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.

A New Genus Of Microsporidian Parasite (Hepatosporidae; Microsporidia) Found In The Oocytes Of Ribbon Worms From The North Pacific Genus Maculaura (Heteronemertea; Nemertea)

An intracellular microsporidian parasite was first observed within oocytes of Maculaura alaskensis, a small pilidiophoran nemertean, commonly found on sandflats along the Pacific coast of North America. Infected oocytes have large vesicles containing dozens to hundreds of diplokaryotic, ellipsoid spores measuring 1.3 by 2.3 μm. A partial small subunit nuclear ribosomal 18S gene sequence isolated from the microsporidian does not match any known microsporidian sequences in the public databases. Phylogenetic analysis groups it with Hepatospora eriocheir in a sister clade to the Enterocytozoonidae. All the known life stages of this parasite are contained within a membranous envelope. This microsporidian was identified in M. alaskensis, Maculaura aquilonia, Maculaura oregonensis, and Maculaura cerebrosa in Coos Bay, Oregon, in M. alaskensis from Newport, Oregon, and in M. aquilonia collected in Juneau, Alaska. This is, to our knowledge, the first species of microsporidia found to directly infect nemertean host cells.

COASTAL WAVE MODELING FOR JETTY REHABILITATION AT COOS BAY, OREGON

Coos Bay Inlet, located on the Pacific coast of southwestern Oregon, is protected by dual jetties constructed in 1928. Because the inlet is exposing to high energy environment, both north and south jetties have deteriorated since the initial construction. Aging, erosion of foundation, lack of effective maintenance, and channel dredging in the past have accelerated the jetty deterioration. To ensure navigation safety, the U.S. Army Corps of Engineers is presently investigating the rehabilitation and redesign of jetties. This paper is focused on numerical storm wave modeling of the existing jetties to provide input forcing information to physical model and redesign of jetties.

JETTY DESIGN USING DUAL LIFE-CYCLE AND PHYSICAL MODELING APPROACH – COOS BAY, OR

The advantages of risk-based methodologies over traditional deterministic analyses have been well documented for the design of coastal projects. The consideration of probabilistic forcing allows for the application of a life-cycle approach that can be used to optimize structure design, including the quantification of uncertainty. Damage progression and functional performance can be assessed over the project’s design life and can be considered in the design process. A lifecycle modeling approach was developed and applied, in conjunction with a 1:55 scale physical model, to the design of the North jetty major maintenance repair in Coos Bay, Oregon.