Controls on Mixed-Energy Sedimentation Across the Miocene Baram Delta Province, NW Borneo

Abstract

Lateral (10–100 km) variations in process dominance in mixed-energy coastal-deltaic systems need not reflect significant allogenic changes in regional or local basin physiography. However, in tectonically active mixed-energy settings, deciphering between the multitude of both autogenic and allogenic-forced process variations is very challenging. We integrate paleogeographic interpretation, paleotidal modeling and stratigraphic facies analysis in order to deconvolve allogenic vs. autogenic controls on two contrasting late-mid Miocene outcrop successions in the Baram Delta Province (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. In tidal channel bodies (4–15 m thick), abrupt vertical changes from sandier facies, through heterolithic facies, to bioturbated mudstones reflect rapid autogenic changes in local sediment supply, with varying degrees of fluvial and marine energy during abandonment. Proximal parasequence sets also contain 4–17 m scale, erosive-based fluvial channel bodies and wave-tide influenced, muddy sandbar deposits. Regional paleotidal modeling indicates that local autogenic deltaic processes controlled wave vs. tidal effectiveness. 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. Paleotidal modeling confirms allogenic-forced changes to an embayed coastline resulted in tidal amplification. We illustrate how numerical modeling of paleo-oceanic processes places important constraints on understanding autogenic vs. allogenic control on sedimentological and stratigraphic architecture in coastal-deltaic successions.

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