Rocky Mountain Section - 69th Annual Meeting - 2017

Paper No. 2-5
Presentation Time: 10:45 AM


BUATOIS, Luis A.1, MANGANO, Maria Gabriela2, DÍEZ-CANSECO, Davinia3, SOLORZANO, Euridice4 and RODRIGUEZ, Williams4, (1)Geological Sciences, University of Saskatchewan, Saskatoon, SK S7K 5E2, Canada, (2)Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2, Canada, (3)Departamento de Estratigrafía and Instituto de GeoCiencias, Universidad Complutense de Madrid, Madrid, AL E-28040, Spain, (4)PDVSA-INTEVEP, Urbanización Santa Rosa, Sector El Tambor, Los Teques, 1070A, Venezuela (Bolivarian Republic of),

Marginal-marine environments comprise a wide variety of coastal settings characterized by rapid environmental perturbations, such as salinity changes, increased sediment discharge and extreme clay flocculation. The brackish-water model is central to our understanding of the ichnologic signature or marginal-marine deposits. However, the importance of freshwater ichnofaunas to delineate the fluvial-tidal transition in both estuarine and deltaic systems has been commonly overlooked. Analysis of a several case studies (e.g. Carboniferous Tonganoxie Sandstone of Kansas, Cretaceous Tremp Formation of Spain, Miocene Oficina Formation of Venezuela) indicates that freshwater trace-fossil assemblages are widespread in the fluvial-tidal setting. In terms of ichnofacies, the fluvial-tidal transition is characterized by a mixed Mermia-Scoyenia ichnofacies. This ichnofauna is characterized by moderate to relatively high diversity of surface trails and trackways in Paleozoic examples, whereas meniscate trace fossils dominate in Mesozoic-Cenozoic examples. Paleozoic ichnofaunas are commonly associated with a wide variety of bedding-surface structures, including tool marks, drainage or seepage rill marks, runnel marks, runoff washouts, foam marks, raindrop impressions, gas escape structures, falling-water marks, and wrinkle marks. In contrast, post-Paleozoic occurrences are characterized by more penetrative and intense bioturbation, with assemblages being typically of low ichnodiversity and dominated by Taenidium or Beaconites. As a result of an increase in depth and extent of bioturbation by the continental infauna through geologic time, the preservation of delicate bedding-plain structures was hampered. Also, overprinting of previously emplaced brackish-water suites became increasingly common, resulting in the formation of composite ichnofabrics that sometimes have defied recognition. Freshwater ichnofacies should be incorporated as a fundamental element of estuarine and deltaic ichnofacies models.