Distal Facies Research Articles

Mineralogical and petrographic features of metakomatiites of the Kostomuksha greenstone structure (Karelia)

Object of study. The paper presents the mineralogical and petrographic study results of metamorphosed and metasomatized komatiites and komatiitic peridotites from the Ozerki soapstone deposit and Pentinsuo prospect, located in the Kostomuksha greenstone structure of the Karelian Craton, Fennoscandian Shield. Material and methods. Surface and drill core samples of various mineral and structural-textural varieties of altered komatiites were studied by optical microscopy, electron microscopy with an attachment for microanalysis, XRD phase, thermogravimetric and chemical analysis. Results. Soapstone formation in the investigated localities displays a multistage alteration and associates with the superimposed alteration of Mg-rich metakomatiite flows and olivine cumulates due to influx of carbon dioxide bearing fluids enriched in calcium and potassium. The chemical composition of initial komatiite (MgO content of the rock) and the degree of its fracture intensity are the main factors controlling soapstone formation. In differentiated lava flows soapstone is formed mainly in high-magnesium cumulate zones characterized by a high content of serpentine. In the flow top and spinifex zones an early amphibole-chlorite-magnetite mineral association is preserved in varying degrees. Talc and carbonate are formed by the decomposition of serpentine and amphibole. The chlorite content in soapstone is controlled by the Al2O3 concentration in the respective flow zones. Conclusion. The carbonate-chlorite-talc is the general natural type of soapstone associated with thin komatiite lava flows of distal volcanic facies. In more thick proximal lava flows the formation of soapstone of chloritecarbonate-talc and talc-carbonate composition is possible. The most prefered prospecting areas for soapstone are fields proximal to the eruptive vent, main lava conduits and subvolcanic analogues of komatiites.

Geomechanical prestack depth migration of the Kraken 3D (Browse Basin, Australia)

Conventional pre-stack depth migration applied to the broadband Kraken 3D Marine Seismic Survey was unable to fully resolve short-wavelength velocity anomalies below the sea floor causing obvious imaging problems and limiting depth conversion and amplitude interpretation. Improved imaging was achieved by initiating tomography using a velocity model built by combining geomechanics with rock physics appropriate for shallow carbonates and mudrocks. 3D gravity modelling using high-resolution bathymetry and compaction trends constitutes a new approach for iteratively building a 3D geomechanical model. Effective stress is derived by applying Terzaghi’s principle within an integration (along depth) involving the model bulk and fluid densities and the vertical component of gravity (all of which may vary spatially, using more refined models). Carbonate and mudrock rock physics models, believed to be appropriate for Neogene sediments along much of the NW Shelf of Australia were derived from abundant core and wireline data acquired during the recent IODP Expedition 356. These models provide the necessary link between effective stress and P-wave velocity with Backus averaging handling the “seismic scale” mixing of different lithologies expected in the Kraken 3D area. Kirchhoff prestack depth migration was revisited from archived preprocessed gathers using the geomechanical model combined with Common reflection angle gathers (CRAM) to initiate tomography. CRAM gathers were very effective in the presence of multiples and complex near surface topography to achieve a better update in tomography. Heavy smoothing of velocities was imposed where sequence stratigraphic interpretation suggests only distal mudstone facies. Tomography in shallower layers was then revisited to restore geologically plausible depth structures and deliver a clear improvement in imaging relative to previous processing efforts.

Material recycling in a sediment‐starved trench recorded in the Early Cretaceous Shiriya accretionary complex, Northeast Japan

AbstractIdentifying and examining geological processes that have occurred in sediment‐starved trenches of ancient non‐accretionary subduction zones exposed on land are still challenging, because such style of subduction is believed to leave scarce rock records. Our new geological mapping, petrography of coarse clastic rocks, radiolarian and zircon U–Pb dating of the Shiriya accretionary complex in Northeast Japan suggest that the younger parts (Sr1 unit) of the complex formed in a sediment‐starved trench. The key observation is that debris derived from the inner trench slope directly overlies pelagic chert in many sections where trench turbidites are lacking. A significant mass of turbidites occurring in other sections are also considered to represent distal facies of the debrites from the adjacent inner trench slope, rather than normal trench‐fill turbidites directly supplied from the continental landmass. These recycled materials from the inner trench slope comprised an imbricate frontal accretionary wedge, together with pelagic siliceous deposits of ocean floor origin. This accretionary wedge of debrite origin suggests that mass wasting on inner slopes of sediment‐starved trenches does not always result in tectonic erosion (removal of materials from the upper plate), but results in material recycling to reconstruct a new frontal accretionary wedge. The spatial dimensions of the recycling cell may be one of the critical differences between accretionary and non‐accretionary margins. Recycled continental materials transported by turbidites from remote landmasses construct frontal wedges in accretionary margins, whereas mass wasting on adjacent slopes and re‐accretion at the trench recycle continental materials in non‐accretionary margins. The transition from an accretionary to non‐accretionary (i.e. sediment‐starved) trench detected in the Shiriya Complex potentially records tectonic response of the inner trench slope to increased friction along the subduction interface.

