Abstract

Erosional features that indicate former sea-level stands on La Orchila Island are wave-cut notches in metamorphic basement rocks, a wave-cut notch just above a low marine terrace, and a relict mangrove swamp. The main limestone terrace consists of a coarse-grained facies (coral and shell rubble) and a fine-grained facies (calcareous and siliceous sand). Other deposits include eolianite (stabilized and cemented calcareous dunes) and salt flats. Beachrock is now forming along the southern and eastern coasts; wave erosion is prominent along the northern and northeastern coasts.

Radiocarbon dates for coral, shell, and beachrock from the terrace range between approximately 8,000 and 41,000 yr B.P. X-ray diffraction and petrographic data indicate alteration and recrystallization of the original aragonite, at least in part, thus rendering the radiocarbon ages too young. Two Th230/U238 dates on coral gave an approximate age of 131,000 yr. Thus, the terrace is most probably of Sangamon age.

The wave-cut notches were probably cut as the island emerged in early Quaternary (?) time. During the Sangamon inter-glacial stage, coral and shell rubble deposited by storms was cemented into beach-rock along the edges of the island. Planation by waves took place during the highest (late Sangamon) sea-level stand. The terrace was exposed during the Wisconsin Glaciation; postglacial sea-level rise did not reach the terrace level because of island uplift.

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