Sedimentary Processes and Geophysical Imaging

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Sedimentary Processes and Geophysical Imaging
Sediment-sequence profiling
Seismic-reflection methods can be used to produce high-resolution, acoustic cross
sections of sedimentary sequences deposited in lakes. The distribution, sequence,
character, and configuration of each unit can then be used to reconstruct many things
about the history of the lake. Deposition features related to lake level, such as beaches
and deltas (see first figure), are especially useful for deciphering hydrological history,
and we have used them extensively in the East African Rift Lakes and other lakes that
experience large fluctuations in level. Changes in lake level can also be inferred from
features such as erosional unconformities and onlap-offlap of strata. Rhythmic vertical
changes in the internal character of seismic sequence can also be related to cyclical
climate changes.
Figure: Submerged and buried Holocene delta in Lake Titicaca. Uppermost of several is
highlighted.
Large scale sedimentation patterns in lakes, such as the transition from delta front to
subaqueous fan to abyssal turbidite basins in Lake Baikal, can be derived from seismicreflection profiles. Such sedimentation pattern yield a variety of information about the
relationship between sedimentation and deformation in tectonically active lake basins. In
many cases, actual fault-displacement history can be observed in tectonic lake basins.
Figure: Fault displaced lake floor and uppermost sediments in Bear Lake, Utah-Idaho.
As mountain glaciers or continental ice sheets advance into and retreat from lake basins,
they leave behind characteristic assemblages of sediments. These sediments are often
preserved in lake basins and can be imaged using seismic-reflection methods. The
sequence and distribution of such deposits provide a great deal of information on the
glacial history of a region.
Figure: Moraine, outwash, and varved glacial-lacustrine deposits in Lake Superior
References
Scholz, C.A., Johnson, T.C., Cohen, A.S., et al., 2007, East African megadroughts between 135 and 75
thousand years ago and bearing on early-modern human origins: Proceedings of the National
Academy of Sciences, v. 104, p. 16416-16421.
Colman, S.M., 2006, Acoustic stratigraphy of Bear Lake, Utah-Idaho-Late Quaternary sedimentation
patterns in a simple half-graben: Sedimentary Geology, v. 185, p. 113-125.
Colman, S.M., Karabanov, E.B., and Nelson, C.H., 2003, Quaternary sedimentation and subsidence
history of Lake Baikal, Siberia, based on seismic stratigraphy and coring: Journal of Sedimentary
Research, v. 73, p. 941-956.
Colman, S.M., Kelts, K., and Dinter, D., 2002, Depositional history and neotectonics in Great Salt Lake,
Utah, from high-resolution seismic stratigraphy: Sedimentary Geology, v. 148, p. 61-78.
Johnson, T.C., Wells, J.D., and Scholz, C.A., 1995, Deltaic sedimentation in a modern rift lake:
Geological Society of America Bulletin, v. 107, p. 812-829.
Johnson, T.C., Halfman, J.D., Rosendahl, B.R., and Lister, G.S., 1987, Climatic and tectonic effects on
sedimentation in a rift-valley lake--Evidence from high-resolution seismic profiles, Lake
Turkana, Kenya: Geological Society of America Bulletin, v. 98, p. 439-447.
Landmesser, C.W., Johnson, T.C., and Wold, R.S., 1982, Seismic reflection study of recessional moraines
beneath Lake Superior and their relationship to regional deglaciation: Quaternary Research, v. 17,
p. 173-190.
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