Tumblagooda sandstone

The Tumblagooda Sandstone is a geological formation deposited during the Silurian or Ordovician periods, between four and five hundred million years ago, and is now exposed on the west coast of Australia in river and coastal gorges near the tourist town of Kalbarri, straddling the boundary of the Carnarvon and Perth basins. Visible trackways are interpreted by some to be the earliest evidence of fully terrestrial animals.

The Tumblagooda is over 1,400 m (4,500 ft) thick: the base is not exposed, but geophysical data (primarily magnetic) indicate the sandstone unconformably overlies a Proterozoic basement. The formation is divided into four facies associations (FAs), numbered stratigraphically, that occur sequentially from bottom to top. These lithified sediments portray an environment dominated by high-energy braided streams flowing into the sea; some parts have been interpreted as reflecting deposition in ephemeral pools controlled by water table rise and fall, but alternative interpretations favour deposition on tidal sand flats.

The lowest facies association in the unit is dominated by trough cross-stratification, deposited by broad, high-energy braided rivers, which formed the outwash plain of an alluvial system.Trace fossils are virtually absent, because the high depositional energy meant burrowing organisms could not survive. The downslope flow was to the north west.

These facies reflect a quieter, more distal environment; the unit is occasionally interrupted by lenses of FA1 sediments. There are two published interpretation of the depositional setting of these facies: they were initially interpreted as tidal sand-flat deposits, an interpretation still followed, but subsequently as continental eolian deposits. The second interpretation is described below. Beds are on the whole thin, planar and well sorted. Beds about 5 centimeters (2.0 in) thick form 2 meters (6.6 ft) units of "bedded sandsheets"—layers of sand blown by the wind—which form a characteristic lithology of this facies. Eolian indicators such as adhesion surfaces and warts are widespread, but may simply indicate regular emergence in an intertidal setting rather than support for eolian deposition and dunes.

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