Sedimentology and tectonic implications of Cretaceous fan‐delta conglomerates, Queen Charlotte Islands, Canada

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Abstract
The Honna Formation, of Coniacian age, consists of several hundred metres of polymictic clast‐supported conglomerate associated with sandstone and mudstone. Five conglomerate facies are recognized: ungraded beds; inverse graded beds; normal graded beds; inverse‐to‐normal graded beds; and parallel‐stratified beds. These facies are interpreted as the deposits of subaqueous cohesionless debris flows and/or high‐density turbidity currents.The depositional environment was a deep‐water, gravelly fan that draped a fault‐controlled, basin‐margin slope. The fan is inferred to have passed upslope directly into an alluvial fan (unpreserved); hence, the name fan delta can be applied to the overall depositional system. This type of fan delta, of which the Brae oilfield in the North Sea is an example, is defined here as a deep‐water fan delta. The lack of a shelf is in marked contrast to other types of fan delta.Three facies associations are recognized in the Honna Formation: subaqueous proximal‐fan conglomerates, distal‐fan turbiditic sandstones, and pro‐fan/interfan mudstones with thin sandy turbidites. The proximal fan is envisaged as an unchannelled gravel belt with a downslope length of at least 20 km; such a long subaqueous gravel belt lacks a known modern analogue. The distal fan was an unchannelled sandy extension of the proximal gravel belt.It is postulated that the Honna Formation accumulated in a foreland basin which migrated westwards from the Coast Mountains where the Wrangellia‐Alexander terrane was colliding with North America. In this model, the Honna fan delta was sourced by a (west‐verging) thrust sheet whose sole‐thrust was the Sandspit Fault immediately to the east.Deep‐water fan deltas appear to develop preferentially when eustatic sea‐level is relatively high, so that the‘feeder’ alluvial fan is small, and gravelly throughout. In petroleum exploration and field development, care should be taken to distinguish deep‐water fan deltas from base‐of‐slope (canyon‐fed) submarine fans, because the two systems differ significantly in terms of coarse‐sediment distribution.