High-Frequency Depositional Sequences and Stratal Stacking Patterns in Lower Pliocene Coastal Deltas, Mid-Norwegian Continental Shelf

Abstract

Extensive deltaic and coastal progradation occurred along the mid-Norwegian continental shelf during the early Pliocene. Thirty-eight well-developed, high-frequency (fourth-order) sequences are identified within the deltaic complex on multifold seismic data. The fourth-order sequences are arranged in four oblique progradational and two sigmoid progradational sequence sets. Deposition of the high-frequency sequences and their stacking patterns probably were in response to high-frequency cycles of relative changes in sea level cycles produced by variable rates of subsidence and uplift, superimposed on high-frequency eustatic cycles within a lower frequency eustatic system. The mixed aggrading/prograding sequence sets are interpreted to represent increased space-added accommodation rates and deposition within third-order highstand systems tracts. Conversely, the progradational sequence sets are interpreted to represent decreasing space-added accommodation rates and deposition within the third-order lowstand systems tracts. The recognition of multiple sequence sets likely reflects the effect of long-term relative fall in sea level (tectonic uplift?) superim osed on high-frequency eustatic cycles. The terminal sequence boundaries of the sequence sets are formed by conjunction of third- and fourth-order falling inflection points on the relative sea level curve. These sequence boundaries represent the timing of maximum subaerial erosion and may represent times of greatest potential for the deposition of reservoir sands.