|Trent River, Photo: Betty Franklin|
The rocks that make up this riverbed today were laid down south of the equator as small, tropical islands. They rode across the Pacific heading north and slightly east over the past 80 million years to where we find them today.
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. And it is massive. At 103 million km2 (40 million sq mi), it is the largest tectonic plate and continues to grow fed by volcanic eruptions that piggyback onto its trailing edge.
This relentless expansion pushes the Pacific Plate into the North American Plate. The pressure subducts it beneath our continent where it then melts back into the earth. Plate tectonics are slow but powerful forces. The island chains that rode the plates across the Pacific smashed into our coastline and slowly built the province of British Columbia. And because each of those islands had a different origin, they create pockets of interesting and diverse geology.
It is these islands that make up the Insular Belt — a physio-geological region on the northwestern North American coast. It consists of three major island groups — and many smaller islands — that stretches from southern British Columbia up into Alaska and the Yukon. These bits of islands on the move arrived from the Late Cretaceous through the Eocene — and continues to this day.
The rocks that form the Insular Superterrane are allochthonous, meaning they are not related to the rest of the North American continent. The rocks we walk over along the Trent River are distinct from those we find throughout the rest of Vancouver Island, Haida Gwaii, the rest of the province of British Columbia and completely foreign to those we find next door in Alberta.
To discover what we do find on the Trent takes only a wee stroll, a bit of digging and time to put all the pieces of the puzzle together. The first geological forays to Vancouver Island were to look for coal deposits, the profitable remains of ancient forests that could be burned to power industry.
Jim Monger and Charlie Ross of the Geological Survey of Canada furthered that work by looking at the complex geology of the Comox Basin in the 1970s. It was their work that helped tease out how and where the rocks we see along the Trent today were formed and made their way north.
We know from their work that by 80 million years ago, the Insular Superterrane had made its way to what is now British Columbia. The lands were forested much as they are now but by extinct genera and families. The fossil remains of trees similar to oak, poplar, maple and ash can be found along the Trent and Vancouver Island. We also see the lovely remains of flowering plants such as Cupanities crenularis, figs and breadfruit.
Heading up the river, you come to a delineation zone that clearly marks the contact between the dark grey marine shales and mudstones of the Haslam Formation where they meet the sandstones of the Comox Formation. Fossil material is less abundant in the Comox sandstones, Here you begin to see fossilized wood and identifiable fossil plant material.
Further upstream, there is a small tributary, Idle Creek, where you can find more of this terrestrial material in the sandy shales. As you walk up, you see identifiable fossil plants beneath your feet and jungle-like, overgrown snarly trees all around you.
|Polyptychoceras vancouverense / BCPA 2003|
The Trent River has yielded some very interesting marine specimens, and significant terrestrial finds. We've found both a wonderful terrestrial helochelydrid turtle, Naomichelys speciosa, and the caudal vertebrae of a Hadrosauroid dinosaur. Walking down from the Hadrosaur site we come to the site of a fossil ratfish find. A little further again and we reach the Polyptychoceras zone and then come to the contact of the two formations. The rocks here have traveled a long way to their current location. With them, we peel away the layers of geologic history of both the Comox Valley and the province of British Columbia.
Photo One: Trent River, Haslam Formation, Trent River. Betty Franklin. Photo Two: Polyptychoceras vancouverense from the Upper Cretaceous (Santonian), Haslam formation, Trent River, Vancouver Island, British Columbia, Canada. 2003 BCPA Calendar.
References: Note on the occurrence of the marine turtle Desmatochelys (Reptilia: Chelonioidea) from the Upper Cretaceous of Vancouver Island Elizabeth L. Nicholls Canadian Journal of Earth Sciences (1992) 29 (2): 377–380. https://doi.org/10.1139/e92-033