2. Name the following sedimentary rocks, and describe the environments in which

Question

2. Name the following sedimentary rocks, and describe the environments in which they might have formed. The sandstone di (8 marks) agram is provided for reference. rains are 78% quartz, 12% feldspar, 10% rock fragments, grain size 0.3 to 2 mm., grains sub-angular. a. G Quartz Quartz arenite (-90% quartz) Compositions of arenite sandstones Arkosic Lithic arenitearenite | (>10% rock fragments) (>10% feldspar) Sandstones with these compositions are rare Feldspar Rock fragments Grains are 96% quartz, 4% feldspar, grain size 0.5 to 1 mm., grains well rounded. b.

Explanation / Answer

As per the figure of the ternary plot (QRF diagram) attached above in which the rocks are marked, we can say that sample "a" is Arkosic arenite and sample "b" is Quartz arenite.

Now the size range for sample "a" is 0.30 mm to 2.0 mm which is equivalent to (as according to Udden-Wentworth grain-size scale for siliciclastic sediment) medium sized sand to very coarse sand and this can be formed by alluvial fans deposition as they are sub angular in nature. While for sample "b" which is 0.50 mm to 1.0 mm which is equivalent to (as according to Udden-Wentworth grain-size scale for siliciclastic sediment) coarse sand and this can be formed by deserts and aeolian (wind) deposition.

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