The Blackspring Ridge Flute Field, south-central Alberta, Canada: : evidence for subglacial sheetflow erosion

The Blackspring Ridge (BSR), located in south-central Alberta, Canada, is dominated by a prominent flute field. Flutes (elongated streamlined depressions) and ridges (elongate streamlined hills) are up to 15 km long and are composed of two material types: in situ bedrock, and in situ pre-Laurentide glaciation fluvial sand and gravel beds. The preglacial beds are Tertiary or early Quaternary in age. The beds are undisturbed, maintain primary bedding structures, and even maintain clast imbrication. No till overlies the gravel beds, although in places large granite boulder erratics lie on the surface, indicating that ice was present in the region in the past. Because the ridges are composed of preglacial materials, they are remnant erosional landforms rather than constructional landforms. Geomorphic and sedimentary evidence favor subglacial meltwater as the erosional agent, rather than ice. We suggest that the elevation of the BSR relative to basal ice would have resulted in confined subglacial meltwater flow, with associated flow acceleration and increased scouring resulting in flute formation. This meltwater stripped away any till cover, leaving behind only a few boulders. Observations at the BSR flute field preclude the possibility that flutes and remnant ridges are the result of deformation of soft clayey beds.     

Utilizing glacial geology in uranium exploration; Dismal Lakes, Northwest Territories, Canada

A field of uraniferous boulders was discovered in a drift-covered valley west of Dismal Lakes. Glacial geological information was combined with boulder location and trace element till geochemical data to model the dispersal of the boulders; and to predict their likely bedrock source. Uraniferous bedrock was eroded by the last, westward flowing glacial ice to cover the area. The debris was englacially transported and subsequently deposited during subglacial melt-out of ice block(s) stagnating below active ice. The distribution of the boulders forms acrude, westward-opening fan centred on the easternmost boulder and oriented with the last ice-flow direction. The largest uranium values from surface till samples (-2 μm fraction) occur 6.2 km east of the main boulder concentration or 1.5 km east of the first boulder occurrence. The likely bedrock source is 6.0 to 6.6 km east of the main boulder concentration.     

Boulder shapes and grain-size distributions of debris as indicators of transport paths through a glacier and till genesis

Debris transported by glacier is derived either supraglacially from nunataks and valley sides or from erosion of the subglacial bed. Debris produced above the glacier by fracturing of rock walls has a dominant coarse fraction with angular boulders. Subsequent englacial or supraglacial transport is relatively passive and little comminution occurs. Debris eroded from subglacial bedrock is initially transported in a basal zone of traction, where particles frequently come into contact with the glacier bed and are retarded by it so that large forces may be generated between particles and the bed and at interparticle contacts. The material introduced into this tractional zone may be subglacial bedrock which has undergone a crushing-plucking event and which has a dominant coarse fraction, or supraglacially derived material which finds its way to the glacier bed. These parent debris assemblages are further comminuted by failure in response to locally concentrated compressive stresses, and attrition at shearing interfaces. Boulders transported through the tractional zone will tend to be rounded and bear several directions of striation. Large boulders embedded in lodgement till will tend to be streamlined with striae parallel to glacier flow and with an abruptly truncated distal extremity, rather like a roche moutonnée. Textural and boulder shape characteristics can be used to help distinguish different types of till.     

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.     

Macro‐ and micro‐sedimentology of a modern melt‐out till – Matanuska Glacier,Alaska, USA

The macro‐ and micro‐sedimentology of a supraglacial melt‐out till forming at the Matanuska Glacier was examined in relationship to the properties of the stratified basal zone ice and debris from which it is originating. In situ melting of the basal ice has produced a laminated to bedded diamicton consisting mainly of silt. Macroscopic properties include: discontinuous laminae and beds; lenses of sand, silt aggregates and open‐work gravel; deformed and elongate clasts of clay; widely dispersed pebbles and cobbles, those that are prolate usually with their long axes subparallel to parallel to the bedding. Evidence for deformation is absent except for localized bending of beds over or under rock clasts. Microscopic properties are a unique element of this work and include: discontinuous lineations; silt to granule size laminae; prolate coarse sand and rock fragments commonly with their long axis subparallel to bedding; subangular to subrounded irregular shaped clay clasts often appearing as bands; sorted and unsorted silt to granule size horizons, sometimes disrupted by pore‐water pathways. Limited deformation occurs around rock clasts and thicker parts of lamina. This study shows that in situ melting of debris‐rich basal ice can produce a laminated and bedded diamicton that inherits and thereby preserves stratified basal ice properties. Production and preservation of supraglacial melt‐out till require in situ melting of a stagnant, debris‐rich basal ice source with a low relief surface that becomes buried by a thick, stable, insulating cover of ice‐marginal sediment. Also required are a slow melt rate and adequate drainage to minimize pore‐water pressures in the till and overlying sediment cover to maintain stability and uninterrupted deposition. Many modern and ancient hummocky moraines down glacier of subglacial overdeepenings probably meet these process criteria and their common occurrence suggests that both modern and pre‐modern supraglacial melt‐out tills may be more common than previously thought.     

