Fluvial System (Facies)
Fluvial System
Stratigraphy & Facies
1. Alluvial Fan Facies
Alluvial fans are cone-shaped to arcuate in plan view, with network of branching distributary channels. The long profile, from fanhead to fantoe, is commonly concave upward; the greatest slope occurs at the fan apex and decreases down the fan. The transverse or cross-fan profile is generally convex upward. Alluvial fan sediments are dominated by gravely deposits, which typically show down-fan decease in grain size and bed thickness and an increase in sediment sorting.
Overall, alluvial fan deposits tend to be characterized by strongly developed thickening and coarsening upward successions, caused by active fan progradation or outbuilding.
1.1 Debris flow dominated fans facies Debris flow dominated fans facies are characterized by lobes of poorly sorted, coarse sediment, commonly with a muddy matrix.
1.2 Stream flow dominated fans facies Stream flow sediments constitute more sheetlike deposits of gravel, sand, and silt that may be moderately well sorted, cross-bedded, laminated, or nearly structureless.
1.3 Proximal fan facies * Texturally immature, very coarse-grained, angular-subrounded clasts. * Conglomerates, matrix supported clasts-supported fabrics. * Unstable minerals present [ (source rock)] = compositionally immature (usually) * Debris flow deposits: massive, un-bedded, very poorly sorted, mud matrix. * Usually there is a confined permanent channel that may contain an intermittent stream.
1.4 Proximal fan facies * Better sorting (sands can be well sorted), fine-grained.
-Sandstones dominate with thin gravel layers in channels.
-Minor mudstones possible. * Unstable minerals still present, but amount depended on climate and source composition. * Main channel branches into many side channels (braided channels).
-Periodic rainfall produces sheet flows over the flat surface and mudflows can occur.
Figure 1.1 General
Stratigraphy & Facies
1. Alluvial Fan Facies
Alluvial fans are cone-shaped to arcuate in plan view, with network of branching distributary channels. The long profile, from fanhead to fantoe, is commonly concave upward; the greatest slope occurs at the fan apex and decreases down the fan. The transverse or cross-fan profile is generally convex upward. Alluvial fan sediments are dominated by gravely deposits, which typically show down-fan decease in grain size and bed thickness and an increase in sediment sorting.
Overall, alluvial fan deposits tend to be characterized by strongly developed thickening and coarsening upward successions, caused by active fan progradation or outbuilding.
1.1 Debris flow dominated fans facies Debris flow dominated fans facies are characterized by lobes of poorly sorted, coarse sediment, commonly with a muddy matrix.
1.2 Stream flow dominated fans facies Stream flow sediments constitute more sheetlike deposits of gravel, sand, and silt that may be moderately well sorted, cross-bedded, laminated, or nearly structureless.
1.3 Proximal fan facies * Texturally immature, very coarse-grained, angular-subrounded clasts. * Conglomerates, matrix supported clasts-supported fabrics. * Unstable minerals present [ (source rock)] = compositionally immature (usually) * Debris flow deposits: massive, un-bedded, very poorly sorted, mud matrix. * Usually there is a confined permanent channel that may contain an intermittent stream.
1.4 Proximal fan facies * Better sorting (sands can be well sorted), fine-grained.
-Sandstones dominate with thin gravel layers in channels.
-Minor mudstones possible. * Unstable minerals still present, but amount depended on climate and source composition. * Main channel branches into many side channels (braided channels).
-Periodic rainfall produces sheet flows over the flat surface and mudflows can occur.
Figure 1.1 General