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STRUCTURAL CHARACTERISTICS OF THE SILLA SYNCLINE (): IMPLICATIONS FOR 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 , 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 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 , 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 , 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 , 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 . 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 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 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 . 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 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 (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 is visible to the left of the road in the foreground (inset). Note that the dip angles in the inset is apparent dip, the eastern limb is steeper.

Stanford Rock Fracture Project Vol. 17, 2006 K-5 offsets ranging from approximately 1 m to 30 m. In syncline, suggesting that the depositional and general the thrust faults are confined within finer underlying structural features are related. grained units. If the formation of the LPA and LSCFZ began In relation to the Silla Syncline, there is a broad before the first appearance of the coarse grained units, zone of thrust faults adjacent to the syncline on its the spacing of these two features would likely be eastern end oriented sub-parallel to the syncline axis, determined by factors other than lithology. From here informally named the Lago Sarmiento Chico Fault Figures 2 and 12 there appears to be a regional spacing Zone (LSCFZ) (Figure 9a). The fault zone generally to large scale folds and thrust faults, as well as coarse forms an antiformal feature, with bedding transitioning grain deposit occurrences. Generally, the spacing from west dipping on the syncline side to east dipping between these large features ranges from 4 to 10 km, on the other. Due to vegetation cover, poor outcrop which is in the same order as the 4 km spacing between exposure, and sharp lithological changes across the the LPA and LSCFZ. This spacing could be the result zone, it was not possible to directly measure the offset of the spacing in the underlying accommodated by the fault zone, but it is likely on the units. scale of tens of meters. This zone is a complex zone of In addition to the focused deposition in the deformation, in which coupled folding and thrust structurally controlled trough, it is possible that channel faulting in fine grained sediments have resulted in migration within the bounding structures could be complicated fault geometries (Figure 9b and c). For caused by a sequential deformation in either limb of the more details on this zone, please see the contribution by newly forming Silla Syncline. In this case, lower order Aydin and Gonzales, this volume. subsidence associated with intermittent slip on the Bed parallel slip occurs in many locations in the bounding fault system could result in migrating channel study area, indicating the continued folding as a result deposition locations. of ongoing regional compression. The change in sediment size from fine-grained to coarse-grained has also been speculated to be the result Interpretations and Discussion of tectonic activity. Deposition prior to the coarse channel deposits in the Silla Syncline was dominantly Coarse grained sediments are confined between the fine-grained. Subsequent to this depositional period, LPA and LSCFZ. Figure 10 is a cross section showing tectonic uplift of the to the west likely initiated the features bounding the coarse grained deposits of the the deposition of the coarse sandstone and Silla Syncline. Note that the coarse grained channel conglomerate deposits (Crane, 2004; Coleman, 2000). deposits are discontinuous in nature, and that single While our structural characterization implies a deposits do not cover the entire width of the syncline. relationship between deformation and depositional Because of the asymmetrical nature of the LPA, we behavior, it may also have implications regarding interpret the fold as being caused by east directed , igneous intrusions as well as present day lake possibly an underlying west-dipping thrust fault. location and orientation. Generally igneous dikes We interpret the confinement of the coarse grained intruded into strike-slip faults while the present day sediments to be caused by the two bounding structures, lakes are located in, and oriented parallel to the strike- the LPA and LSCFZ, and propose that they initiated slip and thrust faults. prior to the deposition of the channel deposits. As the two structures evolved, the space between them would be evolving as a broad flat syncline, which would act as Conclusions a depositional trough or channel. Figure 11 shows the Figure 13 illustrates as series of proposed cross nature of the sediments underlying some of the coarse sections through time, detailing deformation and sandstone and conglomerate packages on the western channel construction that occurred before, during and limb (a) and the eastern limb (b) of the northern Silla after coarse grained sediment deposition in the Silla Syncline. The onlapping behavior indicates that there Syncline. In the early stages of syncline formation, the is an angular relationship between the coarse-grained initial width of the depositional channel was likely units and the finer-grained units below that could controlled by the regional behavior of the underlying indicate ongoing deformation during the time of the large-scale folds and thrust faults within the underlying deposition of different units. While some contacts at Punta Barrosa Formation. At this time, there was not the base of coarse grained units can be interpreted as likely to be a significant surface expression of the mainly erosional, it appears that the contacts of coarse underlying low angle faults or detachments (Figure and fine grained units show evidence for tilting and 13a). deformation before the coarse units’ deposition. In As faulting and the associated folding progressed, addition, these depositional features occur immediately the incipient trough gained promience. At this stage, above the anticline and fault zone bounding the deposition could have begun to be focused through the

