Palaeosols

                                                                     By Jonathan Clarke
Copyright 1999. This may be freely distributed so long as no charges are made and no alterations are made without the prior written consent of the author

Introduction
Many sedimentary successions contain features that have been interpreted as palaeosols.  Because soils require decades for even the most immature to form, and more typically centuries to millennia, these features provide yet another falsification of the theory that the bulk of the sedimentary succession is the result of a single event of about a year�s length, such as Noah�s flood.  The 1990 book by Retallack is perhaps the definitive overview of the subject.  In this work he gives criteria for recognizing palaeosols, a historical review of the development of palaeosols, and reviews their environmental significance. Palaeosols are known from both terrigenous and calcareous successions.  Given the focus of my last two discussions has been on calcareous successions, this one will concentrate mainly on examples from terrigenous successions.
Characteristics of palaeosols
What is a soil?
The soil is the surface layer of the earth that supports plant growth.  Soils are characterized by a number of distinct features.  The first of these is a soil profile.  In a soil profile the soil is divided into a distinct series of soils horizons.  These are typically the O horizon, the leached A horizon, a zone of mineral accumulation (B horizon), and finally, the C horizon, which consists of only weakly altered parental material.  The degree to which each horizon is developed will depend on features such as climate, waterlogging, biota, and time.  A soil in a humid, low lying area is more likely to develop a good O horizon and be water logged.  Such as soil is likely to have a B horizon containing siderite or even sulphides.  A well drained soil in a humid climate is likely to have a well developed profile, whereas that in a more humid climate will be less well developed.  Concentration of soluble minerals such as carbonate are more common in the B horizons of soils of drier climates than those of wetter climates, these will be more likely to have B horizons rich in iron oxides and hydroxides.   Soil formation is accompanied by progressive destruction of the original host fabric, which is variably preserved in the C horizon, and generally absent from the A and B horizons.
Microstructures include peds, glaebules, plasmic structure, and cutans. Peds are blocky, columnar, nodular, mammalliated, or prismatic structures. Individual peds are separated by fractures and voids and show slickensided surfaces. Glaebules are small pellets and nodules that form in the soil.  The soil plasma is the matrix of the soil formed by soluble, mobile minerals. .  The most common types are precipitated coatings on grains known as cutans.  The cutans can be formed silica, carbonate, iron oxides and hydroxides, and clays.
Soils in semi-arid climates are often calcareous and contain extensive precipitated carbonate.  These soils form caliche or calcrete, a durable surface crust of pedogenic carbonate.  Calcrete fabrics are distinctive, and include breccias (often with blackened clasts), crusts, small-scale karst features, and pseudo-anticlines (laminar calcrete that has a series of anticlinal folds developed as a result of lateral expansion of the crust by progressive precipitation of carbonate). Pseudo-anticlinal are also found in non-calcareous soils where they form as the result of the wetting and drying of expandable clays.
Soils developed on calcareous rocks consist of two end members. Calcrete results when precipitation exceeds evapo-transpiration.   It is thus characteristic of arid and semi-arid climates.  When precipitation exceeds evapo-transpiration, leaching results, forming bare karst and terra rossa soils.  Calcretes are thus strongly climate controlled.  Typical water deficits (evapo-transpiration subtracted from precipitation) of �500 to 1,000 mm are most favourable for calcrete formation, although it can also form in more humid environments in there is an abundant supply of carbonate.
Finally, biological activity is a key part of modern soils.   Different plant communities result in different soil types.  Podzols and similar duplex soils (those with two well-differentiated horizons) develop best in well-drained areas under forest cover.  Grasslands are characterised by more homogeneous soils such as solonized brown soils and, chernozerms, and red-brown earths. Root traces are ubiquitous in modern soils, as are the burrows of soil invertebrates.   Plant root traces have different characteristics to both the burrows of terrestrial invertebrates and to those of marine organisms.   Root traces are downward branching and narrowing, smooth walled, tubular structures formed by roots penetrating the soil and its substrate.  After the death of the root, the resulting cavity, often reinforced by precipitation of carbonate, clays, iron oxides, or other minerals around its periphery, may become the site of further vadose precipitation.  The burrows of soil invertebrates produce rough walled, non-bifurcating structures quite unlike root structures.  Marine invertebrate burrows are larger and have a different and usually more complex morphology.
