A NEOPROTEROZOIC MULTIPHASE RIFT SEQUENCE - THE GRAMPIAN AND APPIN GROUPS OF THE SOUTHWESTERN MONADHLIATH MOUNTAINS OF SCOTLAND

The Grampian and Appin groups of the southwestern Monadhliath Mountain s form the earliest known syn-rift sequences of the Scottish central H ighlands. They were likely to have formed in an intracontinental setti ng and represent deposition of mixed elastic and carbonate shallow and deep marine strata. The Grampian Group of the southern Monadhliath Mo untains was deposited during a period of initial basin rifting (NW-SE extension) followed by a phase of thermal subsidence. Syn-rift sedimen ts comprise a 2.5-6km thick turbidite system. Thermal subsidence broug ht about the basinward progradation of shallow marine shelf sediments resulting in the infilling of pre-existing basin topography. The overl ying Appin Group commenced with deposition of a shallow marine sequenc e alternating between nearshore tidal sand and offshore mud deposition . This formed in response to renewed rifting and concomitant subsidenc e. Accelerated rifting resulted in localized footwall uplift and erosi on while sedimentation continued in the hanging-wall areas. Resultant subsidence, perhaps partly thermally driven, caused gradual basin wide ning and produced an onlapping marine sequence. There followed a perio d of progressive elastic deprivation when carbonates were precipitated , and at the onset of anoxic conditions, deposition of organic muds. T he fundamental structural elements responsible for the formation of th e Grampian and Appin group basins were also influential in the orogeni c evolution of the basin-fill. Half-graben fills were deformed to prod uce regionally extensive folds such as the Stob Ban-Craig a' Chail Syn form.