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Regional Studies |
1 British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 8GG, UK
2 Badley, Ashton & Associates, Winceby House, Winceby, Horncastle, Lincolnshire LN9 6PB, UK
Sequence stratigraphical principles, developed largely in Phanerozoic basins, are applied here to a Neoproterozoic metasedimentary succession. The Grampian and Appin groups of the southern Monadhliath represent clastic and carbonate deposition within a multiphase rift basin. Subsequent polyphase deformation has resulted in considerable crustal shortening so that, within a comparatively small area, there is significant lateral variation in basinal settings. The succession is rationalized into a number of transgressive-regressive cycles. The northwestern area is dominated by shelfal metasedimentary rocks which exhibit a hierarchy of sequence boundaries distinguished by abrupt basinward shifts (southwards and southeastwards) in the proximality of marine facies, and onlap. The surfaces between the sequences provide the objective criteria for correlation into the basinal succession of the southeast. This sequence stratigraphical approach gives new insight into Neoproterozoic basin dynamics. Although rifting is confirmed as having a dominant role in creating accommodation for the deposition of at least two ‘supersequences’ thousands of metres thick, the basin depocentre is reinterpreted to have lain to the south and east throughout deposition of the Grampian and Appin groups. The upwards trend towards ‘cleaner’ sand from Grampian to Appin group deposition is suggested to reflect increasing sediment reworking by tides during a second-order transgression. The scale of major sequences means that patterns of deposition are likely to have been basin-wide or greater, so that it may be possible to identify similar transgressive-regressive stratal architectures in structurally isolated and disparate successions. This approach may help resolve the status of some suspect terranes.
