Depositional Environments and Geochemistry of the Shiyantou Siltstone, Yunnan, China  

The Early Cambrian Shiyantou Siltstone of Yunnan Province immediately underlies the Yu’anshan Shale, which contains the Chengjiang Biota.  The Shiyantou occupies a critical stratigraphical interval in the history of life: Small Shelly Fossils appear in the immediately underlying Yuhucun Formation, and the first appearance of trilobites occurs immediately above the Shiyantou, in the lowermost Yu’anshan Shale.  We examined a core drilled through the Yu’anshan and Shiyantou at Haikou, Kunming to better resolve the geochemistry and depositional setting of this sequence. 

Surface exposures of Shiyantou are highly weathered and friable, and appear featureless in outcrop.  Analysis of core material reveals that the Shiyantuo is comprised of amalgamated, 0.4–3.0 cm thick, event beds, which are characterized by alternating quartz silt–black shale laminae, typically several millimetres thick, with significant basal scour between events.  Rarely, ripple cross laminations are present.  Abundant pyrite occurs throughout the Shiyantou, and is typically restricted to thin black shale laminae, whereas authigenic calcite preferentially occurs as a pore-filling phase in thin silt laminae.  Both of these authigenic phases are completely lost from surface exposures due to intensive weathering in the shallow subsurface.  Moderate bioturbation (ichnofabric index 3) of 2–3 cm depth is prevalent throughout the Shyiantou, indicating that oxic bottom waters prevailed at least periodically during the accumulation of the unit.  Pyrite occurs throughout the section as dispersed grains and as larger aggregate clusters.  Petrographic and δ ³⁴S analyses of individual grains isolated from whole rock by HF extraction and δ ³⁴S transects across individual large grains and aggregates provide constraints on the chemical conditions that prevailed in the waters overlying the Shiyantou and during sulfate reduction in the organic rich sediments.