Cross Bedding Research Articles

Microbially induced sedimentary structures in the Paleoproterozoic, upper Huronian Supergroup, Canada

The Paleoproterozoic Gordon Lake and Bar River formations, Huronian Supergroup, contain a variety of sedimentary structures in the Flack Lake area of Ontario, Canada, that have been considered of debatable origin. We identify these structures as microbially induced sedimentary structures (MISS). The preserved MISS are related to microbial mat destruction and decay, and include sand cracks, mat chips, remnant gas domes, pyrite patches, and iron laminae. A biological origin for the fossil structures is supported by their similarities to modern and ancient documented examples of MISS, the sand-dominated nature of the substrate in which they are preserved, and key microtextures identified in thin section. Microtextures include curled, frayed and layered mat chips, carbonaceous laminae, oriented grains, and concentrated heavy minerals. On outcrop scale, the presence of desiccation cracks, flaser and lenticular bedding, and ripples in association with the types of MISS identified in the Gordon Lake Formation support the interpretation of a tidal flat depositional environment. The Gordon Lake Formation contains a greater number and diversity of MISS than the overlying Bar River Formation, as a result of lower energy deposition in the former. The quartz arenite of the Bar River Formation contains fine-grained to pebbly granulestone characterized mainly by tangential and planar cross beds, which is consistent with a tidal channel or sand shoal setting. Although fossil evidence of life is rare in the rocks of the Huronian Supergroup, identification of MISS in the Flack Lake area provides a significant and convincing indication of microbial colonization at the time of deposition.

Application of Supervised Classification and CrostaTechnique for Lithological Discrimination in Parts of South Khetri Belt, Sikar District, Rajasthan

Supervised classification and Crosta technique is widely used for lithological and alteration zone mapping respectively. Landsat ETM+ digital imagery has been used to generate Crosta and Supervised classification image using digital image processing (DIP) technique. In the study, The Crosta technique has been used for the first time in lithological mapping. These techniques were applied to distinguish the litho-contacts between Alwar and Ajabgarh quartzite in the south-western part of North Delhi Fold Belt (NDFB) on the basis of argillaceous and arenaceous nature of the rocks. Litho contacts between Alwar and Ajabgarh groups are varying from quartzite to biotite schist in the Khetri basin whereas in the study area the litho-contacts are Alwar quartzite to Ajabgarh quartzite which are very difficult to classify on naked eye. Therefore, this area has been selected for this study. Several authors have studied the geological setting of Khetri sub basin and classified on the basis of argillaceous, arenaceous nature and primary sedimentary structural features like ripple marks and cross bedding. This is the first attempt in the Khetri sub basin for lithological classification based on remote sensing and digital image processing technique. This study revealed that Crosta image has significant spatial correlation with the lithology discriminated using Supervised classification technique. Lithological variations were clearly demarcated using these technique around Rahunathgarh and Golyana areas. The litho units between these areas were marked under the Group of Ajabgarh quartzite by the GSI people during 1994. However, the present study classified those area under the Group of Alwar quartzite. Similar type of studies can also be carried out where these type of lithological problems arises.

Effective flow properties heterolithic, cross-bedded tidal sandstones: Part 2. Flow simulation

Tidal heterolithic sandstone reservoirs are heterogeneous at the submeter scale because of the ubiquitous presence of intercalated sandstone and mudstone laminae. Core-plug permeability measurements fail to sample a representative volume of this heterogeneity. Here, we investigate the impact of mudstone drape distribution on the effective permeability of heterolithic, cross-bedded tidal sandstones using three-dimensional, surface-based “minimodels” that capture the geometry of cross beds at an appropriate scale. The impact of seven geometric parameters has been determined: (1) mudstone fraction, (2) sandstone laminae thickness, (3) mudstone drape continuity, (4) toeset dip, (5) climb angle of foreset–toeset surfaces, (6) proportion of foresets to toesets, and (7) trough or tabular geometry of the cross beds. We begin by identifying a representative elementary volume of 1 m3 (∼35 ft3), confirming that the model volume of 9 m3 (∼318 ft3) yields representative permeability values. Effective permeability decreases as the mudstone fraction increases, and it is highly anisotropic: vertical permeability falls to approximately 0.5% of the sandstone permeability at a mudstone fraction of 25%, whereas the horizontal permeability falls to approximately 5% and approximately 50% of the sandstone value in the dip (across mudstone drapes) and strike (parallel to mudstone drapes) directions, respectively. Considerable spread exists around these values, because each parameter investigated can significantly impact effective permeability, with the impact depending upon the flow direction and mudstone fraction. The results yield improved estimates of effective permeability in heterolithic, cross-bedded sandstones, which can be used to populate reservoir-scale model grid blocks using estimates of mudstone fraction and geometrical parameters obtained from core and outcrop-analog data.

