27. Magnetostratigraphy of the Cretaceous/Tertiary Geological Boundary1

27. MAGNETOSTRATIGRAPHY OF THE CRETACEOUS/TERTIARY GEOLOGICAL BOUNDARY1

Nikolai Petersen, Institut für Allgemeine und Angewandte Geophysik, Universitát München, Theresienstr. 41, D-8000 Munich, Federal Republic of Germany and F. Heller and W. Lowrie, Institut für Geophysik, Eidgenössische Technische Hochschule Zurich, CH-8093 Zurich, Switzerland

INTRODUCTION The Cretaceous/Tertiary boundary is particularly well preserved and undisturbed in sediments acquired in the The Cretaceous/Tertiary (C/T) geological boundary Cape Basin of the South Atlantic Ocean. The sediments is usually sharply defined. In deep sea sediments, where were acquired by drilling at Site 524 (Leg 73 of the Deep its location can be determined within a few millimeters, Sea Drilling Project), which was located at 29°29.05'S studies have shown that it is characterized by the large- and 3 °30.74'E. The boundary was recognized at a depth scale mortality of the biomass in the oceans (Luterbach- of 203.54 m below the ocean floor on the basis of the er and Premoli-Silva, 1964; Perch-Nielsen, 1979; Hsü et first occurrences of the Tertiary nannofossils Markalius al., 1982). It is also characterized by a distinct enrich- astroporus, M. reinhardtii, Zygodiscus sigmoides, and ment of noble elements, in particular Ir and Os, which Thoracosphaera species (Hsü et al., 1982; Poore et al., has led a number of scientists to postulate that the fau- this vol.). The boundary is also characterized by a mini- nal extinctions were caused by an extraterrestrial event, mum in the CaCO3 content, which decreased from an such as the impact of a comet or a meteorite (Alvarez et average of 40% to about 2% in a 2-cm-thick clay layer al., 1980; Hsü, 1980; Kyte et al., 1980; Smit and Herto- at the boundary. The material in the vicinity of the C/T gen, 1980). boundary is nannofossil marl that is frequently interca- The magnetostratigraphy of the Cretaceous/Tertiary lated by turbidites. The nannofossil marl below the boundary has been studied at several different locations. boundary shows abundant signs of bioturbation, which The boundary has been located within Chron C-29-R as makes it likely that the type of remanent magnetization a result of magnetostratigraphic research on marine pe- carried by the sediment is a postdepositional remanence lagic limestones in Italian sections near Gubbio (Rog- (PDRM). Immediately above the C/T boundary, how- genthen and Napoleone, 1977; Lowrie and Alvarez, ever, no bioturbation is observed. 1977) and near Moria (Alvarez and Lowrie, 1978). The magnetostratigraphy of the C/T boundary was However, measurements on different mammalian investigated in this core in order to complement earlier fossil-bearing continental sections lead in part to contra- studies and to document further its relationship to the dictory results: Lerbekmo et al. (1979), who studied geomagnetic reversal sequence. sediments of the Red Deer Valley, Alberta, placed the C/T boundary within a reversed polarity zone in accord SAMPLING AND MEASUREMENTS with the Gubbio correlation. On the other hand, Butler The overall magnetostratigraphy of the Site 524 core et al. (1977), who studied a section in the San Juan has been determined by Tauxe et al. (this vol.). For their Basin of New Mexico and defined the Cretaceous/Ter- magnetic measurements they used 7-cc specimens taken tiary boundary within a normal polarity zone. This re- at intervals ranging from every 10 cm to every 40 cm, sult was contested by Alvarez and Vann (1979) on the depending on the sediment type. In order to enhance the grounds of possible unconformities. However, Butler's resolution of the magnetostratigraphy in the present in- result was essentially confirmed by Lindsay et al. (1981), vestigation more intensive sampling was carried out. who studied the sections of the San Juan Basin in Cylindrical samples with a volume of 2 cc were taken at greater detail. They reconciled the apparent discrepancy an average spacing of 5 cm. To minimize the distur- with the Gubbio correlation by making the assumption bance of the residual core material by the sampling pro- that the dinosaurs in the San Juan Basin became extinct cedure, a special tool was developed that takes oriented slightly later than the faunal change recognized as the cylinders without affecting the rest of the sediment core. Cretaceous/Tertiary boundary in the Italian marine de- The magnetization of the 2-cc samples was then mea- posits. In other words, they postulated that the C/T sured with a cryogenic magnetometer at the Institut für boundary recognized by vertebrate fossils is younger Geophysik of the Eidgenössische Technische Hoch- than the C/T boundary recognized by the marine inver- schule Zurich. Evenly distributed pilot samples were sub- tebrate fossils at Gubbio. jected to progressive alternating field demagnetization in order to investigate the composition of the natural re-

