The origin and spacing of cross joints: examples from the Monterey Formation, Santa Barbara Coastline, California

Cross joints in the Monterey Formation terminate against a series of pre-existing systematic strike-perpendicular joints. The strike-perpendicular joints act as mechanical layer boundaries during cross joint propagation, and the distance between adjacent strike-perpendicular joints represents a mechanical layer thickness. The fracture spacing index (FSI), or ratio of joint-controlled mechanical layer thickness to cross joint spacing in the Monterey Formation, is approximately 1.3. A model is proposed for cross joint propagation based on the analysis of stress reduction in the vicinity of a newly-formed joint. In this model, cross joint development follows a sequential infilling process as remote tensile stress increases with time. According to the model, the first cross joints (FSI = 0.32) propagate under a remote tensile stress of approximately -14 MPa. A second episode of cross jointing (FSI = 0.65) initiates when remote tensile stress reaches -27 MPa, and a third generation of cross joints (FSI = 1.30) develops at -57 MPa. Joints in each successive episode initiate in the midregion between existing joints where local tensile stress is highest. High remote tensile stresses may develop due to differential horizontal contraction among adjacent stratigraphic beds upon uplift and erosion.