Does Upper Cambrian Dunderberg Shale-Halfpint Carbonate Couplet in Southern Great Basin Qualify as Grand Cycle?

John D. Cooper

In the southern Great Basin, the Dunderberg Shale is the lowest of three members of the Nopah Formation. As a comparatively thin (10-100 + m) but distinctive interval throughout its wide extent, it provides an excellent regional stratigraphic marker within a thick carbonate succession. In this context, the shale represents a widespread shelfal tongue of the outer detrital belt that punctuates the middle carbonate belt. Carbonate interbeds in the Dunderberg decrease in thickness and abundance westward, but provide the index to a spectrum of homoclinal ramp paleodepositional environments ranging from supratidal to deep subtidal, and demonstrate that terrigenous mud was the intruder. Terrigenous mud-carbonate bundles record autocyclic changes in carbonate production as well s clearing shallowing upward cycles related to storm activity and perhaps short-term changes in relative sea level.

Carbonates of the overlying Halfpint Member show a similar spectrum and distribution of facies, but shale is replaced by texturally analogous thin-bedded, silty, peloidal micrite. The regional contact between the two members, which roughly parallels the Dunderbergia-Elvinia trilobite biozone boundary, does not represent a major environmental shift. Instead, the boundary represents the regional shutting down of terrigenous mud deposition, interpreted to be a synchronous event. The Dunderberg-Halfpint couplet, comprising shaly and carbonate half-cycles, would seem to be a logical candidate for a Cambrian grand cycle. However, the lack of a clearly demonstrable diachronous half-cycle boundary that does not record a significant change in depositional setting precludes this assignment. An nvironmental shift did occur, however, later during Halfpint deposition and is expressed by extensive progradation of a thrombolite boundstone facies.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.