Terrestrial Depositional Environments Research Articles

STRATIGRAPHY AND SEDIMENTOLOGICAL EVOLUTION OF THE NEOPROTEROZOIC IKORONGO GROUP OF NORTHEASTERN TANZANIA.

On the northern side of the Ikorongo basin, the Neoproterozoic Ikorongo Group, comprising arenites, arkosic arenites, shales and siltstones, unconformably overlies the Archaean Nyanzian Supergroup of the Tanzania craton. In the study area, the Ikorongo Group is subdivided into three stratigraphic formations namely the Kinenge, Sumuji and Masati Formations. Two new stratigraphic members for the Sumuji Formation are proposed: The lower unit is the shale member which comprises of four alternating horizons of brown and green shales containing mudcracks and ripples and is here reported for the first time to overlie, through a tectonic contact, basement rocks. The upper unit above the shale member is the cross-laminated micaceous siltstone member. Sedimentological studies reveal seven depositional litho-facies and highlight the possibility of hydrodynamic and wave energy fluctuations during deposition of the Ikorongo sedimentary units. The fining upwards trend from the Kinenge sandstones towards the overlying Sumuji shales suggests an energy reduction episode responsible for the deposition of these units whilst the coarsening in grain size, from the Sumuji siltstones upstratigraphy to the Masati sandstones attests to an energy increase. The color variations in the shale member reveal intermittent changes in redox conditions and/or depositional environments during when these strata were laid down. The presence of mud cracks and ripple marks in the rocks of the Sumuji Formation signifies deposition in shallow water environments which had experienced intermittent sub-aerial exposure and desiccation. Fluctuations in hydrodynamic energy, the presence of reddish brown beds and pristine palaeocurrent structures preserved both in space and time within the Ikorongo strata suggest an overall terrestrial depositional environment.

Combining charcoal and elemental black carbon analysis in sedimentary archives: Implications for past fire regimes, the pyrogenic carbon cycle, and the human–climate interactions

This paper addresses the quantification of combustion-derived products in oceanic and continental sediments by optical and chemical approaches, and the interest of combining such methods for reconstructing past biomass burning activity and the pyrogenic carbon cycle. In such context, the dark particles > 0.2 µm 2 remaining after the partial digestion of organic matter are optically counted by automated image analysis and defined as charcoal, while the elemental carbon remaining after thermal and chemical oxidative treatments is quantified as black carbon (BC). The obtained pyrogenic carbon records from three sediment core-based case studies, (i) the Late Pleistocene equatorial Pacific Ocean, (ii) the mid-Holocene European Lake Lucerne, and (iii) the Late Holocene African Lake Masoko, are interpreted as proxy records of regional transportation mechanisms and biomass burning activities. The results show that the burial of dark carbon-rich particles in the 360 kyr-long record from the west equatorial Pacific is controlled by the combination of sea-level changes and low-latitude atmospheric circulation patterns (summer monsoon dynamics). However, the three fold increases in charcoal and BC sediment influxes between 53–43 and 12–10 kyr BP suggest that major shifts in fire activity occur synchronously with human colonization in the Indo/Pacific region. The coarse charcoal distribution from a 7.2 kyr record from Lake Lucerne in Switzerland closely matches the regional timing of major technical, land-use, and socio-economic changes during the Neolithic (between ca. 5.7 and 5.2 kyr BP and 4.9–4.5 kyr BP), the Bronze and Iron Ages (at ca. 3.3 and 2.4 kyr BP, respectively), and the industrialization (after AD 1838), pointing to the key impact of human activities on the sources, transportation processes and reservoirs of refractory carbon during the Holocene. In the tropical Masoko maar lake in Tanzania, where charcoal and BC records are highly sensitive to the local climate and environment, surface runoffs from forested areas and/or aerial transportation over short distances are also important sources for detrital charred particles. However, this 4.3 kyr-long record exhibits a major increase in charcoal and BC sediment influxes between 1.8 and 0.6 kyr BP, synchronously with the regional extent of Late Iron Age and agricultural innovations. Therefore, in both marine and terrestrial depositional environments, the climate- and vegetation-controlled fire regimes appear to be strongly associated to societal changes, or directly affected by human practices. In fact, the anthropogenic effect associated to past human activities (e.g. settlement, agriculture, and metallurgy) has temporarily at least tripled the emissions of pyrogenic carbon in the environment. However, the data from the three Late Pleistocene to Holocene sequences also show that the redistribution of fossil particles by runoff and erosion processes is a significant source of pyrogenic carbon that should be understood as a prerequisite for interpreting sedimentary records of biomass burning.

