Salam Sgm -- INCISED VALLEYS
From AAPG Methods, No.7 (1991)
Width : From less than several miles to many tens of miles
Depth : Ranges from tens to several hundreds of feet.
Incised valleys form and fill in two phases:
1. Phase 1 consists of erosion, sediment bypass through the eroded valleys, and deposition at the lowstand shoreline in response to a relative fall in sea level.
2. Phase 2 consists of deposition within the valleys in response to a relative rise in sea level, generally during the late lowstand or transgressive systems tracts.
The fill may consist of a wide variety of rock types deposited in a variety of environments within the upper reaches of the incised valleys, which include estuarine and braided-stream sandstones, fluvial sandstones showing evidence of significant tidal modification, or coastal-plain sandstones, mudstones or coals. These deposits lie above the sequence boundary
Incised valleys also can be filled with marine mudstones if the rate of deposition of coarse-grained sediment is low relative to the rate of sea-level rise at the end of the lowstand.
Depositional environments and associated rock types within the lower reaches of the incised valleys vary and include lowstand-delta and tidal-flat sandstones and mudstones and beach and estuarine sandstones. Commonly, these shallow-marine strata, in the case of beaches or deltas, form one or more progradational parasequence sets.
INCISED VALLEY or LOCAL CHANNEL?
It is critical to distinguish between incised valleys and local channels, such as distributary channels, in constructing an accurate chronostratigraphic framework.
Incised-valley interpretation is more difficult in a single well log than on a cross section because distributary channels, eroding deeply into underlying deltaic deposits, can juxtapose relatively coarse-grained strata directly on prodelta mudstones mimicking a basinward shift in facies.
In a core or outcrop is that the sequence boundary at the base of an incised valley commonly shows evidence of a hiatus between the times of erosion and deposition. Root zones, soils, or burrowed horizons can form on the valley floor during sea-level lowstand but before the valley is flooded and filled with sediment. A distributary channel is always full of fresh water, or if discharge is low, salt water. It is unlikely that evidence of significant subaerial exposure will occur on a distributary-channel floor. Distributary channels are relatively narrow, incised valleys are commonly several miles wide and should be mapped regionally. Furthermore, widespread incised-valley erosion occurs along a single stratigraphic surface. Deltaic distributary channels usually stack to form multiple horizons.
Distributary channels are encased in delta-plain or stream-mouth bar deposits. Most of the distributary-channel fill is laterally encased in stream-mouth bar deposits. Distributary channels can only step seaward if they have a subaqueous, shallow-water delta platform across which they can migrate. By their nature, distributary channels cannot be encased regionally in deeper-water deposits. Incised valleys commonly are encased in middle-to outer-neritic mudstones because they incise during a relative fall in sea level.
From LOWSTAND ALLUVIAL BYPASS SYSTEMS: INCISED vs. UNINCISED H.W. Posamentier (October 2001, AAPG Bulletin)
Widths of individual channels range from 100 to 250 m. Meander belt widths range from 2 to 6 km. The incised trunk valleys range from 0.5 to 5 km wide and contain channels within them. Valley incision can be traced more than 200 km inboard of the shelf edge.
An incised valley is said to exist where a river has cut into its flood plain sufficiently so that even when it is at flood stage, flow does not overtop the riverbanks. This leaves the formerly active flood plains abandoned and serving as interfluves.
Incised valley systems are most commonly thought to have characterized extensive shelf settings each time a relative sea level fall has occurred. Incised valleys have been described in all types of physiographic settings ranging from active to passive continental margins as well as interior continental basins.
Incised valley systems can be filled with a complex array of depositional systems ranging from open marine to estuarine to fluvial. Incised valley can be the sites of numerous stratigraphic discontinuities that punctuate the stratigraphic architecture. These include tidal ravinement surfaces, base of channel local erositional surfaces, and valley within valley widespread erosional surfaces. Incised valleys are not uncommonly the sites of multiple incision events associated with successive sea level falls.
CAUSES of VALLEY INCISION
1. as a result of sea level or base-level fall
2. as a result of tectonic tilting of alluvial settings
3. as a result of significant decrease in fluvial discharge resulting in formation of underfit streams.
In general, each of these proceses results in a physiography characterized by channels located within valleys cut into formerly active flood basins, though there can exist subtle differences between systems attributed to each cause.
