Planar Cross-bedding Research Articles

Early Late Triassic retro-foreland basin in response to flat subduction of the Paleo-Tethyan oceanic plate, SE Tibet

Syn-subduction basins bear significant implications to understand tectonic evolution of any fossil subduction zone. The late Paleozoic to early Mesozoic (Paleo-Tethyan) tectonics of the eastern and southeastern Tibetan Plateau (i.e., the Sanjiang Orogenic Belt) is featured by ocean-continent subduction systems. A huge pile of volcanic-absent sedimentary succession developed in the middle segment of the Sanjiang orogenic belt, its age and tectonic nature remain unclear. Detailed geological mapping and zircon U-Pb dating results demonstrate that the early Late Triassic volcanic-absent succession comprises the nonmarine Maichuqing Formation in the lower part and the shallow marine Sanhedong Formation in the upper part. The Maichuqing Formation consists of coarse to fine-grained sandstone, siltstone and mudstone with abundant basal erosional surfaces, trough and planar cross-beddings, ripples, mudcracks, and plant fragments. The Sanhedong Formation comprises predominantly bioclastic limestones interlayered with marl, calcareous-muddy siltstone, and calcareous sandstone with abundant bivalve fossils. Syn-sedimentation deformation structures, such as slump folds and associated normal faults are common, suggesting intense tectonism during deposition. Synthesizing sedimentary data, paleocurrent and provenance results, combined with other available data, demonstrate that the volcanic-absent succession deposited within a retro-foreland basin along the rear part of the Permian-Triassic Jomda-Weixi-Yunxian arc in response to flat-subduction of the Paleo-Tethyan Ocean during the early Late Triassic time.

X- Ray Diffraction Studies of Patti Claystone Member of the Campano-Maastrichtian Patti Formation, Southern Bida Basin, North Central Nigeria

X-ray diffraction and field studies were carried out on the Patti Clay Member of the Patti Formation in the southern Bida Basin. The studies involved field collection of samples and XRD laboratory analysis of clay samples in order to determine the dominant clay type, subordinate associated minerals, geologic processes and paleoenvironment of deposition. Field study involved standard litho-description by noting textural properties, facies type, fossil presence, structure and diagenetic effect. Laboratory XRD analysis procedure included sample grinding, XRD recording condition, selection and measurement of reference minerals, quantification of clay minerals using XRD processing software, XPert-Highscore Plus to generate results indicating clay mineral type and other minerals peaks. Lithostratigraphic section is dominated by the alternation of sandstone and claystone layers. The sedimentary structures are planar parallel and cross bedding in nature. The sandstone at Ahoko and along Lokoja-Abuja highway is characterized by liesegang ring and bioturbation structures. XRD results indicated that the Patti Clay Members are dominated by kaolinite with subordinate illite, montmorillonite and zincite in rare cases. The clay members were suggested to be deposited by moderate to low energy of transportation, moderate to low rate of deposition through prograding mechanism in a marginal marine setting. The kaolinite clay was formed by meteoric water reaction with the shallow-seated clay members through leaching of K+, Na+ and silica ions with resultant kaolinite stable at adequate pH that was not acidic.

The late Miocene eolian record at the eastern margin of the Puna Plateau, NW Argentina: Evidence of upper-tropospheric paleocirculation

