Computer modelling of the water resurge at a marine impact:: the Lockne crater, Sweden

When a cosmic impact occurs at sea, the resulting crater can form completely in the water mass or extend into the seafloor. At intermediate to great water depths, the collapse of the water cavity generates a forceful resurge of water along the seafloor towards the central part of the crater. The erosive effects as well as certain conspicuous deposits from resurge flows are known from several craters. This study uses numerical modelling to estimate the magnitude of the resurge in the case of the 455 Myr old marine-target Lockne impact crater, Sweden. The water depth at the Lockne target site was at least 200 m, possibly even as great as I km. Field evidence show that the resurge was strong enough to erode gullies hundreds of metres wide, tens of metres deep, and kilometres long. With computer modelling we studied the behaviour of the flow to get a notion of its magnitude. The calculations required simplification of the reality. We used a fixed seafloor topography and pure water. A rim in the water mass is expected but was neglected in the calculations. The simplification likely led to underestimation of the magnitude of the resurge. The effect of different target water depths was studied. At 200-m target water depth, about 1.2 x 10(11) m(3) of water were needed to fill the Lockne crater, including the water cavity. This took about 2200 s. It has been suggested that the main gully formation occurs when the resurge passes the inner rim of the crater. That phase lasts for about 700 s. With a 500-m target water depth, these processes are 3-4 times faster. The erosive force of the flow (i.e. unit stream power) is 1.9 x 10(5) (Wm(-2)) in the 200-m water depth model. This is of the same magnitude as the Lake Missoula Flood, the greatest catastrophic flooding known on Earth. At 500-m target water, the unit stream power is a magnitude greater. (C) 2002 Elsevier Science Ltd. All rights reserved.