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Sill geometry and emplacement controlled by a major disconformity in the Tarim Basin, China
Earth and Planetary Science Letters 501 (2018) 37–45
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Earth and Planetary Science Letters www.elsevier.com/locate/epsl
Sill geometry and emplacement controlled by a major disconformity in the Tarim Basin, China Zewei Yao a,b,∗ , Guangyu He b,∗∗ , Chun-Feng Li a,c , Chuanwan Dong b a b c
Ocean College, Zhejiang University, Zhoushan 316021, China School of Earth Sciences, Zhejiang University, Hangzhou 310027, China Laboratory of Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China
a r t i c l e
i n f o
Article history: Received 21 November 2017 Received in revised form 9 August 2018 Accepted 14 August 2018 Available online xxxx Editor: T.A. Mather Keywords: saucer-shaped sills magma emplacement layering Tarim Basin seismic data dyke deflection
a b s t r a c t Igneous sills are widely distributed in sedimentary basins worldwide. Sill emplacement has significant impacts on structural and thermal evolutions of sedimentary basins and also provides important insights into volcanic and sub-volcanic processes. Dyke deflection and sill formation are largely controlled by mechanical layering of the host rock, and a number of mechanisms have been proposed. However, most models of sill formation are based much on numerical and analogue modeling. For many natural sills, the key mechanism governing the dyke deflection and sill formation remains to be discussed and, in some cases, controversial. To better understand the control of mechanical layering on sill formation and test the existing models, we study detailed geometries of igneous sills from the central part of the Tarim Basin, China, based on seismic reflection data and borehole data. Nineteen igneous sills which are expressed as packages of high-amplitude reflections are observed, and they were all emplaced in the Upper Ordovician strata at current burial depths of about 5–8 km. The sills can be classified into three geometric types: type 1, saucer-shaped sills; type 2, strata-concordant sills; and type 3, hybrid sills, which have the characteristics of both type 1 and 2. The geometry of the saucer-shaped sills is typical and comprises three distinct parts: a flat inner sill, inclined sheets and, in many cases, flat outer sills. All the bases of the saucer-shaped sills coincide with the disconformity between the Middle and Upper Ordovician strata (the M disconformity), indicating that the disconformity controlled the sill geometry and emplacement depth in the basin. The M disconformity is characterized by a lithological change from underlying limestones to overlying mudstones. We suggest that among the three main mechanisms for the deflection of dykes into sills, namely Cook–Gordon debonding, stress barriers and elastic mismatch, the Cook–Gordon debonding is the dominant mechanism in the study area. This is a mechanism by which a weak contact opens up ahead of a vertically propagating dyke, allowing the dyke to be deflected into a sill when the dyke meets the open contact. This study indicates that the control of mechanical layering of the host rock on sill formation is largely depends on the host-rock lithologies and confirms again that the saucer shape is the fundamental sill geometry under the control of layering. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Igneous sills have been widely observed in sedimentary basins around the world from out-crops (e.g., Galerne et al., 2011; Muirhead et al., 2012; Eide et al., 2017) and seismic reflection data (e.g., Planke et al., 2005; Cartwright and Huuse, 2005; Hansen and Cartwright, 2006; Magee et al., 2016). These intrusions exhibit various shapes, including horizontal sills, laccol-
*
Corresponding author at: Ocean College, Zhejiang University, Zhoushan 316021, China. Corresponding author. E-mail addresses: [email protected] (Z. Yao), [email protected] (G. He).
**
https://doi.org/10.1016/j.epsl.2018.08.026 0012-821X/© 2018 Elsevier B.V. All rights reserved.
iths and saucer-shaped sills (Planke et al., 2005; Jackson et al., 2013). Sill emplacement can have significant impacts on the structural and thermal evolutions of sedimentary basins. Among others, sill emplacement may cause uplift of the host rock, forming forced folds of the overlying strata (Hansen and Cartwright, 2006; Magee et al., 2014), and affect the maturation of the organic matter in the surrounding host rock as well as the hydrocarbon migration and accumulation (Rateau et al., 2013; Spacapan et al., 2018). Sill complexes are also an important part of sub-volcanic plumbing systems (Galland et al., 2014). Understanding the formation of these intrusions is key to assessing the volcanic and sub-volcanic processes (Wilson et al., 2016). Sills form when subvertical dykes or inclined sheets become deflected into a sub-horizontal orientation. However, detailed for-
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mation mechanisms can be complicated and have been discussed for decades (Barnett and Gudmundsson, 2014, and references therein). The first mechanism suggests that sill emplacement is largely controlled by the level of neutral buoyancy, which corresponds to depths where the magma pressure equals the lithostatic pressure (Bradley, 1965; Francis, 1982). However, this point of view is questioned in many circumstances because sills often intruded in a wide range of depth in a sedimentary basin and preferably followed rock-rock interfaces (e.g., bedding planes and disconformities) (Planke et al., 2005; Muirhead et al., 2012; Barnett and Gudmundsson, 2014; Eide et al., 2017). Other three mechanisms, namely Cook–Gordon debonding, stress barriers and elastic mismatch, are proposed to focus more on the controls from local stresses and layering of the host rocks (Barnett and Gudmundsson, 2014, and references therein). Cook–Gordon debonding is a mechanism by which a propagating extension fracture, here a dyke, opens up a weak contact ahead of the fracture tip (Cook et al., 1964; Gudmundsson, 2011). These mechanisms, supported by analogue modeling, suggest that mechanical properties of host rock at boundaries can determine the dyke-to-sill deflection process (Pollard and Johnson, 1973; Kavanagh et al., 2006; Menand, 2008). However, the dominant mechanism has yet to be discussed in real cases. Better geological observations are still needed to refine these theories. Furthermore, under the control of the host rock layering, individual sills are expected to evolve to a saucer shape after their initial horizontal spreading (Pollard and Johnson, 1973; MaltheSørenssen et al., 2004; Thomson and Hutton, 2004; Polteau et al., 2008; Galland et al., 2009; Magee et al., 2013; Chen et al., 2017). The saucer-shaped sill consists of a horizontal sill along a planar discontinuity, stretching outward and upward to inclined sheets, and subsequently to flat outer sills, and such a geometry is considered as a fundamental shape for magma intrusions in sedimentary basins (Thomson and Hutton, 2004; Polteau et al., 2008; Galland et al., 2009). Although this viewpoint is widely accepted, many of the saucer-shaped sills, imaged by seismic data, have cup-shaped bodies (Mathieu et al., 2008), such as those in sedimentary basins along the NE Atlantic margin (Hansen, 2004; Hansen and Cartwright, 2006). The bases of these sills, in accordance with recent field observations from a sill in the Faroe Islands (Walker, 2016), appear to be mildly transgressive through the layering of the host rock, implying that layering is not the primary control on sill geometry. These observations question whether mechanical layering is a fundamental control on the formation of saucer-shaped sills. In this paper, we investigate seismically examples of the igneous sills in the Tarim Basin, northwestern China, to further understand the mechanisms of the deflection of dykes into sills at contacts, and evaluate the control of mechanical layering on the geometry and emplacement depth of saucer-shaped sills. We suggest that the Tarim Basin is a suitable place to undertake the study, because saucer-shaped sills there are typical and are well imaged, their spatial relationships to the dominant layers of the host rock are distinct, geological evolution of the study area is relatively simple and the host sequences are generally flat-lying. We first use high-quality two-dimensional (2-D) seismic data combined with borehole data to document the seismic expression and geometry of igneous sills. We then discuss the main control on sill geometry and growth. For the first time, these intrusions are revealed in seismic data in the Tarim Basin.
