Print this pageCalibrating the Brackish-Water Trace Fossil Model Through Geologic Time*
By
Luis A. Buatois 1, Murray Gingras 2, James MacEachern 3, M. Gabriela Mangano 1, John-Paul Zonneveld 4, George Pemberton 2, Renata G. Netto 5, and Anthony Martin 6
Search and Discovery Article #50017 (2005)
Posted September 15, 2005
*Oral presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005
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1University of Saskatchewan, Saskatoon, SK ([email protected])
2University of Alberta, Edmonton, AB
3Simon Fraser University, Burnaby, BC
4University of Calgary, Calgary, AB
5Unisinos, Sao Leopoldo, Brazil
6Emory University, Atlanta, GA
Abstract
Trace-fossil contents of temporally different brackish-water deposits have been compared, in order to calibrate the assemblages with respect to ichnologic diversity, ichnofossil abundance, bioturbation intensity, and ethology over geologic time. Our comparative analysis reveals that, although the colonization of brackish-water environments was a long-term process spanning the Phanerozoic, the invasion of fully marine organisms into marginal-marine habitats did not occur at a constant rate, but rather, shows overall acceleration marked by five major phases. The first phase (Ediacaran-Ordovician) represents a prelude to the major invasion that typifies the remainder of the Paleozoic. Bioturbation intensities and ichnodiversities are relatively low during this period. The second phase (Silurian-Carboniferous) is marked by the appearance of more varied morphologic patterns and behavioral strategies, manifest by slight increases in ichnodiversity. Ichnofaunas from the third phase (Permian-Triassic) are characterized by the introduction of crustacean-generated burrows, corresponding to the late Paleozoic crustacean radiation, as well as the adaptation of additional groups to brackish-water settings. The fourth phase (Jurassic-Paleogene) is marked by remarkable increases in ichnodiversity and bioturbation intensity. The fifth phase (Neogene-Recent) records the onset of higher abundances and diversities of the modern brackish-water benthos. Differentiation of brackish-water ichnofaunas through time may enhance identification of estuarine incised valley deposits in the stratigraphic record. Recognition of brackish-water ichnofaunas requires familiarity with contemporaneous open-marine counterparts that serve as templates for comparison. Although brackish-water ichnofaunas have changed through the Phanerozoic, some distinctive assemblages and ichnofabrics (e.g. Teichichnus, comprising monogeneric suites or associated with Planolites) have persisted since the Late Cambrian.
