Abstract

The Rio Dell Formation was deposited in a tectonically active area during a time of fluctuating eustatic sea-level. Detailed interpretation of Rio Dell shelf sediments, facilitated by comparison to the nearby, analogous, modern Eel River shelf, has proved a vital tool for decoding the record of these influences. Understanding of the nature and relative roles of processes on the modern and ancient shelf, including those associated with storms and with river floods, not only allows for detailed paleobathymetric reconstruction of the ancient sequence, but also for recognition of relatively subtle effects of syndepositional tectonism.

Shelf sediments of the Rio Dell Formation comprise four depositional sequences bounded by thin, submarine condensed sections. The condensed sections formed during periods of rapid, probably largely eustatic, sea-level rise. The sequences each record progradation of the shelf, predominantly during sea-level highstands. Paleobathymetric reconstruction provides the basis for interpretation of the magnitude of local, relative sea-level changes within and between the sequences.

Subsidence of the Rio Dell shelf apparently played a role in recorded local sea-level fluctuations, as did eustasy. Subsidence ("geohistory") analysis of the sequence indicates that the shelf was subsiding throughout its existence, due not only to sediment loading but also due to tectonism. The analysis points to a period of rapid shelf subsidence, accompanied by onland uplift, between about 1.3 and 1.2 Ma.

An increase in subsidence rates after about 1.3 Ma apparently caused an increase in Rio Dell shelf gradient, which changed the balance between flood and storm processes on the middle shelf. Increased shelf inclination evidently resulted in restriction of storm-wave reworking to a narrower zone extending from the shoreline, whereas distance from shoreline to the shelf area most influenced by river-flood deposition remained unchanged. The resultant increased segregation of flood and storm effects caused a fining of background grain-size distributions at mid-shelf sites.

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