Pebbly Sandstone Research Articles

Provenance of Paleocene–Eocene red beds from NE Iraq: constraints from framework petrography

AbstractThe red-bed deposits in northern Iraq are situated in an active foreland basin adjacent to the Zagros Orogenic Belt, bound to the north by the Iranian plate thrust over the edge of the Arabian plate. The red-bed successions are composed of alternating red and brown silty mudstones, purplish red calcareous siltstone, fine- to coarse-grained pebbly sandstone and conglomerate. The red beds in the current study can be divided into four parts showing a trend of upward coarsening with fine-grained deposits at the top. A detailed petrographic study was carried out on the sandstone units. The clastic rocks consist mainly of calcite cemented litharenite with rock fragments (volcanic, metamorphic and sedimentary), quartz and minor feldspar. The petrographic components reflect the tectonic system in the source area, laterally ranging from a mixed orogenic and magmatic arc in Mawat–Chwarta area to recycled orogenic material rich in sedimentary rock fragments in the Qandel area. The Cretaceous–Palaeogene foreland basin of northern Iraq formed to the southwest of the Zagros Suture Zone and the Sanandaj–Sirjan Zone of western Iran. During Palaeogene time deposition of the red beds was caused by renewed shortening in the thrust sheets overlying the Arabian margin with uplift of radiolarites (Qulqula Formation), resulting in an influx of radiolarian debris in addition to continuing ophiolitic detritus. Mixed sources, including metamorphic, volcanic and sedimentary terranes, were present during deposition of the upper part of the red beds.

The East Africa Oligocene intertrappean beds: Regional distribution, depositional environments and Afro/Arabian mammal dispersals

The East Africa Oligocene intertrappean beds: Regional distribution, depositional environments and Afro/Arabian mammal dispersals

Procesos gravitacionales-fluviales y preservación de vegetación boscosa en un área volcánica: Formación Rancahué (Oligoceno), Andes neuquinos, Argentina

The siliciclastic deposits of the Rancahue Formation (Oligocene, Alumine Basin), formed in the eastern side of the Andean Cordillera (70oW y 39oS) are studied to evaluate the relationship among different depositional systems in volcanic settings and to disclose the processes that contributed to the preservation of a forest developed at the foot of a volcanic relief. In the alluvial volcano-sedimentary record of the Rancahue Formation an upland petrified forest with 33 logs in life position was identified. This flora, with Nothofagoxylon as the main taxon, was developed under temperate and humid climatic conditions. Three facies associations were defined in the study region. Facies association A was accumulated in subaerial talus and foot-talus slopes, and is composed of monomictic (basaltic) boulder and cobble conglomerates formed by debris floods, hyperconcentrated flows, avalanches, landslides and debris flows. The source of these sediments is local and is related to the growth of an intrabasinal basaltic volcanic edifice. Facies association B consists of cross-stratified, plane bedded and massive conglomerates, pebbly sandstones and coarse-grained sandstones accumulated in a regional braided fluvial system with source areas located in the Andean Cordillera. Facies association C is characterised by fine-grained sediments accumulated in subaerial and subaqueous environments, with periodic contribution of ash falls and development of palaeosols. The forest rooted in deposits of facies association C, and the logs preserved in life position are located within the overlying coarse-grained volcaniclastic deposits of facies association A. The burial and preservation of this vegetation was favoured by the highly aggrading rhythm and comparative low erosive power of the gravitational flows that operated in alluvial and colluvial fans of the riparian hydrogeomorphic zone. Conversely, abundant transported and broken trunks included in the deposits of facies association B clearly indicate the strong erosive power of the currents of the associated braided fluvial system.

Pattern of sedimentary infilling of fossil mammal traps formed in pseudokarst at Cerro de los Batallones, Madrid Basin, central Spain

