SEISMITES AND EVIDENCE FOR SYN-DEPOSITIONAL EXTENSIONAL TECTONISM FROM THE NEOPROTEROZOIC BECK SPRING DOLOMITE (PAHRUMP GROUP), SOUTHEASTERN DEATH VALLEY, CALIFORNIA

Seismically induced soft sediment deformation structures are present in 4 horizons in two sections of the Beck Spring Dolomite (< ca. 750Ma) in eastern California. One layer, 1.5m in thickness, is described near the faulted-out top of a section in the northern Saddle Peak Hills (103m above base of the Beck Spring Dolomite) and 3 horizons, 10-40cm in thickness, were observed distributed in the lower and middle members of a section in Ashford Canyon in the southern Black Mountains. Numerous types of structures were observed including fluid-escape features, disrupted bedding, pillow structures, cm-scale faults and disharmonic folds. Deformation features are limited to confined horizons, above and below which deformation is not present.

A series of small (~40m) normal faults are present at the contact of the lower Beck Spring Dolomite-Upper Crystal Spring Formation in the southern Ibex Hills. These normal faults appear to terminate into the base of the Beck Spring and overlying strata are draped and thicken into a small graben formed between normal faults.

Detailed stratigraphic analysis of a siliciclastic-rich section of the Beck Spring Dolomite in the southern Ibex Hills also provides evidence for multiple (>50) rapid subsidence events on a meter scale. Numerous cycles present in this section, and variably present elsewhere throughout the basin exhibit rapid influx of course grained siliciclastic sediments, sharply above dolomitic sediment. This combination of seismites, syn-depositional normal faults and meter-scale rapid subsidence suggest seismically active middle Pahrump Group basin (ca. 750-700Ma), prior to terminal rifting of the western margin of Laurentia during the middle-upper Kingston Peak Formation depositional time (ca. 700-640Ma).