The rise and decline of the Ediacaran biota: palaeobiological and stable isotopic evidence from the NW and NE Lesser Himalaya, India

V. C. Tewari
Geological Society, London, Special Publications, 286, 77-102, 1 January 2007, https://doi.org/10.1144/SP286.6

Abstract

Sediments of Ediacaran age have recently been recognized in the Krol Belt (NW Lesser Himalaya, India) with the discovery of pre-Ediacaran and Ediacaran biotas, carbon isotopic excursions matching the global curve and evidence for one Neoproterozoic glacial event. Other data include evidence for termination of the Krol algal-stromatolitic carbonate cycle in Late Proterozoic and deposition of phosphorite, phosphatic stromatolites similar to Tommotian taxa, and diversification of Small Shelly Fossils (SSF) of Tommotian/Meischucunian Zone 1 in Lower Tal Formation. Recently, Riphean–Ediacaran organic-walled microfossils, stromatolites and sponge spicules (monaxon) have been recovered. The link between the decrease in microbial-induced carbonate sedimentation and the decline of large Riphean stromatolites is recorded in the inner sedimentary belt (Deoban–Gangolihat). The first appearance of complex organic-walled microfossils and acanthomorphic acritarchs is recorded in the Blaini–Infrakrol sediments along with possible Ediacaran soft-bodied metazoans and metaphytes in the Krol Formation. In India, the Late Proterozoic base occurs at the bottom of a pink, microbial limestone in the Blaini Formation, and the Precambrian–Cambrian boundary occurs somewhere in the Lower Tal Formation. Ediacaran metazoans occur above glacial deposits assigned to the Varangian/Marinoan/Blainian and below the first occurrence of SSF (Meischucunian zone I), in the lowermost Cambrian.

Hoffman et al. (1998) proposed the term ‘Snowball Earth’ to describe the concept of Neoproterozoic low latitude glaciation. They suggested that oceans became completely frozen. However, not all researchers would agree (e.g. Etienne et al. 2006), and others suggest that some ocean waters were free of ice, and that primary biological productivity may have collapsed during this period (e.g. Hyde et al. 2000). Indeed, some even infer that continental ice cover was thin and patchy (e.g. Powell et al. 1993). Palaeomagnetic data also supports the view that some Neoproterozoic sequences on different continents accumulated at low palaeolatitudes, for instance in Australia (Chumakov & Elston 1989; Brookfield 1994). They also emphasized that if glacial deposits were not present in the Late Proterozoic sections, there would be no reason to question the validity of the low palaeomagnetic inclinations and palaeolatitudes of accumulation.

Jenkins (1981) proposed the concept of an Ediacarian Period as the terminal Precambrian subdivision. This time period was finally officially termed the ‘Ediacaran’ in 2004 (Gradstein et al. 2004). The Ediacaran stratotype occurs in the Flinders Ranges of South Australia. The Ediacaran Period corresponds to the Vendian Period in the Russian Platform. A supercontinent, Rodinia, including most of the continental crust may have existed at the end of the Mesoproterozoic (1000 Ma) up until around 750 Ma. During the Neoproterozoic, Gondwana consisted of two separate plates: the Eastern (Australia–Antarctica–India) and Western (Africa–South America) plates. These plates collided during the Cambro-Ordovician Pan African orogenies, to which the coeval Ross (Antarctica) and Delamerian (Australia) events are related (Dalziel et al. 2000). In South Australia, the Flinders Ranges host the Adelaidean–Cambrian sequence. The Wilpena Pound Quartzite, well known for its Ediacaran metazoan fossils, underlies the Lower Cambrian archaeocyathan-bearing limestones.

The presence of Ediacaran metazoans and carbonate stromatolitic platform sedimentation has been reported from the Krol Group of the NW Lesser Himalaya in India. In the Tethys Himalaya, the Phe Formation of Zanskar Range consists of Late Proterozoic–Cambrian carbonate platform deposits (Tewari 1998b). The SE coast of present day India was probably connected with East Antarctica at this time (Fig. 1). The Himalayan Cambrian sedimentary succession closely resembles the deposits of the Pacific margin of Antarctica and Australia. Cambrian sequences of northern Victoria Land (Antarctica) and South Australia, both part of Gondwana, have been correlated with the coeval Tethys Himalayan sequence. The Tethyan Early Cambrian sequence of the Zanskar Range is furthermore linked with the Shackleton Limestone Group of Antarctica and Normanville Group of Southern Australia (Casnedi 2002).

Fig. 1.
Fig. 1.

Rodinia supercontinent (after Powell 1996; Yoshida & Arima 2000).

