Sole marks reveal deep-marine depositional process and environment: Implications for flow transformation and hybrid-event-bed models

Abstract

Deposits of sediment gravity flows in the Aberystwyth Grits Group (Silurian, west Wales, United Kingdom) display evidence that sole marks are suitable for reconstructing depositional processes and environments in deep-marine sedimentary successions. Based on drone imagery, 3D laser scanning, high-resolution sedimentary logging, and detailed descriptions of sole marks, an outcrop 1600 m long between the villages of Aberarth and Llannon was subdivided into seven lithological units, representing: a) mudstone-poor, coarse-grained and thick-bedded submarine channel fills, dominated by the deposits of erosive high-density turbidity currents with flute marks; b) mudstone-rich levee deposits with thin-bedded, fine-grained sandstones formed by low-density turbidity currents that scoured the bed to form flute marks; c) channel–lobe transition-zone deposits, dominated by thick beds, formed by weakly erosive, coarse-grained hybrid events, with pronounced mudstone-rich or sandstone-dominated debritic divisions and groove marks below basal turbiditic divisions, and with subordinate amounts of turbidites and debris-flow deposits; d) tabular, medium- to thick-bedded turbiditic sandstones with flute marks and mixed sandstone–mudstone hybrid event beds mainly with groove marks, interpreted as submarine lobe-axis (or off-axis) deposits; and e) tabular, thin- to medium-bedded, fine-grained, mainly turbiditic sandstones mostly with flute marks, formed in a lobe-fringe environment. Both lobe environments also comprised turbidites with low-amplitude bed waves and large ripples, which are interpreted to represent transient-turbulent flows. The strong relationship between flute marks and turbidites agrees with earlier predictions that turbulent shear flows are essential for the formation of flute marks. Moreover, the observation as part of this study that debris-flow deposits are exclusively associated with groove marks signifies that clay-charged, laminar flows are carriers for tools that are in continuous contact with the bed. A new process model for hybrid event beds, informed by the dominance of tool marks, in particular grooves, below the basal sand division (H1 division of Haughton et al. 2009) and by the rapid change from turbidites in the channel to hybrid event beds in the channel–lobe transition zone, is proposed. This model incorporates profound erosion of clay in the channel by the head of a high-density turbidity current and subsequent transformation of the head into a debris flow following rapid lateral flow expansion at the mouth of the channel. This debris flow forms the groove marks below the H1 division in hybrid event beds. A temporal increase in cohesivity in the body of the hybrid event is used to explain the generation of the H1, H2, and H3 divisions (sensuHaughton et al. 2009) on top of the groove surfaces, involving a combination of longitudinal segregation of bedload and vertical segregation of suspension load. This study thus demonstrates that sole marks can be an integral part of sedimentological studies at different scales, well beyond their traditional use as indicators of paleoflow direction or orientation.

Hybrid event bed character and distribution in the context of ancient deep‐lacustrine fan models

“…Increasing mud content likely influenced, or was influenced by, changes in velocity throughout, and along the flows length. These combined factors resulted in segregation into at least three rheologically-contrasting flow components, which deposited H3a-H3c Baas et al (2021). associated the H3a and H3b units (ofHussain et al, 2020) with "a progressive increase in cohesivity as a result of vertical segregation of the suspended load during deceleration", and the examples provided in this study contribute to this discussion.…”

“…14F; sensu Stevenson et al ., 2020), inclusion of finer grained material from initial muddy substrates into flows (i.e. substrate de‐lamination, sensu Fonnesu et al ., 2016), and associated flow deceleration ( sensu Patacci et al ., 2014; Southern et al ., 2015; Baas et al ., 2021), which collectively act to drive mixed flow behaviour during the initiation phase.…”

“…All rights reserved character widely explored (e.g. Davis et al, 2009;Hodgson et al, 2009;Kane et al, 2017;Spychala et al, 2017a;2017b;Bell et al, 2018;Pierce et al, 2018;Baas et al, 2021), their occurrence and variability in deep-lacustrine fan settings is less understood, with there being few published examples (Hovikoski, et al, 2016;Tan et al, 2017;Dodd et al, 2019;Yang et al, 2019;Zhang & Dong, 2020;Shan et al, 2020).…”

“…In particular, the H3 unit is considered as the product of a cohesive debris flow, with upward increase in mud-matrix content associated with longitudinal heterogeneity in the passing flow (Haughton et al, 2003(Haughton et al, , 2009Hussain et al, 2020;Baas et al, 2021). Haughton et al (2009) described H3 as "a complex mixture of sandy mudstone or muddy sandstone with or without mud-clasts, sand injections, 'sand patches', outsized granules and shear fabrics, and often displays a segregation of carbonaceous fragments to the top where they are sometimes laminated".…”

“…Fonnesu et al (2018) divided a succession of HEBs into six 'bed types' (HEB-1 to HEB-6), based on "differences in texture of the muddier and chaotic division", and therefore recognized internal variability within the H3 unit. More recently, H3 has been sub-divided into two sub-units (H3a and H3b) and possible flow processes have been discussed (Hussain et al, 2020;Baas et al, 2021). The observed internal variation of the H3 unit, as illustrated above, suggests that an improved understanding of debrite emplacement processes during mixed flow behaviour events is still required.…”