The Lateral Variability of Sequence Boundaries: A Regional 3-D Seismic Case Study from the Late Cenozoic Southern North Sea

Harding, Rachel; Huuse, Mads; Gawthorpe, Rob

The Late Cenozoic Southern North Sea Delta is a 1000m thick, prograding delta covering the majority of the offshore Southern North Sea Basin, draining Fennoscandia and Northern Europe via the Baltic River System. The delta spans approximately 12 million years and is completely covered by 3-D seismic and exploration borehole data. The PGS Southern North Sea MegaSurvey covers 74,000 km² of the basin and along with a high resolution chronostratigraphic framework, allows for the first time, a continuous 3-D seismic sequence stratigraphic analysis to be undertaken on the Late Cenozoic in the UK and Netherlands offshore.

The lateral extent and character of sequence boundaries along strike within a sedimentary basin is a topic that has been debated in the literature over the last 20 years. The spatially varying subsidence (regional and salt tectonic controls); variable sediment input (grade, volume and direction), changing climatic drivers and connection to the global ocean makes this delta a natural laboratory to test several sequence stratigraphic concepts, including the chronostratigraphic significance of seismic reflections, the lateral variability of sequence boundaries, and eustatic control on sequence development.

The seismic expression of the delta is in the form of clinoforms prograding on to the mid-Miocene Unconformity, initially during the Late Miocene-Pliocene (12.4-2.58Ma) from the northeast; and from the east and south of the Netherlands North Sea from the earliest Pleistocene (2.58Ma) until the Elsterian Glaciation at 0.48Ma.

Several interpreted sequence boundaries have variable expression across the basin. A sequence boundary dated at 2.24Ma has been interpreted regionally, identified as a downlap surface (maximum flooding surface) in the north west of the basin, however landward; the reflection can be traced as a truncation surface, suggesting an erosion. Clinoform break migration analysis of this time period also shows that in relation to the 2.36Ma coastline, the 2.24Ma coastline in the south has regressed basinwards whilst in the north east the coastline has transgressed landward. Similar characteristics have also been identified for a maximum flooding surface at 1.96Ma. This may be explained as relative sea level varies spatially across the basin; and variations in sediment delivery into the basin at different point sources may outpace regional transgressions.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013