Silica diagenesis in Eocene shallow-water ... - Wiley Online Library

Sedimentology (1999) 46, 969±984

Silica diagenesis in Eocene shallow-water platform carbonates, southern Pyrenees JORDI GIMEÂ NEZ-MONTSANT*, FRANCESC CALVET* and MAURICE E. TUCKER *Departament de GeoquõÂmica, Petrologia i ProspeccioÂ GeoloÁgica, Facultat de Geologia, Universitat de Barcelona, 08071 Barcelona, Spain (E-mail: [email protected]) Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK ABSTRACT

Spectacularly developed lower Eocene chert in the Corones platform carbonates of the Spanish Pyrenees is concentrated within a restricted, brackish-water, laminated ostracod-rich facies, which also contains abundant sponge spicules. The chert occurs as nodular, bedded and mottled varieties, and four petrographic types of quartz are developed: microquartz; length-fast (LF) chalcedony; megaquartz; and microspheres. d18O values of chert range from 29á6& to 30á9& (SMOW), which correspond to a broad isotope rank common for biogenic and diagenetic replacement cherts. Calcian dolomite crystals with high Fe and Na are disseminated within the microquartz and LF-chalcedony, but are absent from the megaquartz and host carbonate. The chert is closely associated with desiccation cracks and with interstratal dewatering structures. Load casts are silici®ed, and laminae rich in sponge spicules are convoluted. Early cracks related to dewatering are ®lled by microquartz and quartz cements. Ostracod shells within chert are locally fractured; those in the host carbonate are commonly ¯attened. Late fractures are ®lled by LF-chalcedony and megaquartz. There is much evidence for the dissolution of sponge spicules and their calcitization in the carbonate host rock. Silica for the Corones cherts was derived from sponges during early diagenesis and shallow burial. Early mechanical compaction and sediment dewatering played a major role in sponge spicule dissolution, migration of silica-rich ¯uids and the consequent precipitation of chert. Quartz cements continued to be precipitated into the burial environment. Keywords Chert, Eocene, limestone, Pyrenees, silica diagenesis, sponge

spicules.

INTRODUCTION Carbonate rocks commonly contain silici®ed fossils, burrow structures, evaporites, chert nodules and persistent beds composed of chert. The origin of chert in limestone has been much debated, and there are two major problems: the source of the silica and the timing of silici®cation. Cherts are exceptionally well developed in the carbonate rocks of an Eocene shallow-water platform in the Spanish Pyrenees. Of particular interest is the clear association of silici®cation with desiccation cracks, early interstratal dewaÓ 1999 International Association of Sedimentologists

tering cracks and structures and late fractures, which enables the timing of the silici®cation to be considered within the diagenetic history of the sediment. Dissolution and mobilization of silica in an early and shallow burial diagenetic stage resulted in the silici®cation of interstratal dewatering structures induced by shrinkage and progressive physical compaction of the sediment (upward-injected dykes, cracks, convoluted spicule-rich laminae, etc.) similar to those described by Tanner (1998) in sandstones. In view of this, a model of interaction between silici®cation and other early diagenetic processes requires the 969

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recognition of two main crack types: (1) synsedimentary downward-®lled desiccation cracks; and (2) early interstratal dewatering cracks, downward and upward ®lled (types 1 and 2 interstratal cracks of Tanner, 1998). As will be demonstrated, silici®cation is related to the occurrence of sponge spicules, a common source of silica in other cases (e.g. Geeslin & Chafetz, 1972; Meyers, 1977; Maliva & Siever, 1989; Gao & Land, 1991). The aim of this paper is to document the ®eld occurrence and petrography of the cherts and to describe the silici®cation in the context of the diagenetic history of the Corones carbonates. This study provides a well-documented case history of the complex story of silici®cation in limestone.

GEOLOGICAL SETTING The general structure of the south-eastern Pyrenees is that of a piggy-back sequence of thrust sheets, emplaced on the autochthonous Ebro basin sedimentary succession (Fig. 1a and b). The thrust sheets are divided into upper and lower units, depending on their structural position (MunÄoz et al., 1986; Clavell et al., 1988; VergeÂs & MartõÂnez, 1988; MartõÂnez et al., 1989). The upper thrust sheets (Pedraforca) are mainly composed of Mesozoic rocks and were emplaced ®rst. The lower thrust sheets (Garrotxa antiformal stacks, CadõÂ and Serrat thrust units) are composed of Palaeozoic, Mesozoic and Tertiary rocks (Fig. 1b). The CadõÂ thrust sheet has a width of

Fig. 1. (a) Geological map of the study area in the south-eastern Pyrenees (modi®ed from Losantos et al., 1989). Circled numbers refer to studied sections: 1, BagaÁ; 2, Paller; 3, P. Lillet; 4, Arija; 5, Gombren; 6, Ripoll; 7, Ogassa; 8, St Pau Seguries; 9, Oix; 10, Sadernes; 11, AlbanyaÁ; 12, Terrades. (b) A±B cross-section, line of section shown in (a), showing the main structural units of the south-eastern Pyrenees (MartõÂnez et al., 1989). The Corones carbonate platform outcrops in the CadõÂ thrust sheet and in the Garrotxa antiformal stacks. (c) Lower Ypresian (Cuisian) depositional sequence of the eastern Pyrenees. Silici®cation affects the laminated ostracod-rich facies of the Corones carbonate platform, which forms the transgressive systems tract of the sequence. Delta facies associations represent the lowstand systems tract of the sequence, and the highstand systems tract is composed of shallow to deep carbonate platform deposits. Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 969±984

