The Significance of Textural Character

Terrigenous sediments are most often of siliciclastic origin (i.e., they are comprised of quartz, feldspars and other minerals associated with an igneous origin). In general, they are derived from the erosion of upland or coastal areas and are transported over considerable distances to their ultimate environment of deposition. Terrigenous sediments can also be eroded carbonate that has been lithified and uplifted. In this instance, however, the processes of erosion, delivery and final deposition are generally similar to those of siliciclastic sediments.

In contrast, first-generation carbonate sediments were by and large produced by living organisms in or near to the area where they ultimately came to rest. While this differentiation is somewhat oversimplified, it is still useful for considering the different forces that control the distribution of sediments from each of these two groups. The downplaying of a third group of sediments - hypersaline evaporites and authigenic minerals - is not intended to imply any lack of importance. Rather, it simply reflects their more restricted distribution in nature and a narrower treatment in the published literature.

For terrigenous sediments , a reasonably clear relationship exists between energy level and grain size. Coarser sediments generally reflect higher energy, while quieter areas are dominated by mud. On land, progressively finer sediments are deposited as the slope decreases and the rivers slow down. In the marine environment, energy levels and, therefore, sediment size are controlled by such factors as wave action, exposure, tidal range and water depth. In carbonate environments, however, this relationship is less reliable due to the effect of the in-situ biological origin of sediments. The size of carbonate grains is also strongly controlled by the skeletal architecture of the organisms from which the grains are derived (see fig. 3.6a,b). This strong biological overprint does not mean that physical processes are any less important in carbonate depositional systems. To the contrary, physical factors still impart a strong signature on carbonate grains, but one that is made subtler by biological effects that must also be taken into account. What is required for successful paleoenvironmental reconstruction from carbonate sediments is a careful examination of both textural parameters (size and sorting) and the constituents that make up the deposit.

Sorting is related to uniformity of both the energy regime and sediment supply. For example, wave action is usually the most important controlling factor along open beaches, and sand is moderately sorted, reflecting even energy and supply. In a similar environment, but with nearby alluvial cliffs, beach sediments might be poorly sorted if a significant portion of the material being eroded cannot be moved by ambient processes; the character of the source is overwhelming the ability of physical energy to sort it and . lag gravel and sand are formed.

The interpretation of sorting in carbonate sediments can be problematic. The variable grain shapes (arcuate molluscs vs. elongate spicules) and the high porosity of some particles (i.e., Halimeda or foraminifera with naturally occurring pores and chambers) can make them more susceptible to transport than terrigenous grains of a similar size. Carbonate deposits often have a polymodal size distribution that is related partly to hydraulic equilibrium and partly to the breakdown characteristics of the skeletal organisms that have lived and died within a particular setting. For example, sediment in the backreef may be a mixture of skeletal sand produced by the disarticulation of organisms that live there under normal conditions, gravel or cobbles washed in from the adjacent reef crest during storms, and carbonate mud produced by the biological breakdown of both. The mud and fine sands are in hydraulic equilibrium with quiet conditions at the site most of the time, while the coarser grains are either produced in situ as skeletal debris or are dumped behind the reef during storms. None of these can be removed by weak ambient currents and the resulting sedimentary deposit reflects a myriad of physical and biological agents. While the mixture of grain types makes interpretation of absolute energy level difficult at best, the unique assemblage of grain sizes and types make it difficult to misinterpret the environment of deposition, especially if the deposits from the reef in front and the lagoon behind can be identified.

Whatever the origin of a particular sediment and however mixed the processes have been that brought it to its final resting place, sediment composition and texture will hold important clues to those controlling factors. Whether siliciclastic or carbonate in origin, the physics of transport and deposition are the same. To understand the factors that have contributed to the formation of a particular sedimentary suite, one must become intimate with the complex interplay among sediment composition, sediment texture, and the physical and biological processes related to transport and deposition. Each of these is discussed in greater detail below.