- Email: [email protected]
Terrestrial stromatolites and laminar calcretes: a review
Sedimentary Geology, 65 (1989) 1-13 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
1
Terrestrial stromatolites and laminar calcretes: a review V. P A U L W R I G H T Postgraduate Research Institute for Sedimentology, The University, P. 0. Box 227, Whiteknights, Reading RG6 2AB (U.K.) Received January 24, 1989; revised version received July 13, 1989
Abstract Wright, V.P., 1989. Terrestrial stromatolites and laminar calcretes: a review. Sediment. Geol., 65: 1-13. Laminated carbonates are prominent features of subaerial exposure profiles and many types of calcareous soils including calcretes. Previously much attention was given to differentiating these laminites from marine or lacustrine microbial stromatolites and oncoids. However, recent research has shown that many subaerial laminated carbonates are biogenically formed, mainly, but not wholly, by microbial activity. This paper reviews the criteria for distinguishing terrestrial carbonate laminites from non-terrestrial stromatolites and oncoids, and for differentiating biogenic from abiogenic terrestrial laminites.
Introduction Stromatolites are laminated organosedimentary structures built by the sediment-trapping, sediment-binding, a n d / o r carbonate precipitating activity of microbial communities (Kennard and James, 1986; Burne and Moore, 1987). Oncoids are laminated concentric structures typically of microbial origin (Wright, 1990). Shallow-water limestones commonly contain a variety of finegrained, laminated structures resembling present day stromatolites and oncoids. Such features had been invariably interpreted as microbial in origin. However, a number of studies of subaerial exposure surfaces and of calcrete (caliche) profiles revealed strikingly similar features formed, or so it was thought, by abiogenic processes (Multer and Hoffmeister, 1969; James, 1972; Read, 1974; Verrecchia and Freytet, 1987) (Fig. 1). These studies coincided with the increased awareness amongst sedimentologists of the impor-
University of Reading, P.R.I.S. Contribution No. 021. 0037-0738/89/$03.50
© 1989 Elsevier Science Publishers B.V.
tance of subaerial diagenesis in the stabilization and cementation of carbonate sediments into limestones. It therefore became important to be able to differentiate non-terrestrial (typically peritidal) microbial stromatolites and oncoids from subaerial exposure-related (terrestrial) laminated structures such as laminar calcrete and calcretecoated grains. A further complication arose when a variety of supra-tidal carbonate crusts (pelagosites, coniatolites) and coated grains (coniatoids) were recognized (Purser and Loreau, 1973; Picha, 1978). The distinctions between microbial, non-terrestrial stromatolites and oncoids, and laminar calcretes and calcrete-coated grains was thoroughly reviewed by Read (1976). However, since the publication of that review a number of studies have documented laminar calcrete and coated grains which are biogenic in origin, and in many cases actually microbially formed (Klappa, 1979a; Krumbein and Giele, 1979; Calvet, 1982; Calvet and Julia, 1983; Wright et al., 1988). Thus some laminated stromatolite-like subaerial carbonates (pseudostromatolites) are abiogenic in origin, or
2
V.P. W R I G H T
Fig. 1. (a)A Recent calcrete crust on Tertiary hmestone, Jamaica. Coin is approximately 2 cm in diameter. Similar distinctive crusts are widespread on subaerially exposed carbonate substrates in arid to sub-tropical regions. (b) Calcrete crust separating Upper Jurassic (Upper Oxfordian) coUuvialdeposits (above) and karsted marine limestones of Middle Jurassic age below. From Arrabida area of Portugal. For details see Wright and Wilson (1987). Scale divisions are 2 cm.
are the products of calcified root mats (biogenic pseudostromatolites), while others are microbially formed. This latter group are here termed terrestrial stromatolites ( a n d terrestrial oncoids). Ter-
sediment or rock interface, or within soil profiles. They are most commonly found in association with highly calcareous soils, generally referred to as calcretes (or caliche). The terms stromatolite a n d
restrial stromatolites and oncoids are laminated, microbially formed structures which occur in vadose settings, either forming at the atmosphere-soil or
o n c o i d b e i n g used here following the definitions of K e n n a r d a n d James (1986) a n d B u r n e a n d M o o r e (1987) (see above).
TERRESTRIAL STROMATOLITES AND LAMINAR CALCRETES: A REVIEW
LaminarCalcretes ~
( ~ ~ ~
. abiogenic microbial/lichens rhizolite ~ c y a n o b a c t e r i a
Costed Grains /calcrete abiogenic~ s u p r a t i d a l (vadoid) microbial
Fig. 2. Classification of terrestrial stromatolites and coated grains. The purpose of this paper is to review the literature on biogenic terrestrial laminites and to revise the criteria given by Read (1976) for distinguishing non-terrestrial microbial stromatolites and oncoids from the various types of laminar calcretes and coated grains. Marine vadose laminites (coniatolites and coniatoids) are not discussed in detail, nor are speleothems.
Biogenic terrestrial laminar carbonates Laminar carbonates are very common in both present-day and ancient vadose settings, particu-
3
larly in association with calcrete profiles and other types of calcareous soils or paleosols (Wright et al., 1988). While several workers have stressed the importance of biogenic, including microbial, fabrics in calcretes (e.g. James, 1972; Read, 1976; Harrison, 1977; Freytet and Moissenet, 1983) it is now apparent that many calcretes are actually biogenic in origin and that many laminar calcretes and calcrete-coated grains are specifically microbial in origin. Indeed four distinctly different types of calcareous biogenic laminated structures have been recognized in vadose settings: "desert stromatolites", lichen stromatolites, microbially coated grains (oncoids) and rhizolites (root-formed calcretes) (Fig. 2). The first three types are microbial in origin while the latter is the product of the calcification of roots. Desert stromatolites
Intertidal cyanobacteria are adapted to periods of desiccation and are readily suited to colonize terrestrial settings. Lithophytic cyanobacteria rapidly colonize subaerially exposed carbonate substrates, especially on flat lithified surfaces where moisture collects (Curtis et al. 1976). Such cyanobacterial communities are even common in
Fig. 3. Crytogamicearth (microbial mat) from desert surface at Arches National Park, Utah. For further details of these terrestrial mats see Campbell (1979). Typicallysuch crusts are composedof filarnentouscyanobacteria.Photograph taken in mid-summer.
4
desert areas where higher plants are absent (Campbell, 1979). They are termed desert crusts or cryptogamic earths (Fig. 3). Such mats may have calcite precipitated in them (Campbell, 1979) and can even form laminated calcareous stromatolites termed "desert stromatolites" by Krumbein and Giele (1979). They form at the sediment or soil or rock-atmosphere interface, and would seem to be capable of forming only relatively thin stromatolite deposits. However, cyanobacteria probably play a role in the formation of many types of laminar calcretes but little information is available. Verrecchia (1987) has speculated on the role of cyanobacteria in their formation although microbial material can be easily incorporated into such laminae as a contaminant rather than having been a contributor to their calcification. The term desert stromatolite is inappropriate for if cyanobacteria can be shown to be important in forming many laminar calcretes, only some of these calcretes actually form in deserts sensu stricto. Understandably very few ancient examples of such subaerial stromatolites have been described. Wright (1981) described possible subaerial biogenic crusts from the Lower Carboniferous of
V.P. WRIGHT
South Wales, and Paszkowski (1983) has described similar forms from Lower Carboniferous peridital sequences in Poland. Such crusts are recognized by being intimately associated with calcrete or rendzina profiles and by exhibiting typical stromatolitic growth features such as non-parallel, tufted lamina relief with calcified microbial structures such as calcareous tubes. Despite the lack of recognized fossil examples, terrestrial cyanobacterial communities probably had a very long history. Golubic and Campbell (1979) have speculated that such prokaryotic communities were probably widespread on even the Precambrian land surface and have offered the middle Precambrian form Eosynechococcus moorei H o f m a n n as an analogue for the extant lithophytic form Gloeothece coerulea Geitler. Lichen stromatolites
On exposed carbonate surfaces the first colonizers are cyanobacteria but these are closely followed by fungi with the development of lichens (Curtis et al., 1976). The first lichens are crustose forms which both attack lithified substrates by
Fig. 4. Thick rhizolite laminar calerete from Se~tora de Brezales Formation (Upper Jurassic-Lower Cretaceous), east side of road from Espejon to La Gallrga, 3 km north of Espejon and 500 m north of the chapel of Nuestra Sei~orade Brezales; Camerous Basin, North Spain. These laminar units occur within silicilastic sandflat and wadi deposits. For further details see Wright et al. (1988). Photo courtesy of N.H. Platt, University of Bern.
