Subglacial Shear Research Articles

A Model for Striated Boulder Pavement Formation on Glaciated, Shallow-Marine Shelves: An Example from the Yakataga Formation, Alaska

ABSTRACT Laterally extensive, striated boulder pavements crop out within glaciomarine diamicts of the early Pleistocene Yakataga Formation on Middleton Island, Alaska. Boulder pavements consist of planes of one clast thickness, with faceted upper surfaces showing consistently oriented striation directions, cropping out over several kilometers along strike. Clasts do not have the preferred long-axis alignment nor the characteristic bullet shape of boulders transported at a glacier base and deposited by lodgement processes. In most cases, pavements identify the planar-bounding surfaces of thick (up to 100 m), tabular units of fossiliferous, muddy diamict deposited by the settling of fine-grained sands and muds from suspension, together with ice-rafted debris, in water depths between 20 and 250 m. Diamict facies both underlying and overlying the pavements contain abundant unabraded macro-and microfossils and show no evidence of deformation by subglacial shear processes. Pavements appear to have originated as boulder lag surfaces produced by wave and tidal-current winnowing of diamict on relatively shallow bank areas during episodes of relative sea-level lowering. Lags were subsequently overridden and abraded by a grounding ice shelf. The glacially abraded pavements are thought to record the repeated expansion of a partially floating ice shelf to the edge of the Gulf of Alaska continental shelf during the early Pleistocene. Recognition of boulder pavements formed in this manner may help establish the former presence of ice shelves whose depositional record remains enigmatic.

Depositional controls on the magnetic characteristics of lodgement tills and other glacial diamict facies

Genetic interpretation of massive, unstructured diamict and diamictite facies is a commonly encountered problem faced by sedimentologists. Data are presented concerning the magnetic characteristics, namely, anisotropy of magnetic susceptibility (AMS) and natural remanent magnetism (NRM) of facies deposited by (1) lodgement processes at a glacier base (lodgement till) and (2) subaqueously by pelagic mud deposition and ice-rafting ("rain-out" diamicts). Lodgement tills have an NRM that is distorted around the geomagnetic pole position to form a girdle approximately 90° in length, either transverse or parallel to ice flow direction. This distension appears to be the result of subglacial shear processes because other diamicts, deposited passively by melt-out below stagnant ice and modelled by a laboratory experiment, show a nondeformed NRM clustering around the geomagnetic pole. The AMS data show that lodgement tills have only a weakly orientated magnetic microfabric.Glaciolacustrine "rain-out" diamicts show a precise NRM clustering as in marine and lacustrine muds. This reflects the lack of nongeomagnetic forces acting upon magnetic grains during deposition followed by postdepositional remanence "locking" at depth in the sediment column. These facies show both random AMS fabrics, typical of undisturbed pelagic sediments, and preferred microfabrics resulting from local sediment flow on the lake floor.It is concluded that NRM and AMS offer considerable assistance in genetic studies of massive diamict facies; AMS is particularly useful because the large populations of samples can be rapidly processed. The wider use of this technique by sedimentologists—for investigating other sedimentary facies types—is anticipated.

Characterization of pebble fabrics in modern terrestrial glacigenic sediments

ABSTRACT Pebble fabric data are available from several facies of glacigenic sediments deposited by modern glaciers, where sedimentary processes can be observed or inferred with relatively little ambiguity. Over 100 samples from contemporary environments illustrate fabrics characterizing melt‐out till, deformed and undeformed lodgement till, sediment flow deposits and ice slope colluvium. Lodgement till fabric variability is related to the two‐layer structure of these sediments; a structureless, friable upper layer with low shear strength and high consolidation coefficient, overlying a very compact material of horizontal platy structure. Fabric strength (assessed by eigenvalue analysis) is weaker and pebble dip is more dispersed in the upper structureless horizon. Stronger fabrics in the lower platy horizon may be primary depositional fabrics which are destroyed by subglacial shearing to give weaker fabrics in the upper horizon. Alternatively, upper horizon fabrics may be characteristic of all recently‐deposited lodgement tills, with stronger fabrics developing at depth by dewatering and consolidation. There is a general reduction in fabric strength and an increase in particle dip associated with the transition from melt‐out tills, through undeformed and deformed lodgement tills, to sediment flow deposits and ice slope colluvium. There is, however, considerable overlap in the fabric strengths characteristic of sediment flow deposits and deformed lodgement tills. Fabric data from modern glacial sedimentary facies are used to assist in interpreting the mode of deposition of some Quaternary glacial sediments. Relatively strong fabrics characteristic of melt‐out tills and undeformed lodgement tills are more likely to be diagnostic of genesis than weaker fabrics associated with deformed sediments.

