Lower Cretaceous Organic-Rich Sediments Drilled on Antarctic Continental Margin During ODP Leg 113

S. O'Connell

Lower Cretaceous organic-rich sediments were recovered below a major unconformity at two sites on the eastern Weddell Sea margin during ODP Leg 113.

No age overlap exists between the two sites. A more continuous Cretaceous section was previously recovered at DSDP Site 511 on the Falkland plateau.

Site 692 (2,880 m water depth) is located on a mid-slope bench in Wegener Canyon. Early Cretaceous-age (pre-Albian) sediments extend from 53-98 m below sea floor (mbsf) and are dominated by organic-rich nannofossil claystone. Macrofossils (e.g., belemites and ammonites), thin carbonate lenses (< 1 cm), and water escape structures are abundant. Thin beds of devitrified ash, bioturbation, and graded bedding are present.

Site 693 (2,360 m water depth), 30 km west of Site 692 on the canyon's outer rim, recovered Albian-age organic-rich claystones and mudstones from 416-483 mbsf. Site 693 sediments have lower organic contents than those at Site 692. Glauconite is common in the upper part of the unit. Well-preserved diatoms and diatomite layers suggest high productivity. These sediments differ from Albian-age sediments at Site 511, which consist of open-marine nannofossil ooze.

The sediments were deposited in an anoxic or hypoxic upper bathyal (500-1,000 mbsl) marine environment. Oxygenation occurred later along the Antarctic margin than the Falkland plateau, possibly as the inflow of Atlantic water proceeded south from an opening near the Falkland plateau.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.