LEG 129

Old Pacific Crust/Pigafetta and Mariana Basins

Original attempts to reach the oldest Pacific crust were hampered by a blanket of ubiquitous Cretaceous chert layers and widespread volcanic material further complicated by a lack of seismic data to define these units. Subsequent multichannel seismic profiling indicated that the earlier targets, established solely on the basis of magnetic anomaly interpretations, had been poorly located and succeeded in identifying better prospects. Mapping of magnetic anomalies showed that the eastern parts of the Pigafetta and East Mariana basins are areas where the supposed "Jurassic magnetic quite zone" is bordered to the northwest by possibly the oldest Mesozoic magnetic anomalies ever recognized in the ocean; possibly as old as Late to Middle Jurassic based on extrapolation of the age of the earliest anomaly (M25) so far dated in the Atlantic Ocean. During Leg 129, three sites were drilled in these two basins (Site 800 to Site 802).

Prior to Leg 129, M23 to M25 were the oldest magnetic anomalies dated in the world's oceans (DSDP Atlantic Sites 100 and 105). Sites on younger M-anomalies had yielded a reasonable calibration for the post-M25 anomaly series. At a site in the central Pigafetta Basin (Site 801), located on a magnetic quiet zone southeast of and presumably older than the M25-M37 magnetic lineation sequence, Leg 129 penetrated the Jurassic Pacific. The oldest dated sediment is interbedded with the underlying basalts and lies at the ~ 170 Ma-Callovian/Bathonian boundary.

Magnetic remanence measurements and variations in microfossil abundance and composition show that the Pacific Plate was formed north of the equator during the Middle Jurassic, crossed the equator in a southward direction at the end of the Jurassic, reversed this motion in the mid- Cretaceous and, since then, moved continuously northward. The absence of carbonate in this ridge-crest, equatorial-paleolatitude sedimentary facies suggests that the Late-Middle Jurassic superocean was characterized by carbonate production and/or preservation that was extremely low. The Middle Jurassic basement is composed of interbedded basaltic sheet flows, thin sills, and silicified claystone underlain by pillow basalts exhibiting concave pillow structures, chilled margins, variolitic textures, and microcrystalline interiors. A chrome yellow, silica-cemented hydrothermal concretion overlies the extremely altered pillow basalts.