Download Clay mineral alteration associated with meteorite impact in the

Clay Minerals (1998) 33, 51-64
Clay mineral alteration associated with
meteorite impact in the marine
environment (Barents Sea)
H. DYPVIK
AND R. E. F E R R E L L ,
a
JR.*
Department of Geology, University of Oslo, P.O. Box 1047, Blindern. N 0316 Oslo, Norway, and *Department of
Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803-4101, USA
(Received 13 August 1996; revised 26 November 1996)
A B S T R A C T : More than 50 samples from a Barents Sea borehole near the Mjolnir Structure (an
extraterrestrial impact feature) were used to investigate changes in the clay assemblage associated
with the submarine impact. Seismic evidence, the presence of shocked quartz and a prominent Ir
anomaly restricted the potential impact affected zone to a 10 m interval, straddling the Jurassic/
Cretaceous boundary.
Increased abundance (up to 30 wt%) of a smectite, a randomly interstratified smectite-illite with
85% smectite layers, forms the basis for a two-layer oceanic impact clay model that differs from
published terrestrial cases. The smectite is assumed to represent seawater-altered impact glass from
the ejecta blanket material that was mixed with resuspended shelf sediments by the collision
generated waves. The smectite-rich interval is almost 5 m thick. It is overlain by a coarser unit (~2 m
thick) containing abundant smectite, shocked quartz grains, and anomalous Ix contents at its base.
The smectite-rich interval may have originated as a density/turbidity current, generated by the impact
and the collapse and erosion of the crater rim. Seawater alteration of volcanic glass and changes in
the tectonic regime of the provenance area, or changing oceanic current circulation patterns could
produce similar variations in the clay mineral assemblage. The most compelling evidence for the
possible impact derivation of this clay assemblage is the direct association with the Mjolnir Impact
Structure and associated mineralogical and geochemical anomalies.
The discovery of an Ir anomaly (Alvarez et al.,
1980) and its interpretation as a global catastrophic
event associated with the impact of a large bolide
has produced a great deal of scientific interest and
debate (McLaren & Goodfellow, 1990; Montanari
et al., 1983; Sharpton & Grieve, 1990; Smit, 1982;
Tappan, 1982). Much of the discussion has centred
on the coincidence of an Ir anomaly and the mass
extinctions at the Cretaceous/Tertiary Boundary
(KTB). Many authors have linked the global
changes produced by a large impact with the
major faunal changes occurring at stage boundaries
(McLaren & Goodfellow, 1990). Bolides larger than
1 km in diameter may strike the Earth at a
frequency equal to 4.3 (-I-2.6) • 10 _6 yr -J, and
nearly 82 craters of >30 km diameter should have
been produced in the last 100 million years
(Shoemaker et al., 1990). They may create high
temperature and acidic conditions unfavourable for
living organisms and promote large-scale die-offs.
However, some authors have questioned the exact
stratigraphic correlation of the impact structures
with faunal boundaries and the global consequences
of many impacts (Birkelund & HLkansson, 1982;
Courtillot, 1990; Hansen et al., 1986; Hansen et al.,
1987).
The sedimentary record also contains a number
of components that are interpreted as impact related
(e.g. glass, shocked quartz, spinels, Ir anomalies,
clay minerals). One of the best clay mineralogical
descriptions is of material associated with the KTB
in the Western Interior Basin of North America. It
is the basis for the two-layer model of Pollastro &
Bohor (1993). The lower layer is mainly kaolinitic
9 1998 The Mineralogical Society
52
H. Dypvik and R. E. Ferrell, Jr.
and was formed during the post-depositional
alteration of the silicic melt ejecta blanket layer.
The upper laminated layer was derived from the
fireball and is mostly smectite formed from the
alteration of mafic glass. Their distinctive texture
and impact components distinguish these clays from
other clay beds and tonsteins in the vicinity of the
KTB. The clay mineral differences are the product
of the alteration of different starting materials in the
acidic, organic-rich waters of ancient peat swamps.
Secondary variations in the degree of ordering of
the kaolinite or the illitization of the smectite due to
weathering or burial diagenesis may occur locally.
This two-layer sequence is best developed when the
ejecta and fall-out materials accumulate in terrestrial environments.