AbstractFossil mammal sites of late Miocene age (ca 9 Ma) occur in hourglass‐shaped, non‐interconnected cavities up to 15 m deep, hosted in mudstone (mostly sepiolite), chert and carbonate bedrock in Cerro de los Batallones. This paper provides a model for the sedimentary infilling of the cavities, which functioned as traps for vertebrate faunas and contain one of the richest and best preserved Neogene mammal assemblages of the Iberian Peninsula. Generation of the mammal‐bearing cavities started with the solution of underlying evaporites, which resulted in fissures that were subsequently enlarged by subsurface piping, a process rarely preserved in the ancient sedimentary record. The system of subterranean cavities evolved into a pseudokarst landscape, resulting in doline‐like shafts reaching the ancient land surface. The sedimentary infilling of the cavities comprises both clastic and carbonate lithofacies that were investigated by outcrop observation, standard and scanning electron microscope petrography, mineralogical analysis, and stable isotope geochemistry. Gravel and breccia talus deposits, clast and mud‐supported gravel, pebbly sandstone and mudstone are common detrital infill deposits mostly derived by overflow erosion of bedrock. The deposits containing the mammal bones are marls, and occur both in subsurface cavities and doline‐like depressions. In the underground cavities, marlstone was mainly of clastic origin and accumulated in ponds scattered over the floor of the cavity. In contrast, marlstone deposits in the surface dolines formed mostly as a result of biochemical carbonate deposition in small shallow lakes subjected to fluctuation of the water level. The δ18O and δ13C carbonate values indicate different origins for the two kinds of marls. During the final phases of pipe infill the doline marlstone sealed the mammal sites, usually off‐lapping the adjacent bedrock.

Cementation and blackening of Holocene sands by peat-derived humates: A case study from the Great Dune of Pilat, Landes des Gascogne, Southwestern France

The base sand layers of the aeolian Great Dune of Pilat, which stretches along the coast of Arcachon Bay, have been locally impregnated with a dark brown to black amorphous organic substance of humate composition. The humate-cemented sand forms a well-indurated horizon 40–50cm in thickness that developed immediately beneath the Holocene peaty layer (P1“paleosoil”). The humate, identified by means of FT-IR and Raman micro-spectroscopy, acted both as a cementing agent and as a coloring agent; it formed thin coats and meniscus cements between individual sandstone grains which, in turn, caused the dark, asphaltic-like appearance of the sandstone. Field observations, combined with geochemical analyses, and the presence of identical geochemical compounds recognized in the peat and sandstone humate cement, suggest that the peat-containing low-coalified (Rr=0.2%) fragments of higher, submerged and floating plants and marine algae deposited in a saline and reducing environment served as an obvious source rock for the humate. The humate derived from decaying organic remains that descended from the peat into the permeable sand, where it indurated irreversibly over a period shorter than 3500years.The present-day actions of waves and tides contribute to the erosion of the frontal parts of the humate-impregnated sandstone horizon, resulting in the formation of the dark rounded sandstone slabs and pebbles that are deposited on nearby beaches. This process provides a fine example of recent black pebble formation that derived from pre-existing organic matter-impregnated source rocks. In contrast to their tropical carbonate counterparts, however, the humate-induced blackening of the sandstone pebbles appears to be relatively unstable, as the impregnated fragments bleach when exposed to air and seawater for a few seasons.

Facies analysis of a Toarcian–Bajocian shallow marine/coastal succession (Bardas Blancas Formation) in northern Neuquén Basin, Mendoza province, Argentina

Strata of the Bardas Blancas Formation (lower Toarcian–lower Bajocian) are exposed in northern Neuquén Basin. Five sections have been studied in this work. Shoreface/delta front to offshore deposits predominate in four of the sections studied exhibiting a high abundance of hummocky cross-stratified, horizontally bedded and massive sandstones, as well as massive and laminated mudstones. Shell beds and trace fossils of the mixed Skolithos-Cruziana ichnofacies appear in sandstone beds, being related with storm event deposition. Gravel deposits are frequent in only one of these sections, with planar cross-stratified, normal graded and massive orthoconglomerates characterizing fan deltas interstratified with shoreface facies. A fifth outcrop exhibiting planar cross-stratified orthoconglomerates, pebbly sandstones with low-angle stratification and laminated mudstones have been interpreted as fluvial channel deposits and overbank facies. The analysis of the vertical distribution of facies and the recognition of stratigraphic surfaces in two sections in Río Potimalal area let recognized four transgressive–regressive sequences. Forced regressive events are recognized in the regressive intervals. Comparison of vertical distribution of facies also shows differences in thickness in the lower interval among the sections studied. This would be related to variations in accommodation space by previous half-graben structures. The succession shows a retrogradational arrangement of facies related with a widespread transgressive period. Lateral variation of facies let recognize the deepening of the basin through the southwest.

