A complex mixture of siliciclastic and carbonate sediments in the Lloyd Bay area has been demonstrated by the interpretation of high-resolution reflection seismic profiles and the detailed examination of sediment cores. Several seismically defined sedimentary sequences of late Quaternary age are recognized, corresponding to the stages and effects of postglacial sea level rise, the extent of which was determined by interpretation of coastal onlap and radiocarbon dating of mangrove peat. Petrologic studies of sediment cores revealed four basic lithologic types: sand, mud, Halimeda, and coralgal facies.
The pre-Holocene unconformity has been identified and mapped. It is an erosional surface revealed by both seismic data and petrologic evidence for subaerial exposure.
The Holocene is subdivided into six seismic sequences, thus forming basic time-stratigraphic units. The oldest sequence consists of fluvial channel-fill deposited in response to a rise in base level to approximately -70m (11,000ybp). The overlying sequence represents sediments formed during the initial inundation of the outer shelf to -5Om (10,000ybp), consisting of more widespread channel-fill and mangrove peat on the channel margins. The next sequence contains transgressive sediments formed when the sea transgressed most of the area and reached -23m (9,000ybp). Lloyd Bay formed an estuary at this time, with mangrove deposits on its margins. Initial Halimeda growth started. The overlying sequences represent sedimentation during the final rise of sea level to its present position (6,500ybp), and the following stillstand progradational deposition.
On the present-day sea floor, coralgal facies exist in the vicinity of shelf-edge reefs and mid-shelf platform reefs. Halimeda facies typically covers the outer half of the shelf. Mud facies dominate the rest of the area, and sand facies distribution has diminished to a minimum near mainland promontories. Earlier ln the Holocene, sand facies was deposited ln a larger area from a reworked Pleistocene eolian sand, now forming a palimpsest deposit. The large amounts of siliclastic mud throughout the Holocene, and a significant upwards increase ln carbonate mud content (including the inner Lloyd Bay) suggest that efficient mud transport mechanisms exist, leading to a thorough mixture of terrigenously-derived and carbonate mud. The palaeo-drainage channel in Lloyd Bay is filled with up to 30m of Holocene sediments whereas the general Holocene sediment thickness in the area is of the order of 3-4m.
The Halimeda banks dominate the outer half of the shelf ln terms of depositional rates; more than 3Om have accumulated, in places, during the Holocene. The banks form by growth of the alga Halimeda, and baffling of mud--sized sediment, thereby constructing its own foundation for further growth. Limiting factors appear to be current velocity behind the shelf-edge reefs to the east, wave base at approximately -20m restricting further upwards growth, and poor nutrient supplies, terminating the westward growth of these banks in a mid-shelf position.