ABSTRACT The Puna–Altiplano Plateau of the Central Andes is the second-highest plateau in the world (after Tibet), with a mean elevation of 4000 m.a.s.l. and an arid to hyperarid climate. Uplift of the Puna–Altiplano Plateau has affected lower-level atmospheric circulation, acting as a barrier to humid easterly winds from the Amazon basin and favoring an across-strike precipitation gradient resulting in a humid climate towards the east of the plateau and an arid to hyperarid climate in the orogen's interior. In the modern climate, the Bolivian High anticyclone regulates upper troposphere circulation, but little is known about the high-altitude tropospheric circulation of the past. This work focuses on the eolian record of the San Antonio de los Cobres basin along the eastern border of the Puna Plateau, NW Argentina, with the aim of analyzing its origin and thus elucidating the late Miocene winds. The eolian deposits are constrained by 7.8 Ma (K/Ar and U/Pb) and 6.4 Ma (U/Pb) ignimbrites at the nearly basal and upper contacts, respectively. Based on stratigraphic, sedimentological, and provenance analysis of the eolian units, we have identified three main facies associations (FAs): FA1) cross-stratified sandstones with large- to small-scale tabular, planar cross-bedding and with trough cross-stratification; FA2) sandstones with planar to low-angle stratification associated with thinly laminated ripple sandstone strata; FA3) medium- to coarse-grained massive sandstones associated with pebbly to bouldery, matrix-supported conglomerates and clast-supported conglomerates. The lateral and vertical facies assemblages indicate a dune field confined to topographic depressions dominated by transverse dunes with straight and sinuous crestlines that laterally grade into sandsheets associated with ephemeral streams. Paleoflows, lithotypes, and grain-size determinations indicate a persistent north-northwest provenance and wind velocities of 24–38 km/h (with maximum velocities of 55–75 km/h). The results of our analysis coupled with data from previous studies indicates that, for at least the last ca. 8 Myr, the winds have been blowing constantly from the north-northwest with an intensity similar to the present. This implies that the paleo-atmospheric circulation had a similar pattern to the present-day one. Therefore, we conclude that the upper-troposphere circulation in the Puna Plateau of NW Argentina was already regulated by the Bolivian High anticyclone during the Miocene, generating constant north-northwesterly winds.

Depositional and diagenetic controls on reservoir heterogeneity and quality of the Bhuban formation, Neogene Surma Group, Srikail Gas Field, Bengal Basin, Bangladesh

This study aims to investigate the controls on reservoir heterogeneity and quality of Miocene Bhuban Formation of the Neogene Surma Group at the Srikail Gas Field, Bengal Basin, Bangladesh. The study integrates wireline log data from two wells with core description as well as petrophysical and petrographic data obtained from 24 core plug samples retrieved from three gas bearing zones. The three zones, namely (from shallower to deeper): D-upper, D-lower, and E-sands, were identified from wireline log interpretation. Based on the extracted cores, the sandstones are classified into three main facies association: 1) coarse grained facies association (CFA) composed of sandstone with parallel lamination, trough, and planar cross bedding; 2) medium grained facies association (MFA) comprises of wavy and flaser bedded sandstone; and 3) fine grained facies association (FFA) consisting of siltstone and shaly sandstone. CFA sandstones, mostly abundant in D-lower and E-sands, were deposited in a high energy distributary channel environment with strong tidal influence while the finer-grained sandstones, mainly present in D-upper sand, were deposited in a low energy distributary channel. The results obtained by integrating core plugs and wireline logs show that best reservoir quality rocks are associated with CFA sandstones which are characterized by GR below 105 API, 15–25% porosity, and 30–100 mD permeability. In contrast, FFA sandstones have lower porosity (<10%) and higher shale content (GR > 125 API). The high energy and strong tidal influence on sandstones of CFA resulted in coarser grain size (and thus larger pores), better sorting and less clay content than the finer-grained, high clay bearing, low energy sediments of FFA. Additionally, petrographic and core plug analyses revealed that mechanical compaction is an important factor that reduces the porosity and permeability with increasing proportions of ductile mineral grains. Other less important diagenetic factors include quartz overgrowth and cement. The findings of this study show that the reservoir quality of the studied field is principally controlled by both depositional environment and mechanical compaction.

