Late cenozonic geology of the southern Rocky Mountain trench, British Columbia
Creators: name: Clague, John Joseph
Publisher: name: University of British Columbia
Year: 2011

description: Geologic studies which have provided information on the late Cenozoic history of the southern Rocky Mountain Trench, British Columbia include: (1) the St. Eugene Formation and the origin of the southern Rocky Mountain Trench, (2) the stratigraphy and correlation of Quaternary sediments, (3) the patterns of glacier flow and the origin of late Wisconsinan till, and (4) the sedimentology and paleohydrology of late Wisconsinan outwash. (1) The Tertiary history of the southern Rocky Mountain Trench is inferred from a study of the distribution, stratigraphy, fabric, lithologic composition, structure, and palynology of the Miocene St. Eugene Formation. This unit consists of flood-plain and fan facies and represents the uppermost Tertiary sediments filling block-faulted basins in the Trench. Sediment deposition was probably contemporaneous with faulting, but at least 600 m of displacement along the east side of the Trench postdates deposition of the St. Eugene Formation. The Miocene southern Rocky Mountain Trench was similar in morphology and position to the present Trench, but the Miocene climate was more temperate than the present climate of the area. Palynological results indicate that during the Miocene southeastern British Columbia had abundant summer precipitation and mild, moist winters. (2) Exposed Pleistocene sediments correlate with deposits of the Pinedale Glaciation and the preceding interglaciation. Deposits of three stades and two interstades of the Pinedale Glaciation are recognized. During the earlier interstade the floor of the Trench in southeastern British Columbia was deglaciated, whereas during the later interstade residual ice apparently remained locally along the center of the Trench-CS) Pinedale glacier-flow patterns and till genesis in the southern Rocky Mountain Trench were determined through a study of glacial landforms, till fabric, and till composition. A review of the ways in which directed till fabrics originate indicates that till associated with drumlins accumulated by lodgement of particles due to subglacial pressure melting. Landforms and till fabrics document one major shift in the pattern of glacier flow near the end of glaciation. In mountainous areas till composition is less sensitive as an indicator of such shifts than till fabric. Till composition instead reflects the dominant pattern of ice flow during glaciation. Till constituents decrease with distance from their bedrock sources because of progressive deposition, dilution through sediment mixing, and breakage and abrasion during transport. Other factors which may affect the distribution and relative abundance of constituents in Trench tills include reworking at the ice-sediment interface and transport of constituents englacially and subglacially by meltwater. (4) Late Wisconsinan channeled outwash is coarse, poorly sorted, shows large-scale cross-bedding, and was deposited in channel-bar complexes of high-energy rivers. Peak discharges calculated from channel morphometry and maximum particle size are larger than maximum discharges attributable to uninterrupted summer runoff. Many channels transmitted valleys. An empirical relationship between total volume discharged during documented jbkulhlaups and corresponding maximum instantaneous discharges is applied to one Pleistocene glacier-dammed lake in the study area to show that discharges equal to or larger than those calculated from channel morphometry were attained during jokulhlaups. Some applications of these geologic studies include the prediction of flood magnitudes from self-dumping glacier-dammed lakes, the determination of the source of ore clasts or minerals in till, and a preliminary assessment of the groundwater resources of the southern Rocky Mountain Trench.