Trace fossils from Talchir carbonate concretions, Giridih basin, Jharkhand SSDas∗ and Mahesh Kumar Tripathi Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad 211 002, India. ∗ e-mail: siddharthasankar−[email protected] The carbonate concretions occurring at the bottom of Talchir fissile shale facies preserved signatures of various trace fossils along with a cast of doubtful organisms and cyanobacterial mat structures. The host shale deposited under glacial melt water fed lacustrine condition. The concretions, formed in poorly oxygenated conditions, are either of syndepositional origin and/or deposited a little below the sediment water interface and were later exhumed to the depositional surface due to erosion of soft mud overlying them. The trace fossils are both megascopic and microscopic in nature. The megascopic trace fossils are identified on the basis of their morphology as Monocraterion and Rhizocorallium.Someofthe megascopic structures described remain problematic at present. The microscopic trace fossils are formed due to the activity of marine meiofauna (possibly by nematodes), which, although produced morphologically show similar traces of known larger ichnogeneras but much smaller than them. The discovery of these trace fossils apparently indicate the influx of saline water into a lacustrine domain during the Talchir sedimentation at Giridih basin. Moreover, presence of the above two megascopic trace fossils in the marine lacustrine carbonate concretions may lead researchers to consider their much wider environmental significance than hitherto believed. 1. Introduction bivalves and brachiopods from the Talchir forma- tion of Son-Mahanadi basin (Casshyap and Arora The Talchir Group that marks the beginning of 1994); marine coccoliths from the Talchir forma- Gondwana sedimentation in India is traditionally tion, Ramgarh coal field (Chaudhuri and Mandal considered to be of glacial and fluviolacustrine ori- 1989); trace fossils from the Talchir formation of gin (Blanford et al 1856; Rao 1957; Niyogi 1961; Raniganj and Deogarh coal fields (Guha et al Ghosh and Mitra 1975; Sen 1991; Sen and Banerjee 1994); and marine algal stromatolites and colonial 1991; and others), except for a few well-established cyanophytes from the Talchir formation, Talchir fossiliferous sequences reported from Umaria (Reed basin, Orissa (De 1999) are some of the important 1928) and Manendragarh in Madhya Pradesh findings. Mohanty and Das (1997) reported micro- (Ghosh 1954) and Daltonganj in Jharkhand (Dutta bial mat from Talchir basin, Orissa and inferred 1965). But this concept is under the scanner after periodic influx of saline water into the lacustrine the documentation of various signatures of marine domain based on the stable isotope analysis. The environment from several Gondwana basins of sedimentary structures and trace fossils preserved India. Fossil polyplacophora (Sengupta et al 1999) in the Talchir formation of Sahajuri basin indicates and foraminifers (Pal et al 1994) from Talchir its deposition under glaciomarine deltaic environ- sediments, West Bokaro basin; marine pelecy- ment (Bhattacharya et al 2004; Chakraborty and pods from Ranigunj basin (Chandra 1994); marine Bhattacharya 2005). Chakraborty (1993) indicated Keywords. Trace fossils; Talchir Group; concretions; Giridih basin; environment. J. Earth Syst. Sci. 118, No. 1, February 2009, pp. 89–100 © Printed in India. 89 90 S S Das and Mahesh Kumar Tripathi Figure 1. Geological map of Giridih basin (redrawn after Das 1986; Sengupta et al 1988). marine influence during the deposition of the 2. Geological setting Talchir group of sediments, Giridih basin on the basis of trace fossils preserved in sandstone. The Giridih Gondwana basin, a small WNW- Concretions and nodules are considered very ESE trending basin in eastern India includes important because they often provide ‘frozen Permocarboniferous sediments belonging to the records’ of the condition of the sediment at the Talchir group, the Karharbari and Barakar mem- time of deposition and consolidation processes bers (Sengupta et al 1988). The presence of a large (Raisewell 1971; Selles-Martinez 1996). Concre- inlier of Precambrian rocks within the Gondwana tions are also given lot of importance because sediments is an interesting feature. The inlier is they may often preserve fossils and other bio- bounded by normal faults on all sides. The Talchir genic structures. The concretions/nodules found in group sediments are predominantly present in the the Talchir sediment of different Indian Gondwana western part of the basin (figure 1). The Talchir basins, were predominantly made up of calcareous group directly overlies precambrian gneissic rocks material (Ghosh et al 2002; Das and Goyal 