Journal of Sedimentary Research, 2014, v. 84, 543–551
Research Article
DOI: http://dx.doi.org/10.2110/jsr.2014.46
THE RECOGNITION OF HAILSTONE IMPRESSIONS IN CLAY-RICH SEDIMENT: EXPERIMENTAL RESULTS
AND RELATION TO THE NEOPROTEROZOIC CASE
ZBYSZEK REMIN, TOMASZ KROGULEC, TOMASZ DRELA, AND MICHAŁ SUROWSKI
Faculty of Geology, University of Warsaw, Żwirki i Wigury 93, 02-089 Warszawa, Poland
e-mail: zbyh@uw.edu.pl
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.
INTRODUCTION
Raindrop or raindrop-like imprints are common structures illustrated
in almost every sedimentology textbook, albeit such illustrations
commonly lack detailed descriptions. Structures interpreted as raindrop
imprints have been recorded from different types of sedimentary rock of
various ages, as deep in the past as the Archean (2.7 Ga) volcanic ash of
the Ventersdorp Supergroup, South Africa (Van der Westhuizen et al.
1989; Som et al. 2012). The structures generated by falling hailstones,
with the exception of some shy suggestions (Lyell 1851; Twenhofel 1932;
Fenton and Fenton 1933), are not recognized in the more recent scientific
literature. This is surprising, since the fossilization potential of hailstone
imprints—deeper and larger structures—should be much higher than
those of raindrops. The potential of using hailstone imprints as an
indicator of paleoatmosphere dynamics, past climates, and paleogeography has hitherto been unexplored and, up to now, detailed studies of
hailstone imprints have never been undertaken.
In this study, for the first time, we can clearly show the distinct
differences between hailstone imprints and raindrop imprints on the basis
of experimental data. We also infer that Neoproterozoic impressions
previously interpreted as raindrop imprints are actually hailstone
impressions and thus are able to suggest that the present-day dynamics
of the atmosphere necessary to produce hailstones existed as far back as
the Neoproterozoic–Cambrian transition.
Since the original description by Cunningham (1839), pit-like structures
or small craters have been interpreted as the impressions of falling rain.
However, their origin has been a matter of dispute and has been
repeatedly questioned. Some scientists (e.g., Redfield 1842; Lyell 1851;
Clark 1979; Metz 1981) retained Cunningham’s original concept of a
Published Online: July 2014
Copyright E 2014, SEPM (Society for Sedimentary Geology)
raindrop origin of such structures. Other workers, such as Buckland
(1842), and especially Twenhofel (1932) and Moussa (1974), generally
rejected this interpretation and argued in favor of other factors being
responsible for the production of so-called raindrop impressions. These
workers mainly appealed to the idea of air bubbles rising through the
sediment, resulting in the formation of gas-escape structures. A
comprehensive overview of crater-like structures in sediments is provided
by Moussa (1974).
The aim of this paper is three-fold: 1) to present a detailed description
and comparison of raindrop imprints and hailstone imprints formed
experimentally in clayey sediment; 2) to indicate potential discriminating
features of those impressions in the sedimentary record, based on both
laboratory and field observation; and 3) to discuss why hailstone imprints
have not been recognized previously in the fossil record.
PRESERVATION OF HAILSTONE AND RAINDROP IMPRINTS
There are several conditions that must be met in order to preserve
possible hailstone or raindrop imprints:
1)
2)
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The surface upon which a hailstone or raindrop falls should be of
suitable plasticity—sufficiently soft to be affected by falling droplets
or lumps of ice, but firm enough to retain their shape afterwards.
Sediment that is too soft will collapse and settle back to a flat or
almost flat surface after the impact. Similarly, a substrate that is too
firm will make the impact structures subtle and difficult to interpret
(e.g., lacking the external rim).
The precipitation should be of rather short-term duration, like
summer showers of low intensity. Heavy rain or hail will destroy the
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FIG. 1.—A) Raindrop imprints on a clayey
substrate formed by droplets 5 mm in diameter.
Note their generally circular outline. ER, external rim; ST, spatter tongue; DD, double droplet
print. Raindrop imprints in the upper part of the
photo are formed in a more water-rich sediment,
which caused the collapse of many of the rain
prints, making them less pronounced and more
difficult to interpret and recognize. B) Close-up
of raindrop print showing the spatter tongue.
3)
imprints created, and the rain will cause the sediment to soften to the
extent that it collapses to a flat surface, thereby preventing the
preservation of discrete impact structures.
