Crestal unconformities on an exposed Jurassic tilted fault block, Wollaston Forland, East Greenland as an analogue for buried hydrocarbon traps

Abstract

The stratigraphy of successions exposed in footwall crests of tilted fault blocks is commonly highly complex. Crestal stratigraphy and structure are particularly difficult to unravel in the subsurface due to poor seismic resolution across fault zones, footwall collapse, and coalescing syn- and post-rift unconformities. Crestal ridges are important elements in basin evolution, as they form drainage divides and sediment sources for aprons along footwall scarps and hangingwall deltas. A Middle Jurassic – lowermost Cretaceous footwall crest is exceptionally well exposed in the mountain Stratumbjerg in Wollaston Forland, East Greenland. Rifting and block tilting was initiated in the (?)Bajocian, intensified in the Oxfordian–Kimmeridgian, culminated in latest Jurassic, Volgian, time and faded out in the earliest Cretaceous. The main border faults of the westward tilted blocks trend roughly N–S. The first early syn-rift block was formed in the Middle–Late Jurassic and was 40 km wide. During rift climax in the latest Jurassic it was fragmented into three blocks, each 10–15 km wide. The early syn-rift succession rests on thin Upper Permian evaporites and carbonates or directly on peneplaned crystalline basement. It is composed of the stepwise backstepping marine Pelion, Jakobsstigen and Bernbjerg Formations deposited in progressively deeper water, reflecting the combined effects of increased rifting and long-term eustatic rise. The rift-climax and late syn-rift succession was deposited along the main western basin margin fault scarp. Up to several kilometres thick, it consists of coalesced, mainly conglomeratic, deep-marine–slope-apron fans. Over the block crest this succession unconformably overlies early syn-rift strata, whereas in the deeper parts of the halfgraben, the base of the succession is conformable. A post-rift unconformity was formed in the late Hauterivian, probably during early post-rift emergence and has an irregular, stratigraphically and structurally controlled erosional topography. The eroded early syn-rift, rift-climax and late syn-rift successions were subsequently draped by deep-marine Barremian, and younger post-rift strata. The outcrop example highlights the interplay between large-scale block faulting, minor synthetic intra-block faulting, crestal degradation, and development of rift-climax and post-rift unconformities with pronounced erosional topography. It provides an excellent structural–sedimentological field analogue to deeply buried marine halfgraben settings, which are key elements in many hydrocarbon reservoirs. Its tectonic and stratigraphic development is thus highly similar to a number of large oil fields in the North Sea and the Norwegian shelf.