Channel incision into a submarine landslide: an exhumed Carboniferous

43 Emplacement of submarine landslides, or mass transport deposits, can radically reshape the 44 physiography of continental margins, and strongly influence subsequent sedimentary 45 processes and dispersal patterns. The irregular relief they generate creates obstacles that 46 force reorganisation of sediment transport systems. Subsurface and seabed examples show 47 that channels can incise directly into submarine landslides. Here, we use high-resolution 48 sedimentological analysis, geological mapping and photogrammetric modelling to document 49 the evolution of two adjacent, and partially contemporaneous, sandstone-rich submarine 50 channel-fills (NSB and SSB) that incised deeply (>75 m) with steep lateral margins (up to 51 70°) into a 200 m thick debrite. The stepped erosion surface mantled by clasts, ranging from 52 gravels to cobbles, points to a period of downcutting and sediment bypass. A change to 53 aggradation is marked by laterally-migrating sandstone-rich channel bodies that is coincident 54 with prominent steps in the large-scale erosion surface. Two types of depositional terrace 55 are documented on these steps: one overlying an entrenchment surface, and another 56 located in a bend cut-off. Above a younger erosion surface, mapped in both NSB and SSB, 57 is an abrupt change to partially-confined tabular sandstones with graded caps, interpreted as 58 confined lobes. The lobes are characterised by a lack of compensational stacking and 59 increasingly thick hybrid bed deposits, suggesting progradation of a lobe complex confined 60 by the main erosion surface. The incision of adjacent and partially coeval channels into a 61 thick submarine landslide, and sand-rich infill including development of partially confined 62 lobes, reflects the complicated relationships between evolving relief and changes in 63 sediment gravity flow character, which can only be investigated at outcrop. The absence of 64 channel-fills in bounding strata, and the abrupt and temporary presence of coarse sediment 65 infilling the channels, indicates that the submarine landslide emplacement reshaped 66 sediment transport systems, and established conditions that effectively separated sandfrom 67 mud-dominated deposits. 68

al., 2014) with the generation of large volumes of subglacial sediments (Milliman and Meade,121 1983; Elverhøi et al., 1998). An alternative interpretation of glacial regime proposes a large 122 ice sheet located in the continental interior that was drained by long outlet glaciers. The ice-123 sheet hypothesis is supported by primary glacial deposits identified in the eastern and Interpretation: Lenticular, clast-supported pebbly sandstones bounded by erosion 236 surfaces are interpreted as lag deposits, with clasts carried as bedload transport (Mutti and 237 Normark, 1987;Mutti, 1992). Poorly-sorted, matrix-supported, clay-poor beds suggest 238 deposition from a debris flows (Mutti et al., 2003), with the presence of grooves further 239 indicating passage of cohesive flows, such as debris flows or slumps (Peakall et al., 2020). 240 The position of these lag deposits associated with the composite erosion surface supports 241 the interpretation that FA2A formed through the passage of multiple erosive flows that 242 formed a sediment bypass-dominated zone (Winn and Dott 1977;Mutti and Normark, 1987;243 Gardner et al., 2003;Beaubouef et al., 2004;Stevenson et al., 2015). 244