Effective flow properties heterolithic, cross-bedded tidal sandstones: Part 1. Surface-based modeling

Tidal heterolithic sandstones are commonly characterized by millimeter- to centimeter-scale intercalations of mudstone and sandstone. Consequently, their effective flow properties are poorly predicted by (1) data that do not sample a representative volume or (2) models that fail to capture the complex three-dimensional architecture of sandstone and mudstone layers. We present a modeling approach in which surfaces are used to represent all geologic heterogeneities that control the spatial distribution of reservoir rock properties (surface-based modeling). The workflow uses template surfaces to represent heterogeneities classified by geometry instead of length scale. The topology of the template surfaces is described mathematically by a small number of geometric input parameters, and models are constructed stochastically. The methodology has been applied to generate generic, three-dimensional minimodels (9 m3 [∼318 ft3] volume) of cross-bedded heterolithic sandstones representing trough and tabular cross bedding with differing proportions of sandstone and mudstone, using conditioning data from two outcrop analogs from a tide-dominated deltaic deposit. The minimodels capture the cross-stratified architectures observed in outcrop and are suitable for flow simulation, allowing computation of effective permeability values for use in larger-scale models. We show that mudstone drapes in cross-bedded heterolithic sandstones significantly reduce effective permeability and also impart permeability anisotropy in the horizontal as well as vertical flow directions. The workflow can be used with subsurface data, supplemented by outcrop analog observations, to generate effective permeability values to be derived for use in larger-scale reservoir models. The methodology could be applied to the characterization and modeling of heterogeneities in other types of sandstone reservoirs.

Paleoclimate reconstruction of the quaternary sediments near the Gulf of Aqaba (Southern Jordan)

In this paper, the “Aqaba Formation” formed at the margins of the Aqaba Gulf, during the Quaternary, is sedimentologically studied. This formation consists mainly of clayey beds intercalated with friable silty, medium to coarse-grained sandstones, commonly with planar cross bedding, bioturbated by vertical burrows, and poorly cemented polymict conglomerate. It is also comprised of beds of clay-supported conglomerates occasionally displaying intertonguing relationships. The age of the formation is from Lower Pleistocene. Due to the presence of greenish clay layers within the Aqaba Formation, this means that it was deposited under a stream fresh water environment. It is also noticed that the overlain Pleistocene sediments are rich in Ostracods suggesting a fresh water environment, too. Generally, wetter climatic conditions were prevailing at this period as indicated by the absence of evaporite deposits (e.g., halite and/or gypsum). The clay mineral constituents (kaolinite and illite/smectite intermixed layers) and the absence of palygorskite are supported by the wetter conditions. It is worth to mention that the essential non-clay minerals are quartz and feldspars. This study presents the developments of the margins of the Gulf of Aqaba during Quaternary and reflects the climatic changes during this period.

Meandering river sand body architecture and heterogeneity: A case study of Permian meandering river outcrop in Palougou, Baode, Shanxi Province

Abstract Through actual measurement, careful sand body anatomy and lithofacies analysis of the meandering river sand bodies in the Permian of the Palougou profile in Baode county, Shanxi province, the architectural models of different kinds of meandering river sand bodies are established to characterize their interior sedimentary heterogeneity quantitatively, and predict the remaining oil areas in different kinds of meandering river sand bodies. Based on the outcrop characteristics, such as lithology, grain size, sedimentary structure, color, and so on, eight kinds of lithofacies are identified. By careful anatomy and lithofacies analysis of sand bodies, the bottom trough cross bedding sand body was separated from the classical point bar and named “bottom bar”, and the upper planar cross bedding was named “marginal bar”. Based on fine description of the sedimentary characteristics and superimposed relationships of bottom bar and marginal bar, four kinds of channel sand bodies, namely, lateral migration channel type, chute cutoff channel type, neck cutoff channel type and abandoned channel type, were identified. According to the architecture and heterogeneity characteristics of different types of channel sand bodies, it is concluded the lateral migration channel sand body has weak heterogeneity and little remaining oil, the chute cutoff and abandoned channel sand bodies with a little stronger heterogeneity is richer in remaining oil, and the neck cutoff channel sand body with the strongest heterogeneity has the most abundant remaining oil.

