Early Cretaceous radiolarians of the Northeast Indian Ocean (leg 123 - site 765, site 766 and dsdp site 261) - The Antarctic-Tethys connection


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Early Cretaceous radiolarians of the Northeast Indian Ocean (leg 123 - site 765, site 766 and dsdp site 261) - The Antarctic-Tethys connection
Marine Micropaleontology
Baumgartner P.O.
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During ODP Leg 123, abundant and well-preserved Neocomian radiolarians
were recovered at Site 765 (Argo Abyssal Plain) and Site 766 (lower
Exmouth Plateau). Assemblages are characterized by the numerical
dominance of a small number of non-tethyan forms and by the scarcity of
tethyan taxa. Remarkable contrasts exist between radiolarian assemblages
extracted from claystones of Site 765 and reexamined DSDP Site 26 1, and
faunas recovered from radiolarian sand layers, only found at Site 765.
Clay faunas are unusual in their low diversity of apparently
ecologically tolerant (or solution resistant?), ubiquist species,
whereas sand faunas are dominated by non-tethyan taxa. Comparisons with
Sites 766 and 26 1, as well as sedimentological observations, lead to
the conclusion that this faunal contrast resulted from a difference in
provenance, rather than from hydraulic sorting or selective dissolution.
The ranges of 27 tethyan taxa from Site 765 were compared to the tethyan
radiolarian zonation by Jud (1992) by means of the Unitary Associations
Method. This calculation allows to directly date the Site 765
assemblages and to estimate the amount of truncation of ranges for
tethyan taxa. Over 70% of the already few tethyan species of Site 765,
have truncated ranges during the Valanginian-Hauterivian.
Radiolarian assemblages recovered from claystones at Sites 765 and 261
in the Argo Basin apparently reflect restricted oceanic conditions
during the latest Jurassic-Barremian. Neither sedimentary facies nor
faunal associations bear any resemblance to what we know from typical
tethyan sequences. We conclude that the Argo Basin was
paleoceanographically separated from the Tethys during the Late Jurassic
and part of the Early Cretaceous by its position at higher
paleolatitudes and/or by enclosing land masses.
Assemblages recovered from radiolarian sand layers are dominated by
non-tethyan species that are interpreted as circumantarctic. Their first
appearance in the late Berriasian-early Valanginian predates the
oceanization of the Indo-Australian breakup (M11, late Valanginian), but
coincides with a sharp increase in margin-derived pelagic turbidites.
The Indo-Australian rift zone and the adjacent margins must have been
submerged deeply enough to allow an intermittent influx of
circumantarctic cold water into the Argo Basin, creating increased
bottom current activity. Cold-water radiolarians carried into the Argo
Basin upwelled along the margin, died, and accumulated in radiolarite
layers due to winnowing by bottom currents. High rates of faunal change
and the sharp increase of bottom current activity are thought to be
synchronous with possible pronounced late Berriasian-early Valanginian
lowstands in sea level. Hypothetically, both phenomena might have
been.caused by a tendency to glaciation on the Antarctic-Australian
continent, which was for the first time isolated from the rest of
Gondwana by oceanic seaways as a result of Jurassic-Early Cretaceous
sea-floor spreading.
The absence of most typical tethyan radiolarian species during the
Valanginian-Hauterivian is interpreted as reflecting a time of strong
influx of circumantarctic cold water following oceanization (M11) and
rapid spreading between Southeast India and West Australia.
The reappearance and gradual abundance/diversity increase of tethyan
taxa, along with the still dominant circumantarctic species are thought
to result from overall more equitable climatic conditions during the
Barremian-early Aptian and from the establishment of an oceanic
connection with the Tethys Ocean during the early Aptian.
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06/10/2012 17:23
Last modification date
20/08/2019 15:21
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