Mixed Wave, Tide and Fluvial-Influenced, Tropical Coastal-Shelf Deposition: Miocene-Recent Baram Delta Province, NW Borneo

Abstract

Evaluating the spatial and temporal evolution of ancient, mixed-energy clastic coastal-deltaic systems requires integration of detailed facies analysis with local- to regional-scale geological factors (tectonics, climate, sediment supply, etc.) and basin physiography (e.g. water depth, shape, orientation, etc.), which together determine tide, wave and fluvial effectiveness. This approach is applied to Middle to Late Miocene coastal-deltaic successions in the low-latitude (c. 5° N), tectonically-active Baram Delta Province (BDP) of NW Borneo, utilising outcrop sedimentology, well-constrained paleogeographies and paleotidal modelling. In the eastern BDP, the Belait Formation initially records fluvial-dominated coastal-deltaic progradation above outer shelf to slope mudstones. Thickening- and coarsening-upwards, 4-8 m thick, parasequences are sparsely-bioturbated throughout, contain abundant climbing current-ripple cross-lamination, dewatering structures and soft-sediment deformation, attesting to high rates of current-dominated sedimentation, hyperpycnal flows, substrate instability and mouth bar collapse. The overlying Belait Formation contains abundant, 10-50 m thick, upward-coarsening successions interpreted as prograding storm-, tidal- and river flood-influenced delta front deposits, indicating grteater variability of coastal processes. Seismic evidence for tectonic inversion and paleotidal modeling suggests that a large-scale (10–100 m) stratigraphic change from relatively wave-dominated to more tide-dominated facies successions reflects the development of protected embayments with supressed wave energy. In the western BDP, the Lambir Formation is partitioned into mainly mixed fluvial- and tide- influenced sandstones that erosionally-overlie subordinate wave-dominated (storm-reworked) prodelta to delta front successions. The overlying Miri Formation displays a change towards greater storm-wave energy and the development of storm-flood deltaic parasequences. This reflects a gradual decrease in fluvial and tide effectiveness, consistent with regional paleotidal model results. Numerical modeling of paleo-oceanic processes have been applied in order to better decipher the relative interplay of autogenic vs. allogenic controls on sedimentological and stratigraphic architecture in these variable, mixed-energy clastic coastal-deltaic successions.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016