Heterolithic Beds Research Articles

High-resolution lithostratigraphy and reconnaissance sedimentology of Changotaung structure, Chittagong Tripura fold belt, Bengal Basin, Bangladesh

Unlike other structures in the vicinity of the Chittagong Tripura Fold Belt, the Changotaung anticline is one of Bangladesh's least explored structures. An attempt has been made for the first time to understand and document sedimentary deposits, environments, structure, and tectonic activity based on the high-resolution outcrop and reconnaissance study with the knowledge of broad-brush geology. We found that Changotaung is a symmetrical box-folded structure with an extensive western flank where the amount of dip varies between 11° and 45°. The exposed Cenozoic succession was categorized into three separate sedimentary sequences and correlated with the conventional stratigraphic unit. A first-order simple Markovian approach was presented for the exposed litho-section in an effort to illustrate vertical facies variations in the Upper Surma group. We quantified that heterolithic bed mostly overlies both trough cross-bedding ({P}_{ij}U = 0.706) and parallel laminated bed ({P}_{ij}U=0.955) according to the facies transition probability matrix. According to the results of the stationary distribution, there is a 40% chance of coming across heterolithic beds within the Upper Surma group during any given event that is completely random whereas trough cross-bedding, parallel laminated bed, cumulative sandstone facies, and cumulative shale facies shows around 10.8%, 15.2%, 20.6%, 13.4% probability. We hypothesized, based on the interpretive facies analysis, that the Chittagong Tripura fold belt region's Upper Surma Group underwent three interrelated depositional settings (wave-dominated shallow marine, tide-dominated shallow marine, and fluvio-deltaic distributary).

Reevaluating the process regime in the Sego Sandstone: Sedimentological and ichnological evidence for an underemphasised fluvial signature

AbstractRocks of coastal to shallow‐marine origin are challenging to interpret owing to the complex interplay of various depositional processes. This study reevaluates the relative roles of fluvial, tidal and wave processes in the Upper Cretaceous Sego Sandstone (and subordinately in the underlying Buck Tongue) of the Book Cliffs, USA, a well‐studied ancient coastal to shallow‐marine succession. Detailed sedimentological and ichnological analyses were used to interpret a previously underemphasised riverine signature, consisting of centimetre‐ to decimetre‐thick alternations of sandstone and heterolithic beds inferred to represent flood–interflood periods of variable river discharge. Recognition of a widespread fluvial‐dominated signature across the studied units better agrees with other sedimentological and regional observations in the study area, such as high sandstone–mudstone ratios, largely unidirectional and seaward‐oriented palaeocurrents, and modelled weak tidal conditions in the basin. When considering all of the sedimentological, ichnological and stratigraphic observations together with its regional depositional context, the Sego Sandstone/Buck Tongue system is better explained using a mixed‐energy but fluvial‐dominated deltaic model. This highlights an historical over‐interpretation of tidal processes and subordinate wave processes in the Sego Sandstone and likely in similar units. The widely used approach that emphasises only certain sedimentary features in discerning the process regime from analysis of rocks of inferred coastal to shallow‐marine origin is unrefined and may therefore underrepresent the actual complexity of these systems.

Paleoenvironmental reconstruction of Late Cretaceous rivers, Dinosaur Park Formation, Alberta, Canada

Dinosaur Provincial Park is a UNESCO World Heritage Site with one of the richest vertebrate fossil accumulations globally. The “badlands”-style topography exposes Upper Cretaceous (Campanian) meander-belt deposits of the Dinosaur Park Formation, providing an excellent opportunity for sedimentological and three-dimensional stratigraphic analysis. One of the main unknowns regarding the depositional environments of the Dinosaur Park Formation, specifically the proximity to the fluvial-to-marine transition zone, has been the subject of extensive debate. Our study aims to refine paleoenvironmental reconstructions of Late Cretaceous rivers through facies and facies associations analysis, meander-belt architecture characterization, and paleohydraulic parameter calculations.We investigate a ~7 m thick single-story meander-belt interval of the Dinosaur Park Formation. Six facies, four facies associations, and two architectural elements (both point bars) were identified from 20 stratigraphic sections. In addition, paleohydraulic reconstruction of the formative river channel was completed by estimating bankfull channel depth and width, paleoslope, and backwater length.Contrasting with previous interpretations of deposition (partially or wholly) influenced by tides, these data allow us to demonstrate that the meander-belt deposits of the Dinosaur Park Formation were deposited in a fluvial environment upstream of the fluvial–marine transition zone. Inclined heterolithic strata (IHS) present in the area are of fluvial origin, adding to the growing body of evidence for heterolithic bedding in fluvial point-bar environments. The newly inferred depositional setting for the studied portion of the Dinosaur Park Formation changes our understanding of the paleogeography of the Western Interior Seaway in western Canada and will allow paleontologists to revise the reconstructions of dinosaur habitats and better constrain the taphonomy of these globally significant fossil accumulations. Additionally, this study contributes to the validation and development of a systematic approach to the paleoenvironmental reconstruction of ancient river systems based on paleohydraulic analysis.