Once the sediment dries, the impacted surface must be gently buried
to prevent destruction of the impressions.
All of these preconditions are met in drying puddles and ponds and
along continental (especially areas of fluvial floodplains, lakes) and
marine (mudflats) shorelines, where clayey sediment is cyclically supplied,
forming succeeding planes of suitable plasticity.
Additionally, Recent and fossil raindrop impressions are very often
associated with mudcracks and animal traces, because all of these features
require a similar grain size and substrate consistency for their best
expression and preservation. Both raindrop impressions and ichnofossils,
if preserved, typically occur on the same surface, and hence the presence
of mudcracks and animal traces should draw our attention to where we
should look for possible hailstone imprints in the sedimentary record.
Conditions that favor the preservation of rain and hail impact
structures can be found around almost every drying water puddle filled
with clayey or fine-grained sediment, as well as the margins of lakes and
rivers, drying flood areas, tidal flats, or alluvial plains. Depending on the
size of the puddle, drying may take days or weeks rather than hours. The
zone of moist substrate of suitable plasticity moves towards the center of
the puddle, enabling the formation of rain and hailstone imprints for
quite a long time before final drying. Fine-grained substrates have higher
imprint formation and preservation potential.
ATMOSPHERIC CONDITIONS
In the present-day atmosphere, the size of raindrops can reach up to 5–
7 mm in diameter; larger raindrops seem to be extremely rare as a simple
consequence of atmospheric pressure and surface tension (Willis and
Tattelman 1989). Growing to the maximum size and reaching terminal
velocity, raindrops lose their oval shape and become increasingly
flattened (Spilhaus 1948; Magono 1954). At the terminal velocity and
critical size, they have a tendency to break up, forming two separate
droplets which start to grow again (Dodd 1960; Matthews and Mason
1964). The terminal velocity for the largest water droplets, if not
influenced by wind, is estimated to be approximately 10 m/s21 (e.g.,
Spilhaus 1948; Foote and du Toit 1969).
The formation of hailstones, by contrast, is more complex. Hailstones
are produced within large and thick cumulonimbus clouds containing
liquid water, with the temperature of part of the clouds lying below the
freezing point (Hand and Cappelluti 2011). In such thunderstorm cells,
single or multiple strong updrafts enable the formation of hailstones in the
upper, colder part of the troposphere. Typically, hailstones are a few
millimeters (. 5 mm) to a few centimeters in diameter; however, in
extreme atmospheric conditions within super thunderstorm cells, hailstones
can grow up to ca. 15–20 cm across. Hailstones are irregular in shape rather
than truly spherical balls or lumps of ice. Their surfaces are generally more
or less ornamented by spines, ridges, or other irregularities. The terminal
velocity of hailstone particles is higher than that of raindrops and ranges
from 8–10 m/s21 for a 5 mm hailstone to 15–18 m/s21 for a 20 mm hailstone
(the size used in the experiment) and even much higher for the largest
hailstones (e.g., Khvorostyanov and Curry 2005).
Experimentation
Our experiments imitated the natural fall of raindrops and hailstones
onto a substrate that provides the best chance of preserving the resulting
impact. The focus was on modeling the most typical raindrop and
hailstone impressions produced on a selected clayey substrate (Figs. 1, 2).
Additionally, using graphics software, we simulated how the cross-section
of these structures should look after two-dimensional vertical compaction
by a factor of 50% and 75% (Fig. 3).
To maintain objectivity in producing experimental hailstone and
raindrop imprints, we tried to achieve the same levels of substrate
saturation by water, although the consistency of the substrate varied
slightly between succeeding simulations.
SUMMARY OF METHODS
Two plastic containers [ca. 70 cm 3 40 cm 3 20 cm] were filled with a
15 cm thickness of muddy sediment collected from a dried water puddle in
which recent rain imprints were preserved. The sediment was irrigated
and mixed until a uniformly flat surface was produced using an ordinary
putty knife.
