High-resolution sequence stratigraphy in the early Marsdenian (Namurian, Carboniferous) of the central Pennines and adjacent areas

Abstract

SUMMARY The early Marsdenian (Namurian Series) Readycon Dean Formation of the Pennines and adjacent areas records a complex series of rapid, high-magnitude base-level changes during a short interval of the Pennine Basin’s history. Detailed correlation, using a rapidly evolving ammonoid fauna, enables the details of this complexity to be traced across the basin. Two major base-level rises (flooding events) are seen, one at the base of the Readycon Dean Formation, that led to the establishment of extensive, deep-water, marine shales of the Reticuloceras gracile (R 2a 1) Marine Band and the other that established the R. bilingue (R 2b 2) Marine Band across the region. At the base of this second major transgression is a radioactive, organic-rich dark grey shale (the Denshaw Horizon) that provides a useful marker band. The conditions of intense, basinal anoxia responsible for this horizon have also been observed in a similar basal transgressive shale from the overlying Yeadonian succession. It would appear that stagnation during initial transgression was a recurrent theme in the Pennine Basin’s history. Both marine incursions were followed by the progradation of a single distributary channel sandstone body during highstand conditions: the Readycon Dean Flags and Scotland Flags respectively. Two major sequence boundaries or unconformities also occur in the study interval. The lower sequence boundary shows the development of a 25 m deep palaeo-valley partially infilled with thin beds of sandstone and siltstone of the new Howels Head Flags Member. The valley may have been the feeder channel of the Alum Crag Grits (a thick, turbidite sandstone lobe) but the incision mechanism is enigmatic. The upper sequence boundary is marked by a period of strong erosion (at least 80 m of incision can be demonstrated locally) throughout the basin and, towards the basin margin, substantial portions of the Readycon Dean Formation have been removed. A marked shallowing of facies is associated with this second sequence boundary and it is everywhere overlain by fluviodeltaic facies of the Pule Hill Grit Formation.