Globally, the first appearance datum (FAD) of Oryctocephalus indicus is considered the base of the Wuliuan Stage, Miaolingian Series [Zhao et al. 2019]. In the Cambrian of the Spiti region (Himalaya), the Oryctocephalus indicus biozone is also considered the base of the Miaolingian Series ( ∼509 Ma) [Singh et al. 2016a, 2017a]. This has been well documented from the Parahio and Pin valleys of the southeastern part of the Spiti region [Singh et al. 2016a, 2017a,b]. Although Cambrian rocks are widely distributed in the Himalaya, the Parahio Valley (Spiti region) constitutes the most studied section of the Cambrian deposits [Hayden 1904; Reed 1910; Jell and Hughes 1997; Shah and Paul 1987; Sahni and Sudan 1996; Shah et al. 1988, 1991; Peng et al. 2009; Singh et al. 2014, 2015, 2016a,b, 2017a,b; Popov et al. 2015; Hughes 2016a,b; Gilbert et al. 2016; Yin et al. 2018; Kaur et al. 2019]. However, the Cambrian rocks also occur in the Sumna, Pin and Chandra valleys of the Spiti region [Bhargava and Bassi 1998]. So far, Cambrian body fossils have not been reported from the Chandra Valley. In the Pin Valley, the O. indicus biozone was recently demarcated [Singh et al. 2017b]. The trilobite fauna reported from the Sumna Valley section (at the confluence of Parahio and Sumna valleys) includes the youngest trilobite level, that is, the Iranoleesia butes level [Peng et al. 2009; Singla et al. in press].
The present work pertains to the Sumna Valley, where we demarcate the Oryctocephalus indicus biozone. In addition, a comparative account of the variation in lithologies across the Cambrian Series 2–Wuliuan (Miaolingian) transition in the Parahio and Sumna valleys (southeastern part of the Spiti) is presented to interpret the basin configuration during deposition.
2. Geological setting and lithostratigraphy
The Cambrian deposits in the Himalaya are well known to occur in the Tethyan and the Lesser Himalayan lithotectonic zones, which are separated by the metamorphosed Greater Himalayan Lithotectonic Zone (Figure 1a). The Tethyan Himalayan Zone is bounded in the north by the Indo-Tsangpo Suture Zone and in the south by the South Tibetan Detachment System [Burchfiel et al. 1992; Hodges et al. 1992; Searle and Treloar 1993]. The Spiti region lies in the northernmost part of the state of Himachal Pradesh and constitutes a part of the Tethyan Himalayan Zone [Srikantia 1981; Bhargava and Bassi 1998; Srikantia and Bhargava 2018]. The Cambrian deposits in the Spiti region are well exposed in several localities; for example, Chandra Valley (north-western part of the Spiti region), Ratang Nala (central part of the Spiti region) and Parahio, Sumna and Pin valleys (southernmost part of the Spiti region).
Lithostratigraphically, the Cambrian deposits of the Spiti region are classified under the Haimanta Group [Srikantia 1981; Bhargava and Bassi 1998]. The anchi-metamorphosed sedimentary rocks of the Haimanta Group rest tectonically or non-conformably over the highly metamorphosed rocks of the Greater Himalayan Zone. However, various authors have referred to the tectonic contact as the South Tibetan Fault System [Burchfiel et al. 1992; Hodges et al. 1992; Searle and Treloar 1993; Myrow et al. 2003, 2009; Webb et al. 2007; Kellett and Grujic 2012; Leloup et al. 2015; Lui et al. 2017] even though it has not been properly delineated on a map.
In the Spiti region, the Haimanta Group (?Precambrian–Cambrian) is divided into the Batal and Kunzam La (Parahio) formations [Srikantia 1981; Bhargava and Bassi 1998; Myrow et al. 2006, 2010; Hughes et al. 2018; Srikantia and Bhargava 2018]. Body fossils are not known to occur in the Batal Formation; therefore, its age is controversial and purely based on assumptions about its stratigraphic relationship with the overlying fossiliferous Cambrian Series 2/Stage 4 to Wuliuan (Miaolingian) Kunzam La (Parahio) Formation. The detrital zircon (single grain) from the Batal Formation at Batal locality in Chandra Valley yielded an age of 524 ± 7 Ma [Myrow et al. 2010], suggesting that a part of the Batal Formation is Cambrian in age.
