deep burrows in submarine fan-channel deposits of ...

PALAIOS, 2008, v. 23, p. 223–232 Research Article DOI: 10.2110/palo.2006.p06-127r

DEEP BURROWS IN SUBMARINE FAN-CHANNEL DEPOSITS OF THE CERRO TORO FORMATION (CRETACEOUS), CHILEAN PATAGONIA: IMPLICATIONS FOR FIRMGROUND DEVELOPMENT AND COLONIZATION IN THE DEEP SEA STEPHEN M. HUBBARD1* and MICHAEL R. SHULTZ2 1

University of Calgary, Department of Geoscience, Calgary, Alberta, T2N 1N4, Canada; 2 Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, California, 94583-2324, USA e-mail: [email protected]

ABSTRACT The Glossifungites ichnofacies recognized in Cretaceous strata (Cerro Toro Formation) of the Magallanes foreland basin in southern Chile represents an important discovery in that it extends the stratigraphic utility of firmground trace-fossil suites into thick-bedded, gravityflow deposits of submarine fan-channel environments. The tracefossil suite consists of atypically large Diplocraterion, Skolithos, and Arenicolites, which may reach an inferred length of 7 m. The burrows penetrate muddy, matrix-supported conglomeratic deposits dewatered and consolidated as a result of burial and subsequently exhumed by erosive turbidity currents. In a stratigraphic succession dominated by coarse-grained facies ⬎350 m thick, the burrows are abundant at one stratigraphic horizon correlatable up to 200 km2. This horizon is interpreted as a stratigraphic discontinuity associated with a long-term cessation of coarse-grained, sediment-laden gravity flows into the basin. The colonized surface is the only marker horizon traceable across much of the Magallanes basin study area. INTRODUCTION The objectives of this paper are to (1) document an outcrop example of a deep-water firmground surface characterized by extraordinary trace fossils up to an inferred 7 m in length, (2) review possible mechanisms for firmground development in the deep sea, (3) discuss the potential organisms responsible for the trace fossils, and (4) speculate on the stratigraphic significance of the firm-ground surface. The Glossifungites ichnofacies is a media- (substrate-) controlled tracefossil assemblage, consisting of structures excavated in firm, semiconsolidated sediments (Seilacher, 1964; Frey and Seilacher, 1980). Associated trace fossils are typically robust, unlined with sharp walls, passively filled, vertical to subvertical, and commonly show evidence of in situ organism growth. The Glossifungites ichnofacies is often associated with widespread discontinuities in the rock record (Glossifungites surfaces), interpreted to represent time gaps between the deposition of units and the subsequent colonization by firmground burrowers (Savrda, 1991; MacEachern et al., 1992). This process involves burial of soft sediment, subsequent dewatering, and finally exhumation, resulting in exposure of consolidated sediment (Pemberton and Frey, 1985). As with any significant stratigraphic surface, the utility of a Glossifungites surface as a marker horizon within a stratigraphic package is dependant on its regional extent (Pemberton and MacEachern, 1995; Gingras et al., 2000). Firmground burrows have been well studied in ancient and modern marginal-marine deposits (e.g., Pemberton and Frey, 1985; Savrda, 1991; MacEachern et al., 1992; Gingras et al., 2000, 2001). They have been rarely documented from deep-water (bathyal) successions, however. As a result, the stratigraphic utility of trace fossils associated with the Glossifungites ichnofacies in deep-water environments is not well established.

Hayward (1976) observed a firmground trace-fossil assemblage formed on the wall of an incised submarine canyon that was subsequently buried by coarse turbiditic sand. Anderson et al. (2006) documented a relatively local occurrence of the Glossifungites ichnofacies from the floor of a submarine canyon channel, formed through the exhumation of a firm siltstone layer by erosive turbidity currents. Bromley and Allouc (1992) described borings in modern bathyal hardgrounds formed through surficial lithification of carbonate oozes. Erosion associated with bottomcurrent winnowing during rapid transgression—a period of sediment starvation—provided the mechanism for widespread firmground formation in the Tertiary of the New Jersey slope (Savrda et al., 2001). MacEachern and Burton (2000) assigned an unusual suite of trace fossils from the Viking Formation of Alberta to the Glossifungites ichnofacies. They interpreted the firmground surface to have been excavated above fair-weather wave base and subsequently colonized below wave base following a rapid transgression of the sea. Despite these examples, mechanisms for the generation of regionally extensive firmground surfaces in the deep sea are not well understood, particularly those associated with siliciclastic submarine fan-channel successions. GEOLOGIC SETTING

* Corresponding author.

