GEOLOGICAL JOURNAL Geol. J. 37: 117–134 (2002) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/gj.904
Origin of quartz geodes from Lan˜o and Tubilla del Agua sections (middle–upper Campanian, Basque-Cantabrian Basin, northern Spain): isotopic differences during diagenetic processes JUAN J. GO¤MEZ-ALDAY, FRANCISCO GARCI¤ A-GARMILLA and JAVIER ELORZA* Departamento de Mineralogı´a-Petrologı´a, Universidad del Paı´s Vasco, Bilbao, Spain
Quartz geodes and nodular chert have been found within middle–upper Campanian carbonate sediments from the Lan˜o and Tubilla del Agua sections of the Basque-Cantabrian Basin, northern Spain. The morphology of geodes together with the presence of anhydrite laths included in megaquartz crystals and spherulitic fibrous quartz (quartzine-lutecite), suggest an origin from previous anhydrite nodules. The anhydrite nodules at Lan˜o were produced by the percolation of marine brines, during a period corresponding to a sedimentary gap, with 34S and 18O mean values of 18.8% and 13.6% respectively, consistent with Upper Cretaceous seawater sulphate values. Higher 34S and 18O mean values of 21.2% and 21.8% recorded in the Tubilla del Agua section are interpreted as being due to a partial bacterial sulphate reduction process in a more restricted marine environment. The idea that sulphates may have originated from the leaching of previously deposited Keuper sulphate evaporites with subsequent precipitation as anhydrite, is rejected because the 34S, 18O and 87Sr=86Sr values of anhydrite laths observed at both the Tubilla del Agua and Lan˜o sections suggest an origin from younger marine brines. Later calcite replacement and precipitation of geode-filling calcite is recorded in both sections, with 13C and 18O values indicating the participation of meteoric waters. Synsedimentary activity of the Pen˜acerrada diapir, which lies close to the Lan˜o section, played a significant role in the local shallowing of the basin and the formation of quartz geodes. In contrast, eustatic shallowing of the inner marine series of the Tubilla del Agua section led to the generation of morphologically similar quartz geodes. Copyright # 2002 John Wiley & Sons, Ltd. Received 28 June 2000; revised version received 3 August 2001; accepted 9 August 2001 KEY WORDS anhydrite; calcite, carbon, sulphur and oxygen isotopes; quartz geodes; Campanian; Basque-Cantabrian Basin; Spain
87
Sr/86Sr; synsedimentary diapiric activity;
1. INTRODUCTION Despite many years of controversy regarding the origin of diagenetic quartz geodes (e.g. Chowns and Elkins 1974), it is now generally assumed that they result from selective silicification of evaporite nodules from their borders to their centres. Textural evidence, such as megaquartz characterized by strong undulose, radial extinction, the existence of anhydrite laths included in megaquartz crystals with ‘cubic’ appearance terminations, the growth of spherulitic, fibrous-radial quartz aggregates (quartzine-lutecite), and crusts of zebraic chalcedony are cited as evidence that silica-enriched, interstitial fluids participated in the transformation of earlier anhydrite nodules (Milliken 1979). Accepting this origin, quartz geodes have been reported in sediments of different ages at different places in the USA (Chowns and Elkins 1974; Milliken 1979; Maliva 1987; Ulmer-Scholle et al. 1993; UlmerScholle and Scholle 1994) and Europe (Siedlecka 1976; Tucker 1976; Swennen and Viaene 1986; Swennen * Correspondence to: J. Elorza, Departamento de Mineralogı´a-Petrologı´a, Universidad del Paı´s Vasco, Apartado 644, E-48080 Bilbao, Spain. E-mail:
[email protected]
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et al. 1990; Bustillo et al. 1999). Although typical gem geodes are hollow and lined with euhedral crystals, the name is usually extended by geologists to include similar bodies in which the central void has been occluded, and geodes may therefore be defined as sedimentary nodules with a drusy internal structure (Chowns and Elkins 1974). The frequent occurrence of several types of evaporite pseudomorphs of calcite, dolomite and silica minerals, together with geochemical evidence of infiltration of meteoric waters, such as later replacements and blocky calcite cementation, indicate a complex diagenetic