4. GRAIN-SIZE DISTRIBUTION AND SIGNIFICANCE OF CLAY AND CLAY-SIZED MINERALS IN EOCENE TO HOLOCENE SEDIMENTS FROM SITES 918 AND 919 IN THE IRMINGER BASIN1

Kraig A. Heiden2,3 and Mary Anne Holmes2

ABSTRACT

Lower Eocene to Holocene sediments recovered from Ocean Drilling Program Sites 918 and 919 were studied to determine the grain-size distribution (sand to clay sizes) and mineralogy of the <2 Ám size fraction. The minerals are believed to be of detrital origin. The clay minerals consist of chlorite, smectite, illite, kaolinite, and a mixed-layer illite/smectite. Several non-clay minerals were identified as well, including quartz, plagioclase, alkali-feldspar, amphibole, pyroxene, zeolite, and calcite.

Relative abundances of the clay minerals were semiquantified using an oriented internal standard. Smectite abundances were found to increase with depth, while illite and chlorite abundances decrease with depth.

The Eocene sediments of Site 918 are characterized by a predominance of smectite with some kaolinite and very small amounts of chlorite and illite. This mineral assemblage is indicative of warm climatic conditions at the time of deposition. Oligocene sediments show an increase in chlorite and illite, suggesting that a sediment dam may have existed on the continental shelf, trapping these sediments and preventing their transport into the Irminger Basin, prior to this time. A warming trend in the early to middle Miocene is indicated by increased amounts of kaolinite. Variations in the relative amounts of chlorite and illite at this time may be the result of short-term eustatic sea level changes. Near the end of the Miocene, a cooling trend is indicated by increasing amounts of chlorite and illite relative to smectite abundance. This follows deposition of glauconitic hardgrounds at the top of a chalk unit with coarse sediment deposited above them, often with erosion of the underlying hardground. This interval is believed to coincide with the spillover of Arctic seawater into the Irminger Basin. The hardgrounds indicate hiatuses of short duration (thousands to tens of thousands of years) followed by high density turbidity currents or renewed chalk deposition. Non-clay minerals in the <2 Ám size fraction become common in upper Miocene to Holocene sediments of Sites 918 and 919, as glacial rock-flour that signals the onset of Greenland glaciation. Using R-mode factor analysis, the clay assemblages of Pliocene–Pleistocene age sediments were found to be significantly different from older sediments, with a dramatic transition at 457 mbsf, recording the beginning of significant Greenland glaciation.

1Saunders, A.D., Larsen, H.C., and Wise, S.W., Jr. (Eds.), 1998. Proc. ODP, Sci. Results,152: College Station, TX (Ocean Drilling Program).
2University of Nebraska-Lincoln, Department of Geology, 214 Bessey Hall, Lincoln, NE 68588-0340, U.S.A. mholmes@unlinfo.unl.edu
3Present address: Baker-Hughes, 1610 St. Etienne Rd., Broussard, LA 70518. kheiden@worldnet.att.net