Geology and Setting

Introduction

The Boundary Fault

The Basin Fill

Basin Evolution

Introduction

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

Above: 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).

 

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.

 

Left: Simplified geological map of the Rhynie outlier .

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.

Geological map of the Rhynie area

Above: Simplified geological map of the area north and west of Rhynie village (inset above left).

 

The Boundary Fault

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).

 

Right: 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).

Fractured and mineralised norite

 

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 Sandstone, Quarry 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:

Stratigraphy

Above: Generalised stratigraphy of the Rhynie outlier (left) and the succession in the area northwest of Rhynie village (right) (after Rice et al. 2002). Click on formation names on the left for a review of the lithologies and depositional environments.

 

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:

Stage 1

Above: 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).

Stage 2

Above: 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).

Stage 3

Above: 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).