Biogenic caliches in Texas: The role of organisms and effect of climate

Sedimentary Geology 222 (2009) 207–225

Contents lists available at ScienceDirect

Sedimentary Geology

j ourna l homepage: www.e lsev ie r.com/ locate /sedgeo

Biogenic caliches in Texas: The role of organisms and effect of climate

Jie Zhou ⁎, Henry S. ChafetzDepartment of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA

⁎ Corresponding author.E-mail address: [email protected] (J. Zhou).

0037-0738/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.sedgeo.2009.09.003

a b s t r a c t
a r t i c l e i n f o
Article history:Received 12 January 2009Received in revised form 14 August 2009Accepted 1 September 2009

Keywords:CalicheBiogenicTexasSoil biotaClimate

Biogenic constituents are ubiquitous and abundant in the caliches of Texas. Investigation of 51 calicheprofiles on various host strata (alluvium, limestone, igneous rocks, etc.) across approximately 900 km ofTexas from subhumid east to arid west has shown that 43 of these profiles exhibit prominent biogenicconstituents. These profiles exhibit significant differences in thickness (varying from centimeters to meters)and maturity (varying from I to VI). All of the different caliche facies are composed of low-Mg calcite.Biogenic features generally occur in the upper part of the profiles, including the uppermost portion ofmassive caliche horizons, platy horizons, laminar crusts, and pisoids. The main biogenic caliche facies includerhizoliths (calcified root structures), stromatolite-like laminar crusts, and coated grains. Compared to theabiogenic massive micritic to microsparitic calcite groundmass, biogenic constituents are morphologicallydistinct. These biogenic constituents are composed of several microscopic mineral components, includingcalcified filaments, needle fiber calcite (e.g., single crystalline needles and needle pairs, triangular crystals,and polycrystalline chains of rhombohedrons), spherulites, micro-rods, and nano-spheres. A large number ofcalcified root cellular structures and micro-organisms, e.g., fungal filaments, actinomycetes, and rod-likebacteria, are also present. Plant roots as well as soil biota produce distinctive structures and also enhancelithification by inducing calcite precipitation in the caliches, i.e., biologically controlled or influencedprocesses.Host strata did not significantly influence the abundance nor type of biogenic features in the caliches. Incontrast, climate had an evident effect on the development of biogenic constituents in these caliches in termsof the amount as well as type. The thickness of laminar crusts and grain coatings and the abundance of bioticconstituents within those facies decrease as the climate shifts from subhumid and subarid, in southeast andcentral areas, to subarid and arid, in west and northwest Texas. In addition, root structures and micro-rodsdiminish significantly from the subhumid east to the arid west.

l rights reserved.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Caliche (synonymous with calcrete) is an authigenic calciumcarbonate accumulation which forms mainly in near-surface terres-trial environments. It is mainly distributed in semiarid to arid areas,and is widely used to reconstruct paleoclimatic conditions (Reeves,1976; Alonso-Zarza, 2003). Caliches have been studied for over acentury but the role of organisms in the formation of caliches wasinitially given little attention. In the last three decades, this topic hasreceived considerably more attention and a variety of biotic featureshave been documented (Klappa, 1979a,b, 1980;Wright, 1986; Phillipsand Self, 1987; Phillips et al., 1987; Jones and Ng, 1988; Verrecchiaet al., 1995;Wright et al., 1995; Alonso-Zarza et al., 1998; Kosir, 2004).Because caliches form in a surficial environment, plant roots andassociated micro-organisms are diverse and abundant. Consequently,

there are a great variety of biogenic processes and their resultantproducts involved in the formation of caliche.Wright (1990) classifiedall biogenic constituents as the beta fabrics, which are distinctlydifferent in morphology from the alpha (abiogenic) fabrics. Since1990, additional biogenic components have been documented,including needle fiber calcite (NFC), spherulites, and micro-rods(Jones and Kahle, 1993; Verrecchia an Verrecchia, 1994; Verrecchiaet al., 1995; Loisy et al., 1999). Although plant roots, fungi, algae, andbacteria have been widely accepted as the major contributors to mostcommon biogenic features, the origins of several beta-type fabrics,e.g., NFC, spherulites, and Microcodium, and the specific roles ofdifferent taxa in the formation of these biogenic constituents are notwell understood. In addition, the development of biogenic caliches isinfluenced by climate and host rock, but this has not been extensivelydiscussed in previous studies. In Texas, caliches are widespread andabundant on various types of host strata and form throughout a widerange of climatic conditions. This provides a good opportunity tostudy the characteristics and genesis of biogenic caliches and theinfluence of three major controlling factors, including climate, hostrock, and soil biota.

mailto:[email protected]

http://dx.doi.org/10.1016/j.sedgeo.2009.09.003

http://www.sciencedirect.com/science/journal/00370738

Fig. 1. Distribution of caliche outcrops in Texas on various types of host rock. The distribution of caliche outcrops is divided into five areas based on the underlying lithology (shown in different symbols with the number of outcrops of eachlithology indicated) and climatic conditions indicted by dashed lines and circled numbers: (1) the Gulf Coastal area, (2) south Texas, (3) central Texas, (4) northwest Texas, and (5) west Texas. The solid lines are isohyets, showing the meanannual precipitation (mm).

208J.Zhou,H

.S.Chafetz/Sedim

entaryGeology

222(2009)

207–225

Fig. 2. Idealized well-developed caliche profiles on sandy to muddy (left) and gravelly(right) substrates in Texas. Less well-developed caliche profiles tend to have calcic and/or petrocalcic horizons with or without nodules. Macroscopic biogenic caliches areindicated in the profiles.

209J. Zhou, H.S. Chafetz / Sedimentary Geology 222 (2009) 207–225

2. Geological setting

Texas has a complex tectonic history since the Pre-Cambrian, andthus, it is underlain by a variety of host rocks of significantly differentages. Caliches are ubiquitous in Texas and form on the variousexposed strata. In a small area of central Texas, Pre-Cambrian igneousand metamorphic rocks are exposed whereas large areas of centralTexas and parts of west Texas are underlain by Cretaceous marinelimestone. Parts of west Texas are covered by Tertiary volcanic rocksand northwest Texas (Southern High Plains) has prominent Mioceneand Quaternary eolian accumulations. South Texas, including thecoastal area, has alluvium ranging in age from Tertiary to the present.

Texas also has marked climatic variations across its vast area, inpart due to the proximity to the Gulf of Mexico in the east and adiversified landscape. Mean annual precipitation ranges from1400 mm in the east to 200 mm to the west (Fig. 1). Consequently,it is humid in the east, subhumid to subarid in south and central Texas,subarid in northwest, and subarid to arid in the west.

Caliches are widespread and abundant in Texas. The calicheprofiles range in thickness from a few centimeters to several meters,mostly between 1 and 2 m. Their degree of development also variesgreatly. The deposits may occur simply as discrete nodules dispersedin soil matrix, or as thin, discontinuous laminar crusts immediatelyabove the substrates. Well-developed profiles are common in mostareas of Texas and composed of, from bottom to top, nodular horizon(or locally continuous soft calcic horizons or crack-fills), well-indurated massive caliches (petrocalcic horizons), platy horizons,laminar crusts, and pisoids (Fig. 2). With the exception of petrocalcichorizons in northwest Texas which are of Tertiary age, caliches inmost other areas are directly exposed on the surface, with or withouta top soil, and are of late Quaternary age (West, 1986; Blum andValastro, 1992, 1994; Humphrey and Ferring, 1994; Nordt et al.,1994; Prouty, 1996; Jolley et al., 1998; Monger et al., 1998; Buck andMonger, 1999).

3. Material and methods

Fifty-one caliche profiles were investigated and they formed ondifferent types of host rock separated by as much as 900 km (Fig. 1).For each profile, all caliche facies, including host rocks, were loggedand sampled in the field. The microscopic features of all the calichefacies were studied by standard binocular, petrographic, and scanningelectronic microscopy (SEM) (JEOL 6330F). Freshly broken, non-etched samples were coated with carbon and viewed using anelectron beam of 15 kv and 12 µA. Elemental compositions of calichecomponents were identified by ISIS EDEX as part of the SEM. Inaddition, the bulk mineralogy of each caliche facies was alsodetermined by the X-ray diffraction. Samples were finely ground bymortar and pestle and sieved through a 44 µmmesh. All samples weredried in the oven at 110 °C for 12 h and analyzed using a SiemensD5000 diffractometer run through a 2θ range of 4–32°.

4. Results

Biogenic constituents are ubiquitous in the caliches of Texas on thevarious strata, with or without topsoil at the present day. The calciumcarbonate components of caliches with prominent biogenic constitu-ents are all low-Mg calcite. Among 51 outcrops, 34 exhibit abundantbiogenic constituents, nine exhibit smaller amounts of biogenicconstituents, and only eight of them contain few or no readilyapparent biogenic constituents. The biogenic constituents occur in allcaliche facies, but they tend to concentrate in the upper part of calicheprofiles, which include platy horizons, laminar crusts, pisoids, and theuppermost part of massive caliches (Fig. 2). The caliches without anybiogenic features are generally structureless and composed of micriticto microsparitic, rhombohedral to blocky calcite crystals. Compared to

the abiogenic caliches, biogenic caliches are volumetrically lesssignificant but exhibit much more variation in morphology (Fig. 3).

4.1. Macroscopic features

Biogenic caliches comprise several prominent horizons in theuppermost portions of well-developed caliche profiles and can also beabundant in several minor components of caliches such as nodules,crack-fills, and tepee cement. Themost commonmacroscopic forms ofbiogenic caliches in Texas are rhizoliths, laminar caliche crusts, andpisoid coatings.

4.1.1. RhizolithRhizoliths are organo-sedimentary structures formed by calcite

cementation within or around plant roots (Klappa, 1980). Rhizolithscommonly occur in central and south Texas, and are scattered innorthwest and west Texas. They mostly form in white to moderatebrown, massive to roughly laminated hardpans, white to grayish pinkplaty horizons, and porous white chalky massive caliches. As modernanalogs, living plant roots are common in caliche profiles. Some plantroots extend vertically and horizontally up to several meters and mayform well-developed root mats (Fig. 4A). Rhizogenic calichesgenerally are centimeters to tens of centimeters thick, but somecaliche profiles with abundant root structures in central Texas are upto 1 m thick.

The most prominent macroscopic feature of rhizogenic caliche inTexas caliches is the presence of numerous submillimeter-sized holesor cylindrical channels (root tubules or root molds of Klappa, 1980)with relatively smooth rounded walls in the hardpans (Fig. 4B). Thesetiny pores generally are randomly distributed or form laminated or

Fig. 3. Summary of microscopic features (including abiogenic and biogenic features) of the caliches in Texas (modified from Wright, 1990, 2007). Biogenic features exhibit greatervarieties in morphology than abiogenic features.

