Mud-Bed Thicknesses, Distributions and Cyclicity Along Channel Margins Across the Tidal-Fluvial Transition, Lower Fraser River, BC, Canada

Andrew D. La Croix; Shahin E. Dashtgard; James A. MacEachern

Sedimentation across the tidal-fluvial transition of the lower Fraser River is a function of the interplay of fluvial flow and tidal flux, and the degree of saltwater – freshwater mixing. The local hydrodynamic conditions in the channels determine the distribution of sand and mud, with mud concentrated in the turbidity maximum zone. Vibracores were collected from three channel bars in the tidally influenced reaches of the Fraser River, to assess the lateral distribution and thickness of mud layers and link them to the causative hydrodynamic conditions.

In the freshwater tidal reach near Fort Langley, muds are thin (mm to cm) and laterally continuous for meters¬ – tens of meters. They accumulate in the upper intertidal zone and are planar laminated, reflecting suspension settling during base-flow conditions. Intervals lack cyclic mud deposition, with <5 mud beds/meter in vertical profile. Bioturbation is sparse (BI 0-1), with rare horizontal traces (e.g., Planolites) of deposit-feeders. In the brackish-water, tide-influenced reach at Port Mann, muds are thicker (cm to dm) and span tens – hundreds of meters laterally. They are deposited in the uppermost subtidal and intertidal zones, are planar laminated or show floccule ripples, and are more abundant (~5 beds/meter in vertical profile). Bedding cyclicity is not well expressed, due to the dominance of fluvial processes. Bioturbation is reduced (BI 0-2) and patchily distributed, consisting of vertical dwelling traces (e.g., Skolithos, Polykladichnus) subtending into muds from sand-mud contacts. In the brackish-water, strongly tide-influenced reaches in Canoe Pass, muds are thickest (cm to dm) and extend laterally for hundreds of meters – kilometers. They comprise stacked floccule ripples, with lesser structureless and laminated layers, reflecting dynamic mud deposition. A weak seasonally induced cyclicity occurs, with muds ranging from 5-10 beds/meter in vertical profile. Bioturbation shows BI 0-2, is patchily distributed, and comprises diminutive deposit-feeding (e.g., Planolites, Teichichnus) and dwelling (e.g., Skolithos, Polykladichnus) traces. Burrows occur in both sand and mud beds.

This semi-quantitative comparison of hydrodynamics and mud characteristics is of fundamental importance to the production of unconventional hydrocarbons from tidal-fluvial reservoirs (e.g., McMurray Fm). The study shows that these types of mud layers control heterogeneity and, ultimately, reservoir compartmentalization.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013