lenses in the northwestern Spring Mountains indicates intertidal conditions, and may suggest intermittent marine incursions. The lithologic similarity between conglomerates within the Conglomeratic member and fill within the Caddy Canyon valleys suggests that both deposits were formed by the same fluvial processes.
Other lithologies in the Conglomeratic member also fit into a valley-fill context. The micritic limestone at the base of the Conglomeratic member in the Panamint Range may have formed during rapid sea-level rise after valley incision. This micrite could be a cap carbonate like the carbonates observed above many glacial deposits around the world (Kennedy, 1996 and references therein). The micrite may have precipitated within valleys at other locations as well, but was removed by subsequent fluvial erosion. The diamictite in the Panamint Range is interpreted as a mass wasting deposit which formed through the collapse of a valley wall. A similar origin has been suggested for diamictite within valley- fill in the Neoproterozoic Wonoka Formation of Australia (Christie-Blick and others,
1995).
Erosional truncation of the Rainstorm member and sea level fall Structural complexities preclude the direct observation of large-scale erosional truncation at the base of the Conglomeratic member, but the observed lateral transitions strongly suggest erosional truncation. In the Resting Spring Range, the short distance spanned by the transition suggests erosional truncation of the Rainstorm member and in- filling by the Conglomeratic member rather than a lateral facies change. As suggested by Figure 2-6, erosional truncation in the Resting Spring Range could be as much as 150 m.
Since the valleys incise shallow marine deposits and the valley-fill contains fluvial or fluvially-dominated marine deposits, the valleys were probably formed by sub-aerial rather than submarine erosion. Consequently, the amount of erosional truncation in the Resting Spring Range shows that sea level fell by at least 150 m.
2-27 DISCUSSION
Correlation of the Caddy Canyon and Johnnie valleys
The singularly large size of the Caddy Canyon and Johnnie valleys is the best argument for their correlation. The Caddy Canyon valleys and the Johnnie valleys are larger than any other incised valleys within the post-diamictite Neoproterozoic section in the western
United States. In Idaho and Utah, the next largest incised valley is 60 m deep and occurs at the base of the Geertsen Canyon Quartzite (Levy and others, 1994). The base Geertsen Canyon incision has only been observed at one location in the Portneuf Range, and may be the result of local tectonic activity. Smaller incised valleys are present at multiple horizons within the Caddy Canyon Quartzite and Inkom Formation, but do not correlate between ranges in Idaho and Utah (Levy and others, 1994).
In southern Nevada and southeastern California, no incised valley elsewhere in the section approaches the size of the valley in the Resting Spring Range. Prominent
unconformities occur at the base of the Johnnie Formation (Summa, 1993) and the base of the middle Wood Canyon Formation (Stewart, 1970; Runnegar and others, 1995), but neither unconformity is associated with substantial erosional relief. The Johnnie Formation contains several other sub-aerial unconformities (Summa, 1993), but none of these are associated with substantial erosional relief. When comparing the Neoproterozoic section in Idaho and Utah with the one in southern Nevada and southeastern California, the Johnnie valleys stand out as the only likely correlative of the Caddy Canyon valleys.
Glacioeustatic vs. tectonic origin
of the Johnnie and Caddy Canyon valleys
Incision and filling of the Johnnie valleys requires a substantial drop in relative sea level followed by a substantial sea level rise. This fluctuation in sea level is most easily
explained by a glacioeustatic mechanism in which the formation and melting of glaciers caused a dramatic sea level fluctuation over a relatively short time period. Sea level
oscillations are associated with many glacial deposits including both diamictites in the Kingston Peak Formation (Miller, 1985). Figure 2-9 depicts the regional extent of incised valleys in Idaho, Utah, southern Nevada, and southeastern California. If the Caddy Canyon valleys and Johnnie valleys are time correlative, base level changed along 700 km of the North American margin. A eustatic drop in sea level is the simplest explanation for a change in base level along this great length of the North American margin.
A regional tectonic event could have caused valley incision (Levy and others, 1994 ), but the Johnnie and Caddy Canyon Formations lack direct evidence for faulting. Structural and stratigraphic evidence for Neoproterozoic tectonism is largely confined to the lower Kingston Peak Formation (Walker and others, 1986) and the Perry Canyon Formation (Link, 1983). Evidence for younger tectonism is interpretive. Major thermal subsidence along the western U.S. margin began during the latest Neoproterozoic or early Cambrian (Levy and Christie-Blick, 1991), and may suggest a rifting event at that time. If subtle tectonism occurred during the intervening interval, it is entirely concealed by younger deformation and lack of exposure.
Carbon isotopes in the Noonday Dolomite and Johnnie Formation and Glaciation
As shown in Figure 2-10, carbon isotope values of up to +2.89 per mil occur in the upper four-fifths of the Johnnie Formation in the Nopah Range. These positive values are similar to isotope values which precede major Neoproterozoic glaciations. The positive carbon values are thought to reflect high rates of burial of organic carbon relative to total carbon (Kump, 1991). Burial of large amounts of organic carbon may have reduced atmospheric greenhouse capacity, and, in con junction with 5-10% lower solar luminosity, lead to major glaciations (Hoffman and others, 1998).
The upper four-fifths of the Johnnie Formation also contains two clusters of negative carbon isotope values. Negative values within the uppermost Johnnie Formation and
2-29
Figure 2-9. Ranges containing incised valleys in the uppermost Caddy Canyon Formation (Idaho and Utah) and the uppermost Johnnie Formation (southern Nevada and southeastern California). Geographic distribution of incised valleys in the uppermost Caddy Canyon Formation from Levy and others (1994).