SEQUENCE STRATIGRAPHY OF THE MIDDLE CAMBRIAN SUCCESSION IN EASTERN NEVADA AND WESTERN UTAH: EVIDENCE FOR THE INITIATION AND TERMINATION OF THE HOUSE RANGE EMBAYMENT

Restricted basinal deposits of the Middle Cambrian House Range Embayment (HRE) have been studied extensively in attempts to understand the environmental processes involved in Burgess Shale type fossil preservation, yet the initiation, termination, and depositional history of the fault-controlled HRE have remained unclear. An integrated sequence and chemostratigraphic study is aimed at testing the following hypotheses: (1) the flooding surface associated with down dropping and flooding of the HRE correlates with a surface of uplift and subaerial exposure on the adjacent platform; (2) both the HRE and the shallow-water platform record the same sea-level history but with different stratigraphic expressions; and (3) the exceptional fossil preservation within the HRE relates to differential redox conditions between HRE and adjacent platforms. To test these hypotheses several stratigraphic sections that spanned the HRE both spatially and temporally were studied. Results indicate that formation of the HRE was marked by an abrupt flooding surface within the HRE but was accompanied by shallowing of the adjacent southern platform where a significant subaerial exposure surface slightly post-dated the HRE initiation. This contradictory record, between the platform and HRE, is the result of movement along the HRE's basin-bounding normal fault. Four large-scale sequences can be correlated across the HRE, indicating that deposition across the Middle Cambrian shelf was controlled predominately by climate. Within the HRE, these sequences are expressed as shale-carbonate cycles that record the increase of fall-out carbonate particles from adjacent platforms during regression, while shallow-water platform sequences are separated by karstic unconformities indicating significant base-level fall. Closure of the HRE during the Middle Cambrian is marked by a thick carbonate unit within the HRE (Weeks Formation) that gradually changes to facies types similar to those of the platform. Final termination of the HRE is marked by a major transgressive surface that displays similar features across the entire carbonate shelf. Although still in progress, initial carbon isotopic data reveal a potential carbon isotope gradient across the HRE, consistent with anoxia and sulfate reduction in the restricted basin.