884


Journal of Paleontology 91(5):883–901


Figure 1. Map of the southern Great Basin, showing the facies realms of the inner, middle, and outer shelf (modified from Palmer and Halley, 1979; Sundberg and McCollum, 2000, 2003a; McCollum and McCollum, 2011; Sundberg, 2011; Webster, 2011a). Shaded stars mark the sections investigated: AC, Antelope Canyon; E, Echo Canyon; GR, Groom Range; GS, Grassy Spring; I, Indian Springs Canyon; LC, Log Cabin Mine; M, Montezuma Range; OS, Oak Spring Summit; SM, Split Mountain. White circles represent localities mentioned in the text: BR, Belted Range; GP, Gold Point; WIM, White-Inyo Mountains.


General geology and stratigraphy


The Montezuman–Delamaran succession of the Great Basin reflects the overall flooding of the western margin of the Laur- entian craton (Webster, 2011a). During this time the shelf was spatially and temporally heterogeneous, as documented by the multitude of regional lithostratigraphic units (e.g., Palmer and Halley, 1979; Sundberg and McCollum, 2003b; Webster, 2011a, b; Figs. 1, 2). Based on lithofacies and trilobite dis- tribution patterns, the depositional environment is separated into inner, middle, and outer shelf facies realms (e.g., Stewart, 1970; Palmer and Halley, 1979; Sundberg and McCollum, 2003a; McCollum and McCollum, 2011; Sundberg, 2011; Webster, 2011a; Figs. 1, 2).


Inner shelf facies.—Sections of the inner shelf facies are primarily exposed in the Pioche-Caliente area of eastern Nevada (Fig. 1). The Dyeran-Delamaran boundary interval is represented by the Pioche Formation (Arcuolenellus arcuatus-Mexicella mexicana zones; Sundberg and McCollum, 2000; Sundberg, 2011; Webster, 2011a, c; Fig. 2). Its lower part, the Delamar Member, consists of a succession of bioturbated claystone and


siltstone interbedded with sandstone and conglomerate layers, with carbonate intercalations at the top (Webster, 2011c). The lower cliff-forming portion of the succeeding Combined Metals Member consists of bioclastic oncolitic limestone, nodular limestone, and thin limestone beds. The upper portion of the Combined Metals Member shows a higher siliciclastic content, represented by ribbon limestone, nodular limestone, siltstone, and sandstone intercalations (Webster, 2011c). The base of the Delamaran starts with the Comet Shale Member (Eokochaspis nodosa–Amecephalus arrojoensis zones; Fig. 2), predominated by claystone and siltstone with a few thin limestone beds (Sundberg and McCollum, 2000; McCollumand McCollum, 2011). It is disconformably overlain by the Susan Duster Limestone Member (Amecephalus arrojoensis–Poliella denticulata zones), which consists of a basal bioclastic limestone, an interval of claystone and nodular limestone, and an upper part of nodular-bedded limestone (Sundberg and McCollum, 2003b; Sundberg, 2011). The overlying Log Cabin Member (Poliella denticulata Zone) consists of claystone and siltstone with intercalations of sandstone and bioclastic limestone (Sundberg and McCollum, 2003b; McCollum and McCollum, 2011; Sundberg, 2011). The uppermost part of the Pioche Formation is represented by the Grassy Spring Member


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