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.