of flow deceleration (aggradation rate), govern the style of deposit. Banded sandstones and linked debrites are rarely found juxtaposed together in the same bed because they are distributed preferentially in proximal and distal settings, respectively. Understanding the origins of banding in turbidite sandstones, the conditions under which it forms, and its distribution across deep-water systems and relationship to linked debrites, is important for it to be used effectively as a tool to interpret the geological record.

The lithofacies and depositional paleoenvironment of the Bima Sandstone in Girei and Environs, Yola Arm, Upper Benue Trough, Northeastern Nigeria

The Benue Trough is a contiguous intracontinental basin that formed part of the mega west and central African rift system (WCARS). The Cretaceous sedimentary succession preserved up to 6 km thick. The Aptian-Albian Bima Sandstone non-conformably overlies the Precambrian basement rock and the sandstone is the oldest Cretaceous succession in the Upper Benue Trough. This paper aimed at understanding its lithofacies and depositional environment in Girei and environs within Yola arm of the Upper Benue Trough. From the field studies, 13 lithofacies were identified; massive matrix supported and gravel dominated facies (Gmm), matrix supported gravel (Gmg), clast supported gravel (Gci), gravel stratified (Gh), and gravel stratified trough cross-bedded sandstone (Gt), sand, fine to very coarse grained trough crossbedded sandstone (St), sand, fine, medium, coarse to very coarse grained group planar cross-bedded sandstone (Sp), low angle planar crossbedded sandstone (Sl), sand, fine to coarse grained ripple cross laminated sandstone (Sr), sand, very fine, pebbly coarse grained horizontal bedded sandstone(Sh), Sand, fine to coarse grained massive sandstone (Sm), Convuluted sandstone (Sc), silt and mud (Fsm), and mudstone (Fm). The Bima Sandstone in the study area is sub-divided into proximal alluvial fan facies are composed of Gmm, Gmg, Gci, and Fsm, lithofacies, while the mid and distal fan facies are sequentially characterised by Gh, Gt, St, Sp, Sl, Sr, Sm, Sc, and Fm lithofacies. The alluvial fan facies are matrix supported and are restricted to the basin margin, and it may represent the basal part of the outcrop lithosections, the lack of internal organization of these lithofacies reflects high-strength viscous debris flow and it speaks volume of emergence of stream near the upland, however, the Gci facies probably indicate advancement of stream influence and reduction of gravity, the associated Fsm facies could be a backswamp and/or abandoned channel deposits. The mid fan facies have basal surface that is concave-upward in transverse crossection, well-developed thick amalgamated trough crossbedded sandstone (St) facies, theses attribute represent deep channel deposits, high flow regime, and continuous creation of accommodation space, it also points towards deep perennial sand braided river system. The distal fan facies is composed of amalgamated planar cross-bedded sandstone (Sp) associated with horizontal bedded sandstone (Sh), ripple cross lamination (Sr) and mudstone (Fm) facies with better developed floodplain deposit, pointing towards the influence of steady periodic high flow with topographic difference and it reflects shallow perennial sand-bed braided river. The lithofacies of the Bima Sandstone in Girei and environs, within the Yola arm generally show architectural stacking pattern that is generally thinning and fining upward indicating inactivity and/or fan retrogradation, and the depositional paleoenvironment varies from alluvial to braided river setting. The mid to distal fan facies of the Bima Sandstone can serve as good reservoir for hydrocarbon while the mudstone which is the floodplain deposit can serve as the seals and/or traps for the accumulated hydrocarbon.

Facies Architecture Analysis for Paleo-environment Evaluation in “Tom” Oil Field, Eastern Niger Delta, Nigeria

The cored section of reservoir C, well 4 of the drilled five wells that penetrated three reservoirs A, B and C in “TOM” oil field, Eastern Niger Delta was analysed and described on the basis of lithofacies, sedimentary structures and trace fossil records by using core data and wireline log motifs, with the aim of carrying out thorough geological core analysis to interpret the depositional environment of the oil field. The lithofacies are sandstones with interbedded mudstones and siltstones, the dominant sedimentary structures are parallel to ripple cross laminations, hummocky and swaley cross stratifications, sandy hetherolitics, planar to low angle cross bedding with traces of Teichichnus and Ophiomorpha burrows. The gamma-ray log motifs were noted and used to further constrain the character of the sedimentary facies and depositional environment of the field. A tidal incised – fluvial dominated shallow marine (lower, middle, upper shoreface) comprises of tidal channel sands and tidal flat of the coastal shelf depositional setting in the marginal marine mega depositional environment had been inferred for the “TOM” field.
 Keywords: Shoreface, Reservoir, Lithofacies, Structures

