Benighat Slates Fm

Katmandu region (Lesser Himalayan succession)

Dark bluish-gray to black, soft-weathering, and highly cleaved slates and phyllites represent this formation. The rocks are mainly argillaceous, but there are also subordinate bands of siliceous or finely quartzitic composition. Generally the formation is devoid of quartzites. When weathered, these rocks may display dark green colors due to the presence of chlorite. But such colors are more typical of the Robang Formation. Also, the weathered exposures and scree may display silver-gray, light gray to white, pale green, pinkish, and orange colors. There are some specific zones rich in carbonate content, where they are classified as the Jhiku carbonates or calcareous beds (Stöcklin and Bhattarai 1977, p. 18). The black carbonaceous slates contain much (30–40 %) graphitic matter with some quartz and light mica. They form up to tens of meters thick zones. These graphitic slates weather into limonitic crust, exhibiting yellow, orange, and ochrous colorations, and they are covered with powdery sulfur, presumably from the decomposition of pyrite so common in these rocks. Especially on riverbanks, one encounters white powdery encrustations of bittersalt (Stöcklin and Bhattarai 1977, p. 19). The thickness of the Benighat Slates varies significantly and generally ranges from 500 to 3,000 m. Stöcklin and Bhattarai (1977, p. 19) discovered two boulder-beds within the Benighat Slates in the Hugdi Khola. One of them occurs about 30 m below the base of the Malekhu Limestone Fm. It is a lenticular bed with a thickness varying from 5 to 10 m. In it, the rounded and unsorted boulders range in diameter from7 to 30 or 40 cm, are composed exclusively of quartzite, and are embedded in a green quartzitic or orange to brown calcareous matrix. The second boulderbed lies about 600 m below the base of the Malekhu Limestone and it is 20–30 m thick. The boulders are again made up of poorly sorted quartzite, but they display distinct bedding, and their matrix is quartzitic. The lower boulder-bed passes upwards into 30 m of massive quartzitic phyllite and subsequently to ordinary dark phyllite. Occurring within the Benighat Slates, the Jhiku Carbonates are represented by a variety of lithotypes. Some of them are thinly bedded, very fine-grained, dense, dark gray to black siliceous dolomite interbedded with black slates, whereas others are thin-bedded, platy, fine-grained, yellowish gray to pale green siliceous limestones and dolomites. Others are represented by buff-colored finely quartzitic carbonates, alternating with blue-green phyllite of tens of centimeters in thickness. There also occur a few beds of dark laminated siliceous limestone.