How do soils form?
Soils require three essential ingredients for their formation.  The first is subaerial exposure (they do not form beneath the sea); the other is time (they cannot form overnight). Studies on soils formed in areas with dated falls of volcanic ash or in areas exposed by glacial retreat show that good soil profiles take centuries to millennia to form.  Even the most obvious features of soils, such as a leached A and enriched B horizons, need years to even begin to be evident. The third is that the rate of soil formation must exceed the rate of erosion.  If the rate of erosion is greater than that of soil formation, then soils will not form and the land surface will consist of weathered rock.
Recognising palaeosols
Palaeosols are recognised through many criteria.  The most easy are root or rootlet horizons, sometimes preserved only as impressions.  These are present even in poorly developed soils.  Other important characteristics are the formation of soil profiles and microstructure.
Some authors (like Wright 1982) use the term �pedoderm� and �pedocomplex�.  A pedoderm is a mappable unit of soil, either entire or truncated, whose physical and stratigraphic characteristics permit consistent recognition and mapping.  A pedocomplex is a sequence of soils that lie in close vertical succession but do not overlap.
Palaeosols in time and space
A surface layer of weathered rock would have formed in any area during earth history that underwent subaerial exposure for any length of time and with a relatively low erosion rate.  However the absence of vascular prior to the Silurian (or thereabouts) means that soils in the modern sense are not full developed in older rocks.  However, residual weathering profiles are known from Early Palaeozoic and Precambrian rocks.
Case studies
Early Carboniferous palaeosols, South Wales
Wright (1982) reported on calcrete palaeosols from three separate horizons in the Early Carboniferous Llanelly Formation in Wales.  From the bottom these are the Tyer bont pedocomplex, the Cwm dyar pedoderm, and the Llanelly pedocomplex.
The Tyer Bont pedocomplex overlies the top of the Oolite Group.  The top of the Oolite shows karsted relief of up to 1 m and clay-filled solution pipes.  The complex consists of lenticular sandstones, conglomerates, and clays.  The sandstones and conglomerates strongly channelised and, together with the clays, are interpreted as a fluvial channel and flood plain complex.  The pedocomplex varies between 1 and 6 m in thickness.  Superimposed on the terrigenous sediments are nodular, massive, blocky, and prismatic diagenetic carbonates. These form stratigraphic units and are often truncated by overlying channel units.  The Cwm Dyar pedoderm is developed on top of the Cheltenham Limestone Member.  This member consists of interpreted shallow marine to intertidal limestone.  Palaeosols are common through the sequence and a prominent one near the top is called the Cwm Dyar pedoderm.  The Llanelly pedocomplex is developed within the Gilwern Clay Member.  The host sediments of this complex are an upward fining sandstone interpreted as a high sinuosity stream deposit.  The palaeosols are best developed in the clay rich units.
The clays are smectitic, variably coloured, with red and purple mottles superimposed on a background green colour.   Secondary carbonates grow in this clay matrix.  Clays in the upper part of the Llanelly pedocomplex are somewhat different. A grey clay-rich palaeosol with numerous rootlets and capped by a thin coal occurs at the top of the Llanelly pedocomplex.  Clays in the Llanelly pedocomplex locally show pseudoanticlinal structures reminiscent of gilgai structures in modern smectitic soils.
The nodules are found in the clays and vary in shale and size. Some have gradational contacts with the surrounding sediment, others sharp.  Those with sharp contacts often appear to have shrunk relative to the surrounding sediment.  In size, shape, and texture these nodules closely resemble carbonate glaebules in modern calcretes.
Platey calcretes are found rarely in the Cwm Dyar pedoderm and in the base of the Llanelly pedocomplex.  They consist of limestone plates 1-5 cm thick and 20 cm across separated by clay seams up to 2 cm thick.  These plates are composed of fine-gained carbonate and are often fractured and highly distorted.  Similar plates are known in modern calcretes and the distortion is due to the development of gilgai structures.