In the marine sedimentary sequences associated
the KTB, there are differences of opinion regarding
the significance of the increased quantities of
smectite observed near the boundary. A single,
pure Mg-smectite-rich deposit with a high Ircontent at Sterns Klint, Denmark, (Kastner et al.,
1984) was interpreted to represent material formed
from the alteration of impact glass. Subsequently,
the discovery of multiple beds of Mg-rich smectite
in the same section, led Elliott et al. (1989) to
conclude that the clay minerals were alteration
products of volcanic glass. Robert & Chamley
(1990) suggested that the wide variation of clay
minerals associated with the KTB indicates a global
instability that can not be related to a unique
extraterrestrial event. They interpreted the changes
mainly to represent global changes in sea level and
tectonic activity at that time. Additional examples
from the Betic Cordillera and the BasqueCantabrian Basin were used by Ortega-Huertas et
al. (1995) to argue that the clay mineral variations
associated with the KTB are controlled by erosional
processes in the source areas and the palaeogeography of the depositional basins.
Most of the arguments regarding the nature of
clay mineral changes associated with impact events
are difficult to assess because the details of the
stratigraphic associations are unclear. In the
terrestrial sections which contain most of the data,
the correlation with a particular impact crater is
uncertain. Additional complications arise because
most of the studied sections are in terrestrial
settings with material derived from very distant,
suspected terrestrial impacts. Only four suspected
impact craters of marine origin are reported,
although marine impacts should be several times
more abundant than terrestrial ones because of the
larger target area represented by the oceans. The
Mjolnir Structure, a Late Jurassic submarine crater,
provides an opportunity to assess directly the
changes in clay mineralogy that are produced in
the surrounding continental shelf deposits by the
impact.
STUDY
AREA
The Mjolnir crater was recently discovered by
Gudlaugsson (1993) and geophysical and geological
evidence for an extraterrestrial impact origin have
been presented (Dypvik et al., 1996; Gudlaugsson,
1993). The structure, which is located in the
Barents Sea (73~ 30~ Fig. 1), was formed by
an impact in the Late Jurassic/Early Cretaceous.
The 40 km diameter scar marks a dramatic event.
The presence of shocked quartz records the extreme
pressure (at least 15 GPa) and temperature
(> 1000~
attained within a few seconds of
impact. The approximately 1.5 km diameter bolide
collided with Earth in a shallow shelf sea where the
water depths were between 300 and 500 m. The sea
floor consisted of soft silty clays underlain by
several km of mostly marine claystone and
sandstone units with interbedded thin silt layers.
Samples were collected from a continuous core
(IKU 7430/10-U-01) (Fig. 2) obtained about 30 km
north-northeast of the Mjolnir Impact Structure in
the Barents Sea (Fig. 1). The interval from
67.6-45 m below the sea bed represents clay-rich
sediments of the Hekkingen Formation, that range
in age from Late Kimmeridgian to Early Berriasian.
The first seismic reflector above the chaotic zone of
the impact structure can be projected to intersect
the core at a depth of approximately 46 m (Dypvik
et al., 1996). The location of the samples, a
schematic lithologic description of the core, and
the stratigraphic assignments are presented in
Fig. 2. Dark grey, well-laminated, Kimmeridgian,
fossiliferous, organic-rich claystone units, with
some thin sandy laminae occur in the interval
from the base of the core to 56.5 m. They are
overlain by sandy to silty Volgian claystone,
(56.5-50.0 m) containing some 2 - 5 cm thick,
fining upward beds resembling distal turbidite
deposits. The succeeding, possibly Late Volgian to
Early Berriasian interval can be divided into an
upper and a lower unit. The lower (50.0-47.6 m) is
dominated by dark grey, weakly laminated claystone, while the upper (47.6-40.0 m) contains beds
53
Clay mineral alteration associated with meteorite impact
o
a
o
30 ~
F:G. 1. The approximate location of the Mjolnir structure is marked by the large dot. The smaller dot marks the
location of boring 7430/10-U-01, about 40 km to the North. The Jurassic palaeogeographic setting at the time of
impact is indicated in the inset, based on a reconstruction by Lawver (pers.comm.).
of sandier claystone that are glauconitic in part,
moderately bioturbated, and relatively rich in pyrite
and body fossils. The base of the upper unit is
marked by a 19 cm thick, mudflake conglomerate
composed of three minor fining-upward units.
Shocked quartz (level 47.60-47.00 m, star in
Fig. 2; illustrated in Fig. 3) representing -2% of the
quartz grains observed in thin section and an Ir
(1016 ppt) concentration peak at 46.8 m, more than
15 x higher than the average background value
(Dypvik et al., 1996), have been found. Details of
the mineralogical and geochemical analyses are
presented in Dypvik et al. (1996). There is an
approximately 80 cm thick zone of bioturbated silty
claystone between the first layer containing shocked
quartz and the Ir peak. These mineralogical and
geochemical features, the seismic data, and the
proximity of a crater, are the most direct indicators
that the sampled interval has been influenced by an
extraterrestrial impact. An erosional surface at
45.0 m marks the top of the interval sampled in
detail (Fig. 2).
