Slaty cleavage development and magnetic anisotropy fabrics

Abstract

The shale‐to‐slate transition preserved in the Ordovician Martinsburg Formation at the Lehigh Water Gap, Pennsylvania, provides an opportunity to study the relationship between magnetic anisotropy fabrics and the development of slaty cleavage. Our previous work has indicated that anisotropy of magnetic susceptibility (AMS) does not record changes in finite strain associated with cleavage development in these rocks but instead measures the degree of dissolution and new growth of chlorite. Additional AMS data presented in this paper lend further support to this conclusion. Conversely, anhysteretic remanent magnetization anisotropy (ARMA), which is not affected by paramagnetic chlorite, accurately reflects the strain‐induced rock fabrics associated with cleavage formation. ARMA results show that magnetite dimensional orientations vary from bedding‐parallel in shale samples to cleavage‐parallel in samples with well‐developed slaty cleavage. Samples with weak and pencil cleavage display scattered ARMA orientations which lie in between bedding and cleavage. These intermediate orientations may be due to either passive rotation of magnetite from bedding‐parallel to cleavage‐parallel or (re)crystallization of magnetite. If rotation occurred, grain rotation was highly heterogeneous in the samples with incipient cleavage. The intermediate ARMA orientations may also reflect the varying contribution of two magnetite preferred orientations, a depositional orientation parallel to bedding and a new growth orientation parallel to cleavage.