Elizabeth A. Nesbitt

A NEW PSILONICHNUS ICHNOSPECIES ATTRIBUTED TO MUD–SHRIMP UPOGEBIA IN ESTUARINE SETTINGS

Abstract Psilonichnus lutimuratus n. ichnosp. is described from a Pliocene estuarine-mouth depositional environment (Skolithos ichnofacies) of the Olympic Peninsula, Washington, U.S.A. These simple Y-, I-, and J-shaped, mud-lined burrows occur in situ as dense patches within alternating, wavy-bedded sandstone and mudstone in a storm and flood influenced coastal sequence from an active tectonic margin. The I- and J-shaped traces represent erosional modification of burrow tops during storm-flood events. The new ichnospecies differs from the two other Psilonichnus ichnospecies by the distinct mud-lining of the burrow wall. Comparison with living thalassinoidean shrimp burrows and shrimp ecology allow this new ichnospecies to be attributed to the extant mud shrimp Upogebia. Biological and behavioral characteristics of this living shrimp restrict it to the mouth of the open estuary, and these parameters can be used to narrowly define a shoreline environment in the stratigraphic record.

High-Resolution Architecture and Paleoecology of an Active Margin, Storm-Flood Influenced Estuary, Quinault Formation (Pliocene), Washington

Abstract An exhumed Pliocene estuarine sequence is preserved in exquisite detail in the Quinault Formation of coastal western Washington. The thick fill of the Quinault estuarine basin (∼700 m) represents a single transgressive-regressive system that was punctuated by subduction-related tectonic events, and may reflect erosion and sediment accumulation during accelerated uplift of the Olympic Mountains in the Pliocene. Sedimentation patterns in the ancient Quinault estuary were influenced strongly both by floods on the paleo-Quinault River, and by storm waves on the high-energy, open Pacific coast. Body fossils were derived from both marine and brackish faunal elements. Trace fossil suites indicate that the benthos flourished in the fluctuating physicochemical conditions of an estuary in dynamic equilibrium. An estuarine-specific trace fossil, a new ichnospecies of Psilonichnus that is attributed to burrows of the mud shrimp Upogebia pugettensis, is restricted to inlet-mouth, muddy sandstone facies. The marine-marginal marine transition is well-preserved in Quinault estuarine strata (exposed in the Point Grenville and Cape Elizabeth areas), as evidenced by: (1) distinctive sedimentologic signatures characteristic of lower shoreface, estuarine mouth, central basin, tidal creek/flat, and fluvial subenvironments; and, (2) systematic changes in trace fossil associations in the up-estuary direction, from Skolithos ichnofacies elements to a mixed Skolithos-Cruziana assemblage to Glossifungites associations with rhizome phytoburbation. Hence, Quinault strata record a classic tripartite division of an ancient estuarine system, allowing a fine-scale delineation of subenvironments in an active tectonic regime.

Rapid deterioration of sediment surface habitats in Bellingham Bay, Washington State, as indicated by benthic foraminifera

Foraminiferal assemblages in sediment grab samples were utilized to evaluate the impacts of anthropogenic activities on benthic habitats in Bellingham Bay, Washington State, U.S.A. Seventy-three samples taken in 1987, 1997, 2006 and 2010 yielded 35 species of foraminifera from 28 genera. Assemblage composition and diversity data indicate a marked deterioration between 1987 and 2010, contrary to the published Chemical Index, but analogous to the situation with macrobiota. Correlation of diversity with chemical pollutants and metals did not identify any significant correlations, however, an unrelated but highly relevant study of bottom water dissolved oxygen concentrations and pH in Bellingham Bay suggests eutrophication with accompanying hypoxia and acidification may be part of the cause. Thus, the metrics of contamination alone do not adequately characterize habitat viability, and benthic foraminiferal assemblages provide insight into the health of coastal ecosystems.

Paleoecological analysis of molluscan assemblages from the middle Eocene Cowlitz Formation, southwestern Washington

Molluscan assemblages from upper middle Eocene deposits of southwestern Washington represent a regional biotic system that provides independent evidence of a range of depositional environments in a delta. Four marine molluscan assemblages reflect a subtropical fauna that inhabited an inner neritic to upper bathyal gradient down the prodelta slope. Comparisons with Recent molluscan taxa indicate changing trophic structures from dominantly infaunal filter feeding in the shallow, coarser-grained substrate to dominantly infaunal deposit feeding in the fine-grained mud. A very high diversity of carnivorous gastropods indicates that the biocenosis included a wide array of predominantly errant, soft-bodied invertebrate prey. The most shallow marine assemblages show evidence of dense Turritella patches within infaunal bivalve-dominated associations, indicating mosaics of distribution across a soft-sediment substrate. Low diversity marginal-marine and freshwater assemblages show faunal links with each other. They indicate interdistributary bay and marsh environments that are commonly part of a river-dominated delta.

Paleomagnetism and Counterclockwise Tectonic Rotation of the Eocene-Oligocene Lyre, Quimper, and Marrowstone Formations, Northeast Olympic Peninsula, Washington State, USA

The Eocene-Oligocene Lyre, Quimper, and Marrowstone formations on the northeast Olympic Peninsula of Washington are an important sedimentary sequence whose age has long been controversial. They were thought to span the Eocene-Oligocene transition. In addition, previous paleomagnetic studies on the un- derlying volcanic rocks produced a counterclockwise tectonic rotation. We sampled three sections: West In- dian Island, West Marrowstone Island, and near Woodmans Wharf on Discovery Bay. After both AF and ther- mal demagnetization, all three formations produced a single-component remanence held largely in magnetite which passes a reversal test and is rotated counterclockwise by about 35°. This is consistent with the results from the underlying lower Eocene Port Townsend volcanics. It is also consistent with counterclockwise ro- tations obtained from the lower Eocene Metchosin volcanics and upper Oligocene Sooke Formation on the southern tip of Vancouver Island, showing that the counterclockwise rotation is widespread in the region. Based on magnetobiostratigraphic correlations with other sections in the region, we correlate the Lyre For- mation with late middle Eocene Chrons C15r-C16r (35.0-36.5 Ma). The Quimper Sandstone spans the Chron C15n and probably the lowermost part of Chron C13r (34.6-35.0 Ma), so the Eocene-Oligocene boundary probably lies in the middle of the Quimper Sandstone, as previously suspected. The Marrowstone Shale is correlated with late Chron C13r.