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Micromorphology of Lithified Paleosols
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MICROMORPHOLOGY OF LITHIFIED PALEOSOLS G.J. RETALLACK and V.P. WRIGHT Department of Geological S c i e n c e s , U n i v e r s i t y of Oregon, Eugene, Oregon, 97403, U.S.A. and P o s t g r a d u a t e Research I n s t i t u t e f o r Sedimentology, U n i v e r s i t y of Reading, Reading, RG6 2AB, England.
ABSTRACT L i t h i f i e d p a l e o s o l s show a v a r i e t y of microscopic f a b r i c s s i m i l a r t o t h o s e of t h e rocks t h a t e n c l o s e them. Sedimentary m i c r o f a b r i c s a r e common i n s o i l s and p a l e o s o l s t h a t were weakly developed o r waterlogged. Also p r e s e r v e d i n many p a l e o s o l s a r e s o i l m i c r o f a b r i c s d i s t i n c t from t h o s e of sedimentary r o c k s . I n c l a y e y p a l e o s o l s , a v a r i e t y of b r i g h t c l a y m i c r o f a b r i c s ( s e p i c p l a s m i c f a b r i c of R. Brewer) a r e found. C a v i t y - l i n i n g ( v o s e p i c ) and g r a i n - c o a t i n g ( s k e l s e p i c ) b r i g h t c l a y f a b r i c s may form d u r i n g s e d i m e n t a t i o n and b u r i a l , b u t o t h e r kinds of s e p i c plasmic f a b r i c s a r e t y p i c a l of w e l l d r a i n e d and developed s o i l s and p a l e o s o l s . D e s c r i p t i v e l y s i m i l a r m i c r o f a b r i c s a r e found i n p a l e o s o l s metamorphosed t o lower g r e e n s c h i s t f a c i e s . A t h i g h e r metamorphic g r a d e s , c l a y s a r e a l t e r e d t o micas and t a k e on a h i g h b i r e f r i n g e n c e (omnisepic). M i c r i t i c , n e e d l e f i b e r and d i s p l a c i v e f a b r i c s a r e p r e s e r v e d i n c a l c a r e o u s p a l e o s o l s . Their formation a s o r i g i n a l p a r t s of a s o i l a r e e s p e c i a l l y c l e a r when t h e y a r e a s s o c i a t e d w i t h burrows o r r o o t t r a c e s . Carbonate l a y e r s and nodules a r e l e s s compacted d u r i n g b u r i a l t h a n c l a y e y p a r t s of p a l e o s o l s , and s o p r e s e r v e m i c r o s t r u c t u r e s i n t h e i r o r i g i n a l t h r e e dimensional arrangement. R e c r y s t a l l i z a t i o n of l i m e s t o n e d u r i n g b u r i a l can o b l i t e r a t e s o i l s t r u c t u r e s . Such d e s t r u c t i o n i s n e a r t o t a l i n marble. S o i l micromorphological t e c h n i q u e s and terminology w e r e not designed f o r u s e w i t h p a l e o s o l s now e n t i r e l y l i t h i f i e d and metamorphosed, b u t can c l e a r l y be a p p l i e d t o such m a t e r i a l s a s o p e r a t i o n a l and d e s c r i p t i v e schemes. I n t e r p r e t i n g o r i g i n a l m i c r o s t r u c t u r e s and what t h e y mean f o r a n c i e n t s o i l f o r m a t i o n remains a s i g n i f i c a n t challenge f o r t h e future.
1. INTRODUCTION Micromorphological s t u d i e s have proven t h e i r worth a s a n a d j u n c t t o f i e l d , t e x t u r a l and chemical s t u d i e s of modern s o i l s (Bullock, 1983). For t h e s t u d y of b u r i e d s o i l s whose f i e l d c h a r a c t e r , t e x t u r e and chemical composition have been a l t e r e d by compaction, cementation and o t h e r d i a g e n e t i c changes, micromorpholo g i c a l s t u d i e s a r e even more v i t a l . For example, i t i s n o t p o s s i b l e t o d i s a g g r e g a t e e f f e c t i v e l y f o r g r a i n s i z e a n a l y s i s p a l e o s o l s t h a t have been t u r n e d i n t o rock by b u r i a l and metamorphism. The p r o p o r t i o n s of sand, s i l t and c l a y a r e b e s t determined from p e t r o g r a p h i c t h i n s e c t i o n s , u s i n g s t a n d a r d p o i n t c o u n t i n g techn i q u e s . Compaction and cementation of d e e p l y b u r i e d p a l e o s o l s a l s o a l t e r s t h e i r base s t a t u s ; a measure important t o s o i l c l a s s i f i c a t i o n and i n t e r p r e t a t i o n . N e v e r t h e l e s s , base s t a t u s can b e r e c o n s t r u c t e d w i t h i n broad c a t e g o r i e s by p o i n t
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counting for the abundance of easily weathered base-rich minerals such as calcite, biotite, pyroxene and plagioclase (Retallack, 1983). More important, microscopic features of paleosols, such as clay skins, allow interpretation of similar soil environments and processes as those in which these features form today (Molenaar, 1984; McSweeney and Fastovsky, 1987). A better understanding of the fossil record of soils (Retallack, 1986a), now known as far back as 3300 million years (Lowe, 1983), will depend increasingly on information on the micromorphology of modern soils. A key question for the application of micromorphological studies to paleosols now lithified and metamorphosed is to what extent are original microstructures preserved ? In our experience, descriptive schemes designed for soil micromorphology, such as those of Brewer (1976) and Bullock et al. (1984), can be applied readily to lithified paleosols. Accurate description and communication of thin section features are no longer serious problems for paleopedology, but their is some question whether described structures were formed originally in a soil, by processes that created the parent material of a paleosol or by its alteration during deep burial. In other words, we want to know what does paleosol micromorphology mean in terms of past environments and soil forming processes ?
2. CLAYEY PALEOSOLS Almost all the microfabrics described by Brewer (1976), and a few more besides, have now been found in paleosol claystones (Retallack, 1976,1981,1985, 1986b; Retallack and Dilcher, 1981; Molenaar, 1984; McSweeney and Fastovsky, 1987). Paleosols also show a great variety of igneous, metamorphic and sedimentary textures remaining from their parent materials (Gay and Grandstaff, 1980; Grandstaff et al., 1986; Retallack, 1985). Indeed, one of the uses of micromorphology in sequences of lithified paleosols is to distinguish the paleosols from their enclosing rocks. Paleosols differ from most igneous and metamorphic rocks in the presence of clays, in diffuse stains of iron oxides and in corroded or irregular grain boundaries. These features also are found in hydrothermally altered rocks, but these commonly form veins in igneous or volcanic rocks and include a variety of well-crystallized ore minerals, such as pyrrhotite and galena, that are unstable in most weathering regimes. Paleosols within sequences of sedimentary rocks can be difficult to recognize micromorphologically, especially in coal measures and in rapidly deposited coarse-grained sequences. Many fabrics of weakly developed clayey paleosols are similar to those of sediments unmodified by soil formation. Soils of modern peat swamps and paleosols of coal measures show abundant relict bedding (unistrial plasmic fabric of Brewer, 1976) and cloudy (inundulic) and dull (undulic) fabrics (Retallack and Dilcher, 1981;
643 McSweeney and Fastovsky, 1987). It i s f o r t u n a t e , t h e n , t h a t r o o t t r a c e s a r e so b e a u t i f u l l y p r e s e r v e d under s o i l c o n d i t i o n s of r a p i d b u r i a l o r w a t e r l o g g i n g , because f o s s i l r o o t s provide c l e a r evidence f o r p a l e o s o l s i n c o a l measures. In p e t r o g r a p h i c t h i n s e c t i o n s , c e l l u l a r s t r u c t u r e of p l a n t r o o t s sometimes i s v i s i b l e , e s p e c i a l l y i n p a l e o s o l s t h a t have been p e r m i n e r a l i z e d w i t h s i l i c a o r c a r b o n a t e (Kidston and Lang, 1921; Klappa, 1980; Basinger, 1981). In o t h e r c a s e s only t r a c e s remain of d e c a y - r e s i s t a n t p a r t s of r o o t s , such a s t h e epidermi s and c e n t r a l woody t i s s u e s . Other p a r t s of r o o t s commonly decay and a r e
r e p l a c e d by c l a y ( R e t a l l a c k , 1976). C e r t a i n k i n d s of b r i g h t c l a y ( s e p i c plasmic) f a b r i c a r e d i a g n o s t i c of p a l e o s o l s , a s opposed t o o t h e r k i n d s of r o c k s . This i s a g e n e r a l c l a s s of m i c r o s t r u c t u r e i n which s t r e a k s of h i g h l y o r i e n t e d , and t h u s h i g h l y b i r e f r i n g e n t c l a y s a r e arranged i n a m a t r i x of f l e c k e d , u n o r i e n t e d c l a y ( F i g . 1 ) . The abundance of b i r e f r i n g e n t c l a y i s thought t o i n c r e a s e w i t h p r o p o r t i o n of s w e l l i n g c l a y s ( s m e c t i t e s ) , w i t h p e r i o d i c i t y of w e t t i n g and d r y i n g and w i t h a g e , i n s o i l s t h a t a r e g e n e r a l l y w e l l d r a i n e d (Brewer and Sleeman, 1969). The t h r e e dimensional arrangement of o r i e n t e d c l a y i s u n l i k e l y t o be matched by d e p o s i t i o n a l o r b u r i a l p r o c e s s e s because i t c a l l s f o r a complex, o s c i l l a t i n g and m u l t i d i r e c t i o n a l p a t t e r n of s t r e s s e s . Not a l l s e p i c plasmic f a b r i c s a r e good
Fig. 1. I n t e r s e c t i n g b r i g h t c l a y (clinobimasepic plasmic) f a b r i c i n t h e c l a y e y s u b s u r f a c e (C) h o r i z o n of a p a l e o s o l ( S u l f a q u e p t ) i n t h e mid-Cretaceous (94Ma), upper Dakota Formation, n e a r F a i r b u r y , Nebraska, U . S . A . S c a l e b a r i s 0 . 1 mm.
