Ilonchisay Fm
Type Locality and Naming
Along Ilonchi, N. Nuratau Ridge. P.N. Podkopaev et al. in 1960 (1964) in the volume of Lower Silurian, Ordovician age justified by Z.M. Abdazimova et al., 1969 (12, p. 115-116), Z.M. Abdazimova, 1976 (21, p. 10-11). Stratotype in the Ilonchisaya basin, northern part of the Bogambir mountain.
Synonym: Ilonchisay Formation, Илончисайская св.
Lithology and Thickness
Upper part of the section, corresponding to the Ashgill, consists of sandstones from fine-grained to gravelly, poorly sorted with clastic material (quartz, feldspars, fragments of siliceous and chalcedony rocks, argillites, siltstones, granites, effusives of acidic, intermediate, and basic composition) and siltstones, less frequently lenses of gravelites. The predominant color of the rocks is tobacco-green, greenish-gray. Thickness 100-150m.
Middle Caradocian part of the section is composed of rhythmically interbedded polymictic siltstones, claystone (argillites), and less frequently sandstones of tobacco-green, greenish-gray color. In the siltstones, there is cross-bedding, current marks, and sediment intrusions, various bioglyphs and mechanoglyphs. Thickness 80-100m.
Lower part of the section, corresponding to the upper Llandovery, consists of polymictic, less frequently quartz-mica siltstones of grayish-green, tobacco-green color with interlayers of claystone (argillites). On the surface of the siltstones, there are traces of bottom currents, drag marks, and other various hieroglyphs. Thickness 40-70m.
Relationships and Distribution
Lower contact
The lower boundary is stratigraphically conformable with the Kurbanazin Fm
Upper contact
The upper boundary is not established.
Regional extent
Northern slope of the North Nuratau range, basins of the Ilonchi, Asman, Chagalak, Ustukhan rivers. The Caradoc-Ashgill part of the suite is correlated with the Kair Fm of the S. Nuratau, the middle and upper parts - with the Lyupetsk Fm of the S. Bukantau.
GeoJSON
Fossils
Upper part - Contains graptolites Paraorthograptus pacificus (Rued.), Climacograptus longispinus supernus (Elles. et Wood.), Amplexograptus inuiti (Cox.), A. aff. amplexicalius (Hall.); chitinozoans Conochitina chyadea Jenk., Cyathochitina campanulaeformis Eis., Desmochitina minor f. frinacea Eis., D. aff. dornbuschi Shal., Parachitina curvata Eis., Eisenasckitina (?) costacea (Eis.), Lagenochitina baltica (Eis.), Conochitina schopfi americana Taug.
Middle - Contains graptolites Amplexograptus perexcavatus (Lapw.), A. Fallax (Bulm.), Climacograptus sp.
Lower – Contains graptolites Glyptograptus teretiusculus (His.), G. euglyphus (Lapw.) and others; filamentous microfossils - Muruntaufilum longum Abd. and others.
Age
Depositional setting
Additional Information