Yangyang Wang

Mechanism of the organic matter (OM) accumulation in the Middle Permian Gufeng Formation shale in South China is lack of constraints, which restricts the source rock evaluations and shale gas explorations. To decipher the OM accumulation of the Gufeng Formation, geological and geochemical results related to paleo-environmental variations are presented from the shelf Putaoling section in South China. The OM accumulation in the Gufeng Formation is vertically heterogeneous, shown by a medium total organic carbon (TOC) content (2.3%) in the lower member and a high TOC content (9.6%) in the upper member. The organic-rich shales of the Gufeng Formation are deposited in a complex paleo-environment with restrained water conditions, a warm and humid paleoclimate, a relatively strong chemical weathering, significant hydrothermal activities, a high primary productivity, fluctuating redox conditions, and a relatively high sedimentary rate. Compared to the lower member deposited under anoxic conditions, the upper member is formed in a dominantly euxinic environment with higher productivities. The seawater deoxygenation and the upward-increasing productivity jointly lead to the vertical heterogeneity of the OM accumulation in the Gufeng Formation. Thus, an ‘integrated model’ for the OM accumulation in the Gufeng Formation is established, and which adds to our knowledge that no a single factor or model can explain the OM accumulation in all sedimentary environments.

The coseismic frictional temperature is increasingly recognised as an important factor for the transformation mechanism in shallow crustal faulting. We report changes in the mineralogical and geochemical compositions across the fault zone in the Shaba (SB) exposure of the 2008 Wenchuan earthquake (Mw 7.9), which contained the maximum value of vertical displacement and cut through shale-rich strata by the principal slip surface. Assuming Yb to be an immobile trace element, with respect to those in the adjacent damaged zone and wall rocks, Li, Rb, Cs and most light rare earth elements (LREE) exhibited marked enrichments of 143%, 247%, 475% and >60%, respectively; and a 40% depletion of Sr and <5% depletion of most heavy rare earth elements (HREE) was found, responding to mineralogical transformation such as alteration and dissolution of feldspar and formation of abundant clay minerals. In addition, heterogeneous adsorption of clay minerals could result in fractionation between and within LREE and HREE as well as negative Eu anomalies and positive Ce anomalies. The element enrichment/depletion model of the northern segment of the Yingxiu–Beichuan (YX–BC) fault zone was quite different from the middle/southern segments characterising faults in carbonate and granitic rocks. Through model calculation, the calculated trace element compositions (Li, Rb, Sr, Cs, Ba, La, Sm and Pb) of the fault gouge were obtained at 250 °C, 300 °C and 350 °C, and were produced by frictional heating. By comparing with the observed values, we suggest that high temperatures derived from frictional heating or due to buffering of the slip zone temperature by evaporation of pore water during the earthquake within the principal slip surface in the northern segment of the YX–BC fault zone may not have existed. Mechanical crushing and chemical alteration (not under high-temperature conditions) within the fault zone during the coseismic stage may result in mass removal and element enrichment/depletion.