To produce the raindrop and hailstone impressions, a natural rain
shower and a hailstone fall were imitated. Rain droplets and hailstones of
known size (Table 1) were released from a height of 18 m inside a
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FIG. 2.—Hailstone imprints of different sizes in a clayey substrate. A) Note the funnel-like shape of the largest imprint and the well-expressed external rim. B, C) Closeup of hailstone impact structures. Arrows point to an empty cavity inside the external rim. D) Imprint made in a firmer substrate. Note that the external rim is smaller or
absent (see arrows); SP, splashes; HI5, 10, 20, hailstone imprints made by hailstones 5, 10, and 20 mm in diameter; RI, raindrop imprint made by droplets less than 5 mm
in diameter.
building. Such a height is more than sufficient to allow the rain droplets
and small hailstones (5–20 mm diameter) used in the experiment reach the
terminal velocity. Such conditions allowed us to exclude the influence of
wind, which can markedly affect the maximum velocity of hailstones and
raindrops. The size of the rain droplets used in the experiment was 5 mm,
and the droplets were produced using an automatic pipette. The artificial
hailstones were 5 mm, 10 mm, and 20 mm in size produced in a freezer.
The artificial hailstone and raindrop imprints that were produced were
allowed to dry slowly, typically for a couple of days. They were then
measured and plaster-casts were made of selected examples (Figs. 4, 5).
FIG. 3.—Measurements of raindrop and hailstone imprints. Rd, rain-print depth; RD, rainprint diameter; Hd, hailstone imprint depth; HD,
hailstone imprint diameter; ERh, external rim
height; ERw, external rim width. Note that the
raindrop imprint, in contrast to the hailstone
imprint, can easily be inscribed into a circle,
regardless of the degree of compaction. For a
hailstone imprint this is not possible (dotted
circle); not to scale.
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TABLE 1.— Statistical data of experimental raindrop and hailstone imprints.
Diameter* [mm]
Depth* [mm] %**
Ø 5 mm [n*** 5 143]
Raindrop
12.70
3.52
27.70%
hailstone
Ø 5 mm [n 5 37]
17.55
Ø 10 mm [n 5 17]
32.13
Ø 20 mm [n 5 17]
69.26
10.51
59.90%
20.12
67.60%
59.84
86.40%
External Rim Height* [mm] %**
, 1.00
, 10%
2.75
15.80%
4.62
14.40%
11.01
15.90%
External Rim Width* [mm] %**
, 1.00
, 10%
3.22
18.30%
6.06
18.90%
15.92
23.00%
* – mean value.
** – % of particular feature in relation to diameter.
*** – number of observations.
Interestingly, the original shape of the ice impactor used in the
experiment (ordinary ice shards, rounded cubes, or more or less spherical
ice particles) is generally of no importance on the final shape of the
impact structure. The general oval shape of the imprint, together with a
well-defined external rim, was always repeated independent of the shape
of the ice shard used in the simulation. A pseudo-cubic shape was chosen
to emphasize the nipple-like structure that retains the original shape of
the ice impactor (Fig. 4A–D).
RESULTS
Hailstone and raindrop imprints are impact structures induced by
completely different bodies—solid and fluid respectively. As a result,
different impact geometries should be expected. Our comparative study
reveals remarkable differences between artificial hailstone and raindrop
impressions made under controlled conditions.
Raindrop Imprints
The geometry of a raindrop imprint consists of two elements: i) a
central depression and ii) an external rim. All of the raindrop impressions
are broadly circular in outline, with a rather poorly expressed solid
external rim that surrounds the central depression (Fig. 1). The
depression is shallow and geometrically represents part of a sphere
(Fig. 3). The only exception is when another droplet falls in exactly the
same place as a previous one—in this case the resulting imprint is deeper,
taking the shape of a nipple. Only in this situation can the raindrop
imprint be similar to a hailstone imprint (Fig. 1).
The raindrop impact depth/diameter ratio does not exceed 0.27 in the
experiments conducted and constitutes a significant discriminator to
hailstone imprints, for which the ratio is always higher than 0.6 (Table 1).
The ratio could be smaller, with a decrease in the size of the droplets,
decreased substrate plasticity, or if the rain print is modified by
compaction (Fig. 3).
The height and width of the external rim is variable, but is generally
10% of the diameter or less (Fig. 1, Table 1). When the sediment is
sufficiently slurried but still of suitable plasticity, the raindrop impressions are additionally characterized by spatter tongues that branch out
from the external rim, and typically have a ‘‘tongue-like’’ shape (Fig. 1).
Hailstone Imprints
Hailstone imprints consist of three elements: i) a central depression, ii)
an external rim, and iii) a nipple-like structure at the base of the central
depression. Hailstone imprints generally have a circular or oval shape,
with a well or very well expressed external rim that is high, rounded,
broad (Figs. 2–5), and elevated markedly higher in relation to diameter
compared to raindrop imprints. The external rim surrounds a deep,
funnel-like central depression with more or less irregular, steep walls
terminated by a nipple-like structure (Figs. 4, 5). The depression is never
part of a sphere, and this is the most important distinction between
impressions of raindrops and hailstones (compare Figs. 1 and 2 with
Figs. 3–5).
