A MULTIFACETED APPROACH TO UNDERSTANDING THE DEPOSITIONAL ENVIRONMENT OF DIAMICTON UNITS FROM THE ICELAND SHELF

Interpreting the depositional environment of diamicton units on the Iceland Shelf, specifically distinguishing glacial marine sediment from till, is necessary in order to determine the ice extent onto the shelf during the late Quaternary. This study focuses on the description and analysis of diamicton units recovered in marine cores collected in 1997 and 1999 from the southwestern and northern Iceland Shelf. These cores contain diamicton units that vary in length from 2 meters to over 10 meters. At least 7 properties, including sedimentologic parameters, x-radiograph analyses and fabric analyses, of the diamicton units are measured to facilitate the interpretation of these units. In general, the diamicton units are the lowermost stratigraphic unit in the cores and are characterized by an increase in bulk magnetic susceptibility/mass and a drop in carbon and water content. Sedimentological analyses of the diamicton units alone do not provide a clear understanding of the depositional environment of these units. Micro-fabric analyses and x-radiograph analyses supplement the sedimentology data. Grain size, tilt of the major axes of pebbles, pebble perimeter, and pebble counts were calculated automatically using the NIH Image analysis program. Anisotropy of magnetic susceptibility, degree of anisotropy, and shape were measured on a Kappabridge Apparatus.

The longest core in this study, MD99-2256, contains two diamicton units separated by a sharp contact. Image analysis results of x-rays show an increase in mean grain size and inclination of the long axes of pebbles in the lower diamicton unit. The magnetic parameter T, indicative of shape, reveals a shift from oblate ellipsoids to less oblate ellipsoids at the contact, indicating that the lower unit contains a sheared fabric. A radiocarbon date of >42,000 14C yr B.P. on a shell fragment in the lower diamicton suggests that this unit contains reworked sediments. The sedimentology, image analysis results, magnetic results, and radiocarbon age support the interpretation that this core contains glacial marine sediment overlying glacial till.