The origin of microtextures in fluidized sands: a key recognition criterion for sand injectites, not sand-injection processes

ABSTRACT Sand injectites form when poorly consolidated sand is fluidized and forcibly intruded into a host sediment. Macroscopic features of injectite complexes record their formative processes, but relatively little is known about the differences in microscopic characteristics of injected versus parent sandstones. Microfracturing of grains in sand injectites has been recognized, and is related to grain-to-grain collisions during transport in turbulent flow. This study documents four injectite complexes and uses optical microscopy and point-counting, together with back-scatter and scanning electron microscopy (SEM) to show that grain fracturing occurred in depositional, partially remobilized, and fully injected sands in the four studied injectite networks. Point-count results show an increase in the proportion of microfractured versus non-fractured quartz grains in injected sandstones (between 0 to 60% more) when compared with samples of depositional or partially remobilized sandstones. SEM data show that the microfractures are commonly oriented across multiple adjacent grains and radiating out from grain point contacts. This geometry and the location of microfractures result from post-emplacement mechanisms of grain impingement, dissipation of pore-fluid pressure, and the effect of overburden pressure, rather than being products of flow processes during emplacement. We propose that the effects of overburden pressure are greater on grains in injected sandstones due to more rapid dissipation of pore fluid leading to more abrupt pressure changes than in parent sandstones. Furthermore, the dominance of grain-impingement fracturing in injectites demonstrates that grain-to-grain collision in a relatively low-concentration turbulent flow during emplacement is unlikely. This supports core and outcrop observations, such as layering, preservation of delicate features on injectite margins, and clasts concentrated at the upper and lower margins of the injectites, suggesting that flow during injectite emplacement is dominantly high concentration and laminar.