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Università Degli Studi Di Milano 1 UNIVERSITÀ DEGLI STUDI DI MILANO DOTTORATO DI RICERCA IN SCIENZE DELLA TERRA Ciclo XXXII DIPARTIMENTO DI SCIENZE DELLA TERRA TESI DI DOTTORATO DI RICERCA EARLY CARNIAN AMMONOIDS FROM NEVADA: REVISED TAXONOMY, EVOLUTIONARY TRENDS AND USEFULNESS FOR THE DEFINITION OF THE LATE TRIASSIC TIME SCALE RUBEN MARCHESI Matr. n. R11510 Tutore: Prof. MARCO BALINI Coordinatore: Prof. FERNANDO CAMARA ARTIGAS Anno Accademico 2018-2019 1 2 2 3 Abstract ................................................................................................................................................ 5 Introduction to the biochronostratigraphy of the Upper Ladinian and Lower Carnian ................ 7 1.1 - Basic concepts ............................................................................................................................. 7 1.2 - Evolution of the Tethyan and North American upper Ladinian – Lower Carnian chronostratigraphic scale ...................................................................................................................... 9 1.3 - Carnian Stage GSSP and open problems ................................................................................... 12 Studied localities: geological and structural setting.......................................................................... 15 2.1 - Regional geological and structural setting ................................................................................. 15 2.2 - South Canyon............................................................................................................................. 21 2.3 - China Mountain ......................................................................................................................... 25 Materials and methods ........................................................................................................................ 31 3.1 - General approach to the study of ammonoids ........................................................................... 31 3.2 - Preparation and study of the specimens ..................................................................................... 31 3.3 - Drawing of suture lines .............................................................................................................. 32 3.4 - Other preparation activities........................................................................................................ 32 Taxonomy ............................................................................................................................................. 33 4.1 - Introductory notes ...................................................................................................................... 33 4.2 - Remarks on taphonomy ............................................................................................................. 34 4.3 - Systematic descriptions ............................................................................................................. 36 4.4 - Remarks on the test structure of the Family Trachyceratidae ................................................... 81 4.5 - Evolutionary trends within the South Canyon Trachyceratinae ................................................ 85 Faunistic analysis ................................................................................................................................. 89 5.1 – South Canyon ............................................................................................................................ 89 5.2 - China Mountain ......................................................................................................................... 94 Biochronostratigraphy......................................................................................................................... 97 6.1 - Results........................................................................................................................................ 97 6.2 - Correlation between South Canyon and British Columbia ...................................................... 108 6.3 - Correlation between South Canyon and the Tethys ................................................................ 110 Conclusions ......................................................................................................................................... 117 Appendix and References .................................................................................................................. 119 8.1 - Text Figures ............................................................................................................................. 119 8.2 - Plates ........................................................................................................................................ 145 8.3 - Tables ....................................................................................................................................... 185 8.4 - References ................................................................................................................................ 193 Aknowledgements ............................................................................................................................ 201 3 4 4 5 Abstract The aim of this research is the study of the Lower Carnian (Upper Triassic) ammonoid faunas from Nevada, which have contributed to the definition of the North American Triassic chronostratigraphic scale in the 1960s. These faunas, however, have never been described in detail, both in terms of taxonomy and in terms of stratigraphic distribution in bed-by-bed sampled sections. This research is a contribution for the revision and improvement of the Late Triassic chronostratigraphic time scale. A revised zonation is proposed for the sedimentary succession within the middle member of the Augusta Mountain Formation at South Canyon (New Pass Range, Nevada), type locality of the Desatoyense Zone of the North American chronostratigraphic scale. New Lower Carnian ammonoid faunas from China Mountain (Tobin Range, Nevada) are also described. Silberling & Tozer (1968) introduced the Desatoyense Zone to replace the “Joannites zone” of Johnston (1941), with Trachyceras desatoyense Johnston, 1941, as the index species. The fauna described by Johnston (1941) was collected without accounting for the stratigraphic position of the specimens, and consequently, a range chart could not be provided for the 24 taxa attributed to this zone. An intensive sampling program in the type locality was begun in 2002 by Marco Balini and James Jenks and continued intermittently with other occasional collaborators over a period of several years. Bed by bed collection efforts from 103 fossiliferous beds in six stratigraphic sections have yielded ~3900 specimens, of which 3130 ammonoids, that allow the construction of range charts and recognition of the bioevents on which this revision is based. The systematic study has led to the recognition of 29 taxa, 10 of which are new. These taxa belong to 18 genera, two of which are new: Neoclypites, Sageceras, Perrinoceras, Badiotites, Procladiscites, Arcestes, Joannites, Silenticeras, Frankites, Daxatina, Trachyceras, Genus A, Genus B, Clionitites, Hannaoceras, Sympolycyclus, Lobites and Coroceras. Ammonoids of the Subfamily Trachyceratinae are the most abundant, and provide all the zonal indexes. Frankites sutherlandi and Daxatina occur in the lower part of the succession (sections A and B). The middle- upper part of the succession (section D2, E and lower part of section F) exhibits a different faunal composition and Trachyceras co-occurs with Genus A and Genus B (both Trachyceratinae), which are confined to two narrow stratigraphic intervals. The upper part of section F is characterized by another major faunal change, the onset of Trachyceras with highly indented suture lines that co-occur with Coroceras. With the new data it is possible to subdivide the former Desatoyense Zone into three parts, which are in stratigraphic order: the Daxatina sp. A range zone, the Trachyceras desatoyense interval zone and the Trachyceras sp. B range zone. The Daxatina sp. A zone is correlated with the Canadian Sutherlandi Zone and with the Tethyan Canadensis Subzone. The T. desatoyense and the T. sp. B zones are correlated with the Canadian Desatoyense Zone and with the Tethyan Aon Subzone. The Desatoyense Zone is reported also from China Mountain (Tobin Range, Nevada). This area was investigated in the early 1970s as part of the PhD work of K.M. Nichols (1972). Nichols collected ammonoids from two sites (localities #1251 and #1311, Nichols, 1972), but did not report bed-by-bed collection data, nor described the collection. Nichols & Silberling (1977), referred to the locality and assigned the ammonoid faunas to the Desatoyense Zone. During the 2018 field campaign we visited the area, and noted the location of the fossiliferous interval in the uppermost part of the Smelser Pass Member of the Augusta Mountain Formation, a few meters below the erosional base of the Cane Spring Formation. Two new fossiliferous sites were located and sampled bed-by-bed from exposed beds, and float specimens were collected from the two sites, taking careful note of their stratigraphic position. Stratigraphic sections were measured, and a total of 758 specimens were collected from these two new localities. An attempt was made to find locality #1251 (Nichols, 1972), but was unsuccessful due to the lack of precise locality information.
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