The rocks exposed in the Parahio Valley extend to the southeast in the Sumna Valley. Body fossils of trilobites, brachiopods and small shelly fossils (SSFs) are well known to exist in the Parahio Valley [Hayden 1904; Reed 1910; Jell and Hughes 1997; Peng et al. 2009; Singh et al. 2014, 2015, 2016a,b, 2017a,b, 2019; Popov et al. 2015; Gilbert et al. 2016; Yin et al. 2018; Hughes et al. 2018; Srikantia and Bhargava 2018; Kaur et al. 2019]. An angular unconformity separates the Kunzam La (Parahio) Formation from the overlying Ordovician Thango Formation [Hayden 1904; Bhargava and Bassi 1998; Myrow et al. 2006, 2010]; the latter was deposited after the Kurgiakh orogeny [Srikantia 1977; Xu et al. 2014; Myrow et al. 2016; Singh et al. 2019].
The nomenclature of the Cambrian rocks in the Spiti region is currently a subject of debate [Myrow et al. 2006; Bhargava 2008, 2011; Singh et al. 2014, 2016a; Hughes et al. 2018, 2019; Srikantia and Bhargava 2018]. In the present work, we follow Singh et al.  and adopt the name Kunzam La (Parahio) Formation. The generalised lithostratigraphic scheme of the Spiti region is illustrated in Table 1.
3. Studied section
The Sumna Valley is a north–south-oriented subsidiary valley of the Parahio Valley. It lies between the Parahio (in the northwest) and Pin valleys (in the southeast) (Figure 1b). The studied section lies approximately 3.52 km southeast from the confluence of the Sumna and Parahio rivers. The section is exposed on the southwest facing slope on the bank of the Sumna River (GPS: N 32° 0.291′ and E 77° 57.308′) (Figures 1b and 2). The measured section is 49.5 m thick, and the base of the section lies close to the Sumna River. The section is lithologically divided into five lithounits, I–V (Figures 2 and 4). Lithounit-I, exposed at the base of the section, is 4.0 m of intercalated thin to thickly bedded (0.2–1.1 m) fine-grained, hummocky cross-bedded sandstone and minor shale beds (0.1–0.2 m). The interface of the shale and fine-grained sandstone beds shows little bioturbation (Planolites). Lithounit-II is a 13.8 m thick succession of medium- to coarse-grained sandstone. The sandstone exhibits reactivation surfaces, ripple tops and a few trough cross-laminations. The top 0.56 m of this lithounit is reddish, weathered coarse-grained ferruginous sandstone, which may indicate aerial exposure or a diastem (Figure 3). Lithounit-III is a 5.6 m thick succession composed of blackish to bluish shale (0.7 m), silty shale (0.1–0.3 m) and minor intercalated siltstone and sandstone (0.1–0.2 m). The blackish to bluish shale and silty shale contain the trilobites Oryctocephalus indicus, Pagetia significans and Kunmingaspis pervulgata (Figure 5). The Oryctocephalus indicus biozone is reported only from this unit. Lithounit-IV is a 5.8 m thick succession and consists of alternating silty shale–siltstone and thin beds of fine-grained sandstone. Thin beds of siltstone (0.1–0.3 m) and sandstone (0.1–0.2 m) exhibit ripple surfaces. The shale–siltstone beds contain Pagetia significans and Kunmingaspis pervulgata. Oryctocephalus indicus is not found within this lithounit. Lithounit-V is 20.3 m thick and is made up of medium- to coarse-grained, bedded to massive sandstone (0.5–1.1 m) exhibiting trough cross-beds and ball and pillow structures. Well-preserved fossils have not been recorded from this lithounit though fragments of trilobite Bhargavia prakritika have been observed.
4. Materials and methods
Fossiliferous samples were collected from the Kunzam La (Parahio) Formation. Close systematic sampling was undertaken in order to establish the biostratigraphy of the studied interval. A total of 389 samples of trilobites were collected from the section. For photography, the fossil specimens were washed and dried and coated with magnesium oxide to enhance morphological features. The morphological features were examined under an Olympus SZX16 binocular microscope. The well-preserved selected specimens representing different taxa were photographed using an RSN-9 Stereo Zoom Light Microscope with an attached digital camera. The illustrated trilobites (Figure 5) were deposited in the repository of the Department of Geology, Panjab University under the label CAS/2017/SV.
