J. J. G. Reijmer1, L. Reuning1, and C. Betzler2
1GEOMAR, Kiel, Germany
2Hamburg University, Hamburg, Germany and University of Cambridge, U.K.
ABSTRACT: Sedimentation and Diagenesis of Slope Deposits on a Miocene Carbonate Ramp (Bahamas, ODP Leg 166)
Numerous small-scale depositional cycles are present in the Miocene sediments of seismic sequence m along the western leeward margin of Great Bahama Bank (ODP Leg 166; Eberli et al., 1997, Betzler et al., 1999). These cycles consist of decimetre- to meter-scale alternations between light-grey and dark-grey/black wackepackstones. The light-grey layers are well cemented and nearly uncompacted. They contain planktonic and benthic foraminifera, and bioclasts. Bioturbation in these layers is moderate. The darkgrey wacke-packstones are uncemented, strongly compacted and normally strongly bioturbated. The main components are planktonic foraminifera, fine-grained bioclasts. The dark layers are rich in aragonite and organic carbon and contain around 80 % carbonate. The light layers show low aragonite and organic carbon contents combined with carbonate values of up to 97.5 %. Light ∂13C and slightly negative ∂18O values were observed in the dark uncemented layers while the cemented intervals show heavy ∂13C values and slightly more positive ∂18O values. The differences in ∂18O between light and adjacent dark beds are with approximately 0.5‰ not as pronounced as in the study of Frank and Bernet (2000), who examined cyclic alternations at Site 1006 and 1007 of the same ODP Leg and found isotope differences of up to 2%. The carbon isotope signal between the dark and the light layers shows variations of up to 1.45 ‰. For time equivalent sediments at Sites 1007 Frank and Bernet (2000) record an isotope excursion to higher ∂13C values. They propose the combined effect of the Monterey Carbon Isotope Excursion (Vincent and Berger, 1985; Compton et al., 1990, 1993) and an increased input of platform carbonate to the slope as explanation for this increase. The absolute ∂13C values at Site 1003 are similar to the values in time-equivalent sediments of Site 1007. The relatively high absolute values can therefore be explained with the aforementioned reasons. Both ∂13C and ∂18O co-vary with carbonate content. The Formation MicroScanner images reflect the changes in carbonate mineralogy. The Natural Gamma-Ray Tool shows that variations in siliciclastic content of the examined succession displays a different frequency than the cyclic alternations in carbonate mineralogy.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado