Use of clast shape in determining the sedimentary history of the late devonian keepit conglomerate,Australia

Clasts of the Late Devonian Keepit Conglomerate of northeastern New South Wales, Australia, are predominantly andesites of isotropic nature that lack any apparent initial shape controls. Given adequate time in a particular environment, these clasts could be reasonably expected to develop shapes characteristic of abrasion within that environment.Clasts from both fluvial and marine resedimented conglomerates of the Keepit Conglomerate possess shape characteristics indicative of shaping within a fluvial environment. Maximum projection sphericity, and a plot of sphericity against either the oblate-prolate index or the flatness parameter C/A, were the most useful in indicating fluvial shaping processes. In addition, the $\text{OP}$ Index and the dominant form classes of the Sneed and Folk Sphericity-Form diagram were in support of a fluvial history for the clasts.The occurrence of clasts exhibiting typical fluvial shapes in marine resedimented conglomerates indicates a lack of time and/or insufficient energy in a beach environment for reshaping of the clasts prior to resedimentation into deeper-water submarine-fan environments. This interpretation is consistent with and lends support to the interpretation, based on all other available data, for the Keepit Conglomerate of a history of rapid progradation of fluvial gravels into a relatively low-energy coastline environment, with periodic resedimentation of coarse sediment into deeper water.     

Mega-debris flow deposits from the Oligo-Miocene Pindos foreland basin, western mainland Greece: implications for transport mechanisms in ancient deep marine basins

The turbidite dominated, Oligo-Miocene Pindos foreland basin of western mainland Greece contains two thick (60–72 m), matrix supported conglomerates. The conglomerates are ungraded and contain three clast types: (1) polymict, rounded, extrabasinal clasts (long axes 3–50 cm); (2) tightly folded, intrabasinal clasts (long axes 1–10 m); and (3) tabular, largely undeformed, intrabasinal blocks (long axes 18–300 m). Clasts are isolated within a slit dominated matrix. These chaotic, matrix supported conglomerates are interpreted as mega-debris flow deposits. During transport, extrabasinal clasts were supported by a combination of matrix cohesion and clast dispersive pressure, folded intrabasinal clasts were supported by a combination of buoyancy (Archimedes principle) and clast dispersive pressure. The large tabular clasts were transported by gravity sliding/gliding within the flow on films at high pore fluid pressure. These different clast support mechanisms were active simultaneously within the Pindos mega-debris flow deposits. As a result, the deposits have no systemic vertical stratigraphy, in contrast to many described large scale mass flow deposits. The mega-debris flow deposits are significantly thicker than most described ancient siliciclastic debris flow deposits and provide an ancient analogue for the thick Recent siliciclastic debris flow deposits on continental margins.     

鄂尔多斯盆地西缘砾质冲积扇沉积学特征

晚三叠世鄂尔多斯西缘冲积扇沉积主要由碎屑流、颗粒流、片流及河道砾岩等组成。文中讨论了各类砾岩的沉积学特征,同时指出,不同成因的砾岩,其粒度分布曲线形态及某些粒度参数亦不同.据此本文提出了根据-系列新的粒度分布曲线形态和粒度参数Mdf,Rgs和Mdf/Rgs等甄别冲积扇中不同类型砾岩的设想。     

Modification of sediment characteristics during glacial transport in high-alpine catchments: Mount Cook area, New Zealand

The Mount Cook area in the Southern Alps of New Zealand is heavily glacierized with numerous peaks over 3000 m a.s.l. feeding several large valley glaciers. The region is subject to rapid tectonic uplift and heavy precipitation (up to 15 m per year). This paper describes the clast roundness, clast shape and textural characteristics associated with five glaciers (Fox, Franz Josef, Hooker, Mueller and Tasman) in terms of inputs to the glacier system, transport by the glaciers and reworking following glacial deposition. Inputs include rockfall, alluvial fan and avalanche material delivered to the surface of valley glaciers. Basal debris, where observed at the terminus of two glaciers, consists mainly of incorporated fluvial material. Following deposition, reworking is mainly by subglacial and proglacial streams. The dominant facies are (i) boulder gravel with mainly angular clasts on the steep slopes above the glaciers, (ii) sandy boulder gravel, with mainly angular and subangular clasts, forming lateral and end moraines, and (iii) sandy boulder/cobble gravel with mainly subrounded clasts, and sand, which represent glacially transported sediment reworked by braided rivers. Diamicton is rare in the contemporary glacial environment. Since most sediment associated with glaciers in the Southern Alps lacks unambiguous indications of glacial transport, interpretation of similar sediments in the geological record should not necessarily exclude the involvement of glacial processes.     