The Terminal Proterozoic–Cambrian sequences in the Himalaya are referred to as Blaini–Krol–Tal groups in the Krol belt of Lesser Himalaya (Figs 2 & 3), the Macchal–Lolab groups in Kashmir, the Batal-Kunzam La formations in Spiti, the Martoli Group in Kumaon and Buxa–Miri groups in the NE Lesser Himalaya (Tewari 1998a, b, c, 2002a, b, 2003). Sedimentation in the Lesser Himalaya was terminated by the Pan African Orogeny in the Late Cambrian. The Ediacaran biota recorded in these sequences includes stromatolites, cyanobacteria, acritarchs, algae, sponges and other metazoans, metaphytes (vendotaenids) from the Krol Belt of the Himachal and Uttaranchal Lesser Himalaya (Tewari 1989, 1993, 1994, 1996, 1998a, b, 1999a, b, 2001a, b, 2002a, b, 2003, 2004b; Mathur & Shanker 1989; Shanker et al. 1997; Kumar et al. 1997, 2000; Tewari et al. 2000).

Fig. 2.
Fig. 2.

Geological map of the Blaini–Krol–Tal basin, Lesser Himalaya, India (after Geological Survey of India).

Fig. 3.
Fig. 3.

Lithostratigraphic column, palaeobiological remains and palaeoclimatic events in the Lesser Himalaya, India.

The Deoban–Jaunsar groups in the Lesser Himalaya represents pre–Ediacaran carbonate–siliciclastic sedimentation, and a Mesoproterozoic age is strongly supported by the presence of Early to Late Riphean stromatolite assemblages, and associated microbiota (Table 1).

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Table 1.

C- and O-isotope analyses for Deoban carbonates

The stromatolite assemblage characteristic of the Deoban–Gangolihat dolomite includes Kussiella kussiensi, Colonnella columnaris, Conophyton garganicum in the lower part, Baicalia chandakia, Jacutophyton sp., Baicalia nova, Tungussia sp. in the middle part and Jurusania–Gymnosolen–Minjaria–Inzeria assemblage in the upper part of the sequence (Valdiya 1969, 1980; Tewari 1989, 1996; ⇓ Fig. 5). Cyanobacteria found in the Deoban Chert include Huronispora psilata, Glenobotrydion aenigmatis, Myxococcoides minor, Oscillatoriopsis, Obruchevella, Siphonophycus and Kildinosphaera (Tewari 1989, 2003). Biodiversity of stromatolites and microbiota declined at the end of the Mesoproterozoic. The Jaunsar clastic facies contains no preservation of trace fossils or soft-bodied metazoans. The unconformably overlying Blaini Formation is a glacial unit. The pink cap microbial carbonate of the Blaini Formation may represent the base of the Late Proterozoic (base of the Ediacaran) where simple acritarchs such as the Sphaeromorphida and stratified stromatolites occur.

Fig. 4.
Fig. 4.

Location and geological map of the Ranjit Window (RW), Sikkim, NE Lesser Himalaya, India.

Fig. 5.
Fig. 5.

Carbon isotope chemostratigraphy of the Deoban Group (Meso-Neoproterozoic) Garhwal, Lesser Himalaya, India.

Rodinia began to fragment around 750 Ma, and East Gondwana (India, Australia and Antarctica) separated from West Laurentia. Baltica, Africa and South America were positioned on the other side of Rodinia. Neoproterozoic rifting, breakup of Rodinia and fragmentation of Gondwana, low-latitude glaciation and global warming events have all been recorded from the Lesser Himalaya of India. Carbon and oxygen isotopic variation and chemostratigraphy of the Blaini-Krol-Tal succession strongly suggests that the Lower Tal Formation (δ13C= −4‰ PDB) is of Late Neoproterozoic to Cambrian age. In contrast, sediments of the Krol belt in the Lesser Himalaya are characterized by positive δ13C values (+1 to 6‰ PDB). The presence of multicellular fossils in the Upper Krol is consistent with a possible increase in atmospheric oxygen.

The base of the Late Proterozoic in the Lesser Himalaya occurs in the Blaini Formation. A pink cap carbonate in the Blaini Formation registers a negative δ13C value (−3‰ PDB) and caps glacial sediments that are possibly representative of the Varanger glacial event. Comparison of the available carbon and oxygen isotope curves from other regions of Eastern Gondwana and South China, parts of Siberia and North Africa suggests that the Neoproterozoic–Early Cambrian chemostratigraphy is consistent in isotopic variation, with carbon isotopic signatures similar between Neoproterozoic sediments of the Bambui Group, Central Brazil and the Krol Formation of the Lesser Himalaya, India. In the northeastern Lesser Himalaya of India, the Neoproterozoic sedimentary successions show well developed carbonates, the Buxa Dolomite, with positive C-isotope ratios (δ13C=+3.7 to +5.4 ‰ PDB). The O-isotopic data also demonstrate δ18O values fluctuating within a narrow range between −8.9 and −7.2‰ PDB corresponding with global Late Proterozoic C-isotopic evolution, followed by oscillations during the Precambrian–Cambrian transition in the Lesser Himalaya of Eastern Gondwana.

