Cambrian Stratigraphy and Depositional History of the Northern Indian Himalaya, Spiti Valley, North-Central India

Cambrian stratigraphy and depositional history of the northern Indian Himalaya, Spiti Valley, north-central India Paul M. Myrow† Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA Karl R. Thompson Nigel C. Hughes Department of Earth Sciences, University of California, Riverside, California 92521, USA Timothy S. Paulsen Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA Bryan K. Sell Department of Earth Sciences, University of California, Riverside, California 92521, USA Suraj K. Parcha Wadia Institute of Himalayan Geology, Dehra Dun, Uttranchal 248001, India ABSTRACT facies thicknesses. This paleoenvironmental published stratigraphic or structural evi- reconstruction contradicts previous interpre- dence exists for such an uplift to the south for Recent work on Himalayan tectonics indi- tations of this unit that range from deep-sea either the Greater or the Lesser Himalaya cates that prior to the Cenozoic collision of fl ysch to shallow-marine tidalites. In addi- lithotectonic zones. India and Asia, an enigmatic Cambrian– tion, our paleoenvironmental analysis and Ordovician event may have strongly infl u- paleocurrent data suggest that the upper- Keywords: Cambrian, Parahio Formation, enced the regional geology of the Himalaya. most Lower to Middle Cambrian deposits of India, Tethyan Himalaya, stratigraphy. Stratigraphic and sedimentological analyses the Lesser and Tethyan Himalaya are parts of well-preserved Cambrian deposits are of the same ancient northward-prograd- INTRODUCTION critical for understanding the nature of this ing, fl uvial-deltaic depositional system of the early tectonic event and its infl uence on the paleo-Tethys margin of India. The Himalaya consist of three principal later tectonic evolution of the Himalaya. The An angular unconformity with overlying lithotectonic zones, based on internal stratigra- Parahio Formation, defi ned herein, of the Ordovician conglomeratic rocks has consid- phy and bounding faults, which were activated Parahio Valley, Spiti region, in the Tethyan erable local relief, with meter-scale scours during the Cenozoic collision of India with Asia Himalaya of India, is the best biostratigraph- and a valley fi ll >100 m thick. The scours (Gansser, 1964; LeFort, 1975) (Fig. 1). To the ically resolved section of Cambrian strata in have northeast-southwest orientations, which south, the Lesser Himalaya contains thick Neo- the entire Himalaya. This formation consists parallel both the paleocurrents in the under- proterozoic through upper Lower Cambrian (and of >1350 m of dominantly siliciclastic deltaic lying Parahio Formation as well as published likely younger Cambrian) strata that are uncon- deposits. The formation ranges from upper- paleocurrent readings from the coarse red formably overlain by Permian and younger rocks. most Lower Cambrian (Lungwangmiaoan beds of the overlying Ordovician strata. The central zone, the Greater Himalaya, consists Stage) to middle Middle Cambrian (Hsu- The Cambrian–Ordovician unconformity is of high-grade metamorphic rocks. The northern- chuangian Stage), representing a time span of regional extent, and our recent biostrati- most Tethyan Himalaya contains an extensive of ~5–10 m.y. It contains numerous medium- graphic database indicates that the minimum record of deposition from the Precambrian to scale shoaling cycles that range from storm- hiatus associated with the unconformity in the Eocene. The paleogeographic affi nities of infl uenced offshore deposits to thick trough Spiti is ~15 m.y. Our sedimentological analy- the three zones are the subject of much debate. cross-bedded fl uvial facies. Many thin car- sis and associated paleocurrent data from the Nd isotope and geochronological data have been bonate beds with abundant trilobite fossils Parahio Formation, along with additional used to suggest that the Lesser Himalaya has dis- directly overlie the fl uvial facies and repre- data from units both above and below the tinctly older sedimentary source materials than sent transgressive systems tract deposits. unconformity, indicate that published mod- the Greater Himalaya and the Tethyan Himalaya, The cycles are interpreted to have resulted els portraying foreland basin development at and that the two latter zones together represented from delta-lobe switching, based on a lack of this time with southward-directed thrusting a separate terrane and cover succession, respec- systematic stratigraphic changes in cycle or are problematic. An alternate possibility— tively, that was accreted to India during the early that uplift took place south of the Tethyan Paleozoic (DeCelles et al., 2000). Myrow et al. †E-mail: [email protected]. Himalaya—is also problematic, because no (2003) demonstrated