Bioturbated Mudstones: Critical Interpretations from Under-Appreciated Facies

James A. MacEachern¹, Kerrie L. Bann², and Christopher R. Fielding^3
1 Simon Fraser University, Burnaby, BC
² University of Alberta, Edmonton, AB
³ University of Nebraska-Lincoln, Lincoln, NE

Mudstones comprise a major proportion of the rock record but in most studies are accorded an inordinately small amount of sedimentological expertise. Despite their lithologic, sedimentologic and paleontologic variability, interpretations of clay-rich mudstones are commonly limited to “suspension deposition”, and of siltstones to oscillatory flow or low-energy current transport, depending upon the preserved physical structures. On the contrary, fine-grained facies possess characteristics that record a wide variety of depositional conditions. Ichnological analysis is particularly useful to the resolution of these, because biogenic structures are abundant in mudstones and trace fossil data can be easily collected during their routine description.

Bioturbated mudstones may contain trace fossil suites representative of archetypal ichnofacies, and are especially effective in characterizing abyssal/neritic (Nereites ichnofacies), shelf (Zoophycos ichnofacies), offshore (Cruziana ichnofacies), and backshore (Psilonichnus ichnofacies) deposits. Intergradations between the archetypal ichnofacies are common and reflect similar intergradations between depositional subenvironments. As such, distal and proximal expressions of the Cruziana ichnofacies, and mixed Skolithos-Cruziana ichnofacies permit subdivision of mud-prone marine environments at the facies level.

Stress-induced departures from the archetypal ichnofacies have also been identified, typifying settings such as deltas, estuarine incised valleys, open bays and lagoons, strongly storm-dominated shorelines, and stagnant or stratified water bodies. Distinctive deviations include impoverishment of particular organism behaviors, reduced numbers of trace fossils, reductions in assemblage diversities, reductions in ichnogenera sizes, and occurrences of monogeneric or monospecific suites. These departures from the archetypal highlight a number of environmental stresses that operated in the setting, and refine depositional interpretations of the strata.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005