(B) Amalgamated sandstone beds: 245
Description: Homogeneous, erosively-based sandstone beds (0.5-~4 m thick), with 246 common amalgamation surfaces, comprising white-grey, angular to sub-angular, medium-to 247 well-sorted, very coarse sand and granules (Fig. 3E) that stack to form laterally extensive 248 packages. Typically, beds are structureless with weak normal grading and planar lamination 249 at bed tops. Bed bases occasionally exhibit large, wide flute casts and weakly stratified 250 siltstone clast-rich units that form discrete layers, and some beds contain dish structures. 251 Interpretation: Thick, clean sandstones deposit under high-density turbidity currents and 252 sandy debris flows (Bouma, 1962;Lowe, 1982;Mutti, 1992 Structureless sandstones within the succession can result from deposition from a steady, 258 uniform current (Kneller and Branney, 1995), which may be sufficiently rapid to induce 259 liquefaction (Lowe, 1982;Kneller and Branney, 1995;Peakall et al., 2020) precluding the 260 development of depositional bedforms (Lowe, 1982). there is an abrupt grain-size break to a fine-grained planar and ripple laminated sandstone 266 and siltstone division (Fig. 3F). 267 Interpretation: Very coarse, structureless sandstones were deposited by high density, 268 sand-rich turbidity currents (Lowe, 1982). Dewatering structures form through liquefaction 269 (Mulder and Alexander, 2001;Stow and Johansson, 2002) likely related to rapid deposition 270 (Lowe, 1982;Peakall et al., 2020). Finer grained material was deposited from low-density 271 turbidity currents, with tractional structures formed from reworking by dilute flows above the 272 bed (Allen, 1984; Southard, 1991; Best and Bridge, 1992). The grain-size break is 273 interpreted to reflect the transition from high to low density turbidity current deposition 274 (Sumner et al., 2008), and may indicate sediment bypass (Stevenson et al., 2015). Interpretation: Deposition from upper, dilute parts of turbidity currents (Lowe, 1988). Thin 283 beds suggest low suspension fall out rates. The micro-ripples on the tops of beds are the 284 product of very early stage (incipient) ripples in silts (Rees, 1966;Mantz, 1978), suggesting 285 limited time for tractional reworking and thus a rapidly waning flow. 286

(B) Scoured thin-beds 287
Description: Undulating, fine to coarse, well-sorted sandstone beds with erosive 288 bases, which are commonly truncated by scour surfaces (Fig. 3H). Thinner beds are 289 normally graded, and thicker beds show little grain-size variation, with rare basal siltstone 290 chips (0.5-1.5 cm diameter). Typically, beds of coarser grainsize exhibit cross ripple 291 lamination along with granules dispersed throughout beds that can follow ripple foresets, and 292 finer-grained beds exhibit parallel lamination. Scour-fills are concentrated in granules, and 293 contain siltstone clasts and rare inclined laminae sets. 294 Interpretation: Deposition and tractional reworking by upper dilute parts of turbidity 295 currents (Lowe, 1982). The presence of abundant scour surfaces, infilled by granules 296 indicates sediment bypass, and suggests deposition at a relatively low elevation with respect 297 to the active channel (Hansen et al., 2015). Normal grading and tractional structures 298 overlying these surfaces suggest formation by low-density turbidity currents (Lowe, 1988; 299 Kneller and Branney, 1995). 300

(A) Tabular beds 302
Description: Metre-thick tabular beds with limited basal erosion comprised of white-grey, 303 angular to sub-angular, medium-to well-sorted, coarse sandstone and granules. Beds are 304 weakly normally graded, with rare parallel and current ripple laminations at bed tops and rare 305 isolated siltstone clasts in bed bases. Bed contacts are amalgamated or separated by a 306 defined erosion surface. 307 Interpretation: Normally graded sandstones are interpreted to form from high-density 308 turbidity current deposition (Bouma, 1962;Lowe, 1982 Typically, siltstone divisions are finely-laminated, lack grading, and are thicker than the 318 underlying sandstone layer. Some beds exhibit a sharp grain-size change from sandstone to 319 siltstone, with some normally graded from sandstone to siltstone within 2 cm. 320 4.2 Surface 1 (S1) -Erosion surface 375 In the Southern Sandstone Body (SSB) area, MTD 5 is cut by a >75 m deep, 400 m 376 wide concave-up surface, with stepped margins to the SW and NE (Fig. 4). The SW margin 377 steepens with height (maximum 70°) and exhibits an uneven geometry, before passing 378 westwards to an irregular surface that flattens to sub-horizontal (Figs. 1, 4). This surface is 379 also characterised by clastic dykes marking sand injection into the underlying debrite. The 380 NE margin is faulted (Figs. 1, 2A), with the exposure of S1 on the uplifted block sub-381 horizontal (Figs. 2B-C); the lower portion of this margin is inferred to be a similar gradient to 382 the SW margin. Surface 1 in the Northern Sandstone Body (NSB) area is characterised by 383 smooth margins (to the W and NE). The W margin is steeper and faulted, and the NE margin 384 is more rugose (Fig. 2C). Grooves and other tool marks are present on S1, with depths of up 385 to 0.15 m and a greater width than depth. Palaeoflow from grooves in S1 range from 386  Package 1 (P1; ~20 m thick) directly overlies the lowermost part of S1 in the SSB 397 ( Fig. 4) and comprises a laterally discontinuous basal conglomerate (up to 3 m thick; FA2A), 398 with overlying tabular, commonly amalgamated, very coarse-grained sandstone beds (~17 m 399 thick; FA2b). Multiple erosion surfaces separate FA2A and FA2B, suggesting a phase 400 dominated by sediment bypass. FA2B is present up to the first step in S1 to the SW, where 401 beds onlap S1 at an angle of ~20°. Flute casts on the base of a P1 bed indicate palaeoflow 402 ranges from 265-040°, but predominantly to the NW (Fig. 4). Towards the top of P1, a thin 403 (0.2 m thick) partially preserved unit of ripple laminated fine sandstones (palaeoflow range 404 140-040°) and coarse siltstones is interpreted to represent a period of reduced sediment 405 supply. The sand-rich, commonly amalgamated deposition from high-energy flows, and 406 location within an incisional confining surface (S1) supports interpretation of these deposits 407 as axial channel fills. 408

