Abstract

In Lower Miocene time, a prolonged period of gentle epeirogenic subsidence and transgression resulted in the deposition of a widespread, thick, limestone and shale unit (the 'Kujung Unit I') in the region of the present day Eastern Java Sea. The Lower Miocene paleogeography of part of this region north of Madura Island consists of:

a. a deep water, east-west trending open marine clastic basin in the south (the East Java-Madura Basin)

b. an extensive, east-west positive area of shallow water carbonate deposition to the north (the East Java-Madura Shelf) characterized by a wavewashed, high energy bank edge along the southern margin

c. a shallow basinal area (the Central Depression) of open marine, fine clastic and low energy limestone deposition and

d. a northeast-southwest trending ridge (the JS 1 Ridge) to the northwest of the Central Depression, characterized by extensive shoal water carbonate deposition.

Another basinal area, the East Bawean Trough, was present to the west of the JS-1 ridge.

Shelf margins at the southern edge of the North Madura Shelf and along the northwestern rim of the JS-1 ridge, appear to have been well defined and accompanied by a rapid increase in water depth into the adjacent basinal areas. By contrast, a more gradual transition took place into the Central Depression from the northern edge of the East Java-Madura Shelf and along the southern side of the JS-1 ridge, resulting in both cases in scalloped shelf margins.

Kujung Unit I depositional trends were strongly influenced by the pre-Lower Miocene northeast-southwest structural grain of the Asian Continental margin north of Madura.

South of this, the main structure controlling factor was the east-west grain of the East Java-Madura Basin, a foreland basin associated with the Indonesian Island arc. A major change in sedimentation from carbonates and shales in the north to shales and fine clastics in the south also occurred at the boundary between these two regions.

Extensive bioherm development occurred in the subsiding Central Depression. Six types of bioherm are recognized and classified on the basis of geometry, basinal position and the predominant bio- or lithoconstituent. The six types are divided into four low growth ratio varieties comprising:

a. shelf coral reefs

b. basinal coral reefs

c. basinal mud mounds

d. basin slope pinnacle reefs

And two high growth ratio varities comprising:

a. bank edge foram-algal shoals

b. bank-edge coral reefs

Geometry, lithology, nature of foundation and character of laterally equivalent sediments are determined for each.

Five major facies in the limestone at the Poleng Field have been noted. These are: reef (coral), coral debris, red algal, foram-algal and deep water shelf facies.

Reef growth is explained by postulating an initial regressive phase involving progradation of facies belts and a later transgressive phase during which facies belts retreated inwards towards the reef core.