PRECAMBRIAN SMECTITE OF THE NEOPROTEROZOIC DOUSHANTUO FORMATION, CHINA

The Doushantuo Formation is the oldest of the unusually organic-rich ‘black rock’ series that spans Ediacaran to Cambrian age strata of South China. Black, organic-rich shales (TOC up to 5%), immediately above the Marinoan age cap dolomite, contain distinct horizons of phosphate bearing chert nodules and are interbedded with limestone beds, typically 20 cm thick. In the Yangtze Gorge area, the argillaceous component of these carbonaceous shales are extremely well preserved. Mineralogical analysis, using XRD, reveal that the major clay component of the lower black shale is smectitic, although minor kaolinite is also present. The occurrence of mixed-layered clays in Precambrian rocks is unusual because expandable layers are illitized at relatively low temperatures (~ 120°C) in the presence of potassium. Higher in the sequence, carbonate beds thicken and become dolomitic, acting as a marker between lower smectitic black-shales and upper fossiliferous, phosphatic black-shales. The upper shales are more extensively silicified and the clay component is largely illitized. A coincidental change in feldspar chemistry also takes place between the lower and upper levels. Feldspars of the upper shales are more potassium rich, leading to the hypothesis that the change in clay mineralogy marks a diagenetic front, the position of which is controlled by the availability of potassium.

Determining original clay mineralogy of the Doushantuo has implications for three lines of research on this succession. 1) Explaining preservation of fossils as carbonaceous films. 2) As a proxy for paleoclimate following deglaciation. 3) Determining organo-mineralogical interactions that may have been important during early diagenetic reactions.