Rock physics diagnostics to characterize early diagenetic processes in hemipelagic calcareous ooze in the northern South China Sea margin.

Hemipelagic calcareous ooze is a mixture of biogenic carbonate ooze and detrital terrigenous materials from continental weathering. Understanding its early diagenetic features is critical for geophysical measurement and geo-hazard assessment. However, the early diagenesis of hemipelagic calcareous ooze has rarely been studied due to a lack of samples and in-situ measurement in shallow strata. During IODP Expedition 368, hemipelagic calcareous oozes, up to hundreds of meters thick, were encountered in the shallow subsurface (< 400 m) at two sites (U1501 and U1505) close to the continent-ocean transition zone in the northern South China Sea margin. Here, we use coring and well logging data to perform rock physics diagnostics to characterize early diagenetic processes in hemipelagic calcareous ooze. The calcareous ooze in the study area is primarily chalky marl with the ratio of carbonate to carbonate plus siliciclastic minerals ranging 30% ~ 70%, according to a mineralogy-based lithofaices classification scheme. In the chalky marl interval, pyrite accounts for around 15% of the total mineral composition and this existence may be related to significant post-rift magmatic activity that produced significant amouts of volcanic material. The porosity reduction with depth in the chalky marl interval has three phases and carbonate content is suggested to be the controlling factor affecting porosity reduction. The varying carbonate content in the oozes can result in contrasting mechanical properties and porosities, potentially serving as weak layers for submarine landslides. Additionally, the interface between calcareous ooze and siliciclastic sediment is also a potential failure plane. Therefore, we propose that the development of hemipelagic calcareous ooze has significant implications for seafloor instability on continental margins. [ABSTRACT FROM AUTHOR]