Ice-proximal glaciomarine sedimentation and sea-level change in the inverness area, Scotland: A review of the deglaciation of a major ice stream of the British Late Devensian ice sheet

Abstract

Evidence of both rising and falling relative sea levels and glacitectonic movements is preserved in two formations of raised glaciomarine deposits that were laid down in front of an oscillating ‘grounded’ tidewater glacier in the Inverness Firth. These changes occurred during the latter stages of the disintegration of the Moray Firth ice-stream, one of the major ice streams that drained the British main Late Devensian ice sheet. Most of the glaciomarine deposits antedate a sequence of glacio-isostatically tilted Late Devensian marine shorelines and associated littoral and estuarine deposits. The shorelines began forming at about 13,000 BP and record a progressive fall in relative sea level. A new model for the deglaciation of the Moray Firth region is proposed after a critical appraisal of published accounts of both onshore and offshore Quaternary sequences. The disintegration of the Moray Firth ice stream involved several rapid phases of retreat to pinning points, caused by iceberg calving and triggered by rising global sea level. Each retreat was followed by minor readvances or stillstands, possibly caused by short-lived accelerated periods of glacio-isostatic rebound and concomitant temporary falls in relative sea level. Two such events occurred in the Inverness Firth: the Ardersier Oscillation and the Alturlie Stillstand. Substantial differences (lower relative sea levels, later deglaciation) are apparent between the pattern of ice-retreat in the Moray Firth region and published accounts of the deglaciation of the Irish Sea basin. These differences require a reassessment of some current hypotheses concerning the disintegration of major ice streams associated with high relative sea levels. Furthermore, geological and geomorphological evidence suggesting both rising and falling sea levels in the Inverness area, prior to ca. 13,500 BP, is not fully compatible with recently published computer simulations of the dissolution of the British main Late Devensian ice sheet.