MAGNETITE DISSOLUTION AND NEOCRYSTALLIZATION DURING CLEAVAGE FORMATION - PALEOMAGNETIC STUDY OF THE MARTINSBURG FORMATION, LEHIGH GAP, PENNSYLVANIA

Paleomagnetic study of the shale to slate transition in the Ordovician Martinsburg Formation at Lehigh Gap, Pennsylvania reveals that the re manence of the relatively undeformed shales predates cleavage formatio n, and that the slates were remagnetized during cleavage development. The shales display two components of remanence. Component 1 (C1) was t he only magnetization direction in the majority of the shales, and the first-removed component in 10 shale specimens. Component 2 (C2) is ob served as a single-component remanence with a low (300-400-degrees-C) unblocking temperature range in samples from one shale station and as a second-removed component in 10 shale specimens. The mean directions are (declination/inclination) 347-degrees/-04-degrees (346-degrees/-47 -degrees) in situ (tilt-corrected) for C1 and 80-degrees/-63-degrees ( 135-degrees/-40-degrees) for C2. The pencil slates and well-cleaved sl ates both display single component remanences. The mean directions are 198-degrees/-12-degrees (201-degrees/26-degrees) for the pencil slate s and 199-degrees/24-degrees (225-degrees/54-degrees) for the well-cle aved slates. Using the relationship between cleavage development and r emagnetization in the slates, the timing of cleavage development and f olding, and the regional geology, we can constrain the relative ages o f the remanences as follows: (1) the shale C1 direction is most likely prefolding and represents a primary upper Ordovician direction, (2) t he slate directions are younger in age than the shale directions and a re probably of Permo-Carboniferous (Alleghenian) age. These results sh ow that remagnetization occurred during development of slaty cleavage in these rocks. We interpret the mechanism for this remagnetization to have been strain-induced dissolution and neocrystyallization of magne tite. The additional difference in the characteristic directions of th e pencil slates and well-cleaved slates may be attributed to strain re orientation of the pencil slate remanence after new growth of magnetit e. The unknown temporal relationship between dissolution and neocrysta llization and strain precludes any quantitative attempt to correct rem anence for strain in these samples. Our study shows that paleomagnetic studies of deformed sediments must consider the possibility of dissol ution and neocrystallization of remanence carriers, in addition to str ain-induced rotation.