Trough Cross-bedded Sandstone Research Articles

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

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

THE COMPARATIVE DEPOSITIONAL HETEROGENEITY OF MANCHHAR FORMATION (SIWALIK GROUP), SOUTHERN INDUS BASIN, PAKISTAN

Present study is carried out for detailed description, distribution and interpretation of lithofacies and textural analysis of Manchhar Formation from Gaj River, Manchhar Lake and Lal Bagh Sections of Southern Indus Basin Pakistan. Four different clastic lithofacies are identified during present study, which are conglomeratic sandstone (Gt), shale (Fm), fine to coarse grain flat bedded sandstone (Sh) and fine to coarse grain trough cross-bedded sandstone (St). These Facies suggest that Manchhar Formation is deposited in the Beach to shallow agitated water and fluvial system. Twenty-nine (29) unconsolidated loose and friable sandstone samples were selected for textural analysis. Grain-size parameters such as mean, median, standard deviation, Skewness and Kurtosis were used for transport history, mode and hydrodynamic energy condition to recognize the depositional environment. Stewart Diagram shows deltaic/wave water process takes active role under the influence of wave process. Linear discriminant functions analysis show that the fluvial environment of deposition dominates and influences shallows water and beach environment. Passega diagram reveals that transport mode was rolling and suspension by the tractive current that shows the fluvio-deltaic to deltaic wave with the influence of wave depositional environment of Miocene-Pliocene (Neogene) Manchhar Formation.

The cambrian gog quartzite and deadwood formations of alberta: lessons for subsurface carbon sequestration

Injecting carbon dioxide into reservoirs has been undertaken since the 1940s, mostly as a method to enhance oil recovery. With the recognition of the impact of climate change, interest in carbon sequestration has increased exponentially, with hundreds of projects in progress around the globe. Many of these projects focus on Cambrian quartzites, which meet the requirements for carbon dioxide injection (CCS). Several CCS projects in Alberta, Canada, currently focus on using these reservoirs, including the Basal Cambrian Sandstone and Deadwood Formations, for injection and storage.A series of spectacular outcrops of the Cambrian Gog Quartzite are available for study in the area around Lake Louise in the Canadian Rockies of Alberta. These outcrops provide compelling analogues to other clastic Cambrian formations in the province that have been utilized for CCS. As an example, the Gog Quartzite was used by Shell as a direct reservoir analogue when modeling CO2 injection into the Basal Cambrian Sandstone in the QUEST Project. The Gog Quartzite also has potential to act as a CCS reservoir in its own right and may reach 2 kilometres in thickness.Outcrops of the Gog Quartzite were studied at Sink Lake, Spiral Tunnels and in the vicinity of Lake Louise in Alberta. They include fluvial and shallow marine, high net:gross sandstone beds with stacked trough cross-bedded sandstones. These are commonly interbedded with thin mudstone beds. Sedimentary structures include planar and trough cross-beds, wave ripples and contorted horizons interpreted as seismites with associated dewatering. Some units of the Gog Quartzite are heavily bioturbated with burrows including Arenicolites and Skolithos as well as other marine traces such as trilobite related ichnofacies.The Cambrian Deadwood Formation is also a target for carbon dioxide sequestration and provides the main reservoir for the Aquistore Project in Saskatchewan. It has been targeted by exploration wells in eastern Alberta and a new core study has demonstrated its suitability as a CO2 reservoir. Facies ranging from regolith overlying igneous basement, through alluvial fan and tidal deposits, and out into shallow marine settings with well developed parasequences, are all seen in the cores.The outcrop and core studies provide new data on Cambrian reservoirs which can be applied directly to potential Cambrian CCS targets in the subsurface of Alberta. The data can also be used to identify other potential Cambrian CO2 sinks around the world.

Depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the Upper Cretaceous Shendi Formation in Musawwarat-Naga area, Shendi-Atbara Basin, Sudan

This study examines both vertical and lateral facies changes as well as petrographic characteristics of the outcropping section of the Upper Cretaceous Shendi Formation in the Musawwarat-Naga area with a view of interpreting the depositional environment, provenance, and tectonic setting that prevailed during the deposition of the sediments. Field studies revealed the presence of eight different sedimentary lithofacies. The predominant is the trough cross-bedded sandstone facies (St), representing 75% of the total succession. Four architectural elements were recognized: sandy bedforms (SB), channel fill (CH), gravelly bars and bedforms (GB), and overbank fines (OF). Moreover, the mean value of paleocurrent directions was 285.3°, suggesting a southeasterly located source area. The majority of the sandstones were classified as quartz arenites with subordinate sublithic-arenites. Ternary plots of quartz-feldspar-lithic fragments (QFL) revealed that the tectonic setting is predominantly within the interior craton. It can be suggested that the sediments were sourced from the southeast region and associated with a braided, proximal, fluvial depositional environment. The results of this study provide outcrop analog data that might be useful in exploring hydrocarbons. Specifically, it can enhance the understanding and prediction of the proximal fluvial depositional styles in the subsurface deposits in the Shendi-Atbara Basin and similar rift basins in Sudan and Africa.

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

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

Sedimentary Facies, Architectural Elements, and Depositional Environments of the Maastrichtian Pab Formation in the Rakhi Gorge, Eastern Sulaiman Ranges, Pakistan

An integrated study of sediments was conducted to examine the facies architecture and depositional environment of the Cretaceous Pab Formation, Rakhi Gorge, and Suleiman Ranges, Pakistan. This research focused on analyzing architectural elements and facies, which are not commonly studied in sedimentary basins in Pakistan. To identify lithofacies, outcrop analysis and section measurement were performed. The identified lithofacies were then categorized based on their depositional characteristics and facies associations, with a total of nine types identified within a stratigraphic thickness of approximately 480 m. These facies were mainly indicative of high-energy environments, although the specifics varied by location. Sedimentary structures such as planar and trough crossbedding, lamination, nodularity, load-casts, and fossil traces were found within these facies, indicating high-energy environments with a few exceptions in calm environments. The identified facies were grouped into seven architectural elements according to their depositional environments: delta-dominated elements, including laminated shale sheet elements (LS), fine sandstone elements (SF), planar cross-bedded sandstone elements (SCp), trace sandstone elements (ST), and paleosol elements (Pa); and river-dominated elements, including trough cross-bedded sandstone elements (SCt), channel deposit elements (CH), and paleosol elements (Pa). These architectural elements, along with their vertical and lateral relationships, indicate a transitional fluvio-deltaic environment within the Pab Formation. In conclusion, by interpreting facies and architectural elements, it is possible to gain a better understanding of the depositional history of the formation and the distribution of reservoir units.

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

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

Fluvial architecture of the Upper Cretaceous Nubia Sandstones: An ancient example of sandy braided rivers in central Eastern Desert, Egypt

The present paper focuses on the fluvial architecture and depositional facies model of the Late Cretaceous large sandy braided-river system of the Nubia Formation in the central Eastern Desert of Egypt. Six lithofacies types and two paleosol pedotypes (entisol and vertisol) are arranged in channel and overbank architectural elements that display first- to fifth-order bounding surfaces. Two distinctive fluvial architectural styles are recognized. The Fluvial architecture of the lower member identifies a low sinuous and energetic sandy braided river, in which the channels cover the entire of fluvial landforms with limited overbank areas. The suggested depositional model resembles the classic Platte-type braided river with its broad, low sinuosity streams, rich sand bedload and minor overbank sediments. Recurrent autogenic fining-upward cycles, rarely preserved overbank deposits, dominant entisol pedotype, prevailing trough cross-bedded sandstones, sporadic to absent planar cross-bedded sandstone, and dominant downstream accretion also imply perennial, sandy braided rivers. By contrast, the fluvial architecture of the upper member is characterized by fining-upward cycles, downstream and lateral accretion elements, and commonly preserved overbank deposits (overprinted by the mature vertisol pedotype). This architecture style is ascribed to a perennial sandy braided river characterized by sinuous energetic streams and laterally developed overbank areas, similar to the modern South Saskatchewan River in Canada. The fluvial depositional system and sequences in the Nubia Formation are strongly controlled by Late Cretaceous base-level fluctuations and tectonic pulses of the Laramide Orogeny.