210 J. Zhou, H.S. Chafetz / Sedimentary Geology 222 (2009) 207–225

anastomosing structures due to extension of plant roots or rootlets thatcommonly are outlined by brown micrite. Centimeter-scale roundedholes also exist in the rhizoliths and some of them have walls coatedwith (concentric) brownmicritic layers up to 1 cm thick. Some isolatedcentimeter-sized patches exhibit weakly developed concentric brownmicrite coatings but are completely or partiallyfilledwithbrownmicriteand chalky deposits. Compared to the surrounding groundmass, thebrownmicrite, either massive or concentrically laminated, is dense andwell-indurated, and has fewer siliciclastic grains.

4.1.2. Laminar caliche crustLaminar caliche crusts are one of the most common biogenic

caliche forms in Texas. They develop on all types of substrates andoccur at the top of the caliche profiles unless there are caliche breccias,pisoids, or modern soils on top (Fig. 2).

In general, the laminar crusts are characterized by the alternationof layers of different colors and textures, which are evident in handspecimens (Fig. 4C and D). However, there exist considerablevariations of laminar caliche crusts in terms of color, thickness,induration, and internal structure. The color of laminar crusts variesfrom white, buff, to reddish brown, and gray, dark green, and evenblack. The thickness of the laminar crusts ranges from a fewmillimeters to 10 cm. The development of laminar caliche cruststends to mimic the microtopography of its substrates (e.g., platy ormassive horizons or host rock). The laminar crusts tend to thicken intomicro-low areas and gradually pinch out toward the micro-highareas; this eventually produces a planar upper surface (Fig. 4C).Therefore, the lateral extension of laminar caliche crusts is generallyless than tens of meters. If the underlying caliches are brecciated,laminar crusts can also penetrate downward up to several centimetersand preferentially grow around brecciated caliche clasts, forminganastomosing or roughly concentric patterns.

Other differences between laminar crusts include those withrelatively sparse laminations, whereas some have many closely spacedthin laminae. Contacts between laminations are either distinct orindistinct, which may be outlined by discontinuous dark-coloredpigments (Fe–Mn oxides), or by the difference in the texture of theindividual lamina (Fig. 4E). Laminations can be relatively flat (Fig. 4D),highly undulatory (Fig. 4E), or even stromatolite-like (Fig. 4C). Fenestral-like planar voids tend to occur along the laminations in the upper part ofthe crusts; desiccation cracks may also occur vertically across thelamination and are filled with later generations of micritic calcite.

Commonly, multiple episodes of laminar caliche crusts are super-imposed and form a thick complex caliche crust. Caliche profiles insouth and west Texas, for example, have complex laminar crusts thatare composed of 2 to 3 different sub-crusts. These sub-crusts mayhave similar appearances with intervening thin layers which are richin terrigenous sediment or pisoids (Fig. 4D), or exhibit markedlydifferent colors and textures (Fig. 4E).

4.1.3. Pisoid coatingPisolitic facies were only observed in south andwest Texas, though

they were also reported from the San Angelo (west side of centralTexas) (Turner, 1966) and northwest Texas (Gustavson, 1996).Pisoids usually occur at the top of the caliche profiles, mostlyoverlying or alternating with the laminar crusts (Fig. 4D and E).Well-developed pisolites, a well-cemented caliche hardpan composedmainly of pisoids, only occur in localized areas (e.g., south Texas).However, dispersed millimeter-sized pisoids are common in thecaliche hardpans, commonly alternating with laminar crusts andforming thick (up to 10 cm) complex caliche hardpans.

Pisoids range in size from less than 0.2 mm to a few centimeters indiameter, and the largest one found was 6 cm long. The shape ofpisoids can be spherical, oval, elongated, or even irregular based on

Fig. 4. Biogenic caliche facies on various types of host rock. (A) Caliche hardpan (H) and modern root mats (M) on Cretaceous marine limestone (central Texas). The hardpan isenriched in root structures. (B) Rhizolith on non-gravelly alluvium (south Texas) exhibits roughly laminated structures (open arrows) and numerous sub-millimeter to centimeterscaled holes (solid arrows). (C) Stromatolite-like laminar crust (L) on gravelly alluvium (central Texas). (D) Complex laminar horizon composed of thin laminae (L) alternating withpisoid layers (P) on non-gravelly alluvium. (E) Pisoids (P) and laminar crusts (L) on volcanic rocks (dacite?) (west Texas).


the shape of the nucleus and the development of coatings. As thecoatings grow thicker, the sphericity of the pisoids increases. Themostprominent feature of the pisoids is that they all have coatings withroughly concentric structures. The development of the coatings variessignificantly. The thickness of the coatings ranges from sub-millimeterto several millimeters. Overall the coatings account for only a minorportion of the coated grains (superficial pisoids). The thicknessaround a nucleus is uniform and no obvious downward thickening hasbeen observed. In general, pisoid coatings are composed of filament-rich micrite, microsparitic calcite (including spherulites), and a minoramount of siliciclastic grains.

The composition of the pisoid coatings is generally uniform,whereas the nuclei vary significantly. In general, there are two dif-ferent types of nuclei: one is rock fragments from the substrates oradjacent areas, and the other is in situ brecciated caliche fragments.Both of them are common and some nuclei are composed of both hostrock fragments and caliche fragments (Fig. 4E). Some of the nucleihave corroded margins due to the dissolution and replacement bycalcite and some display desiccation cracks.

Compound pisoids are formed as a result of the coalescence ofseveral pisoids in an admixture with other sediment encased byconcentric coating. Commonly, pisoids are joined and cementedtogether by interstitial micrite, forming honeycomb-like caliches(Fig. 4E) or indurated pisolitic caliche hardpans.

4.2. Microscopic features

There is a variety of biogenic features observed at the microscopicscale (Fig. 3). The large microscopic structures (sub-millimeter to100 μm) are made up of root structures, roughly horizontal laminarstructures, and pisoid coatings. The finer microscopic (100 μm to100 nm) components include alveolar-septal fabric, calcified fila-ments and spores/sporangia, needle fiber calcite (NFC), spherulites,micro-rods, and nano-spheres.

4.2.1. Rhizoliths (root structures)Root structures have been identified mainly by size, external

morphology, and internal structure (Klappa, 1980). The origin of some

Fig. 5. Photomicrographs (A–B, cross-nicols) and SEM images (C–D) of calcified (A–C) and silicified (D) root structures. (A) Abundant roots are largely dissolved and preserved asring-like structures. (B) The well-preserved transverse view of a root structure on the right exhibits cortical tissues with polygonal cells and part of the epidermis. (C) A root moldwith a hollow center and concentric epidermal sections. (D) Longitudinal view of a silicified root displays rectangular root cellular structures.


root structures is still in debate, e.g., Microcodium (Klappa, 1978;Freytet and Plaziat, 1982; Kosir, 2004).

In transverse view, plant roots generally display roughly concen-tric, ring-like structures, mostly 100 to 500 μm in diameter (Fig. 5A, B,and C). Well-preserved root structures are rare in Texas caliches andmost are only partially preserved. The medulla tissues (includingxylem and phloem) are, in many cases, missing and a hole remains inthe center of the structure. In the well-preserved cortical sections, theroughly polygonal root cellular structures are plainly evident (Fig. 5Band D). Generally, the epidermal parts of plant roots are preservedand composed of microsparitic or micritic calcite, needle fiber calcite(NFC), and micro-rods and nano-sized spherical bodies (i.e., nano-spheres).

In longitudinal sections, well-preserved calcified roots display acharacteristic reticulated pattern that principally consists of elongatedrectangular root cells. Well-preserved cellular structures wereobserved in a silicified plant root from west Texas, which is probablyan epidermal part of a root tubule (Fig. 5D). Some partially preservedroot structures, display vascular cellular structures in the center andseem to be composed of several parallel micritic lineaments. In somecases, root traces are only indicated by the tubular holes or cylindricalchannels described previously, 100 to 200 μm in diameter, i.e., rootmolds (Klappa, 1980).

4.2.2. Laminar structures and pisoid coatingsThe horizontal laminae and the concentric coatings of pisoids are

similar in their microscopic morphology, though the macroscopicshapes are different. Most of the laminar crusts with highlyundulatory laminae resemble stromatolites that have formed in theshallow marine environment (Fig. 6A). Vertical columns (or domes)grow in juxtaposition with each other, which may or may not be

separated by vertical voids. The individual columns are 0.1 to 1.5 mmwide and up to 4 mm high. Vertically, the width of the columns is notconstant and the magnitude of arching of the wavy laminae alsovaries. The domes may ramify or coalesce vertically.

The laminae are mainly composed of dark-colored organic-richlayers, e.g., calcified filaments and micritic layers, and light-coloredspherulites and/or microsparitic layers (Fig. 6B). The thickness of theindividual laminae and the density of organic matter or spherulitesalso vary laterally. Silt- to fine-grained sand-sized siliciclastic grainsareminor in volume but very common, interspersed along the laminaeand between the columns.

Similar to the laminar crusts, the laminae in the pisoid coatings arealso composed of dark-colored layers that are enriched in organicmatter andmicrite intercalated with light-colored layers composed ofmicrosparitic calcite (Fig. 6C). The thicknesses of the coatings, thequality of laminations, and the number of laminae within the coatingsvary significantly. Filaments are the major component of pisoidcoatings and some of them may penetrate into the nucleus and formsmall pisoids (Fig. 6D). In contrast to laminar crusts, spherulites (seeSection 4.2.6) are rare in the pisoid coatings. Similar to horizontallaminar crusts, fine-grained siliciclastic particles are also common inpisoid coatings. In addition, there are numerous small pisoids inlaminar crusts or pisolitic facies, which can only be observed underthe microscope. They are mostly 200 to 500 µm in diameter anddisplay well-developed concentric structures (Fig. 7A and B). Some ofthese small coated grains are closely associated with root structures,and their coatings are composed mainly of filaments and NFCs.

4.2.3. PeloidsPeloids (or pellets) are tiny micritic grains without internal

structures. Peloidal grains are up to 300 μm in diameter and vary in

Fig. 6. Photomicrographs (cross-nicols) of laminar crusts and pisoids. (A) Stromatolite-like laminar structures composed of juxtaposed vertical columns (or domes), displaying wavy(or undulatory) morphology. (B) Individual column (or dome) consists of bright-colored spherulite-rich layers alternating with dark-colored organic-rich (filaments) layers.(C) Pisoid has multiple generations of coatings around igneous rock fragment (IRF) with intervening (only locally distributed) microspar laminae. (D) Pisoid composed of well-developed coatings around a reworked caliche fragment (RCF). The dark-colored, concentric coatings contain abundant filaments and some of them penetrate into the nucleus,forming incipient small pisoids (or ooid) (arrow).

Fig. 7. Photomicrographs (cross-nicols) of pisoids that have poorly developed (A) andwell-developed (B) concentric coatings with and without siliciclastic grains in thenuclei.

Fig. 8. Photomicrographs (cross-nicols) of (A) dense peloids randomly distributed in avoid, together with interstitial microspar cement forming a mottled fabric and(B) weakly cemented peloids closely associated with root structures (arrow).



shape from spherical to sub-spherical or even irregular. Some peloidsare dense and resemble the surrounding micritic groundmass.Cementation of a cluster of micritic peloids by microspar produces amottled fabric (Fig. 8A). Some peloids are porous and are intimatelyassociated with root structures (Fig. 8B).

4.2.4. Calcified filaments and calcispheresCalcified filaments are one of the most common biogenic

components that constitute laminar crusts and pisoid coatings. Theyrange in color from brownish to dark green and exhibit differentgrowth structures. They form sub-parallel bundles, irregular inter-twined clusters, or even bush-like patterns.

The filaments in caliches exhibit significant differences in size andshape, and they are calcified in different manners. SEMmeasurementsof 155 filaments found that 33 were calcified tubiform filaments, 56calcified solid filaments, 59 partially calcified to uncalcified filaments,and seven were holes left by the decay of filaments. The externaldiameters (or width if collapsed) of the filaments and the diameter ofholes (if filaments are completely dissolved) are between 0.2 and32 μm, mostly between 2 and 10 μm (Fig. 9), and the length of anindividual filament is up to 250 μm. Most of the filaments are slightlycurved, however some well-lithified rod-like filaments are straight.

Well-developed calcified filaments tend to keep the original tubi-form shapewith a central circular hole filled or empty (Fig. 10A–C). Themorphology of the isodiametric filaments is preserved by the encrus-tation of calcite on the sheaths of the filaments and a tube is left whenthe organic matter decays. The external and internal diameters of thetubiformfilaments range from2 to 22 μmand0.6 to 20 μm, respectively,and the thickness of the encrustation ranges from 0.5 to 5 μm. Crystalsthat precipitated as encrustations around thefilaments display differentcrystal habits and patterns. The sheaths/walls of the filaments generallyact as substrates for the precipitation and growth of crystals. Aroundsome tubiform calcified filaments, calcite crystals with rhombohedralterminations, 1 to 2 μm long, 1 μm or less wide, grow in a radiatingpattern (Fig. 10A). Similarly, some filaments are encrusted with blade-or rod-like crystals, mostly 1 to 5 μm long and 0.3 to 2 μm wide. Theseneedles are oriented perpendicular to the filaments and also produce aradial pattern. The tubiform filaments with radial encrustationsgenerally have a smooth inner wall and relatively thin encrustation.

Fig. 9. Histograph of the diameters of calcified filaments in the Texas caliches. The size range(1979a). For each type of micro-organism, the solid line represents the common range inrepresent the average size for the different taxa.

Some calcified tubiform filaments have relatively thick encrustations. Inthose specimens, calcite crystal habit is not readily visible. Equant (orblocky) calcite crystals, 1 μmor less in size, and curved micro-rods withbulbous heads, up to 1 μm long and 0.2 to 0.4 μm wide, are cementedtogether to mimic the morphology of the filaments. Solid calcifiedfilaments are formed as a result of calcite impregnation and/orencrustation (Klappa, 1979a; Phillips et al., 1987; Verrecchia andVerrecchia, 1994). These solid filaments range in diameter from 0.35to 32 μm, mostly between 4 and 10 μm. Some of the filaments arecomposed of micron-sized calcite crystals with rhombohedral crystalterminations whereas others are composed of bizarre calcite crystalswith a depression in their crystal face centers (Fig. 10C). Similar to thetubiformfilaments, solidfilamentsmay also be encrustatedwith a radialpattern (Fig. 10D), and the constituent crystals of the encrustation areacicular, generally 1 to 5 μm long and 0.3 to 2 μmwide. The center of thesolid filaments is filled with crystals a few hundred nanometers indiameter.

In addition to calcified filaments, partially calcified or uncalcifiedfilaments, 0.2 to 6 μm in diameter, are also common and few of themmaintain their original tubiform morphology. Most of them arecollapsed and/or are partially decayed. Many of this type are stillmainly composed of organic matter rather than minerals with somecrystals scattered on their surface. The existence of some filaments isindicated by smooth walled holes in the caliche that are similar indiameter to those of the calcified filaments.

Among the calcified filaments, there is a group of filaments that area fewmicrons to tens of microns long and less than 1 μmwide, mostlybetween 0.1 and 0.5 μm (Fig. 10E). These filaments have a signif-icantly different size range than most other calcified filaments. Thesefilaments also display a more or less curved shape and/or dichoto-mous branching pattern.

Closely associated with calcified filaments are calcified sphericalbodies, i.e., calcispheres (Wright, 2007). Most of the spheres arespherical to sub-spherical (Fig. 10F), and some have an oval shape.These spheres range from3 to 20 μmindiameter.Well-calcified spheresare encrusted and/or impregnated with rounded to subrounded calcitecrystals. Moderately calcified spheres are slightly collapsed or have ahollow center. Uncalcified or poorly calcified spheres may be partiallydecayed and collapsed and thus lose their original morphology.

s, horizontal lines near the top, for the different taxa are from Kahle (1977) and Klappasize, and the dashed line represents the less common total range, and the solid dots

Fig. 10. SEM images of calcified filaments and calcispheres (spores). (A) A tubiform filament exhibiting smooth inner wall and encrusted with rhombohedral calcite crystals in aradiating pattern. (B) A tubiform filament with a thick encrustation composed of tiny sub-spherical to spherical crystals. (C) A solid filament composed of crystals which do notexhibit regular crystal shape and most of the crystals have depressions in their face centers (arrows). (D) A solid filament encrusted with needle-like crystals displaying a radiatingpattern. The center of the filament is filled with tiny rounded crystals less than 1 μm in diameter. (E) A filament, 0.2 to 0.5 μm in diameter, displays a dichotomous branching patternand co-exists with micro-rods. (F) Calcispheres (or calcified spores (arrow)) commonly occur in a close association with filaments.


4.2.5. Needle fiber calcite (NFC)Needle fiber calcite (NFC) is abundant in the Texas caliches and

commonly associated with root structures (Fig. 11A–F). Most of theNFCs occur in the channels or cracks and cavities. The typical fabricsproduced by NFCs are: (1) open random meshes in the center, or fill,of a pore space (Fig. 11A and C); (2) alveolar-septal fabrics formed byinterconnected sub-parallel bundles in the intergranular pores or roottubules (Fig. 11B and D); (3) grain coating with a roughly concentricstructure; and (4) convoluted fabric or tangentially coating the wall ofvoids to form a bird nest-like shape.

Needle-like crystals were initially defined as having a L:W (length:width) ratio greater than 6 (Folk, 1965). Needle fiber calcite in theTexas caliches displays a variety of crystal forms and awide range of L:W values. Their basic shape is a rod-like crystal that is mostly 0.5 to

2 μm in diameter (Fig. 11C). The length of an individual rod is up to200 μm, mostly between 10 and 50 μm. One or two pairs of rods tendto occur together to form composite fiber crystals (MA-type NFC ofVerrecchia and Verrecchia, 1994) (Fig. 11E). Some rods are not well-developed and have a hollow center, and also commonly, needlesoccur as blades with more or less curved surfaces. The bladed NFC issimilar in size to that of the rods and has either smooth or serratededges. The rod-shaped and blade-like NFC commonly co-exist in thesame caliche facies associated with other biogenic components, suchas root structures and micro-rods (Fig. 11E). In addition to the fibrouscrystals with smooth surfaces, some NFC display rough surfaces, andsome NFC have well-developed overgrowths and thus displayserrated edges (MB-type of Verrecchia and Verrecchia, 1994). Inwest Texas, some of the NFCs that constitute alveolar-septal fabrics

Fig. 11. Photomicrographs (A–B, cross-nicols) and SEM images (C–F) of needle fiber calcites (NFC). In (A) and (C), needle-like calcite crystals are randomly distributed in voids. In (B)and (D), calcite needles are arranged in bundles (arrows) and interconnected into alveolar-septal fabrics. (E) Needle fiber calcite occurs as one or two pairs of needles, co-existingwith micro-rods. (F) A close-up view of needle bundles in (D) (box) show that the needles are composed of many micron-sized crystals.


are composed of 1 μm size anhedral to subhedral calcite crystals(Fig. 11F).

As a variety of NFC, elongated triangular calcite crystals are com-mon in the caliches from central and south Texas and are intimatelyassociated with needle-like crystals (Fig. 12A). This type of calcitecrystal may reach up to 10 μm in width and 40 μm in length. Thetriangular NFCs are mostly symmetrical and some have a rod-shapedNFC that is less than 0.5 μm in diameter that protrudes from the tip ofthe crystal. Most of the triangular NFC crystals have more or lesscurved faces and smooth edges. These triangular crystals commonlygrow from the walls of the voids toward the centers. They occurrandomly in pore spaces, or as a sub-parallel bundle thatmimics a solidfilament (Fig. 12B).

Additional varieties of polycrystalline chains, include tabularrhombohedrons or rhombohedral chains (type I rhombohedralchain). These are peculiar crystal forms that have been consideredas a variety of NFC (Jones and Kahle, 1993). They are common in the

Texas caliches (Fig. 13A–B). The basic form is a tabular rhombohedronwith a relatively smooth surface (Fig. 13A). These rhombohedrons aremostly 8 to 14 μm long and 6 to 8 μm wide. The thickness of thetabular rhombohedrons ranges from less than 1 to 3 μm. Where thetabular rhombohedrons are thicker, linear surface features composedof a series of discontinuous ridges with intervening grooves are moreprominent (Fig. 13B). These roughly parallel ridges consist of manytiny calcite crystals that are 1 μm or less in size. These types ofrhombohedral chains pile loosely and randomly in voids and cracks,or coat their walls, or constitute alveolar-septal fabrics.

The second typeof rhombohedral chain (type II) displays ablade-likeform and is composed of rhombohedral calcite crystals, typically 4 μmlong and 1 μmwide, and rounded to subrounded calcite crystals, 1 μmindiameter (Fig. 13C). This type of rhombohedral chain is up to 170 μmlong and 10 μm wide, and co-exists with many disseminated rhombo-hedral calcite crystals. The constituent calcite crystals are arranged in alinear fashion parallel to the shorter side of the rhombohedral chains.

Fig. 12. SEM images of triangular crystals. (A) Symmetrical and asymmetrical triangularcrystals commonly grow normal to the walls of voids. The asymmetrical crystals haveneedles on one side which protrude at the tips (arrows). (B) Triangular crystals andtheir varieties are arranged into bundles, forming alveolar-septal fabrics in voids.


These types of rhombohedral chains were only observed as coatings onthe surfaces of sand grains in west Texas and are intimately associatedwith calcified filaments which are also composed of 1 μm size roundedto subrounded calcite crystals.

A third type of rhombohedral chain (type III) is composed of calcitecrystals that partially overlap or connect with each other. Theconstituent rhombohedral calcite crystals are 3 to 23 μm long and 2to 8 μm wide, which are significantly larger than the constituentcalcite crystals in the above-mentioned other two types of rhombo-hedral chains. The geometry of the constituent calcite crystals variessignificantly, ranging from tabular to elongate rhombohedrons(Fig. 13D). Most rhombohedral chains of this type occur along thewalls of voids and some of them are associated with other biogeniccomponents, e.g., calcified filaments and more typical NFC forms.

Therefore, there are a variety of NFCs in the Texas caliches. TheseNFCs differ in size, shape, and crystal arrangement, though they have anoverall needle-like appearance. The most common feature for NFCs isthat most of them were observed in the voids and have an intimateassociation with root structures and other biogenic features, such ascalcified filaments and micro-rods and nano-spheres. Conversely, NFCswere not commonly seen in the adjacent well-cemented groundmass,whichmay be because they were obscured by progressive cementationand overgrowth.

