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. Through the description and interpretation of the physical landscape and by use of a detailed construction of the chronology, this study explores the formative processes of three discontinuous but adjacent, Late Quaternary step-terrace sequences.
Terrace remnants on Widden Brook, a sandy alluvial stream in southeastern Australia, were examined and correlated longitudinally to establish their evolutionary history. Three distinct, discontinuous terrace sequences, the Baramul, Widden and Kewarra, were identified within a 26 km reach using sedimentology, topography and chronology. Each terrace sequence occurred within a geomorphically distinct valley setting; an upstream constriction, a valley expansion and a highly constricted downstream section. Combined use 14C and OSL dating techniques allowed for a more rigorous assessment of the alluvial record and indicated that each terrace sequence was formed during the late Pleistocene and Holocene (16.7 ka cal BP – 0.5 ka cal BP). The three terrace sequences identified yielded, continuous chronologies, indicating continuous alluvial deposition throughout the Holocene. A geomorphic model of floodplain abandonment and terrace formation for this valley setting was then constructed.
The confinement of these sequences within the different valley settings and the continuous chronology was a clear indication that localised processes have influenced their formation. 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 non-synchronous, episodic behaviour of incision events, in this catchment, strongly indicates that climate is not a dominant control on terrace formation. The Widden catchment, therefore, provides an ideal landscape to discriminate regional from local controls on terrace formation. These terraces reflect random incision events brought about by intrinsic threshold exceedance. This process was intermittently interrupted or accelerated by large-scale events that stripped sections of the floodplain down to a basal gravel lag. This study emphasises that random geomorphic threshold exceedance must be considered when relating incision events to wide-scale allogenic factors.