Three-dimensional regional-scale hydrostratigraphic modeling based on sequence stratigraphic methods: a case study of the Miocene succession in Denmark

A sound hydrogeological model is the heart of every well-performing groundwater model. A sequence stratigraphic approach has been tested, which has been widely used in the petroleum industry since the 1980s for the construction of three-dimensional hydrogeological models for sedimentary basins. The sequence stratigraphic interpretation has been established for the Miocene succes- sion in western Denmark based on seismic profiles, gamma-ray logs, sediment descriptions and palynological analysis of borehole samples. The up to 400-m-thick Miocene succession consists of deposits originating from deep marine to terrestrial depositional environments. The succession is subdivided into six sequences, corresponding system tracts and distinct lithofacies covering the period from late Oligocene to latest late Miocene. The final model is converted into a binary three-dimensional model that shows the location, geometry and thicknesses of potential deep aquifers, and is prepared for groundwater flow modeling. The model provides a qualified geological description of the connectivity between deep sand forma- tions and the ground surface. By comparison to a traditional lithofacies model based on sediment descriptions of borehole samples, it is shown that the new method gives a more sound geological understanding, which is essential when flow paths and the vulnerability of deep aquifers are evaluated.

Directly Dating Sedimentary Rocks

This contribution seeks to provide a summary of radiometric dating techniques that are currently applicable to Phanerozoic sedimentary deposits and therefore relevant to paleontologists. First we overview the assumptions necessary for radiometric dating and introduce isochrons and concordia diagrams as methods for calculating ages and evaluating isotope systematics. We then focus on the four most promising systems: U-Pb dating of carbonates, Re-Os dating of black shales, and U-Pb and Lu-Hf dating of phosphates. We review expected geochemical behavior of daughter and parent isotopes in the context of both marine and terrestrial depositional environments. A critical evaluation of previously published ages and potential directions for future work are provided.

Characterisation of organic matter in a shallow, reducing, arsenic-rich aquifer, West Bengal

Elevated arsenic in groundwaters extensively exploited for irrigation and drinking water in West Bengal and Bangladesh is causing serious impacts on human health. A key mechanism for the genesis of arsenic (As) in these waters is microbially mediated reductive transformation of arsenic-bearing Fe(III) hydrated oxides. The role of organic C in this process, whether from in situ organic matter (OM), i.e. OM from within the sediment, or from other sources, is widely recognised. Despite this, there is a paucity of data about the characteristics of OM in these As-rich aquifers. Extraction and analysis of the polar and apolar fractions from seven different sediments from a known groundwater As “hotspot” in West Bengal revealed OM characteristic of the original terrestrial depositional environment. However, this was overprinted by abundant hydrocarbons with thermally mature (e.g. petroleum) distributions. These hydrocarbons included abundant high molecular weight n-alkanes and unresolved complex mixtures (UCMs), as well as thermally mature distributions of hopanes and steranes. Additionally, at certain depths (surface sands: 8 and 13 m, deeper sands: 25 m) the OM appeared to be biodegraded, with the preferential removal of petroleum-sourced n-alkanes, suggesting that indigenous microbes within the aquifer can utilise this carbon source. The presence of this previously unreported source of bioavailable organic carbon is of importance as it has the potential to promote microbial activity and subsequent As release in the aquifers.

