Clast-supported Conglomerates Research Articles

Lithofacies Study of Bai Hassan Formation Sequence (Upper Pliocene) in Bekheir Anticline, Zakho Area Northern Iraq

The current study included the Bai Hassan Formation (Upper Bakhtiari) in the Zakho area of northern Iraq. Depending on the grain size class, structures, degree of rounding, sorting, and some other detailed field observations. This formation was divided into eleven lithofacies which are matrix-supported conglomerate, clast-supported conglomerate, laminated sandstone, sandy gravel, planar cross-bedded conglomerate, interbedded siltstone-mudstone, gravely sandstone, interbedded sandstone-conglomerate, lime matrix-supported conglomerate, and caliche lithofacies. These lithofacies are distributed into six coarsening-upward cycles consisting of complete and incomplete cycles, and these lithofacies include sedimentary structures such as imbrication pebbles, bedding plain, load cast, and gravely cross-bedding. The environment of the Bai Hassan Formation represents conjunctive alluvial fan with possible internal lake environments. The study includes the determination of source areas such as sedimentary, igneous, and metamorphic rocks.

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.

Lateral and temporal variations of a multi-phase coarse-grained submarine slope channel system, Upper Cretaceous Cerro Toro Formation, southern Chile

ABSTRACTUnderstanding variations in the sedimentary processes and resulting stratigraphic architecture in submarine channel systems is essential for characterizing sediment bypass and sedimentary facies distribution on submarine slopes. In the Santonian to Campanian Cerro Toro Formation, southern Chile, a coarse-grained slope system, informally known as the Lago Sofia Member, developed in a structurally controlled environment, with complex and poorly established relationships with the surrounding mud-rich heterolithic deposits.A detailed architectural analysis of the most continuous and best-exposed channel system in the Lago Sofia Member, the Paine C channel system, provides insights on lateral facies transitions from channel axis to margin, stacked in a multi-phase sequence of events marked by abrupt changes in facies, facies associations, and architecture.The Paine C channel system is incised into siltstones and claystones interbedded with thin-bedded very fine sandstones, interpreted to be either channel-related overbank or unrelated background deposits. The coarse-grained deposits are divided into a lower conglomeratic unit and an upper sand-rich unit. The lower conglomeratic unit can be further subdivided into three phases: 1) highly depositional and/or aggradational, dominated by thick and laterally continuous beds of clast- to matrix-supported conglomerate, herein named transitional event deposits; 2) an intermediate phase, including deposits similar to those dominant in phase 1 but also containing abundant clast-supported conglomerates and lenticular sandstones; and 3) a bypass-dominated phase, which records an architectural change into a highly amalgamated ca. 45-m-thick package composed purely of lenticular clast-supported conglomerates with local lenticular sandstones. Between the conglomeratic phases, a meter-scale package composed of interbedded thin- to medium-bedded sandstone and mudstone deposits is interpreted to drape the entire channel, indicating periods of weaker gravity flows running down the channel, with no evidence of bedload transport.The upper sand-rich unit is divided into lower amalgamated and upper non-amalgamated phases, and represents a rapid architectural change interpreted to record an overall waning of the system. The sandstone unit laps out onto a mass-transport complex which is interpreted to have been triggered initially at the same time as major architectural change from conglomerates to sandstones.While mindful of the fact that each system is a complete analogue only for itself, we propose a new depositional model for coarse-grained submarine channel systems, in which particular characteristics can provide significant insights into architectural heterogeneity and facies transitions in channelized systems, allowing substantial improvement in subsurface facies prediction for fluid reservoirs.

Detrital zircon U–Pb geochronology and fluvial basin evolution of the Liuqu Conglomerate within the Yarlung Zangbo Suture Zone: A critical geochronometer for the collision tectonics of the Tibetan-Himalayan Orogenic Belt

