Wednesday, October 12, 2011

Gunung Benau (Benau Mount), Type of Sedimentation and Lithology

Consist of interbedded marl and limestone with intercalation of fine grained sandstone conformably overly shoreface sandstone unit. Marl characteristic by grey, limey, frequent calsitic, rich concretions, and pyrite nodules, bioturbated, fossils, minor mica and carbon, concretion. The marl interbedded with light grey, hard, massive, calcite veined, micritic limestone. This well bedded micritic limestone indicate a platform environment. Some build up reef limestone and minor sandstone were also found in this unit.

There also reef limestone with a great amount of fossils including coral, foraminifera, and gastropod. The reef limestone growth in shallow marine environment and interacted with sea level change. Distribution of the reef limestone is not too far and just only found in several spot. It means that the reef can not grow optimally because the sea level change that makes reef can’t catch up or give up to the sea level. The lack of sunshine and nutrient or large influx of terigeneous clastic sediments could also disturbing the growth of reefal limestone.

The limestone is various. In some place, found grey, massive, micritic, very fine-fine grained (calsilutite-calcarenite), no fossil, calsitic, up to 1.5 metres and has a good thin and thick bedding with the marl. The others is fossiliferous limestone, whitish-yellow, texture compact, largely calsite, shells, gastropods, algae, coral. This unit deposited in shelf and shallow water environment. We can interpret the limestone that the massive, micritic limestone is deposited around the outer shelf, near reef, or barrier reef. The major frame building colonies, heads of algae, gastropods, poorly sorted calsirudite, shells, and very rich cavities, hollows is part of reefal (Biohermal Limestone) as a lense or boulder and surrounded by marl and the pelletal packestone, bioclastic wackestone, and limestone is only a local reef development, and growth after shoreface sandstone unit deposited because of drop sea level phase. This local boundstone with limey sand bodies is part of lagoonal or mid-shelf environment.

The middle section contains a tiny section of minor mica, very fine-grained sandstone that interbedded with limestone and marls. This sandstone indicates deltaic/tidal influence in the platform environment. Probably, this environment just occurred in a short time. The whole unit represents platform sedimentation with some localized reef. Sea level change and tectonic process play role during the deposition. The base of this unit is conformably overlies the shoreface unit .The upper contact is an uncomformity with volcanic rock sequence because airfall tuff overlies the marl-limestone unit.


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