Lithofacies Association of Fluvial Deposit, Walat Formation in Cibadak Area West Java

Heterogeneities in fluvial sediment sufficiently vary characteristics based on the sedimentology aspect. The sedimentology analysis is the most important parameter to interpret fluvial architecture. Texture variations and sedimentary structures can define and describe where this sediment was deposited. Walat Formation is one of the fluvial deposit is situated in West java. Based on the observation locations that were carried out, it can be seen the outcrop profile, texture, and sedimentary features of this formation. The first observable lithofacies in this area is facies Gm, wherein this facies comprises pebble or cobble gravel with graded bedding, gravel imbrication, and ripple mark. The second one is facies Gp which comprises gravel stratified with planar cross-beds. The third is facies St, medium to very coarse-grained sandstone with laminae and cross-bed. The fourth is facies Sm, which consists of very fine to coarse-grained sandstone with ripple mark and climbing ripple. Furthermore, facies F1 and C comprises interbedded of sandstone, silt shale, and coal with bioturbation, ripple, and undulatory bedding. The fifth, facies Sh which consists of very fine to very coarse sandstone with sedimentary structures is horizontal lamination. The last but not least, facies Ss comprises fine to coarse sandstone with sedimentary structures small scale scours and cross stratification.

Badenian (middle Miocene) continental paleoenvironment in the Novohrad–Nógrád Basin (Central Paratethys): a volcano-sedimentary record from the Páris-patak Valley in Hungary

Two levels of volcaniclastics, comprising conglomerates, sandstones and mudstones, are interbedded with upper middle Miocene (upper Badenian) andesite pyroclastics near the Hungarian-Slovakian border in the distal region of the Central Slovakian Neogene Volcanic Field. Based on the field sedimentological investigations, the facies of the volcaniclastics (e.g., lateral and vertical grain size changes, sedimentary structures, textures, clast composition), their geometry and field relationships are documented herein with the aim of reconstructing the depositional environment. The silica-cemented volcaniclastics are mostly andesite clasts with only ~ 5% being granitoid, quarzitic, and tuff clasts as well as charred fossil wood fragments. The coarse-grained facies association includes crudely stratified, tabular or lenticular, clast-supported pebble-cobble conglomerates with erosive basal surfaces, b-axis imbrication, alternating with sets of cross-bedding. The fine-grained facies association comprises cross-bedded pebbly to medium-grained sandstone and lenses of tuffaceous clayey siltstone with rare horizontal lamination and water-escape structures. Rip-up mudstone clasts, with diametre up to 1 m, are present in both facies associations, revealing the co-existence of abandoned silty palaeo-channel plugs. Facies associations are arranged in several 0.5-4-m-thick, fining-upwards successions that likely formed in shallow channels as downstream- to laterally accreting longitudinal bars, extensive gravel sheets and bars that migrated in peak flow during floods. Palaeocurrent indicators (i.e., clast imbrication, direction of planar cross-bedding, orientation of petrified wood logs) show bedload transport by traction currents, initially towards ~S, and later towards ~W. Intermittently debris flows also occurred. Cross-bedded sandstones formed as in-channel transverse bars during medium/low discharge. Variation of grain size shows frequent discharge fluctuations during permanently wet conditions in the late Badenian. The 4-5-m-deep, low-sinuosity channels were part of a high-energy, gravel-bed braided-river system on the south-eastern foothills of the Lysec palaeo-volcano. Here, pyroclastics were reworked and redeposited as volcaniclastics during inter-eruption, high-discharge episodes.

SEDIMENTARY ARCHITECTURAL ELEMENTS AND SANDY BRAIDED FLUVIAL SUCCESSIONS IN AJALI SANDSTONE RIDGES, WESTERN AFIKPO BASIN, UTURU, NIGERIA

