Usolka section (southern Urals, Russia): a potential candidate for GSSP to define the base of the Gzhelian Stage in the global chronostratigraphic scale

Conodont species Streptognathodus simulator Ellison, 1941 has been proposed recently to define the Kasimovian-Gzhelian boundary in the global chronostratigraphic scale. The species distributed globally and traditionally has been used as a marker of the base of the Gzhelian Stage in the type sections in Moscow Basin and Urals. Recent studies of conodont taxonomy and biostratigraphy in southern Urals have established the chronocline with ascendant and descendant to Streptognathodus simulator species. Usolka section proposed here as a potential candidate for the GSSP (Global Stratotype Section and Point) to define the global Gzhelian Stage at the FAD of the Streptognathodus simulator within the chronocline Streptognathodus praenuntius Chernykh, 2005 – St. simulator Ellison, 1941 – St. auritus Chernykh, 2005. The chronocline recovered within 2.7 m of beds 4 and 5 at the Usolka section, with all three species described and properly figured. No obvious interruptions in sedimentation are recorded within the Kasimovian-Gzhelian transition there. Several volcanic ash beds are present below and above the proposed boundary, making radiometric calibration highly possible in the near future. Mode of preservation of conodonts with a CAI of around 1.0–1.5 provides excellent basis for the geochemical studies. Accessibility presently is adequate, and this exposure will be improved and maintained permanently for interested scientists. Future access will be guaranteed by means of legislative action to create a scientific preserve.

distribution. Moreover, the taxonomy of the species has been updated recently by B a rrick et al. (2004), who recognize two separate species: St. simulator �sensu stricto�, the concept of which is based on the holotype from Midcontinent Heebner Shale of the Oread cyclothem, and its potential ancestor St. aff. simulator, which occurs in the older Midcontinent Eudora Shale of the Stanton cyclothem and the Merriman-Upper Winchell cyclothem of Texas (H e c k e l et al., 2005). Current studies of upper Paleozoic stratigraphy and biostratigraphy in the Urals during the last few years (C h u v a s h o v et al., 1990, 1993, 2002 2002,2005). The latter publication establishes a chronocline of Streptognathodus praenuntius Chernykh, 2005 -St. simulator Ellison, 1941-St. auritus Chernykh, 2005, which is recovered within 2.7 m of beds 4 and 5 at the Usolka section, with all three species described and properly figured. Besides biostratigraphy and sedimentology, comprehensive geochemical study has been done at Usolka section. Numerous and frequent volcanic ashes occur throught succession in Usolka section (Davy d o v et al., 2002). One volcanic ash layer very close to the boundary has been dated just recently (S c h m i t z et al., 2006). There are several more ashes that potentially will precisely constrain proposed boundary in term of radiometric calibration. Strontium study from conodonts also has been performed in Usolka section (N e e d h a m et al., 2006). We are envisioning studying δ 18 O isotopes from conodonts as well. We propose here the Usolka section as a potential candidate for establishing a GSSP for the base of the global Gzhelian Stage.

Material
The Usolka section is located approximately 120 km southeast of Ufa and about 60 km northeast of Sterlitamak on the northeastern margin of the city of Krasnousolsk, just north of the Usolka River across from the hot-springs resort, in the Bashkortostan Republic of Russia (Fig. 1). This section is exposed along the roadcut on the right bank of the Usolka River in the core of the meridionally striking Usolkian brachianticline.
The section occurs in the axial part of the Belsk depression in the relatively deeperwater portion of the Preuralian Foredeep, and therefore the sedimentary succession there is likely to be relatively undisturbed.
The section starts with dolomitic limestone with chert nodules and rare volcanic ash beds of the Zilim Formation, approximately 10-12 m thick, which is overlain across a covered interval by the predominantly mixed carbonate-siliciclastic succession of the Kurkin (or Kurortnaya) Formation (Fig. 2). During a 2001 field trip, we collected samples and recovered conodonts from volcanic ash near the top of the Zilim Formation. Conodonts there include typical Moscovian Neognathodus and are under current study. The measured thickness between the exposed top of the Zilim Formation and bed 1 of the Kurkin Formation in the section is approximately 5-6 meters.
The Kurkin Formation contains numerous micritic limestone beds and up to 50 volcanic ash layers (Fig. 2) more or less evenly distributed throughout the section (Davyd ov et al., 2002(Davyd ov et al., , 2003. More general information and details on the section can be obtained from several sources (Chuvashov et al., 1991(Chuvashov et al., , 1993Chuvashov & Chernykh, 2002). Chernykh (2005) provided the most comprehensive record of conodont distribution in the section. Because of the undisturbed sedimentary record and abundance of conodonts in the section, higher parts of the Usolka section were proposed as an auxiliary section for the Carboniferous-Permian boundary (Chuvashov et al., 2002), and as a candidate for the GSSP for the base of the Sakmarian Stage (Wardlaw et al., 1999).
The lithology of the Kasimovian-Gzhelian transition is not yet described in great detail, but we plan to re-measure and restudy it in the near future. Our plan is to collect additional conodont samples at a centimeter scale along with samples for fusulinids, smaller foraminifers, and volcanic ash beds where the lithology is appropriate. Below is the latest available description of the Kasimovian-Gzhelian transition (Chuv ashov & Chernykh, 2002), in ascending order of numbered beds (with thicknesses based on Fig. 2