We present new U-Pb detrital zircon ages, depositional history and tectonic model for the Liuqu Conglomerate (LQC) in southern Tibet that represents a critical geochronometer for the collision history of the Tibetan-Himalayan Orogenic Belt. LQC is a ∼5 km–thick, late Mesozoic–Cenozoic molasse deposit occurring strictly within the Yarlung Zangbo Suture Zone (YZSZ) and is tectonically overlain to the north by the Cretaceous Xigaze ophiolite and to the south by the Mesozoic Tethyan Himalaya sequence. It consists of matrix- and clast-supported conglomerates with sandstone intercalations, and its matrix includes poorly to moderately sorted sandstone and mudstone. New U–Pb detrital zircon dating of LQC sandstones has revealed a youngest zircon age of 307 ± 13 Ma and an oldest zircon age of 3362 ± 51 Ma. The age spectrum of zircons displays a prominent peak of ∼935 Ma, two large peaks at ∼516 Ma and 1474 Ma, and two small clusters of ∼2429 Ma and ∼2772 Ma that point to East Gondwana as the likely provenance for the LQC depocenter. The LQC represents fluvial deposits of an axial river system, which developed in an orogen-parallel, transtensional accommodation space within the YZSZ, after the collision of the Late Jurassic-Early Cretaceous Trans–Tethyan arc–trench system with the northern edge of India in the latest Cretaceous. The Indian subcontinent with the accreted Tethyan ophiolites and the intra–suture LQC depocenter arrived at and collided with the active margin of Eurasia during the latest Oligocene (∼23 Ma). The LQC depocenter started receiving clastic material and zircons for the first time from the Gangdese Magmatic Belt and the Xigaze forearc basin to the north by ∼20 Ma. The ensuing continent–continent collision resulted in significant crustal uplift across the collision zone, and in the inversion and rapid exhumation of the LQC strata by the early–Middle Miocene. The depositional and exhumation history of the fluvial LQC formation within the YZSZ involved two discrete collision events during the evolution of the Tibetan-Himalayan Orogenic system.

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.

Petrography, geochemistry and depositional model of Ispikan Conglomerate, Makran Accretionary Prism, Southwest Pakistan

Sedimentology, petrography, and geochemistry of the Ispikan Conglomerate of southwest Makran have been studied to establish its stratigraphic position, age, provenance, and depositional environment. Very thin to massive beds, poor sorting, no fabric, and poor grading are the common features. It is composed of mostly reworked, medium- to coarse-grained pebbly sandstone, siltstone, and discontinuous lenses of matrix- and clast-supported conglomerate. The sandstone is composed of angular to sub-angular, poorly sorted, immature grains having Q72F13L15 as average composition. Pebbles are of mostly metamorphic and acidic igneous origin. Large angular boulders (~1.5m) of sandstone indicate quick debris flow conditions. Geochemical discriminants suggest the derivation of Ispikan sediments from an active continental margin. The Nb and Zr/Th and Ba/Y values indicate the continental island arc setting, whereas a high K/Rb ratio points to the derivation of detritus from an acid and intermediate source. Ispikan Conglomerate is interpreted as an Eocene olistostrome formed after a localized submarine debris flow triggered by slope failure.

Coarse-grained meandering distributive fluvial system of the basal Cedar Mountain Formation, U.S.A.

ABSTRACT The analysis of downstream changes in ancient fluvial systems can better inform depositional models for foreland-basin systems. Herein we analyze the basal deposits of the Early Cretaceous Cedar Mountain Formation of Utah to better understand the variety of fluvial deposits present and to develop a depositional model for the Sevier foreland basin. We also evaluate the long-held interpretation of a braided origin for these deposits and document numerous examples of point-bar deposition in highly sinuous meandering rivers by analysis of large (20 to 60 km2) plan-view exposures. These plan-view exposures allow comparisons between planform and cross-sectional geometries. The study utilizes outcrop data, virtual outcrop models, and satellite imagery to develop a facies model and analyze the architecture of channel bodies in the Buckhorn Conglomerate and Poison Strip Sandstone of the Cedar Mountain Formation. We document downstream (west to east) decreases in lateral channel migration, sinuosity, channel amalgamation, grain size, and percent of fluvial channel facies (conglomerate and sandstone). Fluvial channel deposits occur arranged into larger stratal bodies: multistory–multilateral channel bodies that are dominantly composed of clast-supported conglomerate in the west to a mix of multistory, multilateral, and isolated channel bodies composed of matrix-supported conglomerate in the east. The median width of highly sinuous point bars is similar across the field area (344 m to 477 m), but the inclusion of narrower (median = 174 m), low-sinuosity bar elements in the east indicates an overall reduction in lateral channel migration and sinuosity downstream. Net-to-gross values range from 100% in much of the western outcrops to as low as 38% in the east. Paleocurrent analysis reveals a transverse (west to east) paleoflow for the study interval that merges with axial (south–north) paleoflow near the Utah–Colorado state line. We estimate 104 m3/s-scale discharge and 106 kilometer-scale drainage area for axial rivers based on paleohydraulic analysis which represents a significant part of the Early Cretaceous continental-scale drainage. The observed downstream trends in lateral channel migration, sinuosity, channel amalgamation, grain size, and net-to-gross for the basal Cedar Mountain Formation are consistent with expected trends for sinuous single-thread distributive fluvial systems and are similar to observed trends in the Jurassic Morrison Formation. Medial (Buckhorn Conglomerate) to distal (Poison Strip Sandstone) zones are preserved and span the forebulge to backbulge depozones of a foreland-basin system. Postulated deposits of the proximal distributive fluvial system have been removed during erosion of the foredeep depozone. The easternmost Poison Strip Sandstone and coeval Burro Canyon Formation represent deposits of an axial system at which western-sourced distributive fluvial systems end. Distributive fluvial systems dominate modern foreland basins, and this study suggests that they may constitute a significant proportion of ancient successions.

