Pembrokeshire virtual field trip 

                   structural style and the stratigraphic template

Introduction

The northern limit of Variscan deformation in the United Kingdom lies in Pembrokeshire, SW Wales

 

Vergence of folds and thrust sequences is generally to the NNW, though deformation is commonly accommodated by back-thrusts with local folds verging to the SSE

 

The stratigraphic template strongly influences style of deformation in Pembrokeshire: unit compositions, thicknesses and stratigraphic geometries determine mechanical properties of units and mechanisms of deformation.    

 

Deformed strata of Upper Carboniferous  are generally the most intensely deformed of the units in SW Wales, and form some of the best-known fold-thrust outcrop examples in the United Kingdom.

Summary structural map of Variscan Pembrokeshire (with virtual field localities).

Geological background

Chronostratigraphic chart of Pembrokeshire (modified from Powell, 1989)

 

Facies distributions of Devonian to Carboniferous units are controlled by Silurian/Devonian extensional faults which influenced palaeotopography during deposition.  

 

Inversion of pre-existing extensional features influence structural style and the localisation of thrust faults. Large-offset, laterally persistent structures in Pembrokeshire are interpreted as inverted extensional faults.

 

Where extensional faults are only partially inverted, basement blocks and palaeo-highs act as buttressing blocks and serve to reduce the depth of detachment on contractional faults.   

 

Present-day and partially restored crosss section through E Pembrokeshire (modified from Powell, 1989)

West Angle Bay

Along-strike variation in structure at West Angle (Hancock, Dunne and Tringham, 1982)

South Wales Lower Coal Measures

Trevayne

Monkstone

Saundersfoot

Saundersfoot is one of the most well-known outcrops in the UK and an important site for training future geoscientists. Carboniferous rocks of the South Wales Lower Coal Measures were folded and faulted here during the Variscan Orogeny.

 

Inspect the outcrop and examine the interaction between faults and folds. The detailed virtual outcrops below show a section close to Saundersfoot where a small-scale fold has been decapitated and transported onto another.

 

This relationship implies that folding here occurred before thrusting. This relationship is not clear everywhere: different parts of the outcrop show different deformation histories. In short, deformation here was complex. There does not appear to be any systematic relationship between the timing of folding and faulting.  

Compare the complex deformation structures here to Amroth, just a few miles to the north, where units of a similar age and depositional setting are relatively undeformed.

Broadhaven

One of The Geological Society's 100 Great Geosites. Asymmetric folds in interbedded Carboniferous siltstones, shales and coal horizons are cut here by a number of northward-propagating thrusts. This classic structure has been studied in some detail (e.g. Williams and Chapman,1983) as it provides an excellent outcrop example to examine the interplay between faulting and folding during deformation