Upslope-Migrating Bedforms In A Proglacial Sandur Delta: Cyclic Steps From River-Derived Underflows?

Abstract Backstepping cross-strata on steep foresets of a Gilbert-type proglacial fluviodeltaic system are ascribed to cyclic steps and other associated supercritical bedforms. They provide insight into how sandur river flows transition into the marine realm. These sedimentary structures are located on steep foresets (up to 17°) with corresponding top-lying, flat-based topsets in an upper Pleistocene delta on the North Shore of the Gulf of St. Lawrence, Quebec, Canada. Packages of backstepping cross strata of sand and gravel, lying in the lower part of the delta front outcrop, are organized in 10–20 m spaced pseudo-foresets with a mean slope of 11–12° seawards. Backstepping strata include frequent internal erosion surfaces that onlap upslope on pseudo-foresets and are interpreted as cyclic steps. Narrow, deep, and asymmetrical scours and upslope-climbing cross beds are interpreted as chutes-and-pools and antidunes respectively. Very shallow (< 15 m) depositional paleo-bathymetry is inferred from the preservation of the delta brink. The well-organized stratal pattern in cyclic step to antidune deposits indicates relatively steady and uniform flow patterns. There is insufficient distance for a headscarp large enough to transform to the volume of observed accreted sands or for a flow transformation from a gravitational collapse to net-depositional cyclic steps. These deposits are sandier than the topsets beds and are thus not derived from them, but rather correspond to topset erosional surfaces. The development of cyclic steps from hyperpycnal flows was likely enhanced by tidal drawdown processes. The resulting sediment-laden supercritical flows plunged inertially and evolved into an underflow that generated the cyclic steps on the upper foresets. The cyclic steps have a high aspect ratio and represent an end member of coarse-grained sediment deposited on steep slopes, in contrast to low-gradient, low-aspect-ratio muddy deposits.

Sedimentology and depositional environment of the Kazhdumi Formation Sandstones in the northwestern area of the Persian Gulf

Kazhdumi is one of the Bangestan Group formations aging late early Cretaceous (Albian). Three subsurface sections of the sandstone bearing part of Kazhdumi in the studied area located at northwest of the Persian Gulf are assessed for determining sedimentary facies and depositional environment. Based on the recognized facies, the sandstone bearing zones of Kazhdumi and their relevant paleoenvironments can be addressed as: first one is the B sandstone zone containing thick sandstone layers alternating with shale and siltstone thin layers which are deposited by river influenced conditions because of the characteristics like: cross bedding, oxidized constituents and palesol. The second one, the A sandstone zone which is deposited under a key limestone layer, is composed of sand/sandstone layers with marine features and presents the interaction of marine-river condition in deltaic channels. This zone, is a transition interval showing the changes of river-dominated condition to marine one. The third one is the A sandstone zone deposited above the key limestone layer containing fine grained laminated siliciclastic and chemical sediments. Alternation of distal channel facies and green marine shale of delta containing glauconite, pyrite and organic matter residuals, all confirm the reduction condition in estuarine environment. Applying facies analyses results, isopach maps, porosity distribution maps and sandstone grain size study, the paleocurrent pattern is determined from the southwest to northeast.

Soft Sediment Deformation Structures in the Maastrichtian Patti Formation, Southern Bida Basin Nigeria: Implications for the Assessment of Endogenic Triggers in the Maastrichtian Sedimentary Record

Detailed fieldwork carried out in the southern part of Bida Basin, Nigeria, allowed the documentation of soft sediment deformation structures (SSDS) in the Maastrichtian Patti Formation. The aim of this study is to examine the sedimentary successions, describe and analyse these deformation features, discuss their deformation mechanisms and potential triggers. The Maastrichtian Patti Formation is composed of lithofacies interpreted to have been deposited in tidal and fluvial sedimentary environments. Soft sediment deformation structures recognised in the tidal sediments were clastic dykes, load cast, isolated sand balls, dish-and-pillar structures, convolute lamination, diapiric structures and recumbent folds. Severely deformed cross beds, ring structures, associated sand balls, normal folds and recumbent folds were identified in the fluvial sediments. SSDS recognised were interpreted to have been caused by effects of liquefaction and fluidization. Field observations, facies analysis and morphology of the SSDS indicate that there are relationship between the depositional environments and SSDS. Endogenic processes are considered as the trigger agents and they are represented by rapid sedimentation and overloading, impact of breaking waves, pressure fluctuations caused by turbulent water flow, cyclic stress and current generated by storm waves and changes in water table. The present study did not identify exogenic processes as trigger agent. The occurrence of SSDS in southern Bida Basin strongly favoured a non-tectonic origin but a clear relationship high energy processes in tidal and fluvial depositional environments.

