Revised Tal Group lithostratigraphy and record of biostratigraphically significant Shaanxilithes ningqiangensis from Mussoorie syncline,Lesser Himalaya

Abstract

A succession of bedded earthy argillaceous dolomite (~17 m) overlain by earthy siltstone (~12 m) is recognised in the lower part of the Deo-Ka-Tibba Formation (Tal Group) in the northeastern limb of the Mussoorie syncline, near Bhusti village. The recognition of these two diagnostic units necessitates a reclassification of the Tal Group in the Mussoorie syncline. This succession overlies the massive carbonate unit of the Krol-E (Krol Group) and underlies the Chert-Phosphorite Member of the Deo-Ka-Tibba Formation (Tal Group). Based on close lithological similarity with the sequence in the Nigalidhar syncline (Himachal Pradesh), these lithounits are designated Earthy Dolomite Member (EDM) and Earthy Siltstone Member (ESM), forming the lowermost part of the Deo-Ka-Tibba Formation. The Deo-Ka-Tibba Formation now includes six members: Earthy Dolomite, Earthy Siltstone, Chert-Phosphorite, Argillaceous, Arenaceous, and Calcareous. We also identified the Ediacaran body fossil Shaanxilithes ningqiangensis within the ESM, which indicates its deposition across the Ediacaran–Cambrian boundary interval, similar to that recorded in the Nigalidhar syncline (Himachal Pradesh). The present study establishes a precise litho-biostratigraphic correlation between the Mussoorie and Nigalidhar synclines of the Lesser Himalaya. The discovery of Shaanxilithes ningqiangensis recorded in the Lesser Himalaya is regionally and globally significant, and offers critical insight into the evolution of the dynamic stratigraphy during the transition from the latest Ediacaran to early Cambrian.

INTRODUCTION

The Tal Group succession (latest Ediacaran-early Cambrian) is exposed in the five doubly plunging syncline—Nigalidhar and Korgai (Himachal Pradesh), and Mussoorie, Garhwal, and Nainital (Uttarakhand) (Figure 1A). Since the pioneering studies (Auden, 1934, 1937; Medlicott, 1864; Singh, 1979, 1980), and the subsequent discovery of Cambrian small shelly fauna (Azmi et al., 1981; Brasier & Singh, 1987; Juyal, 1979) as well as trilobites, brachiopods and trace fossils of the Cambrian (Bhargava et al., 1998; Hughes et al., 2005; Jell & Hughes, 1997; Joshi et al., 1989; Kumar et al., 1987), it is widely accepted that the Tal Group represents early Cambrian age (see Singh & Bhargava, 2020 and references therein).

Figure 1.

(A): Location of the Lesser Himalayan NW-SE trending synclines from Krol syncline (NW) to Nainital syncline (SE) and the position of the Mussoorie syncline (area of study), (B): Geological map of the part of the Mussoorie syncline showing the disposition of different geological formations and Garhwal group of rocks (modified after Shanker, 1971).

Moreover, the available literature from these synclines shows a generalised litho-bio-chronostratigraphy of the Tal Group (Hughes et al., 2005), often without adequately addressing its geological extent and variations across the different synclines (Singh & Bhargava, 2020). This generalised litho–bio–chronostratigraphic framework of the Tal Group, now deeply entrenched in the literature, has hindered the regional stratigraphic understanding of this succession in the Lesser Himalaya and continues to pose significant problems for paleogeographic reconstructions (e.g., Khan et al., 2025; Mahmood et al., 2024). Most of the published literature states that the Tal Group occupies the cores of the NW-SE-trending synclines in the Inner Krol Belt of the Lesser Himalaya. A closer examination of these synclinal cores reveals variable rock successions of differing ages. In the Nigalidhar syncline, the Cambrian Tal Group is unconformably overlain by the Ordovician Deona Group, while its core remains undated. In the Mussoorie syncline, the Cretaceous Nilkanth Formation unconformably overlies the Cambrian Tal Group, and Palaeocene–Eocene Subathu Formation, over which rests the klippen of the Garhwal. In the Garhwal syncline, the Cambrian Tal Group is unconformably overlain by the ‘Carboniferous–Permian’ Boulder Slate Formation (Chaturvedi & Talent, 1971; Ganesan, 1971, 1972; Shanker et al., 1973), followed unconformably by the Cretaceous Nilkanth Formation (Bhatia, 1980; Singh, 1979; Tewari & Kumar, 1967), and the Paleocene–Eocene Subathu Formation, which is tectonically overlain by the Garhwal Group of rocks.

