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Geological Society of America Bulletin Downloaded from gsabulletin.gsapubs.org on 10 April 2009 Geological Society of America Bulletin Tectonics of the Neogene Cascadia forearc basin: Investigations of a deformed late Miocene unconformity Lisa C. McNeill, Chris Goldfinger, LaVerne D. Kulm and Robert S. Yeats Geological Society of America Bulletin 2000;112;1209-1224 doi:10.1130/0016-7606(2000)112<1209:TOTNCF>2.0.CO;2 E-mail alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/index.ac.dtl to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes © 2000 Geological Society of America Tectonics of the Neogene Cascadia forearc basin: Investigations of a deformed late Miocene unconformity Lisa C. McNeill* School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Chris Goldfinger College of Oceanic and Atmospheric Sciences, Oregon State University, LaVerne D. Kulm} Corvallis, Oregon 97331, USA Robert S. Yeats Department of Geosciences, Wilkinson Hall, and College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA ABSTRACT sediments. This event may have caused a Cascadia margin. The unconformity is commonly change in sediment provenance on the abyssal angular and therefore easily identifiable in this re- The continental shelf and upper slope of the plain ca. 1.3–1.4 Ma. gion, with stratigraphic control from exploration Oregon Cascadia margin are underlain by an wells and seafloor samples. A younger regional elongate late Cenozoic forearc basin, correla- Keywords: accretionary wedges, Cascadia unconformity, probable latest Pliocene or early tive to the Eel River basin of northern Cali- subduction zone, forearc basins, neotectonics, Pleistocene age, is also recognized on the Oregon fornia. Basin stratigraphy includes a regional submarine fans, unconformity. shelf, but it is less continuous than the late late Miocene unconformity that may coincide Miocene unconformity. with a worldwide hiatus ca. 7.5–6 Ma (NH6). INTRODUCTION In this paper we discuss the age, origin, and The unconformity is angular and probably deformation of the late Miocene unconformity subaerially eroded on the inner and middle The Cascadia continental shelf is underlain by and incorporate this information with the stratig- shelf, whereas the seaward correlative discon- a thick sedimentary sequence extending from the raphy of the forearc basin, adjacent accretionary formity may have been produced by submarine Eel River basin in the south to offshore Vancouver wedge, and abyssal plain to document the Neo- erosion; alternatively, this horizon may be Island in the north. This forearc basin was proba- gene evolution of the Cascadia margin. Of partic- conformable. Tectonic uplift resulting in sub- bly continuous in the early to middle Eocene ular interest are the tectonic and sedimentary in- aerial erosion may have been driven by a (Niem et al., in Christiansen and Yeats, 1992) but teractions between the accretionary wedge and change in Pacific and Juan de Fuca plate mo- has subsequently been deformed and dissected forearc basin and the influence of tectonics and tion. A structure contour map of the deformed into smaller basins, and eroded at its western mar- eustatic sea-level fluctuations on sedimentation, unconformity and correlated seaward reflec- gin. The basin stratigraphy contains several re- erosion, and deformation. We also focus on vari- tor from seismic reflection data clearly out- gional unconformities, suggesting a complex his- ation of deformational patterns across and along lines deformation into major synclines and tory of vertical tectonics, sedimentation, and the margin, and the extent of influence of base- uplifted submarine banks. Regional margin- eustatic sea-level change. An extensive middle to ment lithology on basinal deformation. parallel variations in uplift rates of the shelf late Miocene unconformity was first reported by unconformity show agreement with coastal Kulm and Fowler (1974) and later by Snavely METHODS geodetic rates. (1987), but the extent, origin, and deformation of The shelf basin is bounded to the west by a this surface during the Pliocene and Quaternary Seismic reflection profiles, shelf exploration north-south–trending outer arc high. Rapid have