644 i n d i c a t i o n s of pedogenic p r o c e s s e s . Grain-coating b r i g h t c l a y f a b r i c ( s k e l s e p i c ) can form by r o l l i n g of g r a i n s d u r i n g d e p o s i t i o n o r by compaction of c l a y e y m a t r i x around them d u r i n g b u r i a l , a s w e l l a s by s t r e s s e s g e n e r a t e d around g r a i n s d u r i n g s o i l formation (Fig. 2 ) . Another e x c e p t i o n i s v o i d - f i l l i n g b r i g h t c l a y f a b r i c (vosepic) i n which o r i e n t e d c l a y may f i l l c r a c k s t h a t formed i n t h e o r i g i n a l s o i l o r t h a t were opened by d i s s o l u t i o n a t d e p t h . Voids formed d u r i n g deep b u r i a l a r e c a l l e d secondary p o r o s i t y i n t h e terminology of t h e petroleum b u s i n e s s (Schmidt and McDonald, 1 9 7 9 ) . Voids and f i s s u r e s of secondary p o r o s i t y f i l l e d w i t h c l a y can i n some c a s e s appear s i m i l a r t o c l a y s k i n s and o t h e r c u t a n i c f e a t u r e s , b u t t h e y l a c k t h e l a t e r a l e x t e n t of banding p a t t e r n s s e e n even i n d i s r u p t e d fragments o f s o i l c l a y s k i n s . Thus most ( i n s e p i c , mosepic and masepic), b u t not a l l , k i n d s of b r i g h t c l a y f a b r i c may be d i a g n o s t i c of paleosols. Another important e x c e p t i o n t o t h e u s e of b r i g h t c l a y f a b r i c s a s d i a g n o s t i c of p a l e o s o l s i s t h e formation of woven b r i g h t c l a y f a b r i c (omnisepic) a t h i g h
grades of metamorphism ( g r e e n s c h i s t f a c i e s and beyond). Metamorphic r e a c t i o n s s e r v e t o a l t e r c l a y m i n e r a l s i n t o more h i g h l y b i r e f r i n g e n t micas. A t moderate d e p t h s of b u r i a l (only a km o r s o ) and t e m p e r a t u r e s ( a few hundred d e g r e e s C ) ,
Fig. 2 . Grain-coating b r i g h t c l a y ( s k e l s e p i c plasmic) f a b r i c around p a r t l y plucked c l a y c l a s t s i n t h e c l a y e y s u b s u r f a c e (C) h o r i z o n of t h e Ogi s i l t y c l a y loam p a l e o s o l (Fluvaquentic E u t r o c h r e p t ) i n t h e Oligocene (30 Ma) Brule Formati o n , Badlands N a t i o n a l Park, South Dakota, U.S.A. S c a l e b a r i s 0.1 mm.
smectite is converted to illite with addition of potash and losses of silica, lime and soda (Bethke and Altaner, 1986; and references therein). Paleosols that have suffered illitization at metamorphic grades as high as prehnite-pumpellyite and zeolite facies still show an array of bright clay fabrics similar to those of modern soils. It could be that illitization selectively affects already oriented clay along which fluids can migrate more readily. This idea, suggested by simple observation of paleosol micromorphology affected by moderate grades of metamorphism, needs experimental testing. Paleosols with pervasive metamorphic chlorite, biotite and muscovite formed at high temperatures and pressures (greenschist facies and beyond) have a uniformly high birefringence that is similar to woven bright clay fabric (omnisepic) in soils. Even in such altered paleosols, variations in brightness of micas are suggestive of original less pervasively bright clay fabrics (masepic to insepic), but these features are subtle and difficult to quantify (Retallack, 1986b). Comparisons with modern soils may be limited for such metamorphosed paleosols, but their microscopic study is no less important. 3 . CALCAREOUS PALEOSOLS
Lithified paleosols formed on limestone or by calcification of other sediments in climates with seasonal aridity also show a range of microstructures comparable to those of modern calcareous soils. Much of what is known about the micromorphology of calcareous paleosols has been reported by sedimentary petrographers under the term "vadose alteration fabrics," which have proven important for recognizing ancient exposure and sea level changes in the geological record (Estaban and Klappa, 1983). Another reason so much is known about them is their sheer abundance in the rock record (Allen, 1986; James and Choquette, 1988; Wright, this volume). Pedogenic carbonates have a high preservation potential in the rock record for two main reasons. First, the precipitation of carbonate results in early cementation and so resists later mechanical compaction on burial. The solubility of calcite during deep burial makes it susceptible to pressure solution, but the effects of this process can be easily recognized from stylolites and other features. Second, most pedogenic carbonates are either dolomite or low magnesium calcite (less than 4 mole %Mg).
Both forms are mineralogically
stable during diagenesis. However, other common forms of calcite, aragonite and high magnesium calcite (more than 4 mole % Mg) are readily converted to low magnesium calcite during diagenesis, typically with some loss of microfabric detail. Although these latter forms do occur in present day calcretes (Watts, 1980), they appear to be volumetrically minor. Two broad groupings of pedogenic carbonates can be defined, each with diff-
erent diagenetic susceptibilities. One group (beta calcretes of Wright, this
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volume) consists of biogenic soil carbonates such as rhizoconcretions and related features. Actual roots are relatively rarely permineralized to preserve ultrastructure, but more commonly occur as calcified tubes ( B o r n , 1982; Cohen, 1982). A common form recognized mainly in post-Jurassic calcretes is the struct-
ure known as Microcodium, which consists of small (under 2 mm) radial aggregates of petal-like calcite crystals. Klappa (1978) has compared this form, which is uncommon in Holocene soils, to fungal sclerotia, possibly endomycorrhizae. Its absence in paleosols older than Jurassic may reflect a geologically late appearance or its loss by diagenesis. Needle fiber calcite (lublinite) i s a major contributor to modern calcareous soils and has been recognized in paleosols as old as early Carboniferous (Wright, 1984, 1986). It forms in association with fungal mycelia, but has a relatively low preservation potential (Calvet and Julia, 1983) because the fibers are syntaxially overgrown with micrite-sized rhombs and the acicular form is destroyed (Phillips and Self, 1987). Certainly fossil examples of this structure typically show much alteration (Fig. 3 ) . Finally, many laminar calcretes are biological in origin, due to cyanobacteria (Krumbein and Giele, 1979) or lichens (Klappa, 1979), or to calcified root mats (Wright et al., 1988). The other suite of pedogenic carbonates (alpha calcretes of Wright, this
Fig. 3. Needle fiber calcite in a tubular void remaining from a root trace i n the Heatherslade Geosol of the Early Carboniferous (345 Ma) Gully Oolite at Three Cliffs Bay, South Wales. Scale bar is 50 um long.
