Incorporation of substrate blocks into mass transport deposits: Insights from subsurface and outcrop studies

Mass movements are common on the continental slope, affecting not only the subsequent sea floor morphology but often substantially modifying the underlying deposits. Various styles of substrate interaction have been recognised, representing the various degrees of involvement of the underlying material and its incorporation into the mass movement. This work presents a new style of basal interaction not previously described. Based on the morphology of the basal surface of a mass transport deposit, this can be recognised both in seismic data and in an outcrop analogue. A subsurface example, from an ca 100 m thick mass transport deposit located in Santos Basin, offshore Brazil, displays a basal surface with spoon-shaped scours or scoops. These scoops are of the order of tens up to 400 m in maximum dimension, where masses of underlying sediment have been removed and incorporated into the mass movement. Outcrops used for this work are located in La Rioja Province, Western Argentina, where the study involves a well-exposed ca 200 m thick mass transport deposit that crops out continuously over 7 km. Its basal surface is incised irregularly into the underlying sandstones, incorporating the blocks of sandstone into the mass movement. The striking similarities observed between outcrop examples and the northern Santos Basin suggest that they can be effective analogues, facilitating a comprehensive understanding of mass transport deposit dynamics across diverse basin environments.

and Kneller et al. (2016) have contributed to the understanding of MTDs and their relationships with the enclosing stratigraphy.Furthermore, MTDs play a critical role in petroleum systems as they may modify or remove underlying reservoir, contribute to stratigraphic trapping within the sea floor topography, influence subsequent turbidite systems (Armitage et al., 2009;Bull et al., 2020), and may act both as seals and occasionally as reservoirs.The ways by which mass flows interact with the sea floor and the impact of these flows on the sea floor topography are thus critical for the exploration of deepwater hydrocarbon prospects associated with stratigraphic traps.
Mass transport deposits often show significant internal heterogeneity and deformation, with their internal structural arrangement often systematically partitioned into discrete deformation domains (Lucente & Pini, 2003;Dykstra et al., 2011;King et al., 2011;Ogata et al., 2012Ogata et al., , 2014;;Joanne et al., 2013;Sobiesiak et al., 2016aSobiesiak et al., , 2016b;;Marini et al., 2022;Assis et al., 2023).Sobiesiak et al. (2018) have identified two end-member types of MTD basal contact: free-slip and no-slip.Free-slip contacts occur when the flow detaches from the substrate during translation, either by hydroplaning (Mohrig et al., 1998) or by the liquefaction of substrate (Sobiesiak et al., 2018), and is typically associated with a zone of high strain along the detachment zone at the base of the deposit.Conversely, no-slip contacts occur when the flow adheres to the substrate, and the strain front is located within the substrate itself.In this case, there may be a zone of high strain extending downwards into the substrate (Valdez Buso et al., 2015).Alternatively, the flow can erode the substrate by pushing forward and/ or ploughing into it, peeling back and commonly imbricating strips of substrate (Moscardelli & Wood, 2008;Doughty-Jones et al., 2019).Typically, these strips form by detachment along a particular (presumably weaker) horizon within the substrate, and are bounded also by steep side-walls parallel to the displacement direction, creating what have been described as 'cat claws' or 'monkey fingers' (McGilvery & Cook, 2003;Moscardelli et al., 2006;Sobiesiak et al., 2018).
In some cases, however, large, and often irregular blocks of material are detached from the substrate and incorporated into the flow (Dykstra et al., 2011;Garyfalou, 2015;Clare et al., 2015), often creating a distinctive 'speckly' texture in seismic horizon slices.No-slip basal contacts indicate that coherent mass flows can significantly erode a sandy substrate.Understanding this style of basal interaction has thus implications for predicting potential sub-MTD stratigraphic traps and the sealing capacity of MTDs in deepwater prospects.This paper presents a comparison of two systems, one in the subsurface based on three-dimensional (3D) seismic data and one example from outcrop, both of which show incorporation of material through a style of basal interaction not previously described, and hypotheses on the causes.

