Cenozoic ichthyolith biostratigraphy: Tofino Basin, British Columbia

Abstract

Five new late Eocene – Pliocene ichthyolith zones are defined based on indigenous faunal occurrences in strata that outcrop along western Vancouver Island and samples from six offshore Tofino Basin wells. Five new interval zones are each defined based on distinct transported ichthyolith assemblages. Results are correlated within the framework of established Cenozoic west coast and Arctic foraminifer zones and ichthyolith occurrences in deep-sea core samples and Cretaceous Queen Charlotte Group and Nanaimo Group strata of coastal British Columbia. The use of multiple data sets was important to interpret complex active tectonic margin sedimentation and structures. The integrated ichthyolith and foraminifer biostratigraphy allowed an interpretation of well log data and lithology reports. Also, we reassessed previous offshore seismic data, evaluated fossil preservation and thermal alteration, and distinguished transported from indigenous ichthyolith occurrences. These data allowed a reinterpretation of tectonic segments and associated strata of the Pacific Rim and Crescent terranes and the Cascadia Accretionary Complex onshore and offshore Vancouver Island. Tectonically active phases during the Eocene and Oligocene – early Miocene resulted in transport of Cretaceous to Eocene ichthyoliths from structural highs to lows, indicating proximity of the Pluto I-87 and Zeus D-14 wells to the Tofino Fault between the Pacific Rim and Crescent terranes and suggesting derivation with coeval Hesquiat Peninsula strata. An unconformity above the Eocene Crescent Formation volcanics in Prometheus H-68 and Zeus D-14 wells is correlated with the Pluto I-87 and Apollo J-14 well stratigraphy. An upper Miocene unconformable surface coincides with accretion and uplift of the Cascadia Accretionary Complex.