HOW SIGNIFICANT IS THE UNCONFORMITY AT THE BASE OF THE SAUK SEQUENCE IN AN OFF-CRATON SETTING?

As originally defined, the Sauk Sequence is a cratonic succession of rocks forming the base of the continental-scale Sloss Sequences. In cratonic settings the base of the Sauk is easily recognized as a major unconformity separating Cambrian strata from much older Precambrian rocks. But what is the nature of this contact in more offshore settings where Sauk rocks overlie strata that are relatively conformable?

In the Death Valley area, it is possible to examine in detail this contact from cratonic to miogeoclinal settings utilizing well-studied stratigraphic units. The unit that defines the base of the Sauk Sequence on the craton is a terrestrial, cross-bedded arenite named the middle member of the Wood Canyon Formation (mmWCF), where it rests nonconformably above Proterozoic granites and gneisses. Measured sections show that the mmWCF extends basinward in the same stratigraphic sequence and overlies mixed marine-shelf shales and carbonates of the lower member Wood Canyon Formation (lmWCF).

Previous studies suggested that the three distinct m-thick carbonates forming part of the lmWCF were successively eroded out from the top downward in a cratonward direction from three, to two, to one, and then to zero as the unconformity that defines the base of the mmWCF (and thus the Sauk Sequence) merges onto the craton. Erosion continues entirely through the lmWCF so that in places the mmWCF rests directly on the Stirling Quartzite. Such an expression places demonstrable significance on this as a major erosional surface. However, new preliminary detailed stratigraphic examination of the lmWCF suggests that much of the erosional cut out in the lower member is entirely intra-unit. In one location, the top two carbonate layers are stacked on top of each other omitting a thick section of shale between them and in another location the lmWCF appears to be conformable with the mmWCF, suggesting little erosional significance in a position where it should be dramatic. A distinct chert pebble conglomerate that defines the base of mmWCF in some locations is found at different stratigraphic levels elsewhere, casting doubt on its presence as more than locally important. These features suggest the base of the Sauk Sequence may not be interregionally recognizable in miogeoclinal sections, and in fact may represent a thick zone of distributed erosion.