The mean diameter of the impact crater for the studied size-ranges of
hailstone has a linear relationship to the original size of the hailstone with
a very high correlation coefficient (Fig. 6; r 2 5 0.97). However, the
crater diameter varies from 2.2 to 4.5 times the diameter of the impactor.
This relatively large scatter in data is related to the saturation level of the
target sediment, not with the shape of the ice impactor.
Similarly, the diameter of the crater is highly correlated with its depth
(Fig. 7; r 2 5 0.92) and width of the external rim (Fig. 8; r 2 5 0.94) for
the size-ranges of hailstone used in the experiments (5, 10, and 20 mm).
The depth/diameter ratio ranges from 0.6 (for the smallest hailstones,
diameter (Ø) 5 5 mm) to 0.86 (Ø 5 20-mm-sized lumps of ice; see
Table 1). In some cases, the depth can be as large as or even larger than
the diameter. The latter situation occurs only in soft, plastic substrates
that are thick enough not to affect the maximum depth of the hailstone
penetration. Therefore the minimum sediment thickness for the formation
of undisturbed imprints can be estimated for at least 20 mm for the
smallest hailstone (Ø 5 5 mm), since they penetrated sediment up to
17 mm depth. On the other hand, sediment 10–15 cm thick was enough
for formation of undisturbed impact structures made by a hailstone
20 mm in diameter. However, for the larger ice impactors (50–100 mm),
which occur in nature, the original thickness of sediment should be much
larger (i.e., 20–50 cm or even more for the largest hailstones).
The height of the external rim is similar in all experimental cases, and
varies around 15% of the hailstone imprint diameter (Table 1). The width
of the external rim becomes larger with increasing hailstone diameter,
being 18.3% of the diameter for 5-mm-sized ice particles to a maximum of
23% for the largest hailstones used in the experiment (Ø 5 20 mm;
Table 1).
The splashes that branch out of the external rim are of random
orientation and irregular shape, being spatters of substrate rather than a
continuous spatter tongue typical of raindrop imprints (Figs. 1, 2). In
firmer or coarser sediment, the splashes made by hailstone impacts are
less pronounced or even absent (Fig. 2D), mimicking the morphology of
a rain print.
After melting, the overall shape of the original hailstone, which is
usually irregular, can be retained and cemented in the drying sediment,
forming a specific nipple-like structure at the bottom of the central
depression (Fig. 4). This feature, if preserved, is characteristic only of
hailstone impressions.
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FIG. 4.—Original molds and idealized drawings of hailstone imprints. The imprints are
produced by lumps of ice of the following
diameters: 20 mm (A1, A2); 10 mm (B1, B2; C1,
C2) and 5 mm (D1, D2). Note the nipple-like
structures at the top of each mold, mimicking the
original shape of the hailstone.
Centimeter-size or larger hailstones also may produce an external rim
which can be empty inside (Fig. 2A–C). In such a case, the external rim is
composed of thin layers of ejecta that curve over in such a way that they
touch or almost touch the substrate, leaving a cavity inside and clearly
distinguishing it from the solid external rim characteristic of raindrop
impressions. As the plasticity of the substrate decreases, the formation of
the classical broad and high external rim with an internal cavity (Fig. 2D)
is inhibited—in such a case, the height and width of the external rim is less
prominent and the central depression is shallower. Moreover, on a firm
but plastic substrate, the hailstone impression can only be expressed as a
simple, relatively shallow imprint completely devoid of an external rim
(Fig. 2D). In such cases, the imprint will reflect the original shape of the
hailstone, having a diameter similar to that of the causative ice particle
(Fig. 2D). The only structures that could resemble hailstone imprints are
gas-escape or fluid-escape structures. The latter, however, are characterized by a shallower central depression and a distinctly lower and wider
external rim that is not of impact origin, such as those illustrated by
Dornbos et al. (2007).
Despite significant scatter in the hailstone-impression data, it is clear
that there are measurable differences between hailstone impressions and
raindrop impressions that would allow recognition of hailstone impressions where a suitable substrate was present.
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FIG. 5.—A) Drying mud affected by hailstone
impacts. Note that mudcracks propagate from or
through the imprints. In the rock record this
could significantly hinder their recognition. HI5, 10,
hailstone imprints made by hailstones 5 and
10 mm in diameter; RI5, raindrop imprint caused
by droplet 5 mm in diameter. B) Close-up of
hailstone imprints affected by desiccation cracks.
