Marine and Petroleum Geology

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1 Marine and Petroleum Geology 28 (2011) 761e784 Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: Sediment waves and depositional implications for fine-grained rocks in the Cerro Toro Formation (upper Cretaceous), Silla Syncline, Chile Kirt M. Campion a, *, Barrett T. Dixon b, Erik D. Scott a a Marathon Oil Corporation, 5555 San Felipe Dr., Houston, TX 77056, USA b Anadarko Petroleum Corporation, USA article info abstract Article history: Received 14 January 2010 Received in revised form 30 June 2010 Accepted 7 July 2010 Available online 30 July 2010 Keywords: Sediment waves Turbidite Levee Submarine channels The Cerro Toro Formation contains a variety of lithofacies including pebble and cobble conglomerate, coarse-grained sandstone, thin-bedded mudstone and sandstone, and slumped to chaotic mudstone. Coarse-grained rocks in the Cerro Toro have been described and well documented, whereas characterization and description of mudstone-prone lithofacies are not well documented. These mudstoneprone rocks exhibit two architectural patterns: 1) broad (>200 m) undulating or wavy-bedded elements that laterally terminate by onlap, truncation (toplap) and downlap patterns, and 2) laterally persistent (>400 m), horizontal, thin-bedded mudstone and sandstone. Adjacent to Channel Complex 3 (Paine C Member), these facies exhibit a stratigraphic transition from horizontal to wavy and curved beds concurrent with pronounced aggradation of laterally equivalent, and possibly coeval, channel facies. Sandstone and mudstone beds within the wavy-bedded facies exhibit turbidite lithofacies that include current-ripple lamination (Tc), planar lamination (Tb) massive, graded intervals (Ta), and laminated to structureless silt- and clay-rich beds (Tde). Typically, these beds are a few centimeters thick, but locally, sandstone beds form bedsets over 1 m thick. These thick sandstone bedsets display inclined bedding or lamination associated with mudstone rip-up clasts and are confined to troughs or swales within large-scale wavy-bedded units. Erosion surfaces within this thin-bedded fine-grained lithofacies are spaced vertically at 10e15 m, commonly associated with the crest of curved bedding and display at least 5 m of relief. Based on map patterns, channel-form architectural elements, and stratal-stacking relationships, the coarse-grained rocks in the Cerro Toro have been interpreted as the fill within deep-water channels. In contrast, the thin-bedded mudstone and sandstone lithofacies located adjacent to and eroded into by the channel facies are interpreted as coeval levee facies. Planar, tabular bedsets located at the base of this fine-grained lithofacies are interpreted as initial overbank deposition associated with the upper part of Channel Complex 3, whereas the curved, wavy beds in the fine-grained lithofacies are interpreted as sediment waves developed on the backside of a levee. The curved bedding, lenticular, medium-grained sandstone bedsets, scattered erosional surfaces, and onlap-downlap stratal geometries in the Cerro Toro bear resemblance to sediment waves associated with Quaternary coarse-grained, channel-levee systems. In contrast, sediment waves generated in other deep-water settings, such as upper slope, continental rise or basinal settings, usually are dominated by silt- and very fine-grained sand, very continuous layering (over 1 km), relatively long wavelength (>1 km) and relatively high amplitudes (>10 m). A possible analog for the coarse-grained and fine-grained rocks in the Cerro Toro is the Quaternary Var Ridge and associated fan valley located in the Mediterranean Sea. Like the Cerro Toro, the Var system has coarse-grained material within the fan valley, pebble- to cobble-size clasts, and thin-bedded mud and sand in levees with documented sand bed thickness up to 40 cm and up to medium-grained sand. Architectural details described from the Var Ridge indicated sediment waves make up a significant portion of the levee material. Beds within the sediment waves terminate laterally by onlap downlap, and toplap, similar to the Cerro Toro. The fine-grained material can be linked to deposition from unconfined * Corresponding author. Tel.: þ ; fax: þ addresses: kmcampion@marathonoil.com (K.M. Campion), barrett.dixon@anadarko.com (B.T. Dixon), edscott@marathonoil.com (E.D. Scott) /$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi: /j.marpetgeo

2 762 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 turbidity currents that were generated via overbank flows from the Var fan channel. The development of these architectural elements requires out of channel turbidity currents and relatively thick flows. Growth of the sediment waves in the Var system represents active sedimentation and turbidity currents within a coeval channel rather than development during periods of channel inactivity or degradation. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Most outcrop-based studies of deep-water depositional environments focus on the coarse-grained lithofacies and architectural elements that were deposited in slope channels or basin-floor fans rather than on the mudstone-prone part of the system such as levees (see recent summaries in Weimer et al., 2000 and Nilsen et al., 2007; Flint et al, 2011; Gagnon and Waldron, 2011; Pyles et al, 2011; and Tinterri and Muzzi-Magalhaes, 2011). Local studies of Quaternary deep-water levee systems (Normark et al., 1980; Savoye et al., 1993; Pirmez and Flood, 1997; Migeon et al., 2000, 2001), slope systems (Wynn et al., 2000; Lee et al., 2002), and basin-floor fans (Droz et al., 2003) are an exception where mud-prone parts of the depositional systems are described and characterized. Mudstone-prone parts of ancient deep-water levee systems have a mixed history regarding their recognition and description (see Khan and Arnott, in this issue; Kane and Hodgson, in this issue). Winn and Dott (1979), DeVries and Lindholm (1994), Coleman (2000), Crane (2001), Beaubouef (2004) and Crane and Lowe (2008) describe mudstone-rich rocks in the Cretaceous Cerro Toro Formation in southern Chile with somewhat different interpretations of the mudstone part of the system. Utilizing high-resolution seismic data, levees have been described from a number of modern and subsurface deep-water systems (Normark et al., 1980, 2002; Pirmez and Flood, 1997; Savoye et al., 1993; Migeon et al., 2000, 2001, 2004; Deptuck et al., 2003, 2007 among others). Moderate to large levee systems can exhibit relatively high relief and extensive lateral dimensions which makes their recognition with a range of seismic frequencies viable. Levees associated with some modern deep-water systems such as the Var system in the Mediterranean exhibits relief of up to 400 m (Migeon et al., 2000, 2001). Locally, these studies have documented sediment waves as a typical association on the backside of levees (Normark et al., 1980; Migeon et al., 2000, 2001, 2004; Lewis and Pantin, 2002; Droz et al., 2003; Deptuck et al., 2007). Although these features are commonly observed in modern deepwater depositional systems, large-scale sediment waves have not been readily described from ancient systems in outcrop or the subsurface. Their wavelength (0.4e6 km) and amplitude (w10 m) are such that recognition on conventional, low frequency (<60 Hz) seismic images is a challenge. Most observed sediment waves are at the seafloor or at shallow burial (200e400 m) and are imaged utilizing relatively high-frequency seismic data (>125 Hz) and to image internal geometry of sediment waves requires frequency upwards of 200 Hz data. Criteria for recognition of ancient sediment waves with well logs and core material are lacking, and their identification in outcrop is sparse. Beaubouef (2004), Barton et al. (2007) and Crane and Lowe (2008) identified various architectural elements in the Cerro Toro Formation in southern Chile as the deposit of possible sediment waves. Their descriptions include gross architecture and local characterization of the lithofacies that make up the sediment waves. Under certain conditions, sediment waves within levees may have thick sand beds, but details on structures and extent of these beds are lacking (Migeon et al., 2000, 2001). In large part, sediment waves are not easy to study and identify in outcrop because they are part of mudstone-prone systems that do not form prominent outcrops. Additionally, because of large wavelength and amplitudes, their identification requires large outcrops of mudstone-prone material. Typically, most mudstone-prone outcrops are exposed poorly because of weathering and/or they support heavy vegetation that obscures bedding geometries. The Cerro Toro Formation in Patagonia, Chile has expansive outcrops of mudstone-dominated rocks caused by relatively recent glaciation and the windswept modern setting. These excellent exposures provide continuous views of mudstone outcrops that allow for description and interpretation of bedding not commonly observed in mudstone-prone outcrops elsewhere. The Cerro Toro Formation has been the subject of a number of sedimentological studies since the 1960s. Winn and Dott (1979) interpreted the Cerro Toro as a levee-channel complex. Detailed description of the Cerro Toro stratigraphy in the Silla Syncline followed Winn and Dott s initial work. DeVries and Lindholm (1994), Beaubouef et al. (1996) and Beaubouef (2004) interpreted the Cerro Toro as a levee-channel complex and divided the Cerro Toro in the Silla Syncline into four channel complex sets with associated levee facies. More recent work by Coleman (2000) and Crane and Lowe (2008) dispute the coeval relationship between channel fill and interchannel mudstone in the Cerro Toro in the Silla Syncline. The objectives of this paper are to: (1) present observations on the mudstone and sandstone distribution within the fine-grained lithofacies of the Cerro Toro, including grain size, sandstone body architecture and abundance; (2) describe and document architectural elements believed to be associated with sediment wave development in the Cerro Toro; (3) compare these architectural elements to sediment waves within Quaternary systems in the Mediterranean that bear on the interpretation of mudstone-rich systems in levee settings: and (4) discuss the reservoir implications of overbank sediments in general and sediment waves in particular. The observations presented may provide a useful model for those involved with the analysis and exploration deep-water systems, possibly levee-channel complexes, in the subsurface. 2. Geologic setting Details of the geologic setting of the Cerro Toro Formation in southern Chile are given in Crane and Lowe (2008). In summary the Cerro Toro Formation was deposited in the Magallanes basin during the Late Cretaceous in southern Chile with outcrops extending along the axis of the basin from parts of southern Argentina to Lago Sofia and Cerro Rotunda located near Puerto Natales, Chile (Fig. 1) (Cecioni, 1957; Katz, 1963; Hubbard et al., 2008; Crane and Lowe, 2008). The Magallanes basin developed as an elongate, northesouth-oriented foreland basin that formed to the east of the proto-andean orogenic belt during the latest Jurassic to Early Cretaceous (Dalziel et al., 1974; Wilson and Dalziel, 1984; Biddle et al., 1986). During the Cenomanian and Campanian, shelf systems fed sediments eastward from a proto-andes provenance into the elongate Magallanes basin, where the sediment transport direction turned southward parallel to the basin axis (Scott, 1966). The Cerro Toro includes conglomeratic units, referred to as the Lago Sofia Member (Katz, 1963), named for outcrops located north of Puerto Natales, Chile (Fig. 1). Clasts in the Lago Sofia include volcanic, plutonic, and metamorphic fragments derived from