Hsü, K. J., LaBrecque, J. L., et al., Init. Repts. DSDP, 73: Washington (U.S. Govt. manent magnetization (NRM). Secondary magnetiza- Printing Office). tion were of minor importance in most samples and

657 N. PETERSEN, F. HELLER, W. LOWRIE

could easily be removed by partial alternating field de- ACKNOWLEDGMENTS magnetization in peak magnetic fields less than 160 × 2 This study has been supported by the Deutsche Forschungsgemein- I0 A/m (200 Oe). The nannofossil marl and the inter- schaft. calated turbidites were equally good media for the mag- netostratigraphy. Little difference in magnetic behavior REFERENCES was observed between the two types of sediments, al- Alvarez, L., Alvarez, W., Asaro, F., and Michel, H. V., 1980. Extra- restrial cause for the Cretaceous-Tertiary extinction. Science, 208: though the magnetic stability was slightly lower in the 1095-1108. turbidites than in the marls. Alvarez, L. W., and Lowrie, W., 1978. Upper Cretaceous paleomag- Since the drill location is in the midlatitudes, the in- netic stratigraphy at Moria (Umbrian Apennines, Italy), verifica- clination of the natural remanent magnetization of a tion of the Gubbio section. Geophys. J. R. Astron. Soc, 55:1-17. Alvarez, L. W., and Vann, D. W., 1979. Comment on biostratigraphy sample can be used to identify the polarity. The magnet- and magnetostratigraphy at Paleocene terrestrial deposits, San ic inclination to be expected at the latitude of the drill Juan Basin, New Mexico. Geology, 1:66-61. site is 48°. In the Southern Hemisphere, negative incli- Butler, R. F., Lindsay, E. H., Jacobs, L. L., and Johnson, N. M., nations result from a normal field, positive inclinations 1977. Magnetostratigraphy of the Cretaceous/Tertiary boundary from a reversed field. Comparison of the polarity pat- in the San Juan Basin, New Mexico. Nature, 267:318-323. Hsü, K. J., 1980. Terrestrial catastrophe caused by cometary impact tern in Cores 12 through 29 to the polarity time scale of at the end of Cretaceous. Nature, 285:201-203. LaBrecque et al. (1977), aided by paleontological data Hsü, K. J., He, Q., McKenzie, J. A., Weissert, H., Perch-Nielsen, K., obtained by Poore et al. (this vol.), leads to confident Oberhànsli, H., Kelts, K., LaBrecque, J., Tauxe, L., Kràhenbühl, identification of Chrons C-27 through C-31. By using U., Percival, S. F., Wright, R., Karpoff, A.-M., Petersen, N., Tucker, P., Poore, R. Z., Gombos, A. M., Pisciotto, K., Carman, the LaBrecque et al. (1977) anomaly ages, the sedimen- M. F., and Schreiber, E., 1982. Mass mortality and its environ- tation rate in the vicinity of the C/T boundary was de- 3 mental and evolutionary consequences. Science, 216:249-256. termined to be about 30 mm/10 yr. (Fig. 1). This sedi- Kyte, F. T., Zhou, Z., and Wassan, J. T., 1980. Siderophile-enriched mentation rate appears high by comparison with normal sediments from the Cretaceous-Tertiary boundary. Nature, 288: deep sea cores, a result of the frequent intercalation of 651-656. LaBrecque, J., Kent, D. V., and Cande, S., 1977. Revised magnetic turbidites in the nannofossil marl. polarity time scale for late Cretaceous and Cenozoic time. Geolo- Our measurements show that the Cretaceous/Tertiary gy, 5:330-335. boundary at Site 524 lies well within a zone of positive Lerbekmo, J. F., Evans, M. E., and Baadsgaard, H., 1979. Magneto- inclination, or negative polarity, which can be identified stratigraphy, biostratigraphy and geochronology of Cretaceous- from the overall magnetostratigraphy of Tauxe et al. Tertiary boundary sediments, Red Deer Valley. Nature, 279:26-30. Lindsay, E. H., Butler, R. F., and Johnson, N. M., 1981. Magnetic (this vol.) as the negative polarity interval within Chron polarity zonation and biostratigraphy of late Cretaceous and Pa- C-29. This result is in accord with the Gubbio and leocene continental deposits. Am. J. Sci., 281:390-435. Moria correlations obtained by Lowrie and his cowork- Lowrie, W., and Alvarez, L. W., 1977. Late Cretaceous geomagnetic ers (Fig. 2). No anomalous behavior of the magnetiza- polarity sequence, detailed rock and paleomagnetic studies of the Scaglia Rossa limestone at Gubbio, Italy. Geophys. J. R. Astron. tion inclination, intensity, or susceptibility is observed Soc, 51:561-581. at the C/T boundary itself (Figs. 2 and 3). Luterbacher, H. P., and Premoli Silva, I., Biostratigrafia del limite Cretaceo-Terziario nell'Appennino centrale. Riv. Hal. Paleontol. DISCUSSION Stratigrafla, 70:67-128. The location of the Cretaceous/Tertiary boundary at Perch-Nielsen, K., 1979. Calcareous nannofossil zonation at the Cre- taceous/Tertiary boundary in Denmark. In Birkelund, T., and Site 524 agrees with the magnetostratigraphic studies on Bromley, R. G. (Eds.), Cretaceous-Tertiary Boundary Events, the Gubbio and Moria sections (Lowrie and Alvarez, Vol. 1: The Maastrichtian and Danian of Denmark: Copenhagen 1977; Alvarez and Lowrie, 1978). The relatively long (Univ. Copenhagen), pp. 115-135. time span between the C/T boundary and the next geo- Roggenthen, W. M., and Napoleone, G., 1977. The Gubbio section, Upper Cretaceous-Paleocene magnetic stratigraphy. Geol. Soc. magnetic reversal, deduced in Figure 3 from the sedi- Am. Bull., 88:378-382. mentation rate, makes it unrealistic to assume any caus- Smit, J., and Hertogen, J., 1980. An extraterrestrial event at the Cre- al link between the two events. taceous-Tertiary boundary. Nature, 285:198-200.

