The Lipak Formation (Hayden, 1908) in the Pin Valley comprises mixed siliciclastic and dolomitic sediments in lower levels, richly fossiliferous limestones in middle parts and mudstones in upper levels; the age of the Formation cited in the literature ranges between Devonian to Early Carboniferous (Hayden, 1908). This article reports on detailed bed-by-bed lithostratigraphic sections and conodont biostratigraphy in the Lipak Formation near Mikkim and near Muth in the Pin Valley, Spiti. The actual boundary between the Lipak Formation and the underlying Muth Formation is drawn at the first appearance of dark carbonaceous, argillaceous siltstone and shale interbedded with plant fragments containing sandstone, indicating a distinct change in sedimentation from the mature, unfossiliferous quartz arenites of the Muth Formation, which are interpreted as barrier-island depositional environment (Draganits, 2000). The lowermost part of the Lipak Formation comprises impure, massive quartz arenite, grading into sandstone, intercalated with dark gray siltstone. Some bioturbation occurs on upper bed surfaces, mud chips are found and slightly increasing dolomitic influence is visible. Higher up, the terrigenous clastic influence more and more decreases and gray calcareous sandstone appears. The beds are commonly graded and contain crinoid stems and brachiopods in basal parts; thin siltstone interlayers and increasing bioturbation on upper bedding surfaces are common. These limestone beds are extremely rich in brachiopods, crinoid stems and rarer vertebrate bone, coral fragments and radiolarians and correlates well in the Mikkim and Muth sections. Higher up in the section the fossil contents and grainsize decreases and biocalcarenites dominate, beds with intraformational breccia at the base occur. This succession is followed by finegrained horizontally laminated stromatolitic limestone, intraformational breccia and tepee structures are found. Above a thin oolite horizon a distinct sandstone horizon occurs, followed by graded biocalcarenite beds and several oolite horizons. The subsequent prominent carbonaceous black shale bed sandwiched between ooide containing grainstone indicates an abrupt lithologic change and might represent a Kellwasser Event horizon; no disconformity or subaerial exposure is evident. The biocalcarenite above the black shale resembles those from below, but becomes gradually finer grained and is interrupted by a second sandstone horizon, which is very similar to the lower sandstone interval. The uppermost part of the Lipak Formation is dominated by black mudstone and marls, some thin calcarenite beds occur; macrofossils are scarce. Four meters below the top of the Formation, a irregular bounded bed of striking pink, crystalline calcite is interpreted as horizon of emergence. At the top, the Lipak Formation is truncated by the basal breccias of the overlying Gechang Formation with reworked clasts of mudstone from below. This angular unconformity represents the lack of nearly the whole Carboniferous deposits in this area (Garzanti et al., 1996). The stratigraphically lowermost conodonts originate from 30 m above the base of the Lipak Formation and indicate indubitably Givetian age (e.g. a form of Bipennatus bipennatus, a genus that is not known post-Givetian). This age also represents the minimum age limit for the underlying unfossiliferous Muth Formation. Samples from the uppermost part of the Formation give Famennian, possibly lowermost Tournaisian age. These conodont ages are in a big contrast to the Lipak Formation in Lahaul, where exclusively Tournaisian conodonts are described (Vannay, 1993) and indicate the existence of a considerable relief during its sedimentation.
|Number of pages||2|
|Journal||Journal of Asian Earth Sciences|
|Publication status||Published - 2001|