--> ABSTRACT: Patterns of Turbidite Sandstone Deposition in the Nanpanjiang Basin: Evidence to Constrain the Timing of Plate Collisions during the Assembly of Continental Blocks in South China and Southeast Asia, by Goers, Alexa; Wood, Tanner ; Copley, Lauren; Lehrmann, Dan; Minzoni, Marcello ; Enos, Paul; Lu, Gang; #90142 (2012)

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Patterns of Turbidite Sandstone Deposition in the Nanpanjiang Basin: Evidence to Constrain the Timing of Plate Collisions during the Assembly of Continental Blocks in South China and Southeast Asia

Goers, Alexa *1; Wood, Tanner 1; Copley, Lauren 1; Lehrmann, Dan 1; Minzoni, Marcello 2; Enos, Paul 3; Lu, Gang 4
(1) Trinity University, San Antonio, TX.
(2) Shell International Exploration and Production, Houston, TX.
(3) University of Kansas, Lawrence, KS.
(4) Guangxi Geologic Survey, Guilin, China.

The Nanpanjiang Basin occurs at the intersection of one of the most complex microplate collages in the world. Controversy continues regarding the timing of assembly of tectonic blocks along the Songma and Ailaoshan sutures on the southern border of the south China block. This project evaluates the hypothesis that turbidite sedimentation in the Nanpanjiang Basin was influenced by active Triassic convergence.

Turbidites in the basin include the Early Triassic Shipao Fm. and the Middle-Late Triassic Baifeng, Xinyuan, Lanmu, Bianyang and Laishike fms. Each ranges upward of 700 m and the thickest is nearly 3 km. The turbidites are overwhelmingly VF sand in the northern part of the basin whereas the central and southern parts of the basin also contain F sand size and rarely M sand size. Coarser sand sizes are associated with paleocurrents from the south, and in this area some turbidites exhibit complete bouma sequences with graded A divisions. Successions contain numerous alternations between mud-rich and sand-rich intervals with thickness trends corresponding to proximal/ distal fan components. The northeast basin has conspicuously mud-dominated successions from bypass of sand across the slope whereas the basin center contains sand-dominated successions with amalgamated sands 10’s of m thick indicating ponding into troughs. Spectacularly preserved structures enable robust evaluation of turbidite systems and paleocurrent analyses.

Analysis of 880 paleocurrents indicates two major directions of sediment fill. The northern part of the basin was primarily sourced by the Jiangnan massif in the northeast, and the central and southern parts of the basin were primarily sourced from the south, possibly from the Songma suture zone.

Middle Triassic sands show remarkably similar and relatively mature framework grain compositions dominated by quartz but containing foliated meta lithics, fine sedimentary lithics, polycrystalline quartz, mica, and feldspar. Visual estimates indicate a recycled orogen provenance. In contrast, the Baifeng and Shipao fms. in the southwest and west contain more feldspar and the Shipao also contains volcanic quartz and lithics indicating a magmatic arc provenance.

Our results support the hypothesis of sourcing from the Precambrian massifs in the northern and from an active magmatic arc linked to Triassic convergence along the southern margin of the basin. The hypotheses will be further tested using point counts and detrital zircon analysis.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California