Characterizing locality- and scale-dependent heterogeneity in conglomerate core and associated fluid flow using X-ray CT imaging

Heterogeneous conglomerates preserving matrix and clasts caused the complex distribution of pore spaces and can eventually influence fluid flow. To evaluate the influence of complex heterogeneous features in conglomerate rock on both pore distribution and fluid flow, two conglomerate cores were analyzed using X-ray computed tomography (X-ray CT). Conglomerate cores (Core I and Core II) characterized by different distributions of matrix and clasts represented highly heterogeneous features; in Core I (clast-supported conglomerate), clasts were evenly distributed through the entire core, but in Core II (matrix-supported conglomerate), large-sized clasts were dominantly positioned at the bottom of core. Such heterogeneous features caused differences in volume of pores (Core I: 7,409.54 mm3 and Core II: 17,525.83 mm3). Based on X-ray CT image analysis, the numerical model (single-phase fluid simulation) was implemented to evaluate the permeability of conglomerate at different scales. Especially, due to inner heterogeneous features (e.g., matrix, clasts, and cracks) in conglomerate cores, permeabilities were changed significantly dependent on the selected location and size of sub-domains representing a few centimeters (kh and kv: 1.18×10-12 to 1.01×10-10m2). This suggests that the effect of small-scale heterogeneity should be evaluated carefully when heterogeneous conglomerate cores were analyzed. Additionally, the measurement scale for X-ray CT analysis should be large enough to capture such heterogeneous features (e.g., the size of clasts).

Depositional setting and U-Pb detrital record of rift-related deposits in the Moeda Formation (Minas Supergroup) at the Gandarela and Ouro Fino synclines, Quadrilátero Ferrífero, Brazil

Abstract The Moeda Formation (Caraca Group, base of Minas Supergroup) registers the initial rift stages of the Minas Basin in the Quadrilatero Ferrifero, Southern Sao Francisco craton, SE-Brazil. We present stratigraphic and U-Pb detrital zircon dating analyses of the Moeda Formation that contribute to the comprehension of its sedimentary evolution in the southernmost Gandarela and Ouro Fino synclines (Central-Eastern Quadrilatero Ferrifero). The Moeda Formation sequence consists of a basal proximal alluvial fan with clast-supported conglomerates, grading upward into an alluvial floodplain with quartz arenites, and upper massive sandstones from distal fluvial fans. Because the Moeda Formation is normally limited by similar lithologies at both its base (Nova Lima Group, Rio das Velhas Supergroup) and top (Batatal Formation, uppermost Caraca Group) and stratigraphic inversion has occurred in the study area, U-Pb detrital zircon dating was also extended to these units to differentiate them stratigraphically. The youngest clusters of detrital zircon ages in the Nova Lima Group and Moeda and Batatal formations were 2716, 2777 and 2786 Ma, respectively. Based on our data and relevant literature, the depositional age of the Moeda Formation was interpreted as between 2716 and 2520 Ma with the Mesoarchean continental crust, the Rio das Velhas Supergroup and the Archean TTG complexes as its main source areas.

The late Paleozoic Ice Age along the southwestern margin of Gondwana: Facies models, age constraints, correlation and sequence stratigraphic framework