Mud‐rich low‐density turbidites in structurally‐controlled intraslope mini‐basin: The influence of flow containment on depositional processes and sedimentation patterns (Szczawa, Oligocene, Polish Outer Carpathians)

ABSTRACTThis study demonstrates the effectiveness of sedimentological–statistical multivariate analysis of one‐dimensional‐section in the unravelling depositional processes and sedimentation patterns recorded in a contained and partially ponded succession. Turbidite deposition of a 100 m thick mud‐rich Oligocene‐age sequence at Szczawa, the Polish Outer Carpathians, was primarily controlled by topographic confinement and magnitude of incoming flows. Only the largest turbidite flows were subjected to the true flow ponding, while in smaller volume flows the silty/sandy part behaved as unconfined flow and only muddy suspension cloud developed a ponded character. True ponding of low‐density turbidity currents is interpreted for sandstones that show sedimentary structures attesting to flow reflections and combined‐flow processes, and are associated with abnormally thick co‐genetic mudstones, which in turn results in low sandstone‐to‐mudstone ratio, irrespective of turbidite bed thickness. These features testify to pronounced interaction of flows with confining topography. Tractional structures associated with a considerable proportion of fines, namely banded structure and heterolithic bedding, are interpreted as produced during transitional (turbulent to laminar) flow phase in the condition of mud‐rich turbidite flow confinement. These structures are also considered here as indicators of proximity to source area and location on the slope. The co‐occurrence of banded sandstones, hybrid event megabeds and an olistostrome resulted from flows‐trapping, confined basin geometry, which precluded downslope propagation and further transformation of large‐volume flows. Therefore, a structurally‐controlled contained intraslope mini‐basin is proposed here as the most likely depositional setting. This work provides robust field and statistical evidence for ponding processes of low‐density turbidity currents in a structurally‐controlled mini‐basin. The results of this study are consistent with experimental data and recent field studies on ponded turbidites. Therefore, the Szczawa succession may serve as a new reference example of containment processes of low‐density turbidity currents and represents a valuable depositional model for intraslope turbidite succession.

The influence of bioturbation on reservoir quality: Insights from the Columbus Basin, offshore Trinidad

Nine facies are recognised in two mid-Pliocene cores from the Columbus Basin, offshore east coast Trinidad. Facies successions are consistent with deposition within a deltaic-shoreface continuum. Lower shoreface deposits are heavily bioturbated by Scolicia, Asterosoma, Thalassinoides, Teichichnus, Palaeophycus and other elements of the Cruziana Ichnofacies whereas middle shoreface sandstone beds contain sand-filled burrows of the Skolithos Ichnofacies. Deltaic deposits record coarsening-upward successions with evidence of freshwater influx such as synaeresis cracks, fluid muds, and dispersed organic detritus. Prodelta and distal delta front deposits comprise heterolithic beds and contain sporadically distributed and diminutive structures belonging to the Phycosiphon Ichnofacies. Hummocky cross-stratification is prominent in storm-dominated proximal delta front sandstone deposits that are sparsely bioturbated by Ophiomorpha, Skolithos and other elements of the Rosselia Ichnofacies. Cryptic bioturbation is common at the top of these beds. Thin section examination, core plug analysis, and numerical modelling of the facies were undertaken to better understand fluid flow in bioturbated strata of the Columbus Basin. Whereas lithology and texture remain the primary control on reservoir quality, examples of biogenically enhanced permeability are present in the P3 and P7 cores. The highest permeability values are associated with highly bioturbated middle shoreface deposits that comprise clean, sand-filled burrows. Favourable permeability measurements are also attributed to cryptically bioturbated delta front sandstone beds. The latter display higher permeabilities when compared to unbioturbated and sparsely sandstone beds. Biogenic activity contributed to reduced permeabilities in lower shoreface and distal delta front deposits owing to extensive sediment packing and sediment mixing behaviours. Relationships between permeability and bioturbation intensity indicate that effective permeability in bioturbated intervals of the P3 and P7 cores are best characterised by the arithmetic mean of permeabilities and that simple statistical calculations can aid in the analysis of highly bioturbated facies (BI = 5–6).