margins, including Tianshan mountains, Kunlun mountains and Altyn mountains. The initial formation of the Tarim Basin is related to the breakup of Rodinia in the Neoproterozoic time (Jiang et al., 2017). Its oldest sequences are Sinian/Ediacaran strata exposed in the northwestern and northeastern margins of the basin, but their distribution within the basin is still under discussion (He et al., 2010). After the initial formation, the Tarim Basin received sedimentary deposition almost continuously from the Cambrian to Quaternary, with a total thickness up to 15000 m. However, as a result of multi-phased tectonic events, current thickness of sedimentary rocks varies within the basin (Lin et al., 2012). The basin can be roughly divided into seven tectonic units, including the Kuqa depression, Tabei uplift belt, Northern depression belt, Central uplift belt, Southwestern depression belt, Southeastern uplift belt and Southeastern depression belt (Fig. 1A). These tectonic units can further be subdivided into sub-units (Fig. 1A). Our study area is around the Tazhong uplift in the central part of the Tarim Basin (Fig. 1A). The Lower and Middle Cambrian strata are dominantly composed of dolomite (Fig. 2). In the southern part of the study area, they are supposed to contain interbeded gypsum, acting as an important detachment layer (Yao et al., 2017). The Upper Cambrian to Middle Ordovician sequences consist of dolomite and limestone. During the Late Ordovician, the Tazhong uplift bounded by the Tazhong NO.I fault and Well Zhong2 fault was significantly uplifted (Fig. 1B) and the Upper Ordovician depositional facies in the uplifted area was different from those in adjacent area. In the uplifted area, the Upper Ordovician strata consist of the Lianglitage formation (O3l ) and Sangtamu formation (O3s ) which dominantly contain limestone and mudstone respectively (Fig. 2). Toward the depressed area, the Upper Ordovician sequences gradually turn into the Qiaerbake formation (O3qb ) and Queerqueke formation (O3qq ) (Fig. 1B). The Qiaerbake formation is about 20 m thick and consists of limestone and marlstone (Cai et al., 2011). The overlying Queerqueke formation which comprises the most part of the Upper Ordovician strata is dominantly mudstone and siltstone. From the Silurian to Quaternary, the area was covered by marine and terrestrial sequences up to 6000 m thick. Structural deformation was weak in the northern part of the study area. The igneous sills studied in this paper are mostly observed here and they mostly intruded the Upper Ordovician units or generally followed the boundary between Upper and Lower Ordovician strata (the M disconformity) (Fig. 3). In contrast, structure deformation was relatively strong in the Tazhong uplift and southwestern area, which were subject to detachment faulting and transpressive faulting because of the collision between the Altyn block and the Tarim block (He et al., 2011). Igneous activity within the Tarim Basin is marked by widespread volcanic rocks in the Permian strata. These igneous rocks have been revealed by numerous wells in the basin and field observations in the northwestern corner of the basin (Fig. 2). Among them, the flood basalts are up to 700 m thick and in an area of about 2 × 105 km2 (Yu et al., 2011). They were extruded within a period of 2–3 Ma in the Early Permian, although the absolute age is still controversial (Yu et al., 2011; Zhang et al., 2012). These flood basalts and some other igneous rocks together are named as the Tarim Large Igneous Province (LIP) (Yang et al., 2014). However, the relationship between the igneous sills documented in this study and the Tarim LIP remains to be discussed. 3. Data and methods
2. Geological setting With an area up to 50 × 104 km2 , the Tarim Basin is the largest basin in northwestern China (Fig. 1). The basin is diamondshaped in plan view and bounded by large mountain belts at its
This study uses zero-phase, time-migrated 2-D seismic reflection dataset, which has a total length of about 25000 km and covers an area of about 50000 km2 (Fig. 1A). The grid of 2-D seismic profiles is between 2 × 2 km and 4 × 4 km, permit-
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Fig. 1. (A) Tectonic setting of the Tarim Basin (modified from Lin et al., 2012). The inlet is the topographic image from Consultative Group on International Agricultural Research – Consortium for Spatial Information (CGIAR-CSI), showing the location of the basin. (B) A regional cross section showing the structural context of the study area (modified from He et al., 2009) (see location in Fig. 1 A). TWT = two-way travel time.
ting quasi-3-D imaging of sills. The dominant seismic frequency is about 20–24 Hz. A velocity of 6000 m/s is assumed for sills in this study based on the velocity data from the Tazhong22 well, which penetrated a dolerite sill in this area (Luo, 2006). This amounts to a vertical seismic resolution of sills of about 62.5–75 m and a detectable limit of about 17 m. The sills commonly intruded in a depth of about 4–5 s in two-way travel time. The intruded succession has an approximate velocity of 5000 m/s (Feng, 2008). Thus, the vertical resolution and detection limit of the host rock near the intrusion are about 52–62.5 m and 14 m, respectively. We can judge sill thickness by the ‘tuned reflection packages’ (Smallwood and Maresh, 2002; Magee et al., 2015). The amplitude of sill reflection increases when the sill thickness rises from the detection limit to the vertical resolution, and for larger sill thickness the reflection amplitude remains at a lower value (Smallwood and Maresh, 2002; Magee et al., 2015). Similar to other studies worldwide, four criteria are used to interpret sill reflection: (1) high-amplitude; (2) local transgression; (3) abrupt termination; (4) limited and characteristic spatial geometries, such as a saucer-shaped geometry (Planke et al., 2005). In addition, borehole data from wells Adong1, Shun2 and GL1 are used to constrain the seismic interpretation and physical properties of igneous sills.
4. Seismic and geometric characterization of igneous sills 4.1. Seismic characterization A total of nineteen igneous sills are mapped in the central part of the Tarim Basin in this study, partly shown in Fig. 3. They show packages of continuous, high-amplitude seismic reflections commonly with a peak–trough–peak reflection configuration (Fig. 4), indicating that the sills are expressed as ‘tuned reflection packages’ (Smallwood and Maresh, 2002; Magee et al., 2015). This means that their thicknesses are between the vertical resolution 62.5–75 m and the detection limit ∼17 m. This conclusion is roughly consistent with the drilling data from the Adong1 well which recovered Sills 13 and 11 with a thickness of 55.8 and 47.1 m, respectively (see location in Figs. 1, 3 and 4). Seismic data show that sills all intruded within the Upper Ordovician sequences, which currently dip at very low angles (<2◦ ). The current depths of these intrusions, calculated from the borehole data (e.g., Adong1 well) and seismic data, are about 5–8 km. The hosted Upper Ordovician strata and adjacent strata are generally concordant. There are no large-scale faults or remarkable angular unconformities in the place where sill formed. Reflections of igneous sills are quite distinct in seismic profiles as Upper Ordovician sequences are dominantly characterized by low-amplitude seismic reflections, permitting the sills to be interpreted with great
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4.2. Geometric characterization
Fig. 2. Stratigraphic column showing the Cambrian to Permian strata in the northern part of the study area in the Tarim Basin (modified from Cai et al., 2011 and Lin et al., 2012). Note that igneous sills intruded the Upper Ordovician strata and that the Qiaerbake formation is much thinner than sills.
confidence. However, in many places the M disconformity is also characterized by continuous, high-amplitude seismic reflections, similar to the ‘tuned reflection packages’ of the sills (Fig. 4 and 5). We cannot conclude whether there are strata-concordant sills. But inferring from the pattern of saucer-shaped sills, we interpreted these reflections as igneous sills when they are bounded by inward dipping inclined sheets (e.g., Sill 13 in Fig. 4), or when they are connected to inclined sheets (e.g., Sill 11 in Fig. 4).
Following Planke et al. (2005) and Jackson et al. (2013) on sill classification through seismic observations, igneous sills in the study area can be classified into 3 geometric types (Figs. 5 and 6). Type 1—saucer-shaped sills, which are composed of a flat inner sill, inclined sheets and, in some cases, outer sills (Fig. 6A). Eight saucer-shaped sills are recognized, as the main geometric type in the study area. As some saucer-shaped sills are incomplete, they are subdivided into complete-saucer-shaped sills (Sill 13 in Fig. 4; Sill 7 in Fig. 5A and 5B) and half-saucer-shaped sills (Sill 5 in Fig. 5C and 5D). The inner sills of saucer-shaped sills are flat and coincident with the M disconformity, and abruptly turn into inclined sheets at their periphery. The inclined sheets, in some cases, turn outward into flat outer sills (Sill 7 in Fig. 5B). This transition can be either sharp or progressive. The dip angles of the inclined sheets generally range from 10 to 20◦ , and the transgressive heights are about 500 to 1500 m. The complete-saucer-shaped sills (Sills 13, 9 and 7 in Fig. 3) and their inner sills commonly have an elliptical shape in plan view with an average area of about 404 km2 (121–803 km2 ) and 103 km2 (47–214 km2 ), respectively, whilst the shapes of half-saucer-shaped sills are irregular in plane (Sill 5 in Fig. 3). Type 2—strata-concordant sills (Fig. 6B). It is common that sills propagated upward to high levels through inclined sheets, forming ‘step’ structures (Francis, 1982; Muirhead et al., 2012; Schofield et al., 2012). In contrast to sills that are entirely concordant to bedding planes (Sill 6 in Fig. 5E and 5F), some sills are generally layer-parallel but turn downward to inclined sheets at their periphery (Sill 8 in Fig. 5G and 5H). These inclined sheets are supposed to be attached to strata-concordant sills along the M disconformity. The dip angles of the inclined sheets range from 7 to 10◦ , and the transgressive heights are about 300 to 500 m. These sills are also classified to this type 2 and named as halfstrata-concordant sills. Seismic interpretation shows that strataconcordant sills do not have specific forms in plane and coverage areas vary from about 46 to 471 km2 (average 234 km2 ). Type 3—the hybrid sills that have the geometric characteristics of both saucer-shaped sills and strata-concordant sills (Fig. 6C). Due to the complexity of hybrid sills and limited horizontal resolution of 2-D seismic data, hybrid sills cannot be split into discrete sills. Hybrid sills are also common in the study area (Sills 11 and 12 in Fig. 5I). Similarly, they have flat bases coincidental with the base of the Upper Ordovician sequences. The map areas of the inner sills within the hybrid sills are 17–75 km2 (average 47 km2 ). The dip angles of the inclined sheets of the inner sills range from 7 to 24◦ , and the transgressive heights are about 600 to 2000 m. The plane shapes of hybrid sills are irregular, and their areas are relatively large compared to those of the other two types of sills, ranging from 196 to 2150 km2 (average 1242 km2 ).
Fig. 3. Plane distribution of some igneous sills in the central part of the Tarim Basin (see location in Fig. 1A). Each sill is marked consecutively with a number in a circle.
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Fig. 4. A seismic profile showing seismic characteristics of igneous sills in the Tarim Basin (see location in Fig. 3). Vertical exaggeration factor is about 2.
4.3. Sill junction relationships Three main classes of junction geometries have been recognized in 3D seismic data from the northern Faroe–Shetland Basin (Hansen et al., 2004). Different types of sill junction geometries have also been investigated from our seismic data. Following is a case study from Sills 5, 6, 7 and 8. Seismic profiles show a discontinuity in Sill 8 at the southwestern margin of Sill 7, and the northeastern part of Sill 8 is uplifted (e.g. in Fig. 5B, Fig. 7) (Class C junctions by Hansen et al., 2004). Therefore, we interpret that Sill 7 cross-cut Sill 8 and uplifted the overlying part. Seismic data also show that (1) the northeastern margin of Sill 7 stopped at the southwestern wing of Sill 5 (Fig. 5B); (2) Sill 6 abuts against Sill 5 at its eastern margin (Fig. 5D and 5F); and (3) the edge of Sill 5 links to the base of Sill 8 (Fig. 7). All of these junction geometries can be classified into the Class B junctions (Hansen et al., 2004). However, except for Sill 7 cross-cutting Sill 8, the detailed kinematic and formation sequence of these sills are hard to interpret based on above sill junction geometries, because the Class B junctions can have different kinematic development models (Hansen et al., 2004). 5. Discussion 5.1. Spatial relationship between the M disconformity and the sills The most notable sill features in the Tarim Basin are that all the sills were emplaced in the Upper Ordovician strata and that the bases of saucer-shaped sills generally followed the M disconformity. However, it should be noted that the detailed spatial relationship between the sills and the Qiaerbake formation above the M disconformity is unclear, because the Qiaerbake formation is too thin (c. 20 m) to be discriminated in seismic profiles. Currently, none of wells has revealed their contact relationship. We interpret that bases of saucer-shaped sills followed the M disconformity, because the M disconformity is a regional, planar discontinuity characterized by a lithological change from dominantly limestones to marlstone and mudstone (Figs. 2 and 8). But it is also possible that the sills intruded along the top of, or within, the Qiaerbake formation. Note that the Qiaerbake formation (c. 20 m) is much thinner than the sills (c. 50 m) and that lithology of the Qiaerbake formation changes gradually from limestone in the lower part to marlstone in the upper part as a progressive transition from underlying Yijianfang formation (limestone) strata to overlying Queerqueke (mudstone) formation (Fig. 8). Thus, no matter whether the sills intrude at the top, the base or within the Qiaerbake formation, observations do support that lithologic transition from underlying
limestone to overlying mudstone controlled the deflection of dykes into sills. 5.2. Main controls on sill geometry and emplacement depth The geometry of saucer-shape sills in the Tarim Basin is analogous to the conceptual model of saucer-shaped sills which have three distinct parts: a flat inner sill, inclined sheets and, in many cases, flat outer sills (Chevallier and Woodford, 1999; Polteau et al., 2008). Sills with such a typical saucer shape in the study area, which have been reproduced in a number of numerical and physical simulations with layered host rock (Pollard and Johnson, 1973; Galland et al., 2009; Galerne et al., 2011; Chen et al., 2017), are among the first to be observed in nature. In particular, the saucershaped sills in our study show great resemblance to experimental results of injecting vegetable oil (magma) into silica flour (host rock) with a flexible net (an interface), in which the injected material always follow the flexible net, forming the base of the saucershaped intrusion (Galland et al., 2009; Galerne et al., 2011). Thus, our observations in this study verified the experiment results and conceptual model of saucer-shaped sills, and suggest that the M disconformity mainly controlled the saucer-shaped geometry and the emplacement depth of igneous sills in the Tarim Basin. In addition, the development of strata-concordant sills and flat outer sills at different depths within the Upper Ordovician strata implies that layer interfaces in the host rock other than the M disconformity also affect sill formation (e.g., encouraging the formation of strata-concordant sills). Besides, both numerical and physical modeling indicate that the sharp transition from the inner sill to inclined sheets is controlled by the host-rock layering (Galland et al., 2009; Chen et al., 2017). This is because layering of the host rock can cause asymmetric stresses and the fracture of the overlying strata at the sill termination and the subsequent propagation of inclined sheets (Pollard and Johnson, 1973; Malthe-Sørenssen et al., 2004). Therefore, we suggest that the sharp transition from inclined sheets to flat outer sills in some sills was also controlled by some bedding planes in the host rock. This conclusion corresponds well with the numerical simulation results that flat outer sills and the sharp transition often develop at a layer boundary (Chen et al., 2017). In contrast, as mentioned above, saucer-shaped sills in sedimentary basins along the NE Atlantic margin often show a cupshaped geometry. This means that they commonly lack a flat inner sill concordant to the host-rock layering, a sharp transition from the inner sill to inclined sheets, and flat outer sills (Hansen, 2004; Hansen and Cartwright, 2006). Similarly, sills of such geometries have been reproduced in analogue models using homogeneous media (Mathieu et al., 2008; Galland et al., 2009). It is suggested that
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Fig. 5. Characteristic examples of igneous sills in the Tarim Basin. (A), (C), (E) and (G) are perspective quasi-3-D morphological views of Sills 7, 5, 6 and 8, respectively. (B), (D), (F), (H) and (I) show seismic characteristics and cross-sectional geometries of Sills 7, 5, 6, 8 and 11, respectively. Locations of seismic profiles are marked in Fig. 3. Vertical exaggeration is about 4. Coordinates D, W and N in (A), (C), (E) and (G) corresponds to depth, west and north, respectively. The numbers of the sills refer to those in Fig. 3.
in these places layering of the host rock has not played a dominant role in sill formation, as observed by Walker (2016) in the Faroe Islands. 5.3. Implications for the deflection of dykes into sills There are mainly three mechanisms that control the deflection of dykes into sills, namely Cook–Gordon debonding, stress barri-
ers and elastic mismatch (Fig. 9) (Gudmundsson and Philipp, 2006; Gudmundsson, 2011; Gudmundsson and Løtveit, 2012; Barnett and Gudmundsson, 2014). Elastic mismatch mechanism occurs where there is a contrast in toughness and Young’s modulus of two adjacent layers. This mechanism means that sill is prone to forming at interfaces separating the upper, rigid strata from the lower, soft strata (Fig. 9A) (Kavanagh et al., 2006; Menand, 2011; Gudmundsson, 2011). It is demonstrated that the behavior of a
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Fig. 6. Schematic cross-section diagram showing geometric types of igneous sills in the Tarim Basin (Modified after Planke et al., 2005). Type 1: saucer-shaped sills, which can be subdivided into complete-saucer-shaped sills (A) and half-saucershaped sills (B). Type 2: strata-concordant sills, which can be subdivided into complete-strata-concordant sills (C) and half-strata-concordant sills. (E) Type 3: hybrid sills.
Fig. 7. A NE-trending seismic profile showing sill junction relationships between Sills 5, 7 and 8 (see location in Fig. 3).
dyke (penetration or deflection) at a contact is affected by the Dundurs elastic mismatch parameter α , and the ratio of the energy release rates for dyke deflection along the contact Gd to dyke penetration of the contact Gp (Gd /Gp ) (Fig. 9A) (Barnett and Gudmundsson, 2014; Gudmundsson, 2011). The ratio Gd /Gp is related to the material toughness of the contact and that of the layer above the contact. When the layer above the contact is softer than the layer below (α < 0), dyke deflection occurs only if the material toughness of the contact itself is roughly 26% smaller than that of the overlying layer (Barnett and Gudmundsson, 2014). These conditions are not common in natural cases. By contrast, when the upper layer is stiffer than the lower layer (α > 0), the tendency for dyke deflection at the contact increases (Fig. 9A). However, in this study, the M disconformity separates upper mudstones from lower limestones. The mudstones commonly have smaller Young’s modulus than limestones (Roche et al., 2014), though their exact values in the Tarim Basin are not known currently. According to the elastic mismatch mechanism, deflection of dykes into sills along the M disconformity is quite difficult. Thus, we suggest that the elastic mismatch mechanism is not the primary control on the deflection of dykes into sills. This interpretation is in accordance with recent results of numerical modeling of shallow magma intrusion, which illustrate that elastic mismatch mechanism alone cannot control the deflection of dykes into sills (see case 1 and 2 in Chen et al., 2017). The stress barrier mechanism means that rotation of the principal stresses at boundaries of layers of different stiffness (Young’s modulus) can generate a stress barrier (Fig. 9B) (Gudmundsson and
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Fig. 8. Gamma ray (GR), shallow lateral resistivity (LLS) and deep lateral resistivity (LLD) of the Qiaerbake formation and adjacent strata from the GL1 well (see location in Fig. 1). Note that the gamma ray increases gradually from Qiaerbake formation to Queerqueke formation, and that the carbonate content decreases from Yijianfang formation to Qiaerbake and Queerqueke formations.
