Low-amplitude aeromagnetic anomalies in southeastern Missouri

Aeromagnetic anomalies of less than 200 gammas are associated with topographic relief of exposed Precambrian granitic and volcanic rocks of the St. Francois Mountains. Anomalies over hills of coarsely crystalline granite are as high as 100 gammas, over comparable hills of fine-grained rocks are as much as 200 gammas, and over normal faults or sheared igneous rocks are less than 100 gammas. These anomalies are superposed on larger magnetic features related to pendants of volcanic rock in the roof of a granite batholith and are distinguished from large lateral variations in magnetic intensity by their low amplitude and small areal extent. Analyses of the compound anomalies yield the subsurface configuration of isolated roof pendants of resistant extrusive rock. Application of results to lead-mining areas show that aeromagnetic patterns of low amplitude can guide mineral exploration in the region flanking the Ozark uplift.