A marine origin for the late Mesoproterozoic Copper Harbor and Nonesuch Formations of the Midcontinent Rift of Laurentia

The c. 1.1 Ga Copper Harbor and Nonesuch Formations of the Keweenawan Supergroup exposed along the Canadian-United States shorelines of Lake Superior are part of the surface exposures of the Laurentian Midcontinent Rift. These units have long been considered non-marine in origin and have figured prominently in ideas regarding the evolution of microbial life and the redox conditions of Earth’s ocean-atmosphere system at the close of Mesoproterozoic time. However, these rocks also host hydrothermal metal deposits, the emplacement of which may have compromised primary geochemical signals that are used to underpin those ideas. Here we highlight new sedimentological observations to provide an independent framework for assessing the depositional setting and geochemistry of those strata. We show that the totality of sedimentological features leads to the conclusion that parts of the upper Copper Harbor Formation and the entirety of the Nonesuch Formation were deposited along tide- and wave-influenced shorelines and in shallow-marine settings under evaporitic conditions. Evidence for this interpretation includes the abundance of flaser, wavy, linsen and pinstripe bedding, ubiquity of reactivation surfaces and mud drapes associated with all ripple forms, superposed sets of ripple cross-lamination showing bimodal (herring-bone) sediment transport directions, desiccation cracks and metre-scale hummocky cross-stratification. Further, evaporite fabrics and pseudomorphs after gypsum in the upper 200 m of the Copper Harbor Formation and in numerous stratigraphic positions within the Nonesuch Formation indicate that the water body was saline, not fresh. The emerging palaeogeographic image is one of a large, shallow-marine embayment with fringing sabkha-like shorelines. Ideas about late Mesoproterozoic biospheric evolution and Earth’s surface redox and oxygenation that rely on the Nonesuch Formation and Copper Harbor stromatolites having been deposited within a lacustrine setting require reassessment.

Hinterland setting and composition of an Oligocene deep rift-lake sequence, Gulf of Tonkin, Vietnam: Implications for petroleum source rock deposition

Abstract Most SE Asian oil accumulations are sourced from Paleogene lacustrine mudstones. Even so, the mechanisms controlling formation of these source rocks are not fully understood. Bach Long Vi Island, located in the northwestern South China Sea, offers a unique opportunity to study these mechanisms. A continuous, 800 m thick, organic rich and highly oil-prone, lacustrine section was investigated in outcrop and in a continuously cored section. The Oligocene deep-lake succession formed in the Bach Long Vi Graben that connects the Beibuwan Basin with the Yinggehai-Song Hong Basin. Petrography, detrital zircon U-Th-Pb geochronology and heavy mineral assemblages of sandstone intervals document a locally derived sediment source isolated from the Red River. Orientations of ripple cross lamination in gravity flow sands indicate axial sediment transport along the graben. Combined with the local structural configuration, this suggests that sediments were derived from the Qixi High through a relay ramp located about 10 km south of the island. Rare earth and major elements, detrital zircon ages, sandstone petrography and garnet composition, suggest a dominantly Ordovician – Silurian aged felsic and metasedimentary hinterland. The limited sediment supply from the small hinterland allowed tectonic subsidence to keep pace with graben infill. This promoted the establishment of a long-lived, deep lake favorable for source rock generation. In addition, heavy mineral and geochemical data document an unusually high content of detrital phosphate minerals in the investigated section indicative of a phosphate-rich hinterland. Tropical leaching resulted in an extraordinary high phosphorus input into the lake that catalyzed a high organic productivity and promoted the generation of highly prolific, oil-prone source rocks. The study documents that hinterland size and composition play a primary role in lacustrine source rock deposition. Comparable petroleum source rocks are likely to exist in other ancient rift basins surrounded by small, phosphate-rich hinterlands sheltered from significant sediment input.