1 Introduction
This Groundwater Project book focuses on fractures in siliciclastic rocks, primarily in sandstones and secondarily in sandstone-shale intercalations. This focus is justified by the presence of large aquifers in sandstones of western North America and other regions of the world. The premise of this book is that fractures in these rocks may impart extreme fluid flow behavior, and are therefore critical for groundwater and contaminant flow and their containment. The diagram in Figure 1 shows that fracture types are related to the mechanics and mechanisms of their formation, and in turn, this influences their properties.
Figure 1 – Schematic diagram showing that fracture types are related to their mechanism of formation, which influences the fracture properties. Modified from Zhong and others (2009).
Fracture types that occur in siliciclastic rocks are joints, pressure solution seams (not common in the shallow crust), deformation bands and faults (Figure 2). The simplest form of each of these fracture types is a single fracture with a unique failure and formation mechanism. Although each fracture type has a wide range of physical properties, this book mainly covers the properties most relevant to groundwater science such as permeability, porosity, as well as fracture length and frequency (spatial density).
Figure 2 – Major fracture types in siliciclastic rocks. Unlike carbonates, pressure solution seams are not common in siliciclastic rocks in the shallow crust (thus the gray color), but they are mentioned here for completeness.
Due to their variability and complex interaction with groundwater, fractures in sandstone pose a great challenge to hydrogeologists and engineers. The premise of this book is that even though remote detection methods with limited resolution are available, the direct knowledge of failure structures in siliciclastic rocks, including architecture, distribution and fluid flow properties, is effective for dealing with their impact on groundwater and contaminant flow. In natural materials, groups of fractures typically occur in a hierarchical manner, as shown in Figure 3.
Figure 3 – Common hierarchical character of fracture groups: zones, sets, assemblages and domains, which may include one or more of the individual fracture types presented in Figure 2.
Figure 4 expresses the cross-relationships among fracture types and their common distributions. This book follows an informal approach to the scheme presented by Aydin and Zhong (2017) by introducing data or information at locations in the text based on our anticipation of the needs and desires of readers with diverse interests.
Figure 4 – Linkages between fracture types and their entities.
As an example, Figure 5 displays a gallery of pictures from the Aztec Sandstone at Valley of Fire State Park in southeastern Nevada, USA to highlight the interplay among the various fractures listed in Figure 2 and groundwater. The first image of the gallery (Figure 5a) is a series of deformation bands that compartmentalize an aeolian sandstone with extensive cross-beds trending from bottom left to top right. The image in the middle (Figure 5b) shows joints and sheared joints marked by the yellow color of precipitant materials, which is produced by water percolating along the fractures over geological time. The image on the right (Figure 5c) is an aerial photograph hundreds of meters in dimension covering several faults that exhibit left-lateral offset of the red and light-colored sandstone units for tens of meters.
Figure 5 – A gallery of images from the Jurassic Aztec Sandstone exposed in Valley of Fire State Park, Nevada, illustrating how the fractures interacted with groundwater. a) Compaction bands compartmentalize the medium with various degrees of fluid-rock interaction reactions expressed as colorful haloes that trend from lower left to upper right. These are cross-beds, many of which are over-printed by bed-parallel compaction bands. The lineaments in the direction of the viewer are compaction bands. b) Lineaments in the lower half of the view are joints and sheared joints marked by yellow infill and reaction haloes caused by percolating paleo groundwater. c) Aerial photograph showing red and bluff sandstone units offset for tens of meters by left-lateral strike-slip faults.
This book discusses fractures in siliciclastic rocks as illustrated by figures in the Introduction (Figure 1 to Figure 4), primarily focusing on fractures in sandstones and sandstone-shale sequences (Figure 5) and on their potential role in groundwater storage and flow. Each fracture type has unique formation mechanism, mechanics and properties as illustrated schematically in Figure 1. Understanding fluid flow through subsurface rocks is challenging due to the presence of a wide variety of structural and depositional heterogeneities, as recognized long ago by the pioneering work of N.G.W. Cook and his former colleagues and students at the University of California, Berkeley. On the other side of the San Francisco Bay, at Stanford University, D.D. Pollard, in partnership with A. Aydin and former members of the Stanford Rock Fracture Project, advanced knowledge of the nature and formation of rock fractures in a variety of lithologies and tectonic settings, as well as their impact on fluid flow. The material forming the backbone of this book is from the latter efforts. In this sense, the present book constitutes a Stanford-centric view of rock fractures.
To make the most relevant concepts and illustrations accessible to the readers, some details are provided in boxes at the end of this book, with links to the boxes at appropriate locations throughout the book.
Those who would like to read more on topics of interest may go to the original scientific journals and online publications referenced in this book, and to the Rock Fracture Project and Shale Smear Project data repositories at the Brenner Earth Science Library, Stanford University.