Lithofacies, bounding surfaces and sedimentary architectural elements exposed in two ridges at Uturu being quarried for construction sands were analysed to determine the paleoenvironment of deposition and the factors that control the deposition of sand units. Mainly outcropped is Ajali Formation overlying locally exposed Mamu Formation in Western Afikpo basin. Lithofacies identified include: Trough cross bedded medium- to coarse-grained (St), Planar cross bedded fine- to coarse-grained sandstone (Sp), Small scale planar cross bedded sandstone (SSp), Lenticular mudstone (Fm), Heterolithic sandstone/mudstone (Fsm), Horizontal stratified sandstone (Sh), Cross ripple laminated sandstone (Sr), Reddish muddy sand (Fl), Siltstone (SSm) and Shale (Fsh). The associations of lithofacies and bounding surfaces gave four fluvial and one marine architectural element. The fluvial elements which mainly characterized the Ajali Formation include: Channel-fill (CH), Macroforms Accretion (MA). Flood-Plain Fines (FF) and Channel Abandonment Fines (CAF). Offshore-shoreface fines (OSF) element defined marine Mamu Formation. The profiles of the ridges show dominance of MA followed by CH while FF is limited in occurrence and in some zones pinch-out to lenticular inter-bar mudstone. The MA is characterized by planar cross beddings, reactivation surfaces, internal grading, steep dipping ferruginized accretion surfaces and abrupt flat top which indicate mid-channel bars deposition in typical sandy braided fluvial depositional system. Generally, there is vertical aggradation/ amalgamation of channel deposits and dominance of sheet alluvial architecture. Low rate of channel avulsion, moderate rate of lateral migration and aggradation, variable discharge rate and high rate of sediment supply and subsidence were considered as factors that controlled the deposition and preservations of sand units. This study provided an understanding of mesoscopic heterogeneities and compartmentalization style inherent in hydrocarbon bearing sandy braided reservoirs which can be used as analog model for its development.

Tidal Channel Depositional Complex of the Cretaceous Yolde Formation of the Gongola Sub-Basin, Northern Benue Trough N. E. Nigeria

The facies analysis of the Cretaceous Yolde Formation at Gamajigo village in the Gongola Sub-basin of the Northern Benue Trough indicated fining upward a sequence characterized by the assemblages of five lithofacies that comprises of trough crossbeds, planar crossbeds, massive beds, ripple laminations, and mudstone. This fining upward architecture is defined by trough crossbedded sandstone with erosional contact at base overlain by interbedded sandstone and mudstones often displaying bi-direction paleocurrent patterns and growing mudstones content upwards moving into dominantly mudstones unit. This concurrently demonstrates dissipating energy intensity upwards as such reflecting a tide dominated system clearly of tidal channel depositional complex that most commonly develop in estuaries and tide dominated delta. Though on the account of the presences of Skolithos ichnofacies, a stressed hydrodynamic regime and indicating extension of tidal channel into the open sea may be very likely. This is generally indicative of a tide dominated oceanographic system in the advancing and inundating Cenomanian transgressive sea that define the coastal realm of the Gongola Sub-basin.

Regressive Shoreface Depositional System of the Cretaceous Yolde Formation of the Gongola Sub-Basin Northern Benue trough N. E. Nigeria

The characterization of lithofacies of the Yolde Formation at Briyel town in the Gongola Sub-basin of the Northern Benue Trough indicated coarsening upwards cycles composed transitional succession of six lithofacies that comprises of trough cross-beds, planar cross-beds, massive beds, ripple laminations, parallel laminations and mudstone. This coarsening and thickening upward symmetry is defined by a densely bioturbated mud dominated lower part transitionally grading into heterolithic interval composed thinly bedded sandstone and mudstones both of which developed below fair-weather wave base. This is capped by a succession of moderately bioturbated trough-planar crossbedded sandstone with poly-directional paleocurrent system. These assemblages reflect shoreface deposits and devoid of hummocky cross-stratification therein may account for a moderate wave oceanographic system. Evolution of this setting in the Gongola Sub-basin is indicative of a scenario of a broaden coastline in the coastal palaeogeography of the Yolde Formation typically suppressing due to frictional damping.

Tectonic significance of abrupt immature sedimentation in a shallow cratonic margin basin: The Arkose Level, Mesoproterozoic Paranoá Group