3 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e Fig. 1. (A) General location map of southern Chile. (B) An expanded map showing the distribution of Cerro Toro outcrops and location of Torres del Paine National Park (outlined in box); Cerro Toro outcrops within the Silla Syncline (shown in Fig. 2) are highlighted; modified from Crane and Lowe (2008). a proto-andean cordillera (Scott, 1966; Winn and Dott, 1979). These conglomeratic facies are surrounded and encased by thin-bedded, fine-grained mudstone and sandstone. The Cerro Toro is underlain by deep-water mudstones and sandstones of the Punta Barrosa Formation (AlbianeCenomanian) and overlain by a deep-water sandstone succession, the Tres Paso Formation (Campanian) (Katz, 1963; Scott, 1966; Natland et al., 1974; Winn and Dott, 1979; Fildani and Hessler, 2005; Shultz et al., 2005; Shultz and Hubbard, 2005; Hubbard et al., 2008; and Romans et al., 2011). Outcrops of the Lago Sofia member are well exposed at Torres del Paine National Park and have been studied by Winn and Dott (1979), DeVries and Lindholm (1994), Coleman (2000), Beaubouef (2004), Crane and Lowe (2008), Bernhard et al. (2011). Within the National Park, the Lago Sofia Member is exposed within the Silla Syncline located to the east of Lago Pehoe (Fig. 2). This syncline is a gentle fold that is about 10 km long and 4 km wide with a northesouth axis. Outcrops of coarse-grained sandstone and conglomerate can be traced for distances of 5e10 km with scattered areas of cover, and fairly continuous stratigraphic successions of more than 300 m thickness can be measured. Similarly, mudstone exposures exhibit vertical and lateral continuity along the east and west limb of the syncline with local vegetation cover. 3. Lithofacies and stratigraphy of the Silla Syncline 3.1. Lithostratigraphy Rocks within the Cerro Toro Formation exposed in the Silla Syncline can be grouped into two primary facies associations: (1) a coarse-grained lithofacies association interpreted as channel-fill facies (the Lago Sofia Member); and (2) a fine-grained lithofacies association, interpreted as interchannel or levee facies. Informal nomenclature (Fig. 3) was used by Beaubouef (2004) and Crane and Lowe (2008) for mapping and description of the coarse-grained fill. Beaubouef recognized four channel complex sets (terminology from Sprague et al., 2002), whereas Crane and Lowe referred to each coarse-grained channel-fill unit as a complex designated by a geographic location and letter such as Paine C member. Fig. 4 is a stratigraphic cross sections along the west limb of the Silla Syncline which highlights the stratigraphy of the Paine C member described by Crane and Lowe (2008) or Channel Complex 3 (CC3) defined by Beaubouef (2004). This cross section shows the distribution of coarse-grained lithofacies associations or channel complexes and the fine-grained lithofacies association. Architectural elements associated with the Paine C member or CC3 is the main focus of this paper (Figs. 3 and 4). Coarse-grained rocks within CC3, and mudstone-dominated outcrops at the same stratal level as CC3 are exposed on both the western and eastern limbs of the Silla Syncline (Fig. 2). Both the coarse-grained and fine-grained lithofacies were identified by Winn and Dott (1979), DeVries and Lindholm (1994), Beaubouef (2004), Coleman (2000) and Crane and Lowe (2008). The fine-grained facies was interpreted as coeval levee material to the channel-fill facies by Winn and Dott (1979), DeVries and Lindholm (1994) and Beaubouef (2004), whereas Coleman (2000) interpreted the mudstone as older than the coarse-grained, channel fill and cut into by the channel-fill facies. Crane and Lowe (2008) interpreted the fine-grained facies as interchannel and indicated that the mudstone may be unrelated in a temporal sense to the deposition of conglomerate and sandstone in the channel-fill facies. In their model the fine-grained lithofacies could be deposited as either the overflow from a stratified turbidity current that carried coarse clastic material within a channel, but were not influenced by the channel, or by a low-density turbidity current as background

4 764 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 Fig. 2. Silla Syncline located in the Torres del Paine National Park. Base of CC3 lower (solid black line) is shown on the east side of the syncline, and the base of CC3 upper (heavy dashed line) is depicted on the east and west side of the syncline. Also shown is the base of slumped mudstone beds along the east side of the syncline (dotted black line) and top of the Paine A (solid white line) on the east side of the syncline. The area of sediment waves is outlined by a box on the east side of the syncline; line showing the location of Fig. 4 is labeled. sedimentation. In this latter alternative, fine-grained sediments would have draped the channel system as the channel shut down, and subsequently this fine-grained sediment would be eroded out of the channel setting when high-density turbidity currents were reactivated within the channel setting. This pattern of shut down, mud deposition and re-entrenchment occurs 4e5 times within the CC3 or the Paine C member (Crane and Lowe, 2008) Coarse-grained lithofacies The coarse-grained lithofacies compose remnants of confinedchannel fills. A detailed description of the channel-fill units is provided by Beaubouef (2004) and Crane and Lowe (2008). In summary, these rocks include thick-bedded, clast-supported conglomerate with sandy matrix, mud-matrix-supported conglomerate, and pebbly sandstone along with thin-bedded sandstone and mudstone (Fig. 5). Typically, these rocks exhibit a change in lithofacies from thick-bedded gravels in the axial part of the channel complexes to increasing sandstone and mudstone toward the margin of the channels. The conglomerate and pebbly sandstone are interpreted as traction deposits by Beaubouef (2004) and Crane and Lowe (2008). In contrast, the matrix-supported conglomerates are interpreted as debris flows and slurry flows (Beaubouef, 2004; Crane Fig. 3. Informal lithostratigraphic nomenclature used to subdivide the Lago Sofia member of the Cerro Toro Formation in the Silla Syncline. The focus of this study is on the fine-grained lithofacies that is lateral to the Paine C member or CC3L and CC3U; modified from Beaubouef (2004) and Crane and Lowe (2008). and Lowe, 2008). In outcrop, the contact between the channel and the interchannel facies is an erosional surface. CC3 was split into a lower (CC3L) and upper (CC3U) complex because of different map patterns (shown in Fig. 2 of Beaubouef, 2004). CC3L exhibits a south to southeast trend, whereas CC3U displays a southeast trend. Paleocurrents in the CC3L have a range from about 150 to 180, whereas CC3U has a paleocurrent range of 130e150. The changes in orientation and channel-stacking patterns are indicative of different channel complexes (Sprague et al., 2002; Campion et al., 2005). This change in channel trend is marked by distinct vertical aggradation of channels in CC3U shown in Figs. 4 and 6. The fine-grained facies at a similar stratal level as these aggradational channel complexes is the focus of the remaining discussion Fine-grained lithofacies The stratigraphy of the Cerro Toro Formation is dominated by fine-grained lithofacies and includes thin-bedded, muddy turbidites (Tde beds), thin interbeds of very fine- to fine-grained sandstone turbidites (Tcb beds), lenticular, fine- to medium-grained sandstone beds with scattered mudstone rip-up clasts, thin bioturbated mudstone beds, and local contorted to chaotic mudstone beds (Figs. 7 and 8). Thin-bedded, silt-dominated turbidites are the most abundant rock type in the fine-grained lithofacies and in the northern part of the syncline are about 300 m thick. Locally, these

