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Sedimentology and petrology of the cedar district formation : late cretaceous, southwestern British Columbia. Rahmani, Riyadh Abdul-Rahim

Abstract

The Upper Cretaceous (Campanian) Cedar District Formation of the Gulf Islands and adjacent areas is composed of shale and sandstone, which are present as thick sequences of shale, which may or may not be fossiliferous, and as alternating rhythmic sandstone-shale sequences of the flysch-type. Presence of graded bedding, ripple and convolute laminations, and sole marks in the latter suggest a turbidity current origin. The internal structures of the individual turbidite units correspond largely to the C-E divisions of Bouma (1962) and other authors, and indicate that their deposition took place largely within the lower flow regime. Convolute lamination in the sandstones was formed by oversteepening and deformation of pre-existing ripple lamination and by the deformation of pre-existing plane-parallel lamination by the drag of the overpassing currents. Flute and groove casts and frondescent marks were only found in beds thicker than a foot and a half. Calcareous concretions, most abundant in the shales and occasionally phosphatic, are crossed by organic borings and burrows which are filled with sediments of the surrounding beds. Host rocks of the calcareous concretions tend to thicken around them. The concretions show deformation when present in beds involved in soft-sediment deformation. All these observations suggest their formation in the early stage of diagenesis, probably shortly after, burial. Sandstones of the Cedar District Formation show a gradation from arenites that lack matrix and have a cal-cite cement, to wackes rich in fine-grained matrix. The majority of the wackes and the arenites are feldspathic and arkosic, using the classification of Gilbert (1954). Their composition indicates that the major source was acidic to intermediate igneous and/or low to medium grade metamorphic rock , sedimentary and volcanic rocks were a secondary source. The major source area was possibly a region of high relief that had undergone rapid uplift and erosion, and experienced mainly mechanical weathering. Paleocurrents and lithologic lateral variation indicate that the major source area for the coarse elastics was situated to the east and southeast of the study area. The pre-Jurassic low grade metamorphic rocks of the Cascade Mountains to the east, and the pre-Carboniferous- crystalline rocks of the San Juan Islands to the southeast served as possible source areas for the coarse elastics. Deposition of shaley, fossilif erous parts, of the formation in the southeastern part, of the study area, took place in littoral to upper neritic depths. Turbidite (flysch-type) sequences were deposited in deeper water, below the wave base. The unfossiliferous shale of the central and northern parts of the study area was deposited either at about the same depths as the turbidites, or in deeper water, since thin, delicate, horizontal and-cross laminations are preserved in these rocks. Paleontologic evidence suggests that deposition took place in a somewhat restricted basin having a narrow connection with the open ocean to the west. Paleontologic and mineralogic data suggest that the bottom conditions of the central and northern parts of the basin of deposition were stagnant and reducing. Facies relationships suggest that the basin of deposition had its longest dimension trending SE-NW. Its eastern, southeastern, and southern boundaries were situated between the mainland of British Columbia-Washington and the Gulf-San Juan Islands. Its northern and northwestern boundaries were possibly near the city of Nanaimo and Gabriola Island. To the west, it was connected at least partially to the open ocean. In the southeastern part of the study area, alternation of thick, fossiliferous shale sequences, and sequences which are predominantly turbidites suggests fluctuations in the depth of the basin floor, either due to changes in sea level or to tectonic movements.

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