647 volume) c o n s i s t s of a b i o g e n i c f a b r i c s , t y p i c a l l y r e s u l t i n g from t h e growth of c a r b o n a t e . C a l i c h e n o d u l e s , f o r example, commonly show r e p l a c i v e c r y s t i c f a b r i c s (Wright, 1982), i n which m i n e r a l g r a i n s a p p e a r e t c h e d ( c a r i e s t e x t u r e ) and f l o a t i n g i n a micritic t o m i c r o c r y s t a l l i n e matrix t h a t has replaced an earlier l e s s c a l c a r e o u s m a t r i x ( F i g . 4 ) . I n some cases, f o s s i l c a l i c h e n o d u l e s p r e s e r v e c o n c e n t r i c bands of ( s e a s o n a l ? ) f e r r u g i n i z a t i o n ( R e t a l l a c k , 1985). To t h i s l i s t of o r i g i n a l c a l i c h e m i c r o t e x t u r e s c a n b e added d i s p l a c i v e c a l c i t e t e x t u r e . T h i s h a s been c o n s i d e r e d c o n t r o v e r s i a l b e c a u s e t h e r e i s some doubt t h a t c r y s t a l l i z a t i o n p r e s s u r e can f o r c e a p a r t s o i l m a t r i x (Watts, 1979). However, d i s p l a c i v e f a b r i c s can form by p r e c i p i t a t i o n of c a l c i t e i n c r a c k s i n t h e s o i l . T h i s k i n d of d i s p l a c i v e f a b r i c i s u n l i k e l y t o form under l o a d of b u r i a l , b u t c o u l d form d u r i n g f o r c i b l e i n j e c t i o n s of f l u i d s o r magma, as i n t h e boxwork t e x t u r e s o f many h y d r o t h e r m a l o r e s (Edwards and Atkinson, 1986). In t h i s p a r t i c u l a r c a s e , a s s o c i a t e d v o l c a n i c r o c k s and o r e m i n e r a l s s h o u l d b e a c l u e t o t h e non-pedogenic n a t u r e of d i s p l a c i v e f a b r i c s . There i s more danger i n m i s t a k i n g f o s s i l c a l i c h e f o r a l g a l o r l a c u s t r i n e l i m e s t o n e , which i s what i t h a s been c a l l e d i n some g e o l o g i c a l r e p o r t s . I n t h i n s e c t i o n , t r u l y a q u a t i c carbonates are d i s t i n c t i v e
F i g . 4 . M i c r o c r y s t a l l i n e l i m e mud ( c r y s t i c f a b r i c ) w i t h i n c i p i e n t development of c a r i e s t e x t u r e ( r e p l a c i v e embayments) i n t o s u r p r i s i n g l y w e l l p r e s e r v e d v o l c a n i c s h a r d s , i n c a l i c h e nodule of a c a l c i c (Bk) h o r i z o n of t h e t y p e Samna s i l t y c l a y loam p a l e o s o l ( U s t o l l i c E u t r a n d e p t ) i n t h e Late Oligocene (29 Ma) Sharps Forma t i o n , Badlands N a t i o n a l P a r k , South Dakota, U.S.A. S c a l e b a r i s 0 . 1 mm.
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in their chambered or otherwise complex algal structures and their fossil ostracods, diatoms, clams and snails that may be recognizable even in small fragments (Scholle, 1978). Associated fossil root traces, much less weathered mafic minerals within (compared to beyond) the nodules and evidence of nodules resorted into associated paleochannel sandstones are just a few of the lines of evidence that can be used to identify caliche-bearing paleosols in rock sequences. Carbonate paleosols and parts of paleosols are of special value because they preserve soil textures in their original three dimensional arrangement (Fig. 5 ) , without the crushing that affects associated clayey soil material or sediments after burial (Wright, 1983, 1987). The carbonates themselves are subject to a variety of diagenetic alterations, but only dolomitization and recrystallization will be discussed here. Dolostone, a carbonate rock of the mineral dolomite, is thought to form in modern hypersaline lakes, coastal lagoons and oceans (Zenger et al., 1980) as well as in soils of high base status (Doner and Lynn, 1977). In most cases, originally formed dolomite is micritic or (rarely) microacicular, unlike sugary dolomite of sand-to-silt-sized rhombs that form during burial alteration of non-dolomitic carbonates. Distinguishing between original and burial dolomite is a thorny problem for the history of soils. It remains to be seen whether the abundant dolomitic caliche of Precambrian and Paleozoic paleo-
Fig. 5. Uncrushed marine foraminifer (chambered), brachiopod (curved shell) and fecal pellets (dark) in the surface (A) horizon of the Darrenfelen Geosol of the Early Carboniferous (350 Ma) Cheltenham Limestone, near Llanelly, South Wales. Scale bar is 1 mm.
649 s o l s is a result of their burial alteration, or a signal of changing conditions
of soil formation through geological time (Retallack, 1986a). Carbonates are prone to recrystallization (neomorphism o f F o l k , 1965) at only moderate depths and temperatures of burial. Much of this alteration is related to the inversion of aragonite and high magnesium calcite to low magnesium calcite. However, low magnesium calcite can also recrystallize from very fine grained calcite (micrite) to coarser crystals (more than 4 um, but typically 10-15 um) called microspars. Further burial and metamorphism result in the equigranular mosaic found in marble. The literature on modern and ancient calcretes contains many references to recrystallization because of the presence of calcite crystals in irregular mosaics with irregular crystal boundaries (both characteristic features of recrystallized micrite). However, such fabrics can result from complex series of precipitation and dissolution phases during calcrete growth. Studies of some mosaics using cathodoluminescence show that they are primary and not due to recrystallization (see Wright, this volume).
Many Precambrian and
Paleozoic calcrete nodules show apparent recrystallization features (Fig. 6) but detailed cathodoluminescence work is required to identify the degree of burial related alteration.
Fig. 6 . Recrystallized caliche nodule (upper left) with near-marginal iron stain (neoferran) from the calcic (Bk) horizon of the type Lehigh Gap clay paleosol (Oxic Ustropept) in the Late Silurian ( 4 2 0 Ma) Bloomsburg Formation, near Palmerton, Pennsylvania, U . S . A . Scale bar is 1 mm.
