Spatial, temporal and palaeoecological significance of exhumed firmgrounds and other associated substrate types in Netarts Bay, Oregon, USA

Abstract

Important palaeoecologic and stratigraphic information can be obtained by observing trace assemblages within various substrate types in a modern intertidal marine environment. Overall, there has been a lack of studies conducted concerning quantitative firmness measurements on firmground, stiffground and softground substrates in a modern intertidal setting, and the combined palaeoecological and stratigraphic significance of such substrates. This study assesses the sedimentologic and neoichnologic nature of associated peaty-clay firmground, silty-sand firmground, woodground, stiffground and softground substrates in Netarts Bay, Oregon, USA, to gain a more complete understanding of the potential palaeoecologic and stratigraphic utility of these substrates in an intertidal setting. A north-south-oriented transect was conducted along the eastern margin of Netarts Bay where descriptions of surface and sub-surface biogenic and sedimentary structures were conducted via trenching, boxcoring and epoxy resin casting with results recorded and photographed. A sample of 18 different organisms creating biogenic structures were obtained, stored in glass jars and preserved with an 80% ethyl alcohol solution. Quantitative substrate firmness measurements were conducted on four of the five different substrate types using a modified Brinell firmness test. In Netarts Bay, firmground substrates and associated trace assemblages displaying larger, more robust, vertical traces likely have a higher preservation potential compared with stiffground to softground substrates that have trace assemblages composed of smaller traces that tend to grade into a horizontal position. Firmground substrate in Netarts Bay display more homogenous and firmer profiles (1.28 × 106 Pa to 2.77 × 106 Pa) than those of more heterogeneous stiffground (1.37 × 105 Pa to 2.44 × 106 Pa) and softground (8.60 × 104 Pa to 3.73 × 105 Pa) substrate. The relatively planar Pleistocene-aged firmground substrates and associated in situ woodground substrates at Netarts Bay were found to be laterally extensive and mappable stratigraphically significant surfaces. This is in contrast to the highly variable, non-significant, erosionally derived stiffground and bay-derived softground substrates that are neither extensive nor mappable. Trace assemblages in the peaty-clay and silty-sand firmgrounds studied were representative of a Glossifungites ichnofacies assemblage while associated in situ woodground trace assemblages comprised localized Teredolites ichnofacies assemblages Firmground, Neoichnology, Netarts Bay, Glossifungites, Teredolites.