METHODS
X-ray diffraction (XRD) analyses were run on dried
and crushed whole samples mounted in a sideloading holder. Samples for clay mineralogical
analysis were washed with distilled water to
remove salt, then suspended in a dilute Na
phosphate solution. Following ultrasonic dispersion,
the clay fractions (<2 gin) were extracted by
gravity settling techniques. Oriented films for
XRD were prepared by smearing the clay on glass
plates. X-ray powder diffraction patterns of the airdried, ethylene glycol saturated, and heated (300~
and 550~ for 1 h) materials, as well as the bulk
analyses, were obtained by step-scanning the
interval from 2 ~ 20 to 50 ~ 20 in a Siemens D5000
diffractometer with a Cu tube operated at 40 kV
tt. Dypvik and R. E. Ferrell, Jr.
54
CORE
7430
/ 1 0 - U - 01
claystone / shale
z
siltstone
sandstone
crLLI
.<~r"
UJrr"
.<
s
carbonates
bioturbation
parallel lamination
, 17:
t>l:>
v.c.sand / granules
[3E3
pyrite
>
glauconite
,,,z~
=<'1'"
13C .,~
,,<,>
Iridium
z
,<co
LIJCE
LU
123
J
z
z
uj ~
~_j(,9
.J
LUrr
m
0 84
i
z
,<
C~
-J
0
l
el si vf f m c vc~)
m
w
p
~
b
0
I
I
I
1
I
I
500
ppt
I
Iooo
Ir
FI~. 2. A generalized sedimentological log of core 74307/10 -U-01 is shown. Modified from Geir Elvebakk, IKU.
The 47.6 to 47.00 m interval, from the conglomeratic zone and 40 cm above, is the only zone where shocked
quartz has been identified (star). The highest Ir value is located 20 cm above. The core continues to 67.6 m, with
lithologies similar to those below 55 m in the present log. Tick marks on the stratigraphic section indicate where
samples were obtained for analysis.
Clay mineral alteration associated with meteorite impact
55
Fro. 3. SEM/CL micrograph of a shock metamorphozed quartz grain from level 47.60-47.65 of core 7430/10-U01. The dark lamellae revealed by CL within this single grain were formed by recrystallization of planar fractures
and planar deformation features.
and 30 mA. Examples of composite XRD patterns
for ethylene glycol saturated samples from selected
intervals are presented in Fig. 6.
A simple numerical representation of mineral
abundance in the whole rock sample was derived
from peak height percentage calculations which do
not require mineral reference intensity correction
factors. The minerals used in the calculation and the
d-values of their principal peaks included: chlorite 14 ]k; smectite - 12.5 A; illite - 10 A; kaolinite 7 .~; quartz - 4.26 A; K-feldspar - 3.24 ~,;
plagioclase - 3.18 A; calcite - 3.03/~; dolomite 2.89/k; siderite - 2.79 A; and pyrite - 2.71 A. The
peak height percentage values were also recalculated after subtracting the carbonates and pyrite
from the total in order to assess potential variations
in the original allochthonous mineral components in
the sedimentary environment.
Quantitative estimates of clay mineral abundance
were obtained by comparing NEWMOD simulated
patterns with those produced by the diffractometer.
The parameters used to generate the simulated
patterns are presented in Table 1. An iterative least-
squares minimization procedure reported by Huang
et al. (1993) was used to calculate the fractional
contribution of each simulated pattern to the
observed ones. This procedure systematically
changes the fractional multiplier for each simulated
pattern until R 2, the best-fit estimator comparing the
simulated XRD composite with the actual pattern,
reaches a minimum value.
An example of the calculation procedure is
illustrated in Fig. 4. The pattern for a simulated
medium-crystallite thickness, moderate Fe content
illite ( D M I C A l l ) is presented with a composite
pattern representative of the clay minerals in the
interval from 49.2.0-49.9 m. The simulated peak
intensities must be reduced by a fractional multiplier of 0.5 (equivalent to an approximation of the
weight fraction of illite in the sample) in order to
match the observed illite peaks. The procedure was
repeated to calculate a fractional multiplier for each
of the patterns in the databank. Some of the
reference patterns were rejected during the iterations as they did not match the observed peaks. The
sum of the fractional multipliers for all recognized
56
H. Dypvik and R. E. Ferrell, Jr.
TABLE 1. Parameters used to simulate clay mineral XRD patterns with N E W M O D ,
Symbol
Description
K A O 11
Fine-particle
kaolinite
KAO7
Medium-particle
kaolinite
Layer
types
(fraction)
kaolinite
(0.5)
kaolinite
(0.5)
kaolinite
(0.5)
kaolinite
(0.5)
D M I C A 11
Med.-part.,
moderate-Fe
illite
dimica
(0.5)
dimica
(0.5)
DMICA015
Fine-part.,
moderate-Fe
illite
dmica
(0.5)
dimica
(0.5)
CHLOR1
Med.-part.,
Fe-poor chlorite
tritriehior
(0.5)
tritrichlor
(0.5)
TTCH511
Med.-part.,
Fe-rich chlorite
DSM523
Fine-particle
smectite
CORRE1
Fine-particle
corrensite
S 185R0
Fine-part.
smectite-rich
smectite-illite
tritrichlor
(0.5)
tritrichlor
(0.5)
dismec2gly
(0.5)
dismec2gly
(0.5)
tritrichlor
(0.5)
dismec2gly
(0.5)
dismec2gly
(0.85)
dimica
(0A5)
Ordering
Layer 1
composition
Layer 2
composition
Crystallite
thickness
R = 1
fixed
fixed
low N = 3
high N = 7
R = 1
fixed
fixed
low N = 7
high N = 14
R = 1
1.0 Fe
1.0 K
1.0 Fe
1.0 K
low N = 7
high N = 14
R = 1
0.8 Fe
0.7 K
0.8 Fe
0.7 K
low N = 5
high N = 9
R = 1
R = 1
R = 0
Sil Fe 0.1
Hyd Fe 0.1
Hydx 0.9
Sil Fe 0.1
Hyd Fe 0.1
Hydx 0.9
Sil Fe 1.2
Hyd Fe 2.1
Hydx 0.9
Sil Fe 1.2
Hyd Fe 2.1
Hydx 0.9
0.5 Fe
0.5 Fe
low N = 7
high N = 14
low N = 7
high N = 14
low N = 2
high N = 3
R = 1
R = 0
Sil Fe 0.1
Hyd Fe 0.1
Hydx 1.0
0.1 Fe
0.1 Fe
0.1 Fe
0.9 K
low N = 3
high N = 7
low N = 3
high N = 7
Clay mineral alteration associated with meteorite impact
57
TABLE 1. (contd.)
Symbol
Description
IS5R1
Fine-particle
rectorite
IS6R1
Fine-part. illiterich
illite-smectite
Layer
types
(fraction)
dimica
(0.5)
dismec2gly
(0.5)
dimica
(0.6)
dismec2gly
Ordering
R = 1
Layer 1
composition
Layer 2
composition
0.5 Fe
0.9 K
0.5 Fe
Crystallite
thickness
low N = 5
high N = 11
R = 1
0.5 Fe
0.9 K
0.5 Fe
lowN = 5
0.5 Fe
0.9 K
0.5 Fe
low N = 5
high N = 11
(0.4)
IS7R1
Fine-part. illiterich
illite-smectite
dimica
(0.7)
dismec2gly
(0.3)
IS8R1
Fine-part. illiterich
illite-smectite
dimica
(0.8)
dismec2gly
(0.2)
IS9R1
Fine-part. illiterich
illite-smectite
dimica
(0.9)
dismec2gly
(0.1)
R = 1
high N = 11
R = 1
0.5 Fe
0.9 K
0.5 Fe
low N = 5
high N = 11
R = 1
minerals was then normalized to 1.0. The results are
expressed as the weight fraction of a simulated clay
mineral in the sample. The normalization was
required because the simulations produced sums
that were between 0.85 and 1.1, indicating that the
simulations were not exact and/or non-crystalline
materials were present. Analyses of replicate
samples indicate a precision within _+0.03 wt% of
reported values >10 wt% . Below I0 wt%, the
relative error may be >100%.
The calculated patterns used in the analysis were
selected because they represent the best matches
with observed peak positions, peak widths, and
intensities produced by the Mjolnir samples. The
quartz pattern was obtained by analysis of a
polished plate o f Arkansas novaculite. Two
kaolinites, two micas, two chlorites, one smectite,
one corrensite, a randomly interstratified smectiteillite with 85% smectite layers, and five representa-
0.5 Fe
0.9 K
0.5 Fe
low N = 5
high N = 11
tives of nearest-neighbour ordered illite-smectite
(the percentage of illite layers varying from
5 0 - 9 0 % ) comprised the databank of simulated
patterns (Table 1). These patterns attempt to
simulate changes due to the thickness of the
crystallites, changes in octahedral sheet composition, and interlayering of different ideal clay
mineral layers. Crystallite thickness parameters are
assumed to be directly correlated with minimum
particle size. Hence, the reference materials with
low crystallite thicknesses, between 2 - 5 and/or
below 11, represent fine-particle sized clays.