Response of benthic foraminiferal assemblages to contrasting environments during the Aalenian–Bajocian in the Iberia: a case study from the Talveila section (Iberian Range) and Murtinheira section (Lusitanian Basin)

This paper analyses the composition of the foraminiferal assemblages recorded in two different depositional contexts located in Iberia for the lower Aalenian–lower Bajocian stratigraphical interval: the Talveila section, corresponding to proximal facies within the Iberian Basin (NE Spain), and the Murtinheira section representing distal facies within the Lusitanian Basin (Western Portugal). The obtained specimens (10,736 in total) correspond to 6 suborders, 10 superfamilies, 16 families and 30 genera in Talveila, and 6 suborders, 9 superfamilies, 14 families and 26 genera in Murtinheira. Several biostratigraphic units based on the foraminiferal record and accurately calibrated with the ammonite record were recognized in both sections. Moreover, bioevents based on the first or last appearances of a taxon, significant changes in the abundance of one or several taxa or noticeable changes in diversity of the assemblages were also identified. The palaeoecological analysis shows that the foraminiferal assemblages from both sections were developed in a well-oxygenated and normal salinity shelf environment. The application of diversity indexes indicates that the paleoenvironmental conditions did not remain constant throughout the studied stratigraphic interval; changes recognized in both sections are similar, coeval and correspond to three intervals representing paleoenvironmental conditions more or less favourable for the development of the foraminiferal assemblages. As so, despite the different paleogeographical locations, the development of the assemblages in both sections during the Early Aalenian–Early Bajocian seem to have been conditioned by environmental changes of regional scale, which affected at the same time both the Iberian and the Lusitanian basins.

Carbonate REE + Y signatures from the restricted early marine phase of South Atlantic Ocean (late Aptian – Albian): The influence of early anoxic diagenesis on shale-normalized REE + Y patterns of ancient carbonate rocks

Rare earth elements plus yttrium have been extensively applied in paleoenvironmental studies of ancient carbonate successions due to their fractionation in the marine environment. However, in modern marine anoxic environments, seawater REE + Y signatures can be suppressed in reducing conditions (e.g. stagnant basins and/or early diagenesis) by REE remobilization from several sedimentary components (e.g. detrital siliciclastics, oxides, organic compounds). We present the shale-normalized REE + Y signatures for a transgressive marine carbonate succession of the primitive South Atlantic Ocean (latest Aptian-Albian), in a restricted marine setting with anoxic bottom conditions, to provide and evaluate the REE + Y record deposited in such conditions. Based on well-constrained paleoenvironmental reconstruction through microfacies analysis, three shale normalized REE + Y patterns were identified, varying according to the microfacies associations (MA) and related diagenetic environments: a) light REE-enriched patterns in the more proximal MA associated with burial diagenesis at the base of section; b) flat patterns towards distal MA related to anoxic and sulfidic early diagenesis; and c) middle REE-enriched patterns in MA related to dissolution of FeMn oxy-hydroxide grains under reducing early diagenesis. Cerium anomalies are absent due to stagnant water mass conditions, in accordance with previous studies for the early marine South Atlantic. Y/Ho ratios are lower than modern seawater values, with an increasing trend towards more distal facies. The “non-seawater” shale-normalized REE + Y patterns of the marine carbonates from the primitive South Atlantic Ocean corroborate studies in modern marine reducing environments, in which REE can be remobilized from detrital phases, oxides and organic compounds, suppressing the primary seawater REE + Y signature of authigenic minerals. Caution is suggested for the sole use of carbonate REE + Y signatures as paleoenvironmental proxies, especially in ancient successions lacking proper microfaciological components.