658 MAGNETOSTRATIGRAPHY OF THE CRETACEOUS/TERTIARY BOUNDARY

Sub-bottom depth (m) I 100

Average sedimentation rate = 30 mm/10 yr. 150

Tertiary 200 Cretaceous

250

300 64 65 66 67 68 69 Age (m.y.)

Magnetic time scale (LaBrecque et al., 1977) Figure 1. Sediment accumulation in the vicinity of the Cretaceous/Tertiary boundary versus age for Site 524. Determination of sedimentation rate is based on the correlation of the magnetic polarity zones in the core (taken from the overall magnetostratigraphy of Tauxe et al., this vol.) to the geomagnetic reversal time scale of LaBrecque et al. (1977).

,,659 N. PETERSEN, F. HELLER, W. LOWRIE

A Site 524 B Gubbio c Moria NRM inclination (°) polar latitude (°) polar latitude (°) -80 -40 0 +40 +80 -90 0 +90 -90 0 +90

345 350

200

t

340 345 210

335 340 220 Λ Figure 2. Magnetostratigraphy of the Cretaceous/Tertiary boundary of three marine pelagic sediment sections. A. Site 524, South Atlantic. In the Southern Hemisphere, negative inclinations result from a normal field, positive inclinations from a reversed field. B. Pelagic limestone section at Gubbio, Italy. Data are from Low- rie and Alvarez (1977) and Lowrie (unpublished data). C. Pelagic limestone section at Moria, Italy. Data are from Alvarez and Lowrie (1978) and Lowrie (unpublished data).

660 MAGNETOSTRATIGRAPHY OF THE CRETACEOUS/TERTIARY BOUNDARY

δ 8 NRM NRM Frequency of inclination intensity (A/m) nannofossils Iridium content (ppb) -80 -40 0 +40 +80 10' I02 I0"1

200-

201-

202-

203-

204-

H Nannofossil marl

Turbidite

Figure 3. Detail of the magnetostratigraphy of Site 524 in the immediate vicinity of the Cretaceous/Tertiary boundary. NRM data after alternating field demagnetization in 140 × I02 A/m peak field. Biostratigraphy and iridium data from Hsü et al. (1982). The time interval of 83,000 yr. between the C/T boundary and the next geomagnetic reversal has been determined by assuming a constant sedimentation rate of 30 mm/103 yr. across the boundary (compare Fig. 1).

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