The studied part of the Kunzam La (Parahio) Formation in the Sumna Valley is significant as it bears the Oryctocephalus indicus biozone ( ∼509 Ma), which demarcates the base of the Miaolingian Series [e.g. Zhao et al. 2019; Singh et al. 2016a].
5.1. Oryctocephalus indicus biozone
Along the studied section, the FAD of Oryctocephalus indicus is recorded in blackish to bluish shale, silty shale, siltstone and sandstone intercalations at 17.8 m from the base of the measured section (Section C, Figure 1b and Figure 4). The last appearance datum is recorded at 23.4 m from the base of the section. The Oryctocephalus indicus biozone in the Sumna Valley is 5.3 m thick. Besides the eponymous species, the other trilobite fossils recorded include Pagetia significans and Kunmingaspis pervulgata (Figure 5). Along this section, a thin horizon of argillaceous limestone or nodular limestone, common in the Parahio Valley sections, is absent (see Sections A and B in Figure 1b). This missing limestone horizon contains the Pagetia–Kunmingaspis level [Singh et al. 2016a, 2017b]. Above the Oryctocephalus indicus biozone, Pagetia significans and Kunmingaspis pervulgata are also recorded from 23.4 m to 29.2 m in the section. Poorly preserved fragments of Bhargavia prakritika are recorded in overlying sediments at 29.2 to 49.5 m of the succession.
6. Depositional environment and implication on Cambrian series 2–Miaolingian evolution in Sumna Valley
Based on sedimentological and sequence stratigraphic analysis of the 49.5 m thick successions of the Kunzam La (Parahio) Formation, five lithounits (I–V) are recognised (Figure 4). Lithounits I and II are deposited under the falling stage systems tract (FSST), and they indicate a forced regressive event due to a shift from lower shoreface (lithounit-I) to near shoreface–upper shoreface (lithounit-II) successions (Figure 4). The sequence boundary (SB) lies on the top part of lithounit-II, and it is characterised by a break in sedimentation (a diastem) reflected by the reddish-brown ferruginous, coarse-grained sandstone at the top of lithounit-II (Figure 3). The bluish-black pyritised shale of lithounit-III indicates deposition in a reducing environment during transgression (transgressive systems tract (TST)) of the deeper offshore facies. The maximum flooding surface (MFS) is marked at the contact of the offshore shale (lithounit-III) with an overlying alternation of thin shale–siltstone–sandstone (lithounit-IV). The latter is considered to represent an early stage of the highstand systems tract (HST). The intercalated thin shale–siltstone–sandstone interval is inferred to be formed in an offshore–lower shoreface transition environment. The HST deposits are not very thick, and they represent only the early stage of the HST. Upwards, these are followed by medium- to coarse-grained sandstone (lithounit-V) containing ball and pillow structures and interpreted to be deposited in a near shoreface–upper shoreface environment in an FSST. An SB is recognised in between lithounits IV and V—the recognition is based on the sudden change in depositional environments from offshore–lower shoreface to near shoreface–upper shoreface deposits. Overall, the studied section exhibits dynamic changes in the depositional energy reflected by the presence of two regressive events and one transgressive event within a 49.5 m interval of the Kunzam La (Parahio) Formation, spanning the Cambrian Series 2–Wuliuan (Miaolingian) boundary interval in the Sumna Valley (Figure 4).