650 4. CONCLUSIONS The range of m i c r o f a b r i c s now known from l i t h i f i e d p a l e o s o l s a l r e a d y approaches t h a t of modern s o i l s , d e s p i t e t h e s m a l l number of s c i e n t i s t s t h a t have f o r m a l l y r e p o r t e d t h e i r s t u d y . It i s c l e a r t h a t d e s c r i p t i v e c l a s s i f i c a t i o n s of s o i l micromorphology (such as t h o s e of Brewer, 1976 and Bullock e t a l . , 1984) can be used f o r l i t h i f i e d p a l e o s o l s a s w e l l a s modern s o i l s . How t h e s e v a r i o u s f e a t u r e s a r e t o b e i n t e r p r e t e d i s a n o t h e r m a t t e r t h a t i s f a r from s o l v e d . A p a r t of t h e problem i s a l t e r a t i o n of p a l e o s o l s d u r i n g deep b u r i a l and metamorphism. Some i n f o r m a t i o n on t h e s e changes can be gleaned from t h e e x t e n s i v e l i t e r a t u r e on sediment d i a g e n e s i s and metamorphism. How, o r i f , t h e s e changes on b u r i a l s e l e c t i v e l y a f f e c t p a r t i c u l a r p a r t s of p a l e o s o l s remains t o be s t u d i e d
s e r i o u s l y . Once t h e s e a l t e r a t i o n s t o micromorphology can be f i l t e r e d o u t , t h e remaining f e a t u r e s can be used t o assess how s i m i l a r t o modern s o i l - f o r m i n g p r o c e s s e s were t h o s e of t h e d i s t a n t g e o l o g i c a l p a s t . Micromorphology i s , and i s l i k e l y t o remain, of c r i t i c a l importance t o a s s e s s i n g t h e f o s s i l r e c o r d of soils. 5. ACKNOWLEDGEMENTS G . J . R . thanks paleopedology s t u d e n t s C.R. Feakes, G.S. Smith, J. P r a t t , and G. Thackray f o r t h e i r i l l u m i n a t i n g q u e s t i o n s about p r o b l e m a t i c t h i n s e c t i o n s ,
and t e c h n i c i a n s C a t h e r i n e C l i f t o n and Sandra Newman f o r producing thousands of h i g h q u a l i t y t h i n s e c t i o n s from u n p r e d i c t a b l e s m e c t i t i c c l a y s t o n e s . Much of t h e r e s e a r c h of G.J.R. h a s been funded by t h e U.S. N a t i o n a l Science Foundation, p a r t i c u l a r l y g r a n t EAR 8503232. Research of V.P.W. h a s been s u p p o r t e d by t h e N u f f i e l d Foundation, U.K. This is Reading U n i v e r s i t y , P.R.I.S. C o n t r i b u t i o n 011. 6. REFERENCES Allen, J.R.L., 1986. Pedogenic c a l c r e t e s i n t h e Old Red Sandstone f a c i e s (Late S i l u r i a n - E a r l y Carboniferous) of t h e Anglo-Welsh a r e a , s o u t h e r n B r i t a i n . In: V.P. Wright ( E d i t o r ) , P a l e o s o l s : T h e i r Recognition and I n t e r p r e t a t i o n . Blackw e l l s , Oxford: 58-86. Basinger, J.F., 1981. The v e g e t a t i v e body of Metasequoia rnilleri from t h e Middle Eocene of s o u t h e r n B r i t i s h Columbia. Can. J . Bot., 59: 2379-2410. Bethke, C.M. and A l t a n e r , S.P., 1986. Layer-by-layer mechanism of s m e c t i t e illit i z a t i o n and a p p l i c a t i o n of a new rate law. Clays/Clay M i n e r a l s 34:136-142. Bown, T.M., 1982. I c h n o f o s s i l s and r h i z o l i t h s of t h e n e a r s h o r e f l u v i a l J e b e l Q a t r a n i Formation (Oligocene), Fayum P r o v i n c e , Egypt. Palaeogeogr. Palaeoc l i m . Palaeoec., 40: 255-309. Brewer, R., 1976. F a b r i c and Mineral A n a l y s i s of S o i l s . K r i e g e r , New York: 482 PP. Brewer, R. and Sleeman, J . R . , 1969. The arrangement of c o n s t i t u e n t s i n Quaternn a r y s o i l s . S o i l S c i . , 107: 435-441. Bullock, P . , 1983. The changing f a c e of s o i l micromorphology. In: P. Bullock and C.P. Murphy ( E d i t o r s ) , S o i l Micromorphology, v o l . 1, Techniques and A p p l i c a t i o n s . A.B. Academic, Berkhamsted: 1-18. Bullock, P . , F e d e r o f f , N . , J u n g e r i u s , A., Stoops, G., and T u r s i n a , T . , 1984. Handbook f o r S o i l Thin S e c t i o n D e s c r i p t i o n . Waine Research, Wolverhampton: 152 pp. C a l v e t , F. and J u l i a , R . , 1983. P i s o i d s i n c a l i c h e p r o f i l e s of Tarragona (N.E. S p a i n ) . I n : T.M. P e r y t ( E d i t o r ) , Coated Grains. S p r i n g e r , B e r l i n : 456-473.
65I Cohen, A.S., 1982. Paleoenvironments of root casts from the Koobi Fora Formation, Kenya. J. Sedim. Petrol., 52: 401-414. Doner, H.E. and Lynn, W.C., 1977. Carbonate, halide, sulfate and sulfide minerals. In: J.B. Dixon and S.B. Weed (Editors), Minerals in Soil Environments. soil Science Society of America, Madison: 75-98. Edwards, R. and Atkinson, K., 1986. Ore Deposit Geology. Chapman and Hall, London: 466pp. Esteban, M. and Klappa, C.F., 1983. Subaerial exposure environment. In: P.A. Scholle, D.G. Bebout and C.H. Moore (Editors), Carbonate Depositional Environments. Mem. Am. Assoc. Petrol. Geol., 33: 1-54. F o l k , R.L., 1965. Some aspects of recrystallization in ancient limestones. In: L.C. Pray and R.C. Murray (Editors), Dolomitization and Limestone Diagenesis. Spec. Publ. SOC. Econ. Paleont. Mineral., 13: 14-48. Gay, A.L. and Grandstaff, D.E., 1980. Chemistry and mineralogy of Precambrian paleosols at Elliot Lake, Ontario, Canada. Precambrian Res., 12: 340-373. Grandstaff, D.E., Edelman, M.J., Foster, R.W., Zbinden, E. and Kimberley, M.M., 1986. Chemistry and mineralogy of Precambrian paleosols at the base of the Dominion and Pongola Groups. Precambrian Res., 32: 97-131. James, N.P. and Choquette, P.W. (Editors), 1988. Paleokarst. Springer-Verlag, New York: 416 pp. Kidston, R. and Lang, W.H., 1921. On Old Red Sandstone plants showing structure from the Rhynie chert bed, Aberdeenshire. Part V. The Thallophyta occurring in the peat bed, the succession of plants through a vertical section of the bed and the conditions of accumulation and preservation of the deposit. Trans. R. SOC. Edinburgh, 52: 855-902. Klappa, C.F., 1978. Biolithogenesis of M
652 V a l Onder, South A f r i c a . Precambrian Res., 32: 195-252. R e t a l l a c k , G . J . and D i l c h e r , D.L., 1981. E a r l y angiosperm r e p r o d u c t i o n : Prisca r e y n o k i s i i gen. e t s p . nov. from mid-Cretaceous c o a s t a l d e p o s i t s i n Kansas, U.S.A. P a l a e o n t o g r a p h i c a B179: 103-137. S c h o l l e , P.A., 1978. A c o l o r i l l u s t r a t e d g u i d e t o c a r b o n a t e r o c k c o n s t i t u e n t s , t e x t u r e s , cements and p o r o s i t i e s . Mem. Am. Assoc. P e t r o l . G e o l . , 27: 241 pp. Schmidt, V. and McDonald, D . A . , 1979. The r o l e of s e c o n d a r y p o r o s i t y i n t h e c o u r s e of s a n d s t o n e d i a g e n e s i s . I n : P.A. S c h o l l e and P.R. S c h l u g e r ( E d i t o r s ) , Aspects of D i a g e n e s i s . Spec. P u b l . SOC. Econ. M i n e r a l . P a l e o n t . M i n e r a l . , 26: 175-207. Watts, N.L., 1979. D i s p l a c i v e c a l c i t e : e v i d e n c e from r e c e n t and a n c i e n t c a l c r e t e s : r e p l y . Geology, 7 : 421-422. Watts, N.L., 1980. Q u a t e r n a r y p e d o g e n i c c a l c r e t e s from t h e K a l a h a r i (South A f r i c a ) : m i n e r a l o g y , g e n e s i s and d i a g e n e s i s . Sedimentology, 27: 661-686. Wright, V.P., 1982. Calcrete p a l e o s o l s from t h e Lower c a r b o n i f e r o u s L l a n e l l y Formation, South Wales. S e d i m m t . G e o l . , 33: 1-33. 1983. A r e n d z i n a from t h e Lower C a r b o n i f e r o u s o f S o u t h Wales. Wright, V.P., Sedimentology, 30: 159-179. Wright, V.P., 1984. The s i g n i f i c a n c e o f n e e d l e - f i b r e c a l c i t e i n a Lower Carboni f e r o u s p a l a e o s o l . Geol. J . , 19: 23-32. Wright, V.P., 1986. The r o l e of f u n g a l b i o m i n e r a l i z a t i o n i n t h e f o r m a t i o n o f e a r l y C a r b o n i f e r o u s s o i l f a b r i c s . Sedimentology, 33: 831-838. Wright, V.P., 1987. The e c o l o g y of two e a r l y C a r b o n i f e r o u s p a l a e o s o l s . I n : J. Miller, A.E. Adams and V.P. Wright ( E d i t o r s ) , European D i n a n t i a n Environments. Spec. Iss. Geol. J . , 12: 345-358. Wright, V.P., P l a t t , N.H. and Wimbledon, W.A., 1988. B i o g e n i c l a m i n a r c a l c r e t e s : e v i d e n c e of r o o t m a t s i n p a l e o s o l s . Sedimentology 35: 603-620. Zenger, D.H., Dunham, J . B . and E t h i n g t o n , R.L. ( E d i t o r s ) , 1980. Concepts and Models o f D o l o m i t i z a t i o n . Spec. P u b l . SOC. Econ. P a l e o n t . M i n e r a l . , 28: 320 PP
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MICROMORPHOLOGY OF LITHIFIED PALEOSOLS G.J. RETALLACK and V.P. WRIGHT Department of Geological S c i e n c e s , U n i v e r s i t y of Oregon, Eugene, Oregon, 97403, U.S.A. and P o s t g r a d u a t e Research I n s t i t u t e f o r Sedimentology, U n i v e r s i t y of Reading, Reading, RG6 2AB, England.
