Limestone: The Alamo Primer
Sedimentary Rocks in Devonian

Limestone

Limestone is a sedimentary rock that is comprised mainly of calcium carbonate (CaCO3). The majority of the grains within limestones consist of skeletal fragments from marine organisms that have died and settle to the ocean floor. Limestones can also contain varying amounts of clay, silt, and sand derived from nearshore environments, such as a delta. Some limestones form due to chemical precipitation of calcite or aragonite within hot springs or cave systems. Limestones can also be found within lacustrine (lakes) and evaporite depositional environments. The characteristics of marine limestones change as you move from shallow- to deep-water environments. The shallowest forms of limestone are found within the peritidal zone, the area above low tide and extending upland. Below the low tide line is the subtidal zone, which extends out to the deepest environments and can include a variety of different habitats.

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Peritidal Limestone: Limestone Deposition in Peritidal Environments

Tidal Flats

Peritidal environments are found above low tide within coastal environments and can include coastal lagoons, estuaries, and tidal flats. The dominant influence on these systems is the rise and fall of tides.

Limestones deposited atop tidal flats are often laminated, due to the periodicity of the moving tides bringing in sediment. Often times the limestones deposited here are recrystallized into dolomites. With prolonged periods of exposure, these dolomites can become karsted. Sometimes standing bodies of water can become trapped upland, forming evaporitic minerals.

Tidal Flats

Lagoonal and estuarine settings are often calmer, more restricted environments. With relatively little freshwater input, these environments can have extreme ranges in salinity. The fauna here often have low diversity, since only a few organisms can tolerate such extreme ranges. Carbonate mud can also dominate these calmer settings.

The best representation of peritidal environments associated with the Alamo Impact stratigraphy can be seen in the "Yellow Slope Former" or YSF discussed earlier.

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Supratidal Limestone: Limestone Deposition in Supratidal Environments

Supratidal Environments

Limestone is a sedimentary rock that is comprised mainly of calcium carbonate (CaCO3). The majority of the grains within limestones consist of skeletal fragments from marine organisms that have died and settle to the ocean floor. Limestones can also contain varying amounts of clay, silt, and sand derived from nearshore environments, such as a delta. Some limestones form due to chemical precipitation of calcite or aragonite within hot springs or cave systems. Limestones can also be found within lacustrine (lakes) and evaporite depositional environments. The characteristics of marine limestones change as you move from shallow- to deep-water environments. The shallowest forms of limestone are found within the supratidal zone, the area above high tide and extending upland. Below the low tide line is the subtidal zone, which extends out to the deepest environments and can include a variety of different habitats.

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Subtidal Limestone: Limestone Deposition in Subtidal Environments

Subtidal Environments

Subtidal environments lie below the low tide line and extend to the deepest areas of the oceans. In general, three zones can be recognized: (1) the inner shelf, (2) the outer shelf, and (3) the slope. The inner shelf zone lies closest to the shore and often includes abundant marine life. Organisms burrowing into the ocean floor for shelter or food stir up the sediment and erase any evidence of primary sedimentary structures. Storm activity can also greatly affect this zone, ripping up coarse sediment and marine life that is later deposited as grainstones. The outer shelf zone is usually a deeper, calmer environment. Storm activity is less prevalent here. In many cases, the outer shelf can contain ocean floor highs that can lead to the formation of reef systems. The reef systems will contain a high diversity of marine life and can also lead to the deposition of large skeletal fragments. From the outer shelf we transition into the slope environment, which marks an increase in steepness. The limestones deposited are almost entirely made of muddy carbonate sediment that has been carried off the shelf environment or has settled out of the water column. Gravity flows can develop from the outer shelf-slope transition, depositing coarser material at the base of the slope. In this deeper environment, the majority of invertebrates are small, photosynthetic organisms that float near the top of the water column during life.

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