Presentation on theme: "Desert Environments The specification states that you need to be able to; “describe the deposition in hot desert environments of wadi conglomerates, dune."— Presentation transcript:
Desert Environments The specification states that you need to be able to; “describe the deposition in hot desert environments of wadi conglomerates, dune sandstones and evaporites in playa lakes.”
Desert Environments Strangely the most change in deserts is not brought about by winds, but water. Rainfall is infrequent and when it falls it is usually as a torrential downpour. As the ground has been baked hard - most rain cannot soak into the ground and so runs off (plus it falls too quickly and hard to soak in).
Desert Environments This water quickly enters dried up river beds where it forms a raging torrent. Sometimes it will flow though deep flat bottomed canyons - wadis. These flash floods remove a lot of loose debris and bring in coarser material from surrounding mountains. The sediment is: Coarse grained Poorly sorted Usually rounded (but not always)
Wadi Conglomerate Coarse grained clastic rocks which have rounded clasts are called: CONGLOMERATES They are also poorly sorted. This reflects rapid deposition with rapidly changing energy conditions as found in wadi flash floods. Because they are in desert environments they tend to be red in colour as the Fe is oxidised.
Dune Sandstones As you know desert sands are transported by wind (can be called aeolian deposits). They typically form sand dunes. The windward slope is lower angled <25° and the dip slope is < 40°.
Sand Dunes Sand is driven up the windward side and then falls down the dip slope. This moves sand from the windward side and makes the dune migrate. Former positions of the dip slope are marked by “cross bedding”. This is one of the typical features of desert sandstone deposits.
Cross bedding The top of the cross bed is cut across by the bed above. The base of the cross- bed curves into the bed below. This allows you to work out “way up”.
Desert Sandstones As well as having cross bedding desert sandstones have the following features linked to their transport by wind. Well sorted Well rounded Frosted “Millet seed” texture They will also be red/brown/orange in colour.
Evaporite Deposits 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 The great heat will make the water in the lake do what? Evaporate. The evaporated water is pure H 2 O. 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%
Desiccation Cracks An obvious “sedimentary structure” found in playa lakes is/are: Desiccation cracks/mud cracks. These form when clays in the lake dry up, they shrink and cause the clay sediment to crack. These form typical polygonal shapes in plan view.
Desiccation Cracks In cross section the crack tapers down to a point. The tops are often curled “concave”. This allows you to use these as way up indicators.
Halite Pseudomorphs The evaporite mineral halite (rock salt) typically has a cubic shape. These can form in the mud at the bottom of the playa lake. These get buried and may dissolve away during diagenesis. The crystal shapes remain but they may be filled in by something else. Therefore the crystal shape looks like salt but is not. These are called “pseudomorphs”, pseudo = imitation, morph = shape. They indicate evaporite conditions.