ACRITARCH BIOSTRATIGRAPHY OF THE ORDOVICIANSILURIAN SEQUENCE ALONG SUMNA-RIMKHIM SECTION, TETHYAN GARHWAL HIMALAYA, CHAMOLI DISTRICT, UTTAR PRADESH

Abstract

Rocks of the Lower Paleozoic marine sequence occur in the Tethyan Zone of the Himalaya with exposures in Kashmir, Spiti, (Himachal Pradesh) and Kumaon-Garhwal (Uttar Pradesh) regions of India. Although, these sediments have received attention ofgeologists since long, very little studies have been carried on the palynological aspects so far. Published literature reveals the occurrence of brachiopods, trace-fossils, conodonts and certain other fossil microbiota in the Lower Paleozoic marine sequence. No published record is available on acritarch biostratigraphy of this sequence in the Garhwal region. The present study, therefore, is aimed at investigating these rocks on this aspect, especially in view of well known occurrences of acritarchs in the Ordovician-Silurian rocks from many parts of the world like, United Kingdom, Belgium, Spain, Norway, Arabian Sahara, Africa, United States of America and Canada etc. The study area lies in the Tethyan Zone of the Garhwal Himalaya in and around the village, Sumna, Chamoli district, Uttar Pradesh, India. The geographic coordinates of the region are 30°40' latitude and 80°50' longitude. The area, in general, is poorly accessible due to physiography, hostile climate, and also from the point of view of its proximity to the international border area with China. The study, therefore, was taken up along the mule track between Sumna and Rimkhim covering Ordovician-Silurian sequence outcropping in a stretch of about five kilometers. Two geological field investigations were undertaken during the months of August and September in 1992 and in 1993. The Sumna-Rimkhim section, in the study area, was found to be an ideal section exposing a large part (Ordovician to Silurian) of the Lower Paleozoic marine sequence after a thorough reconnaissance survey. The lithostratigraphic Units (i) (Ordovician to Silurian) namely Garbyang (upper part), Shiala, Yong Limestone and Variegated formations in stratigraphic order were examined along this section of about 680 meters thickness. Seventyone samples of these sediments were collected at measured intervals for the present investigation. Litho-petro-stratigraphic column was prepared after necessary correction through careful computation of the field data. The broad lithology of the section examined constitutes litho-units like siltstone, shale, siltyshale, sandstone and limestone. However, only the sediments of Shiala and Yong Limestone formations (Caradocian to Ludlovian) yielded acritarchs. Samples of Garbyang and Variegated formations (of Llandelian and uppermost Ludlovian ages respectively) were found barren of acritarchs but were found containing other macro-and microfossils. Treatment of samples using the conventional palynological techniques yielded abundant and varied forms of acritarchs with infrequent to rare to common occurrence of chitinozoans, melanosclerites, conodonts, scolecodonts, algae and bryozoa etc. Hence, acritarchs form the principal target for age determination, palynozonation, intercontinental correlation and paleoenvironmental deductions due to the their yield in abundance. Samples were also treated for sedimentological and chemical-sedimentological investigations by adopting conventional laboratory techniques to obtain additional supporting evidences with regard to paleoenvironmental deductions. These detailed studies carried out, under the present investigation, bring out the following significant findings: (1) For the first time, 20 genera and 67 speices of acritarchs have been identified and recorded from the Ordovician-Silurian (Caradocian to Ludlovian) sediments of the Tethyan Garhwal Himalaya, India.

' Taxonomic descriptions of all the acritarch forms have been given. On the basis of first occurrence and last occurrence levels ofselected acritarch species, four distinct acritarch assemblage zones namely Baltisphaeridium longispinosum Var. longispinosum- Multiplicisphaeridium ornatum; Domasia trispinosa-Deunffia monospinosa; Domasia limaciforme-Dactylofusa oblancae; and Leiofusa algerensis-Multiplicisphaeridium osgoodense were erected precisely in stratigraphic order with description of reference section, definition, base and top, significant accessory forms, overseas correlation and age assignment of each zone. Top and base of each zone has been delineated in the following manner: Zone'l : Base of zone I could not be marked because of acritarch barren litho-unit. Top : Last occurrence of Baltisphaeridium longispinosum Var. lonispinosum and first occurrence of Geron sp. cf. G. amabilis. Zone II: Base: The first occurrence of Geron sp. cf. G. amabilis and last occurrence of Baltisphaeridium longispinosum Var. longispinsoum. Top : The last occurrence of Deunffia brevispinosa. Zone III: Base : The last occurrence of Deunffia brevispinosa. Top : The last occurrence ofLeiofusa algerensis and Stellichinatum celestum. Zone IV : Base : Last occurrence of Leiofusa algerensis. Top : Top of zone IV could not be marked because of acritarch barren lithounit. The acritarch biostratigraphic zonation reveals that the Ordovician-Silurian boundary lies within the Shiala Formation which was marked within the lower part of Yong Limestone Formation by earlier workers (Shah and Sinha, 1974 and Khanna et al., 1985). (iii) (2) Paleoenvironmental deductions have been attempted mainly on the basis of acritarchs and other associated fossil microbiotic elements. Other environmental indicators such as limestone/ shale/siltstone/sandstone petrography, clay mineral assemblage, major &trace element and silt contents have also been considered in this study. Occurrence ofacritarchs and other associated fossil microbiotic elements in sediments indicates site of deposition extending mainly from inner to middle to outer Neritic Zone (transitional to marine). Results of overall study reveals that warm water conditions with normal salinity prevailed throughout in the basin of sedimentation. These inferences are supported further by the results obtained after the investigations carried on the sedimentary aspects/parameters. In summary, the record of prolific acritarchs for the first time from the sediments of Caradocian to Ludlovian sequence of the Tethyan Garhwal Himalaya offers excellent opportunity for further detailed study, not only in this region, but also, in Kashmir and Spiti regions of India, which ultimately, would provide the scope of regional correlation within the Indian subcontinent as well.