ABSTRACT L i t h i f i e d p a l e o s o l s show a v a r i e t y of microscopic f a b r i c s s i m i l a r t o t h o s e of t h e rocks t h a t e n c l o s e them. Sedimentary m i c r o f a b r i c s a r e common i n s o i l s and p a l e o s o l s t h a t were weakly developed o r waterlogged. Also p r e s e r v e d i n many p a l e o s o l s a r e s o i l m i c r o f a b r i c s d i s t i n c t from t h o s e of sedimentary r o c k s . I n c l a y e y p a l e o s o l s , a v a r i e t y of b r i g h t c l a y m i c r o f a b r i c s ( s e p i c p l a s m i c f a b r i c of R. Brewer) a r e found. C a v i t y - l i n i n g ( v o s e p i c ) and g r a i n - c o a t i n g ( s k e l s e p i c ) b r i g h t c l a y f a b r i c s may form d u r i n g s e d i m e n t a t i o n and b u r i a l , b u t o t h e r kinds of s e p i c plasmic f a b r i c s a r e t y p i c a l of w e l l d r a i n e d and developed s o i l s and p a l e o s o l s . D e s c r i p t i v e l y s i m i l a r m i c r o f a b r i c s a r e found i n p a l e o s o l s metamorphosed t o lower g r e e n s c h i s t f a c i e s . A t h i g h e r metamorphic g r a d e s , c l a y s a r e a l t e r e d t o micas and t a k e on a h i g h b i r e f r i n g e n c e (omnisepic). M i c r i t i c , n e e d l e f i b e r and d i s p l a c i v e f a b r i c s a r e p r e s e r v e d i n c a l c a r e o u s p a l e o s o l s . Their formation a s o r i g i n a l p a r t s of a s o i l a r e e s p e c i a l l y c l e a r when t h e y a r e a s s o c i a t e d w i t h burrows o r r o o t t r a c e s . Carbonate l a y e r s and nodules a r e l e s s compacted d u r i n g b u r i a l t h a n c l a y e y p a r t s of p a l e o s o l s , and s o p r e s e r v e m i c r o s t r u c t u r e s i n t h e i r o r i g i n a l t h r e e dimensional arrangement. R e c r y s t a l l i z a t i o n of l i m e s t o n e d u r i n g b u r i a l can o b l i t e r a t e s o i l s t r u c t u r e s . Such d e s t r u c t i o n i s n e a r t o t a l i n marble. S o i l micromorphological t e c h n i q u e s and terminology w e r e not designed f o r u s e w i t h p a l e o s o l s now e n t i r e l y l i t h i f i e d and metamorphosed, b u t can c l e a r l y be a p p l i e d t o such m a t e r i a l s a s o p e r a t i o n a l and d e s c r i p t i v e schemes. I n t e r p r e t i n g o r i g i n a l m i c r o s t r u c t u r e s and what t h e y mean f o r a n c i e n t s o i l f o r m a t i o n remains a s i g n i f i c a n t challenge f o r t h e future.
1. INTRODUCTION Micromorphological s t u d i e s have proven t h e i r worth a s a n a d j u n c t t o f i e l d , t e x t u r a l and chemical s t u d i e s of modern s o i l s (Bullock, 1983). For t h e s t u d y of b u r i e d s o i l s whose f i e l d c h a r a c t e r , t e x t u r e and chemical composition have been a l t e r e d by compaction, cementation and o t h e r d i a g e n e t i c changes, micromorpholo g i c a l s t u d i e s a r e even more v i t a l . For example, i t i s n o t p o s s i b l e t o d i s a g g r e g a t e e f f e c t i v e l y f o r g r a i n s i z e a n a l y s i s p a l e o s o l s t h a t have been t u r n e d i n t o rock by b u r i a l and metamorphism. The p r o p o r t i o n s of sand, s i l t and c l a y a r e b e s t determined from p e t r o g r a p h i c t h i n s e c t i o n s , u s i n g s t a n d a r d p o i n t c o u n t i n g techn i q u e s . Compaction and cementation of d e e p l y b u r i e d p a l e o s o l s a l s o a l t e r s t h e i r base s t a t u s ; a measure important t o s o i l c l a s s i f i c a t i o n and i n t e r p r e t a t i o n . N e v e r t h e l e s s , base s t a t u s can b e r e c o n s t r u c t e d w i t h i n broad c a t e g o r i e s by p o i n t
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counting for the abundance of easily weathered base-rich minerals such as calcite, biotite, pyroxene and plagioclase (Retallack, 1983). More important, microscopic features of paleosols, such as clay skins, allow interpretation of similar soil environments and processes as those in which these features form today (Molenaar, 1984; McSweeney and Fastovsky, 1987). A better understanding of the fossil record of soils (Retallack, 1986a), now known as far back as 3300 million years (Lowe, 1983), will depend increasingly on information on the micromorphology of modern soils. A key question for the application of micromorphological studies to paleosols now lithified and metamorphosed is to what extent are original microstructures preserved ? In our experience, descriptive schemes designed for soil micromorphology, such as those of Brewer (1976) and Bullock et al. (1984), can be applied readily to lithified paleosols. Accurate description and communication of thin section features are no longer serious problems for paleopedology, but their is some question whether described structures were formed originally in a soil, by processes that created the parent material of a paleosol or by its alteration during deep burial. In other words, we want to know what does paleosol micromorphology mean in terms of past environments and soil forming processes ?
2. CLAYEY PALEOSOLS Almost all the microfabrics described by Brewer (1976), and a few more besides, have now been found in paleosol claystones (Retallack, 1976,1981,1985, 1986b; Retallack and Dilcher, 1981; Molenaar, 1984; McSweeney and Fastovsky, 1987). Paleosols also show a great variety of igneous, metamorphic and sedimentary textures remaining from their parent materials (Gay and Grandstaff, 1980; Grandstaff et al., 1986; Retallack, 1985). Indeed, one of the uses of micromorphology in sequences of lithified paleosols is to distinguish the paleosols from their enclosing rocks. Paleosols differ from most igneous and metamorphic rocks in the presence of clays, in diffuse stains of iron oxides and in corroded or irregular grain boundaries. These features also are found in hydrothermally altered rocks, but these commonly form veins in igneous or volcanic rocks and include a variety of well-crystallized ore minerals, such as pyrrhotite and galena, that are unstable in most weathering regimes. Paleosols within sequences of sedimentary rocks can be difficult to recognize micromorphologically, especially in coal measures and in rapidly deposited coarse-grained sequences. Many fabrics of weakly developed clayey paleosols are similar to those of sediments unmodified by soil formation. Soils of modern peat swamps and paleosols of coal measures show abundant relict bedding (unistrial plasmic fabric of Brewer, 1976) and cloudy (inundulic) and dull (undulic) fabrics (Retallack and Dilcher, 1981;
643 McSweeney and Fastovsky, 1987). It i s f o r t u n a t e , t h e n , t h a t r o o t t r a c e s a r e so b e a u t i f u l l y p r e s e r v e d under s o i l c o n d i t i o n s of r a p i d b u r i a l o r w a t e r l o g g i n g , because f o s s i l r o o t s provide c l e a r evidence f o r p a l e o s o l s i n c o a l measures. In p e t r o g r a p h i c t h i n s e c t i o n s , c e l l u l a r s t r u c t u r e of p l a n t r o o t s sometimes i s v i s i b l e , e s p e c i a l l y i n p a l e o s o l s t h a t have been p e r m i n e r a l i z e d w i t h s i l i c a o r c a r b o n a t e (Kidston and Lang, 1921; Klappa, 1980; Basinger, 1981). In o t h e r c a s e s only t r a c e s remain of d e c a y - r e s i s t a n t p a r t s of r o o t s , such a s t h e epidermi s and c e n t r a l woody t i s s u e s . Other p a r t s of r o o t s commonly decay and a r e
r e p l a c e d by c l a y ( R e t a l l a c k , 1976). C e r t a i n k i n d s of b r i g h t c l a y ( s e p i c plasmic) f a b r i c a r e d i a g n o s t i c of p a l e o s o l s , a s opposed t o o t h e r k i n d s of r o c k s . This i s a g e n e r a l c l a s s of m i c r o s t r u c t u r e i n which s t r e a k s of h i g h l y o r i e n t e d , and t h u s h i g h l y b i r e f r i n g e n t c l a y s a r e arranged i n a m a t r i x of f l e c k e d , u n o r i e n t e d c l a y ( F i g . 1 ) . The abundance of b i r e f r i n g e n t c l a y i s thought t o i n c r e a s e w i t h p r o p o r t i o n of s w e l l i n g c l a y s ( s m e c t i t e s ) , w i t h p e r i o d i c i t y of w e t t i n g and d r y i n g and w i t h a g e , i n s o i l s t h a t a r e g e n e r a l l y w e l l d r a i n e d (Brewer and Sleeman, 1969). The t h r e e dimensional arrangement of o r i e n t e d c l a y i s u n l i k e l y t o be matched by d e p o s i t i o n a l o r b u r i a l p r o c e s s e s because i t c a l l s f o r a complex, o s c i l l a t i n g and m u l t i d i r e c t i o n a l p a t t e r n of s t r e s s e s . Not a l l s e p i c plasmic f a b r i c s a r e good
Fig. 1. I n t e r s e c t i n g b r i g h t c l a y (clinobimasepic plasmic) f a b r i c i n t h e c l a y e y s u b s u r f a c e (C) h o r i z o n of a p a l e o s o l ( S u l f a q u e p t ) i n t h e mid-Cretaceous (94Ma), upper Dakota Formation, n e a r F a i r b u r y , Nebraska, U . S . A . S c a l e b a r i s 0 . 1 mm.