Facies analysis, aided by X-ray diffraction, QemScan analysis, U-Pb detrital ages and field geological data are used in a detailed sedimentary characterization of the Arkose Level, an immature sedimentary unit that occurs abruptly and discontinuously in the upper Paranoá Group, a sequence of mature marine rhythmites deposited in a shallow cratonic margin basin. This basin was developed during the Mesoproterozoic in the western margin of the São Francisco Craton, central Brazil. The study reveals three facies associations in the Arkose Level: (1) Facies Association I (C), (2) Facies Association II (MC) and (3) Facies Association III (FM). The lithofacies are characterized by plane-parallel lamination and planar cross-bedding, trough cross-stratification, channel-fills, overturned cross-stratification, and convoluted structures. The sedimentological and paleocurrent data acquired in this study leads to the interpretation that this unit represents a record of braided-river deposit, which was influenced by marine reworking in the uppermost portion. The depositional model comprises a transitional continental-marine environment, with channels in the continental domain interpreted as a high-energy braided river system around the margins of a shallow intracratonic basin during the Mesoproterozoic. Compiled and newly acquired LA-ICP-MS detrital zircon U-Pb ages reveal a new source area contribution of 1.1 Ga in the Upper portion of the Paranoá Group. The abrupt deposition of braided-river deposits amid the mature Paranoá basin is most probably associated with local faulting, responsible for exposing the crystalline basement and the sedimentary rocks of previous basins, providing new and near-source areas of crystalline basement rocks. The inferred faulting can be associated to a Stenian unsuccessful rifting event, or to the early stages of opening the Goiás-Pharusian ocean, in the Tonian.

Provenance and diagenesis of Guamá Sandstone, northeastern Pará, Brazil: A Silurian link between the Amazonas and Parnaíba basins

The Guamá Sandstone, exposed in small isolated outcrops in the northeastern region of the State of Pará (Brazil), which are surrounded by Cenozoic sediments, represents a Silurian relic of the Amazonas and Parnaiba basins connected during the Paleozoic and separated only since the Triassic period through the development of the Marajó Basin. Previous outcrop-based studies showed that the Guamá Sandstone is a fine to medium-grained quartzarenite, with high compositional and textural maturity, which was deposited in a coastal marine setting (foreshore/shoreface), correlatable with the environment of the Silurian Nhamundá Formation of the Amazonas Basin. The principal primary sources of the sandstone are associated with rocks formed during the Transamazonico and Brasiliano Cycles according to isotopic dating of zircon. In order to better understand provenance and diagenesis of the Guamá Sandstone core samples of a 50 m deep bore hole, located 6 km north of the town of São Miguel do Guamá, have been studied. The sandstone is usually bioturbated and massive, but planar stratification (foreshore) and cross bedding (shoreface), the latter in the lower part of the profile, are also present indicating a transgressive to regressive coastline. In addition to quartz, the Guamá Sandstone is composed of the following detrital accessory minerals: zircon, tourmaline, rutile, epidote, mica, ilmenite and chromite while kamacite occurred only sporadically indicating, at least for the latter, origin from Fe-Ni-meteorites. Provenance data obtained from the composition of zircon and tourmaline grains, as well as from internal textures of zircon, point to primary igneous and metamorphic source rocks of the Guamá Sandstone. While zircon is principally of magmatic origin, tourmaline grains are derived dominantly from metapelitic, metapsammitic and granitoid rocks. However, pre-existing sediments probably represent, to a lesser extent, the ultimate direct source as suggested by the well-rounded grains of the quartz arenite and worn tourmaline overgrowths. The diagenetic phases of the sandstone include syntaxial quartz overgrowths, microquartz, kaolinite booklets, illite and anatase. The quartz overgrowths are pseudo-zoned and engulf parts of the kaolinite booklets. These may be surrounded by microquartz which usually replaced quartz overgrowths. Illite, which is rare, is probably the product of replacement of kaolinite, and anatase may be related to partial dissolution of ilmenite. The main diagenetic processes including precipitation of quartz and kaolinite began during eodiagenesis, in shallow burial conditions, and proceeded through mesodiagenesis as suggested by multi-episodic quartz zoning and formation of illite. The early precipitation of kaolinite is related to alteration of feldspar and mica caused through flushing of meteoric water during forced marine regression and uplift, probably still in Silurian time. The sources for quartz cementation may include kaolinitization of feldspar and pressure dissolution, the latter associated to concavo-convex and sutured grain contacts, although these types of contact are relatively rare. Therefore, external silica-rich solutions likely occurred.