The study area is located in the Ultima Esperanza District of southern Chile (Figs. 1A–B). The interval studied consists of the Cerro Toro Formation (Santonian–Campanian), part of the overall, upward-shallowing fill of the Magallanes foreland basin (Fig. 2). Studies of benthic foraminifera suggest that the paleobathymetry of the basin was between 1000 and 2000 m during deposition of the strata studied (Natland et al., 1974). Gravity-flow deposits of the Cerro Toro Formation accumulated in a North-South–trending deep-water channel belt present along the axis of the Magallanes basin foredeep (basin-floor ⬃10–20 km in width and ⬎100 km long; see Figs. 1A, C; Hubbard et al., 2008). The formation (⬎2 km thick) includes a thick interval (300–400 m or more) of coarse, conglomeratic fan-channel material encased in bathyal mudstone (Katz, 1963; Scott, 1966). Winn and Dott (1979) interpreted a southward prograding deep-sea fan system fed from a canyon to the north, based on extensive paleocurrent data from the coarse-grained interval. Evidence of turbidity current, traction, and debris-flow processes is present within the conglomeratic strata studied. Trace fossils are rare in conglomerate-dominated facies deposited in the channel axis despite favorable environmental conditions for benthic communities in adjacent areas where overbank flows deposited finegrained sediment. It is probable that gravelly sediments and frequent, immense gravity-flow events hindered infaunal colonization in the channel-floor environment. Overbank units are characterized by a moderate abundance and high diversity of trace fossils, including Alcyonidiopsis, Arenicolites, Chondrites, Gyrolithes, Helminthoida, Helminthopsis, Ophiomorpha, Palaeophycus, Phycosiphon, Planolites, Rosselia, Scolicia, Skolithos, Spirophyton, Spongeliomorpha, Thalassinoides, Zoophycos, radiating burrows on turbidite soles, and various crawling trails

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FIGURE 2—Generalized stratigraphy of the Magallanes basin (adapted from Natland et al., 1974; Wilson, 1991; Fildani et al., 2003; Shultz and Hubbard, 2005).

FIGURE 1—Overview of the Patagonian study area. A) Landsat image of Sierra del Toro and Cordillera Manuel Senõret, Ultima Esperanza District, southern Chile. Conglomerate of the Cerro Toro Formation caps snow-covered peaks (channel outline from Hubbard et al., 2008). Numbers ⫽ measured section locations, including Sierra del Toro, Cerro Castillo, and Ventana Creek. The peak of Cerro Castillo is at 1020 m; the elevation of Lago Sofia is near sea level. B) Location of study area (star) at the southern part of South America. C) Simplified paleogeographic reconstruction of the narrow foredeep of the Magallanes basin shows the confined, deepwater fan–channel complex responsible for deposition of the Cerro Toro Formation.

(Fig. 3). Mud-lined trace fossils (e.g., Palaeophycus and Rosselia) are observed rarely within turbidite sands deposited in the channel axis. Firmground burrows discussed here are the only structures present in graveldominated channel-belt deposits. GLOSSIFUNGITES-ASSOCIATED TRACE FOSSILS The trace-fossil suite consists of common vertical to subvertical Diplocraterion and Skolithos, as well as rare Arenicolites (Figs. 4–5). For each of these trace-fossil types, individual burrow shafts are 0.5 cm in diameter on average, 0.3 to 7 m long, and are sharp walled; spreite of Diplocraterion are exclusively protrusive. The burrow lengths are inferred from their position within thick gravity flow deposits, as discussed later. Trace-fossil density was sensitive to clast percentage within conglomeratic host gravity-flow deposits; reduced densities are observed where extrabasinal clast percentage exceeds 20%–30% of the total rock volume. Burrow shafts follow tortuous paths in areas where extrabasinal

