Interplay of Turbidite and Mass Transport Deposition in a Deepwater Setting: The Lower Cretaceous Britannia Sandstone Formation, UK North Sea

Abstract

Mass wasting processes create depositional relief that acts as localised accommodation, influencing dispersal patterns and deposit heterogeneity of subsequent sediment gravity flows. The Aptian deep-water Britannia Sandstone Formation, North Sea, provides an opportunity to study a succession of MTDs and turbiditic sandstones in a densely drilled area around the Britannia platform. The work is based on integration of well-data from 47 wells from the 35 km² Platform Area of the Britannia Field and detailed sedimentary analyses of 3000 ft (900m) of high quality core from 13 of those wells. This dataset provides a unique subsurface example of the influence that different scales of MTD depositional relief may have on later turbidite deposition. Four mass transport events originated from a structural high located to the north of the study area. In the study area their deposits overly failure surfaces, leaving localised accommodation on their upper surfaces, that was filled by turbiditic sandstone deposits. The MTDs comprise a homogenous, well-mixed siltstone and mudstone matrix with floating sand grains, and a variety of isolated mud and sand clasts up to several meters in size. Isopach maps and well correlations within the Platform Area allow two classes of MTDs to be distinguished on the basis of maximum deposit thickness and depth of evacuated topography, which left different volumes of accommodation space for subsequent events. In the lower reservoir, MTDs 1–2 show an irregular upper topography characterised by up to 60 of metres of relief. Sharp-based, medium to fine-grained, clean, amalgamated turbidite sandstones are deposited onto this deeply evacuated topography. MTDs 3–4 within the middle reservoir represent smaller remobilisations. These deposits have a less pronounced topography, with differential relief of up to 10 metre scale, healed by muddier sandstone beds and subsidiary hybrid event beds (HEBs) that pinch out and lap onto the relatively subtle confining slopes. This depositional trend is discussed in terms of the interaction of a range of flow sizes with a range of scales of local sea floor accommodation space. The documented spatial variation in turbidite lithofacies in this MTD dominated setting illustrates how the facies distribution in MTD-accommodation fill can be quite different from those in conventional architectural and facies models. We are thankful for the data provided and for the core access by BOL (Britannia Operator Ltd.).

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015