644 i n d i c a t i o n s of pedogenic p r o c e s s e s . Grain-coating b r i g h t c l a y f a b r i c ( s k e l s e p i c ) can form by r o l l i n g of g r a i n s d u r i n g d e p o s i t i o n o r by compaction of c l a y e y m a t r i x around them d u r i n g b u r i a l , a s w e l l a s by s t r e s s e s g e n e r a t e d around g r a i n s d u r i n g s o i l formation (Fig. 2 ) . Another e x c e p t i o n i s v o i d - f i l l i n g b r i g h t c l a y f a b r i c (vosepic) i n which o r i e n t e d c l a y may f i l l c r a c k s t h a t formed i n t h e o r i g i n a l s o i l o r t h a t were opened by d i s s o l u t i o n a t d e p t h . Voids formed d u r i n g deep b u r i a l a r e c a l l e d secondary p o r o s i t y i n t h e terminology of t h e petroleum b u s i n e s s (Schmidt and McDonald, 1 9 7 9 ) . Voids and f i s s u r e s of secondary p o r o s i t y f i l l e d w i t h c l a y can i n some c a s e s appear s i m i l a r t o c l a y s k i n s and o t h e r c u t a n i c f e a t u r e s , b u t t h e y l a c k t h e l a t e r a l e x t e n t of banding p a t t e r n s s e e n even i n d i s r u p t e d fragments o f s o i l c l a y s k i n s . Thus most ( i n s e p i c , mosepic and masepic), b u t not a l l , k i n d s of b r i g h t c l a y f a b r i c may be d i a g n o s t i c of paleosols. Another important e x c e p t i o n t o t h e u s e of b r i g h t c l a y f a b r i c s a s d i a g n o s t i c of p a l e o s o l s i s t h e formation of woven b r i g h t c l a y f a b r i c (omnisepic) a t h i g h
grades of metamorphism ( g r e e n s c h i s t f a c i e s and beyond). Metamorphic r e a c t i o n s s e r v e t o a l t e r c l a y m i n e r a l s i n t o more h i g h l y b i r e f r i n g e n t micas. A t moderate d e p t h s of b u r i a l (only a km o r s o ) and t e m p e r a t u r e s ( a few hundred d e g r e e s C ) ,
Fig. 2 . Grain-coating b r i g h t c l a y ( s k e l s e p i c plasmic) f a b r i c around p a r t l y plucked c l a y c l a s t s i n t h e c l a y e y s u b s u r f a c e (C) h o r i z o n of t h e Ogi s i l t y c l a y loam p a l e o s o l (Fluvaquentic E u t r o c h r e p t ) i n t h e Oligocene (30 Ma) Brule Formati o n , Badlands N a t i o n a l Park, South Dakota, U.S.A. S c a l e b a r i s 0.1 mm.
smectite is converted to illite with addition of potash and losses of silica, lime and soda (Bethke and Altaner, 1986; and references therein). Paleosols that have suffered illitization at metamorphic grades as high as prehnite-pumpellyite and zeolite facies still show an array of bright clay fabrics similar to those of modern soils. It could be that illitization selectively affects already oriented clay along which fluids can migrate more readily. This idea, suggested by simple observation of paleosol micromorphology affected by moderate grades of metamorphism, needs experimental testing. Paleosols with pervasive metamorphic chlorite, biotite and muscovite formed at high temperatures and pressures (greenschist facies and beyond) have a uniformly high birefringence that is similar to woven bright clay fabric (omnisepic) in soils. Even in such altered paleosols, variations in brightness of micas are suggestive of original less pervasively bright clay fabrics (masepic to insepic), but these features are subtle and difficult to quantify (Retallack, 1986b). Comparisons with modern soils may be limited for such metamorphosed paleosols, but their microscopic study is no less important. 3 . CALCAREOUS PALEOSOLS
Lithified paleosols formed on limestone or by calcification of other sediments in climates with seasonal aridity also show a range of microstructures comparable to those of modern calcareous soils. Much of what is known about the micromorphology of calcareous paleosols has been reported by sedimentary petrographers under the term "vadose alteration fabrics," which have proven important for recognizing ancient exposure and sea level changes in the geological record (Estaban and Klappa, 1983). Another reason so much is known about them is their sheer abundance in the rock record (Allen, 1986; James and Choquette, 1988; Wright, this volume). Pedogenic carbonates have a high preservation potential in the rock record for two main reasons. First, the precipitation of carbonate results in early cementation and so resists later mechanical compaction on burial. The solubility of calcite during deep burial makes it susceptible to pressure solution, but the effects of this process can be easily recognized from stylolites and other features. Second, most pedogenic carbonates are either dolomite or low magnesium calcite (less than 4 mole %Mg).
Both forms are mineralogically
stable during diagenesis. However, other common forms of calcite, aragonite and high magnesium calcite (more than 4 mole % Mg) are readily converted to low magnesium calcite during diagenesis, typically with some loss of microfabric detail. Although these latter forms do occur in present day calcretes (Watts, 1980), they appear to be volumetrically minor. Two broad groupings of pedogenic carbonates can be defined, each with diff-
erent diagenetic susceptibilities. One group (beta calcretes of Wright, this
646
volume) consists of biogenic soil carbonates such as rhizoconcretions and related features. Actual roots are relatively rarely permineralized to preserve ultrastructure, but more commonly occur as calcified tubes ( B o r n , 1982; Cohen, 1982). A common form recognized mainly in post-Jurassic calcretes is the struct-
ure known as Microcodium, which consists of small (under 2 mm) radial aggregates of petal-like calcite crystals. Klappa (1978) has compared this form, which is uncommon in Holocene soils, to fungal sclerotia, possibly endomycorrhizae. Its absence in paleosols older than Jurassic may reflect a geologically late appearance or its loss by diagenesis. Needle fiber calcite (lublinite) i s a major contributor to modern calcareous soils and has been recognized in paleosols as old as early Carboniferous (Wright, 1984, 1986). It forms in association with fungal mycelia, but has a relatively low preservation potential (Calvet and Julia, 1983) because the fibers are syntaxially overgrown with micrite-sized rhombs and the acicular form is destroyed (Phillips and Self, 1987). Certainly fossil examples of this structure typically show much alteration (Fig. 3 ) . Finally, many laminar calcretes are biological in origin, due to cyanobacteria (Krumbein and Giele, 1979) or lichens (Klappa, 1979), or to calcified root mats (Wright et al., 1988). The other suite of pedogenic carbonates (alpha calcretes of Wright, this
Fig. 3. Needle fiber calcite in a tubular void remaining from a root trace i n the Heatherslade Geosol of the Early Carboniferous (345 Ma) Gully Oolite at Three Cliffs Bay, South Wales. Scale bar is 50 um long.
