An Outcrop Example of Large-Scale Conglomeratic Intrusions Sourced from Deep-Water Channel Deposits, Cerro Toro Formation, Magallanes Basin, Southern Chile, in A

Hubbard, S. M., B. W. Romans, and S. A. Graham, 2007, An outcrop example of large-scale conglomeratic intrusions sourced from deep-water channel deposits, Cerro Toro Formation, Magallanes basin, southern Chile, in A. Hurst and J. Cartwright, eds., Sand injectites: Implications for hydrocarbon exploration 17 and production: AAPG Memoir 87, p. 199 –207. An Outcrop Example of Large-scale Conglomeratic Intrusions Sourced from Deep-water Channel Deposits, Cerro Toro Formation, Magallanes Basin, Southern Chile Stephen M. Hubbard Stephan A. Graham Department of Geological and Environmental Department of Geological and Environmental Sciences, Stanford University, Stanford, Sciences, Stanford University, Stanford, California, U.S.A. California, U.S.A. Brian W. Romans Department of Geological and Environmental Sciences, Stanford University, Stanford, California, U.S.A. ABSTRACT arge-scale vertical to subvertical clastic intrusions (as much as 67 m [219 ft] wide and >100 m [>330 ft] high) are present in Cretaceous strata (Cerro Toro Forma- L tion) of the Ultima Esperanza district, southern Chile. The injectites emanate from the margins of submarine-channel deposits that accumulated at water depths of 1000–2000 m (3300–6600 ft) in the Magallanes foreland basin. The remobilized sediment is very coarse, consisting of sandy matrix conglomerate, muddy matrix conglomerate, and poorly sorted sandstone. The injectite bodies sometimes bifurcate upward and are circular in plan view and, thus, are geometrically analogous in many respects to numerous injection features mapped seismically in the North Sea Basin. The remobilization of coarse sediment was likely induced after the burial of the parent deposit to at least a few hundred meters. The controlling factors on injection are difficult to discern; however, it is probable that the highly energetic process in- volved gas charging of the source body and, potentially, a seismic event trigger associated with the uplift of the Patagonian Andes. Copyright n2007 by The American Association of Petroleum Geologists. DOI:10.1306/1209863M873265 199 200 Hubbard et al. INTRODUCTION The Cerro Toro Formation has proved to be of great interest to sedimentologists and petroleum geologists Clastic intrusions have been described and inter- throughout the last few decades. Conglomeratic gravity- preted for more than 100 yr (e.g., Murchison, 1827; flow deposits including dunes 4 m (13 ft) in height, as Diller, 1889; Newsom, 1903). A recent resurgence in well as gravelly plane lamination present in outcrops of the analysis and interpretation of sandstone dikes and the formation, have improved our general understand- sills in deep-water strata has been triggered largely by ing of deep-water sedimentation mechanics (Winn and the pervasive occurrence of such features in petrolifer- Dott, 1977; Sohn et al., 2002). Interpretations of deep- ous Paleogene deposits of the North Sea Basin (e.g., water channel processes have been made based on ex- Duranti et al., 2002; Molyneux et al., 2002; Huuse et al., ceptional exposures of channel-overbank facies tran- 2003; Huuse and Mickelson, 2004). Significant post- sitions in associated outcrops (Winn and Dott, 1979; depositional remobilization of clastic material through Beaubouef, 2004; Crane, 2004; Hubbard et al., 2004). injection processes adds complexity to both facies dis- These same outcrops are considered to be geometrically tribution and reservoir architecture. Recent advances excellent analogs to hydrocarbon reservoirs from across in three-dimensional (3-D) seismic technology have the globe (e.g., DeVries and Lindholm, 1994; Coleman, made recognition and interpretation of associated fea- 2000; Beaubouef, 2004). tures in the subsurface possible (e.g., MacLeod et al., The area of interest in this study is located approxi- 1999; Huuse and Mickelson, 2004; Huuse et al., 2004); mately 20 km (12 mi) north of the town of Puerto Na- however, characterization of the lithofacies associated tales in southern Chile, along the southern shore of with these large-scale injection features is limited to a Lago Sofia (Figure 1). The Cerro Toro Formation in this few outcrop (e.g., Thompson et al., 1999; Surlyk and region can be divided into numerous distinctive inter- Noe-Nygaard, 2001) and drill-core data sets (e.g., Dixon vals present over a stratigraphic thickness of approxi- et al., 1995; Hillier and Cosgrove, 2002; Purvis et al., mately 2000–2500 m (6600–8200 ft) (Figure 2). Hun- 2002; Duranti and Hurst, 2004). dreds of meters of thin-bedded turbiditic shale and A Cretaceous clastic dike field that crops out in the sandstone characterize the lowermost division of the Magallanes basin of southern Chile represents an