TANIS, A MIXED MARINE-CONTINENTAL EVENT DEPOSIT AT THE KPG BOUNDARY IN NORTH DAKOTA CAUSED BY A SEICHE TRIGGERED BY SEISMIC WAVES OF THE CHICXULUB IMPACT

In the Little Missouri badlands of N. Dakota, an unusual clastic sequence within a river valley separates the Maastrichtian Hell Creek from the Paleocene Ft. Union fm, and is clearly connected to the Chicxulub impact by its content of characteristic glassy impact spherules. An eastward flowing meandering river builds a pointbar, about 11m thick, prograding into the river thalweg at the time of the Chicxulub impact. The clastic event deposit can be roughly divided into two units. The lower unit has a basal lag of conglomerate of ripped-up pebbles, followed by crossbedded sands, including small climbing ripples. This unit is restricted to the lower part of the river thalweg. The upper unit is a dominantly laminated alternation of fine sand laminae and sand flasers -also with climbing ripples- with mud layers, with a decreasing amount of sand upsection. Current indicators, like ripple migration, indicate a dominantly up-river, but also occasionally down-river flow. Both units, in particular the upper unit, contain a mixture of unusually well preserved marine and continental fossils. The unit contains a death assemblage of hunderds of fresh water fishes, sturgeon and paddlefish, with typical 1mm glassy spherules of the Chicxulub impact caught in the gill rakers¹. Other finds include charred tree trunks with amber blebs containing impact glass, dinosaur footprints filled with event deposits, and impact glass at several levels in the event deposit, including the Ir-rich tonstein that caps the event deposit. For the mode of deposition, two options are considered, both triggered by the Chicxulub impact. A tsunami could have reached Tanis from the Gulf of Mexico, but ETA would be days after impact, in conflict with the arrival within an hour of the impact glass spherules. The second option is that water of the remnants of the Western Interior Sea was set in motion in a seiche by a coupling to earthquake M11,5 S- or surface waves arriving in N Dakota within half an hour. In estimating the timing of the sequence of events, the occurrence of the impact spherules in the deposit is critical. The late arrival (?>1hour after impact) of unusually large (4-5mm) impact spherules found in small funnel-like pits in the second unit may play an important role in modeling distal ejecta from large impacts. 1DePalma et al, 2017, Science, in press.