The significance of the Etive Formation in the development of the Brent system: distinction of normal and forced regressions

Abstract

Abstract Recent sequence stratigraphic debate on the Brent system have focused on the interpreted nature of the progradational trajectory (horizontal, slightly upwards or downwards) of the shoreline (Rannoch/Etive Formations) through time, as this gives a direct measure of how late Aalenian-Bajocian relative sea level changed during regression. Early interpretations emphasized the unified shallowing-upward nature of the Rannoch-Etive-Ness depositional system, and implicitly accepted a uniform shoreline progradation, i.e. a shoreline trajectory that was horizontal or slightly rising, implying a stable or slightly rising relative sea level. No irregularities of the trajectory were noted, and unusual shifts in facies, grain size etc. were normally related to autocyclic processes. More recent work has suggested that in some instances there is evidence for more irregular shoreline progradation at certain times, and for fall(s) in relative sea level and forced regression. This evidence comes from incised valleys and deep erosion/subaerial exposure surfaces from the landward (Etive-Ness boundary) and basinward (Rannoch-Etive) reaches of the Brent system respectively. However, it is currently unclear if any of these downshift surfaces recognized in the strandplain/coastal plain and shoreface environments are in time-equivalent strata. Current debate is mostly handicapped by a lack of agreement on the origin and depositional facies of the Etive Formation. There is significant debate about the relative amounts of fluvial, tidal and wave influence detected in the strata of this formation, with some authors arguing for a dominance of fluvial distributaries and mouth-bar deposits, whereas others propose either tidal-channel and inlet deposits or wave-dominated shoreface and strandplain settings. The stratigraphy is impacted by this disagreement. The character and sharp base of the Etive Formation can be argued to be consistent with normal shoreline processes, where wave or tidal conditions can produce significant erosion in the shoreface, without the necessity of any forced regression. Other interpretations, particularly where the Etive Formation is seen in terms of fluvial facies and processes, require a significant basinward shift of the shoreline to explain the Rannoch-Etive superposition, and a fall of sea level to cause the erosive boundary between the two formations. However, there is now ample evidence, including new evidence presented here, that both of the end-member scenarios for the progradation of the Brent system are incorrect. The notion that the overall progradation was entirely a product of normal regression, during stable and/or slightly rising relative sea level, is negated by local evidence of incised valleys, of subaerial exposure and plant growth in lower shoreface strata in the Rannoch Formation, and of repeated erosion surfaces with coarse-grained lags at the base of the Etive Formation. On the other hand, the idea of continuous sea level fall or of a single, late-stage fall, such that there was regional valley incision of the Etive into the Rannoch Formation and that the former is entirely younger than the latter, is negated by local evidence of gradual upward facies change between the formations, of stratigraphic interfingering between the formations, and of time lines passing through the Etive into the Rannoch Formation. It is perhaps not surprising that the system’s overall regressive trajectory varied in time from being forced to being normally regressive, and that further detailed local studies are required before regional generalisations can be made.