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8 Results and Conclusions
8.1 Introduction
8.2 Comparison of bioclast grit
layers and hardgrounds
8.3 Comparison of Mesofossil content
and Bryozoa
8.4 Final conclusion
8.1 Introduction
Three hypotheses were tested about the origin of
the Berg en Terblijt hardground. First, the hardground is
a normal hardground like the other hardgrounds in the Meerssen
member (with erosion before deposition of cover). The second hypothesis
is from Zijlstra, 1996 and comprises the deposition of a stormlayer
directly over the hardground and thus preserving the organisms present.
The third hypothesis is also the deposition of a storm layer on
top of the hardground, but now as a result of the Chicxulub meteorite
impact in the Gulf of Mexico at the end of the Cretaceous.
8.2 Comparison of bioclast
grit layers and hardgrounds
The stratigraphy in the ENCI quarry shows the same
cycli beginning with graded bioclast grit layers, then fine carbonates
and a hardground. These cycli are repeated in the Meerssen
member several times in both locations. At first sight, the Berg
en Terblijt hardground in the Curfs quarry looks very much like
the hardgrounds measured in the ENCI quarry. Some hardgrounds in
the ENCI quarry, however, are much more irregular and bioherm like,
but some others are identical to the Berg en Terblijt hardground
in the Curfs quarry. The structure of the hardgrounds itself shows
no differences too; the same kind of burrows, irregular surfaces
and appearance. Both sections show the same cycli beginning with
graded bioclast grit layers, then fine chalk and a hardground. In
both locations these cycli in the Meerssen member are repeated
several times (see also Zijlstra et al. 1996).
The first difference is the good preservation of
the fossils burried in life position, known to come from the top
of the Berg en Terblijt hardground, which must have been
buried very fast (J. Smit, pers. comm.). Jan Smit found bivalves
with preserved bissus threads. The only large, complete fossil found
during fieldwork is a complete 20 cm. nautilus found in a hardground
in section 3, but not from the Berg en Terblijt hardground.
No complete fossils buried in life position are known from the Meersen
member in the ENCI quarry, only small organisms preserved in burrows.
In the ENCI quarry many pieces of coral were found in stage Ea,
but they have been subject to transportation, although not very
far.
The second difference is the occurrence of the many
claylayers in the Curfs quarry and Geulhemmergroeve. In the ENCI
quarry only one clay lens was found (Stage E). Another difference
is the presence of more thin bioclast grit layers in the Curfs quarry.
The upper part of, for example, stage F in the ENCI quarry
is completely bioturbated. Above the Berg en Terblijt hardground
and claylayer, the level of bioturbation is less (see 4.1
Curfs Quarry and Berg en Terblijt horizon) than the bioturbation
above the studied hardgrounds in the ENCI quarry (homogeneous due
to intensive burrowing).
From comparable sections with a hardground followed
by a bioclast grit layer the roundness of the larger bioclasts and
grainsize was compared. In most sections the fining upward trend
in the bioclast grit layer is clearly visible (i.e. E1, E2, E4,
C1, C4-6 see 4
Detailed field data). Both sections have the same trends, structures
and grainsizes. The roundness of the larger bioclasts is different.
In the Curfs quarry samples C84 and C46 show little rounding in
comparison to the ENCI samples. In samples C5-7 the rate of rounding
is high in comparison to the ENCI samples. The impression that the
bioclast grit layer above the Berg en Terblijt hardground
contains more intact fossils (J. Smit, pers. comm.) maybe true in
some samples but not in all samples.
8.3 Comparison Mesofossil content
and Bryozoa
Between the Berg en Terblijt, the Caster, and the
ENCI samples there are many differences. The large foraminifer Orbitoides
sp. is in both the Caster (50%) and the ENCI sample (80%) very
abundant, unlike the B+T sample (20%). Moreover, in the B+T sample
Siderolites calcitrapoides is very rare (1%) in comparison
to the other samples (almost 9%). Another major difference is the
occurrence of echinoid parts in the B+T sample (20%), while in the
other samples echinoid parts occur 1% or less. The B+T sample contains
also a large number of brachiopods (19%) in comparison to the ENCI
sample (0,3%) and the Caster sample (5,3%). One of the abundant
brachiopods in the B+T sample (Species 60) is not present in the
other samples. In the B+T sample belemnites were found (1%), which
are not present in the other samples.
When the sorted and unsorted Bryozoa are added together,
there is a big difference in Bryozoa percentages from the three
locations. The ENCI sample contains the least with 10%, while the
Caster and B+T sample contain 35% Bryozoa. In the B+T sample from
the Curfs quarry, a large percentage of Bryozoa could not be identified
due to encrusting by algae. Since this encrusting took place on
many Bryozoa, they must have rolled around, over and over again
before deposition. This is also noticeable when looking at the relative
preservation rate of the mesofossils in the three samples. The ENCI
and Caster mesofossil samples contain much better preserved mesofossils
than the B+T sample.
