Broken Ridge and Ninetyeast Ridge
During Leg 121, four sites (Site 752 to Site 755) were drilled on Broken Ridge to test competing
hypotheses about rifting tectonics and lithospheric extension. Operations documented that
sedimentation occurred in increasing water depth before the rifting event, consistent with rift
initiation by intraplate stress. The uppermost strata in the dipping and truncated sequence, pelagic
carbonate ooze, are middle Eocene in age. The youngest sediments deposited on the truncation
surface are upper Eocene oozes containing reworked assemblages of shallow-water foraminifers
with coarse sands and gravels. The winnowed Oligocene to Pleistocene pelagic cap records the
intensity of ocean currents flowing over the ridge. These results indicate > 1,000 m of uplift in
response to a middle Eocene rifting event. The short duration of the rifting event (3-7.5 m.y.) and
low present-day heat flow suggest a mechanical rather than a thermal mechanism for uplift. The
overall response of the ridge to rifting is consistent with theoretical explanations for the uplift of rift
flanks or shoulders as a flexural rebound effect following tectonic unloading of the lithosphere
during extension. The large amounts of volcanic ash documented in the section are similar in
composition to Kerguelen and Ninetyeast lavas, and represent distal fallout of hot spot volcanism a
few hundred kilometers to the southwest. At the K/T boundary, Cretaceous chalks are succeeded
by an ash-rich layer overlain by Paleocene chalks. This ash layer represents an interval of > 1 m.y.
during which the rate of carbonate deposition was as little as 1/8 the prevailing rate immediately
above and below, indicating abnormally low planktonic productivity following an ecological crisis
at the end of the Cretaceous.
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