Alluvial Fans and Braided Streams Warmup

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Badwater alluvial fan, Death Valley, California

Sediment deposited in such settings will be very immature, both compositionally and texturally, because of rapid derivation, short distance of transport, and rapid deposition. Arid climatic settings such as Death Valley also promote mechanical rather than chemical weathering, resulting in the preservation of unstable minerals.

Essay Question

In the stratigraphic record, how would you distinguish an alluvial fan deposit from a braided stream deposit? Ignore aspects of composition and focus on things like texture, sedimentary structure, geometry of the deposit, lateral associations of facies and vertical facies successions.

Multiple Choice Questions

Which of the following statements show(s) faulty geologic reasoning?

A facies model is intended to show all of the variations possible in sediments deposited in a particular environment.

Facies of adjacent depositional environments often interfinger, resulting in variability in the sediments that may not be accounted for by a single facies model

Contemporaneous facies which lie adjacent to one another can stack atop one another as environments shift position through time. Consequently, one can use vertical facies successions to interpret lateral arrangements of depositional environments, even if an unconformity is present in the vertical succession.

Process-response models are at the heart of interpretation of facies associations and depositional environments.

The term facies as originally used by Gressly in 1838 has not undergone an evolution of usage, and still carries the same intent that Gressly intended.

Which of the following statements are true?

Alluvial fan deposits in the stratigraphic record most often show effects of progradation and coarsening upward.

Braided streams deposits commonly have a large volume of mud present in overbank materials

In modern braided stream systems, there is a pattern to the distribution of bar types, such that longitudinal bars are dominant in upstream reaches, and transverse bars are dominant in downstream reaches.

Bar types in braided stream systems have little to do with the sedimentary textures and structures which are present in the deposits.

Debris flows, channelized flow, and sheet flow are all important in alluvial fan settings.