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Saprolite is isovolumetrically weathered bedrock that retains the structure and fabric of the parent rock. This soil parent material forms in areas where crystalline rocks occur at or near the surface of the earth. Saprolite is generally overlain by 1 to 3 m of soil and commonly > 10 m thick. Although saprolite occurs worldwide and often comprises the majority of the regolith, relatively little research has focused on saprolite morphology and genesis. This chapter details various techniques for studying saprolite. Studies of saprolite are hindered by the significant depth of saprolite and the overlying soil, making the acquisition of samples for description and characterization difficult. Samples can be collected and descriptions made from highwalls in quarries. These can be augmented with samples collected with a drill rig, or a modified bucket auger system. A transition zone occurs between soil and saprolite. Identification of the transition zone boundary can be determined by examination of thin sections, and depth distributions of properties such as dithionite-citrate-bicarbonate (DCB) Fe, sand, and clay. Variability in saprolite properties and characteristics are considerable due to inherent differences within the structure, fabric, composition, and grain size of the parent rock, and commonly occurring shear zones and intrusions. Multiple samples within horizons can be used to estimate saprolite variability and better estimate a given parameter. Different systems are used by pedologists, geologists, and engineers to divide, describe, and classify residual regolith materials. Thus, a need exists for a classification system designed to meet the needs of each of the disciplines. Engineering properties of saprolite are considerably different from those of the overlying soil, suggesting that for engineering purposes soil and saprolite should be placed in separate classes. Investigations of saprolite genesis are used to estimate rates of saprolite formation, chemical denudation of the landscape, and isostatic uplift. Rates of saprolite formation, in conjunction with the thickness of saprolite on the landscape, can be used to estimate the maximum age of a landscape.
Weathered rock, a common regolith in many areas unaffected by Pleistocene glaciation, has both lithologic and pedologie characteristics. This paper reviews pedogenic features found in weathered rock substrates and interprets the pedologie processes and environmental roles of this regolith. Lithogenic features such as rock structure, texture, and composition strongly influence weathering and resulting weathered rock characteristics. Joint fractures provide access for infiltrating water and roots, which promote weathering. As rock weathers, it develops microporosity, thereby increasing its water-holding capacity, which further enhances weathering and water availability for plants. Plant roots can penetrate the matrix of saprolite, but in less weathered rock they follow fractures, producing localized organic C concentrations as large or larger than in overlying A horizons. Organic acids and CO2 from decomposing roots promote weathering, and K uptake by living roots causes the transformation of biotite to vermiculite, an important weathering mechanism that extensively fractures rocks. Root exploitation of the saprolite matrix diminishes the importance of lithogenic features by producing channels that more effectively conduct water. Water moving from soil into weathered rock carries colloids which are commonly deposited to form argillans in fractures, abandoned root channels, and intergranular pores within the matrix. These argillans are protected from physical disturbances that affect soils and may be better expressed than those in the solum. In arid and semiarid areas, CaCO3 and opaline silica commonly precipitate within the fractures and porous matrix of weathered rock underlying soils. These features can be used to help interpret past environmental conditions. The weathered rock zone is an important and somewhat neglected part of the soil-rock continuum. Research is needed to better understand how it evolves and functions in the environment.
Pedological studies in thick sedimentary sequences are generally limited to the upper few meters. Field investigation of thick (≤ 50 m) sand deposits on an emergent Pleistocene marine terrace in central California showed morphological differences between the solum at the surface and the deep regolith. Based on morphological and geochemical features, four units were identified within the regolith. Two zones of active pedogenesis occur within three of these units. The surficial unit is in Holocene sand deposits (mixed, thermic, Argic Xeropsamments), and has darkened A horizons, a slightly reddened subsoil, and incipient lamellae at the depth of wetting front infiltration. These lamellae have slightly more clay and Fe oxides than the soil above. Mineral weathering is intense at the surface. The other zone of active pedogenesis is at the base of the regolith, where a lithologic discontinuity above the terrace platform forms an aquitard, and throughflow occurs. Meteoric water percolates through thin regolith deposits above the shoreline angle, and at other locations on the terrace where sediment has been removed by erosion. Percolating water carries clay, organic matter, and solutes to the water table. Weathering is intense within this basal unit. Illuviation of clays and Fe oxides, and precipitation of Fe oxides and silica occur within this unit. As pore space is filled, fractures and channels become paths for saturated water flow. Eluviation of Fe occurs at these sites. Most of the intervening regolith is isolated from current pedogenesis by its great depth and a relatively dry Holocene climate. Well-developed lamellae are preserved as relicts of Pleistocene episodes of soil formation. These lamellae formed by illuviation of clay and Fe oxides, and were sites of silica precipitation. The conceptual model presented here is intended to facilitate understanding of pedogenic and geomorphological evolution of marine terrace deposits, and to assist with the interpretation of groundwater flow in these terrace systems.
if soil depth is defined by the depth of organism activity, then the generalized concept of useful soil depth is much too shallow. While climate and geologic features combine to limit the extent of biologic activity in some soils, this review indicates many instances where such activity continues to great depths. Arbitrarily selecting a 1.5-m lower limit to the solum, we review reports of plant root, root symbiont, and vertebrate and invertebrate activity below this depth. The evidence for plant activity is given by the mere presence of roots as well as observations of water and solute uptake. Water uptake evidence comes from (i) observations of plant roots in the capillary fringe, or at or beneath a water table, (ii) water depletion from unconsolidated regolith with no water table present, and (iii) presence of roots in saprolite and weathered or fractured rock. These observations are numerous and demonstrate root activity to a depth of 40 m under certain conditions. Solute uptake is less readily documented. The few occurrences reported generally range in depth from 2 to 3 m, with one for U at 20 m. Root symbionts (rhizobia and mycorrhiza-forming fungus) also are present at depth (3-34-m) but, like solutes, reports are few. The depth range for faunal activity is from several to 100 m. Clearly, the material below the solum, usually considered not affected by soil-forming processes, is often well inhabited by flora and fauna. The importance of these subsoil volumes is still unclear apart from water withdrawal, but obviously relate to the belowground environment and the adaptation of species to this environment. An excessive concentration of attention in the surface 20 to 40 cm of soil seems often unwarranted.
Recent pressures on community development in Massachusetts require that the soil parent materials be examined in greater detail than has been done in the past. Up to 50% of the soils in Massachusetts are developed in dense glacial till regionally subdivided into the Lower Till of late Illinoian age and the Upper Till of late Wisconsinan age. Twelve till exposures were investigated to relate common morphological features seen in the tills to soil development, hydrology, and potential impact on land use. Depositional features such as sand layers and lenses, contorted silt/clay beds, and shear planes in the tills act as conduits for rapid water (and potential contaminant) movement. Oxidation along joints and fractures in both the oxidized and unoxidized facies in the Illinoian aged Lower Till suggests water movement and redox reactions are ongoing processes and occur several meters below the surface. There is a noticeable increase in both the amount and degree of development of argillans and redoximorphic features within the solum of soils developed on the oxidized Lower Till. The increased development suggests that much of the morphology of the modern soil is not inherited from the till but is due to pedogenesis. We concluded that the brown matrix, oxidized mineral grains, and increased fissility in the oxidized Lower Till result from postdepositional subaerial weathering and that the oxidized Lower Till appears to represent the remains of the Sangamon C horizon. Redox features also are present in the Upper Till generally occurring at textural transitions in the till. The relatively unweathered deeper portions of the Upper Till contain few to no argillans but argillans are common in the solum of soils developed in the Upper Till. These observations attest to the Holocene pedogenic alteration of the surface tills of all ages in Massachusetts.
Pedologists and Quaternary geologists developed different viewpoints almost 70 yr ago regarding concept, designation, and application of zones of weathering, termed horizons by pedologists. A sense of exclusion separated pedologic and geologic domains. The pedologic domain was confined to the upper portion of the earth's surface exhibiting the master horizons of O or A (accumulation), E (eluvial), and B (illuvial) horizons—the solum. In contrast, the domain of Quaternary geology and its interest in soil stratigraphy was focused on a paradigm stressing the subsolum— the realm of the pedologic C horizon and below. We seek to develop a unified paradigm through the introduction of a unified pedoweathering profile (PWP) concept that requires a re-examination of the C horizon concept and the redefinition and use of master horizon D. The PWP contains a C horizon concept of more limited but more precisely defined scope than traditionally used in pedology. In the traditional sense, the interval between the solum and bedrock is designated as C whether modified or not. In the PWP, the C horizon is limited to the modified part of the traditional C that shows pedogenic connection to the overlying solum. The part that is unaltered by pedogenic processes and does not have the hardness of bedrock (R) is recognized as the D horizon. The redefined subsolum horizons are not limited to the glaciated area of the midcontinental USA where these concepts were originally formulated. They exist worldwide in glaciated areas, wetlands, and alluvial, lake, and coastal plains of the past and present. With the growing importance of soil properties at depth, the PWP concept should be useful in paleopedology, soil stratigraphy, geomorphology, sedimentology, hydrogeology, and Quaternary geology.
Properties of earth materials below soil as defined by Soil Taxonomy and above hard rock are important to many land use activities. Although practitioners from several disciplines study and utilize this material in their work, no quantitative system, based on measurable properties of the materials, is available for its classification. Concepts of the various materials are communicated by genetically based names such as alluvium, glacial drift, saprolite and loess, but these terms do not have rigid class definitions. A four-category hierarchical system is proposed for classifying such materials. Concepts of the material's formation are used to guide the structure of the system, but measurable properties are used to define each taxon. Connotative names utilizing a procedure like that of Soil Taxonomy also are proposed for each taxon.