5 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e Fig. 4. Stratigraphic correlations of channel complex set 3 (CC3) in the Silla Syncline showing the distribution of coarse- and fine-grained lithofacies. Distribution of the Paine C is labeled along with the location of Laguna Negra Debrite and key stratal surfaces identified by Crane and Lowe (2008). The strata highlighted in gray are part of the coarse-grained lithofacies and are interpreted as the fill of deep-water, confined channels, whereas areas in white are interpreted as levee or interchannel facies; location of section is shown in Fig. 2; modified from Crane and Lowe (2008). thin-bedded turbidites form thick bedsets (>2 m) that are organized into thickening- and thinning-upward patterns (Fig. 9). Interbedded with the mudstone is thin- to medium-bedded very fine- to fine-grained sandstone with current-ripple lamination (Tc beds), planar lamination (Tb beds) and graded, massive sandstone beds (Ta beds). Other distinctive beds in the fine-grained lithofacies are medium-bedded, fine- to medium-grained massive sandstone with local mudstone, rip-up clasts (Figs. 7 and 10). Usually, these massive sandstone beds and Tbc beds stack to form bedsets that are up to 1.5 m thick with individual beds exhibiting a range from less than 10 cm in thickness to beds greater than 50 cm in thickness. In cross-sectional view, these bedsets have lenticular geometry with lateral extents less than 100 m (Fig. 10). Sandstone beds within the fine-grained lithofacies form about 25e35% of the gross section in close proximity to the coarsegrained channel facies. About 4 km north of the channel facies, near Lago Nordenskjold, sandstone beds form about 15% of the section equivalent to CC3 (Paine C). The decrease in abundance of these sandstone beds away from the axis of CC3 was a relationship noted by DeVries and Lindholm (1994). Within the limits of the outcrop, many of the sandstone beds in the fine-grained lithofacies appear to be disconnected from the coarse-grained, channel-fill facies. Other rock types in the fine-gained lithofacies include slumped mudstone beds to mudstone with a chaotic fabric (Fig. 8). Slumped mudstone beds and chaotic mudstone with scattered sandstone beds are a common lithology in the fine-grained lithofacies, particularly on the east side of the Silla Syncline. Along this margin of the syncline, one large slump zone extends for about 2 km just east of the pinch out of CC3 Upper, and these slumped rocks are less than 75 m from the edge of the channel complex shown in Figs. 2 and 8. The apparent rotation of the slump displayed in Fig. 8B is toward the west, the direction of the CC3 Upper pinch out. 4. Architecture Architectural elements of the coarse-grained and fine-grained lithofacies are varied; the coarse-grained lithofacies that compose channel fills and channel complexes has been described in detail by Beaubouef (2004) and Crane and Lowe (2008) and is not described here. The geometry of thin-bedded turbidites in the fine-grained lithofacies has been given cursory description in the past by Barton et al. (2007) and Crane and Lowe (2008). The vertical stacking and lateral variation of different bedding types in this lithofacies form distinct bedding geometries that include: 1) planar, tabular thinbedded sandstone, siltstone and mudstone (Figs. 7A, 11 and 12); 2) bedsets of thin-bedded sandstone and mudstone that exhibit curved to wavy-bedded geometry (Fig. 13) and local thickeningupward patterns (Fig. 14); 3) lenticular sandstone bedsets with inclined bedding or accretion patterns (Fig. 10); 4) stratal packages of interbedded sandstone and mudstone that exhibit onlapping stratal terminations (Figs. 15, 16, and 18); and 5) chaotic packages of mudstone-dominated lithology (Fig. 8) Plane bed geometry Along the west limb of the Silla Syncline and lateral to CC3U, the fine-grained lithofacies exhibits tabular-shaped beds and bedsets of sandstone and mudstone (Figs. 11 and 12). This outcrop is oriented parallel to sub-parallel to paleoflow direction in CC3L or the Paine C member measured by Beaubouef (2004) and Crane and Lowe (2008). Measured sections displayed in Fig. 4 (Sections 5e8), show a transition from gravel-rich rocks at the base of the section (part of CC3L) to mudstone-dominated rocks at the top of the section. The fine-grained lithofacies at this location is dominated by thin beds of planar-laminated (Tb) sandstone, local very fine-grained sandstone beds displaying current-ripple lamination

6 766 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 Fig. 5. (A) Pebble to cobble conglomerate with medium-grained sandstone matrix in CC3; book is 19 cm (circled). (B) Conglomerate lithofacies in CC3 (Paine C) with imbricated clasts, flow direction to the left; knife is 9 cm for scale. (C) Medium- to coarse-grained sandstone with thin mudstone interbedded along the channel margin setting in CC3; person for scale. (D) Matrix-supported conglomerate in the Laguna Negra Debrite (for distribution see Crane and Lowe, 2008); pocket knife (9 cm) for scale. (Tc), and thin-bedded silt and mudstone (Tde beds) that extend laterally for at least 400 m. Contacts between beds are sharp, but lack significant truncation. Locally, sandstone bedsets exhibit inclined bedding with beds accentuated by mudstone rip-up clasts. These beds were identified by Crane and Lowe (2008; their Fig. 23) who interpreted this bed geometry as constructed by dune migration. Although individual beds exhibit inclined lamination, bedsets in this facies have a tabular geometry and exhibit lateral continuity of at least 400 m Curved-bed geometry Beds that exhibit planar, tabular geometry transition up stratigraphic section to beds that have a curved or wavy geometry (Fig. 13). In contrast to the planar, tabular bedsets that exhibit lateral continuity over 400 m, beds with curved geometries terminate via onlap, downlap, or truncation over distances less than 400 m, and in many beds the distance is less than 100 m (Fig. 13). The transition from planar to curved-bed geometry appears to be gradational in Fig. 6. View toward the northeast of stacked channel complexes in CC3 Upper (CC3U); note the pinch out of a channel along the north shore of Lago Sarmiento Chico (white arrow). This pinch out continues toward the southeast and is exposed along the east limb of the syncline near Lago Pincol. Channels stacked above this pinch out (black arrows at the top of channel fills) exhibit an aggradational, amalgamated to non-amalgamated pattern. This aggradational stack of channel fills (large, vertical arrow) does not extend beyond the valley immediately to the north. To the north, the section is dominated by fine-grained rocks that were deposited out of the channelized setting; this location corresponds to Section in Fig. 4.

7 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e Fig. 7. (A) Thin-bedded muddy turbidites, Tde beds, and thin sandstone beds with planar lamination and current-ripple lamination (Tbc beds) Beds at this location exhibit a planar, tabular geometry. (B) Thin-bedded silt-rich turbidites (Tde beds) located stratigraphically above Fig. 7A. (C) Bioturbated mudstone and siltstone located north of the Park Highway. Bedding is partially preserved but lamination has been disrupted by the burrowing (b); lens cap for scale. (D) Shale rip-up clasts in medium-grained, lenticular sandstone bed. (E) Planar lamination (Tb) in fine-grained sandstone located near Laguna Mellizas; pocket knife for scale (9 cm). (F) Thin-bedded, very fine-grained sandstone with current-ripple lamination (Tc) located in curved beds within the fine-grained lithofacies of the Cerro Toro Formation. two-dimensional views (Fig. 12). However, the possibility exists that all of the beds in the fine-grained lithofacies have curved geometry in three dimensions. In Fig. 13, thin-bedded sandstone and mudstone beds form a curved, convex-up body exposed over a 60 m long outcrop. The beds in Fig.13 have a planar, tabular geometry near the base of the outcrop and gradually increase in amplitude toward the top of the outcrop to about a 1.5 m deflection from horizontal. The total wavelength of the deflection could not be determined. These strata consist of thin-bedded, structureless mudstone with scattered thin-bedded sandstone that displays current-ripple lamination (Tc beds) and/or planar lamination (Tb). Locally, these thin beds are organized into bedsets that are 0.7e2 m thick (Figs. 9 and 14). Trends in bedding are variable from apparent thickeningand coarsening-upward to aggradational bedsets. Bedsets that coarsen upward have a relatively high proportion of sandstone beds that are over 10 cm in thickness, whereas the aggradational bedsets are typically made up of thin-bedded (<5 cm thick) muddy turbidites (Fig. 7B). Within the Cerro Toro, beds that form curved bedsets exhibit inclined sandstone beds that laterally thin and appear to downlap or onlap topography (Fig. 14B). The length of these beds is at least 50 m; longer bed lengths likely exist but could not be measured because of limited outcrop exposure near the site of Fig. 13. Bedset boundaries terminate vertically by toplap or truncation (Fig.14A). The beds in Fig.14 are about 50 cm thicker at the crest of the sediment body and thin toward the margin (northeast) and appear to downlap toward a common surface (Figs. 13 and 14B). Beds that exhibit a convex-upward to wavy pattern are well exposed adjacent to Laguna Mellizas (Figs. 15 and 16) and form an antiformal structure that was recognized by Beaubouef (2004) and Barton et al. (2007). In Figs. 15 and 16, thin-bedded sandstone and mudstone exhibit distinct convex-upward patterns that are at least 100 m wide. From crest to crest, the convex-upward structures in Fig.15 is about 200 m. The amplitude of the larger structure in Fig.15 is about 15 m. This convex-upward structure consists of thin-bedded

8 768 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 Fig. 8. (A) View looking south of slumped mudstone and muddy debrite with scattered sandstone blocks (ss) along the eastern margin of the Silla Syncline. This slumped feature is located near the pinch out of the Paine C or channel complex 3 upper (CC3U). (B) Blow up of inset box showing the slumped margin. This slumped margin extends for about 2 km to the south with apparent direction of movement toward CC3U (west-directed); note person (circled) at top of slump for scale. mudstone and sandstone. Thin sandstone beds display planar lamination (Tb) and current-ripple lamination and are a few centimeters in thickness. Medium-bedded sandstone beds appear to thin and onlap toward the margin of the convex-upward structure (Fig. 16). These pinch outs are positioned at different stratigraphic levels and step toward the northeast, which suggest periodic growth and shift of the sedimentary structure toward the northeast. Fig.17 is a measured section through the margin of the structure, in a swale, and captures the bedding characteristics pictured in Fig. 18. A distinctive sandstone unit near the base of the section consists of fine- to medium-grained sandstone with scattered mudstone rip-up clasts, and includes Ta, Tb and Tc beds that form a bedset that is 1.5 m thick. This unit is covered to the southwest and pinches out to the northeast. The base and top of the bedset are marked by sharp surfaces that exhibit minor erosion (<5 cm). Beds within this unit are inclined to the east-northeast, and over the limited exposure, no lateral facies changes were observed. Most of the other sandstone beds in the measured section are relatively thin and made up of very fine- to fine-grained sandstone with current-ripple lamination (Tc beds). Fig. 18 highlights an erosional surface (10.2 m level in the Fig. 9. (A) Thin-bedded mudstone and sandstone organized into thickening-upward bedset (arrow). The bedset boundary is marked by a surface with toplap (dashed line) which has minor truncation associated with it. (B) Thickening- and coarsening-upward bedset (arrow) capped by medium-grained massive sandstone bed. The bedset boundary is marked by a surface of non-deposition; staff in background is 1.5 m long.

