Hans M. Bolli

Major calcareous nannofossil and foraminiferal events between the middle eocene and early miocene

Abstract The more significant biostratigraphic events during the time interval from beginning Middle Eocene to beginning Miocene with special emphasis on the Eocene-Oligocene boundary are discussed. The microfossils considered are calcareous nannofossils, planktic foraminifera and smaller and larger benthic foraminifera. Areas taken into account include particularly the Caribbean (Trinidad, Barbados), DSDP South Atlantic sections (Cape Basin Site 360, Walvis Ridge Sites 362A, 363, Angola Basin Site 364), the equatorial North Atlantic (DSDP Site 354) and the Southern Alps (Possagno). In the larger foraminifera the treatment is somewhat different, with selected families and genera being considered for their distribution in the eastern and western hemisphere.

Biostratigraphy and Paleotemperatures of a Section Cored on the Nicaragua Rise, Caribbean Sea

The drilling vesselSubmarex of Global Marine was used to drill and core sediments to a depth of 56.4 m on the Nicaragua Rise, between Walton Bank and Jamaica, in 610 m of water. Seismic reflection profiles revealed thick accumulations of layered sediments with some fossil reefs. The sediments consist of undisturbed layers rich in planktonic microfossils alternating with turbidite layers. Absence of older coccoliths indicates that the redeposited material was not appreciably older than the time of redeposition, and oxygen isotopic analysis of benthonic elements shows that this material was derived from a depth not much shallower. The lower portion of the cored section correlates with the Manchioneal Formation of Jamaica. Taxonornic analysis of the calcareous nannoplankton indicates that the level at 2354 cm correlates with the midportions of the eastern equatorial Pacific cores 58 and 62; with the “Nebraskan-Aftonian” boundary of the Gulf Coast; and with the appearance of Hyalinea baltica at Le Castella, southern Italy. This level, therefore, represents the Plio-Pleistocene boundary as officially designated, and an age of about 700,000 years is estimated for the bou ndary. Oxygen isotopic analysis shows important oscillations, with a full glacial-interglacial amplitude, occurring both above and below the Plio-Pleistocene boundary.

Calcareous Spherules from the Albian of DSDP Leg 40, Site 363

Calcisphaerulidae, mostly spherically shaped Pithonella cf. sphaerica, occur in very large numbers in the Albian Cores 26-39 of the Leg 40 Site 363, drilled on Walvis Ridge in the South Atlantic. In the numerous samples investigated from this interval (Bolli, this volume) most of the 44-63 μm fractions consist almost exclusively of Calcisphaerulidae. Often they are also abundant in the 63-177 μm fraction where, however, their size is restricted to not more than about 90 μm. A sample taken in Sample 31-1, 124-126 cm, was found to be equally rich in spherules of about the same size as the Calcisphaerulidae from samples taken above and below this level. Closer examination however showed that almost all of them are not hollow and formed by one or more layers as is typical for the Calcisphaerulidae, but consist of more or less densely arranged radial crystals filling the whole spherule. In comparison with forms described as fossils, the spherules come closest to some Globochaete alpina specimens of Colom (1955). On Plate 3, Figures 17, 19, and 22 he shows more or less spherical specimens filled with radially arranged crystals, similar to those discussed here. The majority of the Globochaete alpina specimens however, figured by Colom and also those originally given by Lombard (1945), are of different shapes and are also smaller in size (about 30 μm). The nature of Globochaete alpina, in particular the specimens with a distinct radial arrangement of crystals as illustrated by Colom, still awaits clarification. Like the spherules described here, they also may be recrystallized forms. Some indications suggest that the spherules are replacements of Calcisphaerulidae. Two distinct types of spherules, with intermediate forms, can clearly be distinguished, which may represent a younger and an older growth stage. One, probably the younger of the two, consists of slender, radially arranged very long crystals in a loose arrangement throughout. The other, comprising the majority of the specimens, consists of tightly packed crystals in the central part, which become progressively more loosely arranged towards the periphery.

Calcareous Organisms Incertae Sedis from the Lower Cretaceous of DSDP Leg 27

Small well-preserved spheres with comparatively large, regularly distributed circular pores occur in the eastern Indian Ocean Sites 260, 261, and 263, (Figure 1) where they are restricted to the Lower Cretaceous, Albian/Aptian. The specimens react very strongly to diluted HC1. Xray examination of two specimens with the Gandolfi camera method, carried out by W. Oberholzer (Zurich) indicates that the tests most likely are made of predominant calcite but may also contain some argonite. Surprisingly, the specimens were found only in virtually non-calcareous sediments where calcareous foraminifera are absent but calcareous nannoplankton is still present, varying from scarce to abundant. Radiolaria and primitive arenaceous foraminifera are also scarce to abundant; dinoflagellates can be abundant and fish remains occur in small quantities. The faunal and floral composition indicates deposition at a depth where calcareous foraminifera were dissolved but calcareous nannoplankton with its higher resistance was largely unaffected. If this environmental interpretation is correct, it is difficult to understand why these highly calcareous, thin-shelled spheres should have been preserved in such good condition. Contamination from samples with higher CaCθ3 content is unlikely because no spheres were seen in many examined samples. The spheres may be compared to certain Radiolaria, in particular some small spumelline forms, possibly also to representatives of the suborder Phaeodarina, e.g., the Recent but much larger Aulonia hexagonia. The possibility that the spheres could be recrystallized radiolarians (Siθ2 replaced by CaCθ3) was therefore taken into consideration. This, however, seems unlikely for (a) they do not closely compare with known Radiolaria; and (b) all samples containing them also carry Radiolaria with siliceous tests of mostly poor preservation. It is difficult, therefore, to believe that only one Radiolaria taxon of several or many taxa present in a sample should have become recrystallized and at the same time remained much better preserved. Also considered was the possibility that the specimens could be artifacts of some sort. All samples of the Leg 27 sediments in which the spheres occur were washed for foraminifera and treated with H2O2. In many of them small tubes of varying shapes were formed by chemical reaction of H2O2 with small pyritic limonitic nodules present in the sediment. These tubes are non-calcareous and do not resemble the spheres here described. The ARGO