Journal of Paleontology, 90(1), 2016, p. 10–30 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2015.74


Cambrian microfossils from the Tethyan Himalaya


Ian R. Gilbert,1 Nigel C. Hughes,1 and Paul M. Myrow2 1Department of Earth Sciences, University of California, Riverside, CA 92521, USA ⟨iangilbert81@gmail.com⟩, ⟨nigel.hughes@ucr.edu⟩ 2Department of Geology, Colorado College, Colorado Springs, CO 80903, USA ⟨pmyrow@ColoradoCollege.edu⟩


Abstract.—Cambrian biostratigraphy of the Indian subcontinent is best documented from the Parahio Formation of the Tethyan Himalaya. Recently established trilobite biostratigraphy shows that the formation encompasses the latest part of unnamed Stage 4 and much of unnamed Stage 5. A variety of small shelly fossils have been recovered via acid digestion of carbonate beds and include tetract and pentact hexactinellid sponge spicules, chancelloriid spicules belonging to Chancelloria sp. and a new species, Archiasterella dhiraji, shells of an helcionelloid comparable to Igorella maidipingensis, a meraspid ptychopariid trilobite, the tubular Cupitheca sp., a poorly preserved hyolith, and an assortment of spinose microfossils of uncertain affinity. These newly recovered microfossils are consistent with the trilobite-based lower and middle Cambrian age determination and do not support a late Cambrian age for the top of the Parahio Formation advocated in some recent literature. The microfossils reported herein significantly expand the known diversity of such fossils from Cambrian strata in the Himalayan region, and allow for comparison of this fauna with others from Gondwanaland and elsewhere. Integration with trilobite data indicate that the stratigraphic ranges of many small shelly fossils described in this study are greater than previously recognized.


Introduction


The well-exposed sections of the Cambrian Parahio Formation in the Parahio Valley, Spiti region, Himachal Pradesh, and in the Purni section of the Zanskar Valley, Ladakh region, of the Indian TethyanHimalaya (Fig. 1) have permitted the establishment of a trilobite-based biostratigraphic zonation for rocks of the later part of Series 2 (Stage 4), and the earlier part of Series 3 (Stage 5) of the Cambrian System (Peng et al., 2009) (Figs. 2–4). This has been an important step in erecting a localCambrian biostratigraphy for the Indian subcontinent and is supplemented by further work on trilobites and other fauna from these and other sections. Carbonate rocks from the Parahio Formation were collected for acid digestion, primarily with the aim of recovering topotype phosphatic brachiopods in order to clarify the systematic concepts of poorly known Cambrian species (Popov et al., 2015). An additional aim was to prospect for microfossils. A particular need with respect to microfossils has been


to assess a report by Bhatt and Kumar (1980) that suggested the presence of conodont and paraconodont specimens assigned to genera such as Oneotodus, Sagittodontus, Furnishina, Problemoconites,and ?Westergaardodina collected from a prominent dolostone bed located near the top of the Parahio Formationinthe ParahioValleysection(seeMyrowet al., 2006a). Such an assemblage is curious because elsewhere these taxa occur at different stratigraphic levels. If correct, it seemingly implies an Early Ordovician minimal depositional age for the top of this unit because the first appearance datum (FAD) of the euconodont Oneotodus is of that age. However, Bhatt and Kumar (1980) themselves suggested a late Cambrian (= Furongian) age


10


for the horizon that was consistent with the earliest biozonation of the Parahio Formation (Reed, 1910), in which the putative presence of the trilobite Dikelocephalus wasusedtoinfer alate Cambrian age for the top of the Parahio Formation in the Parahio Valley (see also Hayden [1904]). This late Cambrian age has long been generally accepted (e.g., Shah and Raina, 1990; Shah et al., 1991), and is advocated in a current textbook (Ramakrishnan and Vaidyanadhan, 2008), despite significant revision to the trilobite taxonomy (Jell and Hughes, 1997; Peng et al., 2009). Hence, a significant discord in age estimates for the upper part of the Parahio Formation at its type section currently exists between this earlier work and that published by Peng and colleagues (2009). This paper aims to explore this discrepancy through a fresh analysis of the microfossil content of the Parahio Formation. Taxonomic reassessment of Reed’s (1910) Dikelocephalus specimensshowedthattheyinfactbelongtoanolder


form


(Jell and Hughes, 1997, p. 15), and on that basis Jell and Hughes suggested the upper part of the Parahio Formation in the Parahio Valleytobemiddle Cambrian inage. Thisconclusionwas reaffirmed by Peng and colleagues (2009), who suggested that the uppermost in situ trilobite bearing rocks, from the Parahio Valley at ~1050m in the section measured by Myrow et al. (2006a), belong to the Ptychagnostus gibbus Zone (i.e., toward the top of Stage 5 of the Cambrian System). The reassessment was basedbothonthe collectionoffossils fromthe upperpartofthe ParahioFormationinthe ParahioValley, andonits correlation to the Zanskar Valley section, where the Parahio Formation is conformably overlain by the Karsha Formation, a thick dolomitic unit of Guzhangian or older age (see Peng et al., 2009).


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188