A paleoenvironmental investigation of San Andres Island, Colombia: a study of carbonate rocks

The modern reef complex of San Andres Island, Colombia, can be divided into five major depositional environments on the basis of sediment type and biota. These are: fore reef, reef, back reef platform, lagoon, and shore. The dominant reef-building organisms are Millipora oomptanata (hydrozoan), Palythoa mammillosa (alcyonarian) , and lime-secreting algae. Windward, or reef, environments are characterized by abundant zoantharian corals, and leeward environments are dominated by alcyonarian corals.

Ancient reef complexes on San Andres Island can be divided into depositional environments similar to those of the modern complex. Both biota and sediment types of the Holocene reef complex are correlative with reef complexes of Pleistocene and Miocene ages. Reefs and rocks of Pliocene age were not found on San Andres Island.

Orientation of ancient reefs, non-carbonate residue from limestones, ancient spur and groove orientations, position of beachrock deposits, and modern reef growth indicate that prevailing winds have shifted from west to east since Miocene time.

X-ray analyses of modern and ancient carbonate sediments indicate that all samples older than Middle Pleistocene have stabilized to low Mg c a l c i t e and dolomite. Modern subtidal sediments from the San Andres area average 70 percent aragonite and 30 percent high Mg calcite with a maximum of 20 mole percent MgCO,. High concentrations of MgCO3 were found only in sediments from reef and shore ("turtle grass") environments and are attributed to the alga Goniolithon.

The paragenesis of limestones from the study area can be summarized in three steps:

1. Initial cementation may occur by precipitation of aragonite in the subaerial and intertidal environments or submarine lithification by "algal binding."
2. Secondary voids may form by the removal of aragonite in the subaerial environment, or aragonite may stabilize to low Mg calcite.
3. Calcite or dolomite may form by direct precipitation, replacement, inversion, or recrystallization.