Hydrogeology WorkshopGeological 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.

•~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.

Hydrogeology WorkshopGeological 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

Upper Mottled Sandstone

(a.k.a. Wilmslow Sandstone)

Transition Beds (between Upper Mottled

Sandstone and Helsby Sandstone)

Helsby Sandstone

Hydrogeology WorkshopGeological 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.

•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.

Hydrogeology WorkshopGeological History (6)

Resulting Stratigraphy (handout)

Date (Ma) Period Epoch

Geology Map Formation No.

Old' clasisification New classification

DescriptionGroup Formation Group Formation

~230

Tria

ssic

f6d

Keuper

Keuper Saliferous Beds

Merc

ia M

udsto

ne G

roup

Wlkesley Halite Mainly rock-salt with this red mudstones

f6c

Middle Keuper Marl Middle Mudstone Mostly red-brown silty mudstone with anhydrite nodules. Silstones in lower part.

f6b

Lower Keuper Sailiferous Beds Northwich Halite Thick beds of red-brown rock-salt, separated by this mudstone horizons.

f6a

Lower Keuper Marl Lower Mudstone Alternating beds of musdstones and siltstones. Gypsum present and thin beds of rock-salt toward top.

f5

Keuper Waterstones Tarporley Siltstones Alternating beds dark red and greean shale (mudstone) and pink-red sanstones (fine grained). Silty toward the top.

~240

f4

Keuper Sandsdtones w/ Keuper Sandstone/Conglomerate.

Sherw

ood S

andsto

ne G

roup

Helsby Sandstone Mainly coarse grained, red-brown, grey/buff sandstones with pebbly bands and comnglomerates.

f3

Bunte

r

Upper Mottled Sandstone Wilmslow Sandstone Fine to medium grained soft red-brown sandstones mottled buff and grey. Some mudstones. Few pebbles.

~245

f2

Pebble Beds Chester Pebble Beds Medium and coarse grained red-brown sandstones. Scattered pebbles and some thin conglomerates.

~250

Permian

ef1

Lower Mottled Sandstone Kinnerton Sandstone Fine to medium grained soft red-brown sandstones. Some thin mudstones near top.

~290+ Carboniferous

d6b

Erbistock Beds Upper Coal Measures

Alternating mudstones and seatearths with a few thin sandstones.

Hydrogeology WorkshopGeological History (7)

Then came the Ice Ages.......