Depositional and paleohydrogeological controls on the distribution of organic matter and other reactive reductants in aquifer sediments

The reactivity of sedimentary reductants is the main control for the natural attenuation of common groundwater contaminants, such as nitrate or chlorinated hydrocarbons. Middle Miocene to Late Pleistocene (0.01–20 My old) aquifer sands of marine, fluvial, fluvio-glacial and aeolian origin were characterized to determine the control of sediment depositional environment, sediment age and paleohydrology on the distribution and reactivity of sedimentary reductants. In addition to the variability in the molecular composition of sedimentary organic matter (SOM), the reduction reactivity of these sediments was determined. Oxygen consumption and carbon dioxide production during sediment incubations indicated that SOM, pyrite and ferrous iron-bearing carbonates were the main reductants. Bulk δ 13C org-values (∼25‰) indicated that terrestrial higher land plants were the main precursor of SOM, regardless of sediment origin. However, pyrolysis-GC/MS analysis showed that SOM in the marine Tertiary sands contained lignin with preserved side-chains, while in the Pleistocene fluvial and fluvio-aeolian sediments, highly degraded lignin and recalcitrant macromolecular aliphatic structures dominated SOM, indicative of aerobic degradation. The higher dynamics of these terrestrial depositional environments as compared with their marine counterpart, likely allows for increased oxygen exposure times and thus more intense aerobic degradation of SOM. The aquifer sediment that fills a large erosional valley created during the Saale ice-age, likely consists of fluvio-glacially reworked marine Tertiary sediments, as supported by carbonate isotopic evidence. The more degraded status and decreased reactivity of SOM in this sediment than in its precursor, is likely due to the increased oxygen exposure during dynamic fluvio-glacial reworking. Despite the highly degraded nature of SOM and the absence of pyrite in the Pleistocene fluvial and fluvio-aeolian sediments, oxygen consumption rates were high during the incubation of sediments from the shallowest part of the aquifer. A reactive ferroan carbonate phase was inferred as the main source of oxidant demand in shallow fluvial and aeolian sands. Depleted oxygen and carbon isotopes indicated that this phase was groundwater derived. Aerobic degradation during sediment deposition appears to control the molecular composition and reactivity of SOM in aquifer sediments and to affect the potential of subsequent diagenetic pyrite formation. In addition, (paleo)hydrological conditions may result in the accumulation of a ferrous iron-bearing carbonate phase precipitated during the exfiltration of Fe(II)-containing anoxic groundwater.

Characterization of the hydrogen isotopic composition of individual n-alkanes in terrestrial source rocks

In order to characterize the H isotopic compositions of individual lipid compounds from different terrestrial depositional environments, the δD values of C-bound H in individual n-alkanes from typical terrestrial source rocks of the Liaohe Basin and the Turpan Basin, China, were measured using gas chromatography–thermal conversion–isotope ratio mass spectrometry (GC–TC–IRMS). The analytical results indicate that the δD values of individual n-alkanes in the extracts of terrestrial source rocks have a large variation, ranging from −140‰ to −250‰, and are obviously lighter than the δD of marine-sourced n-alkanes. Moreover, a trend of depletion in 2H(D) was observed for individual n-alkanes from different terrestrial depositional environments, from saline lacustrine to freshwater paralic lacustrine, and to swamp. For example, the δD values of n-alkanes from a stratified saline lacustrine environment vary from −140‰ to −200‰, δD for n-alkanes from swamp facies range from −200‰ to −250‰, while those from freshwater paralic lacustrine–lacustrine environments fall between the δD values of the end members. The shift toward lighter δD from saltwater to freshwater environments indicates that the source water δD is the major controlling factor for the H isotopic composition of individual compounds. In addition, H exchange between formation water and sedimentary organic matter may possibly be important in regard to the δD of individual n-alkanes. Therefore, other lines of geochemical evidence must be considered when depositional paleoenvironments of source rocks are reconstructed based on the H isotopic composition of individual n-alkanes.

Role of crustal anisotropy in modifying the structural and sedimentological evolution of extensional basins: the Gamtoos Basin, South Africa