Silica diagenesis in Eocene shallow-water platform carbonates 110 km, and its internal structure comprises a broad east±west syncline (Fig. 1a and b). Over the CadõÂ thrust sheet, the Garrotxa antiformal stacks form a structural subunit and include Palaeozoic and Tertiary rocks (Clavell et al., 1988; MartõÂnez et al., 1989). The Serrat thrust sheet is located beneath the CadõÂ thrust (MartõÂnez et al., 1989) and carries an anhydrite and halite succession, which exceeds 1500 m in thickness (Fig. 1b). The uppermost Maastrichtian to middle Eocene foreland succession in the outcropping CadõÂ thrust sheet and the Garrotxa antiformal stacks is composed of a deepening±shallowing set of nine depositional sequences (GimeÂnez-Montsant, 1993; GimeÂnez-Montsant & Salas, 1997; Fig. 1a and b). Sequence 1 (uppermost Maastrichtian± Thanetian) is broadly represented by alluvial systems and is overlain by sequences 2 and 3 (lower Ypresian) constituted by delta and shallow carbonate platform systems. Sequences 4 and 5 (middle±upper Ypresian) record an important basinwide deepening with the development of carbonate slope and platform facies, and sequences 6±9 (uppermost Ypresian±middle Lutetian) are characterized by turbidite, evaporite, delta and alluvial systems. The silici®cation discussed in this paper occurs within a lower Eocene shallow-water carbonate platform succession of sequence 3 (GimeÂnez-Montsant, 1993; GimeÂnez-Montsant & Salas, 1997), bounded above and below by two marine ¯ooding discontinuities and interpreted as a transgressive systems tract (GimeÂnez-Montsant, 1993; Fig. 1c). METHODOLOGY Extensive ®eldwork was carried out over the entire area of the Corones carbonate platform, where 12 stratigraphic sections were studied. Samples of chert for scanning electron microscopy (SEM) and petrographic analysis were etched in concentrated 29 M HF for about 10 min as described by Gao & Land (1991). Eight samples for oxygen isotopic analysis were powdered to 100 mesh and analysed at Geochron Laboratories, Canada. Samples for X-ray diffraction (XRD) were powdered, treated with concentrated 12 M HCl and dried. Microprobe analyses (120) were undertaken with a CAMECA model SX-50, equipped with four WD X-ray spectrometers; operating conditions were 20 kV excitation potential, 15 nA current intensity and 10 lm beam diameter. Petrographic analyses were completed using a Technosyn Cold Cathodoluminescence

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model 8200 MkII operating at 16±19 kV and 350 lA gun current. CORONES CARBONATE PLATFORM The Corones carbonate platform is exposed for 110 km within the CadõÂ thrust sheet (Fig. 1) and in the Garrotxa antiformal stacks, and it increases in thickness from 30 m in the west to 80 m in the east. The Corones platform consists of six lithostratigraphic units, which are made up of four major facies: shallow-water foraminiferal limestone; shelf/coastal plain shale and silt; brackishwater Chara-rich limestone; and laminated ostracod-rich limestone (Figs 2 and 3A). The shallower and more brackish-water facies increase in abundance towards the east, indicating a western source of the marine incursions. A distinctive cherty, ostracod-rich, well-bedded limestone, 9±15 m thick, grades laterally eastwards into Chara and ostracod facies, and westwards and upwards into shallow-water foraminiferal limestone. Another ostracod-rich unit with striking chert, up to 27 m thick, is located in the eastern part of the Corones carbonate platform (Oix to Terrades sections; Fig. 2a), and this also grades westwards into shallowwater foraminiferal limestone. The ostracod-rich units consist largely of laminated ostracod-rich facies, with subordinate molluscan limestone facies. The laminated ostracod-rich facies (Fig. 3A) constitutes grey-black beds up to 2 m thick and is mainly composed of wackestone± packstone with monospeci®c ostracods. The genus Neocyprideis sp. is dominant (G. Carbonel, 1991, pers. comm.) and is a marker of restricted, brackish-water conditions. Desiccation cracks are abundant, and total organic carbon (TOC) reaches 2 wt%. The composition of the organic matter suggests a variable lacustrine in¯uence, which increases towards the east (GimeÂnez-Montsant & Permanyer, 1991). This facies alternates with molluscan wackestone±packstone facies, with beds 0á1±0á5 m thick. The molluscs are monospeci®c and thin-shelled and occur whole or reworked. GENERAL CHARACTERISTICS OF THE CHERT Chert occurs dominantly within the laminated ostracod-rich facies and accounts for up to 20% of the rock. The chert is black and dark grey in

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Fig. 2. (a) Detailed cross-section of the Corones carbonate platform in the south-eastern Pyrenees. The facies pattern shows a progressive restriction from west to east, with the occurrence of laminated ostracod-rich facies and Chara-ostracod facies only in the eastern part of the platform. Circles refer to studied sections in Fig. 1a. (b) Representative stratigraphic section of the Corones carbonate platform (Arija section in a). As shown, silici®cation is associated with the laminated ostracod-rich facies.

colour, and it occurs in three main varieties: (1) nodular; (2) bedded; (3) mottled.

Nodular chert This type is most common (up to 85% of the total chert), with nodules up to 0á1 m thick and more than a metre in length. Their vertical development is controlled by bedding planes. Locally, some nodular chert forms mushroom-like structures with clear lateral contacts (locally sutured) and ¯at bases and tops (Fig. 3B). Where the carbonate host rock has not been ¯attened by compaction, sedimentary lamination continues

undeformed through the nodules and mushroomlike structures. The chert commonly contains relic components (ostracods and sediment) and ghosts of other carbonate grains, providing clear evidence of replacive silici®cation. Laterally, close to the nodules, the carbonate host rock contains abundant calcitized and/or dissolved sponge spicules, 100 lm long.

Bedded chert This chert (up to 10% of total chert) forms beds up to 0á15 m thick and more than 20 m long. Horizontal contacts are sharp, and relics of


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Fig. 3. (A) Typical aspect of the laminated ostracod-rich facies. Cherty ostracod-rich unit (BagaÁ section in Fig. 2a). The marker pen is 15 cm long. (B) Silici®ed mushroom-like structure. When the effect of the compaction is weak, the sedimentary lamination continues through the host rock and the silici®ed substrate, providing clear evidence of the replacive character of the chert. (C) Microquartz variety of chert. Crossed-polarized light. (D) Megaquartz, consisting of inclusion-free crystals with planar and curved contacts. Crossed-polarized light.

ostracods and lamination are common. The bedded chert contains spicule-rich laminae, which are locally convoluted.

