TRIASSIC MARINE FIRMGROUNDS AS EVIDENCE FOR SYN-SEDIMENTARY TECTONISM IN THE SUBSURFACE, MIDDLE TRIASSIC HALFWAY FORMATION, WEST-CENTRAL ALBERTA, CANADA

Marine firmgrounds are formed when unconsolidated sediments are buried, compacted, dewatered, and then subsequently exhumed by wave erosion, allowing these surfaces to exist at the sediment-water interface. Marine firmgrounds observed in multiple Middle Triassic core from west-central Alberta display sharp erosive contacts, vertical unlined burrows, and burrow infill consisting of the overlying sedimentary material.

Firm substrates that have been burrowed by organisms are classified as Glossifungites Ichnofacies assemblages. The method of formation and exhumation of firm substrates may vary, but is generally assigned to be the result of wave erosion during transgression. The recognition and accurate interpretation of firmground substrates can have a profound impact on the sequence stratigraphic significance and facies architecture of the region.

This presentation proposes a tectonic component to the formation of marine firmgrounds within the Middle Triassic Halfway Formation. Firmgrounds seen in core do not hold any major sequence stratigraphic significance, as they appear localized in extent and lack a major facies change across the surface. These firmgrounds also only appear to exist in thin successions of the Halfway Formation. The presence of firmgrounds only in thin successions suggests a decrease in accommodation space, brought on by uplifted block faults. Areas with increased accommodation and thicker successions are thought to be concomitant with these uplifted blocks.

The Middle Triassic Halfway Formation located within the Western Canada Sedimentary Basin of west-central Alberta, Canada consists of a broad range of shoreface siliciclastic and bioclastic deposits, including upper/lower shoreface and tidal channel deposits. The formation of marine firmgrounds within the Middle Triassic may have been the result of localized tectonism – an interpretation that may be relevant to other firmground occurrences.