AbstractThrough the investigation of crustal heterogeneities, sedimentary basin architecture and seismic stratigraphy, we demonstrate how a crust‐scale anisotropy controls the initiation of rifting and the subsequent structural and sedimentological evolution of the Mesozoic Gamtoos Basin, southern South Africa. The results demonstrate that the >90‐km‐long Gamtoos Fault established its length very early in its syn‐rift phase (within ∼5 Ma of rift initiation) before accruing over 6 s (two‐way‐travel time (TWT)), or >12 km, of displacement without any significant subsequent increase in length. In addition, there is no evidence at the resolution of the data of fault segmentation, isolated depocentres nor of intra‐basin faults progressively coalescing during the syn‐rift interval. The early establishment of length resulted in a rapid transition from a terrestrial depositional environment to anoxic, deep marine conditions. The Gamtoos Fault has a 90° bend in the fault trace that we propose is inherited from the underlying structure. Immediately adjacent to the bend the basin‐fill is significantly deformed and a high‐amplitude (>1.7s TWT) monoclinal fold is observed. Previous workers proposed that the fold was a consequence of a complex interplay between compression and extension. Through a restoration of the basin‐fill deformation we produce a model that suggests that the fold is a consequence of the accommodation of extension by the unusual plan‐view trace of the fault. The evolution of the basin does not conform to current fault growth models and it is proposed that its unusual and complex development can be attributed to the underlying crustal‐scale anisotropy, a fact that is likely to be important in other areas in which crustal stretching is superimposed on heterogeneous continental crust.

Glaciomarine deposition around the Irish Sea basin: some problems and solutions

AbstractReconstructing the depositional processes and setting (marine or terrestrial depositional environment) of late Devensian age glacigenic sediments around the Irish Sea basin (ISB) is critical for developing an all‐embracing and consistent glacial model that can account for all observed field evidence. Identifying episodes of marine and terrestrial glacial deposition from field data is considered the first step in achieving this goal. Criteria for distinguishing marine and terrestrial glacial environments in the ISB include the geomorphology, sedimentology and faunal content (biofacies) of the associated deposits.Exposures of glacigenic sediments around the ISB are very diverse in terms of their morphosedimentary characteristics and associated biofacies, and thus inferred depositional processes and setting. Possible reasons for the diverse geological record include the differing geometry of eastern and western ISB coasts, time‐transgressive ice retreat, and differential land rebound effects as a result of forebulge collapse and neotectonics. Poor geochronometric control on ice sheet events has not helped the correlation of ISB events with glacial and climatic events elsewhere. Future investigations of glacial sites around the ISB should use an integrated methodological approach involving a range of geomorphological, sedimentary and biofacies data, and dating control where possible. This will help in developing a more precise and holistic late Devensian glacial model that is constrained rigorously by field geological evidence. Copyright © 2001 John Wiley & Sons, Ltd.

Glaciomarine deposition around the Irish Sea basin: some problems and solutions

Reconstructing the depositional processes and setting (marine or terrestrial depositional environment) of late Devensian age glacigenic sediments around the Irish Sea basin (ISB) is critical for developing an all-embracing and consistent glacial model that can account for all observed field evidence. Identifying episodes of marine and terrestrial glacial deposition from field data is considered the first step in achieving this goal. Criteria for distinguishing marine and terrestrial glacial environments in the ISB include the geomorphology, sedimentology and faunal content (biofacies) of the associated deposits. Exposures of glacigenic sediments around the ISB are very diverse in terms of their morphosedimentary characteristics and associated biofacies, and thus inferred depositional processes and setting. Possible reasons for the diverse geological record include the differing geometry of eastern and western ISB coasts, time-transgressive ice retreat, and differential land rebound effects as a result of forebulge collapse and neotectonics. Poor geochronometric control on ice sheet events has not helped the correlation of ISB events with glacial and climatic events elsewhere. Future investigations of glacial sites around the ISB should use an integrated methodological approach involving a range of geomorphological, sedimentary and biofacies data, and dating control where possible. This will help in developing a more precise and holistic late Devensian glacial model that is constrained rigorously by field geological evidence. Copyright © 2001 John Wiley & Sons, Ltd.

