Site U1512

Abstract

International Ocean Discovery Program (IODP) Site U1512 (34°1.6406′S, 127°57.7605′E) lies in the Great Australian Bight at ~3100 m water depth on the continental slope. A nearby industry well (Jerboa-1) in the Eyre Subbasin on the continental shelf provides a stratigraphic tie along a seismic profile line. The objective for coring Site U1512 was to obtain a continuous Upper Cretaceous record of marine black shales in the Great Australian Bight, including Oceanic Anoxic Event (OAE) 2, which straddles the Cenomanian/Turonian boundary interval. The Site U1512 sediment record will be compared with coeval Expedition 369 sequences cored in the Mentelle Basin to characterize the geochemical and biological responses to extreme global carbon cycle perturbations in different paleoceanographic settings at high southern latitudes. During the Cretaceous, the Great Australian Bight was situated at the eastern tip of a partial seaway (the Australo-Antarctic Gulf [AAG]), with the Naturaliste Plateau in the open ocean at the western gateway that connected the AAG with the southern Indian Ocean. The breakup on the southern margin of Australia occurred at a very slow rate, and rifting is thought to have started in the Cenomanian to Turonian. Plate tectonic reconstructions corresponding to these early stages of rifting are poorly constrained and controversial. The early rift ~15 km thick post-Middle Jurassic sedimentary sequence that accumulated in the Great Australian Bight contains the largest continental margin deltaic sequence deposited during the Late Cretaceous greenhouse period. Accelerated subsidence in the late Albian and through the Cenomanian to Santonian led to the deposition of a thick sequence of marine shales. An overall transgressive phase of sedimentation in the early Paleogene was followed by the establishment of open marine carbonate shelf conditions from the early Eocene onward. The AAG eventually widened to create the Southern Ocean, with a switch to rapid spreading after 45 Ma. Recovery of a mid-Cretaceous dredge sample with as much as 6.9% total organic carbon (TOC) from a location 10 km north-northwest of Site U1512 provided the impetus for coring Site U1512. The latest Cenomanian to earliest Turonian age of the material was based on organic-walled dinoflagellate cyst (dinocyst) assemblages that constitute a typical association for OAE 2 in both hemispheres. Although we cored an additional 130 m below the original target depth, the Cenomanian/Turonian boundary interval was not reached. Despite this, studies of the ~700 m sequence of early Turonian to late Santonian marine claystone will provide new insights into the evolution of Late Cretaceous climate and oceanography in the region of the AAG.