Medium particle sized clays are those with crystallite sizes ranging from 7 to 14 ideal layers.
The values used for clay mineral abundance
provide an improved quantitative representation
(QR of Hughes et al., 1994) of clay mineral
variations, compared to other peak height intensity
methods. The method utilizes the entire XRD
58
H. Dypvik and R. E. Ferrell, Jr.
COMPOSITE
49.2-49.9 m
I-OOZ
I~!
DMICA11
ISIMULATED
I--Z
-
_
||
~',
'
0
'
'
'
I
'
5
9
9
'
I
'
'
'
10
'
|
15
'
'
'
'
I
'
'
'
20
'
I
25
"*
'
'
'
I
30
DEGREES TWO THETA (Cu)
FIG. 4. Comparison of a simulated XRD pattern for a medium-crystallite size, moderate Fe content illite with the
composite pattern for samples from the smectite-rich interval of the core.
pattern and incorporates the possibility that several
minerals with different compositions and crystallite
sizes may contribute to a single peak. The
possibility of human induced calculation and
measurement errors are also minimized by the
application of the least-squares fitting routine.
Nevertheless, the values still fail to meet the
rigorous requirements of a quantitative phase
analysis, because they have not been confirmed
by independent means and standards for calibration
are not available. Differences due to variations in
peak intensities of one of the phases caused by
different mass absorption coefficients for other
minerals in the sample matrix have also been
neglected.
RESULTS
The whole-sample results (Fig. 5) display large
variations due to the presence of variable quantities
of diagenetic and biogenic minerals (pyrite, calcite,
dolomite, siderite). Many of the samples have
considerable quantities of pyrite that formed after
deposition of the detrital silicates. With those
secondary variations subtracted, only minor variations were found in the siliciclastic mineral
composition of the whole-rock samples. However,
an increase in the amounts of K-feldspar and
decrease in plagioclase are seen between 56.8 to
56.3 m, as well as increasing amounts of illitesmectite above 52.5 m.
The composite patterns of Fig. 6, produced by
adding the intensities from six individual samples
within each indicated depth interval, illustrate the
general variation in the clay mineral composition of
the Barents Sea materials. The larger smectite peak
in the samples from the 49.2-49.9 m interval is
readily apparent. The general similarity of the
patterns from intervals below (55.0-56.7 m) and
above (45.2-45.9 m) the smectite enriched zone,
can also be observed.
The changes in clay mineral abundance with
regard to stratigraphic position are presented in
more detail in Fig. 7. Variations in the illite
components of the samples and the smectite-rich
smectite-illite mixed-layered clay minerals are
greatest at a depth of -49 m, near the Volgian/
Berriasian boundary. Samples from depths below
Clay mineral alteration associated with meteorite impact
4~. oo li::FK",."~
fi~,\~
59
~x``~A~!!~!~i;iiiii~ii#i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~#i~`~
.................................
r
I
~x~/////~ilili!iiiiiii!iii!i!i!ili#iiiiiiiiiiiii!ili#i!i!iii!i!#i!iii!i!!~
~?~///////.e~
[] PYRITE
T:.~&q
sl. oo T i i i ~ \ \ ~
t9
........................... ~//////.4iiiiiiiiiii~i~i~i~i~i~i~i
~//////~#i~i~i
:::
[]
SIDERITE
52.45
[] DOLOMITE
53.50
[] CALCITE
54.00
[] PLAGIOCLASE
[]
55.00
~o
K - F E L D S P A R
[] QUARTZ
0
s6 7s s~\\\'q
=~\\\\'~
E
~TA
~.~////////A!i::i:,ii~
k~/////////////~l
~10A
~% .\\',.~
~ / / / / / / / / / / / . d iiii~~~
::::::::
0 ~0 ~
~//////////////A::::::::::::::::::::::i::::::i::iii::iiii!iiiii::::i::::i::i::iiiii~/f~~~
'
[]12A
~i4A
6v.ao !llliiiil.~K-~\\\'~
0 %
,
,
~
20 %
40 %
60 %
~
,
80 %
100 %
% XRD
FIG. 5. XRD peak height percentages o f minerals in the whole sample vs. depth. (14 A = chlorite; 12 ,~ =
smectite; 10 ~, = illite; 7 ,~ = kaolinite).