Mottled chert This chert (5% of total chert) is characterized by irregular silici®ed patches within unreplaced ìslands' of limestone. The silici®ed patches are millimetres to centimetres across and are commonly associated with water-escape structures, which are also silici®ed, as well as fractures and plastic deformation structures within the host rock.

the quartz range from +29á6& to +30á9& SMOW (n 8), and there are no isotopic differences between the chert types. These isotope values occur within the range (+ 22& to + 39&) common for biogenic and diagenetic replacement cherts 1 (Kolodny & Epstein, 1976; Murata et al., 1977; 2 Jones & Knauth, 1979; Matheney & Knauth, 1993; Gao & Land, 1991; Elorza & GarcõÂa-Garmilla, 1993). However, four main petrographic varieties of chert are distinguished: (1) microquartz; (2) length-fast (LF) chalcedony; (3) megaquartz; and (4) microspheres. These quartz varieties occur in both replacive and pore-®lling fabrics.

Microquartz MINERALOGY AND PETROGRAPHY OF THE CHERT Petrography and XRD data con®rm that the cherts are composed exclusively of quartz. d18O values for

This is the most common variety (Fig. 3C) and is characterized by mosaics of equal-sized crystals, up to 35 lm across, with sutured contacts and undulatory extinction. Microquartz mainly replaces bioclasts and carbonate sediment and is


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always the ®rst cement generation to ®ll primary intraparticle porosity (mainly within ostracod carapaces) and early fracture porosity.

Length-fast chalcedony This is a pore-®lling cement, which occurs in two morphological types: (1) pseudobotryoids; (2) isopachous rims. Pseudobotryoidal LF-chalcedony mainly ®lls primary intraparticle porosity (ostracod carapaces) and, locally interparticle porosity. Some ostracod carapaces close to chert nodules and beds are also ®lled by pseudobotryoidal LF-chalcedony. Isopachous rims of LF-chalcedony are up to 100 lm in thickness and ®ll primary porosity and early diagenetic cracks.

Megaquartz This is characterized by mosaics of equal-sized crystals, from 50 to 300 lm in diameter, generally with an equant shape (Fig. 3D). Locally, megaquartz crystals are bladed adjacent to the substrate. This variety always occurs as a late cement, after the microquartz and LF-chalcedony generations, and it ®lls primary and secondary intraparticle porosity of ostracod carapaces, desiccation and early diagenetic cracks and late fractures. Similar pore-®lling sequences have been observed by Mukhopadhyay & Chanda (1972), Meyers (1977) and Noble & Van Stempvoort (1989).

Microspheres Bedded chert contains dense accumulations of spherical structures (Fig. 4A±C), concentrated in thin silici®ed laminae, up to 1 cm thick. These microspheres have a regular size of 20±30 lm. Their internal structure comprises two parts, a central part of cryptocrystalline quartz, 10±15 lm in diameter, and a cortex, 5±10 lm thick, composed of radial ®brous units of cryptocrystalline quartz (Fig. 4B). Locally, the structures are partially dissolved (Fig. 4C), showing an outer envelope. The microspheres are closely associated with sponge spicules, which have been dissolved or are calcitized within the host carbonate rock. Locally, these microspheres occur as geopetal sediment within ostracod carapaces, but they are not related to the late pore-®lling quartz. They are cut by late veins and fractures. The spherical structures resemble the diagenetic quartz microspheres described by Oehler

(1975) and Meyers (1977). In size and shape, they are also similar to lepispheres, although these are usually associated with pore-®lling fabrics and do not show any internal division into a core and a cortex, as distinguished in these spherical structures. The microspheres described here have a characteristic internal structure and size very similar to the spherical spicular structures of some sponges (Cayeux, 1929; Reitner & Keupp, 1991). The partly dissolved and calcitized microspheres in nonsilici®ed limestone are closely associated with calcitized elongate sponge spicules, and so this also supports a biogenic origin. SILICIFICATION AND OTHER DIAGENETIC EVENTS The formation of the chert in the Corones carbonates can be integrated with other diagenetic events, namely: (1) dolomitization; (2) synsedimentary cracking; (3) early fracturing; (4) late fracturing; (5) mechanical compaction; (6) chemical compaction±tectonic compression.

Silici®cation and dolomitization The Corones chert contains disseminated inclusions of rhombic dolomite, from 5 lm to 0á3 mm in size (average 15±20 lm). Under cathodoluminescence (CL), most dolomite crystals have a bright orange colour, and some are zoned (Fig. 4D). Microprobe analysis of the dolomites reveals a calcic non-stoichiometric composition (Ca58±52±Mg42±48CO3). This is typical of early diagenetic dolomites (Carballo et al., 1987; Tucker & Wright, 1990). In addition, some 60% of the dolomites analysed have signi®cant Fe and Na contents (up to 1540 p.p.m. and 1090 p.p.m. respectively); Sr and Mn contents, on the other hand, are below the microprobe detection limit. Some dolomite inclusions have been calcitized, and the calcite is non-luminescent. These inclusions have high, relic Mg contents (range 1500± 6800 p.p.m.; average » 4000 p.p.m.). The abundance of dolomite crystals varies through the chert nodules, although they are usually more common close to the chert±limestone contact and in silici®ed substrates with much relic sediment. The dolomite crystals only occur within the replacive and pore-®lling microquartz and LF-chalcedony or in a zone a few millimetres wide around chert nodules. These dolomite inclusions are absent in the megaquartz



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Fig. 4. (A) Photomicrograph of bedded chert. Elongate sponge spicules are common (arrowed).The abundant spherical structures are also interpreted as sponge skeletal debris (A. San®lippo, 1991, pers. commun.). Plane polarized light. (B) Microspheres showing a core and a cortex of ®brous units of cryptocrystalline quartz. SEM. (C) Partially dissolved microspheres showing remains of outer envelopes (arrowed). SEM. (D) Zoned rhombic dolomite inclusions under CL microscope.

crystals and in the carbonate host rock. In the case of ostracod carapaces ®lled by LF-chalcedony, the shell restricts the growth of the dolomite inclusions. The presence of dolomite inclusions in chert is a common feature, described elsewhere by many authors (Dietrich et al., 1963; Banks, 1970; Geeslin & Chafetz, 1972; Jacka, 1974; 3 Knauth, 1979; Bustillo & Riuz-Ortiz, 1987; Coniglio, 1987; Maliva & Siever, 1989; Noble & Van Stempvoort, 1989; Misik, 1993).