Tidal rhythmites in a deep sea environment: An example from Mio-Pliocene Misaki Formation, Miura Peninsula, Japan

The Mio-Pliocene Misaki Formation, Miura Peninsula, Japan is characterized by alternation of mafic scoriaceous pebbly sandstone, pumiceous sandstone and siltstone, and mudstone formed in a fore-arc basin in an arc–arc collisional zone. The qualitative as well as quantitative evidences of tide during the Misaki sedimentation in the Jogashima area, Miura Peninsula are presented here. The lunar synodic period of ∼28 days/lunar month extracted from the Misaki tidal rhythmite agrees well with the published Miocene tidal rhythmite data. The couplet series of the Misaki tidal rhythmite, however, is often interrupted by downslope resedimentation via turbidity currents, intense penecontemporaneous deformation and bioturbation. Association of deep sea turbidites, mass flow deposits and tidal rhythmite suggest Misaki sedimentation in the Miura Peninsula took place in a submarine canyon setting.

Advances in Arabian stratigraphy: Comparative studies of glaciogenic Juwayl and lower Unayzah strata (Carboniferous – Permian) of Saudi Arabia

ABSTRACT The Juwayl Member of the Wajid Formation, which crops out in the Wajid region of southwest Saudi Arabia, and the Unayzah C and B members of the lower Unayzah Formation in the subsurface of eastern and central parts of the country are all demonstrably late Carboniferous to early Permian in age, based on palynological analysis. Comparative studies of these two stratal units confirm strong genetic similarities between them, suggesting stratigraphic equivalence between the lower part of the Juwayl Member and the Unayzah C member, and the upper part of the Juwayl Member and the Unayzah B member, respectively. The boundary between the lower and upper parts of the Juwayl was not specifically recognized in outcrop, being covered with thick deposits of modern desert sand. The lower Juwayl and the Unayzah C are both interpreted to have been deposited in large glacial valley systems. In the Unayzah C, subsurface isopach mapping suggests down-valley transport from the north, implying that at least some of the ice was situated in localized, upland (alpine) ice caps. Multiple phases of glacial advance and retreat occurred. Glacial outwash sands and gravels (representing retreat phases) were subsequently cannibalized and severely deformed by re-advancing ice and they are identified now only as relict deposits. Glaciotectonic deformation is manifest in the lower Juwayl at outcrop as high to low-angle thrust sheets, and throughout the subsurface numerous shear zones are recognized in the Unayzah C member, both in core as well as in downhole wireline log responses (spectral gamma-ray and image logs). The multiple phases of glacial advance and retreat produced push moraine nappe complexes within which wholesale explosive disruption and redistribution of the sediment is observed. This was a consequence of fluidization related to glacially-induced overpressuring. Potentially significant reservoir heterogeneity arises in the subsurface Unayzah C member in relation to the shear zones, by the creation of reservoir compartments of widely varying extent and with unknown but potentially very poor interconnectedness. The upper Juwayl Member is represented at the outcrop by pebbly sandstones and conglomerates that were laid down upon a braided fluvial, glacial outwash plain. These pass upwards into boulder-bearing siltstones (stratified diamictites) that were deposited in a glaciolacustrine setting. Similar depositional sequences are seen in the subsurface Unayzah B member in the western (basin-marginal) part of the study area. Locally, subsurface data from the Unayzah B in these western areas suggests sustained ice-contact conditions, interpreted as evidence for local sustained “alpine” ice caps throughout Unayzah B time. Farther east, the more basin-central deposits of the Unayzah B member comprise a wide variety of depositional facies, all of which are nonetheless attributed generally to a glaciolacustrine setting. These include: (1) minor ice-contact push moraine deposits indicative of minor glacial re-advance during overall glacial retreat; (2) thick sequences of ice-proximal sublacustrine gravity flow sandstones and massive diamictites; and (3) significantly more ice-distal sublacustrine sandstones and mudrocks, including stratified diamictites and dropstones. In both basin-marginal and basin-central settings, the Unayzah B facies associations everywhere display strong evidence for sustained melting during terminal retreat of the Gondwanan ice sheets. This resulted in sustained infill and deepening and eventual overspilling of the basin-central lakes leading to widespread flooding throughout the basin. This flooding was ultimately manifest also at the basin margins in the west of the study area. A high degree of reservoir heterogeneity occurs within the subsurface Unayzah B member as a result of the wide variety of depositional facies. Reservoir quality correspondingly ranges from excellent to non-reservoir.

Use of Statistical Parameters in the Sedimentological Study of Conglomerate Deposits in Northeastern Part of Akwa Ibom State, Niger Delta Basin, Nigeria

Conglomerates and pebbly sandstone abound along a belt of about 50km long and 11km wide in northeastern part of Akwa Ibom State. Granulometric analysis of 29 samples show the deposits to be poorly to moderately sorted, having sorting coefficient values ranging between 0.63ф to 1.41ф. Pebble morphometric analysis of the conglomerates showed that the mean values of the various morphometric parameters range as follows: flatness ratio (S/L = 0.39-0.58), elongation ratio (I/L =0.64-0.78), maximum projection sphericity (Ψ P = 0.60-0.77), Oblate Prolate index (Ō P = -1.98 to 4.13), coefficient of flatness (39.25-57.86). Roundness index determined for the pebbles through simple comparism with Power’s (1953) roundness chart averaged 0.232. Interpretation of pebble morphometric and granulometric results indicate a fluvial environment of deposition for the deposits.

