Giant current ripples, giant gravel bars, gravel dunes or GCRs[1] are a form of subaqueous dune. They are active channel
topographic forms up to 20 m high, which occur within near-
thalweg areas of the main outflow routes created by
glacial lake outburst floods.[2] Giant current
ripple marks are large scale analogues of small current ripples formed by
sand in streams. Giant current ripple marks are important features associated with scablands. As a landscape component, they are found in several areas that were previously in the vicinity of large glacial lakes.[3]
Giant current ripples can reach a maximum height of 20 metres (66 ft) and reach a maximum length of 1 kilometre (0.62 mi). they occur in ripple fields that can cover an area several kilometers across.[3][6]
As a result of the high energy environment in which they were deposited they consist mainly of pebbles and small boulders, with a small fraction of coarse sand.[6][7] Giant current ripples are whale-back shaped, with large poorly-rounded boulders found near the peak. They are oriented with the long axis perpendicular to the flow direction and with the steep flank oriented against the direction of flow. The sediment is loose and dry, without
loam or
silt. The larger exposed segments are
cross-bedded.[7]
While they are often called ripples, their topography responds to
hydraulic flow as well as
boundary conditions, which means they are more accurately described as a type of
dune.[8]
The physical parameters of the ripples, such as height and
chord distance, can be used to estimate the hydraulics of the flood (e.g., depth and speed).[9]
Another area where Giant current ripples are an important landscape feature is in the Altai Mountains, Russia [3] At least seven
major floods are believed to have happened in this area over a span of 150,000 years during
Marine Isotope Stage 2, or the latter part of the
last glaciation. The lakes that fed these events reached up to 600 km3 (140 cu mi) in capacity. Floodwater depths during the largest of these floods are estimated to have been up to 300 metres (980 ft) and the water would have been moving at a speed of 60 meters per second. This resulted in giant current ripple deposits of up to a hundred meters thick.[14]
Mars
American and British geologists and planetologists have discovered giant current ripple reliefs and other scabland features on Mars surrounding the
Cerberus Fossae, indicating that megafloods have occurred there at some point in the recent past.[15] While they have roughly the same dimensions as on earth when they occur, Giant current ripples appear to be much less common on Mars relative to other scabland features. This is likely a result of differences in the availability of certain types of sediment.[8]
^Baker, Victor R (1973). Paleohydrology and Sedimentology of Lake Missoula Flooding in Eastern Washington. Geological Society of America. p. 58. Special Paper 144.
^Carling, Paul A.; Martini, I. Peter; Herget, Jürgen; Borodavko, Pavel; Parnachov, Sergei (2009-09-24), "Megaflood sedimentary valley fill: Altai Mountains, Siberia", Megaflooding on Earth and Mars, Cambridge University Press, pp. 243–264,
doi:
10.1017/cbo9780511635632.013,
ISBN978-0-521-86852-5
^Megaflooding on Earth and Mars. Devon Burr, Victor R. Baker, Paul Carling. Cambridge, UK: Cambridge University Press. 2009.
ISBN978-0-511-63305-8.
OCLC667005201.{{
cite book}}: CS1 maint: others (
link)