Løtveit, 2012; Barnett and Gudmundsson, 2014). This mechanism often assumes that the magma chamber is circular or ellipsoidal (Fig. 9B) (Gudmundsson and Philipp, 2006; Gudmundsson, 2011) or that the strata are subjected to regional compression or extension (Gretener, 1969; Barnett and Gudmundsson, 2014). This mechanism also suggests that dyke deflection into a sill occurs at an interface bounded by upper, rigid strata and lower, weak strata (Gudmundsson and Løtveit, 2012). However, as discussed above, the M disconformity does not follow this pattern. Moreover, circular or ellipsoidal magma chamber and related uplifts above have not been observed in our study. Regional compression or extension is not strong either, as the strata are generally flat-lying and no large-scale thrust or extensional faults formed in places of sills intrusions. Numerical modeling (Chen et al., 2017) and physical modeling (Galland et al., 2009) also implied that regional stresses are not necessary for dyke-to-sill deflection. Therefore, it is quite possible that stress barrier mechanism is not dominant in our study area either. The Cook–Gordon debonding is a mechanism by which a weak contact opens up ahead of an approaching vertically propagating extension fracture (Cook et al., 1964; Barnett and Gudmundsson, 2014). In a homogeneous, isotropic material, the tensile stress induced ahead of and parallel to an extension fracture is about 20% of the tensile stress ahead of and perpendicular to the crack (Cook et al., 1964; Gudmundsson, 2011). The large tensile stresses induced by a propagating dyke can lead to the opening (debonding) of the weak contact ahead of the dyke. When the dyke eventually meets the open contact, the propagating dyke can be deflected into a sill at the contact. This is a common mechanism for the formation of sills, particularly at shallow depths in the Earth’s crust (Barnett and Gudmundsson, 2014). Thus, by excluding the previous two mechanisms, we suggest that among the three main mechanisms the Cook–Gordon debonding is the dominant mechanism for the dykes-to-sills deflection in the study area (Fig. 9C) and that the M disconformity acts as the weak contact. Recent numerical simulation indicates that the differences in mechanical properties between adjacent layers, among some others, affect whether the Cook–Gordon debonding works, although the critical values are still unconstrained (Zhang et al., 2007; Chen et al., 2017). The mechanical properties include not only the Young’s modulus but also cohesion, Poisson’s ratio, friction angle, etc. In our study, the interfaces below the M disconformity are commonly characterized by a limestone-to-dolomite transition or a dolomite-to-dolomitic limestone transition. The M disconformity as a limestone-to-mudstone transition seems to have the most
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Fig. 9. Schematic illustrations of three mechanisms of the deflection of dykes into sills. (A) Dundurs elastic mismatch parameter α versus the ratio of the energy release rates for dyke deflection along the contact Gd to dyke penetration of the contact Gp (Gd /Gp ), showing domains for possible dyke propagation behaviors: penetration or deflection (modified from Barnett and Gudmundsson, 2014; Magee et al., 2016). (B) A numerical model of a stress barrier (Gudmundsson, 2011). The short, black lines are the directions of principal compressive stress σ1 . Note that the σ1 rotates from sub-vertical to sub-horizontal at the contact between the tuff layer and lava flow. Consequently, the stress barrier is formed. The Young’s modulus of lava flows and tuff layer is 100 GPa and 1 GPa respectively, and the magmatic excess pressure in the magma chamber is 10 GPa (Gudmundsson, 2011). (C) Cook–Gordon debonding mechanism. The weak contact between layer A and B opens up ahead of an approaching vertically propagating dyke (Gudmundsson and Løtveit, 2012).
prominent mechanical contrast among interfaces encountered by the upwelling magma. This implies that this large mechanical contrast allows the Cook–Gordon debonding mechanism to work in the Tarim basin. Thus, in general we suggest that what makes an interface a strong controlling plane depends largely on the lithologies of the host rock involved, but not simply on larger rigidity of the overlying strata as suggested by some experiments using elastic media (e.g., Kavanagh et al., 2006). One interesting observation from our study and some others (e.g., Eide et al., 2017) is that sills are prone to intrude along the boundaries of mudstones. 5.4. Saucer-shaped sill as a fundamental geometric type As mentioned above, sub-horizontal sills would turn into inclined sheets at its periphery and evolve to saucer-shaped sills (Pollard and Johnson, 1973; Malthe-Sørenssen et al., 2004; Mathieu et al., 2008; Galland et al., 2009; Galerne et al., 2011; Chen et al., 2017). This mechanism can be used to interpret the saucershaped sills and other geometric types of igneous sills in our study area. In detail, the half-saucer-shaped sill can be formed when the dip angle of the feeder dyke is less than 90◦ (Pollard, 1973; Barnett and Gudmundsson, 2014), or just when there is heterogeneity across the interface. The strata-concordant sills are always linked to the inclined sheets of saucer-shaped sills, implying that they are the well-developed flat outer sills of saucer-shaped sills. Therefore, the saucer-shaped sill is the basic geometric type of igneous sills in the Tarim Basin. 6. Conclusions Based on seismic data, this paper presents vivid examples of saucer-shaped sills and other geometric types of sills with their distinctive relationships to the host rock layering from the Tarim Basin, China. Some key conclusions are made relating to the geometry of igneous sills and the dominant controls on the sill geometry and emplacement: 1. Nineteen igneous sills showing packages of high-amplitude seismic reflections are observed in the central part of the Tarim Basin. They were emplaced into the Upper Ordovician strata at current depths of about 5∼8 km. 2. Igneous sills can be classified into three geometric types: type 1, saucer-shaped sills; type 2, strata-concordant sills; and type 3, hybrid sills showing geometric characteristics of both type 1 and type 2. 3. The geometry of saucer-shaped sills is typical and comprises three distinct parts: a flat inner sill, inclined sheets and, in many
cases, flat outer sills. The bases of saucer-shaped sills all generally coincide with the M disconformity, which is characterized by a lithological change from underlying limestones to overlying mudstones. 4. The sill geometry and emplacement depth are dominantly controlled by the M disconformity. Some layer interfaces within the Upper Ordovician strata also affect sill formation. 5. Among the three main mechanisms causing the deflection of dykes into sills, namely Cook–Gordon debonding, stress barriers and elastic mismatch, we suggest that the Cook–Gordon debonding is the dominant mechanism in the Tarim Basin because the other two mechanisms do not explain the observations well in this location. 6. What makes an interface a strong controlling plane for sill emplacement depends largely on the lithologies of the host rock involved, but not simply on the scenario where the overlying strata are more rigid than underlying strata. In general, our observations confirm again that the saucershaped sill is the fundamental geometric type of sills under the control of layering. This study also indicates that layer boundaries in the host rock, such as bedding planes and unconformities, could have a profound impact on sill geometry and magma emplacement. Acknowledgements We would like to thank the SINOPEC Northwest Oilfield Company for the permission to publish the seismic and borehole data used in this paper. We gratefully acknowledge reviewers Craig Magee and Joe Cartwright and editor T.A. Mather for their constructive reviews. Olivier Galland is thanked for his valuable comments on an early version of this manuscript. We also thank Tielin Chen of Beijing Jiaotong University, China, for his helpful discussion. Dajun Fang of Zhejiang University is acknowledged for his encouragement during this study. This research was supported by the 13th National Five-Year Sci-Tech Project of China (Grant No. 2017ZX05005-002-002) and National Natural Science Foundation of China (Grant Nos. 41704086, 41761134051 and 91428309). References Barnett, Z.A., Gudmundsson, A., 2014. Numerical modelling of dykes deflected into sills to form a magma chamber. J. Volcanol. Geotherm. Res. 281, 1–11. Bradley, J., 1965. Intrusion of major dolerite sills. Trans. R. Soc. N. Z. 3, 27–55. Cai, Xiyao, Qian, Yixiong, Chen, Qianglu, Chen, Yue, You, Donghua, Yang, Yufang, 2011. Discovery and significance of Qiaerbake and Yijianfang Formations of Ordovician in well GL1, Tarim Basin. Petrol. Geol. Exp. 33 (4), 348–352 (in Chinese with English abstract).
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Distribution and erosion of the Paleozoic tectonic unconformities in the Tarim Basin, Northwest China: significance for the evolution of paleo-uplifts and tectonic geography during deformation. J. Asian Earth Sci. 46 (2), 1–19. Luo, Fuzhi, 2006. Study on Characteristics and Predicting Technology of Igneous Rocks in Tazhong Area of Tarim Basin. Ph.D. Thesis. China University of Geosciences, (Beijing), Beijing, pp. 9–19 (in Chinese with English abstract). Magee, C., Briggs, F., Jackson, C.A.L., 2013. Lithological controls on igneous intrusioninduced ground deformation. J. Geol. Soc. Lond. 170 (6), 853–856.
