Silurian–Devonian active-margin deep-marine systems and palaeogeography, Alai Range, Southern Tien Shan, Central Asia

Abstract

Analysis of Mid-Palaeozoic successions in the northern part of the Alai Range (Kyrgyzstan and bordering Uzbekistan), Southern Tien Shan, Central Asia, has identified a Silurian–Devonian deep-marine depositional system of basin-slope facies-associations. Here, we document the stratigraphy and sedimentology of a region in Central Asia that, through conflict, has become inaccessible for geological research. The turbidite-dominated Pul'gon Formation (Silurian) accumulated in sea-floor depressions and within the inferred basin axis. The large-scale, coarse clastic lenses of the Dzhidala Formation (mostly Devonian) represent the fills of submarine channels, canyons or gullies of the palaeoslope; other slope apron processes include sediment slides, debris flows and olistoliths. The partly time-equivalent condensed sequences of the Mid- to Late Silurian Kursala Formation and Early Devonian Tamasha Formation represent graptolitic mudstone and chert accumulation, respectively, together with thin dolomitic limestones, that accumulated over c . 10–15 Ma, probably on seamounts in the Turkestan (Fergana) Ocean between the continental margin of Kazakhstania and the Alai microcontinent. The graptolitic shale-rich Chakush Formation (early Silurian) is geographically and petrographically different from the Pul'gon and Dzhidala formations and suggests a different provenance (opposing continental margin or seamount talus).