The break-up of Rodinia, East Gondwana and the Lesser Himalayas

The Rodinia supercontinent (Fig. 1) began to fracture around 750 Ma. East Gondwana (India, Australia and Antarctica) separated from West Laurentia (Powell et al. 1993). Baltica, Africa and South America occupied the other side of Rodinia. However, the existence of a Neoproterozoic supercontinent Rodinia is still hypothetical (Yoshida & Arima 2000) and recent geochronological and palaeomagnetic records of South America and Africa show that these cratonic fragments may not have been part of Rodinia (Cordani et al. 2003). The Neoproterozoic evidence for the rifting apart of Rodinia and the fragmentation of Gondwana low-latitude glaciation and global warming events have been recorded from the Lesser Himalaya of India (Tewari 2002a). Carbon and oxygen isotopic variation and chemostratigraphy of the Lesser Himalayan carbonate formations evidently reflect global palaeoclimatic and palaeobiological events (Tewari 2001a, b, c, 2002a, b, 2003, 2004b; Figs 6, 7 and 8). The concept of using C-isotope variation as a global correlator is based upon the assumption that C-isotopic ratios fluctuate in response to changes in the net rate of organic burial, climate variations and availability of atmospheric oxygen.

Fig. 6.
Fig. 6.

Carbon isotope chemostratigraphy of the Gangolihat Dolomite (Meso-Neoproterozoic), Kumaun, Lesser Himalaya, India.

Fig. 7.
Fig. 7.

C-isotope chemostratigraphy of the Uttarkashi Limestone, Garhwal, Lesser Himalaya, India.

Fig. 8.
Fig. 8.

C-isotope chemostratigraphy of the Lameri Limestone, Garhwal, Lesser Himalaya, India.

Terminal Proterozoic glacial deposits have been recorded from most continents, suggesting that palaeoclimatic change on Earth over the last 650 Ma is directly related to atmospheric CO2 fluctuations (Jacobson & Kaufman 1999; Tewari 2001a, 2003). The ‘Snowball Earth’ model suggests that during global glaciation there may be a short-lived change in the C-isotopic ratios of the ocean because of elimination of marine life (not all researchers ascribe to this model, (e.g. Etienne et al. 2006). Hence, the δ13C in marine carbonates will drastically decrease to −6±1% (PDB) (Jacobsen 2001). The deglaciation event is recorded in the C-isotope variation from a pink cap carbonate indicating the end of the Terminal Proterozoic glaciation (Tewari 2001a, 2003). Kaufman et al. (1997) reported δ13C values in cap carbonates varying globally from 0 to −5‰ PDB. The δ13C values of the pink Blaini Limestone also vary from 0 to 3‰ PDB, consistent with the global record. Major palaeoclimatic changes, carbon isotopic fluctuations, biotic extinction-evolutionary events, sea level changes during the Late Proterozoic that are recorded from the Lesser Himalaya, India, are discussed in some detail in this paper.

Secular variations in carbon isotopes, palaeobiological and biogeochemical cycles

Palaeobiological activities on Earth started around 3800 million years ago with carbon as the main element in organic matter (Schidlowski et al. 1975; Eichmann & Schidlowski 1975). The ratio of stable C-isotopes reflects the effect of biological activity in the carbon cycle. Isotopic fractionation of c. 25% between the two carbon species (terrestrial or carbonate carbon, and organic or reduced carbon) has been recorded from rocks c. 3500 Ma old (Eichmann & Schidlowski 1975; Schidlowski et al. 1975, 1983). Carbon isotope stratigraphies of Proterozoic and Early Cambrian sequences have been established in recent years from many different parts of the world (e.g. Knoll et al. 1995; Santos et al. 2000). Carbon isotope excursions through the Neoproterozoic and across the Precambrian–Cambrian boundary have been interpreted to explain evolutionary diversification and extinction events, palaeoenvironmental and palaeooceanic conditions, changes in sea water chemistry and extraterrestrial impacts, and have been used to set up a chemostratigraphy for this time period (see reviews of Brasier et al. 1990, 1996; Brasier 1992; Knoll et al. 1995; Ripperdan 1994; Tucker 1986).