THE APPARENT ABSENCE OF HAILSTONE IMPRINTS IN THE FOSSIL RECORD
Although hailstone imprints seem to have a markedly higher
fossilization potential than raindrop impressions, the former have never
been described. There are several reasons why hailstone imprints have not
been reported from the sedimentary record.
First, the thunderstorms in which hailstones originate are less common
and reflect short-term weather phenomena when compared to rain
showers. The duration of a hailstorm at any given locality is generally less
than a few minutes, while rainstorms may persist for days. Additionally,
hailstorms are often followed by rainfall of different duration, which can
destroy the already formed hailstone impressions. Moreover, the
thunderstorm must form over areas where a substrate of suitable
plasticity occurs, if any impressions are likely to be formed and preserved.
These prerequisite conditions serve to limit the probability of formation
and preservation of hailstone impact structures. Nevertheless, drying
puddles and especially large areas of floodplains or mudflats could
maintain appropriate sediment plasticity for quite a long time, enabling
formation of the impact structure.
Second, raindrop impressions, and possibly hailstone imprints as well,
are commonly associated with desiccation cracks. It is readily observable
in the field that the nucleation of mudcracks generally starts to propagate
from the points where the substrate was disturbed and its original
cohesiveness affected and weakened by, e.g., biogenic structures such as
tracks and trails, objects lying on desiccation planes, or by protruding
gravels. Therefore, even if the hailstone falls on a clayey substrate, the
sediment affected by its impact becomes the best place for mudcrack
nucleation (Fig. 5). Most likely, because the impact craters affect the
cohesiveness of substrate, it allows initiation of mudcrack formation and/
or its propagation through the impact structures. Consequently, hailstone
craters would tend to be destroyed, hindering their preservation and
identification, as exemplified in Figure 5.
Third, hailstone imprints can simply be misinterpreted. If the empty
cavity inside the rim (Fig. 2B, C) becomes filled with sediment, it will
produce a mold of a different shape than expected, i.e., lacking the
impression of external rim, making it difficult to interpret as an impact
structure of hailstone origin. This, together with a funnel-shaped
depression of irregular shape with a nipple-like structure at the bottom,
may even be misidentified as biogenic instead of mechanical.
Fourth, paleoatmospheric conditions may have been significantly
different from those of the present atmosphere. These differences may
have markedly influenced the formation of hailstones, but their influence
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HAILSTONE IMPRESSIONS
FIG. 6.—Scatterplot and regression of the hailstone diameter (5, 10, and 20 mm)
and the diameter of the impact crater based on experimental data.
is unknown. Such factors include: i) different density and different
thickness of the troposphere, suggested for example for the Faint Young
Sun paradox times (e.g., Kasting 1987; Rosing et al. 2010; Som at al.
2012); ii) the presence or absence of greenhouse conditions coupled with
different temperature distributions, such as those that occurred during
Devonian or Permian time; and lastly iii) different paleogeographic
architecture influencing the atmospheric circulation.
Finally, hailstone imprints have never been described in detail and
differentiated from other physical or biogenic structures. It is often the
case that what is neither identified nor described simply ‘‘does not exist.’’
An example of this is the case of hummocky cross-stratification.
Following its recognition and detailed description of the physical
processes involved in its origin (Harms et al. 1975), this structure has
been recorded from thousands of sites of various ages all over the world.
We suggest that a similar situation may present itself in the case of
hailstone impact structures, now that they have been described and
differentiated from other structures.
FIG. 7.—Scatterplot and regression of the crater diameter and the crater depth
based on the experimental data.
549
FIG. 8.—Scatterplot and regression of the crater diameter and the width of the
external rim together with error lines. Blue dots relates to experimental data; red
triangles are original measurements of fossil examples (compare Fig. 9).
NEOPROTEROZOIC HAILSTONE IMPRINTS FROM MAURITANIA
Álvaro (2012) described raindrop imprints from the sabkha deposits
bounded by proximal braided-fluvial and distal shoal complexes of the
Neoproterozoic–Cambrian age of the Adrar of Mauritania (western
Africa; West African Craton). These imprints occur within a shallowingupwards sedimentary cycle of the upper Ediacaran Nouatil Group,
terminated by playa-lake deposits with evaporites. According to Álvaro
(2012), this interval is dominated by shales displaying irregular, decimeterscale cycles bounded at the top by mudflats rich in polygonal desiccation
and microbial shrinkage cracks, raindrop imprints, and halite casts.
