Primary Depositional Fabric Research Articles

Integrating facies, mineralogy, and paleomagnetism to constrain the age and provenance of Paleozoic siliciclastic sedimentary rocks along the northern Gondwana margin: insights from the Araba and Naqus formations in western Gulf of Suez, Egypt

The depositional ages and provenance of the Paleozoic Araba and Naqus Formations along the northern Gondwanan margin in Egypt have remained uncertain due to a lack of index fossils. Resolving this issue is crucial for understanding regional geology during deposition and subsequent tectonic development. We integrate detailed facies analysis, anisotropy of magnetic susceptibility (AMS), paleomagnetism, and mineralogical data to elucidate the genesis and depositional ages of the Araba and Naqus Formations. Petrographic analyses identified seven distinct facies types, providing insights into sedimentary textures, maturity, and sources, with contributions from igneous and metamorphic sources indicated by heavy minerals. X-ray diffraction (XRD) analysis identified accessory minerals such as quartz, goethite, kaolinite, hematite, and anatase. Paleomagnetism isolated two magnetic components (CA and CN) providing the first robust paleo pole positions at Lat. = 70.8° N, Long. = 308.2° E and Lat. = 37.8° N, Long. = 233.1° E, indicating Cambrian and Carboniferous ages for the Araba and Naqus formations, respectively. Thermal demagnetization constrained these dates using established polarity timescales. Mineralogical data indicated that the Araba Formation originated from an igneous source, while the Naqus Formation had a mixed metamorphic-igneous provenance. The integrated AMS and paleomagnetic data reveal evidence of post-depositional deformation. Specifically, the clustering of maximum AMS axes in the NW–SE direction for both formations, suggests the initial presence of a primary depositional fabric. However, prevalent tectonic activity during the Cenozoic appears to have overprinted and modified this fabric through deformation related to rifting of the Gulf of Suez region. Through this novel multi-proxy approach, we have resolved long-standing uncertainties regarding the formations' depositional ages. Our study thereby provides the first chronostratigraphic framework for these strategically important sedimentary units, significantly advancing understanding of regional Paleozoic geology.Graphical abstract

Characterization of Upper Cretaceous Matulla and Wata clastic reservoirs from October field, Central Gulf of Suez, Egypt

The objective of this study was to integrate petrographic thin sections, SEM, XRD, routine core analysis, gas chromatography, and wireline logs to present a comprehensive petrographic and petrophysical assessment of the shallow marine clastic reservoirs of the Coniacian-Santonian Matulla Formation and upper Turonian Wata Formation from the October field, Central Gulf of Suez Basin. The fine to very fine-grained glauconite-bearing subarkose quartz arenites of the Matulla and Wata reservoirs exhibit good intergranular primary porosity with good pore connectivity. Moderate to slight compaction and grain to grain pressure solution resulted in a slightly inhomogeneous packing of framework grains. Feldspar dissolution contributed to the secondary porosity which improved the reservoir quality, while quartz overgrowth and minor calcite cementation are inferred as porosity destroying diagenetic agents. Pore-filling kaolinite and clay phases also had a negative effect on reservoir storage capacity. Routine core analysis indicated > 20% porosity and up to 412 mD permeability in the meso-megaporous sandstones of both the reservoirs. Permeability anisotropy analysis of Matulla samples indicates the dominance of primary depositional fabric and isotropic pores with minor scattered/connected horizontal pores and vertical pore connectivity. Wireline log-based quantitative petrophysical assessments exhibit low shale volume and higher hydrocarbon saturation (up to 80%) on both the reservoirs, which is also supported by the gas chromatography data confirming the presence of oil in the Matulla, and Wata pay zones based on interpreted gas balance, wetness, and character ratios. The study concludes excellent reservoir properties in the Upper Cretaceous clastic intervals of the October field.

