ABSTRACT: The Problem of Using Facies Patterns and Cycle Stacking Patterns to Infer Accommodation and Eustatic Sea Level Histories

BOND, GERARD, C., Lamont Doherty Geological Observatory, Palisades, NY, and MICHELLE A. KOMINZ, University of Texas at Austin, Austin, TX

Vertical changes in water depth reflect only local relative sea level changes, which are influenced by the balance between changes in accommodation space and sedimentation rates. Under certain conditions, no significant water depth change may occur even during a large eustatic cycle, or shoaling may occur during a eustatic rise and deepening may occur during a fall. Recognizing the correct relation between eustasy and vertical facies requires identifying the accommodation change and then determining if that change (not water depth change) correlates regionally.

Using a procedure we developed recently (R2 analyses), we find that in the Middle Cambrian Pierson Cove and Trippe formations, south-central Utah, water depth changes little during a large (third-order?) accommodation cycle. Here, the rate of sediment accumulation and the change in accommodation were essentially equal. In a second example, from the approximately correlative Arctomys and Waterfowl formations, southern Canadian Rockies, a similar-scale accommodation cycle records a water depth decrease as accommodation increases and a water depth increase as accommodation decreases. In this case, the sedimentation rates changed during the accommodation cycle. The eustatic origin of the accommodation cycles is implied by the correlation of the events in several stratigraphic sections in he Cordillera.

The forms of the R2 curves also underscore the problem of using meter-scale cycle stacking patterns to infer accommodation and sea level histories. Sacking patterns tend to identify only the next lower order of cyclicity, and, in the southern Canadian Rockies, the third-order event does not appear in the cycle stacking patterns.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)