UNUSUAL TIDAL SANDBODIES IN THE TRANSGRESSIVE PHASE OF THE BRENT GROUP

The Brent Group is previously thought to be a regressive-transgressive succession. Conventionally, sedimentary environments within the transgressive part were interpreted as coastal plain, shelf, small regressive delta, and wave-dominated estuary. However, the tidal influence is less documented. After comprehensive examination of cores in the Kvitebjorn/Vallemon and Oseberg/Tune areas, four types of tidal deposits are identified: 1) Tidal bars I: overlain by the upper flow regime (UFR) sand flats and stacked fluvial-tidal channel/bar complexes. These combinations are interpreted as the tidal-dominated estuarine deposition, overlying on the regressive Brent Delta. 2) Tidal bars II: interbedded with the bay-head deltaic, salt marsh and tidal flat deposits. These associations indicate the wave-dominated estuarine environment overlying on the tidal-dominated estuary deposits. 3) Stacked tidal dunes I: overlying on the wave-dominated estuarine. It probably deposited at the outer part of the bay. 4) Stacked tidal dunes II: extremely clean and well-sorted; large thickness up to 30 to 40 m; absent trace fossil; and low abundance of mud drapes suggesting high energy. These thick tidal dunes were likely to be deposited in the inlet of the bay environment.

Overall, these tidal sandbodies were deposited in different environments: tidal-dominated estuarine, wave-dominated estuarine, outer bay, and inlet of the bay. Their evolution suggests that in the transgressive phase, the Brent Delta retreated and was drown into the tidal-dominated estuarine environment due to the sea level rise. Under the influence of strong waves, the barrier bar systems were formed in the outer part of the estuarine. Consequently, as the tidal influence reduced, the estuary environment was changed from tidal-dominated into wave-dominated. Tidal bar I and II were deposited in these two estuarine environments, respectively. Subsequently, the estuarine was surrounded by the barrier bars caused by strong waves. The inlet between the barrier bars, with strong energy, connected ocean and bay. The outer part and inlet of the bay considered as the favorable area for the thick stacked tidal dune I and II.