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GCR Display Cores

Section 122-761B-21X-4

Location

Northwestern Australian margin, Wombat Plateau

Regional setting

The northwestern Australian margin is one of the oldest continental margins in the world that has both a relatively low sediment influx and a large biogenic component. As such, it is an ideal margin for integrated sedimentologic, biostratigraphic, paleobathymetric, and subsidence studies.

Leg objectives

• Compare tectonic and seismic sequences with Atlantic passive margins;
• Refine the Mesozoic geological time scale; and
• Characterize of old ocean crust prior to subduction under the Sunda arc.

Lithology

Bioturbated, light gray, clay-rich chalk nannofossil chalk and white nannofossil chalk. Chert (4-15 cm and 125-130 cm). The basal Paleocene section above the sharp change in color is rich in clay and authigenic clinoptilolite (zeolite)

Age

Upper Maastrichtian–Lower Paleocene. K/T boundary has been placed at the sharp change in color and lithology.

Discussion

The nannofossil chalk is the pelagic origin. During the Maastrichtian-Paleocene time the plateau apparently had subsided sufficiently to avoid the influence of sea level fluctuations. Local layers or nodules of porcellanite chert replacing a radiolarian rich foraminiferal chalk and the presence of authigenic zeolite (clinoptilolite) suggest dissolution of skeletal opal, diagenetic silica mobilization, and reprecipitation as authigenic silicate (zeolite) or opal-CT.