Factors controlling joint spacing in interbedded sedimentary rocks: Integrating numerical models with field observations from the Monterey Formation, USA

Local tensile stress normal to a joint is reduced in the vicinity of the joint because such stresses are not transmitted across free surfaces. This stress reduction prevents the formation of new joints in the vicinity of existing joints, and thus influences joint spacing. Lateral extent of this stress reduction shadow increases with joint height, which corresponds to bed thickness for many sedimentary rocks. The linear correlation between joint spacing and bed thickness commonly observed in outcrop is a direct result of this relationship. However, other factors in addition to bed thickness influence joint spacing. We evaluate these factors through both a review of the Hobbs model for joint spacing and a 2D finite element simulation of a crack confined to a lithology-controlled mechanical unit. The stress reduction shadow increases in length with incresaing Young's modulus of the jointing bed, though fracture stress, flaw size, flaw distribution and extensional strain all interact with bed thickness and elastic properties ultimately to control joint spacing. One explanation for the observed decrease in joint spacing with increasing Young's modulus in outcrops of the Monterey Formation is that beds with higher Young's moduli fail at lower magnitudes of extensional strain.