Gariwerdichnus warreni igen. et isp. nov. — probable giant myriapod burrows from Late Silurian fluvial channels in the Grampians Group, Western Victoria, Australia

Large Late Silurian burrows are preserved in fluviatile trough cross-bedded sandstones at Mount Bepcha and Mount William in the Grampians in western Victoria (Australia). These burrows have ventral surfaces with two parallel grooves likely formed by the trace-makers’ paired legs as they burrowed through dunes on braided-river beds, probably after avulsion of sandy channels. The burrows are morphologically distinct from previously described burrows and a new ichnogenus Gariwerdichnus igen. nov. containing a single new ichnospecies G. warreni isp. nov. is erected. The size of the burrows, together with their ridged ventral surface, rounded dorsal surface with large dorsolateral protuberances, and meniscate backfill, point towards giant myriapods as the tracemakers. This conclusion is further supported by the presence of large Diplichnites isp. (including D. gouldi) trackways of comparable size in a stratigraphically higher unit at Mount Bepcha; large D. gouldi are typically attributed to giant myriapods. Early myriapod groups that could have formed G. warreni include eoarthropleurids, arthropleurids, or a Bennettarthra-like organism. Chris Gouramanis [ chris.gouramanis@anu.edu.au ], Research School of Earth Sciences, The Australian National University, Australian Capital Territory, 0200, Australia; Anthony, J. Martin [ geoam@emory.edu ], Department of Environmental Sciences, Emory University, Atlanta, Georgia, 30322, USA; John A. Webb [ john.webb@latrobe.edu.au ], Department of Ecology, Environment and Evolution, La Trobe University, Bundoora, Victoria, 3086, Australia.

Causes and consequences of Oligocene-Miocene relative sea level change in Asia: An example from the Labuan Island (Sabah), Malaysia

Belait Formation: (A) Trough cross-bedded sandstone (fluvial) (B) Interbedded sandstone-mudstone (sandstone dominated) facies overlying mudstone facies (marine). • The Miocene Belait Formation in the Labuan Island formed due to forced regression. • The upper part of the Belait Formation indicates transgression and marine flooding. • The depositional environment changed during the Oligocene-Miocene in Sabah and Sarawak. • This was linked to regional tectonics and/or global relative sea level. • No record of paleoclimate shift like central Asia is inferred in this study. Labuan Island represents the emergent part of a north-eastward plunging anticline. The stratigraphic succession at Labuan is made up of four formations: the Crocker Formation, the Temburong Formation, the Setap shale Formation and the Belait Formation. The sedimentological aspects of the Labuan stratigraphic succession have been studied by several authors but the stratigraphic relationships between the formations is still a matter of conjecture. In this paper we have undertaken an outcrop based (1) sedimentary facies analysis of the Middle Miocene Belait Formation and (2) inferred the mode of stratigraphic sequence building of the Oligocene-Miocene successions. In significant contrast to the tide-storm interactive shallow-marine Belait Formation of Brunei and Sarawak, the Belait Formation of the Labuan island is made up of a lower fluvial and an upper shallow marginal marine to shelf facies association. The sedimentary succession from the Crocker, Temburong, Setap Shale and Belait formations represents a marine regression either due to a relative sea-level fall (forced regression) or to sediment supply exceeding the rate of sea-level rise (normal regression). The ~17 million years unconformity separating the fluvial facies association of the Miocene Belait Formation from the underlying shelfal sedimentary units (Oligocene Temburang and/or Setap shale formations) is a subaerial unconformity that is a consequence of rapid uplift and associated forced regression, and thus indicates the influence of regional tectonics on the mode of stratigraphic sequence building The change in paleocurrent pattern recorded by earlier researchers across the Oligocene-Miocene boundary also indicates tectonic tilt, and a change in sediment dispersal direction because of upliftment. The upper marginal marine facies association of the Belait Formation indicates transgression and the thick shelfal mudstone unit that overlies the marginal marine deposit indicates subsequent maximum flooding. Our sedimentological and stratigraphic analyses reveal that the shift in depositional environment during the Oligocene-Miocene transition is linked to regional tectonics and consequent sea level change and unrelated to paleoclimatic shift.