English

The classification of the sediments into facies and the various environments in which the sediments of the northeastern Voltaian basin were deposited has not been clearly described. Using detailed field mapping together with petrographic studies, six sedimentary facies has been identified; F1 is defined by asymmetric wave formed ripple marks, F2 indicates flute casts on micaceous sandy shales, F3 has bifurcated ripple and climbing ripple marks on argillaceous materials at the middle parts of the scarp, F4 is identified by parallel and cross stratifications observed close to the upper parts of the scarp, F5 has a bipolar herringbone cross beds and planar cross beds on quartz rich sediments at the southern parts of the study area and a distinctive well developed burrows (Skolithos) at the southern half of the study area defines facies F6. The sediments are deposited largely in a shallow marine environment, with subordinate fluvial, flood plain, deltaic, aeolian, tidal flats and shoreface/nearshore environments. Although, there exist a suitable depositional environment for the formation of hydrocarbons, the overlying sediments are not thick enough to give the required temperature and pressure conditions for hydrocarbons development in the northeastern fringes of the basin. Paleosol features evident by pseudokarren, like weathering marking the contact between the middle Oti/Pendjari group and the lower Gambaga group of the study area. The sequence of the area is that of a typical shallowing, thickening and coarsening upward sequence. Key words: Clastic sediments, environments, facies sequence, Gambaga-Nakpanduri, massifs.

WAVE-DOMINATED SHORELINE DEPOSITS IN THE LATE MIOCENE SEDIMENTARY SEQUENCE IN THE MIRI FORMATION NORTH SARAWAK, MALAYSIA

The research area is located in Miri region, north Sarawak. Observation was focused on selected outcrops along road cuts and beach. The distribution of outcrops is widely distributed at a few localities in Miri region such as Tanjong Lobang, Airport road, Hospital road and Oil Well road. Sedimentary sequences can be traced by facies variations as shoreline environment based on the sedimentary structures. Four facies were identified and characterised successfully in the study area based on their sedimentary structures, namely facies 1: planar cross stratified (PcS), facies 2: swaley cross stratified (ScS), facies 3: swaley-Hummocky cross stratified (SHcS), facies 4: Hummocky cross stratified (HcS) and heterollithic facies (He). Facies evaluation deduced that the sedimentation settings resulted as foreshore, upper shoreface, middle shoreface, lower shoreface and tidal (minor occurrence). Wave-dominated in the upward coarsening interval is unambiguous that representing by abundance of hummocky cross stratification and other storm deposits in the sandstone dominated portions, well-defined swaley cross stratified sandstone interval, and up to several meters thick sets of planar cross beds interval interpreted to have been formed by swash waves. Heterolithic units in this wave dominated sequences are as minor occurrence of tidal influence during fair weather .

Asteriacites and other trace fossils from the Po Formation (Visean–Serpukhovian), Ganmachidam Hill, Spiti Valley (Himalaya) and its paleoenvironmental significance

Abstract An assemblage of trace fossils comprising Asteriacites stelliformis, A. quinquefolius, Biformites insolitus, Helminthoidichnites? isp., Lingulichnus isp., Lockeia siliquaria, Palaeophycus tubularis, Planolites isp., Protovirgularia isp. A, Protovirgularia isp. B, Protovirgularia isp. C, Psammichnites isp., Rusophycus isp., and Treptichnus isp. from the Po Formation (Visean-Serpukhovian) exposed along the base of Ganmachidam Hill near the village of Chichong, Spiti Valley in the Himalaya, is described. Storm beds (tempestites) are highly bioturbated. Sedimentary structures such as hummocky cross-stratification (HCS), low-angle planar and trough cross beds, and shallow, slightly asymmetrical gutter casts are observed. The overall trace fossil assemblage indicates the presence of upper shoreface to lower shoreface Cruziana ichnofacies of an open shelf.