Palynology of the Freshwater East Formation (Upper Silurian, Pridoli), Pembrokeshire, South Wales, UK

Upper Silurian (Pridoli) cryptospores and trilete spores are described from the type section of the Freshwater East Formation in Pembrokeshire, South Wales. This formation is the oldest stratigraphical division of the Lower Old Red Sandstone succession south of the Ritec Fault in the Anglo-Welsh Basin. It represents an incised valley fill sequence composed predominately of red dryland alluvial sediments inter-bedded with four tidally-influenced green-grey heterothithic units. The sequence is well known for the occurrence of an early land plant Cooksonia flora. The spore assemblages are dominated by cryptospores (67%), particularly laevigate monads and dyads. Trilete spores are less abundant but are diverse in composition and provide the principal means of accurately dating the sequence. Eleven cryptospore taxa and thirty one trilete taxa are identified. Two new species Cymbohilates richardsonii and Velamisporites edwardsae are erected. The Freshwater East microflora contains a distinctive complex of Chelinospora taxa, here named the Chelinospora lavidensis assemblage. It represents a previously unrecognised assemblage within the Pridoli succession of the Anglo Welsh Basin. An accurate age for this assemblage has been determined by correlation with the Upper Silurian succession of the Cantabrian Mountains in northwest Spain, where a continuous spore succession through the Pridoli Series has independent biostratigraphical age control from chitinozoans. The Freshwater East assemblages are correlated with the upper part of the Chelinospora hemiesferica (H) Biozone, which is dated by the chitinozoans Margachitina elegans and Pseudoclathrochitina carmenchui as early mid Pridoli in age. Rare occurrences of small spiny acritarchs, sphaeromorph acritarchs and prasinophyte phycomata in two of the Freshwater East assemblages support the sedimentological data that the grey-green heterolithic beds were deposited in tidally-influenced fluvial and estuarine environments.

A geomagnetic analysis for lineament detection and lithologic characterization impacting groundwater prospecting; a case study of Buffalo catchment, Eastern Cape, South Africa

A lithostratigraphy analysis commonly adopted for subsurface structures exploration in oil prospecting is deployed for groundwater prognosis, due to the need to address the prevailing water shortage. In this study, the lithology characterization was based on a clustered approach of geomagnetic analysis, geomorphometric analysis, and sequence stratigraphy correlation across the Buffalo catchment, Eastern Cape, South Africa. The result showed the existence of a spatial correlation between the tectonic stress field and the surficial lineaments. Similarly, a significant azimuthal correlation (WNW-ESE) was found between surficial lineaments (203 m–6249 m), subsurface lineaments, and the neotectonic structures. The depth-slicing analysis enables the estimation of the depth of the tectonic stress field, at 382 m, 577 m, 913 m, 1.49 km, and 10.7 km in the subsurface. The sequence stratigraphy correlation based on borehole lithology data enables the identification of the fault system at the contact zones, heterolithic bedding, and the characterization of the hydrostratigraphic domain of the catchment. The study shows that Buffalo hydrostratigraphic structures are dominated by fracture networks, whereby the shallow groundwater resources are possibly hosted by the dolerite-host rock contact zones in the north of the Buffalo catchment. Specific recommendations concerning groundwater management are made to water and environment stakeholders considering the vulnerability of the shallow groundwater system to pollution in the permeable contact zones of dolerite-host rock. The study also recommends the adaptation of surficial lineaments as a morpho-tectonic aid and a hydrostratigraphic analysis tool. The clustered approach demonstrated in this study has important prospects for groundwater exploration in environments with similar geology.

