Deformation Bands Formed by Interrelated Cementation and Brittle Fracture in Porous Sandstones, Hickory Formation, Central Texas: Synkinematic Diagenesis Revealed by Cathodoluminescence Imaging
K. L. Milliken
Porous quartzose sandstones of the Hickory Formation (Cambrian) display prominent anastomosing deformation bands that stand out in relief on weathered surfaces. Enhancement of lithification within bands arises from linked processes of brittle particle size reduction and quartz cementation. In thin section, band margins are abrupt; transition from undeformed sandstone occurs over a distance of a few microns. At band margins quartz-cemented grain debris is observed immediately adjacent to open pores. Localized grain crushing by opening-mode fractures propagated from points of grain contact is the primary deformation mechanism along band margins. Considerable heterogeneity exists in the degree of crushing manifested by different grains in the marginal zone of the band Centra regions of bands have a bimodal particle size distribution in which a few surviving larger grains (generally >100 µm) with few visible fractures are surrounded by a matrix of highly comminuted grain debris (particles mostly in the range of 1 to 10 µm). Absence of transgranular fractures within central parts of bands suggests that displacements there are accommodated by particle rearrangement (grain-boundary sliding) within fine-grained debris, with little further particle comminution, and, possibly, by fractures localized within cement. Authigenic quartz fills intragranular fractures along band margins and also cements intergranular spaces in the central parts of bands, leaving only minor amounts of microporosity. Preferential cementation in deformation bands attests to a p ssible genetic link between deformation and cementation that arises from enhanced silica solubility by particle size reduction and subsequent preferential cement localization on freshly fractured surfaces.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California