Strain analysis and tectonite classification using polymictic metaconglomerates in the Igarra schist belt,southwestern Nigeria

Deformed conglomerates in the Igarra schist belt display contrasting strains between different lithological clast populations. We analysed three different clast populations (pegmatite, metasediment, quartz) across three sites in the Igarra metaconglomerates of Edo state, Nigeria. We calculated finite strain using the R_f/ø method and Flinn graph. Quartz clasts exhibited the least amount of strain, while the pegmatite and metasedimentary clasts had greater strains (pegmatite > metasediment > quartz). The variability in the finite strains gotten from the three sites is controlled by clast composition and probably grain size. The differences in finite strain and maximum elongation direction (λ ₁) in the three sites indicates that the Igarra metaconglomerates was subjected to a heterogenous simple shear deformation which is probably associated with the general transpressional deformation that affected the Pan-African mobile belt considering its (Igarra schist belt) spatial proximity with regional dextral shear zones. Three-dimensional strain analysis in site one indicates a constrictive deformation with the dominance of L tectonites. Spatial analysis of two-dimensional strain suggests a strain gradient where finite strain decreases from north to south.     

Provenance and sediment characteristics of contemporary gravel deposits at Sellicks Beach,eastern shore of Gulf St Vincent,South Australia

Sellicks Beach, located on the eastern shore of Gulf St Vincent, South Australia, is subject to wave-dominated processes and northward longshore transport. During winter, when wave energy is typically vigorous, gravel deposits are exposed across most of the beach, and three step-like berms are well developed. Sand is restricted to a narrow strip that is exposed only at low tide. In contrast, during summer, when wave energy is generally moderate to low, much of the gravel is covered by a thin veneer of sand and only the high berm, on the landward edge of the beach, remains as an obvious feature. Steeply dipping Neoproterozoic to Cambrian strata that outcrop strongly across Sellicks Hill are the original source rocks for the beach gravel; distinctive sedimentary textures, structures and fossils in the cobble-size clasts can be confidently matched with those of the provenance rocks. Much of the sediment entered the modern beach environment as a consequence of coastal erosion of transitional alluvial fan sediments. The oldest alluvial fan sediments are of late Pliocene to earliest Pleistocene age. Mount Terrible Gully provides a conduit for the input of fluvial sediment at the mouth of Cactus Canyon, where clasts as large as boulders accumulate across the beach. Sellicks Beach gravels are subject to longshore transport northwards. Relatively softer clasts, such as those derived from the Heatherdale Shale, are rare beyond Cactus Canyon. In contrast, quartzite clasts are more abundant towards the north. This lithological differentiation is attributed to preferential survivorship of clasts that are physically harder and chemically less reactive. The change in the shapes of clasts northwards, from predominately shingle-like ‘very platy’ and ‘very bladed’ at Cactus Canyon, to more ‘compact’ towards the boat ramp, is in accord with the more massive fabric of the surviving quartzite clasts. At Sellicks Beach, preservation of uplifted, coarse gravels, with entire and comminuted marine molluscan shells, of last interglacial age, provides evidence of neotectonism. At the landward margin of the beach, imbricated gravels in which pore spaces have been infilled with mud, and which show no evidence of modern coastal erosion, may provide evidence of continuing uplift during the recent Holocene. The geological setting, geomorphic framework and modern sedimentary regime at Sellicks Beach combine to provide an exceptionally useful outdoor laboratory for education in field geology.     