clasts are abundant (Figs. 4A–B). Well-sorted, fine- to mediumgrained sandstone is the primary constituent of the burrow fill. Recognition of the trace fossils in the field is enhanced by the contrasting texture of the dark, mud-dominated facies into which the burrows penetrated, and the clean, light-colored passive sand fill likely derived from bypassing turbulent flows (Figs. 4–5). Such characteristics as lithologically distinctive passive fills, sharp burrow walls, and extreme colonization depth of the trace fossils suggest that excavation took place into firm sediment. Robust, vertical burrow construction and maintenance in soft, muddy sediments by organisms is rare owing to difficulties in preventing structural collapse, especially where burrows are excavated deeply (Pemberton and MacEachern, 1995). MacEachern et al. (1992) recognized that passive infilling of a deep, unlined burrow in soft, muddy sediment before it collapses is unlikely. Burrow closure due to slumping or compaction would have occurred because of the narrow shaft diameter of the burrows if the seafloor were not well compacted. The trace-fossil assemblage is assigned to the Glossifungites ichnofacies. NATURE OF COLONIZED DEPOSITS Trace fossils exclusively penetrated muddy, matrix-supported conglomeratic deposits characterized by the presence of large (up to 3 m diameter) rafted intrabasinal mudstone blocks. Both matrix material and rafted blocks were colonized, whereas well-rounded extrabasinal clasts were not (Fig. 4). Deposits were not associated with an originally shifting particulate seafloor, as no physical sedimentary structures are present. Beds of the same facies, sandy-matrix conglomerate, or lenticular sandstone overlie colonized deposits. Discontinuous sandstone and conglomerate lenses are characterized commonly by traction structures, often present filling erosional incisions on top of the burrowed surface. These deposits represent the lags of erosive gravity flows that largely bypassed the area (Hubbard et al., 2008). Within the mud-matrix conglomeratic deposits, individual sedimentation units—deposits of a single sediment gravity-flow event that accumulated over a period on the order of a few hours—can be delineated clearly (Figs. 4E–F). These units are characterized by (1) sharp bases (Fig. 4E) overlain by thin (⬍5 cm) inversely graded intervals locally, (2) a decrease in extrabasinal clast percentage upward with a basal, extrabasinal clast-supported interval typical (Fig. 4F), (3) decreasing maximum extrabasinal clast size upward through the flow unit (Fig. 4F), (4) sandy mud matrix throughout (Fig. 5), and (5) a sharp, broadly undulating to flat top (Fig. 4E). The contact between the basal extrabasinal clast-

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FIGURE 3—Trace fossils of the Cerro Toro Formation in thick-to-thin-bedded turbidites largely deposited in channel-overbank settings. A) Chondrites isp. (9 mm wide pencil). B) Helminthoida isp. C–D) Unidentified burrows on the soles of turbidite beds. E) Large Chondrites isp. F) Skolithos isp. G) Alcyonidiopsis isp. H) Spirophyton isp. I) Phycosiphon isp. J) Spongeliomorpha isp. on the sole of a turbidite. K) Ophiomorpha isp. in a thick-bedded turbidite deposited in the channel. L) Gyrolithes isp.

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supported interval and the overlying matrix-supported interval can be gradational or sharp (Fig. 4E). Sedimentation units range between 3 and 20 m in thickness; those associated with the burrows are 6–9 m thick on average. Recognizing the sedimentation unit top is crucial in determining where the ancient sediment-water interface existed within the stratigraphic column and, thus, how deep individual burrows penetrated. Owing to the narrow diameter and the apparent extreme depth of burrows, individual shafts are only discontinuously exposed on uneven outcrop surfaces (Figs. 4A–D, 5A). The maximum depth of burrow penetration is determined as the vertical distance between the top of the bed and the deepest trace-fossil occurrence within the bed (sometimes the preservation of a U-shaped tube; see Fig. 5D). Burrows do not cross sedimentation unit boundaries. The trace-making organisms were evidently not able to burrow through clast-supported conglomerate at bed bases (Fig. 4F). Notably, burrow density decreases from the bed top downward, into more clast-rich portions of the sedimentation unit (Fig. 4F). There is no palimpsest soft-ground trace-fossil suite in the deposits overprinted by the firmground trace-fossil suite. The presence of extensive biogenic reworking in contemporaneous overbank deposits (Fig. 3) suggests that a condition or series of conditions existed within the channel belt thalweg not conducive to the development of an infaunal-burrowing community. A gravelly seafloor would have been a deterrent to many potential trace-making organisms. Furthermore, the absence of softground trace fossils is attributed, in part, to the 3–20-m-thick sedimentation units that were emplaced over a short time. The tops of beds were, therefore, most likely to be reworked biogenically, however, they inherently had low-preservation potential because of a propensity to be eroded by turbidity currents in the channel environment. Sohn et al. (2002) suggested that the colonized deposits resulted from gravity flows that originated as debris flows, which, through partial dilution, formed high-density turbidity currents at the heads of the flows. Resultant deposits are characterized by clast-supported conglomerate at the base, which was supported by turbulence within the parental gravity flow and a more debris-flow-like, mud-matrix-supported conglomeratic interval supported by sediment cohesion at the top (Sohn et al., 2002). Alternatively, these flows may have been generated from turbidity currents, incorporating mud as they traveled downslope (Winn and Dott, 1979). This is suggested by common rafted fine-grained sediment blocks in various stages of disaggregation at the tops of individual sedimentation units (Hubbard et al., 2008). A downstream proportional increase of mudmatrix conglomerate beds in the channel complex also supports this interpretation (Crane, 2004; Hubbard et al., 2008). Crane (2004) recognized that the sand fraction of one of these sedimentation units from the Cerro Toro Formation was normally graded and, further, that even the upper poorly sorted muddy part of the gravity-flow deposit was rich in sandsized grains. Turbulence was an important process during sedimentation, based on these observations (Crane, 2004, 2007). STRATIGRAPHIC DISTRIBUTION A series of sections were measured through the coarse-grained member of the Cerro Toro Formation at Sierra del Toro and in the Cordillera Manuel Senõret (Figs. 1A, 6). The highest density of firmground trace fossils is observed in a correlative position within the stratigraphic succession at each location (primary surface P1; see Fig. 6). The stratigraphic

succession is characterized by alternating packages of dominantly sandy and dominantly muddy conglomeratic deposits (Fig. 6). These layers can be correlated across much of the study area, using surface P1 as a datum (Fig. 6). The burrowed horizon is relatively widespread, present across a depositional system that covered much of the Magallanes basin foredeep over an area at least 14.5 km long (between Cerro Castillo and Ventana Creek) and almost 6 km wide along the Magallanes basin axis. Walking out the stratigraphic horizon over large distances is not possible in much of the outcrop belt because of glacial erosion of the formation (e.g., between Sierra del Toro and Cerro Castillo; see Figs. 1A, 6). Despite this, the repeated bedset stacking pattern and consistent relative position of the burrowed horizon indicates that the surface is correlatable for greater than 35 km along the basin axis (from Sierra del Toro to Ventana Creek; see Fig. 6). In all but one section, only a single burrowed surface (P1) was identified. At the Cerro Castillo location, P1 and four additional firmground horizons (secondary surfaces S1–4; see Fig. 6) are recognized. Colonization of secondary horizons is notably patchy (lateral continuity typically ⬍100 m); the trace fossils are considerably smaller on average (0.5– 3 m long with shafts often ⬍0.3 cm in diameter; see Fig. 5H) and are present in lower densities (Fig. 5G) compared with the primary burrowed firmground horizon (P1). DISCUSSION Paleoenvironmental Considerations The most likely organisms responsible for the trace fossils in the Cerro Toro Formation are worms, owing to the narrow diameter and inferred extreme length of individual burrow shafts. Species of both annelid and nemetarian worms can grow to lengths that could accommodate the burrows studied (up to several meters long; see Brusca and Brusca, 2003). Furthermore, species from both phyla commonly inhabit burrows. Larval spat likely spread from slopes proximal to the channel belt or were derived from shallow environments (e.g., shelf), spreading across the basin floor via turbidity currents (cf. Hagerman and Rieger, 1981; Pemberton and MacEachern, 1997). The trace makers are interpreted to have thrived in the open niche that the basin floor provided during a cessation of large, coarse-grained (gravelly) sediment-gravity flow events, based on the high-density and well-developed nature of burrows. Filter-feeding strategies are recorded by the burrow morphologies present in the Cerro Toro Formation, consistent with trace-fossil assemblages of the Glossifungites ichnofacies observed in shallow-water settings (Pemberton and Frey, 1985; MacEachern et al., 1992). The inferred extreme penetration depth of the burrows suggests that either (1) organisms burrowed deeply to avoid excessively turbulent gravity flows passing over the seafloor or (2) organisms were also deposit-feeding on organic material within the poorly sorted muddy conglomerate deposits. Trace fossils divert around clasts within the conglomeratic mudstone (Figs. 4A–B; 5E– F), revealing a highly persistent burrowing behavior related possibly to a necessity to forage for food. Organisms may have been forced to modify their feeding strategies periodically, filter feeding during periods where sediment-gravity flows were supplying food and deposit feeding during times of relative quiescence on the seafloor. In settings characterized by fluctuating levels of food availability or varying environmental conditions, organisms have been observed to modify their feeding strategy

← FIGURE 4—Colonized gravity-flow deposits of the Cerro Toro Formation. A) Trace-fossil distribution in clast-rich, matrix-supported gravity-flow deposit (Cerro Castillo, 194–195 m). B) Line drawing of features in (A), emphasizing effect of extrabasinal clasts on burrow-shaft trajectories. C) Top of same bed featured in (A), located approximately 300 m to the south. Note location of photo, at the top of an 8.5-m-thick sedimentation unit in (E). D) Line drawing of area outlined by box in (C). E) Stratigraphic succession consisting of two complete gravity-flow beds characterized by the Glossifungites ichnofacies: P1 ⫽ most thorough, and extensive bioturbation. Burrows inferred up to 7 m deep from the top of this unit (Cerro Castillo Section, 192–201 m). Staff is 1.5 m long. F) Schematic representation of sedimentologic attributes associated with the units in (E). Average grain size, maximum extrabasinal clast size, and clast percentage decreases upward through the transition from clast-to-matrixsupported conglomerate, toward the top of the sedimentation unit.

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(Cadeé, 1984; Shimeta, 1996). Goerke (1971) recognized the ability of polychaete worms to change their feeding method, both deposit feeding and suspension feeding, in response to changing physical or chemical parameters, including food availability. Origin of Firmgrounds in the Deep Sea Firmgrounds are most commonly documented in sediments that were deposited in shallow-water settings (e.g., Pemberton and Frey, 1985; Gingras et al., 2001). Associated surfaces are often considered to be indicative of a regional stratigraphic discontinuity owing to the fact that the formation of firmgrounds in these environments involves either (1) dewatering of mud caused by prolonged periods of subaerial exposure or (2) burial of fine-grained sediments and at least partial dewatering followed by exhumation and colonization by burrowing organisms on the seafloor (Pemberton and Frey, 1985). Exhumation is associated typically with storm activity or relative sea-level fluctuations in shallow marine environments (Pemberton and Frey, 1985; MacEachern et al., 1992). The generation of widespread firmgrounds, and their subsequent colonization is not as well understood in deep-water settings. Incision into semilithified sediment by submarine canyon processes can result in exposure of media suitable for colonization by firmground burrowers (e.g., Hayward, 1976; Anderson et al., 2006). The study area is considered to be well down-system of an area that may be associated with paleosubmarine canyon processes (Scott, 1966; Winn and Dott, 1979). Furthermore, the flat-lying stratal geometry of the burrowed horizon is not consistent with an incised canyon (Fig. 6). Thick successions of constructional channel overbank deposits adjacent to the coarse-grained deposits in the formation (Beaubouef, 2004; Hubbard et al., 2008) are more typical of submarine-fan deposits (Galloway, 1998). In certain bathyal environments, firmground colonization has been documented where sediment (carbonate ooze) consolidation took place through early seafloor diagenesis or cementation (Bromley and Allouc, 1992). The Cerro Toro Formation contains very little carbonate material, so early seafloor cementation is not considered to have been important. Additionally, cemented hard grounds observed by Bromley and Allouc (1992) were ⬍5 cm thick, and therefore, it is unlikely that burrows several meters long would be excavated in sediment consolidated through such a process. Another mechanism for firmground formation is related to a process by which mud-matrix-supported conglomeratic deposits were emplaced in a cohesive, semifirm state. Extrabasinal conglomerate clasts up to 14 cm in diameter are supported within the colonized deposits, evidence that the gravity-flow deposit was highly competent upon deposition (Hampton, 1975; Lowe, 1982). Geotechnical measurements of conglomeratic mudstone on the modern seafloor are not available, but it is plausible that they have shear-strength values within the range and likely at the lower end of other colonized firmgrounds—60–1,000,000 kPa as measured in modern, shallow tidal environments (Gingras et al., 2000). If the process of emplacement of deposits in a firm state represented the method for firmground generation in the Cerro Toro Formation, trace fossils of the Glossifungites ichnofacies would expectedly characterize numerous horizons throughout the stratigraphic succession. Sediment gravity-flow deposits similar to that colonized are present at many elevations in the stratigraphic succession (e.g., at least 15 muddy conglomerate units at Ventana Creek; see Fig. 6), yet few are characterized by Glossifungites trace-fossil assemblages. In this model, the time to gen-

FIGURE 6—Stratigraphic correlations in the conglomeratic member of the Cerro Toro Formation, Cordillera Manuel Senõret. Note each section is located near the paleochannel axis. See Figure 1 for section locations. Location of widespread Glossifungites surface (primary surface, P1), as well as more local burrowed horizons (secondary surfaces, S1–S4) are indicated. Note all photos presented in Figures 4–5 feature trace fossils associated with surface P1, with the exception of Fig. 5C (from S1), Fig. 5G (from S4), and Fig. 5H (from S1).

erate a well-developed Glossifungites ichnofossil suite would be limited to the time it takes for the organisms to spread across the Magallanes basin floor and colonize the firmground. Experimental studies have shown that colonization of deep-water soft ground can take between 2 and 5 years for some polychaete worms (Grassle, 1977; Grassle and MorsePorteous, 1987). Certainly, at similar colonization rates, many more of the muddy conglomerate deposits throughout the stratigraphic succession would have been exposed at the surface for an amount of time conducive to firmground burrowing. For example, the frequency of large gravityflow events from one submarine fan in a tectonically active region over the last 25 ka has been determined to be on the order of hundreds of years (Normark and McGann, 2004). A model whereby colonized deposits were emplaced in a firm state, therefore, is discounted. Large slump events have been observed to leave widespread scars up to 76,000 km2 on the continental slope, at the base of slope, and on basinfloor fans (Bugge et al., 1987). Such events have been documented to expose overconsolidated sediments (e.g., Baltzer et al., 1994), but associated firmground colonization has not been documented previously. It is

← FIGURE 5—Firmground trace fossils of the Cerro Toro Formation. A) Diplocraterion exposed for ⬃50 cm along an outcrop face (Ventana Creek, 175 m). Scale marked at 10-cm intervals. B) Close-up of Diplocraterion from box outlined in (A). C) Detail of sandy spreite associated with a protrusive Diplocraterion (Cerro Castillo, 122.1 m). D) Basal termination of a U-shaped burrow in muddy, matrix-supported conglomerate (Cerro Castillo, 195.5 m). E) Skolithos diverted around clasts (Cerro Castillo, 197.5 m). F) Diplocraterion excavated between clasts in clast-rich portion of muddy conglomerate deposit (Ventana Creek, 174.2 m). G) Isolated Arenicolites in area of low bioturbation (Cerro Castillo, 320.1 m). H) Poorly developed Glossifungites assemblage consisting of diminutive Diplocraterion (Cerro Castillo, 123.1 m). See Figure 1 for section locations.

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FIGURE 7—Sediment gravity-flow deposit characterized by the Glossifungites ichnofacies at Ventana Creek. A) Colonized sedimentation unit. B) Close-up of tractionstructured conglomerate filling an erosional scour at the top of the unit, evidence of the erosive gravity-flows that largely bypassed the area, exhuming the widespread firmground. Note trace fossils are absent beneath lag deposits, indicating emplacement prior to colonization. (C) Sketch of photograph in part (A).

plausible that this mechanism was responsible for widespread firmground exposure in the Cerro Toro Formation. It is likely that the paleodepositional slope in the study area was in the range associated with the origination of slump or slide movement (as low as 0.01⬚; see Prior and Coleman, 1978), based on calculations by Winn and Dott (1979). Glide planes have been recognized to follow planar stratigraphic horizons (bedding planes), such as those associated with the Glossifungites ichnofacies in the study area (Hampton et al., 1996). The head of a slump scarp has not been recognized in the outcrop belt, possibly as a consequence of the feature being broader than the exposure. It is important to note, however, that large-scale slump and slide deposits have not yet been identified within the study interval south of Sierra del Toro (Fig. 1A). Without supporting sedimentological evidence, this model for firmground generation is also not favored. Recently, widespread firmgrounds developed in a Cenozoic slope succession as a result of erosion by bottom currents has convincingly been documented off the east coast of the United States (Savrda et al., 2001). The narrow Magallanes basin was closed ⬍100 km to the north of the study area, however (Munõz Cristi, 1956; Arbe and Hechem, 1984). Bottom currents in modern enclosed basins are not as strong as those in the open ocean and are associated with the erosion and movement of only fine sediment (Roveri, 2002). Although this model cannot be eliminated as a possibility for the creation of the firmground in the study area, it may be unlikely that a contour current could have developed that was able to erode the thick-bedded turbidity current deposits characteristic of the Cerro Toro Formation. An alternative model for firmground generation in deep-sea settings

involves regional-scale, turbidity-current erosion, which sculpted the narrow foredeep of the Magallanes basin. The key elements of this model include (1) widespread deposition of muddy conglomerate deposits, (2) dewatering or consolidation of sediment through burial by sandy turbidity current deposits, (3) exhumation by high-energy, erosive turbidity currents, evidenced by traction-structured sandstone and conglomerate that was deposited by largely bypassing flows that eroded and filled topographic lows (Fig. 7), and (4) colonization of the basin floor by firmground burrowers. The occurrence of three closely spaced Glossifungites surfaces at Cerro Castillo (S2, P1, S3; see Fig. 6) suggests that the basin floor was repeatedly buried and reexhumed locally. Propensity for erosion at Cerro Castillo is likely related to its position in the fan channel system, perhaps associated with a break in slope or a change in channel planform architecture (cf. Pirmez et al., 2000; Hubbard et al., 2008). It is reasonable to conclude that turbulent processes associated with gravity flows in the thalweg of the channel belt were sufficient enough to exhume compacted sediment in the Cerro Toro Formation over great distances, based on analogy from similar deposits in the modern and from the Holocene record (e.g., Normark et al., 1993). This model for firmground generation is preferred and appears consistent with sedimentological observations. Stratigraphic Implications The relatively widespread firmground (P1; Fig. 6) was exposed by a significant, erosive gravity-flow event or series of events that largely bypassed the region, recorded by local lags on the stratigraphic surface (Fig. 7). Only smaller, sand- and mud-dominated sediment-gravity flows passed

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over the study area during colonization of the firmground, evidenced by sandy passive burrow fills (Fig. 4) and the presence of thin, lenticular sandstone beds on the colonized surface locally. Sand-defined spreite (Figs. 5A–C, F) indicate also that sand was available to the trace makers during colonization. Locally exposed and poorly developed Glossifungites assemblages (S1–S4; see Fig. 6) most likely resulted from colonization associated with aerially limited firmgrounds, formed through localized scour of the channel floor. The uniqueness of the P1 surface in the stratigraphic succession and its interpreted mode of formation suggest that it represents an important stratigraphic discontinuity. Gingras et al. (2000) proposed a method for determining the temporal significance of modern, intertidal firmgrounds based on the premise that burrows of the Glossifungites ichnofacies can be associated with a range of seafloor shear strengths. Importantly, in their study, firmgrounds that were temporally most significant (associated with the highest values of media shear strength) were characterized by planar to gently undulating omission surfaces. Further, they noted that burrows associated with excavation into originally less firm sediments show the effects of compaction upon burial (i.e., deformed burrow architecture). The firmground assemblage is interpreted to record colonization of a well-indurated media based on the nature of the low, broadly undulating to flat-lying topographic relief associated with P1 (Fig. 4E) and the lack of compactional deformation of the burrows (Figs. 4–5). From the intertidal environment, the importance of such surfaces is certain, as the most temporally significant colonized firmground assessed by Gingras et al. (2000) represents a stratigraphic discontinuity of 100–200 ka. The temporal significance of the deep-sea firmground (P1) studied is uncertain; however, a discontinuity of similar magnitude during the Late Cretaceous in the Magallanes basin could be attributable to tectonic pulses in the rising Patagonian Andes (Hubbard et al., 2008). The exhumation of firmgrounds in shallow-water environments is tied directly to relative sea-level fluctuations and the corresponding landward or basinward stepping of shoreline processes—transgressive or lowstandregressive erosion (Savrda, 1991; MacEachern et al., 1992; Pemberton and MacEachern, 1995). A direct relationship between sea-level fluctuation and widespread erosion is not necessary in deep-water settings (Normark et al., 1998). In the only comprehensive study of widespread firmground colonization in siliciclastic deep-sea deposits, Savrda et al. (2001) postulated that periods of sediment starvation related to transgressive or maximum highstand conditions, coupled with aggressive bottom currents, led to net erosion and exposure of extensive firmgrounds. Their study focused on slope deposits off the Atlantic margin of North America, where bottom currents are known to be prevalent (Stow and Holbrook, 1984). In the fan-channel deposits of the Cretaceous Magallanes basin, a link between firmground surfaces and sea-level fluctuations is not evident, despite the location of P1 near, or at the top of, a thick package of muddy conglomerate beds (Fig. 6). It is tempting to interpret the alternation of mud- and sand-rich conglomerate packages as a response to cyclic sea-level fluctuations (cf. Posamentier et al., 1991; Normark et al., 1998), with the Glossifungites surface (P1) representing a condensed interval associated with maximum sea-level rise when coarsegrained sediment was sequestered on the shelf and exhumed firmgrounds colonized on the basin floor. This interpretation is difficult to test, however, because channel erosion processes have effectively precluded preservation of widespread, potentially fossiliferous concordant mudstone beds. Furthermore, body fossils are preserved poorly or absent in much of the study area (Katz, 1963; Scott, 1966; Macellari, 1988). The highresolution biostratigraphic or geochemical correlations necessary for validating a sequence stratigraphic framework, thus, are impossible to make with the data available. The utility of trace fossils in defining important marker horizons at a regional scale has been established in numerous basins (e.g., Mortimore and Pomerol, 1991; Olo´riz and Rodrı´guez-Tovar, 2000), including those associated with media-controlled trace-fossil suites (e.g., Bromley and Gale, 1982). In the absence of body fossils and ash beds (Scott, 1966;