The several species of Bryozoa show much differences
in abundance in the three samples. Not all sorted species occur
in all samples. Further on, all samples contain species which are
not in the other samples. In the next comparison only species with
an occurrence of at least 2,5% were taken into account. Species
1 (25%) and 3 (10%) occur frequently in the Curfs sample and much
less in the other two samples. Species 13 occurs frequently in the
ENCI and Caster samples with 17 percent, but much less (9%) in the
B+T sample. Species 26 is extremely numerous in the B+T sample (73%),
in comparison to the other two samples (approximately 20%). Species
27 is only present in the Caster sample with 4%. Another big
difference is shown by species 29, with 15% in the ENCI sample,
1% in the Caster sample, and not present in the B+T sample.
Free standing Bryozoa can be massive (solid),
foliaceous (sheet-like, with zooids on both sides), dendroid (branch-like
or tree-like), or fenestrate (many branches joining and rejoining
to form a net-like shape). The massive forms can live in more turbulent
water than the delicate dendroid Bryozoa. To investigate the differences
between the three samples, the most abundant species were
arranged in groups with the same growth form. For this, only species
with an occurrence of at least 2,5% were taken into account.
The following groups were distinguished: Thin dendroid
(species 10, 26 and 27), moderate dendroid (species 1, 3 and 4),
thick dendroid (species 13), fenestrate (species 14) and free living
Bryozoa (species 29). These eight species include more than 90%
of the total amount of Bryozoa found in each sample.
| Group type |
Number Curfs Caster |
Number Curfs B+T |
Number ENCI |
|
Percentage Curfs Caster |
Percentage Curfs B+T |
Percentage ENCI |
|
|
|
|
|
|
|
|
| Thin dendroid |
15 |
5 |
8 |
|
0,9 |
1,2 |
3,2 |
| Moderate dendroid |
760 |
30 |
68 |
|
44,8 |
7,0 |
27,5 |
| Thick dendroid |
329 |
41 |
47 |
|
19,4 |
9,6 |
19,0 |
| Foliaceous |
447 |
331 |
68 |
|
26,4 |
77,1 |
27,5 |
| Fenestrate |
105 |
22 |
11 |
|
6,2 |
5,1 |
4,5 |
| Free living |
37 |
- |
45 |
|
2,2 |
- |
18,2 |
|
|
|
|
|
|
|
|
| Total |
1693 |
429 |
247 |
|
|
|
|
The results for the Caster sample and the ENCI sample
are roughly the same; only in the ENCI sample a larger number of
free living Bryozoa are present and 15% fewer moderate dendroid
Bryozoa. The B+T sample is very different. While most part of this
sample (77%) are filiaceous Bryozoa, the other types form the minority.
These Bryozoa can withstand much more turbulent water than the dendroid
and fenestrate Bryozoa.
8.4 Final conclusion
In the bioclast grit layer above the Berg en Terblijt
hardground, significantly more dendroid bryozoa are present, indicating
more turbulent waters. Also many of the fossil Bryozoa show encrusting
by algae on all sides in contrast to the ENCI and Curfs-Caster mesofossil
samples. This indicates that many of the components have been on
the surface of the seabed and shifted many times before final deposition.
Also the Berg en Terblijt sample is more rounded by transport
than the ENCI and Curfs-Caster mesofossil samples, although this
is not true in other section-samples. The chalk pebbles in section
1 in the Curfs quarry above the Berg en Terblijt hardground
indicate a relatively energy-rich environment. This layer could
have been deposited in very short time.
The overall impression there would be more intact
fossils in the bioclast grit layer above the B+T hardground, is
not true. The bioturbation between the Berg en Terblijt hardground
and Vroenhoven horizon in the Curfs quarry is less in comparison
to comparable sections from the ENCI quarry.
The well-preserved fossils buried in life position
known to come from the Berg en Terblijt horizon is the primary
argument for a relation with the Chicxulub meteorite impact. Similar
bioclast grit layers were found in the ENCI quarry (Stage Ea), although
no fossils buried in life position were found. Maybe, similar fossils
were found in the ENCI quarry when the stormlayers of Stage E in
the ENCI quarry were deposited without erosion taking place before
deposition.
The deposition of the bioclast grit layer on top
of the Berg en Terblijt hardground must have come suddenly
to bury the organisms in life position. This is probably the effect
of a storm. The only way to determine whether the storm itself was
caused by the Chicxulub meteorite impact is to look for the element
iridium or shocked quartz in or below the bioclast grit layer. From
the three hypotheses, only the first can be rejected.
© 1999
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