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Magee, C., Jackson, C.A.L., Schofield, N., 2014. Diachronous sub-volcanic intrusion along deep-water margins: insights from the Irish Rockall Basin. Basin Res. 26 (1), 85–105. Magee, C., Maharaj, S.M., Wrona, T., Jackson, C.A.L., 2015. Controls on the expression of igneous intrusions in seismic reflection data. Geosphere 11 (4), 1024–1041. Magee, C., Muirhead, J.D., Karvelas, A., Holford, S.P., Jackson, C.A., Bastow, I.D., Schofield, N., Stevenson, C., McLean, C., McCarthy, W., Shtukert, O., 2016. Lateral magma flow in mafic sill complexes. Geosphere 12 (3), 809–841. Malthe-Sørenssen, A., Planke, S., Svensen, H., Jamtveit, B., 2004. Formation of saucershaped sills. In: Breitkreuz, C., Petford, N. (Eds.), Physical Geology of High-level Magmatic Systems. In: Geol. Soc. (Lond.) Spec. Publ., vol. 234, pp. 215–227. Mathieu, L., van Wyk De Vries, B., Holohan, E.P., Troll, V.R., 2008. Dykes, cups, saucers and sills: analogue experiments on magma intrusion into brittle rocks. Earth Planet. 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Contents lists available at ScienceDirect
Earth and Planetary Science Letters www.elsevier.com/locate/epsl
Sill geometry and emplacement controlled by a major disconformity in the Tarim Basin, China Zewei Yao a,b,∗ , Guangyu He b,∗∗ , Chun-Feng Li a,c , Chuanwan Dong b a b c
Ocean College, Zhejiang University, Zhoushan 316021, China School of Earth Sciences, Zhejiang University, Hangzhou 310027, China Laboratory of Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China
a r t i c l e
i n f o
Article history: Received 21 November 2017 Received in revised form 9 August 2018 Accepted 14 August 2018 Available online xxxx Editor: T.A. Mather Keywords: saucer-shaped sills magma emplacement layering Tarim Basin seismic data dyke deflection
a b s t r a c t Igneous sills are widely distributed in sedimentary basins worldwide. Sill emplacement has significant impacts on structural and thermal evolutions of sedimentary basins and also provides important insights into volcanic and sub-volcanic processes. Dyke deflection and sill formation are largely controlled by mechanical layering of the host rock, and a number of mechanisms have been proposed. However, most models of sill formation are based much on numerical and analogue modeling. For many natural sills, the key mechanism governing the dyke deflection and sill formation remains to be discussed and, in some cases, controversial. To better understand the control of mechanical layering on sill formation and test the existing models, we study detailed geometries of igneous sills from the central part of the Tarim Basin, China, based on seismic reflection data and borehole data. Nineteen igneous sills which are expressed as packages of high-amplitude reflections are observed, and they were all emplaced in the Upper Ordovician strata at current burial depths of about 5–8 km. The sills can be classified into three geometric types: type 1, saucer-shaped sills; type 2, strata-concordant sills; and type 3, hybrid sills, which have the characteristics of both type 1 and 2. The geometry of the saucer-shaped sills is typical and comprises three distinct parts: a flat inner sill, inclined sheets and, in many cases, flat outer sills. All the bases of the saucer-shaped sills coincide with the disconformity between the Middle and Upper Ordovician strata (the M disconformity), indicating that the disconformity controlled the sill geometry and emplacement depth in the basin. The M disconformity is characterized by a lithological change from underlying limestones to overlying mudstones. We suggest that among the three main mechanisms for the deflection of dykes into sills, namely Cook–Gordon debonding, stress barriers and elastic mismatch, the Cook–Gordon debonding is the dominant mechanism in the study area. This is a mechanism by which a weak contact opens up ahead of a vertically propagating dyke, allowing the dyke to be deflected into a sill when the dyke meets the open contact. This study indicates that the control of mechanical layering of the host rock on sill formation is largely depends on the host-rock lithologies and confirms again that the saucer shape is the fundamental sill geometry under the control of layering. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Igneous sills have been widely observed in sedimentary basins around the world from out-crops (e.g., Galerne et al., 2011; Muirhead et al., 2012; Eide et al., 2017) and seismic reflection data (e.g., Planke et al., 2005; Cartwright and Huuse, 2005; Hansen and Cartwright, 2006; Magee et al., 2016). These intrusions exhibit various shapes, including horizontal sills, laccol-
*
Corresponding author at: Ocean College, Zhejiang University, Zhoushan 316021, China. Corresponding author. E-mail addresses: [email protected] (Z. Yao), [email protected] (G. He).
**
https://doi.org/10.1016/j.epsl.2018.08.026 0012-821X/© 2018 Elsevier B.V. All rights reserved.
iths and saucer-shaped sills (Planke et al., 2005; Jackson et al., 2013). Sill emplacement can have significant impacts on the structural and thermal evolutions of sedimentary basins. Among others, sill emplacement may cause uplift of the host rock, forming forced folds of the overlying strata (Hansen and Cartwright, 2006; Magee et al., 2014), and affect the maturation of the organic matter in the surrounding host rock as well as the hydrocarbon migration and accumulation (Rateau et al., 2013; Spacapan et al., 2018). Sill complexes are also an important part of sub-volcanic plumbing systems (Galland et al., 2014). Understanding the formation of these intrusions is key to assessing the volcanic and sub-volcanic processes (Wilson et al., 2016). Sills form when subvertical dykes or inclined sheets become deflected into a sub-horizontal orientation. However, detailed for-
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mation mechanisms can be complicated and have been discussed for decades (Barnett and Gudmundsson, 2014, and references therein). The first mechanism suggests that sill emplacement is largely controlled by the level of neutral buoyancy, which corresponds to depths where the magma pressure equals the lithostatic pressure (Bradley, 1965; Francis, 1982). However, this point of view is questioned in many circumstances because sills often intruded in a wide range of depth in a sedimentary basin and preferably followed rock-rock interfaces (e.g., bedding planes and disconformities) (Planke et al., 2005; Muirhead et al., 2012; Barnett and Gudmundsson, 2014; Eide et al., 2017). Other three mechanisms, namely Cook–Gordon debonding, stress barriers and elastic mismatch, are proposed to focus more on the controls from local stresses and layering of the host rocks (Barnett and Gudmundsson, 2014, and references therein). Cook–Gordon debonding is a mechanism by which a propagating extension fracture, here a dyke, opens up a weak contact ahead of the fracture tip (Cook et al., 1964; Gudmundsson, 2011). These mechanisms, supported by analogue modeling, suggest that mechanical properties of host rock at boundaries can determine the dyke-to-sill deflection process (Pollard and Johnson, 1973; Kavanagh et al., 2006; Menand, 2008). However, the dominant mechanism has yet to be discussed in real cases. Better geological observations are still needed to refine these theories. Furthermore, under the control of the host rock layering, individual sills are expected to evolve to a saucer shape after their initial horizontal spreading (Pollard and Johnson, 1973; MaltheSørenssen et al., 2004; Thomson and Hutton, 2004; Polteau et al., 2008; Galland et al., 2009; Magee et al., 2013; Chen et al., 2017). The saucer-shaped sill consists of a horizontal sill along a planar discontinuity, stretching outward and upward to inclined sheets, and subsequently to flat outer sills, and such a geometry is considered as a fundamental shape for magma intrusions in sedimentary basins (Thomson and Hutton, 2004; Polteau et al., 2008; Galland et al., 2009). Although this viewpoint is widely accepted, many of the saucer-shaped sills, imaged by seismic data, have cup-shaped bodies (Mathieu et al., 2008), such as those in sedimentary basins along the NE Atlantic margin (Hansen, 2004; Hansen and Cartwright, 2006). The bases of these sills, in accordance with recent field observations from a sill in the Faroe Islands (Walker, 2016), appear to be mildly transgressive through the layering of the host rock, implying that layering is not the primary control on sill geometry. These observations question whether mechanical layering is a fundamental control on the formation of saucer-shaped sills. In this paper, we investigate seismically examples of the igneous sills in the Tarim Basin, northwestern China, to further understand the mechanisms of the deflection of dykes into sills at contacts, and evaluate the control of mechanical layering on the geometry and emplacement depth of saucer-shaped sills. We suggest that the Tarim Basin is a suitable place to undertake the study, because saucer-shaped sills there are typical and are well imaged, their spatial relationships to the dominant layers of the host rock are distinct, geological evolution of the study area is relatively simple and the host sequences are generally flat-lying. We first use high-quality two-dimensional (2-D) seismic data combined with borehole data to document the seismic expression and geometry of igneous sills. We then discuss the main control on sill geometry and growth. For the first time, these intrusions are revealed in seismic data in the Tarim Basin.