In our opinion, the supposed raindrop imprints illustrated by Álvaro
(compare Fig. 9), are hailstone impressions. Since we had only the photos
of those structures at our disposal (Fig. 9), we could specify neither the
depth of those impact craters nor the height of external rim. Consequently,
we could not determine their relationships to crater diameter. Nevertheless,
we were able to reveal distinct similarities between the experimentally
produced hailstone imprints and fossil impact structures in the Neoproterozoic–Cambrian deposits of Mauritania (Fig. 9).
First, those fossil imprints are of variable size, ranging from 20 to
49 mm (measurements taken from the photo; Fig. 9A–C), clearly too
large to have been formed by falling rain droplets, as previously
interpreted by Álvaro (2012).
The central depressions of these structures (Fig. 9) seem to be deep with
relatively steep walls that clearly resemble the artificially produced
hailstone imprints, especially those formed by 5–10-mm-size hailstone
(compare Fig. 5A, B and Fig. 9).
Since those structures are made in coarser, and most likely less plastic,
sediment (a silty medium), the height and width of the external rims are
expected to be smaller in the Mauritania samples, as predicted from the
experiments. It is confirmed by measurements of the external rim width in
relation to crater diameter (Fig. 8). Although in all Nouatil Group cases
the width of the external rim falls within the range of experimental imprints
(compare predicted zone in Fig. 8), most of them are below the regression
line, implying that external rim widths are smaller than the experimental
rims. Additionally, splashes of the largest Nouatil Group samples (Fig. 9 a
and c) are very similar in nature to those produced during experiments
(compare Fig. 5B). Also, the imprints are not truly circular, but slightly
elliptical (Fig. 9; numbers 1, 4, 6, 8, 9, 11, 12, and 15). This suggests that the
trajectories of those ice impactors may have been affected by wind.
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FIG. 9.—Neoproterozoic–Cambrian example
of fossil hailstone imprints from sabkha deposits
of the Nouatil, Nouatil Group of Mauritania.
Selected HI and approximate size: a) large HI,
diameter (Ø) 5 49 mm; b–d) medium-sized HI,
Ø 5 23 mm, 26 mm and 18 mm respectively.
Photo courtesy of J.J. Álvaro (compare
Álvaro 2012).
PALEOGEOGRAPHIC AND PALOCLIMATIC IMPLICATION
The above lines of evidence indicate that the putative raindrop imprints
of Álvaro (2012) (Fig. 9) may be hailstone impact structures, leading to
interesting inferences concerning the climate and paleogeographic
position of their host sediment in the Neoproterozoic.
The origin of hailstones in the past presupposes specific dynamic
conditions of the atmosphere, similar to dynamics of the Recent
atmosphere. Contemporary hailstorms typically occur in mid-latitude
settings, along mountain ranges, being virtually absent from circumpolar
and circumequatorial (tropics zone) latitudes (see Hand and Cappelluti
2011 for a summary). Therefore, the identification of fossil hailstone
impact structures in sediments could be used as an additional indicator in
the interpretation of the paleoclimate and paleogeographic setting.
According to the newest paleogeographic maps proposed by Torsvik
and Cocks (2013; their fig. 4), Mauritania should have been located
around 70 degrees south latitude (circumpolar). The presence of sabkha
deposits on the one hand and the occurrence of hailstorms in high
southern latitude, far beyond common present occurrences of hailstorms,
on the other hand, suggests significantly different climate zones
distribution and atmospheric circulation in the Neoproterozoic.
ACKNOWLEDGMENTS
Emilia Jarochowska, Krystian Wójcik, Joanna Roszkowska-Remin,
Bogusław Waksmundzki, and Prof. Stanisław Skompski contributed valuable
comments and ideas. Members of the ExTerra Student Scientific Circle are
acknowledged for assistance during the experiments. Special thanks go to
Prof. Javier Álvaro (Centre of Astrobiology (CSIC/INTA), Spain) and Prof.
Guy Plint (University of Western Ontario, Canada) for valuable comments,
which greatly improved the manuscript. Christopher Wood (UK) is thanked
for English review. Thanks go to Editor Prof. James MacEachern and
Associate Editor Prof. John Bloch who served with their careful reviews and
inspiring ideas. The Faculty of Geology, University of Warsaw, is
acknowledged for the financial support.
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Received 15 March 2013; Accepted 29 April 2014.