MSc, Faculty of Science, University of the Witwatersrand, 2011 / The 35.3 million year old, 85 km diameter, Chesapeake Bay impact structure (CBIS) in
Virginia, USA, is one of the best preserved complex marine impact structures on Earth
and is associated with the North American tektite strewn field. Three drill cores
(Eyreville A, B and C) were obtained from the Chesapeake Bay impact structure during
2005-2006 by the CBIS Deep Drilling Project in conjunction with the International
Continental Scientific Drilling Program (ICDP) and the United States Geological Survey
(USGS). The drill cores intersected crystalline basement rocks, impactites, and impactrelated
and post-impact sediments. This study focuses on the impactite sequence of the
Eyreville B drill core. The primary focus has been to examine and understand the
conditions and processes involved in the formation of the crater-fill impactite sequence,
and the provenance of the impactites, through detailed lithostratigraphic, petrographic
and geochemical analysis.
The Eyreville B drill core intersected 154 m of impactites between the depths of 1397.16
and 1551.19 m. The impactite sequence is divided into the upper (1397.16 to 1474.05 m)
and the lower (1474.05 to 1551.19 m) impactite units. The upper impactites are matrixsupported
(23.5 rel% of total clast count) and characterised by suevite, clast-rich impact
melt rock and cataclastic gneiss blocks, whereas the lower impactites are clast-supported
(19.8 rel% of total clast count) and are dominated by polymict impact breccia and
cataclastic gneiss boulders and blocks. The suevites comprise melt and lithic clasts from
sedimentary (predominantly shale and sandstone) and igneous (such as granitoid and
quartz pegmatoid) target rocks in an unsorted matrix composed of mineral (primarily
quartz, feldspar and micas) and lithic clasts. The polymict impact breccias are primarily
composed of metamorphic clasts such as phyllite, mica schist and felsic and mafic gneiss,
and are largely, but not completely devoid of melt clasts. The majority of clasts in the
impactite sequence closely resemble the granitoid, pegmatoid, calc-silicate, amphibolite
and mica schist lithologies found in the underlying basement-derived succession and
megablocks in the overlying sedimentary clast breccia. Overall, the crystalline (igneous
and metamorphic) and sedimentary clasts contribute 62.3 and 20.8 vol%, respectively, of
iv
the total lithic clast composition which is comparable to 58.2 (crystalline) and 26.0
(sedimentary) vol% for the latest published results.
The impactites are generally heterogeneous in terms of their chemical compositions. The
impactite samples display enrichment in FeO+MgO in comparison to the target rock
lithologies, with smaller abundances of K2O and Na2O, with little to no CaO. Throughout
the impactite sequence, the suevites display the largest variety in chemical composition
due to the heterogeneity of the clasts.
The overall abundance of melt clasts varies from 22.1 vol% (of the total clast population)
in the upper impactites to 2.5 vol% (of the total clast population) in the lower impactites.
Melt clasts are generally flattened and elongated and display laminar flow structures
(schlieren), with fractured terminations. Most melts are highly vesiculated and altered to
phyllosilicate minerals. Overall, melt clasts show a general decrease in size with depth.
Observations indicate that no coherent melt sheet was intersected; impact melt rock was
only noted in the impactite sequence at depths between 1402.02 and 1407.49 m and
1450.22 and 1451.22 m. Melt clasts are heterogeneous in terms of their chemical
compositions and are generally SiO2-rich and represent the melting and mixing of
different mineral (quartz, feldspar and phyllosilicates) types derived from the target
lithologies. This finding is comparable to the observations noted in the recent published
literature.
On average, 23.6 rel% of all quartz grains in the upper impactite unit display one or more
PDF (planar deformation features) sets, with this number decreasing to 13.33 rel% for the
lower impactite unit. A general decrease in average shock pressure with depth has been
noted, which is consistent with the decrease in other shock features and melt clast
abundance from the upper to lower impactites. A maximum of 3 PDF sets in the quartz
grains, in the upper impactites, were noted; however, mostly 1 or 2 PDF sets were
observed. Diaplectic glass has been noted in the melt clasts and is present predominantly
in the upper impactites. No PDFs in feldspar grains have been noted.
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A small, low temperature impact-induced hydrothermal system (220 – 300 °C) affected
the material within the crater, which is evident from veins and patches of quartz, calcite,
secondary phyllosilicate minerals (smectite), zeolites, secondary pyrite and chalcopyrite,
as well as other sulphides.
The upper and lower impactites show differing petrographic, geochemical and shock
characteristics, suggesting that they were formed by different mechanisms. The upper
suevites (upper part of the impactite sequence) are composed of fallback debris from the
collapsing ejecta plume or curtain, whereas the suevites (S3 and S2) represent a mixture
of the ground-surge material and fallback debris from the collapsing ejecta plume. The
impact melt rocks are interpreted as either detached remnants of the melt lining the
transient crater or piles of melt derived from fallback debris. It is proposed that the lower
suevites (S1) and polymict impact breccia represent ground-surge deposits at the base of
and behind the advancing ejecta curtain, modified by slumping and mixing of unshocked
material from the outer crater walls. The cataclastic gneiss blocks and boulders slumped
in from the outer transient crater walls and were incorporated into the ground-surge
deposits.
This study of the impactite sequence from the Chesapeake Bay impact structure has
provided new insights into the formation of the impactite sequence as well as that of the
Chesapeake Bay impact structure. Research such as this allows for further understanding
and discussions regarding marine cratering processes (impact processes and impact-generated deposits) and emplacement mechanisms for impact craters. Essentially a study
such as this provides material for further extensive research into the formation of marine
impact craters and comprehensive modelling.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/10430 |
| Date | 19 September 2011 |
| Creators | Jolly, Lauren Cher |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf |
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