margins, including Tianshan mountains, Kunlun mountains and Altyn mountains. The initial formation of the Tarim Basin is related to the breakup of Rodinia in the Neoproterozoic time (Jiang et al., 2017). Its oldest sequences are Sinian/Ediacaran strata exposed in the northwestern and northeastern margins of the basin, but their distribution within the basin is still under discussion (He et al., 2010). After the initial formation, the Tarim Basin received sedimentary deposition almost continuously from the Cambrian to Quaternary, with a total thickness up to 15000 m. However, as a result of multi-phased tectonic events, current thickness of sedimentary rocks varies within the basin (Lin et al., 2012). The basin can be roughly divided into seven tectonic units, including the Kuqa depression, Tabei uplift belt, Northern depression belt, Central uplift belt, Southwestern depression belt, Southeastern uplift belt and Southeastern depression belt (Fig. 1A). These tectonic units can further be subdivided into sub-units (Fig. 1A). Our study area is around the Tazhong uplift in the central part of the Tarim Basin (Fig. 1A). The Lower and Middle Cambrian strata are dominantly composed of dolomite (Fig. 2). In the southern part of the study area, they are supposed to contain interbeded gypsum, acting as an important detachment layer (Yao et al., 2017). The Upper Cambrian to Middle Ordovician sequences consist of dolomite and limestone. During the Late Ordovician, the Tazhong uplift bounded by the Tazhong NO.I fault and Well Zhong2 fault was significantly uplifted (Fig. 1B) and the Upper Ordovician depositional facies in the uplifted area was different from those in adjacent area. In the uplifted area, the Upper Ordovician strata consist of the Lianglitage formation (O3l ) and Sangtamu formation (O3s ) which dominantly contain limestone and mudstone respectively (Fig. 2). Toward the depressed area, the Upper Ordovician sequences gradually turn into the Qiaerbake formation (O3qb ) and Queerqueke formation (O3qq ) (Fig. 1B). The Qiaerbake formation is about 20 m thick and consists of limestone and marlstone (Cai et al., 2011). The overlying Queerqueke formation which comprises the most part of the Upper Ordovician strata is dominantly mudstone and siltstone. From the Silurian to Quaternary, the area was covered by marine and terrestrial sequences up to 6000 m thick. Structural deformation was weak in the northern part of the study area. The igneous sills studied in this paper are mostly observed here and they mostly intruded the Upper Ordovician units or generally followed the boundary between Upper and Lower Ordovician strata (the M disconformity) (Fig. 3). In contrast, structure deformation was relatively strong in the Tazhong uplift and southwestern area, which were subject to detachment faulting and transpressive faulting because of the collision between the Altyn block and the Tarim block (He et al., 2011). Igneous activity within the Tarim Basin is marked by widespread volcanic rocks in the Permian strata. These igneous rocks have been revealed by numerous wells in the basin and field observations in the northwestern corner of the basin (Fig. 2). Among them, the flood basalts are up to 700 m thick and in an area of about 2 × 105 km2 (Yu et al., 2011). They were extruded within a period of 2–3 Ma in the Early Permian, although the absolute age is still controversial (Yu et al., 2011; Zhang et al., 2012). These flood basalts and some other igneous rocks together are named as the Tarim Large Igneous Province (LIP) (Yang et al., 2014). However, the relationship between the igneous sills documented in this study and the Tarim LIP remains to be discussed. 3. Data and methods
2. Geological setting With an area up to 50 × 104 km2 , the Tarim Basin is the largest basin in northwestern China (Fig. 1). The basin is diamondshaped in plan view and bounded by large mountain belts at its
This study uses zero-phase, time-migrated 2-D seismic reflection dataset, which has a total length of about 25000 km and covers an area of about 50000 km2 (Fig. 1A). The grid of 2-D seismic profiles is between 2 × 2 km and 4 × 4 km, permit-
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Fig. 1. (A) Tectonic setting of the Tarim Basin (modified from Lin et al., 2012). The inlet is the topographic image from Consultative Group on International Agricultural Research – Consortium for Spatial Information (CGIAR-CSI), showing the location of the basin. (B) A regional cross section showing the structural context of the study area (modified from He et al., 2009) (see location in Fig. 1 A). TWT = two-way travel time.
ting quasi-3-D imaging of sills. The dominant seismic frequency is about 20–24 Hz. A velocity of 6000 m/s is assumed for sills in this study based on the velocity data from the Tazhong22 well, which penetrated a dolerite sill in this area (Luo, 2006). This amounts to a vertical seismic resolution of sills of about 62.5–75 m and a detectable limit of about 17 m. The sills commonly intruded in a depth of about 4–5 s in two-way travel time. The intruded succession has an approximate velocity of 5000 m/s (Feng, 2008). Thus, the vertical resolution and detection limit of the host rock near the intrusion are about 52–62.5 m and 14 m, respectively. We can judge sill thickness by the ‘tuned reflection packages’ (Smallwood and Maresh, 2002; Magee et al., 2015). The amplitude of sill reflection increases when the sill thickness rises from the detection limit to the vertical resolution, and for larger sill thickness the reflection amplitude remains at a lower value (Smallwood and Maresh, 2002; Magee et al., 2015). Similar to other studies worldwide, four criteria are used to interpret sill reflection: (1) high-amplitude; (2) local transgression; (3) abrupt termination; (4) limited and characteristic spatial geometries, such as a saucer-shaped geometry (Planke et al., 2005). In addition, borehole data from wells Adong1, Shun2 and GL1 are used to constrain the seismic interpretation and physical properties of igneous sills.
4. Seismic and geometric characterization of igneous sills 4.1. Seismic characterization A total of nineteen igneous sills are mapped in the central part of the Tarim Basin in this study, partly shown in Fig. 3. They show packages of continuous, high-amplitude seismic reflections commonly with a peak–trough–peak reflection configuration (Fig. 4), indicating that the sills are expressed as ‘tuned reflection packages’ (Smallwood and Maresh, 2002; Magee et al., 2015). This means that their thicknesses are between the vertical resolution 62.5–75 m and the detection limit ∼17 m. This conclusion is roughly consistent with the drilling data from the Adong1 well which recovered Sills 13 and 11 with a thickness of 55.8 and 47.1 m, respectively (see location in Figs. 1, 3 and 4). Seismic data show that sills all intruded within the Upper Ordovician sequences, which currently dip at very low angles (<2◦ ). The current depths of these intrusions, calculated from the borehole data (e.g., Adong1 well) and seismic data, are about 5–8 km. The hosted Upper Ordovician strata and adjacent strata are generally concordant. There are no large-scale faults or remarkable angular unconformities in the place where sill formed. Reflections of igneous sills are quite distinct in seismic profiles as Upper Ordovician sequences are dominantly characterized by low-amplitude seismic reflections, permitting the sills to be interpreted with great
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4.2. Geometric characterization
Fig. 2. Stratigraphic column showing the Cambrian to Permian strata in the northern part of the study area in the Tarim Basin (modified from Cai et al., 2011 and Lin et al., 2012). Note that igneous sills intruded the Upper Ordovician strata and that the Qiaerbake formation is much thinner than sills.
confidence. However, in many places the M disconformity is also characterized by continuous, high-amplitude seismic reflections, similar to the ‘tuned reflection packages’ of the sills (Fig. 4 and 5). We cannot conclude whether there are strata-concordant sills. But inferring from the pattern of saucer-shaped sills, we interpreted these reflections as igneous sills when they are bounded by inward dipping inclined sheets (e.g., Sill 13 in Fig. 4), or when they are connected to inclined sheets (e.g., Sill 11 in Fig. 4).
Following Planke et al. (2005) and Jackson et al. (2013) on sill classification through seismic observations, igneous sills in the study area can be classified into 3 geometric types (Figs. 5 and 6). Type 1—saucer-shaped sills, which are composed of a flat inner sill, inclined sheets and, in some cases, outer sills (Fig. 6A). Eight saucer-shaped sills are recognized, as the main geometric type in the study area. As some saucer-shaped sills are incomplete, they are subdivided into complete-saucer-shaped sills (Sill 13 in Fig. 4; Sill 7 in Fig. 5A and 5B) and half-saucer-shaped sills (Sill 5 in Fig. 5C and 5D). The inner sills of saucer-shaped sills are flat and coincident with the M disconformity, and abruptly turn into inclined sheets at their periphery. The inclined sheets, in some cases, turn outward into flat outer sills (Sill 7 in Fig. 5B). This transition can be either sharp or progressive. The dip angles of the inclined sheets generally range from 10 to 20◦ , and the transgressive heights are about 500 to 1500 m. The complete-saucer-shaped sills (Sills 13, 9 and 7 in Fig. 3) and their inner sills commonly have an elliptical shape in plan view with an average area of about 404 km2 (121–803 km2 ) and 103 km2 (47–214 km2 ), respectively, whilst the shapes of half-saucer-shaped sills are irregular in plane (Sill 5 in Fig. 3). Type 2—strata-concordant sills (Fig. 6B). It is common that sills propagated upward to high levels through inclined sheets, forming ‘step’ structures (Francis, 1982; Muirhead et al., 2012; Schofield et al., 2012). In contrast to sills that are entirely concordant to bedding planes (Sill 6 in Fig. 5E and 5F), some sills are generally layer-parallel but turn downward to inclined sheets at their periphery (Sill 8 in Fig. 5G and 5H). These inclined sheets are supposed to be attached to strata-concordant sills along the M disconformity. The dip angles of the inclined sheets range from 7 to 10◦ , and the transgressive heights are about 300 to 500 m. These sills are also classified to this type 2 and named as halfstrata-concordant sills. Seismic interpretation shows that strataconcordant sills do not have specific forms in plane and coverage areas vary from about 46 to 471 km2 (average 234 km2 ). Type 3—the hybrid sills that have the geometric characteristics of both saucer-shaped sills and strata-concordant sills (Fig. 6C). Due to the complexity of hybrid sills and limited horizontal resolution of 2-D seismic data, hybrid sills cannot be split into discrete sills. Hybrid sills are also common in the study area (Sills 11 and 12 in Fig. 5I). Similarly, they have flat bases coincidental with the base of the Upper Ordovician sequences. The map areas of the inner sills within the hybrid sills are 17–75 km2 (average 47 km2 ). The dip angles of the inclined sheets of the inner sills range from 7 to 24◦ , and the transgressive heights are about 600 to 2000 m. The plane shapes of hybrid sills are irregular, and their areas are relatively large compared to those of the other two types of sills, ranging from 196 to 2150 km2 (average 1242 km2 ).