Using ichnological relationships to interpret heterolithic fabrics in fluvio‐tidal settings

AbstractIn fluvio‐tidal settings, the sediment is dominantly derived from the river systems. However, the importance of landward tidal transport of sediment in tidally influenced sedimentary environments is difficult to assess, particularly in the rock record. This problem is addressed using two intervals within the Lower Cretaceous McMurray Formation, each representing a distinct inclined heterolithic stratification motif. The ichnological variation between the heterolithic intervals is analyzed to determine which lithosomes are associated with brackish‐water (tidally influenced) colonization windows. From this, the relative fluvial influence responsible for the deposition of the fine and coarse members can be determined. Both of the inclined heterolithic stratification fabrics studied record the deposition of fluvio‐tidal point bars wherein the heterolithic bedding represents variations in river discharge. The first fabric comprises inclined heterolithic stratification in which bioturbation only occurs in mudstone beds. This fabric indicates that deposition occurred in more proximal positions within a fluvio‐tidal system (i.e. the outermost inner to middle estuary or distributary channels). In this example sand deposition is interpreted to represent high‐energy, freshwater dune migration within a fluvial‐dominated setting, whereas mud beds reflect brackish‐water suspension deposition during times of low river discharge. The second fabric, which is interpreted to have developed in more distal depositional positions (i.e. the middle estuary or seaward of the turbidity maximum in deltas), consists of inclined heterolithic stratification with laminated mudstone and bioturbated sandstone. In these inclined heterolithic stratification successions the mudstone beds were deposited under the influence of freshwater and heightened sedimentation rates, whereas bioturbated sandstone was colonized under brackish‐water conditions and in the presence of tidally facilitated sediment transport. In both examples, the bioturbated lithosomes are related to colonization windows that indicate the predominance at that time of marine or tidally influenced processes over fluvial processes.

Effects of Anisotropy and Saturation on Geomechanical Behavior of Mudstone

AbstractThe abundance of mudstone in Earth's crust and its academic and industry applications has led to advancements in the understanding of mudstone deposition and preservation. However, there are few quantitative geomechanical studies on mudstone. To test correlations between anisotropy, fluid saturation levels, and deformability and strength parameters in mudstone, a suite of indirect tensile and unconfined and triaxial compression tests was completed on samples from the Mancos Shale and the Agrio Formation. Discs or plugs were tested with axial loading parallel or perpendicular to bedding, at various confining pressures, and at saturation levels from 0 to 17%. Tensile strength, compressive strength, Young's Modulus, Secant Modulus, stress drop, and energy released exhibit mechanical anisotropy and decrease due to saturation. Tensile strength anisotropy revealed the weakness of heterolithic bedding in mudstone, as tensile fractures commonly propagated along bedding interfaces. Saturation increased mechanical anisotropy for tensile and compressive strength. A compressive strength reduction of 39% to 75% was observed between dry and partially saturated plugs. Lithofacies were used to classify lithologic variations in different types of mudstone. When partially saturated, sandstone‐bearing facies revealed a greater reduction in strength and moduli than clay‐rich mudstone facies. Empirical relationships between partially saturated and dry sample strength and moduli are proposed, to allow for estimation of deformation and failure behavior under in‐situ conditions. The effects that saturation, orientation, and lithofacies have on mechanical behavior are of interest in numerous engineering and academic applications, including fault behavior, induced fracturing of hydrocarbon and geothermal reservoirs, and nuclear waste disposal.

Deepwater to fluvio-deltaic stratigraphic evolution of a deglaciated depocenter: The early Permian Rio do Sul and Rio Bonito formations, southern Brazil

This paper describes facies relationships and stratigraphic architecture of early Permian deglacial deepwater to fluvio-deltaic deposits exposed in the eastern border of the Paraná Basin, southern Brazil. The studied succession is located in the Rio do Sul depocenter and comprises an upward transition between the topmost interval of the Itararé Group (Rio do Sul Formation) and the lowermost unit of the Rio Bonito Formation (Triunfo Member). Geological mapping and facies analysis allowed the recognition of the following facies associations, vertically stacked: thin and thick-bedded turbidites, hyperconcentrated and concentrated density-flow deposits (hyperpycnites), mass-transport deposits, fine-grained heterolithic beds of prodelta, delta-front, shoreface and delta-plain deposits, as well as fluvial beds resting on incised valleys. The stratigraphic analysis through the interpretation of the facies associations, stacking pattern and paleocurrent dispersal trends were enough to define the clinoform stratigraphic architecture, composing a progradational-aggradational pattern. The paleoflows for all facies associations show a main trend towards the south-southwest. In this context, hyperpycnal beds can represent a glacial source from the northeast, because of striated and faceted clasts immersed in the conglomeratic facies, whereas turbidites have been understood as the products of their dilution. Likewise, mass-transport deposits are related to the collapse of the shelf-margin delta, represented by allochthonous blocks composed of deltaic beds within the mass-flow diamictites. Thus, these mass-transport deposits do not evolve downslope to turbidity currents. This configuration was likely for building the relief of the slope system, probably a shelf-edge delta susceptible to mass failure. Paleo-ice flows towards SW recorded in the Rio do Sul Formation, genetic related to fluvio-deltaic deposits of Rio Bonito Formation, highlight the scenario of final stages of the Late Paleozoic Ice Age in the SW of Gondwana and support the previous researches which point to the presence of diachronous ice-spreading centers from paleohighs.