Therefore, when the geological aspects of these NW-SE-trending synclines are examined independently, they reveal variable and non-uniform successions, with certain lithounits absent, unconfirmed, or yet to be documented for specific ages (see Figure 2 in Singh et al., 2025). This lacuna arises from the adoption of a generalised litho-bio-chronostratigraphy of the Tal Group, and overlooking its actual extent in individual synclines, coupled with the assumption that the Tal Group is equally preserved and invariably forms the core of these synclines.

Figure 2.

(A): Google earth image of the studied section near the Bhusti village in northern limb of the Mussoorie syncline, showing the rocks of the Earthy Siltstone and Earthy dolomite members above the massive carbonate of Krol-E formation (Krol Group) and below the Chert-Phosphorite Member of the Deo-Ka-Tibba Formation (Tal Group), (B): Field photograph of the exposure of contact of Earthy Siltstone Member and Chert Member of the Deo-Ka-Tibba Formation at Bhusti village in Mussoorie syncline; (C–D): thinly to -thickly-bedded earthy siltstone which contains the fragments of Shaanxilithes ningqiangensis—an Ediacaran body fossil; (E): measured lithocolumn of the Earthy Dolomite and Earthy Siltstone members (Deo-Ka-Tibba Formation, Tal Group) and location of the Shaanxilithes ningqiangensis bearing level in upper part of Earthy Siltstone Member.

Furthermore, the available literatures on these synclines largely falls into three phases: (a) 1934–1975, when the Tal Group was considered Mesozoic in age, and (b) 1976–1998, when reclassification of the Tal Group and revisions of faunal assemblages began (Bhargava et al., 1998; Dhaundiyal & Kumar, 1980; Jell & Hughes, 1997; Shanker et al., 1993) following the separation of the Cretaceous Nilkanth Formation in Garhwal syncline (Singh, 1976, 1979) and the discovery of small shelly fauna of the Cambrian age from its lowermost strata in the Mussoorie syncline (Azmi et al., 1981; Brasier & Singh, 1987; Juyal, 1979), and (c) 2000–2025, when sporadic studies have been carried out, particularly focusing on body and trace fossils from both previously known and new localities (Desai et al., 2010; Singh et al., 2014, 2015, 2019, 2024, 2025; Tarhan et al., 2013; Tiwari & Parcha, 2006; Tiwari et al., 2013), revisions of faunal assemblages (Hughes et al., 2005; Popov et al., 2015), recognition of arthropod trace fossils of Gondwana affinities (Singh et al., 2019, 2020, 2024), and documentation of the Cambro-Ordovician angular discordance and Ordovician-age trace fossils (Singh et al., 2019). These findings have led to revisions in the lithostratigraphy of the Tal Group (Bhargava & Singh, 2022; Singh et al., 2020, 2019), as well as reinterpretations of zircon age data from the Upper Tal rocks in the Mussoorie region (Myrow et al., 2023).

Due to these lacunae, we initiated a detailed investigation of the Tal Group in the Mussoorie syncline. In the present study, we document a ~29 m thick succession of earthy siltstone and Earthy Dolomite units at the contact between the Krol-E Formation (Krol Group) and the Chert Member of the Deo-Ka-Tibba Formation (Tal Group), exposed on the northern limb near Bhusti village. These two lithounits, now designated as members, not recognised by earlier workers, are additions to the Deo-Ka-Tibba Formation (Tal Group) of the Mussoorie syncline. We also report for the first time the occurrence of the significant Ediacaran body fossil Shaanxilithes ningqiangensis from the Earthy Siltstone Member (ESM) in the Mussoorie syncline, and provide its regional and global correlation and significance. The recognition of two new members leads to a revised lithostratigraphic framework for the Tal Group in the Mussoorie syncline.

GEOLOGICAL SETTING AND LITHOSTRATIGRAPHY

The Mussoorie syncline forms part of the Inner Krol Belt (Bhargava, 1972, 1976) of the Lesser Himalaya (Auden, 1934; Medlicott, 1864; Middlemiss, 1887) (Figure 1A and 1B). The Krol-Tal-Nilkanth-Subathu successions occur within the core of a doubly plunging NW–SE trending syncline dissected by both longitudinal and transverse faults (Shanker, 1971, 1973) and tectonically overlain by the klippen of the Garhwal Group of rocks along the Garhwal thrust (Table 1) (Auden, 1934; Jain, 1972, 1971; Shanker, 1971).

