9.6: Groups, Formations, and Members

Geologists who study sedimentary rocks need ways to divide them into manageable units, and they also need to give those units names so that they can easily be referred to and compared with other rocks deposited in other places. The International Commission on Stratigraphy (ICS) (http://www.stratigraphy.org/) has established a set of conventions for grouping, describing, and naming sedimentary rock units.

The main stratigraphic unit is a formation. A formation is a series of beds that is distinct from other beds above and below, and is thick enough to be shown on the geological maps that are widely used within the area in question. In most parts of the world, geological mapping is done at a relatively coarse scale, so most formations are on the order of a few hundred metres thick. At that thickness, a typical formation would appear on a typical geological map as an area that is at least a few millimetres thick.

A series of formations can be classified together to define a group, which could be as much as a few thousand metres thick, and represents a series of rocks that were deposited within a single basin (or a series of related and adjacent basins) over millions to tens of millions of years.

In areas where detailed geological information is needed (for example, within a mining or petroleum district) a formation might be divided into members, where each member has a specific and distinctive lithology (rock type). For example, a formation that includes both shale and sandstone might be divided into members, one of which is shale, and the other sandstone. If even more detail is required, members may be divided into beds, but that’s only applicable to beds that have a special geological significance. Groups, formations, and members are typically named for the area where they’re found.

The Nanaimo Group: An Example

The sedimentary rocks of the Nanaimo Group on Vancouver Island 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 BC mainland (Figure 9.28). Nanaimo Group comprises a 5000 m thick sequence of conglomerate, sandstone, and mudstone layers. Coal was mined from the Nanaimo Group rocks from around 1850 to 1950 in the Nanaimo region, and is still being mined in the Campbell River area.

Figure 9.28 The distribution of the Upper Cretaceous Nanaimo Group rocks on Vancouver Island, the Gulf Islands, and in the Vancouver area. Source: Steven Earle (2015), CC BY 4.0. View source., modified after Mustard (1994).

The Nanaimo Group is divided into 11 formations (Table 9.3). In general, the boundaries between formations are based on major lithological differences. A wide range of depositional environments existed during the accumulation of the Nanaimo Group rocks, including:

• Nearshore marine for the Comox and Haslam Formations
• Fluvial and deltaic with backwater swampy environments for the coal-bearing Extension, Pender, and Protection Formations
• A deep-water submarine fan environment for the upper six formations.

The differences in the depositional environments are probably a product of variations in tectonic-related uplift over time.

Table 9.3 Nanaimo Group Formations
Age (Ma)	Formation	Lithologies	Depositional Environmemt
~65-66	Gabriola	Sandstone with minor mudstone	Submarine fan, high energy
~66-67	Spray	Mudstone/ sandstone turbidites	Submarine fan, low energy
~67-68	Geoffrey	Sandstone and conglomerate	Submarine fan, high energy
~68-70	Northumberland	Mudstone turbidites	Submarine fan, low energy
~70	De Courcy	Sandstone	Submarine fan, high energy
~70-72	Cedar District	Mudstone turbidites	Submarine fan, low energy
~72-75	Protection	Sandstone and minor coal	Nearshore marine and onshore deltaic and fluvial
~75-80	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
~80-85	Haslam	Mudstone and siltstone	Shallow marine
~85-90	Comox	Conglomerate, sandstone, mudstone (coal in the Campbell River area)	Nearshore fluvial and marine
Source: Steven Earle, with data from Mustard (1994)

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. With the exception of the Haslam formation, they were divided into members, because that was useful for understanding the rocks in the areas where coal was mined.

There is much variety in the Nanaimo Group rocks, and it would take hundreds of photographs to illustrate all of the different types of rocks. Nevertheless, a few representative examples are shown in Figures 9.29-9.31.

Figure 9.29 Nanaimo Group, Spray Formation. Turbidite layers on Gabriola Island. Each turbidite set consists of a lower sandstone layer (light color) that grades upward into siltstone, and then into mudstone. Source: Steven Earle (2015), CC BY 4.0. View source.

Figure 9.30 Nanaimo Group, Pender Formation. Two separate layers of fluvial sandstone with a thin (approx. 75 cm) coal seam in between. Source: Steven Earle (2015), CC BY 4.0. View source.

Figure 9.31 Nanaimo Group, Comox Formation. The metal object is the end of a rock hammer that is 3 cm wide. Almost all of the clasts in this view are well-rounded basalt pebbles cobbles eroded from the Triassic Karmutsen Formation that makes up a major part of Vancouver Island. Source: Steven Earle (2015), CC BY 4.0. View source.

Concept Check: Groups, Formations, and Members

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References

Mustard, P. (1994). The Upper Cretaceous Nanaimo Group, Georgia Basin. In J. Monger (Ed.), Geology and Geological Hazards of the Vancouver Region. Geological Survey of Canada Bulletin 481, 27-95.