Geology and Setting
Introduction
The Boundary Fault
The Basin Fill
Basin Evolution
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).
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).
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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 .
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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.
Above: Simplified geological map of the area
north and west of Rhynie village (inset above left).
| 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).
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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:
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.
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:
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).
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).
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).
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