Microfenestral Porosity as a Compaction Indicator in Micrites and Modern Lime Muds

Zakaria Lasemi, Mark R. Boardman, Philip A. Sandberg

SEM observation of fracture sections and embedded, polished, and etched surfaces of modern lime muds from various shallow and deep marine environments reveals abundant microfenestral porosity. These microfenestrae range between a few microns to tens of microns in diameter and are similar in size and shape to cement-filled voids in uncompacted Plio-Pleistocene microcrystalline limestones described by Lasemi and Sandberg. Microfenestrae are found in all uncompacted lime muds examined, including both calcite and aragonite-dominated types. The fate of microfenestrae during transformation of lime mud to microcrystalline limestone is dependent on the processes involved in pore reduction. If mechanical compaction is the dominant process of pore reduction, then microfenestrae sho ld be reduced or obliterated during burial diagenesis. However, early cementation prior to burial enhances the preservation potential of microfenestrae during lime mud diagenesis. Therefore, recognition of these microfenestrae in microcrystalline limestone should provide a significant clue to the relative roles of compaction and early cementation in fine-grained carbonates.

Microfenestral porosity as a criterion for recognizing the relative effects of compaction and early cementation is particularly useful for microcrystalline limestones which lack larger allochems that could show the effects of compaction breakage or deformation. Furthermore, because allochem breakage and deformation may not necessarily occur during compaction, as suggested by the experimental results of Shinn and others, reduction of microfenestral porosity is perhaps a more reliable criterion of mechanical compaction.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.