Sedimentological and Paleoclimate Modeling Evidence for Preservation of Jurassic Annual Cycles in Sedimentation, Western Gondwana

Abstract The Lajas Formation in the Neuquén Basin, Argentina, consists of a succession of mainly deltaic deposits. In the Middle Jurassic (170 million years ago), the basin was in western Gondwana roughly at the same paleolatitude as its present location (32°–40°S). Decimeter-scale, interbedded, coarser-grained and finer-grained beds in channelized and nonchannelized deltaic deposits have been interpreted as a product of variability in river discharge. The coarser-grained sandstone beds have erosional bases and contain mudstone clasts; internal cross bedding is commonly directed paleoseawards. These beds are interpreted as deposition during river-flood conditions. In contrast, the finer-grained beds are composed of interlaminated sandstone and mudstone, deposited during interflood periods. Bidirectional ripples and millimeter-scale sand–mud laminae suggest the influence of tides. This sedimentological evidence raises the question of whether these cycles represent annual variability in fluvial input. To answer this question, a simulation using the Fast Ocean Atmosphere Model for the Middle Jurassic was run to equilibrium. The model shows that the paleoclimate of the Neuquén Basin was characterized by a strong seasonal cycle, with a wet winter and a dry summer. Model runs suggest that February mean temperatures were 28°C with 4-mm precipitation (±4 mm standard deviation) per month, whereas August mean temperatures were 8°C with 34-mm precipitation (±17 mm standard deviation) per month. The strong seasonal cycles in the simulation, representing 24% of the variance in the precipitation time series, suggest that the sedimentological cycles represent annual variations. The simulation also suggests a Middle Jurassic climate where increased seasonality of precipitation occurred farther poleward than previously thought.

The wet desert system of the Upper Jurassic of the Parnaiba Basin, in the region between Formosa da Serra Negra and Montes Altos, Maranhão State, Brazil

O Mesozoico foi marcado por mudanças geológicas significativas decorrentes da Orogenia Gonduanide, que possibilitou a implantação de sistemas desérticos concomitantemente com expressivos eventos magmáticos. No Brasil, o registro desses eventos é observado na porção central da Bacia do Parnaíba, representados pelos depósitos siliciclásticos da Formação Corda, de idade jurássica, que sobrepõem derrames basálticos da Formação Mosquito. O estudo de fácies e estratigráfico de afloramentos dessa unidade, realizado na região entre Formosa da Serra Negra e Montes Altos, estado do Maranhão, possibilitou reconstituir o paleoambiente e inferir condições paleoclimáticas para essa região da Bacia Parnaíba durante o Jurássico. Foram identificadas quinze fácies sedimentares agrupadas em três associações de fácies (AF), representativas de um sistema desértico: 1) canal fluvial entrelaçado (AF1), caracterizado por conglomerados e arenitos finos a grossos, com grãos e seixos de rochas vulcânicas, com estratificações plano-paralela e cruzada acanalada; 2) lençóis arenosos (AF2), constituídos por arenitos finos a grossos e pelitos subordinados com laminação plano-paralela, laminação cruzada cavalgante, marcas de adesão e gutter casts; 3) campo de dunas (AF3), arenitos finos a médios, com grãos arredondados e estratificações cruzadas tabular e tangencial. Fluxos fluviais efêmeros (AF1), adjacentes ao campo de dunas (AF3), frequentemente retrabalhavam o topo das planícies vulcânicas e, esporadicamente, invadiam os lençóis arenosos (AF2). Comparado aos ergs do Permo-Triássico, o sistema desértico Corda foi mais úmido e menos extenso, precedendo os sistemas fluviais e costeiros de clima mais ameno do Cretáceo da Bacia do Parnaíba