TERRESTRIAL STROMATOLITES AND LAMINAR CALCRETES: A REVIEW
boring and by rhizoid penetration. These activities have been documented in several studies (Jones, 1965; Pomar et al., 1975; Kahle, 1977). Under suitable conditions such lichen crusts may become indurated by CaCO 3, and as they accrete are believed to build laminated "lichen stromatolites" (Klappa, 1979a). These crusts consist of millimetre-scale, organic-rich and organicpoor laminae exhibiting micro-borings, microbial filaments and hyphae, calcite spherulites and other micro-fabrics. Klappa (1979a) described forms from the present-day softs of Spain and Coniglio and Harrison (1983) have documented Pleistocene forms from Florida. Such lichen stromatolites grow very slowly. They should therefore be relatively thin and have a low preservation potential. The superposition during growth would result in early laminae being affected by endolithic activity of later colonizers. As a result earlier features would be degraded into micrite and identifying such zones in the geological record would be very difficult.
5
Fig. 5. Rhizolite laminar calcrete from the Upper Jurassic of Swindon, southern England. This crudely laminated and fenestral calerete overliesbrackish lagoonallimestones. Rhizolite caleretes are composed of denselyinterwovenhorizontalto sub-horizontal rhizocretions representing a calcifiedroot mat. For further details see Wrightet al. (1988).
Rhizofite calcretes Recently Wright et al. (1988) have described laminated calcretes from a variety of paleosol profiles of early Carboniferous, late Jurassic and early Cretaceous ages from southern Britain and north Spain. These laminar forms are up to I m in thickness (Fig. 4), are crudely laminated on a hand-specimen scale (Fig. 5) but in thin section exhibit very fine, contorted, lamination consisting of micritic walled tubes with abundant sub-horizontal tubular fenestrae (Fig. 6) locally containing small faecal pellets and alveolar-septal structures. These authors interpreted such forms as calcified horizontal root mats developed either over indurated surfaces such as lithified shallow marine or lacustrine limestones, or hard-pan calcrete horizons, or in some cases, at the positions of former water tables. They termed them rhizolites. These calcretes, typified by the abundance of calcified root structures (rhizoliths), appear to be relatively common and have been noted by the author in the early Tertiary Bembridge Limestone palustrine sequence of the Isle of Wight (southern England), and as thin crusts in present-day soils
developed on limestone bedrock in west Texas (Fig. 7). They probably form rapidly and may quickly aggrade to form thick horizons (Fig. 4). These are not, strictly speaking, terrestrial stromatolites because evidence of microbial activity is minor or absent. They could reasonably be termed biogenic pseudostromatolites, although the term rhizolite seems more appropriate.
Pedogenic microbially coated grains Coated grains with multiple coats of micrite are very common in calcrete softs and other varieties occur in supratidal deposits (Purser and Loreau, 1973; Esteban and Pray, 1983; Peryt, 1983). Detailed studies by Calvet (1982) and Calvet and Julia (1983) on Quaternary calcrete profiles from Spain, have shown that such coatings are produced by microbial calcification. The laminae, especially the outer most recently formed ones, exhibit abundant calcified microbial tubes of fungi and cyanobacteria, as well as needle-fibre calcite. This unusual form of calcite, consisting of needles
6
v.o. WRIGHT
Fig. 6. (a) Highly fenestral fabric of a rhizolite calcrete horizon. The long axes of the sparry calcite-filled tubular fenestrae are subparallel to the base of the horizon. Note also the larger irregular spar-filled cavities. From the Heatherslade Geosol, Lower Carboniferous, Miskin South Wales (see Wright et al., 1988, for details). Field of view is 11 mm wide. (b) Detail of (a) showing typical microfabric of rhizolite caleretes. Note dark micritic coatings around the spar-filled tubular fenestrae (root moulds), the crudely laminar fabric and areas of alveolar-septal fabric (see also Wright et al., 1988). Field of view is 3 mm wide.
a few m i c r o n s wide a n d u p to 50 g m or m o r e long, is a b u n d a n t in m a n y calcareous soils a n d it has recently b e e n shown to be p r o d u c e d b y fungi (Callot et al., 1985; Phillips a n d Self, 1987). In a d d i t i o n K n o x (1977) a n d Phillips et al. (1987) have also d e t a i l e d the role of m i c r o - o r g a n i s m s in the f o r m a t i o n of calcrete coatings (Figs. 8 a n d 9).
T y p i c a l l y such c o a t i n g s f o r m slowly a n d are relatively thin, i r r e g u l a r l y c o n c e n t r i c a n d exhibit f i l a m e n t - l i k e o u t g r o w t h s (Figs. 10) (Calvet, 1982). Similar micritic o u t g r o w t h s , f o r m i n g i n t e r - g r a n u lar networks, have also b e e n d e s c r i b e d f r o m B a h a m i a n b e a c h r o c k b y Beier (1985). However, thick c o a t i n g s c a n also o c c u r (Fig. 8) a n d are m o r e
TERRESTRIAL STROMATOLITES AND LAMINAR CALCRETES: A REVIEW
7
Fig. 7. Surface view of highly porous rhizolite laminar calcrete horizon from present-day soil profile, Slaughter Canyon, New Mexico.
regular in form. Although such coated grains are commonly termed calcrete pisoids or ooids, their biogenic origin would warrant the use of the term oncoid. Ancient examples of these microbiaUy coated grains have been described from early Carboniferous paleosols by Wright (1986) and from Cambrian
phosphatic paleosols in the Georgina Basin in north Australia (Southgate, 1986). Differentiating between terrestrial stromatolites, stromatolites and abiogenic calcrete structures From the previous discussion it is clear that a variety of biogenic laminated, stromatolite-like
Fig. 8. Pisolitic soil profile from south of Hallett Cove, near Adelaide, South Australia. Note large size of microbially coated grains. Present land surface is at top of photograph, note large reworked calcrete clasts near lens cap.
8
v.P. W R I G H T
Fig. 9. (a) Scanning electron microscope view of microstructure of pisoids from the horizon shown in Fig. 8. Note the presence of calcified tubes (shown in longitudinal section in the centre of the photo). Transverse sections showing columnar crystal forms. (b) Detail of (a). For further details see review of similar features by Phillips et al., (1987).
structures form in s u b a e r i a l w e a t h e r i n g profiles a n d soils. T h u s we are no longer s i m p l y faced with the distinction b e t w e e n l a m i n a r calcretes, or calcrete pisoids a n d s t r o m a t o l i t e s a n d oncoids, as carefully reviewed b y R e a d (1976), b u t we are n o w also faced with distinguishing b i o g e n i c f r o m a b i o genic l a m i n a r calcretes. T h e following sections review the m a j o r char-
acteristics of l a m i n a r calcretes a n d c a l c r e t e - c o a t e d grains a n d the d i s t i n c t i o n s b e t w e e n a b i o g e n i c a n d b i o g e u i c forms.
