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Landslides in the Charlie Lake map sheet, Fort St. John Severin, Jordan M.

Abstract

Many large, deep-seated landslides have occurred along the Peace River Valley and its steep-sided tributary valleys. These failures mostly occur within the Lower Cretaceous Shaftesbury Formation, a horizontally-bedded marine shale and the interglacial silt and clay beds in the Quaternary sequence. A detailed inventory of landslides was compiled in the Fort St. John area, using air photo interpretation and engineering-geological field mapping. The study area is on NTS Map Sheet 94A, comprising 14,000 km² of terrain. Practically all slope activity in the study area occurs on slopes of stream valleys. The observed landslides were separated into a series of failure types, involving bedrock, Quaternary deposits or both. The estimated areas of individual slide units range up to approximately 10,000 m² and volumes to 10,000,000 m². The result of the inventory is a digital landslide map at a 1:20,000 scale and a corresponding database of landslide characteristics. The main focus of the typological classification is the failure mechanism. The classification was reinforced by more detailed field study of selected type cases. There is a prevalence of multi-level failures, approximately 48%, utilizing weak surfaces at multiple levels. This type of failure occurs extensively both in the Quaternary sequence and the Cretaceous bedrock. An activity classification was implemented based on field observations and dated air photographs. The classification comprises landslides that are very active, active, of low activity, ancient, and anthropogenically modified. Results indicate that approximately 90% of the total length of valley slopes in the study area are involved in mass movements. Approximately 15% are occupied by very active or active landslides and 61.0% are Inactive - Dormant. Four areas were chosen to be studied in detail due to their accessibility from Fort St. John, diverse failure mechanisms, and diverse local stratigraphies. A generalized stratigraphic column and landslide model was produced for each area. These were generated from the detailed landslide investigations done.

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