Eocene larger foraminiferal biostratigraphy, depositional history and paleogeography of the Sylhet Limestone of the Mikir Hills of Assam, NE India: Implications for an Open Tethys

Abstract

This study presents results of integrated larger foraminiferal biostratigraphy, lithofacies and stable carbon isotope (13 C org) analysis carried out in the Sylhet Limestone of Dillai Parbat Limestone Mine succession, Karbi Anglong District, Assam. Larger foraminiferal assemblages assign a middle Eocene age to the studied succession of Sylhet Limestone. The Shallow Benthic Zone (SBZ) 13 and SBZ 16-18 and a barren interval in between is recognized in the studied part of the Sylhet Limestone. The SBZ 13 Zone (early Lutetian) is represented by fossiliferous limestone at the base of the section and is inferred to have been deposited in a subtidal environment. The overlying barren interval lies in the arenaceous interval and was likely deposited in a subtidal channel sand shoaling bars. The upper part of the section ismarked by SBZ16-18 (late Lutetian to Bartonian) and was deposited in subtidal channel sand inner neritic zone environments, followed by intertidal marl to subtidal limestone. The organic carbon stable isotopic data reveals a sharp negative excursion at about 34-35m of the succession, which may be correlated with Middle Eocene Climatic Optimum (MECO) and/or vegetation types. This and earlier studies from India and elsewhere indicate that during the middle Eocene, the Neo-Tethys Sea was open and connected through the East and West coasts of India, Northeast India, western Himalaya, and most of the Middle East and southeast Europe. This vast area produced enormous amount of petroleum.

Record of Additional Middle Eocene Vertebrate Remains from the Mikir Hills, NE India: Implications on Paleoenvironment and Paleobiogeography

“…millecaput, N. lehneri, N. praediscorbinus, N. striatus, N. discorbinus, N. praegarnieri, N. cf. ptukhiani, N. vicaryi and Assilina papillata [17]. The horizon containing the vertebrate assemblages described here comes from larger benthic foraminiferal Zone SBZ 16-18 which corresponds to late Middle Eocene age (Bartonian) of [17].…”

“…These beds yield an admixture of faunal assemblages of foraminifers, including larger benthic foraminifers (mainly Nummulites), smaller benthic foraminifers (Quinqueloculina sp., Triloculina sp., Cibicides sp., Turrilina sp., and Lagena sp.) [17] [18] was the first to study in detail the microfaunal assemblages of the area. On the basis of stratigraphic ranges of planktonic foraminiferal species, [19] identified five biozones of middle to late Eocene age from the Kopili Formation of Mikir-North Cachar Hills, Assam: Truncorotaloides topilensis Zone, Truncorotaloides rohri Zone, Globigerapsissemi involuta Zone, Cribrohantkenina inflata Zone, Globigerina gortanii Zone.…”

“…Four larger benthic foraminiferal zones ranging in age from early Eocene to latest middle Eocene occur in the Sylhet Limestone Formation of Mikir Hills [23] along with mammalian lumbar vertebra and fish teeth from Dillai Parbat of Mikir Hills [13]. Lately, [17] delineated Shallow Bethic Zones (SBZ) 13-18 with a barren zone in the middle of the section which suggests that it might correspond to SBZ 14-15.…”

“…ptukhiani, N. vicaryi and Assilina papillata [17]. The horizon containing the vertebrate assemblages described here comes from larger benthic foraminiferal Zone SBZ 16-18 which corresponds to late Middle Eocene age (Bartonian) of [17]. In view of the poor preservation of fossils, specific identifications in some cases have not been attempted, and only tentative identifications are made (Figure 3, Figure 4).…”

“…The Sylhet Limestone in the Mikir Hills is composed of physically and lithologically distinct Sandstone, Shale and Limestone lithofacies associations [17]. These lithofacies are repetitive in nature and occur at different levels forming a 33 m thick succession.…”