Ediacaran carbon-isotope chemostratigraphy and trace element geochemistry of the Blaini–Krol–Tal sequence, Lesser Himalaya

The Neoproterozoic is characterized by global occurrence of thick tidal flat carbonate-siliciclastic sequences and glacial cycles. Carbon and oxygen isotopic variations in these sequences have been used as palaeoclimatic indicators. Neoproterozoic carbonates have been studied worldwide for variations in oxygen and carbon isotope ratios, with special reference to inorganic and organic carbon reservoirs (Schidlowski et al. 1976; Aharon et al. 1987; Brasier et al. 1996; Kaufmann & Knoll 1995; Hoffman et al. 1998; Tewari 1991, 1997; Kumar & Tewari 1995). Low (−3‰ to −5‰) δ13C PDB values have been reported from reddish-pinkish dolomites (cap carbonates) associated with Neoproterozoic deglaciation around the globe, including the Blaini Formation (Bhattacharya et al. 1997; Tewari 1999a, b; Kumar et al. 2000; Santos et al. 2000; Alvarenga et al. 2003).

The carbon isotope chemostratigraphy of the Blaini pink microbial/stromatolitic cap carbonate shows negative values (δ13C −2.7‰ PDB), and the overlying Krol carbonates indicate very high positive values (+6‰ PDB, Fig. 9, Tewari 1991; Kumar & Tewari 1995). Isotopically heavy carbonates (δ13C +6.6% PDB) were deposited in the Upper Krol D stromatolitic facies representing enhanced organic burial. This strong positive isotopic shift is followed by a decrease in δ13C from +2‰ to c. 0‰ in Krol E carbonates. There is a negative shift in δ13C (−2.2‰ to −4‰ PDB) just below the Lower Cambrian (Fig. 9). These δ13C records are in concert with global isotopic variations in Neoproterozoic-Early Cambrian oceans of the world (Tewari 1998c). Bhattacharya et al. (1997) have also reported four depletions in δ13C values from Mussoorie and Garhwal synclines. However, only two peaks are of global significance, the first corresponding to what appears to be the end of the Varanger glaciation of Late Neoproterozoic age. The Blaini pink limestone in the Nainital syncline also shows depleted δ13C values (δ13C=−2.2‰ PDB). The oxygen isotopes (δ18O +20.1 to 28.6‰ SMOW) of the Upper Krol carbonates indicate high oxygen levels associated with possible Ediacaran diversification (Fig. 9). Major Neoproterozoic and Cambrian palaeoclimatic events, isotopic fluctuations and biotic evolution recorded in Lesser Himalaya are summarized in Table 2.

Fig. 9.
Fig. 9.

C-isotope chemostratigraphy of the Blaini–Krol–Tal (Terminal Proterozoic–Lower Cambrian) succession of the Lesser Himalaya, India.

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Table 2.

Major palaeoclimatic and biotic changes across Neoproterozoic–Cambrian Transition in the Lesser Himalaya, India (Tewari 2001a)

Carbon, oxygen and strontium isotopes, trace and rare earth element data are known from the Dhanaulti-Durmala section of the Mussoorie Syncline (Fig. 2; Table 3). The Precambrian–Cambrian boundary has been placed in the Lower Tal Formation (Aharon et al. 1987; Shanker & Mathur 1992; Tewari 1984, 1996, 1999a). The contact between the uppermost Krol Formation (Krol E) and the lower Tal Formation (chert-phosphorite member) is well exposed at Durmala Phosphorite Mine, where phosphorite is being commercially mined. An Early Cambrian stromatolite assemblage including Collumnaefacta vulgaris, Boxonia gracilis, Colleniella, Aldania mussoorica and oncolites are recorded from this locality (Tewari 1984, 1989, 1993, 1996). Small Shelly Fossils of Tommotian (Early Cambrian) age are also reported from phosphatic deposits in the Lower Tal Formation (Brasier & Singh 1989; Brasier et al. 1996). Laser Raman spectroscopy of the Mussoorie Tal Phosphorite indicates the presence of amino acids.

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Table 3.

δ13C, δ18O and Sr-Isotope data of Krol–Tal carbonates from the Mussoorie Syncline, Uttaranchal

The underlying Krol carbonates are tidal flat deposits including stromatolites, and fenestral facies are well developed (Tewari 1984, 2001a, 2002a, b). Carbon, oxygen and strontium isotope data from the Krol Tal carbonates in the Mussoorie syncline are summarized in Table 4. Stable isotope values range from −2.9 to +6.5‰ PDB for δ13C and +20.1 to +28.6‰ SMOW for δ18O respectively; δ13C values are generally positive for Krol C and D, carbonates reaching a δ13C maximum of 6.5‰ PDB. Krol E carbonates have near zero δ13C values. Tal carbonates show negative δ13C values with a δ13C minimum of −2.9‰ PDB. The petrographic study of these carbonates indicates that they preserve primary fabric-like oolites, microbial laminites and micrite. The isotopic signatures are considered unaltered and primary (Aharon et al. 1987). Hence, the positive δ13C values relative to PDB represent high rates of organic carbon burial whereas negative δ13C values and δ13C maxima of 6.5‰ PDB for Krol D dolomite represent increased organic carbon burial. Conversely, 13C minima of −2.9‰ PDB of Lower Tal Formation indicate low rate of organic carbon burial. Possible Ediacaran metazoans and metaphytes are recorded in the Krol Formation (Tewari 1993, 1998b, 2001a, 2002b).

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Table 4.

Trace element distribution in Blaini–Krol–Tal carbonates of the Mussoorie Syncline

87Sr/86Sr data in the Krol sequence vary from 0.7088 to 0.7151 (Table 4). The lowest value of 0.7088 was obtained from the Middle Krol C limestone. Krol C is a cement grade limestone in the Mussoorie Syncline, which has a Sr content of 659 ppm. Veizer et al. (1983) and Burns et al. (1994) have shown that the seawater 87Sr/86Sr value was c. 0.707, during what appears to be the Varanger glaciation, and rapidly rose to 0.709 during Late Ediacaran (to near present day sea water value). The Sr-isotope value of 0.7088 for the Upper Krol carbonates of the Mussoorie syncline may be taken as a near pristine sea water value, confirming that Late Proterozoic sea water values reached to c. 0.709 and corroborating with the earlier findings (Veizer et al. 1983; Burns et al. 1994; Aharon & Liew 1992). According to Aharon & Liew (1992), the high Sr-isotope value of >0.7095 for Krol and Tal carbonate may be due to exchanges with crustal fluids.

Trace element abundances of Blaini–Krol–Tal carbonate rocks are shown in Table 4. All samples from Krol E have high concentration of transition metals (Co, Ni, Cu and Zn), lithophile elements (Cs, Rb, Ba and Pb) and high field strength-cations (Y, Th, U, Zr and Nb), but are depleted in vanadium (Table 5). The strontium content is high for the Lower Tal carbonates. Major oxides, trace elements, and rare earth concentrations are summarized in Table 3.

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Table 5.

REE, Y, Sc and Th distribution in the Blaini and Krol carbonates of the Mussoorie Syncline

Oxides include Al2O3, which varies from 0.01% to 0.72%, K2O which varies from 0.01% to 0.017, SiO2 which varies from 0.02% to 1%, and 74.5% in the black chert straddling the Precambrian–Cambrian boundary. CaO varies from 30.56% to 31.0% in the uppermost Krol and is 9.6% in Lower Tal. MgO varies from 20.97% to 21.0% (0.84% in lower Tal). Na2O varies from 0.033% to 0.34% (0.03% in Lower Tal). Fe2O3 (total) varies from 0.042% to 0.16% (10% in Lower Tal). P2O5 varies from 0.015% to 0.039% (4.0% in Lower Tal). TiO2 varies from 0.001% to 0.022% (0.008% in Lower Tal). MnO varies from 0.004% to 0.035% (0.44% in Lower Tal) (Tables 6 and 7).

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Table 6.

REE distribution in Lower Tal (Precambrian–Cambrian boundary) Formation, Mussoorie Syncline

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Table 7.

REE distribution in Lower Tal Formation, Mussoorie Syncline

Carbon and oxygen isotope chemostratigraphy and palaeobiology of the Buxa Dolomite, NE Lesser Himalaya

Acharyya (1974) subdivided the Buxa Group into two units in the type area based on carbonate content and colour. Namely: (1) the lower Sinchula Formation overlain by (2) the Jainti Formation. The Buxa Group is exposed in Nakshal Khola and in the Jaldhaka river section in the easternmost part of the Darjeeling foothills, eastern Himalaya (Fig. 4). It is also recognized in the Sikkim Himalaya in the Ranjit Window (Raina 1976; Tewari 2004a). In western Arunachal, the Buxa Group is subdivided into two units separated by a pebbly diamictite bed (Shergaon pebble bed). However, the tectonostratigraphic position of the Buxa Group is still not certain in the Arunachal Lesser Himalaya. (Tandon et al. 1979; Tewari 1998a, b, 2002a, b, 2003).