Fácies e Paleoambientes do Membro São Tomé (Formação Emborê) na Porção Emersa da Bacia de Campos

A porcao submersa da Bacia de Campos apresenta um vasto conhecimento geologico acumulado ao longo de decadas de exploracao petrolifera. Todavia, sua porcao emersa e, ainda hoje, carente de estudos estratigraficos devido a escassez de dados de subsuperficie profundos, sobretudo quando relacionado ao Membro Sao Tome da Formacao Embore, uma unidade nao aflorante interpretada como o registro de sistemas deltaicos de alta energia ( fan deltas). Tendo em vista contribuir para o conhecimento da regiao, este estudo teve por objetivo a caracterizacao sedimentologica e paleoambiental dos depositos do Membro Sao Tome que ocorrem em quatro testemunhos de sondagem (2-LF-1-RJ, 2-BG-1-RJ, 2-MU-1-RJ e 2-TO-1-RJ) recuperados na area do Complexo Deltaico do Rio Paraiba do Sul (CDRPS), porcao emersa da Bacia de Campos. Oito facies sedimentares foram caracterizadas (Lm, lamito macico; LAm, lamito arenoso macico; Het, acamamento heterolitico ondulado e/ou lenticular; Ae, arenito com estratificacao cruzada; Am, arenito macico; ALm, arenito lamoso macico; ACm, arenito conglomeratico macico e Ccm, conglomerado fino macico, sustentado pelos clastos), interpretadas em termos de processo deposicional e agrupadas em quatro associacoes de facies (LA1, LA2, FL1 e FL2), permitindo assim a identificacao de paleoambientes de leque aluvial e fluvial. As associacoes LA1 (constituida pelas facies ACm, ALm, Am e LAm) e LA2 (constituida pelas facies ALm, Am, LAm e Lm), que ocorrem nos testemunhos 2-MU-1-RJ, 2-BG-1-RJ e 2-TO-1-RJ, sao interpretadas como o registro de leques aluviais desenvolvidos em areas adjacentes aos principais sistemas de falhas (NE-SW) da regiao norte do estado do Rio de Janeiro. A associacao FL1 (constituida pelas facies ACm, Am, Ae, LAm e Lm), que ocorre no testemunho 2-TO-1-RJ, corresponde a depositos de um sistema fluvial entrelacado. Ja a associacao FL2 (constituida pelas facies Ccm, Am, Ae, Het e Lm), que ocorre nos testemunhos 2-MU-1-RJ, 2-LF-1-RJ e 2-BG-1-RJ, sendo bem caracterizada neste ultimo em funcao de seu maior percentual de recuperacao, e interpretada como o registro de um sistema fluvial do tipo meandrante de baixa sinuosidade ou wandering.

The interpretative significance of ripple-derived sedimentary structures within an upper Neogene fluvial succession of central Poland

Abstract Sedimentary structures discussed in the present study are genetically linked to ripples that consist of pure sand or alternating sand and mud layers. All types of ripple-related structures, such as climbing-ripple cross-lamination and heterolithic bedding, i.e., flaser, wavy and lenticular (nodular), have been identified for the first time in fluvial strata that have been characterised previously as commonly massive. These small-scale bedforms, produced by migrating ripples, have been documented in a fluvial channel of late Neogene age in central Poland. The abundance and co-occurrence of the structures discussed and their spatial distribution provide evidence of their formation under very low-energy conditions, when flow velocity changed markedly, but was often significantly less than 0.5 m/s. Therefore, these ripple-derived sedimentary structures are here recognised as typical of channel fills of an anastomosing river.

