Sedimentary rocks are those formed at or near the Earth’s surface by the deposition (by water, wind or ice), accumulation and lithification of sediment (detrital rock) or by the precipitation from solution (chemical rock) and/or growth in position by organic processes (e.g., carbonate reefs). They are formed at or near the earth’s surface at relatively low temperatures and pressures. They typically occur in layers (strata) separated by bedding planes and differences in composition.
Sedimentary rocks are the most common rocks exposed on the Earth’s surface but are only a minor constituent of the entire crust, which is dominated by igneous and metamorphic rocks. They consist of 75% of all rocks exposed at Earth’s surface. Many of you will work more with sedimentary rocks than with the other rock types.
Sedimentary rocks are formed in a variety of environment where sediment accumulates. Sedimentary environments typically are in the areas of low elevation at the surface, which can be divided into continental, shoreline, and marine environments. Each environment is characterized by certain physical, chemical, and biological conditions, thus sedimentary rocks hold clues to ancient environments and Earth history.
Carbonate rocks are rocks made up of predominately calcium carbonate (CaCO3) and calcium magnesium carbonate (CaMg(CO3)2). Carbonate rocks are difficult to classify, due to the complexity of sources and types of their occurrences.
To determine the rock type of the cutting samples is to immerse it in 10% Hydrochloric Acid (HCl). Typical reaction rates are:
• Limestone – Sample reacts instantly and violently, it will float on top of the acid and move on the surface. It will completely dissolve within minutes and leave the acid frothy. Oil-stained limestones are often mistaken for dolomites because the oil coating the surface of the cutting prevents acid from reacting immediately with CaCO3, and a delayed reaction occurs.
• Dolomitic limestone – Sample reacts immediately, but moderately and is continuous. It will move about in the acid from top to bottom.
• Calcitic dolomite – Sample reacts slowly and weakly at first, but accelerates to a continuous reaction after a few minutes, with some bobbing on the bottom of the dish.
• Dolomite – There is a very slow and hesitant reaction. Bubbles evolve one at a time. The acid may have to be warmed for reaction to proceed. It will leave the acid milky.
Calcimeters can assist in determining the percentages of calcium and dolomite in the rock sample.
The most common stain test for carbonate determination is Alizarin Red S. A drop of Alizarin Red S is placed on the cutting. Limestone will stain a deep red, while dolomite remains unaffected.
Dunham carbonate classification system.
Although there are numerous carbonate classification schemes available. The scheme adopted by the petroleum industry is the “Dunham Classification System”. The Dunham classification is based on the concept of grain support. The classification divides carbonate rocks into two broad groups, those whose original components were not bound together during deposition and those whose original components formed in place and consist of intergrowths of skeletal material. The classification is:
• Mudstone (Mdst) – composed of lime mud (smaller than 20 microns) and less than 10% grains. Mud supported.
• Wackestone (Wkst) – composed primarily of lime mud, with more than 10% grains (larger than 20 microns). Mud supported.
• Packstone (Pkst) – composed primarily of grains, and grain supported. Greater than 10% interstitial mud matrix and occasionally sparry calcite or pore space.
• Grainstone (Grst) – composed of grains, and grain supported. Less than 10% interstitial mud matrix.
• Boundstone (Bdst) – original constituents were bound together and supported in place, by organic growth.
• Crystalline (Xln) – all original textures are lacking due to the effects of recrystallization. Distinct crystal faces, with occasional relics.