Geological characteristics and reservoir properties in the unconventional Montney Formation, southwestern Alberta, Canada

In order to understand the unconventional reservoir characteristics in the Montney Formation, the detailed geological examinations, such as core interpretation, petrographic analysis and the poro-perm measurement (including MICP), have been performed from the Kakwa well, drilled in southwestern Alberta, Canada. The studied Montney section, mostly comprised of fine to coarse siltstone, can be informally subdivided into three distinctive intervals, on the basis of a prominent log marker, inferred to represent a major regional falling and flooding surface. The core description and the log characters have confirmed that the entire section mostly comprises a series of multi-stacked, higher-order parasequences. They are characterized by the coarsening-upward succession, having homogeneous to pinstripe-laminated shale (Facies A), heterolithic siltstone (Facies B), and massive to faintly laminated coarse siltstone (Facies C). The succession is interpreted to be formed in response to a progradational shoreface setting, wherein trace fossils typically show a diminutive and low-diversity suite. The mineralogical analysis has revealed that the cored samples are composed mostly of detrital grains of quartz, feldspar and dolomite with the major cement materials of illite, chlorite and smectite, the ratio of which would be lithofacies-controlled, having a higher proportion of clay and TOC in the more distal facies. Clays, mostly authigenic in origin, commonly occlude primary pores, but disseminated organics contain the pervasive organic nanopores. Pore-throat size distribution shows modal radius (5 to 75 nm) that has a positive correlation with the grain-size variation, but has a negative correlation with respect to the TOC and clay contents. Likewise, the poro-perm system appears to be facies-dependant. The mineralogical and petrographic studies have led to the general conclusions that the poro-perm characteristics are strongly influenced by the pervasive presence of organics and authigenic clays that would fill and occlude the paleopore network. The study results tentatively suggest that the Middle Montney, coarsest and most porous (up to 7%), will have the highest potentiality with respect to the poro-perm system.

Sedimentological and ichnological characterization of muddy storm related deposits: The upper Hauterivian ramp of the Agrio Formation in the Neuquén Basin, Argentina

The lowermost part of the Agua de la Mula Member (upper Hauterivian), Agrio Formation, was studied in distal depositional settings at four locations in the central Neuquén Basin. Detailed sedimentological and ichnological analysis combining field data and microfacies, using petrography, X-ray diffraction, and organic geochemical data allow to recognize two facies associations, representing proximal to distal facies of a mixed, carbonate-siliciclastic homoclinal outer ramp. The distal outer ramp facies association (FA-1) contains mudstones accumulated by suspension settling during periods of high productivity in the water column with calcareous microfossils as fecal pellets and organo-minerallic aggregates. Very thin (<5 mm thick) beds with local skeletal debris and subtle erosional features suggest storm-generated turbidite-like deposits related to bottom currents that interrupted this “low-energy” scenario. Conversely, the proximal outer ramp facies association (FA-2) is mainly composed of illite-rich floccules with terrigenous organic matter derived from the hinterland, probably transported by suspension in the way of benthic fluid mud. The partially bioturbated clay-bearing, silt-rich mudstones are organized into stacked successions of sharp-based, normally graded and very thin (<5 mm thick) beds indicative of erosive waning flows. In these mudstones, the preservation of parallel to curved ripple lamination indicates storm-driven bottom currents that transported mud as bed load into the proximal outer ramp. Some bioturbated mudstones with Phycosiphon incertum demonstrate a close relationship between event bed deposits and oxygenation pulses of the bottom waters by storm-influenced processes.

A new approach to biostratigraphy in the Karoo retroarc foreland system: Utilising restricted-range palynomorphs and their first appearance datums for correlation

The main Karoo Basin (MKB), internationally renowned for its wealth of fossil tetrapods, has been lithostratigraphically subdivided into three discrete regions: two (east and west) proximal facies adjacent to the Cape Fold Belt, and a distal facies, away from the Cape Fold Belt. Because of lithological differences between formations of the proximal and distal sectors of the MKB, it has been difficult to correlate time-equivalent lithostratigraphic units in the different sectors. Palynology provides a tool for refining stratigraphic correlations within the MKB, but only species with restricted ranges are biostratigraphically significant. Restricted-range palynomorph taxa from the Ecca and Beaufort groups indicate the following relationships: the lowermost Vryheid Formation palynoflora can be strongly correlated with that of the Prince Albert Formation, whereas the Whitehill–Collingham formations and No. 2 seam (Vryheid Formation), and the Ripon Formation and No. 4 seam (Vryheid Formation) can be tentatively linked using palynology. The lower–middle Normandien Formation in the north is chronologically equivalent to the Ripplemead member of the Balfour Formation in the south. Although the Ecca–Beaufort Group boundary is known to be diachronous, restricted-range palynomorphs cannot yet confirm that the uppermost Ecca Group in the northern part of the basin was deposited at the same time as the lower Beaufort Group in the south.This study demonstrates that despite diachronous ranges of some taxa, palynology is useful in correlating age-equivalent lithostratigraphic units in the proximal and distal sectors of the basin. A new First Appearance Datum (FAD) palynozonation for the Karoo is presented that is calibrated by the most recent radiometric dates for South Africa, Australia, and South America. The new zones are also correlated with Permian successions in Antarctica, Zambia, Botswana, and India. Future palynostratigraphic work in South Africa must take into account the ranges and FADs of these palynomorph taxa in order to make meaningful biostratigraphic correlations in the Karoo Basin and across Gondwana.

