Coral Reef Project—Papers in Memory of Dr. Thomas F. Goreau. 4.

Contemporaneous Dolomitization of Middle Pleistocene Reefs by Meteoric Water, North Jamaica

Middle Pleistocene reefs of North Jamaica, and elsewhere in the Caribbean, are distinctly different from the 120,000 year-old late Pleistocene “Sangamon” reefs. They are more extensively developed, better lithified, and commonly dolomitized.

In Jamaica, the contact between the two units, the “Sangamon” Falmouth formation overlying the Middle Pleistocene “Yarmouth” (1) Hope Gate formation, is erosional, demonstrating that the diagenesis and dolomitization of the Hope Gate took place before Falmouth time. The Hope Gate dolomite replaces micrite, and fills pores created by the dissolution of aragonite, demonstrating the near contemporaneity of aragonite solution and dolomite precipitation. The dolomites have low Sr (200 ppm) and Na (350 ppm) contents, precluding their precipitation from marine fluids.

Dolomitization took place as CO₂-rich meteoric waters first infiltrated the seaward growing, submarine-cemented reefs, and dolomite precipitated from a fluid whose composition was controlled by the composition of the influxing meteoric water, the amount of mixing with sea water, and more significantly by the composition of the primary reef carbonate. Continued meteoric flushing, after the available magnesium was consumed by dolomitization, resulted in calcites low in Na and Sr and enriched with C¹² and O¹⁶.

Vadose textures superimposed on the intense phreatic-meteoric diagenesis consist only of insignificant amounts of speleothems, despite more than 300,000 years of subaerial exposure.