Petrographic and diagenetic study of siliciclastic Jurassic sediments from the northeastern margin of Africa: Implication for reservoir quality

Northeastern Egypt is one of the most important exposures in the Middle East and North Africa to study Jurassic facies. The present study analyzes 130 thin sections from a subsurface well (Well X) in the northern Gulf of Suez, and from two surface sections at Gebel Maghara (north Sinai) and Khashm Elgalala (North Eastern Desert), Egypt for petrographic and lithofacies analyses. In the present study, detailed petrographic analysis is used as a tool to better understand the diagenetic history of the Early to Late Jurassic siliciclastic sediments; the identified diagenetic elements are inferred concerning the reservoir quality. For this, sidewall cores, scanning electron microscopy and thin sections are used to detail detrital and authigenic mineralogy; these are then used to infer the depositional framework, factors controlling reservoir characteristics, and the operating diagenetic processes. The inferred depositional paleoenvironment is a prograding and retrograding linear siliciclastic shoreline within a shallow marine carbonate platform with coal swamps and occasional cross-cutting rivers. Diagenesis, petrographical characteristics and depositional conditions are the main factors controlling continental and marine reservoir architecture. The diagenetic processes affecting siliciclastic sedimentation are near the surface, shallow to intermediate burial, and deep burial cycles with different pore fluid filling at each stage. The siliciclastic sediments have been categorized into seven depositional lithofacies - calcareous claystone (S1a), carbonaceous claystone (S1b), siltstone (S2), planar and trough cross-bedded sandstone (S3), coarse, well rounded, large scale trough cross-bedded sandstone (S4), ooid sandstone (S5) and coal (C). Grain compaction, various phases of syntaxial quartz overgrowth, carbonate cementation and replacement, creation of dissolution porosity, and clay authigenesis are the most important diagenetic processes that have affected the siliciclastic continental and marine sediments. Many different diagenetic events, both destructive and constructive, have modified porosity. The destructive events include compaction and cementation (of silica, kaolinite, ferroan dolomite, Illite, and anhydride). The constructive events include silica cementation, feldspar dissolution, and ferroan dolomite dissolution. Additionally, present data also suggest that the continental sandstones have excellent reservoir quality whereas the marine sandstones have good to very good reservoir potential. Despite the excellent reservoir quality of the continental sandstone lithofacies, the effective stratigraphic seal is leaking. The interbedded calcareous and carbonaceous claystone lithofacies may provide an excellent stratigraphic seal for these siliciclastic reservoirs. Based on these analyses, a model is proposed that can be used as a template for subsurface Jurassic reservoir characterization and reservoir discrimination.

Stratigraphic Analysis of Gercus Formation in Dohuk area, Northern Iraq

A surface section of the Gercus Formation (Middle-Late Eocene) was studied in Berafat area, Dohuk Governorate, Northern Iraq. The Gercus Formation consists of a mixed siliciclastic sediments, evaporates and carbonate sequences in the studied region, predominantly in the upper and middle parts. Nevertheless, it usually consists of upward-fining carbonate-rich sandstone cyclothems, marl, conglomerate and siltstone along with a gypsum lens and thin micrite carbonate beds. The Gercus Formation was deposited in delta and delta front of occasionally depositional environment which is represented by red-brown claystone and reddish-brown mudstone lithofacies. Cross bedded pebbly sandstone, trough cross-bedded sandstone and laminated cross-bedded sandstone lithofacies are deposited in braided delta environment. Marl lithofacies and gypsum lithofacies are deposited in intertidal and supratidal environments. The sea level fluctuation caused the river base level to occasionally rise and fall. In addition, the process changed from erosion to deposition, while the grain size also changed at different environments from gravel to sand and clays. The Gercus succession of northern Iraq was developed during the Middle-Late Eocene in an active margin basin, where the last stage closure of the New-Tethys and its collision with the Eurasian plate took place between the northeastern Arabian plate. It caused major episodes of uplifts and subsidence along with base level variations due to eustatic ups and downs. Within Gercus Formation, several fourth order cycles can be recognized, reflecting generally asymmetrical cycles, as well as the difference between sediment supply and accommodation space.