A new Oligo-Miocene Mammal-Bearing site from a sedimentary intercalation in the trap basalts of Central Eritrea

We report the findings of two proboscideans from a sedimentary intercalation within the Central Eritrea Tertiary Trap basalt succession. These intertrappean sediments can be traced continuously between Tera’emni and Adi Quala in the Mendefera region, and are interbedded between the underlying Asmara/Aiba-Alaji basalts and the overlying Adi Ugri basalt. This intercalation is up to 50 m thick and consists of a predominant intensely pedogenized red, green and gray mudstone and siltstone with subordinate channelized pebbly sandstone. The newly discovered mammal remains come from the coarse-grained deposits in the Mai Gobro section. Available radiometric datings of the host volcanic rocks constrain the age of the fossils between 24.6 and 22.1 Ma, i.e. at the transition between the Oligocene and the Miocene. The fossil vertebrate remains from Mai Gobro represent two proboscidean families, Deinotheriidae and Gomphotheriidae. The morphological grade of the two Mai Gobro proboscideans would suggest a more derived stage than that of representatives of the same families known from other Oligocene African sited (e.g. Chilga, Ethiopia), suggesting a possible Oligo-Miocene age.

Stonehammer Geopark, New Brunswick, Canada

Stonehammer Geopark of New Brunswick, Canada is one of the newest destinations in the UNESCO‐assisted Global Geoparks Network, and the first in North America. Its rocks tell the amazing billion‐year story of the evolution of eastern North America. Highlights include some of the finest Precambrian stromatolites in the world, a Cambrian site that yielded one of the world's largest trilobites, and a Silurian section with a rich fauna of fish, crustaceans and sea scorpions. At the famous ‘Reversing Falls’ in Saint John, visitors can also see the boundary between the Late Precambrian Brookville Terrane and the western part of the Avalon Terrane, and learn about the late Silurian closure of the Iapetus Ocean during the final stages of the accretion of North America. In and around the city, Devonian pebbly sandstones represent the erosive remnants of the highlands formed as a result of that terrane accretion event, and slightly younger Carboniferous exposures further shed light on the evolution of life at a time when the region lay on the equator. These include early Carboniferous clubmoss forests preserved on the main highway east of Saint John, and late Carboniferous deposits rich in the remains of fossil plants, insects, giant arthropods, and some of the earliest reptiles—the latter recorded by spectacular trackways. Scattered Mesozoic outcrops and Quaternary moraines continue the story to the present day. Positioned at the gateway of Atlantic Canada, Stonehammer Geopark is a strategic part of an emerging geotouristic network that also includes UNESCO World Heritage Sites at nearby Misguasha, Quebec (Devonian coastal ecosystems) and Joggins, Nova Scotia (Pennsylvanian fossil forests). It presents a brilliant opportunity to teach visitors of the extraordinary story of Earth's evolution.

Preliminary results of studies of the Valea Morilor Upper Palaeolithic site (Chişinău, Republic of Moldova): A new camp of mammoth hunters

Preliminary results are presented from the studies of a newly found Paleolithic site – Valea Morilor (Chişinău, Republic of Moldova). The excavations produced unquestionable evidence of mammoth hunting (Mammuthus primigenius Blumenbach, 1799). Excavations of 2009–2010 opened an area of 1264 m2. Three radiocarbon datings (14C) of the fragments of fossil bones of mammots performed in the Gröningen University gave very similar results: in the excavation sector Nr.1 - 20770 ± 90 BP (GrA-46004;Obadă and van der Plicht, 2010); the sector Nr.2 - 20570 ± 80 BP (GrA-52424); and the sector Nr.3 - 20560 ± 80 BP (GrA-52425). Faunal remains are represented by more than 550 mammoth bones (belonging to at least 6 animals), as well as bones of bison (Bison priscus Bojanus, 1827), small mammals [Ochotona pusilla?; Spalax zemni Erxleben, 1777; Lagurus lagurus (Pallas, 1773)] and terrestrial molluscs Helicopsis striata (Mull.), Pupilla muscorum (L.), Vallonia pulchella (Mull.), and Succinea oblonga (Drap.).The assemblage of lithic tools includes 72 objects made of flint, 7 sandstone pebbles, and one made of white quartz. It seems likely that at least one wind-break (shelter) was constructed by the site dwellers. Numerous bones bear traces of cutting, splinting, grinding, and scraping. Darts and spear points found in the vicinity of large bones suggest the weapons deeply penetrated into the animals' soft tissues, and indicate that the Paleolithic inhabitants of the site actively hunted mammoths. A mammoth's elbow bone (ulna) was pierced with some weapon.