9 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e Fig. 10. (A) Stacked lenticular sandstone beds the pinch out laterally by onlap (arrows). These sandstone beds are a mix of Ta and Tb beds with rip-up clasts scattered along bed boundaries; box outlines location of Fig. 10B; (B) Close-up view of lenticular sandstone in inset box. This bed is about 1.5 m thick with inclined beds (foresets) which are highlighted by mud-chip conglomerate (arrows) along bedding surfaces. Direction of foreset inclination is toward the northeast. This outcrop is located northeast of the curved beds shown in Fig. 13; location of outcrop is shown in Fig. 11. measured section) that punctuates development of the curved beds. Above this truncation surface, additional growth of the structure is recorded by convex-upward beds that shift the crest of the antiformal structure to the northeast (Fig. 15; left sideof photo) Lenticular sandstone beds Medium- to fine-grained sandstone beds that exhibit lenticular geometry (Figs. 10 and 19) are a distinctive element within the finegrained lithofacies. These types of beds form bedsets that usually have inclined bedding (Fig. 19), rip-up clasts along bedding surfaces, and consist of medium- to fine-grained, massive graded sandstone (Ta beds) and/or planar lamination (Tb). Within the Cerro Toro these lenticular units consistently exhibit inclined bedding that is oriented to the northeast, between 35 and 80. This orientation contrasts with the southeast orientation measured for current-ripple laminations (w175 ) within the fine-grained lithofacies. Typically, these lenticular beds are associated with the curved and wavy beds; note the relative position of lenticular beds (Fig. 10) and convex-upward beds (Fig. 13) shown in Fig. 11. These beds are usually located in swales, apparently downstream from the crest of convex-upward beds Stratal terminations Abrupt lateral terminations of sandstone and mudstone beds are a common element of the fine-grained lithofacies (Figs. 10, 13, 14, 16 and 20). These stratal terminations exhibit patterns of onlap/ downlap (Figs. 10, 16 and 20), downlap (Fig. 13) and truncation (Figs. 18 and 20). Fig. 20 highlights strata that terminate by onlap. Note in the blow-up photos (Fig. 20B and C) that beds onlap at slightly different stratal horizons rather than along one surface. One surface is difficult to identify at this location, but rather a number of surfaces are associated with development of the sediment bodies. Beds below the zone of onlap exhibit a gradual increase in topography from base to top of the exposed section (Fig. 20B). This increase in topography goes from relatively horizontal beds to beds that exhibit slight curvature. Beds below and above the zone of onlap are dominated by silt-rich turbidites, but sandstone beds and bedsets are more abundant above the zone of onlap.

10 770 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 Fig. 11. Panoramic view looking south along the west limb of the Silla Syncline showing the base of Channel Complex 3 Lower (CC3L) and Channel Complex 3 Upper (CC3U). Paleocurrents from CC3L run parallel to sub-parallel to this limb of the syncline, whereas CC3U obliquely cuts across the syncline (Beaubouef, 2004). The channel-stacking pattern changes between CC3L and CC3U from laterallyamalgamated channels to vertical aggradational stacking in CC3U. The location of the change in stacking is indicated byan arrow at base CC3U. At thisposition, mostof the section isinan interchannellocationand isprimarilythesiteof thin-beddedmudstoneandsandstone. About 500 msouthof thislocation (Fig. 6), sandstoneand gravels form vertically stacked channels. In the foreground, the beds are tabular and planar, whereas in the distance and stratigraphically higher, discontinuous beds are abundant (terminations marked with arrows) and curved to wavy beds are more abundant. White boxes outline figures cited in the text; field of view is about 1 km. The sandstone beds that exhibit a general onlap pattern in Fig. 20 were interpreted as part of a channel margin by both Beaubouef (2004) and Crane and Lowe (2008); however, onlap of the prominent sandstone bed in Fig. 20C resembles the pattern shown by the thick sandstone bedsets in Figs. 10 and 18, which also onlap topography not associated with an erosional surface. Truncation of beds below the onlap zone is unclear, whereas a prominent erosional surfaces might be expected in associated with a channel system. Stratal terminations from onlap and truncation are common in the fine-grained lithofacies. Surfaces associated with erosion (Fig. 20) are difficult to map in the mudstones that dominate the interchannel setting because of incomplete exposure. Local relief on these surfaces is at least 5 m, but the full relief on these surfaces may be greater than 5 m and obscured by cover. Note in Fig. 20 these surfaces are spaced at about 5e12 m in the section and locally are positioned above beds with a convex-upward pattern rather than in a swale. These erosion surfaces are less obvious along the east limb of the syncline, presumably because slumped beds are more common on the east side of the syncline and exposures are less than optimal. Many of the truncation surfaces, but not all, overlie relatively thick sandstone beds, such as in Fig Chaotic bedding Slumped beds to totally disrupted and chaotic bedding styles locally characterize mudstone and sandstone exposed on both the east and west limb of the Silla Syncline. The most dramatic disruption of bedding is displayed along the east limb of the syncline (Figs. 2 and 8) where one slumped zone affects an area that is about 2 km long and 600 m wide adjacent to CC3. The direction of slumping shown in Fig. 8B is toward the margin of CC3. Other slumped beds in the mudstone and sandstone lithofacies are present on the west limb of the syncline but do not affect large areas. 5. Discussion 5.1. Sediment transport and deposition Interpretation of the mode of sediment transport and deposition have been discussed for the coarse-grained lithofacies in the Cerro Toro (Beaubouef, 2004; Crane and Lowe, 2008), but the fine-grained lithofacies and architectural elements described here have been largely neglected. Normal grading observed in numerous beds and the presence of classical Bouma turbidites are important criteria for recognition of deposition

11 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e Fig. 12. Continuation of panoramic view in Fig. 11 looking southeast along the west limb of the Silla Syncline. The base of Channel Complex 3 (CC3) is marked and these rocks transition upwards into the fine-grained lithofacies in the Cerro Toro Formation. The position from channel-related deposition to overbank deposition interpreted by Beaubouef (2004) and Crane and Lowe (2008) is labeled (white arrow). The channel-margin facies contain lenticular beds with scattered gravels. Beds within the fine-grained lithofacies are tabular and laterally continuous over at least 400 m. In the background along the shore of Lago Mellizas, beds are curved to wavy with discontinuous beds most notable within swales (short white arrows). These beds terminate by onlap against topography that has a southeast trend; boxes outline the location of figures referenced in the text; field of view is approximately 800 m in the background. from turbidity currents (Middleton and Hampton, 1973). Deposition from decelerating turbidity currents explains much of the very fine sandstone, siltstone and mud-rich beds in the Cerro Toro, but this mode of deposition does not explain numerous erosion surfaces within the fine-grained lithofacies, local medium-grained sandstone with mud-chip conglomerate, and large curved to wavy bedsets present along the west side of the Silla Syncline. The curved, wavy beds pictured in Figs. 13 and 15 are largewavelength structures (>60 m) with relatively low amplitude of 1.5 m and 15 m respectively. The beds that make up these structures are characterized by turbidites; Ta, Tbc, and Tde beds are the typical Bouma (1962) turbidite associations that make up these structures. The development of these transitions from Ta to Tb beds, for example, is indicative of a decelerating flow with Tb development linked to a decreasing depositional rate that existed for Ta beds (Allen, 1991). Similarly, the deposition of Tc beds represents a deceleration of the current that existed during deposition of Tb beds. The curved structure in Fig. 13 is made up of bedsets that exhibit local thickeningupward patterns (Fig. 14A). These beds appear to be linked to construction by deposition from turbidity currents that decelerated and rapidly deposited massive sand. As topography increased, subsequent flows were affected such that some amount of scour and erosion occurred on the upstream side of the structure, whereas deposition occurred on downstream flank (Fig. 21). This mode of deposition is consistent with flow expansion as the turbidity current moved over the rough surface. Most of the deposition in this case is on the downslope part of the structure, whereas erosion takes place at the crest of the structure. The structure built up 0.5e1 m and then was mantled by muddy turbidites. Although, the outcrop has exposure limitations, the curved structure in Fig. 13 appears to have a symmetrical to slightly asymmetrical geometry with a gentle-dipping, long limb on the northeast side and a relatively steep-dipping side located to the southwest. Based on the direction that beds are inclined (Fig. 14A), the downslope side of the structure is interpreted to the northeast and upslope is to the southwest. This type of symmetrical to asymmetrical structure is similar to sediment waves described from deep-water systems by Migeon et al. (2000, 2001), and Wynn et al. (2000, 2002) among others Curved, wavy beds Thin-bedded muddy turbidites (Tde) and thin- to mediumbedded Ta and Tbc beds make up the curved to wavy beds shown in Figs. 15 and 16. Most of the sandstone beds, particularly beds thicker than 10 cm within these structures, are confined to swales between convex-upward beds displayed in Figs. 16 and 18. This pattern of sandstone distribution is similar to sand distribution located in swales between sediment waves associated with the Var Ridge (Migeon et al., 2001). In the model proposed by Migeon et al. (2001), turbidity currents flowing over sediment waves, erode at wave crest, accelerate downslope and deposit sediment as the flow decelerates near the upstream-facing slope of the next wave. This process results in preferential sand deposition on the upstreamfacing slope and less sand deposition at the crest and downstream