4.2.6. SpherulitesSpherulites, also called calcitic fibro-radial spherulitic polycrystals,

were observed in many environments (see Verrecchia et al., 1995,Table 1). In the Texas caliches, spherulites are common, predomi-nantly in laminar crusts. They preferentially occur as light-colored

layers alternating with dark-colored layers that are enriched incalcified filaments and micrite (Fig. 6B).

The shape and size of spherulites in the Texas caliches are similarto those observed in previous caliche studies (Klappa, 1979a;Boettinger and Southard, 1990; Verrecchia et al., 1995). Spherulitesaremostly spherical or oval, 20 to 50 μm in diameter. But in rare cases,they are up to 100 μm in diameter (Fig. 14A and B). Also, manyspherulites are not well-developed and are hemispherical or fan-shaped. Two or more spherulites may grow into each other and formcompound spherulites. The contact between the spherulites and thesurrounding micritic groundmass can be either sharp or indistinct.

The spherulites are mainly characterized by the radial internalstructure that is formed by straight rod-like crystals and rounded tosubrounded crystals. The straight rod-like crystals are mostly 1 to2 μm long, 0.2 to 0.3 μm in width, and commonly have bluntterminations (Fig. 14C). Most of the straight rods have a constantdiameter, but some become wider toward the rim of the sphericalbodies and some straight ones are up to 0.6 μm in diameter near theirdistal margin. Rounded to subrounded calcite crystals are alsocommon, particular in the center of the spherulites (Fig. 14D), andmost of them are 0.2 to 0.3 μm in diameter, which is similar to thewidth of the straight rods. Micro-rods with a curved shape andbulbous ends are also common in the spherulites.

Well-developed spherulites generally have a hole in the center,around which fibrous calcite needles grow in a radiating pattern.Generally, these holes, 1 to 2 μm in diameter, are partially filled withthe rounded calcite crystals (nano-spheres), 0.2 to 0.3 μm in diameter(Fig. 14D). Within the spherulites immediately adjacent to the holes,rounded calcite crystals are arranged in a linear pattern that is similarto the shape of straight rods.

4.2.7. Micro-rods and nano-spheresMicro-rods refer exclusively to single microcrystalline rods that

are 0.1 to 0.5 μmwide and 1 to 2 μm long. Themicro-rods are differentfrom the straight rods in spherulites in that the straight rods haveperfectly developed straight edges and blunt terminations, thoughthey resemble micro-rods in size. Instead, micro-rods generally havestraight or slightly curved morphology and a bulbous head (Fig. 15A–C). They ubiquitously occur in biogenic caliche facies and are one ofthe major components of these facies.

Commonly, micro-rods are arranged into several spatial patterns:(1) clusters of randomly distributed micro-rods in the voids or crackswith NFC or other biogenic components (Fig. 15A), (2) microbial matscomposed of densely interwoven micro-rods (Fig. 15B), and (3)accumulations in patterns that indicate the existence of rootstructures or filaments. They may also occur scattered randomly inthe micritic caliche groundmass together with other skeletal grains orcalcite crystals.

There are abundant spherical to sub-spherical calcified bodies thatare mostly 0.2 to 0.5 μm and up to 1 μm in diameter (Fig. 15D), i.e.,nano-spheres. They are the major components of the micriticgroundmass for all biogenic caliches in Texas and co-exist intimatelywith other microscopic components such as calcified filaments andneedle fiber calcite. They may occur randomly in voids or matrices orconnect into rods (Figs. 11F and 14D) or strings (Fig. 15D).

5. Interpretation and discussion

5.1. Origins of biogenic caliches

The characteristics of various biogenic features and their originshave been extensively discussed, but some features, such as Micro-codium, needle fiber calcite (NFC), and spherulites, are still not wellunderstood and thus engender intense debates (Klappa, 1978, 1979a,b; Chafetz and Butler, 1980; Phillips and Self, 1987; Jones and Kahle,1993; Verrecchia and Verrecchia, 1994; Verrecchia et al., 1995;

Fig. 13. SEM images of platy rhombohedrons and rhombohedral chains. (A) Thin platy rhombohedrons and rhombohedral chains (type I) (arrows) with smooth surfaces, coat thesurface of voids. (B) Thin platy rhombohedrons (or rhombohedral chains) with prominent grooves and ridges parallel to the short side of the crystals. (C) Elongate rhombohedralchains (Type II) composed of an assemblage of tiny rhombohedral crystals. (D) Thick platy or elongate rhombohedrons and rhombohedral chainsmostly coat the walls of voids or arerandomly scattered in the voids.


Verrecchia et al., 1996; Wright et al., 1996; Loisy et al., 1999; Kosir,2004). Currently, the formation of various biogenic features incaliches has been attributed to six types of biota: plant roots, fungi,algae, cyanobacteria, actinomycetes, and eubacteria (Klappa, 1979b,1980; Esteban and Klappa, 1983; Jones, 1988; Wright and Tucker,1991; Goudie, 1996; Alonso-Zarza, 2003; Kosir, 2004).

5.1.1. Rhizoliths (root structures)Rhizoliths that are characterized by root structures are common,

particularly in central and south Texas. Although most of the root

Table 1Summary of occurrence and development of biogenic caliches in Texas.

Area Coastal South Texas

Host rock (or sediments) Alluvial Alluvial

Climate Moist subhumid to subarid Dry subhumid tosubarid

Mean annual rainfall (mm) 700–1250 550–900Root structure Common in southern part AbundantStromatolitic laminar Common and well-developed in

southern partAbundant and well-developed

Pisoid coating NONE Well-developedCalcified filaments Abundant AbundantNeedle fiber calcite (NFC) Common, associated with root

structuresAbundant, morevarieties

Spherulite Abundant and well-developed inlaminar crusts

Abundant and well-developed

Micro-rod N/A Abundant

Note: NONE — not exist; N/A — no available data. Abundance of each biogenic features is clfeatures is classified into well-developed, moderately developed and poorly developed.

structures are not well-preserved, e.g., partially decayed and alteredduring calcification, the recognition of typical root structures frommany caliche profiles confirms the ubiquitous existence of plant roots.Macroscopically, rhizoliths are characterized by submillimeter-sizedholes which are traces of roots or rootlets (Fig. 4B). Some centimeter-sized holes or isolated patches with brown micritic cement in aconcentric pattern are the traces of the walls of relatively larger roots.The brown micrite is an important part of rhizoliths and closelyassociated with root structures (Jones and Ng, 1988). These areas arebetter cemented than the surrounding white groundmass due to the

Central Texas Northwest Texas West Texas

Limestone, alluvial Eolian Igneous and metamorphic,alluvial

Dry subhumid tosubarid

Subarid Subarid to arid

500–750 400–500 200–450Abundant Rare RareAbundant and well-developed

Abundant and moderatelydeveloped

Abundant and moderatelydeveloped

N/A N/A Moderately developedAbundant Common to abundant Common to abundantAbundant, morevarieties

Rare Moderate to abundant

Abundant and well-developed

Common and poorlydeveloped

NONE

Abundant Abundant Abundant

assified into abundant, common, and rare. The degree of development of each biogenic


enhanced activity of symbiotic soil micro-organisms which co-existedwith plant roots (Jones and Ng, 1988; Alonso-Zarza and Jones, 2007).

Microscopically, fossilized roots are readily observed. The essentialparts of relatively well-preserved root structures include epidermaland cortical tissues. However, the most typical root structures in theTexas caliches are ring-like structures with a hole in center (Fig. 5Aand B), which are the result of preferential calcification of corticalcells. The differential preservation can be explained by themechanismof nutrient acquisition by plant roots from the surrounding soils.Plant roots tend to acquire mineral nutrients by absorbing Ca2+ anddumping 2H+ (Kosir, 2004). The release of a proton (H+) enhancesthe root's capability to obtain nutrients from substrates, and calciteprecipitation in the cortical cells also facilitates nutrient assimilation.

If carbonate precipitation is substantial in living root cells, fossilizedroot structures, such as Microcodium, will be preserved (Kosir, 2004).However, most of the root structures in the Texas caliches are partlycalcified and composedof basic calcifiedbiogenic components, e.g., NFC,filaments, and micro-rods, which are products of calcification of fungi,bacteria, and other micro-organisms. This is because plant rootsgenerally have an intimate relationship with fungi and bacteria in thesoils, such as mycorrhizae (Klappa, 1980). The symbiotic micro-organisms facilitate the absorption of nutrients by the plant rootswhereas the roots, in turn, act as nutrients for these micro-organisms.The roots are consumed and altered by the micro-organisms when theroots are either alive or dead and the root structures are more or lesspreserved due to the instant in situ calcification of and/or by the soilmicro-organisms(Jones andNg, 1988;Alonso-Zarza and Jones, 2007). Insome cases, root cells are completely decayed, leaving only roundedholes. The poor preservation of root structures in the Texas calichesindicates that the calcification of root cells mostly occurred after thedeath of the plants.

Generally, the role of plant roots in the formation of biogenic calichesis: (1) as a major agent which accelerates the weathering of substratesby boring and breaking down the host rock (Klappa, 1980); (2)penetration into host materials which creates significant amounts ofporosity and permeability that facilitate the percolation of CaCO3-richsolution and subsequent calcite precipitation by evapotranspiration toform rhizocretions (Mutler and Hoffmeister, 1968; Jones, 1988); (3)metabolic activity of plant roots which induce the calcification of rootcells (petrifaction) by the exchange of H+ and Ca2+ and other pathways(Klappa, 1980; Kosir, 2004); and (4) plant roots provide substrates andnutrients for the symbiotic micro-organisms, e.g., fungi and bacteria,which may enhance the precipitation of micritic cement within oraround the root structures by their intense activities (Jones and Ng,1988; Wright, 1994; Alonso-Zarza and Jones, 2007).

5.1.2. PeloidsPeloids are common in caliches, particularly associated with other

biogenic components, such as laminar crusts and pisoids (Read, 1974;Adams, 1980; Hay and Wiggins, 1980; Calvet and Julia, 1983; Wright,1983a,b; Jones and Squair, 1989; Alonso-Zarza and Arenas, 2004;Alonso-Zarza and Jones, 2007). Previously, peloids have beeninterpreted as the products of alteration of skeletal grains (James,1972), aggregates of cement (Klappa, 1978), and of fecal origin(Calvet and Julia, 1983; Jones and Squair, 1989; Alonso-Zarza andArenas, 2004). It is highly likely that at least some of the peloids arebacterially induced precipitates, similar to those recognized inmodern reefs (Chafetz, 1986) and travertine deposits (Chafetz andFolk, 1984).

