ABSTRACT The Paleoarchean Antarctic Creek Member of the Mount Ada Basalt, Eastern Pilbara Terrane, Western Australia, includes beds of jasper and white chert composed of sand‐sized silica granules that often contain or are mixed with silt‐sized particles of haematite. Sedimentology of the granule beds can be interpreted in terms of a stratified ocean and the existence of a chemocline roughly coincident with storm wave base between anoxic, iron‐rich deeper waters and a weakly oxidising, well‐mixed, iron‐poor surface layer. Pure silica granules show features suggesting precipitation within the iron‐free surface layer. At depths near and just above the chemocline, a mixing zone developed. Within this mixing zone, haematite was precipitated and widely coated fine silica granules forming distinctive silt‐sized grains termed silica–haematite microgranules. These microgranules as well as discrete haematite particles were incorporated into larger, sand‐sized silica granules showing a more‐or‐less uniform internal distribution of haematite grains. Another distinctive population of silica–haematite granules formed by stirring of the soft, unconsolidated bottom sediment, probably by storms. These granules, which coagulated from the stirred sediment, were so fluid when formed that the included denser haematite particles settled to the lower parts of the still‐soft granules before solidification. Below the chemocline, laminated to banded, haematitic but sparsely granular sediments accumulated. Above the chemocline, deposition occurred in an environment that was commonly wave‐ and/or current‐active but with little significant erosion of the sea floor and no introduction by currents of extrabasinal sediment. These currents may reflect storms, deep circulation or deep tidal currents. The Antarctic Creek Member provides a detailed picture of a possibly widespread but heretofore poorly studied Archean marine sedimentary system dominated by precipitative processes.
Lowe et al. (Mon,) studied this question.