To study the lithological variation and depositional environment across the Cambrian Series 2–Wuliuan (Miaolingian) boundary interval in the southeastern part of the Spiti region, detailed sedimentological studies were carried out along two sections previously described from the Parahio Valley (Sections A and B, Figure 1) [Singh et al. 2015, 2016a, 2017b] and one section (Section C, Figure 1) from the Sumna Valley (Figure 6). Figure 6 depicts a comparison of the lithological variation across the Cambrian Series 2–Wuliuan (Miaolingian) boundary interval in the Parahio and Sumna valleys from the northwest to the southeast direction in the southeastern part of the Spiti region. In the Parahio Valley, the Cambrian Series 2–Wuliuan (Miaolingian) boundary interval is marked based on the Oryctocephalus indicus biozone along two sections, that is, in Section B of the Parahio Valley [Singh et al. 2016a, 2017a] and in Section A of the Kaltarbo locality [Singh et al. 2015; Yin et al. 2018; Kaur et al. 2019]. In the Kaltarbo section (Section A, Figure 1) of the Parahio Valley, the O. indicus biozone is 5.8 m thick (section range 345.7 m–351.5 m). Below this biozone, a horizon of more or less isolated limestone nodules (15–24 cm diameter) occurs encased in shale [Singh et al. 2015]; they were probably formed by diagenetic pressure solution (compaction and dewatering of clay) [cf. [Markello and Read 1981]]. In Section B of the Parahio Valley, the O. indicus biozone is 6.8 m thick (section range 7.74–4.6 m) [Singh et al. 2016a]. Below this biozone, a thin horizon of argillaceous limestone (7.38–7.74 m) exists [Singh et al. 2016a, 2017a]. However, in the Sumna Valley (Section C, Figure 1), the O. indicus biozone is 5.6 m thick (section range 17.8–23.4 m). It directly rests over the reddish-brown ferruginous, medium- to coarse-grained sandstone (Figure 3) and lacks nodular limestone or argillaceous limestone rocks (Figure 6). The thin nodular limestone and argillaceous limestone horizon in the Parahio Valley yield Pagetia, Kunmingaspis and SSFs. Thus, in the southeastern part of the Spiti region in the three sections studied (from northwest to southeast, i.e. Section A, Section B and Sumna Valley Section C; Figure 1), the reddish-brown ferruginous, medium- to coarse-grained sandstone forms the uppermost part of the thick medium- to coarse-grained sandstone (lithounit-II). Compared to Sections A and B (of the Parahio Valley), the absence of carbonates indicates that the Sumna Valley was locally aerially exposed becasue of local topographic high during the Wuliuan (Miaolingian) transgression (Figure 4). The early transgression with argillaceous limestones and nodular limestones filled the lower topography of the Parahio Valley. The subsequent offshore shale bearing the O. indicus and associated trilobites transgressed all over the basin. This implies that the depositional basin topography was not uniform throughout the southeastern part of the Spiti during the Wuliuan (Miaolingian) transgression or the eustatic sea-level change at the basal part of Wuliuan (Miaolingian) [e.g. Zhu et al. 1999; Wang et al. 2006; Gaines et al. 2011]. In the Spiti region, it seems that the Wuliuan (Miaolingian) deposits rest on an undulatory surface formed due to a break in sedimentation or aerial exposure of the rocks of Cambrian Series 2 prior to the Wuliuan (Miaolingian) transgression (Figure 6). The reddish-brown ferruginous, medium- to coarse-grained sandstone of lithounit-II represents a break in sedimentation. It is notable that in the Lesser Himalayan Zone, the Cambrian sedimentation ceased in the late part of Cambrian Series 2/Stage 4 just above the Redlichia noetlingi biozone ( ∼512 Ma) [Singh et al. 2019]. In the Spiti region (Tethyan Himalayan Zone (THZ)), although the Redlichia noetlingi ( ∼512 Ma) is known from a float in the Pin Valley [Hayden 1904; Reed 1910; Jell and Hughes 1997], the exact stratigraphic position with respect to the Haydenaspis parvatya level ( ∼510 Ma) and the O. indicus biozone ( ∼509 Ma) is unknown. Recently, Kaur et al.  have established a stratigraphic distance of approximately 183.4 m between the O. indicus biozone (509 Ma) and the Haydenaspis parvatya level ( ∼510 Ma) in the Spiti region. Finding the in situ occurrence of Redlichia noetlingi in the Spiti region will be helpful in understanding the exact stratigraphic position of the Redlichia noetlingi bearing level with respect to the O. indicus biozone or the H. parvatya level. However, here we propose that in the Spiti region (THZ), the Cambrian Series 2/Stage 4 regressive event affected the region after the deposition of rocks from the Haydenaspis parvatya level ( ∼510 Ma). In addition, a break in sedimentation (diastem) is recognised preceding the Wuliuan (Miaolingian) transgression.
The Oryctocephalus indicus biozone is widely recognised in the southeastern part of the Spiti region, which includes Parahio, Sumna and Pin valleys. This biozone is globally used to demarcate the base of the Miaolingian Series (traditional Middle Cambrian). In the Parahio and Sumna valleys of the Spiti region, this biozone is 5.6 to 6.8 m thick. The uppermost part of the Cambrian Series 2 deposits (immediately below the O. indicus biozone) in the southeastern part of the Spiti region bears a reddish-brown ferruginous, very coarse grained sandstone unit, which indicates a diastem or a break in sedimentation prior to the Wuliuan (Miaolingian) transgression.