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

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

Sedimentary model of K-Successions Sandstones in H21 Area of Huizhou Depression, Pearl River Mouth Basin, South China Sea

AbstractThe nearshore sand bar-sheet sedimentary system of the K-Successions sandstones of the Zhujiang Formation (23.8-16.5 Ma) in H21 Area of Huizhou Depression, Pearl River Mouth Basin has been identified in this research according to lithological characteristics, lithofacies, sedimentary sequence and seismic attributes. Seven lithofacies were recognized: fine-grained massive sandstone (Sm), fine-grained trough cross-bedded sandstone (St), fine-grained planar-bedded sandstone (Sp), fine-grained ripple cross-bedded sandstone (Sr), fine grained horizontally-bedded sandstone (Sh), laminated claystone with interbedded siltstone (Fl) and massive mudstone (Fm). Three sedimentary microfacies were further divided: sand bar, sand sheet and interbar. With the progradation and retrogradation process influenced by sea level changing, delta evolution of K-Successions sandstones in H21 area of Huizhou Depression can be divided into four stages: the early stage of the transgressive systems tract, A/S>1; the medium stage of the transgressive systems tract, A/S>1; the end stage of transgressive systems tract; the stage of the highstand systems tract, A/S<1. Different evolution stages controlled the sandstones distribution.

Mesoproterozoic erg and sand sheet system: Architecture and controlling factors (Galho do Miguel Formation, SE Brazil)

Aeolian systems of the Precambrian time were established in different palaeoclimatic and palaeogeographic conditions than the present. Different morphologies of dunes/draas, as linear, oblique and star forms, are widespread in the recent aeolian systems. However, the ancient aeolian deposits are rarely interpreted as these types of bedforms and usually referred to as transverse dunes. The nature and complexity of bedform morphology in Precambrian aeolian systems, as well as the factors that allow the construction, accumulation and preservation of these environments are still not well defined. Extensive exposures of Galho do Miguel Formation in southeastern Brazil represent a Mesoproterozoic Erg succession and exhibit a distinct facies architecture, which contributed to the understanding of these deposits. Three architectural elements were identified: (i) tangential cross-stratified sandstone with superimposed trough cross-bedded sandstone, (ii) planar-laminated sandstone, and (iii) extensive flat-bedded sandstone dominated by wrinkly laminae. These elements are organized into three succession groups representative of simple transverse dune, oblique compound draa and sand sheet systems. The simple to compound dune/draa system corresponds to the most central parts of the erg, associated with a higher sediment availability and lower water-table level. Conversely, the sand sheet system occurs under low sediment availability and higher water-table level in the marginal portions of the erg. Precambrian climatic oscillations may have changed the aeolian system morphology, allowing the alternation of simple and complex aeolian dunes, resulting in high-frequency changes in sand sheet system, and the remobilization of previously accumulated sediments over large geographic areas.

Geomorphology and clast assemblages of intertidal beachrock: Implications for submerged shoreline preservation

