Late Cretaceous to Paleogene evolution of the Georgia Basin, Southwestern British Columbia

England, Timothy David John (1989) Late Cretaceous to Paleogene evolution of the Georgia Basin, Southwestern British Columbia. PhD thesis, Memorial University of Newfoundland.

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Abstract

Georgia Basin is an elongate forearc basin that initially developed in the Late Cretaceous (Turonian - Maastrichtian). Its origin is ascribed to subduction- related lithospheric downwarping of Wrangellian crust trapped within the arc-trench gap during active convergence on the Pacific margin of North America. The sedimentary record of this period of basin development is the siliciclastic Nanaimo Group. The group is composed of distinct, alternating, dominantly coarse grained and fine grained formations, deposited during times of high and low rates of sediment delivery to the basin, respectively. The main control on grain size is believed to be local tectonic activity. Restored thicknesses exceed 5 km. Study of the group in the western part of the basin on eastern Vancouver Island and adjacent islands reveals a broad spectrum of facies associations, including alluvial, fluvial, paralic, and neritic to mid-bathyal marine deposits. A series of regional facies maps are presented for successive stages of development of the basin. Based on new paleontological data and detailed stratigraphic and structural studies, revisions are proposed to the lithostratigraphic nomenclature of the Nanaimo Group. -- Vitrinite reflectance data show that much of the Nanaimo Group is mature for oil and gas generation. Vitrinite reflectance/depth gradients in the basin range from 0.17 to 0.21 log %R₀/km. Thermal history modeling shows that prevailing geothermal gradients during burial were low (heat flux < 58 mW/m²). Tectonic subsistence was up to 2.7 km in 22 m.y., occurring at essentially constant rates between 100 and 200 m/m.y.. The eastern part of Georgia Basin lies beneath Georgia Strait and/or Cenozoic deposits. The western part of the basin remained buried until mid- to late Eocene after which it was tectonically shortened and uplifted. -- The southwestern part of Georgia Basin is now preserved as a southwest-verging, linked thrust system involving the Nanaimo Group and its Wrangellian basement, termed the Cowichan fold and thrust belt. The thrust system is interpreted to be a leading imbricate fan. The geometry of the belt from plan and profile perspectives illustrates its thick-skinned structural style. Thrusting was most likely in-sequence, creating an estimated minimum 20-30% shortening at the basement/cover interface. Both fault-propagation and fault-bend folding are evident, the former being more common. Kinematic indicators and the geometry of the belt show no evidence of significant transpressional or transtensional displacement fields. -- The sole fault of the thrust system is interpreted to rise from northwest to southeast on a series of deep lateral to oblique ramps and merge with the eastern extension of the San Juan Fault. This explains the rapid westward thickening of the wedge of thrusted Wrangellian basement. In the east, the sole fault is interpreted to locally have permitted the Nanaimo Group to overthrust the San Juan Terrane. Investigation of the Cowichan fold and thrust belt reveals how shortening during westward-progressing terrane accretion is accommodated in a forearc region that is dominated by a large slab of rigid, semicontinental crust.

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