t,O
<>-
\\
\
~
~ ?A K/C
~
A
S
~,-
5A
t~
K/C
z
-~.9
i J
m
m
m
'
0
'
'
'
I
5
'
'
'
'
l
10
'
'
'
'
I
15
'
'
'
'
I
'
'
20
'
'
I
25
'
'
'
'
I
30
DEGREES TWO THETA (Cu)
FIG. 6. XRD patterns o f ethylene glycol saturated samples from selected intervals of the core. The intensities from
six individual samples within each indicated depth interval have been added to produce the composites shown.
60
11. Dypvik and R. E. Ferrell, Jr.
DEPTH
(m)
,(ro,
4,i
.,
i
~"1
.o
,
;
I-
' t
_
q
p9
,i
Pi,
,,.
o9
7C
0
0
DSmS23
9
SI85RO
(
f
0.1
0.2
0 DMICAll IiJ
9
9 DMICA01SI]
t
o
9 :o
y-
A
9
0
P
B D~
o
R1
9 KAO111
CHLOR1
9 IS9RI
9
- 9
QUARhII
9 KAO7
TrCH511
J
0,30
0,1 0 . 2 0 . 3 0 , 4 0 . S 0 . 6 0
0.1
0.7
0.3 0
0.1
0.2
0.30
0.1
0.2
0.30
0.1
0.2
0.3
WEIGHT FRACTION
Fro. 7. Variations in the calculated weight fractions of clay minerals as a function of depth. For explanation of the
symbols, refer to Table 1.
52 m and near 45 m are less smectitic than those in
the intervening section of the core. Changes in the
weight fraction of the other clay minerals and
quartz are not as marked with regard to depth.
Illite is the most abundant clay mineral (Fig. 7)
in the samples from the Barents Sea core. Two
varieties of moderate-Fe content illite with different
particle size distributions best represent the integral
series of 10 A XRD peaks produced by the
samples. The weight fraction of the finer crystallite
size illite varies from ~0.4 to 0.5 in the lower parts
of the core and decreases to a low of ~0.2 in the
interval between 50-47.6 m (DMICA015, Fig. 7).
The weight fraction of the coarser crystallite size
illite ( D M I C A l l , Fig. 7) is generally <0.04 below
50 m, but increases to a maximum of -0.15 in the
50-47.6 m interval.
The weight fraction of smectite-rich smectiteillite (SI85R0, Fig. 7) varies from <0.05 below
50 m to a high of -0.3 in the 50-47.6 m interval.
The changes in smectite abundance are inversely
correlated with those observed for the finer
crystallite illite.
The 9.2 to 9.8 ,~ broad peaks with minimal
ethylene glycol expandability occurring in the XRD
patterns were matched by an R = 1 ordered illitesmectite with differing percentages of illite layers.
The most abundant illite-rich ordered illite-smectites are those with 70% and 90% illite layers
(IS7R1 and IS9R1, Fig. 7). The 90% illite materials
are more abundant than the 70% illite ones below
50 m, are less abundant in the 50-47.6 m interval,
and about equal above 47.6 m. Their combined
weight fraction approaches 0.3 in the interval above
47.6 m.
Changes in the abundance of the other clay
phases are not as notable as those for the illite and
smectite containing materials. The medium crystallite size chlorite (TTCH511) exhibits no recognizable stratigraphic pattern and the medium-crystallite
size Fe-poor chlorite (CHLOR1) is confined to the
interval above 47.6 m. Fine-crystallite size kaolinite
(KAO7) is slightly more abundant in the sections of
the core below 50 m. The coarser kaolinite
( K A O l l ) is only present above a weight fraction
of 0.02 in the interval above 47.6 m. Clay-sized
Clay mineral alteration associated with meteorite impact
quartz is ubiquitous and its abundance (Fig. 7) is
not directly related to any of the recognized
stratigraphic changes. The weight fraction of
quartz is usually <0.1 in the clay-sized materials.
DISCUSSION
There are four major observations that must be
considered in trying to formulate a model for the
origin of the clay mineral alterations observed in
the Volgian and Berriasian sediments near the
Mjolnir crater.
(1) Smectite abundance in the clay fraction
begins to increase at a depth of ~52 m to a
maximum o f - 3 0 wt% in the interval from 50 to
47 m, and then decreases. Smectite abundance
below 52 m and above 46 m is low and usually
<5 wt% of the clay fraction. (2) The mudflake
conglomerate located between 47.6 and 47.4 m
represents a major change in the energy of the
depositional environment. (3) Shocked quartz is
present in the conglomerate and extends upward to
the 46.8 m level in the core. (4) The highest Ir
value was produced by the sample just above the
shocked quartz zone at 46.7 m. Other samples in
the conglomerate and shocked quartz zone have Ir
values greater than twice the average for samples
below 49 m and above 46 m.
There are other geochemical data that must be
considered. Dypvik and Attrep (in prep.) have
found increases in the abundance of Ni, Co, V,
Cr, Zn, and total organic carbon (TOC) that parallel
the increase in the abundance of smectite in the
interval from a depth of 52 m to the base of the
mudflake conglomerate. A dramatic decrease in the
abundance of the siderophile elements and TOC
occurs at the level of the conglomerate and values
remain low to the top of the interval studied.
The changes summarized above can best be
explained by reference to the events associated with
the bolide impact that created the Mjolnir Crater.
The impact forced the water from the area and
vapourized, fractured and melted the sea floor
sediments to form glass that was primarily
deposited as ejecta in, and close to, the crater.
This is similar to the situation for the Montagnis
Crater in Canada (Jansa et al., 1989; Jansa, 1993)
where the ejecta blanket is confined to the area
within two crater diameters of the impact site. The
currents generated by the returning wave surge
resuspended the ejecta materials and mixed them
with resuspended bottom sediments. They floccu-
61
lated, settled to the bottom, and developed gel
strength rapidly. They are now represented by the
smectite enriched materials in the interval from
52 m to the base of the mudflake conglomerate.
The smectite formed by the near-surface alteration
of the impact glass after the restoration of normal
marine conditions. Glass was more abundant near
the top of the interval because of the slower
hydraulic settling velocities of the less dense silicarich glass. Organic matter and the siderophile
elements associated with decaying organic debris
could also be concentrated near the top of this
interval for similar reasons, augmented by material
from organisms killed by the impact (Dypvik &
Attrep, in prep.). This is a stage equivalent to the
ejecta blanket of Pollastro & Bohor (1993), but in
this marine environment larger quantities of
essentially unaltered local sediments are mixed
with the ejecta materials and the blanket is less
extensive.
Subsequently, other marine processes triggered
instabilities in the crater rim or eroded the central
area of the crater, and coarser materials were
distributed by radiating density flows and shelf
currents. These coarser deposits are represented by
the mudflake conglomerate and the other overlying
smectite-rich sediments containing the shocked
quartz and higher concentrations of Ir. They
represent material derived from areas nearer the
crater where the sediments were more directly
influenced by the composition of the bolide. The
basal section of the conglomerate contained about
the same quantity of glass (now smectite) as the
underlying clay-rich interval, but the amount
decreased near the top as erosion progressed
deeper into the metamorphosed section of the
crater. These deposits have some of the same
characteristics attributable to the bolide and ash-fall
layer of Pollastro & Bohor (1993), but have been
transported and deposited from turbid flows. The
affected interval is represented by the sediments
occurring between 47.6 and 46 m.
The changing character of sediments derived
from an impact zone is also described from other
shallow marine impact sites (Jansa et al., 1989;
Newsom et al., 1990; Puura & Suuroja, 1992; Poag
& Aubry, 1995). The impact-metamorphosed basement is generally covered by a lower autochthonous
breccia with shocked quartz. This breccia is often
overlain by an allochthonous breccia with glass,
spherules and mixed lithologies, followed by a
glass-suevite dominated unit. They may be
62
H. Dypvik and R. E. Ferrell, Jr.
reworked by turbidity currents, debris flows and
other erosional processes (Roddy, 1977; McKinnon,
1982; Melosh, 1982; Jansa, 1993; Poag & Aubry,
1995).
The best evidence supporting the bolide hypothesis for the origin of the smectite enrichment in this
sequence is its location. The core is close to a
recognized crater structure and the seismic evidence
(Dypvik et al., 1996) restricts the potentially
affected zone to the interval with high smectite
content. Auxiliary support for the extraterrestrial
origin is provided by the coincidence of the Ir
anomaly and shocked quartz with the smectite zone.
