Injected sand sills in a strike-slip fault zone: A case study from the Pil’sk Suite (Miocene), Southeast Schmidt Peninsula, Sakhalin

This chapter describes sandstone sills intruded into the middle Miocene Pil’sk Suite, which are well exposed along a continuously exposed 8-km (5-mi) section on the southeast coast of Schmidt Peninsula, Sakhalin (Russian Far East). This suite forms part of a relatively thin Neogene succession (probably no more than 1100 m thick [3608 ft]), which unconformably overlies Cretaceous deposits. The succession was deposited on a topographic high, which marks the Mesozoic outer arc high and is now deformed within a large-scale dextral strike-slip fault system, part of the Eurasia–North America plate boundary. The Pil’sk Suite is dominated by organic-rich biosiliceous rocks (opoka, porcelanite, and rare chert). Within this siliceous succession are abundant brown, well-cemented sandstones, subparallel to bedding, with erosional top and bottom contacts. Their changing stratigraphic position makes correlation across distances greater than about 10 m (33 ft) difficult. They are interpreted as having an injected origin.

Trains of upright to gently inclined folds deform the pre-late Miocene succession, including the brown sandstones. The vergence of these structures, and the transport direction of associated thrust faults, is generally toward the southwest. The injected sandstone sills were clearly cemented prior to brittle deformation. Deformation is concentrated into high-strain zones spaced approximately 1 km (0.6 mi) apart, which are probably part of a large flower structure. Almost all the sandstone sills are found within these zones, adding several tens of meters to the stratigraphic thickness. Given the relatively thin post-middle Miocene succession, it is likely that this injection resulted in coeval deformation of the sediment surface.