10.1: Alluvial Systems
Alluvial Fans
Alluvial fans are mounds of coarse grained sediments formed when a confined stream disgorges into an unconfined area. They typically occur along the margins of mountain ranges where bedrock incised channels draining uplands spill out on to broad open valley floors. Alluvial fans occur in areas with significant topographic relief caused by rapid subsidence or uplift (rift basins, foreland basins, fold-and-thrust belts, etc.). They are semi-circular in map view, form mounds in transverse cross section, and form basinward-thinning wedges in lateral cross sections.
Internally, alluvial fans typically include debris flow deposits composed of poorly-sorted, possibly matrix-supported conglomerates, breccias, and/or diamictites and stream flow deposits composed of sorted and stratified sand and gravel. Grain size generally increases toward the mountain front and decreases in a basinward direction. Sediments commonly occur in coarsening upward cycles that record tectonic and sedimentary pulses. (Paleo)flow is generally in a radial pattern away from the source.
Fluvial Deposits
Rivers are areas where overland flow is concentrated in a channel . Floodplains are the areas adjacent to channels that are periodically inundated during floods. Although we commonly think of river channels as u-shaped features, they exhibit a wide range of geomorphic variability and are commonly preserved as complex, amalgamated bodies in the geologic record. The orientation of the outcrop relative to paleoflow direction can also influence one's perception of channel body geometry. A simple channel that appeared u-shaped when viewed perpendicular to paleoflow direction could appear like an elongate sand ribbon if viewed parallel to paleoflow. Modern rivers can be characterized using a variety of criteria including sinuosity, number of active channels, types of bars, and numerous other characteristics. For our purposes, we will take an admittedly oversimplified approach and focus on three points in the continuum of fluvial style: braided , meandering , and anastomosed (aka fixed-multichannel).
Braided Rivers
Braided river systems consist of a broad "river" with an interwoven network of active channels separated by emergent bars . They are typically bedload-choked sand- and/or gravel-dominated systems with bars that typically migrate in a downstream direction. Given the coarse nature of the sediment and the overabundance of bedload, most sediment transport happens during flood stage when the entire river is bankfull and within-channel bars are submerged.
Overall, abundant bedload makes for unstable banks and sand- or gravel deposits with only modest amounts of mud and numerous erosion surfaces. Active channel deposits are much more abundant than floodplain deposits. The Platte River of Nebraska is one of the best known modern braided rivers; well exposed and documented systems include the Castlegate Sandstone (Cretaceous) and the South Bar Formation (Pennsylvanian).
Meandering Rivers
Meandering rivers have a sinuous main channel with a broad, often muddy, floodplain. They typically occur in low gradient areas, carry a mixture of bedload and suspended load, and have more energy than they need to carry the sediment load. They expend the excess energy winding across the floodplain and distributing sediment across it. Internally, the water in channels actually moves downstream in a corkscrew fashion (helicoidal flow) which facilitates erosion on the cutbank and deposition on the opposing pointbar . Sinuosity increases with continued cutbank erosion and pointbar deposition, which gives meandering systems their distinctive sinuous patter. Eventually curvature increases to the point where a reach of the river gets cut off and isolated forming an oxbow lake. The oxbow lake is filled with standing water and eventually fills in with mud and/or organic material.
Overall, meandering river systems generally consist of sandy or heterolithic channel deposits encased within finer floodplain deposits. In a vertical section, a fining-upward trend may be formed by the progression from the (relatively) coarse basal channel deposits, through sandy and heterolithic point bar deposits ( inclined heterolithic strata ), and eventually into finer grained floodplain deposits. The Mississippi River is, by far, the best know modern meandering river; noteworthy ancient examples include portions of the Devonian Catskill Formation of PA and NY and the Cretaceous McMurray Formation of Alberta.
Anastomosed Rivers
Anastomosed river systems have channels that are largely fixed in place without much lateral movement. They are commonly part of a larger drainage network with multiple interconnected channels separated by floodplains. In the geologic record it is difficult to prove that multiple channels were active simultaneously (especially given outcrops of limited lateral extent) so the best one can hope for is to find stable channels - ideally at the same level - isolated within floodplain deposits.
Floodplain Deposits
Floodplain deposits consist of muddy sediment formed from suspension deposition following flooding events and sandy deposits in areas that were closer to higher velocity flows that were capable of moving bedload. Floodplain deposits may show evidence of subaerial exposure (desiccation cracks, raindrop imprints), evidence of plant growth or other terrestrial fossils, and/or evidence of pedogenesis including the destruction of bedding or the development of soil horizons.
Levees are areas of positive topographic relief on the floodplain that form adjacent to channels. They form when channelized flows overtop the river banks and rapidly deposit sediment as the flows become unconfined. Crevasse splays are lobe-shaped sand bodies that form where a channel breaches the levee and deposits a lobe of relatively coarse grained sediment.
Avulsion and Aggradation
Avulsion is the process by which an active channel rapidly shifts to a different location on a topographically lower part of the floodplain. It happens when primary flow breaches a levee and diverts through a crevasse channel, usually during floods. If the new route provides a gradient advantage the active channel may shift to the new course.
Aggradation is the increase in land elevation due to sediment deposition. River channels and levees commonly aggrade together and become elevated above the floodplain. This process eventually raises the downstream elevation and decreases the gradient. If/when the levee is breached during a flood, the active channel will preferentially shift to a the new (steeper gradient) course.
Together, the avulsion and aggradation can profoundly influence the preservation of fluvial deposits. Low avulsion rates or high subsidence rates tend to produce isolated channel bodies; high avulsion rates or low subsidence rates tend to produce highly amalgamated channel bodies. These relationships get much more complex when the processes interact.
Overview
Fluvial deposits are generally preserved as sand or gravel channel bodies that have erosional bases and may be amalgamated. Channel bodies may be encased in finer-grained floodplain deposits. Both river and floodplain deposits can contain evidence of fresh-water and/or terrestrial fossils. Floodplain deposits may show evidence of soil formation and subaerial exposure.
Readings and Resources
- Bryant, M., Falk, P., & Paola, C., 1995, Experimental study of avulsion frequency and rate of deposition. Geology, 23(4), 365-368.
- Harold Fisk's 1944 maps of the Mississippi River: http://www.radicalcartography.net/index.html?fisk
- Leeder, M. R., 2009, Sedimentology and sedimentary basins: from turbulence to tectonics. John Wiley & Sons.
- Miall, A. D., 1977, A review of the braided-river depositional environment. Earth-Science Reviews, 13(1), 1-62.
- Miall, A.D., 1996, The Geology of Fluvial Deposits, Springer, Berlin, 582 p. https://doi.org/10.1007/978-3-662-03237-4
- Miall, A. D. (2010). Alluvial deposits. In N. P. James & R. W. Dalrymple (Eds.), Facies Models 4 (pp. 105–137). Geological Association of Canada.