The enigma of 3400 years BP coastal oolites in tropical northwest Western Australia… why then, why there?
Oolites crop out along the northwestern coast of Western Australia at Port Smith, about 80 km SW of Broome. An oolitic coastal ridge truncated by marine erosion exposes subtidal, intertidal, and supratidal (aeolian) facies. The deposits are firmly indurated and composed of about 75% tangentially and moderately thickly layered, aragonitic ooid grains with over 90% quartz nuclei. Subtidal sedimentary structures are exposed about a metre above the present high tide mark, hinting that sea level may have been somewhat higher when the shoreline was formed. However, the macrotidal range of up to 7 m, and the possibility of cyclonic surges along the coast, precludes unequivocal determinations on this point. Whole-rock amino acid racemisation (AAR) geochronology (epimerisation of isoleucine: D-alloisoleucine/ L-isoleucine or A/I) on each facies of the oolite outcrop averaged 0.106 ± 0.013 ( N = 10). The modern beach contains fewer ooids (∼ 30%), and nearly half of these are stained brown, grey, or black, perhaps as a result of burial, reduction and/or mineralization. A higher (older) mean and large standard deviation in whole-rock amino acid ratio of 0.145 ± 0.067 ( N = 2) supports our inference that ooids on the modern beach were reworked from fossil deposits. Reverse phase chronostratigraphy (RPC) on individual ooid grains holds tremendous promise in this preliminary study. RPC results show a narrow variation of D/ L values (CV = 6-11%), and yield nearly identical D/ L Asp means from light coloured fossil ooids (0.307 ± 0.018 ( N = 17)); light (0.323 ± 0.026 ( N = 12)) and dark coloured ooids (0.298 ± 0.027 ( N = 10)) from the active beach face. When compared to A/I ratios from 14C dated mid-Holocene ooids in the Bahamas, the mean A/I from Port Smith reflects an age of ca. 3500-4500 years that is in agreement with a calibrated AMS 14C age of 3370 ± 50 calendar years BP on the same material. Thus, the ooids were formed, transported, emplaced, strongly cemented, and largely eroded from the beach ridge in only 3400 years. The environmental conditions that underlie a pulse of ooid deposition during a brief period of the mid-Holocene almost certainly involve extensive tidal inlets in the area, a possible mid-Holocene oscillation 1-2 m above present, and a subsequent fall to present level that terminated the process. Apparently, a unique combination of factors produced ooids here, and in only a handful of other sites in Australia and the Indian Ocean.