A pendant bar is a large, streamlined, fluvial bar that is typically composed of gravel which occurs just downstream of a bedrock obstruction within a river channel or floodway that has been scoured by either an outburst flood, megaflood, or jökulhlaup. They are often associated with giant current ripples.[1][2] Malde[3] introduced this to refer to streamlined mounds of gravel deposited by the Bonneville Flood that lie downstream of bedrock projections on the scoured valley floor of the Snake River. They are most common type of bar found within the Channeled Scablands created by the Missoula floods. The obstruction for the initiation of pendant bars in the Channeled Scablands is typically either a knob of basalt or the relict bend of a pre-flood meandering valley.[1][2]

In the Channeled Scablands the larger pendant bars in are up to 2 kilometers (1.2 mi) long and rise 30 m (98 ft) above the floors of the preflood valleys. The bar surfaces lie 30–60 m (98–197 ft) below the high-water surface at the time of their formation.[1][4] During regular annual floods in Central Texas and elsewhere, much smaller, analogous, mid-channel bars form downstream from river channel obstructions, e.g. large boulders of rockfall, or in the lee of barriers of stable trees and logjams.[5]

Occurrence

Pendant bars are associated with the deposits and landforms formed by outburst floods and megafloods on both Earth and Mars. In addition to the Channeled Scablands and the valley of the Snake River, pendant bars have been reported from deposits of the formed by the Maumee Torrent in the Maumee River valley of Indiana;[6] the southern spillway of glacial Lake Agassiz;[7] the Souris spillway near Elcott, Saskatchewan;[8] and Ares Vallis, one of the largest cataclysmic flood channels on Mars.[9]

Internal structure

The pendant bars studied within the Channeled Scablands are composed of well-developed foreset beds. These foreset beds always dips downstream. They consist either of well-sorted subrounded boulders and cobbles or granules and pebbles with an openwork structure. The boulders and cobbles typically exhibit numerous percussion flake scars.[1][4] Also, many of the rounded boulders and cobbles are broken into fragments that Bretz called broken rounds.[4][10]

Origin

Pendant bars form as high-velocity floodwater moves around an obstruction. There, pendant bars accumulate in a zone of localized flow expansion and deceleration downstream of any obstacle which lies within the main flow path. A similar process forms a sand splay, which is much like a shoal but is formed on floodplains or terraces in lower-intensity flooding episodes. Other related fluvial bars that are formed by outburst floods and jökulhlaups are longitudinal, expansion, and eddy bars.[1][2][3]

See also

References

  1. ^ a b c d e Baker, V.R., 1973. Paleohydrology and sedimentology of Lake Missoula flooding in eastern Washington. Special Paper-Geological Society of America 144, 79 pp.
  2. ^ a b c Baker, V.R. 1978a. Large-scale erosional and depositional features of the Channeled Scabland. in The Channeled Scabland, A Guide to the Geomorphology of the Columbia Basin, Washington, edited by Baker, V.R. and Nummedal, D., Prepared for the Comparative Planetary Geology Field Conference held in the Columbia Basin, 5–8 June 1978, p. 81–115, NASA, Washington DC.
  3. ^ a b Malde, H.E. 1968. The catastrophic late Pleistocene Bonneville Flood in the Snake River Plain, Idaho. United States Geological Survey Professional Paper 596, 52 pp.
  4. ^ a b c Baker, V.R., 1984. Flood sedimentation in bedrock fluvial systems. in Sedimentology of Gravels and Conglomerates, edited by Koster, E.H., Steel, R.J. Canadian Society of Petroleum Geologists, Calgary, pp. 87–98.
  5. ^ Baker, V.R. 1977. Stream-channel response to floods with examples from central Texas. Geological Society of America Bulletin, 88(8) 1057-1071 pp.
  6. ^ Fraser, G.S. and Bleuer, N.K., 1988. Sedimentological consequences of two floods of extreme magnitude in the late Wisconsinan Wabash Valley. Geological Society of America Special Papers, 229, pp.111-126.
  7. ^ Fisher, T.G., Lepper, K., Ashworth, A.C., and Hobbs, H.C., 2011, Southern outlet and basin of glacial Lake Agassiz. in Miller, J.D., Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America. Geological Society of America Field Guide. 24, p. 379–400.
  8. ^ Lord, M.L. and Kehew, A.E., 1987. Sedimentology and paleohydrology of glacial-lake outburst deposits in southeastern Saskatchewan and northwestern North Dakota. Geological Society of America Bulletin, 99(5), pp.663-673.
  9. ^ Pacifici, A., Komatsu, G., Pondrelli, M., 2009. Geological evolution of Ares Vallis on Mars: formation by multiple events of catastrophic flooding, glacial and periglacial processes. Icarus. 202,60–77.
  10. ^ Bretz, J.H., 1929. Valley deposits immediately east of the channeled scabland of Washington. II. The Journal of Geology, 37(6), pp.505-541.
Categories