Inclined Heterolithic Stratification in a Mixed Tidal-Fluvial Channel: Middle Arm, Fraser River, British Columbia, Canada

Inclined Heterolithic Stratification (inclined interbeds of sand and mud; IHS) is a commonly observed stratal architecture in mixed tidal-fluvial channels, both in the modern and in the rock record. However, the character of IHS varies along strike depending upon the dominant depositional process within the channel (i.e., fluvial, mixed tidal-fluvial, tidal). To assess the nature of IHS in subequally mixed tidal-fluvial channels, research was conducted on three deposits in the Middle Arm of the lower Fraser River, British Columbia, Canada, including a channel-margin deposit, an in-channel bar, and a point-bar. The channel margin deposit is situated in the most landward position, and the point bar in the most seaward position.

Sedimentologically the IHS in the mixed tidal-fluvial Middle Arm varies as a function of position within the channel. The most landward deposit (8 km inland from the delta front), the channel margin IHS is mud-dominated. Three kilometers seaward of the channel-margin deposit, the in-channel bar is predominantly sand-rich. Finally, the most seaward bar, the point-bar (within the intertidal zone; 4 km from the delta front), shows significant variability with higher sand percentages on the seaward end of the bar. Ichnologically, burrow diversity, burrow density, and trace size increases in the seaward direction. The channel-margin and in-channel bar deposits contain Polykladichnus, Skolithos, Arenicolites, Siphonichnus and Palaeophycus. The point-bar at the downstream end of the channel has the same traces as above, in addition to an abundance of large Siphonichnus produced by large resident bivalves.

By comparing the sedimentologic and ichnologic character of IHS within the various deposits of the Middle Arm to hydraulic conditions in the channel, a number of conclusions are drawn. 1) At the seaward end of the system, where tidal input exceeds fluvial input, sand is more abundant in the seaward direction. 2) At the landward end of the study area, where fluvial input appears to match tidal input, mud is dominant, possibly marking the position of the turbidity maximum zone. 3) In-channel bars tend to be more sand-rich than channel-margin deposits and point bars. 4) The degree of marine (saltwater) influence in the system is best-expressed ichnologically. Higher salinity and prolonged residence time of saltwater within the channel is marked by an increase in trace diversity and density, and by an overall increase in burrow size.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California