Ice Contact Landforms and Sediments

Three types of well-preserved ridges can be recognized at the ice edge in East Antarctic oases: inner moraines, ice-contact fans/screes and thrust-block moraines.

5.4.1 Inner Moraines

Inner moraines form at the margin of the ice sheet where basal debris crops out and accumulates on the ice surface. Beyond the present ice margins older inner moraines often form prominent end moraines. Where an ice core remains in these moraines exposures reveal large recumbent folds with an amplitude of up to 6 m and numerous smaller sheared folds providing evidence of intense compressive deformation within the basal debris zone close to the ice margin (Fitzsimons, 1990, 1997a).

Exposures of the inner moraines reveal massive, matrix-supported diamictons with rare layers of poorly sorted, sandy gravel. Pebble-fabric strengths of the diamictons measured from clast a-axes range from 0.51 to 0.81 and tend to be weaker closer to the surface of the ridges (Fig. 5.6). Directions of maximum clustering are perpendicular to the trends of the ridges and in a few cases oblique to the trends of the ridges (Fig. 5.6).

Figure 5.5 Basal ice at the edge of the ice sheet in Vestfold Hills. A) Slightly deformed stratified basal ice resting on gneiss and unconformably overlain by drift snow. B) Highly deformed basal ice showing a series of tight sheared folds.

The diamictons are accumulations of basal debris that have cropped out on the surface of the ice sheet and subsequently been remobilized by sediment flows. Remobilization has resulted in relatively poorly defined directions of maximum clustering, and slight textural variation is probably related to sorting of sediments in less viscous flows. Stronger pebble fabrics below 1 m depth in the excavations can be interpreted as melt-out till in which the fabric of the basal debris zone has been preserved. The formation of melt-out tills, and the preservation of basal debris fabrics that record ice-flow direction are more likely where the sediment cover exceeds 0.5 m, after which melting slows and the debris is less likely to become saturated and flow.

Figure 5.6 Sedimentary logs of sediments from the crests of inner moraines. The contour interval of the Schmidt nets is two standard deviations. V, and P, and give the azimuth and plunge of the principal eigenvector, S, gives the strength of clustering about the principal eigenvector and R shows the trend of the moraine ridge.

Figure 5.6 Sedimentary logs of sediments from the crests of inner moraines. The contour interval of the Schmidt nets is two standard deviations. V, and P, and give the azimuth and plunge of the principal eigenvector, S, gives the strength of clustering about the principal eigenvector and R shows the trend of the moraine ridge.

5.4.2 Ice-Contact Fans and Screes

Ice-contact fans and screes form sharp-crested cuspate ridge segments up to 20 m high and 500 m long. They form at ice cliffs where melting and sublimation of basal debris results in the fall and/or flow of debris at the foot of the cliff (Fig. 5.7a). Most of these ridges have asymmetrical profiles (Fig. 5.7a) characterized by proximal slopes between 25 and 15° and distal slopes between 15 and 25° (Fitzsimons, 1997b).

Sediments exposed at the crests of ice-contact fans and screes show a range of sedimentary facies, including massive and stratified gravels, horizontally laminated and cross-bedded sands, bouldery gravels with lenses of fine-grained sediment, massive matrix-supported diamictons, stratified diamictons and muds (Figs 5.7b and 5.8). The sediments range from moderately sorted to very poorly sorted, but on average are moderately sorted. Particles up to 0.8 m in diameter are common and occur in a chaotic mixture of diamicton, gravel and well-sorted and stratified sand. Most exposures show that the sediments are well stratified with dips down the distal slope of the moraines at angles of between 5 and 20°. The pebble fabric of diamictons and massive gravels are transverse or oblique to the trend of the ridges (Fig. 5.8) and the clustering about the mean axis ranges from moderate to strong (Sj 0.54-0.86).

Figure 5.7 A) An ice-contact scree forming at the ice margin (left) and two ice-cored ice-contact screes adjacent to the ice margin. Note the supraglacial stream emerging from the contact between basal ice and drift snow. B) Poorly sorted gravel overlain by laminated sand and gravel, and a clast supported diamict exposed in the crest of the ice-contact scree.

Figure 5.7 A) An ice-contact scree forming at the ice margin (left) and two ice-cored ice-contact screes adjacent to the ice margin. Note the supraglacial stream emerging from the contact between basal ice and drift snow. B) Poorly sorted gravel overlain by laminated sand and gravel, and a clast supported diamict exposed in the crest of the ice-contact scree.

Figure 5.8 Sedimentary logs of sediments from the crests of ice-contact screes. The contour interval of the Schmidt nets is two standard deviations. V, and P, give the azimuth and plunge of the principal eigenvector, S, gives the strength of clustering about the principal eigenvector and R shows the trend of the moraine ridge.

Figure 5.8 Sedimentary logs of sediments from the crests of ice-contact screes. The contour interval of the Schmidt nets is two standard deviations. V, and P, give the azimuth and plunge of the principal eigenvector, S, gives the strength of clustering about the principal eigenvector and R shows the trend of the moraine ridge.

The association of diamicton, gravel, sand and the bouldery facies suggests that both alluvial and colluvial processes are important during the formation of the ridges (Fig. 5.7b). The chaotic bouldery lithofacies is interpreted as the product of simultaneous accumulation of alluvial and mass-movement deposits (i.e. large particles fall or roll into alluvial deposits and sediment flows).

5.4.3 Thrust-Block Moraines

Thrust-block moraines form along the lateral margins of outlet glaciers, where ice flows across marine inlets or lakes. The ridges are up to 20 m high with proximal slopes of around 30° and distal slopes of around 25°. As the ice core melts, large tension cracks develop along the ridge crests.

Sediments in thrust-block moraines (Fig. 5.9a) consist of stratified diamictons (Fig. 5.9b), massive diamictons and rare layers of horizontally laminated sands (Fig. 5.9). Many exposures display low-angle thrust faults and sheared zones that consistently dip in an up-glacier direction at angles of between 10 and 25°. The pebble fabric of the diamictons can be divided into a group characterized by weak fabrics associated with stratified diamictons (S1 0.45-0.57) and a group of stronger fabrics adjacent to low-angle faults (S1 0.67-0.85). Massive diamictons frequently contain abundant shell fragments and stratified diamictons occasionally contain beds of shells, some in growth position (Fitzsimons, 1997b).

The distinctive fabric, lamination, and preserved marine shells, sometimes in growth position, suggests the diamictons are glacimarine sediments. Pebble fabrics of attenuated diamictons (faulted and sheared) have similar strengths to deformed lodgement tills described by Dowdeswell and Sharp (1986). The increased fabric strength is interpreted as a consequence of attenuation by shearing either as the blocks were detached or deposited. Preservation of beds of shells and laminations within the diamictons suggests at least some of the sediment may have been frozen during entrainment and transportation and/or that the strain was relatively low.

Low-angle faults, together with slickensides and attenuated diamicts adjacent to the faults, show that the glacimarine sediment has been entrained as a series of blocks with an average thickness of about 0.5 m (Fig. 5.10). The moraines have accumulated as successively older glacimarine sediments were eroded from the floor of the fjord and then deposited on the distal shore.

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