Reports: UR849341-UR8: Patterns of Tectonics, Provenance, and Sediment Fill in Large Complex Foreland Basins: Field, Petrographic and Detrital Zircon Analysis of Siliciclastic Turbidites in the Nanpanjiang Basin of South China

Daniel J. Lehrmann , Trinity University

During the last year my students and I have processed data collected from our 2009-2010 field season, including GIS map compilations, petrographic thin section descriptions, and detrital zircon geochronology. In addition two of my students and I have completed a second field season in south China in June and July of 2011.

The two central hypotheses we are testing are that: 1) Triassic turbidite deposits in the Nanpanjiang basin were sourced by active convergent margin along the southern or southwestern borders of the basin in addition to Archean and Proterozoic massifs in the northern part of the basin, and that 2) the timing and patterns of turbidite fill in the basin impacted the evolution of carbonate platforms within the basin.   The first hypothesis implies that Triassic convergence occurred along either the Aliaoshan or Somgma suture zones bordering south China tectonic block and the adjacent Indochina and Sibumasu the blocks.  The second hypothesis implies that patterns of basin filling impact platform evolution by providing substrates for progradation or foster the lack of such substrates which may result in platform over-steepening and collapse.

Preliminary analysis of paleocurrent indicators and provenance analysis point to at least two major sources of siliciclastic input into the basin: the Jiangnan massif in the northern part of the basin, and a southerly and possibly southwesterly source which may include both a volcaniclastic dominated source (active volcanic arc?) and the Yunkai massif.  Although some samples in the southwestern part of the basin clearly have a volcaniclastic component, most samples have a quartz and lithic rich composition typical of a recycled orogen, pointing to the Precambrian massifs.   Detrital zircon data are still being evaluated to test the ages of detritus input into the turbidite systems and regional stratigraphic data are being compiled to evaluate timing of depocenter fill.  Mapping of carbonate platform margins demonstrates along strike variability in margin architecture with additional episodes and greater progradation over turbidite basin fill proximal to source areas and earlier termination and burial of platforms with turbidites proximal to source areas.

The project has broadened the PI’s geologic background into the areas of turbidite systems, provenance studies and detrital zircon studies. Results from the research have been integrated into an on-campus basin analysis course and into a summer field geology course taught in south China. The project has formed the basin for directed studies projects for three undergraduate students and a senior thesis for a fourth student.  Student projects have included hands on experience in field work in south China, on the LA-ICPMS at the Arizona Laserchron center, and with state of the art petrography and point counting in the PI’s lab a Trinity University.  Students have presented preliminary results of their projects at a university student research symposium and will present at the national meeting of the American Association of Petroleum Geologists in spring.

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