Abstract The Magallanes Basin is a retroarc foreland basin ( Suarez and Pettigrew, 1976 ; Dalziel, 1981 ; Wilson, 1991 : Fildani and Hessler, 2005 ) and the sedimentary sequence preserved in the Andean fold-thrust belt reflects the early extensional phase of basin evolution and the subsequent contractile phase with eventual uplift associated with Andean orogenesis ( Figs. i.1 and i.2 ). In the latest Jurassic, extension associated with the initial breakup of southern Gondwana ( Bruhn, et al., 1978 ; Gust, et al., 1985 ; Pankurst, et al., 2000 ; Calderón et al., 2007 ) culminated in the development of an oceanic backarc basin referred to as the Rocas Verdes Basin ( Katz, 1963 ). Ophiolitic rocks exposed in the Cordillera Sarmiento, south and west of Parque Nacional Torres del Paine, represent the obducted remains of the floor of this backarc basin ( Wilson, 1991 ; Fildani and Hessler, 2005 ; Calderón et al., 2007 ). Compression associated with the onset of the Andean orogeny resulted in rapid uplift along the western basin margin and concurrent foreland subsidence. A deep-water depositional phase, caused by flexural loading of obducted ophiolitic blocks over the attenuated crust, is marked by the turbidites of the Punta Barrosa Formation ( Fig. i.1 ). The overlying shale-rich Upper Cretaceous Cerro Toro Formation represents the climax of deep-water sedimentation. Conglomerate-filled channel systems (channel belt up to 8 km wide) within the Cerro Toro Formation developed along much

Abstract The effect of inherited attenuated crust from the closure of the predecessor backarc basin led to a relatively narrow orogenic belt during the Magallanes foreland development and a short distance from arc to foredeep. The attenuated crust heritage also provided continuous basinal subsidence (contributed to by fold-thrust belt loading and ophiolitic block obduction),which permitted long-lived (>20 my) deep-marine deposition and accumulation of >4000 m of turbiditic sediment that filled the basin axially in a north to south direction ( Figs. ii.1A and C ). Three distinct formations that reflect three distinct phases of deep-water deposition with different stacking patterns are featured in this document: the Punta Barrosa Formation, the Cerro Toro Formation, and the Tres Pasos Formation. These three formations were deposited with contrasting stratigraphic architectures that we relate to two general factors: (1) variability in amount and type of source material (i.e., changes in provenance and/or staging area) and (2) variations in the basin shape throught time. Changes in the source and staging areas are represented by clear sedimentological differences, including the sandstone-and mudstone-dominated Punta Barrosa Formation with banded slurry beds, the conglomeratic channel-fill deposits of the Cerro Toro Formation, and the sandstone packages and mudstone-rich mass transport deposits of the Tres Pasos Formation slope system ( Figs. ii.1B and C ; see following page). Basin morphology controls the general lay-out of depositional systems (e.g., channel dimensions, degree of confinement, dispersal patterns, etc.), which influences the distribution of sediment and resultant stacking patterns. We suggest

Abstract The Tres Pasos Formation is a continental margin-scale progradational mudstone- to siltstone-rich slope succession. The lower part of this formation is characterized by an architecturally variable suite of sandstone-rich channel and sheet elements punctuated by mudstone-rich mass transport deposits. Sandstone-rich sedimentary bodies comprise massive sandstone (mostly high-density turbidity current deposits; S1 and S3 sensu Lowe, 1982 ), intrabasinal mudstone-clast conglomerates, and various finer-grained turbidite facies (i.e., Bouma divisions; Bouma, 1962 ). The sandstone bodies exhibit a highly-variable internal architecture, including complex scour-and-fill features and lateral facies changes. Large-scale mudstone-dominated mass transport deposits (MTDs) typically contain mud-matrix supported debris flow deposits mixed with slide/slump blocks of variable sizes and are common between sandstone packages. The upper part of the Tres Pasos Formation is dominated by turbiditic mudstone and siltstone, hemipelagic mudstone, and sparse scours filled with coarse-grained pebbly material interpreted as by-pass conduits.

Abstract Multiple models have been proposed to account for the origin of channel and out-of-channel deposits in the Cerro Toro Formation at the Silla Syncline. The complex stratigraphic relationship between coarse-grained channel, and fine-grained out-of-channel deposits has been a topic of considerable debate in the scientific literature. Winn and Dott (1979) , Beaubouef et al. (1996) , and Beaubouef (2004) propose aggradational channel-levee models for the channel complexes present in the Parque Nacional Torres del Paine ( Fig. iv.1A, B ). Conversely, Coleman (2000) , Crane (2004) , and Crane and Lowe (2008) have proposed variations on an evolutionary model that invokes erosional confinement of channel sediments in background Cerro Toro Formation mudstone ( Fig. iv.1C, D ). The high-energy axial portion of the best-exposed and most well studied channel complex (Paine C of Crane et al., 2008 ) is a minimum of 4 km wide and the Silla Syncline provides only an oblique cross-section through the complex ( Fig. iv.1D ). If the system is levee confined, the coeval levees must have added substantially to the cross-sectional width of the channel complex; post-depositional structures both east and west of the syncline limit the preserved width to only slightly more than the channel axis in most areas. As a result it is difficult to discern whether the erosional margins preserved in outcrop represent the edges of an incisional channel complex, or the eroded inner margins of extensive levees.

