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Abstract

The sequence stratigraphic analysis of the Lower to Upper Jurassic (Hettangian to Kimmeridgian) succession of the Moray Firth basin synthesizes sedimentological and paleontological data from approximately 400 wells in the offshore domain with scattered outcrops along the eastern coastline of Scotland. The stratigraphic framework produced provides a consistent framework for understanding and predicting basin-wide sedimentary facies distribution.

Three hierarchies of sedimentary cycles are distinguished: (i) major transgressive/regressive (T/R) cycles (with durations between 10 and 50 my), (ii) transgressive/regressive (T/R) facies cycles (with durations between 3 and 10 my), and (iii) regressive/transgressive (R/T) cycles (with durations between 0.5 and 3 my). Two major T/R cycles are identified, which are bounded by three important surfaces of maximum regression. These boundaries, which generally correspond to regionally extensive subaerial unconformities, occur close to the Triassic/Jurassic boundary, the Lower/Middle Jurassic boundary (often termed the “Mid-Cimmerian” unconformity) and the Ryazanian/Valangian boundary (often termed the “Base Cretaceous” unconformity). Each major T/R cycle can be subdivided into component T/R facies cycles, which are also bounded by surfaces of maximum regression. Both types of cycle are synchronous across the basin, although high sediment flux locally affects the timing of peak transgression.

Difficulties exist in objectively defining and correlating most higher-frequency (so-called 3rd-order) sequence boundaries in distal depositional settings. Instead maximum flooding surfaces have proven to be the most reliable, distinctive and easy-to-date surfaces with which to define a rigorous sequence stratigraphic framework. Consequently, higher-frequency cycles are bounded by maximum flooding surfaces, are centered upon transgressive surfaces and hence are termed regressive/transgressive (R/T) cycles. Fourteen R/T cycles are defined, each of which developed simultaneously across the Moray Firth basin. The fact that these R/T cycles can be correlated from both subbasin to subbasin, and from the Inner Moray Firth to Outer Moray Firth basin, indicates that local tectonics had little effect upon controlling the timing of 3rd-order cycle development, even during the Middle and Late Jurassic Epochs when extensional rifting is known to have exerted a pronounced control upon basin development.

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