 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 do 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.