Inland fields of dispersed cobbles and boulders as evidence for a tsunami on Anegada, British Virgin Islands

Marine overwash from the north a few centuries ago transported hundreds of angular cobbles and boulders tens to hundreds of meters southward from limestone outcrops in the interior of Anegada, 140?km east?Cnortheast of Puerto Rico. We examined two of several cobble and boulder fields as part of an effort to interpret whether the overwash resulted from a tsunami or a storm in a location where both events are known to occur. One of the cobble and boulder field extends 200?m southward from limestone outcrops that are 300?m inland from the island??s north shore. The other field extends 100?m southward from a limestone knoll located 800?m from the nearest shore. In the two fields, we measured the size, orientation, and spatial distribution of a total of 161 clasts and determined their stratigraphic positions with respect to an overwash sand and shell sheet deposit. In both fields, we found the spacing between clasts increased southward and that clast long-axis orientations are consistent with a transport trending north?Csouth. Almost half the clasts are partially buried in a landward thinning and fining overwash sand and none were found embedded in the shelly mud of a pre-overwash marine pond. The two cobble and boulder fields resemble modern tsunami deposits in which dispersed clasts extend inland as a single layer. The fields contrast with coarse clast storm deposits that often form wedge-shaped shore-parallel ridges. These comparisons suggest that the overwash resulted from a tsunami and not from a storm.     

Microtextural analysis of a glacially ‘deformed’ bedrock: implications for inheritance of preferred clast orientations in diamictons

Although analysis of clast macrofabrics has been used to differentiate between different types of glacial diamictons and to determine palaeo‐ice flow directions, no account appears to have been made of preferred clast orientations inherited from the parental source material. Clast macrofabrics in tills are typically interpreted as having developed in response to an imposed subglacial deformation and as such provide a link between the sedimentary record and glacier dynamics. They rely on the assumption that any preferred clast orientation is a result of deformation/flow. The results of the micromorphological study of the Langholm Till exposed at North Corbelly near Dumfries (southwestern Scotland) clearly demonstrate that bedrock structure can influence clast orientation (macrofabric) within diamictons. In the lower part of the till, the orientation of elongate clasts preserves the geometry of the tectonic cleavage present within the underlying bedrock. The intensity of this steeply inclined, ‘inherited’ clast fabric decreases upward through the till, to be replaced by a more complex pattern of successive generations of clast microfabrics developed in response to deformation/flow. These results indicate potential limitations of applying clast macrofabric or microfabric analysis in isolation to establish till genesis or palaeo ice‐flow directions. Consequently, due account should be made of other glacial palaeo‐environmental and ice flow indicators, as well as rockhead depth and morphology in relation to the selection of fabric measurements sites. © British Geological Survey/Natural Environment Research Council copyright 2007. Reproduced with the permission of BGS/NERC. Published by John Wiley & Sons, Ltd.     

A shear-diffusion model of till genesis based on the dispersal pattern of indicator rocks in the Grand-Volume Till of central Gaspésie, Québec, Canada

Dispersal patterns of indicator rocks in central Gaspésie reveal that glacial debris is entrained in a basal debris-rich zone of shearing where clast diffusion takes place. The Grand-Volume Till forms a thin till sheet over the high plateaus of Gaspésie Peninsula and resulted from a succession of two Wisconsinan ice flows of distinct orientations (SSE and NE). The lithological composition of this till determined by pebble counts and the three-dimensional dispersal patterns of indicator rocks in it suggest that debris transport occurred principally by simple shear deformation of glacial debris. In addition, the intermixing of clasts at the intersection of two lithologically distinct dispersal trains of SSE and NE orientations, respectively, suggests that extensive mixing takes place during shearing. Physical interactions among the clasts lead to both upward and downward movements which cause the clasts to diffuse across the zone of shearing. This process of shear-diffusion results in continuous incorporation and mixing of the newly encountered rock types during glacial transport.     