history associated with the genesis of quartz geodes. Quartz geodes have also been found in Upper Cretaceous formations of the Basque-Cantabrian Basin, showing similar textural features and later void filling with calcite and/or celestite minerals (Elorza and Rodrı´guez-La´zaro 1984, 1987; Boyce et al. 1990; Elorza and Garcı´a-Garmilla 1993; Go´mez-Alday et al. 1994). Sulphur and oxygen isotopic compositions may be used to indicate the origin of evaporites (marine versus continental) and have revealed recycling from Triassic evaporites in several Mesozoic and Cenozoic sedimentary basins in Spain (Birnbaum and Coleman 1979; Rouchy and Pierre 1979; Ortı´ 1989; Utrilla et al. 1991, 1992; Ayora et al. 1995) and Tunisia (Bechtel et al. 1998). In the English Midlands, the S, C and O isotope compositions of the Upper Triassic sulphate horizons associated with dolomitic Mercia Mudstones (Keuper Marl) suggest a mixed marine and continental origin, where the continental brines were derived from exposed Carboniferous anhydrites (Taylor 1983). However, in the Basque-Cantabrian Basin, little attention has been paid to the question of marine versus non-marine origin and the geochemical characteristics of the precursor sulphates. This is due to the sparse occurrence of quartz geodes in the Upper Cretaceous stratigraphic record. The aim of our work was to characterize, both texturally and geochemically, the middle–upper Campanian quartz geodes at the Lan˜o and Tubilla del Agua sections (Figure 1) and to determine the source of sulphur of the sulphates, as well as the source of geode silica. The existence of active salt diapirs (Villasana de Mena, Ordun˜a, Murguia, Salinas de Rosı´o, Pen˜acerrada and Poza de la Sal) during Upper Cretaceous times in several parts of the Basque-Cantabrian Basin could be linked to synsedimentary uplifts with subsequent local sea-level shallowing and later interaction with meteoric waters. Triassic salt diapirs could also have been a potential source of salt that modified the chemistry of pore-waters. The Pen˜acerrada and Poza de la Sal diapirs are the nearest diapirs to the studied sections. The results obtained confirm a complex diagenetic sequence involving a clear, long-time regression process during the Campanian (Floquet 1991, 1992).
2. GEOLOGICAL SETTING The sedimentary pile at the south of the Basque-Cantabrian Basin composed of marine and continental facies has a total thickness of up to 3000 m and was deposited from Triassic (Keuper facies) to Oligocene continental facies (Ramı´rez del Pozo 1973; Portero and Ramı´rez del Pozo 1979). Quartz geodes of the Tubilla del Agua section are located in a visible horizon (c.10 m thick) of white to yellowish, easily erodible, marly clays within a lithological section crowned by bioclastic limestones with rudists (Hippurites sp. Radiolarites sp. Biradiolites sp.), belonging to the upper part of the regressive Quintanaloma Formation. The age of this formation is middle–upper Campanian (Carreras Sua´rez and Ramı´rez del Pozo 1979; Floquet et al. 1982; Floquet 1991, 1992) (Figure 2A). The marly clays contain a rich microfauna of benthic foraminifers (miliolids, peneroplids, rotaliids) and ostracodes (Oertliella, Limburgina, Kingmaina, Cytherella, Asciocythere). Planktonic foraminifers are very rare and siliceous sponge spicules are preserved only inside the chert nodules (Elorza and Rodrı´guez-La´zaro 1984). The series is slightly folded and dips gently northeast, forming part of the southern flank of the Sedano Syncline (Carreras Sua´rez and Ramı´rez del Pozo 1979). Lan˜o quartz geodes are located in middle–upper Campanian cross-bedded bioclastic grainstone-to-packstone beds of the Vitoria Formation (Amiot 1982). Well-lithified calcarenites are composed of detrital quartz grains, white micas and bioclasts, consisting of echinoderm plates and spines, sponges, gastropods, brachiopods, pycnodonts, bryozoans, benthic foraminifers, ostracods and fecal pellets. Selective silicification mainly affects pycnodont shells and occasional chert nodules appear associated with calcarenites (Figure 2B). Small and irregular calcite geodes lacking an external chert crust are present in the calcarenite beds. The calcarenites were deposited Copyright # 2002 John Wiley & Sons, Ltd.