The influence of the South‐west Indian monsoon on continental deposition over the past 130 kyr, Gujarat, western India

Ocean and ice core records of the intensity of the South‐west Indian Monsoon (SwIM) show rapid shifts in most proxy indicators of climate over the past 125 kyr on decadal to millenial scales. However, the responses of continental environments to such perturbations remain unknown due to the absence of a suitable long‐term continental record. The stratigraphic record from Gujarat, western India, an area where all sedimentological processes are governed by the vagaries of the SwIM, reveals three aggradation phases that represent deposits of seasonal rivers (AP1), ephemeral rivers (AP2) and dust storms (AP3). Based on a review of available dates and new ESR dates on the oldest exposed calcretes from palaeovertisols these phases are assigned the time brackets 135–100 kyr bp (AP1), 100–20 kyr bp (AP2) and 20–6 kyr bp (AP3). These results suggest that continental depositional environments respond in a subdued manner and are separated by thresholds. For climate shifts to effect durable changes in the continental depositional environment, a period between > 5 kyr and 16 kyr is the minimum time required to permanently change the landscape of an area. Alternatively, catastrophic changes in terrestrial depositional environments may also be effected by abrupt climatic shifts that are beyond the tolerance limits of the depositional systems.

The non-destructive determination of REE in fossilized bone using synchrotron radiation induced K-line X-ray microfluorescence analysis

The sensitivity and applicability of the synchrotron radiation induced X-ray microfluorescence (μ-SRXRF) spectrometer at the Hamburg synchrotron laboratory Hasylab for the determination of the distribution of trace concentrations of rare-earth elements (REE) in fossilized bone are discussed and critically compared to those of other trace analytical methods such as instrumental neutron activation analysis (INAA) and LAMP-ICPMS (laser ablation microprobe inductively-coupled plasma mass spectrometry). Measurements were carried out on two bone samples from contrasting terrestrial depositional environments at Olduvai Gorge (Tanzania). Results indicate that the microdistribution of the REE in these biological materials is not homogeneous and that the relative abundance of these elements can provide information on the palaeoenvironment during the fossilization process. The heterogeneous distribution of the REE can be determined in a quantitative and completely non-destructive manner provided the concentrations of individual REE are above 10 μg/g.

The Late Devensian glaciation of the eastern Lleyn Peninsula, North Wales: evidence for terrestrial depositional environments

The glacial deposits of the eastern Lleyn Peninsula record the advance, coalescence and subsequent retreat and uncoupling of Welsh and Irish Sea ice-sheets during the Late Devensian cold stage. During advance a thick sheet of basal diamict was deposited over much of the area, and during retreat and uncoupling, which occurred after 14.5 ka, the eastern part of the area was dominated by the formation of a large sandur system draining from the retreating margin of the Irish Sea ice-sheet. Subsequent stages of retreat are marked by a series of arcuate cross-valley moraines formed either by ice-contact deposition during stillstand or by structural deformation during minor snout oscillation. In the western part of the area sedimentation was controlled by a series of dead-ice ridges running parallel to the retreating ice-margin and this led to the development of a complex assemblage of localised depositional environments including ice-front alluvial fans, marginal sandur troughs and pro-glacial lake basins, all formed under supraglacial ice-marginal conditions. No evidence of glaciomarine deposition is recorded. © 1998 John Wiley & Sons, Ltd.

The Late Devensian glaciation of the eastern Lleyn Peninsula, North Wales: evidence for terrestrial depositional environments

The Late Devensian glaciation of the eastern Lleyn Peninsula, North Wales: evidence for terrestrial depositional environments

Organic petrography and geochemistry of inertinite-rich mudstones, Jakobsstigen Formation, Upper Jurassic, northeast Greenland: Indications of forest fires and variations in relative sea-level