3.2 Ga eolian deposits from the Moodies Group, Barberton Greenstone Belt, South Africa: Implications for the origin of first-cycle quartz sandstones

The 3.2Ga Moodies Group in Barberton Greenstone Belt, South Africa, contains the oldest recognized evidence for eolian processes on Earth. These eolian facies indicate that significant wind regimes existed in the Archean atmosphere. Eolian deposits are developed in the lower Moodies Group in the central part of the belt and consist of stacked inversely graded strata ranging in thickness from 2 to 10mm.Individual strata consist of fine- to medium-grained quartz sandstone at the base that passes vertically into very coarse- to coarse-grained quartz sandstone. Strata make up cross-bed sets up to 3–5m thick. Sets of wind ripple strata are either tabular tangential or in the form of broad troughs. This type of inversely graded stratification is the product of wind-ripple migration. The coarse grain size of the sediment combined with the low-angle of inclination of the strata is best interpreted as the product of sand sheet or dune plinth preservation. Based on the geometry of eolian set boundaries, dunes were probably simple barchan form.The tens-of-meters thick eolian deposits are underlain by braided stream deposits composed of subfeldspathic, pebbly sandstones. The vertical change from braided-stream subfeldspathic sandstones to eolian quartz arenites, highlights the underappreciated capacity of eolian processes to generate first-cycle quartz sandstones on landscapes pre-dating the advent of land vegetation.

Depositional architecture and sequence stratigraphic correlation of Upper Ordovician glaciogenic deposits, Illizi Basin, Algeria

Abstract Upper Ordovician glaciogenic deposits are profoundly important as hydrocarbon reservoirs across North Africa, such as within the Illizi Basin of SE Algeria. In this study we present a new sedimentological and sequence stratigraphic model for Upper Ordovician glaciogenic deposits based on the analysis of core descriptions and wireline logs from 25 wells in the Tiguentourine Field. Within the glaciogenic succession, two ice advance–retreat cycles can be defined, consisting of glaciomarine ice-contact fan deposits and tillites. Deposits of the marine ice-contact fan systems generally show a retrogradational stacking pattern from ice-proximal to ice-distal deposits. This pattern is attributed to the deposition in front of a retreating ice sheet. The proximal marine ice-contact fan deposits consist of massive or low-angle cross-bedded pebbly sandstone. They are interpreted as the deposits of turbulent, high-energy plane-wall jets, emerging from subglacial meltwater conduits. These jet-efflux deposits are up to 60 m thick and interbedded with deposits of cohesive and non-cohesive debris flows. The jet-efflux deposits are overlain by fine-grained, thick-bedded massive sandstone. These mid-fan deposits build up the bulk of the glaciomarine fans and are interpreted as deposits of underflows, generated at the point of flow-detachment, where marine meltwater jets become buoyant and large volumes of sediment fall-out from suspension. In the upper part of the fan succession massive sandstones pass upwards into mud-prone massive sandstones, interpreted as deposits of cohesive sandy debris flows. The most ice-distal deposits are muddy sandstones and mudstones deposited by waning low-density turbulent flows and suspension fall-out. The best reservoir properties within the glaciogenic succession are attributed to the proximal and medial deposits of the ice-contact fans such as coarse-grained jet-efflux deposits and sustained high-density turbulent flow deposits. However, the mud content within the massive sandstones is highly variable and influences the reservoir quality. Both glacial depositional sequences infill 60–175 m deep, elongated depressions, which are interpreted as subglacial tunnel valleys. These tunnel valleys acted as depocentres for the glaciomarine fan deposits. After final deglaciation and post-glacial transgression, organic-rich shale was preferentially deposited in underfilled tunnel valleys.