Neoglacial (<3000 years) till and flutes at Saskatchewan Glacier,Canadian Rocky Mountains,formed by subglacial deformation of a soft bed

Understanding the processes that deposit till below modern glaciers provides fundamental information for interpreting ancient subglacial deposits. A process‐deposit‐landform model is developed for the till bed of Saskatchewan Glacier in the Canadian Rocky Mountains. The glacier is predominantly hard bedded in its upper reaches and flows through a deep valley carved into resistant Palaeozoic carbonates but the ice margin rests on a thick (<6 m) soft bed of silt‐rich deformation till that has been exposed as the glacier retreats from its Little Ice Age limit reached in 1854. In situ tree stumps rooted in a palaeosol under the till are dated between ca 2900 and 2700 yr bp and record initial glacier expansion during the Neoglacial. Sedimentological and stratigraphic observations underscore the importance of subglacial deformation of glaciofluvial outwash deposited in front of the advancing glacier and mixing with glaciolacustrine carbonate‐rich silt to form a soft bed. The exposed till plain has a rolling drumlinoid topography inherited from overridden end moraines and is corrugated by more than 400 longitudinal flute ridges which record deformation of the soft bed and fall into three genetically related types: those developed in propagating incipient cavities in the lee of large subglacial boulders embedded in deformation till, and those lacking any originating boulder and formed by pressing of wet till up into radial crevasses under stagnant ice. A third type consists of U‐shaped flutes akin to barchan dunes; these wrap around large boulders at the downglacier ends of longitudinal scours formed by the bulldozing of boulders by the ice front during brief winter readvances across soft till. Pervasive subglacial deformation during glacier expansion was probably facilitated by large boulders rotating within the soft bed (‘glacioturbation’).     

Sedimentologic evidence for outburst floods from the Laurentide Ice Sheet margin in Wisconsin, USA: implications for tunnel-channel formation

Clast-supported boulder gravel in outwash-fans along the glacial-maximum margin of the Laurentide Ice Sheet in Wisconsin indicates the occurrence of outburst floods. These sediments, with clast intermediate axes of up to 2 m, are located downstream of tunnel channels and were deposited shortly before cessation of glaciofluvial activity on each fan. Since tunnel channels with fans are widespread along the ice-sheet margin in the western Great Lakes region, these outburst floods were probably common. Paleodischarge estimates derived from the boulder deposits are poorly constrained, but values of at least several hundred m³ s⁻¹ are likely. Four potential water sources for the floods exist: an extreme surface-melt event, an extreme precipitation event, drainage of supraglacial lakes, or drainage of stored subglacial meltwater. We focus on the storage of subglacial meltwater behind the ice-sheet margin, as proglacial permafrost was present as ice advanced to its maximum extent, and a frozen-bed zone upstream from the margin probably impeded drainage through groundwater aquifers. Decay of this permafrost ‘seal’ would have eventually allowed trapped water to drain through the tunnel channels. We suggest that the 2-m boulders were entrained in an outburst of subglacial water that enlarged a pre-existing channel cut by ablation-derived flows.     

Structures and textures in till indicating subglacial deposition

Five structural and textural features are discussed: (1) small lenses of sorted material, (2) smudges, (3) small-scale deformations of till matrix and smudges by clasts, (4) clasts consistently striated, and (5) clasts with stoss-and-lee sides. Analyses suggest that these features may be produced by subglacial processes acting in the ice-bed interface. Long axes of small sand lenses and smudges as well as the striation on the upper surface of scattered clasts in lodgement till have a strong preferred orientation in good agreement with the glacier flow direction as indicated by clast fabrics, bedrock striation, and surface fluting of ground moraine. When in traction against the till bed, clasts may plough up till banks. Clasts with stoss-and-lee sides development were also very distinctly oriented as their stoss sides faced significantly up-glacier.
It is concluded (1) that each of the five features discussed is useful as a criterion for subglacial deposition by lodgement, (2) that they indicate important differential movement along the ice-bed interface and therefore suggest a temperate regime in this part of the glacier during the till deposition, (3) that very few orientation measurements of one or more of these features signify the ice movement direction; i.e. a time-saving method to find the paleoflow direction of Pleistocene glaciers, and (4) that taken together with till preconsolidation, mechanical composition, and clast fabric, they may support each other and give good indications of the genesis of Pleistocene tills.     