647 volume) c o n s i s t s of a b i o g e n i c f a b r i c s , t y p i c a l l y r e s u l t i n g from t h e growth of c a r b o n a t e . C a l i c h e n o d u l e s , f o r example, commonly show r e p l a c i v e c r y s t i c f a b r i c s (Wright, 1982), i n which m i n e r a l g r a i n s a p p e a r e t c h e d ( c a r i e s t e x t u r e ) and f l o a t i n g i n a micritic t o m i c r o c r y s t a l l i n e matrix t h a t has replaced an earlier l e s s c a l c a r e o u s m a t r i x ( F i g . 4 ) . I n some cases, f o s s i l c a l i c h e n o d u l e s p r e s e r v e c o n c e n t r i c bands of ( s e a s o n a l ? ) f e r r u g i n i z a t i o n ( R e t a l l a c k , 1985). To t h i s l i s t of o r i g i n a l c a l i c h e m i c r o t e x t u r e s c a n b e added d i s p l a c i v e c a l c i t e t e x t u r e . T h i s h a s been c o n s i d e r e d c o n t r o v e r s i a l b e c a u s e t h e r e i s some doubt t h a t c r y s t a l l i z a t i o n p r e s s u r e can f o r c e a p a r t s o i l m a t r i x (Watts, 1979). However, d i s p l a c i v e f a b r i c s can form by p r e c i p i t a t i o n of c a l c i t e i n c r a c k s i n t h e s o i l . T h i s k i n d of d i s p l a c i v e f a b r i c i s u n l i k e l y t o form under l o a d of b u r i a l , b u t c o u l d form d u r i n g f o r c i b l e i n j e c t i o n s of f l u i d s o r magma, as i n t h e boxwork t e x t u r e s o f many h y d r o t h e r m a l o r e s (Edwards and Atkinson, 1986). In t h i s p a r t i c u l a r c a s e , a s s o c i a t e d v o l c a n i c r o c k s and o r e m i n e r a l s s h o u l d b e a c l u e t o t h e non-pedogenic n a t u r e of d i s p l a c i v e f a b r i c s . There i s more danger i n m i s t a k i n g f o s s i l c a l i c h e f o r a l g a l o r l a c u s t r i n e l i m e s t o n e , which i s what i t h a s been c a l l e d i n some g e o l o g i c a l r e p o r t s . I n t h i n s e c t i o n , t r u l y a q u a t i c carbonates are d i s t i n c t i v e
F i g . 4 . M i c r o c r y s t a l l i n e l i m e mud ( c r y s t i c f a b r i c ) w i t h i n c i p i e n t development of c a r i e s t e x t u r e ( r e p l a c i v e embayments) i n t o s u r p r i s i n g l y w e l l p r e s e r v e d v o l c a n i c s h a r d s , i n c a l i c h e nodule of a c a l c i c (Bk) h o r i z o n of t h e t y p e Samna s i l t y c l a y loam p a l e o s o l ( U s t o l l i c E u t r a n d e p t ) i n t h e Late Oligocene (29 Ma) Sharps Forma t i o n , Badlands N a t i o n a l P a r k , South Dakota, U.S.A. S c a l e b a r i s 0 . 1 mm.
648
in their chambered or otherwise complex algal structures and their fossil ostracods, diatoms, clams and snails that may be recognizable even in small fragments (Scholle, 1978). Associated fossil root traces, much less weathered mafic minerals within (compared to beyond) the nodules and evidence of nodules resorted into associated paleochannel sandstones are just a few of the lines of evidence that can be used to identify caliche-bearing paleosols in rock sequences. Carbonate paleosols and parts of paleosols are of special value because they preserve soil textures in their original three dimensional arrangement (Fig. 5 ) , without the crushing that affects associated clayey soil material or sediments after burial (Wright, 1983, 1987). The carbonates themselves are subject to a variety of diagenetic alterations, but only dolomitization and recrystallization will be discussed here. Dolostone, a carbonate rock of the mineral dolomite, is thought to form in modern hypersaline lakes, coastal lagoons and oceans (Zenger et al., 1980) as well as in soils of high base status (Doner and Lynn, 1977). In most cases, originally formed dolomite is micritic or (rarely) microacicular, unlike sugary dolomite of sand-to-silt-sized rhombs that form during burial alteration of non-dolomitic carbonates. Distinguishing between original and burial dolomite is a thorny problem for the history of soils. It remains to be seen whether the abundant dolomitic caliche of Precambrian and Paleozoic paleo-
Fig. 5. Uncrushed marine foraminifer (chambered), brachiopod (curved shell) and fecal pellets (dark) in the surface (A) horizon of the Darrenfelen Geosol of the Early Carboniferous (350 Ma) Cheltenham Limestone, near Llanelly, South Wales. Scale bar is 1 mm.
649 s o l s is a result of their burial alteration, or a signal of changing conditions
of soil formation through geological time (Retallack, 1986a). Carbonates are prone to recrystallization (neomorphism o f F o l k , 1965) at only moderate depths and temperatures of burial. Much of this alteration is related to the inversion of aragonite and high magnesium calcite to low magnesium calcite. However, low magnesium calcite can also recrystallize from very fine grained calcite (micrite) to coarser crystals (more than 4 um, but typically 10-15 um) called microspars. Further burial and metamorphism result in the equigranular mosaic found in marble. The literature on modern and ancient calcretes contains many references to recrystallization because of the presence of calcite crystals in irregular mosaics with irregular crystal boundaries (both characteristic features of recrystallized micrite). However, such fabrics can result from complex series of precipitation and dissolution phases during calcrete growth. Studies of some mosaics using cathodoluminescence show that they are primary and not due to recrystallization (see Wright, this volume).
Many Precambrian and
Paleozoic calcrete nodules show apparent recrystallization features (Fig. 6) but detailed cathodoluminescence work is required to identify the degree of burial related alteration.
Fig. 6 . Recrystallized caliche nodule (upper left) with near-marginal iron stain (neoferran) from the calcic (Bk) horizon of the type Lehigh Gap clay paleosol (Oxic Ustropept) in the Late Silurian ( 4 2 0 Ma) Bloomsburg Formation, near Palmerton, Pennsylvania, U . S . A . Scale bar is 1 mm.
