Can quantitative and qualitative deep marine depositional models be reconciled?: a case study of the sites member turbidite sequence, Cortina Formation (Upper Cretaceous), Cache Creek, California

Abstract

Two disparate sedimentological and stratigraphic models have been presented for the interpretation of the Upper Cretaceous, Sites Member of the Cortina Formation. This study addresses these discrepancies and proposes a unifying model. The first model is a qualitative-based interpretation of depositional environment by Ingersoll (1978), which focuses on analysis of process facies- and sedimentary body-scale attributes. The second model, presented by Murray et al. (1996), is statistically-based and exclusively focuses on sedimentation event-scale attributes. Ingersoll (1978) interpreted the Sites Member as a series of asymmetric, thickening- and coarsening-upward turbidite packages. However, Murray et al. (1996) interpreted the Sites Member to represent a turbidite sequence in which sedimentary event beds thin-upward as commonly as they thicken-upward. In this study the methods and results of Ingersoll (1978) and Murray et al. (1996) were reproduced and an integrated hierarchical stratigraphic framework is presented. One continuous sedimentological section, totaling 223.2 m, of the middle Sites Member was measured along the northern bank of Cache Creek, California, and was used to classify sedimentary process facies, sedimentation events, and sedimentary bodies. A three-fold hierarchical stratigraphic framework was constructed by correlating upsection changes in these sedimentary attributes. Low-order cyclicity, defined by a regional interpretation of depositional environment, shows that the middle Sites Member represents a component of an overall prograding submarine fan system. Intermediate-order cyclicity, defined by changes in subenvironment, shows that the middle Sites Member represents longitudinal oscillations within the unconfined outer fan environment. High-order cyclicity, defined by statistical analysis, is representative of autogenic compensational lobe stacking. If any of these scales of cyclicity are interrogated in isolation, a unique and misrepresentative interpretation is likely. These three orders of cyclicity combine to form an integrated hierarchical stratigraphic framework that resolves the interpretive discrepancies between Ingersoll (1978) and Murray et al. (1996). Additionally, it was determined that at the event bed-scale, multi-scale and coincident, allogenic and autogenic mechanisms are responsible for hierarchical nested cyclicity. Therefore, event bed thickness is an unreliable criterion for interpretation of profile position and for cycle definition. Instead, grain size is a more consistent proxy for profile position and changes in system energy.