Cambrian rocks in the Lahaul and Spiti district of the state of Himachal Pradesh are well exposed in the Pin, Parahio and the Sumna valleys, the Kunzam La-Takche section and in the Chandra valley. The Parahio valley, the present study area, lies in the southeastern part of the Spiti region and it joins the Pin valley near the village Sagnam, which is situated about 40 km southwest of the Kaza town (Lahaul-Spiti district). Cambrian rocks in the Parahio valley are well exposed between Thango and upstream of the Khemangar khad (Fig. 1). The Cambrian succession in the Parahio valley is highly fossiliferous at certain horizons (Hayden, 1904, Peng et al., 2009 and Reed, 1910).

The Cambrian rocks of the Spiti region belong to the Haimanta Group and comprise the Batal and Kunzam La formations (Bhargava and Bassi, 1998 and Srikantia, 1981; Srikantia and Bhargava, submitted). Hughes et al. (2018) preferred the term Phe and Parahio formations, the latter derived from the earlier term “Parahio Series” described in detail by Pascoe (1959, pp. 580–584), instead of Batal and Kunzam La formations. The Parahio Formation in the Spiti region is highly fossiliferous and interpreted to be deposited in deltaic setting (Myrow et al., 2006a and Myrow et al., 2006b). Recent work in the Parahio valley resulted in formal trilobite biozonation (Peng et al., 2009, Singh et al., 2016a, Singh et al., 2016b, Singh et al., 2017a and Singh et al., 2017b), brachiopod zonation (Popov et al., 2015) and record of small shelly fauna (Gilbert et al., 2016, Singh et al., 2015), acritarchs (Yin et al., 2018), an eldoniid (Hughes, 2016b) and abundant trace fossils (Hughes et al., 2013). The Parahio Formation comprises shale, siltstone, sandstone, quartzite, dolomite and limestone (Myrow et al., 2006a and Myrow et al., 2006b). The Cambrian of the Parahio valley has yielded trilobites, brachiopods, small shelly faunas, an eldoniid, echinoderms and acritarchs (Gilbert et al., 2016, Hayden, 1904, Hughes, 2016a, Hughes, 2016b, Jell and Hughes, 1997, Parcha and Pandey, 2013, Peng et al., 2009, Popov et al., 2015, Reed, 1910, Shah and Paul, 1987, Singh et al., 2014, Singh et al., 2015, Singh et al., 2016a, Singh et al., 2016b, Singh et al., 2017a, Singh et al., 2017b and Yin et al., 2018).

The studied section is located along the left (northern) bank of the Khemangar khad at Kaltarbo locality (Fig. 1) that lies 1.8 km NWW of the Mapo encamping ground in the Parahio valley, southeastern part of the Spiti region (Fig. 1). A nearly 719-m-thick section was measured at the Kaltarbo locality the lower 519 m of the section is illustrated (Fig. 2 and Fig. 3). The GPS coordinates of the base of the section is 32° 2′38.93″N, 077°54′26.76″E, the Haydenaspis parvatya bearing horizon is 32° 2′43.14″N, 077°54′33.21″E, the O. indicus Biozone is 32° 2′45.85″N, 077°54′43.22″E, and the top of the section at 719 m is 32° 2′59.53″N, 077°54′51.47″E (Fig. 1). This is the same section that Myrow et al. (2006a) measured and described as the base (0–760 m) of their composite section of the Parahio Formation (Hughes et al., 2018, fig. 3, p. 4). From Hughes et al. (2018, fig. 3, p. 4) now we are clear that our Kaltarbo section locality (Singh et al., 2015 and Yin et al., 2018) represents the lower part of the section described by Myrow et al. (2006a). Acritarchs and trilobites from the Oryctocephalus indicus Biozone of basal part of the Miaolingian Series (Yin et al., 2018) and small shelly fauna of Cambrian Series 2, Stage 4 (Gilbert et al., 2016 and Singh et al., 2015) are known from this section. Along this section, the Cambrian succession is thick and well exposed along an approachable slope (Fig. 2). Peng et al. (2009) indicate that the PO3 collection which yielded Haydenaspis parvatya level fauna lies at 78.07 m from the base of their section; however, in our section this occur at 162.3 m from the base. We have provided the GPS coordinates of base and top of the measured section and each collection discussed in this work. The base of the measured section in the lower part is covered by debris and a fault is lying below this debris cover. The Khemangar fault separates the Parahio Formation from the underlying Batal Formation which has a different metamorphic grade (Myrow et al., 2006a). Higher up in the section, along an angular unconformity, the Thango Formation (Ordovician) overlies the Parahio Formation. The Parahio Formation along this section comprises shale, siltstone, sandstone, nodular and bedded limestone and massive quartzite (Fig. 3). The lowermost 67 m of the section consists of minor shale intercalations and major fine-grained sandstone intervals which contain abundant traces of Psammichnites gigas and a few specimens of Diplichnites. The overlying 95 m of the section predominantly consist of alternating fine-grained sandstone beds (2–16 m in thickness), and thick siltstone beds (7 to 8 m) at some places and shale horizons (0.5 to 1 m in thickness). This succession upward is followed by low angle cross-bedded sandstone interval (Fig. 2 and Fig. 3), which is succeeded by a limestone bed (0.5 m). The limestone (grainstone) at 162.3 m from the base of the section is sparry, dirty white colored and contains trilobites, small shelly fossils and brachiopods belonging to the Haydenaspis parvatya level. The fossiliferous limestone is followed upward by a nearly 19.7-m-thick siltstone interval (from 162.3 to 182 m) that contains brachiopod fragments. A limestone horizon caps this siltstone interval at 182 m, which yielded a small shelly fauna (Singh et al., 2015). The 182–291-m interval comprises fine-grained sandstone (bedded and massive), thin shale intervals (2 to 6 m in thickness), medium- to coarse grained sandstone intervals (20–25 m), and micaceous siltstone (16–32 m in thickness). The 291 to 345.7 m interval is dominated by thickly bedded to massive medium- to coarse- grained sandstone (4 to 28 m in thickness), which contains poorly preserved indeterminate trilobites at the 335.1 m level. The stratigraphic interval from 345.7 m to 353.4 m has yielded acritarchs and trilobites and includes the 5.8-m-thick O. indicus Biozone (Yin et al., 2018, fig. 2) defining the basal part of the Miaolingian Series, Wuliuan Stage. The trilobite fauna comprises Oryctocephalus indicus, Pagetia significans, Pagetia sp., Kunmingaspis pervulgata and Eosoptychoparia (Danzhaina) sp. (Yin et al., 2018). Upward the succession is dominated by the sandstone and siltstone beds. The entire section contains highly diversified trace fossils of the Cruziana ichnofacies. Myrow et al., 2006a and Myrow et al., 2006b interpreted that these deposits represent the northward-prograding, fluvial-deltaic depositional system of the paleo-Tethys margin of India.

