2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 13
Presentation Time: 11:30 AM


MEYERS, Cassaundra A., Department of Geological Sciences, Florida State University, 108 Carraway Bldg, Tallahassee, FL 32306, ABBOTT, Dallas, Lamont-Doherty Earth Observatory, Palisades, NY 10964, TESTER, Edward W., Department of Physical Sciences, Kutztown University, P.O. Box 730, Kutztown, PA 19530 and BREGER, Dee, Dept. of Materials Science, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, cam07m@fsu.edu

We report here additional evidence supporting 1-2 ca. 1450 yrs. BP impacts in the Gulf of Carpentaria, Australia [1, 2, 3, 4]. Impact effects on marine microfossils have been noted in numerous events, such as the Chesapeake Bay, Eltanin, Ewing, and Alamo impacts [5, 6, 7, 8]. Impact-damaged microfossils may be useful in identifying target formations in marine impacts, as well as investigating impact-related processes such as microspherule and tektite formation.

We selected samples primarily from core GC04 (9.83°S, 135.35°E). We measured the magnetic susceptibility of the core, sieved samples into >150 μm, >63 μm, and >38 μm fractions, and examined the >150 μm samples under a binocular microscope. Samples of obvious microfossils were picked based on a melted appearance, and analyzed with EDAX as CaCO3. We found impact-related materials to 60 cm down, contrary to our calculations of a millimeters-thick ejecta layer. We examined the grains under an SEM for characteristic textures. A few samples were teardrop-shaped or elliptical, transparent, and vitreous in visible light, but analyzed as CaCO3. Splashes of native Fe and TiO2 were observed, some with a stacked appearance. Grains of rutile, melted apatite and ilmenite, zircon, SiO2, tin-rich BaSO4, and FeS were also found on various samples. Three microfossils exhibited quench textures. We identified one of these microfossils as G. ruber [10]; SEM imaging revealed melted on coccoliths, Fe splashes, and grooves. We also noted CaO in a separate sample, a breakdown product of CaCO3.

In addition to melted microfossils in GC04, we found SiO2 with triangular fractures, a candidate for shocked quartz. Microspherules were not abundant in the core, supporting our hypothesis of two low-angle impacts, which produce more distal ejecta [9]. It is our opinion that melted microfossils in the Gulf of Carpentaria are best explained as formed in impact-related processes.

1. Shulmeister & Lees, 1992. Geomo. (5): 521-534 2. Martos et al., 2006. GSA Abs. (38, #7): 299 3. Abbott et al., 2006. GSA Abs. (38, #7): 299 4. Elkinton et al., 2006. GSA Abs. (38, #7): 299 5. Nunes et al., 2002 GSA Abs. (34): 543 6. Edwards & Powars, 2003. Palaios (18): 275-285 7. Morrow et al., 2005. GSA S.P. 384 8. Kyte et al., 2006. AGU Fall Meet., Abs. B13B-1088 9. Tester et al., 2007 (this vol.) 10. Parker, 1962. Micropal. (8, #2): 219-254