Abstract

The southern sector of the New South Wales coast, Australia, delimited by the extent of the Lachlan Fold Belt and southern portion of the Sydney Basin structural provinces, provides an opportunity to examine estuarine depositional complexes in a number of bedrock-controlled valley settings. Sedimentation in these estuaries has progressed since the mid-Holocene under a process regime characterized by a high wave energy shoreface, a microtidal regime, relatively low fluvial inputs, and a relative sea-level stillstand.

A tripartite zonation of estuarine facies is shown to be a consistent feature of estuaries in the study area. The three facies zones are termed: barrier/inlet (Zone A); estuarine lagoon (Zone B); and tidal fluvial (Zone C). Each zone comprises an assemblage of morphostratigraphic units with a distinct sedimentological character interpreted as being intimately linked to the sediment source and the dominant processes operating within the zone.

Zone A sediments, sourced from marine and nearshore environments, are mature, quartz-dominant sands. Bedforms vary from three-dimensional dunes with negligible bioturbation near the estuary mouth, to two-dimensional, moderately bioturbated dunes in the medial portion of Zone A, to intensely bioturbated, incipient dunes at the distal part of Zone A. These variations are attributed to the landward decrease in tidal flow velocity. Zone B sediments are of terrestrial origin and are typically silty to sandy muds deposited in a low energy lagoon. The lagoon floor lacks dune bedforms, and sedimentary structures feature laminae, wavy beds and abundant burrow traces. Zone C sediments are also sourced from the estuary catchment, but are much coarser. Marked variations in textural properties are observed in both the surface and subsurface sediments; they incorporate muds, silty sands and sandy gravels commonly with a high amount of organic detritus. Grain size variations are interpreted as reflecting temporal and spatial fluctuations in the fluvial flow regime.

The sedimentological traits of each zone are found to be common to the two estuaries examined in detail, Wapengo Lagoon and Narrawallee Inlet, the former being partially filled and the latter completely filled. The Narrawallee sequence is interpreted as representing successive episodes, in the late Quaternary, of estuarine sedimentation during sea level highstand and fluvial scouring in the intervening glacial lowstand. Despite the complexity introduced by multiple filling and scouring in Narrawallee, facies zonation and character are maintained.