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Abstract
In Florida’s mantled, eogenetic karst, the sedimentology and mineralogy of siliciclastic and carbonate geological materials greatly impact karst development. Florida’s cover materials are integral parts of the karst development story, and they can present hazards themselves. Cover materials are mostly allochthonous quartz, feldspars, clay minerals, while carbonate rocks consist of calcite with some aragonite and dolomite. Magnesium content of calcite affects diagenetic and karst processes very little; but aragonite dissolves preferentially, and dolostone deposits can be resistant to dissolution. Therefore, it is important to understand the mineralogy of geologically young, carbonate sediments and rocks.
Marine sediments in Florida are predominantly well-sorted, fine-to-medium sands, sourced from areas with limited grain sizes and deposited by waves, currents, and wind. Eolian sands are poorly consolidated with frosted grain surfaces. Grain shapes range from angular shells through variable sands to rounded pebbles and cobbles.
Under-consolidated clay beds in the Miocene Hawthorn Group (Chap. 3) act as confining layers despite high porosities, and the fine-grained sediments have the potential to flow if sinkholes develop. Smectites are common expansive clays in the Hawthorn Group. The Hawthorn clays were significantly altered during two periods of intense weathering in the Late Miocene-Early Pliocene and Late Pliocene-early Pleistocene. The first weathering event formed a prominent paleosol throughout central and northern Florida.
Florida’s carbonate rocks were deposited in broad, shallow seas, creating flat, laterally extensive layers. The overlying siliciclastics, in contrast, represent complex environments resulting in sediment facies with great lateral and vertical variations. This chapter describes the origins and properties of these sediments.
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In this text, the term pore is utilized to describe the intergranular openings between particles in sediments. The term void is used to describe larger, dissolution-created openings.
A distinction is made between adsorption and absorption. If water simply coordinates with the external surfaces of a clay mineral, such as kaolinite, is it said to adsorb onto the mineral. If the water can enter into interlayers, such as in smectites, the water is said to absorb. If the process of hydration is unknown, the term sorption is utilized.
The nontronite in Hawthorn Group clay of the Central Florida Phosphate District is a dioctahedral, iron-rich variety of smectite that is “about midway between montmorillonite and nontronite” (Strom and Upchurch 1985a, p. 118),
Atterberg limits relate to the plastic behavior of clay as the moisture content of the sediment varies. There are two limits that are commonly identified for characterization of the properties of Florida clay. These are (1) the plastic limit, the moisture content below which cohesive sediment ceases to be plastic or moldable, and (2) the liquid limit, the moisture content above which plastic sediment becomes liquid. Note that calculation of these indices and moisture content are calculated as the ratios of the mass of moisture in a sample divided by the mass of the dry sediments.
The disused term attapulgite is sometimes applied to palygorskite. The mineral name attapulgite is derived from deposits located near Attapulgus, Georgia, just over the state line from Florida.
It is important to note that sulfuric acid derived from gypsum and/or anhydrite strata in the subsurface in Florida has likely also contributed to karst development, See Chap. 8 for a discussion of this process.