| Santos Basin
The subsurface study was conducted in the northern Santos Basin, offshore south-eastern Brazil, in an ca 1051 km 2 area located 140 km from the city of Rio de Janeiro (Figure 1A).The Santos Basin is related to the South Atlantic rifting and has a geological history spanning from the Early Cretaceous to modern times.The Turonian to lower Oligocene succession of this basin encompasses a prominent progradational wedge (Juréia progradation) in which numerous MTDs and turbidite systems developed (Moreira & Carminatti, 2004;Carlotto & Rodrigues, 2009;Berton & Vesely, 2016;Yamassaki & Vesely, 2022).Besides overall basin subsidence and high sediment supply, basin physiography and deepwater deposition were also strongly influenced by active salt tectonics of Aptian evaporites (Jackson et al., 2015).The MTD analysed in detail for the purpose of this study is the largest mappable MTD within the Eocene of the Juréia progradation (here named MTD 1) (Figure 1B).

| Paganzo Basin
A suitable area to characterise MTDs interaction at the outcrop scale was identified at Cerro Bola, located in the Carboniferous Paganzo Basin, Western Argentina (Figure 2A).Cerro Bola is located ca 30 km south-west of Villa Union in La Rioja Province (Milana et al., 2010;Dykstra et al., 2011).It forms a prominent west-vergent, doubly-plunging hangingwall anticline to an east-dipping thrust fault (Dykstra et al., 2011) associated with Late Neogene to Quaternary Pampean Range orogenic deformation (ca 4.5 Ma to present; Zapata & Allmendinger, 1996;Jordan et al., 2001).The Carboniferous succession in the area includes MTDs, turbidite lobe complexes and sheets (Liu et al., 2018;Fallgatter et al., 2019), and fluvio-deltaic sediments (Figure 2B), representing deposition at high latitudes during the late Mississippian to Pennsylvanian, Late Palaeozoic Ice Age (Valdez Buso et al., 2020, and references therein).The stratigraphic succession is ca 1000 m thick (Figure 2C) and exposed continuously for 10 km (Figure 2D).This outcrop offers exceptional twodimensional (2D) and 3D exposure for the comprehensive analysis of mass transport interactions (Figure 2E).Three distinct MTDs crop out prominently throughout the area 20554877, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/dep2.283 by University Of Aberdeen The Uni, Wiley Online Library on [10/06/2024].See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions)on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License (Figure 2D).They represent glaciomarine diamictites and subaqueous outwash material, initially deposited close to the grounding line of major ice sheets lying to the east (Valdez Buso et al., 2020, 2021).This material was subsequently gravitationally remobilised towards the westnorthwest (probably during minor glacial re-advances that steepened the depositional wedge) across the floor of a local sub-basin that was perhaps only a few hundred metres deep.The exposures at Cerro Bola represent the body of these MTDs; neither the point of origin (slide scar) nor the downflow termination of the deposits is seen.Both MTD 1 and MTD 2 contain variably deformed sandstone blocks identical to sandstones in the underlying fluviodeltaic units, as well as rafts of siltstone and sandstone turbidite protolith.Both MTD 1 and MTD 2 show evidence of significant interaction with the underlying substrate (Dykstra et al., 2011;Valdez Buso et al., 2015;Fallgatter et al., 2017;Sobiesiak et al., 2016aSobiesiak et al., , 2016bSobiesiak et al., , 2017Sobiesiak et al., , 2018Sobiesiak et al., , 2019)).The primary focus is on MTD 2 due to its superior exposure and clearer geological context.

| Subsurface data
This study utilised a time-migrated, 3D seismicreflection post-stack data volume from the seismic survey BS-500.OpendTect Pro®, a seismic interpretation software package developed by dGB Earth Sciences, was used for seismic interpretation.The study focussed on mapping the top and the base of a MTD (Hz2 and Hz1, respectively) every four in-lines and crosslines across the entire volume (Figure 3A).Le Bouteiller et al.'s (2019) criteria were used for the seismic interpretation of MTDs to characterise the deposit, and to analyse features such as morphology, basal and upper surfaces, slide scar and internal facies distributions (Figure 3B).In addition, seismic attributes such as polar dip, curvature, amplitude, energy, similarity and semblance, among others, were used to elucidate MTD characteristics (Assis et al., 2023).

| 3D Outcrop data
For this study, drones were used to photographically survey the 3D outcrops in Cerro Bola.The photographs were processed using Agisoft Metashape® to build 3D digital models using the SfM-MVS (Structure from Motion-Multi View Stereo) workflow.Agisoft viewer was used for the visualisation of the resulting 3D models, and Stratbox®, a software package provided by Imaged Reality Ltd, was used for characterisation, interpretation and measurements.The 3D models obtained during fieldwork were utilised to characterise and describe both the sandstone blocks and the irregular base of the MTD.

4
MTD 1 is tens of kilometres long and has variable thicknesses, with the greatest thickness (between 74 and 94 ms, ca 100 m thick) observed in its proximal domain near the north-east (see Assis et al., 2023).The predominant flow direction of the MTD is towards the south-east.The basal (Hz1) and upper (Hz2) bounding surfaces were mapped to examine their morphology and internal characteristics of the MTD (Figure 3A,B). 4.1.2| Basal Surface (Hz1)   This surface displays a predominant dip towards the southeast, eroding the underlying strata.A noteworthy erosional characteristic is the presence of quasi-periodic (concave-up depressions), referred to as 'scoops', cut into the subjacent layers (Figures 4A,B and 5A).These scoops are observed in a range of sizes, with the longest dimensions ranging from tens of metres up to 400 m.In addition, the presence of terraces can be observed in conjunction with blocks and a ramp hundreds of metres in size, that is perpendicular to the direction of translation, with a dip that is contrary to the flow direction (Figure 4A,B).This MTD is associated with a landslide scar dipping to the south-east and extending for several tens of kilometres along strike (Figure 4A).

| Internal structure
Mass transport deposit 1 has a predominantly chaotic pattern of reflectors, with low to moderate seismic amplitudes (Figure 3A).Internally it exhibits folds, normal and reverse faults, as well as large blocks (Figures 3A,B and 6), some of them imbricated, and some parallel reflectors, suggesting variable degrees and styles of deformation.Large-sized blocks are observed in the MTD.These large blocks, situated in the north-eastern sector of the proximal domain, are allochthonous, as they are not interacting with the underlying substrate, and are probably translated remnants of the parent material of the MTD (Figure 6A,B).The autochthonous blocks within the MTD are represented by chaotic reflectors in in-lines and speckly textures in Z-slices (Figure 6A,B,C).Seismic attribute analysis reveals significant geometrical and amplitude variations, highlighting the deposit's complexity due to erosion and deposition (Assis et al., 2023). 4.1.4| Upper Surface (Hz2)   Overall, the deposit displays a very rough upper surface.This rugosity tends to be expressed as elongated, arcuate to sinuous ridges and troughs oriented parallel to the parental slope (normal to flow direction).Ridges/troughs are near symmetrical and have wavelengths ranging from 100 to 300 m.In dip-oriented sections, it is possible to observe that this surficial morphology is a consequence of internal folds and imbricated slabs whose orientation tends to be consistent along strike.Protruding blocks within the MTD also give a rugose character to the upper surface (Figure 4C).Possible ponded turbidites (high amplitude, semi-continuous reflectors) can also be observed in the topographic lows of the MTD in interpreted sections (Figures 3A,B and 6A).

| Lower zone
The lower zone of MTD 2 ranges in thickness from 40 to 60 m (Sobiesiak et al., 2016a).The basal contact with the underlying fluvio-deltaic unit consists of a rugose surface, with relief in the order of metres to tens of metres in amplitude and tens to hundreds of metres in wavelength, comparable to the erosional features (scoops) found at seismic scale (Figure 5A,B).This lower zone is distinguished by the presence of sandstone blocks (Figure 7A) embedded within a predominantly silty matrix, with sand content that increases towards the blocks (Dykstra et al., 2011;Garyfalou, 2015).These sandstone blocks can locally constitute as much as 30% of the total volume of the lower zone of the MTD.Their dimensions vary, with some blocks measuring a few metres in length (Figure 7B), while others can reach up to ca 90 m in length and up to 15 m in thickness (Dykstra et al., 2011;Valdez Buso et al., 2015;Sobiesiak et al., 2016a).Internally, the sandstone blocks range from apparently structureless masses to enclaves with undeformed primary structures such as decimetre-scale cross-stratification, similar to structures in the underlying fluvio-deltaic sandstones, from which the blocks are petrographically indistinguishable (Garyfalou, 2015).Some sandstone blocks remain in partial contact with the underlying fluvio-deltaic unit (Figure 5B,C), apparently frozen in the process of removal.Sandy blocks are boudinaged and occasionally folded (Figure 7B), as are the sand stringers derived from them (Figure 7C,D).Also, the streaky layering in the deformed matrix between large boudins of sand may itself be folded.Folding occurs on scales from millimetres to decimetres.