Reservoir Description, Diagenesis and Reservoir Quality of the Deep Marine Sediments in the Propagation Stage of Rift Basin

The exploration and exploitation of the downthrown side of the productive upthrown Pre-rifting reservoirs at the Gulf of Suez marked bring a renaissance to the rift basin Exploration. This work documents the reservoir performance, trap geometry, and structural style of the downthrown deep marine stratigraphic entrapment. In addition, it outlines the requirements necessary beyond the production mitigations and limits. To gain better understanding of Asl Formation (deep marine) reservoir characteristics early in the exploration program, Asl Formation was cored in the north October J-5 well. A continuous 366- foot (112m) core was recovered including the main reservoir interval, which is producing over 12,000 barrel of oil per day. The J-5 Asl Formation has been interpreted as a synthetic, mixed clastic-carbonate turbidity deposit. Three major lithofacies described are recognizable on electric log. These are sandstone, shale and carbonates. The sandstones dominate the lower portion of the core and are the main oil producing section. Sandstone porosity and permeability range from 17 to 27% and 30 millidarcies to 3 darcies, respectively. Compared to the sandstone, the carbonate turbidites have poorer reservoir quality. They dominate the upper portion of the core and are composed of coarse fossil debris, silica framework grains and rock fragments. Horizontal and vertical permeabilities with rock types. Both the sandstone and carbonate sections of the J-5 core have experienced a complex diagenetic history. Deposition of the cored interval was initiated by a convulsive geological event such as flashflood or earthquake. These events would likely generate sufficient energy for mobilization of onshore and/or near shore sediments into multiple gravity flows having regional extent. The reservoir quality is controlled by a combination of primary depositional fabric and subsequent diagenetic modification. This research work can guide future exploration of reservoir-prone deep marine deposits in the Red Sea Rift and provide a reference for the subsurface deep-water reservoir quality analysis in the propagation stage in the Rift Basins.

Quantitative diagenesis: Methods for studying the evolution of the physical properties of tight carbonate reservoir rocks

Conventionally, diagenesis has been studied by making qualitative morphological observations which have been organised into complex classification schemes. Petrophysics, with its many quantitative measurements, now gives us the ability to quantify the effects of the type, degree and timing of complex diagenetic process. The aim of this paper is to examine how different diagenetic processes affect the petrophysical properties of carbonate rocks and to develop quantitative methodologies to describe the results of diagenetic processes. A large number of petrophysical measurements have been made on a suite of 172 core plugs to provide a test data set. Diagenetic modification of the primary depositional fabric was observed in a wide range of measured petrophysical parameters, and that porosity and pore connectedness exert dominant control on all of the electrical and hydraulic rock parameters. This observation has been used to propose a new theoretical framework linking the effect of diagenetic process to petrophysical measureables. Cementation exponent was found to increase with permeability and pore size, especially in recrystallized rocks, and is explained by smaller porosity samples having a better connected pore network. Electrical connectedness was also found to correlate extremely well with hydraulic permeability, showing that these phenomena are linked closely in tight carbonate reservoir rocks. A method for calculating pre- and post-dolomitisation porosity and the degree of dolomitisation from the measured petrophysical and compositional data has also been developed and tested. All electrical and hydraulic properties are related to pore type, allowing cementation exponent to be obtained from optical microscopy/SEM studies or NMR measurements, providing a new approach to estimating cementation exponent in carbonate rocks. This paper also provides a powerful new approach allowing petrophysical changes associated with the type, degree and timing of different diagenetic processes to be tracked quantitatively using a method we have called ‘petrodiagenetic pathways’.

Comparing depositional modes of cave sediments using magnetic anisotropy

Deciphering the origin and depositional history of archaeological and paleontological deposits is fundamental to evaluate artifact and fossil contextualization. We present new rock magnetic data based on the anisotropy of magnetic susceptibility (AMS) that allow the fabric analysis and characterization of depositional modes in cave sediments. This approach was tested on both fossiliferous and sterile deposits for Gran Dolina and Galeria localities from the Atapuerca karstic complex (Spain). Our AMS approach has allowed to determine paleoenvironmental conditions in both sites, as well as to establish where a given sedimentary unit preserves primary depositional fabrics or has possible post-depositional perturbation. In this sense, it is worth emphasizing that even the sampled horizons that have hominid remains Gran Dolina show no signs of fabric disruption or evidence for massive transport of the sedimentary particles.

