Nucleation and development of multi-scale faults in an alternating sandstone and shale turbidite sequence and their effects on groundwater flow and transport

Abstract:

Turbiditic deposits consist of alternating sandstones and shales of various thicknesses. This complex sedimentary architecture produces multiple hierarchies of heterogeneities and controls the development of fractures and faults at various scales. Typically stiff units (e.g., sandstones and siltstones) have a predominantly brittle behavior and produce joints, which propagate across layers characterized by homogeneous mechanical properties and terminate against the boundaries of the so-called mechanical units. Joints provide weak planes for later shearing, leading to the clustering of joint zones and eventually the nucleation of faults. These faults may reach large extents through linkage of neighboring fractures and fault segments. However, the evolution and linkage of the faults is controlled by an interplay between the mechanical properties of the rock units and the fault slip. To evaluate both lateral and vertical continuity of fault zones, which are fundamental for assessing their fluid flow properties, it is desirable to identify what class of heterogeneities may inhibit the fault propagation. This structural framework is also important evidence supporting DFN contaminant transport models used to describe and predict plume behavior at the site.

For this contribution we carried out a multi-scale analysis of the fault and fractures cutting across the turbidite sequence of the Chatsworth Formation (Simi Hills, CA). By means of collecting quantitative structural data for both background and fault-related structures, we established the hierarchy of faults and fractures based on their length, height, width, and offset and spacing where available. Detailed structural maps were integrated with scanline surveys to characterize the architecture of the single structures and identify their stage of development.

Finally, detailed characterization of contaminant distributions from continuous core sampling at the site TCE source and plume corroborate the presence of numerous, multi-scaled, hydraulically active fractures required to match the observed plume migration distance over past decades, thus providing a robust analysis of the hydraulic behavior of structures making up various hierarchical classes within the structural framework.