Stanford Rock Fracture Project Vol. 17, 2006 K-6 newly formed channel (Figure 13b). Once regional of London Philosophical Transaction, series A, v. 300, tectonic activity initiated coarse grained deposition, the p.319-335. syncline became the site for deposition of the coarse Fildani, A., Cope, T.D., Graham, S.A., and Wooden, J.L., grained channel deposits (Figure 13c). 2003, Initiation of the Magallanes foreland basin: Timing of the southernmost Patagonian Andes orogeny revised After the coarse grained units were deposited, by detrital zircon provenance analysis: Geology, v. 31, folding and faulting continued, as evidenced by bed no. 12, p. 1081-1084. parallel slip in the syncline and by the development of Fildani, A., and Hessler, A.M., 2005, Stratigraphic record the LSCFZ (Figure 13d), as well as other thrust faults across a retroarc basin : Rocas Verdes- and folds in the Cerro Toro Formation across the Magallanes Basin, Patagonian Andes, Chile: Bulletin of region. the Geological Society of America, v. 117, no. 11/12, p. Our results support the previous hypothesis put 1596-1614. forward in sedimentological studies by the SPODDS Gust, D.A., Biddle, K.T., Phelps, D.W., and Uliana, M.A., group and others, that the syncline sediments were 1985, Associate Middle to Late Jurassic volcanism and extension southern South America: Tectonophysics, v. structurally controlled. 116, p. 223-253. From this study, there appears to be a significant Hubbard, S.M. 2006, Ph.D. Dissertation, Stanford University, link between depositional behavior and the regional Stanford, California structure of the Magallanes Basin. Based on the Katz, H. R., 1963, Revision of Cretaceous stratigraphy in occurrence of other coarse sandstone and conglomerate Patagonian cordillera of Ultima Esperanza, Magallanes deposits, it is possible that the style of deposition in the Province, Chile: Bulletin of the American Association of Silla Syncline happened elsewhere in the Magallanes Petroleum Geologists, v. 47, no. 3, p. 506-524. Basin as suggested by the distribution of coarse grained Kraemer, P.E., 2003, Orogenic shortening and the origin of units in the form of synclines. the Patagonian orocline (56°S. lat): Journal of South American Earth Sciences, v. 15 p. 731-748. Stanford Project on Deep-Water Depositional Systems Acknowledgements (SPODDS), 2004, Deep-Water Deposits of the This work has been supported by the Stanford Cretaceous Cerro Toro and Tres Pasos Formation, Rock Fracture Project. We would also like to thank the Magallanes Foreland Basin, Chilean . Fieldtrip Guidebook, 44 p. SPODDS group for stratigraphic and sedimentologic Wilson, T.J., 1991, Transition from back-arc to foreland basin insights, as well as logistic advice during this study. development in the southernmost Andes: Stratigraphic record from the Ultima Esperanza District, Chile: Geological Society of America Bulletin, v. 103, p. 98- References 111. Wilson, T.J. 1983, Stratigraphic and structural evolution of Biddle, K.T., Uliana, M.A., Mitchum Jr., R.M., Fitzgerald, the Ultima Esperanza foreland fold-thrust belt, M.G., and Wright, R.C., 1986, The stratigraphic and Patagonian Andes, Southern Chile: Ph.D. Dissertation, structural evolution of the central and eastern Magallanes Columbia University, New York, 360 p. Basin, southern South America, in Allen, P.A., and Winslow, M.A., 1981, Mechanisms for basement shortening Homewood, P., eds., Foreland Basins: International in the Andean foreland fold belt of southern South Association of Sedimentologists Special Publication 8, p. America, in McClay, K.R., and Price, N.J., eds., Thrust 41-63. and : Geological Society [London] Bruhn, R.L., Stern, C.R., and de Wit, M.J., 1978, Field and Special Publication 9, p. 513-529. geochemical data bearing on the development of a

Mesozoic volcano-tectonic zone and back-arc basin

in southernmost South America: Earth and Planetary

Science Letters, v. 41, p. 32-46.

Coleman, J.L., 2000, Reassessment of the Cerro Toro (Chile) sandstones in view of channel-levee-overbank reservoir continuity issues: in Weimer, P. ed., Deep-water reservoirs of the world: Gulf Coast Section, Society of Economic Paleontologist and Mineralogists, 20th Annual Research Conference, p. 252-258. Crane W.H., 2004, Depositional history of the Upper Cretaceous Cerro Toro Formation, Silla Syncline, Magallanes Basin, Chile. Ph.D. Dissertation, Stanford University, Stanford, California. Dalziel, I.W.D., de Wit, M.J., and Palmer, K.F., 1974, Fossil marginal basin in the southern Andes, Nature, v. 250, p 291-294. Dalziel, I.W.D., 1981, Back-arc extension in the Southern Andes: A review and critical reappraisal: Royal Society

Stanford Rock Fracture Project Vol. 17, 2006 K-7 a)

b) Figure 9. a) View to the south along the Lago Sarmiento Chico Fault Zone, with the eastern limb of the Silla Syncline visible on the right of the photograph. b) and c) outcrop exposures within the fault zone showing b) complex fold and fault coupling geometries, and c) low angle faults

c)

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Figure 10. Cross section across the Silla Syncline, showing coarse grained deposits and their relations to the bounding fault cored anticline (west) and complex thrust fault zone (east).

Figure 11a. Angular relationship of sediment layers underlying a conglomerate near Lago Nordenskjold. Location is on the western limb of the Silla Syncline, above the Lago Pehoe Anticline.

Figure 11b. Angular relationships between sedimentary layers at the base of coarse grained units. Location is on the eastern limb of the Silla Syncline, above the Lago Sarmiento Chico Fault Zone.

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b)

c) Figure 12. Schematic cross sections across the Cerro Toro Formation (see figure 2 for cross section locations), showing the periodicity of coarse units and fold axes.

Figure 13. Diagram represent the evolution of the Silla Syncline over time. a) Thrust faults develop in the underlying sediments, but do not show a surface expression. b) Continued fault development results in the formation of folds near the surface including the d) incipient Silla Syncline c) Coarse grained sediments use the developing syncline as a depositional channel d) Folding continues as a result of regional compression. The eastern thrust fault is visible at the surface, while the western thrust fault is not, visible only as an anticline

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