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Submersible-based observations of in-situ fossil tree trunks in Late Cretaceous seafloor outcrops, Orange Basin, western offshore, South Africa

De Beers Marine, P.O. Box 87, Cape Town 8000, South Africa, email: ian.stevenson{at}debeersgroup.com

Marion K. Bamford

Bernard Price Institute for Palaeontological Research, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa, e-mail: bamfordm{at}geosciences.wits.ac.za

A submersible dive undertaken over seabed outcrop situated on the middle shelf off the Namaqualand coast, at a depth of 136 to 140 mbsl, about 32km offshore, revealed an abundance of in-situ fossil tree trunks in exposed Late Cretaceous (Late Coniacian) seafloor outcrop. Based on regional Side Scan Sonar interpretations, it would appear that this outcrop is unique to the dive location, covering an approximate 2km² area of seabed. Integration of these observations with a regional seismic stratigraphic and biostratigraphic framework has enabled more accurate constraints to be placed on the ages of the wood than was previously possible. Two species of fossil wood belonging to the Podocarpaceae were identified from samples collected by the submersible: Podocarpoxylon jago Bamford and Stevenson and P. umzambense Schultze-Motel. Palaeoenvironmental analysis shows that the proximal Coniacian succession for the Orange Basin was typified by a well-vegetated coastal plain, which was periodically waterlogged. The coastal plain experienced repeated periods of progressive drowning of the incised valley systems cutting through the coastal plain and the subsequent development of wave-dominated estuarine incised valley fill systems. The fossil wood recovered from the dive-site is typified by a lack of distinct growth ring development. This implies either a lack of seasonality or sufficient available water all year round, with no low temperatures to stop plant growth. This observation, together with complementary sedimentological and micropalaeontological analysis suggests a temperate palaeo-climate for the area.