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| Titel |
A record of early Silurian glacioeustacy from the Type Llandovery |
| VerfasserIn |
J. R. Davies, R. A. Waters, S. G. Molyneux, M. Williams, J. A. Zalasiewicz, T. R. A. Vandenbroucke, J. Verniers |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067194
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| Zusammenfassung |
| Recent detailed work on the Type Llandovery succession in central Wales, UK has allowed
the erection of a thoroughly revised sedimentary architecture and new sequence
stratigraphical model. Significant new chitinozoan, acritarch and graptolite discoveries have
refined the succession’s potential for international correlation. The new analysis reveals that
Rhuddanian to Aeronian parts of the Type Llandovery succession were fashioned by
movements in marine base level. A series of flooding surfaces are overlain by progradation
sequences (progrades) in which laminated, off-shore mudstones pass vertically and laterally
into bioturbated shoreface sandstones; pentamerid-bearing sandstones characterising the
shallowest phases of deposition. At the tops of the progrades, erosion surfaces recording
emergence merge laterally into the compound unconformities that characterise condensed,
proximal parts of the succession. The progrades comprise a succession of smaller scale
parasquences. They can also be grouped into composite (higher order) sequences and
both these and their component progrades can now be dated using the standard
UK graptolite biozonal scheme. Three composite sequences are recognised that
reached their progradational acme during the acinaces, lower convolutus and upper
sedgwickii-halli graptolite biozones. Significant lower order progrades occurred during the
revolutus, triangulatus and both the middle and upper convolutus biozones. Following a
post-glacial maximum persculptus Biozone highstand, the succession of Llandovery
flooding surfaces record transgressions that peaked during the revolutus, middle
convolutus and lower sedgwickii biozones; significant secondary events are linked
to local first appearances of revolutus and upper convolutus graptolite biozonal
assemblages.
The early Silurian retreat of the Gondwanan-based South Polar ice sheet was interrupted
by episodes of ice re-advance during the Rhuddanian and Aeronian and published
dating supports the confident interpretation of the three higher order Llandovery area
sequences as the culmination of the forced regressions that accompanied these glacial
events. Studies of other Llandovery successions suggest that lower order events
during the triangulatus and convolutus biozones are also widespread and promote
the possibility that all the Llandovery area progrades were far field responses to
glacial re-advance. Recalibrated dating of the Type Llandovery succession makes
this model for a glacioeustatically determined Rhuddanian to Aeronian sequence
stratigraphy available for international testing. Evidence of a eustatic signal is lacking in
Telychian strata in Wales where the impact of any eustatic sea level movements was
enhanced or overridden by the growing influence of intra- and peri-basinal tectonism. |
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