Geological and geophysical investigations of the Chicxulub meteorite
impact structure have been ongoing since its scientific recognition in 1991
Hildebrand et al. 1991). The structure is of important significance because it is
currently the only known impact crater that is linked to a global catastrophe, the
Cretaceous/Tertiary boundary that occurred 65 Ma years ago. Major climatic and
biological changes occurred at this interval that include the disappearance of
70% of all living species, in particular the dinosaurs. A global iridium anomaly
along with the occurrence of shocked quartz grains characterize a thin clay layer
this interval that led to the search for a large meteorite impact crater on
continental crust. A large “volcanic” igneous province identified by oil
exploration boreholes on the NW region of the Yucatán Peninsula was eventually
recognized as a vast impactite deposit associated with a 180 km wide crater. Until
2002, only small grab and chip samples had been described from Chicxulub. This
lack of sampling and, thus, poor understanding of the cratering conditions at
Chicxulub led the International Continental Drilling Program (ICDP) to fund and
drill the Yaxcopoil-1 borehole.
The Yaxcopoil-1 (Yax-1) borehole was drilled 60 km south-southwest
from the center of the Chicxulub meteoritic impact. It intersected 794.63 m of
post-impact cover rocks, 100.31 m of impactites, and 616.03 m of Cretaceous
target rocks, terminating at a final depth of 1510.97 m. The impactite interval, as
well as several selected samples from the Cretaceous target rocks, is the focus of this scientific investigation. In conjunction with this work, the Yax-1 core was
studied by numerous international research groups and is, thus, currently one of
the best studied continuous diamond drill core from an impact crater. This
petrographic and geochemical investigation provides further understanding on the
primary and secondary conditions that influenced the formation of the Yax-1
impactites and selected target rocks.
Five units have been recognized in the impactite interval. These
subdivisions are based on macro- and microscopic observations and are
complemented by geochemical characteristics. Unit 1 (795-822 m) comprises
subrounded melt rock particles that are poorly sorted, yet show a progressive gradation with height, are self supported, show perlitic devitrification texture, and
are generally fine-grained. Unit 2 (823-846 m) and Unit 3 (846-861 m) are
relatively similar, as they both consist of a groundmass-supported breccia with
melt rock particles that are angular, fluidal, and vesiculated in texture. The
groundmass in both units is pervaded by numerous carbonate-veinlets and
decreases in volume towards Unit 3 because of compaction. Unit 2 and Unit 1 are
both altered to a predominantly green colour by the pervasive conversion of
silicate phases from clay minerals. Unit 3 is of a variegated character and is
suggested to be the less altered unit bove Unit 4. Unit 4 (861-885 m) comprises a
massive yet brecciated microcrystalline impact melt rock. It is primarily of a
silicate composition and contains only minor secondary carbonate crystals. All
lithic fragments are of silicate compositions. Unit 5 (885-895 m) shows the
greatest variation in the proportion of melt rock particles and lithic fragments.
The melt rock particles contain numerous microlites that crystallized below the
glass-transition temperature. These are suspended in a carbonate groundmass that
is either of a primary impact melt origin or of a secondary nature. Units 1 and 5
both contain foraminifera fossils and greater proportions of carbonate clasts than
any other units. All unit show shock metamorphic characteristics, i.e., planar
deformation features, ballenquartz, and checkerboard feldspar.
Geochemical results have been obtained by various analytical techniques
in order to constrain cratering and alteration processes at various sampling scales.
Main results reveal that samples from units 1 and 2 have been leached of their
alkali elements, show negative Ce anomalies on a microscopic scale, and show
less major element variation on a bulk sample scale than lower units. The
groundmass in units 1 to 3 comprises a microcrystalline calcite and altered alkali
element-, Ca- and Si- rich cement. In units 2, 3, and 5 melt rock particles are of a
heterogeneous composition. In Unit 1, melt rock particles are highly altered,
therefore volatile rich, and are of a more homogeneous composition than those of
other units. On a bulk sampling scale, the silicate component for the whole
impactite sequence shows remarkable homogeneity. Major and trace element
compositions show that this component and Unit 4 are typical of the upper
continental crust. The carbonate component is more calcite rich than dolomitic and most likely represents strong secondary alteration. No significant sulfur
content was measured compared to published known target rock values. The
contents of the siderophile elements, including Ni, Co, Ir, and Cr, do not indicate
the presence of a significant extraterrestrial component in the Yax-1 impactites.
Cretaceous rocks were also sampled in order to provide compositional
constraints with the impactites and observe any shock related metamorphic
features. Petrographic observations indicate that the Cretaceous rocks in the
Yaxcopoil-1 drill core likely register a multistage deformation history that spans
the period from pre- to post-impact. Contrary to previous studies that claimed
evidence for the presence of impact melt breccia injection veins, no evidence was
found from samples located between 1347–1348 m depth for the presence of melt
breccia.
An emplacement mechanism for the impactite sequence is proposed with
regards to cratering. Unit 5 is interpreted as an early ejecta deposit that was
emplaced following the passage of the initial ejecta curtain during the excavation
stage of cratering. Unit 4 is an allogenic siliceous melt rock body that originated
primarily from the fusion of the silicate crystalline basement. The origin of Unit 4
is based on geochemical and petrographic arguments, i.e., no carbonate
component to the melt could be detected and only igneous/metamorphic
mineral/rock fragments were observed in it. It is suggested Unit 4 was emplaced
as an outward flow of fused crystalline basement rocks from the collapsing central uplift or it may have also been deposited from the fallback of a large melt bomb.
Brecciation occurred post-deposition as fragments fit together like pieces of a jigsaw
puzzle. Units 2 and 3 represent unreworked fallback suevite deposits.
Vesiculated melt rock particles are a testimony of the volatile rich nature of the
collapsing impact plume. Volatiles are believed to have helped disperse the
suevite and inhibited the melt rock particles from undergoing compositional
homogenization. Unit 1 represents a reworked fallback deposit that formed from
the resurge of seawater into the impact basin. Unit 2 is the altered equivalent of
Unit 3 and along with Unit 1 underwent significant post-depositional
phyllosilicate alteration from circulating fluids at the top of the suevite pile.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5753 |
| Date | 14 October 2008 |
| Creators | Tuchscherer, Martin Guillaume |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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