Morphodynamic Modeling of Fluvial Channel Fill and Avulsion Timescales During Early Holocene Transgression, as Substantiated by the Incised Valley Stratigraphy of the Trinity River, Texas

Abstract

The Trinity River system provides a natural laboratory for linking fluvial morphodynamics to stratigraphy produced by sea-level rise, because the sediments occupying the Trinity incised valley are well-constrained in terms of timing of deposition and facies distribution. Herein, the Trinity River is modeled for a range of base-level rise rates, avulsion thresholds, and water discharges, to explore the effects of backwater induced in-channel sedimentation on channel avulsion. The findings are compared to observed sediment facies to evaluate the capability of a morphodynamic model to reproduce sediment deposition patterns. Base-level rise produces mobile locations of in-channel sedimentation and deltaic channel avulsions. For scenarios characteristic of the early Holocene (4.27 mm yr -1), the Trinity fluvial-deltaic system progrades 12 m yr -1, followed by backstepping of 27 m yr -1. Avulsion is reached at the position of maximum sediment deposition (located 105 km upstream of the outlet) after 3,473 model years, based on sedimentation filling 30% of the channel. Under scenarios of greater base-level rise, avulsion is impeded because the channel fill threshold is never achieved. Accounting for partitioning of bed-material sediment to the floodplain influences the timing and location of avulsion over millennial timescales: the time to avulsion is greatly increased. Sedimentation patterns within the valley, modeled and measured, indicate preference toward sandy bed material, and the rates of deposition are substantiated by previous measurements. Although the results here are specific to the Trinity River, the analysis provides a framework that is adaptable to other lowland fluvial-deltaic systems.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017