Laminar calcretes L a m i n a t e d calcretes a r e t y p i c a l l y thin, f r o m a few m i l l i m e t r e s to 10 c m or so in thickness, b u t
T E R R E S T R I A L S T R O M A T O L I T E S A N D L A M I N A R CALCRETES: A REVIEW
A
calcified ~.~fungal'tubes
mycelial s t r a n d \ , ~ . . . . . - - ~ ' - - ~ / ~ A~/ ~needlefibre calcite irregular ~ micritic coatings B
~ intergranular alveolar-septal structure
......perched iqclusions ...... early fractures
=;ane'
9
Kahle, 1977; Robbin and Stipp, 1979) (Fig. 12). The laminae m a y enclose various inclusions such as calcrete clasts, or bioclasts such as terrestrial snails or pupal cases (Read, 1974). However, laminae of particulate matter, such as commonly occur in non-terrestrial stromatorites, are less common. The lamination may be interrupted by root moulds, typically with concentric coatings, differentiating them from simple burrows. In extreme cases, such as in rhizolite crusts, the whole fabric is composed of rhizocretions (Wright et al., 1988). The laminae microstructure is critical for differentiating abiogenic and biogenic laminar calcretes, and it reflects the degree of biological influence on its formation. In abiogenic forms the precipitation of calcium carbonate is primarily
",,I polygonal laminae Fig. 10. Typical morphologiesof small calerete oncoids (microbially coated grains). A. The bridge outgrowth structures are a characteristic feature and represent calcified mycelial bundles. For detailed illustrations see Calvet (1982) and Wright (1986). B. General fabric features common also to non-pedogenic supratidal vadoids.
extreme examples have been recorded, such as rhizolite calcretes up to 1 m in thickness (Wright et al., 1988). However, as pointed out by Read (1976), laminar calcretes are generally much thinner than most stromatolites. Such calcretes follow the underlying topography of the substrate and m a y be vertically inclined or m a y even coat the undersides of overhangs (Fig. 11), a feature rarely seen in non-terrestrial microbial stromatorites. Laminar calcretes typically thicken into depressions and thin over topographic highs (Fig. 12), somewhat the reverse situation to m a n y nonterrestrial stromatolite morphologies. It is in the style of lamination and microstructure that the most important distinctions can be made. Laminar calcretes, regardless of origin, display several typical features. Microunconformities are very c o m m o n (Fig. 12), commonly attributable to phases of dissolution. Lamination is typically due to differences in organic staining and not to actual microstructural differences (Read, 1974;
Fig. 11. Thin ca/crete crust separating Upper Jurassic colluvial deposits (C) and fractured Middle Jurassic marine limestones (L). Note the crust also coats the overhang surfaces (mowed). For locality details see Fig. lb. Scale divisions are 2 em.
10
due to evaporation or CO 2 loss by degassing while in biogenic forms the precipitation of carbonate appears to have been induced by organisms, probably as a result of CO2-uptake during photosynthesis or by the " d u m p i n g " of excess Ca by fungi (Phillips et al., 1987). In abiogenic forms the lamination is typically very fine while the microstructure is dense and micritic. However, microbial remains, calcified or uncalcified, and rootstructures can be incorporated. It must be stressed that such dense, micritic forms represent an end member where biological influence cannot be demonstrated petrographically. Biogenic laminar calcretes are distinguished by having a strongly biogenic microfabric and are usually less evenly and finely laminated than abiogenic forms. The biogenic lamination may sometimes show typical stromatolitic features such as thickening over topographic highs, or even with tufted relief (Wright, 1981). The microfabric is typically wrinkled or contorted and a variety of calcified microbial remains m a y be present attri-
V,P. W R I G H T
butable to calcified fungal hyphae or other microorganisms, or root hairs (James, 1972; Harrison, 1977; Knox, 1977; Klappa, 1979b; Beier, 1985; Phillips et al., 1987). In the case of lichen crusts the lamination may be especially convolute (Klappa, 1979a) and m a y contain organic laminae, irregular fenestral pores, and a variety of spherical structures formed by the calcification of coccoid cyanobacteria or representing perithecial structures. These lichen forms are most readily recognized by their association with abundant microborings and they m a y also show evidence of the concentration of metallic ions (Esteban and Klappa, 1983). Rhizolite crusts also have a very strongly biogenic fabric (Wright et al., 1988), and are composed of a dense, intertwined mat of rhizocretions defined by laminated walls, locally with alveolarseptal structures. In summary, having recognized a laminated carbonate as probably being of terrestrial origin, it is the degree of biological control on the laminae
Fig. 12. Calcrete crust from Bembridge Limestone (Lower Tertiary), Prospect Quarry, Isle of Wight, England. This crust occurs within a series of palustrine limestones with freshwater and terrestrial molluscs. The crust shows several typical calcrete features such as banding due to the degree of organic staining, slight thickening into depressions and small unconformities. The microfabric is a dense micrite showing no biogenic features and the crust is interpreted as an abiogenic form. Some of the clasts in the calcrete profile consist of reworked rhizolite laminar calcrete.
T E R R E S T R I A L S T R O M A T O L I T E S A N D L A M I N A R CALCRETES: A REVIEW
form and microstructure which weill indicate whether it was biogenically or abiogenically formed.
Calcrete-coated grains There is a considerable volume of literature on "vadose" pisoids or vadoids (see review by Peryt, 1983), and two broad categories can be recognized. One type is formed by simple cementation (and often has a radial fabric), and the other type is formed microbially (i.e. terrestrial oncoids). There are many descriptions of the former type, which are commonly associated with supratidal deposits and tepee structures (Dunham, 1969; Esteban and Pray, 1983; Ruiji and Yaosong, 1983). The latter have received less attention but have been carefully described by Calvet (1982) and Calvet and Julia (1983). Calcrete-coated grains are characterised by a variety of features some of which have been reviewed by Read (1976). One very common aspect of calcrete-coated grains is their occurrence in inversely graded horizons. This is a feature also associated with coated grains formed in supratidal environments and its origins have been reviewed by Esteban and Pray (1983). Other features include evidence of early fracturing and re-growth, and association with geopetal "vadose silts", and meniscus cements. Several fabric features indicate that some of the coated grain growth is by in-situ processes not involving any actual movement of the grains. The laminae may show downward elongation (pendant morphology) (or even rarely upward-thickening), perched inclusions on the upper surfaces of the laminae, or the laminae of adjacent coated grains accommodate each other to form polygonal coatings (Esteban and Pray, 1983) (Fig. 10). It is the authors experience that these in-situ features are rarely seen in calcrete-coated grains, where the actual grains are rotated during growth by movement down-slopes (Read, 1974; Arakel, 19820. Calcrete-coated grains have other distinctive features. Firstly the coatings tend to be very fine grained and thin (simple envelopes of micrite), and are typically non-isopachous (James, 1972; Calvet and Julia, 1983; Warren, 1983). Larger forms with many laminae may be more regular in
11
shape (Read, 1974), being sub-spherical to spherical and well rounded (Fig. 8). This reflects the degree of turning during down-slope movement. Forms on longer, steeper slopes are probably more evenly coated than forms which have undergone relatively short transport. One common feature of calcrete pisoids are bridge structures either connecting grains or as simple or branched growths into pore spaces (Fig. 10). These outgrowths of the coating laminae have the same microfabric as the grain cortices and are probably calcified fungal mycelial strands (Calvet, 1982; Calvet and Julia, 1983). Both the coatings and the outgrowths in many recent to Palaeozoic pedogenic coatings contain abundant microbial structures such as calcareous tubes and needlefibre calcite (Knox, 1977; Calvet, 1982; Calvet and Julia, 1983; Wright, 1986; Phillips and Self, 1987; Phillips et al., 1987). Distinguishing calcrete-coated grains from other forms of coated grain will depend on the recognition and evidence of pedogenic processes. Recognizing microbial pedogenic coated grains as against abiogenic forms will depend on the identification of microbial fabrics including the distinctive microbial braces and mycelial outgrowths.
Summary Many laminated carbonates in terrestrial settings are biogenic in origin. Previously they had been considered as largely abiogenic precipitates and referred to as laminar calcretes Or calcrete ooids or pisoids. However, many such forms are the products of root or microbial activity. The latter group, by definition, are stromatohtes and oncoids, and are here termed terrestrial stromatolites and oncoids. Laminar calcretes formed by root activity (rhizolites) are a form of biogenic pseudostromatolite. Distinguishing terrestrial laminites from marine microbial forms is dependent on recognizing associated subaerial exposure-related and pedogenic features in the former. Distinguishing abiogenic terrestrial laminites from both terrestrial stromatolites and oncoids, and rhizolites, depends on recognizing the nature and degree of biological control on the formation of the lamination.
12
Microbial laminites (stromatolites and oncoids) are not restricted to shallow marine, lacustrine or fluvial settings but are important components in many present-day and ancient calcrete profiles.
Acknowledgements I should like to especially thank Sally Phillips (University of Adelaide) for introducing me to the South Australian calcretes figured in this paper. The paper greatly benefited from the constructive comments by Robert Riding and an anonymous reviewer. Richard Lewis is thanked for technical assistance with SEM. Alison Ruegg and Elizabeth Wyeth skilfully typed the final manuscript. This paper is part of a project on paleosols partly supported by the Nuffield Foundation.
