The two main groups 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 (image above). Deposition as bedload was probably generated by stream flood.
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 largish braided river. Field of view across the coastline is about 3 km.
Several, small alluvial fans merge 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).
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.
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, 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 fans merging with gypsum-halite salars, Atacama, northern Chile. Most fans fringe Eocene and younger volcanic cones.
Looking down-slope along inactive parts of fans. Left image shows levees of cobbles and boulders deposited by a debris flow. Atacama, northern Chile. Eocene and younger volcanic edifices in the distance.
Alluvial fan lobes encroaching a gypsum-halite salar, Atacama, northern Chile. Right image shows an elevated fan (left centre) that represents deposition during a phase of higher lake levels; the older fan is now partly degraded. The cuspate and indented distal fan margin record a succession of fan lobes. Grey-brown colours reflect basalt-andesite clast compositions, enhanced by desert varnish.
Cross section through 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. Atacama, northern Chile.
Two distinct debris flow units, separated by thin bedload-deposited sands, or possibly a thin hyperconcentrated flow. Arid alluvial fan, Atacama, 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, Atacama, 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. From a more distal part of an arid alluvial fan, Atacama, 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.
