Possible Suspended-Load Channel Deposits from the Wealden Group (Lower Cretaceous) of Southern England
- J. D. Collinson,
- J. Lewin
Published Online: 29 APR 2009
Copyright © 1983 The International Association of Sedimentologists
Modern and Ancient Fluvial Systems
How to Cite
Stewart, D. J. (2009) Possible Suspended-Load Channel Deposits from the Wealden Group (Lower Cretaceous) of Southern England, in Modern and Ancient Fluvial Systems (eds J. D. Collinson and J. Lewin), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303773.ch30
- Published Online: 29 APR 2009
- Published Print: 7 FEB 1983
Print ISBN: 9780632009978
Online ISBN: 9781444303773
- possible suspended-load channel deposits – Wealden Group, Southern England;
- heterolithic sedimentary units;
- Hastings beds;
- dipping beds;
- crevasse splay units
The Lower Cretaceous Wealden Group of Southern England contains heterolithic sedimentary units ranging from 2 to 10 m thick which cut into varicoloured rootlet- and mudcrack-bearing mudstones similar to those of some recent tropical flood plains. The units display inclined bedding resulting from lateral accretion and have, therefore, been interpreted as river point bars or benches. In addition the units display the following features: (i) sequences generally fine upwards, (ii) they contain considerable amounts of claystone and siltstone either interbedded with very fine-grained quartz sand or beds of sand-grade mudclasts, (iii) the lateral accretion units dip at steep angles, (iv) cross-bedding within the units is mainly of a small scale, and (v) coarse sand- to pebble-sized material is almost entirely intraformational.
These features differ in some respects from the point bars of mixed-load channel systems, which typically form sand-dominated vertical sequences. The Wealden point bars/benches more closely resemble those described from channel systems carrying dominantly suspended loads, found both on mature low-gradient flood plains and tidal flats. This type of point bar is typically very silt/clay rich and the slope (accretion surface) of the point bar or bench is steep owing to the low width/depth ratio of the channel. The thickness of sediment accreted on to the point bar during any one flood cycle appears to have been small, but the rate of point bar migration could have been fairly rapid if floods were frequent.
Variations in the sand content of these point-bar/bench units, and their association with sand-dominated fluvial point-bar units, suggest that Wealden rivers varied in space and time from suspended-load systems to mixed-load systems. The main controls on these variations were probably climate and tectonics, and the consequential alteration of river gradients and sediment supply.
This study provides a rare example of suspended-load point bars or benches in the fossil record. It also highlights the difficulty of distinguishing muddy point-bar deposits from the adjacent flood-plain facies and, in particular, from crevasse splay units which are commonly interbedded with such facies. One must, therefore, consider the overall association of sedimentary facies when interpreting such units.