References Arakel, A.V., 1982. Genesis of calcrete in Quaternary soil profiles, Hurt and Leeman lagoons, Western Australia. J. Sediment. Petrol., 52: 109-125. Beier, J.A., 1985. Diagenesis of Quaternary Bahamian beachrock: petrographic and isotopic evidence. J. Sediment. Petrol., 55: 755-761. Burne, R.V. and Moore, L.S., 1987. Microbialites: organosedimentary deposits of benthic microbial communities. Palaios, 2: 241-254. Callot, G., Guyon, A. and Mousain, D., 1985. Inter-relations entre aiguilles de calcite et hyphes mycrliens. Agronomie, 5: 209-216. Calvet, F., 1982. Constructive micrite envelopes developed in vadose continental environments in Pleistocene eolianites of Mallorca (Spain). Acta Geol. Hispanica, 17: 169-178. Calvet, F. and Julia, R., 1983. Pisoids in the caliche profiles of Tarragona, north east Spain. In: T.M. Peryt (Editor), Coated Grains. Springer, Berlin, pp. 456-473. Campbell, S.E., 1979. Soil stabilization by a prokaryotic desert crust: implications for Precambrian land biota. Origins Life, 9: 335-348. Coniglio, M. and Harrison, R.S., 1983. Holocene and Pleistocene caliche from Big Pine Key, Florida. Bull. Can. Pet. Geol., 31: 3-13. Curtis, L.F., Courtney, F.M. and Trudgill, S.T., 1976. Soils in the British Isles. Longmans, London, 364 pp. Dunham, R.J., 1969. Vadose pisolite in the Capitan Reef (Permian), New Mexico and Texas. Soc. Econ. Paleontol. Mineral., Spec. Publ., 14: 182-191. Esteban, M. and Klappa, C.F., 1983. Subaerial exposure environment. In: P.A. Scholle et al. (Editors), Carbonate Depositional Environments. Am. Assoc. Pet. Geol. Mem., 33: 1054.
V.P. WRIGHT
Esteban, M. and Pray, L.C., 1983. Pisoids and pisolitic facies (Permian), Guadelupe Mountains, New Mexico and West Texas. In: T.M. Peryt (Editor) Coated Grains. Springer, Berlin, pp. 503-537. Freytet, P. and Moissenet, E., 1983. Presence de restes algaires identifiables dans des crohtes calcaires plio-quaternaires du Nord-Est de l'Espagne. C.R. Acad. Sci. Paris, 296: 1563-1566. Golubic, S. and Campbell, S.E., 1979. Analogous microbial forms in recent subaerial habitats and in PRecambrian cherts: Goeothece coerulea Geitler and Eosynechococcus modrei Hoffman. Precambr. Res., 8: 201-217. Harrison, r.S., 1977. Caliche profiles: indicators of near surface subaerial diagenesis, Barbados, West Indies. Bull. Can. Pet. Geol., 25: 123-173. James, N.P., 1972. Holocene and Pleistocene calcareous crust (caliche) profiles: criteria for subaerial exposure. J. Sediment. Petrol., 42: 817-836. Jones, R.J., 1965. Aspects of the biological weathering of limestone pavements. Proc. Geol. Assoc. London, 76: 421-433. Kahle, C.F., 1977. Origin of subaerial Holocene calcareous crusts: role of algae, fungi and sparmicritisation. Sedimentology, 24: 413-435. Kennard, J.M. and James, N.P., 1986. Thrombolites and stromatolites: two distinct types of microbial structures. Palaios, 1: 492-503. Klappa, C.F., 1979a. Lichen stromatolites: criterion for subaerial exposure and a mechanism for the formation of laminar calcretes (caliche). J. Sediment. Petrol., 49: 387-400. Klappa, C.F., 1979b. Calcified filaments in Quaternary calcretes: organomineral interactions in the subaerial vadose environment. J. Sediment. Petrol., 49: 955-968. Knox, G.J., 1977. Caliche profile formation, Saldanha Bay (South Africa). Sedimentotogy, 24: 657-674. Krumbein, W.E. and Giele, C., 1979. Calcification in a coccoid cyanobacterium associated with the formation of desert stromatolites. Sedimentology, 26: 593-604. Multer, H.G. and Hoffmeister, J.E., 1968. Subaerial laminated crusts of the Florida Keys. Geol. Soc. Am. Bull., 79: 183-192. Paszkowski, M., 1983. Algal sedimentary structures in the Dinantian limestones in Krzeszowice area (in Polish, English summary). Prz. Geol., 4: 254-258. Peryt, T.M., 1983. Vadoids. In: T.M. Peryt (Editor), Coated Grains, Springer, Berlin, pp. 437-449. Phillips, S.E. and Self, P.G., 1987. Morphology, crystallography and origin of needle-fibre calcite in Quaternary pedogenic calcretes of South Australia. Aust. J. Soil Res., 25: 429-444. Phillips, S.E., Milnes, A.R. and Foster, R.C., 1987. Calcified filaments: an example of biological influences in the formation of calcrete in South Australia. Aust. J. Soil Res., 25: 405-428. Picha, F., 1978. Depositional and diagenetic history of Pleistocene and Holocene oolitic sediments and sabkhas in Kuwait, Persian Gulf. Sedimentology, 20: 365-389.
T E R R E S T R I A L S T R O MAT OLI TES A N D L A M I N A R CALCRETES: A REVIEW
Pomar, L.,, Esteban, M., Lhmona, X. and Fontarnau, R., 1975. Acci6n de Liquences, Algas y Hongas en la Telodiag6nesis de las Carbonatados de la zona litoral, perilitoral Catalana. Inst. Invest. Geol. Univ. Barcelona, 30: 83-117. Purser, B.H. and Loreau, J.P., 1973. Aragonitic supratidal encrustations on the Trucial Coast, Persian Gulf. In: B.H. Purser (Editor), The Persian Gulf. Springer, Berlin, pp. 343-376. Read, J.F., 1974. Calcrete deposits and Quaternary sediments, Edel Province, Shark Bay, Western Australia. Am. Assoc. Pet. Geol. Mem., 22: 250-282. Read, J.F., 1976. Calcretes and their distinction from stromatohtes. In: M.R. Walter (Editor), Stromatolites. Elsevier, Amsterdam, pp. 55-71. Robbin, D.M. and Stipp, J.J., 1979. Depositional rate of laminated crusts, Florida Keys. J. Sediment. Petrol., 49: 175-180. Ruiji, C. and Yaosong, X., 1983. Vadose pisolites of the Tongying Formation (Upper Sinian System) in southwest China. In: T.M. Peryt (Editor), Coated gains. Springer, Berhn, pp. 538-547. Southgate, P.N., 1986. Cambrian phoscrete profiles, coated grains and microbial processes in phosphogenesis: Georgina Basin, Australia. J. Sediment. Petrol., 56: 429-441. Verrecchia, E., 1987. Le contexte morpho-dynamique des
13 crofites calcaires: apport des analyses s6quentielles l'6chelle microscopique. Z. Geomorphol., N.F., 31: 179-193. Verrecchia, E. and Freytet, P., 1987. Interference pedog6n6se's6dimentation dans les crofites calcaires. Proposition d'une nouvelle methode d'6tude: L'analyse sequentielle. In: N. Fedorof et al. (Editors), Micromorphologie des Sols, Actes de la VII6me R6union Internationale de Micromorphologie des Sols, Paris, Juillet 1985, pp. 555-561. Warren, J.K., 1983. Pedogenic calcrete as it occurs in Quaternary calcareous dunes in coastal South Austraha. J. Sediment. Petrol., 53: 787-796. Wright, V.P., 1981. A subaerial stromatohte from the Lower Carboniferous of South Wales. Geol. Mag., 118: 97-100. Wright, V.P., 1986. The role of fungal biomineralization in the formation of early Carboniferous soil fabrics. Sedimentology, 33: 831-838. Wright, V.P., 1990. Limestone constituents. In: M.E. Tucker and V.P. Wright (Editors), Carbonate Sedimentology. BlackweU, Oxford (in press). Wright, V.P. and Wilson, R.C.L., 1987. A terra-rossa-like paleosol complex from the Upper Jurassic of Portugal. Sedimentology, 34: 259-273. Wright, V.P., Platt, N.H. and Wimbledon, W.A., 1988. Biogenic laminar calcretes evidence of calcified root mat horizons in paleosols. Sedimentology, 35: 603-620.
