Panamint Mountains and alluvial fans interfingering with the Death Valley salt lake.
The two main types of alluvial fans illustrated here are from humid and arid environments. Arid climate fans I have visited or worked on are from Death Valley in eastern California (part of Mojave Desert in the Basin and Range geological province, and the mountains of Atacama, northern Chile. The Atacama examples are about 4000m above sea level. The Death Valley photos were taken in 1996 during an SEPM Research Symposium.
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The Images
Headwaters of active alluvial fans in Tertiary Hills, Northwest Territories. Bedrock here is Paleocene fluvial conglomerate, sand and sub-bituminous coal, that is being recycled by modern alluvial-fluvial drainage. Clast-size range in the fans is determined by the clast sizes in the eroding conglomerates.This small (humid) alluvial fan drains into Peel River, east Yukon. The fan’s outer edge dips its toes in Peel River. Inactive segments of the fan are incised by the river, but active segments provide new gravel, sand and mud to active river side bars (river flow is to the top). Recently active fan channels and flooded swaths are mainly in the central part of the fan, having migrated from the fan edge farthest from the viewer.Incision of a gravel-sand flow unit on the alluvial fan that merges with Peel River (see image above). Deposition as bedload was probably generated by stream flood. Crude imbrication is preserved on the upper surface indicates flow to the right.Part of a large, coastal (humid-cold) alluvial fan complex along the north coast of Yukon (west of MacKenzie River delta). The active channel at this time was itself a braided river. Field of view across the coastline is about 3 km. The coastal segment (top) is characterized by sand and gravel reworked by waves and sea ice.Several, small alluvial fans merge at high angles with the braided stream that drains into the south end of Canon Fiord, Ellesmere Island. Potential paleoflow directions in the fans would be oriented about 90 degrees to indicators in the braided river. This is an arid setting, with most flow during spring and early summer thaw. Incision of a Late Pleistocene (very humid) alluvial fan at Franz Joseph, New Zealand. The gravels are very coarse; boulders up to 3m across. The sediment source is in the immediate background – the western edge of the Southern Alps (here, mostly greenschist). The downstream margin of the fan merges with the coast.Thick, poorly bedded debris flows in Middle Eocene alluvial fans that accumulated outboard of rising thrust belt during the Eurekan Orogeny, Axel Heiberg Island (Arctic Canada). Source rocks consist of various Triassic and Jurassic sandstones and diabase. Bedding dips to the right. These deposits (Buchanan Lake Formation) occur outboard of Stolz Thrust.Death Valley from Dante’s View, looking east towards the Panamint Range (a block-faulted and uplifted metamorphic core complex). Salt flats in mid-view (mostly halite, some gypsum and borax), and a nice succession of (arid) alluvial fans that interfinger with the saline facies. This is one of the classic Basin and Range couplings between fault blocks and intervening basin.Death Valley, looking north from Dante’s View – the fault block here lies immediately east of Panamint Range. Alluvial fans merge with the salt flats. The dimly visible whitish area in the distance is Mesquite Flat sand dunes, near Stovepipe Wells.The view east of Dante’s View, to successive Basins and Ranges.Excellent exposure of Hanauphan fan, Death Valley. There are dozens of debris flow and sheet flood events recorded in this outcrop.Stacked debris flow and sheet flood conglomerates in Natural Bridge fan, Death Valley. The red colour of most sediments here is another testament to the arid environment.Crudely layered debris flow conglomerates in Natural Bridge fan, Death Valley. Most flows developed during flash floods. Person’s elbow for scale, bottom right.Finer grained flow units, Natural Bridge fan, Death Valley. Some of these may have been deposited by hyperconcentrated flows – sand-gravel-mud-water mixtures that have a rheology somewhere between water-bedload deposition and debris flows. A more recent example is shown in the image below.Section through a recent flash flood, hyperconcentrated flow, Death Valley. Texturally, the flow resembles a muddy debris flow; poorly sorted, mud-support of clasts, but the range of clast-sizes is much smaller.Arid alluvial fan distal margin merging with gypsum-halite salar, Alti Plano, northern Chile. Most fans are fed by seasonal erosion of Eocene and younger volcanic cones during spring thaw or flash floods.View of an alluvial fan, approximately along strike. Eocene and younger volcanic cones in the background were part of the caldera system that gave rise to the salar basins. Alti Plano, northern Chile.View down-slope along an inactive parts of a fans. Levees of cobbles and boulders indicate a more recent debris flow. Altiplano, northern Chile. The debris flow probably extended into the salar. Eocene and younger volcanic edifices in the distance.Alluvial fan lobes encroaching a gypsum-halite salar, Altiplano, northern Chile. The cuspate and indented shoreline record a succession of fan lobes. Grey-brown colours reflect basalt-andesite clast compositions, enhanced by desert varnish. The fans are fed by Eocene and younger volcanics.Alluvial fan lobes encroaching a gypsum-halite salar, Altiplano, northern Chile. The elevated fan (left of the access trail) records deposition during a phase of higher lake-salar levels; the older fan is now partly degraded. The cuspate and indented distal fan margin record a succession of fan lobes interfingering with the saline gypsum-halite deposits. Cross section approximately along depositional strike through an inactive fan segment, showing multiple debris flow and sheet flood layers. The outcrop is about 2m high. This is a more distal part of the alluvial fan, and clast sizes are usually less than 15cm. Altiplano, northern Chile.Two distinct debris flow units, separated by thin bedload-deposited sands, or possibly a thin hyperconcentrated flow. Arid alluvial fan, Altiplano, northern Chile. The outcrop sheen is caused by films of halite and gypsum from the nearby salar.Well-bedded ephemeral stream deposits, with a few bedload ripples, some clast imbrication, and scour-and-fill structures around large clasts. Distal, arid alluvial fan, Altiplano, northern Chile.Well bedded-laminated sheet flood sands are probably traction deposits (a few bedload ripples). The intervening poorly-sorted, pebbly sands are hyperconcentrated flow deposits. Section from a distal part of an arid alluvial fan, Altiplano, northern Chile.Desert varnish affects most clasts in Atacama, northern Chile alluvial fans. Darker varnish hues generally indicate older deposits, and longer surface exposure. The relative ages of successive or overlapping fan lobes can often be determined from varnish colour mapping.Large gypsum books on an alluvial fan surface, close to its contact with a salar. The crystals average 5-6cm across. Atacama, northern Chile.
