Meisling, K.*, Darma, W.**, Bergman, S.*, Samsu, D.**, Hertig, S.**, Yough, C.**, and Daly, C.**
*ARCO Exploration Technology & Operations
**ARCO Philippines, 2300 W. Plano Pkwy, Plano, TX 75075


        Tertiary tectonostratigraphic evolution of the Sandakan Basin, South Sulu Sea, Philippines, has been strongly linked to plate tectonic events in Southeast Asia. Paleogene collision of India with Eurasia led to extrusion of Sundaland-Indochina along left-lateral strike-slip faults. Extrusion was linked through oceanic transforms in the South China Sea to deep transtensional basins in Kalimantan, which filled with quartz-rich Eo-Oligocene sandstones. Early to Middle Miocene collision of microplates with SE Sundaland terminated extrusion, resulting in inversion, uplift, and erosion of transtensional basins, development of a magmatic arc in SE Sabah, and flexural subsidence adjacent tectonic depres

sions. The Sandakan Basin developed at the eastern end of one such depression across a thermally-subsiding transform margin separating the Sunda Shield from Oligo-Miocene oceanic crust of the SW Sulu Sea.

        Tectonically-enhanced regression led to progradation of deltaic clastics along the depression to fill the Sandakan Basin with up to 10 km of Neogene sediments, which can be divided into 11 regressive-transgressive sedimentary cycles, bounded by maximum flooding and unconformity surfaces. Each of six Miocene (M1-M6) and five Pliocene-Quaternary (MP, P1, PPl, Pl1, R) cycles is characterized by progradation followed by retrogradation, reflecting the interplay of tectonically-controlled sediment supply, flexural basin subsidence, and eustacy. Cycles stack together to form regressive and transgressive mega-cycles, bracketed by major lowstands and maximum flooding events.

        The first regressive mega-cycle (M1-M4) records Early to Middle Miocene deltaic progradation into the Sandakan Basin in response to tectonic inversion, uplift, and erosion of the quartz-rich Paleogene sandstones of NE Kalimantan. Cycles M1-M4 individually and collectively evolved from agradation to progradation, ending with a thin retrogradational unit. Early fan deltas graded to fluvial-dominated deltas as rapid progradation built a 60-km-wide shelf into the deep marine Sandakan Basin, driven by onshore uplift and erosion moderated by flexural subsidence and eustacy. Coupled tectonic and eustatic regression culminated in a major late-Middle Miocene lowstand (M4), characterized by a widespread sub-aerial unconformity on the shelf, onset of deltaic collapse on shelf-edge normal faults linked to basinal toe-thrusts, and deposition of lowstand turbidites in intra-slope basins. Transgression and flooding of the shelf led to early Late Miocene highstand deposition (M5), culminating in a major flooding event.

        Collision events in E Indonesia in Late Miocene time renewed compression across Kalimantan, diverted source drainages feeding the Sandakan Basin, and resulted in an overall tectonically-enhanced transgression. Late Miocene and Pliocene cycles display weak clastic progradation and were confined to the shelf by local accommodation space associated with active growth faults. Waning clastic influx is reflected in increased shelfal carbonate development. Lowstand unconformities linked to development of sandy turbidites at top Miocene and intra-Pleistocene are interpreted as the result of eustacy and growth-fault reactivation. The tectonostratigraphy of the Sandakan Basin provides a framework for better understanding the interplay of tectonics and eustacy in the structural and stratigraphic evolution of clastic wedges.

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