Beachrock may be considered a reliable indicator of sea level, in addition to a reliable and recognizable source for onshore movement of beachrock clasts by overwash processes associated with extreme events. Key to this is its geomorphology and preservation potential, both of which are poorly understood. Here we examine the processes and deposits associated with beachrock breakdown from Camurupim, NE Brazil and compare its preservation to beachrocks found on continental shelves in South Africa. We study a prominent and continuous 50–60 m-wide beachrock complex that crops out along a 10 km stretch of coastline, and which mostly comprises 1.5 to 2 m-thick trough-crossbedded sandstones. These have generated large joint-bounded blocks that have been transported landwards under extreme energy conditions (based on numerical modelling and comparisons to the contemporary wave climate). Tabular clasts have accumulated as isolated, very coarse boulders on the seaward margin, small clasts in various topographic traps, boulder ridges with large landward- and seaward-dipping clasts on the uppermost part of the outcrop, and subaqueous and subaerial washover fans. Isolated fine meso-boulders emplaced during extreme waves (Hs = ca. 12 m) exhibit fitted textures and pressure-solution contacts, pointing to subsequent rocking and shaking. Smaller clasts in boulder ridges and overwash settings are initially liberated and transported by waves within the annual-decadal frequency spectrum, deposited on the high points of the outcrop. Subsequent movement during less extreme wave action progressively transports the clasts across the surface, where they are often trapped in the fitted fabric on top of the platform. Our breakdown model and overall beachrock preservation scheme is consistent with reported seismic signatures of submerged Holocene beachrock shorelines. Beachrock preservation during transgression is strongly influenced by the original sedimentary facies; thick, trough-crossbedded joint-bound clasts preserve the former shoreline. Clasts from the planar units accumulate as washover fans to landwards of the ridge.

Lithofacies, architectural elements and tectonic provenance of Mio-Pliocene Dupitila Formation in the Cachar Thrust Fold Belt, Northeast India

The post-orogenic terrestrial accumulations of Dupitila Formation (Mio-Pliocene) have been studied for the first time in the Cachar Thrust Fold Belt of Northeast India employing facies and architectural element analysis techniques. Six litho-columns measuring 85 m in thickness were analyzed at the outcrop level with minimum resolutions of 20–30 cm. Seven lithofacies, namely clast-supported horizontally stratified gravel (Gh), matrix-supported planar cross-bedded gravel (Gp), trough cross-bedded sandstone (St), planar cross-bedded sandstone (Sp), horizontally bedded sandstone (Sh), laminated sand–silt–mud (Fl) and massive mud (Fm), were identified. The two lithofacies assemblages, i.e., interbedded gravel–sand and sand–mud associations, characterize the lower and upper divisions of Dupitila Formation, respectively. Identification of five architectural elements, i.e., channel fill complex (CHm), single channel (CHs), gravel bar and bedform (GB), sand bed form (SB) and floodplain fines (FF), was accomplished through lithofacies distribution and arrangement of bounding surfaces. Alternating gravelly and sandy facies succeeded by floodplain fines present an overall thinning and fining upward cycle resembling the gravelly and sandy braided fluvial stream deposits. A semi-humid climatic condition has been envisaged for the formation of lithic to sub-lithic (Q66 F9 R25) Dupitila sandstone. These deposits have suffered early as well as late-stage diagenetic changes. The diversified detritus delivery to the fluviatile Dupitila basin was largely made by a recycled orogen comprising of Shillong massif, eastern Himalaya and the Indo-Burman Range.

Facies analysis and depositional environment of the Campano – Maastrichtian coal-bearing Mamu Formation in the Anambra Basin, Nigeria

Detailed sedimentological logging was carried out within the exposed rock sections of the late Campanian to early Maastrichtian successions of the coal-bearing Mamu Formation, southeastern province of Nigeria. Lithofacies analytical approach was employed in describing the outcropping rock units. The focus was to understand the distribution of these facies and their depositional environments. Eleven lithofacies recognized were found occurring within fining/thinning upward successions typical of retrogradational stacking patterns formed during transgressive episodes and coarsening/thickening succession typical of progradational stacking patterns formed during regressive episodes. These lithofacies were further classified into five facies associations, namely: lagoonal or swamp deposits (FA1); barrier island deposits (FA2); shoreface deposits (FA3); offshore-transition zone deposits (FA4) and; open shelf deposits (FA5). FA1 is dominantly coal and carbonaceous shale deposited in a swamp or lagoonal setting. FA2 is dominantly parallel laminated and cross-bedded fine to medium grained sandstones formed by wave and longshore currents within barrier island complexes. FA3 is dominantly bioturbated sandstone, planar and rippled laminated sandstones, and planar and trough cross-bedded sandstones facies deposited in a shoreface environment, by migrating dunes formed under unidirectional flows above fair-weather wave base. FA4 is mainly wave ripple laminated, hummocky and swaley cross-bedded sandstones deposited by waning oscillatory and combined flows within a storm wave-dominated shelf region, below fair-weather wave base. FA5 is fissile shales and mudstones deposited in an open shelf setting below fair-weather and storm wave base. This study indicates that outcropping units represent lithofacies, which record depositional elements of high frequency transgressive and regressive events in a coastal swamp through shelfal depositional environment. In addition, the coal seams and carbonaceous shale facies (source/seal rocks), and the sandstones, siltstone and heterolithic facies (reservoir rocks) indicate the possible presence of key elements of petroleum system in the Mamu Formation of the Anambra Basin.

Identification and characterization of low resistivity low contrast zones in a clastic outcrop from Sarawak, Malaysia

Zones with low resistivity low contrast (LRLC) are usually overlooked due to the absence of high-end well logging techniques in the older oil/gas fields. The Nyalau Formation outcrop (stratigraphically equivalent to offshore cycles I and II) from Sarawak, offers a good opportunity to identify and characterize LRLC zones through integrated approaches. Seven facies of the Nyalau Formation had been identified, which are: 1) low angle cross-bedded sandstone (CBSS), 2) laminated sandstone (LSS), 3) heterolithic sandstone (HSS), 4) bioturbated sandstone (BSS), 5) clay-rich sandstone (CSS), 6) mudstone (MS), as well as 7) trough CBSS (TCBSS). Apparent resistivity was determined for brine (60 kpm) saturated core plugs at a temperature of 75 °F by employing the sample core induced polarization technique. The measurement of resistivity was calibrated based on offshore salinity, as well as surface and subsurface temperature of the Sarawak. The geological and petrophysical characteristics were analyzed through field observations and lab analyses. The findings of the study revealed that the cutoff value of resistivity found at LRLC zones in Nyalau Formation was 12.6 Ω-m. Accordingly; LSS, BSS, and CSS facies appeared to derive from LRLC zones. The main causes that generate such LRLC zones are laminations, thin bedding, bioturbation, and clay minerals. The dominant effect portrayed by mixed layers of illite/smectite and smectite in LRLC zones could be attributed to their high cation exchange capacity (CEC). Two varying resistivity trends were discovered based on the variances detected in clay types for different facies. A negative correlation was found between resistivity and porosity within high resistivity zones, while a positive relationship within the LRLC zones. Disparity in the resistivity-porosity correlation is deduced to be associated with clay mineralogy. This study suggests that LRLC zones are expected to be present in shallow marine clastic reservoirs of offshore cycles (I and II), where the offshore reflects Nyalau Formation.

Oligocene storm-induced lacustrine deposits in the Yaxi Area of the Jiuxi Basin, northeastern margin of the Tibetan Plateau

Storm-induced deposits in the sedimentary record can now be easily identified on account of a better understanding of processes of unidirectional and oscillatory flow. Thin-bedded sandstone, siltstone, and interlayered mudstone deposits of the Oligocene Jianquanzi Member in the Yaxi Area of the Jiuxi Basin along the northeastern margin of the Tibetan Plateau were formed by shallow lacustrine storm-generated flows. Petrologic analysis reveals that the sandstones exhibit moderate to high compositional maturity but are texturally sub-mature to immature. Based on investigation of drill cores, six storm-deposit lithofacies are identified, including massive or graded sandstone, parallel and trough cross-bedded sandstones, hummocky cross-stratified sandstone, wave-rippled sandstone and siltstone, massive or horizontal bedded mudstone, and structurally deformed sandstone and mudstone. In a vertical succession, these lithofacies occur as three fining-upward lithofacies associations, each of which represents different architectural elements (and depositional processes) of proximal to distal storm deposits. The relatively coarse proximal architectural element is formed by strong density flows and unidirectional-dominated combined flows; the transitional architectural element is dominated by unidirectional- and oscillatory-dominated combined flows to pure oscillatory flows, resembling Tb-e divisions of the classic Bouma sequence; the relatively fine distal architectural element is dominated by unidirectional-dominated combined flows with a very weak oscillatory flow component. The origin of storms responsible for these deposits is attributed to Asian winter monsoons associated with the paleoclimatic and paleogeographic setting.