Daniel J. Lehrmann, Trinity University
During the last year my students and I have processed data collected from our 2011 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 third field season in south China in June and July of 2012.
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.
Analysis of paleocurrent indicators and provenance analysis points to two primary sources of siliciclastic input into the basin: the Jiangnan massif in the northern part of the basin, and a southerly source which may include both an 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 orogeny province, pointing to the Precambrian massifs. Detrital zircon data include: 1) a few concordant zircon ages older than 3000 Ma, 2) widespread peaks across the basin at 1800 Ma and 2500 Ma which may represent derivation from the Khamdian or Yunkai massif, 3) a widespread 900 Ma population likely matching the Jiangnan massif, 3) a peak centered around 400 Ma likely matching the Yunkai massif, and 4) a larger population of zircons dating to around 250 Ma abundant in the southwest part of the basin that is replaced to the northeast by populations with ages of 290-260 Ma.
Detailed mapping of the margins of an isolated carbonate platform have demonstrated diverse of margin architectures along the periphery of the platform and between northern and southern sides of the platform. Variation in architecture resulted from differences in antecedent topography, timing of filling of the basin which modified substrate topography, and windward-leeward effects.
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 five undergraduate students and a senior thesis for a sixth 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 at the south central section GSA meeting and a national AAPG meeting.