| Popis: |
In an earlier study based mainly on a micrite layer in the Pleistocene Miami Oolite in southern Florida, Lasemi and Sandberg (1984) found that microfabric and composition in micrites may be related to the original mineralogy of the precursor lime mud. This chapter takes a broader look at Cenozoic (mainly Plio-Pleistocene) micritic limestones to evaluate the role of original mineralogy in the evolution of microfabric and composition in fine-grained limestones. A number of shallow marine micrites were examined from core or outcrop samples (southern Florida, the Bahamas, and Yucatan, Mexico). Microfabric and compositional data for these samples studied revealed two distinct micrite types (Table 13.1). These types can be interpreted as alteration products of aragonite-dominated precursors (hence, ADP micrites) or calcite-dominated precursors (hence, CDP micrites). The diagenetic microfabrics of monomineralic allochems of known aragonite or HMC (high-magnesium calcite) mineralogy provide strong evidence supporting the inference of mineralogic control on the generation of ADP or CDP micrite microfabrics. The relationship between strontium contents and crystal size in micrites refutes interpretations of aggrading neomorphism of the coarser (“microsparitic”) ADP micrites from the finer CDP micrites. The relatively low strontium contents of CDP micrites also suggest that calcites in the mud precursor were mainly HMC. Taken together, these Cenozoic ADP and CDP micrites can serve as guides for the recognition of original mineralogies in lime mud precursors of ancient fine-grained limestones. |
