We present three new high-precision U–Pb zircon chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) dates from western European Carboniferous diagenetically altered volcanic ash layers (bentonites and tonsteins) that occur interbedded with cyclic siliciclastic sedimentary rocks. The dates constrain the average periodicity of western European late Carboniferous (Arnsbergian to early Langsettian) cyclic sedimentation to between 89 and 124 ka per cycle. This is consistent with the 100 ka ‘short’ eccentricity Milankovitch cycle and supports previous suggestions that the cycles are the product of glacio-eustasy. Cyclostratigraphic tuning of Namurian cycles to the 100 ka eccentricity cycle revises the timing of the global Mississippian–Pennsylvanian boundary to c . 323.9 Ma. CA-ID-TIMS dates also facilitate correlation between different facies belts where biostratigraphical correlations are difficult. Correlation between eastern and western European sequences confirms that the base of the Moscovian stage is located within the Duckmantian (Westphalian B) western European regional substage. Supplementary materials: Zircon separation and concentration procedures, U–Pb zircon CA-ID-TIMS methods and data table, average periodicity calculations for western and central European Namurian and Westphalian sedimentary cycles, and procedures for recalibrating legacy 40 Ar/ 39 Ar dates are available at www.geolsoc.org.uk/SUP18547 .
The Western Irish Namurian Basin (WINB) preserves classic examples of basin floor sequences through to slope deposits and deltaic cyclothems. Despite over 50 years of research into the WINB, its sediment provenance remains highly contested. Sedimentological arguments, including palaeocurrent vectors and palaeoslope indicators have been invoked to propose a sediment source from the NW or the west (i.e. from within Laurentia). These same indicators have been subsequently reinterpreted to reflect a southern provenance. It is not clear from sedimentological arguments alone which interpretation more accurately reflects the infilling of the WINB. Regional-scale constraints on WINB provenance may be obtained with detrital zircon U-Pb geochronology.U-Pb LA-ICP-MS detrital zircon analysis was undertaken on samples from three sandstone units at different stratigraphic levels within the WINB siliciclastic sedimentary fill (Ross Formation, Tullig Sandstone, Doonlicky Sandstone). The samples are dominated by 500-700 Ma zircons, which can be correlated with Cadomian-Avalonian orogenic activity within terranes to the south of the WINB (Avalonia/Ganderia, Armorica and Iberia). In contrast, Eastern Laurentia, to the north of the WINB, was devoid of orogenic activity at this time. WINB samples also yield age populations younger than 500 Ma, and older than 700 Ma. These are not diagnostic of a particular source terrane and thus could be derived from terranes north and/or south of the WINB. WINB detrital zircon age spectra can be reconciled by an Avalonian or combined Avalonian-Laurentian provenance for WINB sedimentary strata. Further research is required in order to distinguish between these two possibilities.
The Dalradian Supergroup contains three distinct glacigenic units, formerly termed ‘Boulder Beds’, which are correlated with widespread Neoproterozoic glaciations. The oldest and thickest unit, the Port Askaig Formation, marks the Appin–Argyll group boundary of the Dalradian Supergroup and has been correlated with the Middle Cryogenian (Sturtian) glaciation. The Auchnahyle Formation, a diamictite-bearing sequence near Tomintoul in NE Scotland, exhibits strong lithological similarities to the Port Askaig Formation. Both these glacigenic ‘Boulder Bed’ units contain abundant dolomite clasts in their lower parts and more granitic material at higher levels. Both metadiamictite units are overlain by thick shallow-marine quartzite units. C isotope data from Appin Group carbonate strata below the Auchnahyle Formation support this correlation. U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) detrital zircon data from the Auchnahyle Formation metadiamictite differ slightly from the Port Askaig Formation, but are similar to detrital zircon spectra obtained from the Macduff Formation, a diamictite unit in the younger Southern Highland Group of the Dalradian Supergroup; both apparently reflect derivation from local basement rocks. No detritus younger than 0·9 Ga is observed, so the data do not constrain significantly the depositional age of the glacial strata. A thin tholeiitic pillow basalt unit in the lower part of the Auchnahyle Formation is geochemically distinct from pre-tectonic metadolerite sills and from basic metavolcanic rocks up-section. A Sturtian (c. 720–700 Ma) age for the Auchnahyle Formation metadiamictite would imply that this basaltic volcanism represents the oldest recorded volcanic activity in the Dalradian Supergroup and is inferred to represent an early, local phase of proto-Iapetan rifting within the Rodinian supercontinent.
High-precision U-Pb zircon CA-ID-TIMS dates from western European late Viséan bentonites. Supplementary Material Mean Cycle Periodicity CalculationsMean cycle periodicity values were calculated employing the same methodology as used by Pointon et al. (2012) for Namurian to early Westphalian siliciclastic cycles within the Pennine and Ruhr basins of northern England and Germany respectively. This is as follows:
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