Chapter 3: Sediment geochemistry and clast stratigraphy of the Port Askaig Formation

Whole-rock elemental geochemical measurements from 317 samples, stone measurements from 211 sites and optical petrographic observations from 147 thin sections have been made on diamictite, sandstone and mudstone samples over the 1100 m thick Port Askaig Formation (PAF) from locations in the Garvellachs, NE Islay and Donegal. The gradual change in stone types upwards - from dolomite-dominated (intrabasinal) to granitic-dominated (extrabasinal) - allows 6 stone suites to be defined. Similar upward changes allow 7 geochemical suites to be recognized. An abrupt change of minerals and elements above D13 indicates a change in the provenance and supply of detrital material at that level. An intra-suite repeated geochemical pattern suggests several cycles over the PAF with increases of clay- and silt-sized particles in the diamictites, followed by abrupt reductions. Marine precipitation added 6-7 times more iron to the Disrupted Beds and Member 2 compared to other levels in the PAF. Increased chemical weathering is present in the Great Breccia (D13). The same patterns of stratigraphic change are recognized at localities over 250 km laterally, indicating that widespread phenomena controlled the deposition of the PAF sediments.