FAILURE MODES IN TURBIDITES OF THE MAGALLANES BASIN, CHILEAN PATAGONIA: A PRELIMINARY ANALYSIS Atilla Aydin and Joseph Gonzales Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 marks the beginning of the development of the Abstract Magallanes Basin as a foreland basin (Figure 1C). Deposition within this tectonic context continued with We investigated failure modes in Upper the Cerro Toro Formation (Figure 1C). The depocenter Cretaceous, deep water turbidites in the Magallanes of the foreland basin was initially located immediately foreland basin in and around the Torres del Paine to the east of the Cordillera and has shifted National Park in Chilean Patagonia with the goal of progressively further eastward as the contractional providing an analog to the mechanisms of deformation deformation has propagated to the east (Winslow, of deep water turbidite reservoirs. A wide variety of 1981). The maximum sediment thickness within the fundamental failure structures are present. Among basin is as great as 7 km (Biddle et al., 1986) and the these are sharp, shear fractures with a normal sense of water depth at deposition is estimated to have been 1 to motion, oriented at about 300 to bedding and 2.5 km (Fildani and Hessler, 2005). representing the earliest failure events in the mudstone- dominated lower Cerro Toro Formation. Cleavage fractures also occur in the mudstone intervals, especially within folds and fault zones. Opening-mode failure is very common in all lithologies, producing networks of joints. Bedding interfaces, marked by mudstone of various thicknesses, prove to be prone to bed-parallel slip. The joints form the bases for sheared- joint mechanism and the strike-slip faulting which is prominent in course-grained units. Faults at low angle to bedding are common in all scales. The most complex faults are associated with coupled processes of folding and faulting. These faults form broad zones with high Figure 1. Geographic and tectonic settings of the degrees of anisotropy. These preliminary results are Magallanes Foreland Basin in southern Chile. (A) promising for identifying the distribution of the Location map and boundaries of the basin (from fundamental failure modes in various lithological units Biddle at al., 1986). (B) Early Cretaceous back-arc basin setting represented by the Tobifera and of deep water turbidites and their impact on faulting Zapata formations. (C) Late Creataceous foreland mechanisms and the resulting fault architecture. basin represented by the Punta Barrosa, Cerro Toro and Tres Pasos (not shown). (B) and (C) from Introduction Wilson, 1991 modified by Fildani and Hessler, 2005, and Crane, 2004. The Magallanes Basin is located at the southern end of South America and covers most of Chilean and The Magallanes basin contains thick sequences of Argentinean Patagonia (Figure 1A). The basin’s Upper Cretaceous, deep water turbidites (Figure 2 and western boundary is demarcated by the Andean 3A) with spectacular outcrops in and around the Torres Mountain belt and the subsidence and ensuing del Paine National Park (Chilean Patagonia). Three contraction of the basin is linked to the rise of the formations (from oldest to youngest), the Punta proto-Andean Cordillera and the convergence beneath, Barrosa, Cerro Toro, and Tres Pasos formations (Figure across the Andean subduction zone (Winslow, 1981; 2 and 3A-the Tres Pasos not shown) record both the Wilson, 1991; Fildani and Hessler, 2005). The sedimentological and structural evolution of the Andean Magallanes Basin deposits rest upon relatively shallow Fold and Thrust belt and provide an excellent analog marin rocks of Late Jurassic/Early Cretaceous age (the for deformation of deep water turbidites. In this paper, Zapata and Tobifera formations in Figure 2A), which we report our preliminary results concerning the failure have been inferred to have been deposited in a back-arc modes of various lithological packages in the older two extensional basin environment (the Rocas Verdes back- formations with an emphasis on the impact of each arc basin in Figure 1B). The arrival of the sediments of failure mode on fault initiation and development, as the Punta Barrosa Formation in the Late Cretaceous well as the resulting fault architecture. Stanford Rock Fracture Project Vol. 18, 2006 J-1 (Figure 4) with a maximum thickness of about 1000m (thinning eastward before pinching out; Wilson, 1991). It shows an overall upward increase in grain size and bed thickness (Katz, 1963; Wilson, 1991; Fildani and Hessler, 2005). The mud/sand ratio is higher in the lower part of the formation (Figure 4A) than in the upper part. Bed thickness in the sandstone packages ranges between 40-150cm (Wilson, 1991; Fildani and Hessler, 2005), although amalgamated composite beds up to 9m thick also exist (Wilson, 1991). From a structural perspective, it is important to note the alternating beds of sandstone and shale (see detailed sections in Figure 4 (B) and (C), which are crucial for the proposed mechanical behavior of the formation. In