Proglacial deposition and deformation in the Upper Carboniferous to Lower Permian Wynyard Formation, Tasmania: A process analysis

The Wynyard Formation of Tasmania, Australia, provides the youngest evidence of grounded ice during the Late Carboniferous–Early Permian in the Tasmania Basin of southeastern Gondwana during the late Paleozoic ice age. Within the Wynyard Formation, four facies associations are recognized: 1) massive diamictite, 2) stratified diamictite, 3) conglomerate and sandstone, and 4) deformed mudstone and fine sandstone. A detailed facies analysis was performed in order to interpret the depositional processes and environments. The massive diamictite facies association contains massive diamictite and sandstone and was deposited primarily by iceberg rain-out in a subaqueous morainal bank setting. The stratified diamictite facies association is composed of stratified diamictite, sandstone, and conglomerate, and was deposited by debris flows and iceberg rain-out, and deformed by glacial pushing. The conglomerate and sandstone facies association is made up of cross-stratified, channelized conglomerate and pebbly sandstone that were deposited by glacial outwash on grounding-line fans. The deformed mudstone and fine sandstone facies association is composed of pervasively deformed pebbly mudstone and fine-grained sandstone that exhibit volcano, flame, and dyke structures. This facies association was deposited by a combination of suspension settling, iceberg rain-out, and soft sediment deformation, in a quiet distal proglacial setting. The facies analysis supports the interpretation that the Wynyard Formation was deposited by a wet-based, tidewater glacier or glaciers. The depositing glacier(s) occupied a ~ 40 km-wide trough in northwest Tasmania. Based on the distribution of the Wynyard Formation and the presence of glacial deposits in other eroded troughs in Tasmania, glaciation occurred in the region within broadly eroded valleys. The Wynyard Formation and overlying mudstone of the Inglis Formation provide a glacial–postglacial signature similar to those in South Africa, Antarctica, and eastern Australia during the latest Carboniferous–Early Permian, therefore the climate warmed in southern Gondwana following this glacial interval. The continued glaciation in eastern Australia was a result of regional topography and oceanographic processes.

Halaqin volcano-sedimentary succession in the central-northern margin of the North China Craton: Products of Late Paleoproterozoic ridge subduction

The Halaqin volcano-sedimentary succession (HVSS) is tectonically situated on top of the largely Late Archean to Early Paleoproterozoic Yinshan terrane in the central-northern margin of the North China Craton. It comprises greenschist- to amphibolite-facies schists, quartzites, marbles and amphibolites, which were originally pelites, pebbly sandstones, sandstones, limestones and volcanic rocks. The presence of tight to isoclinal folds and thrusts of the HVSS indicates tectonic transport from southeast to northwest (present coordinates). U–Pb ages of detrital zircon grains in the clastic rocks suggest provenance from both the Fengzhen belt in the south and the Yinshan terrane in the north. Zircon U–Pb ages of the HVSS volcanic rocks, e.g., 1909 ± 10 Ma, 1894 ± 6 Ma, 1887 ± 6 Ma, and 1884 ± 8 Ma indicate volcanism in the period ∼1910–1880 Ma. Detrital zircon age study of the neighbouring Majiadian Group reveals significant ∼2700 Ma crust growth and ∼2500 Ma metamorphism events in the Yinshan terrane. The HVSS volcanic rocks have a trimodal geochemical distribution demonstrated by SiO 2-contents of 45–50 wt.% (basalts), 53–60 wt.% (andesites), and 70–76 wt.% (dacites–rhyolites). The basalts and andesites are tholeiitic in chemistry, and show distinct depletion in high-field strength elements (HFSE, e.g., Nb, Zr, Ti), compared to neighbouring elements in spider-diagrams. Specifically, low-Mg basalts and andesites (MgO < 5 wt.%) have enriched light REE patterns (La/Yb N = 2.79–7.89), and enriched large ion lithophile elements (LILE). However, high-Mg rocks (MgO > 5 wt.%) have more or less flat REE patterns (La/Yb N = 0.45–1.76) and LILE. ɛNd t ( t = 1890 Ma) values of the basalts and andesites vary from −4.8 to + 0.4. The rhyolites and dacites have strongly enriched light trace element patterns (La/Yb N = 8.49–47.9) and LILE, but depleted HFSE (e.g., Nb, Zr, Ti). The basalts and basaltic andesites most likely originated from the asthenosphere contaminated by melts from the lithosphere, whereas the dacites and rhyolites likely originated from the crust. We propose that the basalts and andesites are extrusive counterparts of the coeval and geochemically similar Xuwujia gabbronorites, and the dacites and rhyolites are upper crustal equivalents of the contemporaneous, chemically comparable Liangcheng granitoids, which are in the originally underlying Fengzhen belt. We suggest that the HVSS was most likely generated by ridge subduction below active continental margin just prior to the final amalgamation of the North China Craton. This ridge subduction contributed to the ultra-high temperature metamorphism in this area. Terminal collision tectonics was responsible for the regional metamorphism and high deformation of the Halaqin succession, when it was transported northwestwards onto the Yinshan terrane.

