MAGNETIC, SEISMIC AND WELL DATA CONSTRAIN STRUCTURAL FEATURES OF SUSITNA BASIN, ALASKA

Susitna basin, about 80 km northwest of Anchorage, is a potential hydrocarbon-producing area but its geology and tectonics are poorly understood. We use magnetic, seismic and well data to better understand its structural history. Three wells show interstratified volcanic and sedimentary rocks of late Paleocene to early Eocene age overlain by younger sandstone, conglomerate, siltstone and coal. High magnetic susceptibilities of the volcanic layers suggest that modification by faults and folds may be expressed in magnetic anomaly maps. Using aeromagnetic data collected in 2000-2012, we applied high-pass filters that enhance anomalies attributable to shallower sources. The filtered magnetic map reveals 0.6-3 km-wide anomalies oriented N30E-N60E throughout the area. Seismic data show that many of these anomalies correspond to structural highs, strongly suggesting the anomalies correspond to anticlines that formed during NW-SE shortening. We note that shallower layers are less deformed, suggesting reduced tectonic activity later. In several places the anomalies show linear NNW-NNE disruptions, some corresponding to reverse and thrust faults in the seismic profiles. Several of the faults appear to cut Miocene and younger strata. Over an area crossing the Yentna River, two NNW-NNE reverse faults bound a ~20-km wide area where the magnetic anomalies become wider and lower in magnitude (< 10 nT), suggesting a deeper magnetic source. In that area the Trail Ridge well shows a ~4 km thick sedimentary package comprising late Paleocene to Eocene strata overlain unconformably by Miocene to Quaternary layers. This area and the faults are truncated to the south by the NW-striking Beluga Mountain thrust fault. Anomalies to the east are less subdued (30-100 nT) and well data show a much thinner sedimentary package. Areas to the west show even stronger magnetic anomalies, suggesting shallow buried igneous and/or metamorphic rocks. The deepest part of the basin thus appears limited in east-west extent. Combined data suggest the region underwent NW-SE shortening expressed as folds that was later modified by E-W and then NE-SW shortening along N-S and NW-SE thrust faults, respectively, perhaps showing effects of oroclinal bending and later collision of the Yakutat block with south-central Alaska.