| সংক্ষিপ্ত: | The characteristics of the transition zone between Upper Sambipitu Fm.
and Lower Oyo Fm. are unique. Generally, the Sambipitu Fm. consists of greenish
brown sandstone and lightgreen shales that showing a cleavage at nearly vertical
angles to the bedding plane. A series of platy limestone and marls are established
unconformably above Sambipitu Formation as part of Oyo Formation.
Meanwhile, a transition zone are exist between both formation that consists of
mixing siliciclastic (especially volcaniclastic) and carbonate with various of
sedimentary structures. These structures such as gradation, parallel lamination,
ripple, planar, trough cross and hummocky cross stratification are indicate more
shallow deposition environment (inner-outer shelf) than the previous publications
(slope). These rocks indicate a difference in origin and depositional environment
as transition zone of the Sambipitu � Oyo Fm.
The Stratigraphic measurements were taken at three river channels that is
the Widoro, Ngalang and Kedungdowo Rivers. Rock geometry, lithology and
sedimentary structure, fossils and paleocurrent were collected to determine the
sedimentary facies and dynamics sedimentation of the transition zone.
Sedimentology and petrography analysis shows that there are 14 kinds of rocks
that form the transition zone which is can be divided into 9 rocks facies.
Volcaniclastic materials that compose the rock is dominated by reworked
pyroclastic materials, whereas epiclastic material occurred in small portion.
Some layers consist entirely of tuff which indicated intense volcanic activity.
Carbonate material consisting mainly of planktonic and bentonic foraminifera.
Mixing siliciclastic and carbonate facies material occur in situ, punctuated
and source mixing. The repetition of transition zone revealed a number of cycles
shallowing up ward (outer shelf � inner shelf). Correlation of the three sections
show that the Ngalang section is the depocenter of the transition zone.
Paleogeography of transition zone is a shelf of active volcanoes distal of with
several small reefs that grew in the shallower environment. Reef can grew well
when volcanic material does not reach their life environment.
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