Supercritical-flow structures on a Late Carboniferous delta front: Sedimentologic and paleoclimatic significance: COMMENT

Ventra et al. (2015) propose a model of hyperpycnal supercritical flow regime deposition, related to a megamonsoonal climate, for a series of interpreted cyclic steps on a Carboniferous delta-front. The new model has major implications for paleoenvironmental reconstructions of this well-studied area, in terms of climate and geographical setting. We contend that fundamental sedimentological observations do not support their proposed model. Ventra et al. interpret that each cross-bedded ‘set’ is the deposit of a single mega-monsoonal flood event; we propose that each set represents multiple events; e.g., their Set 3 is at least 30 discrete events. Event bed sandstones are represented by variable to normal grading, crude cross-stratification and plane-parallel lamination, and a finer-grained top, which is generally ripple-cross laminated (Fig. 1). These are capped by a very thin, silty to fine-grained sand layer with abundant organic material, representing interflood deposition (Hampson, 1997). This repeated motif is only interrupted where bed tops are eroded. Abundant loading, dewatering and flame structures between beds and rip-up clast horizons support the interpretation of discrete depositional events (Fig. 1). A mega-monsoonal origin cannot be invoked for ~20-cm-thick beds. Ventra et al. state that there is a lack of ripple cross lamination or stratification, aside from some upflow dipping crude laminae. Most bed tops are laminated to crosslaminated (Fig. 1). These are interpreted as current ripples, rather than antidunes or climbing ripple backsets, as they have clearly developed tangential foresets. These ripples suggest paleoflow broadly toward the east, in contrast to the overall southwesterly paleoflow. Eastward paleocurrents typically occur on the updip side of the ‘undulatory beds’ of McCabe (1977) and Hampson (1997), suggesting that the exposed section is oblique to depositional dip, rather than parallel as proposed. We infer that there is a significant lateral, as well as downstream, accretion component to these three-dimensional bedforms. The lack of bioturbation between beds cannot be used as evidence for the short duration of events, as there is, in general, a lack of bioturbational structures in the sandy marine sediments of the British Carboniferous Basins. However, the depositional environment as proposed by Ventra (marine delta-front) is at odds with the existing fluvial models, which are based on detailed mapping and correlations (McCabe, 1977; Hampson, 1997). The large-scale architecture shows a progressive decrease in depositional dip that we interpret to reflect healing of scour-related topography. The depth of the scour (minimum of 13 m) corresponds to that observed in major fluvial confluences and is aligned with Hampson’s (1997) interpretation. The overlying section, which Ventra et al. do not discuss, is represented by many smaller-scale trough cross beds, interpreted to represent local shallowing in a fluvial setting associated with filling of the scour. In this stratigraphic unit, features of a similar geometry but at a smaller scale are convincingly shown to represent supercritical flow conditions (Bijkerk, 2014). In summary, we concur with the observations and interpretations of Hampson (1997) that these beds represent the fill of a major scour, potentially associated with a fluvial confluence, which filled in a stepwise manner, by gravity flow in the deepest parts, with periods of minor erosion of the downslope sides of the bedforms (e.g., Ullah et al. 2015). Figure 1. A: Short sedimentological log (from the base of Ventra’s [2015] ‘Set 2’) showing the general character of event beds. B: Character of event beds and finer interbeds (i) and amalgamation surfaces (a). C: Small dune with tangential foresets (f). D: ripple cross lamination (r) with tangential foresets. E: Loading (L) between beds.

Lithofacies, depositional environments and stratigraphic architecture of the Wajid Group outcrops in southern Saudi Arabia

