Macrofossils and their palaeoecology in deltaic sequences of the Lower Carboniferous Yoredale Series, Yorkshire, England

Abstract

Deltaic sequences of the 5-Yard Limestone cyclothem forming part of the Yoredale Series, have been studied at different localities in Wensleydale and Bishopdale, N. Yorkshire. The northernmost, Wensleydale sections contain upward coarsening clastic sequences that are characterized by gradational passages from one lithology into another, suggesting slow deltaic progradation. Highly fossiliferous, calcareous shales, deposited in a shallow, open sea, pass up into less fossiliferous, sometimes evaporitic shales indicating a restricted connection with the sea. The latter show an increase of silt content, ultimately merging into finegrained sandstones of the delta plain facies. A different general picture is found in the Bishopdale sections located in the south. These locations apparently were frequently subject to fluctuations in sediment input and water turbulence, which resulted in periods of abundant algal growth, explosive colonization by the probably opportunistic brachiopod Gigantoproductus and minor transgressions of the sea. It is suggested that current patterns and sediment influx from the north initiated growth of a coastal barrier, causing development of different environmental conditions in the northern and southern part of the area. Due to partial separation from the sea, the northern part changed into a lagoon with variable salinity conditions, while progradation of the delta was slow enough to permit small-scale transgressions over the southern part of the area, eventually followed by deposition of fine-grained, evenly laminated sandstones and medium grained, cross-bedded sandstones characteristic of a seaward prograding barrier beach.