Package 2 (P2) -Aggrading channel-fills 409
In general, Package 2 has a higher proportion of fine-grained material, siltstone 410 clasts, and thinner beds than P1. FA2C dominates P2, and thickens from 3 m in the east to 411 16.5 m above the deepest point of Surface 1, then thins to 6 m in the SW (Fig. 4). P2 is 412 subdivided by the geometry and lateral extent of beds (Fig. 4)  overlying confined heterolithic deposits (T1) in the north (Fig. 1). The base of P2 in contact 419 with S1 and Unit 1 indicates the presence of a palaeo-high during deposition, or a stepped 420 geometry of the NE margin above the level of P1 deposition. Groove data from S1 on the 421 horst block gives palaeoflow readings of 092/272°-172/352°. Above this, lower P2 thins 422 westward from ~9 m to 2.5 m thick, whilst on the eastward side it is cut by the basal surface 423 of P3 (Fig. 4). Upper P2 exhibits multiple concave-up erosion surfaces bounding laterally 424 discontinuous bodies of sandstone-rich deposits (FA2C; Fig. 4), interpreted as smaller-scale 425 channel cuts within the larger-scale S1. 426 The thickest part of upper P2 is to the west. The component beds are more lenticular, 427 with erosion surfaces defining channelised bodies ranging from 1.5-4 m thick. These exhibit 428 a highly aggradational stacking pattern, with limited lateral offset to the east (Fig. 4). 429 Palaeocurrent measurements taken from ripple cross laminations in finer-grained bed caps 430 have a wide range of directions throughout the stratigraphy (062-326°) (Fig. 4), most likely 431 indicating flow deflection from surrounding topography (e.g. Kneller et al., 1991). 432

Package 3 (P3) -Channel widening 433
The base of Package 3 is erosional, and is coincident with a widening of the SSB, 434 marked by a prominent step in Surface 1 on the SW margin (Fig. 4). Because pebbly sands 435 and conglomerates (FA2A) are only present overlying S1 on the step, and not associated 436 with P3 within the channel cut, the step is interpreted to have formed during the initial 437 formation of S1. P3 is ~11 m thick and comprises the same tabular, amalgamated sandstone 438 facies (FA2B) as P1, with no fine-grained bed caps (Fig. 4). It is not possible to identify the 439 lateral and vertical extent of P3 in the SW of the SSB, due to exposure limitations. However, 440 bed thickness, degree of amalgamation, and lack of fine-grained material suggest that flows 441 were still channelised at this point. Package 4 is the most laterally extensive in the study area, and is present in the SSB 444 and NSB (Fig. 4). The base of P4 is marked by an irregular erosion surface (Surface 2 (S2)), 445 overlying P3 and off-axis deposition to the SW (Fig. 4). S2 incises up to 2 m in the east, but 446 the geometry to the SW is unknown, due to exposure limitations. In Log 4, the surface is 447 and siltstones (0.1-0.5 m thick), and is interpreted as a scour-fill (Fig. 4). 466 L4 is continuous across the outcrop and amalgamates with L3 eastwards (Figs. 1, 4