Chemical and physical responses to deformation in micaceous quartzites from the Tauern Window, Eastern Alps

Micaceous quartzites from a subvertical shear zone in the Tauern Window contain abundant quartz clasts derived from dismembered quartz‐tourmaline veins. Bulk plane strain deformation affected these rocks at amphibolite facies conditions. Shape changes suggest net shortening of the clasts by 11–64%, with a mean value of 35%. Quartz within the clasts accommodated this strain largely via dislocation creep processes. On the high‐stress flanks of the clasts, however, quartz was removed via solution mass transfer (pressure solution) processes; the resulting change in bulk composition allowed growth of porphyroblastic staurolite + chlorite ± kyanite on the clast flanks. Matrix SiO₂ contents decrease from c. 83 wt% away from the clasts to 49–58% in the selvages on the clast flanks. The chemical changes are consistent with c. 70% volume loss in the high‐stress zones. Calculated shortening values within the clast flanks are similar to the volume‐loss estimates, and are greatly in excess of the shortening values calculated from the clasts themselves. Flow laws for dislocation creep versus pressure solution imply large strain‐rate gradients and/or differential stress gradients between the matrix and the clast selvages. In a rock containing a large proportion of semirigid clasts, weakening within the clast flanks could dominate rock rheology. In our samples, however, weakening within the selvages was self limiting: (1) growth of strong staurolite porphyroblasts in the selvages protected remaining quartz from dissolution; and (2) overall flattening of the quartz clasts probably decreased the resolved shear stress on the flanks to values near those of the matrix, which would have reduced the driving force for solution‐transfer creep. Extreme chemical changes nonetheless occurred over short distances. The necessity of maintaining strain compatibility may lead to significant localized dissolution in rocks containing rheologic heterogeneities, and overall weakening of the rocks may result. Solution‐transfer creep may be a major process whereby weakening and strain localization occur during deep‐crustal metamorphism of polymineralic rocks.     

The Proterozoic (1.85 Ga) Älvestorp Conglomerate,Bergslagen, Central Sweden

The Älvestorp conglomerates, deposited in an alluvial fan setting, form part of the Svecofennian orogenic belt in west Bergslagen, south central Sweden and are estimated to be as old as c. 1.85 Ga. Reaching a thickness of one kilometre, their architecture and form suggest an alluvial origin. Along sections, massive conglomerates often grade into pebbly mudstones and greenschist facies slates, while pure slates with dolomite concretions and olistolites occur in dark slatey mudstones on the eastern shore of lake Brunnsjön. Inner fan trenches are filled with massive, clast‐supported conglomerates that contain more than 80 percent epiclastic tuffaceous material. The Älvestorp conglomerate is therefore classified as the product of a Proterozoic stream‐flow channel and debris flow, or alluvial fan. The Grythyttan Basin to the north originated by extension after the first of two orogenic stages of Bergslagen.     

Comparative scanning electron microscopy study of oriented till blocks, glacial grains and Devonian sands in Estonia and Latvia

Samples of Middle Devonian (Eifelian age; 387–380 Ma) indurated and non‐cemented sandstone were compared with Pleistocene basal tills in Estonia and Latvia to test a hypothesis that glacial SEM (scanning electron microscopy) microtextures are distinctly different from those produced in a fluvial depositional environment. The deposits of Middle Devonian Aruküla Stage were emplaced in a continental water basin close to sea level and well away from any glacial source. Therefore, the SEM microtextures on quartz grains from the Aruküla Stage should show mainly the effect of stream transport. The basal tills are of Late Weichselian age deposited as ground moraine directly over the sandstone. Additional glaciofluvial and glaciolacustrine samples were included with the tills to determine whether glacial and fluvial‐lacustrine transport could be differentiated by the SEM microtextures. Samples of oriented blocks of till from a limited number of sites were studied without pretreatment to determine whether sand clast orientation could provide a method for determining glacial flow vectors. While there are some microtextural similarities between grains from glacial and glaciofluvial‐lacustrine depositional environments, the vast majority of grains from till deposits (50%–60%) are faceted, sharp edged, angular to subangular, and comprised of numerous and distinct microfeatures including abraded surfaces over microfractures, deep linear and curved troughs (striations), step features, and a preponderance of conchoidal and linear microfractures. Glaciofluvial and lacustrine grains contain abundant abrasion features and v‐shaped percussion cracks that make them very distinct from glacial grains. Fluvial transport produces primarily rounded grains, well abraded, with v‐shaped percussion scars dominating. Thus, it is possible to use microtextural differences between the three sample suites to identify particular depositional environments. Oriented till blocks provide information on sand clast orientation. Although carbonate coatings often obscure sand clasts in untreated blocks, it is possible to determine some microfabric information that can be useful in determining flow direction of the ice.     

