THE EARLY MIOCENE LUTUT FORMATION OF N. CENTRAL JAVA
Lutut Fm. Conglomerate (596 KB). Typical conglomerate facies of the Lutut Formation. Although much coarser grained conglomerates are often found, this specimen illustrates a typical mix of material. Most of the dark clasts are coal fragments, indurated and usually with pyrite. Quartzose lithic clasts are common and coarse grains almost always have inclusions as a consequence of their metamorphic origin. Metasedimentary rock clasts are also common, sometimes with quartz veining or containing clear mica flakes, or as fragments of chert, often radiolarian. Clasts of limestone, more or less recognisable as rounded indurated fragments are also common.
The bioclasts include frequent Nummulites fichteli-intermedius as well as common Tansinhokella and Eulepidina. Recent work by Lunt (in prep), following Muhar (1956) notes that if Nummulites survived as long as the evolution of Tansinhokella it does so only very rarely, so the frequent occurrence of both in the Lutut Fm. is anomalous. Neither of these two genera overlap with the Nephrolepidina ferreroi / crucifera types that are also frequent in this formation. All these genera have very different type of preservation, with the older forms showing strong abrasion, and in the case of Nummulites frequent pyrite inclusion. The Nephrolepidina ferreroi / crucifera and Miogypsina / "Miogypsinodella" types are dark in thin section but very white and powdery in reflected light suggesting neomorphism. They are also the least abraded of the larger foraminifera. Even though these genera have age ranges close to or overlapping the best age of the sediments on planktonic and Sr isotopic dating (mid to later Early Miocene), this suggests that these larger foraminifera have been almost contemporaneously reworked from an un-indurated source.
Lutut sandstone in plain polarised light (612 KB), in crossed polars (508 KB). Example of the soft quartz sand rarely seen outcropping but recovered from the Pencar core, East Lutut area. Laminated fine sandstone from 54 metres in the Pencar core. The sand grains are almost exclusively quartz, the larger grains still showing a polycrystalline nature and most showing strongly strained extinction. Some fragments include clear mica. Between sand laminae are drapes of soft clay that have been largely lost during thin sectioning. These low energy drapes also contain carbonaceous material (un-pyritised, unlike carbonaceous clasts in the coarser facies) as well as planktonic foraminifera.
The planktonic foraminifera are generally small, with a high proportion of juvenile tests. Many specimens are "re-worked" into subsequent sandstone laminae with little abrasion, suggesting a low energy process such as ripples and probably not abrasive turbidite flows (or rippled over-bank sediments derived from such abrasive flows). The low abundance and especially the low diversity and small size of the foraminifera suggests this to have been a relatively shallow marine deposit. It contrasts strongly with the assemblage from the much deeper, overlying Penjaten Fm. (Plate 4).
A coarse sandy carbonate from East Lutut area near the site of the Pencar core (584 KB). . This calcarenite sandstone contrasts with the coarser conglomerates from this area in that there is only a single assemblage of bioclasts, all with the same, unabraded and possibly unaltered preservation. This assemblage is either in-situ or has been reworked from an un-indurated source of an age close to that of the Lutut Fm. itself. In this assemblage are common Nephrolepidina and Miogypsina and a few Miogypsinoides. Rare sections through the juvenile chambers show the species to be M. globulina (sensu Raju 1974), the evolutionary stage typical for the middle to later Early Miocene (Raju 1974, Lunt in prep.) The Nephrolepidina contrasts with the descendant Trybliolepidina seen in the Penjaten Fm. (see below) in that at this older time the embryonic chambers are half the size of the Trybliolepidine forms and lack the embracing geometry of the second chamber around the first.
The non-carbonate sand grains in this sample are polycrystalline, stressed quartz and metasedimentary lithics and rare carbonaceous fragments.
THE PENJATEN Fm. OVERLYING THE LUTUT Fm.
Fossiliferous bed in the Lower Penjaten Fm. (600 KB). This sandy limestone contains many grains of quartz, but unlike the Lutut / Merawu Fm. it also contains coarse crystals of strongly zoned, plagioclase feldspar, in this plate seen weathered and partly replaced by the blue epoxy impregnation. Volcanic ash is present in the matrix. Van Bemmelen (1941) may have included samples such as these within the his Merawu Fm. as he reports andesitic volcanic material in this formation which is not found by us. Note that the Pencar location shown here is overthrust by younger Lutut / Merawu sediments (Pencar core location). The age of the Pencar - Penjaten sample here can be seen to be younger than the Pencar core and outcrop samples as in this image there are specimens of Orbulina, Globorotalia menardii, and Sphaeroidinellopsis, identifiable from gross morphology or wall structure and which indicate an age no older than Middle Miocene (cf. Bolli & Saunders, 1985). Mudstones from the interbedded mudstones yielded nannofossils including Discoaster hamatus and Catinaster coalitus (P.T. Geoservices report) indicating zone NN9 near the base of the Late Miocene. The larger foraminifera in this sample include Trybliolepidina rutteni, which the distinctly large, embracing type embryont, and an absence of Lower T.f Letter stage fossils (sensu Adams, 1970, 1984 and Lunt, in prep). These indicate an age of later Middle or Late Miocene age. The common, diverse and large (mostly adult) planktonic foraminifera contrasts with the planktonic assemblage in the Lutut Fm. (Plate 2) and indicates a significantly deeper environment of deposition for the Penjaten Formation.