5.1.3. Calcified filaments and calcispheresCalcified filaments are one of the most important components that

constitute laminar crusts (Klappa, 1979a; Verrecchia et al., 1995) andthe coatings of pisoids (Calvet and Julia, 1983). They range markedlyin morphology and size, which indicates that they probably arecalcified products of different soil micro-organisms. The common

micro-organisms in soil environments are root hairs, fungi, algae,cyanobacteria, actinomycetes, and eubacteria (James, 1972; Knox,1977; Klappa, 1979b; Calvet, 1982; Coniglio and Harrison, 1983;Phillips et al., 1987; Jones, 1988; Jones and Ng, 1988; Verrecchia et al.,1995;Wright et al., 1995; Loisy et al., 1999; Verrecchia, 2000). In somecases, the biological affiliation of calcified filaments can be differen-tiated by size, shape, and themode of branching (Kahle, 1977; Klappa,1979a). However, there is a large overlap in size andmany similaritiesin morphology between different calcified filaments, which makes itvery difficult to identify them (Calvet, 1982; Jones, 1988; Bruand andDuval, 1999). Moreover, the identification of soil biota simply basedon calcified filaments is limited by the process of calcification, whichmay change the size and shape of the filaments (Jones, 1988).

Based on numerous studies of calcified filaments in calichedeposits (Kahle, 1977; Klappa, 1979a; Jones, 1988; Bruand andDuval, 1999), fungi are the most common species that has beenrecognized. Fungal hyphae have a wide range in diameter, which is upto 13 μm, but mostly between 1 and 4 μm. Their main diagnosticfeatures are Y-shaped branching patterns (Klappa, 1979a) and theirassociation with the calcispheres, which are most likely fungal sporesand/or sporangia (Jones, 1988). In the Texas caliches, it is commonthat filaments up to 10 μm in diameter show typical dichotomousbranches and in many cases, co-exist with the calcispheres (hereinalso interpreted as spores and/or sporangia). They may occur as apartially calcified, isolated filament (hypha), but more commonly areclusters of intertwined filaments (mycelium).

Filaments with a diameter larger than 4 μm without any fruitingbodies are commonly identified as algae, cyanobacteria, or root hairs.Many laminar crusts and pisoid coatings are composed of two or morespecies of biotic filaments, which indicates these caliche facies arelichen stromatolites (Klappa, 1979b; Krumbein and Giele, 1979; Jonesand Kahle, 1985;Wright, 1989; Verrecchia et al., 1991). Cyanobacteriahave been also documented in caliche deposits, especially in thelaminar crusts (Krumbein and Giele, 1979; Verrecchia et al., 1995).Actinomycetes can be positively identified by size and morphology insome caliche hardpans (Fig. 10E). They range in diameter from 0.2 to0.7 μm in this study and they display the same branching pattern asfungi. In contrast, filaments that are less than 1 μm in diameter andwithout evident branching morphology have been identified asthreadlike bacteria (Loisy et al., 1999).

The mechanisms of calcification in or around fungi (Phillips et al.,1987; Verrecchia, 2000; Burford et al, 2006; Verrecchia et al., 2006),algae (Kahle, 1977; Jones and Kahle, 1985; Freytet and Verrecchia,1998), and cyanobacteria (Krumbein and Giele, 1979; Chafetz andBuczynski, 1992; Merz-Preiβ and Riding, 1999; Merz-Preiβ, 2000)have been extensively discussed for a variety of environments,including fresh to hypersaline water and nonmarine to marinesettings. Among them, the biomineralization of fungal hyphae is dueto the reaction betweenorganic acids, particularly oxalic acid, excretedby fungal hyphae, and Ca2+ in the pore solution. This reaction tends toresult in Ca-oxalates or calcium carbonate precipitation (Verrecchia,2000; Verrecchia et al., 2006). Ca-oxalates (CaC2O4·xH2O, x un-known), including whewellite (CaC2O4·H2O) and weddellite (CaC2-O4·xH2O, 2<x<2.5), are metastable forms of Ca-minerals and arereadily transformed into calcium carbonate by bacteria (Verrecchiaet al., 2006). The calcification of algae and cyanobacteria is eitherextracellular or intracellular and themajor pathways are: (1) acting astemplates for nucleation and growth of calcite crystals if the ambientwater is supersaturatedwith respect to calciumcarbonate (Freytet andVerrecchia, 1998;Merz-Preiβ andRiding, 1999;Merz-Preiβ, 2000), (2)inducing the precipitation of calcite in and/or on themucus sheaths bymetabolic activities such as photosynthetic uptake of CO2 and/HCO3

−

(Pentecost and Bauld, 1988; Merz-Preiβ, 2000; Shiraishi et al., 2008),and (3) eubacteria usingdead cyanobacteria as a food source and in theprocess inducing the precipitation of calcium carbonate (Chafetz andBuczynski, 1992; Chafetz, 1994).

Fig. 14. Photomicrograph (A) and SEM images (B–D) of spherulites. (A) A cluster of spherulites from a laminar crust show a variety of geometries, including hemispherical, spherical,and oval. Several spherulites may fuse into a compound form. (B) An oval spherulite displays internal radiating structure. (C) A close-up view shows that the components ofspherulites are mainly straight rods (arrows), mostly 200 to 300 μm in diameter, and a minor amount of micro-rods and nano-spheres. (D) Commonly, a hole occupies the center ofthe spherulites and is partially filled with nano-spheres (arrows). Many nano-spheres are arranged into straight rods, exhibiting a radiating structure.


For the calcified filaments in caliches, the difference inmorphologyis controlled by two major factors, the timing of calcification and themechanism of calcification. In the case of the filaments that maintaintheir original tubiform shape (Fig. 10A and B), the calcification occursprior to or simultaneously with the decomposition of cell walls andthrough in vivo biochemical mechanisms (Phillips et al., 1987),whereas partially decayed or collapsed filaments aremainly preservedthrough post-mortem calcification. The encrustation around thefilaments is controlled either by physicochemical or biochemicalprocesses or both (Phillips et al., 1987; Chafetz and Buczynski, 1992).These processes tend to form tubiform calcified filaments, whereas theimpregnation within the filaments is mainly controlled by biologicalfactors, commonly forming solid calcified filaments (Klappa, 1979a;Phillips et al., 1987). Filaments may also have double layers thatcomprise the encrustation and the impregnation (Fig. 10D).

The constituent mineral crystals display different crystal habits,e.g., rhombohedral, equant, needle, blade, triangular, or other bizarreforms, which are controlled by the chemical composition ofsurrounding microenvironment, rate of crystallization, and geometryof pore spaces within the organic tissues (Klappa, 1979a; Buczynskiand Chafetz, 1991).

5.1.4. Needle fiber calcite (NFC)Needle fiber calcite (NFC) is believed, at least in some cases, to

form through purely abiotic processes, such as intense evaporation(James, 1972; Jones and Ng, 1988; Jones and Kahle, 1993; Borsatoet al., 2000). But more evidence from recent studies indicates thatcalcite needles are very likely of biogenic origin (Verrecchia andVerrecchia, 1994; Verrecchia, 2000; Blyth and Frisia, 2008; Richteret al., 2008; Cailleau et al., 2009).

Currently, it is widely accepted that single crystalline needles areformed by the lysis of fungal sheaths (Phillips and Self, 1987;Verrecchia and Verrecchia, 1994). The major evidence is that thebundles of needles resemble the morphology of fungal colonies thattend to form sinuous bands within pore spaces. In addition, the rod-like crystal habit maymimic the internal structure of fungal filaments.When the organicmatter decays followed by relatively dry conditions,the needles will burst out of the fungal sheaths and form randommeshes within the voids (Fig. 11A and C). The shape and size of singleneedles may vary slightly, e.g., rod-shaped or blade-like. These dif-ferences can be attributed to the different species of fungi or differentparts of fungal sheaths that are calcified. Commonly, single-needlepairs occur together as compound rods, which are probably producedby the calcification of multiple needles within the same sheath(Fig. 11E) or mycelium bundles (Fig. 11D). The needles with roughsurfaces or serrated edges, and triangular crystals (Fig. 11A and B),have been interpreted as the products of further precipitation orepitaxial overgrowth on rod-shaped calcite needles (Jones and Kahle,1993; Verrecchia and Verrecchia, 1994; Cailleau et al., 2009).

However, calcite needles (and needle pairs), can be formedthrough biotic processes other than fungal biomineralization basedon the observations from this study and other recent findings (Olsztaet al., 2004; Blyth and Frisia, 2008; Richter et al., 2008). First, therandomly oriented, delicate needle fabrics within the calcified roottissues and voids are more likely formed in situ rather than throughthe decay of the fungal-like needle bundles. Second, calcite needlesare abundant in the rhizoliths but are rare, if not completely absent, inthe fungi-rich laminar crusts. Third, calcite needles can be microbiallyinduced without fungi in cave environments (Cañaveras et al., 2006;Blyth and Frisia, 2008; Richter et al., 2008). In addition, needle calcite

Fig. 15. SEM images of micro-rods and nano-spheres. (A) A loose cluster of micro-rods with slightly curved shapes and bulbous heads are randomly distributed in voids. (B) Micro-rods are densely cemented into a interwovenmat, coating the surface of a void. (C) Micro-rods with bulbous heads are cemented together into an elongate plate (arrow), co-existingwith filaments encrusted by radiating needles. (D) Nano-spheres occur randomly distributed within a groundmass or in voids, and as shown, some are arranged into string-likeaccumulations.


crystals can grow through the progressive crystallization of micron tonanometer size, highly concentrated droplets (Olszta et al., 2004).Needles composed of stacked nano-spheres in west Texas (Fig. 11F)may be formed through this process.

Therefore, single crystalline needles may occur not only withinfungal sheaths, but also within the tissues of plant roots and otherorganisms. The formation of single needles or needle pairs may notnecessarily be species-specific, but rather may be controlled by theorganicmacromolecules (polymer) in the pore solutions. Bacteria couldalso play a vital role in the formation of some needle-like crystals.

The formation of tabular rhombohedrons or rhombohedral chains(Fig. 13A–D) was attributed to rapid precipitation caused by rapidevaporation of pore fluid (Jones and Ng, 1988). The occurrence ofpolycrystalline chains (P chains) was also interpreted as a result ofpurely abiotic processes (Verrecchia and Verrecchia, 1994). However,the effect of biota cannot be ignored because these rhombohedronsand needle-like chains all occur in the voids and cracks generated byplant roots and other organisms. These complex crystals are closelyassociated with other biogenic components, e.g., calcified filaments,triangular crystals, and single needles. In contrast, in the adjacentabiogenic groundmass, no polycrystalline chains were observed.

The needles with serated edges or triangular needle-like crystals arebelieved to be formed by subsequent constructive growth aroundneedle calcite (Jones and Ng, 1988). Similarly, Cailleau et al. (2009)suggested that the polycrystalline rhombohedral chains are also theresult of the overgrowth of calcite needles. However, the tabularrhombohedrons or rhombohedral chains may also form by thecalcification of biofilms or Extracellular Polymeric Substances (EPS)because most of these crystals coat the walls of pore spaces or occur insub-parallel bundles. Biofilms and EPS, a kind of gelatinous material

excreted by micro-organisms, are common in the microbial sediments(Verrecchia et al., 1993; Decho, 1994; Jones, 1995; Preat et al., 2003;Decho, 2000); they are also abundant and generally occur on the surfaceof the voids in the Texas biogenic caliches. Most of the rhombohedronsor rhombohedral chains appear to be formed through the calcificationofEPS because they are exceptionally thin (<1 μm) compared to theirlength (8–14 μm) and width (6–8 μm), though they display, more orless, rhombohedral terminations. Additionally, Type II rhombohedralchains are composed of isolated rhombohedrons and nano-spheres thatalso comprise the co-existing filaments, which also indicate a possiblebiotic origin. The type III polycrystalline chains (Fig. 13D), however, co-existwith isolated typically abiogenic rhombohedral calcite crystals andmay reflect a significant role of abiotic processes.

