non clastic rocks

41 70 0
  • Loading ...
1/41 trang

Thông tin tài liệu

Ngày đăng: 30/11/2016, 18:07

 The specification states that you need to be able to ”Identify, describe and explain the origin of nonclastic 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).” Limestones 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  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 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 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 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 - km depth "Carbonate Compensation Depth" because at that depth the P is so great that carbonates are redissolved  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)   The specification says:  Describe the deposition in deltaic environments of delta top (topsets) to form coal, sandstones of the delta slope (foresets) and shales to form offshore deposition (bottomsets)  Understand deltaic deposition in cyclothems Deltas and Coal All coal was once living matter, typically plants  The Coal Measures in the Carboniferous formed from large equatorial forests  During that period the conditions were perfect for the formation of coal  Low relief High water table Anaerobic conditions Slow subsidence In plan view you can see that deltas are divided into:  Distributary channels  Swamps/flood plain  Distributary channels: These diverge from the main river channel and distribute the water and sediment over the delta top  The channel will be channel shaped and will have medium/coarse sediment in the channel but usually coarse/medium sandstone  It will have cross bedding/lamination due to current ripple formation  Delta Sediments     These beds are called topset beds and can be seen on the cross-section (A) These deposits get finer as the delta progresses into the sea as the energy drops At the front slope of the delta there are finer sandstones with cross-lamination, these are Foreset beds (B) Where the delta front levels out onto the seabed the sediment is a finer still possibly siltstones or even marine limestones and these are the Bottomset beds (C) Swamps:     In between the distributary channels are swamps and slightly elevated areas with trees/vegetation These swamps are stagnant water and are therefore anaerobic/anoxic This means that the vegetation does not break down and so peat can accumulate This peat ultimately forms coal on burial/diagenesis Cyclothems These occur where there is a repeated sequence of sedimentary rocks  The most common kind of cyclothem occurs where deltas advance (prograde) and retreat (retrograde) repeatedly  Why would deltas this:  Sea level changes Changes in the sediment supply Isostasy Cyclothems The cyclothem shows the growth of the delta then it’s decay  The coal and seat earth (fine sand with roots which represents the soil in which the plants grew) formed  Where did they form?  On the delta top in the swamps  The delta retreats and erodes until limestones form offshore in clear sea (bottom set beds)  Cyclothems The delta then advances (progrades) the limestones get covered by fine sediments such as marine shales at the end of the delta (still bottom set beds)  This is followed by slightly coarser siltstones which are part marine part fresh water (still bottom set)  Then as the delta advances further the delta slope with its fine sandstones appears (foreset beds)  Because the sands are transported by a river current it is cross bedded/laminated  Cyclothems Again the delta advances so that the delta top appears  This produces coarse cross bedded sandstones in the distributary channels (top set)  Then we are back to the coal and seat earth in the swamps  The whole thing then repeats  Types of Coal    The types you need to know are: Lignite Bituminous Anthracite The type depends on the amount of compaction/burial and diagenesis it has undergone having started off as peat As you move from lignite to anthracite the rank increases Coal As the coal is buried:  The rock is compacted  Pore space is reduced  Volatiles and water is lost  Therefore the C remains and increases in %  In addition to carbon, coals contain hydrogen, oxygen, nitrogen and varying amounts of sulphur  High-rank coals are high in carbon and therefore heat value, but low in hydrogen and oxygen  Low-rank coals are low in carbon but high in hydrogen and oxygen content  Lignite  Lignite is a soft, brown or black coal Bituminous Coal  Bituminous coal, ranked second highest, Anthracite  The highest rank, and hardest coal, is anthracite [...]... 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... means that the vegetation does not break down and so peat can accumulate This peat ultimately forms coal on burial/diagenesis Cyclothems These occur where there is a repeated sequence of sedimentary rocks  The most common kind of cyclothem occurs where deltas advance (prograde) and retreat (retrograde) repeatedly  Why would deltas do this:  Sea level changes Changes in the sediment supply Isostasy
- Xem thêm -

Xem thêm: non clastic rocks , non clastic rocks , non clastic rocks

Gợi ý tài liệu liên quan cho bạn

Nhận lời giải ngay chưa đến 10 phút Đăng bài tập ngay