Ra. Pickrill, SHALLOW SEISMIC STRATIGRAPHY AND POCKMARKS OF A HYDROTHERMALLY INFLUENCED LAKE, LAKE ROTOITI, NEW-ZEALAND, Sedimentology, 40(5), 1993, pp. 813-828
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.