Diapir and dome structures on a scale of meters to kilometers are widespread in Earth's continental crust and represent an important tectonic element of cratons, orogenic belts, and sedimentary basins. These structures advect heat from lower to higher crustal levels, often producing pronounced prograde contact metamorphic aureoles. This standard thermal situation is violated by the up to 8 km in diameter migmatitic domes and diapirs of metasedimentary rocks that penetrate the world's largest layered intrusion, the Bushveld Complex. These domes and diapirs rose with an average rate of about 8 mm/yr and were characterized by an unusual inverted thermal structure, with the cores of the structures 200–300 °C colder than the rims. Numerical modeling supports the interpretation that the process was triggered by the emplacement of an 8-km-thick, hot, dense mafic magma over a cold, less dense sedimentary succession, resulting in a dramatic lowering of the viscosity of the sediments during contact metamorphism and partial melting. Dome and diapir nucleation is interpreted to have been defined by the formation of initial anticline-shaped disturbances related to fingered lateral injection of the Lower Zone of the Bushveld intrusion between the felsic roof and sedimentary floor sequence. The partially molten, mobile, but relatively cold domes and diapirs promoted cooling of the giant magma chamber, rapidly bringing cooler material into higher crustal levels, and freezing the surrounding magmas. We argue that our work has a more general significance as similar thermal structures should be a widespread feature associated with partially molten mantle diapirs (“cold plumes”) generated in the proximity of subducting slabs. These structures are likely responsible for rapid upward melt transport above subduction zones and for the associated volcanic activity. The exposed structures observed in the Bushveld Complex provide a unique opportunity to study the “cold” diapir/plume phenomenon, thus leading to a broader recognition and understanding of this geological process.

Three main features of paleosols are useful for distinguishing them from enclosing rocks: root traces, soil horizons, and soil structures. Fossil root traces are best preserved in formerly waterlogged paleosols. In oxidized paleosols their organic matter may not be preserved, but root traces can be recognized by their irregular, tubular shape, and by their downward tapering and branching. Often root traces are crushed like a concertina, because of compaction of the surrounding paleosol during burial. The top of a paleosol may be recognized where root traces and other trace fossils are truncated by an erosional surface. Root and other trace fossils are not useful for recognizing paleosols of middle Ordovician and older age, since large land organisms of such antiquity are currently unknown. Soil horizons usually have more gradational boundaries than seen in sedimentary layering. Commonly these gradational changes are parallel to the truncated upper surface of the paleosol. Some kinds of paleosol horizons are so lithologically distinct that they have been given special names; for example, cornstone (Bk) and ganister (E); the letter symbols are equivalent horizon symbols of soil science. Compared to sedimentary layering, metamorphic foliation, and igneous crystalline textures, soil structure appears massive, hackly, and jointed. The basic units of soil structure (peds) are defined by a variety of modified (for example, iron-stained or clayey) surfaces (cutans). Peds may be granular, blocky, prismatic, columnar, or platy in shape. Concretions, nodules, nodular layers, and crystals are also part of the original soil structure of some paleosols. Complications to be considered during field recognition of paleosols include erosion of parts of the profile, overlap of horizons of different paleosols, development of paleosols on materials eroded from preexisting paleosols, and the development of paleosols under successive and different regimes of weathering.

Coupled, one-dimensional transport equations for CO 2 and O 2 in pre-Middle Ordovician soils have been developed. The value of the ratio ΔP O 2 /ΔP CO 2 , where ΔP O 2 is the difference between the O 2 pressure in the atmosphere and at any level in a particular soil profile, and where ΔP CO 2 is the difference between the CO 2 pressure in the atmosphere and at the same level in the same soil profile, depends on the relative importance of O 2 and CO 2 transport by diffusion and transport by advection in soils. The transport equations were solved in closed form for a particularly simple soil model. Numerical solutions were obtained for more complex soil profiles, and the sensitivity of the P O 2 profile in soils to changes in a variety of parameters was explored. The results indicate that free oxygen has been present in the atmosphere at least since early Proterozoic time. The range of atmospheric oxygen pressure permitted by the available paleosol data is large. Our currently preferred range of the oxygen pressure in the atmosphere between ca. 2.5 and 1.8 Ga is between 5 × 10 −4 and 1 × 10 −3 atm.