Silici®cation and synsedimentary cracking Desiccation cracks are common within the Corones limestones (Fig. 5), and silici®cation is usually associated with them. An unambiguous subaerial desiccation crack has to exhibit some key features, listed by Tanner (1998), to differentiate them from interstratal cracks, also observed

in the examples studied here. The studied desiccation cracks have tapered V-shaped pro®les (Fig. 5) and are orthogonal in plan view. Millimetre-scale gypsum pseudomorphs are uncommon but do occur within sedimentary laminae immediately below some desiccation cracks, and were clearly related to subaerial evaporation, as shown in other case studies (Donovan & Foster, 1972; Tanner, 1998). Figure 5 shows a desiccation crack, 5 cm deep, with its upper surface cut and eroded by a silici®ed spicule-rich lamina. The upper half of the crack is ®lled with sediment, not found above the crack and also eroded by the top spicule-rich lamina, and with intraclasts from the walls of the crack. All of these characteristics invalidate a later interstratal origin quoted for early diagenetic downward-®lled cracks (type 2 crack of Tanner, 1998). On both sides of the subaerial crack, the host rock is affected by


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Fig. 5. (a) Drawing of a polished slab consisting of alternations of light and dark beds of chert with some relic limestone, showing the interaction between desiccation cracks and injected silica dykes. Upward ¯uid injection also used vertical conduits of subaerial cracks to migrate, evidence of an almost synchronous timing of subaerial exposure and silica mobilization in the interstitial ¯uid. (b) Detailed sketch of desiccation crack ®ll in centre of slab in (a). The upper part of the crack ®ll contains evaporite pseudomorphs related to the subaerial exposure. See text for further explanation.

microfractures and bedding-parallel apophyses. The lower part of the crack is partly ®lled by larger, millimetre-sized intraclasts, included in a matrix of microquartz with abundant relic sediment. The remaining porosity is ®lled by a characteristic pore-®lling sequence, which is, in order of precipitation: (1) a discontinuous rim of microquartz; (2) an isopachous rim of LF-chalcedony; (3) megaquartz, which occludes practically all the remaining porosity; (4) local baroque dolomite, which has an irregular, corroded contact with previous precipitates.

Silici®cation and early fracturing Early fractures are generally associated with ¯uidescape structures, and these are themselves commonly silici®ed. In Figs 6±9, multiple dykes of microquartz, more or less perpendicular to bedding, are shown cutting the limestone beds. Other microfractures ®lled by microquartz extend up from silici®ed layers but do not pass completely through the host rock. The lower parts of these silica dykes display features of ¯uid escape and injection, and preserve evidence of the upward



Fig. 6. Polished slab (a) and interpretation (b) showing àrrowhead' crack and convoluted spicule-rich lamina. The ®lling of the àrrowhead' shows a geopetal pattern. The lower part of the crack is replaced and ®lled by microquartz, and the upper part is ®lled by up to three later generations of quartz (see text). A silica nodule is developed close to the convoluted spicule-rich lamina, suggesting a close genetic relationship between convolution of the spicule-rich lamina and mobilization of silica-rich ¯uids and precipitation of the chert nodule.

¯ow of silica-rich ¯uid. This indicates that a silica-rich ¯uid used these vertical cracks as conduits to migrate upwards from a very permeable substrate. The silica dykes display a slight plastic deformation, and they are similar to the type 2 interstratal crack of Tanner (1998), considered to be the result of layer-parallel contraction caused by compaction. Locally, some injection dykes used previous desiccation cracks as vertical conduits to migrate upwards (Fig. 5a). This process has been observed in clastic sediments (Oomkens, 1966), in which wet sand has intruded upwards into desiccation cracks. Tanner

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(1998) considered this relation to indicate interstratal shrinkage of downward-®lled cracks (type 1 crack) and upward-injected cracks (type 2 crack). However, in the present case study, this paragenesis is demonstrated by some differential movement features, noted above, between the downward-®lled subaerial cracks and type 1 interstratal cracks of Tanner (1998). Mottled chert is associated with silici®ed ¯uid-escape structures and calcitized spicule-rich host sediment with early fracturing and intraclasts (Fig. 9B); this is evidence for a genetic relation between this chert type and the incorporation of silica into migrating ¯uids during interstratal dewatering. Àrrowhead' cracks (Figs 6 and 7) deform the silici®ed host rock in a more or less ductile fashion, causing or accentuating the deformation of convoluted spicule-rich laminae. These cracks are up to 1 cm in size and are developed more or less vertically, pointing stratigraphically upwards. The lower parts of some arrowhead cracks are ®lled by microquartz and include intraclasts and relic host sediment. Their upper parts are ®lled by a pore-®lling sequence, similar to that of the desiccation cracks, of microquartz, LF-chalcedony and megaquartz. The lower microquartz ®ll to the arrowhead cracks suggests a geopetal ¯oor overlain by silica cement. Shape, pore-®lling sequence and relationship to convoluted laminae suggest that the arrowhead cracks formed by the escape of interstitial ¯uid and the shrinkage of the sediment. In plan view, the silici®ed arrowhead cracks are incomplete polygons with a trilete or spindle shape. In cross-section, these cracks vaguely resemble gypsum pseudomorphs. Some spicule-rich laminae are strongly convoluted and are adjacent to zoned silica nodules and arrowhead cracks (Figs 5±7). The convoluted laminae show irregular bases and tops and lateral thinning and pinching-out; this indicates a loss of silica by dissolution. Moreover, some of this convolution is clearly induced by the arrowhead cracks cutting spicule-rich laminae. Contrasting with these structures, the initial growth of nodular chert did not disturb the sedimentary lamination. Its formation, similar to the structures of ¯uid migration related to silica dykes, had to originate within permeable sediment, which allowed the free circulation of silica-rich ¯uid. Moreover, convoluted spicule-rich laminae restricted the vertical growth of adjacent chert nodules. This is evidence for the convolution of spicule-rich laminae taking place before or synchronous with the development of the chert nodules.


