Terminal Distributary Channels of Fluvial-Dominated Delta Systems

Cornel Olariu and Janok Bhattacharya
University of Texas at Dallas, Richardson, TX

Terminal distributary channels are the most distal channelized features of a distributive system and can be both subaerial and subaqueous. Examination of modern and ancient deltas indicate that deltaic terminal distributary channels are narrow and shallow features with low topography, and are rarely incised. Deltaic terminal distributary channels are intimately associated with mouth bar deposits and are infilled by aggradation and lateral or upstream migration of the mouth bars. Terminal distributary channel deposits have characteristic sedimentary structures of unidirectional effluent flow but may also show evidence of reworking by waves and tides. The modern and ancient examples of deltas that we present show typical coarsening upward delta front facies successions but do not contain deep distributary channels, as have been routinely interpreted in many ancient deltas. We show that delta front deposits are typically capped by small terminal distributary channels, the cross sectional area of which represents a fraction of the main fluvial “trunk” channel. Sediment dispersion into the basin through multiple terminal distributary channels will result in an overall lobate shape of the delta. “Trunk” distributary channel deposits typically display a fluvial character and may overlie smaller terminal distributary channels within delta front deposits. Formation and evolution of mouth bars and terminal distributary channels is controlled by variation in river discharge, composition of bedload and relative variation between river and basin energy. Commonly, deltas have multiple distributaries at different scales, but the basin depth relative to channel depth and river discharge control the number of terminal distributary channels.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005