Characteristic magnetizations of paleomagnetic samples from ODP Leg 103

Lower Cretaceous and Tithonian sediments were sampled for magnetostratigraphy at six holes (Holes 638B, 638C, 639A, 639D, 640A, and 641C). Magnetic polarity chrons were assigned to polarity zones using biostratigraphic constraints. In Holes 638B and 638C, polarity Chrons M3 and perhaps Ml are present in the upper part, M4 and M5 are apparently absent, M6 through M9 are poorly represented, M10 and M10N are apparently absent, and Ml 1 and M12 are tentatively assigned to the lower part. Strata in Holes 638B and 638C dip toward the south. In Hole 639A, polarity Chron M13 is well documented and M12A may be present. In Hole 639D, polarity Chrons M19 through M21 may be present, but the data is poor. Hole 640A had inadequate recovery to identify polarity chrons. In Hole 641C, polarity Chron MO is well documented and occurs significantly above the nannofossil marker of the Barremian/Aptian boundary; comparison to Italian magnetostratigraphy implies that this nannofossil datum is timetransgressive.

Cretaceous calcareous nannofossils of ODP Leg 103 holes

Ocean Drilling Program Leg 103 recovered Lower Cretaceous sediments from the Galicia margin off the coast of Iberia. The high diversity and abundance of assemblages makes this excellent material for the study of Early Cretaceous calcareous nannofossils. With the exception of a hiatus between the upper Hauterivian and lower Barremian, nannofossil distributions form a continuous composite section from the lower Valanginian to lower Cenomanian sediments recovered at the four sites. The sedimentation history of this rifted continental margin is complex, and careful examination of the nannofossil content and lithology is necessary in order to obtain optimum biostratigraphic resolution. The Lower Cretaceous sequence consists of a lower Valanginian calpionellid marlstone overlain by terrigenous sandstone turbidites deposited in the Valanginian and Hauterivian during initial rifting of this part of the margin. Interbedded calcareous marl and claystone microturbidites overlie the sandstone turbidites. Rifting processes culminated in the late Aptian-early Albian, resulting in the deposition of a calcareous, clastic turbidite sequence. The subsequent deposition of dark carbonaceous claystones (black shales) represents the beginning of seafloor spreading, as the margin continued to subside to depths near or below the CCD. The diversity, abundance, and preservation of nannofossils within these varied lithologies differ, and an attempt to distinguish between near shore and open-marine assemblages is made. Genera used for this purpose include Nannoconus, Micrantholithus, Pickelhaube, and Lithraphidites. In this study, six new species and one new subspecies are described and documented. Ranges of other species are extended, and an attempt is made to clarify existing, yet poorly understood, taxonomic concepts. A technique in which a single specimen is viewed with both light and scanning electron microscopes was used extensively to aid in this task. In addition, further subdivisions of the Sissingh (1977) zonation are suggested in order to increase biostratigraphic resolution.