2007). andinturnareoverlainbyKarharbarimember. Based on the trace element and isotopic studies The boundary between the Talchir group and the Ghosh et al (2002) suggested fresh water lacustrine Karharbari member is gradational to the south depositional environmental condition for the con- but a N–S trending fault separates them towards cretions of West Bokaro, Ramgarh (Damodar val- the east. The northern boundary of Talchir group ley basin), and Talchir (Mahanadi valley basin). against Precambrian rocks is marked by prominent The carbonate concretions of various shapes and faulting while the western boundary is an erosional sizes are found in the Talchir Group sediment, unconformity. Giridih basin (Damodar valley), Jharkhand. Das The Talchir group sediments which are mainly and Goyal (2007) studied these concretions for exposed in and around the Sooknee Nala (western their various crack morphologies. These carbon- part) comprises basal boulder conglomerate facies ate concretions have preserved various megascopic succeeded by khaki-green fissile shale facies, and trace fossils, doubtful impressions of body fossils, an upper sandstone facies (table 1). The carbon- and various signatures of microscopic trace fossils. ate concretions, occurring at the bottom part of The objectives of this paper are to describe the fissile shale facies, are well exposed at the bed in above findings and correlate them to a possible the north-western part of Sooknee Nala (plate 1A). environmental set up during the Talchir group For detail geologic setting please see Sen (1991). sedimentation at Giridih basin. The glacial melt water-fed lacustrine and turbidity Trace fossils from Talchir concretions 91 Table 1. Lithofacies variations at the study area. Lithofacies Description Trace fossils C. Sandstone Massive, matrix supported conglomerate Not reported. with angular clasts. It laterally as well as vertically changes to fine grained, yel- lowish, feldspathic sandstone with spo- radic pebbles. Sandstone shows parallel lamination, cross-lamination, and con- volute lamination. B. Shale Khaki-green fissile shale with dropstones. Burrows in the sandstone: Zoophycos, The lower part is massive greyish black Skolithos, Planolite and Teichichnus diamictite with negligible clast con- (Chakraborty 1993). Trace fossils tent. The diamictite passes upward into in the carbonate concretions (this bluish-grey laminated calcareous silt- paper). stone which is often sandy. The carbon- ate concretions occurring at the bottom of fissile shale and above siltstone. Some of the concretions show dropstones. The shale facies also contains few stringers of fine grained laminated sandstone and siltstone. The sandstone shows ripple- drift lamination. Striations and chatter marks preserved on the siltstone. The upper part of the facies is weakly strati- fied and interlayered with rhythmites. A. Boulder conglomer- Massive, poorly sorted conglomerate with Horizontal and vertical burrows in silt- ate (Diamictite) clayey and sandy matrix. The clasts are stone (Sen 1991). Burrows are not dominantly cobble size with occasional classified. boulders. Boulders are often faceted. The conglomerate passes upward in to a bluish-grey fine grained laminated cal- careous siltstone. The siltstone in places sandy and cross-laminated. current deposition is suggested for the shale litho- coloured, fine grained, crinkled layer and relatively facies (Sen 1991). lighter grey coloured, fine grained layer (plate 1C). These layers have developed due to seasonally changing water depths corresponding to fluctua- 3. Carbonate concretions tions in light intensities which triggered the stacked buildup of sets of dark and light laminae. The dark The concretions occur as laminated bodies of horizontal layers are rich in filamentous cyanobac- varying shapes and sizes. The laminations of the terial mat formed during the winter, whereas the shale could be traced in continuity to the con- light layers rich in sediment formed during the sum- cretions. The concretions are grey-black coloured, mer (Gerdes 2007). The deformed laminae are of dense and compact with diameters varying between syncompactional origin with the host shale. At 4 cm and 50 cm. On the basis of their morphology small depths with continuous deposition sediment Das and Goyal (2007) classified them into three compacts rapidly and growing concretion develops categories: cabbages like growth, domed up growth deformed laminae (Raisewell 1971). The concre- structure with laminae are warped towards periph- tions are predominantly made up of calcite and ery, and discoid bodies with concentric structure at quartz with little feldspar, pyrite and clay min- plan view. The vertical axis in all the above varie- erals (illite and chlorite). The insoluble