Differentiating Freshwater & Brackish-Water Successions Using Trace Fossils

John-Paul Zonneveld¹, Murray K. Gingras², and Stephen T. Hasiotis^3
1Regional Geoscience, Geological Survey of Canada, Calgary, AB, Canada
²Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada
³Geology, University of Kansas, Lawrence, KS

Trace-fossil assemblages typical of mid-latitude brackish-water settings represent infaunal communities of low diversity & 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.

Temperate zone aquatic continental trace fossil assemblages are commonly depauperate due to the temporary nature of these environments & the high depositional energy of many fluvial channels. Traces include shallowly tiered Planolites, Arenicolites, rare, short Rhizocorallium, shallow Skolithos & meander-form trails produced by gastropods, annelids & arthropods. Fish & bivalves produce distinct forms commonly under-reported in freshwater settings. Absent are long, vertical crayfish burrows which are dug deeply only in subaerial settings.

Primary differences between fresh- & brackish-water trace-fossil assemblages in mid-latitudinal settings include: 1) deeply penetrating shafts are absent in freshwater, subaqueous settings; 2) large crustacean burrows are common in brackish-water settings & absent in subaqueous freshwater settings; 3) Planolites is a very shallow tier trace fossil in fresh-water settings, but represent deeper tiers in brackish water; 4) such traces as Cylindrichnus, Gyrolithes, & annelid-generated Teichichnus have not been reported from continental deposits, 5) clam-associated behaviors are commonly deeply infaunal in marine salinities & shallow to epifaunal in freshwater (Siphonichnus & Lockeia respectively); 6) Wood clasts in brackish water may display borings consistent with the Teredolites Ichnofacies. Those in freshwater settings exhibit borings distinctive from those included in the Teredolites ichnofacies (i.e. insect & woodpecker borings).

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas