Inherited Depositional Topography Control on Shelf-Margin Oversteepening, Readjustment, and Coarse-Grained Sediment Delivery to Deep Water, Magallanes Basin, Chile

Introduction

A shelf-margin is characterized in depositional-dip profile from shallow-dipping topset, across steeper foreset (clinoform), to shallow-dipping bottomset (Figure 1). Shelf-margin profiles that approximate clinoforms are generally considered to represent progradation of graded slopes (Hedberg, 1970; Ross et al., 1994; Steel and Olsen, 2002; Johannessen and Steel, 2005; Patruno et al., 2015; Hodgson et al., 2018). Graded margins are defined as margins where erosional and depositional processes are in equilibrium, resulting in topographically smooth slope profiles that prograde basinward (Hedberg, 1970; Ross et al., 1994). Conversely, Hedberg (1970) described out-of-grade slopes, wherein the shelf margin and upper slope are zones of net erosion, sediment bypass, and mass wasting, whereas the lower slope and basin are associated with substantial deposition. The shelf margin is a gateway for the transport of coarse-grained sediment into deep water, and it is commonly attributed to either external forcings (i.e., allogenic controls) promoting sediment supply (Carvajal and Steel, 2009; Kertznus and Kneller, 2009) or internal dynamics of the system (i.e., autogenic controls), such as compensational stacking as a result of delta-lobe switching (Olariu and Bhattacharya, 2006; Muto et al., 2007; Straub et al., 2009; Hajek and Straub, 2017). Ross et al. (1994) noted that inherited depositional topography can control the evolution of later shelf margins, potentially fostering development of oversteepened upper slopes and promoting mass wasting and/or coarse-grained sediment delivery to deep water (Figure 1). Inherited depositional topography does not clearly fit into the either/or, allogenic/autogenic dichotomy of controls on shelf-margin depositional evolution because underlying deposits might have been the product of a wholly distinct set of conditions.

Studies of shelf-margin depositional systems have commonly focused on the prediction of down-slope deep-water fans through stratigraphic analysis (Plink-Björklund et al., 2001; Prather et al., 2017), analysis of external controls (Carvajal and Steel, 2009), or investigation of shelf-edge deposits (i.e., wave-, tide-, or river-dominated process regime) (e.g., Dixon et al., 2012b; Cosgrove et al., 2018). The controls on transitions between slopes with a graded profile and out-of-grade configurations that promote coarse-grained sediment transfer to deep water have remained understudied (Gomis-Cartesio et al., 2018). Slope readjustment comprises the processes of erosion, sediment bypass, and marine onlap of submarine-fan deposits in response to changing basin physiography (Ross et al., 1994). However, the detailed sedimentology and stratigraphy of major shelf-margin readjustments, like those described by Ross et al. (1994) across a large-scale shelf-margin transect, have not been well documented. Furthermore, comparably little work has been focused on the sedimentologic record along the entirety of the transition from shallow-marine to deep-water depocenters along basin margins of high relief (>500 m) (Dixon et al., 2012a; Poyatos-Moré et al., 2019). Deciphering the details of siliciclastic basin margins is challenging as a result of their large scale (several hundreds of meters to kilometers of relief; McMillen, 1991; Helland-Hansen, 1992; Hubbard et al., 2010; Patruno et al., 2015). These high-relief shelf-margin systems are most commonly observed in seismic-reflection data, characterized by >10 m of vertical resolution (e.g., Pinous et al., 2001; Houseknecht et al., 2009), with important insight drawn from stratigraphic modeling (e.g., Uličný et al., 2002; Burgess et al., 2008; Gerber et al., 2008).

This study focuses on the characterization of outcropping shelf-margin stratigraphy, with an emphasis on the record of punctuated out-of-grade conditions related to inherited depositional topography in the Upper Cretaceous Tres Pasos and Dorotea formations, southern Chile. The unique outcrop perspective of seismic-scale clinoforms (i.e., >1000 m relief) provides the opportunity to investigate the stratigraphic architecture developed in response to shelf-margin oversteepening. This is augmented with documentation of the down-dip sedimentological variability along out-of-grade surfaces that were associated with significant coarse-grained sediment bypass to the deep basin.

Geologic Setting

In this study, we examine deposits of the Upper Cretaceous Dorotea and Tres Pasos formations that crop out in southern Chile (Figure 2; Hubbard et al., 2010; Daniels et al., 2018). The units were deposited in the Cenomanian–Maastrichtian Magallanes retroarc foreland basin, located at the southern end of the 7000 km-long Andean Cordillera along the southwestern margin of the South American plate (Figure 2A; Dalziel, 1981; Wilson, 1991; Fildani and Hessler, 2005; Daniels et al., 2019).

The Magallanes Basin contains 3–5 km of Upper Cretaceous deep-water strata, deposited during a prolonged period of elevated subsidence (Romans et al., 2011; Bernhardt et al., 2012). The foreland basin is underlain by thinned continental crust, attributed to a precursor extensional back-arc basin (Rocas Verdes Basin; Romans et al., 2010; Fosdick et al., 2011). This attenuated continental crust promoted high subsidence during thrust loading and led to long-lived deep-water conditions in the retroarc setting (Natland et al., 1974; Hubbard et al., 2010; Fosdick et al., 2014). Deposition of unconfined turbidites of the Cenomanian-Turonian Punta Barrosa Formation marks the onset of deep-water sedimentation (Fildani et al., 2003; Malkowski et al., 2017), followed by the Coniacian-early Campanian Cerro Toro Formation, a mudstone-dominated succession with a conglomeratic channel system situated along the length of the foredeep axis (Figure 2D; Crane and Lowe, 2008; Hubbard et al., 2008; Jobe et al., 2010). The subsidence rate waned and the basin underwent a ∼2 Myr long period during which mass-failure processes dominated, presumably due to a change in hinterland dynamics and/or basin uplift (Daniels et al., 2018, 2019). Subsequently, the basin filled with the Campanian-Maastrichtian Tres Pasos and Dorotea formations, characterized by shelf-margin clinoforms that prograded along the basin axis, from north to south (Romans et al., 2009; Hubbard et al., 2010; Schwartz and Graham, 2015). The Tres Pasos Formation is a mudstone-dominated slope succession with intercalated sandy turbidite systems and mass-transport deposits (Smith, 1977; Shultz et al., 2005; Armitage et al., 2009; Auchter et al., 2016); these strata are overlain by genetically linked deltaic deposits of the Dorotea Formation (Arbe and Hechem, 1984; Macellari et al., 1989; Covault et al., 2009; Hubbard et al., 2010; Leppe et al., 2012; Schwartz and Graham, 2015; Schwartz et al., 2017; Manriquez et al., 2019). The deep-water basin (>1000 m relief) and basin axially oriented depositional systems (>40 km long slopes) promoted the development of shelf-margin clinoforms similar in scale to those of continental margins (Figures 2E,F; Carvajal and Steel, 2009; Hubbard et al., 2010; Romans et al., 2011).

Methodology

The study area is located at approximately 51° 00′ S and 72° 30′ W, adjacent to the Chile-Argentina border (Figure 2). We focus on a 15 km-long exposure of a high-relief shelf-margin depositional system at Cerro Cazador (Figure 2C). At this location, we record south-southeast paleoflow, which is approximately parallel to the N/NW-S/SE south trend of the outcrop (Figure 2C; Smith, 1977; Shultz and Hubbard, 2005; Bauer, 2012; Daniels et al., 2018). The correlation between deltaic and slope strata can be physically traced in the outcrop belt over several kilometers in many cases (Figures 2F, 3, 4). Relatively flat strata of the upper part of the Dorotea Formation at the top of the succession are preserved across the entire study area and are used as a datum, providing the basis for estimations of shelf-margin relief.

High-resolution satellite imagery draped on a digital elevation model was used to correlate the outcrop to the regional stratigraphic framework of Hubbard et al. (2010) to the south, which comprises a series of southward-prograding clinoforms and clinothems >35 km long and 900–1000 m thick (Figures 2E,F). Hubbard et al. (2010) were the first to interpret these clinoforms by tracing the base of thick (40–100 m), sandstone-rich packages exposed as prominent ridges 10–35 km long in the outcrop belt. These composite sandy units are described as “basal clinothem packages” in this study, which directly overlie erosional surfaces that share characteristics of sequence boundaries (Mitchum et al., 1977; Houseknecht et al., 2009). Although clinoforms can be defined by relatively continuous shelf-margin mudstone-prone strata that potentially represent zones of maximum flooding (Galloway, 1989; Steel et al., 2008; Houseknecht et al., 2009), the large scale of the Tres Pasos-Dorotea clinoforms in the Magallanes Basin and sparse exposure of vegetated mudstone-prone strata make delineation of flooding surfaces unreliable over long distances. As such, it is likely that many more clinoforms are present in the outcrop belt than are detected and reported here; the identified clinoforms likely bound multiple clinothems (e.g., Patruno et al., 2015). Thus, these strata could be considered as clinothem sets or compound clinothems, but for simplicity we refer to them as clinothems throughout.

The dataset collected for this study includes 2980 m of measured stratigraphic section, which represents the foundation for facies analysis. Facies variations along individual clinoforms are documented from measurements of grain size, sedimentary structures, bed contacts, and trace fossils (summarized in Table 1). Detailed stratigraphic correlations are made by physically tracing surfaces as well as beds, and confirmed through the use of ground-based and aerial photo mosaics. Where tracing surfaces is not possible due to inaccessible topography, regional correlations (5–40 km) are made with high-resolution satellite data (<1 m resolution).

Tres Pasos and Dorotea Stratigraphy and Sedimentology

Regionally, the fill of the Magallanes Basin is characterized by southward prograding shelf-margin strata exposed for >100 km north-south distance (Romans et al., 2010; Daniels et al., 2018). We consider the clinoforms to be high relief, based on comparison to the compilation of Carvajal et al. (2009). In this section, we describe and interpret key characteristics of the Tres Pasos-Dorotea shelf-margin system, aspects of which were first broadly presented by Hubbard et al. (2010). Key components are the 40–100 m-thick basal clinothem packages (Figure 3), which variably extend at least tens of kilometers from Cerro Cazador (north) to Arroyo Hotel (south) (Figure 4). These sandstone-rich basal packages are overlain by fine-grained successions hundreds of meters thick, which compose the bulk of clinothem strata. Toward the north, clinothems are capped by resistant sandstone-rich (topset) deposits; these deposits generally pinch out southward (basinward).

The strata include a section of thick, mass-transport-deposit (MTD)-dominated strata (Chingue unit), and three shelf-margin units (i.e., Figueroa, Oveja, and Puma; Figure 3). The MTD-dominated Chingue unit and the progressively younger shelf-margin units (Figueroa through Puma) correspond to the four stages of basin evolution described by Daniels et al. (2018). Although the shelf-margin strata correlates farther to the south, our emphasis on clinothems associated with abundant slope sandstone focused our analysis on the Figueroa and Puma shelf-margin units (Figure 4). Due to the large scale of the shelf-margin strata and limited outcrop extent, only the youngest shelf-margin units (upper Figueroa, Oveja, and Puma) are exposed from topset through distal slope; the bases of the Figueroa and Chingue units are characterized exclusively by slope strata in the portion of the outcrop belt studied. There is a lack of evidence for unconfined submarine fan deposits at the distal end of the outcrop belt. It is plausible that unconfined deposits have not been encountered because a significant toe-of-slope break at the transition to the basin floor did not exist in the basin; both non- and deep-marine foreland basins commonly promote formation of long channel systems due to a preferred tectonic slope along their axes, as well as lateral confinement (e.g., Graham et al., 1975; Burbank, 1992; Malkowski et al., 2017; Sharman et al., 2018).

Like other outcrops of large-scale shelf-margin systems, the Tres Pasos-Dorotea deposits at Cerro Cazador provide primarily a two-dimensional (2-D) perspective of the ancient basin margin, limiting interpretations of stratigraphic evolution in 3-D. Considerable along-strike variability of continental margins on the modern seafloor (e.g., Olariu and Steel, 2009; Ryan W. B. et al., 2009) suggests that the same variation ought to be expected in the stratigraphic record of the Magallanes foreland basin margin (e.g., Martinsen and Helland-Hansen, 1995; Driscoll and Karner, 1999; Jones et al., 2015; Madof et al., 2016; Paumard et al., 2018; Poyatos-Moré et al., 2019).

The Chingue Unit

The Chingue deep-water unit is up to 900 m thick and exposed for 5 km along depositional dip (Figure 4). Due to the large thickness and areal extent of the stratigraphic unit, deposits of only a relatively short segment of the entire paleo-slope crop out at Cerro Cazador. A basal sandstone-rich Chingue stratigraphic package is 50–75 m thick (Figure 3) and consists of an upward coarsening and bed thickening succession (Facies F1, F3–5; Figures 5A,B) capped by a chaotically bedded deposit (Facies F2) and a sandstone-filled channel form (Facies F3; Table 1). The basal sandstone-rich Chingue stratigraphic package is lenticular in a depositional-dip perspective, and overlies a zone of growth faulting and abundant sandstone intrusions (Figure 5A). Overlying this basal package is ∼800 m of chaotically bedded strata (Figure 4).

Shultz and Hubbard (2005) attribute the basal sandstone-rich package of the Chingue deep-water unit to ponding in localized accommodation created by growth faulting on an unstable slope. The capping MTD and channel fill are interpreted to record healing of the accommodation and basinward stepping of a channel system down slope (cf. transient fan of Adeogba et al., 2005). Overlying this basal sandstone-rich section, the hundreds of meters of chaotically bedded deposits are attributed to long-lived mass wasting (Facies F2; Figures 5C,D; Table 1; Daniels et al., 2018). The propensity of MTDs is interpreted to record an extended period of out-of-grade slope processes, during which the upper slope was oversteepened and prone to regular failure (Figure 4; Ross et al., 1994; Prather et al., 2017). These MTD-dominated strata reach their maximum thickness at Cerro Cazador and thin southward (Figure 4).

The Figueroa Shelf-Margin Unit

The Figueroa shelf-margin unit is up to 600–700 m thick and exposed for >40 km along depositional dip (Figure 4). The basal Figueroa clinothem package is 75–125 m thick (Figure 3), primarily consisting of sandstone-filled channel forms (Facies F3-F4) encased in mudstone (Facies F1) and chaotically bedded deposits (Facies F2) (Figures 6A–C, 7A). Channel forms are discontinuously exposed along the dip-oriented outcrop belt (Figure 6A). The base of this sandstone-rich succession defines the Figueroa clinoform, which has at least 900–1000 m of relief (Figures 2F, 4; Hubbard et al., 2010; Daniels et al., 2018). Although the paleoslope gradient cannot be accurately measured in the uplifted stratigraphic succession, physical correlation in the outcrop belt demonstrates that it is steepest in the most proximal locations preserved (see Cerro Cazador locality in Figure 4). The topset deposit is not preserved in the outcrop belt for this clinoform segment.

The basal Figueroa clinothem package reflects a transition to increased sand delivery to the slope, supported by the presence of a 300 m-thick section of sandy slope turbidites at Laguna Figueroa (Figures 4, 8; Hubbard et al., 2010). We postulate that a southward prograding delta reached the break-in-slope associated with the upper surface of MTD-dominated stratigraphy in the Chingue unit (Figure 8). The basin margin was oversteepened, and a prolonged period of coarse-grained sediment transfer off the shelf edge was initiated (cf. Ross et al., 1994; Figure 1C). A number of drivers for the sedimentation pattern in the basin could be drawn upon; however, the inherited basin topography evident from the stratigraphic analysis is suggestive that it was a dominant driver of off-shelf delivery of coarse-grained sediment.

The upper part of the Figueroa clinothem, stratigraphically overlying the sandstone-rich basal clinothem package (Figure 3), is 300–500 m thick at Cerro Cazador. To the north, it is capped by a series of upward-coarsening packages that are collectively up to 100–150 m thick (Facies F9–10, F12, F14) (Figure 8 and Table 1). These packages pinch-out southward into widespread concordant fine-grained strata with isolated sandstone bodies (Facies F1, F4-F5) (Figures 6D–F) and localized MTDs (Figure 7B). The coarsening-upward packages are characterized by widespread evidence for shallow-marine conditions, including hummocky cross stratification, symmetrical ripples, abundant wood debris, and a low-diversity trace fossil suite (Figure 9; Table 1; MacEachern et al., 2005).

Upper Figueroa clinothem stratigraphy is interpreted to record the southward transition of deltaic topset deposits into mudstone-prone prodelta and slope strata (Figures 3, 4).

The Oveja Shelf-Margin Unit

The Oveja shelf-margin unit is 300–400 m thick (Figures 3, 10A–C), consisting of a 30–50 m thick upper section with 5–20 m thick upward-coarsening and bed-thickening successions with carbonaceous mudstone (F14), lenticular coarse-grained sandstone (F7), medium-grained sandstone (F10, F13), and hummocky cross-stratified sandstone (F9) (Figures 7C, 11). This upper section transitions southward to laterally extensive (km-scale), upward-coarsening and bed-thickening packages 20–60 m thick composed of interbedded fine-grained sandstone and mudstone (Facies F1, F4) (Figures 10F,G, 11). The most distal deposits at the base, which make up most of the thickness of this unit, are primarily fine grained (Facies F1), punctuated by thin and lenticular sandstone-filled channel forms characterized by variably oriented stratification, including backsets (Facies F5-F6) (Figures 10D,E and Table 1). Notably, the upper half of this unit is exceptionally exposed along a series of unvegetated mountainside outcrops, which display seismic-scale stratal geometries characterized by basinward-dipping and basinward-thinning packages overlain by flatter units (i.e., apparent toplap) (Figures 10A, 11). Collectively, these strata record the progradation of Oveja shelf-margin clinoforms.

We interpret the sharp lithologic change from deltaic sandstone of the upper Figueroa clinothem to overlying mudstone of the basal Oveja clinothem to reflect a backstep of the shelf (Figure 8; Houseknecht and Shank, 2004; Henriksen et al., 2011). The emplacement of distal slope facies on the proximal deltaic deposits (Figures 3, 4, 10A–C) indicates a landward shift of facies above the Figueroa clinothem. The magnitude of apparent backstep (i.e., >10 km) and landward shift of facies could represent a basin-wide event and reorganization of the shelf-slope system (Figure 1C; cf. Ross et al., 1994). Although the 3-D character of shelf-margin migration and its driving mechanism are speculative, orogenic activity in the adjacent Andean Cordillera and associated basin subsidence and transgression of the shoreline could have influenced stratigraphic sequence development (cf. Cant and Stockmal, 1989; Laskowski et al., 2013). Correlation between events in the fold-and-thrust belt (Fosdick et al., 2011) and the timing of this major landward retreat of facies cannot be constrained with current data. However, Daniels et al. (2018) showed that this transgression took place at ∼75 Ma, which corresponds to a time of significant global sea-level rise (Kominz et al., 2008).

We interpret that sustained sediment supply promoted progradation of the Oveja basin margin at Cerro Cazador (Figures 8, 11). The 300–400 m-relief clinoforms prograded toward the abandoned, or relict shelf edge break-in-slope of the underlying high-relief Figueroa shelf-margin unit (Figure 8E). Regardless of potential external (e.g., Carvajal et al., 2009) or internal (e.g., Gerber et al., 2008) drivers on the accommodation/supply regime, as the relict shelf edge was encountered, a period of slope instability and failure (i.e., out-of-grade conditions) led to truncation of the Oveja clinothem topset and initiation of the Puma shelf-margin unit (Figure 8F).

The Puma Shelf-Margin Unit

The basal 20–100 m thick Puma basal clinothem package (Figure 3) is correlated for 35–40 km along paleoslope (Figure 4). In the most proximal location on Cerro Cazador (Figures 11B, 12), sandstone-dominated strata (Facies F8, F10, F13) overlie undulatory surfaces with up to 4 m of relief. Basinward, upward-coarsening packages (F8, F10-F11, F13–14) are truncated by multiple concave-up surfaces with 5–60 m relief that are up to 150 m wide (minimum estimate based on partial strike view; Figures 12, 13A–D). These surfaces define channel forms composed of mostly heterolithic deposits (F14) including lenticular sandstone beds with rare pebble lags (F7) (Figures 7D, 13C). More distally, the basal Puma clinothem package largely comprises fine-grained deposits (F11, F13, F14) that are truncated by concave surfaces with 1–5 m relief, which are overlain by similar interbedded facies (Figures 13E,F).

Evidence for mass wasting and slump-induced concave-up surfaces at the shelf margin at Cerro Cazador (Figures 13A–D) mark the position of the Puma shelf edge (Figure 8). As the Oveja basin margin prograded to the abandoned, relict shelf edge of the underlying Figueroa shelf margin unit (Figure 8), the shelf-margin relief increased and a period of slope readjustment ensued (Figure 8E). Regional mapping shows that just basinward of the associated shelf-edge delta, the paleoslope was at its steepest (Figure 4). The Puma margin records a basinward shift of facies and the introduction of abundant sand and gravel into the basin (e.g., Plink-Björklund and Steel, 2005; Houseknecht et al., 2009). Reimchen et al. (2016) demonstrated that 35 km basinward from the mapped shelf margin on Cerro Cazador, the Puma basal clinothem package is composed of conglomerate- and sandstone- dominated submarine channel deposits >60 m thick (Figure 4). After a period of enhanced coarse-grained sediment delivery to deep water, deposition of mudstone-prone slope facies ensued as the shelf-edge trajectory began to rise sharply (Figure 4). Slope strata are largely dominated by mudstone above the Puma clinoform across the Cerro Cazador study area.

The Stratigraphic Record of Sediment Transfer Along High-Relief Clinoforms

The outcropping stratigraphic record at Cerro Cazador shows evidence of net-depositional slope processes, including voluminous fine-grained strata that compose the Chingue unit, and the Figueroa, Oveja and Puma shelf-margin units (Figure 4). We interpret that a significant proportion of the slope strata is a result of hemipelagic or channel-overbank sedimentation of fine-grained material (Normark et al., 1993; Deptuck et al., 2003; Kane and Hodgson, 2011; Poyatos-Moré et al., 2016).

Shelf-edge to upper-slope conduit development directly overlying the Puma clinoform on Cerro Cazador, including mass-wasting deposits overlain by channel forms up to 60 m thick, records initiation of a zone of predominantly coarse-grained sediment bypass (Figures 13A–D; cf. Nemec et al., 1988; Anderson et al., 1996; Jones et al., 2013). The channel fills are largely dominated by heterolithic thin- to thick-bedded strata with very thin sandstone and pebble lags that are among the coarsest deposits on the slope (Figure 7D). Such features are interpreted to provide a template for channelized sediment transfer to deep water (Mayall et al., 1992; Porebski and Steel, 2003; Ridente et al., 2007; Sylvester et al., 2012; Gomis-Cartesio et al., 2018; Gales et al., 2019).

Extensive tracts of basal clinothem packages, between the shelf edge and lower slope, are characterized by abundant cross-cutting erosional surfaces with >10 m of relief, as well as the complex superposition of channel fills, mass-wasting deposits, ponded sandstones and lag deposits (Figure 14). Collectively, these deposits highlight the protracted development of stratigraphic surfaces in response to a variety of processes including erosion, sediment bypass, mass-wasting and deposition from gravity flows (Hodgson et al., 2016). Bed-scale evidence for high-energy currents that bypassed a portion of their sediment load along the basal sections of the Figueroa and Puma clinothems include: (1) conglomeratic lag deposits; (2) cross bedding; and (3) mudstone drapes that mantle erosion surfaces, which are interpreted to record deposition from the tails of largely bypassing turbidity currents (Table 1; cf. Mutti and Normark, 1987; Stevenson et al., 2015). Sedimentary body-scale evidence for prolonged sediment transfer across the Magallanes Basin slopes are represented by mass-wasting deposits (F2) including large rafted sediment blocks (Figure 13B), channel forms with evidence for a polyphase history of multiple cut-and-fill events (5–20 m thick) (Figures 6A–C, 7A), and channel forms filled primarily with thinly interbedded turbidites (Figures 13E,F; cf. Hubbard et al., 2014; Stevenson et al., 2015).

Perhaps the most significant record of sediment bypass along the Figueroa and Puma basal clinothem packages in the Cerro Cazador study area is the increase in overall proportion of sandstone and conglomerate deposits from upper slope strata downslope 30–40 km to the south, in the vicinity of Arroyo Picana and Laguna Figueroa (Figures 4, 8; Macauley and Hubbard, 2013; Hubbard et al., 2014; Pemberton et al., 2016; Reimchen et al., 2016). Coarse-grained lower-slope to basin-floor deposits are commonly linked to laterally extensive (kilometers), deep (10’s of meters) incisions and canyons developed along the coeval shelf margin (e.g., Anderson et al., 1996; Johannessen and Steel, 2005; Ryan M. C. et al., 2009; Henriksen et al., 2011; Sylvester et al., 2012).

Discussion: Depositional Topography Control on Slope Readjustment

The Campanian shelf-margin to slope depositional system at Cerro Cazador is dominated by mudstone across the 2-2.5 km thick and >60 km long transect (Figures 2F, 4). Two prominent sandstone-rich packages mantle clinoform surfaces (i.e., Figueroa and Puma basal clinothem packages), standing out amongst the fine-grained stratigraphic backdrop. As such, they represent an opportunity to investigate the topographic controls on coarse-grained sediment delivery across a high-relief basin margin in the context of well-defined stratigraphic architecture, sediment-routing history, and source-area evolution (Romans et al., 2010, 2011; Fosdick et al., 2011; Bernhardt et al., 2012; Daniels et al., 2018).

We document ∼8 Myr of coarse-grained sediment transfer beyond the shelf margin in the deep-water Magallanes Basin (Daniels et al., 2019). Ross et al. (1994) proposed a series of mechanisms associated with shelf-margin readjustment that lead to enhanced coarse-grained sediment delivery beyond the shelf margin (Figure 1); however, the predicted stratigraphic architecture has yet to be widely linked to an outcrop record.

At Cerro Cazador, an early phase of instability is recorded by the Chingue unit, which comprises growth faults, extensive sandstone intrusions, and chaotically bedded deposits indicative of unstable slope conditions (Figures 4, 5). These strata accumulated over 2-2.5 Myr and are mapped beyond the study area for up to 100 km north-south along depositional dip (Daniels et al., 2018). The extensive period of mass wasting resulted in a southward-thinning wedge of dominantly MTDs up to 900 m thick at Cerro Cazador, where it then thins to <500 m southward over a distance of only ∼20 km. We speculate that the thickest composite slope sandstone accumulation (∼300 m) in the basin resulted from progradation of the Figueroa basin margin to the break-in-slope at the top of the Chingue interval where MTDs begin to thin rapidly to the south (Figure 8). This observed change in basin margin architecture associated with prominent antecedent topography combined with the introduction of voluminous sand and gravel to the basin is consistent with the interpretation that the clinoform slope oversteepened, initiating the long-lived (up to >2 Myr; Daniels et al., 2018) Figueora channel system. Up slope, the Figueroa system is characterized by substantial erosion and sediment bypass, which transitions down slope to hundreds of meters of submarine channel strata that is comparable to channel deposits in continental margin strata (Macauley and Hubbard, 2013; Fowler and Novakovic, 2018; Pemberton et al., 2018; Jackson et al., 2019). The generation of this oversteepened slope is similar to a mechanism proposed by Ross et al. (1994), wherein steep slopes develop as a result of fluctuating carbonate and siliciclastic deposition and/or tectonic deformation. In mixed siliciclastic and carbonate systems, following carbonate platform development, deltas reach the relict carbonate platform edge and the oversteepened slope leads to prolonged coarse-grained sediment delivery to deep water (see Scenario B; Figure 1C). Using a modeling approach, Uličný et al. (2002) showed that 2-D stacking patterns of shelf-slope-basin clinoform systems are highly sensitive to initial depth (i.e., water depth at onset of deposition). Their study focused on tectonic (fault) inheritance whereas we emphasize topography inherited from the previous depositional phase. In the Chilean basin, instead of development of a break in slope at a relict carbonate platform edge, a morphologically similar scenario was established through the emplacement of mass-transport deposits prior to development of the Figueroa shelf-margin (Figure 8A).

The large-magnitude backstep on top of the Figueroa basin margin set the stage for an additional, protracted phase of slope instability and submarine-fan development (Figure 8E). This backstep is followed by the progradation of the Oveja basin margin to the abandoned, relict shelf edge of the Figuoera basin margin. The corresponding change in shelf-margin physiography led to a period of oversteepening that resulted in erosion of Oveja shelf-margin strata and initiation of the coarse-grained basal Puma clinoform package (Figures 8F,G). Similar to Figueroa strata, submarine-channel systems that are compositionally and architecturally similar to examples documented from continental margins are mapped along the system (Reimchen et al., 2016). The observed stratigraphic architecture at Cerro Cazador is consistent with scenario A of Ross et al. (1994), as shown in Figure 1C.

The erosional Figueroa and Puma clinoforms mark the disruption of a graded shelf-margin profile (Figure 4). Antecedent topography inherited from preceding phases significantly influenced the development of subsequent shelf margins including oversteepened upper slopes, and the transfer of coarse-grained sediment to deep water. Whether the inherited topography was a result of initial configuration (e.g., tectonic) or from predecessor clinoform shelf-margin position, the break in the slope profile caused the system to reach a disequilibrium state.

