Bathymetry on a Carboniferous reef

Abstract

Broadhurst, F. M. & Simpson, I. M.: Bathymetry on a Carboniferous reef. In the Lower Carboniferous deposits of Castleton, Derbyshire, well bedded shelf limestones, apparently of shallow water origin, pass northwards into a marginal tract of poorly bedded apron-reef limestones which dip at about 30° downwards and away from the ‘shelf’. Geopetal infillings of shell cavities in the apron-reef indicate only minor movement since deposition, and the observed dip must be due to deposition on a sloping sea floor. At certain times this sea floor was colonised by stromatolitic algae and corals such as Lithostrotion at the apron-reef crest where there was minimum water depth, followed to progressively increasing depths by (1) a fauna dominated by the coral Michelinia, (2) a fauna of small brachiopods, bryozoa, molluscs, trilobites, and other organisms and (3) a fauna dominated by Pseudamussium. At other times crinoidal debris was the dominant component of the apron-reef, when an alignment of crinoid stems parallel to the dip of the slope occurred at low levels, but a random orientation at the apronreef crest. Other sediments on the apron-reef are apparently devoid of macrofossils. Volcanic activity occurred during the development of the apron-reef, and it is suggested that uplift of the shelf area preceded the subaerial flow of a lava tongue which reached and plunged down the apron-reef slope and into the sea.