COMPARISON OF POROSITY DEVELOPMENT IN MODERN AND ANCIENT CARBONATE ROCKS
Allochem types and rock textures are remarkably similar. Modern biosparites, deposited as aeolianites, are dominated by ooids, peloids, and fragments of a variety of marine organisms including red algae, green algae, benthic foraminifera, mollusk fragments, and, more rarely, coral and echinoderms. Pennsylvanian biosparites and biomicrites, deposited on an open shelf, contain allochems that are of similar size, type, and pre-diagenesis mineralogy and include peloids, ooids, foraminifera, mollusk fragments, and chaetidids.
In the modern samples diagenesis has rapidly resulted in delicate dissolution of individual cortices in ooids, precipitation of meteoric cement, limited marine cement, and in some cases a clay mineral. Pore types in the modern samples include interparticle, intraparticle and vuggy. Diagenesis in the ancient samples is more extensive with partial dissolution and replacement of ooids, partial silicification of allochems and matrix, and filling of pore space with large, blocky late spar. Pore types include interparticle, intraparticle and vuggy with a large amount of porosity lost to late cementation. In the Rattlesnake-142 core, porosity is greatest in cycles interpreted as high stand deposits. Porosity development, particularly the large cavern systems in the modern rocks, is interpreted as forming in association with fresh water lens during sea level high stand.