Planar Cross-bedded Sandstone Research Articles

Tidal-influenced shoreline interpretation of Miocene Tanjong Formation in Imbak Canyon, Sabah, Malaysia

The Tanjong Formation of Imbak Canyon, Sabah is the Early Miocene aged shallow marine unit that crops out in the Tongod district. Tanjong Formation has previously been interpreted to have been deposited in a shallow marine environment without tidal influence. This paper reinterpreted the depositional environment with emphasis on tidal deposits through newly logged sections encountered during 2019 Imbak Canyon Expedition. Proximity-driven strike orientation correlations were employed to interrelate the logged geological sections. The eight facies recognized are 1) Mudstone Facies (M), 2) Lenticular Facies (LF), 3) Wavy Facies (WF), 4) Flaser Facies (FF), 5) Hummocky Cross-Bedded Sandstone Facies (HCS), 6) Swaley Cross-Bedded Sandstone Facies (SCS), 7) Planar Cross-Bedded Sandstone Facies (PCS), and 8) Structureless Sandstone Facies (SS). Three interpreted facies associations deduced are 1) intertidal deposits, 2) middle to lower shoreface, and 3) offshore deposits. We conclude that Tanjong Formation was deposited in a macrotidal open coast setting with significant tidal imprint signatures. The identification of tidal influence within a wave-dominated system is an infrequent occurrence, yet it holds significant potential for enhancing the refinement of shallow marine models.

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

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

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

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

Facies analysis, petrography and textural characteristics of the onshore Paleogene-Neogene Lawin Basin, Perak, Peninsular Malaysia: Insights into palaeodepositional environment and provenance

Sedimentological features of the onshore Paleogene-Neogene Lawin Basin are of importance in better understanding the basin evolution and geodynamics. The N-S trending Lawin Basin, which is situated within the Western Belt of Peninsular Malaysia, is an extensional basin that formed during the sinistral movement of the Bok Bak Fault zone. This study evaluates the palaeodepositional environment, reveals the provenance, and interprets the transport dynamics of the mini-basin by integrating facies, petrographic, and grain size analyses. The basin lithologies can be broadly divided into conglomerate and sandy facies, with the former consisting of clast-supported conglomerate, matrix-supported conglomerate, cross-stratified pebble conglomerate, and bedded pebbly conglomerate, whereas the latter comprises massive pebbly sandstone, horizontally bedded sandstone, planar cross-bedded sandstone, and ripple laminated sandstone. The lithologies can be grouped into three main facies associations, including massive conglomerate, stratified conglomerate, and fluvial sandstone deposits, which reflect alluvial fan (debris/gravity flow deposits), braided gravel bar, and channel bar elements, respectively. Petrographic data indicate that the conglomerates are clast- and matrix-supported and were derived from a proximal source. The QFL range of detrital framework grains reveals that sandstones are composed of quartz (43–80 vol%), feldspar (9–54 vol%), and lithic fragments (2–16 vol%). The sandstones are coarse-grained, poorly-sorted, and sub-rounded to sub-angular in texture. They are arkose, sublitharenite, and lithic arkose in composition. QtFL/QmFLt ternary diagrams show that the sandstones are primarily derived from a provenance area comprising continental block material, including uplifted basement rocks and transitional continental deposits. This study proposes that the sandstones had a nearby granitic source (Bintang) and a debris source derived from recycled sedimentary rocks (the Baling Group sediments). Grain size parameters ranges (mean, −2.37 to 0.74 Φ; sorting, 1.22 to 2.07 Φ; skewness −0.21 to 0.91 Φ, kurtosis, 0.66 to 1.98 Φ, and average mean-sorting index of −0.24) are consistent with immature to sub-mature sedimentation deposited within a high-energy fluvial environment.

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.

Depositional facies, sequence stratigraphy and reservoir potential of the Eocene Nanka Formation of the Ameki Group in Agu-Awka and Umunya, southeast Nigeria

A sedimentological investigation was carried out in Agu-Awka and Umunya localities to interpret the facies, depositional environment, sequence stratigraphy, reservoir potential, and architecture of the outcropping Eocene Nanka Formation of the Ameki Group in southeast Nigeria. Petrographic analysis reveals that the sediments are composed of predominantly subangular to subrounded recycled quartz grains with a minor amount of rock fragments. It indicates that the sediments are texturally submature and mineralogically mature. Lithofacies analysis indicates that the formation is composed of fine to coarse-grained, trough and planar cross-bedded sandstone with clay-drape units, interpreted to be a subtidal bar facies, and a minor mudstone interval with sand to muddy heterolith interpreted to be a tidal mudflat facies. This suggests that the formation is of mixed environments, such as a tidally influenced high-energy fluvial and low-energy marsh to lagoonal settings. The lower sand unit of the subtidal bar facies (fine to coarse-grained cross-bedded sandstone with clay-drape) represents a highstand systems tract. The sharp base sandy heterolith lithofacies that forms the upper unit of the subtidal facies, and the overlying tidal mudflat facies that comprises a bedded mudstone and overlying wave rippled sandy heterolith intervals represent a transgressive systems tract. The reservoir quality of the sandstone facies is predicted to be moderate to good based on textural statistical analysis. The various relationships of the sedimentological parameters of the outcrops revealed by this study may assist and contribute to the prediction and understanding of the reservoir stratigraphic heterogeneity and properties of the subsurface depositional facies of the Nanka Formation.

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

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

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

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

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.

Storm and tide influenced depositional architecture of the Pliocene–Pleistocene Chad Formation, Chad Basin (Bornu Sub–basin) NE Nigeria: A mixed fluvial, deltaic, shoreface and lacustrine complex

Lithofacies analyses of the upper part of the Chad Formation (Bama Ridge Complex) in the Bornu Sub–basin of the Chad Basin indicated four facies associations; fluvial, deltaic, shoreface and lacustrine sequences. The fluvial sequences are composed of fining upward cycles with successive occurrence of planar crossbedded sandstone facies displaying unimodal paleocurrent system and rare mudstone facies typical of braided river system. The deltaic succession consists of both fining and coarsening upwards cycles with the former depicting fluvial setting of an upper delta plain while the later suggestive of mouth–bar sequences. The setting displays a polymodal current system of fluvial, waves, storms and tides that were primarily induced by complex interactions of seiches and lunar tides. Similar current systems devoid of fluvial patterns were reflected in the coarsening upward packages of the shoreface sequences. Lacustrine succession composed of thick bioturbated mudstone facies generally defines the base of these coarsening upward profiles, giving a fluvio–lacustrine geomorphic relief where complex interaction developed the deltaic and shoreface facies along its shorelines. Clay mineral fractions of the formation are dominantly kaolinitic, indicating a predominantly humid tropical–subtropical climatic condition during their deposition. This climatic regime falls within the African humid period of the early–mid Holocene that led to the third lacustrine transgression of the Lake Mega-Chad, whereas the subordinate smectite mineralization points to aridification that characterizes most of the post humid period to recent.

Depositional environment of the Fort Member of the Jurasic Jaisalmer Formation, (western Rajasthan,.

Lithofacies and granulometric analysis were carried out to decipher the depositional environment of the Fort Member of the Jurassic Jaisalmer Formation. Based on field data nine lithofacies have been identified including trough cross-bedded sandstones, planar cross-bedded sandstones, matrix supported conglomerates, thinly bedded siltstone and sandstones, herringbone cross-bedded sandstones, wave rippled sandstones, laminated sandstones, hummocky cross-bedded sandstones, limestones and shales. Granulometric analysis of the sandstones samples has been carried out for their statistical and textural parameters. Bivariant plots of textural parameters such as graphic skewness versus graphic standard deviation and skewness versus standard deviation confirm the high energy (beach) origin of sandstones. These results suggest a wide spectrum of marine environments ranging from inner shelf to upper shoreface for the Fort Member sandstones.

Structures and sequences in early Permian fluvial Barakar rocks of peninsular India Gondwana basins using binomial and Markov chain analysis

Cyclic characteristics of bed forms (sedimentary structures) of early Permian fluvial Barakar rocks are studied statistically, using quasi-independence Markov chain and entropy. The hierarchy of sedimentary structures confirms that the corresponding bed forms do not represent random depositional event and follow a definite pattern of Markovian mechanism in a predictable cyclic arrangement. The preferential upward transition of sedimentary structures that can be derived for the Barakar sandstone is scoured surface (Ss) → horizontal bedding (Sh) → planar cross-bedded sandstone (Sp) → trough cross-bedded sandstone (St) → ripple cross-lamination (Sr) → parallel lamination (Fl) → scoured surface (Ss). The sequence is an asymmetrical cycle and characteristic of the lateral accretion and aggradations of fluvial channels. Entropy analysis corroborate the above inference, and E (pre) vs E(post) plots for each sedimentary structure correspond to the type B category, suggesting lower and upper truncated asymmetrical cycles. Hydro-dynamically, the Barakar cycles represent a vertical sequence of bedding types which implies a steady upward decrease in the intensity of flow—from the upper flow regime in the lower part, the lower flow regime of moderate to high intensity in the middle part, and the lower flow regime of low to very low intensity in the upper part.

Sedimentary and petrofacies analyses of the Amasiri Sandstone, southern Benue Trough, Nigeria: Implications for depositional environment and tectonic provenance

This study was undertaken to determine the depositional environment, provenance and tectonic setting for the Turonian Amasiri Sandstone, southern Benue Trough, Nigeria, using lithofacies analysis and re-appraisal of petrography of the sandstones. Local stratigraphy and field relationships show a thick succession of shales alternating with elongate/parallel sandstone ridges extending eastwards from Akpoha to Amasiri through Itigidi and Ugep to Apiapum areas. Lithofacies analysis reveals 9 lithofacies suggestive of storm (mass flow) and tidal shelf processes. These include dark grey to black laminated shale/silty mudstones, bioturbated mudstones, coquinoid limestones, very fine-grained bioturbated sandstones with shell hash/debris in places and limestone rip-up clasts, massive and chaotic sandy conglomerate with rip – up clasts, fine to medium-grained, parallel laminated sandstone, hummocky cross-stratified, massive, medium to coarse-grained sandstones, medium to very coarse-grained, planar cross-bedded sandstone, with clay-draped foresets and Ophiomorpha burrows, and coarse-grained trough cross-bedded sandstone. Petrofacies analysis identifies the sandstones as feldspathic and arkosic arenites. Ternary plot of framework mineralogy indicates derivation from an uplifted continental block related to the nearby Oban Massif and Cameroon Basement Complex.

A new Cenomanian vertebrate tracksite at Tamajón (Guadalajara, Spain): Palaeoichnology and palaeoenvironmental implications

A new Upper Cretaceous vertebrate tracksite has been discovered at Tamajón (Iberian Ranges, Guadalajara, Spain). The track level is a relatively smooth and slightly undulating sandy ferruginous crust, corresponding to an erosive surface at the base of a small meandering channel. It is incised into the underlying planar cross-bedded sandstones of coastal bars located at the middle-upper part of the Utrillas Formation (middle-upper Cenomanian). The site shows an extraordinary concentration of vertebrate tracks, among which numerous sets of two to five isolated digit impressions (“swim tracks”) and, at least, two trackways referred to crocodyliforms, and a single tridactyl footprint probably produced by a theropod dinosaur can be recognized. There are also several long traces (epichnial grooves) revealing sharp direction changes (up to 90°) which seem to be fish fin traces (Undichna unisulca), although crocodyliforms (tail marks) and/or fish invertebrates cannot be rejected as possible tracemakers. Some crocodyliform tracks reveal a thin raised rim, due to the displacement of the sediment by the pressure produced by the feet. Several impressions are moderately deformed by small sediment slides, only preserving their deepest part (claw marks). This is clearly indicative of a soft substrate with a high degree of plasticity and water content at the time of the track registration. Nevertheless, the sediment was hard enough to preserve manus and pes print morphologies and also possible crocodyliform tail and/or fish fin traces. Small rhizoliths can also be recognized and may belong to herbaceous wetland vegetation. The morphology of the palaeochannel, the sedimentary context and the track preservation seem to indicate that the tracks were impressed in a shallow channel located near the coast, under wet conditions and in different moments of time. This discovery represents the first occurrence of vertebrate ichnites in the Utrillas Formation, a stratigraphic unit where osteological and ichnological remains are relatively scarce, and it confirms that some crocodyliforms lived in near coast channels during the deposition of this unit.

Siliciclastic deposit of the Nahr Umr Formation, sedimentological and depositional environment studies, central and southern Iraq

The depositional facies and environments were unraveling by studying 21 subsurface sections from ten oilfields in the central and southern Iraq and a large number of thin sections of the Nahr Umr (siliciclastic deposit) Formation (Albian). This formation is mainly composed of sandstone interlaminated with minor siltstone and shale, with occurrence of thin limestone beds. Nahr Umr Formation is subdivided into three lithostratigraphic units of variable thicknesses on the basis of lithological variations and log characters. Mineralogically and texturally, mature quartz arenite and sandstones are the common type of the Nahr Umr Formation. The sandstones are cemented by silica and calcite material and have had a complex digenetic history. Compaction, dissolution, and replacements are the main diagenetic processes. Prodelta, distal bar, distributary mouth bar, distributary channel, over bank, and tidal channel are the main depositional environments recognized for the Nahr Umr Formation, within the studied wells. This formation was deposited in shallow marine and fluvial–deltaic environments and exhibit progradational succession of facies. Eight sedimentary facies that have been identified in the Nahr Umr Formation include claystone lithofacies, claystone siltstone lithofacies, lenticular-bedded sandstone–mudstone lithofacies, wavy-bedded sandstone–mudstone lithofacies, flaser-bedded sandstone–mudstone lithofacies, parallel and cross lamination sandstone lithofacies, trough cross-bedded sandstone lithofacies, and planar cross-bedded sandstone lithofacies. The depositional model of the Nahr Umr Formation environment was built based on the lithofacies association concepts.

Análise faciológica da Formação Alter do Chão (Cretáceo, Bacia do Amazonas), próximo à cidade de Óbidos, Pará, Brasil

Cretaceous deposits of the Alter do Chao Formation, exposed on the banks of the Amazon river, near the town of Obidos - PA, record a succession of sandstones, conglomerates and mudstones. Seven lithofacies have been recognized which comprise massive conglomerate (Cm), trough cross-bedded sandstone (St), planar cross-bedded sandstone (Sp), climbing cross-laminated sandstone (Sl), massive mudstone (Mm), bioturbated mudstone (Mb) and deformed mudstone (Md). These facies are organized in fining-upward cycles, 1 to 6m thick, and have been grouped into two associations: channel fill and overbank deposits belonging to a fluvial meandering system. The channel-fill deposits include lenticular gravel bars composed of facies Cm and St, sandy bedforms with facies St, Sp and Sl, and lateral accretion bar, consisted of facies St, Sp and Mm, but with distinct migration with regard to that of the sandy bed forms. While paleocurrent measurements from facies St and Sp of sandy bedforms indicate a preferred orientation of paleoflow to SW and a subordinate flow to S, the lateral accretion bar migrates to E/ESE. The following overbank elements have been recognized based on their geometric relationships: muddy floodplain deposits including abandoned channel plug, crevasse splay channel filled with facies St and Sp and natural levee deposits composed of facies St, Sl and Mm. While the muddy channel plug overlies sandstones, the crevasse channel fill is in contact with mudstones of the floodplain and also cuts through fine-grained sandstones and mudstones interpreted as natural levee deposits. Adjacent muddy lenses in the same stratigraphic level on sandstone but with smaller width than that of the abandoned channel are attributed to swales, filled during floods, capping lateral accretion bar.

Análise faciológica da Formação Alter do Chão (Cretáceo, Bacia do Amazonas), próximo à cidade de Óbidos, Pará, Brasil

