Physical and biological functioning in Proterozoic rivers: evidence from the archetypal pre-vegetation alluvium of the Torridon Group, NW Scotland

Abstract

In modern rivers, vegetation affects hydrological, geomorphological and sedimentological functioning, so extant fluvial systems can provide only partial analogues for those rivers that operated before the evolution of land plants. However, pre-vegetation rivers were the norm for the first 90% of Earth's history and so a better understanding of their sedimentary product can provide insights into both the fundamental underlying mechanisms of river behaviour and the ways in which fluvial processes operated on ancient Earth. In addition to a short review of the history of research into pre-vegetation alluvium, this paper presents a fieldwork-based case study of the later Proterozoic Torridon Group, which contains some of the most extensive and easily accessible exposures of pre-vegetation alluvium worldwide. Three alluvial architectural deposits have been recognized: (1) channel-bedform deposits (c. 80%); (2) barform deposits (c. 20%); and (3) out-of-channel deposits (≪1%). Channel-bedform deposits have erosional bases and most frequently stack vertically to form thick multistorey channel-bedform sequences. The preferential preservation of these deposits, which record the deepest parts of river channels, suggests that channel migration had a dominant control on preservation in the Torridon Group. Less frequently, channel-bedform deposits pass upwards into a genetically related barform deposit. Barform preservation in these instances is interpreted to be due to channel avulsion, which protected the barforms from reworking. Channel-bar thickness, measured from the basal erosional surface of a channel-bedform deposit to the top of its associated barform deposit, indicates minimum water depths of 1.7 to 8.0 m. Downstream-accreting barform deposits are most frequent, but lateral and upstream modes of accretion are also well represented. Dominant southeastward-palaeoflow directions imply that the Torridonian rivers were sourced from the Grenvillian Mountain Belt. The preserved architectural deposits and narrow dispersal of palaeocurrent data are explained by interpreting the Torridon Group as the alluvium of dominantly low-sinuosity rivers, with signatures recording autogenic fluvial adjustments. In the few rare instances where out-of-channel deposits are preserved, they contain fossil evidence for microbial mats, which prove that not all Proterozoic river systems were wholly abiotic. The overall characteristics of the Torridon alluvium, in terms of its ubiquitous highly tabular beds of sand-grade or coarser material, make it an archetypal example of pre-vegetation alluvium as known globally.Thematic collection: This article is part of the SJG Collection on Early-Career Research available at: https://www.lyellcollection.org/cc/SJG-early-career-research

Sustained fluvial deposition recorded in Mars’ Noachian stratigraphic record

“…With regard to channel and barform facies, evidence of relatively stable, deep-channelled drainage on pre-vegetation Earth is being increasingly reported 37 . Such findings are helping to dispel notions that pre-vegetation rivers were ubiquitously wide and shallow 49 , an observation that can now be extended to Mars (Fig. 2).…”

A new stratigraphic framework for the early Neoproterozoic successions of Scotland

“…The Stoer and Torridon groups occur west of the Moine Thrust, are only weakly deformed and have been the foci of detailed sedimentological studies as exemplars of pre-land-plant fluvial systems preserved as several-kilometre-thick successions of mostly trough and planar cross-bedded feldspathic sandstone (e.g. Selley 1969;Stewart 1982Stewart , 1988Stewart , 2002Nicholson 1993;Owen and Santos 2014;Ielpi and Ghinassi 2015;McMahon and Davies 2020). They also contain well-preserved, organic-walled microfossils postulated to be one of the earliest non-marine eukaryotic biotas (Strother et al 2011;Brasier 2012, cf.…”

Pre‐Vegetation, Single‐Thread Rivers Sustained by Cohesive, Fine‐Grained Bank Sediments: Mesoproterozoic Stoer Group, NW Scotland

“…Despite the increasing recognition of planform variability in pre‐Silurian rivers (Ganti et al., 2019; Ielpi et al., 2016; Ielpi & Rainbird, 2016; McMahon & Davies, 2020; Santos & Owen, 2016), it remains unclear how unvegetated channel banks were sufficiently stable to foster relatively deep channels, in which flow likely consistently exceeded the threshold stress of sediment motion (Ganti et al., 2019; Lapôtre et al., 2019). This knowledge gap, with an apparent domination of channel facies relative to floodplain facies in pre‐Silurian fluvial strata, has led to the assumption that pre‐vegetation, single‐thread rivers were rare and/or limited to smaller streams (Davies et al., 2017, 2020; Long, 2011; McMahon & Davies, 2019).…”