Abstract

As most dolomites are alteration products of limestones a brief review of the nature of limestones is given in which it is shown that the deposition of limestones is analogous to the deposition of terrigenous rocks in that primary textures are determined largely by environmental conditions at the site of deposition. Limestones, however, are usually local or intrabasinal in origin and depend on organic activity for accumulation. The resultant lime coquinas, sands, silts and muds, as well as the indurated equivalents, are subject to alteration, either in the form of "straight" recrystallization (calcite to calcite) or in the form of dolomitization. (Other alteration processes such as chertification are not discussed here.)

Three general modes of origin for dolomites are: unconformity dolomitization, structurally controlled dolomitization (both of these usually occurring late in the history of the rock), and stratigraphic dolomitization. Studies involving penecontemporaneous stratigraphic dolomitization may give us a key to dolomitization in general. This is illustrated particularly in the penecontemporaneous dolomitization of lime muds wherein a progression may often be observed revealing individual steps in the alteration of calcilutite to dolomite: in a given calcilutite randomly oriented dolomite crystals are seen to increase in abundance to the point at which the entire rock becomes a sucrosic dolomite.

Two types of dolomitic mottling in limestones are discussed. First there occurs a depositionally controlled type of mottling in which thinly alternating zones of dolomite and limestone present a laminated appearance. An early time for the development of this type of dolomitization is deduced from the fact that plastic deformation sometimes appears to have taken place during sedimentation and following selective dolomitization of some of the laminae; the dolomitized layers reacted plastically while the calcite layers were being deposited. In general, depositionally controlled mottling is a result of incomplete or differential dolomitization. A second type of mottling, a "patchy" variety, may be attributed to the activity of burrowing organisms. While the calcite (or aragonite) muds or sands are being deposited certain organisms such as anastomosing foraminifera or tubular algae or worms may be actively reworking the materials of deposition. The organic material in the burrows may become dolomitized and the resultant rock will be mottled in a patchy manner.

The precise mechanism causing muds to become dolomitized may not be responsible for penecontemporaneous dolomitization of bioclastics and calcarenites, etc., or for later dolomitization of lithified limestones. However, we often observe shells or indurated limestones (the latter in unconformity or structurally controlled dolomitization) which have been altered to sucrosic dolomite, so that the general manner in which alteration occurs may be analogous to the manner in which calcite muds have been dolomitized; i.e., crystals form randomly replacing a portion of a calcite shell fragment, for example, increasing in number until the crystals touch and form a framework. Later dissolution of the interstitial calcite will then produce a sucrosic dolomite.

"Compact" crystalline dolomite is a type of dolomite in which the crystals are oriented in such a fashion that they fit together closely or compactly. Usually coarser than the sucrosic type, the crystals of "compact" dolomite may have formed more slowly than the crystals of the generally finer sucrosic type so that more time was available for the crystals to grow in an oriented fashion. Secondary outgrowth in sucrosic dolomites may also produce a compact type of dolomite.

Some pertinent things we can say about dolomitization at the present time are:

1. Dolomitization often destroys original textures of limestones.

2. Dolomitization is a great equalizer as regards its effects on textures of limestones. A completely dolomitized rock usually has a very uniform texture. If it is a lime mud that has been dolomitized the general texture will have been made coarser. If it is a calcarenite or calcirudite that has been dolomitized then the newly created (dolomite) texture will be finer.

3. As far as porosity is concerned, dolomitization usually tends to increase or create porosity rather than to destroy it.