Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Late Quaternary Upper Slope Deepening (Fining) Upward Sequences Offshore the Great Barrier Reef, IODP 325 Expedition
(1) Earth Science MS-126, Rice University, Houston, TX.
(2) School of Geosciences, The University of Sydney, Sydney, NSW, Australia.
(3) Institut fuer Geowissenschaften, J. W. Goethe-Universitaet, Frankfurt am Main, Germany.
(4) Institute of Geophysics, School of Ocean Earth Science and Technology Hawaii, Honolulu, HI.
(5) Department of Ocean Engineering, University of Rhode Island, Narrangansett, RI.
(6) Geology Department, Western Washington University, Bellingham, WA.
(7) British Geological Survey, Edinburgh, United Kingdom.
Hole 325-M0058A (58A) is the deepest hole at 172 m in terms of current water depth, drilled during IODP 325 Expedition in spring 2010 along a seven drill site transect SE of Cairns offshore the Great Barrier Reef. The 41.4 m-long sedimentary sequence recovered in Hole 58A is mainly composed of three unconsolidated green mud sections intercalated with two distinct sandy intervals. In the muddy parts of the core, planktic foraminifera are very common in three levels characterized by highest reflectance values (or the lightest colors) combined with the lowest magnetic susceptibility values. The upper sand/grainstone section, at least 2 m thick, consists of fine to medium sand with large rock fragments, as big as cobble-sized, of well cemented grainstone and visible fragments of mollusks, bryozoa, coralline algae, echinoids, ‘Larger’ benthic foraminifera, and serpulids. The lower sand section is about 7 m thick and characterized by fine to medium sand. The observed lithologic cyclic pattern in Hole 58A is clearly illustrated in the color reflectance and the paleomagnetic magnetic susceptibility data.
The cyclic variations observed up the sedimentary section in Hole 58A are interpreted to represent deepening (fining) upward sequences, corresponding to the last two and one half glacial-interglacial cycles from Marine Isotope Stage MIS-7 to MIS-1. During glacial intervals, as Last Glacial Maximum and during MIS-6, a live coralgal reef had to be established in close vicinity of Hole 58A where the water depth was approximately 40 m at that time and was shedding coarse neritic material towards the site of Hole 58A. The low values of color reflectance, and the high values of the paleomagnetic magnetic susceptibility data can be explained by input of siliciclastics during intervals of sea level lowstands when the Queensland continental shelf was mostly exposed. Once the deglaciations (MIS-2 to 1, and MIS-6 to 5) were initiated, the coralgal reefs had to migrate westward and upward to keep up with the 120 m sea level rise. Coarse grain export from the reefs diminished, and only fine grain sediment produced on the reefs reached the location of Hole 58A, while proportions of pelagic sediment increased. Once the Queensland shelf was re-flooded, siliciclastic sediments, as today, were kept along the Australian shoreline and the sediments at Hole 58A became more carbonate-rich, explaining the high color reflectance values and the low magnetic susceptibility values.