Abstract:
Thin, well-defined condensed sections containing authigenic green minerals of the verdine facies occur in forereef depositional sequences of the Great Barrier Reef (GBR), Queensland, Australia. Pristine and reworked verdine grains within these sections formed in a mixed carbonate-siliciclastic depositional setting that accumulated during the last 1.5 m.y. in relatively warm waters. Seismic reflection, physical properties, and oxygen isotope data reveal that cyclic deposition occurred at ODP Site 821. Authigenic verdine grains are concentrated and most abundant along maximum flooding surfaces at the top of transgressive systems tracts and are also concentrated along marine flooding surfaces within transgressive systems tracts. These are notably "pristine" and commonly occur as relatively well-preserved green clay minerals that infill primary intraparticle porosity, especially within foraminifer tests. There are also verdine grains that show the effects of reworking and transport. These are usually not contained within foraminifer tests like the pristine verdine, but are abraded to fragmented, less abundant, and typically occur within highstand systems tracts. The reworked nature of these particles is attributed to a higher-energy depositional setting where loss of accommodation space, seaward progradation, and sediment reworking occur. Chemical data from electron microprobe analyses reveal low K (sub 2) O concentrations and X-ray diffraction data possibly indicate variations in the degree of verdine mineral maturity. Verdine is ferric-rich, but because it contains iron in both its oxidized and reduced states, we suggest that it forms in association with mildly reducing, suboxic solutions from which both ferric and ferrous iron may be supplied. Evolution of verdine is terminated shortly after burial. The relatively rapid formation of verdine facies minerals may have implications on reverse weathering and may play an important role in large scale biogeochemical cycling in the oceans. This facies may provide a sink for reverse weathering reactions involving Fe, Mg, and K in clay mineral formation. Although it has been previously suggested that in situ formation of verdine minerals is rare in water depths exceeding ca. 60 m and, due to its poor preservation potential, is rare in pre-Holocene sediments, we document here its formation at paleo-water depths of about 200 m within repetitive moderately condensed sections spanning the past approximately 1.5 m.y.