AEROMAGNETIC DATA PROVIDE MAP-VIEW CONSTRAINTS ON SEDIMENT PROVENANCE: APPLICATION TO COOK INLET AND CIRCLE, ALASKA

Magnetic field maps reflect abundances of Fe- and Ti-rich minerals such as magnetite, titanomagnetite and hematite, and typically show a strong response to igneous or metamorphic bodies rich in these minerals. Sediments derived from such rocks often have enhanced magnetic properties as well, suggesting that magnetic survey data can be used to recognize and map associated pathways, complementing mineralogical and geochemical approaches to provenance studies. However, because of their lower volume and greater exposure to weathering, such sediments usually generate low-amplitude, short-wavelength, noise-like anomalies that are difficult to discern and often filtered during processing. This effect is compounded if the magnetic sensor is flown far from the magnetic source because representative anomalies become attenuated and indistinguishable from system noise. We have developed an approach that enhances visualization of short-wavelength anomalies and have applied it to two regions where low-altitude aeromagnetic surveys have been conducted, Cook Inlet and Circle, AK.

Cook Inlet is bordered by rocks of the igneous and metamorphic rocks of the Alaska and Aleutian Ranges to the west, the metamorphic and mélange Chugach Mountains to the southeast, and the igneous and metamorphic Talkeetna Mountains further to the northeast. Clusters of short-wavelength magnetic anomalies are aligned with mapped glacial deposits in several areas. Differences in the magnitudes of anomaly variations are likely related to the sources of the glacial deposits, with those to the west showing the greatest variation, and those to the southeast showing minimal variation. These differences are consistent with comparisons between previous petrologic analyses and magnetic susceptibility measurements of local sediments. The Circle quadrangle southwest of the Tintina fault zone contains exposed quartzitic and mafic schists intruded by granite and mafic rocks, as well as gold-bearing alluvial gravel. Magnetic field maps exhibit broad highs over the mafic schists and some intrusive bodies while short-wavelength anomalies are associated with various streams and in some cases extend to exposed bedrock. We compare anomaly locations to previous analyses of stream sediments to constrain the sources of associated sediments.