- Copyright © 2014, SEPM (Society for Sedimentary Geology)
Based on experimental impacts of hailstone and rain droplets on clayey sediment, distinct differences between their imprints are shown. The key difference that unequivocally differentiates these impact structures is that raindrop imprints are always part of a sphere, even after different degrees of compaction, whereas hailstone imprints never have such a shape, forming a more or less regular funnel, terminated by a “nipple-like” structure that usually retains the original shape of the hailstone. We show and discuss the reasons why hailstone imprints have never been reported in the sedimentary record, albeit their fossilization potential seems to be much higher than that of raindrop imprints, which are commonly reported from various types of sedimentary rocks of widely differing ages. Inferred hailstone imprints are illustrated from the fossil record for the first time, from the Neoproterozoic–Cambrian transition of western Africa.
The origin of paleohailstones required specific dynamic conditions of the atmosphere similar to those of the Recent atmosphere. Present-day hailstorms are generally restricted to specific geographic regions, e.g., mid-latitude, along mountain ranges, and they are largely absent from circum-polar and circum-equatorial areas. Correspondingly, hailstone impact structures preserved in the sedimentary record may possess interpretational value, regarding fundamental questions concerning the dynamics of the paleoatmosphere and perhaps paleogeography of ancient continents.