Limestone beds can have massive, platey, blocky and prismatic textures. The prisms in the prismatic beds are oriented vertically and are sometimes cylindrical in shape.  The beds are often fractured and filled by calcite cement.  This is a very early feature as it is also found in reworked limestone pebbles in the channel units.  Clays between fractures in the limestone beds may show curved slickensided surfaces that are at a high angle to bedding.  These are different from the tectonic slickensides that are straight and at a low angle to bedding.  The limestone beds are often brecciated; varying from jigsaw fit breccias to ones that are completely disrupted.  Sand, clay, or calcite fills the spaces between the clast veins.   In thin section the limestone beds are very fine-grained and clay rich.  The clay occurs as small seams and as included grains.  Internal cavities with internal sediment are common.  The limestone beds closely resemble those found in modern calcrete hardpans. Slickensides are also common in modern soils and are formed by the shrinkage and expansion of clays.  The difference in orientation between these slickensides and those of undoubted tectonic origin strongly suggests a pedogenic origin for these features.  The prismatic to blocky texture in many of the beds closely resemble calcareous peds in size, shape, and texture.   The microfabric also closely resemble that of modern calcretes.
Mississippian calcretes, Appalachian Basin
The Middle and Late Mississippian Slade Formation of northeastern Kentucky contains nine recognised palaeosol complexes (Ettensohn et al. 1988).  In their study of the lower part of the Formation Ettensohn et al. (1988) described the characteristics of five of these, developed on or in the St Louis, Ste Genevieve, Mill Knob, Warix Run, and Cave Branch Members.  These palaeosols are developed on both sides of the Waverley arch, a syn-sedimentary tectonic uplift.  The authors distinguish between interformational palaeosols developed on the top of units and representing relatively extended periods of exposure, and intraformational palaeosols that develop during relatively brief facies changes or sea level changes.  The palaeosols are separate on the flanks of the Waverley arch but converge and amalgamate across the top.   Interformational palaeosols on the St. Louis and Ste. Genevieve Members are locally removed by localised down cutting and the valleys infilled by the Warix Run Member.
The best-developed soils are the interformational palaeosols that represent longer periods of subaerial exposure.  Those developed intraformationally are far less well expressed.  Interformational palaeosols developed on the St. Louis, Ste. Genevieve, and Mill Knob members generally show truncated profiles with only the lower B (zone of soluble mineral accumulation, typically carbonate in these soils) and C horizons (weathered rock) typically preserved.  Rarely, however, the A horizon is preserved.  Intraformational palaeosols are not as well zoned, with C horizons penetrated by plant roots the extent of development.
Interformational and intraformational soils of the St. Louis, Ste. Genevieve, Warix Run and Mill Knob Members are all characterised by carbonate accumulation in the B horizon (where present) and calcrete formation.  They are thus indicative of semi-arid conditions.  The Cave Branch Member that overlies the Mill Knob Member has many characteristics of a terra rossa, which form only under more humid conditions.  These features indicate an increase in rainfall higher in the Slade Formation compared to the lower parts.
Modern calcretes contain a range of characteristic fabrics.  These include; laminar calcretes, pseudo-anticlines, small scale karst, and breccias.  All these features can be observed in the palaeosols of the Slade Formation.  A diverse range of smaller scale pedogenic features are also present.  The most common  type are cutans of carbonate, iron oxides and hydroxides, clays, and silica.  Grains include amorphous nodules of carbonate, blackened pebbles (a feature very common in modern calcrete and caliche), and vadoids, carbonate grains formed by multiple generations of cutans.
Pleistocene or Holocene carbonates undergoing subaerial exposure show a progressive and destructive replacement of the original sedimentary grains and cement by fine-grained carbonate or micrite.  This subaerial micritisation progressives along porous horizons, small cavities or fractures.  Extensive micritisation of the original carbonate grains and cements is a common feature of the palaeosols in the Slade, and is only found within them, rather than pervasively with the rock units as a whole.