1
Terrestrial stromatolites and laminar calcretes: a review V. P A U L W R I G H T Postgraduate Research Institute for Sedimentology, The University, P. 0. Box 227, Whiteknights, Reading RG6 2AB (U.K.) Received January 24, 1989; revised version received July 13, 1989
Abstract Wright, V.P., 1989. Terrestrial stromatolites and laminar calcretes: a review. Sediment. Geol., 65: 1-13. Laminated carbonates are prominent features of subaerial exposure profiles and many types of calcareous soils including calcretes. Previously much attention was given to differentiating these laminites from marine or lacustrine microbial stromatolites and oncoids. However, recent research has shown that many subaerial laminated carbonates are biogenically formed, mainly, but not wholly, by microbial activity. This paper reviews the criteria for distinguishing terrestrial carbonate laminites from non-terrestrial stromatolites and oncoids, and for differentiating biogenic from abiogenic terrestrial laminites.
Introduction Stromatolites are laminated organosedimentary structures built by the sediment-trapping, sediment-binding, a n d / o r carbonate precipitating activity of microbial communities (Kennard and James, 1986; Burne and Moore, 1987). Oncoids are laminated concentric structures typically of microbial origin (Wright, 1990). Shallow-water limestones commonly contain a variety of finegrained, laminated structures resembling present day stromatolites and oncoids. Such features had been invariably interpreted as microbial in origin. However, a number of studies of subaerial exposure surfaces and of calcrete (caliche) profiles revealed strikingly similar features formed, or so it was thought, by abiogenic processes (Multer and Hoffmeister, 1969; James, 1972; Read, 1974; Verrecchia and Freytet, 1987) (Fig. 1). These studies coincided with the increased awareness amongst sedimentologists of the impor-
University of Reading, P.R.I.S. Contribution No. 021. 0037-0738/89/$03.50
© 1989 Elsevier Science Publishers B.V.
tance of subaerial diagenesis in the stabilization and cementation of carbonate sediments into limestones. It therefore became important to be able to differentiate non-terrestrial (typically peritidal) microbial stromatolites and oncoids from subaerial exposure-related (terrestrial) laminated structures such as laminar calcrete and calcretecoated grains. A further complication arose when a variety of supra-tidal carbonate crusts (pelagosites, coniatolites) and coated grains (coniatoids) were recognized (Purser and Loreau, 1973; Picha, 1978). The distinctions between microbial, non-terrestrial stromatolites and oncoids, and laminar calcretes and calcrete-coated grains was thoroughly reviewed by Read (1976). However, since the publication of that review a number of studies have documented laminar calcrete and coated grains which are biogenic in origin, and in many cases actually microbially formed (Klappa, 1979a; Krumbein and Giele, 1979; Calvet, 1982; Calvet and Julia, 1983; Wright et al., 1988). Thus some laminated stromatolite-like subaerial carbonates (pseudostromatolites) are abiogenic in origin, or
2
V.P. W R I G H T
Fig. 1. (a)A Recent calcrete crust on Tertiary hmestone, Jamaica. Coin is approximately 2 cm in diameter. Similar distinctive crusts are widespread on subaerially exposed carbonate substrates in arid to sub-tropical regions. (b) Calcrete crust separating Upper Jurassic (Upper Oxfordian) coUuvialdeposits (above) and karsted marine limestones of Middle Jurassic age below. From Arrabida area of Portugal. For details see Wright and Wilson (1987). Scale divisions are 2 cm.
are the products of calcified root mats (biogenic pseudostromatolites), while others are microbially formed. This latter group are here termed terrestrial stromatolites ( a n d terrestrial oncoids). Ter-
sediment or rock interface, or within soil profiles. They are most commonly found in association with highly calcareous soils, generally referred to as calcretes (or caliche). The terms stromatolite a n d
restrial stromatolites and oncoids are laminated, microbially formed structures which occur in vadose settings, either forming at the atmosphere-soil or
o n c o i d b e i n g used here following the definitions of K e n n a r d a n d James (1986) a n d B u r n e a n d M o o r e (1987) (see above).
TERRESTRIAL STROMATOLITES AND LAMINAR CALCRETES: A REVIEW
LaminarCalcretes ~
( ~ ~ ~
. abiogenic microbial/lichens rhizolite ~ c y a n o b a c t e r i a
Costed Grains /calcrete abiogenic~ s u p r a t i d a l (vadoid) microbial
Fig. 2. Classification of terrestrial stromatolites and coated grains. The purpose of this paper is to review the literature on biogenic terrestrial laminites and to revise the criteria given by Read (1976) for distinguishing non-terrestrial microbial stromatolites and oncoids from the various types of laminar calcretes and coated grains. Marine vadose laminites (coniatolites and coniatoids) are not discussed in detail, nor are speleothems.
Biogenic terrestrial laminar carbonates Laminar carbonates are very common in both present-day and ancient vadose settings, particu-
3
larly in association with calcrete profiles and other types of calcareous soils or paleosols (Wright et al., 1988). While several workers have stressed the importance of biogenic, including microbial, fabrics in calcretes (e.g. James, 1972; Read, 1976; Harrison, 1977; Freytet and Moissenet, 1983) it is now apparent that many calcretes are actually biogenic in origin and that many laminar calcretes and calcrete-coated grains are specifically microbial in origin. Indeed four distinctly different types of calcareous biogenic laminated structures have been recognized in vadose settings: "desert stromatolites", lichen stromatolites, microbially coated grains (oncoids) and rhizolites (root-formed calcretes) (Fig. 2). The first three types are microbial in origin while the latter is the product of the calcification of roots. Desert stromatolites
Intertidal cyanobacteria are adapted to periods of desiccation and are readily suited to colonize terrestrial settings. Lithophytic cyanobacteria rapidly colonize subaerially exposed carbonate substrates, especially on flat lithified surfaces where moisture collects (Curtis et al. 1976). Such cyanobacterial communities are even common in
Fig. 3. Crytogamicearth (microbial mat) from desert surface at Arches National Park, Utah. For further details of these terrestrial mats see Campbell (1979). Typicallysuch crusts are composedof filarnentouscyanobacteria.Photograph taken in mid-summer.
4
desert areas where higher plants are absent (Campbell, 1979). They are termed desert crusts or cryptogamic earths (Fig. 3). Such mats may have calcite precipitated in them (Campbell, 1979) and can even form laminated calcareous stromatolites termed "desert stromatolites" by Krumbein and Giele (1979). They form at the sediment or soil or rock-atmosphere interface, and would seem to be capable of forming only relatively thin stromatolite deposits. However, cyanobacteria probably play a role in the formation of many types of laminar calcretes but little information is available. Verrecchia (1987) has speculated on the role of cyanobacteria in their formation although microbial material can be easily incorporated into such laminae as a contaminant rather than having been a contributor to their calcification. The term desert stromatolite is inappropriate for if cyanobacteria can be shown to be important in forming many laminar calcretes, only some of these calcretes actually form in deserts sensu stricto. Understandably very few ancient examples of such subaerial stromatolites have been described. Wright (1981) described possible subaerial biogenic crusts from the Lower Carboniferous of
V.P. WRIGHT
South Wales, and Paszkowski (1983) has described similar forms from Lower Carboniferous peridital sequences in Poland. Such crusts are recognized by being intimately associated with calcrete or rendzina profiles and by exhibiting typical stromatolitic growth features such as non-parallel, tufted lamina relief with calcified microbial structures such as calcareous tubes. Despite the lack of recognized fossil examples, terrestrial cyanobacterial communities probably had a very long history. Golubic and Campbell (1979) have speculated that such prokaryotic communities were probably widespread on even the Precambrian land surface and have offered the middle Precambrian form Eosynechococcus moorei H o f m a n n as an analogue for the extant lithophytic form Gloeothece coerulea Geitler. Lichen stromatolites
On exposed carbonate surfaces the first colonizers are cyanobacteria but these are closely followed by fungi with the development of lichens (Curtis et al., 1976). The first lichens are crustose forms which both attack lithified substrates by
Fig. 4. Thick rhizolite laminar calerete from Se~tora de Brezales Formation (Upper Jurassic-Lower Cretaceous), east side of road from Espejon to La Gallrga, 3 km north of Espejon and 500 m north of the chapel of Nuestra Sei~orade Brezales; Camerous Basin, North Spain. These laminar units occur within silicilastic sandflat and wadi deposits. For further details see Wright et al. (1988). Photo courtesy of N.H. Platt, University of Bern.
