The Architecture of Prograding Sandy-Gravel Beach Ridges Formed During the Last Holocene Highstand: Southwestern British Columbia, Canada

摘要

This study was conducted to establish the sedimentary architecture of a beach-ridge system that developed in response to rising and then stabilizing sea level in a macrotidal setting. The beach ridges document a shift from transgressive to highstand depositional conditions. A beach-ridge complex is being deposited on the Boundary Bay tidal flats that flank the Fraser River delta in southwestern British Columbia. The delta onlapped an offshore island, during the Holocene, and partially incorporated it into its delta plain; sand and gravel eroded from this island is the sediment source for the beach ridges. Four depositional–geomorphic environments are identified: beach ridges, lagoon and swales, washover fans, and tidal flats. The beach ridges prograded over the tidal flats of the Fraser delta when sea level stabilized and the late-Holocene highstand was established. The very earliest beach ridges were deposited about 5200 to 4800 years BP, but most were formed after 2250 years BP with the establishment of the Holocene highstand. Beach-ridge progradation rates varied between 1.6 m a–1 to 0.48 m a–1. Ground penetrating radar (GPR) was used to provide continuous two- and three-dimensional images of the facies in order to delineate the depositional architecture of the sandy-gravel beach ridges. We supplemented the customary use of well logs and cores for ground truthing GPR lines with cone penetration testing (CPT), which provided a detailed suite of logs invaluable for correlating radar reflections with subtle lithologic variations unidentifiable in core. A synthesis of geomorphic environments, lithologic logs, and radar facies is used to construct a depositional model of the beach-ridge complex.