Dilyuvial-accumulative terraces

Dilyuvial terraces and ramparts are thick, usually about 240 m (up to 340 m) above riverbanks, of strata clearly layered, cleanly washed with gravel , gravel, and wood deposits, including both layers and lenses of sand and sandy loam , and angular boulders and blocks of very variegated petrographic composition. Dilyuvial strata accumulated during a sharp drop in the energy of flows or under conditions of reverse flows in the zones of erosion shadow directly below the radical protrusions, bends of the main valley, or in large extensions of the valleys. The surfaces of all diluvial terraces are generally inclined to the root sides of the valleys, often complicated by small isometric depressions that fix sections of underwater whirlpools . The edges of the terraces are clear and are emphasized by the coastal berm.

Due to later erosion and shedding, the diluvial strata are now very distinct terrace-shaped sculptural forms with well-defined eyebrows and clearly defined ledges having a drop close to the angles of repose (rest angles) for the corresponding fractions. Therefore, diluvial terraces and ramparts, strictly speaking, are not river terraces , it is the thickness of the diluvium, which has a terrace-like shape.

To the foothills, the heights of diluvial-accumulative terraces are reduced to 100, and in the region of Gorno-Altaysk , to 60 meters (the famous “Maiminsky shaft” dividing the valleys of the Katun River and its right tributary of the Maima River ). In their structure, the proportion of well-washed coarse-grained sands increases, and the roundness of gravel and pebbles increases ^[1] .

The term “bar” in the understanding of J. H. Bretz is a swath-like or terrace-like thickness of a clastic material, superpowerful for the “normal alluvium ” of a given valley, having an original morphology . Deposits of “bars” were discharged by super-flows in suitable areas along the paths of lake water flow with a sharp drop in the transporting ability of diluvial flows . In English literature, the term " giant gravel bar " is widely used today ^{[2] [3]} .

However, due to the obvious inconvenience of this term, caused by the notion of “bar” widely known in Russian marine geomorphology ^[4] , A. N. Rudoi , the author of the theory of diluvial morpholithogenesis , uses the term “suitable in meaning and content” for such thicknesses and forms of relief diluvial-accumulative terrace, rampart ”, proceeding from the general name of exogenous processes , as a result of which these shaft-shaped forms were formed ^{[1] [5] [6] [7]} .

The deposits of zones of natural screens, the mechanism of formation of which was described in detail by A. A. Chistyakov ^[8] , were studied in the conditions of large Siberian rivers by S. A. Laukhin ^[9] , who proposed calling them limon-like facies .

Precipitation of the upper levels of diluvial terraces and ramparts is really similar to deposits of low-flowing lakes, on the basis of which, within the limits of at least the so-called Yalomansk depression , they were considered as such. Other ideas about the genesis of the strata that compose high levels of diluvial terraces (kama terraces, anomalously powerful alluvium strata under tectonic subsidence, on the basis of which, in fact, the “Yaloman tectonic hollow” was distinguished) indicate a great interest of specialists in these strata and an unusual mechanism their occurrence.

As in the case of stagnant sediments of diluvial flows from the North American Late Quaternary glacial-dammed lake Missoula , fine-grained layers of silt and polymictic sands alternate in these strata with coarser, mostly gravel-gravel horizons, confined to the main channels of the Kata river channel . The mechanical composition of sediments becomes thinner up the tributary valleys, with distance from the main valleys. The advanced layers plunging at angles of 6–8 ° under the edge of the Small Yaloman River indicate water splashes from the river valley. Chewy.

At the same time, horizons of sand , gravel and pebbles , with a thickness of about 25 cm, alternate with interbeds of silt, with a thickness of about 10 cm. Often there are horizons of loam and silt with stall textures. Deposits of stagnant water are unconditionally blocked by boulder-Galernik layers with the inclusion of small blocks. These relatively coarse-grained formations are the result of the energy pulsations of the flow passing up the valleys of the tributaries. The same powerful currents up the valleys of the Small Yaloman and Inya rivers during floods along the main channel could also generate fracture textures in silty horizons.

In the deposits of all high diluvial terraces, the exceptionally poor roundness of the macadamous-gritty fraction dominating in the structure of the strata, and its motley petrographic composition attracts attention. The last two circumstances, in particular, indicate that the clastic material in the stream was transported mainly in a suspended state, which was first noticed 25 years ago by A. N. Rudoi ^[5] . Large, up to several meters in diameter, angular blocks are enclosed in these deposits without breaking layering .

The formation of deposits of diluvial terraces and ramparts of diluvial origin occurred as a result of either one powerful superfluid, or, more likely, as a result of several diluvial flows. Judging by the thickness of the deposits, the minimum depths of diluvial flows in the Chui and Katun valleys were at least 250 m, flow velocities were tens of meters per second, and the flow rates always exceeded 1 million m³ / s. This correlates well with the data obtained from the calculations. New absolute dates obtained by modern methods also indicate the astronomical age of diluvial flows with flow rates of more than 10 million m³ / s - between 15 and 7 thousand years ago ^{[10] [11]} . The maximum flow rate of the diluvial flow was set at 18 million m³ / s, which exceeds the flow rate previously considered to be the maximum on the Earth during the breakthrough of Lake Missula ^[12] .

In Central Altai diluvial ramparts and terraces are known as “high terraces” of Chui and Katun. Russian geologist Pyotr Chikhachev admired their amazing size and perfect, seemingly man-made, shape more than 150 years ago ^[13] . Since then, the genesis of “high terraces” has been the subject of fierce debate ^[14] , which is beginning to fade today due to its evidence ^{[15] [16]} .

• A.N. Rudoi . The doctrine of diluvium - general provisions (neopr.) . Knol . Date of treatment January 30, 2011. Archived on April 15, 2012.
• A.N. Rudoi . Giant gravel bars - diluvial-accumulative shafts (“high terraces”) (Russian) . Knol . Date of treatment February 8, 2011. Archived April 23, 2012.
• A.N. Rudoi . Scablands. Scabland: a brief overview (Russian) (inaccessible link - history ) . Knol . Date of treatment February 8, 2011. (unavailable link)
• Keenan Lee. The Altai Flood.
• Keenan Lee. The Missoula Flood.
• Channels scablands