Modern soils are characterised by peds that give soil a characteristic blocky, columnar, or crumbly structure.  They are best developed in the B horizon of soils, the zone of strongest alteration.  The interpreted B horizon in the palaeosols of the Slade are also characterised by ped-like structures.  Voids and slickensided surfaces separate those of the Slade from each other. Columnar, prismatic, mammalliated, nodular, polyhedral and blocky peds are present.
Trace fossils, formed by roots and soil invertebrates are also a common feature in modern soils. Root traces in the palaeosols of the Slade are very common and closely resemble those of modern soils.  Small burrows are also common in the palaeosols of the Slade.  Many are back-filled by many small peloids, these may be faecal pellets.  Similar pellets are common as interstitial fill between larger grains in the palaeosols.
Pennsylvanian underclays
Fossil soil horizons are common beneath coal successions (Diessel 1992).  These are variously described as seat earths, ganisters, fireclays, and underclays. Their genesis may be related or unrelated to the overlying coal.  In the second case transported plant matter may be deposited on top of a pre-existing soil or the coal forms after a rapid change in surface environment.   Where the coal and the soil are directly related, the coal can be regarded as an exceptionally well developed O horizon.
Underclays are well known from beneath the Early Pennsylvanian coals of the Appalachian, Illinois, and Mid-western Basins of the United States and have been studied for more than 40 years. Huddle and Patterson (1961) provide a good example of such an early study.  These authors list a number of criteria why they believed that the underclays beneath the Pennsylvanian coals were palaeosols.  These criteria included including roots, profile development, microstructure, and mineralogy.  Root traces in the underclays include the large structures known as Stigmaria, Radictes, and rootlets.  Palaeosol profile development is poor, as would be expected in the water logged ground conditions beneath a coal-forming environment.  However, roots were generally observed to have destroyed the parental substrate fabric beneath the coal.  These zones show a rubbly texture with slickensided surfaces along the fractures.  Siderite is observed in these underclays, consistent with precipitation from reduced pore water.
A more detailed and more recent study of underclays beneath the Upper Elkhorn coals of eastern Kentucky and the Lower Kittanning Coals of Pennsylvania is that of Gardner et al. (1988).  These authors confirm the conclusions of Huddle and Patterson (1961) and also give a nice taxonomy of underclay models.  They show that there is a progressive description upwards through the palaeosol of minerals inherited from the substrate.  There is a parallel increase of secondary minerals such as kaolinite up through the palaeosols.  One would predict that palaeosols formed on palaeotopographic highs would show a greater degree of weathering.  This is exactly what is observed; palaeosols on palaeotopographic highs have higher kaolinite and lower chlorite percentages compared to the parental material than those elsewhere.  In contrast, palaeosols formed near the basin centre show the least degree of weathering using the same index (lowest kaolinite and highest chlorite).
Finally, YEC literature on underclays commonly cite the study of Schultz (1958) who cast some doubt on whether underclays were true palaeosols, suggesting that they had an allochthonous (transported) origin.  Thus the YEC literature uses the study of Schultz to demonstrate that they are not, in fact, palaeosols. However, Schulz focused on only the plastic underclays (those rich in kaolin), one of several types of seat earth.  He later pointed out to Huddle and Patterson that his conclusions that underclays were allochthonous applies only to the plastic underclays and not to others. Indeed, Huddle and Patterson were sceptical of Schultz�s conclusions even with respect to the plastic underclays, considering the textural, mineralogical and root evidence indicated that they too formed as in situ palaeosols.
[Editor's note: vertical tree trunks have been found coming out of the bottom of the Pittsburgh coal seam and extending into the strata above showing that the the tree is best interpreted as having grown in situ and been rooted in the sediments under the coal--grm. see A. T. Cross, �The Geology of Pittsburgh Coal,� Second Conference on the Origin and Constitution of Coal, Crystal Cliffs, Nova Scotia, June 1952, pp 32-99, p.76]
Pennsylvanian palaeosols in a marine carbonate succession, Arizona
The Black Prince Limestone was deposited during the same biostratigraphic interval as the Pennsylvanian coals described above.    This 50-90 m thick unit was described by Goldhammer and Elmore (1984) as consisting of at least 6 upward shallowing cycles of carbonate sediments, each capped by a palaeosol.  Because palaeosols cap sedimentary cycles, they formed during relatively brief periods and are equivalent to the intraformational palaeosols of the Slade Formation. These palaeosols are of two types, calcrete and terra rossa.
