Whole-rock chemical analyses indicate that the Onondaga Formation along a Rochester-Syracuse (New York) transect has a marked eastward increasing trend in K/Al ratio, which correlates with an increase in authigenic magnetite content and degree of diagenetic illitization. An eastward increase in the content of some authigenic minerals including K-feldspar, quartz, and Fe-rich chlorite in the rocks is indicated by petrographic and X-ray diffraction studies. These observations suggest a link between remagnetization and potassic alteration of Fe-bearing silicates in the Onondaga. Two hypothetical reactions are proposed in which magnetite authigenesis is related to the breakdown of Fe-bearing smectite in the presence of potassium. Products of these reactions include authigenic K-feldspar, illite, quartz and chlorite, as well as magnetite. Our results suggest that remagnetization was triggered by a basin-wide migration of K-rich fluids during the Alleghenian Orogeny.