The Rarity of Bifurcation in Shallow Lake Deltas: Implications for Reservoirs and Overall Delta Processes

Abstract

Examination of deltas forming in natural and man-made lakes reveals that non-bifurcating single-channel deltas are significantly more common than bifurcating lobate deltas. We propose that the de-facto delta morphology is characterized by a fluvial channel forming a single-thread, progradational channel into the basin without bifurcation as a basinward extension of the river. In turn, the bifurcation of the delta is an artifact of basin controls not intrinsic to the river system. This linear delta morphology is the result of sediment grain-size segregation within the river channel caused by suspended load mud outpacing bedload sand. Fine-grained, muddy sediment accumulates at the river mouth and sand lags behind as mid-channel and side-attached bars. As the delta channel advances to some critical length, only mud reaches the delta mouth. Bifurcation is inhibited by the absence of sand available to form mouth bars that divert flow. During high-discharge flow events, upstream sand storage is transported downstream and deposited into the basin as thin, laterally continuous overbank sand sheets called blowout wings. Lobate deltas may form following avulsions due to the high concentration of sand derived from breached levees driving bifurcation where the levee breach occurs at the sand-rich part of the channel. Once lobate deltas lengthen and can segregate suspended load from bedload, the delta channel lengthens and returns to a single-channel morphology. This process creates non-bifurcating delta regardless of sand load in the river, and therefore is the default delta morphology. This work documents modern examples of these processes occurring at the Red River Delta, Lake Texoma, TX, the Denton Creek Delta, Grapevine Lake, TX, and in tie channels of the Grijalva River System, Tobasco State, Mexico. Outcrops of the Kayenta Formation in Warner Valley, Utah serve as an ancient analogue to these modern fluvio-lacustrine processes, displaying a lake assemblage dissected by fluvial channels that are connected by laterally extensive sand wings. Channel assemblages develop as a statistically clustered fairways centered on a primary channel axis. Blowout wings increase the potential static reservoir connectivity of the fluvial bodies by connecting otherwise isolated channels through a network of laterally extensive wings. These modern processes and resultant reservoir architecture are representative of shallow lacustrine environments without strong wave influence.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018