Petroleum retention in the Mandal Formation, Central Graben, Norway

Upper Jurassic organic matter-rich, marine shales of the Mandal Formation have charged major petroleum accumulations in the North Sea Central Graben including the giant Ekofisk field which straddles the graben axis. Recent exploration of marginal basin positions such as the Mandal High area or the Søgne Basin has been less successful, raising the question as to whether charging is an issue, possibly related to high thermal stability of the source organic matter or delayed expulsion from source to carrier.The Mandal Formation is in part a very prolific source rock containing mainly Type II organic matter with <12 wt.-% TOC and HI < 645 mg HC/g TOC but Type III-influenced organofacies are also present. The formation is therefore to varying degrees heterogeneous. Here we show, using geochemical mass balance modelling, that the petroleum expulsion efficiency of the Mandal Formation is relatively low as compared to the Upper Jurassic Draupne Formation, the major source rock in the Viking Graben system. Using maturity series of different initial source quality from structurally distinct regions and encompassing depositional environments from proximal to distal facies, we have examined the relationship between free hydrocarbon retention and organic matter structure. The aromaticity of the original and matured petroleum precursors in the Mandal source rock plays a major role in its gas retention capacity as cross-linked monoaromatic rings act on the outer surface of kerogen as sorptive sites. However, oil retention is a function of both kerogen and involatile bitumen compositions. Slight variations in total petroleum retention capacities within the same kerogen yields suggest that texture of organic matter (e.g. organic porosity) could play a role as well.

Carbonate microfacies and chemostratigraphy of a late Aptian–early Albian marine distal section from the primitive South Atlantic (SE Brazilian continental margin): Record of global ocean-climate changes?

Late Aptian–early Albian limestones from the eastern Brazilian continental margin record the early evolution of the South Atlantic Ocean. In Tethyan and North Atlantic domains, a planktic foraminiferal turnover and organic-rich deposits related to Oceanic Anoxic Event 1b (OAE 1b) point to major ocean-climate changes through this interval. Coeval organic-rich deposits of the South Atlantic Ocean have been interpreted as the product of restricted circulation rather than attributed to a global event. However, previous investigations of the early marine phase of South Atlantic lack data from more distal facies, making correlations to global events difficult. Here, we present C, O, and Sr isotopes, elemental geochemistry, TOC and pyrolysis data, as well as a microfacies analysis of an upper Aptian–lower Albian distal section from the Campos Basin (southeastern Brazil). Our focus is on the paleoenvironmental characterization of and the possible association between organic-rich deposits and major perturbations related to Aptian–Albian transition. Five microfacies associations (MA) were identified in the informal units I and III, which were deposited in the neritic region on a carbonate ramp. Organic-rich deposits were described in unit III, composed of planktic-dominated wackestones interbedded with black shales, in a distal dysoxic to anoxic environment. The carbonates 87Sr/86Sr ratios showed a drastic increase (0.7072–0.7074), interpreted as enhanced chemical weathering, supported by the increase of continental input to the top of section. This trend was accompanied by a long-term δ13Ccarb negative excursion, which were assigned to the latest late Aptian–early Albian interval of the isotope reference curves, in accordance with the described occurrence of Colomiella recta. This scenario matches those proposed for the late Aptian–early Albian transition and OAE 1b set, as an enhanced greenhouse stage, pointing to the influence of the referred ocean-climate changes on the deposition of organic-rich deposits of the early South Atlantic Ocean. This investigation gives more evidences that these perturbations were a widespread event, as a product of broad-scale disturbances in the global carbon cycle which also controlled organic deposition and preservation on restricted settings.

