Oceanic island arc stratigraphy in the Caribbean region: don't take it for granite

Abstract

A popular misconception of island arcs is that they consist mostly of intrusive rocks belonging to the granite family, and volcanic rocks. Study of exhumed Caribbean island arcs indicates that they are composed mostly of sedimentary rocks such as epiclastic volcanic materials, and true plutonic and volcanic rocks are relatively minor. A simple model for the stratigraphic development of oceanic island arcs is proposed based on observations in the Caribbean region and intuitive reasoning. The lowermost part of the model arc is oceanic crust (oceanic arc depositional system 1, or OADS I). Built atop of the oceanic crust is the arc sequence, including volcaniclastic strata, lava flows with feeder dikes and plutons. Oceanic arc strata consist of a lower pyroclastic-absent volcanic unit (OADS II) formed by submarine volcanism, and an upper unit (OADS III) bearing volcanic, pyroclastic and epiclastic rocks (referred to here as the pyroclastic-present association or unit). Both OADS II and III may contain intercalated pelagic and/or biogenic sediments such as limestones and cherts. Submarine volcanic rocks include pillowed and sheet lava flows, lithic and hyaloclastic breccias and resedimented lithic and hyaloclastic debris. OADS III can be recognized by the presence of shallow-water fossils, stream- or beach-worn volcanic clasts and other epiclastic materials even in deep-water environments where such materials are resedimented. Shallow-water materials and epiclastic strata are never found in OADS II. Thickness of the units vary in accordance with initial water depth of oceanic crust prior to arc formation, amount of thermally induced uplift during early arc rifting, subsidence due to loading, and differential basinal subsidence or rates of volcanic aggradation. Examples of arc sequences from Aruba, Bonaire, Curaçao, Grenada, Martinique, the Virgin Islands and Puerto Rico show significant variability. Aruba and Curaçao show incomplete sequences of arc growth and may reflect arc development on thick oceanic crust (for example, a plateau). Submarine volcanic arc facies associations (OADS II) occur only in the northern Virgin Islands (Water Island Formation) and Bonaire (Washikemba Formation). Typically, great structural thicknesses (5–10 km) of arc-related strata are preserved, although condensed arc sequences built upon accreted terranes are present locally (southwest Puerto Rico, Grenada). On Grenada, the active magmatic platform is underlain by overthrust forearc(?) basin strata, indicating that magmatic platforms may grow by means other than volcanic aggradation.