MAGNETIC GLACIAL AND FLUVIAL SEDIMENTS IN THE HOLITNA BASIN AND OTHER CENOZOIC BASINS IN ALASKA - RELEVANCE FOR THE AEROMAGNETIC SEARCH FOR HYDROCARBONS

A high-resolution aeromagnetic survey was flown in the Cenozoic Holitna basin in interior Alaska for coal bed methane exploration. Interpretation of the survey shows that many short-wavelength magnetic highs are caused by magnetic sediments derived from both igneous rocks of the Wrangellia composite terrane (WCT) and Tertiary granodiorite to tonalite superimposed on the WCT. The magnetic sediments were transported as glacial till and outwash. At their greatest (pre-Wisconsin) advance, glaciers crossed the Denali fault and deposited glacial till in the pull-apart Holitna basin, producing magnetic highs aligned with the fault. The short-wavelength magnetic highs occur as peaks on a low-amplitude (100-200 nT) regional magnetic high in the Holitna and other interior basins. Isolated igneous outcrops within the basins cause only local weak magnetic anomalies. Thus igneous rocks within or beneath the basins are not a compelling explanation for the regional magnetic high over the basins. In contrast, regional magnetic anomalies over igneous rocks in the WCT have high amplitudes (500-2500 nT). I conclude that the regional magnetic high in interior Alaska is partially caused by magnetic sediments derived from the WCT. Late Cenozoic uplift (5 km at Denali) of magnetic igneous rocks provided abundant magnetic sediments. Glaciers were larger and more active on the southern flank of the Alaska Range and Wrangell Mountains. Abundant magnetic sediments were transported to the south via the Susitna and Matanuska Valleys to Cook Inlet basin, all of which have higher amplitude (200-400 nT) magnetic highs than do the interior basins. The presence of magnetic sediments in the interior and southern basins preclude the effective use of magnetic analysis to determine depth to magnetic basement. The term “magnetic basement” is inappropriate in non-cratonic Alaska, where continental crust is under construction. Magnetic analysis is useful, however, for tracking magnetite from its igneous sources to its sedimentary sinks. If carbon derived from Mesozoic and Cenozoic coal bearing strata eroded from the uplifted Alaska Range and Wrangell Mountains followed similar paths as sedimentary magnetite, then magnetic analysis may help locate coal bed methane and basin-centered hydrocarbons contained in magnetic clastic sediments.