Sedimentology of the Essaouira Basin (Meskala Field) in context of regional sediment distribution patterns during upper Triassic pluvial events

Upper Triassic continental clastics (TAGI: Trias Argilo-Greseux Inferieur) in the Essaouira Basin are largely restricted to the subsurface, which has limited analysis of the depositional environments and led to speculation on potential provenance of the fluvial systems. Facies analysis of core from the Meskala Field onshore Essaouira Basin is compared with tentatively time-equivalent deposits exposed in extensive outcrops in the Argana Valley, to propose a process orientated model for local versus regional sediment distribution patterns in the continuously evolving Moroccan Atlantic rift during Carnian to Norian times. The study aims to unravel the climatic overprint and improve the understanding of paleo-climatic variations along the Moroccan Atlantic margin to previously recognised Upper Triassic pluvial events.In the Essaouira Basin, four facies associations representing a progressive evolution from proximal to distal facies belts in a continental rift were established. Early ephemeral braided river systems are succeeded by a wet aeolian sandflat environment with a strong arid climatic overprint (FA1). This is followed by the onset of perennial fluvial deposits with extensive floodplain fines (FA2), accompanied by a distinct shift in fluvial style, suggesting increase in discharge and related humidity, either locally or in the catchment area. The fluvial facies transitions to a shallow lacustrine or playa lake delta environment (FA3), which exhibits cyclical abandonment. The delta is progressively overlain by a terminal playa with extensive, mottled mudstones (FA4), interpreted to present a return from cyclical humid-arid conditions to prevailing aridity in the basin.In terms of regional distribution and sediment source provenance, paleocurrent data from Carnian to Norian deposits (T5 to T8 member) in the Argana Valley suggest paleoflow focused towards the S and SW, not directed towards the Meskala area in the NW as previously suggested. A major depo-centre for fluvial sediments is instead located in the southern Argana Valley, possibly the Souss Basin. To effectively source the reservoir sandstones found in the Meskala Field, a more local provenance area has hence to be envisaged. Despite this, the direct comparison of the genetic evolution of sedimentary sequences in the Argana Valley and Essaouira Basin shows a similar progression from dominantly arid ephemeral depositional environments to humid perennial sedimentation, returning to prominent arid conditions. This suggests climatic control in both regions, where an enhanced humid signal drives perennial fluvial flow in otherwise arid dominated sequences. On a regional scale, this is suggested to record the impact of strong Triassic pluvial events previously recognised in other basins along the Central Atlantic margin during the Carnian to Norian periods.

Holocene tephra succession of Puyehue-Cordón Caulle and Antillanca/Casablanca volcanic complexes, southern Andes (40–41°S)

Puyehue-Cordón Caulle and Antillanca volcanic complexes are two of at least 50 active frontal arc volcanoes that define the 1400km-long Southern Volcanic Zone of Chile. Holocene tephra deposits in Chile and Argentina (40–41°S) up to 100km east of these volcanoes comprise at least five voluminous (~1 to ~8km3) pyroclastic-fall layers that preceded several recently deposited Cordón Caulle pumice fallouts. Field observations of proximal, medium, and distal facies of the deposits, in conjunction with geochronology and geochemistry of the volcanic complexes, indicate that three fall layers are derived from Puyehue volcano (Puyehue 1 and 2, and Mil Hojas), whereas two are sourced from the Antillanca complex (Playas Blanca-Negra, and Nahuel Huapi Tephra), 20km to the south. The oldest tephra (calibrated 14C age 10.49±0.12ka, 2σ), found only at medium-distal facies, is deposited directly on granitic moraine boulders and consists of deeply weathered, orange dacitic pumice lapilli. The next prominent tephra at ~7ka comprises dacitic pumice and its age is equivalent to a rhyodacitic dome exposed in the Puyehue summit crater. Above these deposits there are phases of a complex eruption consisting of a conspicuous compositionally-zoned tephra. It also comprises a pyroclastic density current, together with lithic rich and scoriaceous fallout deposits. Mineralogical, geochemical, and Sr isotope evidence, plus the isopach maps, confirm that this sequence of eruptive events is sourced from Antillanca at 1932±68yrBP. The total volume of this eruptive sequence exceeds 8km3, making it the largest Holocene eruption from either volcanic complex. This eruption was likely responsible for the destruction of an ancestral Antillanca volcano and the formation of a 4.5km diameter caldera. A distinctive younger unit in the region is a voluminous rhyodacitic pumice fall (calibrated 14C age 1.11±0.07ka), above which a series of several alternating dark lithic and pumice lapilli beds accumulated. Correlation with proximal deposits indicates that the 1.11ka eruption was derived from Puyehue and destroyed ~3km3 of rhyodacitic domes at this volcano summit. Historic explosive activity at the nascent Casablanca volcano and along Cordón Caulle, including the 2011–2012 eruption (~1km3 of uncompacted pumice), the largest from this fissural zone, emphasizes an increased risk for volcanic hazards in central Chile and Argentina.