addition, the formation underwent a very low-grade metamorphism (Fildani and Hessler (2005), perhaps due to anomalously high temperature introduced by Figure 2. Geological map of the Torres del Paine nearby intrusions and high overburden pressure. National Park and its vicinity. Locations of major Although the porosity of the sandstone beds in this study sites, Park Highway, Tyndall Bridge road cut, formation has not been determined, it is thought to be Lago Grey road cut along with Park Headquarter are low. also marked. Figure 3. (A) Upper Jurassic to Upper Cretaceous rocks stratigraphy of the Magallanes Basin. (B) The Cerro Toro section at the Silla Syncline. (C) Figure 4. (A) Stratigraphic column of the Punta Heirarchical packages within the Cerro Toro Fm. (D) Barrosa Fm. (the lower 400m) and (B) and (C) detail Detailed section at the lower part of the formation packaging of sandstone/mudstone sequences. (A) across the western limb of the Syncline. (A) From From Wilson, 1991; and (B) and (C) from Fildani and Wilson, 1991; and (B) to (D) from Crane, 2004. Hessler, 2005. Upper Cretaceous turbidites Cerro Toro Formation The Punta Barrosa Formation is capped by the Punta Barrosa Formation mud-rich turbidites of the Cerro Toro Formation, which This formation marks the initial sediment arrival suggests a paleobathymetry of approximately 2000 m into the Magallanes foreland basin associated with the (Fildani and Hessler, 2005). The thickness of this Andean Orogeny (Wilson, 1991; Fildani et al. 2003). formation is estimated to be 1100 to 2500 m (Katz, Recent detrital zircon analyses by Fildani et al. (2003) 1963; Wilson, 1991; Crane 2004). The age of the Cerro indicate that the age of the Punta Barrosa Formation is Toro Formation, based on fossil evidence, has been about 92-95 Ma (Turonian). The formation is made up established as middle-to-upper Senonian (Katz, 1963), of alternating sandstone and mudstone turbidites but it might be younger, based on recent zircon analysis Stanford Rock Fracture Project Vol. 18, 2006 J-2 of the underlying Punta Barrosa Formation. The road cut”, the “Park Highway at the Silla Syncline”, lithology of the Cerro Toro Formation is predominantly and the “Lago Grey road cut” (Figure 2). mudstone and thin-bedded, fine-grained sandstone turbidites, intercalated with coarse-grained sandstone and conglomerate units (Crane, 2004). Crane identified three intervals of matrix- and clast-supported conglomerate and coarse-grained sandstone (Figure 3B and C), classifying them as the 4th order depositional architectural elements. He interpreted these elements as channel systems, located in incised submarine valleys. The mudstone units, interspersed with thin beds of siltstone and fine-grained sandstone, between the coarser elements of the 4th order, have thicknesses ranging from 100 to 350 m. The top mudstone- dominated unit has been eroded in the study area. Crane (2004) interpreted the mudstone-dominated intervals as late-stage channel filling or deposition from unconfined, low density turbidity currents. A detailed, measured section by Crane (Figure 3D) illustrates Figure 5. (A) and (B) Summary of the Cerro Toro and packaging within the lower part of the Cerro Toro Punta Barrosa formations and the fundamental Formation, west of the Silla Syncline. The Tres Pasos lithological elements making up these formations. Formation represents the third and youngest unit of (C) and (D) Fundamental failure modes and deep water turbidite deposition in the Magallanes properties of the faults developed from the failure Basin, which is not being considered in this study. structures. Timing and magnitude of deformation Shear fractures The development of a fold and thrust belt is The most conspicuous type of failure structures generally considered to be contemporaneous with are sharp (very thin), inclined slip surfaces with deposition. See Contribution K (in this volume) for a beautifully striated and polished surface markers. These detailed analysis of the relationship between the sharp discontinuities occur predominantly in the deposition and structural deformation of the Cerro Toro mudstone packages of the lower Cerro Toro Formation Formation. Here, we note only that the overall (Figure 6A) and are limited in size. contraction reflected by the folds and reverse/thrust faults in the region is enormous. Winslow (1981) estimated it at 30 to 160 km, while Kraemer (2003) estimated it at 110 km at lat 50° S and 300 to 600 km at lat 56° S. Failure modes The diagrams in Figure 5 summarize the major lithological elements making up the Punta Barrosa and Cerro Toro formations and their packaging patterns (5A and B). Also identified in the figure are the failure modes within each lithological unit or assemblage of units (Figure 5C). In this section, we will briefly describe each failure mode and related fault development, as well as the geometric and physical characteristics of the structural products (Figure 5D). We will also briefly assess the potential impact of these Figure 6.