Pemberton, S. George
University of Alberta, Edmonton, Alberta, Canada

Abstract: Stratigraphic Applications of the Glossifungites Ichnofacies: Delineating Discontinuities in the Rock Record

The Glossifungites ichnofacies is a substrate-controlled assemblage of trace fossils, characterized by robust, sharp-walled (locally scratch-marked), unlined, passively filled, vertical to sub-vertical domichnia, excavated into semiconsolidated (firmground) substrates. The assemblage is dominated by the ichnogenera Diplocraterion, Skolithos, Arenicolites, firmground Thalassinoides/Spongeliomorpha?, firmground Gastrochaenolites, Psilonichnus, and Rhizocorallium, which typically crosscut the softground, resident trace fossil suite.

The Glossifungites assemblage demarcates discontinuity surfaces that reflect pauses in sedimentation, generally accompanied by erosion. Colonization of erosionally exhumed substrates and excavation of domiciles occur overwhelmingly within marine or marginal marine settings. Many of these discontinuity surfaces correspond to boundaries of stratigraphic significance.

Lowstand surfaces of erosion marked by Glossifungites assemblages reflect settings lying in marine or marginal marine positions immediately succeeding erosion related to sea level lowstand conditions. These settings are largely limited to seaward margins of some incised valley fills, incised submarine canyons, and lowstand (forced-regression) shorefaces. More favorable to firmground development and colonization are marine flooding surfaces (parasequence boundaries) with accompanying transgressive erosion. Under such conditions, generation of the discontinuity surface occurs within a marine setting, permitting almost immediate colonization by opportunistic organisms. Examples of amalgamated (co-planar) lowstand erosion and transgressive erosion surfaces have also been recognized. Those are produced where lowstand conditions expose or exhume the substrate but the marine conditions permitting colonization do not occur until the following transgressive phase.

In addition, the emplacement of the Glossifungites ichnofacies can enhance the permeability and vertical transmissivity of a relatively impermeable matrix. Permeability enhancement develops when burrows into a firmground are filled with sediment from the overlying strata. If the lithology contrasts with the encapsulating firmground substrate, anisotropic porosity and permeability is developed.

AAPG Search and Discovery Article #90929©1998-1999 AAPG Distinguished Lecturers