Fires and storms—a Triassic–Jurassic transition section in the Sichuan Basin, China

The Upper Triassic Xujiahe and Lower Jurassic Zhenzhuchong formations of the Sichuan Basin, China, are important sources of plant fossils and windows into the intervening extinction event. However, there is an on-going debate as to whether the environment represented by the Xujiahe and Zhenzhuchong formations was continental or included an important marine component. We studied the Xujiahe–Zhenzhuchong section near Qili Town of Xuanhan County, in the east of the basin and report hummocky and swaley cross-stratification in the Xujiahe Formation. This, along with minor Skolithos and heterolithic bedding, provides strong evidence for shallow marine conditions and favours an interpretation as the deposits of a wave-dominated coast. It also suggests common and extreme storm activity, possibly hurricanes, at what was a mid-latitude (c. 34–40°N) location in the Late Triassic. Charcoal is found in most samples throughout the section. The predominant fossil wood morphology is consistent with Xenoxylon. Together, the sedimentological evidence of storms and fire suggests a highly disturbed environment.

Geological interpretation of channelized heterolithic beds through well test analysis

Well test analysis is a valuable tool to measure the dynamic response of a reservoir through determination of the hydraulic connectivity and effective permeability of the reservoir. Analytical models in well test analysis, are developed based on a simple geological structures, to provide reasonably good approximations for the description and performance of such reservoirs. Nevertheless, most prolific reservoirs such as channelized systems consist of sedimentological features with high degrees of heterogeneity that influence the pressure transient response where using conventional analytical models may result in misleading interpretations. The focus of the current study is on reservoirs which depositional environment corresponds to a main channel feature incising into heterolithic beds in lateral continuity. Analysis of the pressure response demonstrated that it can be used as a tool to predict the equivalent isotropic horizontal permeability of the channel. We explored that the ratio of well test permeabilities between the radial flows can lead to the identification of a secondary geological body next to channel. Thus, it can be used to find the distance of the interface between channel and heterolithic. The results of this study showed that particular features of pressure and its derivative curves from a channel-heterolithic system are useful well testing signatures for reservoir characterization. Therefore, we proposed an algorithm for the recognition of pressure trends and the development of relationships to be used for well test interpretation of heterogeneous oil and gas reservoirs.

Carbonate drowning successions of the Bird\u2019s Head, Indonesia

Drowning unconformities and their related strata are important records of key tectonic and environmental events throughout Earth’s history. In the eastern Bird’s Head region of West Papua, Indonesia, Middle Miocene strata record a drowning unconformity present over much of western New Guinea, including several offshore basins. This study records platform carbonate strata overlain by mixed shallow- and deep-water units containing benthic and planktonic foraminiferal assemblages in several outcrop locations across the eastern Bird’s Head region. These heterolithic beds are interpreted as drowning successions that are terminated by a drowning unconformity. We define a succession exposed along the Anggrisi River in the eastern Bird’s Head as a stratotype for carbonate platform drowning in the Bird’s Head, analogous to similar faunal turnovers identified in its offshore basins. Detailed facies analyses, biostratigraphic dating, and paleoenvironmental interpretations using larger benthic and planktonic foraminifera collected from the Anggrisi River succession help to constrain the drowning event recorded onshore as beginning in the Burdigalian and ending in the Serravallian. The cause of platform drowning in the Bird’s Head is attributed to a reduction in the rates of carbonate accumulation due to the presence of excess nutrients in the depositional environment. Already foundering carbonate platforms due to environmental deterioration were left vulnerable to submergence and eventually succumbed to drowning. Low rates of carbonate production were outpaced by the rate of relative sea-level rise caused by high-amplitude oscillations in global glacio-eustatic sea-level change and/or regional tectonic subsidence. The duration of the drowning event across the entire Bird’s Head region is interpreted to have lasted a duration of approximately 9.5 My, between 18.0 and 8.58 Ma. This has implications when interpreting timings of sedimentary basin fill across western New Guinea and in other basins where carbonate platform drowning is recorded.