Greenalite precipitation linked to the deposition of banded iron formations downslope from a late Archean carbonate platform

Banded iron formations (BIFs) were deposited as deep-water facies distal to the late Archean Campbellrand carbonate platform, Transvaal Supergroup, South Africa. They are traditionally interpreted to have formed from iron oxides/hydroxides and silica that precipitated when upwelling hydrothermal water enriched in Fe2+ and silica mixed with cooler, shallow seawater enriched in dissolved oxygen. The ferric oxides/hydroxides were then converted to hematite or, in the presence of organic matter, reduced and incorporated into siderite. New high-resolution microscopy of BIFs from distal facies of the Campbellrand platform questions that interpretation, revealing the presence of abundant greenalite nanoparticles, interpreted to represent primary precipitates from ferruginous seawater, consistent with recent results from equivalent-aged BIFs of the Hamersley Group, Western Australia. The particles locally define primary sedimentary lamination, and are engulfed in diagenetic chert that preserves shrinkage structures, considered to have formed during dehydration and recrystallization of amorphous silica cement. The replacement of greenalite nanoparticles by coarser grained oxide, silicate and carbonate minerals, with destruction of primary textures and structures, indicates that iron-silicate muds were originally much more widespread. Based on our findings we propose a new model for the deposition of the BIFs, involving iron silicate precipitation from seawater followed by diagenetic silica cementation on the seafloor. We suggest that ferrous iron and silica were transported in hydrothermal plumes sourced from acidic vent fluids. Upon mixing with cooler, more alkaline seawater, the solubility of ferrous iron and silica fell to levels that favored rapid nucleation and the precipitation of iron-silicate nanoparticles over vast areas of the seafloor. The close association between iron and silica during the deposition of BIFs may be explained by the precipitation of iron-silicate nanoparticles and possible sorption of silica onto the surfaces of sinking nanoparticles. Furthermore, on the seafloor, iron-silicate nanoparticles presumably acted as nucleation sites for dissolved silica, promoting early diagenetic silica precipitation and cementation of the iron-rich muds. Our results indicate that iron silicates were important primary precipitates and suggest that changes in alkalinity rather than redox state played a key role in the precipitation of the precursor sediments of BIFs.

Sedimentology and ichnology of Neogene Coastal Swamp deposits in the Niger Delta Basin, Nigeria

AbstractOften analyses of depositional environments from sparse data result in poor interpretation, especially in multipartite depositional settings such as the Niger Delta. For instance, differentiating channel sandstones, heteroliths and mudstones within proximal environments from those of distal facies is difficult if interpretations rely solely on well log signatures. Therefore, in order to achieve an effective and efficient interpretation of the depositional conditions of a given unit, integrated tools must be applied such as matching core descriptions with wireline log signature. In the present paper cores of three wells from the Coastal Swamp depositional belt of the Niger Delta are examined in order to achieve full understanding of the depositional environments. The well sections comprise cross-bedded sandstones, heteroliths (coastal and lower shoreface) and mudstones that were laid down in wave, river and tidal processes. Interpretations were made from each data set comprising gamma ray logs, described sedimentological cores showing sedimentary features and ichnological characteristics; these were integrated to define the depositional settings. Some portions from one of the well sections reveal a blocky gamma ray well log signature instead of a coarsening-upward trend that characterises a shoreface setting while in other wells the signatures for heteroliths at some sections are bell blocky in shaped rather than serrated. Besides, heteroliths and mudstones within the proximal facies and those of distal facies were difficult to distinguish solely on well log signatures. However, interpretation based on sedimentology and ichnology of cores from these facies was used to correct these inconsistencies. It follows that depositional environment interpretation (especially in multifarious depositional environments such as the Niger Delta) should ideally be made together with other raw data for accuracy and those based solely on well log signatures should be treated with caution.

