Stratigraphy and Faunal Assemblages of the Upper Cretaceous Sediments of Meghalaya Shelf: Palaeoenvironmental Implications

The Upper Cretaceous — Early Palaeogene sediments of Meghalaya, mostly restricted along the southern part are represented by the Mahadek and Langpar formations and host marine invertebrate fossils. The Cretaceous-Palaeogene (K/Pg) mass extinction boundary is recorded within the Langpar Formation based on foraminiferal zonation. The present study is restricted to the Upper Cretaceous sediments, where the mega-invertebrate fossils were studied and attempt was made to study the lithological variation together with faunal assemblages in order to review the stratigraphy. Two marker horizons were identified from the Late Maastrichtian sediments that have characteristic lithology and fauna and are helpful in regional correlation. The Mahadek Formation, a medium to coarse grained calcareous sandstone unit with two characteristic echinoid genera Gongrochanus sp. and Stygmatopygus sp. and the bivalve Agerostrea sp. represent one marker horizon. The study indicates that it is the transition zone in the upper shelf environment. The other marker horizon within the lower part of the Langpar Formation, consist of carbonaceous shale and calcareous subarkose intercalation host heteromorph ammonites Eubaculite sp., Glyptoxoceras sp. etc. and prolific gastropods along with burrows. This indicates a comparatively deeper shelf environment. The presence of pyrite frambroids of size less than 5µm from the marker horizon of the Langpar Formation indicate suboxic to anoxic conditions and their presence give substantial evidence for the toxicity in the environment due to anoxic condition and thus the role of anoxia during the K/Pg mass extinction is strengthened by the record. Several sections were studied and the abundant echinoids (Gongrochanus sp. and Stygmatopygus sp.) were observed in Mawsynram (Phlang Mawsyrpat and Phud Japung area), whereas prolific Agerostrea sp. in Noh Kalikai, Sohkha and Muktapur-Amlarem sections and heteromorph ammonites along with gastropods in Sohra and Pynursla sections. The shell bed successions of echinoids and bivalves provide evidence of local episodic storm events in the upper shelf. The lithology and faunal assemblage together with the tectonic set up of the basin led to conclude that the reason for the difference in faunal diversity was probably due to the difference in the palaeo-depth of the basin.