Northern Territory Western Australia Lithic Gravel Ridges

Ridges composed entirely of lithic gravel and occasional coral fragments are common along the Kimberley Coast of northwest Western Australia, close to the border with the Northern Territory. These ridge sequences form gravel barriers and often impound back-barrier lagoons in embayments along sections of coast dominated by steep rock cliffs (Fig. 9). They are particularly common along the western side of Cambridge Gulf north of Wyndham in the east Kimberley region. The gravel ranges in size up to 1.6-m diameter (A-axis) and 1.4 m (B-axis) and have been deposited into sequences of up to 9 ridges paralleling the shore. At La Crosse Island

Fig. 9 Lithic gravel ridges at La Crosse Island, Cambridge Gulf, Western Australia

(Fig. 9) offshore from the mouth of the Ord River, gravel ridges have been deposited in every embayment and thereby form a discontinuous sequence that surrounds the island. The ridges at two surveyed sites each on opposite sides of the island extend up to 5 m AHD, and radiocarbon samples on coral fragments embedded with the core of ridges from each of seven ridges from these sites show that they were deposited between approximately 5,000 yrs B.P. until recently (Nott, 2000). The radiocarbon samples do not show a progressive increase in age with distance inland, suggesting that the ridges are regularly overtopped and reworked by marine inundations.

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