Lecture SR7-Non- Clastic Sedimentary Rocks

• The specification states that you need to be able to ”Identify, describe and explain the origin of non-clastic sedimentary rocks using observation of colour, fossil content, mineral composition and texture; ironstone, evaporites (gypsum, halite), limestones (micritic, oolitic, fossiliferous, chalk), coals (lignite, bituminous and anthracite).”

• This is the largest non-clastic group and they consist mostly of carbonate minerals:

• 1)    Calcite CaCO3

• 2)    Dolomite Ca,Mg (CO3)2

• 3)    Plus organic remains preserved as carbonate skeletons.

• Originally these minerals form as carbonate mud which then slowly turns into solid rock via diagenesis.

• During this process some of the original physical and chemical properties can change slightly and frequently calcite can change to dolomite.

Limestones

• In the geological past shallow seas were widespread and limestone could be deposited over 1000's km2.

• Organisms with carbonate skeletons occur throughout the world, so in theory carbonate sediments can be deposited anywhere e.g. seas and oceans.

• However, they do not occur everywhere, there are several factors that influence and therefore control the deposition of carbonates:

• T of water• Salinity• Water depth• Amount of silica input• Limestones tend to form in

warm seas.

• These conditions are proved by the presence of index fossils such as corals.

• Therefore most limestones form in tropical/sub-tropical belts 0 - 30 north or south of the equator.

• Most limestones formed since the Cambrian have formed in these latitudes.

• The limestone forming organisms are also affected by salinity and depth of the water and therefore tend to live at depths up to 200 m (where sunlight can penetrate).

• If the sea is too saline then animals do not survive as well so limestones tend to occur in normal salinity ranges.

• This depth allows algae to photosynthesise and animals to thrive in continental shelf areas in particular.

• Occasionally carbonate deposits can be found in environments deeper than 200 m e.g. abyssal plains.

• However, the organisms forming these deposits would not have lived there.

• They were floating or swimming organisms which once dead sank into the deeper water and slowly accumulated as oozes (very fine carbonate sediments).

Carbonate Compensation Depth• Limestones cannot form below 3 - 5 km depth

"Carbonate Compensation Depth" because at that depth the P is so great that carbonates are re-dissolved.

• Carbonates will also not form if there is a large influx of silica material or debris from the land.

• This affects the survival and growth of limestone forming organisms and inhibits the growth of the grasses that trap and fix the carbonate mud in place.

• Therefore limestones form either at a distance from land or else close to land but not undergoing a lot of erosion (low lying land masses).

• A wide range of different types of limestone exist:

a) Chemical

b) Detrital (minor)

c) Bioclastic

d) Biological

Micrite and Sparite• A matrix can often be

present especially in clastic or biological limestones, helping to hold the fragments together.

• The matrix can be:

a) A mud that the clasts/fossils fell into (micrite).

b) A later infilling cement (sparite).

CHEMICAL FORMS OF LIMESTONE:

• Limestone/Micrite:• This is mostly

precipitated CaCO3.

• As a sediment it resembles a pale very fine mud.

• Once it is compressed into a rock it is very fine grained, grey/cream in colour.

Oolitic limestone/Oolite: • They have a very obvious

texture of almost perfectly spherical grains.

• Individual grains are called ooliths and are rounded and spherical.

• They form in shallow water marine conditions usually in the tidal zone.

• Dissolved CaCO3 in sea water gradually precipitates around a nucleus: shell or clast.

Oolitic limestone/Oolite 2:• As the tides roll the grains

backwards and forwards they get an even distribution of CaCO3 around the nucleus.

• Eventually it forms a series of concentric layers (oolith).

• The warmer the sea water the more CaCO3 can be dissolved. The better the chance of ooliths forming.

• When the grains reach a certain size they become too heavy to move and therefore they stay in place eventually they are cemented together.

• These rocks are very well sorted.

 Chalk:• This is a very fine grained

limestone with a micrite texture.

• No grains can be seen and it is even difficult with a hand lens.

• They are rich in calcite usually > 90% and sometimes as high as 99%.

• As a sediment it was a calcareous ooze consisting of the skeletons of planktonic organisms:

Chalk• Coccoliths calcareous algae• Foraminifera micro-organisms• As they died they settled out

from the water and as they were pelagic (free swimming or floating) creatures.

• They settle out in a quiet water environment and over time large accumulations of these skeletons can accumulate.

• It is thought that chalk formed in a shelf environment well away from land where there were no currents to disturb the ooze/mud.

Shelly limestone:

• Broken shell fragments from various types of fossils = death assemblage indicates a high energy environment, typically just below the tidal zone.

Reef or coral limestone: • Reefs can form from coral

or algae usually in warm shallow clean water.

• They are usually located near to the continent.

• Reef limestones are very useful in stratigraphy because they indicate the edges of the continental shelf and are therefore a useful indicator of a past environment = index fossils.

Evaporite Deposits (playa lakes)

• The flash floods running down wadis often run into temporary (ephemeral) lakes in the desert.

• These are called playa lakes (salt lakes).

• The water that goes into the lake has many ions in solution.

• From where?

• Chemical weathering.

Evaporite Deposits (playa lakes)• The great heat will make the water in

the lake do what?

• Evaporate.

• The evaporated water is pure H2O.

• This leaves behind the ions in solution which will be deposited as minerals.

• As the remaining water gets more concentrated less soluble minerals will form.

• There is a sequence of evaporite minerals.– gypsum begins to precipitate when the

volume is reduced to 30%

– halite after reduced to 10%

– and Mg and K salts after 5%

Restricted Basins

• Restriction of exchange with the open ocean, in a semi-enclosed basin, is necessary to drive the salt content high enough for precipitation to begin.

• Sea water comes in (but cannot escape) and evaporates and so the water gets denser and sinks.

• More water comes in but again evaporates and sinks driving up the salt concentration.

• Such restricted bodies of water are:

(1) coastal lagoons;(2) salt seas on the shelves(3) early rift oceans in the deep

sea.

Sabkha Environments• These occur where deserts

meet the sea.• Because of the heat water is

drawn up through the sand dunes by capillary action.

• This causes a flow of sea water along and up the dunes.

• When this sea water reaches the surface it evaporates leaving behind evaporites.

• Typically gypsum forms (desert rose).

• There is increased evaporite deposition in the shallow sea too.