The lowest identified trilobite level at 162.3 m along the 719-m-thick continuous section of the Parahio Formation in the Kaltarbo locality (Parahio valley) contains Haydenaspis parvatya and Prozacanthoides lahiri (Fig. 4), which belong to Haydenaspis parvatya level of Peng et al. (2009). The faunal elements of Haydenaspis parvatya level are Haydenaspis parvatya, Prozacanthoides lahiri, Probowania bhatti, and Mufushania nankingensis. The eodiscoid Pagetia significans and the ptychoparid Kunmingaspis pervulgata first appear in the 345 to 345.7 m interval in limestone horizon (nodular in appearance) and the trilobite fauna of the Oryctocephalus indicus Biozone is recorded from 345.7 to 351.5 m (5.8 m) (Singh et al., 2015 and Yin et al., 2018). Acritarchs, filamentous algae and the cryptospore-like microfossils have been recorded from 343.7–353.1 m interval (Yin et al., 2018). At 335.1 m, six poorly preserved specimens of ptychoparid trilobites are recorded from sandstone, but their preservation precludes identification even at the generic level. Hughes (2016b) reported a Burgess Shale type fossil eldoniid from O. indicus bearing beds (Hayden horizon 2) in this section.

Peng et al. (2009) recognized the Haydenaspis parvatya level (Series 2, Stage 4) at about 78.07 m above the base of their measured section at Parahio valley, Spiti. They suggested that the level is equivalent to the upper part of the as yet-unnamed Stage 4 of the Cambrian Series 2. They recognized Kaotaia prachina biozone at 439.44 m above the base of their section that contains the trilobite Kaotaia prachina, Bhargavia prakritika and Xingrenaspis parthiva. They did not find indicus in situ even though it had been collected from this site previously. They were, however, able to estimate that it likely occurs at about 200 m in the section based on correlating their known occurrence levels with those of Hayden, who first found O. indicus. Singh et al. (2016b and 2017a) reported the Bhargavia prakritika from 12.1 m above the O. indicus biozone in the Parahio valley section and 7.3 m above the O. indicus Biozone in the Pin valley. Singh et al. (2014) also recorded Yuehsienszella (GPS coordinates 32° 2′43.23″N, 077°54′29.24″E) nearly 237 m below the O. indicus Biozone. Hughes (2016a) and Hughes et al. (2018) considered Yuehsienszella of Singh et al. (2014) to be Bhargavia, probably B. prakritika.

Hughes (2016a) estimated the position of O. indicus Biozone as ∼286 m (fig. 15, p. 453), and also as 400 m (p. 453) from the base of section of Peng et al. (2009) based on the find of Singh et al., 2014 and Singh et al., 2016b). Our work along a continuous section in which trilobite faunas of Haydenaspis parvatya level and the O. indicus Biozone are located (Singh et al., 2015 and Yin et al., 2018) suggests that the stratigraphic distance between the Haydenaspis parvatya level and the O. indicus Biozone is 183.4 m (Fig. 3 and Fig. 5). Thus, we suggest that in lithocolumn of Peng et al. (2009) the O. indicus Biozone is near the 262 m stratigraphic level (Fig. 5), which is close to the one of the estimated positions, i.e. ∼286 m of Hughes (2016a).

Regarding Bhargavia prakritika, which has been recorded in various lithologies, silty-shale, siltstone, fine-grained sandstone, coarse-grained sandstone and limestone indicate that this species in the Parahio Formation has a long stratigraphic range. Peng et al. (2009) reported this species from the limestone horizon (PO15 collection) from the K. prachina Biozone. Singh et al. (2016b) reported this species from the silty-shale and fine-sandstone in the Parahio Formation in the Parahio valley and from coarse sandstone in the Pin valley (Singh et al., 2017a). If Hughes (2016a) and Hughes et al. (2018) view is correct regarding the Yuehsienszella, which is reported nearly 237 m below the O. indicus biozone (Singh et al., 2014), as Bhargavia prakritika it indicates that B. prakritika is a long-ranging faunal element in the Parahio Formation. Another example of a long-ranging taxon in the Parahio Formation is Kunmingaspis stracheyi, which occurs from Paramecephalus defossus Biozone to the Iranoleesia butes level covering more than 280 m interval (Peng et al., 2009). It is presently unknown whether either Haydenaspis parvatya or Oryctocephalus indicus range beyond the beds in which are found in this section or in others in the Parahio Valley.