The Process of Sand Injection: Internal Structures and Relationships with Host Strata (Yellowbank Creek Injectite Complex, California, U.S.A.)

The Yellowbank Creek Injectite Complex (YCIC) has excellent exposure of the external geometries and internal structures in a large-scale sandstone injectite. The YCIC has an irregular geometry that can be defined neither as a sandstone dike nor as a sill. Substantial erosion occurred along the margins between the YCIC sandstone and host mudstones that includes scoured basal margins and a scalloped upper margin that cuts up to 5 in into the overlying host mudstones. Erosion of host mudstone has incorporated significant volumes of mudstone clasts into sandstones at the margins of the injectite. Using microtextural data corrasion is identified as a typical process by which mudstone is eroded during sand injection. Corrasion occurs when particles of mudstone are either cut or broken from a cohesive bed as the result of high-velocity turbulent flow of sand grains acting as tools. Corrasion together with scouring and the general erosion of the host mudstone imply high-velocity turbulent flow conditions and confirm that the fluidized sand had a sufficiently low viscosity, during sand injection, to allow turbulence.

Internal structures include laminae of more tightly packed sand grains than occur in the surrounding sandstones, and are interpreted to have formed by flow and deposition of injecting sands during waning of flow, thus reflecting an overall reduction of pore-fluid pressure. Deformation of laminae into folds defines bands, which are aligned parallel to the margins of the sandstone. The force driving deformation is likely to have been shear stresses induced between the injected sandstone and the more rigid host mudstone. The principal style of deformation is hydroplastic. Oversteepened laminae and pipes are interpreted to have formed due to post-sand-injection consolidation and resulted in only minor mobilization of sand.