Unit 3 -Overlying turbidites (FA5) -Lobe fringe deposits
474 Unit 3 is correlated from the SSB to the NSB to form a high aspect ratio package 475 (Fig. 1). The contact between Unit 2 and Unit 3 (FA5) is characterised by an abrupt transition 476 from thick-bedded and amalgamated coarse sandstones (L5) to coarse siltstones (Fig. 4). 477 Unit 3 forms a 2-5 m thick coarsening-and thickening upwards package from siltstones 478 interbedded with cm-scale sandstones to increasingly thick (up to 10 cm) sandstone beds 479 (Fig. 4). Palaeocurrents from current ripple lamination range from 020-080°NE, with grooves 480 The NSB-fill is up to 60 m thick (Fig. 6), with 2 m of lenticular, conglomeratic beds 493 (FA2A) overlying S1. Above this ~40 m of very coarse sandstones (0.4 to ~3 m thick; FA2B) 494 is present that thin and become less amalgamated towards the margins. The presence of 495 high-density turbidity current deposits confined by S1 supports an interpretation of axial 496 channel sandstones. Above this, S2 is overlain by a 0.2 m thick, laterally-discontinuous bed 497 of FA2A in the east (Fig. 6). P4 (L2-L5) of Unit 2 is correlated from the SSB, with Units 3 and 498 4 also present overlying the NSB. P4 has a similar stacking pattern, with four distinct 499 sandstone beds (FA4A) intercalated with FA4B and FA4C. FA4B is present in the central 500 part of the outcrop, with a 360° spread of palaeocurrents, but absent to the NE. L2 to L4 are 501 tentatively correlated across the area. L5 and Unit 3 were walked out and correlated across 502 faults using their distinctive lithology and bed architecture. Unit 3 (~3 m thick) contains 503 palaeocurrents ranging from 357°-084°. Overall, P4 thins and onlaps towards the NE margin 504 of MTD 5, before passing into the subcrop (Fig. 1). 505

