Date of Award
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At Hongyan Section, the Triassic Yangtze Platform margin architecture is preserved in the western part of the Nanpanjiang Basin. A syncline exposes a continuous two-dimensional cross section through the margin. Stratigraphic correlations with spectral gamma ray logs demonstrated that the Hongyan margin evolved from an Upper Permian platform margin dominated by diverse skeletal grainstone to an Induan, broad ramp profile with ~1.5° slope. The ramp consists of prograded ooid-shoals that change to bioturbated lime mudstone with slump folds and debris flow breccias on the mid ramp and laminated lime mudstones on the outer ramp. In the Olenekian, a more abrupt bank profile developed as the reef steepened, with a barrier of ooid shoals and a restricted lagoon and peritidal interior. During the Middle Triassic (Anisian), the platform developed a progressively steepening Tubiphytes microbial-cement reef-rimmed margin. The margin reached up to 250 m relief with slope clinoforms ~35°. Slope facies changed from debris-flow breccia to talus and calciturbidites, and siliciclastic turbidites of the Xinyuan formation buried the slope.
Hongyan section traverses the basin, platform margin, and interior. Analysis of spectral gamma-ray logs and elemental geochemistry (U, Mo, V) shows onset of basin anoxia in the Early Triassic, continuation of an anoxic basin with redox fluctuations at the end of the Early Triassic, and basal Anisian and oxic conditions later in the Middle Triassic. Spectral gamma ray profiles show generally low values and low Uranium (U) content across the Permian-Triassic boundary and in the Lower Triassic (Induan). This notable Uranium depletion probably resulted from a global Oceanic Anoxic Event (OAE) related to the end-Permian mass extinction and depletion of seawater U due to widespread sedimentation of U in the world’s anoxic oceans. Spiky, high gamma ray intervals with elevated U and Potassium (K) within the Lower Triassic (Olenekian) and Middle Triassic (Anisian) reflect locally developed anoxia and siliciclastic influx.
Carbonate factory types shift from a relatively high proportion (6%) of skeletal contents in the Upper Permian to abiotic (oolite and micrite) (98%) in the Induan, to microbial and abiotic (11% and 54%, respectively) in the Olenekian, to a return of higher skeletal content (6%) in addition to microbial crusts and cement in the Anisian. This return of biotic contents in the Middle Triassic may reflect the recovery from the end-Permian extinction and seawater anoxia. However, the shift to a more abrupt bank profile in the Olenekian indicates that the change in contents preceded the biotic recovery from the end-Permian extinction, suggesting that seawater redox conditions may have had a greater role on margin architecture than biotic evolution had. These trends mirror those seen in the Great Bank of Guizhou.
Previous models predict an increase in marine cement during anoxic conditions; in my study it was found that marine cement levels decreased during periods of anoxia (Induan and Olenekian). I hypothesize that this resulted from greater carbonate saturation during these periods, resulting in greater abiotic micrite production (similar to whitings), which occluded pore space and thus inhibited marine cementation. Future work may evaluate whether seawater redox changes and carbonate saturation state may be able to demonstrate very small-scale secondary porosity or cementation not visible in this study.
Stepchinski, Leanne M., "Controls on Carbonate Factory Type (Abiotic, Microbial, Skeletal) on the Hongyan Margin of the Yangtze Platform, South China" (2015). Geosciences Student Honors Theses. 14.