Pennsylvanian

© Craven & Pendle Geological Society

Geological History

Pennsylvanian times saw a return to a wetter equatorial climate typified by the rapid advance of fluviatile and deltaic sandstones into local subsiding basins. Occasionally the delta top was flooded with marine waters (marine incursions).

Such flooding events are characterised by marine bands which commonly contained goniatites: marine faunas very useful for dating and correlating rocks. Yeadonian, Marsdenian and Kinderscoutian strata are exposed in the Cliviger Valley.

By Pennsylvanian (Coal Measures) major rivers, distributary channels and freshwater lakes provided an ideal environment for coal swamps. Sandstone deposition was typically fluviatile characterised by thinner sandstone beds.

As deltas prograded soils formed and swamp vegetation became established; characterised by coal-bearing cyclic sequences containing discrete marine bands. Complimentary to the marine incursions were the predominance of non-marine bivalves, also used for correlation purposes.

Insects and winged insects appeared in the middle Namurian. The swampy equatorial climate of the Coal Measures supported diverse communities of non-marine bivalves.  Whilst on land a rich and diverse flora became established typified by common plants such as Calamites, Sphenophylum and the giant "clubmosses" such as Lepidodendron, Sigillaria and true ferns such as Cordaites. The Carboniferous Period came to an end when general desiccation (failure of coal seams and the spread of red beds) began in the north and spread southwards.  So began the Permian Period.

Classic Outcrop

Lancaster Fells : Eskew Beck

The Kirkbeck Formation which is exposed in Eskew Beck and at Low Bentham Wier, is one of the most important Pennsylvanian outcrops in Britain. Here you can see Alportian (H2b1) and Kinderscoutian (R1b3) wave influenced shallow marine wave and storm influenced sequences with unusual shelly faunas and foraminifera.

Below I have modified slightly the Carboniferous succession dominated by the Millstone Grit Group and ammonoid zonations. The modifications are in the STAGE and SUBSTAGE parts of the table to reflect the new official names of the Carboniferous System.