GEOLOGICAL CONTEXT OF PEAT DEPOSITION IN CUMBEBIN SWAMP, NORTHEASTERN NEW SOUTH WALES: A GEOLOGICAL FRAMEWORK FOR CARBON SEQUESTRATION STUDIES

Throughout the Twentieth Century, human activities have heavily impacted coastal wetlands. Drainage and filling, the clearing of vegetation, and pollution are a few of such activities that have contributed towards the loss of these habitats.

Cumbebin Swamp is a low-lying, estuarine/alluvial flood plain that is situated southwest of the township of Byron Bay along the northern New South Wales coastline. It is remnant of what was once a regionally-significant wetland system. The dominant lithological groups present in the Cumbebin Swamp area are of Quaternary origin and overlie Paleozoic metamorphic bedrock. Soil types present in Cumbebin Swamp include massive black and gray coastal clays that have been derived from a combination of alluvial materials originating from basalts, sandstones, metamorphic rocks and marine sediments. Some areas of the swamp contain overlying acidic peats.

The purpose of this study has been to determine the geological origin and evolution of Cumbebin Swamp, and to provide a geological framework for carbon sequestration studies.

Vibracore equipment was used to collect a 53 cm-long, 12 cm-diameter wetland core. Swamp sediments have been characterized in terms of thickness, location within the swamp stratigraphy, and environment of deposition. Detailed geology and soils maps of Cumbebin Swamp and surrounding coastal plain have been compiled using NSW Department of Mineral Resources data and ArcView 8 software.

Aerial photography reveals evidence of older, relict coastal embayments, and the kidney-shaped appearance of the swamp suggests that it may correspond with an estuary or lagoon environment.

Eight distinct layers of sediment have been identified throughout the wetland core. Acidic, water-saturated, oxygen-poor conditions are responsible for peat deposition since the last high sea level (which peaked 1 m APSL and occurred 6.5 ka), and this sediment was found up to 20 cm below the swamp surface. The presence of a reddish-brown, oxidized layer of sand 10-17 cm in depth suggests that two periods of peat deposition may have taken place. Fine-grained quartz sand was found 20-34 cm below the swamp surface and overlies a 6-cm thick bed of angular quartz gravel. This material may represent weathered Paleozoic rocks. A 10 cm-thick layer of impenetrable coffee-rock was found at the base of the core.