TERRESTRIAL STROMATOLITES AND LAMINAR CALCRETES: A REVIEW
boring and by rhizoid penetration. These activities have been documented in several studies (Jones, 1965; Pomar et al., 1975; Kahle, 1977). Under suitable conditions such lichen crusts may become indurated by CaCO 3, and as they accrete are believed to build laminated "lichen stromatolites" (Klappa, 1979a). These crusts consist of millimetre-scale, organic-rich and organicpoor laminae exhibiting micro-borings, microbial filaments and hyphae, calcite spherulites and other micro-fabrics. Klappa (1979a) described forms from the present-day softs of Spain and Coniglio and Harrison (1983) have documented Pleistocene forms from Florida. Such lichen stromatolites grow very slowly. They should therefore be relatively thin and have a low preservation potential. The superposition during growth would result in early laminae being affected by endolithic activity of later colonizers. As a result earlier features would be degraded into micrite and identifying such zones in the geological record would be very difficult.
5
Fig. 5. Rhizolite laminar calcrete from the Upper Jurassic of Swindon, southern England. This crudely laminated and fenestral calerete overliesbrackish lagoonallimestones. Rhizolite caleretes are composed of denselyinterwovenhorizontalto sub-horizontal rhizocretions representing a calcifiedroot mat. For further details see Wrightet al. (1988).
Rhizofite calcretes Recently Wright et al. (1988) have described laminated calcretes from a variety of paleosol profiles of early Carboniferous, late Jurassic and early Cretaceous ages from southern Britain and north Spain. These laminar forms are up to I m in thickness (Fig. 4), are crudely laminated on a hand-specimen scale (Fig. 5) but in thin section exhibit very fine, contorted, lamination consisting of micritic walled tubes with abundant sub-horizontal tubular fenestrae (Fig. 6) locally containing small faecal pellets and alveolar-septal structures. These authors interpreted such forms as calcified horizontal root mats developed either over indurated surfaces such as lithified shallow marine or lacustrine limestones, or hard-pan calcrete horizons, or in some cases, at the positions of former water tables. They termed them rhizolites. These calcretes, typified by the abundance of calcified root structures (rhizoliths), appear to be relatively common and have been noted by the author in the early Tertiary Bembridge Limestone palustrine sequence of the Isle of Wight (southern England), and as thin crusts in present-day soils
developed on limestone bedrock in west Texas (Fig. 7). They probably form rapidly and may quickly aggrade to form thick horizons (Fig. 4). These are not, strictly speaking, terrestrial stromatolites because evidence of microbial activity is minor or absent. They could reasonably be termed biogenic pseudostromatolites, although the term rhizolite seems more appropriate.
Pedogenic microbially coated grains Coated grains with multiple coats of micrite are very common in calcrete softs and other varieties occur in supratidal deposits (Purser and Loreau, 1973; Esteban and Pray, 1983; Peryt, 1983). Detailed studies by Calvet (1982) and Calvet and Julia (1983) on Quaternary calcrete profiles from Spain, have shown that such coatings are produced by microbial calcification. The laminae, especially the outer most recently formed ones, exhibit abundant calcified microbial tubes of fungi and cyanobacteria, as well as needle-fibre calcite. This unusual form of calcite, consisting of needles
6
v.o. WRIGHT
Fig. 6. (a) Highly fenestral fabric of a rhizolite calcrete horizon. The long axes of the sparry calcite-filled tubular fenestrae are subparallel to the base of the horizon. Note also the larger irregular spar-filled cavities. From the Heatherslade Geosol, Lower Carboniferous, Miskin South Wales (see Wright et al., 1988, for details). Field of view is 11 mm wide. (b) Detail of (a) showing typical microfabric of rhizolite caleretes. Note dark micritic coatings around the spar-filled tubular fenestrae (root moulds), the crudely laminar fabric and areas of alveolar-septal fabric (see also Wright et al., 1988). Field of view is 3 mm wide.
a few m i c r o n s wide a n d u p to 50 g m or m o r e long, is a b u n d a n t in m a n y calcareous soils a n d it has recently b e e n shown to be p r o d u c e d b y fungi (Callot et al., 1985; Phillips a n d Self, 1987). In a d d i t i o n K n o x (1977) a n d Phillips et al. (1987) have also d e t a i l e d the role of m i c r o - o r g a n i s m s in the f o r m a t i o n of calcrete coatings (Figs. 8 a n d 9).
T y p i c a l l y such c o a t i n g s f o r m slowly a n d are relatively thin, i r r e g u l a r l y c o n c e n t r i c a n d exhibit f i l a m e n t - l i k e o u t g r o w t h s (Figs. 10) (Calvet, 1982). Similar micritic o u t g r o w t h s , f o r m i n g i n t e r - g r a n u lar networks, have also b e e n d e s c r i b e d f r o m B a h a m i a n b e a c h r o c k b y Beier (1985). However, thick c o a t i n g s c a n also o c c u r (Fig. 8) a n d are m o r e
TERRESTRIAL STROMATOLITES AND LAMINAR CALCRETES: A REVIEW
7
Fig. 7. Surface view of highly porous rhizolite laminar calcrete horizon from present-day soil profile, Slaughter Canyon, New Mexico.
regular in form. Although such coated grains are commonly termed calcrete pisoids or ooids, their biogenic origin would warrant the use of the term oncoid. Ancient examples of these microbiaUy coated grains have been described from early Carboniferous paleosols by Wright (1986) and from Cambrian
phosphatic paleosols in the Georgina Basin in north Australia (Southgate, 1986). Differentiating between terrestrial stromatolites, stromatolites and abiogenic calcrete structures From the previous discussion it is clear that a variety of biogenic laminated, stromatolite-like
Fig. 8. Pisolitic soil profile from south of Hallett Cove, near Adelaide, South Australia. Note large size of microbially coated grains. Present land surface is at top of photograph, note large reworked calcrete clasts near lens cap.
8
v.P. W R I G H T
Fig. 9. (a) Scanning electron microscope view of microstructure of pisoids from the horizon shown in Fig. 8. Note the presence of calcified tubes (shown in longitudinal section in the centre of the photo). Transverse sections showing columnar crystal forms. (b) Detail of (a). For further details see review of similar features by Phillips et al., (1987).
structures form in s u b a e r i a l w e a t h e r i n g profiles a n d soils. T h u s we are no longer s i m p l y faced with the distinction b e t w e e n l a m i n a r calcretes, or calcrete pisoids a n d s t r o m a t o l i t e s a n d oncoids, as carefully reviewed b y R e a d (1976), b u t we are n o w also faced with distinguishing b i o g e n i c f r o m a b i o genic l a m i n a r calcretes. T h e following sections review the m a j o r char-
acteristics of l a m i n a r calcretes a n d c a l c r e t e - c o a t e d grains a n d the d i s t i n c t i o n s b e t w e e n a b i o g e n i c a n d b i o g e u i c forms.
