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_Journal of Structural Geology 32 (2010) 1187–1200_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Forward modeling synsedimentary deformation associated with a prograding steep-sloped carbonate margin_ _Phillip G. Resor a,*, Eric A. Flodin b_ _a Department of Earth and Environmental Sciences, Wesleyan University, 265 Church Street, Middletown, CT 06459, USA_ _b Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, CA 94583, USA_ _article info_ _Article history: Received 1 February 2008 Received in revised form 9 April 2009 Accepted 27 April 2009 Available online 8 May 2009_ _Keywords: Carbonate deformation Synsedimentary deformation Geomechanics Permian Capitan reef Guadalupe Mountains_ _abstract_ _Differential compaction associated with prograding and aggrading steep-sloped carbonate margins leads to penecontemporaneous and post-depositional modifications of stratal geometries and tensile and shear stress concentrations that might result in brittle deformation. In an effort to investigate controls on these deformation processes, we employ a step-wise gravity loaded elastic model that captures prefailure displacement and stress field patterns for a depositional geometry based on the Permian Capitan depositional system, Guadalupe Mountains, West Texas and New Mexico, USA. We consider four model geometries with varying progradation to aggradation (P A) ratios, from strongly prograding (P A ≈ 10) to strongly aggrading (P A ≈ 0.1). The strongly prograding case (P A ≈ 10) is used for sensitivity analysis that investigates the effects of varying rock mechanical properties of basin and platform facies. Model results yield relatively consistent patterns of deformation and stress that include: (1) a region of enhanced subsidence centered near the platform margin, (2) basinward displacement of the platform margin that decreases down slope, and (3) positive maximum Coulomb stress and positive (tensile) stress, both in-plane and out-of-plane, near the platform margin and in adjacent slope and platform facies. The patterns of deformation for the strongly progradational model are strikingly similar to present day stratal geometries of the Capitan depositional system that are often inferred to be primarily depositional in origin. Model results suggest that these geometries are established immediately upon deposition and may therefore affect the stratal architecture of the margin, but significant additional deformation also occurs during subsequent platform growth. We interpret the regions of positive Coulomb stress and tensile stress as areas likely to fail by faulting or jointing, respectively. This inference is corroborated by field observations of early-formed brittle deformation features in the Capitan margin. Our geomechanical models of the Capitan margin suggest that early-formed deformation is an integral part of the general steep-sloped carbonate system._ _© 2009 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Prograding carbonate platforms often aggrade hundreds of meters above their associated basin floors and develop high-relief margins with moderate to steep slopes. Carbonate platforms thus exert significant vertical loads on underlying fine-grained slope and basin sediments and also have the potential to extend horizontally in association with the slope face. These processes may cause significant synsedimentary deformation that can create additional accommodation space, modify depositional geometries, and generate faults and joints. Predictive models of these processes_ _* Corresponding author. Tel.: +1 860 685 3139; fax: +1 860 685 3651. E-mail addresses: presor@wesleyan.edu (P.G. Resor), eflodin@chevron.com (E.A. Flodin)._ _0191-8141 $ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2009.04.015_ _have the potential to improve understanding of early fracture formation that may be important in hydrocarbon migration and storage and interpretation of reef ecology and stratal architecture._ _Compaction-induced differential subsidence likely plays a significant role in the development of carbonate platform geometries and sequence stratigraphy (Hunt et al., 1996; Playford, 1984). Experimental studies have demonstrated that carbonate sediments may undergo significant compaction during burial. Mud-rich shallow water carbonates may lose more than 50% of their volume during the first w200 m of burial (Goldhammer, 1997; Shinn et al., 1977; Shinn and Robbin, 1983). Carbonate sands, while compacting less at shallow depths due to limited potential for grain reorientation and dewatering, may experience significant volume loss due to grain breaking and pressure solution at burial depths greater than a few hundred meters (Fruth et al., 1966; Goldhammer, 1997). Down-hole investigations (Audet, 1995; Budd, 2001, 2002; Halley and Schmoker, 1983; Heydari, 2000) have corroborated these experimental