Danielsen and Over—Lower Silurian conodonts, Shine Jinst region, Mongolia 1 P C 99°20E 44°24N Q D S Yamaan Us D O Q Ulaanbaatar O Tsahildag zadgai P
C C D
Harganakhudag Bayansair
Indert Indert
D D
D D
Tsakhir Govialtai
D
Chuluun Tsagaankhaalga
D-S S
S O
Amansair Sharchuluut Gashuunovoo
Formation with the boundary located at the base of the persistent carbonates. At the type Sharchuluut (Rozman and Minjin, 1988), ~30km southeast of Yamaan Us spring, two members are recognized, the lower Govi Member composed of green sandstones, siltstones, and yellow limestones containing crinoids, brachiopods, trilobites, corals, stromatoperoids, bryozoans, conodonts, and gastropods, which is considered early–mid Llandoverian based on brachiopod and coral remains; and the upper Yamaan Us Member composed of lower green siltstones, and upper red-brown, gray limestones containing crinoids and brachiopods of late Llandovery–early Wenlock age (Minjin et al., 2001). At Yamaan Us, the Sharchuluut consists of white, gray, red-brown limestones and green siltstones with crinoids, stromatoporoids, corals, bryozoans, chitinozoans, conodonts, and upper Llandovery–lower Wenlock tabulates (Minjin et al., 2001).
Material and methods
Sixty meters of the upper Zalaa and lower Sharchuluut formations were measured and described across the contact; 19 carbonate samples were collected (Table 1; Fig. 2) and processed for conodonts using a buffered 10% formic acid solution, and lithium metatungstate for heavy liquid separation of the remaining residue. Conodonts were then picked from the processed sample and imaged using a scanning electron micro- scope. The Zalaa and Sharchuluut formations were sampled at Yamaan Us (section “Yamaan Us-2” of Minjin et al., 2001) in the western portion of the Shine Jinst region (Fig. 1). The measured section began in the upper Zalaa Formation (Fig. 2). The lower 32m of the section consisted of green-gray shales with fossiliferous carbonate interbeds and lenses. Carbonates
Tsagaanbulag Zalaa
Figure 1. (1) Geologic map of the Shine Jinst region showing the location of Yamaan Us, Pulkovo1942 datum updated to Monref97 GPS coordinates. (2) The Zalaa Formation overlain by the Sharchuluut Formation at Yamaan Us. Dashed line = formation contact, ‒ 0 = 0 m in Figure 2.
are typically green, weathering to grayish brown, and contain crinoids, trilobites, brachiopods, and orthoconic nautiloids. Overlying this shale unit, the Zalaa grades from green shales to pink and white, weathering to yellow-brown, recrystallized crinoidal grainstones interbedded with red and green shales. This transitions back to green-gray shale with interbedded greenish gray and reddish brown, weathering to red-brown, carbonates for ~5.5 m. The base of the Sharchuluut is recog- nized by the persistent carbonates consisting of reddish brown crinoidal grainstone with rare shale (Figs. 1b, 2). Conodonts were found throughout the section, although never in great abundance. Additional fauna in the thin carbonate beds included ostracodes, proetid trilobites, rhynchonellid brachio- pods, and monoplacophorans. The thicker carbonates in the uppermost Zalaa and lower Sharchuluut contained abundant crinoidal remains.
Results and discussion
Conodont fauna, age, and zonation.—Thirteen samples produced conodonts with yields ranging from 1 to 46 specimens per kilogram. The remaining samples were barren. Seven species representing six genera were identified from the 13 samples; these include Distomodus kentuckyensis Branson and Branson, 1947, Ozarkodina hassi (Pollock, Rexroad, and Nicoll, 1970), Panderodus serratus Rexroad, 1967, Panderodus unicostatus (Branson and Mehl, 1933a), Pranognathus siluricus (Pollock, Rexroad, and Nicoll, 1970), Pseudolonchodina expansa (Armstrong, 1990), and Pseudoo- neotodus beckmanni (Bischoff and Sannemann, 1958); Icrio- della was tentatively identified. The conodont taxa recovered fromthe Zalaa and Sharchuluut formations occur in lower Silurian (Llandovery) strataworldwide.
D-S Q D S S D S D Tsakhir D D D D D C 5 km 2
79
0
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