12 772 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 Fig. 13. (A) Exposure of curved beds in the fine-grained lithofacies that display a convex-upward pattern. Beds thin and pinch out toward the northeast. (B) Line drawing of bed geometry. The sandstone beds thicker than 10 cm are highlighted in gray. These sandstone beds thin and pinch out toward the northeast, whereas the beds are covered toward the southwest. Triangles depict stacked beds that form bedsets which are linked by similar lithology and terminate by an erosion surface or abrupt change in bed thickness; bar is 2 m and serves as a reference point between photo and drawing; boxes highlight close-up photos; see Figs. 11 and 12 for outcrop location. slope of the sediment wave. Sandstone beds in Figs. 15 and 16 exhibit pinch outs at different stratigraphic levels and different points along the crest of the curved structure, whereas mudstone beds drape the crest and swale of the structure. These sandstone beds do not fill an incision or channel into older sediments, so the pinch outs are interpreted as onlap points along pre-existing topography. The observation that the pinch outs occur at different points along the curved structure indicates growth of the structure concurrent with deposition. Examination of contacts between beds shows that many of sandstone beds in Figs. 17 and 18 have a sharp base and sharp top. Migeon et al. (2001) identified graded beds with sharp bases and tops as typical bed types in the Var inversely asymmetrical sediment waves. In the case described by Migeon et al. (2001), they also identified inversely graded beds which were not seen in the Cerro Toro. Migeon et al. (2001) estimate the turbidity current flow velocity for transport of sandy material with a grain size range between about 0.1 and 0.3 mm is 1.17e1.51 m/s in the Var levee system. This velocity is reached as turbidity currents move over sediment waves in the Var levee. The grain size of turbidite sandstone beds in the Cerro Toro (Fig. 17) is similar to the grain size of sandy turbidites from core in sediment waves from the Var, so a similar order of magnitude for flow velocity is envisioned for turbidity currents in the Cerro Toro overbank setting, at least for transportofsandsize material. This flow velocity is relatively high for deposition of mud but does explain the numerous erosion surfaces identified in this outcrop section (Figs. 18 and 20) as well as other sections in the Cerro Toro along the west limb of the syncline. Presumably, the mudstone that drapes or mantles some of these curved beds may be linked to decelerating turbidity currents which were derived from the upper part of stratified flows moving down the CC3 channels. Crane and Lowe (2008) proposed this kind of mechanism to explain much of the finegrained lithofacies in the Cerro Toro Lenticular beds Lenticular beds made of fine- to medium-grained sandstone are abundant in the Cerro Toro (Figs. 10 and 18e20). These types of beds become increasingly common above the CC3L on the west limb of the Silla Syncline. They appear to be present in the swales between curved to wavy beds in the Cerro Toro; hence there abundance is thought to be linked to an increase in topographic features in the overbank setting of the Cerro Toro. The structures that built the topography are interpreted as sediment waves. Most of these lenticular sandstone beds exhibit inclined beds or lamination that is directed to the northeast, about Flows that produced these features were of sufficient velocity to erode mudstone and move medium-grain sand (0.25 mm). Estimates by

13 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e Fig. 14. (A) Bedding patterns associated with curved bedsets in Fig. 13. Note beds (highlighted with dashed white lines) are inclined toward the left (northeast), the apparent flow direction, and thin toward the right (southwest). These beds thin because of toplap and truncation to the southwest. Beds labeled with a G are graded from fine-grained sandstone at the base to very fine-grained sandstone at the bed top. (B) Sandstone beds (at arrow) can be traced from the southwest until they thin and disappear toward the northeast, possibly by downlap. Note sharp, nonerosive base of the bed in the middle of the photo (marked with arrows). These beds are graded with fine-grained sandstone at the bed base and very fine-grained sandstone at the top which transitions into silt; see Fig. 13 for outcrop location. Migeon et al. (2001) for this range of grain size are about 1.5 m/s. Most of these lenticular sandstone beds exhibit traction features which appear to be linked to a migrating bedform. Crane and Lowe (2008) suggested dune migration to explain similar features. Deposition of these lenticular beds is believed to be caused by turbidity currents that began to decelerate at the change in gradient between the downslope side of a sediment wave and the upstream side of the next sediment wave. The relatively coarse grain size of sandstone in these lenticular beds is likely derived from energetic turbidity currents that spilled out of nearby channels, presumably CC Transition from tabular to curved beds The transition from tabular to curved beds above CC3L along the west limb of the Silla Syncline corresponds to a change in the style of sedimentation in the overbank or interchannel area. The tabular beds are interpreted here as the first stages of overbank deposition from CC3. At the scale of the outcrop on the west limb of the Silla Syncline, this deposition took place on a surface with little topography and aggraded as the channel complexes in CC3 filled and aggraded. Channels in CC3U exhibit much more vertical aggradation and preservation of mudstone between channels than those in CC3L. A comparison of the channel stacking in CC3U (Fig. 6) with the pattern in CC3L (Figs. 11 and 12) shows a change from laterally amalgamated channels in CC3L to vertically amalgamated channels in CC3U. This change in channel-stacking pattern is displayed in measured sections (compare Section with Sections 5e7 in Fig. 4). This change in channel stacking is concurrent with a number of stratigraphic changes in the fine-grained lithofacies including: 1) development of curved and wavy beds; 2) development of lenticular sandstone bedsets with inclined bedding (most displaying a northeast flow direction); and 3) the development of a number of erosion surfaces in the fine-grained lithofacies. These developments in stratal patterns indicate that the overbank or interchannel area in the fine-grained lithofacies was periodically the site of highvelocity, turbidity currents that could cause local erosion as well as transport medium-grained sand.

14 774 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 Fig. 15. View of curved wavy beds along the south shore of Laguna Mellizas. The beds exhibit several pinch outs along the margins of convex-upward structures (highlighted with white dotted arrow). Sandstone beds pinch out (white arrows) along the margins of the convex-upward structures indicating the presence of a structure at the time of deposition; pinch out at different stratigraphic levels indicate active growth of the structure during deposition. The crest of the convex beds shifts to the northeast (left) with deposition; line drawing highlights pinch out of relatively thick sandstone beds (shaded gray). Barton et al. (2007) referred to this area as part of an inner levee; box outlines location of Fig. 16; location of this outcrop is shown in Fig. 12; field of view is approximately 200 m. Fig. 16. Curved, convex-upward bedding forming an antiformal sedimentary structure. A surface with minor erosion is highlighted by a dashed white line at about 11 m above the water level. Below this erosion surface, sandstone beds (labeled A and B ) pinch out at different stratigraphic positions along the margin of the curved beds indicating growth of the structure occurred between deposition of the sandstone beds. The crest of the curved beds shifts to the northeast (left) with deposition; location offigure shown in Fig. 15; field of view is approximately 75 m.

