MASS WASTING AND DETRITAL CARBONATE DEPOSITION, CEPU BLOCK, EAST JAVA

摘要

Carbonates in the Cepu block, East Java Basin, were deposited from the Latest Eocene-Earliest Oligocene through the Early Miocene, and are comprised of shallow platform, reef build-up, and detrital carbonate facies. Build-ups grew on differentially subsiding horst blocks formed since Eocene rifting. The Banyu-Urip to Sukowati trend is on the northern edge of the Cepu horst block; the Jambaran to Alas-Tua trend is on the southern edge. Both trends exhibit evidence on 3-D seismic data of erosion and mass wasting which removed a significant volume of the original platform and build-ups.The diachronous top carbonate envelope (TCE) is a prominent seismic reflector caused by strong impedance contrast between a dominantly clastic section above (Tuban Shale) and carbonates below. The TCE is a time transgressive carbonate subcrop surface. On south-facing windward margins, steep, and scalloped-shaped build-ups reflect erosion and mass wasting. On leeward sides, detrital aprons are shallower-dipping and constructional. Seismic and well data suggests that detrital material was transported up to 20km south off the platform and into the adjacent Dander graben via paleo-low seaways localized by normal faults relay ramps.A Rupelian to Chattian-aged detrital apron onlaps the Jambaran build-up. To the south, the Dander-1 well contains over 2000ft of "Kujung" carbonates with age ranges from Rupelian to Chattian, consistent in age and stacking patterns with the Jambaran detrital section. Dander-1 well log character and limited core suggests the Kujung is also detrital in origin and deposited in a local graben, later inverted during the Pliocene. The end of the Rupelian was marked by a significant 3rdorder relative sea level fall, which may have been a trigger for mass wasting as an episodic or continuous process via higher order lowstands and highstands, storms, and tectonics, which resulted in build-up removal from the Cepu platform.