Texture and structure of resedimented conglomerates: examples from Książ Formation (Famennian—Tournaisian), southwestern Poland

The Famennian-Tournaisian conglomerates and sandstones of the Ksiaz Formation are interpreted as marine resedimented deposits. Matrix- and clast-supported conglomerate beds are equally common, and two textural sequences (motifs) have been recognized: (I) matrix-rich conglomerate → pebbly sandstone → sandstone, and (II) clast-supported conglomerate → sandstone. Variation in clast type partly controls motif type, and therefore, to some extent, matrix percentage in the conglomerates generally. Grading is extremely common in both clast- and matrix-supported conglomerates: inverse (19%), inverse-to-normal (14%) and normal (26%). The studied succession, itself part of a 4 km thick, fan delta, basin-fill sequence, is organized into large (110–150 m) and small-scale (5–30 m) sequences, both of which show (1) upward coarsening and thickening, (2) upward trend of sandstones and pebbly sandstone → matrix-rich conglomerates → clast-supported conglomerates and (3) a less clear upward tendency of massive and normally graded beds → inversely graded beds. Variation in matrix percentage in beds is therefore also partly controlled by fan processes, during the progradation of fan bodies and lobes. It is predicted that individual resedimented conglomerate beds or motifs show general downfan trends in thickness, texture and structure opposite to those evident in the vertical sequences.     

Anatomy of a Forearc Submarine Fan: Upper Eocene — Oligocene Andaman Flysch Group,Andaman Islands,India

Detailed facies analysis in the upper Eocene—Oligocene Andaman Flysch Group reveals fourteen different facies, grouped into three different associations. These facies associations represent two anatomical divisions of a submarine fan, viz. inner fan and depositional lobe in middle fan. Paleocurrent pattern, high percentage of quartz in the sandstones and outer arc derived clasts in the inner fan channel conglomerates suggest dual sediment supply in this fan. A longitudinal geometry of the basin has been inferred where juxtaposition of sediments of different proximality deterred development of any definite sequence pattern. A tectonically induced active sediment supply in a rising sea level stand is thought to be responsible for development of the fan.     

A terrestrial-shallow marine transition in the archean opemisca group east of Chapais,Quebec

The Stella and Hauy Formations of the Archean Opemisca Group in the eastern Chapais Syncline feature numerous fluvial to shallow marine transitions throughout the stratigraphic sequence. During the first depositional phase (Stella Formation), relatively high gradient alluvial fans prograde into a shallow marine environment. The coarse clastic sediments reflect a Scott-type depositional environment. The initial depositional environment is envisaged to be a high-energy, wave- and storm (flood) -dominated coastline bordered by coastal alluvial fans. Shallow marine deposits consist of coarse- to very coarse-grained sandstones and intraclast and pebble to cobble conglomerates. The prevalent shallow marine sediments are related to storm and/or flood deposition.Terrigenous sediments of the Donjek-type depositional environment prevail during the second depositional phase (Hauy Formation). The shallow marine sediments, although still wave- and storm (flood) -dominated, are finergrained, and when correlated with the terrigenous sediments imply a lower gradient and a maturing of the coastline. Furthermore, contemporaneous volcanism of andesitic (shoshonitic) to basaltic composition, occurs during this evolutionary stage. The sedimentary basin gradually evolved during the two depositional phases.The repetition of terrigenous and shallow marine sediments throughout the stratigraphic sequence, coarsening- and fining-sequences in the conglomerates, and the alluvial fan setting strongly suggest a fault-bounded basin margin. Tectonic uplift of the hinterland initiates alluvial fan progradation onto the shelf, whereas quiescent periods are documented by marine transgression. The clast composition of the conglomerates and the alluvial fan environment suggest local derivation of clasts.The synvolcanic Chibougamau Pluton and the Lake Dore Complex acted as a stable basement during deposition of the Opemisca Group. The small landmass served as a stable platform for shallow marine deposition, even when tectonically influenced. A late phase intraplutonic (cratonic) development of an initial backarc basin system (internal zone of the Abitibi greenstone belt) is proposed as the overall depositional setting of the Chapais basin.     