A Tide-Dominated Beach from the Cambro-Ordovician Cabos Formation of Northwestern Spain

Abstract: Beaches on which tidal processes play a significant role, as opposed to purely wave-dominated beaches, are little known from the rock record due to a lack of suitable sedimentological criteria and facies models. Cambrian strata of the Cabos Formation in Asturias, Spain, with a wealth of tide-generated sedimentary structures and a close similarity to modern tide-dominated beaches, provide an opportunity to advance our understanding of ancient tidal beaches. Our field-based analysis of the Bres and Taramundi members identifies three lithofacies: Facies 1—simple bedsets of fine sandstone, divided into (1A) sandstone with tabular and wedge-shape geometries and horizontal to low-angle laminae, (1B) sandstone with tabular, wedge-shape, and wave-form geometries and decimeter-scale planar cross-bedding, and (1C) sandstone with a channel form and erosional base; Facies 2—composite bedsets of mudstone and fine sandstone; and Facies 3—intraformational breccias composed of (3A) rip-up clasts, (3B) sandstone clasts, and (3C) skeletal clasts. The numerous tidal structures testify to bed-load transport by unidirectional and bidirectional tidal currents, alternating deposition from bed load and suspended load, emergence during the ebb stage of tidal cycles, tidal scour, and regular fluctuations of tidal-current velocity. In contrast, structures indicative of swash–backwash, oscillating waves, and storms are much less conspicuous. This study argues that the Cabos sandstone and mudstone accumulated on a tide-dominated, wave-influenced beach distinguished by four morphologic zones: a flat sandy supratidal beach (Subfacies 1A), a sandy intertidal beach characterized by small 2-D dunes of a ridge-and-runnel system (Subfacies 1B), cross-shore channels (Subfacies 1C), and the subtidal nearshore with a mix of mud and rippled sand (Facies 2). Rare storm processes on the tidal beach are suggested by the breccias (Facies 3). As increasingly more local examples like the Cabos are recognized in the rock record, sedimentologists can begin to construct a generalized facies model for this largely unexplored sedimentary environment.

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.

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.

Morphologic and hydrodynamic controls on the occurrence of tidal bundles in an open-coast macrotidal environment, northern Gyeonggi Bay, west coast of Korea

Tidal dunes with well-defined rhythmic tidal bundles are documented from the lower intertidal zone of an open-coast macrotidal environment in Gyeonggi Bay, Korea. Based on combined morphologic, sedimentologic and hydrodynamic datasets, this study aims to characterize the factors that govern the temporal and spatial variability of tidal bundles in a non-barred, unconfined macrotidal environment. The tidal dunes are flood-asymmetric and of longer wavelength (10–20m) with small ebb caps on the upper bank, and symmetric to slightly ebb-asymmetric and of shorter wavelength (5–10m) with larger ebb caps on the lower bank. The upper-bank dunes are characterized by more steeply dipping flood-directed planar cross-beds and thinner mud drapes than the lower-bank dunes. Each tidal bundle consists of a single mud drape that is stratified to cross-stratified, rich in silt and very fine sand. It overlies ebb-directed ripples and represents dynamic mud deposition during the ebb tidal phase. The presence of strong rotary currents (up to 0.25m/s) and low suspended-sediment concentration of flood currents prevent deposition of mud drapes during the high-tide slack-water period. The distinct asymmetry in the water elevation at which the velocity peaks during the ebb and flood phases results in the preferential preservation of flood-directed cross-beds in the lower intertidal zone, where the ebb current – although stronger than the flood currents – is of shorter duration and hence unable to reverse the dune profile. The pronounced time–velocity asymmetry at the higher elevation combined with the distinct velocity peak asymmetry leads to a better preservation of hierarchical tidal cycles in the upper-bank dunes. The present study suggests that the persistent occurrence of single, stratified to cross-stratified mud drapes, which reflect dynamic mud deposition during the ebb phase, and the dominance of flood-directed cross-beds are diagnostic features of tidal bundles in the intertidal zone of unbarred, open-coast macrotidal environments. A proposed model for mud drape deposition provides a new perspective on the origin of tidal bundles together with useful criteria for reconstructing the paleo-depositional setting.

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.

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.