Table 1.

Generalised Ediacaran-Eocene lithostratigraphy of the Mussoorie syncline (Lesser Himalaya, India)

Initially, in the Mussoorie syncline, the Tal Group was classified into the Lower Tal Formation, comprising the Chert, Argillaceous, Arenaceous, and Calcareous members, and the Upper Tal Formation, consisting of the Quartzite and Limestone members (Shanker, 1971, 1973). Later, Shanker et al. (1993) reclassified the Tal Group into the Deo-Ka-Tibba Formation, which encompasses the Chert, Argillaceous, Arenaceous, and Calcareous members, and the Dhaulagiri Formation, which includes members A, B, C, D, and E (Table 1). This classification is commonly employed by various workers in the Mussoorie syncline (Bhargava & Singh, 2022; Hughes et al., 2005; Joshi & Tiwari, 2014; Singh et al., 2014; Tiwari & Parcha, 2006; Tiwari et al., 2013).

The body fossils record from the Tal Group of Mussoorie syncline are very poor and restricted to the Chert Member (Deo-Ka-Tibba Formation) and the Shale Member (Dhaulagiri Formation). The Chert Member has yielded small shelly fauna assignable to the Anabarites–Protohertzina Zone of Terreneuvian age, correlating with Meishucunian successions in Yunnan, South China (Azmi et al., 1981; Brasier & Singh, 1987; Hughes et al., 2005; Juyal, 1979). The Lower Cambrian trilobite Xela mathurjoshi is known from the grey-greenish shale of Member B of the Dhaulagiri Formation (Jell & Hughes, 1997; Mathur & Joshi, 1989). Abundant trace fossils are known from the upper part of the Arenaceous Member (Deo-Ka-Tibba) and the members A and B of Dhaulagiri Formation (Joshi & Tiwari, 2014; Singh et al., 2014, 2025; Tiwari & Parcha, 2006; Tiwari et al., 2013). Since fossils are not known in younger members of the Dhaulagiri Formation, the ages of the Members C to E remain uncertain. Myrow et al. (2010, 2023) reported a detrital zircon profile for the Member E of the Dhaulagiri Formation that is statistically indistinguishable from those of the Ordovician Misri Banda Quartzite and the Shian Formation (also known as the Thango Formation) of the Tethyan Himalaya. Equivalent rocks in the Nigalidhar syncline contain trace fossils Cruziana of Ordovician age (Singh et al., 2019).

A generalised lithostratigraphic framework of the Mussoorie syncline is summarised in Table 1.

STUDIED SECTION AND GEOLOGICAL OBSERVATIONS

The studied section is exposed along the roadside near Bhusti village, forming a part of the northern limb of the Mussoorie syncline (Figure 2A–2E). The studied outcrops are exposed at a latitude of 30.395240° and a longitude of 78.210843 (Figure 1A and 1B). The ~29 m thick succession is well exposed in a road cut. It consists of the Bedded Earthy Dolomite Member (EDM) (~17 m) overlain by the ESM (~12 m) (Figure 2E). This succession occurs directly above the massive dolomite of the Krol-E Formation (Krol Group) and below the Chert-Member (Deo-Ka-Tibba Formation, Tal Group) (Figure 2E). The village of Bhusti is situated over the Chert Member (Figures 2 and 3). The newly identified members are described below.

Figure 3.

(A): Field photograph showing the contact between the Chert Member and Earthy Siltstone Member of the Deo-Ka-Tibba Formation near Bhusti village; (B–D): Phosphorite streaks and nodules within the Chert-Phosphorite Member of the Deo-Ka-Tibba Formation.

Earthy Dolomite Member

The massive carbonates of the Krol-E Formation are overlain by thin, argillaceous dolomite beds that appear earthy in colour on weathered surfaces and grey on fresh exposures, interbedded with subordinate earthy siltstone and mudstone (shale). Though laterally variable and non-uniform, this succession attains a maximum thickness of about 17 m. The EDM is analogous to the unit described from the Nigalidhar syncline in the Lesser Himalaya (Bhargava & Singh, 2022; Bhargava et al., 1998). In the Nigalidhar syncline, the EDM has been considered part of the uppermost Krol-E (Bhargava & Singh, 2022; Bhargava et al., 1998; Tarhan et al., 2013). However, the presence of earthy, argillaceous material (siltstone and shale) in the Mussoorie section suggests a distinct phase of sedimentation compared to the Krol-E of the Krol Group. Accordingly, in the present study, we assign this member to the basalmost part of the Tal Group, representing the lowermost unit of the Tal transgression, indicative of increased terrigenous influx. In the Nigalidhar syncline, the late Ediacaran tubular fossil Shaanxilithes first appears within the EDM (Bhargava & Singh, 2022; Tarhan et al., 2013); however, no specimens or fragments of Shaanxilithes tubes were observed in the Mussoorie syncline within this unit.

