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Jurassic limestones from Holes 544A and 547B were sampled to determine their magnetic stratigraphy and to compare their magnetic properties with those of Italian limestones of similar age and facies. The recovery at both holes was insufficient for determining a recognizable pattern of magnetozones. However, the presence of reversals at Hole 544A indicates that these limestones were not deposited within the Jurassic "quiet zone" of prolonged normal polarity (Oxfordian-Callovian). Thermal and alternating field demagnetization of these varicolored limestones indicates either single component magnetizations or, in some cases, the superimposition of antiparallel components with differing blocking temperature and coercivity spectra. INTRODUCTION The sampling at Hole 544A was conducted in order to obtain the maximum possible sample density to resolve a magnetic stratigraphy. In contrast, the sampling at Hole 547B was directed toward the collection of varicolored samples of differing lithology for a study of variations in magnetization history. The samples were subjected to thermal and alternating field demagnetization treatments and their magnetization was measured with a cryogenic magnetometer. HOLE 544A The Jurassic limestones at Hole 544A (Unit III) are pale reddish brown (10R 5/4) in color with pale yellowish brown (10YR 6/2) and yellowish gray (5Y 7/2) mottling. Forty-three samples were taken from Cores 544A13 to 544A-16 (Fig. 1). The recovery was too poor, and therefore the sampling density too low, to establish a continuous magnetic stratigraphy for these Jurassic rocks. However, the inclination data obtained clearly define several reversals (Fig. 1). The samples were thermally demagnetized in progressive 50°C steps from 100 to 550°C, with additional steps of 575 and 600°C for some samples. The resulting demagnetization data are interesting and will be discussed in some detail. Many of the samples, such as 544A-13-1, 12 cm (Fig. 2A), show single component decay of the magnetization vector to the origin of the orthogonal projection. The large drop in intensity of the magnetization vector between 300 and 35O°C in this particular sample is not a general characteristic. The reversed interval at the base of Core 544A-13 (Fig. 1) is characterized by behavior similar to that of 544A-13-1, 110 cm (Fig. 2A) where the polarity of the magnetization does not change until demagnetization temperatures of 450°C are reached. This Hinz, K., Winterer, E. L. et al., Init. Repts. DSDP, 79: Washington (U.S. Govt. Printing Office). 2 Address: Department of Geology, University of Florida, Gainesville, FL 32611. temperature suggests that the normal magnetization component in this sample is not viscous in origin, because it is unlikely that a viscous remanent magnetization (VRM) carried by hematite (or any other mineral) could survive demagnetization temperatures in excess of 35O°C (Dunlop and Stirling, 1977). The normal overprint apparent in this and other samples that feature a high blocking temperature reversed component is probably chemical in origin (CRM) and reflects hematite growth during both a reversed and a subsequent normal polarity interval. The presence of antiparallel hematite CRM's has been observed in rocks of similar facies from Italy (Channell, Freeman, et al., 1982) where it has been demonstrated that the higher blocking temperature component records the older polarity interval. The reversed samples at the top of Core 544A-14 become reversed after demagnetization at 100°C (544A-14-1, 27 cm; Fig. 2A), but the normal overprint is present up to 250°C. The lower reversed interval in Core 544A-14 (544A-14-2, 14 cm; Fig. 2A) exhibits this same normal overprint but up to temperatures of 400°C. Another sample from the same reversed interval (544A-14-2, 93 cm; Fig. 2B) shows similar behavior with the normal overprint present at temperatures up to 450° C. The polarity of the magnetization components is not always unambiguous. For example, the behavior of 544A-14-3, 63 cm (Fig. 2B) suggests the presence of a high temperature reversed magnetization component, but the vector endpoints are not well defined due to the low magnetization intensities at these high demagnetization temperatures. Another example of ambiguity is seen in 544A-15-3, 88 cm (Fig. 2C) where the vector endpoints appear to be missing the origin of the orthogonal projection and a reversed magnetization component is not clearly defined. Reversed magnetizations are occasionally apparent at low demagnetization temperatures (544A-15-2, 29 cm; Fig. 2C) but usually reversed magnetizations are only achieved at demagnetization temperatures of 200°C or above. As mentioned above, the pervasive normal overprint seen in samples which feature a high blocking temperature reversed magnetization component is very probably

Magnetization of Varicolored Pelagic Limestones from Holes 544A and 547B