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

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 inc...

Role of forced regression in controlling Brent Group reservoir architecture and prospectivity in the northern North Sea

“…18). The model in Figure 18 infers incision not just during Etive time (Olsen & Steel 2000), but also during deposition of the Rannoch (see also Hampson et al 2004). We envisage a basin floor sloping to the north (Fig.18a), with sediment reworking and redistribution promoting seawards build out of the delta to the north (Fig.…”

“…Mjos et al 1998;Hampson et al 2004). To test these schemes, detailed semi-regional evaluations that bridge the gap between field-scale descriptions and basin-scale evaluations are required (Olsen & Steel 2000).…”

3D seismic imaging of lower delta-plain beach ridges: lower Brent Group, northern North Sea

“…Erosion of this dome during the Middle Jurassic resulted in northward progradation of a large delta system (commonly called the 'Brent Delta') northwards along the axis of the Viking Graben (Underhill & Partington 1993). Within the study area the deposits of this delta are ascribed to the Brent Group, which comprises five lithostratigraphic units: the Oseberg (Broom Formation in the UK sector), Fjellanger et al 1996;Olsen & Steel 2000), which pass upward and landward into coal-bearing, lower coastal-plain deposits of the Ness Formation (e.g. Ryseth et al 1998;Fig.…”

“…Olsen & Steel 2000;Løseth & Ryseth 2003); the reader is referred to those studies for more details. However, a brief description of the key sedimentary facies in the Etive Formation is provided here to allow a link to be made between geophysical (seismic) and geological observations presented below.…”

“…The sedimentary structures, abundant rooting and occurrence above foreshore (lower Etive Formation) and below delta-plain (Ness Formation) deposits suggests that the upper part of the Etive Formation was deposited in an overall 'backshore' environment (e.g. Graue et al 1987;Fjellanger et al 1996;Olsen & Steel 2000; see also general summary on backshore facies by Bhattacharya & Walker 1992).…”