not been documented in detail. Cranswick well chronology, dated seafloor samples and uplift and possible eustatic sea-level fall re- and Piper (1992) produced a regional isopach cores, and Ocean Drilling Program (ODP) and sulted in the formation of the late Miocene un- map for the entire forearc basinal sequence on the Deep Sea Drilling Project (DSDP) drill site conformity. Basin subsidence and outer arc Oregon margin. The late Miocene angular uncon- stratigraphy (Fig. 1) were used to map and inter- high uplift effectively trapped sediments formity is easily identifiable in seismic reflection pret the late Miocene unconformity within the within the basin, which resulted in a relatively profiles across the Oregon continental shelf; how- forearc basin. The unconformity and seaward starved abyssal floor and narrower Pliocene ever, it is less continuously traceable on much of correlative reflector were traced on seismic pro- accretionary wedge, particularly during sea- the Washington shelf, and between central and files throughout the central and northern Oregon level highstands. During the Pleistocene, the southern Oregon. In these regions, the uncertainty margin to produce a structure contour map. A outer arc high was breached, possibly con- of identifying and tracing the unconformity is due proprietary data set of multichannel migrated tributing to Astoria Canyon incision, the pri- to poor stratigraphic control, nonangularity, fault- seismic reflection profiles used for the map was mary downslope conduit of Columbia River ing, or uplift and erosion. We therefore restricted collected in two acquisition phases: (1) 1975, 46- our study, including the construction of a structure channel, and (2) 1980, 96-channel, which form a *E-mail: [email protected]. contour map of the unconformity, to the central network of north-south and east-west profiles GSA Bulletin; August 2000; v. 112; no. 8; p. 1209–1224; 10 figures. 1209 MCNEILL ET AL. 125 W 124 131 49 128 124 120 P-0072 Columbia River 46 N Figure 1. Tectonic setting of JF DSDP-176 Juan de Ql Washington Cascadia subduction zone (in- Fuca Qn Plate G Nehalem set) and study area indicating Gu Bank 46 Astoria W data sets used to identify late Canyon Nehalem Miocene unconformity and de- Ridge 175 Juan de Fuca 176 Bay termine stratigraphy of the fore- 174 arc basin. Inset shows locations Blanco FZ 892 Tillamook C R of relevant Ocean Drilling Pro- WV Bay gram (ODP) and Deep Sea 43 Drilling Project (DSDP) sites. Pacific Netarts Plate Rogue Bay Ridge Oregon WV—Willamette Valley, CR— Canyon Coast Range, ERB—Eel River Gorda basin, Washington submarine Bench Cascade Plate ERB California canyons: JF—Juan de Fuca; 100 km Gorda MFZ Eel 40 173 Canyon Ql—Quillayute; Qn—Quinault; 45 G—Grays; Gu—Guide; W— P-0103 Siletz Willapa. Main figure shows mul- Bay tichannel seismic (proprietary, Netarts Embayment Fig. 4 solid) and single-channel data Stonewall Bank (dashed) used to trace the uncon- ODP Fig. 8b formity and locates dated sam- 892 Yaquina Bay ples at industry exploration wells, P-0093 along single channel seismic lines Fig. 8c Fig. 5 Alsea (discontinuous samples along Bay dashed lines) and at individual sample sites (triangles). Main structural features of the central P-0087 Cascadia forearc basin are also Fig. 8a labeled in italics. The Cascade OREGON Bench (Kulm and Fowler, 1974) Heceta P-0130 Bank Siltcoos 44 extends between Heceta to Ne- Bank halem Bank along the upper slope of the Netarts Embayment. Umpqua The shelf edge is defined here and River Siltcoos in subsequent figures as a topo- Embayment graphic break identified on seis- mic profiles. Bold lines represent 0 50 km seismic profiles illustrated in sub- shelf sequent figures. edge multichannel seismic single channel seismic profile reflection profile (with occasional seafloor samples) dated seafloor P-0072 industry exploratory well sample across the shelf and upper slope (Fig. 1). Addi- identifying the unconformity include tracing surface. The traced unconformities were digi- tional seismic profiles (both single channel from areas without angular truncation, regions where tized and converted to xyz values in UTM (Uni- Shell Oil Company (Fig. 1) and multichannel the
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