Fossil soils in alluvial red beds from the Juniata Formation, near Potters Mills, in central Pennsylvania, provide evidence of soil-forming processes during Late Ordovician time. Paleogeographic and facies considerations indicate that the fossil soils formed on flood plains west of the Taconic uplift. Most studies of paleosols of this age or older have considered soils developed on metamorphic or igneous basement rock. Alluvial fossil soils provide evidence of conditions during shorter intervals of weathering without problems of overprinting by successive and different weathering regimes. They can be recognized by the presence of trace fossils and the development of soil horizons and structures. Problems associated with such fossil soils include establishing the nature of the parent material and distinguishing clay formation in the soil from originally deposited fining-upward cycles. These difficulties can be overcome by comparing paleosols in different stages of development, as indicated by degree of ferruginization, density of trace fossils, amount of clay, and abundance and size of caliche nodules. In modern soils, caliche forms in alkaline conditions under which TiO 2 is stable. Gains and losses of oxides (measured in grams per cubic centimeters) relative to TiO 2 in a strongly developed paleosol were compared with those of a weakly developed paleosol, taken to approximate the compositional range of the parent material. Concentration ratios indicate significant soil development of the strongly developed paleosol beyond the compositional range of the weakly developed paleosol. There was depletion of SiO 2 and enrichment of Fe 2 O 3 , Al 2 O 3 , and K 2 O relative to TiO 2 . Anomalous enrichment of K 2 O has been documented in other fossil soils. Both x-ray diffraction studies and a strong correlation between K 2 O and Al 2 O 3 are evidence that most of the potassium is contained in sericitized illite.

Subaerial exposure surfaces in the Middle and Upper Mississippian Slade Formation of northeastern Kentucky are largely composed of cutanic concentrations of micritic calcite within the former Ccam horizons of caliche soils. The association of this material with soil horizons and structures, as well as with abundant root traces, strongly indicates a pedogenic origin. In fact, the contribution of plants and small soil organisms was far greater than has been previously recognized. The caliches occur as “interformational” profiles on disconformities separating lower Slade members and as “intraformational” profiles within three lower Slade units. Paleoexposure was related to position on a structurally active margin of the Appalachian Basin and to episodes of regional and local regression. The caliches resulted from soil and ground-water conditions in a semi-arid climate characterized by seasonal rain and drought and an overall net moisture deficit. Growth of roots, desiccation, and displacive crystallization broke up parent limestones, allowing access of vadose waters and creating framework (skeleton) grains that were easily transformed into a mobile plasma fraction by solution. Solution of carbonate grains and eluviation of carbonate-bearing solutions primarily occurred during the moist rainy season, whereas illuviation rapidly followed the onset of drought. The calcium carbonate was deposited largely as internal, laminar plasma concentrations called cutans, which have been incorrectly referred to as “crusts” in previous work on the Slade. Accumulation of these cutanic laminae formed indurated laminar calcrete deposits near the bases of the caliche profiles. These calcretes may be of physicochemical or rhizocretionary origin, depending on conditions of exposure. More diffuse, irregular calcretes apparently developed along avenues of porosity and were formed by plasma separation, the in situ micritization of other limestone textures. Although climate in the Meramecian and earliest Chesterian epochs was the major factor responsible for caliche formation, the length of exposure and the type of carbonate lithology controlled the nature and thickness of caliche profiles. “Intraformational” profiles are always thin and immature, representing short-lived exposure on porous lithologies like calcarenite. Conversely, “interformational” profiles are always mature or composite and represent longer periods of exposure on more impermeable lithologies such as calcilutite. Impermeable lithologies were important, because they prevented migration of soil waters and plasma below the soil profile. By late Early Chesterian time, the climate had become more humid, and the latest formed caliches were partially destroyed by solution, creating a leached, clayey residual soil on top of earlier caliche soils. On structurally elevated areas, where exposure was long and drainage was good, this period of humid pedogenesis resulted in composite terra rossa paleosols produced from the humid weathering of older caliche profiles.