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Figure 1. Simplified geological map showing the geographic location of the Tubilla del Agua (A) and Lan˜o (B) sections in the BasqueCantabrian Region, northern Spain.
on a shallow platform with high-energy structures; the platform became deeper towards the east (Go´mez-Alday et al. 1994). One of the problems concerning the determination of the diagenetic sequence at both the Tubilla del Agua and Lan˜o sections is the apparent controversy over the sedimentary environment. Thus marine fauna are indicative of a shallow platform environment (Portero and Ramı´rez del Pozo 1979; Floquet et al. 1982; Elorza and OrueEtxebarria 1985; Floquet 1991), whereas these geodes may initially have been anhydrite nodules which grew in a continental sabkha environment with high evaporation rates (Chowns and Elkins 1974; Tucker 1976). However, silicified anhydrite nodules cannot be used as unmistakable indicators of a sabkha environment, since they only provide evidence of hypersaline pore-waters during early diagenesis (Elorza and Rodrı´guez Lazaro 1984; Maliva 1987). Structural evidence shows that the Pen˜acerrada salt diapir was active during Campanian time in the Lan˜o zone, as suggested by Ramı´rez del Pozo (1973) and Go´mez-Alday et al. (1994). Diapiric uplift was responsible for local shallowing which reinforced the general eustatic shallowing of the open-platform sedimentary series, whereas the Tubilla del Agua section shallow platform lacks evidence of active synsedimentary diapirism (Carreras Sua´rez and Ramı´rez del Pozo 1979). Copyright # 2002 John Wiley & Sons, Ltd.
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Figure 2. Lithological sections of the Tubilla del Agua (Quintanaloma Formation) and Lan˜o (Vitoria Formation) sites showing the stratigraphic position of the quartz geodes. Key: 1, calcite geodes; 2, quartz geodes; 3, fibrous quartz; 4, rudists; 5, pycnodonts; 6, gastropods; 7, sponges; 8, bioclasts; 9, lacazine; 10, burrows; 11, desiccation cracks; 12, nodular chert; 13, erosive surface (e.s.); a, bioclastic limestones; b, marl to marly clays with lacazine; c, rudist beds; d, marls with lacazine; e, marly clays; f, limestone and dolostone alternations; g, limestone and dolostone alternations with cross-bedding; h, continental siliciclastic sands; i, coarse-grained bioclastic calcarenite; j, fine- to medium-grained bioclastic calcarenite.
3. ANALYTICAL TECHNIQUES Several thin-sections of quartz geodes were prepared for standard transmitted light petrography and carbonate staining with Alizarin Red S and potassium ferricyanide (following Dickson 1965). First, the quartz geodes were crushed in order to liberate the anhydrite inclusions. Isotopic analyses were carried out on BaSO4, prepared from anhydrite that had been dissolved using warm (c. 80 C) 2.5 M HCl for 30 minutes. The solution was filtered and the sulphate precipitated with the addition of 5% BaCl2. After about 2 hours, BaSO4 precipitation is complete. The precipitate, ready for isotopic analysis, was carefully washed two or three times in distilled water and subsequently dried. Isotopic analyses were performed at the Laboratorio de Iso´topos Estables of Salamanca University (Spain). A method based on that described by Robinson and Kusakabe (1975) was employed for sulphur, although the extraction line was based on the principles of Coleman and Moore (1978). 34S=32S ratios were determined on SO2 gas Copyright # 2002 John Wiley & Sons, Ltd.