The southwestern margin of South America offers a complete record of the Late Paleozoic Ice Age (LPIA) that affected the Gondwana supercontinent. The tripartite division of LPIA glacial episodes has been refined with the help of new radiometric dates and biostratigraphic (flora and fauna) zonations in recent years to five shorter-lived discrete events: 1. Latest Devonian-earliest Tournaisian, 2. Tournaisian, 3. Visean, 4. Serpukhovian-Early Bashkirian, 5. late Pennsylvanian-earliest Permian. These glacial events are capped by postglacial transgressive deposits with marine fauna. The unbalanced preservation potential of the glacial deposits, skewed toward the glaciomarine sediments, provides an uneven stratigraphic record with few cases of continental glacial sedimentation, confined to the Serpukhovian-Early Bashkirian event, and numerous examples of glacial sedimentation in marine environments. Glacial sedimentation in marine settings has been grouped in two main facies associations: a valley-glacier-retreat (fjord) facies association and a submarine-retreat (glaciomarine apron) facies association in open-marine areas.Transitional facies, correspondent to those formed by the flooding of valleys during postglacial transgressions, are widely distributed along the Protoprecordillera in western Argentina, where paleofjord successions are well exposed particularly in western Paganzo Basin, and mapped in subsurface in the Tarija basin. A general paleofjord model includes (from base to top) the following stages: (i) Incision of paleovalley and deposition of subglacial diamictites in ice contact deltas, (ii) Early Transgressive stage characterized by resedimentation of subglacial material by subaqueous sediment gravity flows and slumps in proglacial settings, (iii) Maximum flooding (late transgressive stage) dominated by black shales or laminated mudstones related to a marine incursion that flooded valleys; normal marine or brackish conditions may dominate this stage and (iv) Highstand: progradation of a fluvial-deltaic system including in some cases Gilbert-type deltas. In glaciomarine apron environments, the basal facies includes massive clast-supported conglomerates, with few striated and polished clasts, followed by fining-upward successions including thinly bedded diamictites with ice-rafted debris (IRD) and locally contorted sandstone masses in diamictite beds, indicative of mass-emplacement mechanisms. The presence of inter- and intratill pavements suggests glacial advance/retreat fluctuations along the basin margins.Deglaciation sequences, reflecting deposition mainly during the retreat of ice sheets, ice caps and alpine glaciers and successive deglaciation, can be used as operational tools for the analysis of glacial successions in southwestern Gondwana. They are characterized as rather simple upward-fining successions in open marine settings as exemplified in most of the Calingasta-Uspallata Basin,Claromecó Basin (and adjacent Ventana foldbelt, VFB) and central portions of the Paraná and Karoo basins. In more proximal areas (i.e., paleofjords) this vertical trend is commonly punctuated with deltaic wedges fed by nearby provenance areas.The late Paleozoic glacial-related successions of southwestern Gondwana exhibit a common tripartite motif, equivalent to second-order sequences with estimated durations of 10–80 Myr. The lower section corresponds to glacial and glacially-influenced diamictites; the middle interval is initiated with postglacial transgressions. The lower and middle intervals correspond to the deglaciation sequence as described and identified in several basins of Gondwana. Finally, the upper term includes coastal progradation, followed in some places by continentalization, accompanied in many sectors by increasing aridization. Examples of second-order sequences can be identified in the thick late Paleozoic successions of the Paraná and Karoo basins and in the VFB. Thinner second order sequences can be identified in the Calingasta-Uspallata, Rio Blanco, Paganzo and San Rafael basins. In the Paganzo and San Rafael basins the middle interval is also punctuated by short lived marine ingressions. The basal sequence boundary is commonly an abrasion surface (glacial erosion surface, GES) developed on bedrock. Deglaciation sequences are assigned to third order sequences made up, when present, of a thin lowstand system tract (LST) of subglacial deposits followed upward by thick glaciomarine and glacially influenced sediments. These facies are part of a thick transgressive systems tract (TST) that culminates with marine shales that reflect interglacial or postglacial conditions during ice retreat. Thus, the deglaciation sequences are proposed to be third order sequences made up of LST-TST or exclusively TST.

Stratigraphy and facies architecture of the Neoproterozoic syn- and inter-eruptive succession: An example from Gabal El Urf, Northeastern Desert, Egypt

A new integrated stratigraphic, sedimentological and geochemical study was carried out on the Gabal El Urf deposits to clarify their facies architecture, sequence stratigraphic framework, and evolutionary stages. The Gabal El Urf in the Northeastern Desert of Egypt is a well-preserved example of the Neoproterozoic volcano-sedimentary deposits. It comprises inter- and syn-eruptive facies that were generated by diverse depositional processes including debris flows, sheet floods, lacustrine flood flows, and pyroclastic ash falls/density currents. These facies correspond to alluvial fan, aeolian -fluvial, lacustrine, and pyroclastic facies associations that are arranged in four depositional cycles. They reveal variations in accommodation to sediment supply (A/S) ratio. Unconformities and lithofacies assemblages define six stages of volcanism and sedimentation and three unconformity-bounded tectonic sequences that should be allied to tectonic subsidence and basin uplift. These sequences may be subdivided in Low-Accommodation Systems Tracts (LAST) having negative A/S (A/S ratio close to zero) and High-Accommodation Systems Tracts (HAST) with fluctuation in A/S ratio between zero and +1. Sequences 1 and 3 are characterized by clast-supported conglomerates and pyroclastics, overlain by fluvial sand bodies and debris flow deposits, respectively denoting a LAST (positive A/S ratio close to zero). Sequence 2 is marked by pyroclastic ash fall facies at the base, passing to lacustrine deposits that are interlayered with fan-delta conglomerates at the top, defining a HAST (A/S ratio >1). The defined sequences are superimposed by thick fissure-fed ignimbrites which are capped by a co-ignimbritic breccia lag deposits with concentrated pyroclastic fountains. Clastic petrofacies of the El Urf succession indicate an adjacent miscellaneous rock sources involving volcanic, plutonic, and metamorphic terrains during the chief sedimentation processes in rift-related paleotectonic regime. The low to moderate magnitude of the CIA and PIA together with low SiO2/Al2O3 ratio indicate sediment recycling and moderate weathering of the protolith near uplifted basin flank(s) under dominant semi-arid to arid conditions. The development of the El Urf basin is akin to the post-amalgamation Ediacaran volcano-sedimentary basins that are widely distributed in the northern part of the Arabo-Nubian Shield, and is a key tectono-stratigraphic marker for the Cryogenian-Ediacaran shift along a major unconformity surface.

Provenance shift through time in superposed basins: From Early Cryogenian glaciomarine to Late Ediacaran orogenic sedimentations (Araçuaí Orogen, SE Brazil)

Records of Precambrian glaciation have been reported from southeastern Brazil for over a century. We present sedimentological, stratigraphic and isotopic (U-Pb and Lu-Hf on detrital zircons, C and O on carbonates) studies on diamictite-rich to diamictite-free successions of the Araçuaí Orogen, the Brazilian counterpart of the Araçuaí – West Congo Orogenic System (AWCO). From base to top, the Chapada Acauã Formation (Macaúbas Group) includes a diamictite-rich unit, with lenses of graded sandstone and clast-supported conglomerate, that gradually passes to graded sandstone, pelite with sparse oversized clasts and rare carbonate lenses on top, representing mass flow and turbidity current deposits of submarine fan, followed by finer-grained turbiditic sedimentation with iceberg discharges along fan fringes, from glaciomarine to post-glacial scenarios. The Salinas Formation comprises a deep-sea sand-mud sequence composed of pelites and pelitic wackes. The Chapada Acauã Formation was deposited in-between 750 and 667 Ma. It shows wide spectra of zircon ages (typical of continental rift to passive margin settings) that start around 3.2 Ga and display main age peaks indicating sediment provenance from the Rhyacian-Orosirian basement (εHf(t) = +14.6 to −18.5) and Early Tonian anorogenic rocks (950–880 Ma; εHf(t) = −3.2 to −23.2) for the diamictite-rich lower unit and, for the upper unit, also in Early Cryogenian anorogenic rocks (ca. 715 Ma, εHf(t) = −9.6). In contrast, the Salinas Formation shows most ages in-between 676 and 620 Ma and a maximum sedimentation age around 551 Ma, with εHf(t) from +6.9 to −18.2, unravelling an important shifting of sediment provenance to sources located in magmatic arcs and collisional granites of the Araçuaí and Ribeira orogens. The Early Cryogenian age and negative Hf signature for the glaciomarine Chapada Acauã Formation point to sediment sources in the 720–670 Ma anorogenic igneous rocks found in AWCO and adjacent Congo – São Francisco Craton, and suggest correlation with the Sturtian global glaciation.

Provenance of the Ediacaran Salinas Formation (Araçuaí Orogen, Brazil): Clues from lithochemical data and zircon U-Pb (SHRIMP) ages of volcanic clasts

Abstract Salinas Formation occurs in a large region of the Northern Aracuai orogen, Southeastern Brazil. It includes turbiditic wackes (> 10% matrix) to arenites (< 10% matrix), pelites and clast-supported conglomerates, metamorphosed from the biotite zone of the greenschist facies to the sillimanite zone of the amphibolite facies. Salinas Formation lies unconformably on the top of or in tectonic contact with the Macaubas Group, and hosts Cambrian granitic intrusions dated between 540 and 500 Ma. Aiming to unravel sediment provenances for the Salinas basin, we present a detailed lithochemical (45 samples) study on low-grade rocks preserved from the regional deformation, which are found in the type area of the Salinas Formation. In addition, we compare them with deformed and more metamorphic rocks of similar composition but located in other basin sectors. The lithochemical data indicate limited chemical weathering in the sediment sources, good correlations with the mineralogical compositions in respect to the variable amounts of metamorphic minerals typical of pelitic (micas, garnet, and other peraluminous silicates) and psammitic (feldspars, quartz) fractions. The main provenances of sedimentary protoliths are clearly related to continental magmatic arc and active continental margin environments. U-Pb (SHRIMP) analyses performed on zircon grains from clasts of intermediate to felsic volcanic rocks, extracted from a clast-supported metaconglomerate, yield concordant zircon Pb206 /U238 ages from ca. 579 Ma to ca. 697 Ma, with most of the ages in the interval of 587 to 630 Ma. This indicates that the main primary sediment source is the Rio Doce magmatic arc (630-580 Ma), in very good agreement with the arc-related lithochemical signature. Eleven zircon crystals yield a Concordia age of 620 ± 10 Ma, representing an important Ediacaran volcanic episode in the tectonic evolution of the Aracuai orogen.