The statistics of counting clasts in rudites: a review, with examples from the upper Palaeogene of southern California, USA

The relative proportions of gravel sized particles of different lithology in rudaceous sedimentary rocks are generally determined in the field by clast counting. Clast counts are usually carried out qualitatively in order to assess sedimentary provenance. However, a review of the statistical aspects of clast counting suggests that this technique also can be applied quantitatively, and to investigate a variety of other objectives during basin analysis. Geographical and stratigraphical changes in the relative proportions of clasts can be quantified statistically and used to characterize sediment dispersal patterns in space and time, respectively. Statistical comparisons between clast assemblages can be used as a tool to match up rock units. This approach may help to constrain tectonic or suspect-terrane models, or to document sediment recycling. Both counting and sampling errors contribute to the total probable error of a clast count. Sampling error results from the uneven distribution of clasts in outcrop, perhaps caused by selective sorting. Counting error arises from a count of some number less than the total number of clasts in the population. Sampling and counting errors can be minimized by counting in closely spaced subsets, and by counting a total of at least 400 clasts, respectively. Thus, a useful procedure is to count four closely spaced subsets of 100 each, and combine the results for a total of 400. Point counting should not be used because differences in particle size produce biased results. A better method is to count all clasts above some minimum size within a specified area of outcrop. Analysis of upper Palaeogene non-marine conglomerates composing part of the Sespe Formation in California, using confidence intervals, hypothesis testing, analysis of variance, ratio analysis and varietal studies, demonstrates that useful statistics can be derived by counting clasts.     

Geometry and dynamics of braided channels and bars under experimental density currents

Submarine channels convey turbidity currents, the primary means for distributing sand and coarser sediments to the deep ocean. In some cases, submarine channels have been shown to braid, in a similar way to rivers. Yet the strength of the analogy between the subaerial and submarine braided channels is incompletely understood. Six experiments with subaqueous density currents and two experiments with subaerial rivers were conducted to quantify: (i) submarine channel kinematics; and (ii) the responses of channel and bar geometry to subaerial versus submarine basin conditions, inlet conditions and the ratio of ‘flow to sediment’ discharge (Q _w/Q _s). For a range of Q _w/Q _s values spanning a factor of 2·7, subaqueous braided channels consistently developed, were deeper upstream compared to downstream, and alternated with zones of sheet flow downstream. Topographic analyses included spatial statistics and mapping bars and channels using a reduced‐complexity flow model. The ratio of the estimated depth‐slope product for the submarine channels versus the subaerial channels was greater than unity, consistent with theoretical predictions, but with downstream variations ranging over a factor of 10. For the same inlet geometry and Q _w/Q _s, a subaqueous experiment produced deeper, steeper channels with fewer channel threads than its subaerial counterpart. For the subaqueous cases, neither slope, nor braiding index, nor bar aspect ratio varied consistently with Q _w/Q _s. For the subaqueous channels, the timescale for avulsion was double the time to migrate one channel width, and one‐third the time to aggrade one channel depth. The experiments inform a new stratigraphic model for submarine braided channels, wherein sand bodies are more laterally connected and less vertically persistent than those formed by submarine meandering channels.     

Sedimentary petrology and geochemistry of siliciclastic rocks from the upper Jurassic Tordillo Formation (Neuquén Basin, western Argentina): Implications for provenance and tectonic setting

