Autogenic, Allogenic and Mixed-Process Controls on Stratigraphic Architecture in the Baram and Champion Delta Systems, NW Borneo

Abstract

Exceptional rates of subsidence (up to 3000 m/Ma) and sediment supply (from a tectonically-active hinterland) have preserved a >6 km stratigraphic record of Mio-Pliocene coastal-deltaic sedimentation in the Baram Delta Province (BDP) of NW Borneo. Equivalent present-day coastal systems (e.g. Baram, Trusan and Padas deltas) display substantial decadal-kyr morphological changes. Deconvolving allogenic vs. autogenic controls on the evolution of these systems will be illustrated in two contrasting late-mid Miocene outcrop successions in the BDP: (1) the Lambir Formation (western BDP), and (2) the Belait Formation (eastern BDP). The Lambir Formation records deposition during rapid early coastal-deltaic progradation and comprises fluvio-tidal sandstones that are sharp-to-erosionally juxtaposed on wave-dominated (storm-reworked) prodelta to delta front successions. Single and multi-storey channel bodies comprise: (i) sand-dominated bars and dunes (2–9 m thick); (ii) laterally migrating, elongate tidal bars (inclined heterolithic strata, 1–6 m thick); and (iii) mud-dominated carbonaceous hetereolithics (1–2 m thick). Abrupt vertical changes from sandier facies to bioturbated mudstones reflect rapid autogenic changes in local sediment supply and fluvial energy. Proximal parasequence sets also contain 4–17 m scale, erosive-based fluvial channel bodies and wave-tide influenced, muddy sandbar deposits, representing local flooding or wave ravinement during channel abandonment. The Belait Formation was deposited under significant tectonic influence within a narrow (5–20 km), fault-bounded embayment (Berakas Syncline). This sub-basin configuration and its high rate of accommodation creation formed an effective sediment trap, with high aggradation and a steeply rising shelf trajectory. Abundant upward coarsening successions are interpreted as prograding storm- and river flood-influenced delta front deposits. Storm-reworking of tidal bars and intercalated tidal sand bodies further indicate mixed-energy processes. However, larger-scale (10–100 m) partitioning of stratigraphic architecture into relatively tide- and wave-dominated successions suggests temporal changes in process dominance, in response to allogenic-forced changes in shoreline geometry. The different sedimentological and physical stratigraphic expressions of these two time-equivalent and geographically-adjacent coastal-deltaic successions will be discussed in relation to autogenic and allogenic processes.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015