Architecture analysis of a river flood-dominated delta during an overall sea-level rise (early Pliocene, SE Spain)

The sedimentary record of delta-front to prodelta transition that developed during the early Pliocene (Bajo Segura Basin, peri-Mediterranean Betic Cordillera Basin) represents an unusual infilling of an incised valley cutting downwards into late Messinian pelagic marls. The complete delta succession, less than 65 m thick, consists of four upwardly fining and thinning sequences several metres thick. Each sequence consists of basal conglomeratic and coarse-grained sandstone beds (delta-front deposits) evolving upwards to alternating sandstones and silty-marl beds (prodelta deposits). The retrogradational stratal succession pattern is interpreted as having been deposited during an overall sea-level rise. Facies association is characterised by quasi- to planar-laminated sandstones and pebbly sandstones alternating with clast-supported boulders to pebbles, sheet-like climbing cross-laminated sandstones with abundant plant remains, and epsilon cross-stratified and channelized gravels and sandstones. These are interpreted as having been deposited by highly concentrated flows interpreted as hyperpycnal turbidity flows from high (cohesionless debris flows and sinuous submarine channels) to moderate (sheet-like climbing cross-laminated sandstones) discharge river-flood events. Slabs of sediment from unstable banks, outsized clasts which roll onto thin beds from channel mouths, slumps and intraformational breccias in delta front are other signatures (including a lack of bored clasts) which indicate high-energy and rapid deposition at the river mouth, encouraging the delta-front gravitational instability failures. Major changes in the discharge of the river-fed delta could have been caused by brief catastrophic, heavy-precipitation events coupled with quasi-stationary convective orographic rainfall typical of the modern Mediterranean.

Jurassic volcanic and sedimentary rocks of the La Silla and Todos Santos Formations, Chiapas: Record of Nazas arc magmatism and rift-basin formation prior to opening of the Gulf of Mexico

Stratigraphic relationships, detrital zircon provenance, U-Pb and 40 Ar/ 39 Ar geochronology, and trace element geochemistry in volcanic and sedimentary rocks of the Sierra homocline of central Chiapas near La Angostura reservoir in Mexico document an extensive pulse of Early–Middle Jurassic arc magmatism in rocks that overlie and intrude the Permian–Triassic Chiapas massif. Upper Jurassic rift-basin strata unconformably overlie the volcanic rocks and the massif. A Pliensbachian U-Pb (zircon) SHRIMP (sensitive high-resolution ion microprobe) age from porphyritic andesite (191.0 ± 3.0 Ma), Early to Middle Jurassic 40 Ar/ 39 Ar dates from andesitic dikes, U-Pb grain ages of detrital zircons in overlying strata (196–161 Ma), and previously reported K-Ar dates indicate that subduction-related magmatism occurred in the western portion of the Maya block from Early to latest Middle Jurassic time. We assign the volcanic rocks to the La Silla Formation, which correlates with the informal Pueblo Viejo andesite of the Cintalapa and Uzpanapa regions to the northwest. La Silla magmatism predates opening of the Gulf of Mexico Basin. The Todos Santos Formation, which overlies La Silla Formation, was deposited in extensional basins during the early stages of gulf opening. We recognize a lower El Diamante Member of the Todos Santos, consisting of red fluvial sandstone, mudstone, and minor conglomerate containing primarily volcanic-lithic detritus; this member is characterized by a nearly unimodal Jurassic detrital zircon age population that indicates a Callovian or younger depositional age. Volcanic activity continued into the upper part of the El Diamante Member, but with a more mafic character. We also recognize an upper member, which we term the Jerico Member. This member is characterized by thickly bedded, coarse-grained pebbly arkose intercalated with several thick intervals (tens of meters) of conglomerate and pebbly sandstone. Sandstone petrology indicates a source in the granitic rocks of the Chiapas massif, with a tendency to show deep-seated sources and a diverse zircon population in the upper part of the section. The upper Todos Santos Formation in the study area is gradational into the overlying San Ricardo Formation (Kimmeridgian–Tithonian). The La Silla Formation was deposited in volcanic-complex environments, with a clear lack of differentiated volcanic rocks. Fluvial strata of the El Diamante Member were deposited in a mud-rich sinuous river system. The Jerico Member was deposited in large, sand-rich fluvial systems, which probably represent deposits of rift-axis trunk streams; conglomerate facies were deposited in adjacent and interfingering alluvial fan systems. We suggest that the stratigraphic record of the western Maya block records a transition from volcanic arc to intra-arc basin and subsequently to rift basin during Pliensbachian to Oxfordian time.

