Taphonomy of the middle Cambrian (Drumian) Ravens Throat River Lagerstätte, Rockslide Formation, Mackenzie Mountains, Northwest Territories, Canada

Abstract

The Middle Cambrian (series 3, Drumian, Bolaspidella Biozone) Ravens Throat River Lagerst€ atte in the Rockslide Formation of the Mackenzie Mountains, northwestern Canada, contains a Burgess Shale-type biota of similar age to the Wheeler and Marjum formations of Utah. The Rockslide Formation is a unit of deep-water, mixed carbonate and siliciclastic facies deposited in a slope setting on the present-day northwestern margin of Laurentia. At the fossil-bearing locality, the unit is about 175 m thick and the lower part onlaps a fault scarp cutting lower Cambrian sandstones. It consists of a succession of shale, laminated to thin-bedded lime mudstone, debris-flow breccias, minor calcareous sandstone, greenish-coloured calcareous mudstone and dolomitic siltstone, overlain by shallow-water dolostones of the Broken Skull Formation, which indicates an overall progradational sequence. Two~1-m-thick units of greenish calcareous mudstone in the upper part exhibit soft-bodied preservation, yielding a biota dominated by bivalved arthropods and macrophytic algae, along with hyoliths and trilobites. It represents a low-diversity in situ community. Most of the fossils occur in the lower unit, and only the more robust components are preserved. Branching burrows are present under the carapaces of some arthropods, and common millimetre-sized disruptions of laminae are interpreted as bioturbation. The fossiliferous planar-laminated calcareous mudstone consists of chlorite, illite, quartz silt, calcite and dolomite and is an anomalous facies in the succession. It was deposited via hemipelagic fallout of a mixture of platform-derived and terrestrial mud. Geochemical analysis and traceelement proxies indicate oxic bottom waters that only occasionally might have become dysoxic. Productivity in the water column was dominated by cyanobacteria. Fragments of microbial mats are common as carbonaceous seams. Complete decay of soft tissues was interrupted due to the specific sediment composition, providing support for the role of clay minerals, possibly chlorite, in the taphonomic process. □ Burgess Shale, Cambrian, Lagerst€ atte, Mackenzie Mountains, taphonomy.Julien Kimmig [jkimmig@gmail.com], and Brian R. Pratt [brian.pratt@usask.ca],

Coprolites in the Ravens Throat River Lagerstätte of Northwestern Canada: Implications for the Middle Cambrian Food Web

“…On the other hand, some BST deposits do show that bioturbation was not as rare as often assumed (Caron et al 2014). Thin sections and sawn surfaces oriented perpendicular to bedding reveal local, millimeter-sized domains of disturbed lamination due to small horizontal and vertical burrows that have been compacted during burial by about 80-85% (Kimmig and Pratt 2016), an estimate that accords with other evidence in BST Lagerstätten (Whittington 1985).…”

“…Elemental composition of about 20 rock samples was determined using a JEOL JXA-8600 Superprobe with backscattered scanning electron microscope (Kimmig and Pratt 2016). However, elemental mapping was not performed on the carbonaceous specimens because they have relief, which would give distorted quantitative results.…”

“…Sedimentary structures like ripples and scoured surfaces are absent. The lithology, geochemistry, and ichnology indicate deposition on a deeper water ramp under an oxic to disoxic water column (Kimmig and Pratt 2016). Along with relatively rare soft-bodied organisms, articulated and disarticulated ptychoparioid trilobites (including Elrathia), agnostoid arthropods (Hypagnostus), and hyoliths (Haplophrentis) are sporadically present.…”

“…BIOTURBATION A single specimen of a diffuse, centimeter-sized Rhizocorallium-like trace visible on one bedding plane and some simple burrows appearing as furrows under one arthropod carapace (fig. 8A-8D in Kimmig and Pratt 2016) give the impression that bioturbation is uncommon in the Lagerstätte interval. This would be consistent with the rarity of ichnofossils in other BST deposits (e.g., Gaines and Droser 2010;Mángano 2011), which has been taken as evidence for anoxic to dysoxic bottom waters.…”

Coprolites in mid-Cambrian (Series 2-3) Burgess Shale-type deposits of Nevada and Utah and their ecological implications

“…The length of at least 85 mm in some of the Morphotype 1 burrows, together with the relatively rare horizontal burrows in both the Pioche and the Spence, indicate the environment in the Great Basin during the Cambrian might have, at times, been well oxygenated and allowed for bioturbation at a larger scale than in some other BST deposits (Garson et al 2012, Kimmig & Pratt 2016. However, it is also possible that individualistic taphonomic or diagenetic pathways present in other BST deposits did not allow for the preservation of the kinds of burrows found in the Great Basin deposits.…”