Abstract Outcrop accessibility: difficult Outcrop Coordinates: 51.0852°S, 72.6541°W Refer to outcrop 7 on location map The Wildcat channel complex is up to 5–6 km wide and 330 m thick, making it the largest exposed on Sierra del Toro. Hubbard and Shultz (2008) suggest that this channel complex is stratigraphically equivalent to the upper two-thirds of the conglomeratic portion of the Cerro Toro Formation at Cerro Castillo, located due south of Sierra del Toro adjacent to the south shore of Lago del Toro. A portion of the eastern Wildcat channel margin was characterized by Hubbard et al. (2008b) (outcrop featured in Fig. 7.2E , referred to as “Sarmiento Vista” by Hubbard et al., 2008b) . The eastern margin of the Wildcat channel complex is well exposed on the northeast face of the mountain, displaying 200 m of conglomeratic channel fill thinning until its final pinchout ( Figs. 7.1B and 7.2 ). This exposure allows for a direct correlation of beds along a ~3 km transect oblique to depositional strike (1.5 km when projected parallel to strike; Fig. 7.3 ). The slope of the margin averages approximately 7.5° over this distance, although it is partitioned into a steep ‘wall’ at its base, near the channel complex axis, and a shallower ramp nearer the final pinchout ( Fig. 7.3 ). As a result, the eastern margin is characterized by a “steerhorn” cross-sectional profile. Paleoflow measurements taken at the margin of the channel complex

Abstract Outcrop accessibility: variable Outcrop Coordinates: 51.4716°S, 72.6508°W Refer to outcrop 8 on location map Conglomeratic deposits of the Cerro Toro Formation attributed to deposition in a large-scale basin axial channel belt are well exposed in the Cordillera Manuel Señoret, north of Puerto Natales between Cueva del Milodon and Lago Toro. The conglomeratic succession is informally called the “Lago Sofia Member” because of the presence of Laguna Sofia in this area ( Scott, 1966 ). The large injectite complex that is the focus of Chapter 9 is present in the Cordillera Manuel Señoret on the south shore of Lago Sofia ( Fig. 8.1A ). The channel belt is characterized by low sinuousity, demonstrated with outcrop mapping and the collection of extensive paleocurrent data ( Fig. 8.1A ). Outcrop exposures of the channel belt margin are present on the north faces of Cerro Benitez and Cerro Mocho ( Fig. 8.1B, C ). Because of poor accessibility, the margin at Cerro Mocho cannot be easily visited. A levee interpretation for fine-grained material adjacent to the channel belt is based on a series of key observations: (i) paleocurrent measurements indicative of flow divergence (50° to 100°) measured from out-of-channel units as compared to those taken from inside the channel belt container; (ii) progressive thinning of sandstone beds away from the channel belt margin; (iii) evidence that the eroded channel margin was poorly indurated, including a stepped profile and injection of channel fill into out-of-channel strata; (iv) deposits that record slumping on the

Abstract Outcrop accessibility: easy Outcrop Coordinates: 51.5394°S; 72.6291°W Refer to outcrop 9 on location map The injectites at Lago Sofia were first reported by Scott (1966) and Winn and Dott (1979) . The first comprehensive evaluation of the features was done by Schmitt (1991) , with a more recent summary published by Hubbard et al. (2007) , who noted the geometrically analogous nature of the injectites to similar features in the North Sea. The injectites at Lago Sofia are exposed in 3-D, characterized by a near vertical outcrop of channel deposits in the lowest part of the succession. The middle part of the succession where the injectites are most prevalent is exposed on an ancient terrace, allowing for plan-view mapping of the injectite complex. The upper part of the stratigraphic succession is exposed sub-vertically, permitting visualization of the cross-cutting relationships between injectites and host strata ( Fig. 9.1 ). A series of three channel bodies (C1–C3) are laterally offset and progressively younger eastward. Each channel body is associated with injected bodies at its margins. Both margins of C1 experienced sediment remobilization and injection. Channel C2 experienced injection solely on its western margin; the associated injectite, I3, bifurcates and is observed to cross-cut >125 m of overlying stratigraphy. Channel C3 is the smallest, as is its accompanying injectite (I4).