The Upper Jurassic Tordillo Formation is exposed along the western edge of the Neuquén Basin (west central Argentina) and consists of fluvial strata deposited under arid/semiarid conditions. The pebble composition of conglomerates, mineralogical composition of sandstones and pelitic rocks, and major- and trace-element geochemistry of sandstones, mudstones, and primary pyroclastic deposits are evaluated to determine the provenance and tectonic setting of the sedimentary basin. Conglomerates and sandstones derived almost exclusively from volcanic sources. The stratigraphic sections to the south show a clast population of conglomerates dominated by silicic volcanic fragments and a predominance of feldspathic litharenites. This framework composition records erosion of Triassic–Jurassic synrift volcaniclastic rocks and basement rocks from the Huincul arch, which was exhumed as a result of Late Jurassic inversion. In the northwestern part of the study area, conglomerates show a large proportion of mafic and acidic volcanic rock fragments, and sandstones are characterised by a high content of mafic volcanic rock fragments and plagioclase. These data suggest that the source of the sandstones and conglomerates was primarily the Andean magmatic arc, located west of the Neuquén Basin. The clay mineral assemblage is interpreted as the result of a complex set of factors, including source rock, climate, transport, and diagenesis. Postdepositional processes produced significant variations in the original compositions, especially the fine-grained deposits. The Tordillo sediments are characterised by moderate SiO₂ contents, variable abundances of K₂O and Na₂O, and a relatively high proportion of ferromagnesian elements. The degree of chemical weathering in the source area, expressed as the chemical index of alteration, is low to moderate. The major element geochemistry and Th/Sc, K/Rb, Co/Th, La/Sc, and Cr/Th values point to a significant input of detrital volcanic material of calcalkaline felsic and intermediate composition. However, major element geochemistry is not useful for interpreting the tectonic setting. Discrimination plots based on immobile trace elements, such as Ti, Zr, La, Sc, and Th, show that most data lie in the active continental margin field. Geochemical information is not sufficiently sensitive to differentiate the two different source areas recognized by petrographic and modal analyses of conglomerates and sandstones.     

Debris flow deposits of early miocene age,deadman stream,Marlborough, New Zealand

Detailed analysis is presented of a conformable succession of Early Miocene conglomerates and sandstones lying between massive marine mudstones. The coarse sediments reflect deposition by a spectrum of subaqueous debris-flow mechanisms during an early pulse of tectonism that ultimately resulted in Plio-Pleistocene eversion of the Kaikoura Mountains.Sparse pebbly mudstones and rare sandy conglomerates show disoriented clasts and reflect high-viscosity flows and slurry-creep flow mechanisms. Other deposits have little mud matrix, hence appear to reflect low-viscosity flow processes. The largest clasts in these have a preferred planar orientation, probably reflecting dispersive grain pressure, and a preferred long-axis orientation parallel to flow direction. Common sorted sandstones and some conglomeratic sandstones show diffuse parallel lamination; with rare exceptions neither grading nor traction structures are present. Other conglomeratic sandstones show trough cross-bedding which we attribute to entrained bedload movement during intersurge episodes of debris flow.Microfossil data from the mudstones indicate sedimentation in an environment of outer neritic to upper bathyal aspect. Most detritus is abraded, suggesting derivation from terrestrial or inner neritic sources, but angular calcilutitic clasts and irregular sandstone and mudstone clasts and rafts were probably derived from submarine erosion between the emergent source area and the site of accumulation. Deposits generally appear to infill broad shallow channels. Paleocurrent and fabric analysis indicate a markedly uniform flow direction throughout succession, and suggest that the locus of channeling remained relatively fixed in space throughout accumulation of hundreds of metres of superimposed, commonly amalgamated debris-flow deposits. Although lateral control away from the measured sequence is limited, we infer that the locus of deposition lay shoreward of any submarine canyon or fan.     

CO2 sequestration and extreme Mg depletion in serpentinized peridotite clasts from the Devonian Solund basin, SW-Norway

The conglomerates of the Solund Devonian basin of SW-Norway contain numerous (locally up to 20 vol.%) peridotitic clasts with concentric mm- to 10-cm thick zones of varying red to black color. The peridotite clasts show a clear, alteration-related textural evolution. The least-altered rocks are partly serpentinized peridotites, showing a typical mesh texture with veins of serpentine, magnesite and Ni-rich magnetite surrounding olivine (Fo₉₁) relicts and its Mg-depleted, clay-like alteration product (deweylite assemblage). In the more advanced ophicarbonate stage, the mesh cells contain calcite, silica and are surrounded by talc. In the final stage, quartz, calcite, and hematite dominate the mineralogy and occur together with minor amounts of chromite, talc, Cr-chlorite, and Cr-hydroandradite. In tandem with this textural evolution is a decrease in MgO from 40 to 2.5 wt% and a CaO increase from 1 to 35 wt%. All peridotite clasts are characterized by high Cr and Ni concentrations. The chemistry and the textural evolution show that the clasts formed by an extreme Mg-mobilization from the peridotite, with development of secondary porosity and subsequent precipitation of calcite. MgO removed from the clasts after burial is in part consumed by replacement reactions in the sediment matrix around the clasts where Mg-free minerals (e.g., almandine) are replaced by Mg-bearing minerals (e.g., talc). Calculated apparent ⁸⁷Sr/⁸⁶Sr ratios of the clasts at 385 Ma (0.7124-0.7139), corresponding to the inferred age of sediment deposition and incipient clast alteration, indicate interaction with diagenetic basinal fluids. We explain the reaction history as a three stage process involving (a) partial serpentinization of olivine in an oceanic environment (b) breakdown of olivine relicts to the deweylite assemblage resulting in mobilization of MgO under (near-) surface conditions in a tropical Devonian climate and (c) further Mg-mobilization and replacement of the deweylite assemblage by calcite and quartz after diagenesis. Sedimentary basins with abundant weathered peridotite represent potential sites for a permanent CO₂ storage by formation of calcite in a low-temperature environment.     

