Neoichnology and Sedimentology of Tidal Bars at Tillamook Bay, Oregon

Recent studies demonstrate the importance of small-scale ichnological and sedimentological characterization in estuaries. Neoichnological observations were recorded from the bar surfaces, and 20 sediment cores were collected on modern tidal bars at Tillamook Bay, Oregon. Other observations were drawn from core x-rays, grain size analyses, total organic carbon (TOC) analyses, and observed sand-mud ratios. Tillamook Bay is a mesotidal estuary with a mean tidal range of 1.7 m and a diurnal range of 2.3 m. The tidal influence affects the mixing of both fluvial and ocean water, resulting in a high salinity range, from <0.5 ppt in the inner estuary at low tide, to 28 ppt in the outer estuary during both low and high tide. The relatively small area of approximately 34 km² of Tillamook Bay makes it an ideal place for observing local sedimentological and neoichnological spatial variations.

Poorly sorted, coarse and medium-grained sands mixed with mud are observed at the river mouths in the inner estuary, as well as in the inner to middle estuary transition zone. Inclined heterolithic stratification (IHS) is observed locally in the inner estuary. In this portion, bioturbation diversity and intensity are low.

The middle estuary is characterized by dominantly mixed fine sand and mud. Bioturbation intensity in the IHS deposits, present in the middle estuary, increases away from tidal channels and upwards into current and wave-rippled, bioturbated tidal flat deposits. Middle estuary bioturbation consists of a high-diversity, high-abundance suite of Skolithos, Thallasinoides, Arenicolites, Planolites, Palaeophycus, and Siphonichnus. Bioturbation intensity is moderate to high in the middle estuary, with a bioturbation index (BI) commonly ranging from 2 to 5 BI.

Outer estuary bioturbation diversity is moderate. The BI ranges from 0 to 3. The sand bars in the outer estuary are smaller in total volume relative to the inner and middle estuary bars due to a larger tidal prism in this portion. However, both tides and waves influence the outer estuary bars and, as a result, the bars contain clean, well-sorted medium-grained sands with very low mud content, observed as low-relief dunes and low-relief megaripples.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California