Jones, Colin Michael (1977) The sedimentology of carboniferous fluvial and deltaic sequences: the Roaches grit group of the South-west Pennines and the Pennant sandstone of the Rhondda valleys. Doctoral thesis, Keele University.

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Abstract

This thesis describes two Carboniferous fluvio-deltaic sequences in which river channel fills are dominant, the Roaches Grit Group (Namurian R2b) in the south-west Pennines and Lower Pennant Measures (Westphalian C) in the Rhondda Valleys. In addition, sedimentation in non meandering sandy rivers is reviewed and discussed.
Non meandering rivers can be classified into straight and two braided types. In the former, alternate bars attached to the channel sides give rise to solitary sets of cross bedding which are eroded during falling stage. In braided rivers channel fill characteristics vary with discharge regime. These range from regimes with pronounced short lived flood peaks with rapid rising and falling stages, probably producing relatively uniform cross-bedded fills, to those which show limited variation between high and low stage and where high stage sediments are mainly reworked. Multiple channel braided rivers should have channel abandonment sequences showing alternatin1 periods of bedform movement and ponded water.
Palaeocurrents vary with regime and some braided rivers may show a variance similar to that of meandering rivers.
The Roaches Grit Group is an overall coarsening upwards sequence 375m thick at maximum, with Deep Water, Delta Slope, Delta Top and Delta Margin Associations. Within these, 20 lithofacies are described and discussed. The lowest Deep Water Association is dominated by turbidites which generally thicken up the sequence. The Delta Slope shows evidence of syn-sedimentary faulting. Density currents deposited much of the sediment and turbidity currents cut channels into the slope. Large fluvial channel fills dominate the Delta Top Association and large solitary sets of cross-bedding within these were probably produced by alternate bars. Sandwaves occurring in shallower parts of the channel produced cosets of tabular cross-bedding, and smaller forms superimposed during falling stage produced convex-up erosion surfaces within individual sets. There was little low stage sedimentation and a discharge regime with a pronounced flood peak plus a prominent falling stage period is envisaged.
Palaeocurrents are towards the north-west and the delta prograded into a nw-se trending deep water trough, filling this part of the basin for the first time. The petrography and sedimentology however suggest a northerly sediment source. Tectonic movement along the southern margin of the basin diverted the river and at the Roaches led to channel fills being successively offset towards the north.
The Lower Pennant Measures, 550m thick at maximum, contain Fluvial Channel and Delta or Crevasse~delta Associations. 13 lithofacies are described and discussed. The Delta Association, largely restricted to the lower Lynfi Beds, consist of small, laterally variable coarsening upwards and fining upwards sequences, recording the filling of shallow, bays. In most of the succession this early progradational phase is reworked by the succeeding large braided river. This produced thick, laterally extensive sandstones composed of stacked channel fills with low order vertical facies sequences.
Key sections in the association show channel fills uninterrupted by major erosion. These contain major channel base, conglomerate. main sandstone, alternating beds, siltstone and seat-earth/coal components. The conglomerate records concentration in the thalweg, whilst the cleaner, main sandstone component was deposted in topographically higher parts of the channel. Sequences in this show little order and no upwards fining. They were largely determined by discharge changes with larger bedforms being commonly washed out during falling stage. The overlying alternating beds fine upwards and formed during progressive channel abandonment. Widespread coals record periodic diversions of the coarse sediment supply before a further phase of progradation.

Item Type: Thesis (Doctoral)
Subjects: Q Science > QE Geology
Divisions: Faculty of Natural Sciences > School of Geography, Geology and the Environment
Depositing User: Lisa Bailey
Date Deposited: 21 May 2019 15:50
Last Modified: 21 May 2019 15:50
URI: http://eprints.keele.ac.uk/id/eprint/6364

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