The Recognition of Hailstone Impressions In Clay-Rich Sediment: Experimental Results and Relation To the Neoproterozoic Case

Abstract

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.

Fossil hailstone imprints in Mesoproterozoic (1.7 Gyr) quartzite of the Tombador Formation (Chapada Diamantina Group), Brazil

“…Interestingly, there are dozens of quartzite flagstones from the same quarry along the same walkway described here that bear considerably smaller, raindrop patterns (approximately 3-5 mm in diameter and ~1 mm deep), which are quite distinct from hailstone impressions (Figure 4) and do not show significantly raised borders (Remin et al, 2014;Metz, R., 1981). It is interesting to note that the hailstone indentations in a large quartzite flagstone (Figure 2A) (approximately 70 x 80 cm) occupy only about two-thirds of that slabwhich is curious, because hailstorms would not be expected to have sharp "boundaries".…”

Palaeoenvironmental and sequence stratigraphic analyses of the Jurassic Datta Formation, Salt Range, Pakistan

“…Such conditions are common in the upper flow regime during flooding in the upper part of the channel margin environments, natural levees, crevasse splays, and other flood plain deposits (Miall 1985;Willis 1989;Boggs 2009). The presence of mudcracks, bioturbation, and raindrop imprints suggest subaerial conditions (Remin et al 2014). These inferences strengthen the deposition of this lithofacies in the channel margin near floodplain environments.…”

Experimental rain prints and gas escape structures as a framework for interpreting circular imprints in shales of the Pottsville Formation (Pennsylvanian, Alabama)

“…The results of these experiments suggest that rain prints would be rare in saturated sediment and absent in subaqueous conditions and might be less common in muddy sediment. However, rain prints with other key features, such as spatter, have been observed to form in mud (Remin et al, 2014); rain prints also occur, albeit rarely, in Pottsville shales (cf. Buta & Crocker, 2005; Buta & Kopaska‐Merkel, 2016), suggesting that these experiments did not capture the full range of natural conditions.…”

“… Variable pit size across a surface, generally between 5 and 20 mm, although hail prints have the potential to produce a larger range of sizes (Remin et al, 2014). Correlation between pit diameter and depth, as larger features represent impact of larger droplets. Prints occur only in concave epirelief and convex hyporelief. Structures commonly overlap. Spatter is common (Remin et al, 2014), although it need not occur on every impact mark. Surfaces with rain prints may contain other subaerial exposure features. …”

“…• Variable pit size across a surface, generally between 5 and 20 mm, although hail prints have the potential to produce a larger range of sizes (Remin et al, 2014 (Remin et al, 2014), although it need not occur on every impact mark. • Surfaces with rain prints may contain other subaerial exposure features.…”