Annual Moraine Ridges at Skàlafellsjökull, South-East Iceland

AbstractMoraine ridges have formed annually at Skálafellsjökull, south-east Iceland since about A.D. 1912. These ridges are asymmetrical with a steeper distal slope, and their surface morphology reflects their internal structure. Most ridges are formed at the glacier margin, and they form a series of concentric arcs about it. Their plan form reflects small details of the ice-front morphology. Some ridges are composed of boulders released from the ice by ablation and swept together by glacier re-advances, but most have a core of deformed till. Those which were ice-cored have a surface veneer of resedimented debris consisting of ice-slope colluvium and sediment-flow deposits. Lodgement tills at Skálafellsjökull have a two-layer structure, with a weak upper horizon apparently formed by dilation during subglacial shearing of the till. Ridges are composed of this sheared material and overlie undisturbed lodgement till at depth. Subglacial shearing results in a discharge of till towards the glacier margin and its accumulation there when glacier retreat ceases in winter. Till may also be extruded from beneath ice at the margin as a result of loading by the ice, and subsequently bulldozed into a ridge by a winter readvance of the glacier.

Annual Moraine Ridges at Skàlafellsjökull, South-East Iceland

AbstractMoraine ridges have formed annually at Skálafellsjökull, south-east Iceland since about A.D. 1912. These ridges are asymmetrical with a steeper distal slope, and their surface morphology reflects their internal structure. Most ridges are formed at the glacier margin, and they form a series of concentric arcs about it. Their plan form reflects small details of the ice-front morphology. Some ridges are composed of boulders released from the ice by ablation and swept together by glacier re-advances, but most have a core of deformed till. Those which were ice-cored have a surface veneer of resedimented debris consisting of ice-slope colluvium and sediment-flow deposits. Lodgement tills at Skálafellsjökull have a two-layer structure, with a weak upper horizon apparently formed by dilation during subglacial shearing of the till. Ridges are composed of this sheared material and overlie undisturbed lodgement till at depth. Subglacial shearing results in a discharge of till towards the glacier margin and its accumulation there when glacier retreat ceases in winter. Till may also be extruded from beneath ice at the margin as a result of loading by the ice, and subsequently bulldozed into a ridge by a winter readvance of the glacier.

The Origin and Microfabric of Lake Superior Red Clay

ABSTRACT Two red-clay units, the Hanson Creek and Douglas members of the Miller Creek Formation, occur in Wisconsin along the bluffs of Lake Superior. The red clay is nonlaminated, consists of 60 to 90 percent clay, contains few pebbles, and occurs dominantly basinward of the shorelines of ancient glacial lakes. These characteristics have led previous workers to interpret the red clay as either lake sediment or subglacial till. Glacial striations, flutes, macrofabric, an areal distribution characteristic of till, grain-size trends, shear planes, and outcrop characteristics indicate that the red clay is subglacial till and not lake sediment. Microfabric analysis of the red clay reveals a preferred orientation of elongate sand grains. This preferred orientation is related to ice-flow direction but in a way different from that shown by most analyses of till microfabric. Nearly half of the 25 samples analyzed have the major mode of sand-grain trends oriented transverse to regional ice-flow direction as indicated by flutes, striations, and macrofabric. In the remaining samples, the sand-grain trends are parallel to ice flow. The development of a transverse or parallel orientation may be influenced by clast content and subglacial shearing. Linear streaks of silt and clay, referred to as microfoliation, occur in the matrix of the red clay. The microfoliation bands are planes that strike transverse to ice flow and dip in the up-ice direction, The structure of the microfoliation and its orientation relative to ice flow suggest that they may be shear planes caused by overriding ice. The microfoliation shows a more consistent relationship to ice flow than the sand-grain trends.

