Simplified geologic map and cross sections of central and east Anchorage, Alaska

Abstract

This geologic map and preliminary cross sections of central and east Anchorage, Alaska, are based on previous mapping, limited new photointerpretation, and available subsurface data. Using PC-based Geographic Information System (GIS) software, the existing geologic map has been updated and simplified by adding recent fill deposits and combining units of similar genesis, composition, and age that are also recognizable in the subsurface. The GIS database consists of a USGS geologic map and over 4,000 geotechnical boreholes and water-well logs provided by numerous public and private sources. Geologic cross sections were developed by using GIS to project graphic lithologic logs into scaled vertical layouts along selected lines. Stratigraphic units were manually correlated using the log sections as guides. Identification and correlation of subsurface units are somewhat hampered by complex glacial geology, sparseness of deep boreholes, and significant variation in lithologic descriptions among many drillers. Although these limitations result in some generalized, undifferentiated geologic units, the differences among interpreted units are of the level desired by the geotechnical user community for highlighting engineering and seismic behavior.