The basal siliciclastic Silurian-Devonian Tacaratu formation of the Jatobá basin: Analysis of facies, provenance and palaeocurrents

The Silurian-Devonian Tacaratu Formation represents the basal siliciclastic sequence of the Jatobá Basin and is the focus of this study. Provenance and palaeocurrent studies are fundamental to the analysis and understanding of the surface diagenetic evolution and tectonic evolution of sedimentary basins. Field work (stratigraphic logs and sedimentary structures) and laboratory studies (petrography) were used for characterization this sedimentary fill. The Tacaratu Formation is comprised of sandstones varying from medium-to coarse-grained with conglomeratic levels. Seven main lithofacies have been identified and described: poorly stratified, matrix to clast-supported conglomerates; trough cross-stratified conglomerates; cross-stratified clast-supported conglomerates; cross-stratified pebbly sandstones; coarse-to medium-grained sandstones with trough cross-stratification; coarse-to medium-grained sandstones with planar cross–stratification; and horizontally stratified sandstones. The compositional analysis of 20 sandstone samples revealed homogeneity and compositional regularity among the samples. Fifty per cent of the sedimentary rocks were classified as quartz arenite, 40% as sub-litharenite, and 10% as subarkose. The provenance of these sandstones indicates a stable craton origin, with tectonic events at the margin of the basin. Diagenesis has evolved from eodiagenesis through mesodiagenesis to telodiagenesis. Palaeocurrent readings show a consistent trend to the NNW and a secondary mode to the NE. Palaeocurrent readings point to the São Francisco Craton as the main source area - more precisely the resistant areas of the Borborema Province.

Evolution of monogenetic rift-related alkaline magmatism in south Egypt: Insight from stratigraphy, geochronology, and geochemistry of the Natash volcanics

The Mesozoic Natash volcanics are rift-related monogenetic volcanic complex that erupted in south Eastern Desert of Egypt. The architecture of their building has evolved through three phases namely: (1) tuff cone consisting of explosive -dominated pyroclastics that stand for an intensity eruption peak; (2) the effusive lava-conquered phase, characterized by prevalent basalt-trachyte-rhyolite fissure eruptions; and (3) late-stage subvolcanic plugs/domes. The first phase began with the emplacement of spatter-rich ignimbrites followed by grain-supported lapilli-tuffs and hyaloclastics in a shallow-water environment (phreatomagmatic phase) with a vent-opening phase. This initial phase is followed by deposition of clast-supported conglomerates of lahar facies that represent significant erosional period, separating the pyroclastics and effusive phases. The second phase comprises multiphases displaying compositional stratification (primitive beneath evolved) of pahoehoe Hawaiian and rubbly vulcanian lavas, overlying the pyroclastic deposits. The third phase is the youngest and comprises plugs of aphyric trachytes. The volcanics occasionally have vesicle sheets, cylinder-elongate vesicles, gas blister, and inflation clefts, reflecting degassing, rupturing, brecciation/quenching history, and pressurized flows. U-Pb zircon ages from two samples of trachyte volcanics gave concordant age of 92 ± 0.9 Ma. One trachyte plug yielded concordant age of 86.6 ± 1.3 Ma. These ages represent two events belonging to the major Senonian volcanic activities. The investigated volcanics have alkaline affinity and display a continuous composition ranging from basalt to trachyte and rhyolite. Their chemical compositions display linear, curvilinear, and scattered trends on the Harker diagrams matched with the petrographic data and reflect comagmatic genesis for the entire sequence. Crystal fractionation of olivine + pyroxene + plagioclase + Fe-Ti oxides + apatite plays a role in the volcanic evolution. The origin of the mafic rocks is derived from deep mantle source in the garnet facies via low degree of partial melting. Such magma source is affected by fluid-rich metasomatism generated by dehydration subducted crust on the EM I, EMII, and HIMU mantle reservoir during Neoproterozoic time. The basalt-trachyte-rhyolite suites and their pyroclastics from the Natash area, together with Mesozoic alkaline volcanics from the surrounding terrains in Africa, constitute a remnant of extensive magmatisms, having large extension, high eruption rate, and OIB-type chemical composition.

The Ediacaran volcanosedimentary succession of Gabal Abu Had, North Eastern Desert, Egypt: geological study, facies analyses, and depositional setting

