THE OLD RED SANDSTONE OF SCOTLAND: EVIDENCE FOR A GANGES-SIZED RIVER DRAINING THE CALEDONIAN HIGHLANDS OF NORWAY

The Old Red Sandstone is one of the world’s most famous formations as it lies atop James Hutton’s unconformity at Siccar Point. Provenance studies of the conglomeritic sedimentary rocks in the Old Red Sandstone of Scotland’s Midland Valley point to mostly local sediment sources. However, the presence of large-scale cross-stratification, and at one locality a fossil bar over 10 m thick, suggests deposition in a large river system originating from a distant but unknown source. Stream flow indicators point to sources in or beyond what is now the North Sea and the North Atlantic Ocean. Using zircon U/Pb and white mica ⁴⁰Ar/³⁹Ar ages we established a provenance profile for these river sediments. We then compared this provenance profile to units throughout Scotland as well as from northern England to test for potential local sourcing. We found no appropriate source for the zircons in northern England. However, detrital zircon age spectra contain a number of overlapping populations, many of which correlate with ages found in the basement rocks of the Scottish Highland terranes, including a key 1.65 Ga signal. Preliminary white mica ⁴⁰Ar/³⁹Ar ages of ~410 Ma in the Old Red are too young to be sourced from these Scottish terranes. Taken together, our data are compatible with data obtained from coeval sedimentary basins in Norway where they too exhibit a pronounced U/Pb peak at 1.65 Ga and a ⁴⁰Ar/³⁹Ar peak of ~ 410 Ma, matching what we found in the Old Red Sandstone. The deposits are taken to represent a Ganges-sized river system (>20 m water depth) that drained the Norwegian Caledonian highlands, and carried sediment across what would eventually become the North Sea to Scotland.