I. Carbonates common in tropical zones (within 30° of equator) and in some lakes. Results from direct chemical precipitation and, strongly, from the influence of biologic processes.

.The composition of most carbonates lies somewhere between calcite (CaCO₃) and dolomite (CaMg(CO₃)₂. For example, most contain some magnesium, but not as much as pure dolomite.

II. Chemistry: the key here is that any process that removes CO₂ (gas) from normal seawater (pH=8.4) tends to drive up pH, encouraging deposition of carbonate. Such processes include: increasing temperature, evaporation, and pH.

III. Most carbonate comes from growth and death of critters who make their hard parts out of carbonate. Over 90% of carbonates formed in modern environments are thought to be biological in origin and form under marine conditions. Distribution of most carbonate is directly controlled by environmental parameters favorable for the growth of the calcium carbonate secreting organisms. These parameters include temperature, salinity, substrate, and presence/absence of siliciclastics.

IV. Sedimentation Rates:

Modern: reefs: 3+ m/ka, open sea/lagoon: ~1m/ka

Ancient average: 0.4 m/ka

So: although they are deposited fast when critters are alive and thriving, there must be a lot of time that they don't grow, that is critters are inhibited by things like salinity, etc (mentioned above).

V. Classification Schemes:

A. Bob Folk - emphasizes what carbonate looks like in thin section and what sorts of particles are seen. The presence or absence of carbonate mud (micrite) likely reflects secondary, diagenetic, change in chemistry.

B. Bob Dunham - cares less about the particles and interprets the % mud to reflect environmental conditions during deposition.