ABSTRACT

Integrated morphologic, petrographic, and mineralogic analyses of sediment grains of the miliolid foraminifer Archaias and the green alga Halimeda from shallow marine environments in Florida, the Bahamas, and Belize document pervasive syndepositional recrystallization in these grains. Scanning electron microscopy shows that original skeletal rods and needles recrystallize on the sea floor to a variety of equant micritic fabrics. In thin section, this textural alteration corresponds to progressive micritization, with loss of the familiar golden brown color exhibited by fresh miliolids and Halimeda and gradual development of gray cryptocrystalline carbonate. X-ray diffraction and electron microprobe studies show that this textural alteration may be accompanied by mineralogical changes from Mg-calcite to aragonite or from aragonite to Mg-calcite. Our findings support petrographic studies that reported syndepositional recrystallization in a wide variety of carbonate grains about thirty years ago, but which have been largely ignored. Together, these studies indicate that carbonate recrystallization is a widespread process of early diagenesis.

Extensive recrystallization in shallow tropical seas challenges basic principles regarding the setting and timing of textural and mineralogic alteration of metastable carbonate grains. In addition, recognition of recrystallization as an important process of micritization contradicts prevailing theories that micritization results solely by infilling of microborings. Finally, our findings raise fundamental questions concerning the nature of carbonate crystal structure, mechanisms of crystal growth, and driving forces for recrystallization. Answering these questions will require sedimentologists to step beyond traditional bounds into fields such as biomineralization and materials science.