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Structural Characteristics of the Silla Syncline (Chile

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Structural Characteristics of the Silla Syncline (Chile STRUCTURAL CHARACTERISTICS OF THE SILLA SYNCLINE (CHILE): IMPLICATIONS FOR FAULT CONTROLLED DEPOSITIONAL BEHAVIOR Joe Gonzales and Atilla Aydin Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 Abstract Detailed structural characteristics of the Magallanes Foreland Basin, a compressional foreland basin located in southern Chile has not previously been documented. In this paper, we study the Silla Syncline, a 4 km wide fold composed of fine-grained mudstone, coarse sandstone and conglomerate deposist of the 2000 to 2500 m thick deep water Cerro Toro Formation. The syncline is bounded on its western flank by an anticline, and on its eastern flank by a broad zone of thrust faults and the associated folds which are oriented sub-parallel to the syncline axis. Previous sedimentological research has suggested a link between the location and orientation of the coarse grained sediments of the Silla Syncline, and the surrounding structural elements. From the lack of coarse-grained deposits outside of the syncline, it appears that deposition took place in a confined trough or channel. We propose that this confining channel was defined by preexisting thrust- cored anticlines, initiated in underlying formations, possibly within the Punta Barrosa Formation, which allowed for focused sedimentation. To support our proposal, we examined the nature of sediments directly beneath the coarse grained deposits, the nature of the bounding anticline and fault zone. Our results suggest that in the Magallanes Basin, folding and thrust faulting play a role in determining the depositional behavior of coarse grained sediments. Introduction Figure 1. a) Location of Magallanes Basin. b) and c) basin evolution diagrams, Outcrop exposures of deep water clastic sediments modified from Fildani and Hessler, 2005, are useful analogs for modern day reservoirs, and and Wilson, 1991. understanding how these sediments deform can provide information on reservoir properties, such as fluid flow. Toro Formation from the detailed studies by the The Magallanes basin, located in southern Chile (Figure Stanford Project on Deep-Water Depositional Systems 1a) exposes deep water clastic sediments. Structural (SPODDS) group at Stanford University and relate the aspects of the basin have been studied at a regional characteristics to the surrounding structures. scale (Dalziel, 1981; Winslow, 1981; Wilson, 1991; Past researchers (Coleman, 2000; Crane, 2004; Kraemer, 2003), but little work has been done at the Hubbard, 2006) have speculated that the depositional outcrop scale. In this study, we focus on the Silla locations of the coarse grained deposits in the Silla Syncline, a 4 km wide fold located in the Magallanes Syncline were controlled by surrounding structural Basin. The syncline is composed of coarse sandstone features, but have not explored this link in detail. From and conglomerate deposits, with a surrounding our characterization we hope to document that a lithology of fine sandstone and mudstone. We preexisting fold and thrust fault zone surrounding the reexamined the depositional characteristics of the Cerro Silla Syncline controlled the location and geometry of the sediment deposition of the syncline. Stanford Rock Fracture Project Vol. 17, 2006 K-1 Figure 2. Geologic map of the Silla Syncline and surrounding area. Modified from Wilson, 1991. undergoing extension related to the breakup of Geologic Setting Gondwana (Bruhn et al., 1978, Gust et al., 1985). This Regionally, the study area is centered about 51°S extension resulted in the development of a back arc and 73°W, and is located in the eastern part of the basin, and ophiolitic rocks present south and west of Andean orogenic belt. The orogenic belt is oriented N- Parque Nacional Torres Del Paine are thought to be S at the above latitude, and bends towards the SE near remnants of this basin (Dalziel et al., 1974, 1981). The the southern end of South America. The study area is change from extensional to compressive environments located in the Parque Nacional Torres Del Paine, is believed to be the eastward progression of Andean adjacent to Lago Nordenskjold, Lago Pehoe, and Lago orogenic activity. Sarmiento (Figures 2). The park is approximately 110 The Magallanes foreland basin was filled in the km north of the town of Puerto Natales, and Late Cretaceous to Early Tertiary, with approximately 7 approximately 400 km north of the city of Punta km of sediments (Crane, 2004; Wilson 1983, 1991). Arenas. Figure 3 is a stratigraphic column showing the details The sediments in the Silla Syncline are part of the of the sediments for the Late Cretaceous. The Cerro Cerro Toro Formation, which is present