The clay mineralogical changes described above are
clearly not necessarily unique products of a bolide
impact. Thus far, no vestiges of the parent glass to
compare with glasses of known volcanic or impact
derivation have been found. However, we should
not expect to find much original glass in these Late
Jurassic sediments, as the alteration of glass to
smectite proceeds rapidly (Grieve et al., 1977; Pohl
et al., 1977; Newsom et al., 1990; Sigurdsson et al.,
1991; Puura & Suuroja, 1992). The smectite
produced is dioctahedral, and the associated trace
elements are within the ranges expected for clays
originating in an oxygen-poor, marine environment
(Dypvik & Attrep, in prep.). The smectite is
comparable to the 95% smectite-5% illite mixedlayered clay mineral found as an alteration product
of glass in the Ries Crater (Newsom et al., 1990).
Other potential sources for the increased smectite
in the proposed impact affected interval are difficult
to evaluate. Burial diagenesis usually destroys
smectite, and in this case the sediments have not
been buried to significant depths. Local tectonic
influences on the types of sediment supplied to the
continental shelf are not very likely as the area is
distant from the palaeoshoreline and there are no
reports of other major Late Jurassic tectonic
disturbances in the this area of the Barents Sea.
Lower Cretaceous dolerites occur to the North of
Mjolnir, on the islands of Svalbard and Franz Josef
Land. Recent, unpublished age determinations show
the dolerites of Svalbard to be post-Berriasian (S.
Dahlgren, pers.comm., 1996), while those of Franz
Josef Land are post-Barremian (A. Andresen, pers.
comm.), in both cases younger than the Mjolnir
impact.
The available literature contains no references to
the clay mineral composition of comparable
sediments in this section of the ocean or the
adjacent parts of the continents that would permit
correlation of this smectite-rich zone with processes
affecting the normal distribution of sediments. Thus
the smectite probably does not have a detrital or
volcanic origin.
Nevertheless, some enigmas remain. Why did a
wave with the energy to resuspend so much
sediment not leave any evidence of basal scour?
Why do the siderophile elements and TOC not have
a distribution that mimics that of smectite? If it is
assumed that the mudflake conglomerate represents
the base of the wave-surge induced scour following
the impact, then all of the impact derived material
is confined to the interval between 47.7 and 46 m
and the differences in the minor element composition of the materials above and below the contact
could be explained by the development of anoxic
sea bottom conditions prior to the impact. This
would, however, make it impossible to account for
the secondary smectite below the conglomerate
unless the extreme temperatures associated with the
impact were able to facilitate an in situ alteration of
clay minerals at depths at least 4.5 m below the
sediment/water interface. This would seem unlikely
considering the poor thermal conductivity of
sediments and the distance from the impact site.
The sequence near the Mjolnir crater may be
correlated with the Jurassic/Cretaceous unconformity (t~-hus, 1991) reported in other cores from the
Barents Sea by Arhus et al. (1990). It is often
expressed by Valanginian coquina beds directly
succeeding black Volgian shales or occurs within
lithologically homogeneous looking VolgianValanginian black shale successions. This unconformity could be the result of severe wave/current
erosion of the sea bed, processes related to the
Mjolnir impact.
CONCLUSIONS
Clay minerals occurring in a core near the Mjolnir
Crater provide a distinct record that can be
attributed to the impact of a bolide in a marine
environment. Two distinct zones can be recognized
by changes in the abundance of a smectite-rich
smectite-illite formed from the alteration of impactgenerated glass. The clay mineral zones are not as
distinct as those of purported impact origin
occurring on the continents. The lower, partly
anoxic, zone contains materials that are essentially
equivalent to the ejecta blanket of the Pollastro &
Bohor (1993) two-layer model, but smectite, rather
than kaolinite, is the major alteration product in the
Clay mineral alteration associated with meteorite impact
marine environment. The original glass is diluted by
the admixture of resuspended sea floor sediments.
In the upper zone, shocked quartz and Ir occur with
varying amounts of smectite. The sedimentary
structures suggest that these deposits originated
partly as density flows/currents originating from the
collapse/erosion of the crater rim or core of the
crater and are not directly equivalent to the ash-fall
deposits of the terrestrial model. The clay mineral
changes are not unique indicators of an impact and
other criteria are required to distinguish these
deposits from ones derived from the sea floor
alteration of volcanic ejecta or contributed by
erosional and related tectonic events typical of
many sedimentary basins.
ACKNOWLEDGMENTS
We are grateful for the analytical help of W. LeBlanc,
B.L. Berg and I.A. Hansen. S.T. Gudlaugsson and F.
Tsikalas kindly helped with the making of Fig. 1. This
study has been supported financially by the NorwayAmerica Association, Saga Petroleum a.s. and the
Research Council of Norway. IKU Petroleum research
took the studied core 7430/I0-U-01 and kindly placed
core material at our disposal.
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