EARLY DOLOMITIZATION OF PLATFORM CARBONATES AND THE PRESERVATION OF MAGNETIC POLARITY

Results from a combination of techniques are presented to evaluate the nature of magnetization in shallow-water platform carbonates which ha ve undergone recrystallization during early calcification and dolomiti zation. Magnetic grain separates, coercivity spectra, modified Lowrie- Fuller tests, magnetization efficiency, and magnetostratigraphic const raints indicate that the ultrafine-grained magnetite is preserved duri ng early burial geochemical regimes, inversion from aragonite/high-mag nesium calcite to low-magnesium calcite, and even pervasive dolomitiza tion. These single-domain crystals are thought to occur as interacting multigrain clusters, some of which may exceed 1 mum in diameter. Thes e large clusters may help prohibit magnetic reorientation during diage nesis. Furthermore, during both fabric preserving and fabric destructi ve dolomitization, the ultrafine-scale replacement process restricts r eorientation of the clusters, thus preserving depositional or early po stdeposition magnetic orientation. This early dolomitization (matrix s tabilization) may even help protect and extend the subsurface lifespan of the original polarity.