SAMBIPITU CONGLOMERATES DEPOSITED IN MIXED CARBONATE-VOLCANICLASTIC ENVIRONMENT ALONGSIDE AN EXTINCT VOLCANIC ARC DURING THE ONSET OF THE WONOSARI CARBONATE PLATFORM

The Sambipitu Fm. conglomerates, in south central Java, were deposited in a dynamic environment, flanking the edifices of an extinct volcanic arc. The conglomerates are composed primarily of andesite clasts with volcaniclastic sand matrix but also contain limestone clasts of varying lithologies.

Carbonate platforms initiated on top of irregular topography, on and adjacent to eroded volcanic edifices of the extinct volcanic arc, after seas transgressed into the lows surrounding the edifices. Concurrently volcaniclastic conglomerates (interpreted to be alluvial fan facies) were being shed off the eroding volcanic highs into intervening lows and creating a gradational (inter-tonguing) contact with marine volcaniclastic comglomerates bearing carbonate clasts (interpreted to be fan-delta facies).

Two end-member hypotheses for the volcanic clasts in the conglomerates is that they are: 1) allochthonous detritus shed from erosion of older, previously lithified, carbonates exposed by fault scarps within the volcanic edifices; or 2) they are coral and associated carbonate clasts that formed offshore coeval with the deposition of fan-deltas and were transported to the shoreline and incorporated into the fan-delta deposits.

Although the carbonate clasts display different lithologies such as coral boundstone, skeletal packstone and rodolith rudstone, petrographic analysis reveals no definitive evidence of burial diagenesis or disparate clast age to indicate sourcing of clasts from of preexisting limestones. However, minor burial (< 1km) may have occurred. This scenario makes it unlikely that carbonate clasts were shed from an uplifted fault block. Instead, carbonate clasts were most likely derived from coeval offshore carbonate reefs and associated environments and were mixed with volcaniclastics fanglomerate facies as seas transgressed.

The Sambipitu Fm. conglomerates contain bore holes, significant encrustation by red algae, marine foraminifera, vadose, and marine cements. These factors are indicative of deposition in a fan-delta environment (alluvial fans entering the sea). The depositional system creates a unique opportunity to study rocks that are neither carbonates nor volcaniclastic, but somewhere in between.