A boulder beach formed by waves from a calving glacier; Eqip Sermia, West Greenland

A large-scale boulder beach close to a tidal glacier was examined at Eqip Sermia, Disko Bugt, West Greenland, in 1989. Photographs from 1912 and 1929 show an advance of the glacier of more than 1.5 km beyond its present location. Lateral and terminal moraines were formed in the sea, and subaerial parts and their positions can be detected from the old photographs. Today the outermost part of this moraine system has disappeared totally, except for about 1 km of the lateral moraine. The distal 300 m of the still existing moraine apparently has been displaced and transformed into a shape that, in plan view and cross-section, resembles a barrier spit. The material of the boulder beach consists mainly of coarse clasts with boulders of 1 m to more than 1.5 m in diameter. Distributions of clast sizes and sediment structures on the barrier surface also suggest wave and overwash dynamics as being the responsible agents. Located in the inner part of a fiord system, the fetches are restricted and thus normal waves are very small. Large waves generated by glacier calving, and/or sea-ice action, are therefore the only processes that can explain the geomorphology and clast distribution of this coastal feature.     

Transformation,partitioning and flow–deposit interactions during the run‐out of megaflows

Four megabeds (I to IV) were recognized throughout the Cerro Bola inlier, a glacially influenced depositional area of the Carboniferous Paganzo Basin, south‐western La Rioja Province, Argentina. Such anomalous thick beds are associated with the collapse of an unstable basin margin after periods of large meltwater discharge and sediment accumulation. Failure of these previously deposited sediments triggered mass flows and associated turbidity currents into the basin. Megabed I is up to 188 m thick and was deposited during a transgressive stage by re‐sedimentation of ice‐rafted debris. Also part of the transgressive stage, Megabeds II, III and IV are up to 9 m thick and are associated with a dropstone‐free period of flooding. Megabeds were subdivided into three divisions (1 to 3) that represent a spectrum of flow properties and rheologies, indicative of a wide range of grain support mechanisms. These divisions are proposed as an idealized deposit that may or may not be completely present; the Cerro Bola megabeds thus display bipartite or tripartite organization, each division inferred to reflect a rheologically distinct phase of flow. Division 1 is a basal layer that consists of clast‐supported and matrix‐supported, pebble conglomerate, rarely followed by weak normally graded to ungraded, very coarse‐ to coarse‐grained sandstone. This lower interval is interpreted to be the deposit of a concentrated density flow and is absent in bipartite megabeds. Division 2 is represented by a mud‐rich sandstone matrix with dispersed granule to pebble‐size crystalline and mudstone clasts. It also includes fragments of sandstone up to boulder size, as well as rafts of cohesive muddy material and wood fragments. Division 2 is interpreted to be a result of debris‐flow deposition. A debrite‐related topography, resulting from the freezing of high yield strength material, captures and partially confines the succeeding upper division 3, which fills the topographic lows and pinches out against topographic highs. Division 3 is rich in mudstone chips and consists of very coarse‐grained, dirty sandstones grading upward to siltstones and mudstones. It is interpreted to be a deposit of a co‐genetic turbidity current. Spectral gamma ray and petrographic analyses indicate that both debrite and co‐genetic turbidite have high depositional mud content and are of similar composition. One of the megabeds is correlated with an initial slump‐derived debris flow, which suggests that the mass flow becomes partitioned both at the top, generating a co‐genetic turbidity current and, at the base, segregating into a concentrated density flow that seems to behave as a gravelly traction carpet.     

Mass flow sedimentology within the HYC Zn–Pb–Ag deposit,Northern Territory,Australia: evidence for syn-sedimentary ore genesis