ABSTRACT The Wajid Group is a Palaeozoic siliciclastic succession of southern Saudi Arabia. In the outcrop belt it is ca. 500 m thick, whereas in the subsurface, the thickness increases to more than 4,500 m. The siliciclastic sediments have great reservoir potential for hydrocarbons and for groundwater. Although they represent one of the most important aquifers of the Arabian Peninsula, neither their sedimentologic, lithostratigraphic, nor their reservoir characteristics are satisfactorily known. In this study, a detailed description of lithology and sedimentology is given and the Wajid Group sediments are interpreted in terms of depositional environment and facies architecture. Thirteen lithofacies (LF 1 to LF 13) have been recognised, most of them composed of different subfacies. These lithofacies are grouped into 9 lithofacies associations (LF-A1 through LF-A9). LF-A1 through LF-A3 and LF-A7 represent shallow-marine siliciclastic environments. The remaining lithofacies associations describe periglacial environments of the Hirnantian (Late Ordovician) and Permian Gondwana glaciations. Except for a few pro-glacial fluvial deposits, fluvial successions and aeolian sediments are absent in the outcrops of the Wajid Sandstone. Five formations are recognised in the Wajid Group: the Dibsiyah, Sanamah, Qalibah, Khusayyayn, and Juwayl formations. They are all separated by major unconformities. The Dibsiyah Formation represents a vast sand-sheet complex with core and margin facies formed under shallow-marine conditions. These marine conditions enabled an abundant fauna to proliferate and leave its traces in the form of Skolithos piperock and Cruziana sp. A late Cambrian to Early Ordovician age is inferred for these deposits from regional considerations. The Sanamah Formation records the Late Ordovician Hirnantian glaciation with coarse sandstones and conglomerates. A variety of glacier-induced sedimentary structures are present. The internal succession is composed of three major sediment packages reflecting three ice advance-retreat cycles. The latest of these cycles is overlain by a few metres of marginal-marine sediments of the Qalibah Formation. The Khusayyayn Formation was deposited probably during Early Devonian times. It also represents a sand-sheet environment characterised by the dominance of mega-scale and giant cross beds and bed sets. A marine depositional environment is assumed from scarce Skolithos sp., and because nearly all indicators of a braided river system are absent. The Juwayl Formation of Permian age was deposited at the interface of the Late Palaeozoic Gondwana ice shield with a large lake that may have covered most of southern Arabia and adjacent areas. Proglacial sandstones and conglomerates were deposited close to the glaciers, whereas fine-grained sediment with dropstones, boulder pavements and a wide spectrum of soft-sediment deformation are characteristic of the lake environment. While the two glacial successions and the Khusayyayn Formation can rather confidently be attributed to the geological time scale, either through seismic correlation or biostratigraphically, the Dibsiyah Formation has not yet been biostratigraphically well dated.

Petrographic and geochemical study of the Maastrichtian Ajali Sandstone, North Central Nigeria

The Upper Cretaceous Ajali Sandstone is an extensive stratigraphic unit of the Anambra Basin in southern Nigeria. It consists of friable, white cross-bedded sandstones exhibiting a fining upward sequence. Structures such as bioturbation, planar and herringbone cross beds indicate variability in depositional environment ranging from fluviatile to deltaic. Petrographic and geochemical compositions (major and trace elements) of these sandstones have been investigated to determine their provenance, tectonic setting and weathering conditions. Fifteen sandstone samples examined are rich in quartz but poor in feldspar and lithic fragments. Texturally, the framework grains of the sandstones are fine- to medium-grained, sub-angular to sub-rounded, moderately sorted to poorly sorted, positively skewed and leptokurtic. The sandstones are texturally immature as depicted by their sub-angular edges of grains, but mineralogically mature in terms of high percentage of quartz. The high chemical index of alteration (CIA) values (71.0–99.2%) for these sandstones suggests that they were derived from highly weathered rocks in the source area. The composition of the major oxides in the sandstones revealed that SiO 2 (49.1–99.7%), Al 2 O 3 (0.2–30.3%), Fe 2 O 3 (0.4–1.8%) and TiO 2 (0.06–3.2%) were the most abundant elements in all sandstone samples indicating a high detrital quartz and clay mineral content. The Al 2 O 3 /TiO 2 ratios (1.47 to 12.48), Ti/Zr (6.48–18.63) and Zr/Cr (2.24–22.36) suggest that the sandstones were derived from variable basement complex rocks, including some contribution from mafic or ultramafic components. Inferences from the plots of K 2 O/Na 2 O versus SiO 2 indicated a passive margin tectonic setting for the sandstones. The high loading of CaO and Na 2 O in more than 50% of the samples is indicative of terrigenous input, mainly in the form of carbonates and silicates. These results are generally consistent with a derivation of the sandstones from adjacent igneous and metamorphic basement complexes (Cameroon–Adamawa highlands and Oban Massif) while the extensive and blanket-like geometry of the Ajali Sandstone is indicative of an excellent reservoir for groundwater in the Anambra Basin.

