The Diagenetic Role of Brittle Deformation in Compaction and Pressure Solution, Etjo Sandstone, Namibia

Abstract

Scanned-cathodoluminescence (CL) imaging of the quartz-rich, porous Etjo Sandstone from northern Namibia shows that brittle deformation has played a major role in developing arcuate and interpenetrated grain contacts. Such contacts, previously interpreted to result from pressure solution, are seen in scanned-CL images to arise primarily from rearrangement of fragments formed by brittle deformation. Brittle deformation dominates compaction and produces extensive microfractures that heal with authigenic quartz cement. The volume of intragranular authigenic cement is significant and represents a previously unrecognized sink for silica in sandstones. True pressure solution is minor in the Etjo and is generally limited to contacts between brecciated fragments and unfractured, detrital grains. In addition to this pressure solution, silica may also be mobilized from the dissolution of comminuted fragments near grain contacts. However, the amount of silica imported into grains is substantially larger than that which appears to come from dissolution sites. Grain overlap can no longer be considered to arise from simple pressure solution, and the volume of authigenic quartz measured in sandstones must include intragranular fracture-filling cement as well as overgrowths and pore-filling cement.