Off-axis deposition on elevated surfaces
506 Three distinct sedimentary successions overlie steps in Surface 1, with one between 507 the NSB and SSB (T1) (Fig. 1) and the others on small (10 m-wide) concave-up steps (T2 508 and T3) to the west of the SSB (Figs. 1, 4). These deposits share similar depositional 509 architectures and processes. 510 4.7.1 T1 511 T1 is located between the SSB and NSB, is ~15 m thick, and directly overlies S1 and 512 MTD 5, (Figs. 1, 7A is cut by S2 (Fig. 4). overlie S1, which are interpreted as bypass-dominated deposits. The overlying T3 535 stratigraphy is subdivided into a lower and upper succession (Fig. 8). Lower T3 comprises 536 six 1-3 m thick coarsening-and thickening-upwards units (T3.1-3.6) of fissile-(FA3A; Fig.  537 7D) and scoured-thin-beds (FA3B). The lowermost unit (T3.1) (Fig. 8) Fig. 8) is 541 characterised by multiple cm-deep scour surfaces that are orientated broadly W-E/NW-SE 542 (090/270°-140/320°) and mantled with granules (Fig. 7F). The scour-fills comprise a matrix 543 of medium-grained sandstone, with siltstone chips concentrated close to the scour surface 544 but dispersed throughout. Scours are 3-5 cm in length (Fig. 7F) throughout. In summary, the scours and grooves are orientated approximately W-E to NW-555 SE, whilst the ripples show an almost 360 range of palaeocurrents (Fig. 4). 556 The base of Upper T3 is marked by a 20 cm thick granule-rich, very coarse-grained 557 sandstone with an erosional base overlain by abundant siltstone clasts (Fig. 8). Six overlying 558 beds (0.14 -1.16 m thick) are normally graded from very coarse to fine sandstone or coarse 559 siltstone with parallel lamination. This interval is overlain by a distinctive bed containing 560 convex-upwards, low-angle lamination, inclined towards the main conduit (Fig. 7E) combined flow bedform. This is followed by six erosively-based normally graded and locally 566 planar laminated sandstone beds, then a sandstone-dominated interval with multiple erosion 567 surfaces mantled by siltstone clasts. This succession is cut by a surface overlain by extra-568 and intra-basinal small pebbles to large cobbles (up to 30 cm diameter) (Fig. 8). Overlying 569 beds form fining-upwards packages of normally graded coarse to fine-grained sandstones, 570 before the deposition of P4. 571 The elevated location of T1, T2 and T3 above the main conduit, the absence of 572 physical bed-scale connections with axial deposits, the highly variable palaeocurrents 573 The stepped geometry, and pebbly sandstones and conglomerates (FA2A) overlying S1 595 at the base of P1 and on the SW margin under P3 in the SSB, under the lowermost package 596 of the NSB, and underlying T2 and T3 on elevated surfaces suggests that S1 deepened 597 through multiple phases of erosion and sediment bypass. The SW expression of S1 in the 598 SSB is characterised by onlap and localised erosion, indicating limited modification of S1 599 and MTD 5 during P1. This supports the formation of the composite S1 during an initial P1 represents the first stage of fill within the SSB, and is characterised by coarse-611 grained amalgamated sandstones above the lowest part of the S1 surface (Fig. 9A). 612 Overlying this, P2 is characterised by a wider grainsize range, fine-grained bed caps, and 613 smaller-scale channelised bodies (Fig. 9B). During P2, flows eroded into the remobilised T1, 614 and were able to deposit either side of the palaeohigh between the SSB and the NSB. The 615 increase in fine grained material could either reflect a change in sediment source character, 616 or a change in flow parameters resulting in reduced flow velocity and potential to bypass that 617 affected grainsize sequestration along a system. The increase in finer material allowed 618 formation and stabilisation of channels banks within the larger-scale conduit bounded by S1 619 (Peakall et al., 2007). The change in bed geometry may also reflect reduced confinement 620 leading to lower local velocities and deposition and preservation of finer grainsizes, allowing 621 elementary channels to form and migrate (Figs. 4, 9B). P3 is marked by tabular sandstone 622 beds (Fig. 9C), with the greater bed thickness, coarser grain-size and level of amalgamation 623 suggesting deposition from larger, higher energy flows, capable of bypassing finer-grained 624 sediment down-dip (Kneller and Branney, 1995). The thin-bedded sandstone-siltstone couplets (FA4B) in the lower section of P4 are 671 interpreted as ponding of distal lobe fringe deposits, with increasing bed thickness and 672 amalgamation of beds L1-L4, and decreasing volumes of fine-grained material suggesting 673 progradation of a lobe complex. Erosional features in the lower portions of a lobe, such as 674 the surface that truncates L1, are typically erosive products of larger flows (Fig. 4) which 675 suggests sufficient space within S1 to allow 'unconfined' deposition at this point. Sand-rich 676 hybrid beds similar to those seen in La Peña have been observed in areas proximal to the 677 lobe axis (Fonnesu et al., 2015;Brooks et al., 2018). A similar configuration is supported by 678 deposition of L5, which is characterised by amalgamated sandstone beds indicating a lobe 679 axis. 680 5.1.5 Avulsion, lobe switching and back-stepping 681 The contact between Unit 2 and Unit 3 is a sharp change (Fig. 4, Log 9, 8 and 3, Fig.  682 6 top) from axial lobe to distal lobe fringe deposits, indicating a sudden change within the 683 system. This corresponds with a change in palaeocurrent direction from the NW to the NE. 684 The most likely mechanism for rapid abandonment of a lobe is upstream avulsion (Prélat et 685 al., 2010;Macdonald et al., 2011). Therefore, Unit 3 represents distal lobe fringe deposition 686 of a new lobe, or a phase of abandonment. 687