Abstract Outcrop accessibility: extremely difficult Outcrop Coordinates: 50.7048°S, 72.7361°W Refer to outcrop 10 on location map The Tres Pasos Formation (1000–1500 m thick) records the transition from bathyal water depths of the upper part of the Cerro Toro Formation to the shallow-marine and deltaic facies of the overlying Dorotea Formation. The lower part of the Tres Pasos Formation exposed at Cerro Divisadero (600 m thick) is the northernmost (most landward) of the Tres Pasos outcrop locations in this guidebook. Four sandstone-rich packages that range in thickness from 20 to 80 m are recognized and mapped across a ~3 km long outcrop transect. The north-south trending outcrop is generally parallel to depositional dip (basinward to the south), although ridge-and-gully topography provides three-dimensionality at a smaller scale (10s to 100s of meters). The four sandstone-rich units are separated by intervals of shale and siltstone tens of meters thick. Unit 1 is partially covered and minor post-depositional structure (reverse faults and associated drag folds) disrupt detailed architectural characterization of Unit 4. Units 2 and 3 are, therefore, the focus of the panels in this guidebook. Refer to Romans et al. (2008) for detailed presentation and interpretation of the Tres Pasos Formation at Cerro Divisadero.

Abstract Outcrop accessibility: very difficult - Overview accessibility: easy Outcrop Coordinates: 50.8166°S, 72.6947°W Refer to outcrop 11 on location map Outcrops of the Tres Pasos and Dorotea formations in northern Ultima Esperanza District highlighted in this field guide ( outcrops 10 , 12 , and 15 ) are remote and extremely difficult to access. One way to appreciate the general characterstics of the strata is from a distance along a dirt road on the west side of the north-south flowing Rio Zamora, which has carved a canyon into the Tres Pasos Formation and upper Cerro Toro Formation. From within the boundaries of Parque Nacional Torres del Paine, take the road to “Laguna Azul” and continue heading north ( Fig. 11.1A ) (note: at the time of publication, this road went through private land with unlocked gates; please always respect private land and communicate your intentions to stay on road and view outcrops from afar). The outcrops are on the other side of Rio Zamora from this road; do not attempt to cross the river. From north to south, the outcrop belt straddles three mountains: Cerro Divisadero, Cerro Mirador, and Sierra Contreras. Abundant paleocurrent data from the Tres Pasos Formation in this area indicate that basinward was to the south-southeast ( Shultz et al., 2005 ; Romans et al., 2008 ). The transect displayed in this river canyon is therefore generally parallel to depositional dip, providing an opportunity to see a dip-parallel slope cross-section. Some east-west tributary

Abstract Outcrop accessibility: moderate Outcrop Coordinates: 51.1537°S, 72.4725°W Refer to outcrop 13 on location map Limited exposures of sandstone are present in the Tres Pasos Formation between Sierra Contreras and Laguna Figueroa; an important exception is a 60 m thick succession dominated by coarse-grained turbiditic units at El Chingue Bluff. The stratigraphic section is characterized by an upward coarsening of grain size, and the vertical progression of facies indicating increasing energy. The top of the bluff is defined by the thick sandstone package; beds at the base of this sandstone package are truncated by a normal fault at the southern end of the outcrop, and lap out towards the north onto the tilted hanging wall. Stratal thickening and thinning across the fault, and unfaulted overlying deposits, have led to the interpretation that the fault was active during sand deposition (a growth fault), and that it created accommodation space for the partially ponded sand body ( Shultz and Hubbard, 2005 ). This, and other growth faults in the underlying strata, have contributed to the development of an intraslope minibasin model for deposition. The sandstone package is overlain by a thick succession of mass transport deposits, recording the progradation of the slope across the area. Sandstone dikes are extensive, paralleling growth fault planes. Shultz (2004) has speculated that these dikes were injected downwards, sourced from the coarse sandstone present in the top of the measured section.

Abstract Outcrop accessibility: easy to moderate Outcrop Coordinates: 51.3696°S, 72.4210°W Refer to outcrop 14 on location map A thick sandstone package (~350 m) characterizes the Tres Pasos Formation outcrop between Laguna Figueroa and Arroyo Picana along the Puerto Natales-Cerro Castillo highway ( Fig. 14.1 ). Overlying this sandstone package is a succession of mudstone-dominated strata on the order of 1000 m thick. Sierra Dorotea is capped by deltaic deposits of the Dorotea Formation. In the satellite image presented in Figure 14.1 , the northern part of Sierra Dorotea, Cerro Sol and Cerro Cazador are featured. The interpreted stratigraphic architecture consists of large-scale slope surfaces (Chingue, Figueroa, and Puma) building southward (the image is oriented along depositional dip). Sandstones of the Tres Pasos Formation at Laguna Figueroa are interpreted as base-of-slope deposits. Conversely, the corresponding topset units are attributed to the shallow-marine Dorotea Formation. Individual slope surfaces are locally identified as resistant ridges in the satellite image (surface length ~30–35 km; surface stratigraphic relief ~1600 m; slope angle ~2.5–3 degrees). A schematic diagram of the architecture in the region is presented in Figure 14.3 , incorporating the location of the El Chingue Bluff slope minibasin fill within the regional context ( Outcrop 13 ). Architectural elements along the lower part of slope surfaces include mud clast-filled bypass channels, channelized sheets and sandstone-filled channels, featured on the following pages.