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Fig. 7. (a) Polished slab (drawing shown in b) constituted by alternations of dark bands of bedded chert and light bands of host rock (limestone) and (b) interpretation of polished slab (mirror-image of a) etched in concentrated HF. A limestone bed is intruded by abundant silica dykes and underlain by convoluted spicule-rich laminae, which show marked lateral variations in thickness. Chert nodules grew close to the convoluted spicule-rich laminae. Rectangle shows area in Fig. 8.

Fig. 8. Polished slab etched in concentrated HF, displaying the internal structures; limestone beds are darker. Convolution of spiculerich lamina is related to important development of silica nodules, providing evidence on the timing of the silica nodules and the silica removal necessary for liquefaction and convolution of the lamina. White arrow indicates an àrrowhead' crack. Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 969±984


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Fig. 9. (A) Polished slab etched in concentrated HF showing a silici®ed injection structure (silica dyke) produced by upward ¯uid escape. Bar scale 1 cm. (B) Interpretation of (A). The silica dyke has a central conduit (crack) and an outer part silici®ed by diffusion within a permeable substrate. Note that the sedimentary lamination is not deformed by the upward migration band of the silica dyke, although it is variably silici®ed, indicating that ¯uid migrated through a permeable sediment. The central crack of the dyke extends below, affecting the convolution of spicule-rich lamina. (C) Polished slab displaying mottled chert. Close to mottled chert, host rock displays cracks produced by the early interstratal shrinkage of the sediment and migration of ¯uids. Host rock shows a clotted texture because of the calcitization of abundant spicules. Scale bar 5 cm.

Silici®cation and late fracturing Late fractures cutting both the chert and the limestone are associated with the development of secondary porosity affecting chert, host rock, and earlier diagenetic structures (Fig. 10). Fracture ®lls contain up to three generations of cement, in the order: (1) isopachous LF-chalcedony; (2) megaquartz; (3) ferroan calcite and/or baroque dolomite, which may contain hydrocarbon inclusions. Moreover, the contact between the ®rst two generations of cement and the third generation is an irregular dissolution surface (Fig. 10A and B).

Silici®cation and mechanical compaction As a result of the later diagenetic lithi®cation of the carbonate host sediment, the timing of the silici®cation of sedimentary structures and components can be related to compaction of the host sediment. Some ostracod carapaces occurring within chert nodules show an incipient fracturing, which took place before the silici®cation. This early fracturing also affects the ®rst generation of microspar rim cement, which lines the intraparticle porosity of the ostracod carapaces. Moreover, the silici®ed rock also displays loadcast structures (Fig. 10C), clearly indicating

silici®cation after early mechanical compaction, when the carbonate sediment was still in a plastic state.

Silici®cation and chemical compaction±tectonic compression In the CadõÂ thrust sheet, burial diagenesis was characterized by the effects of lithostatic pressure and tectonic compression, inducing both vertical and horizontal stress. The effect of these processes was enhanced by the incompetent nature of the laminated ostracod-rich facies, resulting from its ®ne lamination and relatively high clay mineral and organic matter content. The main structures produced by chemical compaction are pressure dissolution seams and stylolites, the latter with residues up to a few millimetres thick. Microbrecciation occurs locally. Alternation between more competent laminae (generally with a grain-supported fabric and composed of ostracod shells) and less competent laminae (generally with a mud-supported fabric and rich in organic matter and clays) results in a more intense ¯attening of components and matrix in the incompetent laminae, with dissolution of the carbonate fraction. Ostracod carapaces in chert and those ®lled by LF-chalcedony in


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Fig. 11. (A) Differential chemical compaction strongly affects the host rock. The original sedimentary fabric in the chert, basically undisturbed, shows some compacted grain contacts and grain breakage resulting from an initial physical compaction. Rhombic crystals are dolomite. A perpendicular calcite vein only affects the chert nodule. (B) Polished slab showing common tectonic structures (microfolding, thrusting, foliation) after the formation of silica nodules.

Fig. 10. (A) and (B) Late fractures affecting a chert nodule. The fractures are ®lled by a ®rst generation of LF-chalcedony, then megaquartz and, ®nally, baroque dolomite and/or ferroan calcite with common inclusions of hydrocarbons (arrows in B). Crossed-polarized light. (C) Silici®ed load-cast structure. Chert is dark.

unsilici®ed host sediment are relatively uncompacted or only affected by a weak fracturing predating an initial mechanical compaction (Fig. 11A). However, non-silici®ed ostracod cara-

paces and carbonate host rock surrounding a nodule are ¯attened and commonly have pressure dissolution shadows. The subsequent dissolution of the carbonate increased the clay fraction and gave a reddish colour to the affected zone. Clearly, undisturbed silici®ed ostracods and sediment indicate that the diagenetic processes leading to silici®cation occurred before chemical compaction, a timing relationship noted elsewhere (Geeslin & Chafetz, 1972; Namy, 1974; ElShahat, 1977; Eley & Jull, 1982; Maliva & Siever, 1989). Moreover, the chert is cut by veins up to 2 mm across, ®lled by calcite and/or dolomite cement. They are oriented perpendicular to the bedding plane and extend out of the chert for several millimetres (Fig. 11A). These veins are


Silica diagenesis in Eocene shallow-water platform carbonates interpreted as the result of lithostatic pressure causing fracturing of the silici®ed rock, which would have been much more competent and brittle than the carbonate host rock (Fig. 12, C1). Similar fractures have been identi®ed by Geeslin & Chafetz (1972) and Bustillo & Ruiz-Ortiz (1987). Tectonic compression produced a characteristic metre-scale, concentric folding within the laminated ostracod-rich facies of the Corones carbonates, as well as foliation, fractures and minor thrusts (GimeÂnez-Montsant & VergeÂs, 1991; GimeÂnez-Montsant, 1993; Fig. 11B). Microfolding resulted in recrystallization of grain-supported laminae, whereas intense tectonic deformation of mud-supported laminae resulted in the dissolution of carbonate and local formation of zoned dolomite crystals, 30±40 lm in diameter. This type of dolomite, related to the deformation (cf.

