Source area reconstruction using heavy mineral, mineral chemical, geochronological and palaeocurrent data: Carboniferous sandstones of the Scottish midland valley

Le résumé fourni par la source

Integration of several data sets (heavy mineral analysis, rutile and apatite trace element geochemistry, zircon, rutile and apatite geochronology, and palaeocurrent measurements), in conjunction with published sedimentological and petrographic data, has enabled a reconstruction of source areas supplying Late Devonian and Carboniferous sandstones in the eastern part of the Scottish Midland Valley. There is evidence for both first-cycle and recycled detrital sources, and for temporal variations in sediment supply patterns. These temporal shifts in provenance have enabled the succession to be divided into six units of different character (Packages 1 to 6). Late Devonian sandstones (Package 1) have comparable characteristics to Lower Palaeozoic sandstones of the Scottish Southern Uplands Block to the south, and are interpreted as recycled from similar material. Differences in apatite morphology testify to a shift from aeolian deposition in the Knox Pulpit Formation to fluvial deposition in the overlying Kinnesswood Formation. A recycled source is also inferred for the overlying Strathclyde Group (Visean) sandstones (Package 2), on the basis of depletion of weatherable mineral components, but palaeocurrent evidence and mineral chemical data indicate a change in source area location and sediment input direction. Recycling from an Upper Old Red Sandstone source (Late Devonian) in the Moray Firth to the north is considered most likely. Input from a more distant northern source, delivered by the ‘Pennine River' appears in the later part of Package 2, and becomes dominant at the base of the Limestone Coal Formation (Namurian) (Package 3). The Pennine River was a large fluvial system that is known to have supplied much of the Carboniferous succession in the Pennine Basin of Yorkshire and Northumberland. The new rutile trace element and geochronological data acquired in this study confirm that the source region included parts of East Greenland where high-grade (granulite facies) metapelites formed during the Caledonian orogenic cycle are located. The Pennine River was also the dominant source during Coal Measures (Westphalian) deposition (Package 5), but supply was temporarily reduced during the intervening Passage Formation (later Namurian) (Package 4), which is dominated by recycled Upper Old Red Sandstone detritus similar to that present in the older Strathclyde Group. Pennine River supply diminished in the latest exposed Coal Measures (Package 6), which has characteristics indicating increasing involvement of sourcing from the Scottish Highlands (Dalradian and Loch Ness Supergroups).

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