Wartono Rahardjo
Department of Geology, Gadjah Mada University
The so called Southern Mountains of Central Java is a mountainous regioin located in the southeastern part of Central Java. It is bounded to the west, north and east by depressions, which are the Yogya-Bantul, Solo Zones and Wonogiri-Baturetno depressions respectively. To the south the mountains is bounded by the Indian Ocean. Clastic sedimentary rocks constitute the stratigraphically lower part of the mountains, overlain by mostly bedded carbonates. The clastics were produced mainly by gravity depositional processes. Total thickness of the clastics is about 4000 meters. The age of the rocks ranges from Oligocene to Middle Miocene. Formations comprising the Southern Mountains from the oldest to the youngest are :
This Formation is composed of conglomerates, sandstones and mudstones of marine origin. It can be subdivided into two parts which are the lower and the upper part. Previous investigators such as Bothe (1929, in van Bemmelen, 1949) and Sumarso & Ismoyowati (1975) called the lower part of this Formation as Kebo Beds. This part is composed of interbeds between sandstones, siltstones and mudstones which exhibit turbiditic characters. There are several of conglomeratic sandstones with clay clasts as major constituent. Three layers of andesitic lava are present in the middle part of this Kebo Beds. The upper part of this Formation, previously called the Butak Beds, consists of stacked interbeds of conglomeratic sandstones which vertically grade into siltstones or mudstones. A greenish color of the sandstones due to chloritic alteration product is characteristic for this upper part. The Kebo-Butak Formation as a whole attains thickness of more than 800 meters. It is interpreted that the whole column was the product of mostly lower submarine fan deposition with several interruptions of mid fan type deposition (Rahardjo, 1983). The age of this Formation is Late Oligocene (Sumarso & Ismoyowarti, 1975; van Gorsel et al., 1987).
Overlying the Kebo-Butak Formation is the Semilir Formation. It consists of light-colored and light-weight dacitic tuffaceous sandstones siltstones and claystones with several lenses of tuffaceous pebble conglomeratic sandstones. Most of the fragments of the sandstones and conglomeratic sandstones are acid pumice which gives a spotty nature to the rock. At the outcrop this Formation exhibits well bedded feature, abudant with sedimentary structure characterizing turbidites. The lack of microfossils in most of the finer layers indicates that they were deposited in a very deep environment, probably exceeding the depth of the CCD. A rare occurrence of Globigernoides primordius in this Formation, however, points out an age of Early Miocene (van Gorsel et al., 1987). Total thickness of this Formation is close to 1000 meters.
The Nglanggran Formation differs from the preceeding Formations for it consists mostly of coarser volcanic materials such as andesitic breccias which grade upward into pebble congglomerates and andesitic sandstones. It exhibit a poorly developed bedding. In the field they form tops of cliffs, or the more pronounced undulating of ridges, due to their resistancy to erosion. The coarser part of the breccia consists mostly of andesitic lava blocks imbedded in medium to coarse grained sandstone matrix. Between the breccias, andesitic lava intercalations which have endured autoclastic brecciation process are ocassionally found . Nglanggran Formation is interpreted as product of rapid submarine debris flow, caused by slumps on Early Miocene andesitic volcanoes. The contact between this Formation with the older Semilir Formation is very sharp with scouring features. This phenomenon generates a debate whether it is an unconformity or not. This topic still needs more clasification by understanding more on the nature of deposition of the Nglanggran before conclusion is to be made. On the other hand, the contact between the Nglanggran and overlying Sambipitu Formation is more definitely shown in Putat area . Here the Nglanggran gradually grades into well bedded turbiditic sandstones of Sambipitu Formation.
This Formation consists of thick series of well bedded marine sandstones, siltstones and claystones with well defined turbiditic characters. It differs from the previous Formations in having less or completely lack of volcanic constituents. In Widoro River section , volcanic materials are still found in the lower part of the Formation, while upsection they are replaced by quartz dominated sandstones with increasing carbonate content. Within the more calcareous part, coral fragments and larger forams are abundant, indicating that at the time of their deposition, the basin recieved materials from a shallow water carbonate shelf. This is probably due to backward encroaching slumps occurred during low sea level stand. Toward the top, there are slump structures and cross-bedding features which used to be a dune form, produced by some kind of traction current flowing in a relatively deep marine environment. Eventually the sandstones gradually grade into limestones of the Oya Formation. Figure ...... : Schematic Stratigraphic column of Souther Mountains along Piyungan - Wonosari Road
STOP 1 : East of Piyungan village, Km. 16 of Yogyakarta-Wonosari Route. Roadsite outcrop of stacked series of fine - grained tuffaceous sandstones of Semilir Formation. Average bed thickness is 50 cm exhibiting turbiditic characters. Typical sedimentary structures include graded bedding with small clay balls, thin parallel and ripple-laminated intervals.
STOP 2 : Duren River Roadside and riverbed outcrops of coarse-grained volcanic breccia of Nglanggran Formation. Several meters thick fining-upward cycle can be recognized. Bedding is poor. Most of the coarse clasts are andesites and basaltic - andesite blocks giving an appearance of rough surface. This deposit is interpreted as the product of a submarine debris flows, probably from a nearby active volcanic centre (van Gorsel at al., 1987).
STOP 3 : Putat River Riverbed outcrop showing transition from Nglanggran Breccia to the overlying Sambipitu sandtones. The upper part of Nglanggran consists of three cycle of fining upward gradation from coarse breccia to finely laminated slitstones and claystones, some contains marine microfossils. The upper breccia beds show a change from andesitic clasts to pumice fragments, a feature commonly found on the Semilir Formation.
STOP 4 : Widoro River Riverbed outcrop of a thick series of sandstones comprising the Sambiptu Formation. The lower part of the section exhibits a uniformly bedded fine sandstones, claystones and marls. Upsection the sandstones shows a peculiar features of "flysch-type" deposits, with bottom part often crossed the underlying layers. The bedding in the lower part is thick coarse sandstones with abundant clasts of laminated claystones with marine fauna. Toward the top section, there are sandstone beds with change thickness over a short distance. There is an interval of alternating glauconitic sandstones and claystones which gradually become a cross-bedded sandstones with thin bed of calcarenites at the top. At the transition between the Sambitu with the overlying carbonates of Oya Formation, there is a 5 to 10 meters thick contorted-bed interval, which may reprensent slumping or diapirism (van Gorsel, 1987).
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