A multi-proxy study of anthropogenic sedimentation and human occupation of Gledswood Shelter 1: exploring an interior sandstone rockshelter in Northern Australia

Rockshelters contain some of the most important archives of human activity in Australia but most research has focused on artifacts and cultural context. This study explores geomorphological and geoarchaeological approaches for understanding a sandstone rockshelter in interior northern Australia: Gledswood Shelter 1. At this site, magnetic susceptibility and micromorphology techniques were integrated with bulk sedimentology, soil chemistry and geochronology to better understand the record of human impact and site formation processes. The micromorphology studies indicate that primary depositional fabrics, such as graded bedding or laminations, are absent, and sediment structural development is low throughout the entire sequence, with most samples exhibiting a high degree of post-depositional mixing. The sediment magnetic susceptibility analysis reveals magnetic changes coinciding with human occupation, a result of anthropogenic burning. Specifically we highlight that combustion features are prevalent in this sandstone shelter and provide critical insights into the human usage of the shelter.

Current‐aligned dewatering sheets and ‘enhanced’ primary current lineation in turbidite sandstones of the Marnoso‐arenacea Formation

AbstractTurbidite sandstones of the Miocene Marnoso‐arenacea Formation (northern Apennines, Italy) display centimetre to decimetre long, straight to gently curved, 0·5 to 2·0 cm regularly spaced lineations on depositional (stratification) planes. Sometimes these lineations are the planform expression of sheet structures seen as millimetre to centimetre long vertical ‘pillars’ in profile. Both occur in the middle and upper parts of medium‐grained and fine‐grained sandstone beds composed of crude to well‐defined stratified facies (including corrugated, hummocky‐like, convolute, dish‐structured and dune stratification) and are aligned sub‐parallel to palaeoflow direction as determined from sole marks often in the same beds. Outcrops lack a tectonic‐related fabric and therefore these structures may be confidently interpreted to be sedimentary in origin. Lineations resemble primary current lineations formed by the action of turbulence during bedload transport under upper stage plane bed conditions. However, they typically display a larger spacing and micro‐topography compared to classic primary current lineations and are not associated with planar‐parallel, finely laminated sandstones. This type of ‘enhanced lineation’ is interpreted to develop by the same process as primary current lineations, but under relatively high near‐bed sediment concentrations and suspended load fallout rates, as supported by laboratory experiments and host facies characteristics. Sheets are interpreted to be dewatering structures and their alignment to palaeoflow (only noted in several other outcrops previously) inferred to be a function of vertical water‐escape following the primary depositional grain fabric. For the Marnoso‐arenacea beds, sheet orientation may be linked genetically to the enhanced primary current lineation structures. Current‐aligned lineation and sheet structures can be used as palaeoflow indicators, although the directional significance of sheets needs to be independently confirmed. These indicators also aid the interpretation of dewatered sandstones, suggesting sedimentation under a traction‐dominated depositional flow – with a discrete interface between the aggrading deposit and the flow – as opposed to under higher concentration grain or hindered‐settling dominated regimes.

Pore fabric assignment using electrical conductivity of some Albian–Cenomanian sequences in north Eastern Desert, Egypt

The electric pore fabric concept has been testified in many previous studies, mostly concerned with sandstone rock samples. The present study is a further trial to testify the reliability of the proposed concept on carbonate and clastic facies. The studied facies are representative for the Albian–Cenomanian Raha and Abu Qada Formations at the western side of the Gulf of Suez in Egypt. A total of 37 samples have been studied petrophysically by measuring electric conductivity in 3-D to rank their petrophysical potentiality and to detect their electric pore fabric. The studied samples are grouped into four facies, namely: (1) facies 1 (calcareous sandy siltstones) and (2) facies 4 (glauconitic quartz arenite) as clastic facies as well as (3) facies 2 (bioclastic limestones, and dolostones) and (4) facies 3 (clayey to sandy limestones) as carbonate facies. Porosity values of the studied carbonate facies are very good due to dolomitization and dissolution, whereas porosity values of the clastic facies are poor due to their high clay content. The formation resistivity factor values are low to medium values due to the reducing effect of the clay content and the very good porosity values. Archie’s parameters a and m are mostly related to the formation resistivity factor and to some extent to the mineralogical composition as well as the electric tortuosity. The electric pore fabrics of the studied facies are slightly anisotropic, mostly as primary depositional fabrics affected by slight foliation rather than lineation. Presence of a slight fracturing system can be revealed by assigning the minimum formation resistivity factor through the vertical direction perpendicular to the bedding plain. However, the fracturing system is still not strong enough to affect the electric pore anisotropy by increasing the electric pore lineation as a secondary fabrics’ indicator.

