A multi-proxy assessment of terrace formation in the lower Trinity River valley, Texas

[摘要] A proposed null hypothesis for fluvial terrace formation is thatinternally generated or autogenic processes, such as lateral migration andriver-bend cutoff, produce variabilities in channel incision that lead to theabandonment of floodplain segments as terraces. Alternatively, fluvialterraces have the potential to record past environmental changes fromexternal forcings that include temporal changes in sea level andhydroclimate. Terraces in the Trinity River valley have been previouslycharacterized as Deweyville groups and interpreted to record episodic cutand fill during late Pleistocene sea level variations. Our study useshigh-resolution topography of a bare-earth digital elevation model derivedfrom airborne lidar surveys along ∼  88 linear kilometers of the modernriver valley. We measure both differences in terrace elevations and widthsof paleo-channels preserved on these terraces in order to have twoindependent constraints on terrace formation mechanisms. For 52 distinctterraces, we quantify whether terrace elevations fit distinct planes –expected for allogenic terrace formation tied to punctuated sea level and/orhydroclimate change – by comparing variability in a grouped set ofDeweyville terrace elevations against variability associated with randomlyselected terrace sets. Results show Deweyville groups record an initialvalley floor abandoning driven by allogenic forcing, which transitions intoautogenic forcing for the formation of younger terraces. For these differentterrace sets, the slope amongst different terraces stays constant. For 79paleo-channel segments preserved on these terraces, we connected observedchanges in paleo-channel widths to estimates for river paleo-hydrology overtime. Our measurements suggest the discharge of the Trinity River increasedsystematically by a factor of ∼  2 during the late Pleistocene.Despite this evidence of increased discharge, the similar down-valley slopesbetween terrace sets indicate that there were likely no increases insediment-to-water discharge ratios that could be linked to allogenic terraceformation. This is consistent with our elevation clustering analysis thatsuggests younger terraces are indistinguishable in their elevation variancefrom autogenic terrace formation mechanisms, even if the changingpaleo-channel dimensions might, viewed in isolation, provide a mechanism forallogenic terrace formation. Methods introduced here combine river-reach-scale observations of terrace sets and paleo-hydrology with localobservations of terraces and paleo-channels to show how interpretations ofallogenic versus autogenic terrace formation can be evaluated within asingle river system.

[学科分类] 土壤学