Formation, dispersion and accumulation of terra rossa on the Cayman Islands

Abstract

AbstractUnconsolidated and lithified terra rossa fills surface depressions, joints, fractures and cavities throughout the karst landscapes on the Cayman Islands. The terra rossa is formed from kaolinite, halloysite, vermiculite, hydrotalcite, böhmite, gibbsite, goethite, hematite, anatase, quartz, calcite, dolomite, halite and X‐ray amorphous material. Locally, phosphate derived from bird guano and biofragments is present. Lithified terra rossa is characterized by numerous generations of fractures that are lined with clay and filled with multiple phases of calcite cement. There are no obvious geographic or stratigraphic differences between the terra rossa found on the three islands. A residual origin for the terra rossa is unlikely given that the Oligocene to Pleistocene limestone/dolostone bedrock contains <1% non‐carbonate material. Detrital sediments cannot be the source because none developed on these islands, given that they are surrounded by deep oceanic waters. The terra rossa probably originated largely from wind‐blown volcanic dust or Saharan dust. Various geochemical ratios and rare earth element profiles indicate that that the Cayman terra rossa has a strong affinity to Saharan dust that is frequently blown into the Caribbean region each year. The Cayman terra rossa is characterized by SiO2/Al2O3 ratios (<1.1) that are significantly lower than those associated with terra rossa on other islands, such as Jamaica and Barbados. The low SiO2/Al2O3 ratios of the Cayman terra rossa is due, at least in part, to the fact that quartz is rare to absent. X‐ray amorphous material is a common component in many of the Cayman samples. Although difficult to date precisely, the formation of terra rossa on the Cayman Islands appears to have been an ongoing process for at least the last 5 Myr. This time frame is consistent with the origin of the Sahara Desert that came into existence with aridification of North Africa about 5 Ma.