Arizona Geological Survey OPEN-FILE REPORT OFR 08-06

Arizona Geological Survey
Cosmogenic Dating The use of isotopic extraction and measurement techniques to estimate
the age of particles at the Earth’s surface from the natural rate of accumulation of certain
isotopes generated through spallation processes caused by cosmic‐ray bombardment.
Cosmogenic dating directly dates particles in a geologic deposit, but uncertainties may arise
from prior exposure to cosmic radiation (i.e., the particle moved from a position on the Earth’s
surface to a different exposed position) or through erosion. Cosmogenic 10 Be dating,
summarized here, theoretically can be used to date deposits millions of years old but in practice
is limited by the erosion rate of particles and their deposits.
Debris Flow A two‐phase fluid of less than 40% water and more than 60% sediment, generally
very poorly sorted, that typically results from slope failures in the desert Southwest. Debris
flows are considered to be non‐Newtonian flows that are dominated by particle‐particle
friction.
Debris‐Flow Deposit Typically, a poorly sorted deposit of particles along a watercourse.
Debris‐flow deposits can be difficult to distinguish from streamflow deposits or colluvium, but
several characteristics can be definitive: (1) the presence of poorly‐sorted sediment with a
surface subparallel to a channel, (2) the general lack of strongly imbricated particles in the
deposit, (3) the presence of boulder trains extending upstream from an obstruction, (4) large
particles (e.g., boulders) resting on a bed of fine‐grained sediment and not in contact with other
large particles within a deposit, (5) the general lack of stratification in deposits. Many of these
characteristics may also be present in hyperconcentrated deposits.
Holocene
time.
The Holocene generally is considered to be the last 11,000 years of geologic
Hyperconcentrated Flow Deposits These deposits, while recognizable from certain
stratigraphic characteristics, result from only vaguely known sediment transport processes that
may result from pulses of debris flow and (or) runout facies downstream of debris‐flow
deposition, or they may result from flash‐flood sediment transport unrelated to debris flows.
Sediment concentration in hyperconcentrated flow generally is in the 40‐60% range by weight.
When associated with debris flows, hyperconcentrated flow deposits have a characteristic
arrangement of stratigraphic layers, which can be relatively thin, that show alternating
deposition of poorly sorted sediment (but with smaller particles) with relatively well sorted
sand or gravel lenses.
Imbrication Three‐dimensional arrangement of pebbles, cobbles and boulders in a
sedimentary deposit that indicates either streamflow deposition or reworking by streamflow.
Imbricated deposits may appear to be flat‐lying and overlapping as if tiled. Gravel particles dip
upstream, opposite to the flow direction.
Landslide This term is somewhat ambiguous when applied to desert environments. In
more humid environments, a landslide is a deep‐seated slope failure of large spatial extent that
typically moves along a saturated failure plane fed by ground water; the overlying sediments
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