650 4. CONCLUSIONS The range of m i c r o f a b r i c s now known from l i t h i f i e d p a l e o s o l s a l r e a d y approaches t h a t of modern s o i l s , d e s p i t e t h e s m a l l number of s c i e n t i s t s t h a t have f o r m a l l y r e p o r t e d t h e i r s t u d y . It i s c l e a r t h a t d e s c r i p t i v e c l a s s i f i c a t i o n s of s o i l micromorphology (such as t h o s e of Brewer, 1976 and Bullock e t a l . , 1984) can be used f o r l i t h i f i e d p a l e o s o l s a s w e l l a s modern s o i l s . How t h e s e v a r i o u s f e a t u r e s a r e t o b e i n t e r p r e t e d i s a n o t h e r m a t t e r t h a t i s f a r from s o l v e d . A p a r t of t h e problem i s a l t e r a t i o n of p a l e o s o l s d u r i n g deep b u r i a l and metamorphism. Some i n f o r m a t i o n on t h e s e changes can be gleaned from t h e e x t e n s i v e l i t e r a t u r e on sediment d i a g e n e s i s and metamorphism. How, o r i f , t h e s e changes on b u r i a l s e l e c t i v e l y a f f e c t p a r t i c u l a r p a r t s of p a l e o s o l s remains t o be s t u d i e d
s e r i o u s l y . Once t h e s e a l t e r a t i o n s t o micromorphology can be f i l t e r e d o u t , t h e remaining f e a t u r e s can be used t o assess how s i m i l a r t o modern s o i l - f o r m i n g p r o c e s s e s were t h o s e of t h e d i s t a n t g e o l o g i c a l p a s t . Micromorphology i s , and i s l i k e l y t o remain, of c r i t i c a l importance t o a s s e s s i n g t h e f o s s i l r e c o r d of soils. 5. ACKNOWLEDGEMENTS G . J . R . thanks paleopedology s t u d e n t s C.R. Feakes, G.S. Smith, J. P r a t t , and G. Thackray f o r t h e i r i l l u m i n a t i n g q u e s t i o n s about p r o b l e m a t i c t h i n s e c t i o n s ,
and t e c h n i c i a n s C a t h e r i n e C l i f t o n and Sandra Newman f o r producing thousands of h i g h q u a l i t y t h i n s e c t i o n s from u n p r e d i c t a b l e s m e c t i t i c c l a y s t o n e s . Much of t h e r e s e a r c h of G.J.R. h a s been funded by t h e U.S. N a t i o n a l Science Foundation, p a r t i c u l a r l y g r a n t EAR 8503232. Research of V.P.W. h a s been s u p p o r t e d by t h e N u f f i e l d Foundation, U.K. This is Reading U n i v e r s i t y , P.R.I.S. C o n t r i b u t i o n 011. 6. REFERENCES Allen, J.R.L., 1986. Pedogenic c a l c r e t e s i n t h e Old Red Sandstone f a c i e s (Late S i l u r i a n - E a r l y Carboniferous) of t h e Anglo-Welsh a r e a , s o u t h e r n B r i t a i n . In: V.P. Wright ( E d i t o r ) , P a l e o s o l s : T h e i r Recognition and I n t e r p r e t a t i o n . Blackw e l l s , Oxford: 58-86. Basinger, J.F., 1981. The v e g e t a t i v e body of Metasequoia rnilleri from t h e Middle Eocene of s o u t h e r n B r i t i s h Columbia. Can. J . Bot., 59: 2379-2410. Bethke, C.M. and A l t a n e r , S.P., 1986. Layer-by-layer mechanism of s m e c t i t e illit i z a t i o n and a p p l i c a t i o n of a new rate law. Clays/Clay M i n e r a l s 34:136-142. Bown, T.M., 1982. I c h n o f o s s i l s and r h i z o l i t h s of t h e n e a r s h o r e f l u v i a l J e b e l Q a t r a n i Formation (Oligocene), Fayum P r o v i n c e , Egypt. Palaeogeogr. Palaeoc l i m . Palaeoec., 40: 255-309. Brewer, R., 1976. F a b r i c and Mineral A n a l y s i s of S o i l s . K r i e g e r , New York: 482 PP. Brewer, R. and Sleeman, J . R . , 1969. The arrangement of c o n s t i t u e n t s i n Quaternn a r y s o i l s . S o i l S c i . , 107: 435-441. Bullock, P . , 1983. The changing f a c e of s o i l micromorphology. In: P. Bullock and C.P. Murphy ( E d i t o r s ) , S o i l Micromorphology, v o l . 1, Techniques and A p p l i c a t i o n s . A.B. Academic, Berkhamsted: 1-18. Bullock, P . , F e d e r o f f , N . , J u n g e r i u s , A., Stoops, G., and T u r s i n a , T . , 1984. Handbook f o r S o i l Thin S e c t i o n D e s c r i p t i o n . Waine Research, Wolverhampton: 152 pp. C a l v e t , F. and J u l i a , R . , 1983. P i s o i d s i n c a l i c h e p r o f i l e s of Tarragona (N.E. S p a i n ) . I n : T.M. P e r y t ( E d i t o r ) , Coated Grains. S p r i n g e r , B e r l i n : 456-473.
65I Cohen, A.S., 1982. Paleoenvironments of root casts from the Koobi Fora Formation, Kenya. J. Sedim. Petrol., 52: 401-414. Doner, H.E. and Lynn, W.C., 1977. Carbonate, halide, sulfate and sulfide minerals. In: J.B. Dixon and S.B. Weed (Editors), Minerals in Soil Environments. soil Science Society of America, Madison: 75-98. Edwards, R. and Atkinson, K., 1986. Ore Deposit Geology. Chapman and Hall, London: 466pp. Esteban, M. and Klappa, C.F., 1983. Subaerial exposure environment. In: P.A. Scholle, D.G. Bebout and C.H. Moore (Editors), Carbonate Depositional Environments. Mem. Am. Assoc. Petrol. Geol., 33: 1-54. F o l k , R.L., 1965. Some aspects of recrystallization in ancient limestones. In: L.C. Pray and R.C. Murray (Editors), Dolomitization and Limestone Diagenesis. Spec. Publ. SOC. Econ. Paleont. Mineral., 13: 14-48. Gay, A.L. and Grandstaff, D.E., 1980. Chemistry and mineralogy of Precambrian paleosols at Elliot Lake, Ontario, Canada. Precambrian Res., 12: 340-373. Grandstaff, D.E., Edelman, M.J., Foster, R.W., Zbinden, E. and Kimberley, M.M., 1986. Chemistry and mineralogy of Precambrian paleosols at the base of the Dominion and Pongola Groups. Precambrian Res., 32: 97-131. James, N.P. and Choquette, P.W. (Editors), 1988. Paleokarst. Springer-Verlag, New York: 416 pp. Kidston, R. and Lang, W.H., 1921. On Old Red Sandstone plants showing structure from the Rhynie chert bed, Aberdeenshire. Part V. The Thallophyta occurring in the peat bed, the succession of plants through a vertical section of the bed and the conditions of accumulation and preservation of the deposit. Trans. R. SOC. Edinburgh, 52: 855-902. Klappa, C.F., 1978. Biolithogenesis of M
652 V a l Onder, South A f r i c a . Precambrian Res., 32: 195-252. R e t a l l a c k , G . J . and D i l c h e r , D.L., 1981. E a r l y angiosperm r e p r o d u c t i o n : Prisca r e y n o k i s i i gen. e t s p . nov. from mid-Cretaceous c o a s t a l d e p o s i t s i n Kansas, U.S.A. P a l a e o n t o g r a p h i c a B179: 103-137. S c h o l l e , P.A., 1978. A c o l o r i l l u s t r a t e d g u i d e t o c a r b o n a t e r o c k c o n s t i t u e n t s , t e x t u r e s , cements and p o r o s i t i e s . Mem. Am. Assoc. P e t r o l . G e o l . , 27: 241 pp. Schmidt, V. and McDonald, D . A . , 1979. The r o l e of s e c o n d a r y p o r o s i t y i n t h e c o u r s e of s a n d s t o n e d i a g e n e s i s . I n : P.A. S c h o l l e and P.R. S c h l u g e r ( E d i t o r s ) , Aspects of D i a g e n e s i s . Spec. P u b l . SOC. Econ. M i n e r a l . P a l e o n t . M i n e r a l . , 26: 175-207. Watts, N.L., 1979. D i s p l a c i v e c a l c i t e : e v i d e n c e from r e c e n t and a n c i e n t c a l c r e t e s : r e p l y . Geology, 7 : 421-422. Watts, N.L., 1980. Q u a t e r n a r y p e d o g e n i c c a l c r e t e s from t h e K a l a h a r i (South A f r i c a ) : m i n e r a l o g y , g e n e s i s and d i a g e n e s i s . Sedimentology, 27: 661-686. Wright, V.P., 1982. Calcrete p a l e o s o l s from t h e Lower c a r b o n i f e r o u s L l a n e l l y Formation, South Wales. S e d i m m t . G e o l . , 33: 1-33. 1983. A r e n d z i n a from t h e Lower C a r b o n i f e r o u s o f S o u t h Wales. Wright, V.P., Sedimentology, 30: 159-179. Wright, V.P., 1984. The s i g n i f i c a n c e o f n e e d l e - f i b r e c a l c i t e i n a Lower Carboni f e r o u s p a l a e o s o l . Geol. J . , 19: 23-32. Wright, V.P., 1986. The r o l e of f u n g a l b i o m i n e r a l i z a t i o n i n t h e f o r m a t i o n o f e a r l y C a r b o n i f e r o u s s o i l f a b r i c s . Sedimentology, 33: 831-838. Wright, V.P., 1987. The e c o l o g y of two e a r l y C a r b o n i f e r o u s p a l a e o s o l s . I n : J. Miller, A.E. Adams and V.P. Wright ( E d i t o r s ) , European D i n a n t i a n Environments. Spec. Iss. Geol. J . , 12: 345-358. Wright, V.P., P l a t t , N.H. and Wimbledon, W.A., 1988. B i o g e n i c l a m i n a r c a l c r e t e s : e v i d e n c e of r o o t m a t s i n p a l e o s o l s . Sedimentology 35: 603-620. Zenger, D.H., Dunham, J . B . and E t h i n g t o n , R.L. ( E d i t o r s ) , 1980. Concepts and Models o f D o l o m i t i z a t i o n . Spec. P u b l . SOC. Econ. P a l e o n t . M i n e r a l . , 28: 320 PP
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