Miocene Initiation of Crustal Shortening in the North Qilian Shan: Stratigraphic Evidence From the Cenozoic of the Hexi Corridor
Abstract
New sedimentalogic, stratigraphic, and compositional data from the Paleogene-Neogene stratigraphic succession exposed in the Hexi Corridor and within the North Qilian Shan, west of the cities of Yumen and Jiuquan, provide evidence to support initiation of crustal shortening in the North Qilian Shan fold-thrust belt during the Miocene. The stratigraphy is composed of four lithostratigraphic units. Stratigraphically lowest is Unit 1, containing red, often nodular, overbank mudstone and fine to very coarse sandstone channel deposits. Unit 1 grades into Unit 2, a massive well-imbricated pebble to cobble conglomerate. Unit 3 consists of cobble conglomerate and lies in angular unconformity over both Units 1 and 2; it is conformable with overlying conglomeratic Unit 4, which contains a progressive unconformity. The onset of conglomerate deposition at the base of Unit 2 suggests an increase in depositional energy, which we interpret as the result of proximal orogenesis. In addition, Units 2 and above contain strongly northeast trending paleocurrents, indicating paleoflow transverse and away from the Qilian Shan. Clast lithologies that match sources in the North Qilian Shan, and sandstones with detrital framework modes indicate a recycled orogen source. In contrast, Unit 1 paleocurrents are more variable with a generally northward flow and the composition of sandstones and clasts are more diagnostic of a continental block source. These data strongly suggest Units 2 and above have a provenance in the North Qilian Shan, while Unit 1 does not. Detrital zircon age determinations from Unit 1 are also inconsistent with a source in the North Qilian Shan but suggest a provenance in hinterland regions within the Central/South Qilian Shan. In sum, these results are all consistent with proximal uplift of the North Qilian Shan during deposition of the gradational transition from Unit 1 to Unit 2. Uncertainty remains about the significance of potential sediment sources in an older South/Central Qilian Shan, however these results demonstrate unambiguous shortening in the Qilian Shan before the end of the Miocene, and possible much earlier.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.T41C1581B
- Keywords:
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- 8104 Continental margins: convergent;
- 8108 Continental tectonics: compressional;
- 8111 Continental tectonics: strike-slip and transform;
- 8169 Sedimentary basin processes;
- 8175 Tectonics and landscape evolution