The lower-middle Oxfordian Jakobsstigen Formation, Wollaston Forland, northeast Greenland, consists mainly of stacked coarsening-upward successions of offshore to shoreface heteroliths, sandstone and rare foreshore sandstones. The units are separated by thin, laterally extensive sheets of terrigenous carbonaceous mudstones, which have been subjected to organic petrographic and geochemical studies. The mudstones are thermally immature, with maturities corresponding to R 0 in the range 0.35–0.50%. The mudstones contain very high proportions of allochthonous inertinite, subordinate huminite, char and negligible proportions of liptinite. Inertinite reflectance distributions are markedly bimodal, with maxima at approximately 1.73 and 4.91% R m. Both pyrolysis yields and solvent extract yields are low. The distributions of n-alkanes are markedly light-end skewed and show a pronounced predominance of even-numbered compounds in the lower carbon number range. Biomarker-distributions feature a dominance of C 29-steranes, slight enhancement of extended hopanes and αββ-steranes, low proportion of tricyclic triterpanes and very low hopane/sterane ratios. Sedimentological, organic petrographical and geochemical evidence suggests that the regular alternation between marine and terrestrial depositional environments during deposition of the Jakobsstigen Formation was related to low-amplitude, high-frequency changes in relative sea-level and local climate. The mudstones were deposited during early rise of relative sea-level in shallow, flat-bottomed lakes or lagoons on a broad coastal plain. The lakes acted as traps for fine elastic sediment and for predominantly windborne inertinite, generated by wildfires in the hinterland. High rates of evaporation rendered the lakes mildly saline, hampering their colonization by vegetation other than cyanobacteria and halophilic microorganisms. Similarly, saline porewaters excluded higher plant vegetation from emergent areas. Upon continued rise of the relative sea-level, the lakes were gradually flooded and their deposits became covered by sandy shallow marine sediments. The larger areas covered by shallow marine waters during periods of high relative sea-level led to a more humid local climate and to lower frequency of wildfires. During falling relative sea-level, the marine deposits were eroded and partially removed and the cycle subsequently repeated upon renewed rise in relative sea-level. Hence, minor changes in relative sea-level gave rise to the regular alternation of two vastly different depositional environments, as well as to marked variations in local climate.

96/04638 Report on the activities of the International Committee for Coal and Organic Petrology during the period 1991–1995

An interdisciplinary investigation of a section through the early Paleocene Ardley coal zone at Wabamun, Alberta reveals that there are distinct vertical variations in the palynological, petrographical and geochemical properties reflecting changes in the original depositional environment. Palynological assemblages are dominated by Taxodiaceaepollenites hiatus revealing that the original coal-accumulating mires were inhabited by taxodiaceous vegetation. Subordinate palynomorphs include varied gymnosperm pollen other than T. hiatus, Laevigatosporites spp (fern spores of polypodiaceous affinity), Stereisporites spp (mosses including Sphagnum) and Osmundacidites sp. (Family Osmundaceae). Coal petrography reveals that these coals are inertinite-rich. Abundant fusinite (pyrofusinite), along with semi-fusinite and inertodetrinite comprise up to 65% of the maceral assemblages. Huminite macerals are predominantly eu-ulminite B with lesser eu-ulminite A, densinite and geli-huminite (gelified huminite lacking any remnant cell structure). Liptinite macerals include resinite, sporinite and cutinite.Thinner, stratigraphically lower seams at Wabamun (seams Nos 6-3) consist predominantly of bright coal with high huminite contents. Palynofloral assemblages are comprised of T. hiatus with few abundances of other palynomorphs, suggesting that the original depositional environment was low-lying, dominated by taxodiaceous vegetation, and frequently flooded as evidenced by numerous shale partings, high ash contents and relatively high sulphur percentages. The thick No. 2 seam displays a distinct dulling-up petrographic profile resulting from increasing inertinite contents. Coupled with increased abundances of Stereisporites spp, this may be evidence of localized ombrogeny within the original Wabamun mires, and may support a genetic relationship between Stereisporites sp. and inertodetrinite. The No. 1 seam consists of abundant fibrous (fusain-rich) coal. Modes of origin of this fibrous coal include extreme oxidation due to prolonged drought and possibly forest fires. Elevated ash contents within the Nos 1 and 2 seams may be a result of this oxidation of organic matter and concentration of the inorganics, as well as airborne volcanic ash as evidenced by numerous bentonite bands within these coals. Sulphur contents are extremely low, as low as 0.05% reflecting a terrestrial depositional environment.

