Sedimentology, stratigraphy, and structure of the late cretaceous rocks of Mayne and Samuel Islands, British Columbia Public Deposited

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  • An ancient delta complex is partly recorded by the upper seven formations of the Cretaceous Nanaimo Group exposed on Mayne and Samuel Islands of southwestern British Columbia. Features especially suggestive of deltaic sedimentation here are: upward-coarsening marine to fluvial sequences, cyclic repetition of facies, subaqueous slumping, fluvial-marine interfingering, facies changes, lobate geometry, and variations in thickness along strike. Clastics were derived from a varied terrane of the Paleozoic and Mesozoic basement both on Vancouver Island and farther southeast in the San Juan Islands of Washington. These pre-Cretaceous rocks largely include chert, limestone, pyroclastics, granitics , and low-grade metamorphics. Swift, short-headed streams from nearby mountains to the east and south carried detritus to a subsiding marine embayment adjacent to the southeast coast of Vancouver Island. Locally on Mayne Island, the composite section is at least 8, 000 feet thick. The oldest unit, only partially exposed, is the Extension- Protection Formation, made up of interbedded conglomerates and arkosic sandstones. Interfingering with it is the largely superjacent Cedar District Formation comprising a recurrent, marine turbidite facies. It contains a molluscan fauna as is also known in the similar younger strata. Overlying it, the DeCourcy Formation, a second fluvial unit of sandstone to conglomerate, shows a lobate development ascribed to distributary switching. This is the top member of an upward-coarsening, marine to fluvial deltaic cycle. Immediately above, a nearly identical cycle begins with the Northumberland Formation, composed of repetitiously graded turbidites. Overlying them is the Geoffrey Formation, the final part of the second exposed deltaic cycle. Within it, sandstones enclose especially coarse conglomerates, suggesting a change from an easterly to a proximal southerly source. Marine encroachment followed which allowed deposition of the Spray Formation, the third exposed monotonous turbidite sequence. The next higher unit is the fluvial Gabriola Formation which terminated Nanaimo Group deposition and completed the final deltaic cycle. Its upper contact is not exposed. The study area lies on the north flank of the northwesterly trending Trincomali Anticline, which was folded during early Tertiary. Faults have offset this structure locally and at least two separate events are suggested.
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