DEVONIAN SILICICLASTIC ROCKS OF THE BABÍ LOM LOCALITY (SOUTHERN MORAVIA, CZECH REPUBLIC): SEDIMENTARY ENVIRONMENT RECONSTRUCTION AND PROVENANCE STUDY

Sandstones and conglomerates, which northwards of Brno agglomeration build a hill (ridge) named Babi lom are strongly diagenezed (anchi-metamorphozed) and tectonically deformed. Their regional distribution and stratigraphic position between the Cambrian(?) clastics and Upper Devonian limestones, as well as their appearance meant that they age is considering as Middle Devonian. They are distinguished as the so-called „Basal Devonian Clastics“. Almost all primary sedimentary features are well preserved and distinct. In the metasediments of the Babi lom hill were distinguished both the facies and structures typical of alluvial fans and rivers. Among others they are channel lags with imbricated grains, cross beddings and debrites. Within sedimentary association was established the way-up direction what clearly evidences a coarsening upwards sequence. Assuming the lack of rotation of series of sediments in the X-Y plane, the palaeotrasport was toward the north while major supply with material was from the east.

Nature of the Campano-Maastrichtian Sub-Basins in the Gongola Basin, Nigeria

Geological mapping was carried out and outcrop sections were examined alongside palaeocurrent directions from the greater part of the Campano-Maastrichtian outcrops in the Gongola Basin in Nigeria. Palaeocurrent measurements were obtained from asymmetrical ripples, cross bedding and dune bedforms within the channel-filling sub-facies of the Arowa Formation, and the foresets of the tabular cross bedding displayed in the Duguri formation. A north-westerly to a northerly paleocurrent direction was determined for the Arowa formation, indicating that the open sea was located to north-north-west, while a unimodal palaeocurrent directions characterize the Duguri Formation. These currents transported sediment into the Dukku, Akko and Bashar sub-basins with an overall centripetal pattern. The main structural features of the western part of Gongola Basin from E-W, are the N-S, NE-SW and NW-SE trending faults. These series of faults controll the basin subsidence and deposition of the Campano-Maastrichtian succession in the Dukku, Akko and Bashar sub-basins.The lateral and vertical facies variation within the Campano-Maastrichtian succession are in consistence with the northerly retreat of the sea.

Wellbore stability model for shale gas reservoir considering the coupling of multi-weakness planes and porous flow

Irregular wellbore collapse phenomena and accidents frequently occur during drilling operations in Longmaxi shale gas reservoir. Considering shale formation with natural cross beddings and fractures, we propose a multi-weakness plane instead of a single weakness plane failure model. Shale samples obtained from the Lower Silurian Longmaxi Strata of Sichuan Basin are investigated based on characterization and analysis of mineralogy, pore structure, sliding failure condition, and rock mechanics to study the impact of porous flow on jointed shale masses. Results show that Longmaxi gas shale is a brittle and fracture-prone material with poor hydrating capacity and extremely low permeability in rock matrices. Reduction of rock strength under porous flow may contribute to changes in intensity parameters of the weakness planes. Therefore, considering the failure of multi-weakness planes under porous flow, we present a wellbore stability model for shale gas reservoir. Two types of weakness plane distribution patterns are examined to discuss the effect of the occurrence, numbers, and water saturation of weakness planes. The results demonstrate that the number of weakness planes, difference in weakness plane occurrence, and diverse water saturation levels significantly affect wellbore stability during drilling.

Lithofacies, Parasequence Stacking, and Depositional Architecture of Wave- To Tide-Dominated Shorelines In the Frontier Formation, Western Wyoming, U.S.A

