FORAMINIFERAL ASSEMBLAGE ZONES IN OREGON TIDAL MARSHES - RELATION TO MARSH FLORAL ZONES AND SEA-LEVEL

Foraminiferal assemblages from three surface transects across estuarine marshes on the central Oregon coast reflect vertical floral zones based on vascular plants. Discriminant analysis distinguishes assemblages of modern mudflat, low marsh, and high marsh zones using the types of percentage data found in fossil assemblages. Upland samples are barren of foraminifera. The high marsh is dominated by Trochammina macrescens f. macrescens and T. inflata. The highest part of the high marsh, above mean higher high water, is usually marked by increases in the percentage of T. inflata and Haplophragmoides sp. Miliammina fusca is found throughout the high and low marsh. The low marsh may be divisible into two subzones: an upper subzone with T. macrescens f. macrescens and T. inflata and a lower subzone with Ammotium salsum and Ammobaculites exiguus. Mudflat assemblages contain Reophax nana, M. fusca, A. salsum, A. exiguus, and low numbers of calcareous species. Assemblages resemble those studied in other cool-temperature tidal marshes, but the elevational range of the high marsh foraminiferal zone in Oregon is greater than the range of equivalent zones in the tidal marshes of eastern North America. Large ranges on low and high marsh assemblage zones limit the precision with which changes in paleosea levels can be estimated from fossil assemblages in Oregon.