Sedimentary Rhythms and Climatic Forcing of Pleistocene-Holocene Mixed Carbonate/Siliciclastic Sediments off the Great Barrier Reef

Abstract

Sediments recovered from the upper-slope and outer-shelf Great Barrier Reef transect (Ocean Drilling Program Leg 133, Sites 819 through 821) contain a high-resolution, 1.5-m.y. record of mixed siliciclastic and carbonate sedimentation. More than 30 multimeter-scale couplets of fining-and coarsening-upward sediments having varying proportions of carbonate and authigenic glauconite are present. These couplets stack into cyclic bundles observed in carbonate and magnetic susceptibility data that, in turn, vary systematically with respect to systems tract development deduced from sequence-stratigraphic analysis. At Site 821, these bundles and their variations clearly track relative changes in sea level for the past ~ 1.48 m.y. The data presented suggest that these various levels of cyclicity are expressed as the result of high sedimentation rates along the margin and that the dominant control on their development has been climate variations that modulated variations in sea level and terrigenous influx. The average duration of many of these cycles is in the Milankovitch waveband and is related to both fourth-(102-443 ka) and fifth-order (34-88 ka) cycles of sea-level change. These cycles are examined with respect to their lithology (color, percentage of total foraminifers, bioclasts, siliciclastics, authigenic glauconite, nannofossils, and tunicates), sequence stratigraphy, downhole log characteristics, magnetic susceptibility, and percentage of carbonate. Time series and Fourier transform results from high-resolution magnetic susceptibility and sonic velocity data suggest strong orbital forcing of the sedimentary cycles, although with differing intensities for different depositional packages and time frames. Magnetic susceptibility/carbonate cycles are interpreted as pulses of deposition of fine-grained siliciclastics during times of maximum flooding (along downlapped surfaces) and subsequent highstand progradation. These pulses dilute carbonate and contain fine-grained magnetically susceptible detrital minerals.

Gloria Knolls Slide: A prominent submarine landslide complex on the Great Barrier Reef margin of north-eastern Australia

“…In the GKS study area, no long sediment cores are directly available to assess the presence of weak layers. proportions of carbonates and siliciclastics, from clay-rich sediments with numerous silt intercalations to carbonate-rich bioclastic wackestones (Glenn et al, 1993). Interestingly, the presence of a slump appears with strong amplitude mounded reflectors at the top of the progradational seismic sequence 6 in site 819 (Davies et al, 1991b).…”

Milankovitch Cycles from Fourier Analysis of Logs from Sites 865 and 866

“…They used the wavelength of the eccentricity-modulated signals to determine sedimentation rates. Glenn et al (1993) investigated the translation of climate modulation by Milankovitch-like forcing into sedimentary cycles in mixed carbonate and siliciclastic sediments using sonic logs from Site 821 off of the Great Barrier Reef, Australia. Climate modulation at that site incorporates such factors as lags in ocean-climate response to orbitally produced changes in insolation, as well as independent changes in sediment supply (e.g., turbidites) and tectonic subsidence.…”