Stratigraphic Response of Wave-Dominated Estuaries to Different Relative Sea-Level and Sediment Supply HistoriesQuaternary Case Studies from Nova Scotia, Louisiana and Eastern Australia

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Abstract
The stratigraphic organization of facies within estuarine valley fills for three wave-dominated coasts illustrates the importance of relative sea-level behaviour and sediment supply. The study areas differ primarily in their Holocene sea-level history and relative significance of clastic sediment sources. Lawrencetown Lake on the eastern shore of Nova Scotia has experienced sea-level rise of 30 to 40 cm/century since early Holocene time and has yet to reach the point of maximum transgression. Sediment is sourced primarily from eroding drumlin headlands, beaches and drowned barriers. Fluvial sediment input is negligible. Valley stratigraphy is characterised by transgressive flood-tidal delta and nearshore-bar facies overlying central basin facies. Bounding surfaces include a lowstand unconformity beneath glacial deposits, a transgressive surface at the base of central basin deposits, an inlet diastem and a shoreface ravinement surface. These surfaces are all stratigraphically separated in the valley fill. Bay-head delta and floodplain facies, and associated bounding surfaces are poorly developed. The maximum flooding surface has yet to form. Locally, abundant supplies of sediment from drumlins and drowned barriers results in periodic shoreline progradation, despite rising sea level. Shoreline transgression has continued at varying rates along the western sector of the Louisiana coast since about 9 ka, in response to episodic, relative sea-level rise. Sea level is presently at relative stillstand, although the mean water level in Lake Calcasieu is rising at 60 cm/century. Sediment sources include local, low-gradient coastal-plain rivers and distributaries of the Mississippi River that discharge sediment into westward-flowing longshore currents. Subsurface data from Lake Calcasieu reveal a 30-to 55-m deep valley filled predominantly with facies of fluvial origin, including a lowstand wedge, two aggraded fluvial deposits in the transgressive systems tract, transgressive marsh and transgressive to highstand central basin deposits. The contact between lower and upper central basin sediments represents the maximum flooding surface. Modern fluvial channel and bay-head delta facies are incised into aggraded central basin deposits at the head of the estuary. A channel diastem occurs at the contact between bay-head delta and central basin facies. At the seaward end of Lake Calcasieu, the shoreface ravinement surface truncates central basin deposits and extends into the estuary. The presence of a 15 km wide chenier plain indicates that transgression has been punctuated by shoreline progradation. Sea-level along the New South Wales coast has maintained a stillstand position since about 6.5 ka and estuaries lie close to the highstand position. Marine sediment input was high during the early to mid-Holocene, but inner-shelf supplies of sediment became depleted by 3 ka BP. South-coast streams have relatively steep gradients and drain catchments composed of coarse-grained lithologies. The Wapengo Lagoon and Narrawallee Inlet valley fills display regressive characteristics resulting from bay-head delta progradation across central basin deposits, and in Narrawallee, the river delta interfingers with tidal-inlet facies. The shoreface ravinement surface terminates at the mouths of both estuaries, and inlet diastems appear inactive. Bay-head delta diastems, however, continue to grow seaward. Multiple estuarine sequences are preserved in both valleys due to incomplete scouring during the last sea-level lowstand.

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