The palaeosols interpreted as being ancient calcretes consist of brown, laminated micrite that disconformably overlies unaltered marine limestones.  The laminae alternate between massive micrite and fenestral micrite containing detrital grains.  Vertical and inclined fissures and vugs cut the laminated micrite and are then lined by it.  The fenestrae are randomly oriented and superficially resemble root molds. The laminated micrite occurs as sheet-like deposits in the form of small, laminated domes and saucer-like depressions.  The grains in the fenestral laminae are predominantly micritised skeletal grains. The calcretes would have formed under semi-arid conditions.
Palaeosols interpreted as terra rossa are 0.1->1 m thick and are composed of grey limestone clasts in a reddish matrix.  They vary from <1 cm to 50 cm in size and vary from subspherical to elongate and rounded to angular. The clasts are composed by the same lithologies as make up the unaltered limestones between the palaeosol horizons, including fossiliferous grainstones, non-fossiliferous lime mudstones and (rarely) laminar micrite (from the calcrete).  Primary textures are well preserved in these nodules.  In rare cases the clasts consist of silicified limestone.  Clast boundaries are often sharp, with truncation of both grains and early cements.  Concentric fractures are common.  Some clasts however have gradational boundaries with the matrix.  The matrix consists of poorly sorted fossils, limestone fragments, and silt to clay sized particles mainly made up of insoluble material with some carbonate.  The interpreted terra rossa consists of two facies.  Type 1 consists of brecciated parental limestone where the fractures consist of anastomosing, dendroid, and planar fractures. Type 2 consists of crudely bedded units in which clasts make up as little as 5%.  Terra rossa form as residual deposits, with type 1 forming completely in situ and type involving some reworking.  The amount of dissolution required to form a terra rossa can be calculated by comparing the amount insoluble material in the parental limestone with that in the terra rossa.  In the Black Prince Limestone, 10-20 m of limestone would need to be dissolved to yield the average 1 m thickness of insoluble matrix in the terra rossa.  Thus the cycles overlain by terra rossa have been considerably reduced by dissolution, to the extent that more than one cycle may have been complete destroyed.  This is indicated by the rare presence of laminar micrite clasts in some of the terra rossa horizons. These would have been derived by solution collapse of an entire cycle mixing relict clasts of calcrete with other lithologies into a younger terra rossa.
Triassic palaeosols, New South Wales
Greg Retallack (1976, 1977) described a succession of at least 16 successive palaeosols exposed along the New South Wales (Australia) coastline, north of Sydney.  They occur in the Upper Narrabeen Group (Triassic) of the Sydney Basin.  The succession was interpreted by Retallack as a prograding fluvial system, with fluvial sediments (Bald Hill Claystone) that are drowned lagoonal deltaic sediments (Garie Formation) and then braided river sediments (Hawkesbury Sandstone).   These palaeosols contain O, A, B, and C horizons.  A range of microstructures are also well preserved.