An important goal of sedimentary geoscientists is to gain insights into the controls on sedimentation and the creation of the depositional record for prediction in analogous settings. In an effort to communicate to colleagues and the broader geoscience community, we commonly attempt to place our interpretations in the context of allogenic versus autogenic controls (e.g., Beerbower, 1964; Paola et al., 2009). Allogenic controls are external to the sediment-routing and depositional system, including changes in eustatic sea level, tectonics, climate, and initial geometry of the receiving basin, whereas autogenic controls are internal to the sedimentary system, including intermittent sediment storage producing episodic, spatially discontinuous sedimentation (Romans et al., 2016; Hajek and Straub, 2017) and landward retreat of deltaic prisms in response to changing area and slope during deposition (Muto and Steel, 2002). However, some controls do not clearly fit into an either/or, allogenic/autogenic dichotomy. The disruption of graded shelf-margin profiles in response to inherited depositional topography during basin evolution is one such control. For example, although receiving-basin geometry can be considered an allogenic control governed by underlying tectonic configuration (Nelson and Kulm, 1973; Mutti and Normark, 1987), subsequent basin filling creates the depositional topography across which sediment transport and deposition occur to create stratigraphic architecture (e.g., Burgess et al., 2008). In this way, both external tectonic underpinning and internal dynamics of sedimentation govern resultant topography, which, in turn, influences the occurrence and position of shelf margins prone to deliver coarse sediment to the basin. Beyond understanding controls on margin evolution through major slope readjustments, there are implications for reservoir prediction associated with hydrocarbon exploration or carbon storage (see Ross et al., 1994); for example, the thickest, coarsest turbidite accumulations onlap erosional shelf-margin clinoforms and develop downstream of paleotopographic escarpments (Figures 8C,G). Additionally, with sufficient data coverage (outcrop or subsurface) the prediction of stacking patterns and facies in the context of inherited topography has the advantage of such features being potentially preserved and identified in the physical stratigraphic record without invoking assumptions about external forcings that are difficult or, in some cases, impossible to constrain.

Conclusion

A large-scale shelf-to-slope depositional system featuring basin margins with >1000 m paleobathymetric relief and slope lengths >40 km is preserved in outcropping Cretaceous strata of Chilean Patagonia. The depositional architecture observed in these outcrops is analogous to seismically imaged clinoforms of slope systems worldwide. The strata are characterized by evidence for the development of two key coarse-grained sediment-delivery systems that resulted in fairways of sandstone-dominated submarine-channel deposits >100 m thick in the deep-water basin: the Figueroa and Puma clinoforms. The stratigraphic position and architecture of each leads to an interpretation that they formed during major slope readjustments, across initially oversteepened depositional topography, which resulted in transient out-of-grade slope conditions. Strata directly overlying the Figueroa and Puma clinoforms (i.e., basal clinothem packages) are sandstone-rich, recording phases of enhanced coarse-grained sediment delivery to the deep-water basin. The 35 km long and up to 2.5 km thick stratigraphic section represented by the two slope systems was largely associated with mudstone, deposited over 3–4 Ma. The only two large-scale sandstone accumulations in the 2-D outcrop belt are recorded by the Figueroa and Puma basal clinothem packages, demonstrating the importance of major slope readjustments in margin evolution. Pre-existing depositional topography controls the locations of subsequent shelf margins and coarse-grained sediment delivery to deep water. Understanding this control has predictive value in analogous settings that were subjected to intermittent oversteepening and transient phases of coarse-grained sediment bypass to deep water.

Data Availability Statement

All datasets generated for this study are included in the article/supplementary material.

Author Contributions

DB did the fieldwork as a graduate student and wrote the initial draft. SH envisioned the project, secured the funding, assisted with fieldwork, and edited the manuscript. JC and BR assisted in the field and with manuscript preparation, including formulating the purpose, and organization of the manuscript.

Funding

Support for this research was provided by the Chile Slope Systems Joint Industry Project, members of which include Anadarko, BG Group, BHP Billiton, BP, Chevron, ConocoPhillips, Equinor, Hess, Maersk, Marathon, Nexen-CNOOC, Shell, and Repsol. Additional financial support was provided by the Natural Sciences and Engineering Research Council (Grant number RGPIN/341715-2013 to SH).

Conflict of Interest

DB was employed by company Chevron Canada Limited.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The results presented benefited substantially from discussions with Andrea Fildani (Deep Time Institute), who first visited Cerro Cazador in 2008. Andrew  Madof (Chevron USA) provided insightful input that shaped some of the ideas presented in this work. Fieldwork was assisted by Ryan Macauley, Sean Fletcher, Kerrie Bann, Keegan Raines, Erin Pemberton, and Kirt Campion. We thank Mr. Mauricio Alvarez Kusanovic and Ms. Hella Roerhs Jeppesen for graciously allowing us to access their land. The comments from Frontiers reviewers GH and NP improved the clarity of the manuscript immensely and are much appreciated. Reviews of an earlier version of this manuscript by Drs. Sverre Henriksen (Equinor), Ron Steel (UT Austin), and Chris Jackson (Imperial College London) significantly improved the focus of the manuscript – we appreciate their significant efforts on our behalf. JC acknowledges support of the Quantitative Clastics Laboratory sponsors.

References