Therefore, the formation of NFCs in the Texas caliches is largelycontrolled by biotic processes, but abiotic processes are involved inlocalized environments.

5.1.5. SpherulitesThere is an intimate association between the spherulites and the

filamentousorganismsbecause calcifiedfilaments are abundant and thelaminations in the crusts are mainly produced by the preferentialdistribution of filaments. Verrecchia et al. (1995) claimed thatcyanobacteria are responsible for the formation of the spherulites inthe laminar crusts based on the recognition of cyanobacterial remains incaliches and the experimental production of spherulites by cyanobac-terial filaments. However, this interpretationwas questioned byWrightet al. (1996) and Mees (1999). The major disagreement is that, ifspherulites are produced by cyanobacteria, spherulite-bearing laminarcrusts must form at the soil–air surface (photic zone), but this is notnecessarily true because spherulite-rich laminar crusts (or root mats)


could form under a top soil 20 to 40 cm thick (Wright et al., 1996) andspherulites were also observed in subsurface cracks (Mees, 1999).Instead,Wright et al. (1996) suggested that the origin of the spherulitesin the caliches is closely associated with bacterial activity. Additionally,calcite spherulites can be synthesized in the lab when mucilaginousbacteria or biofilms are abundant in the solution (Braissant et al., 2003;Chekroun et al, 2004; Parraga et al., 2004). In this study, it was observedthat spherulites are composed of rod-shaped crystals, micro-rods, andnano-spheres, all of which are closely associated with biotic activities(see Section 5.1.5). Therefore, the formation of spherulites ismost likelyaproductof bacterial activity in anorganic-enrichedmicroenvironment.

5.1.6. Micro-rods and nano-spheresMicro-rods are characterized by their size and shape, and they

commonly are associated with plant roots, fungi, and other micro-organisms. These sinuous rods with bulbous heads are thought to becalcified bacteria (Boquet et al., 1973; Phillips and Self, 1987;Verrecchia and Verrecchia, 1994; Loisy et al., 1999). The rod-shapedbacteria are recognized as bacilliform and threadlike bacteria (Loisyet al., 1999), and the calcified nano-spheres are nano-sized coccoidalbacteria (Folk, 1993; Verrecchia and Verrecchia, 1994; Folk, 1999;Folk and Chafetz, 2000; Folk and Lynch, 2001).

Laboratory experiments amply demonstrate that bacteria in thesoils and other geological environments can induce the precipitationof calcite directly or indirectly (Boquet et al., 1973; Buczynski andChafetz, 1991; Monger et al., 1991; Chafetz and Buczynski, 1992;Chafetz, 1994; Braissant et al., 2003; Chekroun et al., 2004; Parragaet al., 2004). There are manymechanisms that have been suggested toexplain the role of bacterial mediation in the precipitation of calciumcarbonate (Castanier et al., 2000; Chafetz and Buczynski, 1992). Innatural geological environments, commonly two or more of theprocesses are active at the same time. No matter what process(es) areresponsible, themetabolic activity of bacteria may eventually increasethe pH and lead to the supersaturation of the ambient solution withrespect to calcite or aragonite, which tends to induce the mineralprecipitation (Knorre and Krumbein, 2000).

The bacterially induced calcite crystals commonly display charac-teristic crystal forms, such as spheres and dumbbells (Buczynski andChafetz, 1991; Chafetz and Buczynski, 1992; Folk and Chafetz, 2000;Braissant et al., 2003). However, the ‘bizarre’ crystals induced bymicrobial activity, together with calcified microbial bodies, may beprogressively cemented and fused intomicrosparitic or sparry crystalswhose shape resembles that of abiotically precipitated calcite crystals.

Fig. 16. This figure shows the relationship between macroscopic features and the co-existicomposed of several types of microscopic features. The solid lines represent the common reresponsible micro-organisms for each type of microscopic features is also indicated.

This gives rise to difficulty in differentiating biogenic calcite fromabiogenic calcite crystals simply based on crystal habit (Loisy et al.,1999; Cailleau et al., 2005). For the micro-rods in the Texas caliches,they commonly accumulate as open random meshes in pore spaces(Fig. 15A). Due to further cementation and/or recrystallization, theymay also occur as better cemented interwoven mats (Fig. 15B) andmonocrystalline plates (Fig. 15C) or rhombohedrons.

Similarly, nano-spheres may coalesce into clusters and eventuallyinto a dense groundmass. Commonly, nano-spheres are arranged intorod-like polycrystals (Figs. 11F and 14D) or even rhombohedral-likeshape, which could eventually recrystallize into monocrystallinecalcite rhombohedrons. Commonly, nano-spheres are intimatelyassociated with micro-rods and other skeletal grains. Bacteria mayact as a catalyst in the calcification of mucus (Chafetz and Buczynski,1992) and the glutinous material, and the encapsulation of eubacteriainto micron-sized calcite crystals.

Based on this study, together with numerous previous studies, it isobvious that there exists a complex relationship between the large-scale and fine-scale biogenic features. Commonly, each type of large-scale feature, such as calcified root structures and laminar crusts, iscomposed of a specific set of fine-scalemineral components, includingfilaments, NFC, and nano-spheres (Fig. 16). Similarly, the relationshipbetween the biogenic features and soil biota is also complex. Oneorganism can be responsible for several different biogenic features. Inturn, similar biogenic features may also be produced by differentorganisms through different metabolic processes (Fig. 16).

Although many biogenic features have been recognized and themechanisms through which biota induce the precipitation of calciteand generate distinctly different morphological features have beenextensively discussed, the influence of organisms in the formation ofcalichesmay be underestimated due to: (1) the preservation potentialof biogenic features is low (Wright et al., 1988), (2) biogenic featuresmay transform into abiogenic features by additional abiotic precip-itation during the short post-formation period, and (3) there is a poorunderstanding of the genetic processes by which organisms areinvolved in the formation of caliches.

5.2. Effect of climate

Climate has long been considered as one of the most importantfactors that control the development of caliches, such as theoccurrence and maturity of caliches (Alonso-Zarza, 2003). Biogeniccaliches can form in a wide range of climatic conditions. Nodules from

ng and constituent microscopic features. Generally, one type of macroscopic feature islationships and the dashed lines represent the less common relationships. The possible


a humid area near Houston with a mean annual rainfall of 1250 mm,for example, exhibit abundant biogenic features, including calcifiedfilaments and micro-rods. Biogenic caliches fromwest Texas and NewMexico, where there is a mean annual precipitation of less than200 mm, are also well-developed, and include laminar crusts andpisoids (Machette, 1985; Verrecchia et al., 1996; Monger andGallegos, 2000). Overall, biogenic caliches are less strongly influencedby climate than abiogenic caliches. Thick rhizogenic caliche can formdirectly on substrates (Wright et al., 1995) and thick laminar calichecrusts can form in subhumid as well as desert areas withoutsignificant accumulation of abiogenic caliches (Machette, 1985;Verrecchia et al., 1996). Nevertheless, biogenic caliches have beenfound to be affected by climatic conditions in terms of amount as wellas type in a regional area (Table 1).

First, rhizoliths or calcified root structures are much betterdeveloped in south and central Texas where there is a dry subhumidto subarid climate than in northwest and west Texas where there is asubarid to arid climate. Root structures are abundant and tend to formlaterally continuous root mats in south and central Texas, but aresparse in northwest and west Texas.

Second, the thickness and abundance of biogenic features areprogressively less well-developed in the profiles as climate becomesmore arid from south and central Texas to northwest and west Texas.Biogenic caliches are up to 1 m thick within profiles in south andcentral Texas, but only prominent in hardpans and uppermostportions of petrocalcic horizons that are, at most, several centimentersthick in northwest and west Texas. Similarly, the biogenic compo-nents (beta fabrics) are less abundant in the macroscopic biogeniccaliche facies from east to west. Instead, abiogenic components (alphafabrics), such as euhedral to equant calcite microspar, are increasinglycommon to the west. In addition, calcified filaments, needle fibercalcite, and spherulites display a greater abundance and variability insouth and central Texas than in the drier northwest and west Texas.

The influence of climate, especially precipitation, on the develop-ment of biogenic caliches is due to the availability of soil moisture,which controls the density of vegetation cover and thus theabundance of roots in the soils (Alonso-Zarza, 2003). Plant roots arean important food supply for soil microbes, such as fungi andeubacteria (Klappa, 1980). Compared to subarid to arid areas (i.e.,northwest and west Texas), subhumid to subarid areas (i.e., centraland south Texas) have higher amounts of annual rainfall and the soilenvironment is comparatively moist and favorable for the develop-ment of dense, relatively large root systems. As a part of soil systems,these roots and the symbiotic microbes play an active role in theformation of caliches and tend to bemore easily preserved in direct orindirect ways. Soil moisture becomes gradually less as soil waterpercolates downward, which contributes to the preferential distribu-tion of biogenic features in the upper parts of caliche profiles.

Other factors must also be considered, such as the age of biogeniccaliches, sedimentation (the input of terrigenous sediment), andcomposition of the host rock, when comparing the growth of biogeniccaliches under various climatic regimes. Older biogenic caliches, suchas the Tertiary petrocalcic horizons in northwest Texas, are likely todisplay lower degrees of biogenic evidence due to poor preservationwith age. High rates of sedimentation can also “choke” or “dilute”biogenic caliches. Although substrates could be influential for thedevelopment of biogenic caliches (Klappa, 1980; Kosir, 2004), thereare no significant differences observed in this study for the biogeniccaliches formed on different types of host rocks, including marinelimestone, calcareous andnon-calcareous alluvium, and igneous rocks.

6. Conclusion

Biogenic (beta-type) caliches of late Quaternary age are ubiquitousand abundant in Texas. They develop on various types of host rock andunder significantly different climatic conditions. Most of the biogenic

caliches occur in the uppermost portions of caliche profiles andconstitute several characteristic horizons (or caliche facies) such asplaty horizon, laminar crusts, and pisoids. Macroscopically, biogeniccaliches include rhizoliths (calcified root structures), laminar crusts,and pisoid coatings. Each of these megascopic structures arecomposed of a variety of microstructures or components, such asalveolar-septal fabrics, calcified filaments, peloids, spherulites, needlefiber calcite (NFC), and micro-rods and nano-spheres. This study,together with numerous previous studies, indicate that thesediversified features are produced by several major species of biota,including plant roots, fungi, cyanobacteria, actinomycetes, andeubacteria. These organisms induce the precipitation of calcite indirect or indirect ways through their metabolic activity and produceconstituent crystals with characteristic morphology.

