Shallowing upwards sedimentation in a coral reef lagoon great barrier reef of australia
Journal of Sedimentary Petrology 59(6): 1036-1051
Sediments in the 5-25 m deep lagoon of Davies Reef, a typical mid-shelf reef in the Central Great Barrier Reef, are described, based on data gleaned from 19 soft sediment cores (75 mm diameter, up to 7.5 m penetration), 22 surface sediment samples and air lift surveys. The Holocene sediment pile, which averages 5-10 m thickeness, is entirely skeletal in origin and has a very poorly sorted, coral gravel-rich base which grades up into poorly sorted muddy sands with infaunal molluscan gravel. These muddy sands (mainly derived from the reef rim as opposed to being produced in situ on the lagoon floor) have been intensely bioturbated by callianassid shrimps, leaving the infaunal molluscan gravel patchily distributed and the bivalves disarticulated and preferentially oriented sub-horizontally, concave-up. There are no vestiges of primary depositional sedimentary structures such as bedding or cross-bedding. Radiocarbon dating suggests that significant accumulation of muddy-sand in the center of the lagoon did not begin until about 2,500-3,000 yr BP, and that sediment accumulation rates have increased through time from an average of 1.4 mm/yr between 2,400 and 640 yr BP, to 3.4 mm/yr over the past 640 years. Interpreting these data, the coral gravel-rich base to the sediment pile represents a period of prolific coral growth across the top of Davies Reef Platform following flooding by the Holocene transgression but before the outer reef rim caught up with sea level to form a lagoon. When the rim did reach sea level (about 3,000-4,000 yr BP?), is started to shed large quantities of mud and sand grade sediment into the center of the reef platform and this, combined with decreased water circulation, led to the demise of coral growth in the newly formed lagoon. As the area of reef rim at sea level increased, so too did the supply of sediment to the lagoon, which explains the observed increase in sediment accumulation rate through time. Using previously published data to aid speculation into the future development of the lagoon, a shallowing-upwards model for reef lagoon sedimentation is developed. In this model, a complete sequence consists of fining-upwards base (reflecting the gradation from coral gravel up into bioturbated muddy sands) and a coarsening-upwards top (reflecting a gradation from bioturbated muddy sands up into a zone of shallow subtidal coral rubble and patch reefs, then inter- and supratidal windward storm shingle ramparts and leeward sand cays).