CARBONATE RECRYSTALLIZATION IN SHALLOW MARINE ENVIRONMENTS - A WIDESPREAD DIAGENETIC PROCESS FORMING MICRITIZED GRAINS

Integrated morphologic, petrographic, and mineralogic analyses of sedi ment grains of the miliolid foraminifer Archaias and the green alga Ha limeda from shallow marine environments in Florida, the Bahamas, and B elize document pervasive syndepositional recrystallization in these gr ains. Scanning electron microscopy shows that original skeletal rods a nd needles recrystallize on the sea floor to a variety of equant micri tic fabrics. In thin section, this textural alteration corresponds to progressive micritization, with loss of the familiar golden brown colo r exhibited by fresh miliolids and Halimeda and gradual development of gray cryptocrystalline carbonate. X-ray diffraction and electron micr oprobe studies show that this textural alteration may be accompanied b y mineralogical changes from Mg-calcite to aragonite or from aragonite to Mg-calcite. Our findings support petrographic studies that reporte d syndepositional recrystallization in a wide variety of carbonate gra ins about thirty years ago, but which have been largely ignored. Toget her, these studies indicate that carbonate recrystallization is a wide spread process of early diagenesis, Extensive recrystallization in sha llow tropical seas challenges basic principles regarding the setting a nd timing of textural and mineralogic alteration of metastable carbona te grains. In addition, recognition of recrystallization as an importa nt process of micritization contradicts prevailing theories that micri tization results solely by infilling of microborings. Finally, our fin dings raise fundamental questions concerning the nature of carbonate c rystal structure, mechanisms of crystal growth, and driving forces for recrystallization, Answering these questions will require sedimentolo gists to step beyond traditional bounds into fields such as biomineral ization and materials science.