Ichnological analysis of the Upper Miocene in the ANH-Tumaco-1-ST-P well: assessing paleoenvironmental conditions at the Tumaco Basin, in the Colombian Pacific

Tumaco is a frontier basin located on the SW Colombian Pacific coast. It is composed of a thick siliciclastic sequence up to reach 10,000 m-thick. In recent years, the National Hydrocarbon Agency-ANH has promoted new exploration wells in order to understand the sedimentary dynamic and its relationship with petroleum systems. One of them, the ANH-Tumaco-1-ST-P well has ∼3000 m (12,000 feet). We carried out sedimentological, geochemical, and micropaleontological detailed analyses with special attention to the ichnology on a 55 m-cored interval (from 1695.3 to 1640.4 m = 5563–5382 ft) in order to assess paleoenvironmental conditions. Beds are composed of green and gray mudrocks interbedded with lithic sandstones and fine-grained tuffs. Calcareous microfossil assemblages defined by the recovery of Uvigerina carapitana, Uvigerina laviculata, Uvigerina pigmaea, Globigerina woodi, Globigerionoides obliquus, Discoaster bellus gr., Catinaster coalitus, Reticulofenestra pseudoumbilicus and Sphenolithus abies indicated a Tortonian age, between CN6/CN7 biozones. Six sedimentary facies were identified: (1, 2) massive and laminated mudrocks, (3, 4) massive and normal-graded sandstones, (5) heterolithic beds, and in some cases (6) sandstones with soft-deformation structures. These rocks were accumulated in a shallowing platform-prodelta environment with continuous volcanic influence. Ichnotaxonomic analysis, conducted for the first time in the Colombian Pacific, allowed the identification of eighteen ichnogenera: Alcyonidiopsis, Asterosoma, Chondrites, Conichnus, Cylindrichnus, Diplocraterion, Ophiomorpha, Palaeophycus, Phycosiphon, Planolites, Rhyzocorallium, Schaubcylindrichnus, Scolicia, Siphonichnus, Taeinidum, Teichichnus, Thalassinoides, and Zoophycos. The ichnological association belongs to the archetypal Cruziana ichnofacies and its “distal” expression. By integrating lithofacies and ichnological results, two segments have been distinguished: 1) the lower one (1695.3–1677.1 m) dominated by massive and laminated mudrocks and 1 to 6 ichnofabric indexes and, 2) the upper segment (1677.1–1640.4 m) characterized by the increase of the sandstone facies, and ichnofabric indexes from 1 to 5. Along these segments, four ichnofabrics have been identified: 1) Phycosiphon-Planolites, 2) Planolites-Thalassinoides, 3) Zoophycos-Chondrites, and 4) Phycosiphon-Thalassinoides, reflecting stable sedimentary conditions with low-moderate energy, low-moderate sedimentation rates, and dominant aerobic facies. Minor variations in these parameters (sedimentation rate, energy conditions, substrate consistency, oxygenation and organic matter), however, induced changes in the macrobenthic tracemaker community, and consequently in the registered trace fossil assemblages.

Palaeocurrent and facies analysis of Ajali Sandstone in Western Anambra basin, Nigeria

Outcrops of the Campanian - Maastrichtian Ajali Sandstone in Ayogwiri, Fugar and Orame in the Western Anambra Basin allow for the recognition of palaeocurrent fabric and facies characterization of the formation. Detailed outcrop logging was carried out and production of graphic logs was made for each outcrop location. The Ajali is extensively cross-stratified with different types of cross-bedding, such as the planar, trough, and herring-bone cross-beddings. Palaeocurrent directions of the cross-beds were taken using the compass-clinometer to elucidate the palaeocurrent direction of the environment of formation and rose diagrams were constructed for each bed. The Ajali Sandstone is made up of two basic facies. The basal facies, is made up of thinly laminated, heterolithic beds of the shoreface environment. The upper facies consists mainly of cross-bedded sandstones of a tidally influenced shallow marine environment. The major palaeocurrent direction of the Ajali Sandstone in the studied area was in the NE direction which infers the sediment provenance source to be the uplifted Abakiliki Anticlinorium. However, subordinate direction to the south occurs in Orame1.

