Kevin L. Treptau¹, James A. MacEachern², Peter S. Mustard²

(1) Simon Fraser University, Burnaby, BC
(2) Simon Fraser University

ABSTRACT: An Integrated Sedimentological-Ichnological Analysis of a Distal Submarine Fan Succession: Upper Cretaceous Cedar District Formation, Nanaimo Group, Southwest British Columbia, Canada

The Campanian Cedar District Formation is a coarsening and thickening upward succession of mudstone, siltstone, and fine-grained sandstone deposited under deep marine conditions. Integrated sedimentological-ichnological analyses are based on the examination of 13 outcrop sections, ranging from 30 to >300 meters thick.

Detailed sedimentology reveals the main process of deposition was via easterly derived turbidity currents.  Bouma TCE and TBCE divisions dominate the overall succession, with compacted mud-sand ratios ranging from 2:1 to 4:1.  The dominant Bouma C divisions are typically rippled to convolute-laminated fine-grained sandstone a few centimeters in thickness.  Bouma TE and TDE divisions are more common in the basal units, whereas Bouma TABCE divisions are increasingly predominant upward.  However, the muds are dominantly turbidite derived throughout; hemipelagic muds are rare.

The pre-turbidite assemblage is characterized by a low diversity suite of the Zoophycos ichnofacies. There is a curious absence of graphoglyptids. The post-turbidite assemblage is dominated by a varied suite including Neonereites, ?Scolicia, Ophiomorpha, Thalassinoides, Taenidium, and Zoophycos.

Deposition occurred within distal prograding submarine fan complexes. Water depths are uncertain, but at least outer shelf or deeper. The post-turbidite ichnological assemblage may reflect activities of both resident organisms and those swept basinward by gravity flows.  The absence of graphoglyptids may indicate higher sedimentation rates and/or more abundant food resources than typical of abyssal settings.  High frequency turbidity flow events could account for this by precluding thick hemipelagic accumulations.  Alternatively, deposition may have occurred in a setting more proximal than might be expected from the sedimentological evidence alone.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado