Grampian Highlands Field Guide: Prologue

From Earthwise
Revision as of 14:00, 29 July 2021 by Dbk (talk | contribs) (1 revision imported)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigation Jump to search
This page is part of a category of pages within the Grampian Highlands Field Guide.
Author: J R Mendum, BGS

Prologue*[edit]

When the Open University Geological Society (Jane Schollick — NW England) asked if I would lead a week-long field trip to the Grampian Highlands based at Kindrogan Field Centre, I took some time before replying. Basically, I was apprehensive about how and whether I could sensibly demonstrate the complex geology of the Dalradian Supergroup, and particularly those elements of Grampian orogenesis that affected this part of Scotland in late Cambrian to mid Ordovician times. I did assent and then gave considerable thought as to how to present the geology both in the field and on paper to try to maximise the benefit and enjoyment of the week. This work has had generally positive outcomes for me, but inevitably not without a little pain.

This prologue has two diagrams appended, P1 showing the Dalradian stratigraphy, and P2 showing the overall geological structure. It will not attempt to summarise the rather detailed geology of the Dalradian Supergroup, or the Grampian and other orogenic events. To cover that option I recommend you obtain a copy of the lengthy introductory ‘chapter’ to the Geological Conservation Review volume on the Dalradian Rocks of Scotland, one of a series of separate papers published by the Geologists’ Association in 2013. This paper undoubtedly contains more information than you may wish to have or to know; I suggest you treat it as a reference document, dipping into for entertainment and enlightenment when you feel in the mood.

The basic facts are that the Dalradian succession was deposited in rift basins near the margins of the Rodinian supercontinent and at later times on the continental margin of Laurentia between around 720 Ma and 515 Ma. During accumulation of the sedimentary and volcanic sequence stretching and thinning of the crust was generally slow and irregular. Note that nowhere else on the Laurentian margin is such a comprehensive succession preserved. Rift-drift transition (ocean-crust formation) probably took place around 570 Ma, but subsequently, from Cambrian times through to the end Silurian, plate tectonic movements and related orogenic events dominated in this area of the globe. Collision of continental and oceanic fragments and island arcs with the continental ‘foreland’ undoubtedly occurred. Possible plate geometries for these tectonic events, which were undoubtedly rapid at times, are the stuff of imagination, as most of the evidence has been subducted, faulted out, or is effectively undecipherable.

The later Caledonian history of faulting and igneous intrusion is also complex but this excursion guide only covers these events in passing. Hence, although the Grampian Highlands do contain much evidence of their complex history, the resolution and understanding of the full geological picture, in space and time, remain tantalisingly out of reach. We do have a much improved picture in 2014, but realistically we cannot ever get the full story.

Knowing that Field Studies Council centre at Kindrogan was used for many years for the field excursion for SXR 339, I was keen to look at the material covered by the course and the approach taken by the Open University in dealing with the complex geology of the Grampian Highlands. My impression was that the course presented a necessarily simplified version of the geological history, with the attendant field excursion ranging from hands-on mapping to laboratory work, and including visits to some of the ‘classic’ localities. While this was practically informative and undoubtedly touched on some of the deeper problems, it should have merely served to whet the appetite of the attendees (and leaders even). Patently, one can spend several lifetimes looking at/studying the Dalradian rocks of the Grampian Highlands, and there will still be further basic questions to answer. Indeed several geologists have spent most of their lives dedicated to Dalradian rocks (e.g. Jack Treagus).

To try to get around this dilemma on this excursion I have chosen to focus only on certain aspects of the Dalradian geology and the Grampian Event, hopefully in a bit more depth. This field guide provides the background to the geology of specific areas and/or geological features that range widely from Neoproterozoic turbiditic (bouma) sequences (in the Loch Laggan area) through to the Loch Lomond Re-advance and glacial meltwater features. Some days follow on from previous ones, providing a longer continuous across-strike traverse to gain a larger scale picture of the regional geology (e.g. Days 2 and 3). Other days will visit parts of the Grampian Highlands that do not directly link, geologically or geographically. The aim is to look a little more closely at some of the questions posed by the rocks, to think about the consequences, and hopefully discuss some of the implications, either on the rocks, in the bus, or later. To this extent if you become bored with examples of folding and refolding — apologies! Given reasonable weather the Highlands (and their rocks) are still a good place to be.

File:GHFGfigP1.jpg
Figure P1    Stratigraphy of the Appin, Argyll and Southern Highland Groups (Dalradian Supergroup) in the central and eastern parts of the Grampian Highlands (modified from Stephenson and Gould, 1995).
File:GHFGfigP2.jpg
Figure P2    Block diagram showing the major fold structures in the Grampian Highlands. Brittle Faults have been omitted. Note that the colours refer to structurally defined areas to aid their description in the Regional Guide. The numerous abbreviations for the structures are also given in the guide. (from Stephenson and Gould, 1995, modified from Thomas, 1979).

*  A prologue or prolog is a word of Greek origin (πρόλογος prologos), referring to the opening to a play or story that establishes its setting and gives background details, commonly related to an earlier tale, along with other miscellaneous information. It was the invention of Euripides and its importance in Greek drama was very great; it sometimes almost took the place of a romance, to which the play itself succeeded, alternatively it was effectively an explanatory first act. It was later criticized as a useless impertinence, standing as a barrier between the audience and their enjoyment of the play, but for Athenian audiences it was useful and pertinent, supplying just what they needed to make the succeeding scenes intelligible. It is a device commonly used today at the start of plays, operas, and even large or long sporting events, e.g. Le Tour de France.