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The depositional age and stratigraphic correlations of metamorphosed and variably deformed rocks of Mount Everest are poorly known because of limited recovery of diagnostic fossils. Detailed study of Cambrian and Ordovician strata from along the length of the Himalaya has produced a coherent stratigraphy that stretches from northern India to Tibet. Our work also demonstrates that the North Col Formation rocks (= Everest series), between the Qomolangma and Lhotse detachments of the South Tibetan detachment system, still locally preserve sedimentary textures and primary stratigraphy that match those within Cambrian strata ~1100 km to the west in northern India. This demonstrates a coherency of depositional systems and stratigraphic architecture for Cambrian deposits along much of the Himalaya Tethyan margin. It also allows, for the fi rst time, identifi cation of precise depositional ages of several units in the Everest region, in particular, the Yellow Band carbonate and directly underlying siliciclastic strata, which are both shown to be late Middle Cambrian in age. Detrital zircon data presented herein for a sample from these siliciclastic strata contain a similar age spectrum to those from Middle Cambrian strata in northern India, as well as grains as young as ca. 526 Ma, both of which support the depositional age and continuity of depositional systems along the length of the Himalaya. Highly fractured rocks of the Ordovician lower Chiatsun Group in the hanging wall of the South Tibetan detachment system in Nyalam, 75 km to the west of Everest, correlate with Ordovician strata of the Mount Qomolangma Formation on Mount Everest. Our correlations indicate that the base of the summit pyramid of Everest, the foot of the "Third Step," is composed of a 60-m-thick, white-weathering thrombolite bed. The top of this ancient microbial deposit crops out only 70 m below the summit of Mount Everest.

Precise biostratigraphic constraints on the age of the Tal Group are restricted to (1) a basal level correlative with the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone of southwest China, (2) a level near the boundary of the lower and upper parts of the Tal Group correlative with the early Tsanglangpuan Stage (Drepanuroides Zone), and (3) an interval low in the upper part of the Tal Group correlative with later in the Tsanglangpuan Stage (Palaeolenus Zone). These correlations are based on small shelly fossil and trilobite taxa. Other chronostratigraphic constraints include the marked negative δ13C isotopic excursion coincident with the transition from the Krol Group to the Tal Group. This excursion is used as a proxy for the Precambrian–Cambrian boundary in several sections worldwide and, if applied to the Lesser Himalaya, indicates that the boundary is at or just above the base of the Tal Group. The upper parts of the Tal Group may be of middle or late Cambrian age and might form proximal equivalents of sections in the Zanskar–Spiti region of the Tethyan Himalaya. Both faunal content and lithological succession are comparable to southwest China, furthering recent arguments for close geographic proximity between the Himalaya and the Yangtze block during late Neoproterozoic and early Cambrian time. Trilobites from the uppermost parts of the Sankholi Formation from the Nigali Dhar syncline are described and referred to three taxa, one of which, Drepanopyge gopeni, is a new species. They are the oldest trilobites yet described from the Himalaya.

A well-preserved Cambrian section in the Zanskar Valley of northern India has previously been interpreted to record the transition from a passive to an active tectonic margin related to Cambrian-Ordovician orogenesis. This interpretation has been used to support the tectonostratigraphic interpretation of other successions across the Tethyan Himalaya. Our detailed paleoenvironmental analysis significantly revises the tectonic and depositional history of these Cambrian deposits: no definitive record of impending Cambrian-Ordovician orogenesis is recorded in these late Middle Cambrian rocks.A critical transition from an , 125-m-thick, stromatolite-bearing carbonate deposit, the Karsha Formation, into shale and sandstone of the Kurgiakh Formation, was interpreted to represent tectonically induced drowning of a carbonate platform. Siliciclastic strata of the Kurgiakh Formation were thought to record deep-water flysch deposition in a tectonically active foreland basin next to an arc-trench system. This interpretation was based on sandstone beds with classic Bouma sequences. We show that these event beds in the Kurgiakh instead contain hummocky cross-stratification, quasi-planar lamination, and combined-flow ripple stratification, all of which reflect deposition in shallow-marine, storm-influenced environments. Thus, although the Karsha carbonate platform may have been drowned, it did not culminate in deep-sea flysch deposition, and this in turn eliminates a major line of evidence linking Kurgiakh deposition to the onset of Cambrian-Ordovician orogenesis. Other aspects of Cambrian-Ordovician deposits of northern India also shed doubt on the proposed link between Kurgiakh sedimentation and the Cambrian-Ordovician orogenic event. First, our improved biostratigraphic database suggests that the transition from the Karsha carbonate to the Kurgiakh Formation may have predated the main phase of Cambrian-Ordovician orogenesis, as recorded by overlying Ordovician molasse, by as much as 20-30 My. Second, published data from the Ordovician molasse indicate northward paleocurrents, which are parallel to those recorded by siliciclastic deposits of the Parahio Formation below the Karsha, and thus are at odds with standard models of foreland basin development for the Cambrian-Ordovician event.Our sedimentological analysis of depositional cycles of the Parahio Formation indicates that these strata record storminfluenced environments from offshore marine to shoreface to fluvial settings. This is at odds with previous paleoenvironmental interpretations that ranged from deep-sea flysch to intertidal deposits. Paleocurrent data for marine and fluvial facies of the Parahio Formation in both Zanskar and the Spiti Valley to the south indicate northeast sediment transport. This supports the view that the Parahio and overlying carbonate of the Karsha Formation record the ancient northern passive margin of India during the Cambrian and that these strata may be distal equivalents of the younger Cambrian deposits of the Lesser Himalaya.