14
Journal of Paleontology 90(1):10–30
Figure 4. Stratigraphic relationship between trilobite zonation, section, height (meters) in section, collection name, and Hayden (1904)/Reed (1910) levels and the microfossil-bearing material of the Parahio Formation in the Zanskar Valley (Ladakh region) and Parahio Valley (Spiti region) sections. Here trilobite zones are given as encompassing unzoned intervals between the fossiliferous horizons indicated in Figures 2 and 3. Hayden’s (1904) levels 2, 4, and 13 have not yielded microfossils described herein, but their position is given for clarity.
The residue was then sieved to isolate various size fractions from which microfossils were isolated using a binocular microscope. Choice specimens were mounted on scanning electron microscope (SEM) blanks using a conductive carbon tape and acetone solution. These were coated with platinum and palladium using a Cressington 108 AUTO sputtering device. High-resolution images were obtained using the XL30 FEG SEM at the Central Facility for Advanced Microscopy and Microanalysis (CFAMM) located at the University of California, Riverside (
http://micron.ucr.edu/). An attached EDAX device was used to determine elemental composition using EDS.
Geological setting
The material described herein was collected from the Tethyan Himalaya, the northernmost of four major lithotectonic zones that comprise the central part of the Himalaya, just south of the Indus-Tsangpo suture zone, which is the boundary of the Lhasa block of Tibet (Myrow et al, 2006a) (Fig. 1). The Spiti and Ladakh regions are adjacent, and the Parahio Formation, which has been recognized in both, consists of thick siliciclastic deltaic deposits that contain relatively thin, fossil-bearing carbonate beds, which represent transgressive systems tract deposits developed over marine flooding surfaces (Myrow et al., 2006a; Myrow et al., 2006b). It is these carbonate beds from which the material examined herein was collected. The Parahio Formation was deposited along the ancient passive margin of India during the Cambrian (Myrow et al., 2006a, 2006b), and maps and GPS coordinates specifying the locations of sections and samples are available in these publications and others (e.g., Peng et al., 2009).
Biostratigraphy
Biostratigraphic implications of the microfossil finds.—The morphologies of hexactinellid sponge spicules described herein are quite simple, thus making assessment beyond hexactinellid affinity impossible. As a group, hexactinellid sponges are thought to range from the late Ediacaran to the present day (Gehling and Rigby, 1996), and this is all the temporal constraint that these Parahio Formation fossils provide. Chancelloria is known to range from low in the
pre-trilobitic lower Cambrian to the lower part of the upper Cambrian (Mostler and Mosleh-Yazdi, 1976; Mostler, 1980). Given this, the Parahio Formation Chancelloria suggest that the youngest depositional age of these rocks is pre-Prochuangia Zone (i.e., early late Cambrian), and that they are no older than the base of the Cambrian. This result suggests that no part of the Parahio Formation is younger than the Proagnostus bulbus Zone. Specimens from elsewhere belonging to Archiasterella dhiraji n. sp. occur both in the lowest trilobite bearing Cambrian (Bengtson, Conway Morris, Cooper, Jell, and Runnegar, 1990) and also in Stage 4 (Skovsted and Peel, 2007), so they are consistent with the occurrence of Chancelloria but favor a slightly older youngest age. Igorella maidipingensis likewise has a lower Cambrian
occurrence, ranging from the Nemakit-Daldynian Stage to the Tommotian stages of the Siberian Platform, Yangtze platform, and Iran (Devaere et al., 2013; Parkhaev and Demidenko, 2010). The genus Cupitheca is also found in the lower Cambrian of Antarctica, China, Australia, Spain and Kazakhstan (Mambetov in Missarzhevsky and Mambetov, 1981; Zhou and Xiao, 1984; Bengtson in Bengtson et al., 1990; Wrona, 2003; Malinky and Skovsted, 2004; Kruse et al., 2004; Jensen et al., 2010).
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