SEDIMENT STORAGE, TRANSPORT TIMES AND FINGER-PRINTING MULTI-DEFORMED SOURCE REGIONS USING DETRITAL MUSCOVITE AR/AR LASERPROBE AGES

The relationship between tectonics and sedimentation, and the timescales involved in source block exhumation, erosion and sediment transportation can be examined using Ar/Ar ages from detrital muscovites. The single grain fusion approach yields an age for each detrital muscovite grain analysed. The range of ages from each sample can reveal whether there is predominantly one or a mixture of source blocks supplying detritus, when it/they exhumed, and the time interval between source exhumation and sediment deposition.

Analysing micas from different stratigraphic levels allows an assessment of changing sediment sources through time. Detrital white micas from early-Triassic to late-Cretaceous sands in the Norwegian Sea were derived from the nearby Caledonian front. Caledonian molasse was stored in intramontane basins for up to 290 Ma, until the area of deposition experienced uplift in the Permo-Triassic and re-deposited the detritus in it’s present location. Preliminary data from Cretaceous-Paleocene sands/muds/siltstones from Kangerlussuaq, SE Greenland, suggest that sediment is predominantly derived from Caledonian source, with some input from a Precambrian source region. More interestingly, the white micas from, for example, the Turonian sandstones, span much greater age ranges than those from Turonian muds, the same is true of Maastrichtian and Cenomanian sand and mud pairs.

Where muscovite ages reveal wide apparent age ranges, analysing discrete regions of individual detrital muscovite grains using the UV laserprobe can reveal further information. A recent single grain fusion study of muscovites from Upper Silurian sands from SW Wales suggested that the source region must have been Avalonian (~ 550 Ma). However, UV laserprobe analyses of areas within individual detrital muscovite grains from the same sample revealed that muscovite cores were much older (up to 870 Ma), with younger grain edges (down to 435 Ma). A Grampian Highland/Moine source is more consistent with these data, and invoking East Avalonia as a source region would have resulted in an entirely false appraisal of Cambrian/Ordovician sediment supply and transportation, and Silurian sedimentation.