Geology of northeast Wales: Mesozoic
Triassic Period: 251-199
Ma (million years ago)
Desert-type environments prevailed as the climate became warmer and
drier. The Permian and succeeding Triassic rocks (often lumped
together as Permo-Triassic) include fluvial pebble beds and aeolian
(wind-blown) sandstones, all with a distinctive red colour caused by
the presence of iron oxide and indicative of deposition on land. The
Permo-Triassic sediments were deposited on an eroded surface of
Carboniferous and
older rocks during a time of rifting that resulted in the fault-bounded
Vale of Clwyd. Adjacent to northeast Wales, deepening sedimentary
basins
were developing in Cheshire and off the North Wales coast: here there
were
extensive salt-lakes and thick sequences of lacustrine sediments.
The strong geothermal gradients associated with these nearby deep
basins, coupled with tensional stresses in the local crust, created a
driving force and pathways respectively for mineralising fluids. The
fluids were generated, within the depths of the basins, by water
expulsion during sediment compaction under deep burial. The fluids
migrated up out of the basins into the older uplifted Carboniferous
strata, and especially the Lower Carboniferous limestones, where they
precipitated veins and irregular bodies of valuable minerals along
faults and palaeokarstic (cave) features.
The mineralisation was dominated in northeast Wales by calcite,
together with
the lead and zinc ores, galena and sphalerite, fluorite, quartz and a
variety of scarcer minerals. Such mineralisation occurs in many
Carboniferous Limestone outcrops within the UK and is either referred
to as Pennine-style or, more correctly, Mississippi Valley Type.
Mining
for lead and zinc was widespread, especially during the 19th Century,
and remains of the mines are often encountered, especially on Halkyn
Mountain and in the Minera district. In total, since 1845 (when
compulsory recording of mineral production was introduced), the
district has produced over 550,000 tons of lead ore, 2.4 million ounces
of silver and over 300,000 tons of zinc ore. Localised, but
nevertheless interesting, occurrences of iron, cobalt, nickel and
copper ores have also been exploited, particularly in the Prestatyn
district and (with respect to copper) around the village of Llanfair
Talhaern.
Jurassic
Period: 199-145 Ma (million years ago)
Cretaceous
Period: 145-65 Ma
(million years ago)
Strata
belonging to these periods are absent from NE Wales although, over the
border, an area of Lower Jurassic marine sedimentary rocks near
Whitchurch indicates that basin subsidence and sedimentation continued
into the Jurassic, in a closely adjacent area. Samples of chalk (Upper Cretaceous) can
occasionally be found in Irish Sea Ice glacial deposits in the Wrexham
area and some areas of the Irish Sea are floored by chalk. Indeed,
there is strong evidence that the Chalk Sea covered much of the UK,
including NE Wales, but the following 65 million years of erosion -
especially after major uplift early in the Tertiary - removed the chalk
in most places.
*******************************************************************************************
Previous:
Lower Palaeozoic (Ordovician
and
Silurian)
Upper
Palaeozoic (Devonian,
Carboniferous and Permian)
Next:
Cenozoic
(Tertiary and Quaternary)
Click on any section to learn more!
|
|
Above: Geological timescale, with the Mesozoic Era highlighted.
|
Above: Red-beds - Permo-Triassic dune
sandstones exposed at Ffordd Llanrhydd, Vale of Clwyd
|
Above: Fluorite:
purple crystals to 1.5 cm, from a mineral vein in the Halkyn district.
|
|
|
