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REVISITED R. Fox1 and K. Ernstson2. 1Practical Geophysics, 6832 Triumph Lane, West Jordan UT 84084, USA
( 2Faculty of Philosophy I, University of Würzburg, D-97074 Würzburg, Germany
Introduction: Thirty years ago, Dr. Robert E. Co-
henour, recipient of a PhD in Geology from the Uni-
versity of Utah in 1957, published a paper titled, “The
Great Salt Lake Astrobleme” (GSLA) [1]. His interest
in impact geology was initiated by a geologic enigma,
introduced during his earlier years as a geologic stu-
dent, an enigma related to the “Northern Utah High-
land”, a geologic feature devoid of approximately
10,700 meters of Paleozoic sediments. These sedi-
ments occur in the surrounding mountain ranges, but
abruptly terminate at the margin of the ‘highland’, ex-
posing primarily Precambrian rocks overlain by early
Tertiary rocks. The missing section of rocks was pro-
fessed to be the source of heat and pressure that pro-
duced the, now exposed, underlying Precambrian
Farmington Canyon metamorphic complex. When
asked, the professor could give no explanation for the
missing 10.7 km sedimentary section. This simple fact
was the driving force behind Dr. Cohenour’s research
into impact geology. His research, initiated in the late
1950’s, resulted in a paper titled, “The Asteroidal Im-
pact Theory and Some Geologic Evidence for Asteroi-
dal Impacts on Earth" [2]. This paper presented a logi-
cal argument for terrestrial impacts and their associated
effects at a time when there was little knowledge about
or interest in terrestrial impact geology. This research
provided the encouragement for Dr. Cohenour to later
postulate the GSLA as the most logical explanation for
the missing Paleozoic section and other associated
geological enigmas. His 1987 paper was virtually ig-
nored by the local geological community, a community
seemingly unfamiliar with and disinterested in impact
Revisitation of the GSLA was induced by an
awareness of recent and ongoing geological investiga-
tions (e.g., [3]), misguided by a lack of acceptance and
appreciation its effects. Over a period of two years,
2016-2017, using the hypothesized GSLA as a guide,
field evidence has been collected from in and about its
perimeter that consistently supports its validity. Here
we report on this new field evidence with a focus on
peculiar geologic settings well known from many other
impact structures, on mesoscopic deformation features
and on microscopic evidence of shock metamorphism
as diagnostic of meteorite impact. In light of this, a
broader discussion of the conventional geologic mod-
els opposing the 30 years old impact model remains
undone for now, apart from ascribing the “Northern
Utah Highland” Precambrian to the suggested central
uplift of the complex impact structure showing some
similarity to the Manson (Iowa) impact crater.
Fig. 1. Map of proposed impact structure. Google
Earth. The elliptical shape is attributed to Basin and
Range deformation.
Results - Impact deformation from macro- to
micro-scale: Breccias are a significant constituent of
impact structures due to the established con-
tact/compression, excavation and modification stages
of impact cratering. The GSLA is no exception, and
the richness of megabreccias, monomictic, polymictic
and dike breccias in the field may even be called char-
Megabreccias. In general they are characterized by
great extesion and by large-sized components (mega-
blocks), occur at best with gigantic landslides and oth-
erwise are typical for larger impact structures (e.g.,
[4]). Fig. 2 shows impressive details of megabreccia on
Antelope Island belonging to the postulated central
Fig. 2. Megabreccia in the GSLA. Antelope Island.
Fig. 3. Monomictic breccias in the GSLA.
Monomictic breccias. Due to frequently enormous
confining pressures typical voluminous monomictic
breccias in impact structures show a grit brecciation,
which have been called monomictic movement brecci-
as and in many impact structures are considered diag-
1407.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
nostic of impact deformation [5]. Various aspects of
GSLA monomictic breccias are shown in Fig. 3.