Earthy Siltstone Member

The EDM gradually grades upward into the siliciclastic-rich ESM. The ESM, about 12 m thick, is composed predominantly of light grey, massive-to-thick-bedded siltstone that displays an earthy colour on weathered surfaces and locally contains thin streaks of ash beds. The ESM is analogous to the equivalent unit described from the Nigalidhar syncline (Bhargava & Singh, 2022; Bhargava et al., 1998). In the Nigalidhar syncline, the ESM also includes grey, red, and green shale, limestone, argillaceous chert, and sporadic phosphatic nodules, which are notably absent in the Mussoorie syncline. Volcanic components such as palagonite fragments (Hans & Singh, 1994) and characteristic trace elements have been reported from the Nigalidhar syncline. The upper part of the ESM in the Mussoorie syncline contains abundant, though poorly preserved, ribbon-shaped fragments of Shaanxilithes (Plate 1).

PLATE 1.

(1–7): Fragmentary and ribbon-shaped Shaanxilithes from the Earthy Siltstone Member (ESM) of the Deo-Ka-Tibba Formation (Tal Group), Mussoorie syncline, Lesser Himalaya.

REVISION OF TAL GROUP LITHOSTRATIGRAPHY AND REGIONAL CORRELATION

Since the present study focuses on the contact between the Krol and Tal groups in the Mussoorie syncline, the lithostratigraphy of this interval is discussed in detail. The nature of the contact between the Krol and Tal groups in the Mussoorie syncline has long been debated. Auden (1934) and Saxena (1969) regarded it as an unconformity, whereas Shanker (1971, 1975) described a locally developed ‘transition zone’ at the base of the Tal Group, comprising argillaceous limestone (often phosphatised) interlayered with thin streaks of phosphatic rocks—brecciated in places—and chert. Banerjee and Narain (1976) similarly recognised this ‘transition zone’ and considered it part of the Lower Tal Formation (now Deo-Ka-Tibba Formation of Shanker et al., 1993). Subsequent workers largely ignored this transition zone, instead treated the Chert Member as the base of the Tal Group (Azmi et al., 1981; Bhargava & Singh, 2022; Brasier & Singh, 1987; Hughes et al., 2005; Joshi & Tiwari, 2014; Shanker, 1973; Shanker et al., 1993; Singh et al., 2014, 2025; Tiwari & Parcha, 2006; Tiwari et al., 2013).

The newly identified ~29 m thick succession between the Krol-Tal groups differs from the ‘transition zone’ of Shanker (1971) and Banerjee and Narain (1976) in that it comprises two distinct lithounits: an Earthy Dolomite and an overlying earthy siltstone. The EDM is argillaceous, whereas the ESM consists of light grey, massive-to-thickly-bedded siltstone (weathering to an earthy colour), and interbedded with grey shale (minor) (Table 2). The occurrence of these two well-preserved lithounits has so far remained unrecognised in the Mussoorie syncline. Similar lithological units have also been described from the Nigalidhar syncline (Bhargava et al., 1998) at the Krol-Tal transition, where they are identified as the EDM (Krol Group) and ESM (Tal Group). Due to the recognition of these units in the Mussoorie syncline, along with their analogy to the Nigalidhar syncline, the lithostratigraphic framework of the Krol-Tal transition in the Mussoorie syncline is revised in this communication (Table 2). The revised lithostratigraphic framework of the Tal Group and its correlation are given in Table 2. Accordingly, the Deo-Ka-Tibba Formation now comprises six members: the Earthy Dolomite, Earthy Siltstone, Chert, Argillaceous, Arenaceous, and Calcareous members (Table 2).

Table 2.

Revised Tal Group lithostratigraphy of the Mussoorie syncline (Uttarakhand) and its correlation with the Nigalidhar syncline (Himachal Pradesh), Inner Krol Belt, Lesser Himalaya.