Clay mineral analyses of underclays directly below the Upper Elkhorn Coals (eastern Kentucky) and the Lower Kittanning Coal (western Pennsylvania) demonstrate systematic lateral and vertical variations that include changes in the kaolinite-illite ratio, mica loss ratio, weathering ratio, apparent thickness of mica, and distribution of chlorite. Clay mineral analyses of associated, unweathered shales indicate that approximately 30 percent of the regional variation in Lower Kittanning underclay mineralogy is inherited from the parent material. The remaining variation is attributed to in situ pedogenesis. Petrographic analysis of thin sections from a fluvial sandstone subjacent to the Upper Elkhorn underclay suggests that position of the ground-water table controlled pedogenesis. Two distinct alteration zones separated by a diffuse, subhorizontal boundary are present in the sandstone: an upper zone characterized by kaolinization of feldspars, dissolution of chlorite and detrital dolomite, and absence of siderite; and a lower zone characterized by ferron dolomite replacement of both detrital feldspar and detrital dolomite, and authigenic pore fillings of chlorite and siderite. These systematic changes in sandstone and underclay mineralogy are consistent with a pedogenic model in which the process of podzolization was controlled by position of the ground-water table and topography. In this model, the main phase of organic material accumulation occurred above the underclay after water-table levels intersected the land surface as a result of compaction, subsidence, or marine transgression. Regional gravity and structure data in western Pennsylvania further suggest that syntectonic movements were the fundamental controls on regional topography and ground-water levels, and thus, pedogenesis.

Nodular calcareous paleosols are common in the upper member of the Upper Triassic Dolores Formation in the San Juan Mountains of southwestern Colorado. These soils are developed in reddish brown, very fine-grained sandstone and siltstone of a sand-sheet facies that was deposited by eolian and aqueous processes on the margins of a large Triassic erg. Characteristics of these paleosols include nearly complete destruction of physical sedimentary structures, extensive mottling associated with burrows and root trace fossils, poorly sorted textures, and abundant carbonate nodules. Vegetative stabilization of the sand sheet is recorded by trace fossils of long, monopodial root systems, and fine networks of rootlets. Distinctive purple pigmentation of the large root mottles appears to have been produced by more coarsely crystalline hematite, which precipitated in the presence of root-derived organic compounds. Faunal bioturbation in these soils takes the form of meniscate and structureless burrows of the Scoyenia ichnofacies. The meniscate burrows are common in recent soils and pre-Holocene paleosols, and probably represent sediment reworking by arthropods. Carbonate nodules in these soils are composed of micrite and microspar, and they contain sparry calcite crystallaria and septaria. These glaebules occur as individual “floating” entities and as stacked columns. Burrows cross-cut some nodules, indicating that at least some of the pedogenic carbonate accumulations were relatively unlithified at the time of deposition.

Paleosols are well preserved beneath Upper Cretaceous continental sediments from the eastern Gulf Coastal Plain to the northern Mississippi Embayment. These buried soils are remnants of land surfaces that weathered during Cretaceous time and formed on several rock types, including Precambrian(?) metamorphic rocks and Paleozoic limestone and sandstone. Most soil profiles have distinctive morphologic features and exhibit pedogenic horizon zonation with ferruginous and clay-rich B horizons grading downward to saprolite and parent material within 7 m. Organic matter is not preserved, but pedotubules, which we interpret as fossil roots or burrows, are present in many paleosols. Illuvial and residual concentrations of Al 2 O 3 and Fe 2 O 3 occur in upper soil horizons where kaolinite and halloysite are the principal clay minerals. A variety of precursor minerals, including 2:1 lattice clays, muscovite, feldspar, and biotite, are present in the lower part of the soil profiles. Variations in pedogenesis were caused by differences in parent material and by local geomorphic factors such as paleoslope and drainage. The rates of paleosol development and the residence times of the various land surfaces are uncertain, but the times of burial and preservation range from Cenomanian to early Maestrichtian. Chemical and mineralogic characteristics of these paleosols are similar to those of modern soils of tropical to subtropical climates; however, the fossil soils may be polycyclic, and their pedogenic characteristics could have been modified by postburial diagenesis. Climatic reconstructions based on paleobotanical, lithostratigraphic, and paleotemperature data from Cretaceous sediments support our interpretation that these paleosols formed in a warm, humid environment on well-vegetated piedmont surfaces.

