Reconnaissance geology and geologic hazards of the offshore Coos Bay basin, Oregon

Abstract

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 g~ accumulations cover areas of as much as 12.0 km , and combined geophysical and ':II

Systematic characterization of morphotectonic variability along the Cascadia convergent margin: Implications for shallow megathrust behavior and tsunami hazards

“…The uplift associated with sea mount subduction may be partly responsible for the increase in outer wedge gradient within subregion 3b. Deformation within the inner wedge of region 3 is dominated by eastnortheast-directed compres sion and dextral translation as suggested by the orientation of structures mapped on the continental shelf (Clarke et al, 1985;Snavely, 1987;Goldfinger et al, 1992;McNeill et al, 2000).…”

Distribution of Methane Plumes on Cascadia Margin and Implications for the Landward Limit of Methane Hydrate Stability

“…Johnson et al (2019) hypothesized that the shallow peak on the outer shelf of Washington, in particular the emission fields parallel to the shelf edge near the submarine canyon heads off Washington (Figure 2), is due to listric faulting and diapirism within a period of intra-seismic extension. There is also evidence for extensive diapirism and listric faulting at shelf and slope depths for portions of the northern Oregon (this study and Snavely, 1987) and southern Oregon margin (Clarke et al, 1985), although existing data are too sparse to generalize to the entire USCM (Figures 1-5). The FEMHS…”

“…The bubble streams are sited near/ on a truncated 1 km diameter cone rising 80 m from a base at 500 m water depth. There is a diapir-like structure beneath this feature (westernmost side of Figure 10 of Clarke et al, 1985), and this morphology is interpreted as a constructional feature built by fluidized, gas-rich, over-pressured sediment flowing from a central vent (Brown, 1990;Dimitrov, 2002). An ROV dive (H1674) in 2018 (Figure 4B) identified extensive carbonate deposits and seeps on the flank and summit (Baumberger et al, 2019).…”

“…An ROV dive (H1674) in 2018 (Figure 4B) identified extensive carbonate deposits and seeps on the flank and summit (Baumberger et al, 2019). A cluster of bubble emission sites is also located on a previously active diapir located 12 km SW of the mud volcano site at 480-750 m depth (Clarke et al, 1985).…”

Marine Geohazards: A Bibliometric-Based Review

“…However, during the 1990s, the number of articles that involved the seafloor surface and subsurface mapping were predominant, and there was ample use of acoustic methods for multi-beam bathymetry and high seismic resolution, see Figure 15. Geological sampling and geotechnical measurements were rare [46,47,50,51]. During the second and third phases, this scenario evolved in the sense that geological sampling, geotechnical measurements, and reviews became more common.…”