Quaternary fluvial tufas of Sarıkavak area, southwestern Turkey: Facies and depositional systems

Abstract In the Quaternary Sarikavak fluvial tufa deposits, situated in south-western Turkey, different facies have been identified and their mutual relationships have been unravelled. This tufa formation is located at the northern part of Acigol Graben. The development of the Sarikavak tufas represents typical fluvial and fluvio-palustrine environments. In total eight tufa lithofacies have been differentiated, namely: moss tufa facies, oncolitic tufa facies, stromatolitic tufa facies, phytoclastic tufa facies, intraclastic tufa facies, extra-formational clastic tufa facies, sapropelitic tufa facies, and palaeosol. Preliminary U/Th datings indicate that these deposits formed prior to 219 ka and continued until 80 ka (MIS 7 till 5). δ13C signatures (0.78‰ to −1.63‰ V-PDB) best can be explained by limestone dissolution processes or respiration from C4 plant vegetation into the soil or both. For that reason, the Sarikavak tufa deposits can be classified as ‘travitufa’ which precipitated in a distal facies system of travertine occurrences. δ18O values (−7.49‰ to −10.83‰ V-PDB) of the whole data set reflect some significant changes, which is interpreted in terms of temperature variations reflecting warm and cold periods among MIS 7 (interglacial), MIS 6 (glacial) and MIS 5 (interglacial). The development of the Sarikavak tufa deposits are related to the Maymundagi fault that borders the Acigol Graben in the north. This fault and its synthetic fault segments play a pivotal role with regard to the palaeo-flow direction of water and control the basin topography. These structural structures are likely responsible for all active and ancient springs in the study area, as well as their respective carbonate precipitates.

Evidence for a petroleum subsystem in the Frontier Formation of the Uinta–Piceance Basin petroleum province

Anomalous carbonate horizons with intercrystalline hydrocarbon residue, cone-in-cone structures, and calcite “beef” veins in adjacent sandstone beds record potential evidence for hydrocarbon generation and seepage in the middle to upper Turonian Frontier Formation from the Uinta Basin, Utah and Colorado. Eight carbonate occurrences, all encountered within distal delta-front facies (thin-bedded sandstones and siltstones), were sampled at outcrop locations from the southern and eastern margins of Dinosaur National Monument. Seven petrographic facies (PF1–PF7) were identified using standard petrographic and cathodoluminescence microscopy: PF1, large and small botryoids and fans; PF2, yellow-brown spherules; PF3, microcrystalline spar cement; PF4, blocky spar; PF5, prismatic spar; PF6, drusy mosaic spar; and PF7, dolomite. Facies PF1–PF3 are synsedimentary phases comprising a large percentage of carbonate horizon volume, whereas PF4–PF7 are late-stage fabrics. The δ13C values of PF1–PF3 (−9.9‰ to −20.0‰) are consistent with contributions from biogenic methane seepage during deposition and early diagenesis. Brecciated PF1 fabrics and blowout depressions within sandstone horizons further indicate significant methane generation during deposition and early burial. Late-stage fabrics contain δ13C (−8.0‰ to −17.3‰) and δ18O (−6.5‰ to −13.5‰) values consistent with progressive burial, during which intercrystalline hydrocarbon residue, cone-in-cone structures, and calcite beef veins were formed by the thermal maturation of organic matter from enclosing distal delta-front facies. Together, these features reveal the potential for the thin-bedded facies of the Frontier Formation distal delta front to serve as a potentially viable petroleum subsystem previously unrecognized in the Uinta–Piceance petroleum province.

Morphodynamics and facies architecture of streamflow-dominated, sand-rich alluvial fans, Pleistocene Upper Valdarno Basin, Italy

Abstract The overwhelming documentation of coarse-grained alluvial fans dominated by mass flow contrasts with the scarce accounts of finer grained, traction-dominated alluvial fans. To fill this gap, we present sedimentological and architectural data from a set of sand-rich, streamflow-dominated Pleistocene fans flanking the eastern Upper Valdarno Basin, Italy. The routing of sand-rich sediment resulted from the fast, intense weathering of the feldspar-rich, carbonate-deprived sandstone bedrock underlying the fan catchments. Although capable of entraining large boulders, high water discharge sustained tractional reworking along the proximal facies belts, hindering mass flow. The medial facies belts have a channelized, braided planform and are dominated by processes hardly distinguishable from those characterizing fluvial environments. Along the distal facies belts, extensive overbank tracts are composed of terminal splays and crevasse lobes, the spatial arrangement of which controlled the evolution of through-going tributary channels connected to the axial basin drainage. This study focused on the sedimentary processes, stratal architecture and morphodynamics of the alluvial fans and considered the effect of bedrock inheritance on their development. The results underline how granulometry and mass flow dominance are not distinctive of alluvial fan sedimentation per se and indicate how the critical detection of piedmont, radial palaeomorphology is crucial in the identification of ancient alluvial fans.

