1. Western Australian Museum, Francis St, Perth WA 6000
2. Research School of Earth Sciences, Australian National University,
ACT 0200
Barrow Island is an emergent anticlinal structure situated near the inner margin of the Exmouth Plateau, northwestern Australia. It rises to 60 m asl and is currently separated by a 60 km shallow water strait (to 15 m) from the Onslow coast. The surficial carbonate exposure includes minor ?Eocene Giralia calcarenite, confined to the centre of the island, and various subunits of the mid- to late Miocene Trealla Limestone equivalent, forming the major plateau surfaces and exposures in coastal cliffs. Various Plio-Pleistocene calcarenite and aeolianite units are present around the margin of the island. The Trealla Limestone equivalent supports a spectacular palaeokarst which is best seen in section in 10-30 m high cliffs along the west coast. Its features range from sediment-filled solution tubes and grikes, to large, infilled dolines and partially filled phreatic cavern systems. Most palaeokarst features are infilled by cemented red breccias, occasionally with associated speleothem. Today, Barrow Island supports a skeletal soil mantle and a low hummock-grass steppe dominated by Triodia wiseana. Small pockets of Eucalyptus spp. and mangroves are present in sheltered valleys and coastal embayments respectively.
Vertebrate fossils have been recovered from a total of six major sedimentary units including cave- and doline-fill breccias, and a coastal aeolianite. Palaeomagnetic determinations conducted on several separate breccias, combined with interpretations of coastal geomorphic history, indicate that breccia formation commenced in the Gilbert Chron, but that most of the larger deposits accumulated during the Gauss Chron. A later phase of sedimentation into partially re-excavated caverns occurred during the early to mid? Pleistocene, and continues on a small scale through to the present.
The earliest vertebrate fauna, of possible late Gilbert Chron age, has yielded a variety of both browsing and grazing kangaroos (including Sthenurus sp.), two species of ringtail possums, and single species of bandicoot, marsupial mouse and thylacine. Despite the presence of other small mammals, there is no evidence of rodents in the deposit. The faunal suite points to a mosaic vegetation of grasslands and woodland/shrubland. Several faunas of presumed Gauss age contain abundant remains of bandicoots, bettongs, small wallabies, a brush-tailed possum, marsupial mice (Sminthopsis) and a variety of murid rodents (Pseudomys spp., Zyzomys sp., a new genus of water rat), along with a megadermatid bat. The absence of ringtail possums or browsing kangaroos suggests a disappearance of remnant forest/shrubland habitat by that time. The time interval between these and the earlier fauna is also of great interest insofar as it may include the arrival of murid rodents in Australia.
Two faunas of probable early to mid-Pleistocene age are found in direct stratigraphic association. The earlier fauna, preserved in a remnant cave floor deposit with associated speleothem, contains a diverse suite of mammals including both large and small macropodids, bettongs, two genera of bandicoots, at least four species of dasyurids, a brush-tailed possum, a ghost bat (Macroderma sp.), other microchiropteran bats, occasional reptiles and birds, and at least nine species of murid rodents including representatives of the 'newcomer' genus Rattus. The diversity of this fauna greatly exceeds that found on Barrow Island today but is similar to that recovered from archaeological deposits of late Pleistocene to early Holocene age on the nearby Montebello Islands; it is presumed to relate to a sea level low stand. The overlying fauna, derived from a silicified aeolianite, has yielded a small fauna that includes a large species of Isoodon distinct from that found on the island today.
The Barrow Island faunal succession provides the first record of vertebrate evolution through the late Tertiary and Quaternary of northwest Australia, and a unique opportunity to examine the long-term evolution of an insular fauna.