The base of this Late Proterozoic carbonate sequence is represented by glaciomarine-fluvial diamictite-pebble beds. Other sedimentary facies include microbial buildups, oncolites, oolites, fenestral-textured sediments and microstromatolites, suggestive of shallow marine tidal flat depositional environments for the Chillipam, Rupa, Dedza, Menga and Panging limestones. Acharyya (1974) interpreted the Buxa Dolomite as shallow subtidal to intertidal deposits. Fossils include organic-walled microfossils, vase-shaped microfossils and micro metazoans, in addition to stromatolites, all suggestive of a Late Proterozoic age (Figs 13 and 14). Positive C-isotopic values (+3.7 to +5.4‰ PDB) of the Buxa Limestone (Fig. 10) relate to known global palaeobiological events. High positive values of δ13C from +2.8‰ PDB to +4.2‰ PDB characterize the Chillipam Dolomite and the Dedza Dolomite in the West Kameng of the Arunachal Lesser Himalaya (δ13C=+5.8‰ PDB). Additional microbiota have been recorded from the Subansiri area of the Arunachal Lesser Himalaya (Tewari 2005).

Fig. 10.
Fig. 10.

C-isotope chemostratigraphy of the Menga (Buxa) Limestone (Terminal Proterozoic), Arunachal, Lesser Himalaya, Northeastern Himalaya (East Gondwana).

The stable carbon and oxygen isotope data from the Buxa Dolomite (Meso-Neoproterozoic) cropping out in the Ranjit river valley, western Sikkim, NE Lesser Himalaya, India (Figs 4 and 11). The Buxa Dolomite comprises stromatolitic dolomite, cherty dolomite, intraclastic-oolitic dolomite, and minor siliciclastic sediments (Fig. 11). The stromatolite assemblage and the recently discovered microbiota from the Buxa Dolomite in Ranjit window, Sikkim Lesser Himalaya suggest a Meso-Neoproterozoic age (Tewari 2003, 2004a, b). The δ13C (PDB) varies in a narrow range from −1.4‰ to +1.0‰. (Fig. 11) Oxygen isotope values grade from 19‰ to 23.9‰ (δ18O SMOW). The lower part of the Buxa Dolomite showing a mostly positive trend may be the result of an increased rate of organic matter burial in a shallow carbonate platform. The δ13C and δ18O values of the Buxa Dolomite are summarized in Table 8.

Fig. 11.
Fig. 11.

C-isotope chemostratigraphy of the Buxa Dolomite, Ranjit Window (Meso-Neoproterozoic), Sikkim, NE Lesser Himalaya, Northeastern Himalaya (East Gondwana).

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Table 8.

δ13CPDB and δ18OPDB of the Buxa Dolomite, Ranjit Window, Sikkim, NE Lesser Himalaya

The recorded stromatolite assemblage includes: Kussiella kussiensis, Clonnella columnaris, Conophyton garganicum, Baicalia sp., Tungussia sp., Jurusania sp., Gymnosolen sp., Minjaria sp., Kalpanaella n. sp., Nucliella, Colleniella, Conistratifera, Aldania sp. and Collumnaefacta sp. (Tewari 2004a). The microbial assemblage from cherts includes Obruchevella sp., Eomycetopsis sp., Myxococcoides sp., Huronispora sp., Melanocyrillium sp. etc (Tewari 2004b).

Based on isotope data combined with sedimentological and palaeobiological data, the Buxa Dolomite appears to have been deposited on a carbonate platform, well connected with the ocean, the environment favourable for the formation of microbialites and cyanobacterial microbial mats, indicating that the environment of deposition was shallow marine (peritidal/subtidal to intertidal).

Discussion and conclusions

The Ediacaran radiation of metazoan and metaphytic multicellular life seems to have occurred after a change from ‘Snowball Earth’ to global warming in the Neoproterozoic period (Fig. 12). A major global decline of Mesoproterozoic stromatolites, planktonic acritarchs and other prokaryotic biota has been recorded on Earth, related to Neoproterozoic glaciation (Hoffman et al. 1998; Tewari 1993). Radiation of new acanthomorphic acritarchs, stromatolites, multicellular brown algae, vendotaenids, Tyrosotaenia, Krolotaenia and Ediacaran metazoans like Cyclomedusa, Charniodiscus, Dickinsonia, Spriggina and Pteridinium was recorded widely, and possibly in the Krol Group of the Lesser Himalaya (Fig. 3) (Knoll & Walter 1992; Kaufman & Knoll 1995; Mathur & Shanker 1989; Shanker et al. 1997; Tewari 1993, 1999a, b; Kaufman et al. 1997). Ediacaran animals originated prior to the main Varanger glaciation and diversified. The Cambrian explosion of shelled organisms is recorded in the Tal Group of rocks of the Lesser Himalaya (Fig. 12) and in the Tethyan sequences of Spiti-Zanskar and Uttaranchal Himalaya (Kumar et al. 1997; Tewari 1998b). Tewari (2001b) has reported Late Proterozoic organisms and microstromatolites in the Menga (Buxa) Limestone of the northeastern Himalaya. In this sequence, there is a well-developed diamictite, stromatolitic carbonate sequence (Buxa Dolomite/Chillipam Formation) exposed in the West Kameng district of the Arunachal Lesser Himalaya, similar to the Blaini diamictite and Krol Formation in sedimentary facies and microbial buildups (Tewari 2003). The Precambrian–Cambrian boundary may exist in this section (Tewari 1998a).

