physiographic division of india(4)

RED TACHYLATIC  BASALT

 By hydrothermal alteration the black colur of the fresh tachylytic basalt is changed to red and hydrothermally altered red tacylytic basalt are much more common than unaltered black tachylytic basalt.

If Red Tachylatic  basalt  confined condition i. e. below the cover of rock then it is very hard compact suitable for any foundation.

If Red Tachylatic  basalt  unconfined condition i. e. exposed to atmosphere it decompose weather very fast & to get friable loose material is called Red bole. (Soil)

 Engg. Point of view it must be below the cover of rock.

Dykes in Deccan Trap: - Over large areas the D. T. Lava have been intruded by basaltic dolerite dykes. They are numerous in Tapi & Narmada valley. Saurashtra, North konkan , Dhule Jalgaon. Few dykes in Pune, Nasik, Ahemadnagar, Jalana , Aurangabad dist. Most dykes have a considerable lateral extent about tens of km. Their thickness varies from half meter to 50 m.

Dykes having prismatic joints due to which if joints open wide & inter connected leakage may occur in foundation of structure .It may required to excavate , grouting & back filling impervious material along trench of dyke . Most of the dams in Maharashtra crossing no. of dykes very few are leaky.

Inter-Trappean beds.:- Deccan trap formation are made up of basaltic flow . These basaltic flow were not erupted continuously but in between two successive basaltic flow there is long time internal gap. During this time gap between two flows there was presence of some fresh water lakes. Due to which there was river origin deposit. These deposits are very few in lateral extent & their thickness varies from 3’ to 10’. It contains plant and animal fossil. Deposit in both two flows are called Inter trappean beds. Composition  may be chert, impure lime stone, clay. Many plant remains & fresh water mollusca, shells are well preserved .Similarly the fossils of frog , fishes are found. Such deposit occurs near Malbar hill, Worli, Nagpur, Dhule etc.