Conditions and criteria for “grain growth” in metals are not followed in diagenesis of most sedimentary carbonates; thus the concept should not supersede “recrystallization” as first documented by H. C. Sorby for both petrographers and metallographers. An inclusive term, “Neomorphism,” is herein proposed to embrace the following processes where gross composition is constant: Inversion (aragonite to calcite), Recrystallization (calcite to calcite), and Strain-recrystallization (strained calcite to unstrained calcite). Neomorphism and its daughter processes may be aggrading or degrading; the former may be Porphyroid (a few crystals growing to replace a passive groundmass), or Coalescive (nearly all crystals are consuming or being consumed). In these processes driving forces and physical conditions (porosity, solutions, etc. ) vary considerably.
Diagenetic calcite has the following attributes: (1) Origin—Precipitated (passively) (P), Displacive (D), or Neomorphic (N); (2) Shape—Equant (E), Bladed (B), or Fibrous (F); (3) Dimensions—Aphano-crystalline (1) to Extremely Coarsely Crystalline (7); (4) Foundation—Syntaxial Overgrowth (O), Crust (C), or Random (R). Numerous combinations are possible, expressed symbolically as for example P. E3 (precipitated, finely crystalline equant spar), or N. B5O(neomorphic coarsely bladed overgrowth—for example, starting from a trilobite and replacing micrite).
Three phases of neomorphism are discussed. All micritic limestones have undergone Porphyroid Neomorphism (?), probably from 2μ needles or plates to 2μ subequant polyhedral blocks of calcite, involving digestion of the vast majority of original mud particles of similar length but much more slender than the polyhedra.
In some limestones, neomorphism bursts through the “micrite curtain” to form microspar. Normal micrite measures 1½-2½μ; a frequency saddle exists at 3-4μ before another peak frequency at 5-6μ (microsparite). This volumetrically very important type of neomorphism is probably a coalescive process; it results in uniformly-sized grains, usually of simple loafish form, and is most frequent in limestones with shale interbeds, probably not in brackish environments as has been claimed previously.
In freakish limestones, diagenesis may go still further, producing pseudospar or fibers largely by Porphyroid Neomorphism. These may mimic closely the appearance of normal pore-fill calcite; criteria of grain shape, orientation, uniformity, and boundaries are equivocal. The only firmly diagnostic criteria are those based upon grosser fabric relations such as transection of allochems, occupation of large areas unsupported by allochems, or presence of undigested inclusions.
Figures & Tables
Dolomitization and Limestone Diagenesis
In its broadest sense, diagenesis encompasses those natural changes which occur in sediments or sedimentary rocks between the time of initial deposition and the time — if ever — when the changes created by elevated temperature, or pressure, or by other conditions can be considered to have crossed the threshold into the realm of metamorphism. Deciphering the nature of diagenetic processes, and the time or times when they took place, is of critical importance for adequate geological interpretation. Papers in this volume were presented at a symposium on the diagenesis of carbonate rocks held in Toronto, Canada on May 20, 1964 at the joint meeting of AAPG and SEPM.