Nonsynchronous, episodic incision: evidence of threshold exceedance and complex response as controls of terrace formation

Terrace sequences can represent regional or continental scale factors such as climatic fluctuations, neotectonic activity, and base-level change. However, they can also reflect random incision events brought about by local scale, geomorphic threshold exceedance, and subsequent complex response. This study explores the formative processes of three discontinuous, but adjacent, late Pleistocene to late Holocene step-terrace sequences in southeastern Australia. Correlation of river terrace fills was undertaken by comparing terrace remnants based on topography, morphology, sedimentology, stratigraphy, and chronology. A geomorphic model of floodplain abandonment and terrace formation for this valley setting is presented. Most of southeastern Australia has shown no evidence of tectonic uplift during the late Quaternary. Bedrock bars on the Hunter River isolate the study reach from downstream base-level changes. The nonsynchronous, episodic behavior of incision events in this catchment strongly indicates that climate is not a dominant control on terrace formation. With the exclusion of climatic fluctuations, tectonic uplift and base-level change as causes of incision, catastrophic floods, and the exceedance of geomorphic thresholds emerge as the dominant controls of terrace formation.