Lithostratigraphy and biostratigraphy of the Pin formation type section (Spiti, NW India)

Goel and Nair (1977) introduced the term 'Pin limestone' after the outcrop of the formation on the right side of the Pin river 1.6 km South of village Muth (Pin valley, Spiti). Srikantia (1974) introduced 'Takche Formation' after a valley east of Kunzam La in an unpublished survey report; the first published reference of the term 'Takche Formation' represents a short note in Srikantia (1977). Although both names have been published in the same year, the use of Pin Formation seems advantageous, as the Takche type locality is not well developed (Bhargava and Bassi 1998) and strongly faulted (Hayden 1904). Due to the variable lithology the use of the more general term 'Pin Formation' is strongly recommended. Rich faunas have already been described from the Pin formation (e.g. Reed, 1912; Bhargava and Bassi, 1998). Hayden (1904) concluded Caradocian age for parts of the lower limestone horizon, dated by trilobites and brachiopods. The calcareous upper levels are tentatively dated into the Llandoverian to Wenlockian by Hayden (1904); Reed (1912). In our investigation we carried out a detailed bed-by-bed profile of the complete type section and revised the biostratigrapy by new conodont data. The lower boundary of the Pin formation represents a sharp, but conformable contact of thick-bedded, light gray quartzites above characteristic brick-red fine-grained sandstone of the underlying Shian formation (Goel and Nair, 1977). The upper boundary is a sharp contact of pinkish gray dolomite, overlain by white quartz arenite of the Muth Formation. This definition of the boundaries corresponds with Hayden (1904) 'Silurian limestone'. The contact to the Muth formation above represents a disconformity in the distal sections at Mikkim and Muth (Draganits, 2000), but an erosional unconformity in the proximal Baba La section. The Pin formation reaches a thickness of some 290 m; in general within the first 230 m two large-scale thickening and coarsening upward cycles are recognizable. The first cycle is consists of some 100 m of dolomitic sandstone with conspicuous light gray sandstone at the base. Thin shale inter-layers, mud chips and bioturbation on the upper bed surfaces are common. The contact to the overlying 50 m thick, gray limestone is gradual and marked by an increasing content of bioclasts. Coral buildups occur mainly in the middle part of the limestone, the upper part comprises nodular limestone. The boundary to the second cycle is relatively sharp. The second cycle starts with dolomitic siltstone grading into dolomitic sandstone; both together being some 25 m thick. The contact to the overlying 55 m limestone is gradual. Coral buildups (Bhargava and Bassi, 1998) are abundant in the upper part. The uppermost 60 m of the Pin formation consist of dolomitic sandstone with shale/siltstone inter-layers. The siliciclastic influence increases towards the top. High content of crinoidal fragments, increasing silicification towards higher levels and strong bioturbation are characteristic.