15 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e Fig. 17. Measured section through lower portion of curved beds in Figs. 16 and 18. The sandstone body in the lower 1.5 m of section exhibits wavy lamination (Tw) massive beds, planar lamination (Tb), contorted lamination (C) and current-ripple lamination (Tc). This sandstone beds laps out about 5 m laterally (northeast) from this section location. The medium-bedded sandstone bed at 9 m in the section, pinches out laterally by a combination of onlap and truncation across the top of the bed. An erosion surface is present at 5.8 m in the section and is manifest by a slight change in dip across the surface. An additional erosion surface is located at about 10.2 m in the section and extends across much of the field of view in Figs. 16 and Modern systems and modeling 6.1. Quaternary systems Studies of Quaternary deep-water settings (Normark et al., 1980, 2002; Savoye et al., 1993; Migeon et al., 2000, 2001; Lee et al., 2002; Wynn et al., 2002; Posamentier, 2003; and Deptuck et al., 2003, 2007) have provided seismic profiles, map patterns, and core data that are useful for interpretation of ancient deep-water systems. The architecture of different elements in channel-levee systems (Migeon et al., 2000, 2001; and Deptuck et al., 2003, 2007) iswell displayed and can be related to stratal architecture observed in the Cerro Toro. Savoye et al. (1993) and Migeon et al. (2001) described sediment waves built on the Var Sedimentary Ridge located in the Mediterranean Sea offshore from the coast of Nice, France (Table 1; Fig. 22). These sediment waves occur on the backside of a welldeveloped levee associated with the Var channel (Fig. 22). Highresolution seismic lines over these sediment waves are shown in Figs. 23 and 24. Migeon et al. (2001) describe three types of sediment waves on the Var; asymmetric with smooth downstream flanks and short, steep upstream flanks, symmetrical which are smooth and exhibit continuous seismic reflectors across the sediment wave, and inverse asymmetric waves with smooth upstream flanks and short, steep downstream flanks. Sedimentation is slightly different in the three types of sediment waves; the asymmetric waves exhibit higher upslope growth rate resulting in continuous, parallel reflections on seismic lines, whereas sedimentation is low

16 776 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 Fig. 18. Lenticular sandstone beds along the margin of the curved to wavy beds shown in Fig. 16. These beds include different turbidite lithofacies including massive, graded layers (Ta), planar (Tb) and current-ripple (Tc) laminations. Sandstone beds pinch out (black arrows) by onlap and by truncation/toplap (see inset). Many of the sandstone beds and bedsets have a sharp base and top; not a gradation into siltstone or mudstone. The sandstone at the base of section is 1.5 m thick. Select beds are highlighted with dashed white line to emphasize layering and discontinuities in the section. The location of the stratigraphic section shown in Fig. 17 is marked; outcrop located next to Laguna Mellizas. on the downslope flank resulting in local erosion or downlap of stratal layers (Migeon et al., 2001). The asymmetric waves contain the most sandstone of the three types of sediment waves. Weak acoustic penetration on the upstream part of asymmetric waves indicates that they are likely sand prone, whereas the downstream flank is likely mud prone (Migeon et al., 2001). Based on core data (see Fig. 24 for core locations), sand content decreases from 60 to 35% between the upstream and downstream flanks (Migeon et al., 2001). The asymmetry of the wave results from a major asymmetry of deposits between the upstream and downstream flanks. Only a few seismic reflections can be followed from one flank to the other. Truncated seismic reflections observed near the crest and on the downstream flank indicate strong erosion, whereas deposition seems to occur mainly on the Fig. 19. (A) Lenticular sandstone beds with local scour and erosion at the base of the bedset. These beds are internally massive and locally exhibit planar, inclined lamination. Rip-up clasts of mudstone are common in these beds and commonly are scattered along bed boundaries. Direction inclined bedded or foresets is toward the northeast; location of outcrop is shown in Fig. 11.

17 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e Fig. 20. (A) Exposure of lenticular sandstone beds that pinch out by onlap against topography and by erosional truncation. Onlap points are marked by arrows and labeled with an O, erosional edges are marked with arrows and an E. Surfaces with onlap do not display erosion. (B) Blow up of outcrop in box shows that beds are curved and sub-parallel to the onlapping strata above the large vertical arrow and that beds do not onlap one surface, but a number of beds form a composite onlap pattern; prominent bed (arrow) is about 1 m thick; (C) Blow up of sandstone bed pinch out showing abrupt termination. Lamination within this sandstone bed is inclined to the left (black dashed line), north to northeast, a pattern similar to other lenticular sandstone beds examined in the fine-grained lithofacies within the Cerro Toro; note bed truncation (white arrow) above the sandstone pinch out. upstream flank. Symmetrical sediment waves with relatively continuous layers on seismic sections are interpreted as the deposit from vertical aggradation (Migeon et al., 2001) and are mud prone. The inverse asymmetric waves described by Migeon et al. (2001) are thought to represent downslope growth and deposition. Draping sediment predominates with local erosion and truncation of seismic reflections on the downslope flank of the sediment wave. Based on core data, sand content is relatively low (<35%) in these sediment waves and increases slightly in the downstream part of the inverse asymmetric sediment wave (see Migeon et al., 2001; Fig. 13). Internally, stratal patterns observed on seismic lines over sediment waves from the Var include onlap and downlap patterns developed from migration and aggradation of the sediment wave (Figs. 23 and 24). Additionally, truncation surfaces are developed in the sediment waves at various stratal levels (Figs. 23 and 24). These erosion surfaces are common in the sediment waves, and these surfaces are thought to be developed through erosion by highenergy, turbidity currents that move over the levee surface and vary in deposition versus erosion as they move over the wave field (Migeon et al., 2001). Commonly, erosion is observed at the crest and on the downslope side of the waves (Migeon et al., 2001). Such

18 778 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 Fig. 21. Schematic drawing depicting geometry of local thickening- and coarsening-upward bedsets (vertical arrow) in the fine-grained facies of the Cerro Toro Formation. Direction of flow based on bed inclination is to the left. These bedsets appear to build topography that affects deposition and erosion. The upslope zone exhibits erosion or non-deposition, whereas Ta and Tde beds are deposited on the downslope side of the structure. a high-energy event was documented after a submarine landslide in the Var canyon and collapse of a segment of the Nice Airport in 1979 (Savoye et al., 1993). Sediment fed into the Var canyon generated a thick, high-velocity sediment gravity flow that moved downslope and spilled out of the fan valley and over the levee surface breaking a telephone cable on the levee (see Fig. 22 for location of cable break). Velocity estimates of turbidity currents moving over the sediment wave field were between 5 and 6 m/s by Migeon et al. (2001) and slightly higher, 9.8 m/s, by Savoye et al. (1993). Cores recovered from the Var Sedimentary Ridge provide information on the turbidite lithofacies distribution in sediment waves (Migeon et al., 2000, 2001). These cores have shallow penetration and record a small part of the levee history. However, the cores penetrated sand beds, particularly on the upstream side of the sediment waves, where medium-grained sand was recovered with bed thickness up to 40 cm (Migeon et al., 2001). Lithofacies in the cored sediments include turbidites with massive sands (Ta), planarlaminated sands (Tb), and current-ripple laminated sands as well as muddy turbidites, particular Te beds. The sand content declines in the cores toward the downstream part of the sediment wave from about 60 to 30% sand. These percentages record only the uppermost few meters of the sediment wave, so the sand percent may vary over different thickness intervals through one of the sediment waves. The Var channel is floored with pebbles and cobbles that locally form gravel dunes (personal communication B. Savoye, 2007). The grain size in the system is similar to the grain size within the CC3 (Paine C) described by Beaubouef (2004) and Crane and Lowe (2008). In contrast, cored sediment waves contain abundant fineto medium-grained sand beds, similar in grain size to the Cerro Toro fine-grained lithofacies, but in general, more sand is present in the Var than is observed in the Cerro Toro fine-grained lithofacies. Descriptions of sediment waves from areas other than levee settings are common (see Wynn et al., 2000; 2002; Lee et al., 2002). Wynn et al. (2000) described sediment waves from the continental slope and rise in an unchannelized, open slope area. They interpreted the sediment waves as formed by unconfined turbidity currents that transported mostly mud with some volcaniclastic material rather than bottom currents because of the large number of turbidite beds in core, wave crest orientation, and lack of bottom current sedimentation. The waves they described had characteristics similar to sediment waves in levee systems (Wynn et al., 2000). Inspection of seismic lines from Wynn et al. (2000), a mud-prone system and Migeon et al. (2000), a mud and sand system, indicate greater reflection continuity in the mud-prone system that Wynn et al. (2000) examined than the Var levee studied by Migeon et al. (2000). Possibly, more erosion and less reflection continuity are likely in a levee system than in a continental rise setting Numerical modeling Numerical modeling by Lee et al. (2002) has several implications for the stratigraphy of sediment waves. Different depositional rates or seafloor erosion likely exist for different grain size fractions from the suspended load of a turbidity current breaching its canyon or levee walls and flowing down a series of numerical steps (sediment waves). Their modeling suggests areas of seafloor erosion are located on the downslope side of waves and deposition should be focused on the flats or upslope side of waves. It is challenging to link their observations to the Cerro Toro, but it is clear that deposition of the coarsest-material in the overbank setting is located in the swales of curved, wavy beds. Preferential erosion on the flanks of structures is unclear, but erosion near the crest of convexupward beds exists at several stratigraphic levels in the Cerro Toro fine-grained lithofacies (Fig. 20). Another observation from the numerical experimentation by Lee et al. (2002) is the importance of seafloor roughness in influencing the drag on the overriding turbidity currents as manifested by its effective drag coefficient. They suggest that the initial seafloor roughness is important if turbidity currents are able to generate a field of sediment waves. In the Cerro Toro, initial overbank sedimentation is characterized by beds that exhibit relatively high lateral continuity, and the interpretation here is that these beds Table 1 Summary table of physical characteristics of the Var Channel and Var Sedimentary Ridge (statistics from Savoye et al., 1993; Migeon et al., 2000, 2001). Location Western Mediterranean Sea; offshore from Nice, France Shelf characteristics High relief, onshore setting with a4e5 km wide shelf Input sediment size Cobble- to clay-sized detritus Water depth over levee 2000e2600 m Upper Var Channel gradient >11e8% Middle fan valley gradient 4e<1% Levee gradient 3e4% Levee height e north side Up to 50 m Levee height e south side Up to 400 m (range of 30e400 m) Sediment wave field 1500 km 2 Sediment wave amplitude 5e60 m (average is 20 m) Sediment wave wavelengths 0.6e7 km (average is 2.2 km) Notable events A submarine slide and collapse of a portion of the tarmac from the Nice airport in 1979 and cable break offshore on the southern levee flank. Estimated range of turbidity 3.70e9.8 m/s current velocities over sediment wave field