throughout Toro Formation conformably overlies the Punta Barrosa much of the Magallanes Basin (Figure 2). The Formation. The thickness of the Cerro Toro Formation Magallanes Basin has been characterized as a retro-arc is estimated at 2000 to 2500 m (Katz, 1963; Wilson, foreland basin (Wilson, 1983), related to the Andean 1991). The age of the formation, based on fossil Orogeny (Figure 1b and c). Prior to the Orogeny, in the evidence is middle to upper Senonian (Katz, 1963), but late Jurassic to the early Cretaceous, the area was might be younger based on recent zircon analysis of the Stanford Rock Fracture Project Vol. 17, 2006 K-2 Figure 3. Stratigraphic column of the Late Cretaceous sediments in the Magallanes Basin a) Figure 5. Schematic diagram of depositional setting in the Late Cretaceous, showing the Silla Syncline as a feeder channel to the larger Magallanes Basin. From Hubbard, 2006. underlying Punta Barrosa Formation (Fildani et al., 2003). The lithology of the Cerro Toro Formation is dominantly mudstone and thin bedded fine sandstone turbidites (Figure 4a) containing several large sequences of conglomerate and coarse sandstone deposits (Figure 4b) (Crane, 2004). In addition to turbidites, the formation shows evidence for debris and slurry flows (Crane, 2004). Sediment composition and depositional style indicate that the Cerro Toro Formation was deposited in b) deep water, with a paleobathymetry of approximately 2000 m (Fildani and Hessler, 2005). Crane (2004) interpreted the formation to represent filled channel systems located in incised submarine valleys. Throughout its evolution, the general morphology of the basin is that of a north-south oriented axial basin (Wilson, 1991). Paleocurrent data throughout the basin show transport directions to the south and southeast (Wilson, 1991; Crane, 2004). The Silla Syncline, located on the western slope of the basin has been interpreted as a feeder channel to the basin floor (Hubbard, 2006; Crane, 2004; SPODDS, 2004; Figure 5). Crane (2004) also identified several distinct channel deposit members within the syncline, indicating that the Figure 4. a) typical fine grained sediments depositional channel was migrating during in the study area, and b) typical coarse sedimentation, although the paleocurrent data indicates sandstone and conglomerate deposits. that the transport direction was similar for all channels. Stanford Rock Fracture Project Vol. 17, 2006 K-3 Figure 6. Structural map of the study area showing major folds, thrust faults, and strike-slip faults. Coarse grain rock locations take from Crane, 2004. largest of the mapped folds, with a width of up to 4 km. Structural Characterization Crane (2004) identified three coarse grained channel Figure 6 is an aerial photograph that shows the deposit members of the Cerro Toro Formation that give structural elements of the Silla Syncline and the fold its large wavelength. surrounding area. Fold axes are colored black, thrust West of the syncline, following the eastern edge of faults are red, right-lateral strike-slip faults are blue, Lago Pehoe is an anticline, here informally named the and left-lateral strike-slip faults are green. Lago Pehoe Anticline (LPA) (Figure 8), a smaller fold Additionally, areas of coarse grained rock within fine sandstones and mudstones. The anticline is (conglomerates and coarse sandstones) are highlighted generally asymmetric, with the eastern limb dipping in yellow, and complex zones of contractional steeper than the western limb. deformation are highlighted in red. The strike-slip East of the Silla Syncline, the lithology is faults are a younger feature than the other elements and dominantly mudstone and fine grained sandstone are not the focus of this study. Elements related to layers, interpreted as turbidites (Crane, 2004). In folding and thrust faulting are described below: addition to thrust and reverse faults, these sediments display higher order folds, with smaller wavelengths Folding than the Silla Syncline. In many cases, these folds are associated with thrust faults at their axes Fold amplitude and wavelengths in the region appear to be controlled in part by the changing Thrust Faulting lithology of the formation, with large wavelength folds (up to 8 km) involving in the coarse grained units, and The change in lithology from the Silla Syncline smaller wavelength folds occurring in the fine grained eastward also marks the appearance of numerous north- units. Fold axes typically trend north-south. The Silla south trending thrust faults and associated folds. The Syncline (Figure 7), plunges to the north and is the thrust faults occur at different scales, with visible Stanford Rock Fracture Project Vol. 17, 2006 K-4 Figure 7. View to the south along the Silla Syncline Figure 8. View to the south along the Lago Pehoe Anticline. The fold axis
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