Fig. 12.
Fig. 12.

Global correlation of the Blainian-Krolian and Talian (Neoproterozoic to Lower Cambrian) stages of the Lesser Himalaya, India, East Gondwana, (after Brookfield 1994 with slight modification).

Fig. 13.
Fig. 13.

Neoproterozoic microfossils from the Buxa Dolomite, NE Himalaya. (a–b) Obruchevella parva Reitlinger 1959, BSIP Slide No. 13; (c) Volyniella valdaica (Shepeleva 1967), BSIP Slide No. 13; (d) Palaeolyngbya catenata Hermann 1974, emend Butterfield et al. 1994, BSIP Slide No. 13; (e) Glenobotrydion aenigmatis Schopf 1968, BSIP Slide No. 13; (f) Vetronostocale equale sp. nov., BSIP Slide No. 13; (g) Palaeoanacystes suketensis Zhang 1984, BSIP Slide No. 13; (h and l) Paratetraphycus giganteus Zhang 1984, BSIP Slide No. 13; (i) Huronispora robusta Mendelson & Schopf 1980, BSIP Slide No. 13; (j) Oscillatoriopsis robusta Mendelson & Schopf 1980, BSIP Slide No. 13; (k) Syphonophycus typicum Hermann 1974, BSIP Slide No. 11; (m) Myxococcoides minor Schopf 1968, BSIP Slide No. 13; (n) Eosynechococcus rugosum (Maithy) Hofmann & Jackson 1994, BSIP Slide No. 13; (o) Siphonophycus rugosum (Maithy) Hofmann & Jackson 1994, BSIP Slide No. 13; (p) Polythrycoides lineatus German 1974 emend Timofeev et al. 1976, BSIP Slide No. 13; (q and r) Vetronostocale amoenum Schopf and Blacic, emend Xu & Awramik 2001, BSIP Slide No. 13; (s) Oscillatoriopsis breviconvexa Schopf & Blacic 1971, BSIP Slide No. 13; (t) Oscillatoriopsis rhomboidalis Sivertseva 1989, BSIP Slide No. 13.

Fig. 14.
Fig. 14.

Neoproterozoic microfossils from the Buxa Dolomite, NE Himalaya. (a) Trachyhystrichosphaera aimica Hermann 1981, BSIP Slide No. 13; (b) Lophosphaeridium rarurm Timofeev 1959, BSIP Slide No. 13; (c) Vandalosphaeridium reticulatum Vidal 1981, BSIP Slide No. 13; (d) Margominuscula rugosa Naumova 1960, BSIP Slide No. 13; (e) Margominuscula simplex Pychova 1973, BSIP Slide No. 13; (f) Archaeohystrichosphaeridium semireticulatum Timofeev 1959, BSIP Slide No. 13; (g) Micrhystridium lanatum Volkova 1969, BSIP Slide No. 13; (h) Granomarginata vetula Saluja et al. 1972, BSIP Slide No. 13; (i) Micrhystridium ampliatum Wang 1985, BSIP Slide No. 13; (j) Melanocyrillium hexodiadema (Bloesser) cf Angochitina milanensis, BSIP Slide No. 13; (k) Lophosphaeridium jansoniusii Saluja et al. 1971b, BSIP Slide No. 13; (l) Leiosphaeridia visingsa Zang in Zang & Walter 1995, BSIP Slide No. 13; (m) Navifusa segmentatus Prasad & Asher 2001, BSIP Slide No. 13; (n) Trachysphaeridium robustum Yin & Li 1978, BSIP Slide No. 13; (o) Archaeohystrichosphaeridium cellulare Timofeev 1959, BSIP Slide No. 13; (p) Polythrycoides lineatus German 1974 emend. Timofeev et al. 1976, BSIP Slide No. 13; (q) Vetronostocale amoenum Schopf and Blacic, emend Xu & Awramik 2001, BSIP Slide No. 13; (r) Vetronostocale amoenum Schopf & Blacic, emend. Xu & Awramik 2001, BSIP Slide No. 13; (s) Oscillatoriopsis breviconvexa Schopf & Blacic 1971, BSIP Slide No. 13.

