FORCEFULLY INJECTED CLASTIC DIKES AND SILLS ASSOCIATED WITH THE K/T IMPACT TSUNAMI

Tsunami deposits associated with the K/T impact are characterized by deposits of coarse sand and conglomerate generated by the passage of the wave. At several K/T boundary localities bordering the Gulf of Mexico, multiple layers of sand have been interpreted as evidence for repeated wave arrivals caused by reflection of the impact-generated tsunami or multiple waves generated by complex processes associated with crater formation. The K/T section exposed along the Tombigbee River at Moscow Landing in western Alabama is unusual in that contains abundant sand dikes and sills, some of which contain large (tens of centimeters) clasts of Cretaceous limestone. The dikes and sills originated in the post-impact basal Clayton sand and were injected downward into the underlying Cretaceous Prairie Bluff chalk. Several features of the dikes and sills indicate that they were forcefully injected as opposed to having passively filled an open fissure. The formation of a sill requires that the overlying rock is lifted to accommodate the fill material. Most of the observed sills are sand-filled and are several centimeters thick and 10-20 centimeters long. Although vertical clastic dikes may have filled open fissures, one 0.5 meter thick dike dips about 45° and therefore must have been injected with enough force to have partially lifted the overlying block of rock. Evidence of forceful injection of this dike is also provided by a 20-centimeter thick zone of sheared chalk adjacent to the dike. Passive infilling would not shear the walls of the host rock. Forceful injection of clastic dikes is a common feature of earthquake-induced liquefaction, but these kinds of structures form by upward injection of pressurized sand, not downward. The mechanism by which the clastic dikes were injected at Moscow Landing is unclear, however one possibility is that the sand was forced into the underlying sediments by a breaking tsunami wave. Maximum wave heights were about 200 meters in this area. The abrupt transition from open ocean to shallow shelf would have caused the wave to break and the impact of the cascading water could have forced sand into the underlying sediments.