912 SASTRUGI

Taylor, G. and Eggleton, R.A. (2001) Regolith Geolog)' and , Chichester: Wiley.

GRAHAM TAYLOR

SASTRUGI Sharp irregular ridges, mounds or dunes. They form on ice sheets, ice caps, sea ice and tundra (typ­ ical of and Greenland), and are com­ Plate 117 Close-up photograph of mottles on a posed of ice and compacted snow. Originating wave cut platform at Darwin, Australia. The from the Russian word zastrugi, they are formed by hammer provides scale. These mottles are up to aeolian and the deposition of drifting snow 15 em across and are composed of haematite, (Gow 1965), and typically are I-2m long and providing the red colour, cementing a saprolitic 10-15cm in height (though exceptional cases can matrix of kaolinite and quartz. The intervening reach 1.5 m in height and hundreds of metres longl. bleached saprolite is identical, except that it does Sastrugi align longitudinally with the predominant not contain haematite wind direction, making it possible to infer the prevailing wind direction at the time of sastrugi Secondary overprints may affect the appear­ development from their configuration. They often ance of the weathering profile and the saprolite. form after a blizzard on the hard ice surface, The most widely observed modification is the for­ becoming larger and harder as the blizzards blow mation of iron oxihydroxide aggregations called across them throughout the winter months. mottles. Mottles (Plate 117) may extend through­ Sastrugi are usually found in the lee of obstacles but out the profile, but are mostly observed in the are also known to exist in open conditions. upper collapsed saprolite. Another common fea­ ture in profiles developed mainly on felsic rocks is Reference a bleaching or the removal of Fe-oxihydroxides from the lower parts of the saprolite and saprock. Gow, A.j. (1965) On the accumulation and seasonal stratification of snow at the South Pole, journal of This zone is referred to by many as a pallid zone, 5, 467-477. particularly in what are described by some geo­ morphologists as a 'lateritic profile'. Further reading Figure 141 (p. 909) shows idealized examples of how some physical and mineralogical properties Wanen, S.G. and Brandt, R.E. (1998) Effect of surface roughness on bidirectional reflectance of Antarctic of saprolite change through a profile. One point snow, journal of Geophysical Research - E: Plalle!; of interest here is the commonly observed addition 103 (Ell), 25,779-25,788. of quartz to the upper parts of saprolite developed from basalt, which of course contains no quartz STEVE WARD when fresh. This indicates the addition of 'foreign' material to saprolite profiles may occur either by overtopping of the saprolite or by other deposits SCABLAND from aeolian accession. This is a very common feature of most saprolite profiles but is most read­ A scabland is an erosional landscape formed by a ily observable over weathered mafic rocks. catastrophic flood and is generally applied to the effects of Jokulhlaups. It was first introduced by References Bretz (1923) to describe the erosion and stripping of the Basalts by floods from Becker, G.F. (1895) Gold fields of the southern Appalachians, Annual Report of the United States Missoula in eastern , Geological Survey, Part Ill. Mineral Resources of the USA. Bretz adopted a term that had been used United States, Metallic Products 16, 251-331. by local farmers to describe the 'scabby' terrain: Pillans, B., Tonui, E. and Idnurm, M. (1999) 'The terms "scabland" and "scabrock" are used Palaeomagnetic dating of weathered regolith at Norrhparkes Mine, NSW, in G. Taylor and c.F. Pain in the to describe areas where (eds) Regolith '98, New Approaches to an old denudation has removed or prevented the accu­ Continent, Proceedings, 237-242, Penh: CRC LEME. mulation of a mantle of soil, and the underlying SCABLAND 913

Bedforms in the Channeled Scabland

Scoured in rock Scoured in Depositional sediment

Macroforms (scale Pool and riffle sequence, Large-scale Longitudinal bars controlled by Quadrilateral residual streamlined residual (a) Pendant bars channel width) forms in channel forms (b) Alternate bars Anastomosis (c) Expansion bars Eddy bars Mesoforms (scale Longitudinal grooves Scour marks Large-scale controlled by Pot-holes transverse ripples channel depth) Inner channels (giant current Cataracts ripples) Microforms Scallop pits Not preserved Small-scale ripple stratifica tion (restricted to slack water facies)

