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A GRAVITY ANOMALY IN THE RIES IMPACT CRATER EJECTA BLANKET: SECONDARY OR PRIMARY CRATERING?

Conference Paper

A GRAVITY ANOMALY IN THE RIES IMPACT CRATER EJECTA BLANKET: SECONDARY OR PRIMARY CRATERING?

A GRAVITY ANOMALY IN THE RIES IMPACT CRATER EJECTA BLANKET: SECONDARY OR
PRIMARY CRATERING? K. Ernstson, Faculty of Philosophy, University of Würzburg, D-97074 Würzburg,
Germany (kernstson@ernstson.de)
Introduction: Secondary cratering has been dis-
cussed in planetary impact research for a long time and
is understood as a striking phenomenon of large im-
pacts. Secondary craters occur when excavated materi-
al is ejected from the formation of primary craters and,
when landing outside (sometimes inside) the primary
craters, produce their own craters. On other planets and
their moons the phenomenon can be well studied as
part of landscape formation. In the Ries crater, the so-
called "ballistic erosion" and "secondary cratering"
have been discussed early and have been described in
detail in connection with drilling in the ejecta blanket
[1], where the "secondary wasting", which is widely
used today as an expression of the abrasion of local
material and an intensive mixing on ejecta emplace-
ment could well be observed. Here, I report on a gravi-
ty survey that sheds some light on an unexpected fea-
ture of the Ries crater ejecta blanket (Fig. 2, 3).
. Fig. 1. Location map.
Fig. 2. Geological general map of the Ries impact
crater and field for the gravity survey in the ejecta
blanket. Modified from Geological Map of Bavaria,
1:500,000.
The gravity survey: Extensive gravity measure-
ments in the Ries impact structure have been per-
formed as early as in the sixties [2], and later only a
few student courses for some local improvement of the
existing gravity data followed.
Fig. 3. Typical aspect of the Ries crater ejecta (Bunte
Breccia on top of autochthonous Jurassic limestones).
Gundelsheim quarry.
Fig. 4. The Bissingen gravity Bouguer anomaly.
The discovery of the exceptional, kilometer-sized
gravity anomaly within the ejecta blanket was a matter
of fortuity related with a gravity campaign for hydro-
geological purposes. The survey comprised well over
200 gravity stations the distribution of which is shown
in Fig. 4. Data processing with the usual reductions led
to the Bouguer gravity map in Fig. 4 of a basically
distinct negative anomaly. Because of the numerous
short-wavelength gravity anomalies due to dislocated
megablocks within the Bunte Breccia ejecta, a stronger
low-pass filtering was used for simple modeling (Fig.
5), and a gravity profile was taken from this map for a
very simple 2.5D model calculation the result of which
is also shown in Fig. 5. For lack of more specific den-
sity data a straightforward modeling has produced a
two-layer density distribution that assumes a mass
deficit responsible for the gravity anomaly. Because of
this simple assumption the shape of the negative mass
follows more or less the shape of the gravity curve.
This reveals a steplike slope of the central depression
1227.pdf51st Lunar and Planetary Science Conference (2020)
with a depth of about 200 m. This value depends of
course on the density difference, but -0.25 g/cm3 corre-
sponds well with earlier measured seismic velocities
for ejecta and autochthonous Jura limestones [3].
Fig. 5. Low-pass filtered Bissingen anomaly, gravity
profile and 2.5D modeling.
Discussion: What does the low density filling of the
roughly bowl-shaped structure consist of? A filling
with Ries ejecta masses makes sense, if one uses pre-
viously published thicknesses of the Bunte Breccia
masses for comparison. While at the Bissingen dis-
tance of 20 km from the crater center average thick-
nesses of 50 - 30 m (primary and secondary wasting
ejecta) are given [4], >100 m have often been found
[5], and in a pre-Ries erosion channel, seismic and
geoelectric measurement resulted in up to 200 m
mighty Bunte Breccia [3]. This is exactly the order of
the thickness of the mass deficit in the Bissingen cra-
teriform structure. However, a comparable pre-Ries
erosion channel which abruptly has deepened on such
