As we know from previous post explained about sedimentary environments, at least we know we have 6 different types on the whole of sediment; alluvial fan sediments, fluvial deposits, delta deposits, lake deposits, barrier island deposits, and lagoon deposits. Now this article will explain about fluvial deposits, mostly known as river sediments.
Fluvial deposits definition
Fluvial deposits are deposit sediments as the result of rivers. In fact, the sediments include deposits of braided streams, meandering rivers, and an astomosing streams. The figure below shows the fluvial facies of meandering rivers.
Types of fluvial deposits
There are different types of fluvial deposits and reservoirs. The two end-member depositional types are braided-river and fluvial-river deposits. A third type, incised-valley fill, can contain either or both of these end members within the confines of the valley. In addition, fluvial deposits near the mouths of the valleys may become reworked by estuarine and tidal processes, which ultimately produce a different set of reservoir properties. The geometry, size, and reservoir characteristics of each fluvial type depend upon transportational, depositional, and postdepositional (diagenetic) processes that are controlled by several external variables, including geographic location, sediment source areas (provenance), climate, and degree of tectonic activity.
Braided-river fluvial deposits tend to be relatively coarse-grained and consist of gravel and sand, with little to no mud. Because of this, the beds tend to be laterally continuous over much or all of the width of the braid plain, although the presence of some shale beds may disrupt the continuity locally.
By contrast, meandering-river fluvial deposits tend to be finer-grained, more lenticular, and partially or completely encased in floodplain shales. Depending upon the deposit’s degree and type of postdepositional compaction and cementation, its porosity and permeability can be quite variable. However, in general, braided-river facies are more porous and more permeable than are meandering-river facies.
Braided streams fluvial deposits
Braided streams have branched channels because the river channel is not very stable. This usually occurs with steeper stream gradients and an abundant supply of sediments. Braided streams favor then deposition of coarse sediments containing coarse sand and gravel with litle clay and silt. Meandering rivers move in loops, with the greatest velocity at the outer bank where erosion occurs and lower velocity at the inner bank where deposition occurs. The fining-upward sequence from sand to silt to clay is typical of meandering river facies.
This sequence is the result of the water velocity decreasing as the river, over a given spot, migrates from the outer bank to the inner bank. In the same way, along the U.S. Gulf Coast is a typical example of bayous and slough in regions of very low stream gradient and with subsidence.
Meandering river fluvial deposits
Meandering river sediments vary from coarse to fine grains upwards, as shown in figure below. During the basin deposit process, thick sediments may form due to the continuous development of the meandering rivers. In the meandering river sedimentary system, channel sands act as the skeleton in the rock mass. Generally, it forms laterally many strips of sandstones surrounded by the flood basin deposits. In the lateral direction some layers in the strata were thick, thin, or even disappear . Vertically, for the most part, the sandstones array or overlap each other in lens shape, and lithology varied cyclically. Therefore, in the meandering river deposit system, fine grained sediments always surround sand sediments. These made rock mechanical properties anisotropic and heterogeneous.
The rocks in fluvial deposits are sandstone, siltstones, shales, mud stones, and claystones and have the following geomechanical behaviors:
- The rocks are stratified layers and interbedded and alternated with soft and hard layers.
- The sandstones are weak in weathering resistance, and the strength changes gradually from the bottom to the top.
- The strength of the sandstone in which the sandstone becomes thinner and near the dead-end is the lowest.