The O horizons are represented by ferruginised leaf litter.  The leaves in the litter are ragged, irregularly ferruginised and have shallow scribbling on their surfaces which resemble the trails of invertebrates in modern leaf litters. Other leaves are preserved only a skeletons and naturally macerated fragments. Ferruginised sticks and twigs may also present.  In other localities the litter is sufficiently thick and the organic matter sufficiently well preserved for it to be a cuticle coal or carbonaceous shale. The O horizon is underlain by a type of palaeosol called ganister.  These are silica cemented, medium to fine sandstone beds less than 1.5 m thick.  The ganisters contain numerous radiating clumps of roots and associated rootlets.  More or less vertical and unbranched tubular burrows with circular or elliptical cross section.  They are distinct from the root and rootlet traces and closely resemble the burrows of insects, crustaceans, and earth worms.  Cicada-like insects are known from the Triassic in the area.  Very rare larger burrows with elliptical cross section of 7 X 12 cm are possibly vertebrate burrows.  Fossil lungfish and amphibians are known from the succession and may be responsible for these large burrows.  Holes 15-20 cm across in the top of ganisters may represent cradle knolls formed by the fall of small trees.  Smaller egg-cut depressions in the ganister may represent individual  plant growth. The leached, silica-rich character of the ganisters and their association with roots, rootlets, and burrows, indicates that they form the A horizons of palaeosols.  The B horizons of palaeosols in the Upper Narrabeen Group contain concentrations of iron oxides and hydroxides in the more oxidised profiles and siderite in the more reduced ones. Some of these nodules are found as clasts in channel sandstones, confirming that they formed during very early diagenesis. Various ped structures, including prismatic and blocky, are present in some palaeosols. The contact between the A and B zones are marked by vermicular mottles, probably after plant roots.  The interpreted palaeosols show enrichment in copper compared to the substrate sediments, as do modern soils.
The palaeosols form 6 well defined associations, the Long Reef, Avalon, Turrimetta, South Head, St Michael�s, and Warriewood series.  Each of these formed in a different depositional setting, under different soil hydrological conditions, and with different plant assemblages.  The Long Reef series is found in the fluvial sediments of the Bald Hill claystone.  The soils are weakly differentiated, with a simple A and B horizon, and are heavily ferruginised.  At least 6 to 8 stacked palaeosols are present.  The characteristics of the palaeosols indicate that they formed in well drained country.  This is consistent with the fluvial depositional setting of the host sediments.  It is also consistent with the plant fossils.  Although there are no well preserved plant fossils conifer pollen is common in the Formation, however.  Minor Falcisporites (from Dicroidium) is also present. The reconstructed vegetation is that of a conifer forest with a Dicroidium understorey.  The Avalon series are developed on a distributary complex of point bar ridges, crevasse splay levees, and silty flood plains.  They are characterised by thin O horizons, bleached A horizons, and grey, clay-rich B horizons with siderite nodules.  Cradle knolls are common in the upper surface of the palaeosols.  The palaeosols suggest formation in a low lying area with a well drained upper portion and a water logged lower part of the soil.  This is consistent with the depositional setting of the sediments.  The palaeobotanical evidence is also agrees with this. There is a diverse flora (Neocalamites, Asterotheca, Cladophlebis, Gleichenites, Chiropteris, Taeniopteris, and several species of Dicroidium. The small cradle knolls, small wood fragments (none more than 11 cm across), and the small size (50 cm or less) of the radiating root networks suggest that individual plants were small, presenting a swampy woodland association.  The Turrimetta series occur just above the Long Reef palaeosols.  They have formed on fluvial clays of the Bald Hill and lower Garie Formations.  The Turrimetta series have a thick organic rich layer (O horizon) that overlies a grey A horizon and a mottled red vermicular B horizon that also contains sideritic nodules.  The structure of this soil is complex, indicating initial well drained conditions forming the A and B horizons changing to more water logged conditions that formed the O horizons and the sideritic nodules that overprint the B horizons.  This is consistent with the change from the subaerial fluvial Bald Hill Formation to the lagoonal Garie Formation.  The palaeobotanical association indicates large conifers, Dicroidium, and Neocalamites.  This suggests that the boggy low land was covered by a swamp forest flora.  The South Head series are developed on siltstones and shales of the Garie Formation draping cross bedded sandstones identified as point bar and channel sand dune deposits.   They are possibly also partly clayey levee bank deposits.  The palaeosols are poorly developed, with some pink to grey weathering, weak ped development and numerous carbonaceous roots.  The colour mottling suggests partial water logging and development of gley fabrics. The plant fossils are dominated by small, shrubby fossil plants, especially Taeniopteris and Dicroidium.  This suggests a shrubby woodland to heath land vegetation in a moderately drained fluvial setting.  The St Michael�s series of palaeosols consist of a ferruginised A horizon and a grey clay rich B horizon with abundant insect burrows.  This soil series formed on a distributary complex of point bar ridges, crevasse splay levees, and silty flood plain, similar to the Avalon soils, but somewhat better drained and therefore more oxidised.  This suggests a more elevated setting, perhaps on low levees.  The palaeoflora consists of Neocalamites, Asterotheca, Cladophlebis, Gleichenites, Chiropteris, Taeniopteris, and Dicroidium. The vegetation was therefore a similar woodland.  Finally the Warriewood series consist of very thin and poorly developed A and B horizons forming on silt and clay.  The plant fossil assemblage varies from Dicroidium and similar forms to almost monospecific association of logs, rhizophores, and leaves of the lycopod Pleuromeia.  The abundance of Pleuromeia remains in prodelta deposits suggests it fringed interdistributary bays.  The Warriewood series thus may represent a catena from relatively well drained Dicroidium woodland to swampy Pleuromeia meadows.