In contrast to host rock, climate has a significant effect on thedevelopment of biogenic caliches in terms of quantity as well asvariety. Calcified root structures and other biogenic caliches are betterdeveloped in subhumid to subarid areas (south and central Texas)than in subarid to arid areas (northwest and west Texas).

Acknowledgements

The authors are grateful for the assistance of Drs. James Meen andKaroline Muller of the Texas Centre for Superconductivity at theUniversity of Houston in the use of the XRD and SEM instruments. Theauthors are also thankful for the time and effort of two anonymousreviewers and the editor, Brian Jones. Their constructive criticismshave greatly improved this manuscript.

References

Adams, A.E., 1980. Calcrete profiles in the Eyam Limestone (Carboniferous) ofDerbyshire: petrology and regional significance. Sedimentology 27, 651–660.

Alonso-Zarza, A.M., 2003. Palaeoenvironmental significance of palustrine carbonatesand calcretes in the geological record. Earth-Science Reviews 60, 261–298.

Alonso-Zarza, A.M., Arenas, C., 2004. Cenozoic calcretes from the Teruel Graben, Spain:microstructure, stable isotope geochemistry and environmental significance. Sedi-mentary Geology 167, 91–108.

Alonso-Zarza, A.M., Jones, B., 2007. Root calcrete formation on Quaternary karsticsurfaces of Grand Cayman. Geologica Acta 5, 77–88.

Alonso-Zarza, A.M., Sanz, M.E., Calvo, J.P., Estevez, P., 1998. Calcified root cells inMiocene pedogenic carbonates of the Madrid Basin: evidence for the origin ofMicrodium b. Sedimentary Geology 16, 81–97.

Blum, M.D., Valastro, S., 1992. Quaternary stratigraphy and geoarchaeology of theColorado and Concho Rivers, west Texas. Geoarchaeology 7, 419–448.

Blum, M.D., Valastro, S., 1994. Late Quaternary sedimentation, lower Colorado River,Gulf Coastal Plain of Texas. Geological Society of America Bulletin 106, 1002–1016.

Blyth, A.J., Frisia, S., 2008. Molecular evidence for bacterial mediation of calciteformation in cold high-altitude caves. Geomicrobiology Journal 25, 101–111.

Boettinger, J.L., Southard, R.J., 1990. Micromorphology and mineralogy of a calcareousduripan formed in granitic residuum, Mojave Desert, California, USA. In: Douglas, L.A.(Ed.), Soil Micromorphology: A Basic and Applied Science. Developments in SoilScience 19. Amsterdam, Elsevier, pp. 409–415.

Boquet, E., Boronat, A., Ramos-Cormenzana, A., 1973. Production of calcite (calciumcarbonate) crystals by soil bacteria is a general phenomenon. Nature 246, 527–528.

Borsato, A., Frisia, S., Jones, B., Van der Borg, K., 2000. Calcite moonmilk: crystalmorphology and environment of formation in caves in the Italian Alps. Journal ofSedimentary Research 70, 1179–1190.

Braissant, O., Cailleau, G., Dupraz, C., Verrecchia, E.P., 2003. Bacterially inducedmineralization of calcium carbonate in terrestrial environments: the role ofexopolysaccharides and amino acids. Journal of Sedimentary Research 73,485–490.

Bruand, A., Duval, O., 1999. Calcified fungal filaments in the petrocalcic horizon ofEutrochrepts in Beauce, France. Soil Science Society of America Journal 63, 164–169.

Buck, B.J., Monger, C.H., 1999. Stable isotopes and soil-geomorphology as indicators ofHolocene climate change, northern Chihuahuan Desert. Journal of Arid Environ-ments 43, 357–373.

Buczynski, C., Chafetz, H.S., 1991. Habit of bacterially induced precipitates of calciumcarbonate and the influence ofmediumviscosity ofmineralogy. Journal of SedimentaryPetrology 61, 226–233.

Burford, E.P., Hillier, S., Gadd, G.M., 2006. Biomineralization of fungal hyphae withcalcite (CaCO3) and calcium oxalate mono-and dehydrate in Carboniferouslimestone microcosms. Geomicrobiology Journal 23, 599–611.

Cailleau, G., Braissant, O., Dupraz, C., Aragno, M., Verrecchia, E.P., 2005. Biologicallyinduced accumulations of CaCO3 in orthox soils of Biga, Ivory Coast. Catena 59,1–17.


Cailleau, G., Verrecchia, E.P., Braissant, O., Emmanuel, L., 2009. The biogenic origin ofneedle fibre calcite. doi:10.1111/j.1365-3091.2009.01060.x.

Calvet, F., 1982. Constructive micrite envelope developed in vadose continentalenvironment in Pleistocene eolianites of Mallorca (Spain). ACTA GEOLOGICAHISPANICA 17, 169–178.

Calvet, F., Julia, R., 1983. Pisoids in the caliche profiles of Tarragona (NE Spain). In: Peryt,T.M. (Ed.), Coated Grains. Springer, Berlin, pp. 456–473.

Cañaveras, J.C., Cuezva, S., Sánchez-Moral, S., Lario, J., Laiz, L., González, J.M., Saiz-Jiménez, C., 2006. On the origin of fiber calcite crystals in moonmilk deposits.Naturwissenschaften 93, 27–32.

Castanier, S., Le Metayer-Levrel, G., Perthuisot, J.-P., 2000. Bacterial roles in theprecipitation of carbonateminerals. In: Riding, R.E., Awramik, S.M. (Eds.), MicrobialSediments. Springer-Verlag, Berlin, pp. 32–39.

Chafetz, H.S., 1986. Marine peloids: a product of bacterially induced precipitation ofcalcite. Journal of Sedimentary Petrology 56, 812–817.

Chafetz, H.S., 1994. Bacterially induced precipitates of calcium carbonate: habit andmineralogy. In: Krumbein, W.E., Paterson, D.M., Stal, L.J. (Eds.), Biostabilization ofSediments. Oldenburg Press, Oldenburg, pp. 149–163.

Chafetz, H.S., Buczynski, C., 1992. Bacterially induced lithification of microbial mats.Palaios 7, 277–293.

Chafetz, H.S., Butler, J.C., 1980. Petrology of recent caliche pisolites, spherulites, andspeleothem deposits from central Texas. Sedimentology 27, 497–518.

Chafetz, H.S., Folk, R.L., 1984. Travertines: depositional morphology and the bacteriallyconstructed constitutents. Journal of Sedimentary Petrology 54, 289–316.

Chekroun, K.B., Rodriguez-Navarro, C., Gonzalez-Munoz, M.T., Arias, J.M., Cultrone, G.,Rodriguez-Gallego, M., 2004. Precipitation and growth morphology of calciumcarbonate induced bymyxococcus xanthus: implications for recognition of bacterialcarbonates. Journal of Sedimentary Research 74, 868–876.

Coniglio, M., Harrison, R.S., 1983. Holocene and Pleistocene caliche from Big Pine Key,Florida. Bulletin of Canadian Petroleum Geology 31, 3–13.

Decho, A.W., 1994. Molecular-scale events influencing the macroscale cohesiveness ofexopolymers. In: Krumbein, W.E., Paterson, D.M., Stal, L.J. (Eds.), Biostabilization ofSediments. Oldenburg Press, Oldenburg, pp. 135–148.

Decho, A.W., 2000. Exopolymer microdomains as a structuring agent for heterogeneitywithin microbial biofilms. In: Riding, R.E., Awramik, S.M. (Eds.), MicrobialSediments. Springer-Verlag, Berlin, pp. 9–15.

Esteban, M., Klappa, C.F., 1983. Subaerial exposure environment. In: Scholle, P.A.,Bebout, D.G., Moore, C.H. (Eds.), Carbonate Depositional Environments. AAPGMemoir 33, 1–53.

Folk, R.L., 1965. Someaspects of recrystallizationof ancient limestones. In: Pray, L.C.,Murray,R.C. (Eds.), Dolomitization and LimestoneDiagenesis: SEPMSpecial Publication, vol. 13,pp. 14–48.

Folk, R.L., 1993. SEM imaging of bacteria and nannobacteria in carbonate sediments androcks. Journal of Sedimentary Petrology 63, 990–999.

Folk, R.L., 1999. Nannobacteria and the precipitation of carbonate in unusual environ-ments. Sedimentary Geology 126, 47–55.

Folk, R., Chafetz, H.S., 2000. Bacterially induced microscale and nanoscale carbonateprecipitates. In: Riding, R.E., Awramik, S.M. (Eds.), Microbial Sediments. Springer-Verlag, Berlin, pp. 40–49.

Folk, R.L., Lynch, L.F., 2001. Organic matter, putative nannobacteria and the formation ofooids and hardgrounds. Sedimentology 48, 215–229.

Freytet, P., Plaziat, J.C., 1982. Continental carbonate sedimentation and pedogenesis, lateCretaceous and early Tertiairy of Southern France. Contributions to Sedimentology12 213 pp.

Freytet, P., Verrecchia, E.P., 1998. Freshwater organisms that build stromatolites: asynopsis of biocrystallization by prokaryotic and eukaryotic algae. Sedimentology45, 535–563.

Goudie, A.S., 1996. Organic agency in calcrete development. Journal of Arid Environ-ments 32, 103–110.

Gustavson, T.C., 1996. Fluvial and Eolian Depositional Systems, Paleosols, andPaleoclimate of the Late Cenozoic Ogallala and Blackwater Draw Formations,Southern High Plains, Texas and New Mexico. The University of Texas at Austin,Bureau of Economic Geology, Report of Investigations No. 239, 62 pp.

Hay, R.L., Wiggins, B., 1980. Pellets, ooids, sepiolite and silica in three calcretes of thesouthwestern United States. Sedimentology 27, 559–576.

Humphrey, J.D., Ferring, R.C., 1994. Stable isotopic evidence for latest Pleistocene andHolocene climatic change in north-central Texas. Quaternary Research 41,200–213.

James, N.P., 1972. Holocene and Pleistocene calcareous crust (caliche) profiles: criteriafor subaerial exposure. Journal of Sedimentary Petrology 42, 817–836.

Jolley, D.M., Grossman, E.L., Tilford, N.R., 1998. Dating calcic soils under marginallithologic conditions using a soil developmental index: an example from theStockton Plateau, Terrell County, Texas. Environmental & Engineering Geoscience4, 209–223.

Jones, B., 1988. The influence of plants and micro-organisms on diagenesis in caliche:example from the Pleistocene Ironshore Formation on Cayman Brac, British WestIndies. Bulletin of Canadian Petroleum Geology 36, 191–201.

Jones, B., 1995. Processes associated with microbial biofilms in the twilight zone ofcaves: examples from the Cayman Islands. Journal of Sedimentary Research A65,552–560.

Jones, B., Kahle, C.F., 1985. Lichen and algae: agents of biodiagenesis in karst brecciafrom Grand Cayman Island. Bulletin of Canadian Petroleum Geology 33, 446–461.

Jones, B., Kahle, C.F., 1993. Morphology, relationship, and origin of fiber and dendriticcalcite crystals. Journal of Sedimentary Petrology 63, 1018–1031.

Jones, B., Ng, K.-C., 1988. The structure and diagenesis of rhizoliths from Cayman Brac,British West Indies. Journal of Sedimentary Petrology 58, 1002–1007.

