Mapping the floor of Lake Mead (Nevada and Arizona): Preliminary discussion and GIS data release, USGS Open-File Report 03-320

SEISMIC FACIES

Figure 10. Seismic-reflection profiles collected normal to the Colorado River thalweg.

The seismic profiles show numerous reflectors in the post-impoundment sediment (Fig. 10). Many of the reflectors can be traced throughout the entire western part of the lake, but the strength of individual reflectors is variable (Twichell and others, 2002). On profiles collected normal to the thalweg, reflector strength varies laterally (Fig. 10 B, C, D). Piston cores penetrated the upper 3-5 m of the post-impoundment deposit and recovered numerous silt and very-fine sand beds in areas where reflector strength is strong. Cores contain clay and rare, thin, silt beds in areas where reflector strength is weak (Twichell and others, 2003). Clay layers separate the silty beds in the cores. Many of the reflectors coincide with the thin beds of sand or silt, but some reflectors may be caused by reverberation between closely spaced beds. Cores show that sediment in the coarser beds becomes finer to the west, and that the number and thickness of coarse beds decreases to the west. The westward decrease in reflector strength and grain size in the 3-5 m long cores suggest that reflector strength is controlled mainly by the amount of coarser sediment. Because of this relation, we used reflector strength as a proxy for mapping the distribution of fine sand, silt, and clay along the profiles.

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