Facies (2010) 56:179–193 DOI 10.1007/s10347-009-0202-8 123 ORIGINAL ARTICLE An isolated carbonate knoll in the Timor Sea (Sahul Shelf, NW Australia): facies zonation and sediment composition Claudia Wienberg · Hildegard Westphal · Eva Kwoll · Dierk Hebbeln Received: 8 May 2009 / Accepted: 30 August 2009 / Published online: 17 September 2009  Springer-Verlag 2009 Abstract This paper constitutes a Wrst detailed and systematic facies and biota description of an isolated car- bonate knoll (Pee Shoal) in the Timor Sea (Sahul Shelf, NW Australia). The steep and Xat-topped knoll is character- ized by a distinct facies zonation comprising (A) soft sediments with scattered debris and scarce sponges, hydro- zoans and crinoids (320–210 m water depth), (B) hard- ground outcrops (step-like banks, vertical cliVs) that are mainly colonized by octocorals and sponges (210–75 m), and (C) the summit region (75–21 m) where the slopes merge gently into the Xat-topped summit that is densely colonized by massive and encrusting zooxanthellate corals and the octocoral Heliopora coerulea. In contrast, the sedi- ments recovered from the summit are dominated by the green alga Halimeda, subordinate components are corals, benthic foraminifers, mollusks, and coralline red algae. Thus, the sediments are classiWed as chlorozoan grain assemblage. However, non-skeletal grains (fecal pellets, ooids) are almost completely absent. This discrepancy between the living biota and the sediment composition could reXect a disruption by the severe tropical cyclone Ingrid that hit the northern Australian shelf in March 2005, just before the sampling for this study took place (September 2005). Keywords Tropical carbonate sediments · Chlorozoan facies · Facies zonation · Sediment composition · Halimeda · Tropical cyclone · Pee Shoal · Timor Sea · Sahul Shelf Introduction and objectives Since the mid-20th century, it has been known that along the shelf break in the Timor Sea numerous seabed struc- tures exist that had been postulated to be of biological ori- gin (van Andel and Veevers 1967; Hovland et al. 1994). These structures occur within the Vulcan sub-basin at the northwestern edge of the Sahul Shelf (NW Australia), and are separated from the coral reefs of the Ashmore Platform (Teichert and Fairbridge 1948) in the west, the Sahul Shoals in the north, and the Halimeda-dominated Big Bank Shoals in the east, the latter being intensely studied in terms of facies distribution (Heyward et al. 1997). The Wrst study focusing on the seabed features between Ashmore Platform and Big Bank Shoals (van Andel and Veevers 1967) described them as banks with steep slopes (12–33°) and Xat tops. Dredge samples revealed that the green alga Halimeda constitutes the majority of the sediment and is associated with foraminifers, bryozoans, and coralline algae. More recent seismological studies have revealed the knoll-like shape of these structures with a circular to oval shape (Hovland et al. 1994), and that they occur as clusters. Their bases are located in water depths down to 350 m. Their tops are elevated above the sea Xoor between 20 and 225 m while their diameters range between 100 and 1,000 m (Hovland et al. 1994). Seismic proWling implied that most of these knolls are related to deep-seated faulting accompanied by seepage of hydrocarbons, and that they have formed as a result of in situ accumulation of skeletal material (Hovland et al. 1994). Nevertheless, to date, little is known about the total number of knolls, their origin, internal structure and com- position, and in particular about the sediment coverage and biogenic assemblages associated with them. Due to their isolated occurrence in water depths down to 350 m, biota C. Wienberg (&) · H. Westphal · E. Kwoll · D. Hebbeln MARUM, Center for Marine Environmental Sciences, University of Bremen, Leobenerstrasse, 28359 Bremen, Germany e-mail: cwberg@marum.de