Fluvial Deposits and River Classification
Rivers
a. General Concepts
Fluvial deposits comprise about 30% of all sedimentary deposits on earth. They can form successions of several kilometers in thickness, especially in distal environments such as alluvial and coastal plains.
The channel pattern of rivers can be single or multiple and is described according to the sinuosity index, which is the thalweg length divided by the valley length.
b. Classification
i) Single-Channel Straight Streams
Single-channel straight streams have a sinuosity index of less than 1.5. They are rare in nature and are usually developed over short distances only.
ii) Single-Channel Meandering Streams
Single-channel meandering streams have a sinuosity index of more than 1.5. They develop on low gradients in perennial rivers with a high suspension load and cohesive banks, which are commonly formed by clay plugs deposited in oxbow lakes. Vertical accretion of the levees causes the riverbed to build up above the surrounding flood plains. When a levee is breached and avulsion occurs, the river may thus follow a completely different course, with the result that the channel deposits typically form shoestring sands with a width: thickness ratio of less than 25:1. The basal contact is deeply eroded, asymmetric, and concave-upward. Due to the high sinuosity of the active channel, paleocurrent directions will show a variation of more than 180º and are commonly bimodal or polymodal when plotted on a rose diagram. Sedimentary structures are dominated by lower flow regime types such as trough and planar cross-lamination, with common epsilon cross-lamination and lateral accretion surfaces. Mesocycles are well developed. Contacts are sharp at the base and gradational with the flood plain mudstone at the top. Oxbow lakes filled with organic-rich clays, levees, and crevasse splays are common, while exhumed meander belts may be discerned in ancient fluvial deposits.
iii) Multichannel Braided Streams
Multichannel braided streams have a sinuosity index of less than 1.5. They normally develop along higher gradients in ephemeral rivers with a high bottom load (sand and gravel) with easily eroded banks. Braided rivers are therefore wide, shallow streams divided by islands and sandbars, which are frequently altered or destroyed because the active channels change their courses regularly. The width: thickness ratio of braided river deposits is high (25:1 to 100:1) due to the rapid lateral migration of the channels. They therefore form tabular sands with low basal relief. Paleocurrent directions have a smaller range (less than 180º) and rose diagrams show a unimodal (sometimes bimodal) pattern. Sedimentary structures include a larger percentage of upper flow regime types (upper plane beds, antidunes). Mesocycles are only well developed in certain types of braided rivers. Contacts are sharp at the base and top, and levees, crevasse splays, and exhumed meanders are absent. Oxbow lakes are also absent, but abandoned anabranches do occur. They are straighter in plan and are represented in the rock record by mudrock lenses with erosional basal and upper contacts.
Miall (1978) distinguished between 6 types of braided rivers. The Trollheim, Scott, and Bijou Creek types are proximal rivers, whereas the Donjek, South Saskatchewan, and Platte types are characteristic of distal environments. However, most rivers are combinations of these types.
iv) Multichannel Anastomosing Streams
Multichannel anastomosing streams may have a sinuosity of more than 1.5, i.e., the individual channels can show meandering, braided patterns. They develop on low gradients and are similar to meandering rivers in that the channels are relatively deep and narrow, being separated from each other by stable flood plains. The channels are commonly symmetrical-convex in cross-section, with gravel and sand showing trough cross-lamination, ripple lamination, and slump structures at the base. The channels are eventually filled with silt and mud showing abundant wavy, ripple, and lower plane lamination. In modern environments, anastomosing rivers are characterized by lush vegetation forming marshes between the channels, so that the latter do not migrate much. Rapid, vertical deposition is therefore typical, leading to the construction of thick (up to 80m) sand shoestrings in the floodplain muds. Levees and crevasse splays may also be present.
c. Sub-environments and Processes
i) Meandering Rivers
are characterised by point-bar deposits. In a river meander, the highest current velocity is reached on the outside of the bend, so that the coarsest sediment and sedimentary structures representing the highest flow regime are found here. Due to erosion of the outside (concave) bank and deposition on the inner (convex) bank or point bar, the channel migrates in the direction of the former. The result is a fining-upward sequence of clay pebble conglomerate followed by sandstone with upper plane lamination, trough and tabular cross-lamination, ripple-laminated siltstone and finally