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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Petrology
Vol. 32 (1962)No. 4. (December), Pages 657-697

Petrology, Origin and Deposition of the Highest Lower Old Red Sandstone of Shropshire, England

J. R. L. Allen

ABSTRACT

The Abdon and Woodbank series are the youngest Lower Old Red Sandstone (Lower Devonian) formations known in Shropshire, England. The rocks present are intraformational conglomerates, lithic sandstones, argillolithic sandstones, siltstones, dolomicrites, dolomitic calcirudites, sandy calcarenites and algal biolithites. Exotic pebbles occur which indicate derivation from Archaean, Ordovician, Silurian, and early Lower Old Red Sandstone formations lying to north and west. Sandstone composition is dependent on grain size, the coarsest being richest in rock fragments. The argillolithic sandstones and siltstones also contain clay minerals, which occur either as discrete aggregates or as an intergranular paste, the amount increasing as grain size falls. The clastic detritus underwent size-so ting during transport, and the rocks at the depositional site appear to reproduce size-composition relationships characterizing the source debris. Rock composition controlled compaction and cementation, and roundness varies normally.

The conglomerates lie on scoured surfaces. The sandstones show cross-bedding, scour-and-fill, and primary current lineation which indicate currents from the northwest, confirming the position of the source inferred from other data. The beds are grouped largely into cyclothems, thus:

                               argillolithic siltstone
                               sandstone
intraformational   lithic
conglomerate       sandstone   algal biolithite
                               siltstone

The Abdon and Woodbank series were probably deposited by rivers on a floodplain on which there were ephemeral lakes and mudflats. The conglomerates and sandstones were laid down in the distributary channels. The siltstones were deposited largely in shallow lakes and on mudflats where flocculated clays could settle. By analogy with the modern Colorado Delta floodplain, the cyclothems are demonstrated to indicate successive diversions of the main distributary streams as the result of crevassing. The Abdon and Woodbank series are synorogenic, and were accumulated during the early phases of the main Caledonian (Middle Devonian) mountain-building in Wales.


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