Diagenetic controls on fault-zone architecture and permeability structure
Diagenetic controls on the evolution of fault-zone architecture and permeability structure: Implications for episodicity of fault-zone fluid transport in extensional basins Randolph T. Williams1,†, Laurel B. Goodwin1, and Peter S. Mozley2 Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87801, USA
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ABSTRACT Diagenesis demonstrably changes both the mechanical and hydrologic properties of sedi ments. However, the effect of these changes on the distribution and types of structures that develop in fault zones over time, and their impact on fault-zone fluid flow has not previously been systematically investigated. We explored the impact of diagenesis on the mechanical and hydrologic properties of initially unlithified sand cut by a syndeposi tional normal fault in an extensional basin. Field, microstructural, and geochemical data document a diagenetic record of initially continuous fluid flow through a hangingwall damage zone up to 10 m wide. In this zone, initial deformation via particulate flow is recorded by a foliation defined by a grain shape preferred orientation, preserved by subsequently precipitated, pore-filling calcite cement. A transition to episodic fluid flow is demonstrated by calcite veins that crosscut the previously cemented, foliated sandstone within relatively narrow (≤5 m from the fault core) segments of the older damage zone. Unlike the widespread record of particulate flow, veins are restricted to the vicinity of a mapped relay zone between overlapping fault segments, and an inferred, partially covered relay zone. Vein microstructures record repeated fracture opening and seal ing. Veins in breccia zones have δ13C values as high as +6.0‰, suggesting degassing of CO2- and/or CH4 -charged fluids. These data collectively suggest that relatively early dam age zone cementation strengthened and stiff ened the foliated damage zone, affecting the localization of brittle fractures and fluid flow in relay zones. Our results highlight system atic changes in the character and locus of rtwilliams@wisc.edu
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deformation and fluid flow during the devel opment of normal faults and provide a basis for predicting how these structures may act to trap or transmit fluids during the develop ment of extensional basins. INTRODUCTION Fault-zone architecture and permeability structure influence a variety of processes of societal interest, including contaminant transport, trapping versus leakage of hydrocarbons, the localization of ore deposits, and induced seismicity (e.g., Cox et al., 1991; Cox, 1995; Knipe, 1993, 1997; Guglielmi et al., 2008; Faulkner et al., 2010; Yielding et al., 2010). Research focused on better understanding the controls on these processes implicitly recognizes the fact that architecture and permeability structure must change over time, but this research has rarely been designed to investigate how these changes influence the spatial and temporal distribution of fluid flow during fault development (cf. Eichhubl et al., 2009). The research we describe in this contribution represents an attempt to fill part of this gap in knowledge through explicit evaluation of the impact of progressive diagenesis on the hydromechanical behavior of a fault that initiated in unconsolidated sediment. The concept that fault-zone diagenesis will both record and change flow pathways over time is not new (e.g., Knipe, 1993). Our documentation of this record of a normal fault in a syntectonic extensional basin, however, represents the first attempt to understand both where within the fault zone fluid flow was initially localized and how and why the locus of flow changed over time. This work provides a starting point for understanding where and how we might expect these processes to occur in the present in the subsurface. The impact of a fault on fluid flow in a sedimentary basin depends on a variety of factors,
including the juxtaposition of rocks and sediments of differing permeability (e.g., Yielding et al., 2010); smearing of low-permeability clay in the fault core (e.g., Vrolijk et al., 2015); and the hydrologic properties of fault rocks relative to surrounding protolith (e.g., Antonellini and Aydin, 1994; Evans et al., 1997; Rawling et al., 2001; Eichhubl et al., 2009). All of these will change as the fault accumulates displacement over time. At any given point in time, the spatial relationship between these elements can be described in terms of fault-zone architecture: a fault core, where the majority of displacement is accommodated, flanked by damage zones of subsidiary structures such as fractures, which are bounded on either side by host rock or protolith (Chester and Logan, 1986). The relative thicknesses and types of structures and their spatial arrangement within each of these architectural elements ultimately determine whether a given fault will act as a barrier, conduit, or barrier-conduit system with respect to fluid flow (Caine et al., 1996). The types of structures formed will depend on deforma tion mechanisms, which are determined by the mechanical properties of architectural elements, which in turn can vary in space (structural position in the fault zone) and time (in response, for example, to diagenesis during syntectonic fluid flow and/or changes in stress during burial; e.g., Knipe, 1993; Davatzes et al., 2003; Boles et al., 2004; Johansen et al., 2005; Eichhubl et al., 2009; Riley et al., 2010; R otevatn and Bastesen, 2014). Explicitly incorporating these spatial and temporal variations into conceptual models of fault-zone permeability structure would improve our ability to predict the subsurface distribution and potential migration of petroleum, ore-mineralizing fluids, and contaminants in sedimentary basins. Understanding the evolution of fault-zone permeability structure in a syntectonic basin poses a particular challenge. Faults that form