Tertiary palaeosols, South Dakota
Greg Retallack also studied Late Eocene through to Oligocene soils from the Badlands of South Dakota (Retallack 1983).  A succession some 130 m thick contained no fewer than 87 palaeosols.  Apparently it is the multicoloured nature of these palaeosols that cause the spectacular colours of the Badlands National Park.  The host succession comprise the White River and Arikaree Groups.  Interpretation of their depositional environment has been controversial, but the best current understanding is that they represent an alluvial plain.
The palaeosols fall into a number of distinct associations or series, like those of the Triassic of the Sydney Basin.  These associations consist of soil profiles, mineralogy, and structures that closely resemble a range of soils from modern environments.  The resemblance is sufficiently close that the author was able to identify them as chernozerms, podzols, red-brown earths, solonized brown calcareous soils, alluvial soils, and weisenboden soils.  Although plant remains are poorly preserved and generally absent from these oxidising environments, the soil type enables the vegetation type to be reconstructed and, in conjunction with the mineralogy and soil zonation, the palaeoclimate to be understood. Associated with the soils are vertebrate fossils assemblages that show some dependence on the soil type. The flat-lying nature of the succession and the heavily dissected badlands topography allows accurate determination of palaeorelief along soil surfaces, in some cases as much as 25 m of palaeorelief being present.
Chernozerm and grey-brown calcareous soils are typical of grasslands of warm-cool temperate, seasonally dry, to semi arid climates.  The fauna consists of rabbits, beavers (not all extinct beavers were forest and riverside dwellers), and deer-like ruminants.  Solonized brown soils form in more wooded grassland areas, such as a savanna, in warm-cool temperate, seasonally dry, to semi arid climates.  The vertebrate assemblage included deer like ruminants, horses, rhinoceros, beavers, rabbits, and squirrels.  Red-brown earths are woodland soils of subtropical to warm temperate, seasonally dry, sub humid to humid climates.  The vertebrate assemblage on these soils includes rhinoceros, horse, titanotheres, turtles, rabbits, and squirrel like rodents.  The swales of river bars (recognised from cross-bedded alluvial sand deposits) have poorly developed alluvial soils.  One reason they are so poorly developed is that they support only shallowly rooted herbaceous plants.  No vertebrate remains were reported from these soil associations.  Podzols are very easily recognised duplex soils that form beneath woodland to forest cover in subtropical to warm temperate, seasonally dry, sub humid to humid climates.  The vertebrate assemblage consists of tapirs and horses (at that time browsers rather than grazers). Wiesenboden soils are typical of episodically waterlogged soils in wooded areas, such as tree-covered streamside swales (where they are developed on channel margin sediments) or seasonally swampy tree-covered lowland interfluves, or seasonally swampy tree-covered lowland interfluves further away from the channels.  These rather swampy soils are associated with fossils of lizards, moles, rodents, rabbits, ruminants, horses (browsers again) and pigs.