Anisotropy of magnetic susceptibility (AMS) studies of Campanian–Maastrichtian sediments of Ariyalur Group, Cauvery Basin, Tamil Nadu, India: An appraisal to Paleocurrent directions

Oriented samples of sediments from Ariyalur Group, Cauvery Basin, south India, were studied for low field anisotropy of magnetic susceptibility (AMS) measurements to unravel the magnetic fabrics and paleocurrent directions. The results of AMS parameters of the sediments indicate primary depositional fabrics for Sillakkudi, Ottakovil and Kallamedu sandstone formations and secondary fabric for Kallankurichchi limestone formation. The obtained low degree of anisotropy (P j ), oblate shape AMS ellipsoid and distribution of maximum (K 1) and minimum (K 3) susceptibility axes on equal area projection confirm the primary sedimentary fabric for Sillakkudi, Ottakovil and Kallamedu Formations. In the case of ferruginous, lower arenaceous, Gryphaea limestone and upper arenaceous limestone beds of Kallankurichchi Formation have recorded more than one fabric. The observed AMS parameters like shape factor (T) (prolate to oblate), q value and random distribution of minimum (K 3) and maximum (K 1) susceptibility axes are supported for secondary fabrics in Kallankurichchi Formation as a result of post-depositional processes. Based on petrographic studies, it can be established that K 1 AMS axis of biotite mineral could represent the flow direction. The established paleocurrent direction for Sillakkudi is NW–SE direction while Ottakovil and Kallamedu Formations recorded NE–SW direction. Overall the paleoflow directions observed for Ariyalur Group is NE–SW to NW–SE.

Magnetic fabric analyses as a method for determining sediment transport and deposition in deep sea sediments

Abstract Anisotropy of magnetic susceptibility (AMS) analyses and the characteristic relationship between the q-value (magnetic lineation/foliation) and the imbrication angle (β) of the minimum susceptibility axes from the bedding plane (q–β diagrams) provide insight into the paleocurrent direction and depositional processes. During Integrated Ocean Drilling Program (IODP) Expedition (Exp) 333 sediments were cored from within the Nankai Trough accretionary prism at Site C0018, the depositional center for six mass transport deposits (MTDs). We analyzed specimens from the two subunits, Ia and Ib, and the six mass transport deposits identified during Exp 333. The sediments that are unaffected by MTD emplacement record episodic changes in paleocurrent direction and depositional processes including current, slope gravity, viscous suspension, and grain collision over the past 1.24 Ma at Site C0018. Magnetic fabric analyses proved useful in identifying sediment disturbance and the direction of movement at the time of emplacement, especially within MTDs 1, 2, 5, and 6. The magnetic fabric analyses were also useful in characterizing undeformed sediments within MTD 3. These undeformed sediments likely record primary depositional fabrics. We propose two additional fields, deformation/disturbance and tilting, on the q–β diagram in order to account for steep imbrication angles and relatively high q-values.

Characterizing the relationship between petrophysical trends and diagenetic cementation patterns: A case study from a carbonate Arab Reservoir in the Dukhan Field, State of Qatar

The Upper Jurassic Arab-C Reservoir in the Dukhan Field is the product of a stable epicontinental shelf subjected to continuous relative sea-level variations, with the preserved sediments representing a complex of syndepositional carbonate lithologies and textures deposited in subtidal, intertidal and sabkha environments. Though the depositional units are spatially correlated within a sequencestratigraphic context, petrophysical observations do not conform to the primary depositional fabric, and a cross-cutting relationship is observed.

