To understand the ground we are walking upon, we need to peel back the layers of time. Hornby has a total stratigraphic thickness of 1350 m of upper Nanaimo Group marine sandstone, conglomerate and shales.
These sediments are what we search through to find wonderfully preserved fossils where outcrops along the beachfront proffer them partially exposed.
Four formations underlie the island from oldest to youngest, and from west to east: the Northumberland, Geoffrey, Spray and Gabriola.
During the upper Cretaceous, between ~90 to 65 Ma, sediments derived from the Coast Belt to the east and the Cascades to the southeast poured seaward to the west and northwest into what was the large ancestral Georgia Basin. This major forearc basin was situated between Vancouver Island and the mainland of British Columbia.
The Nanaimo Group as a whole represents largely coarse-grained units deposited in deep-sea fan systems. In this environment, deeper channels continuously cut through successive shale and sandstone bodies. The channels funnelled density currents into the basin, while also building levee deposits.
During the upper Cretaceous, between ~90 to 65 Ma, sediments derived from the Coast Belt to the east and the Cascades to the southeast poured seaward to the west and northwest into what was the large ancestral Georgia Basin. This major forearc basin was situated between Vancouver Island and the mainland of British Columbia.
The Nanaimo Group as a whole represents largely coarse-grained units deposited in deep-sea fan systems. In this environment, deeper channels continuously cut through successive shale and sandstone bodies. The channels funnelled density currents into the basin, while also building levee deposits.
Turbidity currents travelled down the channels, and also overtopped the levees spilling across backslope areas. Think of it as a thoroughfare of motion by our softer sediments. The sequential sediment formations, from significantly coarse-grained sandstones and conglomerates to fine silts and shale units of the Nanaimo Group, are considered to be partly due to eustacy, but more significantly related to relative sea-level changes induced by regional tectonics in an active forearc setting.
The eustatic sea level is the distance from the centre of the earth to the sea surface. An increase of the eustatic sea-level can be generated by decreasing glaciation, increasing spreading rates of the mid-ocean ridges or more mid-oceanic ridges.
The Northumberland Formation is made up of a massive, dark-grey mudstone that is locally interlaminated and interbedded with siltstone and fine-grained sandstone. There are abundant calcium carbonate concretions, parallel and current ripple laminations, clastic dikes and folded layers due to slumping. In the Gulf Islands to the south, this formation has yielded abundant and diverse foraminifera, tiny single-celled organisms, members of a phylum of amoeboid protists, that tell us these marine paleodepths formed at 150-1200 metres below the ocean surface.
The Northumberland Formation is made up of a massive, dark-grey mudstone that is locally interlaminated and interbedded with siltstone and fine-grained sandstone. There are abundant calcium carbonate concretions, parallel and current ripple laminations, clastic dikes and folded layers due to slumping. In the Gulf Islands to the south, this formation has yielded abundant and diverse foraminifera, tiny single-celled organisms, members of a phylum of amoeboid protists, that tell us these marine paleodepths formed at 150-1200 metres below the ocean surface.
The more resistive Geoffrey Formation consists of thick-bedded sandstone and conglomerate. It is highly channelized, and some sandstone has exposed parallel and ripple laminations. The Spray Formation exposed on the east end of the island is a massive olive-grey mudstone with interlaminations of sandstone.
Furthest to the east, the youngest exposures on Hornby Island are from the Gabriola Formation, which outcrops on the eastern peninsula. This is again a thick-bedded and channelized sequence of conglomerates and massive sandstone with minor mudstone interbeds. South, in the Gulf Islands, this formation has contained ammonites, gastropods and pelecypods. Paleowater-depth from foraminiferal assemblages has been set at 200 m.
Katnick, D.C. and P.S. Mustard (2001): Geology of Denman and Hornby Islands, British Columbia (NTS 92F/7E, 10); British Columbia Geological Survey Branch, Geoscience Map 2001-3.
England, T.D.J. and R. N. Hiscott (1991): Upper Nanaimo Group and younger strata, outer Gulf Islands, southwestern British Columbia: in Current Research, Part E; Geological Survey of Canada, Paper 91-1E, p. 117-125.
Photo: Heidi Henderson aka the Fossil Huntress — Palaeo Sommelier / Explorer-in-Residence