Paleosols formed on overbank deposits of the lower Eocene Willwood Formation in the Bighorn Basin, Wyoming, can be differentiated on the basis of pedogenic maturity. The least mature soils generally formed close to the channel margin where sediment accumulation rates were rapid, whereas the most mature soils developed on distal flood plains where accumulation rates were significantly slower. The term pedofacies is introduced to delimit laterally contiguous bodies of sedimentary rock that differ in their ancient soil attributes as a result of distance from areas of relatively rapid sedimentation. Vertical successions of Willwood overbank deposits show three orders of pedofacies sequences. Simple sequences consist of one or more paleosols bounded below and above by crevasse-splay deposits. They were generated by slow and sporadic alluviation and soil modification that were periodically interrupted by more rapid crevasse-splay deposition. On a larger scale, compound pedofacies sequences are composed of multi-story paleosols sandwiched between channel sandstones. Pedogenic maturity of the paleosols progressively increases and then decreases upward in response to episodic channel avulsion. Development of compound and simple pedofacies sequences was largely controlled by local patterns of deposition that produced vertical variability in the rate of sediment accumulation. Superimposed on these smaller-scale cycles are pedofacies megasequences that are hundreds of meters thick. Megasequences show a distinct upward change in the overall maturity of their constituent compound sequences. They provide evidence for changes in the rate of sediment accumulation produced by allocyclic processes including varied tectonic activity. Comparison between Willwood deposits in the northern and central parts of the Bighorn Basin reveals that areally differing sediment accumulation rates, and thereby basin subsidence rates, can also be interpreted from large-scale pedofacies sequences.

Many Quaternary fan and terrace deposits of the arid and semiarid regions of the Mojave Desert and the subhumid (Mediterranean) Transverse Ranges, southern California, have similar parent material and are relatively well dated, thereby enabling evaluation of the influence of changes in climate on the rates and processes of soil development. Within soils of a given chronosequence, soil age and many morphologic, mineralogic, and chemical properties are strongly related, reflecting primarily an evolutionary, time-dependent trend of continuous soil development during Quaternary time. Comparison of soil development in Holocene deposits of similar age of arid, semiarid, and subhumid regions indicates that increases in the amount of effective soil moisture explain observed systematic differences in the rates and processes of soil development. Accordingly, major climatic changes at the end of Pleistocene time have resulted in the development of soil profiles that are, to a certain extent, polygenetic. This change in climate caused a decrease in the depth of leaching, causing accumulation of secondary carbonate and locally gypsum, materials derived from incorportion of eolian fines, in argillic B horizons of Late Pleistocene soils. Increases in dust influx and availability of calcareous dust, caused by the widespread development of playas and decreases in vegetation at the end of the Pleistocene, have probably also influenced rates of soil development. In subhumid regions, the leaching environment favors development of noncalcic profiles during Quaternary time. This leaching environment may approximate the leaching environment experienced during late Pleistocene time by soils that are forming presently in a semiarid climate that favors accumulation of calcium carbonate. Late Pleistocene and older soils in presently semiarid regions also possess thick, clay-rich B horizons with authigenic clay and iron-oxide minerals, features that indicate soil development during past periods of increased effective moisture. The relatively rapid development of argillic horizons in latest Pleistocene soils of currently subhumid regions is attributed to attainment of a threshold of soil formation, rather than the Pleistocene-to-Holocene climatic change. The threshold is caused by the accumulation of large amounts of silt and organic matter, a process that causes decreases in permeability, increases in available water-holding capacity, and ultimately increases in the rate and magnitude of chemical weathering. Soil chronosequence studies in California demonstrate that rates and processes of soil development may be influenced by changes in climate, in a direct fashion by causing changes in leaching regime, or in an indirect fashion by causing variation in the pattern and intensity of eolian activity.