Early-Middle Silurian Sedimentary Successions and Their Response to Tectonism and Eustatic Fluctuations: A Case Study from the Outcrops in Keping Area, Tarim Basin

The Silurian, influenced by the Late Caledonian movement, is a significant change stage of palaeoenvironments and palaeotectonics in Tarim basin. The transformation from Pre-Silurian marine carbonate dominant deposits into Silurian marine clastic dominant deposits is bounded by the basin-scale unconformity surface at the bottom of Silurian. It is important to recognize the character of the tectonic movement and its depositional response by studying the sedimentary successions and mineral and lithological feature of sandstone in this period. Therefore, the outcrops in Keping area, where the Silurian depositional sequence is continuous and of perfect exposure, were selected for this study. Based on the detailed outcrop observation, sampling and laboratory tests, the sedimentary environments and successions of the Early-Middle Silurian, and its responses to sea-level changes are reconstructed, and then sequence stratigraphy units are divided. Continental shelf, deltas and tidal influenced shoreline etc. are recognized from the Lower-Middle Silurian in Keping area. Large scale deltas deposits have been founded in the Lower Silurian. The study results of sandstone framework mineralogy composition, compared with Dickinson's framework modes, reveal that the Early-Middle Silurian sandstone were mainly derived from the provenance of collision orogenes, forebulge, and continental block. It directly reflects that the orogene around the Tarim plate's impact was due to the Late Caledonian compression during the Early Silurian. As a result, the north and the south of the Tarim plate were uplifted and a large denudation area was formed. Sedimentary successions analysis reveals the tectonics and eustatic fluctuation in the period of the Early-Middle Silurian. During the early stage (Kepingtage stage) of the Early-Middle Silurian, tectonic activity was intensive and eustatic fluctuation was obvious under the influence of the Late Caledonian movement. As a result, large scale delta systems with lower maturity and higher lithic content sandstone on the background of continental shelf environments developed, which reflects that as a result of rapid uplifting, there was an abundance of source input from the north of Tarim block. During the later stage of the Early-Middle Silurian, tectonic activities decreased and got stabilized, and low-amplitude eustatic fluctuation was dominant. Tidal action was obvious, and tidal flat, tidal channel, small deltas, and lagoon were developed, which sandstone was charactered by higher maturity and lower lithic content. Sedimentary successions of the Early-Middle Silurian indicate that tectonic activities decreased, and the depression area was filled with sediments.