Geology
by Mark Tringham
This is an excerpt from The Cave Pearls of Meghalaya, Volume 1, Pala Range and Kopili Valley, Edited by Thomas Arbenz. Copyright is reserved.
Meghalaya mostly comprises a relatively stable high structural block, called the Shillong Plateau, which is cored by Precambrian granites, gneisses and schists. Around its southern flanks a younger sedimentary sequence was formed, which includes some limestones. The high block formed a promontory of land that became surrounded by ocean from early Cretaceous times onwards, after the break-up of the Gondwanaland super-continent and separation of India from neighbouring continental land-masses such as Australia and Africa. During Cretaceous to Paleogene times the Tethys Ocean lay to the north of Meghalaya, in the area now occupied by the Himalayas and the proto-Indian Ocean was to the south.
The Paleogene sedimentary sequence is called the Jaintia Group and this is up to about 1.4 km thick with a mixture of limestones, sandstones, shales and coals (Samanta & Raychaudhuri 1983). The caves of Meghalaya are nearly all formed in these limestones. This sedimentary sequence thins and onlaps northwards onto the emergent Shillong Plateau.
Within the Jaintia Group the limestones occurs as 3 main layers, all of which are Eocene aged. Most recent publicationsrefer to the three main limestones present as the Lakadong, Umlatdoh and Prang Limestone Members and these, together with intervening sandstones, shales and coals, make up the Shella Formation. The Shella Formation is underlain by the Langpar Formation and overlain by the Kopili Formation. These 3 formations make up the Jaintia Group.
The limestone members can be distinguished by their differing fossil content, with indicative foraminifera present. For example, the fossil content of the Prang Limestone includes Orbitolites, Alveolina, Nummulites, Assilina, Discocyclina and Asterocyclina together with gastropods, echinoids and bivalves (Samanta & Raychaudhuri 1983). In the central and western Khasi Hills a full sedimentary sequence occurs, with all 3 limestones present, while around the eastern Jaintia and Cachar Hills the sequence is reduced to only two members, with the Prang Limestone Member above and the coaly Narpuh Sandstone below. The Prang Limestone is widespread, with an intermittent outcrop, mostly 4km to 8km wide and stretching a length of approximately 325 km from the Assam Mikir Hills in the NE to the Garo Hills in the west. The limestone also occurs in the shallow subsurface, concealed beneath a capping of Kopili Shales and other rocks and many caves can be followed from the limestone outcrop into these concealed areas.
The Shella Formation limestones, sandstones, shales and coals are interpreted to have been deposited in shallow marine to lower coastal plain tropical environments. The Shillong Plateau provided an eroded hinterland with fluvial transport of clastic sediments, comprising sand, silt and clay, south and south-westwards across the region. The limestones generally are grey to dark-grey and yellowish-grey shaly and micritic, with abundant shelly remains. They are mostly thick to massively bedded and nodular, with thin marly or shale bed partings. The great abundance of shelly foraminifera and algae indicate a prolific shallow marine environment, well suited to both benthic and pelagic shelly organisms. It was likely formed over a large shallow marine shelf sea, parallel to an Eocene shore-line, during a warm period when calcareous organisms could proliferate.
