THE SEDIMENTOLOGIC, DIAGENETIC AND STRATIGRAPHIC SIGNIFICANCE OF GLOSSIFUNGITES- AND TRYPANITES- DEMARCATED DISCONTINUITY SURFACES IN AN UPPER TRIASSIC MIXED SILICICLASTIC-CARBONATE SUCCESSION, NORTHEASTERN BRITISH COLUMBIA, CANADA

Discontinuity surfaces, delineated by abrupt changes in lithology, chert / phosphate granule / pebble lags, substrate-controlled trace fossil assemblages, and pronounced diagenetic alteration of underlying, and occasionally overlying, strata are exceptionally common in the Upper Triassic (Carnian) Baldonnel Formation at Williston Lake, northeastern British Columbia, Canada. Many of these surfaces are characterized by well-developed, low- to moderate-diversity trace fossil assemblages attributed to the Glossifungites ichnofacies. Less commonly these surfaces are overprinted by sparse, low-diversity trace fossil assemblages characteristic of the Trypanites ichnofacies. The nature and ichnotaxonomic composition of substrate-controlled ichnofacies in the Baldonnel Formation varies both laterally (between outcrop sections) and vertically (stratigraphically) within the Baldonnel Formation. Fabric selective (interparticle, intraparticle and biomoldic) and non-fabric selective (vuggy and burrow) porosity are commonly best developed in bioclastic packstone and grainstone beds immediately beneath discontinuity surfaces, particularly those characterized by Glossifungites assemblages.

It has previously been suggested that the abundance of Glossifungites demarcated discontinuity surfaces in the Baldonnel Formation negates their stratigraphic utility. However careful analysis of these surfaces, with due attention paid to taxonomic composition, trace fossil density and the nature (composition and diagenetic attributes) allows the sedimentologic, diagenetic and stratigraphic significance of each surface to be accurately identified.

Evidence of ecological succession is provided by the presence of softground, firmground and hardground trace fossil assemblages in the same samples. Complex depositional / diagenetic relationships occurred wherein substrata characterized by moderately diverse Skolithos or proximal Cruziana ichnofossil assemblage were buried, subsequently exhumed, colonized by firmground-preferring infauna, lithified, colonized by hardground-preferring infauna, and reburied. Diagenetic alteration occurred via fluid penetration through burrows during, and shortly after, hardground formation.