Controls on halogen concentrations in sedimentary formation waters

AbstractChlorine is the most abundant halogen in sedimentary formation waters with concentrations from <100 to >250000 mg/l. Bromine is the second most abundant halogen at <1 mg/l to >6000 mg/l with iodine from <0.1 mg/l to >100 mg/l and fluorine from <0.1 mg/l to 30 mg/l. Chlorine and bromine show a strong systematic covariation suggesting that they are subject to the same controlling mechanisms. Fluorine only shows relatively high concentrations at elevated chlorine and bromine concentrations showing that fluorine, chlorine and bromine are possibly controlled by the same processes. Iodine does not correlate with any of the other halogens indicating that unique processes control iodine.Key geological parameters that influence chlorine and bromine (and possibly fluorine) concentrations are the presence of salt in a basin, the age of the reservoir unit and the kerogen-type within the main hydrocarbon source rock in a basin. The presence of salt in a basin shows that sea water was evaporated to halite saturation producing connate waters with high concentrations of chlorine and bromine. The presence of salt also leads to high salinity waters through water-salt interaction during burial and diagenesis. Tertiary reservoirs typically have much lower chlorine and bromine concentrations than Mesozoic or Palaeozoic reservoirs. The age of the reservoir unit may simply reflect the different amounts of time available for formation water to interact with salt. The dominance of type II marine kerogen in a basin leads to higher bromine concentrations. This may reflect the dominance of a marine influence in a basin which is more likely to lead to salt deposition than terrestrial depositional environments. Iodine concentrations are independent of all these parameters. Other geological parameters such as depth of burial, temperature, basin forming mechanism and reservoir lithology have no influence upon halogen concentrations.Key processes that affect halogen concentrations are sea water evaporation and dilution, water—salt interaction and input from organic sources. Chlorine and bromine data lie close to the experimentally-derived sea water evaporation trend showing that sea water evaporation may be an important general control on halogens. Sea water dilution is probably responsible for most low salinity formation water chlorine and bromine concentrations for the same reason. Sea water dilution can occur either by meteoric invasion during burial, or following uplift and erosion, or by diagenetic dehydration reactions. Water can interact with salt in a variety of ways: halite dissolution by congruent processes, halite recrystallization by incongruent processes, sylvite dissolution or recrystallization and halite fluid inclusion rupture. Halite dissolution will lead to high chlorine and relatively low bromine waters because halite contains little bromine. In contrast, halite recrystallization will lead to bromine-enhanced waters because NaBr dissolves preferentially to NaCl. The occurrence of dissolution or recrystallization will depend on the water rock ratio, greater volumes of water will lead to more dissolution and waters with higher Cl/Br ratios. Sylvite is usually rich in bromine so dissolution will lead to bromine-enhanced waters. Primary aqueous inclusions in halite contain high bromine concentrations so that rupture, during deformation or recrystallization, will lead to bromine-enhanced formation water. A combination of these processes are responsible for the very limited range of Cl/Br ratios although congruent halite dissolution must have a limited role due to the absence of waters with high Cl/Br ratios.Iodine is strongly concentrated in organic materials in the marine environment; oils and organic rich-source rocks have high I/Cl and I/Br ratios relative to sea water or evaporated sea water. All formation waters are enriched in iodine relative to sea water implying that there has been input from organic matter or interaction with oil. However, hydrocarbon source rock type in a basin has no discernible effect upon iodine concentrations.

Organic petrology of an early Paleocene coal zone, Wabamun, Alberta: palynology, petrography and geochemistry

An interdisciplinary investigation of a section through the early Paleocene Ardley coal zone at Wabamun, Alberta reveals that there are distinct vertical variations in the palynological, petrographical and geochemical properties reflecting changes in the original depositional environment. Palynological assemblages are dominated by Taxodiaceaepollenites hiatus revealing that the original coal-accumulating mires were inhabited by taxodiaceous vegetation. Subordinate palynomorphs include varied gymnosperm pollen other than T. hiatus, Laevigatosporites spp (fern spores of polypodiaceous affinity), Stereisporites spp (mosses including Sphagnum) and Osmundacidites sp. (Family Osmundaceae). Coal petrography reveals that these coals are inertinite-rich. Abundant fusinite (pyrofusinite), along with semi-fusinite and inertodetrinite comprise up to 65% of the maceral assemblages. Huminite macerals are predominantly eu-ulminite B with lesser eu-ulminite A, densinite and geli-huminite (gelified huminite lacking any remnant cell structure). Liptinite macerals include resinite, sporinite and cutinite. Thinner, stratigraphically lower seams at Wabamun (seams Nos 6-3) consist predominantly of bright coal with high huminite contents. Palynofloral assemblages are comprised of T. hiatus with few abundances of other palynomorphs, suggesting that the original depositional environment was low-lying, dominated by taxodiaceous vegetation, and frequently flooded as evidenced by numerous shale partings, high ash contents and relatively high sulphur percentages. The thick No. 2 seam displays a distinct dulling-up petrographic profile resulting from increasing inertinite contents. Coupled with increased abundances of Stereisporites spp, this may be evidence of localized ombrogeny within the original Wabamun mires, and may support a genetic relationship between Stereisporites sp. and inertodetrinite. The No. 1 seam consists of abundant fibrous (fusain-rich) coal. Modes of origin of this fibrous coal include extreme oxidation due to prolonged drought and possibly forest fires. Elevated ash contents within the Nos 1 and 2 seams may be a result of this oxidation of organic matter and concentration of the inorganics, as well as airborne volcanic ash as evidenced by numerous bentonite bands within these coals. Sulphur contents are extremely low, as low as 0.05% reflecting a terrestrial depositional environment.

