The fourth shale macroband of the Dales Gorge Member of the Brockman Iron Formation in the Hamersley Basin in Western Australia contains a 2.49 billion years-old impact ejecta layer (Hassler and Simonson,2001). The S4 layer is the least studied Paleoproterozoic impact layer in the Hamersley Basin. It displays textures and replacement minerals not seen in other layers. Analysis of this layer will help our understanding of the processes that form impact ejecta, especially in the Paleoproterozoic.
It is believed that these ejecta settled in a deep basin environment; laterally extensive beds and mud-sized grains are evidence of the deep basin setting. Samples were chosen based on density of ejecta within the layer. Furthermore, the ejecta exhibited good internal textures. Also, samples were taken from two outcrop sites, Dales Gorge and Yampire Gorge, and one core site, Tom Price.
This impact ejecta layer is made up of two distinct populations that are coarse to very coarse sand sized grains, an angular population and a splash form population. Splash forms are circular, teardrop or oval shaped grains while angular grains usually have boxier shapes. Evidence, in the form of specific crystal morphologies, such as swallow-tail terminations and bowties, is present to support the hypothesis that the target rock was basaltic. Broken splash forms with rims that have devitrification textures are present. Analysis shows that these grains devitrified before breakage occurred. This observation helps to understand the processes that form the splash 'forms. Mineral compositions vary between the outcrop sample and the core sample. In the former, oxidized tilpnomelane and potassium feldspar are the secondary minerals. However at Tom Price, the secondary mineral is unoxidized stilpnomelane. Different diagenetic environments are obviously at work between the DG/YG sites and the Tom Price. A comparison of the S4 layer with other studied impact ejecta shows that composition, size and internal textures varied from ejecta layer to ejecta layer.