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Williams et al. as sediment is deposited (growth faults) cut unlithified sediments in the uppermost basin fill and bedrock at depth and can juxtapose units of different mechanical and hydrological properties over time. In addition, the hydromechanical properties of the sediments themselves will change through progressive diagenesis, and these changes will affect the evolution of faultzone deformation and permeability. Consider, for example, the dominant deformation mechanism in faults in unlithified sediments: distributed particulate flow, which is accomplished by grain boundary sliding and rotation, with limited grain breakage (Heynekamp et al., 1999; Rawling and Goodwin, 2003, 2006). This process reduces porosity through shear-enhanced compaction and leaves a signature of foliation and lineation defined by grain shape preferred orientation (Goodwin and Tikoff, 2002; Rawling and Goodwin, 2006). The disruption of sedimentary layering via particulate flow in poorly lithified damage zones generally increases fault-parallel permeability relative to the protolith but decreases cross-fault permeability, resulting in an anisotropic permeability structure (Rawling et al., 2001). As porosity is further decreased during burial, compaction, and cementation, high-porosity sedimentary rocks experience more localized failure, commonly forming cataclastic shear deformation bands in fault damage zones. Though these typically reduce cross-fault permeability (Sigda et al., 1999), their overall impact on fault-zone permeability is relatively small in heterolithic sedimentary sequences (Rawling et al., 2001). With sufficient cementation and porosity reduction, however, sedimentary rocks will fail via transgranular fracturing. Interconnected fracture networks in damage zones can increase faultparallel permeability by three orders of magnitude or more (Evans et al., 1997; Flodin et al., 2004). Thus, both the magnitude and longevity of fault-parallel permeability, or in petroleum terms, a fault’s potential to leak, are ultimately controlled by damage-zone deformation mechanisms, which are expected to change as faulted sediments are progressively lithified. To assess the impact of syntectonic fault-zone diagenesis on the evolution of fault-zone architecture and permeability structure in extensional basins, we conducted field, microstructural, and geochemical analyses of the Loma Blanca fault zone in the Socorro Basin of the Rio Grande rift, New Mexico, USA. We tested the following two hypotheses: (1) The degree of cementation is the primary control on damage-zone deformation mechanisms where faults cut high-permeability sand layers deposited during fault activity, and (2) spatial and temporal variations in damagezone deformation mechanisms control the loca-
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tion, magnitude, and episodicity of fault-parallel fluid migration during fault-zone evolution. Our results allow us to confirm these hypotheses. In the Loma Blanca fault zone, preferential calcite cementation records initially continuous fluid flow through an area where the hangingwall damage zone was composed of nonindurated or weakly cemented sand. This damage zone is relatively wide, and it is characterized by a steeply dipping foliation that records particulate flow prior to and/or during cementation. The resulting cemented “sand tectonite” is cut by transgranular fractures only in the vicinity of relay zones between fault segments. These fractures occupy a relatively narrow portion of the previously cemented damage zone. Microstructures in the fractures are interpreted as recording episodic fluid flow during repeated fracture opening and sealing. Calcite cement distribution and structures therefore record a shift from distributed deformation and flow in hanging-wall damage zone sands prior to lithification, to fracture and flow localized in the vicinity of relay zones following precipitation of pore-filling cement. This record of spatiotemporal changes in deformation mechanisms focuses attention on the mechanical evolution and hydrologic significance of relay zones during the growth and linkage of normal faults in extensional basins. GEOLOGIC SETTING The Loma Blanca fault is a north-striking, down-to-the-east, basin-margin normal fault situated ~5 km east of the western margin of the Socorro Basin, the southernmost axial basin of the Rio Grande rift (Fig. 1A; Chapin and Cather, 1994). The modern-day Socorro and La Jencia Basins formed a continuous basin (the Popotosa Basin) until the late Miocene– early Pliocene, when the Lemitar Mountains were uplifted through the floor of the Popotosa Basin. The sediments of the Popotosa Basin record two separate episodes of rapid extension, which occurred in the late Oligocene and mid-Miocene (Cather et al., 1994). During these episodes, increased displacement rates on basinmargin faults caused lakes and therefore playa and lacustrine deposits to migrate from the basin center toward the master fault. Thus, finegrained mudrock is intercalated with coarsergrained clastic deposits throughout the basin. Total extension in the Popotosa Basin is ~50% (Chapin and Cather, 1994). The Loma Blanca fault is composed of at least three distinct segments along a 23 km trace and juxtaposes hanging-wall Quaternary allu vium against footwall Pliocene fluvial sands at the surface (Cather et al., 1994). Machette (1982) documented a history of seismic activity
in the Quaternary based on fault scarp morphology and offset terrace gravels, which record recurrent surface ruptures in the late Pleistocene. Although displacement magnitudes observed at the surface are relatively modest (~2 for shape preferred orientation analysis, and at least 100 individual grains were measured in each sample (cf. Shelley, 1995; Cladouhos, 1999).
