SHALLOW SEISMIC STRATIGRAPHY AND POCKMARKS OF A HYDROTHERMALLY INFLUENCED LAKE, LAKE ROTOITI, NEW-ZEALAND

Lake Rotoiti formed 11 850-20 000 years ago when lava flows dammed the drainage system through the Okataina caldera, one of the major rhyoli tic centres of the intracontinental, back-arc, Taupo Volcanic Zone of North Island, New Zealand. The lake morphology reflects this complex o rigin, with remnants of the old caldera rim, the lava dam and relict r iver system being preserved. High resolution seismic reflection profil es (7 kHz) show the drowned river system channelled inflowing water in to the lake basin, scouring and depositing sediments and maintaining a n active sublacustrine channel in the former river valley. Airfall tep hras are preserved in lake sediments. The tephrachronology, establishe d from the surrounding catchments, can be correlated with the siesmic stratigraphy for eight major eruptions over the last 11 850 years. Hyd rothermal fields underlying the lake generate gaseous sediments which mask seismic penetration. This is indistinguishable from seismic maski ng caused by biogenic gas accumulation within the sediments. Venting o f hydrothermal gases at the surface creates pockmarks. From the seismi c stratigraphy beneath pockmarks seven stages of pockmark genesis have been identified which show pockmarks grow, decay, migrate and persist over several thousand years.