The Buxa Dolomite of the Northeastern Himalaya of Bhutan, Sikkim, Darjeeling and Arunachal Pradesh has been traditionally correlated with the Mesoproterozoic (Riphean) Shali–Deoban–Gangolihat carbonate belt of the NW Himalaya, on the basis of lithological similarities and stromatolite content (Valdiya 1969, 1980; Tewari 2003). Acharyya (1974), to the contrary, considered the Buxa Dolomite to be older than Late Palaeozoic but younger than the Shali-Deoban stromatolitic carbonates. This paper suggests correlation of Buxa, Chillipam and Dedza dolomites of the northeast Himalaya to the Late Neoproterozoic on the basis of microstromatolites/stromatolites, microbiota and carbon isotope stratigraphy (Tewari & Sial 2003, 2004). Recently discovered microbialites, organic walled microfossils, sponge spicules and extensive carbonates (Tewari 2001a, b, c, 2002a, 2003, 2004a) (Fig. 10) from the Menga-Mara section in Subansiri Valley indicate a Late Proterozoic age for this sequence. Ten representative samples (MM1 to MM10, Fig. 10) of dolomite were analysed for C- and O-isotopes. The C-isotopic ratios are significantly positive and quite consistent with δ13C (carbonate carbon) values varying from +3.7 to +5.4‰ PDB. δ18O values fluctuate between −8.9 and −7.2‰ PDB. The consistency of the carbon isotopic record from the Buxa Dolomite indicates that the isotopic data are representative and the signatures are unaltered. The significantly positive C-isotopic results are in concert with global Late Proterozoic isotopic patterns (Aharon et al. 1987; Kumar & Tewari 1995; Tewari 1991, 1997, 1998c, 2001a, 2002a; Tewari & Sial 2004; Bhattacharya et al. 1996). A close relationship of δ13C enriched carbonate rocks with peaks in stromatolite diversity and biomass can be attributed to the enhanced bioproductivity.

The Late Proterozoic–Early Cambrian sequence of the Uttaranchal Central Lesser Himalaya has been studied in detail for C-isotope chemostratigraphy (Aharon et al. 1987; Tewari 1991, 1997, 2001a, b; Kumar & Tewari 1995; Tewari & Sial 2004; Bhattacharya et al. 1996)—the first such attempt in the northeastern Himalaya. The Buxa Dolomite positive isotopic excursions are strikingly similar to those of the Krol Formation (Krolian) of the Uttaranachal (Central Lesser) Himalaya. It is interesting that the Doushantuo carbonates of the Yangtze Platform, Southern China (Shen & Schidlowski 2000) display high positive δ13C values similar to the Krol–Buxa belt. The Krol–Buxa–Doushantuo carbonates were deposited after a Neoproterozoic glaciation (Tewari 2001b, c, 2002a, 2003) and have very similar sedimentary facies, biota, and carbon-isotope chemostratigraphy (Tewari 2001a, b, 2002). The Lesser Himalayan sequences of India show similar palaeobiological and stable isotopic patterns to other Ediacaran sequences; further detailed investigation shows promise of correlation worldwide.

Acknowledgments

I am thankful to M. Schidlowski, Biogeochemistry Department at Max Planck Institute für Chemie, Mainz, Germany and A. N. Sial, NEG–LABISE, Pernambuco University, Recife, Brazil for carbon isotopic analysis of the carbonates and discussions. Professors R. J. F. Jenkins, University of Adelaide, Australia, S. Turner, Monash University, and M. Yoshida, Gondwana Institute of Geology and Environment, Hashimoto, Japan are thanked for critical reviews of the manuscript and valuable suggestions. The author is grateful to P. Vickers–Rich (Australia), M. Fedonkin (Russia), J. Gehling Project leaders of I.G.C.P.-493 for discussions on Ediacaran and Vendian biota. M. Tucker, University of Durham, England is thanked for discussions in the field. K. S. Valdiya, Chairman and B. R. Arora, Director, Wadia Institute of Himalayan Geology, Dehradun, India provided facilities and permission to publish the paper. This is a contribution to the I.G.C.P. Project 493 on ‘Rise and Fall of the Vendian Biota’. G. Chauhan ably typed the article.

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