688
Multiple mechanisms can form terraced surfaces within submarine channel systems 689 (Hansen et al., 2015), which then act as sites for subsequent deposition. The presence of 690 FA2A on steps on S1 immediately below T2 and T3 suggest that these surfaces were once 691 the location of much higher energy and coarser-grained flows that mainly bypassed 692 sediment basinward, compared to the overlying deposits. This, coupled with S1 cutting down 693 10 m over a width of 18 m (a gradient of ~55°), and this elevation difference between T3 and 694 the SSB (Fig. 4) suggests formation of the terraced surface was through bend cut-off by 695 entrenchment (Hansen et al., 2015), with T3 deposited in the older elevated and abandoned 696 channel cut. The spread in palaeocurrent data in lower T3 (Fig. 4)  siltstone chip-rich intervals increase upwards in the upper T3 succession (Fig. 8). This 728 change could record: a) higher aggradation of channel-axis deposits relative to terrace 729 deposition, allowing increasingly coarse grainsizes and deposition of thicker beds, or b) 730 increasing flow magnitude through time, possibly through system progradation, or c) some 731 combination of the two. The overall pronounced coarsening-up succession of T3 suggests 732 that the terrace deposits may largely reflect bed thalweg aggradation, rather than increasing 733 flow magnitudes. Given that turbidity current velocity decreases exponentially with height, 734 once above the height of the velocity maximum, then even large increases in flow magnitude 735 are unlikely to be able to produce major scour surfaces and deposition of granules and 736 siltstone chips (up to 1.5 cm in size) on highly elevated terraces (sensu Babonneau et al., 737 2004). Given the overall bed stacking with repeated minor coarsening-up cycles, the lower 738 part of the terrace is most easily explained as recording successive phases of channel 739 thalweg aggradation during the infill phase. If related to initial downcutting and formation of 740 S1, then there would need to be six progressively larger phases of bed aggradation within 741 the channel, followed by renewed downcutting. The abrupt change between the deposits of 742 the lower and higher terrace suggests that there was a major phase of channel aggradation 743 at this point, which may have been accompanied by increased flow size. 744

Relative Timing of the Southern Sandstone Body and Northern Sandstone Body
Faulting in the centre of the study area largely prevents tracing of stratigraphic surfaces 747 between the SSB and the NSB. The relationship between P2 and T1 provides the oldest 748 observational constraints available of the temporal evolution of SSB and NSB. The rotation, 749 deformation, and incision of T1 (Fig. 7) suggests it was originally more extensive. The 750 instability and remobilisation is likely related to a phase of erosion prior to deposition of P2. 751 This indicates that the NSB was active prior to the deposition of P2, which is supported by 752 the different depths of incision of the NSB and SSB. The NSB incision is ~15 m shallower 753 than the SSB, and had they been contemporaneous, the SSB would have had a significant 754 gradient advantage over the NSB, with the majority of flows transported through the SSB. 755 This may suggest that the NSB incised and filled prior to the incision by the SSB (Fig. 10Ai). 756 The two channels may have formed from an updip avulsion, or by two separate channel 757 systems (Fig. 10Aii) Alternatively, it may be the case that the NSB and SSB were coeval. Subtle variations in 771 channel morphology and thalweg gradient can influence flow velocity, and thus the erosion-772 deposition threshold (Kneller, 1995;Stevenson et al., 2015). A steeper gradient in the SSB 773 would result in more sediment bypass through this channel, whilst deposition occurred in the 774 NSB (Fig. 10Bi). When available accommodation within the NSB was filled, all flows would 775 be diverted down the SSB (Fig. 10Bii), which begins to aggrade (Fig. 10Biii) Here we present the first study of two exceptionally well-exposed erosional channel 812 systems (the NSB and SSB) that incised into a thick megaclast-bearing debrite. We also 813 document the formation and flow-scale evolution of a seismic-scale outcrop, using 814 sedimentological analysis, geological mapping and photogrammetric modelling. We 815 demonstrate the ability of flows to progressively incise >75 m into an underlying MTD, a 816 debrite, with remarkably steep margins (up to 70°). The evolution from erosion-and 817 sediment bypass-dominated to deposition-dominated is marked by aggradational stacking of 818 sand-rich channel-fill, exhibiting a high degree of homogeneity. Above this, stepped changes 819 in confinement coincided with a change in intrachannel architecture to laterally-migrating 820 channel bodies, follows by tabular, highly-aggradational fill. Furthermore, we examine the 821 sedimentological and stratigraphic evolution of two types of depositional terrace: an 822 entrenchment terrace, and the first outcrop example of a terrace deposit situated in a bend terrace is characterised by a lack of discernible bed thickness and grainsize trends but 1328 exhibits a higher degree of erosion and greater bed thicknesses. 1329