Analysis
In the described succession (Fig. 2), bed 1 was not sampled for either conodonts or foraminifers and therefore its age cannot be determined. However, because conodonts recovered 6 meters below the top of the Zilim Formation are Moscovian Neognathodus, bed 1 is in a transitional position between the Moscovian and Kasimovian stages. Beds 2 and 3 have yielded the typical Kasimovian conodonts Id. sagittalis Kozitskaya and Streptognathodus cancellosus (Gunnell), and thus belong to the Kasimovian Stage. Bed 3 was not properly sampled in the past, so we will re-sample it in greater detail. In the lower part of bed 4 (4-1) at 4.7 meters above the base of the section (mab), the conodont species are more advanced than in bed 3, with St. zethus Chernykh & Reshetkova and St. pawhuskaensis Harris & Hollingsworth, of which the former marks the base of the regional Virgilian Stage in North America (Heckel, 2004). The newly described species St. praenuntius Chernykh, 2005, which closely resembles St. simulator, also occurs in this level (Fig. 2). The fusulinid assemblage that is found at the same level is usually characteristic of the late Kasimovian, although some species range up into the early Gzhelian. In the upper part of bed 4 (4-2), starting from 4.9 mab, conodonts St. elegantulus and St. simulator occur. The latter species, as mentioned above, is the traditional index for determining the base of the Gzhelian Stage in the Moscow Basin and the Urals and is recently proposed index of the base of the global Gzhelian Stage (Heckel et al., 2005). Upwards, in bed 5 at 6.4 mab, the conodont assemblage is very similar to that from bed 4-2, except that new and more advanced forms that resemble St. simulator occur. These forms were recently described as a new species St. auritus Chernykh, 2005 (Figs. 2-3). No fusulinids were found in beds 5 or 6, and the fusulinids that were recovered from bed 7 (8.2 mab) are poorly preserved. In bed 8 (10.8 mab), the typical Gzhelian species Rauserites stuckenbergi and Daixina rugosa were recovered among other fusulinid species (Fig. 2).

Discussion
The traditional base of the Gzhelian in the Moscow Basin was proposed by Nikitin (1890) at the base of the Rusavkino unit in a limestone near Gzhel village that disconformably overlies the Troshkovo unit of the Kasimovian (Ivanova & Khvorova, 1955). Makhlina et al. (1979) recognized four sed- imentary cycles within the Rusavkino. However, more recently the Rusavkino has been divided into three cycles: Lower, Middle, and Upper Rusavkino (Heckel et al., 2005).   Chernykh et Reshetkova, 1987. 11 -IGG U15-20;12 -IGG U15-30 13-15 Streptognathodus praenuntius Chernykh, 2005. 13 -U13-23;14 -IGG U13-11;15 -IGG U13-11; (note that scale bar for this specimen slightly reduced) this interval and the data on conodont evolution suggest that there are no documented breaks in sedimentation during this transition, and allow the Usolka section to be considered as a candidate for the Global Stratotype Section and Point (GSSP) for the base of the Gzhelian Stage. St. praenuntius in the established chronocline possess strong similarity to St. simulator, but differs from the latter in a less eccentric and less well developed trough along the medial line of the platform element (Fig. 2). The phylogenetic evolutionary relation between Streptognathodus praenuntius and St. simulator is proposed because: (1) these two species appear in successive order, and (2)   Schwageriniformis (Tumefactus)? sp.. 7 Schwageriniformis schwageriniformis mosquensis (Rosovskaya). 2 -USO-4-11BCh; 7 -USO-4-4BCh.

5.
A better relationship among conodont, fusulinid, and ammonoid biozonations and other fossil groups must still be worked out.