SEQUENCE STRATIGRAPHY OF THE LOWER-MIDDLE CAMBRIAN BOUNDARY INTERVAL, SOUTHERN GREAT BASIN

Historically, the mixed siliciclastic and carbonate rocks of the Lower-Middle Cambrian boundary interval have not been correlated with precision because of a lack of faunal control and the presumed absence of high-resolution chronostratigraphic units. A several-year study of the boundary interval along a nearshore to offshore transect in the southern Great Basin resulted in the recognition of key surfaces which were critical to the application of sequence stratigraphic concepts. Maximum sea level rises appear to coincide with the extinction of endemic trilobite species and the reduction or absence of an infauna within a shale facies, coupled with the introduction of cosmopolitan trilobites and Burgess Shale faunas into shelfal environments.

Sequence boundaries were expressed as subtle disconformities, although a wide range of criteria needed to be utilized. Karst surfaces and pebble beds were the exception, not the rule, and many of the subtle disconformities were based on a sharp lithologic contact just above a low-angle truncation of the underlying highstand systems tract.

We recognized four relatively thin stratigraphic sequences within the Lower-Middle Cambrian boundary interval of the southern Great Basin. Each of these sequences has its own facies characteristics, and thus illustrates that a wide range of criteria must be used in the evaluation of sequences, even within a passive margin environment.