Australia: The Land Where Time Began

A biography of the Australian continent 

The Rhynie Chert – Stratigraphic setting and taphonomy

The host rocks of the Rhynie Chert date to the Early Devonian (Pragian; about 396 Ma), and are among a sequence of sedimentary rocks that were deposited by streams, rivers and lakes during the Devonian, at a time when Scotland, much of northern Europe, Greenland and North America were combined in a single large continent, Laurasia, that was located between 0^oS and 30^oS. The Devonian rocks at Rhynie are surrounded by the older rocks of the Dalradian metamorphic and plutonic igneous rocks from the Ordovician (77,78). The Rhynie sediments were deposited in a basin that was relatively narrow and trended northeast-southwest within these older rocks. The basin is a half-graben with a fault that was active at the time of deposition marking the western edge and the sedimentary rocks lie unconformably on basement rocks marking the eastern edge. The palaeoenvironment that is believed to have been present at the time of deposition of the chert is one of rivers and lakes that deposited a complex of sands that were cross-bedded in regions of high flow and muds on the floodplains and in shallow ephemeral lakes. Hydrothermal activity was centred on the fault system and this altered the subsurface rocks in the vicinity of the fault, and sinter was deposited around the hot springs and geysers at the surface in the Rhynie area. Strata within the basin were caused to dip towards the northwest by later Earth movements, while near Rhynie village the chert-bearing rocks are folded into a syncline which plunges to the northeast.

The condition of the organisms at the time of fossilisation (the degree of decay) and the degree and timing of replacement by silica both determine the variability of the preservation of the biota. The array of preservation types ranges from the 3D internal anatomy of those organisms that had been completely silicified at or soon after death that were preserved exquisitely, to organisms that had been preserved as compressed, coalified strips that were unidentifiable that had been silicified following decay and burial. Gut contents have been preserved, even delicate structures such as book lungs are silicified perfectly, in a few known instances in which the whole bodies (cf. moulted skins) of arthropods have been found. It is clear that some fossil arthropods are moults, such as one that was found which had a leg in the opisthosoma (abdomen) of a trigonotarbid, the leg moult having lodged in the shed skin of the opisthosoma during moulting. The plant fossils that have been found are in a variety of orientations, such as upright stems with terminal sporangia (spore cases) and rhizomes that are horizontal, which have been assumed to be in growth position, to prostrate straws which have collapsed and decayed and become part of a layer of leaf litter. Arthropod remains have been found in some sporangia (80). Though as these sporangia were among horizontal plant debris, it is believed most likely that these arthropods had entered empty (dehisced) sporangia when they were lying on the ground, possibly as a sheltering place to moult, and so not entering the sporangia prior to dehiscing to feed on the spores.

Silica precipitates as sinter, which is an amorphous, hydrated form of opaline form called Opal-A, in hot-spring environments. Opal-A is precipitated when the hydrothermal solution, which is supersaturated with respect to Opal-A, erupts at the surface as a geyser or hot-spring, which then cools. Factors such as pH, and the presence of dissolved minerals, organic matter and living organisms such as cyanobacteria may also affect precipitation, as well as temperature drop and evaporation of the hydrothermal fluids. Permineralisation is a process by which plant material is silicified by permeation and filling of voids, which contrasts with the direct replacement of cell walls, which is petrification, during which the organic structure is the template for the deposition of silica. The common soluble form of silica, silicic acid, polymerises with the loss of water to form opaline silica. Hydrogen bonding between the hydroxyl groups in the silicic acid, and the cellulose and lignin within the organic tissue, is involved in the nucleation of amorphous silica on wood and plant material. The histology of the plant is preserved by precipitation of silica within the cells and openings between cell walls. Complete silicification requires a regular, high influx of silica-containing solution, though the initial phase of silicification of organic matter may occur in a matter of days. Rapid, pervasive silicification in the Rhynie hydrothermal area was possible because of the regular outflow of solutions from hot-springs and geysers, and the silicification occurred before there was any significant cellular decay, and it was this process that resulted in the exquisite preservation of the plants in the Rhynie chert. Sinter is a light, porous substance which displays little similarity to chert rock. During burial and over time, however, the amorphous silica phase Opal-A becomes unstable and changes gradually into the crystalline form, quartz, which is more stable. As sinter transforms Opal-A to quartz, percolating fluids containing silica may precipitate still more crystals in voids and fractures in the rock so that the chert that results retains almost none of its original porosity.

Sources & Further reading

1. Selden, Paul & Nudds, John, 2004, Evolution of Fossil Ecosystems, Manson Publishing.