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Abstract

Biostratigraphically well-calibrated exposures of Lower Jurassic rocks in the Wessex, Bristol Channel, Cleveland and Hebrides basins have been remeasured and interpreted in the context of sequence stratigraphy. The aim has been to see whether the stratigraphy and facies are compatible with the hypothesis that relative sea-level changes were synchronous across all these basins. The Lower Jurassic Series can be subdivided into four large-scale (so-called 2nd-order) lithologic cycles, with durations of approximately 3–10 my, that appear to be synchronously developed in all onshore British basins; the cyclic changes in facies become more extreme as the cycles young. Candidate maximum flooding surfaces in the large-scale cycles, identified on the basis of distal starvation, or facies successions indicative of maximal accommodation space in proximal areas, occur in the lower semicostatum zone (Lower Sinemurian), obtusum–oxynotum zones (upper Sinemurian), lower jamesoni zone (lower Pliensbachian) and falciferum–bifrons zones (lower Toarcian). Candidate sequence boundaries in the large-scale cycles, defined on the basis of major unconformities or facies successions indicative of minimal accommodation space in proximal areas, are recognized in the upper turneri zone (mid-Sinemurian), mid-raricostatum zone (upper Sinemurian), basal margaritatus zone (mid-Pliensbachian) and levesquei zone (upper Toarcian).

In general, at the large scale, the Lower Jurassic Series of the Dorset area of the Wessex Basin shows the most distal pattern of sediment accumulation, in which condensed sections (limestone or mudrock) correspond to relative sea-level rise or highstand and expanded sections (mudrock or sandstone) correspond to relative sea-level fall or lowstand. In contrast, the Lower Jurassic Series of the Skye, Pabay and Raasay areas of the Hebrides Basin exemplify the proximal pattern of sedimentation in which expanded sections (sandstone and mudstone) correspond to relative sea-level rise or highstand, and condensed sections (sandstone) correspond to relative sea-level fall or lowstand. The Yorkshire coast successions of the Cleveland Basin exemplify an intermediate setting. Significant divergence from this pattern is evident in the Toarcian (and through the Middle Jurassic) deposits over which interval the style of accumulation in the Hebrides is intermediate between that of the Wessex Basin and that of the Cleveland Basin. This indicates a reduction of clastic supply or increase in creation of proximal accommodation space in the Hebrides area relative to Yorkshire that began in the early Toarcian.

Lithologic cyclicity at the scale of ammonite zones and subzones (so-called 3rd-order) is recognized in a variety of facies; durations are inferred to be approximately 0.5 to 3 my. In a manner that contrasts with the large-scale cycles, the medium-scale cycles become more weakly expressed upwards through Lower Jurassic successions. The link between medium-scale sedimentary cycles and relative sea-level change is more interpretative than is the case for the large-scale cycles. There are few surfaces that have a definitive expression in all basins considered here; those that do are: candidate maximum flooding surfaces in the lyra and taylori subzones, and at the stokesi–subnodosus subzonal boundary (all major); and candidate sequence boundaries in the mid-jamesoni zone (moderate), and at the base of the stokesi subzone (major). Similarly, there are few surfaces that appear strongly localized, the best examples being candidate sequence boundaries in the subnodosus and gibbosus subzones, which are developed mainly in the south and north respectively. In hemipelagic-dominated, distal facies, there is evidence to suggest that stratigraphic condensation is a consequence of relative sea-level fall rather than rise; relative sea-level rises in these settings appear to have generated erosion surfaces. A new relative sea-level curve is presented with medium- and large-scale cycles shown that are compatible with all the successions considered in this study.

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