a short distance from NW and which also has no real
runoff should be eliminated. The explanation remains
that the deepening and subsequent refilling occurred
during the Ries event itself, and I refer to the "ballistic
erosion" and "secondary cratering" from the Introduc-
tion and to [5]. Not to be completely excluded and not
yet discussed for the Ries environment is a direct im-
pact of a small companion projectile possibly separated
from the main impact body, which at a diameter of
perhaps 100 - 200 m has produced an independent
Bissingen crater, which was filled up immediately
afterwards by the ejecta masses originating from the
main crater. This might explain the rounded shape
better, and the nearly symmetrical gravity profile with
the steplike slope may remind of a kind of an inner
ring. A certain uncertainty is caused by the unsymmet-
rical extension of the negative gravity anomaly to the
southwest, which could not be further measured within
the scope of the project.
Both models, which are here discussed, have some
appeal for the Ries crater research. There has never
been much discussion about secondary cratering and
ballistic erosion at the Ries impact. After the detection
of a more or less bowl-shaped structure several kilo-
meters in size, obviously filled with ejecta up to depths
of about 200 m, the impact-mechanical question of the
secondary cratering arises concerning both the second-
ary projectile ejection from the primary crater (starting
location, angle and speed) and the associated landing
mechanism, and in particular the role of the secondary
projectile as a component of the total ejection mass
with the consequence of secondary impact and filling.
This is postponed here for the time being in favor of
the discussion of the second model. The fact that the
Ries projectile did not fall from the sky alone is due to
the existence of the small Steinheim impact crater as
generally accepted, and there have also been consid-
erations of accompanying impacts much further east
and west [6-9].
Conclusions: Gravimetry has long been an im-
portant tool in the investigation of impact structures in
terms of structural investigations, mass estimations and
energy considerations. The Ries crater is no exception.
What is new, however, is that gravity measurements
with large impact structures can look into the under-
ground in more detail, from where, if at all, only point-
by-point information from deep boreholes is available.
Although this can make important contributions to the
understanding of impact processes, as the NASA bore-
holes in the Vorries not far west of Bissingen have
shown, these drillings have completely escaped that in
the immediate vicinity a structural feature exists,
which can make important contributions to the under-
standing of ejecta emplacement and deposits. Accord-
ingly, a coupled impact with projectiles differing by
one order of magnitude is conceivable at a distance of
only 20 km. Details of the impact sequence for the
possible small companion - influence by the large main
impact, the individual phases of contact and compres-
sion, excavation and modification - must remain unan-
swered for the time being. Therefore, it might be inter-
esting to significantly expand the existing area of grav-
ity measurements.
References: [1] Hörz, F. et al. (1983) Reviews of
Geophysics, 21, 1667-1725. [2] Kahle, H.-G. (1969) Z.
Geophys., 33, 317-345. [3] Bader, K. and Schmidt-
Kaler, H. (1977) Geologica Bavarica, 75,401-410. [4]
Zhu, M.H. and Wünnemann, K. (2013) 44th LPSC,
Abstract #1921. [5] Hörz, F. (1982) Geol. Soc. Amer.,
Spec. Pap., 190, 39-55. [6] Rutte, E. (1971) Geoforum,
7, 84-92. [7] Rutte, E. (2003) Land der neuen Steine,
110 p., Regensburg (Univ.Verlag). [8] Ernstson, K. et
al. (2019) 50th LPSC, Abstract #1370. [9] Hofmann, F.
(1978) Bull. Ver. schweiz. Petroleum-Geol. u. -Ing.,
44, 17-27.!
1227.pdf51st Lunar and Planetary Science Conference (2020)
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References: [1] Hörz, F. et al. (1983) Reviews of Geophysics, 21, 1667-1725. [2] Kahle, H.-G. (1969) Z. Geophys., 33, 317-345. [3] Bader, K. and Schmidt-Kaler, H. (1977) Geologica Bavarica, 75,401-410. [4]