Vertical trends within the prograding Salt Wash distributive fluvial system, SW United States

AbstractProgradation is an important mechanism through which sedimentary systems fill sedimentary basins. Although a general progradational pattern is recognized in many basins, few studies have quantified system scale spatial changes in vertical trends that record fluvial system progradation. Here, we provide an assessment of the spatial distribution of vertical trends across the Salt Wash distributive fluvial system (DFS), in the Morrison Formation SW, USA. The vertical distribution of proximal, medial and distal facies, and channel belt proportion and thickness, are analysed at 25 sections across approximately 80 000 km2 of a DFS that spanned approximately 100 000 km2. The stratigraphic signature of facies stacking patterns that record progradation varies depending on location within the basin. An abrupt and incomplete progradation succession dominates the proximal region, whereby proximal deposits directly overlie distal deposits. A more complete succession is preserved in the medial region of the DFS. The medial to distal region of the DFS are either simple aggradational successions, or display progradation of medial over distal facies. Spatial variations in facies successions patterns reflects preservation changes down the DFS. A spatial change in vertical trends of channel belt thickness and proportion is not observed. Vertical trends in channel belt proportion and thickness are locally highly variable, such that systematic up‐section increases in these properties are observed only at a few select sites. Progradation can only be inferred once local trends are averaged out across the entire succession. Possible key controls on trends are discussed at both allocyclic and autocyclic scales including climate, tectonics, eustasy and avulsion. Eustatic controls are discounted, and it is suggested that progradation of the Salt Wash DFS is driven by upstream controls within the catchment.

Sedimentology and Palaeontology of the Beaufort Group in the Free State Province Supports A Reciprocal Foreland Basin Model for the Karoo Supergroup, South Africa

Various different models have been proposed for the style of fill of the Karoo Basin in South Africa. One of these proposes that the Karoo Basin behaved as a partitioned entity, with reciprocal fills on either side of a hinge line separating proximal and distal facies. Detailed sedimentological, palaeontological, stratigraphic and geophysical data for the Beaufort Group in the central Free State lends credence to this model. In the study area fossils assignable to the Dicynodon , Lystrosaurus and Cynognathus assemblage zones occur in the Balfour, Katberg and Burgersdorp formations respectively. This data shows that certain parts of the Beaufort Group stratigraphic succession that are present in the southern part of the Karoo Basin, are absent in the central Free State, indicating either a depositional hiatus or a period of erosion at the beginning and the end of Lystrosaurus Assemblage Zone, as well as the end of Cynognathus Assemblage Zone times. During these depositional hiatuses or periods of erosion, deposition or preservation of the rock record was restricted to the proximal sector (south of the hinge line). Study of the geophysical data has also allowed for the definition of a basement high north-east of Thaba Nchu, which probably accounts for the provenance of the informally named Musgrave unit (upper Balfour Formation), which also has an anomalous south-westerly palaeocurrent direction.

A Marine Incursion In the Lower Pennsylvanian Tynemouth Creek Formation, Canada: Implications For Paleogeography, Stratigraphy and Paleoecology

Abstract We document the occurrence of a marine bed, and its associated biota, in the Lower Pennsylvanian (Langsettian) Tynemouth Creek Formation of New Brunswick, and discuss its implications for paleogeography, stratigraphy, and paleoecology. This is only the second marine interval found in the entire Pennsylvanian fill of the Maritimes Basin of Canada, the other being recently found in the broadly same-age Joggins Formation of Nova Scotia. Evidence for the new marine transgression comprises an echinoderm-rich limestone that infills irregularities on a vertic paleosol surface within the distal facies of a syntectonic fluvial megafan formed under a seasonally dry tropical climate. Gray, platy ostracod-rich shales and wave-rippled sandstone beds that directly overlie the marine limestone contain trace fossils characteristic of the Mermia Ichnofacies, upright woody trees, and adpressed megafloras. This association represents bay-fills fringed by freshwater coastal forests dominated by pteridosperms, cordai...