Gabal Abu Had is an exposure of a volcanosedimentary succession in the North Eastern Desert Basement Complex. This succession includes intercalation of two major rock units, which are Dokhan Volcanics and Hammamat Group with different styles of formation, deposition environments, and genesis. Gabal Abu Had succession (GHS) is a northward dipping, c. 700-m-thick volcanosedimentary succession that rests on metavolcanic and old granitoid rocks with erosion unconformity. The lower part of GHS is dominated by volcaniclastic mass flow deposits and andesitic lava with interbedded gravely sandstone, whereas the upper sequence is composed of pyroclastic flow deposits including welded to no welded ignimbrite intercalated with gravely sandstone and massive clast-support conglomerate toward the top. Facies analysis study of GHS presented eight lithofacies types, which grouped into five lithofacies associations. The GHS basin started with effusive eruption of silica-poor volcanic center, which produced andesitic lava. A part of lava underwent hyaloclastic fragmentation due to the presence of fluvial water in places producing the volcaniclastic mass flow deposits. Later, an explosive silica-rich volcanic center affected the GHS basin and created the pyroclastic plain deposits (ignimbrite and bedded tuff). The fluvial braided river is still in action since the first eruption, producing gravely sandstone, which is intercalated with the volcanic sequence. The upper GHS is characterized by thick, massive, and clast-supported conglomerate (well rounded clasts up to 100 cm) of alluvial fan facies. Several silica-rich and silica-poor subvolcanic intrusions were emplaced in the GHS. The GHS development displays a cycle from low- to high-energy sedimentation under humid climatic conditions, in addition to extension and down faulting of basin shoulders. In comparison with Gabal El Urf, located to the north of GHS and was studied by El-Gameel (2010), the GHS is a lava-rich succession rather than Gabal El Urf succession which is mainly pyroclastic rich.

Depositional settings and evolution of a fjord system during the carboniferous glaciation in Northwest Argentina

Fjord systems, represented by glacial diamictites and postglacial transgressive shales, formed in the basins of western Argentina during the late Carboniferous Gondwana glaciation. Well exposed fjord deposits of the Guandacol Formation were studied in the Loma de Los Piojos region (Protoprecordillera), where they fill a 2.9 km wide paleovalley with steep side walls and a relatively flat floor. The valley cross-cuts Lower Devonian sandstones and Mississippian mudstones and sandstones, and provides evidence of glacial abrasion, including striated pavements and glacial microtopography (grooves, ridges, and striae). Based on the analysis of seven sedimentary logs, eight sedimentary facies in the valley fill were recognized: (A) Massive diamictites; (B) Laminated mudstones with dropstones; (C) Stratified diamictites; (D) Clast-supported conglomerates and sandstones; (E) Deformed diamictites, conglomerates and sandstones; (F) Folded diamictites; (G) Mudstones interbedded with sandstones, and (H) Stacked and amalgamated sandstones. These sedimentary facies are grouped into two principal facies assemblages that represent different stages of the paleovalley fill. Assemblage 1 is composed of diamictites (Facies A, C and F), laminated mudstones with dropstones (Facies B), and conglomerates (Facies D and E), which represent glacially influenced sedimentation in the paleovalley. Assemblage 2 represents the paleovalley fill when glacial influence ceased, and comprises laminated mudstones interbedded with sandstones (facies G) and stacked sandstone beds (facies H) that mostly record deltaic sedimentation. Stratigraphic relationships, plant fossils found in the paleovalley walls and palynological assemblages recovered in mudstones of facies D help to establish an early Pennsylvanian age for both the incision and the filling of the paleovalley. The studied paleovalley records an exceptional example of the western Gondwanan glacial to postglacial transition. Due to the continuous stratigraphic succession within the paleovalley as well as palynological, megafloristic and radiometric data, this example provides a complete framework of the late Carboniferous postglacial evolution in western Gondwana.

Geological investigation of river terraces along the Modi Khola valley, Parbat district, western Nepal Lesser Himalaya

Fluvial terraces are remnants of the former floodplain of a stream or river. Wide river valley with spectacular terraces are the well-known places of the Himalaya because of their unique morphology and history of origin. The Modi Khola and Kali Gandaki valleys in west Nepal, Lesser Himalaya are characterized by a number of spectacular river terraces. In the present study, the river terraces along the Modi Khola valley were mapped at 1:25,000 scale and their lithological and sedimentological characteristics were investigated. The river terraces along the Modi Khola are distributed in at least three levels. They are named as Godam Formation (Higger Terrace), Badagaun Formation (Middle Terrace) and the Modibeni Formation (Lower Terrace). The Godam Formation is standing out at an elevation between 860 m and 1100 m. The upper surface of the terrace is extremely rugged with karstic features. The Godam Formation is characterized by matrix-supported, calcite cemented conglomerate with angular to sub-angular clasts of calcareous shale, laminated siliceous limestone and quartzite. The Badagaun Formation (Middle Terrace) is distributed at an altitude from 700 to 860 m on both sides of the Modi Khola valley. It is deeply weathered forming a very thick (almost 3-5 m) residual soil (mostly lateritic soil). Karst features such as caves and sink holes are common in this terrace. This formation is mainly composed of matrix-supported conglomerate with granule-, cobble- and boulder-sized clasts of calcareous shale, siliceous limestone and rarely quartzite. The Lower Terrace (Modibeni Formation) is fluvial deposit consisting of rounded and subrounded clast-supported conglomerate. Both the Upper and Middle Terraces have been interpreted as cohesive debris flow facies derived from the Annapurna Range. Presence of three levels of terraces indicates at least three phase of uplift in the area in the Holocene time. The debris flow deposit in the upper and Middle Terraces indicates catastrophic outbursts from natural dams or large rock-ice avalanches in the Annapurna Range triggered probably by megaearthquakes as in Pokhara valley as suggested by Schwanghart et al. (2015).

