Sedimentology of lower Cretaceous sediments from the east Antarctic margin

Lower Cretaceous organic-rich sediments were recovered at Sites 692 and 693 on the eastern Weddell Sea margin during ODP Leg 113, below a major unconformity.
Site 692 (2875 m water depth) is located on a mid-slope bench in Wegener Canyon. Early Cretaceous age (Neocomian, probably Berriasian-Valanginian) sediments extend from 53 to 98 meters below seafloor (mbsf) and are dominated by strongly laminated, organic-rich nannofossil claystone and mudstone with peloids, calcispheres, and radiolarians. Macrofossils (e.g., belemnites and ammonites), thin lenses and nodules containing carbonate-fluorapatite (< 1 cm diameter), and fluid-escape structures are abundant. Thin beds of devitrified ash and poorly developed graded bedding are present.
Site 693 (2360 m water depth), 30 km west of Site 692 on the outer canyon rim, yielded Albian age organic-rich claystones and mudstones from 416 to 484 mbsf. Site 693 sediments have organic contents lower than those at Site 692. Glauconite is common, and three thin limestone beds are present in the upper part of the unit. Well-preserved diatoms, radiolarians, and diatomite layers suggest that the sediments were deposited under conditions of high productivity. Ashclay layers with well-preserved glass shards indicate volcanic activity at the time of deposition.
The sediments at both sites were deposited in an upper bathyal (500-1000 mbsl) marine environment under primarily dysaerobic conditions. At Site 692 the sediment may have experienced periods of anoxia. The sediments at Site 693 record the beginning of oxygenation along the Antarctic margin during the Albian at about the same time as on the Falkland Plateau.