SAWMILL SINK, ABACO, BAHAMAS: A MULTI-ENVIRONMENT FOSSIL PRESERVATION SYSTEM

Caves in limestones are well known as fossil repositories. Caves developed in very young limestones can contain fossils of an approximately similar age in both the cave wall rock as well as the sediment infill as a result of the small time window between limestone deposition and dissolution. Caves in coastal environments, as a result of sea-level change, alternate cyclically between vadose and phreatic conditions, which create unique but ephemeral fossil capture and preservation conditions.

Sawmill Sink, Abaco, Bahamas, is a large progradational collapse structure or blue hole that intersects dissolution conduits at depth, but is also open to the land surface above. The wall rock contains Quaternary subtidal fossils, primarily mollusks and corals, while the cave void space contains at least two separate Late Quaternary vertebrate fossil assemblages. During glacioeustatic sea-level lowstands in the late Pleistocene, Sawmill Sink provided a refuge that hosted bat colonies and several owl roosts. Sea level rise in the Holocene preserved fossil remains in these deposits from subsequent disturbance by insects, burrowers, and scavengers. The owl roosts consist of a large and varied small vertebrate sample, dominated by land birds, actively transported by barn owls to a central collection site. As sea level approached modern elevations in the late Holocene, the fresh-water lens approached the surface opening. The opening collected large amounts of plant material, which in turn concentrated at the halocline where decay produced a thick anoxic mixing zone. A talus pile of collapse debris created a collection surface for organic material within that mixing zone. The blue hole acted as a traditional passive pitfall trap, capturing large vertebrates, notably crocodiles and tortoises, which accumulated on the surface of the talus pile, and were entombed in organic peat deposits.

Sawmill Sink is unique in that a talus pile acted as a stable platform on which skeletons of large vertebrates could collect, while the coincident anoxic mixing zone contributed to the superb preservation by limiting degradation of the fossils by scavenging, oxidation, and disturbance. The result is the best preserved Quaternary sample of large vertebrates from the entire West Indian region.