The Permian-Triassic boundary in Wyoming: the case of the disappearing paraconformity

Long-standing interpretations of paleontologic, sedimentologic, and stratigraphic evidence from Permian-Triassic marine sequences in western Wyoming have suggested an interruption in deposition of several million years' duration between the two systems, even though physical evidence of unconformity is subtle and somewhat equivocal. We postulated that an unconformity of this duration should be more pronounced in paralic and non-marine facies in central and southeast Wyoming than in adjacent inner-shelf marine facies in westcentral Wyoming. Therefore, we correlated an erathem boundary-bearing sequence from westcentral Wyoming (where it is faunally controlled) to southeast Wyoming (where it is non-fossiliferous) and studied this sequence for evidence of hiatus. The correlations were made using surface sections, surface gamma-radiation logs, and subsurface log suites.In southeast Wyoming, the lithostratigraphic equivalent to the systemic boundary in westcentral Wyoming is located within a redbed-evaporite sequence traditionally interpreted as having accumulated in paralic and/or terrestrial depositional environments. Physical evidence of disconformity at this surface in southeast Wyoming is no greater, and is in places less, than at several other horizons within the boundary-bearing sequence. Also, petrologic examination of the terrigenous clastic units below, through, and above the boundary-bearing sequence in central and southeast Wyoming suggests notable stability of the depositional environment. Southeastward stratigraphic thinning of various units within this boundary-bearing sequence is demonstrable; however, compelling evidence of regional truncation is not evident, and the stratigraphic thinning appears to be due to primary depositional processes rather than post-depositional erosion during hiatus.We interpret slow, episodic, yet generally continuous deposition of evaporite and siliciclastic units in southeast Wyoming across the Permian-Triassic boundary. If true, then conventional biostratigraphic estimates of the duration of a hiatus separating Permian inner and middle-shelf carbonate facies from overlying Triassic siliciclastics in western Wyoming appear to be overly long, and may need re-evaluation.

Facies studies of coal and coal-bearing strata

Abstract Prior to 1960 most discussions on the depositional environment of coal-bearing strata were related to theories of the origin of cyclothems. Work during the 1960s on modern deltas, particularly the Mississippi, led to the interpretation of the classic cyclothem as deltaic in origin, with coals originating in delta-top swamps. During the 1970s and early 1980s, detailed models of depositional environments were developed for coal-bearing strata. Coals have now been described in association with virtually all types of terrestrial depositional environments. There are surprisingly few published examples of facies analyses of coal-bearing strata. Detailed measured sections and meticulous descriptions of sedimentary structures are necessary for geologists to make independent assessments of published environmental interpretations and to make comparisons between various coal-bearing formations. The coal industry could benefit from more predictive facies models, which will require greater integration of coal and clastic facies studies. In most mires close to areas of active clastic deposition, ash content of peat is too high for them to be precursors of coal. Most coal must have originated as peats in raised mires or in low-lying mires well removed from active clastic environments. In the latter case, a significant hiatus in deposition may be represented by the contact of coal with underlying sediments. Subsurface studies, which allow an understanding of the three-dimensional character of coal-bearing strata, may provide clues as to the type of swamp in which a coal was deposited. Recent studies in the Upper Cretaceous of Alberta, for example, provide clear evidence that coals originated in mires well removed from contemporaneous clastic depositional systems.