THE SEDIMENTARY HISTORY OF A DELTA LOBE FROM A YOREDALE (CARBONIFEROUS) CYCLOTHEM

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Abstract
Summary: A delta lobe has been recognised in the Great Limestone (Yoredale) cyclothem over an area of approximately 700 square kilometres in the northern Pennines (Weardale to Stainmore). The lobe occupies the entire clastic interval of the cyclothem and has a complex history of sedimentation involving four distinct phases. The coastal plain phase includes small-scale fluvial channels, and a mudstone-spiculite chert association deposited in a shallow, shoreline bay. This phase preceded lobe development in the Stainmore Trough and was terminated by a localised transgression. In the upstream and intermediate lobe area, the progradational phase consists of small-scale coarsening upwards sequences which represent the repeated infilling of an extensive interdistributary bay by levees, crevasse splays, sand spits and minor mouth bars. Small-scale (crevasse) channels and larger-scale (distributary) channels are associated with these sequences. Farther south the progradational phase comprises a single large-scale coarsening upwards sequence which represents progradation of a river-dominated (mouth bar) shoreline over offshore muds. During the abandonment phase, a thin marker horizon accumulated, comprising coal in the upstream area (widespread marsh development) and fossiliferous sandstone, mudstone and limestone towards the shoreline (reworking and normal sedimentation). This horizon reflects slow deposition rates related to conditions of minimal sediment supply and subsidence. In the post-abandonment phase, sediment supply recommenced and a barrier island—lagoon complex was established on the abandoned lobe. The lagoon facies include small-scale coarsening upwards sequences formed during infilling, in one case by washover fans. The barrier is made up of medium-scale, wave-dominated coarsening upwards sequences which demonstrate contrasted migration trends equivalent in time along the barrier.