Cretaceous deposits of the Alter do Chão Formation, exposed on the banks of the Amazon river, near the town of Óbidos - PA, record a succession of sandstones, conglomerates and mudstones. Seven lithofacies have been recognized which comprise massive conglomerate (Cm), trough cross-bedded sandstone (St), planar cross-bedded sandstone (Sp), climbing cross-laminated sandstone (Sl), massive mudstone (Mm), bioturbated mudstone (Mb) and deformed mudstone (Md). These facies are organized in fining-upward cycles, 1 to 6m thick, and have been grouped into two associations: channel fill and overbank deposits belonging to a fluvial meandering system. The channel-fill deposits include lenticular gravel bars composed of facies Cm and St, sandy bedforms with facies St, Sp and Sl, and lateral accretion bar, consisted of facies St, Sp and Mm, but with distinct migration with regard to that of the sandy bed forms. While paleocurrent measurements from facies St and Sp of sandy bedforms indicate a preferred orientation of paleoflow to SW and a subordinate flow to S, the lateral accretion bar migrates to E/ESE. The following overbank elements have been recognized based on their geometric relationships: muddy floodplain deposits including abandoned channel plug, crevasse splay channel filled with facies St and Sp and natural levee deposits composed of facies St, Sl and Mm. While the muddy channel plug overlies sandstones, the crevasse channel fill is in contact with mudstones of the floodplain and also cuts through fine-grained sandstones and mudstones interpreted as natural levee deposits. Adjacent muddy lenses in the same stratigraphic level on sandstone but with smaller width than that of the abandoned channel are attributed to swales, filled during floods, capping lateral accretion bar.

보령지역 충남 분지 중생대 초기 하조층의 퇴적상 분석

충남 보령 지역 트라이아스기 후기 하조층은 충남분지의 최하위층으로서 하부는 주로 각력암이나 역암, 중부는 역암, 상부는 역암과 사암이 호층을 이루고 있다. 하조층은 입자지지 수평층리 역암, 입자지지 균질 각력암, 기질지지 균질 역암, 기질지지 점이층리 역암, 역질 사암, 평행 엽층리 사암, 균질 사암, 정상 점이층리 사암, 역전점이층리 사암, 판상 사층리 사암, 곡상 사층리 사암, 저경사 사층리 사암 그리고 이암의 13개의 암상으로 구성되고 이들은 4개의 상조합을 이룬다. 상조합 I은 테일러스나 상부 선상지 삼각주에서 퇴적된 것으로 보이는 입자지지 및 기질지지 균질 각력암으로 구성되어 있다. 상조합 II는 기질지지 균질 역암과 수평층리 입자지지 역암으로 구성되고 상부선상지 삼각주에서 퇴적된 것으로 보인다. 상조합 III은 기질지지 균질 역암, 평행엽층리 사암 및 균질 사암으로 구성되고 중부선상지 삼각주에서 퇴적된 것으로 보인다. 상조합 IV는 균질 역질 사암, 수평엽층리 사암과 균질 사암으로 구성되고 하부선상지 삼각주에서 퇴적된 것으로 보인다. 전체적으로 보아 하조층은 초기는 테일러스와 상부 선상지 삼각주 환경에서 퇴적되고, 중기는 상부 선상지 삼각주와 중부 선상지 삼각주가 교호한 환경에서 퇴적되었으며, 말기에는 중부 선상지 삼각주와 하부 선상지 삼각주가 교호하는 환경에서 퇴적된 것으로 해석된다. Facies analysis of the Late Triassic Hajo Formation, the lowest stratigraphic unit in the Chungnam Basin, shows that the lower part is composed mainly of breccias or conglomerates; the middle part, conglomerates; and the upper part, conglomerates and sandstones. The formation consists of 13 facies, which include horizontally stratified clastsupported conglomerate, clast-supported massive breccia, matrix-supported massive breccia or conglomerate, matrixsupported graded conglomerate, massive pebbly sandstone, horizontally laminated sandstone, massive sandstone, graded sandstone, inversely graded sandstone, planar cross-bedded sandstone, trough cross-bedded sandstone, low angle crossbedded sandstone, and massive mudstone. These are grouped into 4 facies associations (FA). FA I consisted of clastsupported and matrix-supported massive breccias presumably deposited in the talus or upper fan delta environment. FA II consists of matrix-supported massive conglomerate and horizontally stratified clast-supported conglomerate of cobble size and it seems to have been deposited in the upper fan delta environment. FAIII consisted of matrix-supported massive conglomerate of pebble size, horizontally laminated sandstone and massive sandstone may have been deposited in the middle fan delta environment. FAIV consists of massive pebbly sandstone, horizontally laminated sandstone and massive sandstone and presumably was deposited in the lower fan delta environment. In general the Hajo Formation is interpreted to have been deposited at the talus/upper fan delta environment in early stage; it might have been deposited in the alternating environments of upper and middle fan delta in middle stage; and it seems to have been deposited in alternating environments of middle and lower fan delta in late stage.