Laminar calcretes L a m i n a t e d calcretes a r e t y p i c a l l y thin, f r o m a few m i l l i m e t r e s to 10 c m or so in thickness, b u t
T E R R E S T R I A L S T R O M A T O L I T E S A N D L A M I N A R CALCRETES: A REVIEW
A
calcified ~.~fungal'tubes
mycelial s t r a n d \ , ~ . . . . . - - ~ ' - - ~ / ~ A~/ ~needlefibre calcite irregular ~ micritic coatings B
~ intergranular alveolar-septal structure
......perched iqclusions ...... early fractures
=;ane'
9
Kahle, 1977; Robbin and Stipp, 1979) (Fig. 12). The laminae m a y enclose various inclusions such as calcrete clasts, or bioclasts such as terrestrial snails or pupal cases (Read, 1974). However, laminae of particulate matter, such as commonly occur in non-terrestrial stromatorites, are less common. The lamination may be interrupted by root moulds, typically with concentric coatings, differentiating them from simple burrows. In extreme cases, such as in rhizolite crusts, the whole fabric is composed of rhizocretions (Wright et al., 1988). The laminae microstructure is critical for differentiating abiogenic and biogenic laminar calcretes, and it reflects the degree of biological influence on its formation. In abiogenic forms the precipitation of calcium carbonate is primarily
",,I polygonal laminae Fig. 10. Typical morphologiesof small calerete oncoids (microbially coated grains). A. The bridge outgrowth structures are a characteristic feature and represent calcified mycelial bundles. For detailed illustrations see Calvet (1982) and Wright (1986). B. General fabric features common also to non-pedogenic supratidal vadoids.
extreme examples have been recorded, such as rhizolite calcretes up to 1 m in thickness (Wright et al., 1988). However, as pointed out by Read (1976), laminar calcretes are generally much thinner than most stromatolites. Such calcretes follow the underlying topography of the substrate and m a y be vertically inclined or m a y even coat the undersides of overhangs (Fig. 11), a feature rarely seen in non-terrestrial microbial stromatorites. Laminar calcretes typically thicken into depressions and thin over topographic highs (Fig. 12), somewhat the reverse situation to m a n y nonterrestrial stromatolite morphologies. It is in the style of lamination and microstructure that the most important distinctions can be made. Laminar calcretes, regardless of origin, display several typical features. Microunconformities are very c o m m o n (Fig. 12), commonly attributable to phases of dissolution. Lamination is typically due to differences in organic staining and not to actual microstructural differences (Read, 1974;
Fig. 11. Thin ca/crete crust separating Upper Jurassic colluvial deposits (C) and fractured Middle Jurassic marine limestones (L). Note the crust also coats the overhang surfaces (mowed). For locality details see Fig. lb. Scale divisions are 2 em.
10
due to evaporation or CO 2 loss by degassing while in biogenic forms the precipitation of carbonate appears to have been induced by organisms, probably as a result of CO2-uptake during photosynthesis or by the " d u m p i n g " of excess Ca by fungi (Phillips et al., 1987). In abiogenic forms the lamination is typically very fine while the microstructure is dense and micritic. However, microbial remains, calcified or uncalcified, and rootstructures can be incorporated. It must be stressed that such dense, micritic forms represent an end member where biological influence cannot be demonstrated petrographically. Biogenic laminar calcretes are distinguished by having a strongly biogenic microfabric and are usually less evenly and finely laminated than abiogenic forms. The biogenic lamination may sometimes show typical stromatolitic features such as thickening over topographic highs, or even with tufted relief (Wright, 1981). The microfabric is typically wrinkled or contorted and a variety of calcified microbial remains m a y be present attri-
V,P. W R I G H T
butable to calcified fungal hyphae or other microorganisms, or root hairs (James, 1972; Harrison, 1977; Knox, 1977; Klappa, 1979b; Beier, 1985; Phillips et al., 1987). In the case of lichen crusts the lamination may be especially convolute (Klappa, 1979a) and m a y contain organic laminae, irregular fenestral pores, and a variety of spherical structures formed by the calcification of coccoid cyanobacteria or representing perithecial structures. These lichen forms are most readily recognized by their association with abundant microborings and they m a y also show evidence of the concentration of metallic ions (Esteban and Klappa, 1983). Rhizolite crusts also have a very strongly biogenic fabric (Wright et al., 1988), and are composed of a dense, intertwined mat of rhizocretions defined by laminated walls, locally with alveolarseptal structures. In summary, having recognized a laminated carbonate as probably being of terrestrial origin, it is the degree of biological control on the laminae
Fig. 12. Calcrete crust from Bembridge Limestone (Lower Tertiary), Prospect Quarry, Isle of Wight, England. This crust occurs within a series of palustrine limestones with freshwater and terrestrial molluscs. The crust shows several typical calcrete features such as banding due to the degree of organic staining, slight thickening into depressions and small unconformities. The microfabric is a dense micrite showing no biogenic features and the crust is interpreted as an abiogenic form. Some of the clasts in the calcrete profile consist of reworked rhizolite laminar calcrete.
T E R R E S T R I A L S T R O M A T O L I T E S A N D L A M I N A R CALCRETES: A REVIEW
form and microstructure which weill indicate whether it was biogenically or abiogenically formed.
Calcrete-coated grains There is a considerable volume of literature on "vadose" pisoids or vadoids (see review by Peryt, 1983), and two broad categories can be recognized. One type is formed by simple cementation (and often has a radial fabric), and the other type is formed microbially (i.e. terrestrial oncoids). There are many descriptions of the former type, which are commonly associated with supratidal deposits and tepee structures (Dunham, 1969; Esteban and Pray, 1983; Ruiji and Yaosong, 1983). The latter have received less attention but have been carefully described by Calvet (1982) and Calvet and Julia (1983). Calcrete-coated grains are characterised by a variety of features some of which have been reviewed by Read (1976). One very common aspect of calcrete-coated grains is their occurrence in inversely graded horizons. This is a feature also associated with coated grains formed in supratidal environments and its origins have been reviewed by Esteban and Pray (1983). Other features include evidence of early fracturing and re-growth, and association with geopetal "vadose silts", and meniscus cements. Several fabric features indicate that some of the coated grain growth is by in-situ processes not involving any actual movement of the grains. The laminae may show downward elongation (pendant morphology) (or even rarely upward-thickening), perched inclusions on the upper surfaces of the laminae, or the laminae of adjacent coated grains accommodate each other to form polygonal coatings (Esteban and Pray, 1983) (Fig. 10). It is the authors experience that these in-situ features are rarely seen in calcrete-coated grains, where the actual grains are rotated during growth by movement down-slopes (Read, 1974; Arakel, 19820. Calcrete-coated grains have other distinctive features. Firstly the coatings tend to be very fine grained and thin (simple envelopes of micrite), and are typically non-isopachous (James, 1972; Calvet and Julia, 1983; Warren, 1983). Larger forms with many laminae may be more regular in
11
shape (Read, 1974), being sub-spherical to spherical and well rounded (Fig. 8). This reflects the degree of turning during down-slope movement. Forms on longer, steeper slopes are probably more evenly coated than forms which have undergone relatively short transport. One common feature of calcrete pisoids are bridge structures either connecting grains or as simple or branched growths into pore spaces (Fig. 10). These outgrowths of the coating laminae have the same microfabric as the grain cortices and are probably calcified fungal mycelial strands (Calvet, 1982; Calvet and Julia, 1983). Both the coatings and the outgrowths in many recent to Palaeozoic pedogenic coatings contain abundant microbial structures such as calcareous tubes and needlefibre calcite (Knox, 1977; Calvet, 1982; Calvet and Julia, 1983; Wright, 1986; Phillips and Self, 1987; Phillips et al., 1987). Distinguishing calcrete-coated grains from other forms of coated grain will depend on the recognition and evidence of pedogenic processes. Recognizing microbial pedogenic coated grains as against abiogenic forms will depend on the identification of microbial fabrics including the distinctive microbial braces and mycelial outgrowths.
Summary Many laminated carbonates in terrestrial settings are biogenic in origin. Previously they had been considered as largely abiogenic precipitates and referred to as laminar calcretes Or calcrete ooids or pisoids. However, many such forms are the products of root or microbial activity. The latter group, by definition, are stromatohtes and oncoids, and are here termed terrestrial stromatolites and oncoids. Laminar calcretes formed by root activity (rhizolites) are a form of biogenic pseudostromatolite. Distinguishing terrestrial laminites from marine microbial forms is dependent on recognizing associated subaerial exposure-related and pedogenic features in the former. Distinguishing abiogenic terrestrial laminites from both terrestrial stromatolites and oncoids, and rhizolites, depends on recognizing the nature and degree of biological control on the formation of the lamination.
12
Microbial laminites (stromatolites and oncoids) are not restricted to shallow marine, lacustrine or fluvial settings but are important components in many present-day and ancient calcrete profiles.
Acknowledgements I should like to especially thank Sally Phillips (University of Adelaide) for introducing me to the South Australian calcretes figured in this paper. The paper greatly benefited from the constructive comments by Robert Riding and an anonymous reviewer. Richard Lewis is thanked for technical assistance with SEM. Alison Ruegg and Elizabeth Wyeth skilfully typed the final manuscript. This paper is part of a project on paleosols partly supported by the Nuffield Foundation.