A Synthesis of Depositional Trends In Intertidal and Upper Subtidal Sediments Across the Tidal–Fluvial Transition In the Fraser River, Canada

Abstract Sedimentological, neoichnological, palynological, and geochemical trends from upper subtidal and intertidal positions on channel bars in the lower Fraser River, Canada are synthesized into a single, coherent framework. From these data we define criteria for determining depositional position in shallow water depths in tide-influenced rivers. Three sedimentological trends are observed from the river mouth, through the locus of mud deposition (within the turbidity maximum zone (TMZ)), and into the freshwater-tidal zone. (1) The recurrence (per meter) and thickness of mud beds increase towards the TMZ and tapers in both the landward and seaward directions. (2) Muddy current ripples and graded current ripples are most abundant in the TMZ; they are less common with decreasing brackish-water influence and are absent in the freshwater river reach. (3) Heterolithic bedding (i.e., flaser, wavy, and lenticular) is common in the TMZ, less common seaward, and absent from the freshwater realm. In addition to the sedimentological trends, four ichnological trends parallel decreasing water salinity. With decreasing salinity, there is: (1) a decrease in bioturbation intensity from BI 2–3 to BI 0–1; (2) a decrease in the abundance of bioturbated beds; (3) a marked decrease in the diversity of traces from 5–6 forms to 1–2 forms; and, (4) a decrease in the diameter and length of traces. Traces are rare to absent in the tidal freshwater zone. Palynological and geochemical trends generally follow ichnological trends but are less obvious. Neither dinocyst abundance nor geochemical signature can be used to determine relative position in a tide-influenced river channel, although dinocyst abundances greater than 1% indicate significant marine influence, and 0–1% marine dinocysts indicate tidal influence. Although it is not feasible to determine exact depositional position within the tidal–fluvial transition, our results suggest that it is possible to determine where sediments were deposited relative to the TMZ. In turn, predicting relative depositional position can assist in unraveling stratigraphy and in recognizing nested channels in architecturally complex sedimentary successions.

Characteristics of tidal-bore deposits and facies associations in the Qiantang Estuary, China

Abstract The characteristics of tidal-bore deposition in the Qiantang Estuary are studied on the basis of core strata and grain-size data, and lateral and longitudinal associations with other sedimentary facies are synthesized. A typical depositional package by tidal bores and associated flows is generally initiated with (1) an undular/planar erosion base, overlain by (2) a massive sandy bed, and (3) a sandy bed with parallel laminations or some thinly heterolithic beds. The depositional units, especially the parallel laminations, are prone to deformation into convolute bedding, flames, and boiled sand patches with water-escape structures. In the middle estuary, tidal-bore deposits (TBDs) at the main channel and the lower tidal flat transit laterally into alternative beds of TBDs andz heterolithic beds at the middle tidal flat, and then toward incomplete tidal rhythmites on the higher tidal flat. TBDs are generally coarser and less sorted than tidal sandy deposits (TSDs), and tidal muddy deposits (TMDs) have finer grains, less sorting and lower positive skewness than either TSDs or TBDs. Therefore, bivariate plotting of size parameters is a useful tool to differentiate between these three genetic sedimentary bed types. Along the axis, three facies divisions are obvious and consist of: (1) linear depositional ridges and erosion troughs at the outer estuary, (2) TBDs at the middle estuary, and (3) coarse fluvial deposits at the upper estuary. The tripartite facies model of the Qiantang Estuary is similar to other well-known tide-dominated estuarine facies models, but it is the first to stress tidal-bore deposition in the sedimentary facies using detailed discriminative textural and structural characteristics. The approaches offer the potential to better understand tidal-bore processes and their important role in sediment dispersion and facies formation within modern and ancient macrotidal to hypertidal estuaries.

Two new non-haptotypic palynomorph taxa from the middle Permian Upper Gharif Member, Oman

Two non-haptotypic palynomorph taxa, Schizosporis? pennyi n. sp. and Tetraporina forbesii n. sp., are described and illustrated from Subunit B of the Upper Gharif Member immediately underlying the base of the Khuff Formation at the northern Huqf outcrop, Oman. Tetraporina forbesii also occurs in heterolithic beds within the lowest part of the Khuff Formation in the Barik-36 oil well. The forms are assigned to the Wordian-Capitanian Oman and Saudi Arabia Palynological Biozone 6 (OSPZ6) on the basis of their occurrence with Florinites? balmei, and other taxa characteristic of OSPZ6. They were previously assigned to informal taxa and are likely freshwater zygospores, and their prevalence in the Subunit B section of the Upper Gharif Member may indicate impermanent stagnant ponds in a coastal plain or estuarine palaeoenvironment.