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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Gurmeet Singh posted an Question
October 05, 2020 • 18:01 pm
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30 points
30 points
- IIT JAM
- Geology (GG)
Please explain last three lines sir.
JUD Classification of Sandstone Several schemes are proposed for classification of sandstones, but the most commonly used schemes include both texture (the presence and amount of either interstitial matrix i.e., clasts with diameters finer than 0.03 millimetre or chemical cement) and mineralogy (the relative amount of quartz and the relative abundance of rock fragments to feldspar grains). Although these classifications were not intended to have tectonic significance, the relative proportions of quartz, feldspar, and fragments are good indicators of the tectonic regime. It is possible to discriminate between stable cratons (rich in quartz and feldspar), orogens (rich in quartz and fragments), and magmatic arcs (rich in feldspar and fragments). tisation of sandstone is shown below in figure. Sandstones are first subdivided into two 450 matrix) consist of a sand-size
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Rahul kumar Best Answer
The last three lines are related to provenance (source) settings inferred from the clastic sedimentary rocks. 1. Dominance of Quartz indicates the stable craton settings. It means that the prolonged weathering conditions have modified the mineralogy and leftover with stable minerals only. 2. Quartz and lithi fragments indicate orogenic conditions. In orogenic area, slope conditions inhibit the intense weathering and hence it results into presence of lithic fragments along with Quartz. 3. Feldspar and fragments is diagnostic assemblage of magmatic rocks. Weathering of magmatic rocks result into small lithic fragments along with feldspar. In absence of prolonged weathering conditions, unstable minerals remain within the clastic rocks.
Sir what is stable croton settings.
and sir why magmatic rocks do not considering- Qtz
Stable craton condition means the flat lying region which is stable and very far from plate boundaries.
Quartz will be there in case of magmatic rocks but the abundance of feldspar and lithic fragments is relatively higher in case of magmatic rock provenance than in other tectonic settings (stable craton, orogen etc.). This is why we don't talk about Quartz in case of magmatic arcs.