Geology of the Karangbolong Mountains
Introduction
The Karangbolong Mountains are situated in the southern coastal plain of Central Java, to the southwest of of the Lokuloh region and thus outside the Serayu Basin. The southern coastal plain only is interrupted by the Karangbolong promontory and by some small outcrops located on the coast at about 6 km east of the mouth of the Serayu River. The latter outcrops mainly consist of coarse breccias and are indicated of fig. The Karangbolong Mountains are the physiographical and structural equivalent of the Southern Mountains of West and East Java (Van Bemmelen, 1949). These have subsided below sea level between Cilacap and the Opak River near Yogyakarta, with the exception of the horst-like transverse structure of Karangbolong. A preliminary geological map of the area and two cross-sections and a longitudinal section were compiled, based upon data collected in the field and on geological airphoto interpretation. Five main geological units have been distinguished on the map. The deposits of the “volcanic Facies of the Halang Formation” mainly occur in the southern part of the area. These predominantly consist of andesitic breccias. Exposures of the deposits of the “Marine Facies of the Halang Formation” mainly are found in the northern section. These mostly consist of marly sandstones and marls. Rocks of an “Intrusive Phase” comprise the third unit. The “Karangbolong Limestones” mostly consist of typical reef limestone, which mainly occur in the central part of the area. The “Alluvial Deposits” east and westwards of the Karangbolong Mountains mainly consist of the alluvial sands and silts. The data given in these paragraphs primarily have to be regarded as a contribution to future investigations. Another reason for the geological survey carried out in the area is that the geological evolution of the Karangbolong promontory and the southern coastral plain closely is linked with the development of the Serayu Basin.
Description of geological units
The deposits of the “volcanic Facies of the Halang Formation” consist of tuffaceous andesitic breccias, lahar deposits and some tuffites, and mainly andsitic lava flows. The flows have a glassy texture, indicating their quick cooling as pillow lavas. The breccias mostly are deeply weathered. A distinctive layering only is recognizable in some outcrops near the Indian Ocean. At the coast the tuffaceous breccias mostly consist of very coarse, angular boulders, inbedded in the tuffaceous deposits and alternating with some layers of less coarse breccias. The dip of these layers is at the western coast about 30 degrees southwest and at the opposite side ca. 30 degrees southeast. The dips and strikes give the impression that the breccias were formed on the slopes of a volcano. They however probably are caused by more recent tectonic movements. These deposits are entirely comparable with the beds of the Volcanic Facies of the Halang Formation exposed withinthe Serayu Basin. In the northern part of the Karangbolong area the deposits are overlain by the beds of the Marine Facies ot the Halang Formation. The deposits of the latter unit are not found beetwen the deposits of the Volcanic Facies and the reef limestones of Karangbolong in the southern section of the area.
Magma intruded through the feeder channels of the eruption centres of the Halang Volcanoes. Hypabyssal sills and dikes were not found, but 11 volcanic necks presently are exposed in the southern part of the Karangbolong area. The necks consist of dioritic and augite-andesitic rocks and situated more or less in two rows with a north-south direction. Some quartz also occurs in the rocks. The necks protrude through the deposits of the Volcanic Facies of the Halang Formation, but are younger than the reef limestone deposits. The volcanic necks therefore most probably can be correlated with the rocks of the “Maung Intrusive Phase of the Halang Formation” exposed in the Serayu Basin.
Exposures of the deposits of the “Marine Facies of the Halang Formation” mostly occur in the northern part of the Karangbolong Mountains. The deposits consist of tuffaceous marly sandstones and marls. The marly sandstones are fine-grained and mainly consist of plagioclase, hornblende, augite and guartz. They contain molluses. The marls usually hold very few tuffaceous elements and are less distinctively layered. These deposits can be aced into the South Serayu Mountains where they form the upper beds of the Marine Facies of the Halang Formation, which are the equivalent of the Third Marl-Tuff Horizon of Harloff (1933). In the Karangbolong area they probably comprise the uppermost beds of the formation. The deposits are slightly folded, the anticlinal axes predominantly having a northwest-southeast direcion. In the central section of the area these deposits only occur locally and with a different facies. In such outcrops the beds mostly consist of sapropel and some lignite, while oil shales also occur. These lagoon-type deposits were formed at the southernmost perimeter of the exposures of the beds of the Marine Facies. A weathered breccia layer with a thickness of a few metres occurs in some places between the sapropel beds and the covering reef limestones. These breccias consist of erosion products of the deposist of the Volcanic Facies of the Halang Formation. In the southern part such lagoon-type deposits do not occur.
The “Karangbolong Limestones” mainly occur in the central section of the area. Some rather extensive outcrops are also found in the southern part. The limestones overlie unconformably the breccias of the Volcanic Facies of the Halang Formation in such places. The limestones were formed transgreesively on top of both the volcanic and marine Halang beds. An angular unconformity between the deposits of the Marine Facies and the limestones appears to occur in some places. The lack of recognizable layering in the reef limestones makes it however difficult to establish this with certainty. The exposed rocks almost entirely consist of typical reef limestones. They mostly are fine-grained, hard and pure and contain Lepidocyclina as well as corals and Echinoidea (Heine, 1930). On several levels the limestones contain white chalk blanks and chalk marls, which consist of erosion products of the reef. The limestones reach a thickness of more than 100 m at the perimaters of the central section of the Karangbolong Mountains. The thickness in the central part locally probably amounts to about 200 m. The reef limestones are easily soluble under tropical conditions, resulting in the formation of extensive cave-systems. Phosphate deposits occur in many of these caves and locally are quarried. Some outcrops of limestones also contain manganese ores (Keil, 1930).
The coastal plains west and east of the Karangbolong Mountains are bordered on the landward slide by the South Serayu Mountains. The coastal area is composed of beach ridges with alternating silty flats. The recent beach sediments mostly consist of fine-grained sands and silty sands. The sand and silt-sized particles are transported by e.g. the Ijo River which enters the Indian Ocean at the western perimeter of the Karangbolong promontary and the Centang River which flows along the eastern side. The sand and part of the silt-sized particles are washed up the beach, thus causing coastal accretion. The sands and silts however rapidly are converted into clays by tropical weathering processes. The clay sized particles are transported further offshore by longshore currents. Narrow strips of fluviatile deposits occur along streams draining the southern part of the Karangbolong area.
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