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_Journal of Structural Geology 32 (2010) 1231–1249_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Controls on syndepositional fracture patterns, Devonian reef complexes, Canning Basin, Western Australia Edmund L. Frost III *, Charles Kerans John A. and Katherine G. Jackson School of Geosciences, Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712, USA Article info Article history: Received 25 November 2007 Received in revised form 27 April 2009 Accepted 29 April 2009 Available online 20 May 2009 Keywords: Syndepositional fracture Neptunian dike Stratigraphic architecture P A ratio Canning Basin Abstract Syndepositional fractures are an important feature of high-relief carbonate systems and exert a profound control on many facets of platform evolution and reservoir development. Based on data collected from the Canning Basin’s Devonian reef complexes, this study characterizes syndepositional fracture patterns as a function of variations in: lithofacies, depositional position, stratigraphic architecture, and mechanical stratigraphy. Fracture parameters, such as extension and fracture intensity, are documented to vary strongly as a function of lithofacies. The highest syndepositional extension values occur in the microbial facies of the Famennian platform margin, with extension values three times higher than observed in equivalent Frasnian strata. Position along the depositional profile exerts a strong control on fracture patterns, with an approximate two-fold increase in syndepositional extension and fracture intensity typically observed from the platform interior to the platform margin. Syndepositional fracture intensity is shown to vary systematically with changes in platform-margin trajectory, with high fracture intensities observed in strongly progradational platforms and decreased fracture development in aggradational and retrogradational platforms. Evidence for the temporal evolution of the mechanical stratigraphy of the Devonian reef complexes is presented, with early-lithified strata effectively behaving as a single, large-scale (50–150 m) mechanical unit during syndepositional fracture development, while secondary fractures become increasingly affected by bed-scale (0.25–5 m) mechanical heterogeneity introduced by progressive diagenesis. The results presented here potentially provide a tool for predicting fracture characteristics (e.g., intensity, orientation, location, and vertical extent) from limited subsurface data and provide a method for characterizing syndepositional deformation in other systems. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Large, syndepositional, opening-mode fractures, often referred to as neptunian dikes, represent an important style of deformation in many carbonate platforms throughout the geologic record. These syndepositional fractures create a laterally and vertically extensive, early fluid-flow system that strongly affects platform-wide diagenetic patterns (Kerans, 1985; Hurley, 1986; Whitaker and Smart, 1997; Melim and Scholle, 2002; Hunt et al., 2002; Jones and Xiao, 2006), the spatial distribution of karst enhanced permeability (Smart et al., 1988; Kosa and Hunt, 2006; Guidry et al., 2007; Baceta et al., 2007) and subsequent carbonate reservoir properties (Collins et al., 2006). Moreover, with burial or later tectonism, * Corresponding author: ConocoPhillips Subsurface Technology, 600 N. Dairy Ashford - PR 3060, Houston, TX 77079, USA. Tel.: +1 281 293 2511. E-mail addresses: ned.l.frost@conocophillips.com (E.L. Frost III), ckerans@mail.utexas.edu (C. Kerans). 0191-8141 $ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2009.04.019 syndepositional fractures are often preferentially reactivated, and their distribution relative to later stress orientations can have a significant affect on ensuing deformation patterns. Fracture development in carbonates is typically attributed to stress associated with regional tectonic deformation (Price, 1966; Nelson, 1985) or burial diagenesis (Marrett and Laubach, 2001), with fracture characteristics (e.g., spacing, orientation, and termination) ascribed to variations in bed-scale mechanical properties (Narr and Suppe, 1991; Bai and Pollard, 2000; Underwood et al., 2003; Shackleton et al., 2005). Large-scale syndepositional fracture networks present a paradox as they develop prior to significant burial and often in the absence of coeval regional deformation (Playford, 1984; Hunt and Fitchen, 1999; Kosa and Hunt, 2005). Moreover, syndepositional fractures often crosscut numerous bed-scale (0.25–5 m), ‘‘apparent’’ mechanical layers, implying that the mechanical units that control these features occur at the sequence to formational scale (50–150 m); a scale of mechanical stratigraphy not commonly considered in fracture studies. As a result, syndepositional fractures are typically not predicted in settings that have 1232 E.L. Frost III, C. Kerans Journal of Structural Geology 32 (2010) 1231–1249 not experienced appreciable tectonic deformation and the enhanced vertical connectivity related to their formation is often underestimated; often leading to errant characterization of carbonate platform paragenesis and reservoir properties. This study utilizes outcrop-and remote-sensing-scale fracture attribute data collected from the Canning Basin’s Devonian reef complexes to address the following questions related to syndepositional fracture development. (1) Do syndepositional fracture patterns vary as a function lithofacies and position along a depositional profile? (2) Does a relationship exist between syndepositional fractures patterns and variations in stratigraphic architecture? (3) At what scale do mechanical units occur in early-fracture systems? And (4) do these mechanical units vary temporally with continued diagenesis? 