The hills, flood plains, sand dunes, and headlands that characterize the vicinity of the Coos estuary are the result of a complex series of coastal geomorphic events. The shoreline, landforms, and soils of the area reflect the interactions of tectonic plates over the past 50 million years, changes in sea level, and local weakness in the earth’s crust, along with more human recent land use.
South Slough NERR lies along a geologic fold, or syncline, which bears its name. Due to this formation, the watershed’s eastern and western sides are of distinct geologic types, with different elevations and gradients. The eastern shore formation, which rarely exceeds 250 feet, is typical of the larger Coos estuary watershed. Its highly erodable Quaternary marine terraces of unconsolidated to semi-consolidated sand, silt, and clay are gently sloping, and worn down along creek beds to sandstone and siltstone overlain by loamy sand and sandy and silty loam. The western side’s Empire Formation, with scattered Quaternary terraces, is unique to the South Slough. Its hard, impermeable marine sandstone rises 370 feet above sea level, in a long, steeply sloping north-south ridge. These western slopes are mantled with sandy and silty loam and loamy sand.
Sediments in the South Slough watershed and estuarine tidal basin are derived from several sources including terrestrial runoff, oceanic deposition, and biotic origins. The predominant soil type throughout the upland forested areas is silt loam of the Templeton-Salander group with medium to high runoff and erodability. Sandy marine terraces of the Bullards-Bandon-Blacklock group are also present on the northeastern slopes of the watershed. Soils in the tidally flooded salt marshes are classified as rich organic histosols (Haagen, 1989), which typically consist of compacted clay, sand, and fine mud in alternating layers with mineral sand/silt and organic peat materials. Tidal flats in the South Slough estuary are composed of mud flats and sand flats. The organic content of the mud flats is relatively high and they occur in areas of the estuary that experience low tidal energy. Sand flats, in contrast, occur in areas of high tidal energy and have a much lower organic content.
Core sediment samples from several marsh locations in the estuary indicate a buried layer of coarse-grain sand that overlies organic material (Peterson and Darienzo, 1989; Nelson et al., 1998), providing evidence that South Slough was inundated by a tsunami about 300 years ago (Satake et al., 1996).
Due to its marine origins, the South Slough drainage area includes small deposits of black sands where ancient waves and currents concentrated heavy minerals (iron, chromium, minor amounts of gold, titanium, zirconium, platinum and garnet). Two to four thousand feet beneath the Coaledo
Formation a U-shaped seam of coal encircles the Slough.