Cryptic burrow traces in black shales – a petrographic Rorschach test or the real thing?

Abstract

In black shale studies, apparent lack of bioturbation is commonly taken as an indication of anoxic bottom waters. Yet, modern oxygen-stressed environments show that even at suboxic levels (0.0 to 0.2 ml l À1 oxygen), microscopic eukaryotic benthic organisms live in the uppermost millimetres to centimetres of the substrate. Known as meiofauna, these organisms disturb the primary fabric as they move through the sediment. These modern examples inspired students of the rock record to report sub-millimetre irregularwiggly features in black shale thin sections as meiofaunal burrows. Preparation flaws in thin section manufacture can cause reduced light transmission that results in darkened regions when viewed in transmitted light, and one wonders whether micro-burrows reported in the literature are indeed burrows, or alternatively artifact-induced optical illusions. Examining shale thin sections of Archean to Tertiary age showed: (i) that burrow-like features are common regardless of age; and (ii) that burrow identification varies significantly between observers. When transposing locations of presumed traces onto scanning electron microscope images of the same field of view (diamond polished surface), no evidence of fabric modification at locations of presumed burrows was observed. Microfabric within and outside 'burrows' was the same, and larger fabric elements traversed 'burrows' uninterrupted. Once both sides of the rock slice were polished, presumed burrows could no longer be observed, indicating that damage to the underside of the thin section caused the mistaken identification of micro-burrows. Because burrows emplaced in water-rich muds must undergo substantial compaction-induced 'flattening' and deformation prior to lithification, geometries of presumed burrows that disagree with a plausible compaction regime provide an additional filter to separate potential burrows from artifact-induced features. Recognizing meiofaunal disruption is an important aspect for accurate palaeoenvironmental interpretation of black shales. Yet, initial assessment via optical microscopy needs to be verified through scanning electron microscope-based examination of shale fabrics.

Ichnology of Muddy Shallow-Water Contourites From the Upper Jurassic–lower Cretaceous Vaca Muerta Formation, Argentina: Implications for Trace-Fossil Models

“…Biogenic structures in fine-grained deposits are notoriously difficult to identify and classify (see Schieber et al 2021). Also, identifying trace fossils in core presents its own challenges (see Pemberton et al 2001).…”

Reply to the Discussion by Al‐Mufti on “On the origin and significance of composite particles in mudstones: Examples from the Cenomanian Dunvegan Formation” by Li et al. (2021), Sedimentology, 68, 737–754

“…If raw images were given to, say, ten individuals with a set of instructions on how to mark particles, we are confident that there would be ten different versions of particle arrangements. When in search for patterns, humans invariably ‘find a way’ (Schieber et al ., 2021), but whether such perceived patterns have a basis in reality begs serious scrutiny. As detailed below, from our understanding of mud‐aggregate generation, transport, deposition and burial, the particles as marked by Al‐Mufti (2022) cannot be considered intraclastic aggregates.…”