Investigating sub-seismic sedimentary intrusions in the Middle Jurassic to Lower Cretaceous Agardhfjellet Formation (Svalbard)

Abstract

Meso-scale (sub-seismic) sedimentary injectites are inferred to play an important role in controlling subsurface fluid flow as documented in many hydrocarbon plays at various scales. Detailed characterisation of such units, usually unresolvable at the seismic scale, can be directly achieved at outcrop scale. In this framework, two sedimentary injection complexes have been analysed in the middle Jurassic-lower Cretaceous Agardhfjellet Formation exposed at Deltaneset (central Spitsbergen) at different stratigraphic levels. The upper complex comprises two main isolated, decimetres-thick clastic dykes characterized by different orientation and consolidation, tapering out vertically (up- and downward) within a stratigraphic thickness and a lateral extension of more than 50 m and 200 m, respectively. The lower complex is coarser-grained, made up by a network of interconnected dykes and sills, branching off from isolated lenticular bodies, interpreted to be linked to seafloor extrusion structures (sand volcano). Petrographic and micromorphologic analysis were used to identify the possible source of the remobilized material for both the upper and lower complexes within the over- and under-burden formations. Our results reveal that such granular material is likely sourced by the underlying coarse-grained lithologies of the late Triassic to middle Jurassic Wilhelmøya Subgroup. The lower complex was firstly emplaced during the Late Jurassic at shallow burial conditions, while the upper complex developed at higher confinement pressure, probably during the Late Cretaceous, with the progressive reworking of the same granular material. Field data allow detailed characterisation of complex structural-stratigraphic architectures of sedimentary intrusions, which can be used to constrain their spatial-temporal relationships with subsurface fluid flow.