Zakaria Lasemi, Mark R. Boardman, Philip A. Sandberg
Transformation of porous (65%) lime muds to low-porosity (< 5%) microcrystalline limestones (MCL) is known to be caused by cementation or compaction. However, differentiation of the results of these processes in MCL has been a difficult task, largely due to the microcrystalline nature of these rocks. Some megascopic criteria have previously been determined from both experiment and observation for lime muds which contain organic matter or clays sufficient to produce (micro) stylolitic seams. Apparent lack of shell breakage has also been used as evidence for early cementation in MCL. Recent experimental data show that shells within lime muds may survive considerable compaction. Thus far, no clear criteria exist that can be used as compaction indicators in allochem-poor M L or in MCL which lack organic material and/or clay.
In this study we report the presence, in modern lime muds and ancient MCL, of microfenestral pores which are potentially useful in distinguishing early cementation from compaction. Detailed SEM investigation of polished, etched sections of modern lime muds shows that these microfenestrae range from 2 to 30 µm in diameter and are commonly not recognizable in thin section. Microfenestrae are not facies dependent; they are found in both calcite and aragonite-dominated lime muds from both shallow (supratidal to subtidal) and deep-marine environments. Microfenestrae in ancient MCL are recognizable as cement-filled regions, similar in size and shape to microfenestral pores in modern lime mud. SEM examination of our experimentally compacted modern lime muds shows that very little compac ion (as low as 100 psi) usually results in obliteration of these microfenestrae. As a result, the presence of microfenestrae in some ancient MCL strongly suggests that early cementation occurred prior to compaction of the mud precursors. Variations in presence and relative abundance of microfenestrae among Phanerozoic MCL suggest that in some rocks early cementation was important, whereas in others compaction processes have dominated porosity reduction.
AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.