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Stratigraphy, structural geology and petroleum potential of Cretaceous and Tertiary rocks in the central Graham Island area, Queen Charlotte Islands, British Columbia Indrelid, Jarand


Mapping at 1:25 000 scale on the central Graham Island has shown that Cretaceous strata are more widely distributed than previously known. This study examines the stratigraphy and structural geology of the Cretaceous rock sequence, and also addresses the petroleum potential of these units. At the base is the Cretaceous sandstone unit. This unit is divided into three lithofacies. The massive sandstone lithofacies is a coarse grained, dark green to greenish black, massive sandstone. Parts of this lithofacies contains up to 50 % glauconite. The grey sandstone lithofacies is finer grained and has better defined bedding than the massive sandstone. It is frequently found with interlayered sandstone, siltstone, and shale. The third sandstone lithofacies is characterized by pervasive bioturbation. All three lithofacies are texturally immature, contain angular quartz and feldspar, and are rich in chlorite clay. The Cretaceous sandstone unit is interpreted as a transgressive sequence deposited on a storm dominated shelf. Conformably overlying the sandstones are the massive friable shale and silty shale of the Cretaceous shale unit. Intervals with increased input of storm generated sandstone layers are found throughout the unit. Spherical and elliptical calcareous concretions up to over 1 m across are common. The Cretaceous shale unit represent a continuation of marine transgression with deepening of the sedimentary basin. Turbidites forming the Skidegate Formation are interbedded with the upper part of the shale unit. This formation consists of interbedded shale, siltstone, and fine grained sandstone. Sedimentary structures are often well developed on bedding surfaces. The rocks of this unit are distal turbidites and levee deposits of a submarine fan. Coarse clastic rocks of the Honna Formation are interbedded with the Skidegate Formation. This formation is dominated by pebbly conglomerates and coarse grained sandstones. The clast material in the conglomerate lithofacies is mainly derived from units present on the islands. The sandstone lithofacies consists of indurated, bluish, medium- to coarse-grained sandstone. This formation is richer in quartz and biotite than any other Cretaceous sandstones of the central Graham Island. The Honna and Skidegate formations are the result of deposition from a submarine fan system that was initiated in Late Cretaceous time. Deposition of shale continued after the deposition of the submarine fan-related formations terminated. The Cretaceous rock sequence is overlain by Tertiary volcanic and sedimentary rocks. Volcanic rocks occur throughout the area, and sediments of the Skonun Formation are exposed in north. Three major periods of deformation are recorded in the Cretaceous units. The first event was a Late Cretaceous to Early Tertiary northeast directed compression, resulting in northwest trending folds and thrust faults. The deformation was highly localized to areas were weakness zones existed in the older basement rocks. Two periods of Tertiary block faulting activity are recognized. The first resulted in northwest-trending faults, parallel to older structures. Later Tertiary block faulting developed northeast trending faults, which are the youngest macroscopic structures in the area. The Cretaceous rock sequence does not contain any promising hydrocarbon source or reservoir rocks. The TOC, S1, and S2 values from Rock-Eval® pyrolysis are low for all units, and the organic material present is mostly gas prone. Visual porosity is generally poor, as a result of chlorite pore-filling clay and calcite cement.

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