U. S. Geological Survey, United States

Processes that build continental-margin stratigraphy on time-scales of >20 kyr have been investigated. Eustatic sea-level exerts a major influence on sedimentation, but the Eel River margin shows that its effects can be interwoven with those of tectonism. Rapid Oligocene subsidence along the Cascadia subduction zone resulted in a foundered forearc basin. Regression and sedimentary reconstruction began in the Pliocene, and up to 1 km of sediment has accumulated since then, with rotating faults, synclines, anticlines and regional uplifts marking plate interactions. Fourteen seismic unconformities along structural highs can be traced into synclines. Many are ravinements formed during rising sea level, and approximately 70-100 kyr cyclicity suggests a glacio-eustatic signal. Incised channels formed during regressions over the past approximately 360 kyr, when rivers drained into Eel Canyon. In contrast, the New Jersey margin has long been dormant tectonically, providing clearer access to a eustatic imprint. Lack of Paleogene sediment supply resulted in a carbonate ramp prior to development of Oligocene deltas. With little accommodation space to allow aggradation, clino-forms prograded approximately 100 km seaward, reaching the shelf break by Late Pleistocene. Coastal-plain drilling recovered approximately 15 Oligocene and Miocene highstand deposits, which correlate with glacio-eustatic oscillations. Beneath the mid-to-outer shelf, incised valleys have been preserved, and clinoform strata suggest reworking of lobate deposits. Four Late Pleistocene sequences reveal no hiatuses at sequence boundaries, and no correlations between glacio-eustatic oscillations and stratal architecture. Stratal discontinuities are a common feature in margin sediments and provide objective means of interpreting the geological record. Continuous coring is essential to understand the processes that create stratal architecture.