GLACIOTECTONIZED QUATERNARY SEDIMENTS AT DINAS-DINLLE, GWYNEDD, NORTHWALES, AND THEIR BEARING ON THE STYLE OF DEGLACIATION IN THE EASTERN IRISH SEA

The sedimentology and structure of Late Quaternary sediments exposed i n a coastal section at Dinas Dinlle, North Wales, are described. The d eposits form two small hills, the northern one being more prominent an d containing northward dipping units of gravel and diamicton bounded b y day-lined thrust faults. The southern hill comprises complex folded sands and gravels overlying a basal till surface. High-resolution seis mic surveys along the beach, 50 m west of the cliffs, show a roughly h orizontal rockhead lying some 35 m below beach level, and demonstrate that thrust faults penetrate the entire Quaternary sequence to a detac hment immediately above rockhead. Thrust units in the exposed cliff se ction show excellent correlation with the seismic sequence 50 m to the west, indicating a strike of 080 degrees/260 degrees. Balanced cross- section techniques were applied to an initial simple layered sequence comprising a lower (Irish Sea) till overlain by a coarsening upwards s and to gravel unit, followed by upper till (mixed lithology, including Welsh and northern provenance), and finally an upper gravel unit. Sho rtening of approximately 54% is estimated within the thrust sequence. It is concluded that the two isolated hills at Dinas Dinlle are erosio nal remnants of a formerly more extensive push moraine extending westw ards into Caernarfon Bay which has now been truncated by coastal retre at. Deglaciation of the area was interrupted by ice sheet readvance to wards 170 degrees across a proglacial surface. (C) 1997 Elsevier Scien ce Ltd.