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Wanless, 1979), is non-luminescent and clearly very different from the dolomite inclusions in the chert described earlier. This folding commonly deforms horizontal dissolution seams and stylolites and ¯attened laminae between chert nodules, predating most of the burial compaction (Fig. 12, C2). This relative timing is supported by regional data (MunÄoz et al., 1986; MartõÂnez et al., 1989; GimeÂnez-Montsant & Salas, 1997; MartõÂnez et al., 1997); the Corones carbonate platform is part of the CadõÂ thrust sheet, which suffered uplift and compression beginning around 49 Ma. Before this, it was subjected to ¯exure and downwarping by the Pyrenean thrust belt and a high sedimentation rate (up to 0á5±2 m ky)1; GimeÂnez-Montsant & Salas, 1997), which caused a progressive compaction of the stratigraphic column for some 2±3 Myr at the very least.

Fig. 12. Diagram summarizing the main diagenetic stages. (A) Very shallow-water sedimentation of spicule-rich laminae and ostracodrich facies was accompanied by recurrent subaerial exposures with desiccation cracks and growth of vadose gypsum below erosive surfaces. (B) In early and very shallow diagenetic conditions, biogenic silica dissolved and mobilized into interstitial water at the time of a progressive physical compaction. Interstratal dewatering resulted in upward migration of silica-rich ¯uids through injection dykes, which also used desiccation cracks as alternative vertical pathways for migration. Bedding planes acted as vertical permeability barriers con®ning silica-rich ¯uid, which was trapped into nodular growths and `mushroom-like' structures. Convolution, pinching-out and thinning of individual spicule-rich laminae formed by dissolution of silica, which was mobilized and trapped into adjacent silica nodules. (C) Progressive burial increased vertical stress (r1), resulting in the compactive ¯attening and carbonate dissolution of the sediment surrounding silica nodules (C1). In the silica nodules, the lithostatic stress was expressed by vertical calcite veins in concert with a horizontal stretching (r3). Later, regional Tertiary compression and uplift (C2) produced a change into a horizontal stress (r1), with the tectonic deformation of the carbonate rock and chert bands. Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 969±984

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DISCUSSION The silica in the Corones cherts is considered to have been derived from sponge spicules, which were originally composed of metastable amorphous opal-A (Calvert, 1971; Fig. 12A). Dissolution took place early in a highly permeable clayey-carbonate sediment with abundant pore ¯uid. When the pore ¯uids became saturated, silica was precipitated as zoned chert nodules, and interstratal ¯uid-escape structures, related to the injection of silica dykes, were also silici®ed (Fig. 12B). In this context, convolution of spiculerich laminae remains a problem. Dewatering of spicule laminae during initial compaction could result in their differential convolution, although the associated mechanism for increasing pore ¯uid pressure is unclear. On the other hand, liquefaction is the mechanism responsible for convolution in clastic sediments (Lowe, 1975; Hiscott, 1979), in which an increasing pore ¯uid pressure can be caused by earthquakes and high sedimentation rate. Although liquefaction of individual spicule-rich laminae interbedded with undisturbed host sediment seems unlikely here, there is no alternative process for increasing pore ¯uid pressure that would be capable of convoluting water-saturated sediment. From the evidence of partial dissolution (corroded contacts and pinching-out of laminae; Figs 5a and 6) and the close association with arrowhead cracks, silica dykes and adjacent chert nodules, a change in the liquid±solid state, through dissolution of biogenic silica, could increase the local pore-water pressure, causing liquefaction on this very small scale. Subsequent convolution would decrease the pore ¯uid pressure by releasing the excess of silica-rich ¯uid trapped in adjacent nodules. The association of ìnstantaneous' liquefaction and gradual silica dissolution suggests that there is a critical value or threshold in dissolution of the silica to initiate the process. The close relationships of the silici®ed convoluted spicule-rich laminae with arrowhead cracks, silica dykes and adjacent chert nodules suggest that they formed very close together in time and were then the sites of silica precipitation. Moreover, the preferential horizontal growth of larger chert nodules indicates strong vertical variations in permeability induced by the bedding. The `mushroom-like' structures would have been generated by lateral migration of silicarich ¯uids through the sediment and discontinuous and patchy precipitation in local traps and nucleation zones (Fig. 12B).

Mottled chert is also associated with ¯uidescape structures. Dissolution and convolution of spicule-rich laminae accentuated deformation in the sediment and caused migration of ¯uids, resulting in irregular patches of chert and host rock ìslands'. Although Bustillo & Riuz-Ortiz 4 (1987) interpreted mottled chert in Jurassic turbidites as forming in a later diagenetic, Mg-rich environment, all the evidence here supports a very early diagenetic origin. Bioturbation could also have destroyed the original sedimentary fabric and produced a mottled pattern. However, bioturbation is rare in the Corones laminated ostracod-rich facies and cannot account for all the mottled lithofacies. Fluid-escape structures caused by the movement of interstitial silica-rich ¯uids are the result of an internal arrangement of the sediment induced by sediment loading, consolidation, shrinkage and compaction (Lowe, 1975; Hiscott, 1979). The incipient fracturing of ostracod carapaces included in chert, the silica dykes injected through the host sediment, the arrowhead cracks related to convoluted spiculerich laminae and the silici®ed load-casts all suggest that mechanical compaction was taking place when the silica-rich ¯uids were being mobilized. Concentrations of dolomite crystals close to the chert±host rock contact in replacive and pore®lling early microquartz and LF-chalcedony (but never in late megaquartz and LF-chalcedony) clearly suggest selective dolomitization during early diagenetic interaction between the silicarich ¯uids and the host sediment before the latter's silici®cation. In addition, high Fe and Na contents and non-stoichiometry support the early diagenetic origin of the dolomite. The magnesium for the dolomite could be derived from high-Mg calcite within the laminated ostracod-rich facies. Supporting an early diagenetic origin, Misik (1993) suggested that similar rhombic dolomite inclusions in chert formed in sediment with a high water content. Kastner et al. (1977) related the origin of dolomite in chert to the release of magnesium during the transformation of opal-CT to quartz. Subaerial exposure surfaces in the laminated ostracod-rich units probably also had some in¯uence on silici®cation. In the ®rst place, these surfaces imply discontinuous sedimentation, which would have modi®ed the onset and effects of mechanical compaction. Secondly, evaporation would have taken place at the subaerial exposure surfaces, and this could have accelerated the process of ¯uid escape through


