The evolution of a sand-rich basin-fill sequence in the Pendleian (Namurian,E1c)of North-West England

The Bowland and Lancaster Fells Basins of north-west England contain a Pendleian El, sequence up to 650m thick, most of which is coarse clastic material. Each of the major stratigraphic units in this sequence, namely the Pendle and Grassington Grit Groups, represents a phase of clastic input to the basins. Initially, the basins were deep marine troughs with water depths of several hundreds of metres. Their northern margins were steep submarine slopes or relict fault scarps rising up onto the partially emergent Askrigg and Lake District Blocks. Within the basins there was significant topographic relief caused by differential compaction over buried Dinantian fault blocks. The Pendle Grit Group, comprising the Pendle Grit and Pendle Shale Formations, represents the development of a sand-rich submarine fan/slope system in these confined basins. The sediment for the fans was probably supplied by a fluvial source to the north-west. Intra-basinal relief within the basins strongly influenced the initial development of the fan system but was later swamped by sedimentation. Only the Waddington Fell High remained present throughout deposition: the "low-seeking" turbidite sediments of the Pendle Grit Formation pinch-out over this structure. A period of uplift or eustatic sea-level fall at the end of Pendle Grit deposition resulted in the development of a minor unconformity on the southern margin of the Askrigg Block and across reactivated intra-basinal highs. This unconformity heralds a major change in the palaeogeography of north-west England and the beginning of Grassington Grit Group deposition. The fluvial clastic input moved eastwards, supplying sediment directly into shallow water on the Askrigg Block. This resulted in rapid progradation of a coarse clastic dominated braid-delta system over the Askrigg Block and, subsequently, out into the Bowland Basin. Water depths still increased rapidly across the fault controlled boundary between these two palaeogeographic features and this led to a change in the depositional processes on the braid-delta system. The resulting differences between Grassington Grit Group facies sequences across this palaeogeographic boundary form the basis for recognition of two Formations, namely the Grassington Grit and Warley Wise Formations. Detailed sedimentological study of the sand-rich fans in the Pendle Group shows that they are dominated by in-channel deposition: lobes and basin plain deposits are very rare. A channel hierarchy has been recognised in the fan sediments, based on the presence of erosion surfaces of different magnitude and extent. First-order erosion surfaces bound channel-complexes up to 1000m in width and 100m deep. These features were cut by infrequent, high energy turbidity currents. They were filled by progradation of a coarse-grained turbidite sand-body deposited from smaller, more frequent turbidity currents trapped in the first-order channel. During this process, second-order channels were cut and filled in the prograding sand-body. Individual beds within the fan system provide evidence for lateral migration of turbidity currents during deposition and also for prolonged flow events. The flow mechanics of such flows are qualitatively examined and their evolution with time and space over the fan system is discussed. A new facies model for sand-rich fans is presented, based on the sedimentological features seen in the Pendle Grit Formation. The El, basin-fill sequence was buried to several kilometres depth by end early Westphalian times. It then underwent rapid uplift. The paragenetic sequences in sandstones from this sequence are related to the maximum burial depth and the amount of subsequent uplift: deeply buried sandstones developed illite cements and, if affected by meteoric flushing during uplift, also have extensive pore-filling kaolinite. This relationship allows qualitative predictions of the reservoir quality of the Ei, sandstones to be made from an estimate of their burial histories.