Chapter 15 The Witvlei Group of East-Central Namibia

Abstract The Witvlei Group is preserved in two regional synclinoria in the Gobabis-Witvlei area of east-central Namibia and as isolated outcrops 90 km SW of Rehoboth, itself some 200 km south of that area. It consists of mixed, coarse- to fine-grained siliciclastic and carbonate strata deposited in deep- to shallow-marine, and locally non-marine, settings along the post-rift continental margin of the Kalahari Craton prior to the onset of foreland basin sedimentation recorded by the overlying terminal Neoproterozoic–Cambrian Nama Group. No direct age constrains exist for the Witvlei Group, but it post-dates c. 800 Ma rift-related rocks and pre-dates the c . 548 Ma base of the Nama Group, thereby placing it as Cryogenian to Ediacaran in age. The Witvlei Group consists of three main units, from oldest to youngest, the Blaubeker, Court and Buschmannsklippe Formations. The Blaubeker Formation is highly variable in thickness and can be as much as 1000 m thick. It consists mostly of massive, polymict diamictite and, in the area of the type locality, contains conglomerate and pebbly sandstone beds. The diamictic strata combined with the presence of numerous faceted and striated clasts provide the evidence for glaciogenic influences on sedimentation. The highly variable thickness pattern likely reflects the infill of palaeo-valleys formed by the deep erosion and scouring of bedrock by ice, and the conglomerates and pebbly sandstones record glacial outwash processes. The Tahiti Formation is a locally developed, fine-grained sandstone above the Blaubeker Formation. It is poorly exposed and its exact stratigraphic relationship to the Blaubeker rocks and overlying Court Formation remains to be determined. The Blaubeker rocks are overlain sharply by the basal unit of the Court Formation, the Gobabis Member. This Member is from 20 to 60 m thick and consists mostly of dark and light grey laminated dolostones that display a δ 13 C carbonate profile that rises from values of −4‰ in the lowermost beds to values of 5‰ in the topmost. The Gobabis Member is conformably overlain by the shales, marls and thin limestones of the Constance Member followed by quartzites of the uppermost unit of the Court Formation, the Simmenau Member. The basal unit of the Buschmannsklippe Formation is the light to tan and pink grey dolostone of the Bildah Member. Its basal contact is sharp everywhere, and it is gradationally overlain by a coarsening (shoaling) upward succession from shales, thin limestones (some exhibiting formerly aragonitic fans) and fine sandstones of the La Fraque Member, to interbedded quartzites and stromatolitic and cherty dolostones of the Okambara Member. The δ 13 C carbonate profile for the Buschmannsklippe rocks shows that the basal beds of the Bildah Member begin at –4‰, followed by a decline to –6‰ in the lower La Fraque limestones and then a rise to –3‰ in the dolostones of the Okambara Member before being truncated by the base of the regionally unconformably overlying basal Weissberg Quartzite Member of the lower Nama Group. Although no glacial sediments have been recognized below the Bildah Member, its lithofacies character, stratigraphic position and C-isotopic profile are compatible with and strikingly similar to younger Cryogenian cap carbonates. Thus, the Witvlei Group arguably contains both the older and younger cap carbonates of Neoproterozoic time, but only the older Cryogenian glacial deposit.

Application of Markov chain analysis to a fining-upward fluvial succession of the Early Permian Warchha Sandstone, Salt Range, Pakistan

Markov chain analysis is applied to the cyclic properties and degree of ordering of lithofacies in the Early Permian (Artinskian) Warchha Sandstone. The 30 to 155 m-thick Warchha Sandstone is well exposed in the Salt Range, Pakistan and dominantly composed of sandstone, siltstone and claystone succession. Seven lithofacies have been identified on the basis of geometry, gross lithology and sedimentary structures. Lithofacies are cyclically arranged in a fining-upward pattern. A complete cycle starts with pebbly sandstone accomplished by thin layer of basal conglomerate and terminates with claystone. The non-stationary first order Markov chains have been applied statistically on vertical transition of facies using outcrop data of the Warchha Sandstone succession. Later, a chi-square test was applied to test the dependency between any two facies states in facies transitions. The results of this study reveal that the sediments of the Warchha succession were controlled by Makovian mechanism and as a whole represent fluvial succession deposited in a predictable cyclic arrangement of lithofacies.