The significance and timing of sheetflood vs braided channel deposition on lacustrine fan deltas, Junggar Basin, NW China

ABSTRACTThe fan-delta reservoir play has become an important exploration target within the Junggar Basin, especially in the Mabei area within the Mahu Sag, where a fan-delta oil and gas pool has been recently discovered. The sedimentary characteristics, distribution patterns and formation mechanisms of the fan-delta clastic bodies in Lower Triassic Baikouquan Formation (T1b) were studied using seismic, well log and core data accompanied by a flume tank experiment and a modern analogue depositional study. The T1b in the Mabei area is composed of a fan delta consisting of fan-delta plain (including debris flow deposits, sheetflood deposits, braided channel deposits, and floodplain deposits), fan-delta front (including subaqueous reworked sheetflood deposits and distal sheetflood deposits) and muddy lake deposits. The sheetflood deposits, characterised by moderately sorted conglomerates with brown matrix, form during the peak flooding. They are widespread and sheet-like, occupying the major portion of the fan-delta plain. The braided channel deposits are characterised by well-sorted and clast-supported conglomerates and pebbly sandstones, formed later during the falling flow stage. Owing to the decline in volume and velocity, and the formation of continuously braided fluid flow, the sediments of the flood period are reworked, generating the braided channels. Laterally, the braided channels occur as belts of clastic bodies surrounded by continuous sheetflood deposits. Vertically, the braided channels are interbedded between the sheetflood deposits. The subaqueous reworked sheetflood deposits are characterised by greyish-green, well-sorted and clast-supported conglomerates, whereas the distal sheetflood deposits are characterised by well-sorted sandstones, intercalated with mudstone. The subaqueous reworked sheetflood and distal sheetflood deposits are a distal partly subaqueous extension of the main sheetflood deposits, albeit reworked by basinal currents and waves. The distal sheetflood deposits form on distal fringes of the sheetfloods and are more thoroughly reworked by longshore and wave currents. The braided channel, subaqueous reworked sheetflood and distal sheetflood deposits can form high-porosity reservoirs. These findings challenge the common view and suggest that the channelised facies on the fans are not the main flood events; rather, the more extensive sheetfloods are the major flood events.

Sedimentary Facies and Architecture of a Gigantic Gravelly Submarine Channel System in a Cretaceous Foredeep Trough (the Magallanes Basin, Southern Chile)

The Lago Sofia conglomerate in southern Chile is a deep-marine gravelly deposit, which is hundreds of meters thick and kilometers wide and extends laterally for more than 100 km, filling the foredeep trough of the Cretaceous Magallanes Basin. For understanding the depositional processes and environments of this gigantic deep-sea conglomerate, detailed analyses on sedimentary facies, architecture and paleoflow patterns were carried out, highlighting the differences between the northern (Lago Pehoe and Lago Goic areas) and southern (Lago Sofia area) parts of the study area. The conglomerate bodies in the northern part occur as relatively thin (< 100 m thick), multiple units intervened by thick mudstone-dominated sequences. They show paleoflows toward ENE and S to SW, displaying a converging drainage pattern. In the southern part, the conglomerate bodies are vertically interconnected and form a thick (> 400 m thick) conglomerate sequence with rare intervening fine-grained deposits. Paleoflows are toward SW. The north-to-south variations are also distinct in sedimentary facies. The conglomerate bodies in the southern part are mainly composed of clast-supported conglomerate with sandy matrix, which is interpreted to be deposited from highly concentrated bedload layers under turbidity currents. Those in the northern part are dominated by matrix- to clast-supported conglomerate with muddy matrix, which is interpreted as the products of composite mass flows comprising a turbidity current, a gravelly hyperconcentrated flow and a mud-rich debris flow. All these characteristics suggest that the Lago Sofia conglomerate was formed in centripetally converging submarine channels, not in centrifugally diverging channels of submarine fans. The tributaries in the north were dominated by mass flows, probably affected by channel-bank failures or basin-marginal slope instability processes. In contrast, the trunk channel in the south was mostly filled by tractive processes, which resulted in the vertical and lateral accretion of gravel bars, deposition of gravel dunes and filling of scours and channels, similar to deposits of terrestrial gravel-bed rivers. The trunk channel developed along the axis of foredeep trough and its confinement within the trough is probably responsible for the thick, interconnected channel fills. The large-scale architecture of the trunk-channel fills shows an eastward offset stacking pattern, suggesting that the channel migrated eastwards most likely due to the uplift of the Andean Cordillera.