Geology and setting

Geology and setting

The geology of the Rhynie area was originally mapped and described by Geikie in 1878 with further work by Wilson and Hinxman (1890) and Read (1923). The most recently revised map was published in 1993: BGS Sheet 76W. However, it has only been in recent years following the drilling programmes undertaken by the University of Aberdeen that the geology and particularly the sedimentary succession or lithostratigraphy is better constrained (Rice et al. 2002).

The sediments which host the Rhynie chert, as mentioned in previous pages, are Early Devonian (Pragian) in age and form a small part of what is commonly known as the Old Red Sandstone or ORS. The ORS comprises a sequence of continental sediments deposited by streams, rivers and lakes throughout the Devonian Period when Scotland, much of northern Europe, Greenland and North America formed a large continent called Laurussia or the 'Old Red Continent', located between 0o and 30o south of the equator. During the Early Devonian (Pragian) the Rhynie area was situated in the subtropics around 28o south of the equator (see inset below).

Palaeogeographical map of Laurussia or the 'Old Red Continent' during the Early Devonian around 400Ma. At this time the Rhynie area (R) was situated approximately 28o south of the equator (Copyright owned by Dr Ronald Blakey, Northern Arizona University).

Simplified geological map of the Rhynie outlier .At Rhynie, the sediments which host the chert form an outlier surrounded by a basement of Dalradian metamorphic and Ordovician plutonic igneous rocks (see inset left). The Rhynie sediments were deposited in a relatively narrow, northeast - southwest trending basin set within these older rocks.

The basin in the south and the northern most region is a half graben,  the western edge  marked by a low angle (at the current level of erosion) extensional fault system which was active at the time of deposition during the Early Devonian (see below), whereas at the eastern edge of the basin the sediments lay directly on the basement rocks, the contact representing an unconformity. The central to northern area, containing the Rhynie chert locality is now believed to represent a completely fault-bounded strike-slip basin caused by regional trans-tensional forces (Rice and Ashcroft in press). Within the basin the sedimentary rocks are tilted, generally dipping towards the northwest, though the chert-bearing rocks in the area to the northwest of Rhynie village are folded into a syncline, the axis apparently plunging to the northeast.

The cherts are associated with shales and sandstones towards the top of the sedimentary succession and are only known to occur in situ in the vicinity of the village of Rhynie itself, quite close to the faulted western margin of the basin (see inset below). Results from the drilling programmes suggest much of the sedimentary succession here is heavily disrupted by faulting, with the area being divided into a number of small faulted blocks along the main fault zone. The main features including the sedimentology and lithostratigraphy are described below.

Simplified geological map of the area north and west of Rhynie village

The Boundary Fault

A sample of core from below the fault zone at Rhynie showing green-coloured heavily fractured and hydrothermally altered basic igneous rock, norite (N) with fractures mineralised and sealed by chert (C).The fault zone forming the western boundary of the basin was cored during the 1997 drilling programme. At the base of the borehole 97/2 and lying between the Devonian sediments and the Ordovician basic igneous rocks (norite) which  make up the basement in the area around Rhynie village, the fault zone is heavily mineralised. This fault which surfaces to the west of Rhynie probably acted as one of the main conduits for the flow of hydrothermal fluids from deep reservoirs to the hot spring surface expressions. The fault zone comprises a multi-phase breccia of basic igneous rocks set in a matrix of chert and cut by calcite and quartz veins. The igneous rocks immediately below this fault are also intensely hydrothermally altered, fractured and mineralised (see inset right).

The Basin Fill

There is a variety of rock types that comprise the fill of the Rhynie basin including primarily sedimentary rocks (mainly sandstones and shales) and volcanic rocks. Traditionally the sequence of rocks has been divided into three main lithostratigraphic units: the Tillybrachty SandstoneQuarry Hill Sandstone and Dryden Flags formations; the latter unit in the Rhynie area hosting the cherts (see inset below). See Rice et al. (2002) for details of the stratigraphy. Click on the formation names in the inset below for an outline of the lithologies encountered in each and their inferred environments of deposition.

Generalised stratigraphy of the Rhynie outlier (left) and the succession in the area northwest of Rhynie village (right) (after Rice et al. 2002).

Tillybrachty Sandstone Formation

This lithostratigraphic unit comprises the lower part of the sequence of the basin fill and is laterally variable in character. In much of the basin the unit is dominated by coarse-grained clastic rocks, primarily locally derived sandstones and conglomerates. In the area of Rhynie itself much of the unit appears to comprise a significant thickness of tuffs with interbedded sandstones (Rice et al. 2002). Some of the tuffs appear to represent direct air-fall deposits, but most appear to have been water-lain and were probably derived from eroding ash cones.

Thin section of andesite showing vesicles (v) and plagioclase phenocrysts (p). Most of the plagioclase and mafic minerals have been hydrothermally altered (scale bar = 1mm). Note: The blue colour in the vesicles is dyed epoxy resin to show void space in the thin section.Concurrent volcanic activity is also recorded in the rocks of the Tillybrachty Sandstone Formation in the form of extrusive andesite lava flows. The andesites show many primary textures that were preserved at the time the lava cooled and solidified such as vesicles and phenocrysts, however, due to later passage of hot fluids through the lavas the original minerals that make up the rock have been highly altered (see inset left).

The sedimentary rocks of the Tillybrachty Sandstone Formation appear to represent localised deposition in an alluvial environment. The conglomerates and coarse-grained sandstones were probably initially deposited as alluvial fans at the basin margins with sediment being distributed further into the basin by sheet flood events. Localised volcanic activity is also evident by the presence of andesitic lava flows and tuffs.

Quarry Hill Sandstone Formation

This lithostratigraphic unit succeeds the Tillybrachty Sandstone Formation. It occurs throughout much of the basin and is particularly distinctive in the south west of the area where it outcrops in the disused sandstone quarries on Quarry Hill (see inset below). This formation primarily comprises stacked, well bedded, current rippled, laminated and cross-stratified sandstones rich in feldspar grains. In this particular area these sandstone beds form large-scale channelised deposits commonly exhibiting erosive bases with mud rip-up clasts and are occasionally interbedded with scoured thin shale horizons.

Exposure of the Quarry Hill Sandstone Formation at Quarry Hill, Rhynie. The hammer (arrowed) is 40cm long.

In the Quarry Hill area the sandstones locally yield casts of transported plant debris, some of which are particularly large (see inset below). 

Base of a sandstone bed in the Quarry Hill Sandstone Formation at Quarry Hill, Rhynie, showing fossilised plant debris (P).

Of these plant remains, and perhaps one of the most enigmatic, may tentatively be assigned to the genus Prototaxites (belonging to an extinct group of Palaeozoic plants called the nematophytes). When the area was actively being quarried in the late 19th and early 20th centuries, these plant fossils were occasionally found in the form of logs preserved as sandstone casts. One specimen, recorded by Newlands (1913) as "a plant fragment" and nearly 1.3 m in length, is now housed in the collection of the Department of Geology and Petroleum Geology, University of Aberdeen (see inset below).

Sandstone cast of a probable nematophyte 'log' from the quarries at  Quarry Hill, Rhynie, as reported by Newlands (1913) (scale bar = 10cm).

Trace fossils have also occasionally been found on bed surfaces including Diplichnites, a preserved trackway about 5cm wide, probably created by a large myriapodous arthropod (see inset below).

The under surface of a sandstone bed from Quarry Hill showing the trace fossil Diplichnites. This is the cast of a trackway, most likely made by a large myriapodous arthropod, such as a millipede or arthropleurid. The shape of the footprints suggests the direction in which the animal was moving was from left to right in this picture

Close-up of Diplichnites. Notice a second trackway made by a smaller individual crossing the bottom right corner of the sandstone slab.

The "White Sandstones Unit" encountered in Borehole 97/2, north west of Rhynie village, is a 50m interval of hydrothermally altered white, laminated, rippled and cross-stratified sandstones with minor shale interbeds. It is thought to represent at least part of the formation in this area (see insets below). Locally the sandstones are heavily cemented by calcite. In this borehole its basal contact is with a hydrothermal breccia above the main fault zone and, with increasing shale content, passes upwards into the more mud-rich beds of the Dryden Flags Formation explained below.

Typical features of the White Sandstones Unit in Borehole 97/2

Vertical burrows (B) cutting ripple laminated sandstone. Current ripples (C) in sandstones with dark carbonaceous debris.Graded sandstone bed with sharp erosive base (S) fining upwards into more argillaceous current rippled sandstone (C). Ripped-up mud clasts (M) at the base of a cross-stratified bed.

The overall environment that seems to be represented by this unit is one dominated by shallow fluvial channels of a river system flowing towards the northeast along the axis of the basin. The occasional shale horizons may in part represent poorly preserved overbank and floodplain deposits.

The lateral change from the more stacked, well-bedded sand-rich lithofacies at Quarry Hill to the dominantly thinly bedded sands and minor shales in the area northwest of Rhynie village suggests the presence of larger-scale fluvial channels in the Quarry Hill area.

Dryden Flags formations

The Rhynie cherts occur within the Dryden Flags Formation which forms the upper part of the of the sequence of the basin fill. It is a sequence of thinly bedded micaceous sandstones (traditionally termed 'flags') and shales, that were previously recognised as the Dryden Flags and Shales and in part may represent a lateral equivalent to the Quarry Hill Sandstone Formation (see Rice et al. (2002) for details of the stratigraphy described below).

The succession of the Dryden Flags Formation in the Rhynie area, following the drilling programmes by the University of Aberdeen and recent studies, can now be divided lithologically into four discrete units. The lowest unit, informally termed the "Shales and Muddy Sandstones Unit" grades up from the "White Sandstones Unit" below and comprises primarily thickly bedded (up to 1m thick), erosively based massive muddy sandstones interbedded with thin shales and siltstones which show abundant soft-sediment deformation features. This unit passes upwards into a thick sequence of dark shales with thin fluidised and disrupted sand and silt laminae, informally termed the "Lower Shales Unit" (see inset below left).

Laminated shales with fluidised and disrupted sandy and silty laminae in the "Lower Shales Unit" (Dryden Flags Formation) Massive chert with very well preserved axes of Aglaophyton major (A) in the "Rhynie cherts Unit" (Dryden Flags Formation).

Above the shales lies the chert-bearing part of the sequence, informally termed the "Rhynie Chert Unit". Comparing borehole and trench data the chert beds (see inset above right) are laterally impersistant (Trewin & Wilson 2004). One borehole (19c) studied by Clare Powell exhibited 53 individual chert beds over a depth interval of 26m, many of which are plant-bearing (Powell et al. 2000b) (for a more detailed review of the chert morphologies and textures see the following section on Chert Textures). The chert horizons are variably interbedded with silicified compacted sandstones with organostylolites, together with thin shales and siltstones similar in lithology to the units below and above. By comparing the biota content and textures of the discontinuous plant-bearing chert beds of the Rhynie Chert Unit with Recent plant-bearing sinters and modern hot spring environments at Yellowstone National Park, USA, Trewin and Wilson (2004) suggest that much of the Rhynie chert succession represents sinter deposition on the distal, cooler reaches of a hot spring outwash apron.

The uppermost lithological unit in the Dryden Flags Formation is informally termed the "Upper Shales Unit" and is at least 13m thick. It comprises primarily green-coloured laminated shales (locally exhibiting desiccation cracks) variably interbedded with impersistant thin sandstone beds that often display sharp to erosive bases, mud rip-up clasts, current ripple laminae and graded bedding. Patchy calcite cementation is present in the sandstones. Similar lithologies host the Windyfield chert, some 700 m to the northeast of the Rhynie chert locality, and therefore it has been suggested the Windyfield chert probably occurs within this younger unit (Rice et al. 2002; Rice & Ashcroft in press).

The overall environment represented by the rocks of the Dryden Flags Formation appears to be an alluvial plain setting with mainly overbank deposition on floodplains and in ephemeral lakes with occasional sheet-flood events. The cherts and their interbedded sediments representing localised deposition of subaerial and subaqueous sinters from hot spring and geyser activity and the accumulation of soils on floodplains (see inset below).

Environmental reconstruction of the Rhynie area during the deposition of the Dryden Flags Formation. The background setting is an alluvial plane with an axial river system (middle distance), floodplain with patchy soil development and plant growth (center foreground), shallow ephemeral lakes (right foreground) and local eroding volcanic ash cones (far distance). An active geyser vent is seen in cross-section (left foreground) with sinter deposits and localised freshwater ponds. Black bands represent earlier, buried sinters, sealed vents and fractures (after Trewin 1994).

Basin Evolution

The Rhynie basin was formed as a result of trans-tensional forces affecting the Dalradian metamorphic and Ordovician igneous rocks in the northeast Grampian area following the closure of the Iapetus ocean. Based on present knowledge (Rice et al. 2002) the evolution of the Rhynie basin can be described in diagrammatic form.

Regional crustal extension and initiation of half-graben basin. Locally derived sands, conglomerates and tuffs with basin-wide andesitic lava flows (after Rice et al. 2002).

Continued crustal extension and subsidence. Dominantly a fluvio-lacustrine environment with mature cross-bedded sands deposited in an axial river system  and argillaceous sediments deposited on floodplains and in shallow ephemeral lakes. Hydrothermal activity begins, the faulted basement-sediment contact acting as the main conduit. Hydrothermal alteration of subsurface rocks in vicinity of fault zone and deposition of sinters at the surface in the Rhynie area (after Rice et al. 2002).

Continuing crustal extension and subsidence to present geology. Slices of early basin fill are preserved as slices in the basin margin fault zone (after Rice et al. 2002).