Eocene fan delta-submarine fan deposition in the Wagwater Trough, east-central Jamaica

ABSTRACT The Wagwater Trough is a fault-bounded basin which cuts across east-central Jamaica. The basin formed during the late Palaeocene or early Eocene and the earliest sediments deposited in the trough were the Wagwater and Richmond formations of the Wagwater Group. These formations are composed of up to 7000 m of conglomerates, sandstones, and shales. Six facies have been recognized in the Wagwater Group: Facies I-unfossiliferous massive conglomerates; Facies II—channelized, non-marine conglomerates, sandstones, and shales; Facies III-interbedded, fossiliferous conglomerates and sandstones; Facies IV—fossiliferous muddy conglomerates; Facies V—channelized, marine conglomerates, sandstones, and shales; and Facies VI—thin-bedded sheet sandstones and shales. The Wagwater and Richmond formations are interpreted as fan delta-submarine fan deposits. Facies associations suggest that humid-region fan deltas prograded into the basin from the adjacent highlands and discharged very coarse sediments on to a steep submarine slope. At the coast waves reworked the braided-fluvial deposits of the subaerial fan delta into coarse sand and gravel beaches. Sediments deposited on the delta-front slope were frequently remobilized and moved downslope as slumps, debris flows, and turbidity currents. At the slope-basin break submarine fans were deposited. The submarine fans are characterized by coarse inner and mid-fan deposits which grade laterally into thin bedded turbidites of the outer fan and basin floor.     

The liquid limit of peat and its application to the understanding of Irish blanket bog failures

Catastrophic failures of blanket bogs, involving the escape and outflow of large volumes of semi-liquid basal peat, are well-known phenomena in Ireland but have only very rarely been reported from elsewhere in the world. Their precise causes and mechanisms are as yet unclear. The liquid limit (w _L) was identified as a potentially useful indicator of the susceptibility of peat to such failure because peat has extremely high natural water contents and, as an index property, w _L takes no account of the properties or structures of highly heterogeneous intact peat. However, the usual procedure for determining the w _L of peat is not fully standardised. Prepared samples will normally include potentially highly reactive particles of disrupted fibres and wood fragments that would not be present in such freshly disintegrated form in the field. This paper presents results from w _L determinations of peat obtained from the scar margins of three bog failures in northwest Ireland, using four different test procedures including a method involving wet-sieving of the peat to separate the humified <425-μm fraction for testing without incorporating artificially fragmented particles of fibres. The sampled peat was classified as H₈–H₁₀ according to the von Post humification scale. The fibre contents varied between the sites, but the ash contents were <3% in all but one test sample, and bulk densities (dry and field-wet) of the peat from all three sites were almost identical. w _L results from the wet-sieving method were 708–785%, compared with 633–980% from the standard method. The highest measured field water contents exceeded the wet-sieved w _L for all three of the field sites. Tests of cone penetration into intact peat cores demonstrated the influence of the reinforcing effect of in situ fibres. The results strongly suggest the need to adopt a fully standardised procedure for determining the w _L of peat. Additional shear vane measurements of intact and remoulded peat from a bog failure in Northern Ireland indicated a very high ‘strength sensitivity’. This leads to the suggestion that a slight disturbance of basal peat can lead to a loss of strength that rapidly propagates as local stresses change and cause further remoulding as water contents exceed w _L.