Abstract The Frontier Formation (Late Cretaceous) of western Wyoming contains a range of wave- and tide-dominated deltaic facies. Two examples are investigated in detail: one in which the tide-dominated facies occur in a protected bay, and the second in which the tide-influence deltas occur on the open shelf. The first example is from the Oyster Ridge Sandstone Member of the Frontier Formation, exposed along the north–south oriented Hogsback Thrust. The facies becomes progressively more tide-dominated to the north progressing from shoreface parasequences in the south, to mixed-influence deltas, to south-prograding ebb-tide-dominated deltas, to flood-tidal bars in the far north. The tide-dominated delta and flood-tidal bars have broadly similar parasequences: Basal facies are bioturbated mudstone to muddy sandstone that grade upward into cross-bedded and current-rippled sandstones with common mud drapes. All of these facies are interpreted to have been deposited in a north–south oriented bay formed in a foredeep basin, bounded on the west by the thrust front, and on the east by a broad, low uplift, and open to the ocean to the south. Tidal facies predominated in the bay, where deposition was not influenced by waves and where tides may have been enhanced due to tidal amplification. The second example comes from the subsurface First Frontier Member of the Frontier Formation, also in western Wyoming. Approximately 700 closely spaced wells, including several with cores, were used to describe and map this system, which is composed of a series of northeastward-prograding delta lobes. The facies stacking pattern observed in core is similar to the flood-tidal bars described from the outcrop. Maps of net sandstone show distinct linear sandstone bodies oriented northeast–southwest that are interpreted as tidal sandstone ridges similar to features in modern tide-dominated deltas. Significantly, regional well-log correlations do not show any evidence of a barrier that would have protected the system from wave influence. These deltas appear to have been tide-influence on the open shelf. This and other studies of ancient tidal deltas reveal some common elements. Tidal deltas are characterized by coarsening-upward parasequences with abundant current ripples, large-scale cross beds, and mud drapes. Prodelta faces may be either well laminated or bioturbated, and in the former case may preserve evidence of tidal cyclicity. Sandy delta-front facies commonly have steeply dipping clinoforms and may form shore-normal elongate tidal bar systems. Delta-plain facies are rarely preserved, but where present are composed mostly of tidal-flat and tidal-channel deposits. The tide-dominated delta parasequences and mid-estuary tidal bars can have very similar stacking patterns, so that single cores or well logs alone are not sufficient to distinguish the setting or sandstone geometry. In this study the stacking patterns of the Oyster Ridge Member tidal bars and tidal deltas appear very similar to First Frontier Member tidal deltas, and all have bioturbated prodelta facies, which contrasts sharply with tidal deltas from central Wyoming and elsewhere. Thus stacking patterns alone are not predictive of depositional setting (shelf vs. estuary) or parasequence geometry (delta vs. tidal bar).

Combination of electromagnetic, geophysical methods and sedimentological studies for the development of 3D models in alluvial sediments affected by karst (Ebro Basin, NE Spain)

Abstract An integrated analysis was carried out in a selected quarry of the oldest terrace of the Ebro River, where a wide gravel unit is characterized by large-scale cross bedding outcrops. This unit has been interpreted as a lake with marginal deltas. Previous sedimentological studies have pointed out that braided rivers have dominated in the area during the Early Pleistocene. The presence of a lake, which in stable conditions (without subsidence) could be hardly developed in a braided fluvial setting, supports that a topographic depression pre-existed. The delta gravels suggest a depth of at least 5 m for this depression. Established models from broadband multifrequency electromagnetic survey and ground penetrating radar (GPR) show that the analyzed area was affected by a long-term karstic subsidence, which generated a doline field in which topographic depressions were filled by marls and marginal gravels, and subsequently affected by minor collapses during different subsidence reactivation episodes. A 3D model of the evolution and meaning of the doline field and its interaction with sedimentation has been developed. This model can be applied in other areas where high-resolution geophysical 3D models are difficult to establish because of the limitations of the geophysical surveys due to the presence of interbedded mudstones, subsiding depressions filled by human activities or near surface water levels.

Petrography, mineralogy and geochemistry of Cretaceous sediment samples from western Khorat Plateau, Thailand, and considerations on their provenance

At Mo Hin Khao on the western flank of Khorat Plateau, Thailand, the Phra Wihan Formation reveals litharenite and sublitharenite with some subarkose and arkose. A cuesta in the eroded sedimentary sequence exhibits spectacular rock pillars of considerable geotourist potential. The rock sequence is high in silica (SiO2 67–98wt%) and contains quartz, mica, magnetite, chert fragments and accessory minerals such as zircon and tourmaline and amphibole species. These accessory minerals suggest felsic rocks, such as granite, granodiorite and pegmatite, were sources for the sandstones. Geochemical analyses of the sedimentary sequence suggest that source rocks may lie in the passive continental margin, before sediment transport and deposition in the Khorat Basin by rivers flowing across a large flood plain. Many depositional sequences/episodes formed thick beds of cross bedded clastic rocks. A high average maturity index (>5) indicates sedimentary reworking/recycling. Chemical Index of Alteration (CIA) values range from 47 to 98, suggesting variable chemical weathering within the source area rocks, largely representing moderate to high degrees of weathering. The average CIA value of these sediments (78) suggests that relatively extreme alteration factors were involved.