Silica diagenesis in Eocene shallow-water platform carbonates interstratal shrinkage and consolidation of the sediment. In this respect, it is signi®cant that the spicule-rich laminae are better preserved immediately beneath exposure horizons. In addition, the subsequent marine ¯oodings after the exposure events could have altered the pore water pressure and contributed to pore water movement, sponge-spicule dissolution and silici®cation. Finally, it is worth noting that the chalcedony present is the length-fast variety. This suggests that the silici®cation is not related to the dissolution and replacement of evaporites or to saline ¯uids, in which length-slow chalcedony would be the expected quartz type (cf. Folk & Pittman, 1971).

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ACKNOWLEDGEMENTS The authors would like to thank A. San®lippo for identi®cation of spherical spicular structures, and G. Carbonel for classi®cation of some ostracod fauna. Discussions with K. Bitzer and R. Salas are much appreciated. Helpful reviews were provided by E. Fellman, W. Meyers and R. Siever. J. Andrews and I. Jarvis are thanked for their editorial advice. Economic support was provided by the Comissionat per Universitats i Recerca de la Generalitat de Catalunya (1995 SGR 00195), Project PB92-0862-CO2-01 (DIGICYT) and a Postdoctoral Fellowship from the Spanish Ministry of Science and Education. REFERENCES

CONCLUSIONS 1 Silici®cation within the Eocene Corones carbonates of the Spanish Pyrenees is concentrated within laminated ostracod-rich limestones, and the silica was derived largely from the dissolution of sponge spicules. 2 The onset of silica dissolution and mobilization took place during very early diagenesis, and was related to increasing mechanical compaction. Interstratal dewatering structures, including convoluted laminae, injection dykes, load-casts and fractures, were produced by the migration of the silica-rich ¯uids and are silici®ed by microquartz or contain silica cements of length-fast chalcedony or megaquartz. 3 Dolomite crystals within the cherts are nonstoichiometric (CO3Ca58±52±Mg42±48) with high Fe and Na contents. They were precipitated in the earliest geochemical interchanges between silicarich ¯uids and the host sediment, before chert formation. 4 Silica continued to be precipitated into the shallow burial diagenetic environment, and length-fast chalcedony and megaquartz were precipitated in fractures produced by mechanical compaction. However, silici®cation was completed before the onset of chemical compaction and tectonic compression, when the chert, but especially the host carbonate, was affected by pressure dissolution and cut by calcite±dolomite-®lled veins. 5 The Corones carbonates provide a good case study of the complex processes of silici®cation and chert formation in a limestone succession.

Banks, G. (1970) Nature and origin of early and late chert in the Leadville Limestone, Colorado. Bull. Geol. Soc. Am., 81, 3033±3048. Bustillo, M.A. and Riuz-Ortiz, P.A. (1987) Chert occurrences in carbonate turbidites: examples from the Upper Jurassic of the Betic Mountains (southern Spain). Sedimentology, 34, 611±621. Calvert, S.E. (1971) Nature of silica phases in deep-sea cherts of the North Atlantic. Nature, 234, 133±134. Carballo, J.D., Land, L.S. and Miser, D.E. (1987) Holocene dolomitization of supratidal sediments by active tidal pumping, Sugarloaf Key, Florida. J. Sedim. Petrol., 57, 153±165. Cayeux, L. (1929) Les Roches SeÂdimentaires de France ± Roches Siliceuses. Impr. Nationale, Paris. Clavell, E., MartõÂnez, A. and VergeÂs, J. (1988) Morfologia del basament del Pirineu oriental: evolucioÂ i relacioÂ amb els mantells de corriment. Acta Geol. HispaÂnica, 23, 129±140. Coniglio, M. (1987) Biogenic chert in the Cow Head Group (Cambro-Ordovician), western Newfoundland. Sedimentology, 34, 813±823. Dietrich, R.V., Hobbs, C.R.B. and Lowry, W.D. (1963) Dolomitization interrupted by silici®cation. J. Sedim. Petrol., 33, 646±663. Donovan, R.N. and Foster, R.J. (1972) Subaqueous shrinkage cracks from the Caithness Flagstone Series (Middle Devonian) of northeast Scotland. J. Sedim. Petrol., 42, 309±317. Eley, B.E. and Jull, R.K. (1982) Chert in the Middle Silurian Fossil Hill Formation of Manitoulin Island, Ontario. Bull. Can. Petrol. Geol., 30, 208±215. Elorza, J. and GarcõÂa-Garmilla, F. (1993) Chert appearance in the Cueva-BedoÂn carbonate platform (upper Cretaceous, northern Spain). Geol. Mag., 130, 805±816. El-Shahat (1977) Petrography, and Geochemistry of a Limestone-Shale Sequence with Early and Late Lithi®cation: The Middle Purbeck of Dorset, England. PhD Thesis, University of Southampton.


