SURFACE TEXTURAL ANALYSIS OF QUARTZ SAND GRAINS FROM ODP SITE-918 OFFTHE SOUTHEAST COAST OF GREENLAND SUGGESTS GLACIATION OF SOUTHERN GREENLAND AT 11 MA

Shipboard analysis of the 1183-m sedimentary section recovered at Site 918 in the Irminger Basin during Ocean Drilling Program Leg 152 revea led material of glacial origin (diamictons, ice-rafted debris (IRD) an d dropstones) as deep as 543 m below sea floor (bsf). The sediment con taining the deepest dropstone was biostratigraphically dated shipboard as approximately 7 Ma, pushing back the date for the onset of glaciat ion on southern Greenland by 5 Ma. Thin layers of fine sand were found as much as 60 m deeper in the core, raising the possibility of an eve n earlier date for glaciation. To determine the sedimentary history of these deeper sand layers, the surface textures on quartz grains from eleven cores bracketing the interval of interest were analyzed by scan ning electron microscope. The results suggest that the grains in the 6 0-m interval below the deepest dropstone have a glacial history. At th at level, an Ii-Ma Sr-isotope date was obtained from planktonic forami nifers. This late Miocene timing is supported biostratigraphically by both nannofossil and foraminifer assemblages, indicating a new minimum age for the onset of glaciation on southern Greenland and in the Nort h Atlantic. (C) 1997 Elsevier Science B.V.