IMPACT SPHERULES FROM THE CRATERS KANMARE AND TABBAN IN THE GULF OF CARPENTARIA

            Two crater candidates have been identified in the Gulf of Carpentaria as impact events that were the likely source of megastsunami waves responsible for the Holocene aged chevron dunes found on Groote Island and Van der Lin Island [1].  Using deep-sea cores from the Gulf of Carpentaria we show that the impact ejecta found in these samples is consistent with the proposed locations of the two source craters, Kanmare and Tabban.

            We measured the magnetic susceptibility of the samples at various depths to determine the likelihood of finding impact material within the sample.  We then sieved the samples into four size fractions (>250 microns,  >150 microns, > 63 microns, and > 38 microns).  Using a light microscope, we looked through the sieve fractions to find possible tektites, impact spherules, impact glasses, impact breccias, and impactor fragments.  We analyzed the picked items using the SEM and X-ray analyzer.  For some samples, we placed spherules on slides and measured their diameters using a petrographic microscope.

            We've found magnetic iron oxide impact spherules in three cores.  Most are round with quenched surfaces; some are tektite-shaped and appear ablated.  These spherules occur with pieces of basement rock, including siderite.  Impact glasses and merrillite, an extraterrestrial mineral, have also been found in these cores [2].  The size distribution of spherules from a sample in MD29 shows peaks at 85 microns and 125 microns. According to Melosh and Vickery the melt droplet size distribution from an impact should peak around the mean [3].  The location of core MD29 is such that it should contain impact ejecta from both proposed craters.  The two peaks indicate that two impacts are needed for making spherules with this size distribution.  The peaks at 85 microns and 125 microns can be attributed to 12km and 18km craters, respectively.  This implies that the size distribution and location of impact spherules is consistent with the location and proposed sizes of the Kanmare and Tabban craters.

[1].Abbott et al., 2006-this volume.  [2]. Elkinton et al., 2006-this volume

[3] Melosh, H.J., Vickery, A.M. (1991). Melt droplet formation in energetic impact events. Nature,350, 494-497.