8/2/2019 Tectonic Evolution of the Bristol Channel Borderlands
Chapter 1
7/10
CHAPTER ONE
Tectonic Evolution of the Bristol Channel borderlands
INVERSION TECTONICS
Page 1-13
Post-Caledonian Devonian extension created new faults as well as
reactivating old faults, which controlled the sedimentation
and final thickness of the Old Red Sandstone. These extensional
faults were also inverted during Variscan compression to
form structures such as the Benton and Ritec Thrusts.
Field Examples
The following examples provide mesoscopic field evidence for
reactivation in SW Dyfed:
Example 1: Druidston Haven (Grid Reference 860 170)
Fig. 1.14 shows a moderately south-dipping fault with dip slip
slickenside lineations which juxtaposes black Ordovician
shales in the footwall and siltstones, shales and ironstones of
the Millstone Grit in the hangingwall.
The structure has a complex polyphase history which begins with
post-Ordovician-pre-Namurian folding and faulting which
affected the rocks in the footwall. A major post-Namurian
extensional phase then down-faulted the Millstone Grit against
the
deformed Ordovician shales. The extensional fault was then
reactivated by positive inversion. This is indicted by the
distinct
hangingwall anticline within the Millstone Grit.
CHAPTER ONE
Tectonic Evolution of the Bristol Channel borderlands
INVERSION TECTONICS
Page 1-14
The structure probably represents at least three phases of
tectonism including Caledonian compression, Late Namurian
extension and Variscan compression.
Example 2: Porth LLeuog near Whitesands Bay (Grid Reference 732
270)
Fig. 1.15 shows the boundary between Arenigian strata in the
northerly hangingwall of a steeply dipping fault and
Tremadocian strata in the footwall to the south.
The fault clearly shows post-Arenigian extension, but mesoscopic
folds and shear indicators in the hangingwall of the main
fault suggest compression. It is possible therefore that
positive inversion had also taken place along the Whitesands
Bay
section.
