A technique for quantification of quaternary ground tilting as recorded by alluvial fill terraces: An example from the northern Gulf of Mexico coastal plain

Abstract

To quantify the degree of Quaternary structural tilting, we examined topographic slopes of treads of alluvial fill terraces. Like strath terrace treads, fill terrace treads can serve as a structural datum if restored to their pre-erosion geometry. Their slope degree and direction can then be compared to the slope of the modern floodplain to assess neotectonic tilting. On the northern Gulf of Mexico coastal plain, a ~45km-wide flight of seven Pleistocene alluvial fill terraces of the Ouachita River records a preferred southwestward migration of the river, and previous authors attribute this lateral migration to Quaternary structural tilting. The original geometries of Ouachita River terrace treads are needed as structural datums, but the treads are dissected by tributary streams. On younger treads, we observe concordant elevations of tributary divides and stronger soil development on divides than on tributary valley flanks. This indicates that divides are minimally eroded and preserve parts of the original treads. In order to restore terrace treads to their pre-dissection geometries, we constructed a topographic envelope model using elevations of tread remnants on tributary divides. This model effectively ‘fills’ the tributary valleys on the treads. We extracted each restored tread from the model and determined the tread's best-fit plane. We used three planes with R2 values >0.9 as datums. Consistent with westward tilting, directions of topographic slope of middle/late Pleistocene treads are rotated 80o toward the south-southwest, and treads have slopes that increase with terrace age from 0.017° to 0.09° (0.3 to 1.6 m/km). This quantitative estimate of late Quaternary westward regional tilting reveals a 10-fold increase in slope relative to the Holocene floodplain (0.14 m/km).