Early dolomitization of platform carbonates and the preservation of magnetic polarity

Abstract

Results from a combination of techniques are presented to evaluate the nature of magnetization in shallow‐water platform carbonates which have undergone recrystallization during early calcification and dolomitization. Magnetic grain separates, coercivity spectra, modified Lowrie‐Fuller tests, magnetization efficiency, and magnetostratigraphic constraints indicate that the ultrafine‐grained magnetite is preserved during early burial geochemical regimes, inversion from aragonite/high‐magnesium calcite to low‐magnesium calcite, and even pervasive dolomitization. These single‐domain crystals are thought to occur as interacting multigrain clusters, some of which may exceed 1 μm in diameter. These large clusters may help prohibit magnetic reorientation during diagenesis. Furthermore, during both fabric preserving and fabric destructive dolomitization, the ultrafine‐scale replacement process restricts reorientation of the clusters, thus preserving depositional or early postdeposition magnetic orientation. This early dolomitization (matrix stabilization) may even help protect and extend the subsurface lifespan of the original polarity.