The Middle Jurassic Vardekløft Formation of East Greenland—analogue for reservoir units of the Norwegian shelf and the Northern North Sea

Abstract

The Mesozoic rift basin of East Greenland was uplifted during the Tertiary, and provides excellent exposures of Mesozoic sediments of the same age and type as those occurring deeply buried in the shelf seas around the northern North Atlantic. This paper outlines the preliminary results of field work on the Middle Jurassic Vardekløft Formation, which is the best exposed analogue to the main reservoir rocks of the Northern North Sea and the Norwegian shelf. The formation is up to 600 m thick and is of Boreal Bajocian–Callovian age. The sediments were deposited during a period of sea-level lowstand followed by an overall rise. The base of the formation is an important sequence boundary and the top is a maximum flooding surface, separating the formation from the overlying downlapping Olympen Formation and its correlatives. The Vardekløft Formation consists of shallow marine sandstones of the Pelion Member, which pass basinward into offshore siltstones of the Fossilbjerget Member. The boundary between the two members is strongly diachronous, younging sourcewards toward the north along the basin axis, with a major northward displacement (> 100 km) in the Pompeckji Zone. Internally, the formation is composed of up to 30 coarsening-upward units. The thickness of the units appears to vary in cyclical fashion upward through the member. The units typically consist of a lower, commonly very thin, fine-grained sandstone or siltstone, a middle fine-grained laminated to rippled sandstone, and an upper medium- to very coarse-grained cross-bedded, multi-storey sandstone, topped by a marine flooding surface. This facies succession reflects deposition during overall decreasing water depth. The widespread lateral continuity of the lower, commonly erosional, boundary of the multi-storey sandstone facies suggests that the top part was deposited during relative sea-level fall. This is supported by the high degree of reworking and abundance of coarse-grained sandstone and pebble conglomerate. Different types of flooding surfaces are recognized, showing distinct proximal–distal change in amount of winnowing and erosion. Three orders of relative sea-level changes which probably had significant influence on sandbody distribution and geometry can be recognized. The longest cycle had a duration of c . 20 Ma. It corresponds to the Vardekløft Formation and Olympen Formation which form a crude transgressive–regressive cycle deposited during initial lowstand and succeeding rise, and during highstand and early lowstand, respectively. An intermediate cyclicity is recognized in the stacking pattern of the coarsening-upward units, suggesting cyclical changes in the rate of long-term sea-level rise. A minimum rate of relative rise characterized the Middle and Upper Boreal Bathonian Ishmae–Variabile Zones. High rates of relative sea-level rise characterized the Boreal Bajocian Borealis–Indistinctus Zones and the Lower Callovian Apertum–Calloviense Zones. Finally, the individual coarsening-upward units represent short-term cycles of relative sea-level changes. Comparison with contemporaneous reservoir units such as the Rannoch, Etive and Tarbert formations of the Brent Group in the Northern North Sea shows many similarities in depositional motifs, including vertical and lateral changes in facies development and stacking patterns.