Laterally continuous mass-flow deposits are an important feature of the HYC stratiform sediment-hosted Zn–Pb–Ag deposit, which reveal more about the HYC mineralising system than has been previously recognised. Mass flow deposits are interbedded with sheet-like mineralised lenses in a carbonaceous dolomitic siltstone host rock. Sedimentological processes of mass-flow deposit emplacement are proposed that constrain stratiform mineralisation to the top metre of the sediment pile, based on mass-flow geometry and detailed clast petrology. Four distinct sedimentary facies are identified within the mass-flow units: framework-supported polymictic boulder breccia; matrix-supported pebble breccia; and gravel-rich and sand-rich graded turbidite beds. The boulder breccias are weakly reverse graded and show rapid lateral transition into the other facies, all of which are distal manifestations of the same sedimentary events. The flow geometry and relationships between these facies are interpreted to reflect mass-flow initiation as clast-rich debris flows, with transformation via the elutriation of fines into a subsequent turbulent flow from which the turbidite and matrix-supported breccia facies were deposited. All the mass-flow facies contain clasts of the common and minor components of the in-situ laminated base-metal mineralised siltstone. Texturally these are identical to their in-situ counterparts, and are clearly distinct from other sulphidic clasts that are of unequivocal replacement origin. In the boulder breccias, intraclasts may be the dominant clast type and the matrix may contain abundant fine-grained sphalerite and pyrite. Dark coloured sphaleritic and pyritic breccia matrices are distinct from pale carbonate-siliclastic matrices, are associated with high abundance of sulphidic clasts, and systematically occupy the lower part of breccia units. Consequently, clasts that resemble in-situ ore facies are confirmed as genuine intraclasts that were incorporated into erosive mass flows prior to complete consolidation. Disaggregation and assimilation of sulphidic sediment in the flow contributed to the sulphide component of the dark breccia matrices. The presence of laminated sulphidic intraclasts in the mass-flow facies constrains mineralisation at HYC to the uppermost part of the seafloor sediment pile, where this material was susceptible to erosion by incoming clast-rich mass flows.Editorial handling: N. White     

SEDIMENTATION IN THE RIFT ZONE OF THE JUAN DE FUCA RIDGE

The sediments of the upper Swartkops River are almost exclusively gravels and boulder beds derived from the Cretaceous Uitenhage Group and the Paleozoic Cape Supergroup rocks. Many of the cobbles and boulders are second-cycle clasts, the great majority of which are quartzitic in composition. Pebble size and shape were examined and fabric analysis was performed on samples from 22 sites in the study area. Pebble imbrication planes dip consistently upstream at angles of 20? to 50? and pebble long axes generally are aligned normal to the flow direction. Clasts in the braid-plain deposits range from a few millimeters to tens of centimeters (large boulders over a meter in diameter are not uncommon). Pebble roundness ranges from 0.2 to 0.9 (averaging 0.43) and sphericity values range from 0.3 to 0.9 (averaging 0.59). The gravel clasts are angular to well-rounded, but are predominantly subrounded. Zingg diagram plots show a majority of discoidal pebbles, but there is a diversity of shapes reflecting the complex source area from which some resedimented clasts originated.

Channel and bar morphology is complex, with gravel bars often merging laterally and longitudinally with main and secondary channels. Both channels and bars are terraced stepwise downstream and across the braid plain. Bar tops are armored by both small and large clasts, whereas channels may be lined with cobbles or boulders, but often exhibit small pebble lags. Algal mats occur as fresh curtains in all standing pools of water and dried crusty deposits on pebbly substrates in inactive channels.

Imbrication studies demonstrate conclusively that pebble imbrication is the most meaningful indicator of flow direction in a gravel deposit and is far more reliable than rare cross-bedding encountered in bar-top sands, where bedforms often migrate laterally rather than downstream. The Swartkops braid-plain gravels resemble the ancient deposits of the Ventersdorp Contact Reef, both deposits being characterized by boulder-rich gravels, poor clast sorting, resedimented pebbles from a proximal fault-bounded source, and algal mats. Although heavy minerals are lacking in the Swartkops, trapping of fines by algal filaments appears to occur during low-flow conditions.     

Hirnantian glacial and deglacial record in SW Djado Basin (NE Niger).

Pluridisciplinary fieldwork highlights features generated by an extended ice-sheet in the Djado Basin during the Hirnantian. Two glacial palaeovalley systems associated with glacial pavements and separated by thin glaciomarine interstadial series are revealed. Rigid glacial pavements characterised by abrasion erosion are differentiated from soft glacial pavements characterised by soft-bed deformation. Glacial pavements are associated with subglacial bedforms such as megaflutes, flutes and meltwater channels. They are also associated with clastic dykes and glaciotectonic structures such as deformed flutes, subglacial folds and duplex structures. This record demonstrates that ice was warm-based and flowed rapidly on the highfluid- pressure soft substrate, as for ice streams. The erosional glacial landscape is typical of areal scouring, and the depositional sediment-landform assemblage corresponds to subglacial processes. These data afford a reconstruction of glacial events which is consistent with the two polyphased low-frequency glacial cycles inferred in previous studies. During interstadial and postglacial stages, grabens, normal faults, radial extensional microfaults and extensional dihedrons were generated by extensional tectonics during glacio-isostatic rebound. In sectors highly affected by this tectonics, doleritic dykes reflect a basal crust fusion increase induced by adiabatic decompression.     

