The Cretaceous-Tertiary transition on the shallow Saharan Platform of Southern Tunisia


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The Cretaceous-Tertiary transition on the shallow Saharan Platform of Southern Tunisia
Keller G., Adatte T., Stinnesbeck W., Stueben D., Kramar U., Berner Z., Li L.Q., Perch-Nielsen K.V.
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A multidisciplinary approach to the study of a K/T boundary section on
the Saharan Platform based on planktic and benthic foraminifera,
calcareous nannofossils, lithology, stable isotopes, mineralogy and
geochemistry reveals a biota stressed by fluctuating hyposaline, hypoxic
littoral and nearshore environments, productivity changes, and a
paleoclimate altering between seasonal warm to temperate and warm/humid
conditions. Benthic foraminifera indicate that during the last 300 kyr
of the Maastrichtian (CF1, Micula prinsii) deposition occurred in a
inner neritic (littoral) environment that shallowed to a near-shore
hyposaline and hypoxic environment during the last 100-200 kyr of the
Maastrichtian. These conditions were accompanied by a seasonal warm to
temperate climate that changed to warm/humid conditions with high
rainfall, by decreasing surface productivity, and significantly
decreasing planktic and benthic foraminiferal species richness. The K/T
boundary is marked by an undulating erosional contact overlain by a 10
cm thick sandstone layer which is devoid of any exotic minerals or
spherules. Their absence may be due to a short hiatus and the fact that
the characteristic clay and red layer (zone PO) are missing. During the
earliest Danian (Pla), low sea-levels prevailed with continued low
oxygen, low salinity, high rainfall, high erosion and terrigenous
sediment influx, accompanied by low diversity, low oxygen and low
salinity tolerant species. These environmental conditions abruptly ended
with erosion followed by deposition of a phosphatic siltstone layer that
represents condensed sedimentation in an open (transgressive) marine
environment. Above this layer, low sea-levels and a return to
near-shore, hyposaline and hypoxic conditions prevailed for a short
interval [(base of Plc(2))] and are followed by the re-establishment
of normal open marine conditions (inner neritic) comparable to the late
Maastrichtian. This marine transgression is accompanied by increased
productivity, and the first diversified Danian foraminiferal assemblages
after the K/T boundary event and represents the return to normal biotic
marine conditions. Though the WT Seldja section represents one of the
most shallow marginal sea environments studied to date for this
interval, it does not represent isolated or atypical conditions. This is
suggested by the similar global trends observed in sea-level
fluctuations, hiatuses, as well as faunal assemblages. We conclude that
on the Saharan platform of southern Tunisia, longterm environmental
stresses beginning 100-200 kyr before the K/T boundary and related to
climate, sea-level, nutrient, oxygen and salinity fluctuations, were the
primary causes for the eventual demise of the Cretaceous fauna in the
early Danian. The K/T boundary bolide impact appears to have had a
relatively incidental short-term effect on this marine biota.
Création de la notice
28/09/2012 11:02
Dernière modification de la notice
20/08/2019 14:46
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