Lithologic, mineralogical, and petrophysical characteristics of the Eau Claire Formation: Complexities of a carbon storage system seal

Concerns about potential climate change related to greenhouse gas emissions have spurred researchers across the world to assess the viability of geologic storage of CO2. In the Illinois Basin in the United States, the Cambrian Mount Simon Sandstone has been targeted as a reservoir for carbon capture and storage (CCS). In this CCS system, the Eau Claire Formation is expected to serve as the primary seal to prevent upward migration of the CO2 plume; however, little work has been done to specifically determine how well it will function as a seal. Although the lateral extent and thickness of the Eau Claire Formation, along with its generally low permeability, certainly make it a prime candidate to serve in this capacity, the primary depositional fabric and mineralogy, which are the fundamental controls on the petrophysical charter of this unit, remain poorly constrained. Therefore, the purpose of this study is to investigate the lithologic, mineralogical, and petrophysical properties of the Eau Claire Formation in an effort to characterize its potential as a functional seal in a CCS system. Sixty-six core-derived Eau Claire Formation samples from seven wells within the Illinois Basin are described using a combination of petrography, reflectance spectroscopy, x-ray diffraction, geochemical, and petrophysical analyses. These analyses show that the Eau Claire Formation contains five different lithofacies (sandstone, clean siltstone, muddy siltstone, silty mudstone, and shale) with fine-scale heterogeneities in fabric and mineralogy that greatly influence the petrophysical properties. Porosity, permeability, and entry-pressure data suggest that some, but not all, lithofacies within the Eau Claire Formation have the capability to serve as a suitable CCS seal. Abundant authigenic minerals and dissolution textures indicate that multiple generations of past fluid-rock interactions have occurred within the Eau Claire Formation, demonstrating that much of the formation has behaved as a fluid conduit instead of as a seal. Minerals that would be potentially reactive in a CCS system (including carbonate, glauconite, and chlorite) are common in the Eau Claire Formation. Dissolution of these and other phases in the presence of carbonic acid could potentially jeopardize the sealing integrity of the unit. Although complexities in the sealing properties exist, the dynamics of the CCS system and the potential for precipitation of new minerals should allow the Eau Claire Formation to serve as an adequate seal.

A 210Pb sediment budget and granulometric record of sediment fluxes in a subarctic deltaic system: The Great Whale River, Canada

To elucidate how modern river discharge conditions of the Great Whale River (GWR) are represented in the marine sedimentary record, eight box and gravity cores were examined in terms of 210Pb and 137Cs radiochemistry, granulometry and physical sedimentary structures. These data were analyzed to provide insights into sedimentary processes and patterns at the study site. Sediment accumulation in the study area appears to be a relatively steady process over time-scales of 50–100 yr, allowing biological activity to overprint the primary depositional fabric. Subtle differences between 137Cs and 210Pb sediment accumulation rates (SARs) suggest an offshore shift in the locus of fine sediment deposition during the past ∼150 yr, which may be a result of ongoing climatic warming leading to decreasing sea-ice coverage and a more energetic marine environment. Under present day conditions 23% (40,000 t/yr) of the discharged sediment appear to accumulate in a 25 km2 area off the river mouth. The remaining 77% (136,000 t/yr) are either deposited further offshore, possibly along the northeastern shore as a result of Hudson Bay's counterclockwise circulation, or dispersed into the Hudson Bay system. Grain diameter frequency analyses suggest that environmental processes controlling sediment transport and deposition vary over decadal time scales. Although, we cannot define an exact cause for this pattern, these shifts may be related to variations in river discharge, wave climate, possibly due to windier conditions or less sea-ice dampening, bioturbation or a combination of all. This suggests that also longer term river discharge signals are preserved in the marine sedimentary record offshore the Great Whale River. In summary, no major change in sediment discharge over the past ∼150 yr was observed. However, the offshore shift in the locus of sediment deposition suggests, that a warming climate will lead to more energetic marine conditions, less sea-ice coverage, and an increased offshore transport of terrestrial matter.