Glacitectonic deformation in the Chuos Formation of northern Namibia: implications for Neoproterozoic ice dynamics

The Chuos Formation is a diamictite-dominated succession of Cryogenian age, variously interpreted as the product of glaciomarine deposition, glacially related mass movement, or rift-related sediment remobilisation in a non-glacial environment. These interpretations have wide ranging implications for the extent of ice cover during the supposedly pan-global Neoproterozoic icehouse. In the Otavi Mountainland, northern Namibia, detailed analysis of soft-sediment deformation structures on the macro- and micro-scale support glacitectonic derivation in response to overriding ice from the south/south-east. Overall, the upward increase in strain intensity, predominance of ductile deformation features (e.g. asymmetric folds, rotational turbates and necking structures, clast boudinage, unistrial plasmic fabrics) and pervasive glacitectonic lamination support subglacial deformation under high and sustained porewater pressures. In contrast, soft-sediment structures indicative of mass movements, including flow noses, tile structures, and basal shear zones, are not present. The close association of subglacial deformation, abundant ice-rafted debris and ice-contact fan deposits indicate subaqueous deposition in an ice-proximal setting, subject to secondary subglacial deformation during oscillation of the ice margin. These structures thus reveal evidence of dynamic grounded ice sheets in the Neoproterozoic, demonstrating their key palaeoclimatic significance within ancient sedimentary successions.     

Inverse modeling of velocities and inferred cause of overwash that emplaced inland fields of boulders at Anegada, British Virgin Islands

A combination of numeric hydrodynamic models, a large-clast inverse sediment-transport model, and extensive field measurements were used to discriminate between a tsunami and a storm striking Anegada, BVI a few centuries ago. In total, 161 cobbles and boulders were measured ranging from 1.5 to 830?kg at distances of up to 1?km from the shoreline and 2?km from the crest of a fringing coral reef. Transported clasts are composed of low porosity limestone and were derived from outcrops in the low lying interior of Anegada. Estimates of the near-bed flow velocities required to transport the observed boulders were calculated using a simple sediment-transport model, which accounts for fluid drag, inertia, buoyancy, and lift forces on boulders and includes both sliding and overturning transport mechanisms. Estimated near-bed flow velocities are converted to depth-averaged velocities using a linear eddy viscosity model and compared with water level and depth-averaged velocity time series from high-resolution coastal inundation models. Coastal inundation models simulate overwash by the storm surge and waves of a category 5 hurricane and tsunamis from a Lisbon earthquake of M 9.0 and two hypothetical earthquakes along the North America Caribbean Plate boundary. A modeled category 5 hurricane and three simulated tsunamis were all capable of inundating the boulder fields and transporting a portion of the observed clasts, but only an earthquake of M 8.0 on a normal fault of the outer rise along the Puerto Rico Trench was found to be capable of transporting the largest clasts at their current locations. Model results show that while both storm waves and tsunamis are capable of generating velocities and temporal acceleration necessary to transport large boulders near the reef crest, attenuation of wave energy due to wave breaking and bottom friction limits the capacity of storm waves to transport large clast at great inland distances. Through sensitivity analysis, we show that even when using coefficients in the sediment-transport model which yield the lowest estimated minimum velocities for boulder transport, storm waves from a category 5 hurricane are not capable of transporting the largest boulders in the interior of Anegada. Because of the uncertainties in the modeling approach, extensive sensitivity analyses are included and limitations are discussed.     

Slump blocks, intraformational conglomerates and associated erosional structures in Pennsylvanian fluvial strata of eastern Canada

Pennsylvanian fluvial channel sandstones in New Brunswick and Nova Scotia contain numerous examples of eroded mudstone surfaces, including in situ mudstone beds, boulders and slumped blocks. The eroded surfaces bear a variety of structures including linear scours, flutes, longitudinal furrows and rill marks. A block of interchannel mudstone up to 40 m in extent, displays a basal slip-plane, slump-related deformation and evidence of intense corrasion on a channel floor. Mudstone clasts from small pebbles to boulders over 4 m long are common immediately above channel-base erosion surfaces and represent a lag. Clasts over 20 cm diameter are commonly fluted, occasionally on all sides, suggesting clast rotation. Rill marks occur on large mudclasts and in situ mudstone surfaces and indicate emergence and erosion by surging water or surface runoff. Preservation of the delicate erosional structures depended on a highly cohesive mud substrate and subsequent rapid burial. A previous interpretation of the mud blocks and their surficial features as the result of mud intrusion is inconsistent with the field evidence.