984


Folk, R.L. and Pittman, J.S. (1971) Length-slow chalcedony: a new testament for vanished evaporites. J. Sedim. Petrol., 41, 1045±1058. Gao, G. and Land, L. (1991) Nodular chert from the Arbuckle Group, Slick Hills, SW Oklahoma: a combined ®eld, petrographic and isotopic study. Sedimentology, 38, 857±870. Geeslin, J.H. and Chafetz, H.S. (1972) Ordovician aleman ribbon cherts: an example of silici®cation prior to carbonate lithi®cation. J. Sedim. Petrol., 52, 1283±1293. GimeÂnez-Montsant, J. (1993) AnaÂlisis de cuenca del Eoceno Inferior de la Unidad CadõÂ (Pirineo Oriental): El sistema deltaico y de plataforma carbonaÂtica de la FormacioÂn de Corones. PhD Thesis, University of Barcelona. GimeÂnez-Montsant, J. and Permanyer, A. (1991) Variations in organic matter content related to carbonate platform cycles: evidence for cyclic productivity. In: Advances and Applications in Energy and the Natural Environment (Ed. by D.A.C. Manning), pp. 30±34. Manchester University of Press, Manchester. GimeÂnez-Montsant, J. and Salas, R. (1997) Subsidence analysis in thrust tectonics. Application to the southeastern Pyrenean foreland. Tectonophysics, 282, 331±352. GimeÂnez-Montsant, J. and VergeÂs, J. (1991) Los pliegues de la FormacioÂn Corones (Unidad CadõÂ): origen tectoÂnico versus origen sinsedimentario. I Congr. Esp. Terciario. Vic. Abstracts volume, 151±154. Hiscott, R.N. (1979) Clastic sills and dykes associated with deep-water sandstones, Tourelle Formation, Orcovician, Quebec. J. Sedim. Petrol., 49, 1±10. Jacka, A.D. (1974) Replacement of fossils by lengthslow chalcedony and associates dolomitization. J. Sedim. Petrol., 44, 421±427. Jones, D.L. and Knauth, L.P. (1979) Oxygen isotopic and petrographic evidence relevant to the origin of the Arkansas Novaculite. J. Sedim. Petrol., 49, 581±597. Kastner, M., Keene, J.B. and Gieskes, J.M. (1977) Diagenesis of siliceous oozes I. Chemical controls on the rate of opal-A to opal-CT transformation ± an experimental study. Geochim. Cosmochim. Acta, 41, 1041±1059. Knauth, L.P. (1979) A model for the origin of chert in limestone. Geology, 7, 274±277. Kolodny, Y. and Epstein, S. (1976) Stable isotope geochemistry of deep sea cherts. Geochim. Cosmochim. Acta, 40, 1195±1209. Losantos, M., AragoneÂs, E., BeraÂstegui, X., Palau, J. and PuigdefaÁbregas, C. (1989) Mapa geoloÁgic de Catalunya: Barcelona, Departament de PolõÂtica Territorial i Obres PuÂbliques, Generalitat de Catalunya, 1:250 000, 1 sheet. Lowe, D.R. (1975) Water escape structures in coarsegrained sediments. Sedimentology, 22, 157±204. Maliva, R.G. and Siever, R. (1989) Nodular chert formation in carbonate rocks. J. Geol., 97, 421±433. MartõÂnez, A., VergeÂs, J., Clavell, E. and Kennedy, E. (1989) Stratigraphic framework of the thrust geome-

try and structural inversion in the Southeastern Pyrenees, La Garrotxa area. Geodinamica Acta, 3, 185±194. MartõÂnez, A., Rivero, L. and Casas, A. (1997) Integrated gravity and seismic interpretation of duplex structures and imbricate thrust systems in the southeastern Pyrenees (NE Spain). Tectonophysics, 282, 303±329. Matheney, R.K. and Knauth, L.P. (1993) New isotopic temperature estimates for early silica diagenesis in 5 bedded cherts. Geology, 21, 519±522. Meyers, W.J. (1977) Cherti®cation in the Mississippian lake Valley Formation, Sacramento Mountains, New Mexico. Sedimentology, 24, 75±105. Misik, M. (1993) Carbonate rhombohedra in nodular cherts. Mesozoic West Carpathians. J. Sedim. Petrol., 63, 275±281. Mukhopadhyay, A. and Chanda, S.K. (1972) Silica diagenesis in the banded hematite jasper and bedded chert associated with the iron ore group of Jamda-Koira Valley, Orissa, India. Sedim. Geol., 41, 113±135. MunÄoz, J.A., MartõÂnez, A. and VergeÂs, J. (1986) Thrust sequences in the eastern Spanish Pyrenees. J. Struct. Geol., 8, 399±405. Murata, K.J., Friedman, I. and Gleason, J.D. (1977) Oxygen relations between diagenetic silica minerals in Monterey Shale, Temblor Range, California. Am. J. Sci., 277, 259±272. Namy, J.N. (1974) Early diagenetic chert in the Marble Falls Group (Pennsylvanian) of central Texas. J. Sedim. Petrol., 44, 1262±1268. Noble, J.P.A. and Van Stempvoort, D.R. (1989) Early burial quartz authigenesis in Silurian Platform carbonates, New Brunswick, Canada. J. Sedim. Petrol., 59, 65±76. Oehler, J.H. (1975) Origin and distribution of silica lepispheres in porcelanite from the Monterey Formation of California. J. Sedim. Petrol., 45, 252±257. Oomkens, E. (1966) Environmental signi®cance of sand dykes. Sedimentology, 7, 145±148. Reitner, J. and Keupp, H. (eds) (1991) Fossil and Recent 6 Sponges. Springer-Verlag, Berlin. Tanner, P.W.G. (1998) Interstratal dewatering origin for polygonal patterns of sand-®lled cracks: a case study from late Proterozoic metasediments of Islay, Scotland. Sedimentology, 45, 71±89. Tucker, M.E. and Wright, V.P. (1990) Carbonate Sedimentology. Blackwell Scienti®c Publications, Oxford. VergeÂs, J. and MartõÂnez, A. (1988) Corte compensado del Pirineo oriental: geometrõÂa de las cuencas de antepaõÂs y edades de emplazamiento de los mantos de corrimiento. Acta Geol. HispaÂnica, 23, 95±106. Wanless, H.R. (1979) Limestone response to stress: pressure solution and dolomitization. J. Sedim. Petrol., 51, 27±36.

Manuscript received 3 June 1996; revision accepted 2 October 1998.
