Abstract:
Aside from its significance in establishing the impact-mass extinction paradigm, the Chicxulub crater will probably come to exemplify the structure of large complex craters. Much of Chicxulub's structure may be mapped' by tying its gravity expression to seismic-reflection profiles revealing an [~]180 km diameter for the now-buried crater. The distribution of karst topography aids in outlining the peripheral crater structure as also revealed by the horizontal gradient of the gravity anomaly. The fracturing inferred to control groundwater flow is apparently related to subsidence of the crater fill. Modelling the crater's gravity expression based on a schematic structural model reveals that the crater fill is also responsible for the majority of the negative anomaly. The crater's melt sheet and central structural uplift are the other significant contributors to its gravity expression. The Chicxulub impact released [~]1.2 x 1031 ergs based on the observed collapsed disruption cavity of [~]86 km diameter reconstructed to an apparent disruption cavity (Dad) of [~]94 km diameter (equivalent to the excavation cavity) and an apparent transient cavity (Dat) of [~]80 km diameter. This impact energy, together with the observed [~]2 x 1011 g global Ir fluence in the Cretaceous-Tertiary (K-T) fireball layer indicates that the impactor was a comet estimated as massing [~]1.8 x 1018 g of [~]16.5 km diameter assuming a 0.6 gcm-3 density. Dust-induced darkness and cold, wind, giant waves, thermal pulses from the impact fireball and re-entering ejecta, acid rain, ozone-layer depletion, cooling from stratospheric aerosols, H2O greenhouse, CO2 greenhouse, poisons and mutagens, and oscillatory climate have been proposed as deleterious environmental effects of the Chicxulub impact with durations ranging from a few minutes to a million years. This succession of effects defines a temperature curve that is characteristic of large impacts. Although some patterns may be recognized in the K-T extinctions, and the survivorship rules changed across the boundary, relating specific environmental effects to species' extinctions is not yet possible. Geochemical records across the boundary support the occurrence a prompt thermal pulse, acid rain and a [~]5000 year-long greenhouse. The period of extinctions seems to extend into the earliest Tertiary