Cretaceous-Paleogene Biomagnetostratigraphy of Sites 752-755, Broken Ridge: A Synthesis

Abstract

Broken Ridge, in the eastern Indian Ocean, is a shallow-water volcanic platform which formed during the Early to middle Cretaceous at which time it comprised the northern portion of the Kerguelen-Heard Plateau. Rifting during the middle Eocene and subsequent seafloor spreading has moved Broken Ridge about 20°N to its present location. The sedimentary section of Broken Ridge includes Turonian-lower Eocene limestone and chalk with volcanic ash, an interval of detrital sands and gravels associated with middle Eocene rifting and uplift, and a middle-late Oligocene unconformity overlain by a thin section of Neogene-Holocene pelagic calcareous ooze. This paper summarizes the available post-cruise biostratigraphic and magnetostratigraphic data for the Cretaceous-Paleogene section on Broken Ridge. The synthesis of this information permits a more precise interpretation of the timing of events in the history of Broken Ridge, in particular the timing and duration of the middle Eocene rifting event. Paleontologic data support rapid flexural uplift of Broken Ridge in response to mechanical rather than thermal forces. Other highlights of the section include a complete Cretaceous/Tertiary boundary and an opportunity for first-order correlation of Paleogene diatom stratigraphy with that of the calcareous groups.Weissel, J., Peirce, J., Taylor, E., Alt, J., et al., 1991. Proc. ODP, Sci. Results, 121: College Station, TX (Ocean Drilling Program).

The effect of long-term low-temperature exposure on apatite fission track stability: A natural annealing experiment in the deep ocean

“…Fig. 9 shows AFT ages plotted against the biostratigraphic ages of the same samples (Cepek and Wind, 1979;Mazzei et al, 1979;McNulty, 1979;Okada and Thierstein, 1979;Salvatorini and Cita, 1979;Pospichal et al, 1991;Erba and Covington, 1992;Chaisson and D'Hondt, 2000;Kameo and Bralower, 2000; Ca 9.740 9.746 9.909 9.844 9.823 9.798 9.961 9.986 9.833 9.523 9.565 9.705 9.848 9.826 9.624 9.747 9.860 9.720 9.690 9.807 9.790 9.812 9.684 P 6.049 5.991 5.935 5.955 5.958 5.961 5.894 5.874 6.023 6.138 6.107 6.076 6.028 6.036 6.112 6.054 6.019 6.081 6.095 6.044 6.054 6.048 6.097 and Sanfilippo, 2000). Within the ±1r-error limits, all AFT ages are identical with their deposition ages, indicating that track shortening of about 10% does not affect fission track ages.…”

Taxonomy and magnetobiochronology of Tribrachiatus and Rhomboaster, two genera of Calcareous nannofossils

“…Our placement of this datum, between Samples 752A-17-1, 45 cm and 752A-17-2, 45 cm (Figure 13), is significantly lower than previously reported. A major unconformity occurs between Cores 12 and 13 (Pospichal et al, 1991) on Site 528 data, although the magnetostratigraphy of Site 528 has large uncertainty as discussed above. Although the FO of T. orthostylus is not a marker in the zonations of Martini (1971) and Okada and Bukry (1980), Figure 14 shows that the datum is consistently located very near the top of Chron C24r, with an age of 53.7 Ma in the polarity time scale of Wei (1995a).…”

“…We see a number of problems with this model: 1) The abundance of M. decussata sharply decreases across the Cre taceous/Tertiary boundary, and the species is generally very rare and sporadic in the younger sediments. When this species occurs in significant numbers in Cenozoic sediments, it is usually ac companied by other Cretaceous taxa (see detailed discussion by Pospichal, 1995). This suggests that M. decussata was most likely reworked into the Cenozoic sediments rather than actually having lived in the Cenozoic.…”