DISCERNING FRESH- AND BRACKISH-WATER TRACE FOSSILS AND IDENTIFYING THE TRANSITION BETWEEN THE TWAIN

Trace-fossil assemblages typical of mid-latitudinal brackish-water settings represent infaunal communities of low diversity and high population density, comprising trophic generalists. Brackish-water, trace-fossil assemblages are commonly dominated by Planolites, Cylindrichnus, Teichichnus, or Gyrolithes. Thalassinoides, Ophiomorpha, Skolithos, Arenicolites, and Palaeophycus are present but less abundant. Wood clasts in brackish water may display borings consistent with the Teredolites Ichnofacies.

Temperate zone continental settings contain subaerially exposed and aquatic environments, which range from fresh, saline, alkaline, hypersaline, to brackish water. Aquatic trace fossil assemblages are commonly depauperate because of temporary nature of aquatic continental environments and the high depositional energy expended in many fluvial channels. Freshwater-aquatic continental traces emplaced in subaqueous habits are consistent with shallowly tiered Planolites, Arenicolites, very rare short Rhizocorallium, shallowly penetrating Skolithos, and various meander-form trails produced by gastropods and arthropods. Bivalves also produce distinct forms in freshwater settings. Conspicuously absent are long, vertical crayfish burrows: these are dug deeply only in subaerial settings. In continental settings, bored wood includes traces produced by beetles, bees, moths, mayflies and woodpeckers, whose borings are distinctive from those of the Teredolites Ichnofacies.

The main differences between fresh- and brackish-water trace fossil assemblages in mid-latitudinal settings observed in this study are: 1) deeply penetrating, lined or unlined shafts are absent in fresh-water, subaqueous settings; 2) large-diameter crustacean burrows are common in brackish-water settings, but only descend deeply from subaerial exposure surfaces in continental settings; 3) Planolites is a shallow tier trace fossil in fresh-water settings, but represents a shallow- to medium-depth tier in brackish water; 4) such traces as Cylindrichnus, Gyrolithes, and annelid-generated Teichichnus are not reported from continental deposits, and; 5) borings preserved in wood clasts are discernibly different in continental versus brackish-water settings.