19 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e Fig. 22. Location map of a segment of the Var Channel and sediment wave crests (dark black lines), seismic lines, cable break, and core distribution discussed in text. Measured wavelength of different sediment waves measured is around 1e3 km, and the length along the crest is measured in 7e20 km; modified from Migeon et al. (2001). were deposited on a relatively featureless surface. However, deposition of sandstone turbidite beds on this surface began to build topography as displayed by the transition from relatively horizontal, flat beds to curved, convex-upward beds in Fig. 13 and by the thickening-upward bedset in Fig. 14A. The development of this kind of bed roughness may have contributed to the formation of sediment waves, local erosion surfaces and the localized deposition of lenticular beds in the Cerro Toro Sediment waves in the Cerro Toro Fig. 23. Segment of seismic profile V11 illustrating architecture of sediment wave on the Var Ridge. Arrows indicate stratal terminations. Highlighted surfaces are defined by onlap above the surface and truncation or toplap below the surface; vertical scale is in milliseconds (125 mse100 m); approximate length of seismic line is 3 km; for location of line see Savoye et al. (1993) and Migeon et al. (2001). Large-wavelength structures (>60 m) within the fine-grained lithofacies of the Cerro Toro are interpreted as sediment waves that formed on the backside of a levee. Key elements leading to this interpretation include: 1) Convex-upwards structures (Fig. 13). These structures are relatively large wavelength and low amplitude which internally display growth to the northeast (Fig. 15); 2) Curved, wavy growth structures (Figs. 15 and 16) that display development of convex-upward beds. Beds onlap these structures along different stratal surfaces indicating growth during deposition; 3) Erosion surfaces along the crest and backside of structures (Figs. 18 and 20); and 4) Deposition of sandstone-rich turbidites as lenticular beds located within swales. Paleocurrents measured from inclined beds in these lenticular units (Figs. 10 and 19) exhibit a northeast orientation which is at a high angle to paleocurrents measured in CC3 by both Beaubouef (2004) and Crane and Lowe (2008); and 5) Grain size of lenticular beds is medium-grained sandstone (0.25 mm); the relatively coarse grain size of these lenticular beds is anomalous to most of the fine-grained lithofacies in the Cerro Toro. This kind of grain size has a common association with sediment waves in levee settings, whereas sediment waves in continental rise or unchannelized slopes

20 780 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 Fig. 24. Segment of seismic reflection profile NIC 34 showing seismic response of a large sediment wave and coring location points from the Var Sedimentary Ridge. Prevailing flow direction is to the southeast (large arrow). Apparent growth of the sediment wave is to the northwest or up paleoslope toward the Var channel. The sand-prone part of the system, based on core data, is at core location KN127; location of seismic line shown in Fig. 22; resolution is about 2e3 m; modified from Migeon et al. (2001). typically are dominated by silt and pelagic material (see summaries in Normark et al., 1980; Wynn et al., 2002; and Lee et al., 2002) Barton et al. (2007) indicate a northerly direction of growth for sediment waves located below the Nordenskjold member of the Cerro Toro, and Crane and Lowe (2008) measured a northeast direction for a sediment wave or dune. All of the thick sandstone bedsets that display inclined bedding along the west limb of the Silla Syncline, exhibit paleocurrents consistent with a northeast flow direction. This north and northeast direction of transport most likely represents paleocurrents at a high angle to the nearby channels in the CC3 rather than parallel or toward the channel setting. In contrast, Crane and Lowe (2008) measured a number of current-ripple laminations in the fine-grained lithofacies described here. Their results show flow directions for current ripples to the southeast or about a 90 difference with the lenticular sandstone beds. A possibility exists that paleocurrents based on current ripples represent the regional slope and flows that mantled the sediment waves. If sediment waves covered the overbank area, then flow path(s) over this surface are likely to be complicated; some turbidity current flows are likely to follow the swale between sediment waves, whereas the most energetic flows are likely to flow directly over sediment wave crests, likely away from the channels. Sandstone content in structures interpreted as sediment waves is less than 35% (Fig. 17) in the Cerro Toro. Measured sections through the fine-grained lithofacies near Laguna Mellizas, 300 m south of the Park Highway and about 1.5 km north of the Park Highway indicate a relatively low sandstone content in all sections consistent with observations by Beaubouef (2004) and Crane and Lowe (2008) for the fine-grained lithofacies. Given the low sandstone content, sediment waves in the Cerro Toro are most similar to the symmetrical and inverse asymmetric sediment waves; however, sandstone beds appear to pile up on the upstream side of the structure in Fig. 15, which is consistent with the asymmetric sediment waves described by Migeon et al. (2001). The low sandstone content is probably caused by infrequent, high-energy flows spilling out of CC3 channels onto the levee. Sediment waves have been interpreted for part of the Cerro Toro in the past, most notably Barton et al. (2007) and Crane and Lowe (2008); however, structures interpreted as sediment waves by Barton et al. and Crane and Lowe are smaller than the structures identified here. Beaubouef (2004) identified the antiformal structure in Fig. 15 and interpreted this feature as part of the overbank levee. The number and extent of sediment waves is believed to be abundant around the Silla Syncline. The wavelength of sediment waves may vary from a few hundred meters to several kilometers and the amplitudes exhibit a range from a few meters to 20 m or more (see summaries in Normark et al., 1980; and Wynn et al., 2002). Given that the width of the Silla Syncline is 4 km, lowwavelength sediment waves have the best chance of recognition in this setting, whereas the long-wavelength structures could be larger than the syncline. The structure in Fig. 15 has a wavelength of about 200 m which is on the low side for sediment waves described in the literature. The amplitude of this structure is at least 15 m, which is not an uncommon value, but the full structure is not exposed. Flow direction for this structure, based on inclined bedding in the lenticular beds on the southwest flank is toward the northeast. Given the shape of the antiformal feature and general sandstone content (w35%), this structure is most like the symmetrical sediment waves described by Migeon et al. (2001) 7. Depositional summary Deposition of Cerro Toro mudstone, sandstone and conglomerate was initiated in the Silla Syncline region prior to deposition of rocks associated with CC3 (Paine C member). Older rocks such as the Pehoe Member have been interpreted as the deposits of turbidity currents that are associated with channel and overbank deposition (see Bernhard et al., 2011). The depositional setting for CC3 (Paine C) and the Laguna Negra debrite was in a slope setting which fed into the main Cerro Toro gravel-rich channels to the east of the Silla Syncline. The initial phase of deposition was marked by a debris flow, the Laguna Negra debrite which was directed toward the south (Crane and Lowe, 2008). It is unclear what controlled the distribution of the debrite; no clear channel confinement is recognized. Possibly, some structural control forming a structural low played a role (Crane and Lowe, 2008). Subsequent channel fills in CC3 are well documented by Winn and Dott (1979), Beaubouef (2004) and Crane and Lowe (2008). Thick, high-density turbidity currents transported coarse-grained gravel material through the channel system that existed during CC3 time. These flows were likely stratified with the upper, low-density part of the flows overflowing the channel and spreading out onto the overbank setting. These stratified flows would have deposited two lithofacies; the coarse-grained lithofacies, consisting of conglomerate and sandstone with minor mudstone, were deposited in confined channels, and the fine-grained lithofacies, mudstone and sandstone, were deposited in the overbank setting. At least periodically flows were of sufficient turbulence and velocity to move mediumgrained sand out of the channel. Once out of the channel, this sand

