1. Hydrogeology Workshop
Geological History (3)
Permian and early Triassic times
• (~290 – 230 Ma);
• Surrounded by high ground;
• Arid/desert climate @ 15 - 20 degrees north of the equator;
• Land of braided rivers and flash floods.
2. •~15 – 20 degrees north of the equator, arid/desert
climate;
•Eroding higher ground to the south and west;
•Subsiding basin north-south oriented basin;
•Huge amounts of material washed down, often as
flash floods;
•Braided rivers system, flowing from south to north,
meanders across the basin floor;
•Pretty inhospitable to life, even ignoring that 95% of
species had just gone extinct.
3. Hydrogeology Workshop
Geological History (4)
Keep your eye on the
blue line.
• Hills eroding;
• Extension faults cause the
basin to subside;
• Sediments fill subsiding
basin.
All of
this and this
ends up in here
7. Hydrogeology Workshop
Geological History (5)
Middle Triassic (~230 - 220 Ma)
•Lower relief and low-lying basin
receives mainly fine grained material
(silt and mud);
•Lakes form and evaporate, leaving
gypsum & anhydrite deposits.
•Periodic marine incursions.
Isolated areas of saline water
evaporate to leave extensive
deposits of rock-salt.
8. •Lower topographic relief, sluggish river system only capable of
transporting fine-grained material silt/mud;
•Lakes form and evaporate;
•Periodic incursion of seawater, followed by evaporations;
•Mudstones and siltstones with evaporites, including significant rock-
salt deposits.
9. Hydrogeology Workshop
Geological History (6)
Resulting Stratigraphy (handout)
Date (Ma) Period Epoch
Geology Map
Formation No.
Old' clasisification New classification
DescriptionGroup Formation Group Formation
~230
Triassic
f
6d
Keuper
Keuper Saliferous Beds
MerciaMudstoneGroup
Wlkesley Halite Mainly rock-salt with this red mudstones
f
6c
Middle Keuper Marl Middle Mudstone Mostly red-brown silty mudstone with anhydrite nodules. Silstones in lower part.
f
6b
Lower Keuper Sailiferous Beds Northwich Halite Thick beds of red-brown rock-salt, separated by this mudstone horizons.
f
6a
Lower Keuper Marl Lower Mudstone Alternating beds of musdstones and siltstones. Gypsum present and thin beds of rock-salt toward
top.
f
5
Keuper Waterstones Tarporley Siltstones Alternating beds dark red and greean shale (mudstone) and pink-red sanstones (fine grained). Silty
toward the top.
~240
f
4
Keuper Sandsdtones w/ Keuper
Sandstone/Conglomerate.
SherwoodSandstoneGroup
Helsby Sandstone Mainly coarse grained, red-brown, grey/buff sandstones with pebbly bands and comnglomerates.
f
3
Bunter
Upper Mottled Sandstone Wilmslow Sandstone Fine to medium grained soft red-brown sandstones mottled buff and grey. Some mudstones. Few
pebbles.
~245
f
2
Pebble Beds Chester Pebble Beds Medium and coarse grained red-brown sandstones. Scattered pebbles and some thin
conglomerates.
~250
Permian
ef
1
Lower Mottled Sandstone Kinnerton Sandstone Fine to medium grained soft red-brown sandstones. Some thin mudstones near top.
~290+ Carboniferous
d
6b
Erbistock Beds Upper Coal
Measures
Alternating mudstones and seatearths with a few thin sandstones.