Tectonics and paleogeography of a post-accretionary forearc basin, Coos Bay area, SW Oregon, USA

ABSTRACT This field guide reviews 19 sites providing insight to four Cenozoic deformational phases of the Cascadia forearc basin that onlaps Siletzia, an oceanic basaltic terrane accreted onto the North American plate at 51–49 Ma. The field stops visit disrupted slope facies, prodelta-slope channel complexes, shoreface successions, and highly fossiliferous estuarine sandstones. New detrital zircon U-Pb age calibration of the Cenozoic formations in the Coos Bay area and the Tyee basin at-large, affirm most previous biostratigraphic correlations and support that some of the upper-middle Eocene to Oligocene strata of the Coos Bay stratigraphic record represents what was differentially eroded off the Coast Range crest during ca. 30–25 Ma and younger deformations. This suggests that the strata along Cape Arago are a western “remnant” of the Paleogene Tyee basin. Zircon ages and biostratigraphic data encourages the extension of the Paleogene Coos Bay and Tyee forearc basin westward beyond the Fulmar fault and offshore Pan American and Fulmar wells. Integration of outcrop paleocurrents with anisotropy of magnetic susceptibility data from the middle Eocene Coaledo Formation affirms south-southeast to north-northwest sediment transport in current geographic orientation. Preliminary detrital remanent magnetism data show antipodal directions that are rotated clockwise with respect to the expected Eocene field direction. The data suggest the Eocene paleo-shoreline was relatively north-south similar to the modern shoreline, and that middle Eocene sediment transport was to the west in the area of present-day Coos Bay. A new hypothesis is reviewed that links the geographic isolation of the Coos Bay area from rivers draining the ancestral Cascades arc to the onset of uplift of the southern Oregon Coast Range during the late Oligocene to early Miocene.

Developing landslide chronologies using landslide-dammed lakes in the Oregon Coast Range

ABSTRACT The Oregon Coast Range is a dynamic landscape that is continually shaped by shallow and deep-seated landslides that can have disastrous consequences to infrastructure and human lives. Searching for evidence of potentially coseismic mass wasting is incredibly difficult, particularly when historical observations are limited. Landslide-dammed lakes with submerged “ghost forests” in the Oregon Coast Range present the unique opportunity to establish landslide chronologies with subannual accuracy when dendrochronology is applied. This field guide will visit the unique landslide-dammed Klickitat Lake and explore a drowned ‘ghost forest’ to discuss methods used to establish a prehistoric landslide chronology in western Oregon, USA. After exploring the lake and exposing its geomorphic secrets, the guide will end with a stop on Marys Peak, a mafic volcanic intrusion composed of gabbroic dikes and pillow basalt that forms the highest point in the Oregon Coast Range. With the landscape of western Oregon laid out before us, we will discuss short- and long-term geomorphic evolution of the Oregon Coast Range and Willamette Valley.

Structural styles, deformation, and uplift of the Olympic Mountains, Washington: Implications for accretionary wedge deformation

The Olympic subduction complex is the exposed subaerial Cascadia accretionary wedge in the Olympic Mountains of Washington State. Uplift of the mountains has been attributed to two competing models: margin-normal deformation from frontal accretion and underplating, and margin-parallel deformation from the clockwise rotation and northward movement of the Oregon Coast Range block compressing the Olympic Mountains block against the Canadian Coast Range. East-northeast−oriented folds and Quaternary thrust faults and paleostress analysis of faults in the Coastal Olympic subduction complex, west of the subduction complex massif, provide new evidence for north-south shortening in the Coastal Olympic subduction complex that fills a large spatial gap in the north-south shortening documented in prior studies, substantially strengthening the block rotation model. These new data, together with previous studies that document north-south shortening in the subduction complex and at numerous locations in the Coast Range terrane peripheral to the complex, indicate that margin-parallel deformation of the Cascadia forearc has contributed significantly to uplift of the Olympic Mountains. Coastal Olympic subduction complex shallow-level fold structural style and deformation mechanisms provide a template for analyzing folding processes in other accretionary wedges. Similar-shaped folds in shallow-level Miocene turbidite sediments of the Coastal Olympic subduction complex formed in two shortening phases not previously recognized in accretionary wedges. Folds began forming by bed-parallel flow of sediment into developing hinges. When the strata could no longer accommodate shortening by flexural flow, further shortening was taken up by flexural slip. Similar-shaped folds in the deeper accretionary wedge rocks of the subduction complex massif have a well-developed axial-surface cleavage that facilitated shear folding with sediment moving parallel to the axial surface into the hinges, a structural style that is common to accretionary wedges. The pressure-temperature conditions and depth at which the formation of similar folds transitions from bed-parallel to axial-surface−parallel deformation are bracketed.

Rainfall triggers more deep-seated landslides than Cascadia earthquakes in the Oregon Coast Range, USA

The coastal Pacific Northwest USA hosts thousands of deep-seated landslides. Historic landslides have primarily been triggered by rainfall, but the region is also prone to large earthquakes on the 1100-km-long Cascadia Subduction Zone megathrust. Little is known about the number of landslides triggered by these earthquakes because the last magnitude 9 rupture occurred in 1700 CE. Here, we map 9938 deep-seated bedrock landslides in the Oregon Coast Range and use surface roughness dating to estimate that past earthquakes triggered fewer than half of the landslides in the past 1000 years. We find landslide frequency increases with mean annual precipitation but not with modeled peak ground acceleration or proximity to the megathrust. Our results agree with findings about other recent subduction zone earthquakes where relatively few deep-seated landslides were mapped and suggest that despite proximity to the megathrust, most deep-seated landslides in the Oregon Coast Range were triggered by rainfall.

Dendrochronological dating of landslides in western Oregon: Searching for signals of the Cascadia A.D. 1700 earthquake

Large-magnitude earthquakes and hydrologic events in mountainous settings commonly trigger thousands of landslides, and slope failures typically constitute a significant proportion of the damage associated with these events. Large, dormant deep-seated landslides are ubiquitous in the Oregon Coast Range, western United States, yet a method for calculating landslide ages with the precision required to diagnose a specific triggering event, including the A.D. 1700 Cascadia earthquake, has remained elusive. Establishing a compelling connection between prehistoric slope instability and specific triggers requires landslide ages with precision greater than that provided by 14C dating of detrital materials. Tree-ring analysis is the only known method capable of determining landslide age with this precision. Dozens of landslide-dammed lakes in western Oregon present an opportunity to use tree rings from drowned snags, or “ghost forests,” to establish the year of death, and thus landsliding. We cross-dated tree-ring indices from drowned Douglas fir trees with live tree-ring records from the Oregon Coast Range that exhibit synchronous, time-specific patterns due to regional climate variations. Our analyses determined that the landslides responsible for creating Wasson and Klickitat Lakes occurred in A.D. 1819 and 1751, respectively. The 14C dates from selected tree rings and landslide deposit detritus are consistent with our tree-ring analysis, although the ages exhibit high variability, revealing the limitations of using 14C dating alone. Because dendrochronology provides annual precision for landsliding, sampling of tree rings at additional landslide-dammed lakes throughout the Oregon Coast Range can be used to constrain the potential effects of ground motion and major storms on Cascadia landscapes.