GeoConvention 2012: Vision 1 Core Examples from Modern Estuarine Tidal Bars, Tillamook Bay, Oregon Rares Bistran* University of Alberta, Edmonton, Alberta, Canada rares.bistran@ualberta.ca David Herbers, Murray Gingras, John-Paul Zonneveld, and S. George Pemberton University of Alberta, Edmonton, Alberta, Canada Introduction Tidal sand bars are ideal estuarine morphologic features to use as modern analogues in understanding the complex sediment distribution of ancient estuarine environments, many of which host large hydrocarbon resources (e.g. McMurray Formation in Alberta). Tidal bars may accrete both laterally and vertically, resulting in complex stratal geometries and sediment distributions. Heterolithic Stratification and Inclined Heterolithic Stratification (Thomas et. al, 1987) are common characteristics of inner and middle estuary tidal bars at Tillamook Bay. Due to changes in sediment source, and the interplay between tidal and fluvial currents and sediment reworking, large variations appear in the sedimentological and ichnological character of Inclined Heterolithic Stratification (IHS). At Tillamook Bay, IHS is observed in the inner and middle estuary, being the most prominent in the middle estuary bars. In the outer estuary bars, IHS is largely absent and not recognized. Study Area Tillamook Bay is located on the coastline of the state of Oregon, United States, approximately 100 km west of the city of Portland (Figure 1). It is a restricted bay, separated from the Pacific Ocean by the 5 km long Bayocean Peninsula. The bay is approximately 10 km long in a northwest-southeast direction and 3.4 km wide, but averages only 2 meters depth over a 34 km 2 area. It is a mesotidal estuary, with a mean tidal range of 1.7 m and a diurnal range of 2.3 m. Five major rivers, comprising a watershed of 1,546 km 2 , deliver sediment to Tillamook Bay. Four of the rivers enter the bay at the southeast part and deliver the vast majority of mud and sand found within the inner and middle estuary. The outer estuary consists of clean, well sorted sands, both of marine and terrestrial origin (Komar, 1997, and Komar et. al. 2004). The inner estuary is characterized by dominantly muddy channel fills and alternating layers of medium sand and mud at the river mouths. More complex sediment distributions are characteristic of the middle estuary mixed sand and mud IHS; the middle estuary deposits are also the most intensely bioturbated. The clean, well sorted sands of the outer estuary are sparsely bioturbated. Methods A total of twenty-four cores were collected in the summer of 2011. Core locations are indicated in Figure 2, with the exception of three cores, which were collected further upstream in the inner estuary river channels, and lie outside of the map area. Nine cores were selected as examples for this core presentation, and their locations are indicated in Figure 2. Seven of the nine cores were selected from locations in the inner and middle estuary, due to the complexity of deposits found there, and therefore the higher amount of information that can be discussed. Cores were collected through vibracoring using