Fig. 3. Plane distribution of some igneous sills in the central part of the Tarim Basin (see location in Fig. 1A). Each sill is marked consecutively with a number in a circle.
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Fig. 4. A seismic profile showing seismic characteristics of igneous sills in the Tarim Basin (see location in Fig. 3). Vertical exaggeration factor is about 2.
4.3. Sill junction relationships Three main classes of junction geometries have been recognized in 3D seismic data from the northern Faroe–Shetland Basin (Hansen et al., 2004). Different types of sill junction geometries have also been investigated from our seismic data. Following is a case study from Sills 5, 6, 7 and 8. Seismic profiles show a discontinuity in Sill 8 at the southwestern margin of Sill 7, and the northeastern part of Sill 8 is uplifted (e.g. in Fig. 5B, Fig. 7) (Class C junctions by Hansen et al., 2004). Therefore, we interpret that Sill 7 cross-cut Sill 8 and uplifted the overlying part. Seismic data also show that (1) the northeastern margin of Sill 7 stopped at the southwestern wing of Sill 5 (Fig. 5B); (2) Sill 6 abuts against Sill 5 at its eastern margin (Fig. 5D and 5F); and (3) the edge of Sill 5 links to the base of Sill 8 (Fig. 7). All of these junction geometries can be classified into the Class B junctions (Hansen et al., 2004). However, except for Sill 7 cross-cutting Sill 8, the detailed kinematic and formation sequence of these sills are hard to interpret based on above sill junction geometries, because the Class B junctions can have different kinematic development models (Hansen et al., 2004). 5. Discussion 5.1. Spatial relationship between the M disconformity and the sills The most notable sill features in the Tarim Basin are that all the sills were emplaced in the Upper Ordovician strata and that the bases of saucer-shaped sills generally followed the M disconformity. However, it should be noted that the detailed spatial relationship between the sills and the Qiaerbake formation above the M disconformity is unclear, because the Qiaerbake formation is too thin (c. 20 m) to be discriminated in seismic profiles. Currently, none of wells has revealed their contact relationship. We interpret that bases of saucer-shaped sills followed the M disconformity, because the M disconformity is a regional, planar discontinuity characterized by a lithological change from dominantly limestones to marlstone and mudstone (Figs. 2 and 8). But it is also possible that the sills intruded along the top of, or within, the Qiaerbake formation. Note that the Qiaerbake formation (c. 20 m) is much thinner than the sills (c. 50 m) and that lithology of the Qiaerbake formation changes gradually from limestone in the lower part to marlstone in the upper part as a progressive transition from underlying Yijianfang formation (limestone) strata to overlying Queerqueke (mudstone) formation (Fig. 8). Thus, no matter whether the sills intrude at the top, the base or within the Qiaerbake formation, observations do support that lithologic transition from underlying
limestone to overlying mudstone controlled the deflection of dykes into sills. 5.2. Main controls on sill geometry and emplacement depth The geometry of saucer-shape sills in the Tarim Basin is analogous to the conceptual model of saucer-shaped sills which have three distinct parts: a flat inner sill, inclined sheets and, in many cases, flat outer sills (Chevallier and Woodford, 1999; Polteau et al., 2008). Sills with such a typical saucer shape in the study area, which have been reproduced in a number of numerical and physical simulations with layered host rock (Pollard and Johnson, 1973; Galland et al., 2009; Galerne et al., 2011; Chen et al., 2017), are among the first to be observed in nature. In particular, the saucershaped sills in our study show great resemblance to experimental results of injecting vegetable oil (magma) into silica flour (host rock) with a flexible net (an interface), in which the injected material always follow the flexible net, forming the base of the saucershaped intrusion (Galland et al., 2009; Galerne et al., 2011). Thus, our observations in this study verified the experiment results and conceptual model of saucer-shaped sills, and suggest that the M disconformity mainly controlled the saucer-shaped geometry and the emplacement depth of igneous sills in the Tarim Basin. In addition, the development of strata-concordant sills and flat outer sills at different depths within the Upper Ordovician strata implies that layer interfaces in the host rock other than the M disconformity also affect sill formation (e.g., encouraging the formation of strata-concordant sills). Besides, both numerical and physical modeling indicate that the sharp transition from the inner sill to inclined sheets is controlled by the host-rock layering (Galland et al., 2009; Chen et al., 2017). This is because layering of the host rock can cause asymmetric stresses and the fracture of the overlying strata at the sill termination and the subsequent propagation of inclined sheets (Pollard and Johnson, 1973; Malthe-Sørenssen et al., 2004). Therefore, we suggest that the sharp transition from inclined sheets to flat outer sills in some sills was also controlled by some bedding planes in the host rock. This conclusion corresponds well with the numerical simulation results that flat outer sills and the sharp transition often develop at a layer boundary (Chen et al., 2017). In contrast, as mentioned above, saucer-shaped sills in sedimentary basins along the NE Atlantic margin often show a cupshaped geometry. This means that they commonly lack a flat inner sill concordant to the host-rock layering, a sharp transition from the inner sill to inclined sheets, and flat outer sills (Hansen, 2004; Hansen and Cartwright, 2006). Similarly, sills of such geometries have been reproduced in analogue models using homogeneous media (Mathieu et al., 2008; Galland et al., 2009). It is suggested that
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Fig. 5. Characteristic examples of igneous sills in the Tarim Basin. (A), (C), (E) and (G) are perspective quasi-3-D morphological views of Sills 7, 5, 6 and 8, respectively. (B), (D), (F), (H) and (I) show seismic characteristics and cross-sectional geometries of Sills 7, 5, 6, 8 and 11, respectively. Locations of seismic profiles are marked in Fig. 3. Vertical exaggeration is about 4. Coordinates D, W and N in (A), (C), (E) and (G) corresponds to depth, west and north, respectively. The numbers of the sills refer to those in Fig. 3.
in these places layering of the host rock has not played a dominant role in sill formation, as observed by Walker (2016) in the Faroe Islands. 5.3. Implications for the deflection of dykes into sills There are mainly three mechanisms that control the deflection of dykes into sills, namely Cook–Gordon debonding, stress barri-
ers and elastic mismatch (Fig. 9) (Gudmundsson and Philipp, 2006; Gudmundsson, 2011; Gudmundsson and Løtveit, 2012; Barnett and Gudmundsson, 2014). Elastic mismatch mechanism occurs where there is a contrast in toughness and Young’s modulus of two adjacent layers. This mechanism means that sill is prone to forming at interfaces separating the upper, rigid strata from the lower, soft strata (Fig. 9A) (Kavanagh et al., 2006; Menand, 2011; Gudmundsson, 2011). It is demonstrated that the behavior of a
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Fig. 6. Schematic cross-section diagram showing geometric types of igneous sills in the Tarim Basin (Modified after Planke et al., 2005). Type 1: saucer-shaped sills, which can be subdivided into complete-saucer-shaped sills (A) and half-saucershaped sills (B). Type 2: strata-concordant sills, which can be subdivided into complete-strata-concordant sills (C) and half-strata-concordant sills. (E) Type 3: hybrid sills.
Fig. 7. A NE-trending seismic profile showing sill junction relationships between Sills 5, 7 and 8 (see location in Fig. 3).
dyke (penetration or deflection) at a contact is affected by the Dundurs elastic mismatch parameter α , and the ratio of the energy release rates for dyke deflection along the contact Gd to dyke penetration of the contact Gp (Gd /Gp ) (Fig. 9A) (Barnett and Gudmundsson, 2014; Gudmundsson, 2011). The ratio Gd /Gp is related to the material toughness of the contact and that of the layer above the contact. When the layer above the contact is softer than the layer below (α < 0), dyke deflection occurs only if the material toughness of the contact itself is roughly 26% smaller than that of the overlying layer (Barnett and Gudmundsson, 2014). These conditions are not common in natural cases. By contrast, when the upper layer is stiffer than the lower layer (α > 0), the tendency for dyke deflection at the contact increases (Fig. 9A). However, in this study, the M disconformity separates upper mudstones from lower limestones. The mudstones commonly have smaller Young’s modulus than limestones (Roche et al., 2014), though their exact values in the Tarim Basin are not known currently. According to the elastic mismatch mechanism, deflection of dykes into sills along the M disconformity is quite difficult. Thus, we suggest that the elastic mismatch mechanism is not the primary control on the deflection of dykes into sills. This interpretation is in accordance with recent results of numerical modeling of shallow magma intrusion, which illustrate that elastic mismatch mechanism alone cannot control the deflection of dykes into sills (see case 1 and 2 in Chen et al., 2017). The stress barrier mechanism means that rotation of the principal stresses at boundaries of layers of different stiffness (Young’s modulus) can generate a stress barrier (Fig. 9B) (Gudmundsson and
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Fig. 8. Gamma ray (GR), shallow lateral resistivity (LLS) and deep lateral resistivity (LLD) of the Qiaerbake formation and adjacent strata from the GL1 well (see location in Fig. 1). Note that the gamma ray increases gradually from Qiaerbake formation to Queerqueke formation, and that the carbonate content decreases from Yijianfang formation to Qiaerbake and Queerqueke formations.