The Origin of the Millimeter-Scale Lamination in the Neoproterozoic Lower Beck Spring Dolomite: Implications for Widespread, Fine-Scale, Layer-Parallel Diagenesis in Precambrian Carbonates

Abstract The Neoproterozoic lower Beck Spring Dolomite exhibits a prominent, millimeter-scale, light–dark, planar lamination commonly interpreted as a primary depositional fabric. Microscopic examination reveals that the dark laminae are composed of micrite and contain a micrometer-scale, wavy fabric, whereas the light laminae are composed of relatively coarse-crystalline pseudospar that engulfs, surrounds, and crosscuts the dark laminae. Nearly all of the light laminae surround, or occur in proximity to, elongate (long axis fabric-parallel), spar-filled cavity structures. The average δ18O values for dark and light laminae are −1.8‰ (std. dev. 1.6) and −3.0‰ (std. dev. 1.3) VPDB, respectively. Cavity-filling cements are even more depleted in 18O, with an average isotopic value of −6.2‰ (std. dev. 1.1). These characteristics suggest that the coarse-crystalline, light laminae are the result of the preferential, aggrading neomorphism of a thinly laminated, micritic host rock and that the (now cemented) cavity structures acted as conduits for the fluid responsible for recrystallization. The oxygen isotope distributions are consistent with progressive recrystallization by an 18O-depleted fluid, likely of meteoric origin. Therefore, we interpret the prominent millimeter-scale, light–dark layering in the lower Beck Spring Dolomite to result from widespread, laminae-parallel, diagenetic processes—perhaps controlled by fine-scale layer-parallel differences in initial porosity and permeability—and not from primary sedimentary processes. Extensive, layer-parallel diagenesis on such a fine scale is rare in Phanerozoic shallow marine deposits, due to disruption by bioturbation. However, before the advent of metazoan burrowing, fine-scale, layer-parallel fluid flow would have been more common. Thus, the diagenetic fabric of the lower Beck Spring Dolomite may represent a mode of carbonate diagenesis restricted to the Precambrian. Similar diagenesis may also occur in Phanerozoic environments devoid of vertical bioturbation.

Tufas and travertines of the Mediterranean region: a testing ground for freshwater carbonate concepts and developments

AbstractActively forming travertine and tufa sites are widespread in temperate and warm climates, such as the Mediterranean region, but also develop in other climatic regimes from the Arctic to equatorial latitudes. The terms travertine and tufa are sometimes applied indiscriminately today; however, each has a specific definition. Most studied deposits are active or of Quaternary (especially Holocene) age. There are fewer Tertiary and even less late Mesozoic examples on record. In part, this is because of a recognition problem as these deposits commonly preserve as fragmentary erosional remnants. Physico‐chemical precipitation of calcium carbonate is commonly considered to be the main mechanism contributing to tufa and travertine deposition. Rapid cooling of waters further aids the process in thermal (travertine) sites. However, biomediation of calcium carbonate, associated with prokaryote–microphyte biofilms, is now recognized to be equally important in slow flowing and near static sites. Much of this precipitation occurs in close association with cyanobacterial, heterotrophic bacterial and diatoms. The Mediterranean region figures significantly in the development of travertine and tufa research. Consequently, this article employs key literature on Mediterranean examples to illustrate developments and concepts in freshwater carbonate research. Modern work started in the 1950s and defined many previously unknown outcrops. Many such sites became the targets for early radiocarbon dating studies. Since the late 1970s, a number of laboratory‐based and field‐based classification schemes have been proposed. Some schemes were based on associated macrovegetation types; others were based on petrological characteristics or on geometry and sedimentary facies characteristics. Of late, the sedimentological modelling has benefited greatly from the use of shallow geophysical techniques in order to better define the internal geometries of the deposits. Two distinct research strands were developed during the 1990s, both aimed at a better understanding of environmental change. The first considered biocomponents. The research was multifaceted and ranged from exploring the role of micro‐organisms in cement and lime mud precipitation to biostratigraphic analysis of contained pollen and invertebrate remains; these provided proxy environmental information which has helped to define past climate variability in the region. The second research strand focussed on geochemical signatures as a proxy tool for defining environmental change. This definition was made possible by new petrological studies which have greatly aided the recognition of primary depositional fabrics. This improved knowledge has reduced the sampling dangers of diagenetic contamination inherent in earlier geochemical studies. Lithofacies modelling has also brought to light important early post‐depositional neomorphic changes, which obscure primary fabrics, and complex diagenetic histories have been revealed. Stable isotope signatures of carbon and oxygen have also been used to define palaeo‐water temperature. Fine‐tuning of depositional time frames has also been achieved from absolute dating techniques employing radiocarbon and uranium/thorium.

Rapid cold water formation and recrystallization of relict bryophyte tufa at the Fall Creek cold springs, Alberta, Canada

The Fall Creek Tufa is a relict, mid- to late-Holocene freshwater carbonate spring deposit that is formed almost entirely of calcite-encased Cratoneuron commutatum gametophytes. The spring is presently active, emitting 13 °C meteorically derived waters, but it is not precipitating calcite. Wood embedded in the youngest part of the tufa yielded a radiocarbon (14C) date of 310 ± 50 years BP. The oldest parts of the deposit could not be dated by radiocarbon methods because of a lack of embedded organics. The Fall Creek Tufa, however, probably began forming ~5000 years BP in response to the same wet climatic conditions that initiated calcite precipitation at Miette Hot Springs in Jasper National Park and the Cave and Basin Hot Springs in Banff National Park. The relict deposit is composed largely of calcite cements that were precipitated around the bryophytes, with lesser amounts of internal sediments. The tufa formed through a repeated four-stage developmental process that involved (I) encrustation, (II) encapsulation, (III) cavity occlusion, and (IV) diagenetic alteration. These stages were temporally and spatially variable. Although a young deposit, much of it has undergone extensive aggrading recrystallization, whereby crystal size has increased as adjacent calcite crystals were sutured together. Epifluorescence micro s copy shows that primary depositional fabrics were masked, but not destroyed, during recrystallization. Narrow ranges of δ18O (VPDB, Vienna Pee-Dee Belemnite standard) values (–18.4‰ to –17.7‰) and δ13C (VPDB) values (–1.2‰ to 0.5‰) indicate that the calcite was in isotopic equilibrium with the water that it precipitated from, and that diagenesis did not reset the stable oxygen and carbon isotopic compositions.

A quantitative examination of modern sedimentary lithofacies formation on the glacially influenced Gulf of Alaska continental shelf

Continental-shelf lithofacies are described from a series of cores collected in the northern Gulf of Alaska, a high-energy paraglacial shelf experiencing rapid rates of sediment accumulation. Short-lived tracers ( 234Th and chlorophyll- a) indicate that during the annual peak in fluvial sediment input (summer), biologic sediment mixing coefficients in the surficial seabed are generally lower than other coastal environments (<20 cm 2 yr −1) and mixing extends downward <10 cm. 210Pb geochronology indicates that sediment accumulation rates (time scales of 10–100 yr) are 0.1–3 cm yr −1. The measured bioturbation and accumulation rates lead to predictions of moderate to bioturbated lithofacies, as observed. Primary depositional fabric is preferentially preserved where sediment accumulation rates >2 cm yr −1 and non-steady sediment deposition occurs. Depositional fabric is also observed in strata at 50–100 m water depths and is similar in appearance to beds that may form through deposition of wave-induced fluid-mud flows, which have been observed forming on other shelves with moderate to high wave energy. Five general lithofacies can be identified for the study area: inner-shelf sand facies, interbedded sandy mud facies, moderate-to-well-bioturbated mud facies, gravelly mud facies, and Tertiary bedrock facies. The moderate-to-well-bioturbated mud facies is areally dominant, representing over 50% of the shelf area, although roughly equal volumes (∼0.4 km 3) of strata with some preservation of primary fabric are annually accumulating. Lithofacies on this paraglacial shelf generally resemble mid- and low-latitude allochthonous shelf strata to a much greater degree than Holocene glacimarine strata formed on shelves dominated by icebergs and floating ice shelves. Paraglacial strata may be differentiated from non-glacial shelf strata by lower organic carbon concentrations, a relatively lower degree of bioturbation, and increased preservation of primary depositional fabric.

