Limestones - Warm-Up

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Essay Question

Various limestones and sandstones can be compared to one another on the basis of the three major constituents contained in the rocks: grains, matrix and cement. Describe grains, matrix and cement in limestone, and discuss their implications for origins of the rock. In your discussion, show how grains, matrix and cement might be interpreted in ways similar to grains, matrix and cement in sandstone. Consider such things as source, transport, and depositional history of the sediment.

Then, discuss how grains, matrix and cement in limestone must be interpreted very differently from these same components in sandstone. In your discussion, it will be important to focus on the fact that limestone is derived from a source within the basin, and the implications that this origin has for interpretation.

Multiple Choice Questions

Which of the following statements is (are) true?

because aragonite is rhombohedral and calcite is orthorhombic, the aragonite lattice is more "open", permitting ions like Mg++ to substitued for Ca++

limestones have formed at relatively equal rates throughout Phanerozoic time, because the organisms that are largely responsibile for their precipitation have been around since the beginning of the Cambrian

MgCO3 may be present in calcite in amounts as large as 20 mole% or more

adding CO2 to water will eventually produce carbonate ions by dissociation of bicarbonate ions, thereby enabling Ca++ to combine with the carbonate to secrete calcite and hence produce limestone

carbon dioxide degassing is not especially important in the precipitation of calcimum carbonate and the formation of limestone

carbonate rocks are not likely to form in environments where large amounts of siliciclastic sediment (especially mud) are being deposited

Limestones must often be interpreted differently from sandstones because they are made of components derived from within the basin of deposition, and because organisms play a very significant role in their formation. Which of the following is true regarding the interpretation of limestones compared to sandstones?

grain shape in limestones may reflect the shape of skeletal grains rather than hydrodynamic processes operating in the environment of deposition

because calcium carbonate minerals are very reactive in pore waters that are not in equilibrium with the grains or crystals, original depositional fabrics may be obliterated more than is the case with sandstones

mud may be trapped in a grain-supported limestone, even though currents may be present, because the irregular shapes of skeletal grains may act as traps or baffles to preserve the mud

intragraular porosity is much more common in limestones than in sandstones, because many skeletal grains have internal cavities not filled with inorganic crystalline material as part of the morphology of the living organism.

grain size and sorting in many limestones may be a function of the paleoecology of the depositional environment, rather than the hydrodynamics of the environment

limestones seldom give information regarding external sources, except in the most general of ways (e.g., accumulation of carbonates generally occurs under conditions of tectonic quiescence, because such conditions favor very little input of siliciclastic detritus to the depositional basin)