The stratigraphy, structure, and sedimentology of the cretaceous nanaimo group, Galiano Island, British Columbia

Graduation date: 1977Presentation date: 1976-06-03Four formations of the Cretaceous Nanaimo Group crop out on Galiano Island and on Parker and Gossip Islands, nearby. These formations are: early Campanian Geoffrey Formation which encloses a tongue of the Northumberland Formation; early Campanian to late Maestrichtian Spray Formation; and Maestrichtian Gabriola Formation. The stratigraphic succession has a maximum composite thickness in excess of 6, 400 feet.\ud The formations are the product of two types of deposition: 1) non-marine fluvial, and 2) nearshore to offshore marine. The nonmarine\ud and marine deposits intertongue throughout the sequence resulting in two cycles of upward-coarsening grain size, interpreted as having formed in a short-headed stream-delta.\ud Paleocurrent direction data and modal analyses of the formations indicate the particles composing the rocks, predominantly quartz, feldspar, mica, and volcanic rock fragments, were derived from the pre-Cretaceous rocks of the coastal mountains of Vancouver and Saltspring Islands that lay to the south-southwest. The Galiano area is structurally simple. The area, located\ud entirely on the northeast flank of the Trincomali Anticline, consists of strata that form a simple homocline dipping northeast. Faults are not common. The fault system trends N. 200 E. and consists of steeply\ud dipping normal faults, southeast blocks down relative to northwest blocks. The fracture system, consisting of a set trending N. 240 E. with dips near vertical and a set trending N. 450 W. dipping 800 south with dips near vertical and a set trending N. 450 W. dipping 800 southwest is ubiquitous but has little topographic expression. It was analyzed to establish the stress system responsible for the faults and fractures in the area. The results suggest that the fractures were formed by compression in a north-northwest--south-southeast direction