Polymictic breccias. In impact stuctures they char-
acteristically originate in the rapidly proceeding stages
of cratering - excavation, ejection, landing of ejecta
and their heighly energetic mixing with the local target
material. This is the reason for the formation of brecci-
as-within-breccias right up to multiple breccia genera-
tions normally not observed in geological processes.
The abundant breccia generations (e.g., Fig. 4) encoun-
tered in the GSLA are strong evidence for the postulat-
ed impact event.
Fig. 4. Polymictic breccias in the GSLA (Harkers
Dike breccias (or breccia dikes). They are a promi-
nent feature in impact structures [6] frequently allow-
ing detailed reconstruction of the cratering process.
Fig. 5 compiles a few examples of monomictic and
polymictic dikes and dike systems exposed in the
GSLA emphasizing a probable impact signature.
Fig. 5. Delineation of breccia dikes in the GSLA.
Mesoscopic deformation. The Mountain Dell Can-
yon deposit (Fig. 6) is interpreted as part of an ejecta
blanket that reasonably is expected to exist around a
big and relatively young impact structure. High confin-
ing pressures acting within very short times during
excavation, ejection and landing of target rocks leads
to high-pressure/short-term deformations in particular
observed with hard components embedded in a soft
matrix. Impressive examples have been described for
the Ries impact crater and the large Spanish Azuara
and Rubielos de la Cérida impacts (e.g. [4]) pointing
out that such extreme deformations, nevertheless, leave
the affected cobbles and boulders coherent, factors
incompatible with slow tectonic stress.
Fig. 6. Typical high-pressure/short-term deformation
from the Mountain Dell Canyon suggested impact
ejecta deposit.
Shock metamorphism. So far, the investigations of
the GSLA have mainly focused on the field evidence
for the proposed giant impact event as exemplified
before. In contrast lab analyses and polarizing micros-
copy for shock identification are still preliminary. Typ-
ical planar deformation features (PDF) in quartz are
widespread and in part extremely abundant and tightly
packed in the quartzite rocks (Fig. 7). Multiple sets of
PDF in a grain are common, and in thin sections more
than 80 % of all quartz grains may show PDF. Very
often, sets of closely spaced planar features are mark-
edly bent or kinked, which may be interpreted as tec-
tonically produced. Since they are closely linked to
undulatory extinction and kink banding in the quartz
grains (Fig. 7) a crystallographic orientation is indicat-
ed pointing to primary PDF shock deformation and a
later deformation of the quartz grains in the ongoing
cratering process. Multiple sets of planar fractures (PF)
in quartz meanwhile considered as diagnostic of im-
pact shock [8], are also abundant.
Fig. 7. Multiple sets of PDF in quartz. To the left:
Kink bands in quartz grains with linked PDF orienta-
Conclusion: Dr. Cohenour's original idea of the
GSLA was solely (but convincingly) based on topo-
graphic, structural and stratigraphical considerations.
His 30 years old model, using textbook knowledge of
meteorite impact cratering as an established geologic
process, is an unerring guide to geologic characteristics
uniquely associated with impacts. There is little doubt
that the GSLA should be considered a probable large
impact structure that formed around the K-Pg bounda-
[1] Cohenour, R.E. (1987) Utah Geol. Ass. Publ., 16,
133-150. [2] Cohenour, R.E. and Sharp, B.J. (1987)
Utah Geol. Ass. Publ., 16, 105-132. [3] King, J.K. and
Willis, G.C. (2000) Misc. Publ. 00-1, Utah Geol. Surv.
[4] Ernstson, K., et al. (2002). Treballs del Museu de
Geologia de Barcelona, 11, 5-65. [5] Reiff, W. (1978)
Meteoritics, 13, 605-609. [6] Lambert, P. (1981) In:
R.B. Merrill, P H. Schultz (eds.) Lunar Planet.
Sci. Proc. 12A, 59-78. [7] French B.M. (1998) Trac-
es of catastrophe, 120 p., Houston (LPI). [8] French,
B.M. and Koeberl, C (2010) Earth-Sci. Rev., 98, 123-
1407.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
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