ROCK-MAGNETIC SIGNATURE OF GLACIMARINE SEDIMENTS FROM THE ALEXANDER, YAKUTAT AND SOUTHERN MARGIN COMPOSITE ACCRETED TERRANES OF COASTAL ALASKA

Rock-magnetic properties were used to distinguish glacimarine sediments within continental terranes in Southern Alaska. Rock-magnetic characteristics of 363 discrete samples from the Yakutat, Alexander and Southern Margin composite (SMC) terranes were experimentally determined at the Institute for Rock Magnetism at the University of Minnesota. Glacial erosion and rapid deposition into fjords within each terrane yielded minimally altered sediments where primary mineralogy controls the magnetic signature. Low-field magnetic susceptibility (c) was measured using a kappa bridge. A 100mT peak field and a .05mT bias field were applied to impart an anhysteretic remanent magnetization (ARM) to the samples. A saturated isothermal remanence (SIRM) and a subsequent back isothermal remanence (BIRM) were imparted using 1T and –300mT fields, respectively. Hysteresis properties were determined using a vibrating sample magnetometer and low-temperature demagnetization of the SIRM was conducted using Quantum Design’s magnetic property measuring system (MPMS). Susceptibility (c) ranged widely from 9 x 10⁸ m³/kg in SMC samples to 1 x 10⁶ m³/kg in Alexander samples. The ratio of c/SIRM was very high for SMC samples indicating that room temperature susceptibility is dominated by paramagnetic minerals. S-ratios (BIRM/SIRM) were near 1 for Alexander and Yakutat samples predicting that magnetite is the dominant carrier in those sediments while some of the SMC samples were significantly different from 1 suggesting the magnetic carrier is dominated by canted-antiferromagnetic minerals. The coercivity force (Hc), remanent coercivity (Hcr), saturation magnetization (Ms) and remanent saturation (Mrs) were calculated from hysteresis loops. A plot of Mrs/Ms and Hcr/Hc revealed that most samples fell into the pseudo-single domain category. None plotted as single-domain or superparamagnetic and a few from the Alexander and Yakutat terranes were questionably multi-domain. The distinctive rock-magnetic signatures apparently do not result from local magnetite-rich bodies but pervade each terrane and are explained by the dominance of metasedimentary rocks in the SMC terrane and foliated granitic, mafic volcanic and various metamorphic rocks in the Alexander and Yakutat terranes.