that previously published GSA Bulletin; March/April 2006; v. 118; no. 3/4; p. 491–510; doi: 10.1130/B25828.1; 15 fi gures. For permission to copy, contact [email protected] 491 © 2006 Geological Society of America MYROW et al. Lesser Himalayan data were skewed by the al., 1992; Vannay and Grasemann, 1998; Dezes the Spiti region and considered the formation sampling of only older Precambrian strata and et al., 1999; Wyss et al., 1999; Vannay et al., to be of fl ysch origin. An intertidal to tidal-fl at that samples from younger units of similar 2004). The northern boundary of this belt is the environment has been invoked for the upper part depositional age (late Early Cambrian) from the south-dipping Cenozoic Great Counter Thrust, of the section (Bhargava et al., 1982; Bhargava Tethyan Himalaya and the Lesser Himalaya are which marks most of the Indus-Tsangpo suture and Bassi, 1998; Srikantia and Bhargava, 1998, similar to each other and to the protolith of the zone, separating rocks of the Indian and Eur- p. 227), and also for intervals within the lower Greater Himalaya. This eliminated the bulk of asian plates to the south and north, respectively part (Bhargava et al., 1986). evidence for an exotic origin of Greater Hima- (Yin et al., 1999; Yin and Harrison, 2000). The thickness of the Parahio Formation has laya and Tethyan Himalaya rocks. Other tectonic The sections of the Parahio Formation long been debated. Hayden (1904, p. 13) noted models of the early tectonic history of the Hima- described in this study (Fig. 3) are on the south- that in the Parahio Valley “numerous folds laya are based on reconstructions of the northern west limb of the Muth Syncline, 10–15 km west can be seen thus proving that the enormous Indian margin during the Cambrian and Early of the village of Mikkim, on steep slopes on the thickness [of the Parahio Formation] is only Ordovician (Garzanti et al., 1986; Gehrels et north side of the Parahio River, near the confl u- apparent.” He concluded that the entire sec- al., 2003), including an enigmatic tectonic event ence of the Khemangar River and the Debsa tion between the Khemangar River Fault and represented in northern India by a prominent Khad River (Fig. 2). This section was fi rst stud- the Cambrian–Ordovician unconformity is not Cambrian–Ordovician boundary unconformity. ied by Hayden (1904). The uppermost part of more than ~600–900 m thick, but noted that this However, the sedimentological and stratigraphic the section was measured southeast of the vil- estimate was “merely approximate” (Hayden, context of the Cambrian Tethyan sedimentary lage of Thango on the east side of the Sumna 1904, p. 15). Other estimates for the thickness succession, on which such reconstructions rely, River (also known as the Kinlung Nala) just of the entire formation in the Parahio Valley is poorly known. above where it joins the Parahio River. are <400 m (Parcha, 1996; Fig. 2), 1080 m The Cambrian Parahio Formation of the Spiti The base of the Parahio Formation and its (Bhargava and Bassi, 1998), ~2000 m (Fuchs, Valley region of northern India, which lies within contact with the Batal Formation is not exposed 1982), 2700 m (Kumar et al., 1984; Bhargava the Tethyan Himalayan fold-thrust belt that within the area. The units are juxtaposed by the et al., 1986), and 3350 m (Srikantia, 1981). Our formed along the northern Indian passive margin Khemangar River Fault, which is partly covered detailed bed-by-bed measurement of the section during deformation in the Eocene (Searle, 1986; by the river. The fault strikes north-northwest, revealed that structural complications were not Steck et al., 1993; Corfi eld and Searle, 2000; dips steeply to the east, and displays a normal an issue and that a nearly complete measured Wiesmayr and Grasemann, 2002), is particularly sense of stratigraphic offset (Figs. 2, 4). The ori- section was possible. Our section of the Parahio important in this regard. It is a thick and well- entation of the Khemangar River Fault is similar Formation (Fig. 3) is 1352 m thick. An addi- preserved sedimentary Cambrian succession that to a series of north-striking Cenozoic extensional tional ~100–200 m of poorly exposed Parahio has never been subjected to a detailed sedimen- faults ~30 km to the southeast in the Sutlej Val- Formation exists at the base of the section but tary analysis. The formation represents the oldest ley area (Vannay et al., 2004). However, the fault was not measured. The sedimentology of this Tethyan sedimentary rocks with abundant and is also parallel to north-northwest–striking out- interval is directly comparable to that of the rest well-preserved fossils and sedimentary structures. crop-scale strike-slip faults that we measured in of the measured section. Chronostratigraphic

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