ex- formation, deposited during a period of relative quies- cellent analog to some of the intrusive bodies in the cence in the Magallanes basin foredeep. A laterally North Sea Basin. The objectives of this chapter are to discontinuous conglomerate body 75 m (246 ft) thick (1) describe and interpret the associated lithofacies of sharply overlies the fine-grained interval, suggesting the intrusions, (2) document geometric relationships a drastic change in sediment source area and deliv- between the large-scale injectites and their source ery of coarse detritus into the basin. After a pause in deposits, and (3) discuss the utility of these features as coarse-grained sedimentation (recorded by the accu- reservoir analogs. mulation of 150 m [492 ft] of thin- to thick-bedded turbidite beds), more than 500 m (1640 ft)of conglomeratic beds accumulated across the basin foredeep. As flow into the basin waned overall, 300 m (984 ft) of STUDY AREA AND gravity-flow deposits accumulated, ranging from thin- BACKGROUND GEOLOGY bedded turbidites, erosive sandy high-density turbidity- current deposits, and conglomeratic units with dunes During the Late Cretaceous, deep-water sediments 4 m (13 ft) high. The formation is capped by a thick of the Cerro Toro Formation accumulated in the Magal- package (hundreds of meters) of fine-grained bathyal lanes foreland basin of southern Chile. The elongate deposits. basin was characterized by a north–south-trending foredeep adjacent to the rising Andean Cordillera to the west. Based on paleocurrent data indicating north- to-south transport, Winn and Dott (1979) envisioned SEDIMENTARY FACIES a southward-prograding deep-sea fan system fed from a canyon to the north into an extensive, basin-axial The strata of interest in this study include the channel belt that was present for more than 100 km lowermost conglomeratic interval of the Cerro Toro (62 mi) from the north of the Torres del Paine to the Formation and the overlying thin-bedded turbiditic south of Puerto Natales (Figure 1). Lateral conduits into units (beneath the 500-m [1640-ft]-thick conglomer- the southward-flowing fan-channel complex sourced di- atic interval; Figure 2). A detailed measured section rectly from the Andean Cordillera to the west are pos- through this interval is presented in Figure 3. The litho- tulated to have also contributed significant sediment facies recognized within this interval include concor- into the basin (Crane and Lowe, 2001; Sohn et al., dant, bed-parallel deposits, as well as discordant cha- 2002). otic deposits (nonparallel to bedding). Following the An Outcrop Example of Intrusions Sourced from Deep-water Channel Deposits 201 terminology used by Duranti and Hurst (2004), the bases of some structureless units (Figure 5). Sharp, ero- bed-parallel deposits are referred to as stratified facies, sive bases are characteristic, uncommonly associated and those deposits discordant with bedding ori- with mudstone rip-up clasts 1–2 cm (0.4–0.8 in.) in entation are considered unstratified facies. The litho- diameter. facies identified within the interval studied include Stratigraphically, sedimentation units of L2 are ob- (1) stratified thin-bedded sandstone and mudstone, served to increase in regularity at the top of the detailed (2) stratified thick-bedded sandstone, (3) stratified measured section, prior to the onset of conglomeratic conglomerate, (4) unstratified conglomerate, and (5) deposition (Figure 3). unstratified poorly sorted sandstone. Interpretation Lithofacies 1: Stratified Thin-bedded The thick-bedded sandstone units of L2 record Sandstone and Mudstone deposition from low-density (traction structured beds) Description or erosive high-density (structureless) turbidity cur- rents. Normally graded, structureless beds are inter- Stratified thin-bedded sandstone and mudstone preted as S3 divisions of Lowe, (1982); inversely graded constitute lithofacies 1 (L1). Individual sandstone beds bases of units are indicative of traction carpet depo- range from less than 5 to 40 cm (2–15 in.) in thick- sition (S1 divisions of Lowe, 1982). This lithofacies is ness and are generally tabular for hundreds of meters. transitional, recording gravity-flow energy intermedi- They are composed of fine- to lower medium-grained ate between those flows, which deposited L1 (smaller, sandstone, normally graded and characterized by par- low-density flows) and L3 (conglomeratic flows). tial (TbÀc divisions typically) Bouma sequences. Soft- sediment deformation is present locally within sandstone beds (Figure 4). Mudstone interbeds are considered Lithofacies 3: Stratified Conglomerate to be genetically linked to sandstone beds. Description Sedimentation units of L1 are present lateral to, overlying, and underlying packages of conglomeratic Lithofacies 3 (L3) consists of stratified

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