As an example of the detailed paleoenvironmental reconstruction possible, the Scenic Member of the Brule Formation (early Late Oligocene) consists of palaeochannels incised into older Cretaceous sediments (with palaeosols along the contact).  The channel sands indicate broad, low relief river systems passing laterally into flood plain silts.  The Scenic Member is some 35 m thick in the Pinnacles area and contains some 23 successive palaeosols.  Red-brown earths are found on the levee bank facies, indicating the development of woodland in these relatively well-drained and well watered localities.  The channels themselves, characterised by unstable substrates, have only alluvial soils that form with herbaceous vegetation, well able to adapt to such substrates.  The flood plains have solonized brown soils typical of wooded savanna vegetation, as would be expected in drier areas further away from the water courses.
Discussion
A global flood model for the rock record must explain the features of palaeosols outlined in the above review.  In particular it must account for the following features:
Why these features are found only in the stratigraphic position where such features are to be expected (along unconformities, at the tops of upward shallowing cycles, for example) and not throughout the rock succession.
Why the features of fully developed palaeosols coincide in biostratigraphic time with the appearance of land plants and animals.
Why coals are only associated with palaeosols dominated by leaching, water logging and relatively reduced conditions, not ones with extensive good drainage, accumulation of carbonate, and oxidation.
Why features with the characteristics of palaeosols are most common in sediments whose interpreted depositional environments are precisely those where palaeosols would be expected, namely terrestrial and shallow marine sediments.
Only two options appear to be available to advocates of a global flood model for these successions.  Either these palaeosols are exactly that, formed by subaerial exposure and the resulting interaction of physical, chemical, and biological processes, or they are not palaeosols and only look that way then in fact they are the result of subaquatic deposition and subaquatic and burial diagenesis.
If they are palaeosols then flood geologists must be able to explain (without appeal to ad hoc miracle) why and how these features that normally require decades to millennia to form, developed in minutes to days.  This would include the growth of trees and other plants, together with the activities of the soil fauna (which somehow had to have survived the flood until that time), and how characteristic assemblages terrestrial vertebrates would move to each soil type.
If they are not palaeosols, then flood geologists must explain how sedimentary processes can mimic a wide range of features.  These include: the appearance of soils in these lithologies, down to and including roots, soil invertebrate structures, soil horizons and microstructures, vadose textures, and geochemical and mineralogical signatures.  They must also explain why these features vary together with the terrestrial flora and fauna, the palaeogeography, and the sedimentary facies in a way consistent with a palaeosol origin.
Also, flood geologists must explain why these horizons are not single or rare features.  They are common and often occur repeatedly in geological successions.
Finally, saying that these deposits containing palaeosols formed after the flood does not help.  Some, such as those of the Appalachian Basin, are part of very thick successions kilometres thick that have been deformed, uplifted, eroded, and then overlain by younger deposits that too have then been eroded.  Some of these also will contain palaeosols.  This would still have to occur within a few years or decades for the Pennsylvanian palaeosol bearing succession.   A few extra years or even decades will not solve the problem of either the sedimentary facies of the palaeosols.
If supporters of flood geology do so for reasons of their faith or Biblical interpretation, then they are unlikely to be swayed by such data.  However, if they hold their position because they believe it is supported by the geological evidence, then they must take these data and their most likely interpretation seriously.
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References
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ETTENSOHN, F. R., DEVER, G. R., and GROW, J.  1988.  A paleosol interpretation for profiles exhibiting subaerial exposure �crusts� from the Mississippian of the Appalachian basin.  Geological Society of America Special Paper 216: 49-79.
GARDNER, T. W., WILLIAMS, E. G. and HOLBROOK, P. W.  1988.  Pedogenesis of some Pennsylvanian underclays; groundwater, topographic and tectonic controls. Geological Society of America Special Paper 216: 81-101.
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RETALLACK, G. J.  1983.  A paleopedological approach to the interpretation of terrestrial sedimentary rocks: the mid-Tertiary fossils soils of Badlands national Park, South Dakota.  Geological Society of America Bulletin 94: 823-840.
RETALLACK, G. J. 1990.  Soils of the past. Unwin Hyman, Boston, pp 213-215.
SCHULTZ, L. G.  1958.  Petrology of underclays.  Geological Society of America Bulletin 69: 363-402.
WRIGHT, V. P.  1982. Calcrete palaeosols from the Lower Carboniferous Llanelly Formation, South Wales.  Sedimentary Geology 33: 1-33