INTERPRETATION OF AN AEROMAGNETIC SURVEY OF THE AMCHITKA ISLAND AREA, ALASKA. Geological Survey Professional Paper 707.

An aeromagnetic survey of about 1,800 square miles of Amchitka Island and the adjacent insular shelf has provided information on Tertiary volcanic, intrusive, and sedimentary rocks. This includes identification of rocks that cause anomalies and the lateral extents, structures, and approximate depths of those rocks. Near proposed drill sites, anomalies were examined for features that might be related to faulting. The survey was facilitated by data on the magnetic properties of 347 rock specimens collected from surface exposures and 216 from drill cores and by plots of 25 miles of ground magnetic traverse. The data on magnetic properties furnished bases on which anomalies were related to geologic features; ground surveys classified near-surface rocks as either lava or breccia. The total magnetization of volcanic breccia, tuff breccia, volcanic sandstone, and siltstone averages about 7.0X104 gauss, an effective direction generally being along the earth's magnetic field. This value is designated as the "ambient magnetization level" for the area. The prominent anomalies come directly from lava flows and thick sills that have total magnetizations which differ from the ambient level for the island. Anomalies also come indirectly from large bodies of intrusive rock that have altered and destroyed the magnetite content of overlying flow rocks. The average for 219 surface and 81 core specimens of lava is 1 4 . 2 ~ 1 0 ~ gauss induced intensity and 1 2 . 8 ~ 1 0 ~ gauss remanent intensity. Lavas of the Chitka Point Formation have normal remanent polarities and produce positive anomalies. The basalt lavas of the Banjo Point Formation, as well a s the pillow lavas and breccias of Kirilof Point in the upper part of the Amchitka Formation, have both normal and intermediate polarities and produce positive and negative anomalies. Individual breccia samples from the Banjo Point Formation and the lower part of the Amchitka Formation have significant values of remanent intensity, but directions vary so greatly from sample to sample that a thick section of breccia does not give a magnetic anomaly. Although dikes and small sills have total magnetizations well above the ambient level, their thicknesses are too small to give a significant effect a t the datum plane 1,600 feet above sea level. The normal polarity of the White House Cove intrusive and the reversed polarity of the East Cape intrusive confirm that these intrusives are separate features, emplaced a t different geologic times. Computation of the effects of sheetlike models shows tha t the steeper gradients of theoretical anomalies are positioned near the ends of flows or sills that have been terminated by faulting. Drill sites were selected in areas away from gradients considered to be fault related. Nearly all prominent anomalies over land and many over water can be reasonably interpreted and can be correlated with known geologic features. Anomalies and geologic data suggest that the magma of the Chitka Point Formation originated in a large volcanic center on western Amchitka Island and eastern Rat Island. Faults that are well delineated by aeromagnetic contours on Amchitka do not appear to extend very f a r seaward, and marked submarine trenches that have the same general trend are not well defined magnetically.