2. Geologic setting 2.1. Regional setting The Canning Basin is Australia’s largest sedimentary basin covering approximately 430,000 km² and containing over 15,000 m of Ordovician through Cretaceous-age strata (Brown et al., 1984). The Canning Basin is bounded to the east by the Proterozoic Kimberley Block and to the west by the Achaean Pilbara craton (Figs. 1 and 2). Basin development initiated in the Ordovician with broad intracratonic down-warping, followed by active rifting in the Middle Devonian through the Early Carboniferous and the development of the deeply subsiding NW–SE-oriented Fitzroy Trough (Fig. 1). Extensional values of 50_ are reported within the Fitzroy Trough during the Late Devonian to Early Carboniferous (Drummond et al., 1988; Shaw et al., 1995) with the principal horizontal regional stress oriented NE–SW during the Late Devonian (Craig et al., 1984). Throughout the Late Devonian, carbonate deposition was widespread along the eastern margin of the Canning Basin on the shallow Lennard Shelf, with the reef complexes fringing the mountainous Precambrian Kimberley Block, as well as smaller, isolated, emergent Precambrian topography (e.g., Oscar Range; Fig. 2). The 10–50 km-wide shallow Lennard Shelf is flanked to the northeast by the mountainous Precambrian King Leopold foldbelt of the Kimberley Block and to the southwest by large-scale Late Devonian extensional fault systems (e.g., Oscar Fault and Pinnacle Fault) and the Fitzroy Trough. Two major reef-building sequences are recognized within the Devonian reef complexes, (1) the Givetian–Frasnian Pillara Sequence and (2) the Famennian Nullara Sequence (Fig. 3; Playford, 2002, and references therein). The Pillara Sequence developed during basin-wide transgression and is characterized by long-term backstepping with the development of pinnacle reefs and high-relief platforms with steep escarpment margins. Platforms of the Pillara Sequence were constructed by a consortium of stromatoporoids, corals, and calcimicrobes. Minor progradation occurred in the latest Frasnian, followed by the Frasnian Famennian (F F) mass extinction event and brief subaerial exposure of the reef complexes. The subsequent prograding Famennian reef complexes of the Nullara Sequence are composed primarily of high-energy grainstones with high-relief microbial margins (Hurley, 1986; Stephens and Sumner, 2003; Frost, 2007). Platform thicknesses range from approximately 300 to 600 m for the Frasnian Pillara sequence (Hurley, 1986) and 150–200 m for the Famennian Nullara Sequence (Hurley, 1986; Chow et al., 2004; Frost, 2007). Extensive early-marine cementation and microbial activity played an essential role in the evolution of both the Frasnian and Famennian reef complexes, allowing for the development of stout, wave-resistant reef rims and steep platform-margin escarpments. Early lithification also created brittle facies that were prone to syndepositional fracturing (Playford, 1980, 1984; Kerans et al., 1986). The Devonian reef complexes were subjected to burial depths of approximately 2000 m by the Middle Carboniferous (Playford et al., 1989; Wallace et al., 1991; Frost and Kerans, 2009). During this time, the reef complexes were subjected to pressure solution, secondary fracturing, and gentle basinward tilting (Fig. 2; Hurley and Lohmann, 1989; Wallace et al., 1991). Regional uplift associated with the Alice Springs Orogeny in the late Carboniferous coupled with glaciation in the Late Carboniferous to Early Permian removed many of the younger sediments overlying the Devonian reef complexes creating a broad glacial peneplain (Figs. 2 and 4; Eyles et al., 2001; Playford, 2002), along which an extensive paleokarst network developed along Late Devonian joints and fractures within the reef complexes (Playford, 2002). Permian glacial sediments of the Grant Group subsequently infilled much of the paleokarst network and buried the reef complexes creating a regional angular unconformity. During Permian burial, the reef complexes were again subjected to pressure Ключевые слова: platform margin, platform interior, secondary fracture, goldhammer, australia, mechanical unit, scanlines table, hunt, remote-sensing data, carbonate platform, microbial lens, cement, microbial, unconsolidated slope, reef complex, fracture pattern, setting, set fracture, sedimentary geology, data, early-lithication pattern, lesser degree, syndepositional fracturing, stratigraphy, structural, aapg, intensity, platform-margin, eds, large-aperture fracture, oscar, burial, laubach, observed, lohmann wallace, compaction, platform evolution, frost iii, crosscutting relationship, study area, fischer, early, gorge, suppe bai, jackson school, syndepositional fault, one-dimensional scanlines, carbonate, journal, petroleum, antecedent topography, journal sedimentary, study, rock, microbial binding, economic paleontologists, texas, lithologic boundary, vertical extent, progressive diagenesis, structural geology, reservoir, long axis, mechanical property, secondary, extensive fracturing, playford, kosa, basin, devonian, basinal stratum, tectonic, time higher, mechanical, frasnian, eld mapping, gravitational, iii kerans, gravitational instability, submarine, western australia, oscar fault, ground-based scanlines, fracture attribute, bedding boundary, syndepositional fracture, kerans, canning basin, gravitational process, syndepositional deformation, stratigraphic, location, fracture development, kerans journal, margin, oscar range, western, highly variable, iii, range, fracturing, diagenesis, stratigraphic architecture, mechanical stratigraphy, syndepositional, journal structural, geologic, bedding, developed, windjana gorge, mechanical contrast, fitchen hunt, sedimentology, bulletin, fracture network, sequence, scanline, structural evolution, weakly developed, triassic, windjana, deformation, temporal evolution, oriented parallel, growth fault, limestone range, depositional facies, controlled, canning, regional, aapg bulletin, journal structural geology, geological, ward, frost kerans, forereef, classic face, devonian reef, result presented, facies, price, pattern, sequence stratigraphy, morown cliffs, fracture spacing, fracture intensity, australian, face, differential compaction, complex, rling, platform-derived sediment, syndepositional extension, hurley, growth, figs, marine cement, slope, relationship, stratum, sequence boundary, george, reef platform, frost, tinker, geology, backreef stratum, james, fitchen kosa, fault, unit, set, fracture, fitzroy trough, platform-margin trajectory, early deformation, condence interval, upper, continued burial, famennian, strong control, harris, platform-margin escarpment, mechanical layer, progradational platform, nelson, sandstone, table, sedimentary, marine, architecture, bed, control, basinal sediment, purser, dominant style, generally restricted, reef, scale, geological society, sediment, backreef, ratio, frasnian reef-at, platform, boundstone, ginsburg, aperture, marrett, burial diagenesis, extension, topography, development, pollard underwood