Inclined heterolithic stratification in a mixed tidal-fluvial channel: Differentiating tidal versus fluvial controls on sedimentation
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| Authors: | Johnson, SM; Dashtgard, SE |
| Year: | 2014 |
| Journal: | Sediment. Geol. 301: 41-53 Article Link (DOI) |
| Title: | Inclined heterolithic stratification in a mixed tidal-fluvial channel: Differentiating tidal versus fluvial controls on sedimentation |
| Abstract: | Tidal and fluvial processes control deposition and determine the sedimentological and ichnological character of sediments in the mixed tidal fluvial Middle Arm, lower Fraser River, Canada. Sedimentological trends that define the mixed tidal fluvial zone include: 1) mud beds present from the intertidal zone to the base of the channel, 2) a downstream increase in the number of mud beds, and 3) a lateral mud sand mud profile developed in the intertidal zone of each bar. Non-rhythmic deposition of sand under conditions of elevated river discharge is apparent, although sand beds are interbedded with cm- to dm-scale mud beds deposited during periods of low river discharge and increased tidal influence. In rare cases, mm- to cm-scale rhythmically alternating sand and mud lamina are deposited in successions of 12-14 beds (tidal rhythmites). Ichnologically, burrow density, trace size, and homogeneity in the vertical distribution of traces increase in the seaward direction. Burrows in all of the bars are evenly distributed in the vertical profile through substrates of similar grain size; however, infaunal distributions vary as a function of grain size and subaerial exposure. Muddier sediments and substrates that experience less subaerial exposure display a higher degree of bioturbation (bioturbation index [BI] 2-5). Sand beds are generally bioturbated to a lesser degree (BI 0-1) except in the lower delta plain, where higher degrees of bioturbation (BI 3-5) can be produced. The sedimentological and ichnological character of sediments in the tidal fluvial Middle Arm showcases subtle, but important differences between tide-influenced, river-dominated inclined heterolithic stratification (IHS) and mixed tidal fluvial IHS. In particular, more homogeneous and diverse burrowing in both mud and sand beds, more rhythmic sand mud interbedding, mud deposition to the base of the channel, and the development of a mud sand mud profile along the length of the bar are considered indicative of heightened tidal influence and sustained brackish-water conditions. (C) 2013 Elsevier B.V. All rights reserved. |
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