Source: From Baker (1978a) rock is exposed or covered largely with its own 7 m high and 18 to 130 m in chord length. They are coarse, angular debris' (Bretz 1923: 617). composed predominantly of gravel and boulders. Slackwater deposits accumulated in low velocity areas including re-entrants to the major valleys and Channeled Scabland in pre-flood tributary valleys. The formal physiographical region known as the The scablands were formed by discrete out­ 'Cltanneled Scabland' is located in the northern bursts from a range of sources. These included portion of the Columbia Plateau in eastern an enormous subglacial reservoir that extended Washington, USA and comprises all area of over much of central British Columbia (conserva­ 2 approximately 40,000 km . It is a spectacular tive estimates of water volume are 105 km 3) channel complex eroded deeply into and (Shaw et al. 1999) and Glacial . basalt bedrock. The large flood discharges spilled This lake impounded 2,184 km 3 of water during ol'er pre-flood divides into adjacent valleys and its maximum extent (O'Connor and Baker 1992). produced the effect of channels dividing and The last major period of scabland flooding is rejoining to form anastomosing (see ANABRANCHfNG placed a pproximarely between 18,000 and AND ANASTOMOSING RIVER) complexes. These 13,000 years BP. Facies analysis of sedimentary dil"ide crossings are several hundred feet above sequences suggest that there may have been as floors. many as forty floods (Waitt 1985) each separared A typical scabland complex includes erosional by decades or centuries. Shaw et al. (1999) and depositional forms. Baker (1978a) has adopted propose that rhere were fewer floods and that a hierarchical classification of bedforms for the many of the variations in the sedimentary Channeled Scabland (see Table 41). The erosional sequences can be ascribed to pulses within a flood landforms include grooves, pot-holes, rock basins, caused by the input of multiple sources of flood­ inner channels and cataracrs. Bretz et al. (1956) waters during rheselong duration flows (up to ascribed several scabland features to differences 1,000 days). between various basalr flows. Cataracts, such as High water marks along the scabland channels Dty Falls, formed as one group of basalr flows was have been used to reconstruct the maximum flood stripped from underlying resistant flows. Where stages and water surface gradients. These include they were exposed by the floods, the columnar eroded channel margins, depositional features, ice jointed basalts exerted a strong joint control and the rafted ERRATICS and divide crossings. Discharges action by floodwater yielded boulders as large as 21.3 X 106 m3 sec-I were conveyed >30 m diameter. The most spectacular of the depo­ through the channel scabland (Baker 1978b). sitional forms are the streamlined channel deposits, Some constricted channels reached velocities as some superimposed by 0.5 to high as 30 m sec-I. These high velocities were 914 SCANNING ELEGRON MICROSCOPY

patterns, expanding and contracting reaches asso­ ciated with flow constrictions, streamlined hills, inner channels with recessional headcuts, pendaRl forms (bars or erosional residuals) on the down current sides of flow obstacles, longitudinal grooves, irregular 'etched' zones on channel tlOO15 and scour marks around obstacles (Baker 1982).

References Baker, V R. (1978a) Large-scale erosional and deposi­ tional features of the Channeled Scabland, in VR. Baker and D. Nummedal (eds) The Channeled Scabland, 81-115, Washington, DC: NASA. -- (1978b) Paleohydraulics and hydrodynamics of Scabland floods, in V.R. Baker and D. Nummedal (Wsl The Channeled Scabland, 59-79, Washington, DC: NASA. --(1982) The Channels of Mars, Austin: Univcrsil1' of Arizona Press. . Bretz, J.H. (1923) The of lhe Columbia Plateau, Journal of Geology 31, 617-649. Bretz, j.H., Smith, H.T.U. and Neff, G.E. (19561 Channeled Scabland of Washington; new data and interpretations, Geological Society of AII/eried Bulletin 5, 957-1,049. Burr, D.M., Grier, j.A., McEwen, A.S. and Kesztheiyi,P. (2002) Repeated aqueous flooding from lhe Cereberus Fossae: evidence for very recently e~'t3n~ deep groundwater on Mars, Icarus 159,53-73. Malin, M.e., Edgett, K.S., Carr, M.H., Danielson, G.E., Davies, M.E., Hartmann, W.K., et al. 120011 M21-01914, Malin Space Science Systems Mall Orbirer Camera Image Gallery (http://www.I11S1S. Plate 118 Portion of MOC image M2101914 com/moc...ga Ilery/). (http://phorojournal.jpl.nas.1.go\1). O'Connor, J.E. and Baker, V.R. (1992) Magnitudes and which is centred near 7.89°N, 153.95°E, pixel implicarions of peak discharges from Glaci,jl Lakr resolution is 4.4 m. (Malin et at. 2001). This Missoula, Geological Society of America Bul/elm image shows anastomosing channel pattern and 104,267-279. multiple streamlined forms in a flood channel Shaw, J., Munro-Stasiuk, M., Sawyer, B., Beaney, C, emanating from a fissure. For detailed description Lesemann, j., Musacchio, A., Rains, B. and see Burr et at. (2002) Young, R.R. (1999) The channeled scabland: backlo Brerz? Geology 27, 605-608. Waitt, R.B.]. (1985) Case for periodic, colossal jokulh­ possible because of the combination of great flow laups from glacial Lake Missoula, depth (60 to 120 m) and very steep water surface Geological Society of America Bulletin 96, gradients 2 to 12m/km. 1,271-1,286. On Mars, data from orbiting satellites have detected stripped zones on the floors of autflow channels in the Chryse Basin. These anastomosing complexes are 100 km wide and flow over 2,000 SCANNING ELECTRON km across the planet's surface. They are usually MICROSCOPY initiated from collapsed zones. Other examples show multiple and asynchronous flows emanating The Scanning Electron Microscope (SEM), some­ from geothermal fissures in recent Martian history times used in association with Energy Dispersift (see Plate 118) (Burr et al. 2002). By analogy to the Spectrometry (EDS), has been used for srudyil1 scablands on Earth, it is gen,rally accepted that the the surface textures (and chemistry, with EDS) of Martian outflow channels were also formed by sediments (especially quartz grains) since 1962. catastrophic floods. Martian outflow channels The use of the SEM has had many implications inc.lude a distinctive assemblage of scabland for geomorphology, including the determinatioo landforms: regional and local anastomosing of the origin of depositional landforms, the