Winn and Dott, 1979), the correlative Glossifungites surface (P1; see Fig. 6) represents the only potential chronostratigraphic marker horizon within the ⬎350-m-thick conglomeratic section of the Cerro Toro Formation in the Cordillera Manuel Senõret. SUMMARY AND CONCLUSIONS 1. A relatively extensive firmground surface is preserved within Late Cretaceous deep-water sedimentary rocks of the Magallanes basin in southern Chile, recognized by the presence of trace fossils of the mediacontrolled Glossifungites ichnofacies. The colonized horizon is characterized by the trace fossils Diplocraterion, Skolithos, and Arenicolites, which penetrated the paleomedia up to an inferred 7 m, based on the presence of trace fossils near the bases of sedimentation units 6–8 m thick. Colonization was associated with a muddy, matrix-supported conglomeratic unit that averages 6–9 m thick. The burrows were excavated by worms that adopted both filter- and deposit-feeding strategies in response to varying environmental conditions on the Magallanes basin floor, during a time period when high-energy, gravelly gravity flows ceased to pass through the channel environment. 2. The surface represents the first documented example of the Glossifungites ichnofacies associated with a succession of thick-bedded gravity-flow deposits in a base-of-slope and basin-floor-fan channel environment. Firmground formation is interpreted to have occurred as follows: (a) widespread emplacement of muddy, conglomeratic gravity-flow deposits, (b) dewatering or firming through burial by turbidites, and (c) exhumation of sediment by erosive turbidity currents. 3. The widespread stratigraphic horizon characterized by trace fossils of the Glossifungites ichnofacies represents an important stratigraphic discontinuity within the Cerro Toro Formation. The surface represents the best marker horizon available within the study area. ACKNOWLEDGMENTS We are grateful to the Stanford Project On Deep-Water Depositional Systems, a consortium of energy companies that includes Amerada Hess, Anadarko, ChevronTexaco, Conoco-Phillips, ENI-AGIP, ExxonMobil, Husky Energy, Marathon, Nexen, Occidental Petroleum, Petrobras, and Roho¨l-Aufsuchungs AG, for financial support of our research. Brian Romans (Stanford University) and Marcelo Solari (Universidad de Chile, Santiago) ably assisted with fieldwork. Drs. Steve Graham and Don Lowe (Stanford University) reviewed an earlier version of the manuscript, and their insights are much appreciated. Input from PALAIOS reviewers (Rick Beaubouef and an anonymous reviewer), as well as Associate Editor John-Paul Zonneveld and Editor Stephen Hasiotis, improved the clarity and focus of the paper. REFERENCES ANDERSON, K.S., GRAHAM, S.A., and HUBBARD, S.M., 2006, Facies, architecture and origin of a reservoir-scale sand-rich succession within submarine canyon fill: Insights from Wagon Caves Rock (Paleocene), Santa Lucia Range, California: Journal of Sedimentary Research, v. 76, p. 819–838. ARBE, H.A., and HECHEM, J.J., 1984, Estratigraphia y facies de depositos marinos profundos del Cretacio Superior, Lago Argentino, Provincia de Santa Cruz: Congreso Geologico Argentino Actas v. 5, p. 7–41. BALTZER, A., COCHONAT, P., and PIPER, D.J.W., 1994, In situ geotechnical characterization of sediments on the Nova Scotian Slope, eastern Canadian continental margin: Marine Geology, v. 120, p. 291–308. BEAUBOUEF, R.T., 2004, Deep-water leveed-channel complexes of the Cerro Toro Formation, Upper Cretaceous, southern Chile: American Association of Petroleum Geologists Bulletin, v. 88, p. 1471–1500. BROMLEY, R.G., and ALLOUC, J., 1992, Trace fossils in bathyal hardgrounds, Mediterranean Sea: Ichnos, v. 2, p. 43–54. BROMLEY, R.G., and GALE, A.S., 1982, The lithostratigraphy of the English chalk rock: Cretaceous Research, v. 3, p. 273–306. BRUSCA, R.C., and BRUSCA, G.J., 2003, Invertebrates: 2nd ed., Sinauer Associates, Sunderland, Massachusetts, 936 p. BUGGE, T., BEFRING, S., BELDERSON, R.H., EIDVIN, T., JANSEN, E., KENYON, N.H., HOL-

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ACCEPTED AUGUST 2, 2007