| Middle zone
As described by Sobiesiak et al. (2016aSobiesiak et al. ( , 2016b)), the transition between the lower and middle zones of MTD 2 occurs over ca 15 m.This transitional contact is characterised by a vertical decrease and eventual disappearance of sand grains within the silty matrix.Within the middle zone, which ranges in thickness from 50 to 90 m, there is a prevalence of blocks and large enclaves of stratified siltstone and sandstone dispersed within a matrix predominantly composed of silty material.The siltstone enclaves contain thin to medium bedded, normally-graded fine-grained sands to silts (turbidites), often folded; they are interpreted as part of the protolith of the MTD.Although sandstone blocks are still present in the middle zone, they are less frequent and generally smaller in size than those found in the lower zone.

| Upper zone
The upper zone of MTD 2 ranges in thickness from 40 to 60 m (Sobiesiak et al., 2016a(Sobiesiak et al., , 2016b)).Within this zone, the green siltstone matrix contains clasts derived from the metamorphic basement cropping out along the basin margins (as in Sierra de Maz, 15 km to the north-west; Valdez Buso et al., 2015).The occurrence of sandstone and siltstone blocks in the upper zone is less frequent than that of those in the lower zone, and these blocks are generally smaller in size.The upper zone is characterised by the presence of thrusts and large-scale folding, as documented by Dykstra et al. (2011) and Sobiesiak et al. (2016a).

| DISCUSSION
Relationships between MTDs can be differentiated according to the degree of involvement with the substrate.With free slip, displacement is focussed along the basal surface of the MTD, and there is minimal substrate involvement, as is the case with hydroplaning debris flows (Mohrig et al., 1998;Sawyer et al., 2012;De Blasio et al., 2012), or softening of the shear layer by the incorporation of sea water (Elverhøi et al., 2005).Seismic studies have revealed features associated with basal erosion variously described as scours (Nissen et al., 1999;Posamentier & Kolla, 2003), glide tracks (Prior et al., 1984;Nissen et al., 1999), grooves (Bull et al., 2009;Posamentier & Kolla, 2003), striations (Bull et al., 2009;Gee et al., 2005), ramp and flat systems (Bull et al., 2009;Omosanya & Alves, 2013), megascours (Moscardelli et al., 2006), and downflow-diverging plan-view features described as cat claws (Moscardelli et al., 2006) or monkey fingers (McGilvery & Cook, 2003).Two different styles of substrate interaction can be differentiated (Sobiesiak et al., 2018).In one case, limited or zero slip along the basal surface of the mass movement (either the basal failure surface below the sea floor or, in the case of frontally emergent mass movements, at the sea floor itself; Frey-Martinez et al., 2006) produces grooves or striations, or leads to the delamination of the substrate, and the imbrication of stratigraphically discrete slices of material (Moscardelli et al., 2006).Such cases are often easily discerned in seismic data, both in section (Gong et al., 2014), and as 'pressure ridges' visible in horizon slices or visualisations of the upper surface of the MTD (Bull et al., 2009), and 'cat-claws' at the basal surface.
In the other case, of highly erosive mass movements (Sobiesiak et al., 2016a(Sobiesiak et al., , 2019)), substrate material is incorporated into the body of the MTD.This is visible in outcrop, as in the case presented here, as identifiable blocks of substrate material, generally completely enclosed within the MTD matrix.In seismic data, blocky MTDs are identified by speckly or granular texture in horizon slices (Figure 6A).It is generally not possible to differentiate seismically between included substrate and fragments of protolith, although mass balance arguments indicate that the volumes involved may be considerable (Nugraha et al., 2022).However, the mechanism by which this material is incorporated into the mass flow has not previously been evident.The seismic example shown here demonstrates a previously undescribed form of basal surface that, rather than showing delamination of substrate in slip-parallel strips, consists of irregular to ovoid depressions or scoops.In the outcrop example and in the 3D model (Figure 5A,B,C) shown here, a comparably irregular basal surface is seen that demonstrably represents the incorporation of blocks of substrate material into the MTD.Blocks of substrate have become detached, sometimes only partially, with varying degrees of internal deformation (implied by the local to pervasive absence of primary sedimentary structures), and incorporated into the MTD.This implies limited or zero slip along the interface between the mass movement and the substrate, as also indicated by the local ductile strain within the substrate.
The delamination style of substrate deformation/incorporation depends on the presence of weak layers to facilitate slip, implying well-stratified substrate.The irregular shapes of the blocks and the irregularity of the interface between the MTD and the substrate in the cases described here are possibly a reflection of the rather isotropic nature of the highly sandy fluvio-deltaic substrate, lacking any clear internal horizons of easy slip along which planar detachments might form.The incorporation and disaggregation of sand from the margins of the blocks into the MTD matrix show that they were still ductile and uncemented.Nonetheless, the discrete form and the preservation of primary structures within some of them indicates a degree of coherence.This is suggestive of a significant rheological contrast between the flow and the substrate.

| CONCLUSIONS
It is possible to infer that the irregular basal surface (Hz1) of the Eocene MTD 1 in Santos Basin with its numerous concave-up erosion features ('scoops') was generated by the lifting up or plucking of material from the underlying stratigraphy over a broad area in a quasi-periodic fashion.These features are directly comparable to the basal interactions of MTD 2 with the substrate at Cerro Bola, representing a style of basal interaction that has not previously been described.It is suggested here that this basal surface geometry may be widespread where the substrate is relatively homogenous, perhaps especially where the sediments are semi-consolidated.

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I G U R E 1 (A) Study area location in northern Santos Basin (based on GeoANP, 2021 and NERUS, 2021 data).(B) Crosssection showing the stratigraphic position of the studied MTD 1.

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I G U R E 2 (A) Study area location in western Argentina.(B) Geological map of the Cerro Bola area (modified from Dykstra et al., 2011).(C) Stratigraphic column showing the position of MTD 2 (modified from Dykstra et al., 2011).(D) Cerro Bola anticline showing the different MTDs and fluvio-deltaic intervals.(E) The different zones of MTD 2.

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I G U R E 3 (A) In-line showing the mapped horizons.(B) Interpreted surfaces and MTD characteristics using Instant Cosine Phase attribute.

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Base and top surfaces of MTD 1. (A) Polar dip attribute showing the slide scar, ramp and basal erosional features as the concave up scoops.(B) Most Negative Curvature attribute showing, in another perspective, the basal erosions of this MTD.(C) Polar Dip Attribute showing different features of the upper surface (MTD top), as blocks and ridges.20554877, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/dep2.283 by University Of Aberdeen The Uni, Wiley Online Library on [10/06/2024].See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions)on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

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I G U R E 5 (A) In-line section showing the Eocene MTD 1 and its erosive base.The basal surface, depicted in red, defines a 2D view showing the different scoops.(B) 3D model and view of the basal erosion of MTD 2 in Cerro Bola outcrops.The size of the scoops is around 200 m and they substantially affect the underlying strata.(C) Outcrop photography showing the underlying fluvio-deltaics 2 cut by MTD 2.

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I G U R E 6 (A) Z-slice showing the location of figures B and C. Notice the size of the ramp and the allochthonous block.(B) In-line showing the ramp, faults within the mass transport and ponded turbidites on top of MTD 1. (C) In-line showing in detail one allochthonous block and several autochthonous blocks as well as the scoops or erosional concave up features.

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I G U R E 7 (A) Snapshot taken from a 3D model of the lower zone of the blocky MTD 2. (B) Boudinaged sandstone blocks within the MTD matrix.(C) Sandstone streaks sheared from the margins of a sandstone block (centre).(D) Sandstone blebs and streaks derived from blocks.