Alluvial fansStratigraphic recordGEOL 342 Sedimentation and StratigraphyLecture 8: Alluvial fans24 February 2005Assoc. Prof. A. Jay KaufmanTerrestrial sedimentary environmentsIn the past descriptions of sedimentary units dominated our thinking, but over thepast 30 years or so sedimentologists adopted an approach that is more genetic to studymodern depositional environments. This viewpoint allows us to predict what types ofsediments and stratigraphic sequences are formed. It is important to ask what processesare involved in depositional systems. Facies models generalize sedimentary features inmodern depositional systems, but distill out local variability.Non-marine environments are poorly preserved because they sit above base levelof erosion, the level on Earth’s surface above which sediments must eventually erode,and below which they are deposited. Most of the time the base level is near or at sealevel.What environments are thus preferentially preserved in the geologic record?How are terrestrial environments then preserved?Alluvial fansThe most proximal and coarse grained of sedimentary environments is the alluvialfan. These are found next to mountain belts and are the products of two main depositionalprocesses: debris flows and sheet flows. The deposits of alluvial fans reflect theseprocesses.In the modern, alluvial fans are bodies of very course grained sediment. Theyhave steep upper surfaces, ranging from 16– 1.5, with the slope decreasing towards thebasin. The slope magnitude depends on the fan’s provenance (muddy, gravelly) andtectonic setting. Due to this steep slope, alluvial fans are always upper flow regime andcommonly spercritical (F > 1). In contrast, river (fluvial) environments have grades of0.5– 0.01. 1ClassificationAlluvial fans can be subdivided into two main categories:Name Primary facies Secondary Facies ProvenanceDebris-dominatedDebris-flowparaconglomeratesChannelizedorthoconglomeratesGranitic, volcanic, pelitic,carbonateWater dominatedSheet-floodorthoconglomeratesChannelizedorthoconglomerates,debris-flowparaconglomerates Some metamorphic(gneisses, quartzites),quartz areniticRock avalanche (minor)MonomicticbrecciasPolymictic breccias AnyFan DeltaSee above See above DEPOSITED IN WATERA debris flow occurs when all sizes of sediment ranging from boulders to clay thatis saturated with water moves en mass and is rapidly deposited with little to nostratification (except in the case where multiple debris flow sheets are stacked). Debris flows occasionally preserve reverse grading, especially near their bases.In the upper reaches of the fan they form lobate, tabular bodies of uniform thickness. Amud flow is a class of debris flow with mainly fine-grained particles that can move atrapid rates (up to 10 km/hr) also forming narrow lobes.2Alluvial fans are useful scientifically for two reasons: they are always near therangefront and they are very coarse. Since they’re near the rangefront, they usually reflectnearby tectonic uplift. They also are very strong indicators of what portions of theadjacent basement were exposed at a given time. Since they are very coarse, this makesprovenance determination very easy. Alluvial fans represent a high risk of naturalhazards in the form of landslides and are studied for this reason.Alluvial fans are generally restricted in area, typically being no more than 1-10km from their source. They form when there is a sudden change in a stream from anarrow confined channel with a steep gradient to the broad flat of a valley, which causesa sudden drop in the hydraulic power of the stream, and hence deposition.These are cone-shaped deposits of very coarse sediments, generally lacking finersilts and clays. They are built up by successive sheet-flow (sand-dominated) or debris-flow deposits, most often in desert environments, but also noted in humid, glacialenvironments as well (outwash fans in front of melting glaciers). When alluvial fanscoalesce along mountain fronts they form a bajada.3The upper surface of an alluvial fan is dominated by braided streams, whichtypically have wide and shallow anastomosing channels that form in the upper reaches ofstreams where slope is greater and where flowing water is often choked with moresediment than the fluid can carry. During the highpoint in a flood water escapes the mainchannel and creates a sheet flood of well-sorted sand or fine gravel with little or no silt orclay (midfan sheets are typically well-sorted, well stratified, and cross-bedded).At the intersection point on an alluvial fan the main channel shallows to thesurface of the existing fan causing sheet floods that form lobes of coarse boulders,cobbles, and sand called sieve deposits, due to their lack of fine grained material. Sievedeposits are coarser near the front of the lobe as fine material is winnowed away by watercoursing through the sieve.Alluvial fan deposits (sometimes known as fanglomerates) typically coarsenupwards as the fan spreads out from the mountain range. That is the grain size in a fangenerally decreases towards the valley. Surface of alluvial fans are covered withradiating systems of braided stream channels; each new flood cuts a new channel and fillsexisting channels with coarse gravels.4Stratigraphic recordThe formation of alluvial fans requires rapid uplift (rising fault scarps, forexample) in tectonically active areas. The sequence of coarsening upward cross-beddedsandstones, fanglomerates, and unsorted debris flow deposits occurs due to progradationof the fan out into the distal valley. Most alluvial fans are distant from the oceans, butsome spill into the shoreline producing fan deltas. These deposits are generally limited inlateral extent, but their thickness can be considerable (up to 1000s of m in some basins ifsubsidence is persistent). Sediments can be very immature and angular with abundantcoarse rock fragments and feldspars. Sheet flood deposits are typically oxidized, soredbeds are common. Fossils are not generally preserved in the coarse-grained facies ofan alluvial fan.What type of sandstone would be predicted to form in alluvial