Jones, B., Squair, C.A., 1989. Formation of peloids in plant rootlets, Grand Cayman,British West Indies. Journal of Sedimentory Petrology 59, 1002–1007.

Kahle, C.F., 1977. Origin of subaerial Holocene calcareous crusts: role of algae, fungi andsparmicirtization. Sedimentology 24, 413–435.

Klappa, C.F., 1978. Biolithogenesis of Microcodium: elucidation. Sedimentology 25,489–522.

Klappa, C.F., 1979a. Calcified filaments in Quaternary calcretes: organo-mineral inter-actions in the subaerial vadose environment. Journal of Sedimentary Petrology 49,955–968.

Klappa, C.F., 1979b. Lichen Stromatolites: criterion for subaerial exposure and amechanism for the formation of laminar calcretes (caliche). Journal of SedimentaryPetrology 49, 387–400.

Klappa, C.F., 1980. Rhizoliths in terrestrial carbonates: classification recognition,genesis and significance. Sedimentology 27, 613–629.

Knorre, H.V., Krumbein,W.E., 2000. Bacterial calcification. In: Riding, R.E., Awramik, S.M.(Eds.), Microbial Sediments. Springer-Verlag, Berlin, pp. 25–31.

Knox, G.J., 1977. Caliche profile formation, Saldanha Bay (South Africa). Sedimentology24, 657–674.

Kosir, A., 2004.Microcodium revisited: root calcification products of terrestrial plants oncarbonate-rich substrates. Journal of Sedimentary Research 74, 845–857.

Krumbein, W.E., Giele, C., 1979. Calcification in a coccoid cyanobacterium associatedwith the formation of desert stromatolites. Sedimentology 26, 593–604.

Loisy, C., Verrecchia, E.P., Dufour, P., 1999. Microbial origin for pedogenic micriteassociated with a carbonate paleosol (Champagne, France). Sedimentary Geology126, 193–204.

Machette, M.N., 1985. Calcic soils of southwestern United States. Weide, D.L. (ed.), Soiland Quaternary Geology of the Southwestern United States. Geological Society ofAmerica Special Paper 203, 1–21.

Mees, F., 1999. The unsuitability of calcite spherulites as indicators of subaerial exposure.Journal of Arid Environments 42, 149–154.

Merz-Preiβ, M., 2000. Calcification in cyanobacteria. In: Riding, R.E., Awramik, S.M.(Eds.), Microbial Sediments. Springer-Verlag, Berlin, pp. 50–56.

Merz-Preiβ,M.,Riding, R., 1999. Cyanobacterial tufa calcification in two freshwater streams:ambient environment, chemical thresholds and biological processes. SedimentaryGeology 126, 103–124.

Monger, H.C., Daugherty, L.A., Lindemann,W.C., 1991.Microbial precipitation of pedogeniccalcite. Geology 19, 997–1000.

Monger, H.C., Gallegos, R.A., 2000. Biotic and abiotic processes and rates of pedogeniccarbonate accumulation in the southwestern United States — relationship toatmospheric CO2 sequestration. In: Lal, R., Kimble, J.M., Eswaran, H., Steward, B.A.(Eds.), Global Climate Change and Pedogenic Carbonates. Lewis Publishers, Florida,pp. 273–289.

Monger, H.C., Cole, D.R., Gish, J.W., Giordano, T.H., 1998. Stable carbon and oxygenisotopes in Quaternary soil carbonates as indicators of ecogeomorphic changes inthe northern Chihuahuan Desert, USA. Geoderma 82, 137–172.

Mutler, H.G., Hoffmeister, J.E., 1968. Subaerial laminated crusts of the Florida Keys.Geological Society of America Bulletin 79, 183–192.

Nordt, L.C., Boutton, T.W., Hallmark, C.T., Waters, M.R., 1994. Late Quaternaryvegetation and climate changes in central Texas based on the isotopic compositionof organic carbon. Quaternary Research 41, 109–120.

Olszta,M.J., Gajjeraman, S., Kaufman,M.,Gower, L.B., 2004. Nanofibrous calcite synthesizedvia a solution-precursor-solid mechanism. Chemical Material 16, 2355–2362.

Pentecost, A., Bauld, J., 1988. Nucleation of calcite on the sheaths of cyanobacteia using asimple diffusion cell. Geomicrobiology Journal 6, 129–135.

Phillips, S.E., Milnes, A.R., Foster, R.C., 1987. Calcified filaments: an example of biologicalinfluences in the formation of calcrete in South Australia. Australia Journal of SoilResearch 25, 405–428.

Phillips, S.E., Self, P.G., 1987. Morphology, crystallography and origin of needle-fibrecalcite in Quaternary pedogenic calcretes of South Australia. Australia Journal ofSoil Research 25, 429–444.

Parraga, J., Rivadeneyra, M.A., Martin-Garcia, J.M., Delgado, R., Delgado, G., 2004.Precipitation of carbonates by bacteria from a saline soil, in natural and artifitial soilextracts. Geomicrobiology Journal 21, 55–66.

Preat, A., Kolo, K., Mamet, B., Gorbushina, A.A., Gillan, D.C., 2003. Fossil and subrecentfungal communities in three calcrete series from the Devonian of the Canadian RockyMountains, Carboniferous of northern France and Cretaceous of central Italy. In:Krumbein, W.E., Paterson, D.M., Zavarzin, G.A. (Eds.), Fossil and Recent Biofilms — ANatural History of Life on Earth. Dordrecht, Kluwer, pp. 291–306.

Prouty, J.S., 1996. Late Pleistocene-Early Holocene karst features, Laguna Madre, SouthTexas: a record of climate change. Transactions of the Gulf Coast Association ofGeological Societies 46, 345–351.

Read, J.F., 1974. Calcrete deposits and Quaternary sediments, Edel Province, Shark Bay,Western Australia. In: Logan, B.W., Read, J.F., Hagan, G.M., Hoffman, P., Brown, R.G.,Woods, P.J., Gebelein, C.D. (Eds.). AAPG Memoir 22, 250–282.

Reeves Jr., C.C., 1976. Caliche: Origin, Classification, Morphology and Uses. EstacadoBooks, Lubbock. 233 pp.

Richter, D.K., Immenhauser, A., Neuser, R.D., 2008. Electron backscatter diffractiondocuments randomly orientated c-axes in moonmilk calcite fibres: evidence forbiologically induced precipitation. Sedimentology 55, 487–497.

Shiraishi, F., Bissett, A., de Beer, D., Reimer, A., Arp, G., 2008. Photosynthesis, respiration andexopolymer calcium-binding in biofilm calcification (Westerhöfer andDeinschwangerCreek, Germany). Geomicrobiology Journal 25, 83–94.

Turner, R.D., 1966. Cenozoic pisolitic limestone and caliche near San Angelo,West Texas[Unpublished M.S. Thesis]. Houston, University of Houston, 46 pp.

Verrecchia, E.P., 2000. Fungi and sediments. In: Riding,R.E., Awramik, S.M. (Eds.),MicrobialSediments. Springer-Verlag, Berlin, pp. 68–73.


Verrecchia, E.P., Verrecchia, K.E., 1994. Needle-fiber calcite: a critical review and aproposed classification. Journal of Sedimentary Research A64, 650–664.

Verrecchia, E.P., Ribier, J., Patillon, M., Rolko, K.E., 1991. Stromatolitic origin for desertlaminar limecrusts: a new paleoenvironmental indicator for arid regions.Naturwissenschaften 78, 505–507.

Verrecchia, E.P., Dumont, J.-L., Verrecchia, K.E., 1993. Role of calcium oxalatebiomineralization by fungi in the formation calcretes: a case study from Nazareth,Israel. Journal of Sedimentary Petrology 63, 1000–1006.

Verrecchia, E.P., Freytet, P., Verrecchia, K.E., Dumont, J.-L., 1995. Spherulites in calcretelaminar crusts: biogenic CaCO3 precipitation as a major contributor to crustformation. Journal of Sedimentary Research A65, 690–700.

Verrecchia, E.P., Freytet, P., Verrecchia, K.E., Dumont, J.-L., 1996. Spherulites in calcretelaminar crusts: biogenic CaCO3 precipitation as a major contributor to crustformation-reply. Journal of Sedimentary Research A65, 1041–1044.

Verrecchia, E.P., Brassaint, O., Cailleau, G., 2006. The oxalate-carbonate pathway in soilcarbon storage: the role of fungi and oxalotrophic bacteria. Biochemical Cycles 12,289–310.

West, L.T., 1986. Genesis of soils and carbonate enriched horizons associated with softlimestones in central Texas [Unpublished Ph.D. Dissertation]. Texas A & MUniversity, College Station, pp. 221.

Wright,V.P., 1983a.A rendzina fromthe LowerCarboniferousof SouthWales. Sedimentology30, 159–179.

Wright, V.P., 1983b. Morphogenesis of oncoids in the lower Carboniferous LlanellyFormation of South Wales. In: Douglas, L.A. (Ed.), Soil Micromorphology: a Basic

and Applied Science. Developments in Soil Science, vol. 19. Elsevier, Amsterdam,pp. 424–434.

Wright, V.P., 1986. The role of fungal biominerization in the formation of EarlyCarboniferous soil fabrics. Sedimentology 33, 831–838.

Wright, V.P., 1989. Terrestrial stromatolites and laminar calcretes: a review. Sedimen-tary Geology 65, 1–13.

Wright, V.P., 1990. A micromorphological classification of fossil and recent calcic andpetrocalcic microstructures. In: Douglas, L.A. (Ed.), Soil Micromorphology: a Basicand Applied Science. : Developments in Soil Science, vol. 19. Elsevier, Amsterdam,pp. 401–407.

Wright, V.P., 1994. Paleosols in shallow marine carbonate sequences. Earth-ScienceReviews 35, 367–395.

Wright, V.P., 2007. Calcrete. In: Nash, D.J., McLaren, S.J. (Eds.), Geochemical Sedimentsand Landscapes. Blackwell, Oxford, pp. 10–45.

Wright, V.P., Tucker, M.E., 1991. Calcretes: an introduction. In: Wright, V.P., Tucker, M.E.(Eds.), Calcretes. : IAS Reprint Series, vol. 2. Blackwell, Oxford, pp. 1–22.

Wright, V.P., Platt, N.H., Wimbledon,W.A., 1988. Biogenic laminar calcretes: evidence ofcalcified root-mat horizons in paleosols. Sedimentology 35, 603–620.

Wright, V.P., Platt, N.H., Marriott, S.B., Beck, V.H., 1995. A classification of rhizogenic (root-formed) calcretes, with examples from the Upper Jurassic–Lower Cretaceous of Spainand Upper Cretaceous of southern France. Sedimentary Geology 100, 143–158.

Wright, V.P., Beck, V.H., Sanz-Montero, M.E., 1996. Spherulites in calcrete laminarcrusts: biogenic CaCO3 precipitation as a major contributor to crust formation-discussion. Journal of Sedimentary Research 66, 1040–1041.

Documents

Biogenic caliches in Texas: The role of organisms and effect of climate