Geology of Aves Island (Venezuela) and subsidence of Aves Ridge, Caribbean Sea

Abstract

Aves Island, the only outcrop of Aves Ridge above sea level, is a calcareous sand island on a beach-rock platform surrounded by a fringing reef. The highest part (3.72 m a.s.l.) consists of a storm terrace. A comparison of geological maps of the island in 1968 and 1983, revealed the following changes: (1) the southern storm terrace, a prominent feature in 1968, has disappeared; (2) the northern storm terrace has been reduced in area; (3) beach rock crops out more extensively in 1983; (4) removal of beach sand in the northeastern part of the island has exposed probably older, weathered beach rock; and (5) shingle beaches, reported in 1957, absent in 1968, were again present in 1983; they may be a relict feature underneath the beach sand. These changes are attributed to erosion by hurricanes and storms; during the interval between hurricanes, normal beach and shore processes act on the island. Present-day sedimentary and erosional instability may also be in part due to extensive guano mining during the 19th century. A seismic survey of Aves Island platform has revealed two sedimentary units overlying the acoustic basement. These two units are interpreted as unconsolidated, water-saturated sand (Surficial Unit), and slightly compacted to compacted sediments (Middle Unit). The acoustic basement probably consists of consolidated sedimentary rocks. On the basis of dredged rocks, the rate of subsidence of Aves Ridge can be estimated as 3.8 or 6.0 × 10 −5 m yr −1. This rate is insufficient to explain historical changes in size and sedimentary distribution of Aves Island and platform. Using these rates, the acoustic basement cropped out above sea level 1.1–1.7 Ma ago; the last outcrop of this basement during an interglacial sea-level stand must have been between 0.25 and 0.40 Ma ago.