The "T.a" Letter Stage was first defined on faunas that were later revised by Geof Adams (1970) as T.a 3. The older, T.a 2 & T.a 1 faunas are very rare in the Indo-Pacific area as they pre-date major plate re-organisation and sedimentary basin forming events. This series of events occurred at about 44 or 45 mybp or mid-Middle Eocene. The top of T.a is virtually on the Middle to Late Eocene boundary (if defined on the mass extinction datums of planktonic formas at the top of P14) between 38 and 39 mybp. [Note that some workers use the top Bartonian Stage for the top Middle Eocene, an event that occurs about halfway through P15, about 37 mybp].
T.a carbonate faunas are not uncommon where sediments of this age crop-out or are drilled, as the new basins were being transgressed by marine conditions. However there are few major biohermal developments of this age in the region, probably as there was an overwhelming influx of clastic sediments at that time.
The images shown here are reduced to about 1/5 their original resolution and highly compressed as JPG files. Original images available on request, as TIF files from 120 to 400 MB in size
- Halmahera, East Indonesia (516 KB). This limestone is typical of T.a, Middle Eocene limestones in eastern Indonesia, both on the low latitude locations such as Halmahera and on the high latitude Australian carbonates. In these areas the carbonates can often be dominated by Nummulites, often assigned to either N. javanus or N. boninensis, with just a few small Discocyclina specimens. One can be sure this assemblage is T.a firstly as the same beds contain a few specimens of Planocamerinoides / Assilina and Alveolina, but in this image the Nummulites have strong dimorphism between the two generations and the "A" or macrospheric generation has an enlarged proloculus, sometimes over 500 microns, with few whorls, - similar to N. bagelensis from the T.a of Java.
- Worowari area - 'Assilina' limestone (388 KB). Limestone with what has usually been called Assilina but is now placed in Planocamerinoides. This limestone clearly qualifies to be what has always been regarded as T.a Letter stage, from the 'Eocene' outcrop at Worowari, north Central Java. In fact the outcrop here is Early Oligocene; a soft pebbly mudstone olistotrome with frequent boulders of Middle Eocene limestone, sandy limestones and conglomerates. In this image there is one large microspheric (B generation) Planocamerinoides, numerous small macrospheric (A generation), discocylinids and one Nummulites bagelensis identified by the lens cross-section and very large initial chamber (proloculus).
- Karangsambung Nummulitic limestone (312 KB). Limestone from underneath the geological field camp at Karangsambung, Central Java. In the lower right part of the image is part of the very large Nummulites javanus (B or microspheric generation) which is about 2 cms in full diameter. Note the thin-walled last whorl which is a gerontic stage lacking the marginal canal and probably associated with reproduction before death. The smaller but macrospheric specimens seen over most of the image are the corresponding A generation usually given the name Nummulites bagelensis, Bagelen being the district name of this location under pre-war adminstration.
- Nummulitic limestone from Jiwo (408 KB). Limestone from the Jiwo Hills, near Yogyakarta. This bed has abundant Nummulites bagelensis and adjacent beds have common N. javanus. Within the fine grained fraction of this sample planktonic foraminifera can be seen, including many probably Acarinina spp. and a few almost certain Morozovella specimens with their distinctive peripheral keel. These form identify the T.a Letter Stage and an age in P14 or older. Also in this sample are specimens of Pellatispira, a genus somethings thought to not range as old as this. These are simple morphotypes, the more complex species being restricted to T.b