Event sedimentation, bioturbation, and preserved sedimentary fabric: Field and model comparisons in three contrasting marine settings

A model for the emplacement, bioturbation, and preservation of fine-scaled sedimentary strata has been used to evaluate the impact of interacting physical and biological sedimentary processes on preserved sedimentary fabric in three depositional settings. The settings, which have contrasting hydrodynamic, geological, and biological characteristics, are Eckernförde Bay, German Baltic Sea; the Eel Shelf, northern California, USA; and the northern Gulf of Alaska continental shelf, USA. We have compared field measurements of sedimentation and bioturbation (from 7Be, 234Th, 210Pb, and 137Cs measurements), and observations of sedimentary fabric derived from core X-radiographs with model predictions of preserved sedimentary fabric. The one-dimensional model is forced by fluctuations in the instantaneous sedimentation rate, which deposits sediment with primary sedimentary fabric. Primary sedimentary fabric is then modified by bioturbation, which is represented as a first-order depth-limited reaction term, the rate of which is derived from radioisotopic and time-series estimates of bioturbation depth and intensity. Model output includes depth in seabed and corresponding preservation quotient, a measure of relative primary versus biogenic fabric. Although measured rates and depth scales of sedimentation and bioturbation vary across the three study areas by more than a factor of ten, model results strongly resemble actual sedimentary fabric in core X-radiographs in each case. Our results support and expand on concepts that invoke episodic sedimentation and depth-dependent bioturbation as important competing factors in the preservation or destruction of primary depositional fabric, and also suggest that radioisotopic estimates of bioturbation rates can adequately portray natural conditions.

Cinnabar, livingstonite, stibnite and pyrite in Pliocene silica sinter from Northland, New Zealand

AbstractSilica sinter masses in the southern portion of the Pliocene Puhipuhi geothermal field of Northland, New Zealand, have recrystallized to microcrystalline quartz and moganite but many primary depositional fabrics of the sinters can still be recognized. Finely disseminated cinnabar, acicular stibnite, pyrite framboids and minor livingstonite are distributed through both massive sinter and stromatolitic fabrics with sulphide mineralization extending from fractured rocks about former spring vents into less disturbed sinter layers. The deposition of sulphides in the sinters is part of a continuum of mineralization resulting from the former hydrothermal regime and which extends to depth in the extinct geothermal system. Periodic changes in the hydrology, such as repeated fracturing following fracture sealing facilitated episodic sulphide deposition. Mercury is considered to have travelled in the liquid phase with antimony and precipitated directly as cinnabar. Remobilization of the sulphides, along with the recrystallization of the sinter masses, have produced complex textural relations. The multifaceted paragenesis of the sulphides is reflected in the range of their minor and trace element compositions revealed by electron microprobe analyses.

Strain partitioning of deformation mechanisms in limestones: examining the relationship of strain and anisotropy of magnetic susceptibility (AMS)

In order to investigate the relationship between rock strain and anisotropy of magnetic susceptibility (AMS), strain partitioning and AMS analysis was conducted at 35 sites from two stratigraphically adjacent Paleozoic limestone units in the Patterson Creek and Wills Mountain anticlines in the central Appalachian orogen of West Virginia. In addition, anisotropy of anhysteretic remanent magnetization (AARM) was conducted on selected samples to examine the role of preferentially oriented magnetite on the AMS fabric. Strain is partitioned into bed-normal shortening due to compaction solution strain (≤35.0% shortening), bed-parallel shortening due to tectonic solution strain (≤13.3% shortening), calcite twinning strain (≤5.8% shortening), and grain-boundary-sliding (≤26.7% shortening). The AMS fabrics in the rocks were found to be a result of a complex interaction between rock lithology, deformation mechanisms, and strain magnitude. Although all the rocks have experienced the same deformation conditions, six different AMS fabrics are exhibited. Each of the different AMS fabrics is a composite fabric resulting from the overprinting of three components: (1) an inherent primary depositional AMS fabric that is attributed to preferentially oriented phyllosilicates in the rock matrix; (2) a diagenetic and/or compaction AMS fabric formed during burial that is due to preferentially oriented phyllosilicates in solution structures and in the rock matrix; and (3) a tectonic AMS fabric that was imparted on the rocks by layer-parallel-shortening deformation prior to folding, and is also attributed to preferentially oriented phyllosilicates in solution structures and in the rock matrix, as well as twinning of ferroan calcite.