Textural Evolution During Experimental Lime-Mud Diagenesis

S. O. Moshier, K. M. McManus

Diagenesis of metastable carbonate mud involves reorganization of material and pore space accomplished through mineralogic stabilization, cementation, and compaction. Popular interpretations of textural evolution during lime-mud diagenesis are influenced by the 1961 experiments and pioneering use of transmission electron microscopy by Hathaway and Robertson. Important implications included early wet inversion (replacement) of aragonite by calcite retaining needle form, import of CaCO₃, and needle fattening by aggrading neomorphism to form sorted, nonporous crystal mosaics. Those 1965 experiments by Folk were conducted, however, under abnormally simulated burial conditions (10-20 km).

Low temperature and pressure transformation experiments (50°-100°C; 1 bar) were performed using distilled water and synthetic mud composed of 5 × 20-µm aragonite needles (95%) and 10-µm calcite rhombs (5%). SEM photomicrographs of impregnated, polished, and etched materials document simulated lime-mud diagenesis in a geochemically closed, shallow-subsurface meteoric system. At 50% conversion, aragonite needles have undergone partial to complete dissolution, and CaCO₃ is redistributed to calcite cements grown in open pore spaces. Calcites consist of 15-µm crystals and crystal aggregates that are faceted, contain multiple zones and overgrowths, and appear to engulf or embay adjacent particles of corroded aragonite. The calcite produced after 100% conversion consists of a bimodal distribution of crystal sizes (2-5 and 10-15 µm). Crystal aggregates exhibit both curved and straight compromise boundaries, and many contain aragonite inclusions. There appears to be no evidence for pseudomorphic replacement of aragonite, needle fattening, or aggrading neomorphism of calcite cements. These observations conform to recent interpretations of Holocene lime-mud and ancient micrite textures.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.