References Arakel, A.V., 1982. Genesis of calcrete in Quaternary soil profiles, Hurt and Leeman lagoons, Western Australia. J. Sediment. Petrol., 52: 109-125. Beier, J.A., 1985. Diagenesis of Quaternary Bahamian beachrock: petrographic and isotopic evidence. J. Sediment. Petrol., 55: 755-761. Burne, R.V. and Moore, L.S., 1987. Microbialites: organosedimentary deposits of benthic microbial communities. Palaios, 2: 241-254. Callot, G., Guyon, A. and Mousain, D., 1985. Inter-relations entre aiguilles de calcite et hyphes mycrliens. Agronomie, 5: 209-216. Calvet, F., 1982. Constructive micrite envelopes developed in vadose continental environments in Pleistocene eolianites of Mallorca (Spain). Acta Geol. Hispanica, 17: 169-178. Calvet, F. and Julia, R., 1983. Pisoids in the caliche profiles of Tarragona, north east Spain. In: T.M. Peryt (Editor), Coated Grains. Springer, Berlin, pp. 456-473. Campbell, S.E., 1979. Soil stabilization by a prokaryotic desert crust: implications for Precambrian land biota. Origins Life, 9: 335-348. Coniglio, M. and Harrison, R.S., 1983. Holocene and Pleistocene caliche from Big Pine Key, Florida. Bull. Can. Pet. Geol., 31: 3-13. Curtis, L.F., Courtney, F.M. and Trudgill, S.T., 1976. Soils in the British Isles. Longmans, London, 364 pp. Dunham, R.J., 1969. Vadose pisolite in the Capitan Reef (Permian), New Mexico and Texas. Soc. Econ. Paleontol. Mineral., Spec. Publ., 14: 182-191. Esteban, M. and Klappa, C.F., 1983. Subaerial exposure environment. In: P.A. Scholle et al. (Editors), Carbonate Depositional Environments. Am. Assoc. Pet. Geol. Mem., 33: 1054.
V.P. WRIGHT
Esteban, M. and Pray, L.C., 1983. Pisoids and pisolitic facies (Permian), Guadelupe Mountains, New Mexico and West Texas. In: T.M. Peryt (Editor) Coated Grains. Springer, Berlin, pp. 503-537. Freytet, P. and Moissenet, E., 1983. Presence de restes algaires identifiables dans des crohtes calcaires plio-quaternaires du Nord-Est de l'Espagne. C.R. Acad. Sci. Paris, 296: 1563-1566. Golubic, S. and Campbell, S.E., 1979. Analogous microbial forms in recent subaerial habitats and in PRecambrian cherts: Goeothece coerulea Geitler and Eosynechococcus modrei Hoffman. Precambr. Res., 8: 201-217. Harrison, r.S., 1977. Caliche profiles: indicators of near surface subaerial diagenesis, Barbados, West Indies. Bull. Can. Pet. Geol., 25: 123-173. James, N.P., 1972. Holocene and Pleistocene calcareous crust (caliche) profiles: criteria for subaerial exposure. J. Sediment. Petrol., 42: 817-836. Jones, R.J., 1965. Aspects of the biological weathering of limestone pavements. Proc. Geol. Assoc. London, 76: 421-433. Kahle, C.F., 1977. Origin of subaerial Holocene calcareous crusts: role of algae, fungi and sparmicritisation. Sedimentology, 24: 413-435. Kennard, J.M. and James, N.P., 1986. Thrombolites and stromatolites: two distinct types of microbial structures. Palaios, 1: 492-503. Klappa, C.F., 1979a. Lichen stromatolites: criterion for subaerial exposure and a mechanism for the formation of laminar calcretes (caliche). J. Sediment. Petrol., 49: 387-400. Klappa, C.F., 1979b. Calcified filaments in Quaternary calcretes: organomineral interactions in the subaerial vadose environment. J. Sediment. Petrol., 49: 955-968. Knox, G.J., 1977. Caliche profile formation, Saldanha Bay (South Africa). Sedimentotogy, 24: 657-674. Krumbein, W.E. and Giele, C., 1979. Calcification in a coccoid cyanobacterium associated with the formation of desert stromatolites. Sedimentology, 26: 593-604. Multer, H.G. and Hoffmeister, J.E., 1968. Subaerial laminated crusts of the Florida Keys. Geol. Soc. Am. Bull., 79: 183-192. Paszkowski, M., 1983. Algal sedimentary structures in the Dinantian limestones in Krzeszowice area (in Polish, English summary). Prz. Geol., 4: 254-258. Peryt, T.M., 1983. Vadoids. In: T.M. Peryt (Editor), Coated Grains, Springer, Berlin, pp. 437-449. Phillips, S.E. and Self, P.G., 1987. Morphology, crystallography and origin of needle-fibre calcite in Quaternary pedogenic calcretes of South Australia. Aust. J. Soil Res., 25: 429-444. Phillips, S.E., Milnes, A.R. and Foster, R.C., 1987. Calcified filaments: an example of biological influences in the formation of calcrete in South Australia. Aust. J. Soil Res., 25: 405-428. Picha, F., 1978. Depositional and diagenetic history of Pleistocene and Holocene oolitic sediments and sabkhas in Kuwait, Persian Gulf. Sedimentology, 20: 365-389.
T E R R E S T R I A L S T R O MAT OLI TES A N D L A M I N A R CALCRETES: A REVIEW
Pomar, L.,, Esteban, M., Lhmona, X. and Fontarnau, R., 1975. Acci6n de Liquences, Algas y Hongas en la Telodiag6nesis de las Carbonatados de la zona litoral, perilitoral Catalana. Inst. Invest. Geol. Univ. Barcelona, 30: 83-117. Purser, B.H. and Loreau, J.P., 1973. Aragonitic supratidal encrustations on the Trucial Coast, Persian Gulf. In: B.H. Purser (Editor), The Persian Gulf. Springer, Berlin, pp. 343-376. Read, J.F., 1974. Calcrete deposits and Quaternary sediments, Edel Province, Shark Bay, Western Australia. Am. Assoc. Pet. Geol. Mem., 22: 250-282. Read, J.F., 1976. Calcretes and their distinction from stromatohtes. In: M.R. Walter (Editor), Stromatolites. Elsevier, Amsterdam, pp. 55-71. Robbin, D.M. and Stipp, J.J., 1979. Depositional rate of laminated crusts, Florida Keys. J. Sediment. Petrol., 49: 175-180. Ruiji, C. and Yaosong, X., 1983. Vadose pisolites of the Tongying Formation (Upper Sinian System) in southwest China. In: T.M. Peryt (Editor), Coated gains. Springer, Berhn, pp. 538-547. Southgate, P.N., 1986. Cambrian phoscrete profiles, coated grains and microbial processes in phosphogenesis: Georgina Basin, Australia. J. Sediment. Petrol., 56: 429-441. Verrecchia, E., 1987. Le contexte morpho-dynamique des
13 crofites calcaires: apport des analyses s6quentielles l'6chelle microscopique. Z. Geomorphol., N.F., 31: 179-193. Verrecchia, E. and Freytet, P., 1987. Interference pedog6n6se's6dimentation dans les crofites calcaires. Proposition d'une nouvelle methode d'6tude: L'analyse sequentielle. In: N. Fedorof et al. (Editors), Micromorphologie des Sols, Actes de la VII6me R6union Internationale de Micromorphologie des Sols, Paris, Juillet 1985, pp. 555-561. Warren, J.K., 1983. Pedogenic calcrete as it occurs in Quaternary calcareous dunes in coastal South Austraha. J. Sediment. Petrol., 53: 787-796. Wright, V.P., 1981. A subaerial stromatohte from the Lower Carboniferous of South Wales. Geol. Mag., 118: 97-100. Wright, V.P., 1986. The role of fungal biomineralization in the formation of early Carboniferous soil fabrics. Sedimentology, 33: 831-838. Wright, V.P., 1990. Limestone constituents. In: M.E. Tucker and V.P. Wright (Editors), Carbonate Sedimentology. BlackweU, Oxford (in press). Wright, V.P. and Wilson, R.C.L., 1987. A terra-rossa-like paleosol complex from the Upper Jurassic of Portugal. Sedimentology, 34: 259-273. Wright, V.P., Platt, N.H. and Wimbledon, W.A., 1988. Biogenic laminar calcretes evidence of calcified root mat horizons in paleosols. Sedimentology, 35: 603-620.