The document describes the geological history of an area from the Permian to Triassic periods, approximately 290 to 230 million years ago. The area was located 15-20 degrees north of the equator in a desert climate, surrounded by higher ground. Braided rivers flowed from south to north, carrying large amounts of eroded sediment from the higher ground which was deposited in the subsiding basin to the north. This created a succession of sandstone and mudstone formations. During the Middle Triassic, the landscape became more subdued and fine-grained sediments like mudstone and siltstone were deposited along with evaporites such as gypsum and halite deposits from isolated saline lakes.

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

4. Upper Mottled Sandstone
(a.k.a. Wilmslow Sandstone)

5. Transition Beds (between Upper Mottled
Sandstone and Helsby Sandstone)

6. Helsby Sandstone

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.

10. Hydrogeology Workshop
Geological History (7)
Then came the Ice Ages.......