{"id":386,"date":"2018-08-24T15:11:49","date_gmt":"2018-08-24T19:11:49","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/physicalgeologyh5p\/chapter\/9-6-groups-formations-and-members-2\/"},"modified":"2021-07-26T01:09:00","modified_gmt":"2021-07-26T05:09:00","slug":"9-6-groups-formations-and-members-2","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/physicalgeologyh5p\/chapter\/9-6-groups-formations-and-members-2\/","title":{"raw":"9.6 Groups, Formations, and Members","rendered":"9.6 Groups, Formations, and Members"},"content":{"raw":"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\/<\/a>) has established a set of conventions for grouping, describing, and naming sedimentary rock units.\r\n\r\nThe main stratigraphic unit is a formation<\/strong>. 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.\r\n\r\nA series of formations can be classified together to define a group<\/strong>, 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.\r\n\r\nIn areas where detailed geological information is needed (for example, within a mining or petroleum district) a formation might be divided into members<\/strong>, 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.\r\n

The Nanaimo Group: An Example<\/h1>\r\nThe 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.\r\n\r\n \r\n\r\n[caption id=\"attachment_382\" align=\"aligncenter\" width=\"500\"]\"\"<\/a> Figure 9.28<\/strong> 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.<\/a>, modified after Mustard (1994).[\/caption]\r\n\r\nThe 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:\r\n