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Quaternary alluvium, colluvium, and terrace deposits. Sierra Ladrones Formation (lower Pliocene - lower Pleistocene) Popotosa Formation (upper Oligocene - upper Miocene) Tertiary volcanic and intrusive rocks (Eocene - Oligocene)
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Mississippian - middle Eocene carbonates Proterozoic Rocks Major faults. Dashed where concealed or inferred, bar and ball on downthrown block.
Figure 1. (A) Simplified geologic map of the Socorro Basin of the Rio Grande rift show ing outcrop relations and the major fault zones: Loma Peleda fault zone (LPFZ), Loma Blanca fault zone (LBFZ), and Socorro-Canyon fault zone (SCFZ). The La Jencia Basin (not shown) is located immediately west of the Lemitar Mountains. Inset shows position of the Rio Grande rift in New Mexico; star shows position of Socorro Basin. Map is modi fied from Cather et al. (1994). Box shows approximate extent of 1:12,500 map shown in B. (B) Map of the trace of the Loma Blanca fault in the study area showing that the fault is composed of two distinct segments that link in the South fracture zone. The fault includes a splay in the northern map region. The fault is inferred to be poorly cemented where poorly exposed (dashed and dotted lines). Boxes show the locations of the North and South fracture zones (described in text).
A subset of samples was selected for stable isotope analysis to provide constraints on the fluid source of calcite cement associated with each category of secondary structure. Approximately 10 mg aliquots of sample powder were collected from thin-section billets using a lowspeed Dremel with a diamond bit to provide petrographic constraints on aliquot mineralogy. Two distinct aliquots were collected from individual billets for a subset of samples to assess within-sample variability. Powdered samples from cemented sand injectites (described in later sections) were magnetically separated to remove minor MnO2 cements prior to analysis. Stable isotope analyses were conducted at the University of Wisconsin–Madison Stable Isotope Laboratory using a five-collector Finnigan MAT 251 mass spectrometer and at the University of Michigan Stable Isotope Laboratory using a Finnigan MAT 253 triple-collector isotope ratio mass spectrometer coupled to a MAT Kiel IV preparation device. Analytical uncertainties determined by repeated analyses of National Bureau of Standards (NBS) carbonate 19 are ~0.05‰ for d18O and d13C. Analyses of distinct aliquots from single samples indicate that within sample variability is on the order of 0.3‰; aliquot stable isotope values are reported as the mean of the two analyses. RESULTS
Microstructural Analysis We selected a subset of samples for microstructural analysis using optical petrography, secondary electron (SE) and backscattered-electron (BSE) imaging. Samples were impregnated with blue-dyed epoxy to facilitate quantification
of sample porosity. Secondary electron and BSE images were obtained on a Hitachi S3400 scanning electron microscope (SEM) operated at 15 keV and 60 nA at the University of Wisconsin– Madison. Photomosaics of cemented, foliated damage-zone samples were used to quantify grain shape preferred orientations in the plane
Fault-Zone Structure The Loma Blanca fault is composed of two soft-linked segments in the study area (Fig. 1B). The fault dips steeply east (~75°–85°) and has a curvilinear trace with strikes ranging
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Williams et al. grains (e.g., Goodwin and Tikoff, 2002) is commonly visible. This foliation is best developed in an ~10–20-cm-wide zone immediately adja cent to the fault core (Fig. 2B). Rarely, it is overprinted by cataclastic deformation bands, although these do not stand out by either color or relief in the damage zone and are therefore difficult to see. The foliation strikes parallel to the fault, but it is shallower in dip, with measured values of 68°–75°E. The orientation of the foliation relative to the fault is similar to that recorded in the Sand Hill fault zone in the Albu querque Basin, and it is consistent with normal shear (cf. Heynekamp et al., 1999; Rawling and Goodwin, 2006). Sand injectites and veins filling opening mode fractures are restricted to two large, 200–400-m-long, well-cemented outcrops at the north and south ends of the map area where they crosscut the sand foliation. The occurrence of these structures defines the North and South fracture zones of the Loma Blanca fault (Fig. 1B).
from 350° to 020°. A large fault splay is located ~1 km south of the fault’s intersection with the Rio Salado (Fig. 1B). Where well cemented, the exposed portion of the hanging-wall damage zone is 2–10 m wide and stands out in marked topographic relief (Fig. 2A). Mapped exposures of the fault therefore approximate the along-strike extent of fault-zone cements (Fig. 1B). The footwall damage zone is uncemented and poorly exposed throughout strike, and the width of this architectural element could not be determined. Clays are generally absent from the fault core, although a thin (
