Stratigraphy and sedimentology of the Cretaceous Nanaimo group, Saltspring Island, British Columbia Public Deposited

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  • Clastic sedimentary rocks of the Nanaimo Group on Saltspring Island, British Columbia are Late Cretaceous in age, overlie a deformed and intruded Paleozoic basement complex, and consist of eight formations. These eight formations, from oldest to youngest, are the Comox, Haslam, Extension-Protection, Cedar District, De Courcy, Northumberland, Geoffrey, and Spray Formations. The Formations vary greatly in thickness, consist of multiple members, and show complex intertonguing relationships. A composite section of minimum thickness totals approximately 9,000 feet. In the study area, the Nanaimo Group is tentatively divided into two subgroups, the lower Nanaimo Group - Comox through Extension-Protection Formations - and upper Nanaimo Group - Cedar District through Spray Formations. The lower Nanaimo Group consists of a series of three transgressive sequences, each of which overlies an erosional surface and shows an upward-fining of grain size and progression from nonmarine or nearshore marine to offshore marine deposition. The upper Nanaimo Group can be divided into a total of five progradational cycles which show upward-coarsening from pro-delta mudstone to delta-front sandstone, and locally to fluvial-marine channel conglomerate. Prodelta muds were probably deposited at lower neritic to upper bathyal water depths. The Comox Formation was deposited as a delta or fan delta with a braided fluvial system and flanking littoral sandstones including beaches which contain heavy mineral placers and tidal creek channels. The Extension-Protection Formation was deposited along a piedmont plain with laterally contiguous marine turbidites. The Cedar District and De Courcy Formations intertongue through a stratigraphic interval of at least three thousand feet. Four Cedar District to De Courcy progradational cycles are characterized by an upward progression from offshore marine mudstone and distal turbidite to proximal turbidite or/and fluidized sediment flow and locally to a laminated and cross-bedded sandstone and channelized conglomerate lithofacies. The sequences are indicative of repeated superposition of prodelta, delta-front slope, and fluvial-marine environments of deposition. The Northumberland and overlying Geoffrey Formation comprise a very thick progradational sequence complete with a thick and extensive conglomeratic interval in the Geoffrey Formation. Mineralogy and Paleocurrent data of the Comox Formation suggest derivation from low to intermediate grade metamorphic and intermediate to basic igneous rocks located a short distance to the southwest. The overlying sandstone-conglomerate formations record a decreased contribution from metamorphic and basic igneous rocks and increased contribution from intermediate to silicic plutonics and volcanics. De Courcy paleocurrent data suggest derivation from the east: sandstone compositions indicate the major source was an uplifted region of intermediate plutonic rocks which was rapidly eroded and subject to mainly mechanical weathering. The Geoffrey Formation shows an abrupt increase in intermediate to silicic volcanic debris possibly derived from the Bonanza Volcanics located to the south and southwest on Vancouver Island. Post-depositional mineralogic changes include widespread cementation by calcite, laumontite, and quartz. The assemblage laumontite plus prehnite is locally developed in the Comox Formation. Laumontite occurs in all overlying sandstone formations. A green diagenetic matrix was derived from the in situ alteration of volcanic rock fragments and is particularly abundant in the Geoffrey Formation. Tertiary faulting and folding of the study area formed the Kulleet Syncline, Trincomali Anticline, a system of northwest-trending strike faults and a system of north-trending transverse faults.
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