Løtveit, 2012; Barnett and Gudmundsson, 2014). This mechanism often assumes that the magma chamber is circular or ellipsoidal (Fig. 9B) (Gudmundsson and Philipp, 2006; Gudmundsson, 2011) or that the strata are subjected to regional compression or extension (Gretener, 1969; Barnett and Gudmundsson, 2014). This mechanism also suggests that dyke deflection into a sill occurs at an interface bounded by upper, rigid strata and lower, weak strata (Gudmundsson and Løtveit, 2012). However, as discussed above, the M disconformity does not follow this pattern. Moreover, circular or ellipsoidal magma chamber and related uplifts above have not been observed in our study. Regional compression or extension is not strong either, as the strata are generally flat-lying and no large-scale thrust or extensional faults formed in places of sills intrusions. Numerical modeling (Chen et al., 2017) and physical modeling (Galland et al., 2009) also implied that regional stresses are not necessary for dyke-to-sill deflection. Therefore, it is quite possible that stress barrier mechanism is not dominant in our study area either. The Cook–Gordon debonding is a mechanism by which a weak contact opens up ahead of an approaching vertically propagating extension fracture (Cook et al., 1964; Barnett and Gudmundsson, 2014). In a homogeneous, isotropic material, the tensile stress induced ahead of and parallel to an extension fracture is about 20% of the tensile stress ahead of and perpendicular to the crack (Cook et al., 1964; Gudmundsson, 2011). The large tensile stresses induced by a propagating dyke can lead to the opening (debonding) of the weak contact ahead of the dyke. When the dyke eventually meets the open contact, the propagating dyke can be deflected into a sill at the contact. This is a common mechanism for the formation of sills, particularly at shallow depths in the Earth’s crust (Barnett and Gudmundsson, 2014). Thus, by excluding the previous two mechanisms, we suggest that among the three main mechanisms the Cook–Gordon debonding is the dominant mechanism for the dykes-to-sills deflection in the study area (Fig. 9C) and that the M disconformity acts as the weak contact. Recent numerical simulation indicates that the differences in mechanical properties between adjacent layers, among some others, affect whether the Cook–Gordon debonding works, although the critical values are still unconstrained (Zhang et al., 2007; Chen et al., 2017). The mechanical properties include not only the Young’s modulus but also cohesion, Poisson’s ratio, friction angle, etc. In our study, the interfaces below the M disconformity are commonly characterized by a limestone-to-dolomite transition or a dolomite-to-dolomitic limestone transition. The M disconformity as a limestone-to-mudstone transition seems to have the most
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Fig. 9. Schematic illustrations of three mechanisms of the deflection of dykes into sills. (A) Dundurs elastic mismatch parameter α versus the ratio of the energy release rates for dyke deflection along the contact Gd to dyke penetration of the contact Gp (Gd /Gp ), showing domains for possible dyke propagation behaviors: penetration or deflection (modified from Barnett and Gudmundsson, 2014; Magee et al., 2016). (B) A numerical model of a stress barrier (Gudmundsson, 2011). The short, black lines are the directions of principal compressive stress σ1 . Note that the σ1 rotates from sub-vertical to sub-horizontal at the contact between the tuff layer and lava flow. Consequently, the stress barrier is formed. The Young’s modulus of lava flows and tuff layer is 100 GPa and 1 GPa respectively, and the magmatic excess pressure in the magma chamber is 10 GPa (Gudmundsson, 2011). (C) Cook–Gordon debonding mechanism. The weak contact between layer A and B opens up ahead of an approaching vertically propagating dyke (Gudmundsson and Løtveit, 2012).
prominent mechanical contrast among interfaces encountered by the upwelling magma. This implies that this large mechanical contrast allows the Cook–Gordon debonding mechanism to work in the Tarim basin. Thus, in general we suggest that what makes an interface a strong controlling plane depends largely on the lithologies of the host rock involved, but not simply on larger rigidity of the overlying strata as suggested by some experiments using elastic media (e.g., Kavanagh et al., 2006). One interesting observation from our study and some others (e.g., Eide et al., 2017) is that sills are prone to intrude along the boundaries of mudstones. 5.4. Saucer-shaped sill as a fundamental geometric type As mentioned above, sub-horizontal sills would turn into inclined sheets at its periphery and evolve to saucer-shaped sills (Pollard and Johnson, 1973; Malthe-Sørenssen et al., 2004; Mathieu et al., 2008; Galland et al., 2009; Galerne et al., 2011; Chen et al., 2017). This mechanism can be used to interpret the saucershaped sills and other geometric types of igneous sills in our study area. In detail, the half-saucer-shaped sill can be formed when the dip angle of the feeder dyke is less than 90◦ (Pollard, 1973; Barnett and Gudmundsson, 2014), or just when there is heterogeneity across the interface. The strata-concordant sills are always linked to the inclined sheets of saucer-shaped sills, implying that they are the well-developed flat outer sills of saucer-shaped sills. Therefore, the saucer-shaped sill is the basic geometric type of igneous sills in the Tarim Basin. 6. Conclusions Based on seismic data, this paper presents vivid examples of saucer-shaped sills and other geometric types of sills with their distinctive relationships to the host rock layering from the Tarim Basin, China. Some key conclusions are made relating to the geometry of igneous sills and the dominant controls on the sill geometry and emplacement: 1. Nineteen igneous sills showing packages of high-amplitude seismic reflections are observed in the central part of the Tarim Basin. They were emplaced into the Upper Ordovician strata at current depths of about 5∼8 km. 2. Igneous sills can be classified into three geometric types: type 1, saucer-shaped sills; type 2, strata-concordant sills; and type 3, hybrid sills showing geometric characteristics of both type 1 and type 2. 3. The geometry of saucer-shaped sills is typical and comprises three distinct parts: a flat inner sill, inclined sheets and, in many
cases, flat outer sills. The bases of saucer-shaped sills all generally coincide with the M disconformity, which is characterized by a lithological change from underlying limestones to overlying mudstones. 4. The sill geometry and emplacement depth are dominantly controlled by the M disconformity. Some layer interfaces within the Upper Ordovician strata also affect sill formation. 5. Among the three main mechanisms causing the deflection of dykes into sills, namely Cook–Gordon debonding, stress barriers and elastic mismatch, we suggest that the Cook–Gordon debonding is the dominant mechanism in the Tarim Basin because the other two mechanisms do not explain the observations well in this location. 6. What makes an interface a strong controlling plane for sill emplacement depends largely on the lithologies of the host rock involved, but not simply on the scenario where the overlying strata are more rigid than underlying strata. In general, our observations confirm again that the saucershaped sill is the fundamental geometric type of sills under the control of layering. This study also indicates that layer boundaries in the host rock, such as bedding planes and unconformities, could have a profound impact on sill geometry and magma emplacement. Acknowledgements We would like to thank the SINOPEC Northwest Oilfield Company for the permission to publish the seismic and borehole data used in this paper. We gratefully acknowledge reviewers Craig Magee and Joe Cartwright and editor T.A. Mather for their constructive reviews. Olivier Galland is thanked for his valuable comments on an early version of this manuscript. We also thank Tielin Chen of Beijing Jiaotong University, China, for his helpful discussion. Dajun Fang of Zhejiang University is acknowledged for his encouragement during this study. This research was supported by the 13th National Five-Year Sci-Tech Project of China (Grant No. 2017ZX05005-002-002) and National Natural Science Foundation of China (Grant Nos. 41704086, 41761134051 and 91428309). References Barnett, Z.A., Gudmundsson, A., 2014. Numerical modelling of dykes deflected into sills to form a magma chamber. J. Volcanol. Geotherm. Res. 281, 1–11. Bradley, J., 1965. Intrusion of major dolerite sills. Trans. R. Soc. N. Z. 3, 27–55. Cai, Xiyao, Qian, Yixiong, Chen, Qianglu, Chen, Yue, You, Donghua, Yang, Yufang, 2011. Discovery and significance of Qiaerbake and Yijianfang Formations of Ordovician in well GL1, Tarim Basin. Petrol. Geol. Exp. 33 (4), 348–352 (in Chinese with English abstract).
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