54 6.5 Groups, Formations, and Members — Physical Geology – 2nd Edition

Figure 6.5.1 The distribution of the Upper Cretaceous Nanaimo Group rocks on Vancouver Island, the Gulf Islands, and in the Vancouver area. [Image Description]

The sedimentary rocks of the Nanaimo Group provide a useful example for understanding groups, formations, and members. During the latter part of the Cretaceous Period, from about 90 Ma to 65 Ma, a thick sequence of clastic rocks was deposited in a foreland basin between what is now Vancouver Island and the B.C. mainland (Figure 6.5.1). The Nanaimo Group strata comprise a 5000-metre-thick sequence of conglomerate, sandstone, and mudstone layers. Coal was mined from Nanaimo Group rocks from around 1850 to 1950 in the Nanaimo region, and even more recently in the Campbell River area.

The Nanaimo Group is divided into 11 formations as described in Table 6.6.  In general, the boundaries between formations are based on major lithological differences. As can be seen in the far-right column of Table 6.6, a wide range of depositional environments existed during the accumulation of the Nanaimo Group rocks, from nearshore marine for the Comox and Haslam Formation, to fluvial and deltaic with backwater swampy environments for the coal-bearing Extension, Pender, and Protection Formations, to a deep-water submarine fan environment for the upper six formations.
Table 6.6 The formations of the Nanaimo Group[1]
[Skip Table 6.6]
Approximate Age (Ma) Formation name Lithologies Depositional Environment
65 to 66 Gabriola Sandstone with minor mudstone Submarine fan, high energy
66 to 67 Spray (Fine grained) Mudstone/sandstone turbidites Submarine fan, low energy
67 to 68 Geoffrey Sandstone and conglomerate Submarine fan, high energy
68 to 70 Northumberland (Fine grained) Mudstone turbidites Submarine fan, low energy
70 De Courcy Sandstone Submarine fan, high energy
70 to 72 Cedar District (Fine grained) Mudstone turbidites Submarine fan, low energy
72 to 75 Protection Sandstone and minor coal Nearshore marine and onshore deltaic and fluvial
75 to 80 Pender Sandstone and minor coal Nearshore marine and onshore deltaic and fluvial
80 Extension Conglomerate, with minor sandstone and some coal Nearshore marine and onshore deltaic and fluvial
80 to 85 Haslam (Fine grained) Mudstone and siltstone Shallow marine
85 to 90 Comox Conglomerate, sandstone, mudstone (coal in the Campbell River area) Nearshore fluvial and marine

The five lower formations of the Nanaimo Group are all exposed in the Nanaimo area, and were well studied during the coal mining era between 1850 and 1950. All of these formations (except Haslam) have been divided into members, as that was useful for understanding the rocks in the areas where coal mining was taking place.  Within some of those members even some individual beds have been named if they were of specific importance to the mining industry.

Although there is a great deal of variety in the Nanaimo Group rocks, and it would take hundreds of photographs to illustrate all of the different types of rocks, a few representative examples are provided in Figure 6.5.2.


Alternating layers of light and dark rock of various textures

Figure 6.5.2a Representative photos of Nanaimo Group rocks. Turbidite layers in the Spray Formation on Gabriola Island. Each turbidite set consists of a lower sandstone layer (light colour) that grades upward into siltstone, and then into mudstone. (See Figure 6.4.2 for detail.)


A tall, light coloured rock wall with a black streak running through the middle.

Figure 6.5.2b Two separate layers of fluvial sandstone with a thin (approx. 75 centimetres) coal seam in between. Pender Formation in Nanaimo.

Figure 6.5.1 image description: A map showing that Nanaimo Group rocks are present along the east coast of Vancouver Island from Nanaimo to Campbell River, farther inland in areas around Port Alberni and Duncan, on much of the Gulf Islands and a bit in the Vancouver area. [Return to Figure 6.5.1]

Media Attributions

  • Figure 6.5.1: Redrawn based on Mustard, P., 1994, The Upper Cretaceous Nanaimo Group, Georgia Basin, in J. Monger (ed) Geology and Geological Hazards of the Vancouver Region, Geol. Survey of Canada, Bull. 481, pp. 27-95. © Steven Earle. CC BY.
  • Figure 6.5.2abc: © Steven Earle. CC BY.
  1. [Based on data in Mustard, P., 1994, The Upper Cretaceous Nanaimo Group, Georgia Basin, in J. Monger (ed) Geology and Geological Hazards of the Vancouver Region, Geol. Survey of Canada, Bull. 481, p. 27-95.] ↵

<!– pb_fixme –>

<!– pb_fixme –>
<!– pb_fixme –>

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Physical Geology by Cynthia Krutsinger is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book