BIOGEOGRAPHY OF SOME EOCENE LARGER FORAMINIFERA, AND THEIR APPLICATION IN DISTINGUISHING GEOLOGICAL PLATES

ABSTRACT

Indo-Pacific Eocene carbonate sediments can be divided into two groups based on the presence of certain larger foraminifera. One of these faunal groups is associated with the Sundaland craton, the geological core of western Indonesia and is also found on low latitude Pacific islands as well as low latitude western Tethyan regions. The second fauna is found on the Australian Plate, and the micro-plate terrains have been derived from it since the Eocene.

This correlation leads to the hypothesis that the Middle and Late Eocene Sundaland fauna, identified by the three, probably related genera: Assilina, Pellatispira, and Biplanispira [hereafter abbreviated to "APB"] indicate a low latitude, shallow marine fauna, able to cross oceanic migration barriers but restricted from migrating far outside the tropics. In contrast, the fauna identified by the genus Lacazinella, which has about the same stratigraphic range as the APB lineage, is thought to be a higher latitude fauna centered on the Australian continent.

This faunal difference occurred at a time of maximum separation of the Sunda and Australian plates. Therefore, subsequent Tertiary collision of these plates can be identified by the present complex distribution of previously separate faunas, and a tectonic suture can be plotted for the collision area in Eastern Indonesia/Papua, which generally agrees with other geological data.

To partially test this hypothesis some Eocene limestone samples from Christmas Island were collected and studied. This oceanic guyot is now slightly closer to Indonesia than Australia, but in Eocene times it would have been much further south. In this location neither APB or Lacazinella were found. Instead the fauna is dominated by small Discocyclina and Grzybowskia, the first record of the latter genus in the Indo-Pacific area. The few known species of Grzybowskia are recorded from the Boreal region of Europe, in strata usually assigned to the Late Eocene. This new record suggests Grzybowskia is, somewhat like Lacazinella, a higher latitude form but with representatives on both sides of the Eocene equator.

A new species, Grzybowskia jasoni sp. nov. is described.

Peter Lunt. Lundin Oil & Gas B.V., Plaza Great River 8th Floor, Jl. HR. Rasuna Said Kav X-2 No. 1, Jakarta 12950
Indonesia.

KEY WORDS: foraminifera, Eocene, palaeogeography, biogeography, Christmas Island, Indonesia, southeast Asia, Ocean Drilling Program