Date of Award

5-2014

Document Type

Thesis campus only

Abstract

The Yangtze Platform was a vast carbonate platform bordering the Nanpanjiang Basin on the southern end of the South China Block from the Late Paleozoic till its demise in the Late Triassic. The modern location of the platform is in southern Guizhou Province, China.

The Middle-Late Triassic development and termination differed laterally along the margin of the Yangtze Platform. The northeast portion of the platform developed a progradational margin and terminated with a shift from shallow-water, peritidal carbonates to shallow-water siliciclastics. It is believed that this portion of the platform terminated because it was overwhelmed by the clastic influx. The southwestern portion of the platform developed an aggradational and back-stepping margin and terminated with the shift from sedimentation of shallow-water, peritidal carbonates to deeper-water, pelagic carbonates and then to very deep-water argillaceous limestone and shale. It is believed that the southwestern portion terminated due to drowning by rapid tectonically-induced subsidence.

Evidence of a significant tectonic influence includes syndepositional faults throughout the shallow-water limestone that die out at the boundary with the overlying deeper-marine limestone, subsidence analysis showing northward migration of increased subsidence rates from the M. Triassic (Ladinian) to U. Triassic (Carnian), significantly different thicknesses of the deeper-water limestone in nearby sections and volcanic and flysch sedimentation that indicate foreland basin development through convergence of a volcanic arc.

This study uses geochemistry of the SW Yangtze Platform termination sequence in order to determine the chemistry of the seawater chemistry during termination. Field gamma ray measurements of radioactive elements (U, Th and K) in addition to ICP-OES and ICP-MS analysis of rock samples allowed the use of proxies to infer the siliciclastic influx (%Al, Si/Al, Ti/Al and ppm K), paleoproductivity (ppm P, Ni, Ba, Cd, and Zn) and paleoredox conditions (U, V, Mo, Cr, Co) during deposition of the termination sequence. Average siliciclastic, nutrient and redox proxy concentrations rise throughout the termination sequence with significantly higher concentrations in the deep-water facies. Within the gradational transition zone between the peritidal limestone and deep-water limestone, there are shifts in the siliciclastic, nutrient and redox proxy concentrations where higher concentrations correlate to the deep-water limestone facies and lower concentrations correspond to the peritidal limestone facies.

Integrating previous findings with the new geochemical data, it is interpreted that rapid tectonic subsidence was a major cause of termination but an additional mechanism is required to explain the significant variation and magnitude of nutrient and redox proxy concentrations that covary with facies type within the transition zone. This additional mechanism is likely land-ward derived fluxes of excess nutrients which aided rapid tectonic subsidence in the termination of the southwestern Yangtze Platform.

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