SHAANXILITHES NINGQIANGENSIS AND ITS BIOSTRATIGRAPHIC SIGNIFICANCE

Shaanxilithes ningqiangensis (Xing et al., 1984) is an elongate, ribbon-like body fossil characterised by its distinctive annulated morphology (see Wang et al., 2021). Its specimens in the ESM occur as flattened, ribbon-shaped bodies (Plate 1 (1–2)) with closely spaced transverse annulations, or as disc-shaped, flat, broken segments (Plate 1 (3–7)), preserved on multiple bedding planes. Widths range from less than 1 mm to about 3 mm, while the preserved lengths vary from 30 mm to over 80 mm. The surface displays closely spaced transverse annulations producing a finely segmented appearance; in some cases, annulations are faint, irregular, or discontinuous, reflecting taphonomic overprinting (Plate 1 (1, 5, 7)). Margins are generally smooth, albeit jagged or irregular edges also occur (Plate 1 (2–4)), suggesting variation in preservation and morphology.

Globally, Shaanxilithes ningqiangensis has been documented from China, Siberia, India, and Namibia, making it a potential index fossil for late Ediacaran successions (Darroch et al., 2016; Ding et al., 1992; Hua et al., 2004; Li et al., 1997; Lin et al., 1986; Meyer et al., 2012; Rogov et al., 2012; Shen et al., 2007; Tarhan et al., 2013; Wang et al., 2021; Weber et al., 2007; Wood et al., 2017; Xing et al., 1984; Yang & Zheng, 1985; Yang et al., 2005, 2013; Zhang, 1986; Zhang et al., 2015; Zhao, 1995; Zhu et al., 2017; Zhuravlev et al., 2009). Taphonomic and geochemical studies support the interpretation of Shaanxilithes as an organic-walled macrofossil (Trahan et al., 2013) rather than a trace fossil (Xing et al., 1984).

Tarhan et al. (2013) briefly described Shaanxilithes from the Nigalidhar syncline, occurring within the EDM (considered the uppermost part of the Krol Group) and the ESM of the Shaliyan Formation (Tal Group; Bhargava et al., 1998). They noted that, as Shaanxilithes occurs within both of these members and lies beneath a heavily chertified horizon—interpreted as a major sequence boundary marking the geologically abrupt drowning of the Krol carbonate platform (Jiang et al., 2003) and regionally designated as the Ediacaran–Cambrian boundary (Bhargava et al., 1998)—these fossils likely predate the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone (SSF Fauna 1). However, owing to the absence of biostratigraphic or radiometric age constraints below this level, a definitive age assignment for Shaanxilithes in the Nigalidhar syncline remains uncertain, despite its clear stratigraphic position within the uppermost Krol and lowermost Tal units. The present record of Shaanxilithes below the Chert-Phosphorite Member in the Mussoorie syncline is significant, as it constrains the age of the equivalent horizon in the Nigalidhar syncline. The Chert-Phosphorite Member in the Mussoorie syncline contains the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone (SSF Fauna 1; Brasier & Singh, 1987) of Terreneuvian age. In the Mussoorie syncline, the Shaanxilithes occurs below the Chert-Phosphorite Member, a position similar to that of the Nigalidhar syncline (Table 2). The stratigraphic position of Shaanxilithes suggests that in the Nigalidhar syncline, the Ediacaran–Cambrian boundary lies within the Chert Member of the Shaliyan Formation.

CONCLUSIONS

This study presents two major contributions:

Along the Krol-Tal contact in the Mussoorie syncline (Lesser Himalaya), newly recognised Earthy Dolomite and ESMs are a distinct departure from the previously described ‘transition zone’. The recognition of these members and their correlation with equivalent units in the Nigalidhar syncline necessitated a revision of the lithostratigraphic framework of the Tal Group and a redefinition of the Deo-Ka-Tibba Formation to include six constituent members, namely the Earthy Dolomite, Earthy Siltstone, Chert, Argillaceous, Arenaceous, and Calcareous.

The discovery of Shaanxilithes ningqiangensis in the Mussoorie syncline, at horizons equivalent to those in the Nigalidhar syncline, provides a valuable basis for litho- and biostratigraphic correlation between the two synclines in the Lesser Himalaya. In the Mussoorie syncline, the stratigraphic position of Shaanxilithes predates the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone (SSF Fauna 1), suggesting a comparable stratigraphic relationship in the Nigalidhar syncline.