21 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e could be transported via turbidity currents over the sediment wave field with local deposition in swales between sediment waves Deposition of fine-grained rocks Channel complex set 3 exhibits an evolution in its history from a south-flowing depositional unit in the lower part of the system, CC3L, to a southeast-flowing system in the upper part of the unit, CC3U. Additionally, the systems change from a laterally amalgamated channel complex in CC3L to a vertical aggradational system in CC3U (note change in stacking arrangement in Fig. 4). Associated with this channel aggradation is deposition of a thick mudstonerich unit that persists from just north of the channel-margin area near Lago Sarmiento Chico to Lago Nordenskjold, a distance of about 5 km. Presumably, the fine-grained interval near Lago Nordenskjold was more distant from the channel because sandstone percent is less in this area than in the vicinity south of the National Park Highway and Lago Mellizas. Initially, turbidity currents moving out of channel would have spread out over a relatively featureless overbank depositing tabular, horizontal beds linked to channels in CC3L. This overbank setting clearly developed topography that is expressed in a number of outcrops by sediment waves with broad, curved geometries (Figs. 13, 15 and 16; also see Barton et al., 2007). This vertical change in bed architecture from tabular, horizontal beds to wavy, curved stratal geometry along the west limb of the syncline is inferred to correspond to growth of a levee complex associated with CC3U. As overbank deposition developed in the Cerro Toro, topographic relief between channel and overbank likely increased. With an increase in height, less sand would have escaped out of the channel to spill onto the levee, an observation consistent with the vertical decrease in sand content above the channel facies observed in Figs. 11 and 12. Low-density turbidity currents spilling out of the channel would have followed the prevailing slope, which was toward the south to southeast (Scott, 1966; Crane and Lowe, 2008). Presumably, turbidity currents flowing out of the channel upslope from the Silla Syncline would follow local gradient as well, toward the south to southeast. With sinuous channel geometry, flow direction on the overbank would parallel channel paleoflow, and locally, flow direction might trend toward or away from the channel. This kind of geometry is set up in the Var system where the lower Var channel deflects to the south because of salt diapirs along the eastern part of the basin (see Savoye et al., 1993). Additionally, topography on the overbank likely deflects low-energy turbidity currents such that a variety of flow directions might be measured. Although a different deep-water system, turbidity currents flowing out of the Var channel travel southeast over the levee surface (Fig. 22) toward the lower Var channel resulting in flow directions that merge downslope. The orientation of the channel relative to regional slope and slope on the levee is key to reconciling paleocurrent measurements. The evolution of CC3 from laterally amalgamated to aggradational channel stacking is similar to the pattern displayed in the Benin-Major channel described by Deptuck et al. (2003, 2007). This channel exhibits distinct vertical aggradation and coeval growth of a levee system. In the Benin system, an inner and outer levee exists. The outer levee is older than most of the aggradational channel fill (see summary slide by Deptuck et al., 2007, their Fig. 9) The possibility exists that both an inner levee and outer levee system exists for the Cerro Toro (See Barton et al., 2007), but this is difficult to test with the exposures in the syncline. Sediment waves are associated with the Benin Major and are most prevalent near the outer bend of channels (see Deptuck et al., 2007, their Fig. 2). The numerous erosional surfaces in the fine-grained facies of the Cerro Toro, supports the idea that periodically turbulent flows moved over the overbank area with enough energy to erode mudstone and move medium-grained sand into sediment waves. Generation of these high-energy flows would seem easiest through a linkage to flows moving through active, coeval channels, which could have been located several kilometers away. Such flows overflowed the Var channel and maintain enough energy to break a telephone cable located 15 km down the slope from the main channel (Migeon et al., 2001) Sequence of events for levee construction Fig. 25 is a summary diagram that shows the general stacking of strata in CC3 as well as the Laguna Negra debrite. Other summary diagrams are shown in Beaubouef (2004) and Crane and Lowe (2008) for channel complex set 3 (Paine C). The key difference here is the linkage with sediment wave development and vertical aggradation of channel complex 3 upper. Key events in the depositional history include: 1. Debrite deposition e The Laguna Negra debrite moved downslope from north to south. It is difficult to identify any confining channel; relief on the base of the debrite is minimal; however, the relief may increase in an eastewest or depositional-strike view. The debrite thins and pinches out to the east. 2. Deposition of CC3L. This channel complex flowed to the south to southeast. The channel is offset to the west from the Laguna Negra debrite in the north near Lago Nordenskjold, but overlies the debrite south of the park highway and trends to the southeast in the southern part of the syncline. The debrite may influence the location of CC3L. Potentially, the debrite created relief on the seafloor that directed turbidity currents toward topographic low points. 3. Concurrently with, or more likely before, deposition of CC3U gravels, construction of levees began. Aggradation of the channel complex associated with CC3U signals a change in the depositional system. Construction of confining levees along the northeastern and eastern side of the system seams likely near L. Mellizas and L. Nordenskjold. 4. Deposition of CC3U. This channel complex has a slightly different orientation than CCL and cuts across the Silla Syncline from northwest to southeast. The east side of the syncline is likely the site of an outer bank of a sinuous CC3U channel complex. Coarse-grained rocks are located in close proximity to the erosional cut. Plugging of this channel complex with gravel and sand, marked the gradual shut down of the system until rejuvenation with deposition of Channel Complex Set 4, the Nordenskjold member of Crane and Lowe (2008) 7.3. Reservoir implications of levee system Locally, levee systems have been developed as hydrocarbon reservoirs (Rossen and Sickafoose, 1994), but in general these lownet-to-gross units have not been pursued as reservoirs. The potential for reservoir sands in an overbank system is variable. The volume of sand and continuity of sandstone beds displays a change as the levee system evolves. Based on outcrops in the Cerro Toro, the first overbank beds appear to spill out of the channel and have potential for lateral continuity. The abundance of these beds also appears to be greater early in the development of the overbank than later as the levee develops and constructs high walls that are likely to confine sand within the channel. In terms of predictability, commonly, channel-levee systems exhibit a slight coarsening down channel. This kind of relationship has been documented by Droz et al. (2003) in the Zaire, by Manley et al. (1997) in the Amazon and by Migeon et al. (2000) in the Var. In large part this relationship is

22 782 K.M. Campion et al. / Marine and Petroleum Geology 28 (2011) 761e784 Fig. 25. Summary diagram showing the general stacking arrangement of strata in channel complex set 3 or the Paine C member. Debrite deposition marked the initial phase of coarse-grained sediment moving into the Silla Syncline. Subsequent deposition was marked by channel fills that contained pebble and cobble conglomerate. Note transition from axial gravels to sand and mud toward the margin of channels. Overbank material associated with CC3L has tabular beds. In contrast, overbank material with the upper part of CC3U has lenticular sand bodies and well-developed sediment waves. Note change in channel-stacking pattern from lateral migration in CC3L to vertical aggradation in CC3U. a consequence of progradation of the levee system and vertical construction. Sediment waves are observed in most studies of deep-water turbidite systems in basin floor, toe of slope and slope settings, and are a typical architectural element of the levees (see summaries in Normark et al., 1980; Wynn et al, 2002; and Droz et al., 2003). Thick sandstone beds (1e1.5 m) associated with sediment waves such as those in the Cerro Toro fine-grained lithofacies could be misleading if they were penetrated in a well bore. In outcrop these beds appear to have limited lateral continuity because they occupy the trough between sediment waves. The extent of these sandstone beds parallel to the wave crest is likely greater than perpendicular to wave crest; however, data on bed extent parallel to wave crest is lacking. Migeon et al. (2001) infers from seismic response that sand trends along the sediment wave front may be quite continuous, a relationship that could not be confirmed in this study. Thin-bedded sandstone beds associated with the levee are deposited from decelerating flows along sediment waves (covering upslope, downslope and crest) associated with various types of sediment waves (asymmetrical, symmetrical and inverse asymmetric sediment waves). These sandstone beds are generally thin, 10 cm or less, and appear to terminate via downlap and toplap over relatively short distances (10 s of meters) in both a downslope and upslope direction. The extent of these beds parallel to the sediment wave crest may be extensive based on continuity of seismic reflections in these kinds of waves described by Migeon et al. (2001). Recognition of levee systems utilizing seismic data is relatively straightforward (Deptuck et al., 2007). However, recognition of sediment waves in the subsurface using seismic would be difficult because of the low amplitude of the sediment waves (w10 m) and conventional seismic data is relatively low frequency (25e30 Hz) which means resolving beds that are less than 20 m thick is a challenge. However, utilization of log and core data along with seismic could help the interpreter recognize or infer the existence of sediment waves. Recognition criteria for the thin-bedded rocks that make up sediment waves in core include: 1) They are made up of a mix of sandstone turbidites including Ta, Tb, and Tc beds and muddy turbidites, Tde beds; bioturbation is localized and sparse to absent in sandstone beds. Sandy or muddy debrites were not recognized in any of the outcrops examined, but thin slurry beds (Lowe and Guy, 2000) or linked-debrite beds (Talling et al., 2004) could be deposited in this setting. Migeon et al. (2001) interpreted various beds as debrites that were cored in the Var. Pelagic sediments are relatively sparse in the Cerro Toro, but in high-relief levee systems where only clayed material is stripped from turbidity currents moving through the channel, the percentage of pelagic material may be high. If the core contains abundant pelagic material along with abundant bioturbation, the section is likely associated with a levee system that has shut down or is part of an unchannelized system; 2) interpreted sediment waves in the Cerro Toro consist of sandstone and mudstone turbidite beds that form two types of bedsets. The first type is 1e2 m thick and typically exhibits a thickening and slight coarsening-upward trend (Fig. 17). The second type of bedsets are aggradational, form a drape facies that is about 2 m thick, and is made of muddy turbidites with local bioturbation; no thickening- or thinning-upward trends were observed in this type of bedset (Fig. 8B). Erosional surfaces are relatively abundant and may punctuate the stacking patterns; 3) Bedset boundaries are relatively sharp at both the top and base. Gradual changes in lithofacies between bedsets with sandstone and those dominated with mudstone are uncommon. Wynn et al. (2000) suggested the use of dipmeter logs to discriminate sediment waves. Although the dip angles associated with these structures are relatively low (5 or less), this method may be enhanced through use of a combination of dipmeter with core, particularly if bedset packaging could be recognized in the core. Most of the sandstone beds identified in the levee system of the Cerro Toro are not well connected vertically, which raises potential issues with migration as well as production from these kinds of reservoirs. Silt- and clay-rich turbidites drape or mantle large parts of the sediment waves which potentially isolate the sandstone beds, and most of the sandstone is very fine- to fine-grained and locally medium-grained in the levee setting. These fine grain sizes may have reasonable porosity, but can experience relatively rapid permeability decline with compaction and any cementation. Production rates from this kind of reservoir would likely not be high, but rates may be sustainable for a relatively long time. Presumably, these reservoirs would make better gas reservoirs than oil reservoirs.