results, demonstrating significant porosity and permeability loss associated with mechanical and chemical compaction during the burial of carbonate sediments. Boundstones and early cemented sediments such as hard grounds, however, may retain much of their original volume, due to the presence of a relatively rigid framework. Lateral variations in carbonate facies and cementation may thus lead to significant variations in compaction across a carbonate shelf and slope._ _The effects of differential compaction of carbonate sediments may include tilting of beds, the development of internal unconformities and sediment onlap (Hunt et al., 1996; Rusciadelli and Di Simone, 2007), and synsedimentary jointing (Devaney et al., 1986; Frost and Kerans, 2009; Guidry et al., 2007; Playford, 1984; Stanton and Pray, 2004) and faulting (Hunt et al., 2002; Kosa and Hunt, 2005). These effects are most clearly demonstrable in examples where uniform originally flat-lying strata overlie lateral variations in carbonate facies and are warped down over more compactable units (e.g. Anderson and Franseen, 1991). Differential compaction has also been suggested as a mechanism for generating basinward dips of platform strata associated with a number of carbonate platforms (Hunt and Fitchen, 1999; Longley, 1999; Rusciadelli and Di Simone, 2007; Saller, 1996). Alternatively, it has been suggested that basinward dipping strata may represent the original depositional geometry (e.g. Hurley, 1989; Kerans and Tinker, 1999; Osleger, 1998)._ _Efforts to model the effects of differential compaction have been largely conceptual or have treated compaction as a one-dimensional process driven purely by overburden mass (e.g. Longley, 1999; Saller, 1996). In situations of rapid sedimentation lateral fluid flow may become important (Dugan and Flemings, 2000) and reasonable consideration of compaction requires coupling between compaction and fluid flow (Bitzer, 1999). Furthermore, early cementation of carbonate strata (Grammer et al., 1993) may lead to lateral transmission of compaction-related stresses and strains within the cemented sedimentary layers. Modeling compaction of rapidly prograding carbonate platforms is thus likely to require at least two-dimensional mechanical considerations._ _Extension or collapse associated with carbonate platform margins may also be a significant cause of synsedimentary deformation (e.g. Bosellini, 1998; George et al., 1995; Hine et al., 1992) and may create joints and sediment-filled dikes in recently deposited sediments (Playford, 1984). Carbonate slopes are typically steeper than siliciclastic slopes and have concave-up profiles (Kenter, 1990; Schlager and Camber, 1986). Rapid cementation may lead to slopes that exceed the angle of repose for loose sediment (Grammer et al., 1993). Steep carbonate slopes are thus metastable; supported by cohesion rather than frictional contact. Self erosion may further steepen slopes and generate the concave-up profile (Schlager and Camber, 1986). A variety of mechanisms have been suggested for triggering carbonate slope failure including sea level fall, seismic activity, storm or tsunami waves, and the development of overpressure (e.g. Bosellini, 1998; George et al., 1995; Spence and Tucker, 1997). Rusciadelli et al. (2003) used a two-dimensional finite difference approach to model the collapse of the Cretaceous Maiella platform margin. These authors explored the effects of loading due to sea level fall and seismic events, but did not explicitly incorporate effects of differential compaction into their model._ _In this paper we use two-dimensional finite element modeling to explore the integrated effects of loading due to carbonate platform growth and steep slope angles on the synsedimentary deformation of carbonate platforms. The geometry and facies distribution of our model is based on the Capitan depositional system of West Texas and New Mexico, where the paucity of tectonic deformation, excellent outcrop exposure, and abundant previous work provide good constraints on platform to basin geometry and facies distribution. We use an elastic rheology in order to explore the distribution of stresses and displacements prior to failure. Model results suggest that steep-sloped carbonate reef complexes are inherently unstable and that differential displacements and stress concentrations, both differential and tensile, play an important role in the evolution of these systems. While we focus our attention on the Capitan system, conclusions drawn from examination of model results are relevant to the general case of steep-sloped carbonate margins._ _2. Geologic background_ _The Permian Capitan depositional system has been the topic of numerous publications since the seminal work of King (1948). Here we briefly summarize the geologic elements pertinent to modeling and understanding synsedimentary deformation of the system._ Ключевые слова: e, r, o