OCEAN OXYGENATION AND ORIGIN OF FOSSIL-BEARING CHERT BANDS IN LOWER CAMBRIAN STRATA OF SOUTH CHINA

The lower Cambrian strata of the Yangtze Gorges area, and northern Hunan Province, South China, are composed of the Yanjiahe, Liuchapo and Niutitang formations that contain exquisitely preserved 3-dimensional macro- and microfossils (acritarchs, cnidaria, sponge spicules, microbes) within chert bands. The chert horizons are fundamental in the collection of these important microfossils, but the origin of silica needed for widespread chert formation is poorly understood in this region. Pyrite framboids (p.f.), early diagenetic microcrystalline aggregates of pyrite, have been useful tools in understanding ocean oxygenation in Paleozoic to modern sediments, including in the Late Ediacaran Doushantuo Formation. Larger framboids indicate oxic to dysoxic conditions, forming in situ below the sediment-water interface. Smaller framboids are size-limited because of aggregation within an anoxic water column. In addition to p.f. size, the total concentration of rare earth elements (REE), REE ratios (e.g., La/Yb), and major and trace element ratios (e.g., Si/Ge, REE/Fe) have been demonstrated to aid in determining sediment source (terrigenous, hydrothermal, or biogenic) to help us understand the origin of the abundant chert bands in the lower Cambrian.

To better understand the environmental and diagenetic context of these fossiliferous strata, cherts have been sampled from 17 localities spanning more than 300 km from the carbonate shelf to deep basin environments across the South China Craton. Pyrite framboids are quantified in terms of diameter, deformation, degree of oxidation (pyrite or hematite), and recrystallization (later diagenetic crystallization). Samples were also analyzed for REE’s and major and trace elements, including Fe, Si, Ge, Mg, Ca, Mn, Al, and Ti) using laser ablation inductively-coupled plasma mass spectrometry. Preliminary data favors oxic to dysoxic bottom waters on the Yangtze platform (p.f. mean size = 8.47-12.31 microns), with isolated occurrences of anoxia (p.f. mean size = 4.78-5.21 microns). Preliminary REE data do not support a biogenic origin of the chert bands. Based on preliminary REE and major and trace element data, it is difficult to differentiate between hydrothermal and terrigenous input to the basin, likely due to effects of later alteration and weathering.