High and low resolution sequence stratigraphy in lithological prediction—examples from the Mesozoic around the northern North Atlantic

Abstract

The Mesozoic around the northern North Atlantic was characterized by a succession of extensional tectonic events separated by periods of regional subsidence governed by thermal contraction. Sedimentation was controlled by tectonic subsidence, sediment and water loading, isostatic compensation, sediment influx, climate, and eustatic and relative sea-level variations at several scales. A series of examples from the Upper Triassic–Lower Cretaceous from East Greenland and the Northern North Sea have been selected to illustrate the relative importance of the controlling factors on depositional systems of different temporal and spatial scale. A long-term eustatic rise, superimposed by short-term cycles thus started in the Hettangian and culminated in the Kimmeridgian, whereas the main tectonic events include uplift of a major dome in the Early Jurassic, subsidence of the dome in the Middle Jurassic, and domal collapse and rifting in the Late Jurassic. The syn- to early post-rift Wollaston Forland Group (Volgian–Valanginian) is an example of a strongly tectonically controlled coarse-grained deep-water system deposited on tilted fault blocks. Deposition of the partly contemporaneous Fynselv Member (Raukelv. Formation) took place during uniform regional subsidence mainly governed by thermal contraction; relative sea-level variations appear to be the dominant factor controlling the sedimentary architecture. The two successions differ dramatically in geometry, facies and sedimentary processes. The tilted fault block example seems, however, to include the same number of roughly contemporaneous large-scale cycles as the unit deposited in the uniformly subsiding basin. This suggests that a relative sea-level signal can be recognized also in the tectonically controlled deep-water succession. The effects of sea-level control on contemporaneous shallow marine successions are illustrated by the Middle Jurassic Vardekløft Formation and the Brent Group of the Northern North Sea. The two units show great similarity and contemporaneity in terms of facies, parasequence stacking patterns, and systems tract successions. This allows formulation of a combined age and lithological model, which can be used in prediction of spatially separated systems tracts, some of which may contain unrecognized reservoir bodies. The similar development is highly suggestive of eustatic control. However, both regions were affected by large-scale doming, and the uniform development of the two systems may thus be controlled by relative sea-level changes reflecting large-scale tectonic processes. Finally, a high resolution sequence stratigraphic analysis of the lacustrine Rhaetian–Sinemurian Kap Stewart Formation is presented to demonstrate a hitherto unrecognized play type. Correct identification of sequence stratigraphic elements in this formation allows prediction of significant deltaic reservoir sandstones interbedded with black organic-rich lacustrine mudstones with excellent source rock potential. Combined low and high resolution sequence stratigraphy can be used in large-scale lithological prediction and comparison between systems situated far apart, and in detailed within-system predictions on a reservoir scale.