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| Titel |
High but balanced sedimentation and subsidence rates (Moodies Group, Barberton Greenstone Belt), followed by basin collapse: Implication for Archaean tectonics |
| VerfasserIn |
Christoph Heubeck, Donald R. Lowe, Gary R. Byerly |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250037064
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| Zusammenfassung |
| Archaean tectonophysical models distinguish between thick, rigid and thin, mobile crust;
from these the major mechanisms and rates for continental growth are derived. Archaean
sedimentary rocks, preserved in metamorphosed and highly deformed greenstone belts, can
contribute to constrain these models by estimating subsidence rates, derived from the
combination of facies changes and precise age dates.
Largely siliciclastic strata of the Moodies Group form the topmost unit of the Barberton
Supergroup of the Barberton Greenstone Belt (BGB), South Africa, represent one of the
world’s oldest unmetamorphosed quartz-rich sedimentary sequences, and reach ca. 3500m
thick (Lowe and Byerly, 2007). Large parts of the Moodies Group were deposited in apparent
sedimentary continuity in alluvial, fluvial, shoreline and shallow-marine environments
(e.g., Eriksson, 1979; Heubeck and Lowe, 1994). Distinctive sources and variations
in facies indicate that Moodies deposition occurred at times in several basins. In
several now tectonically separated regions, a regional basaltic lava (unit MdL of
Anhaeusser, 1968) separates a lower unit (ca. 2000m thick and possibly representing an
extensional setting) from an upper unit (ca. 1500m thick and characterized by progressive
unconformities, rapidly changing facies, thicknesses, and sandstone petrographic
composition).
Single zircons separated from a felsic air-fall tuff of the middle Moodies Group and
immediately overlying the basaltic lava in the Saddleback Syncline were dated on
the Stanford-USGS SHRIMP RG. Out of 24 dated grains, two near-concordant
groups have mean ages of 3230,6+-6,1Ma (2Ï; n=9) and 3519+-7 Ma (2Ï; n=9),
respectively. We interpret the former age as representing the depositional age of the
tuff, the latter as representing inherited zircons from underlying Onverwacht-age
basement.
The interpreted depositional age of the Moodies tuff is indistinguishable from numerous
similar ages from felsic and dacitic volcanics at the top of the underlying Fig Tree Group
(Schoongezicht Fm.; Byerly et al., 1996), implying that ca. 2000m of Moodies sandstones
and subordinate siltstones and conglomerates were deposited in not more than a few (0-6)
Ma. Their comparatively low degree of facies variation and lithological change implies a
balance between rates of sediment supply and of subsidence, creating thick stacked
units. Ferruginous shales and thin BIFs of the upper Moodies Group suggest that
background “Fig-Tree-style” sedimentation continued during Moodies time but
was mostly overwhelmed by the apparently brief but massive influx of medium-
to coarse-grained quartzose sediment. Because two progressive unconformities,
marking Moodies basin uplift and onset of renewed overall BGB shortening, occur
only 50 m above this dated unit, they are likely of a similar age and imply that
dominant NW-SE-directed shortening in the BGB began shortly after 3230+-6
Ma.
The combination of these new data with published information thus suggest
that the Moodies Basin formed after 3225+-6 Ma (i.e., at the earliest at 3231) but
was already largely filled and began to be deformed by 3231+-6 (i.e., at the latest
by 3225). Moodies deposition thus happened geologically nearly instantaneously
following the end of Fig Tree volcanism, took very little time and deposited large
volumes of sediments on a rapidly subsiding basement just prior to large-scale BGB
deformation.
REFERENCES
Byerly, G.R., Kroner, A., Lowe, D.R., Todt W., Walsh, M.M., 1996, Prolonged
magmatism and time constraints for sediment deposition in the early Archean Barberton
greenstone belt: Evidence from the Upper Onverwacht and Fig Tree groups: Precambrian
Research, 78, p. 125-138.
Eriksson, K.A., 1979, Marginal marine depositional processes from the Archaean
Moodies Group, Barberton Mountain Land, South Africa: Evidence and significance:
Precambrian Res., 8, p. 153-182.
Heubeck, C. and Lowe, D.R., 1994, Depositional and tectonic setting of the Archaean
Moodies Group, Barberton Greenstone Belt, South Africa: Precambrian Res., 68, p.
257-290.
Lowe, D.R., and Byerly, G.R., 2007, An overview of the geology of the Barberton
Greenstone Belt and vicinity: Implications for early crustal development; in: M.J. von
Kranendonk, R.H. Smithies and V.C. Bennett, eds., Earth’s Oldest Rocks. – Elsevier
(Developments in Precambrian Geology), vol. 15, p. 481-526. |
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