Model for glaciotectonism

Dislocated, thrusted, and folded masses of soft bedrock and drift have been described throughout the glaciated portions of central North America and northern Europe, and the origin of these surficial deformations has mainly been settled in favor of glacioteconic causes. The model for g]aciotectonic deformation described here consists of two stages: (1) initial stage of pro glacial or ice-marginal thrusting and stacking of large floes during glacier advance over permafrozen terrain, and (2) subsequent stage of subglacial shearing and penetrative deformation of previously-thrusted floes, when ice overrides the area. Ice thrusting takes place along a basal permafrost decollement, where high pore-water pressure is de­veloped, and floes are pushed as permafrozen slabs forward and upward in front and beneath the toe of the advancing glacier to form a topographic ridge (Stauchmoranen or kuppelbakke ). Once the ice overrides the ridge of thrusted floes, subglacial deformation by basal shearing may create glaciodynamic melange. Most glaciotectonic disturbances are thought to develop in a largely frozen state, and the possibility of large-scale subglacial thrusting beneath thick ice or far back from the ice margin is generally believed negligible. The majority of severe glaciotectonic deformations in central North America and northern Europe are found in transitional zones, which separate the central glaciated zones characterized by deep glacial erosion from marginal zones, where glacial deposition was the rule. Glacial thrusting provides a mechanism for stripping sedimentarycover off the Canadian and Fennoscandian shields during repeated Pleistocene glaciations.

Till fabric reorientated by subglacial shear

Abstract Till fabric in a dislocated till from a coastal cliff in western Sjaelland, Denmark, is decribed. Based on analyses of different parts of a recumbent syncline, possible causes for the formation of the dislocations and the internal till fabric are discussed. It is concluded that the present till fabric is developed from an earlier fabric, which was reorientated subglacially due to inhomogeneous shear in connection with the transgression of a later glacier.

Subglacial Shearing and Crushing, and the Role of Water Pressures in Tills from South-East Iceland

Till exposed at the retreating margin of Breidamerkurjökull has a two-layer structure; an upper horizon of high void ratio and with a high concentration of fines, and an underlying, denser, less fine-grained horizon with a platy structure. It is suggested that the upper horizon has undergone continuous subglacial shearing in which dilation and interparticle crushing have occurred, the latter having produced the high concentration of fines, and that much grain crushing generally attributed to glacial transport may well be post-depositional in origin. The lower horizon is thought to have lain below the level at which shear stresses imposed by the overriding glacier were attenuated to values below the strength of the till, and thus to have undergone compaction only.Under normal circumstances, the frictional strength of the till will rise under increasing ice loads whereas the basal shear stress will remain constant, so that till deformation will be largely a marginal phenomenon. However the effective pressure in the till is reduced by the existence of a subglacial water pressure.Equations are developed which show how the effective pressure in subglacial beds depends not only on the shape and size of the glacier but also on the compressibility of the beds and their hydraulic transmissibility at zero effective pressure. A low hydraulic transmissibility gives a low effective pressure, which is conducive to deformation of subglacial sediments.Implications for glacier surging, drumlin formation and till fabric orientation are discussed.

The Nature and Rates of Post-Depositional Changes in Recently Deposited Till from South-East Iceland

Investigations of the pattern of variation in texture and structure of an extensive lodgement till currently being exposed by glacier retreat reveal the roles of three groups of processes. These are depositional processes which influence the sorting of the till, post-depositional processes of sub-glacial shearing and crushing which increase the silt concentration immediately below the glacier sole and reduce the density of the till by dilation due to shear, and soil-forming processes operating beyond the glacier margin.Once the till is exposed the structure of the dilatant layer is destroyed by compaction due to drainage, percolating water carries fines down through the profile and wind action produces stone pavements by deflation near to the glacier and loess accumulation further away. The surprising speed at which these processes operate and the extent to which the sedi-mentological character of the till is changed after glacier retreat have important implications for both pedologists and glacial geologists.