Structural study of Cactus Crater

Abstract

The detailed structure of Cactus Crater, a 105 m diameter nuclear explosion crater formed in water-saturated carbonate rock of Eniwetok Atoll, is delineated using the high to low Mg calcite diagenetic transition as a stratigraphic tracer. Outside Cactus, this transition is observed as a discontinuous horizon which appears to be depressed, possibly as a result of the cratering event, near the crater. Beneath the crater, this transition occurs gradually over a 4.5 ± 0.5 m interval, leading to the following conclusions: material inside Cactus Crater underwent primarily in situ brecciation and mixing, the maximum depth of the excavation cavity is 20 m below sea level, and a fallback breccia lens, if it exists, has a maximum thickness of l m. A central uplift of 4.5 ± 0.5 m is inferred from the observation that the transition interval occurs at a 4 to 5 m greater depth at 1/2 crater radius than in the center. The excavation process, deduced from the Mg calcite transition as well as gamma well log data, involves high velocity injection of strongly shocked material to form the excavation cavity lining. The in situ brecciation and mixing appears to be a turbulent process, probably facilitated by fluidization of the carbonate rock. Based on the Mg calcite transition patterns beneath the crater floor, dynamic rebound is inferred as the modification mechanism for Cactus Crater. Using the Melosh Bingham model for dynamic rebound, a maximum strength of ~1 bar is inferred for the cratered carbonate medium. This strength value is representative of clays, such as those in which Snowball, a chemical explosion crater having dimensions and features similar to Cactus', was formed. Comparisons between Cactus and meteorite impact craters are also presented.