Mineralogy and Early Diagenesis of Carbonate Sediments

Abstract

ABSTRACT The mineralogy and important trace elements of Recent carbonate sediments and environmentally comparable Pleistocene carbonate rocks have been studied. All of the rocks and most of the sediments employed were obtained in southern Florida. Additional sediment samples came from the Bahama Banks, the South and East China Seas, and the Persian Gulf. Most sediments were found to consist of aragonite, high-magnesium calcite, and low-magnesium calcite. Neither dolomite nor vaterite was encountered. Shallow water carbonate sediments contain an average of about 70 percent of unstable forms of CaCo3 with aragonite predominating and high-magnesium calcite dominant over low-magnesium calcite. Deep water carbonate sediments are composed predominantly of low-magnesium calcite, and high-m gnesium calcite dominates aragonite. Compositional differences between shallow and deep water carbonates are believed mainly to result from differences in the importance of contributions of skeletal material by certain groups of organisms in the two environments. The mineralogy of the Pleistocene carbonate rocks studied showed them to consist mainly of low-magnesium calcite and indicated that under near-surface conditions in nature a stability sequence runs as follows: low-magnesium calcite>aragonite>high-magnesium calcite. Diagenesis of carbonate sediments is accompanied by an important loss in the level of abundance of the elements, magnesium, strontium, barium, and manganese. These elements, including magnesium, are lost from the chemical system and do not appear to form new min rals in place, although they may later do so at some other place. Since the chemical system is not closed, the significance of important volume changes which accompany diagenesis can not yet be assessed.