The offshore Coos Bay basin underlies the continental shelf and upper slope between Heceta Bank and Coquille Bank, Oregon, and comprises a succession of middle Eocene and younger principally marine sedimentary rocks 5,000+ m thick. Three acoustic units have been defined within this basin on the basis of seismic-reflection and sampling data: acoustic unit 1, inferred to be Quaternary in age and to be a maximum of about 500 m thick, is widespread on the continental shelf and slope; acoustic unit 2., inferred to be late Miocene to late Pliocene in age, is exposed on Heceta Bank, on the inner shelf between Cape Blanco and Cape Arago, and, locally, in breached anticlines; and acoustic unit 3, inferred to comprise strata of Eocene to middle Miocene age, is exposed locally in breached anticlines on the inner and middle continental shelf and on the middle of the continental slope. Recurrent major episodes of deformation are evidenced by regional unconformities of late Pliocene to early Pleistocene, early late Miocene, and middle to late Eocene age; these unconformities probably reflect significant changes in the history of plate convergence along this margin.
Potential geologic hazards to offshore development include groups of short north- to northwesttrending faults that cut Quaternary (locally Holocene) deposits on the continental shelf between Cape Arago and Heceta Bank. Seismic activity appears to be low within the offshore basin area; however, a significant possibility exists of damage from tsunamis, sea-floor mass movement, and liquefaction resulting from earthquakes in adjacent regions, notably the northern California continental margin. Unstable sea-floor conditions resulting from recent subaqueous slides and flows appear to be uncommon, although the presence of locally steep slopes and thick accumulations of unconsolidated sediment with unknown engineering characteristics make site-specific studies of sea-flool stability advisable. Buried zones as large as 190 km in area and 2.00 to 2.50 m thick of disrupted and rotated acoustic reflectors appear to record episodes of seafloor failure during Pleistocene low stands of sea level. Several folds showing sea-floor relief, one of which appears to be diapiric, reflect tectonic instability of the modern upper continental slope along the west margin of the basin. Acoustic anomalies suggestive of shallow gas accumulations cover areas of as much as 12.0 km, and combined geophysical and geochemical evidence indicate that gas is being vented at the sea floor in two localities. At one locality, 2.5 km west of Cape Arago, samples contained high methane concentrations and equivocal indications of thermogenic hydrocarbon gas. Reduction of the bearing strength of the enclosing sediment by such gas accumulations enhances the possibility of failure; if these accumulations are of thermogenic origin, they may reflect an overpressured zone at depth.