Quantifying Compaction and Cementation in Deformation Bands in Porous Sandstones

Abstract

Combined electron microbeam imaging techniques can be used to quantify cementation and compaction processes in deformation bands and their surrounding host rocks. Mixed secondary and backscattered electron signals can be used to definitively identify pore space, whereas scanned cathodoluminescence can be used to discriminate between detrital and authigenic quartz. Classic deformation bands from three porous sandstone units, the Cambrian Hickory Sandstone of central Texas (two bands and two host rocks), the Pennsylvanian Tensleep Sandstone of Wyoming (one band and one host rock), and the Pennsylvanian Weber Sandstone of northwestern Colorado (one band and one host rock), were examined using these imaging techniques. Cathodoluminescence images demonstrate that bands develop through a combination of grain-scale brittle processes and cementation. Point counting of scanning electron microscopy image mosaics reveals that the intergranular volume in deformation bands is higher than is apparent from transmitted light microscopy. Cementation equals or exceeds compaction as a cause of porosity decline in both deformation bands and host rocks. No evidence exists for significant pressure solution during band development. The intergranular volumes of host rocks in the range of 31–38% suggest that all of these samples have experienced burial of 2 km (1.2 mi) or less. Contrasts in the compactional and cementational states of bands and surrounding host rocks possibly reflect the differing availability of quartz nucleation surfaces in these different parts of the rock. Preferential emplacement of cement in the bands can lead to divergent paths of compactional behavior in bands relative to host rocks during the postkinematic phase of their burial history.