Sedimentation in a Fjord-Head Delta, Bella Coola, British Columbia

Abstract

<p>The Bella Coola delta is a laterally confined, steep-fronted, coarse-grained fjord-head delta in North Bentinck Arm in tile Coast Mountains of British Columbia. River mouth morphodynamics are directly related to the large tidal range (3.9 m mean range, 5.9 m large range). The tide changes the location and nature of the zone of marine-riverine interaction, producing a high tide and a low tide outlet. Frictional forces dominate at high tide as the effluent disperses over the delta plain through four morphodynamic zones and a radial bar. During low tide, frictional, inertial and buoyant forces and an internal hydraulic jump control the formation of five morphodynamic zones and a lunate-middle ground bar.</p> <p>The subaqueous delta consists of a sandy delta front with slopes of 2.2 to 15° and a silt-clay prodelta sloping at 0.5 to 2.2°. Debris flow or turbidity current chutes and their depositional zones, shallow and deep-seated rotational slides, growth faults and folds are the primary morphologic components. Sediment instability and deformation are mainly a result of gravitational forces, earthquake loading, retrogression, oversteepening of distributary mouth bar fronts, and excess pore pressures produced by gas generation, depositional loading and rapid tidal drawdown.</p> <p>The sediment package of the Bella Coola delta and North Bentinck Arm consists of two seismic sequences units, a lower probably Pleistocene sequence and an upper delta - related sequence. A glacial moraine facies and a glaciomarine facies characterizes the lower unit. The upper sequence delta plain sediments are well stratified sands and gravels deposited by distributary channels, with some rippled sands and peat. Delta front sediments probably consist of sand and mud laminae deposited from suspension, debris flow sands and gravels, graded sand and slit associated with turbidity currents and undisturbed bedding in slide blocks. Massive silts and clays deposited from suspension and graded sand and silt turbidities compose the prodelta. Rotational slides, growth faults with displaced beds and roll-over anticlines, and folds related to creep processes have deformed prodelta sediments.</p>