Burrowed Stiffgrounds on Estuarine Point Bars: Modern and Ancient Examples, and Criteria for their Discrimination from Firmgrounds Developed Along Omission Surfaces

Allostratigraphic studies based on the examination of outcrop and core have benefited greatly from the application of substrate-specific ichnofacies. The Glossifungites Ichnofacies, in particular, has been widely employed in the recognition of stratigraphic bounding surfaces within Cretaceous deposits of the Western Canada Sedimentary Basin. Glossifungites Ichnofacies-demarcated surfaces are not limited to erosional discontinuities of allocyclic origin, however. Two examples of the recurring development of the Glossifungites Ichnofacies within accretionary bank deposits of marginal-marine settings, one modern and one ancient, are presented and discussed.

Tidal point bars along the Shepody River of southern New Brunswick are dominated by the accumulation of fine-grained sediment in a macrotidal regime. Depositional surfaces are generally bioturbated and incipient trace fossils can be observed in underlying layers. Bioturbation of the silty mud occurs seasonally, emphasizing the heterolithic fabric. The upper 5-20 cm of the substrate is soft to fluid, and overlies comparatively stiff, but not fully compacted mud (i.e., stiffground). The firmer sediment readily supports open and unlined burrows, whereas burrows in the soft surficial layer are maintained only with mucous linings.

Analogous Glossifungites Ichnofacies-demarcated stiffground surfaces have been observed within heterolithic, mudstone-dominated accretionary bank deposits of the Aptian McMurray Formation in northeast Alberta. A distinct trace fossil suite subtends from the tops of many mudstone beds, with burrow fills consisting of silt and very fine-grained sand sourced from the overlying bed. While a veneer of soft sediment may have been present above the cohesive mud at the time of burrowing, little record of such is preserved. Seasonal variations in estuarine water circulation and sediment texture are inferred to have led to the generation and exposure of the stiff substrate.

These stiffground surfaces differ greatly in (paleo)environmental setting and stratigraphic occurrence from firmgrounds developed along omission surfaces. As a result, it is important to be able to differentiate between stiffground and firmground occurrences when assessing the significance of a Glossifungites Ichnofacies-demarcated surface. Firmgrounds show evidence for significant erosion along, and/or facies dislocation across, the burrowed surface. This implies that a significant shift in depositional setting has taken place between pre- and post-firmground conditions. Stiffgrounds, conversely, show evidence for only minor erosion, and they may be common within continuous sediment packages. Additionally, significant post-colonial compaction takes place with stiffgrounds, as the surface is developed along a substrate that has been subject to only minor compaction prior to exposure.