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employing a dedicated VG-Isotech SIRA-IITM mass spectrometer. Repeated analysis, including chemistry, of several international and internal reference standards gave an average reproducibility of better than 0.18% for sulphides and 0.23% for sulphates. Isotopic results are given in the usual delta notation relative to the internationally accepted Canyon Diablo Troilite (CDT) standard. NBD-123 gave a value of 34S ¼ 16.93% in the laboratory. The extraction of CO2 for oxygen isotopic analysis following standard techniques was performed by burning c. 10 mg of BaSO4 at 1100 C in excess spectrographic-grade graphite. Any CO produced was converted to CO2 by high voltage discharge. CO2 was analysed in a second VG-Isotech SIRS-IITM mass spectrometer to avoid the problems associated with sulphur contamination. The quality of the results was monitored by means of repeated analysis of both internal and international (NBS-127) standards under identical analytical conditions. The average precision obtained was 0.2%. Isotope results are given in the usual delta notation relative to Standard Mean Ocean Water (SMOW). During the time it took to complete the results, NBS-127 gave 18O ¼ 9.18% (SMOW). Two chemical procedures were employed for Sr separation. For the first procedure, 20 ml of water purified using a Milli-Q (Millipore) system (resistance: 18.3 M cm) was added to 2 g of pulverized sample (quartz þ anhydrite/ calcite inclusions). The mixture was heated to c. 25–30 C for 10 days. A 7 ml sample of the mixture was evaporated and redissolved in 2 ml of HCl 2.5 N and introduced into the ion exchange column with AG50W resin. Seven days later, an additional 8 ml of the mixture was evaporated and redissolved in 2 ml of HCl 2.5 N and introduced into the same type of column. This procedure is useful to preferentially dissolve sulphates and other salts. In the second procedure, 15 ml of HCl 6 N was added to 2 g of the pulverized sample. The mixture was heated c. 25–30 C for 10 days. A 7 ml sample of the mixture was evaporated and redissolved in 2 ml of HCl 2.5 N and then introduced into the columns. Seven days later, an additional 7 ml of the mixture was evaporated and redissolved in 2 ml of HCl 2.5 N and similarly introduced into the columns. This procedure, in contrast to the former, is more suitable to preferentially dissolve carbonates. The isotopic analyses were performed using the VG54 multicollector mass spectrometer at the Universidad Complutense de Madrid (CAI de Geocronologı´a y Geoquı´mica Isoto´pica). 87Sr=86Sr values are normalized to 86 Sr=88Sr ¼ 0.1194. Repeated analyses of the NBS-987 standard gave 86Sr=88Sr ¼ 0.710267 0.000002 (2m for n ¼ 66). The oxygen and carbon isotopic composition of carbonates was determined using a VG SIRA-9 mass spectrometer at the Laborato´rio de Iso´topos Esta´veis (Universidad de Recife, Brasil). Extraction of CO2 from carbonates was carried out according to the method described by McCrea (1950). The results are expressed in delta notation in per mil values relative to the Peedee Belemnite (PDB) standard. Reproducibility for both 18O and 13C was better than 0.1%. 4. GEODE MORPHOLOGY, PETROGRAPHY AND ISOTOPIC VALUES 4.1 Tubilla del Agua geodes Quartz geodes from the Tubilla del Agua section are spherical to sub-spherical and have external rounded protuberances similar to the head of a cauliflower (Chowns and Elkins 1974). They vary from 1 to 15 cm in diameter within an individual marly level. Most are hollow, and small geodes 1–2 cm in diameter are usually completely filled with milky white quartz. Collapsed examples include an inhomogeneous admixture of sparry calcite and quartz crystals. Inside the geodes, quartz crystals of irregular growth in hexagonal prisms are crowned by rhombohedra (Figure 3A). In some cases, the geodes present partially superimposed masses of carbonate of concretional appearance with a maximum thickness of several millimetres. Small compact nodules of microcrystalline chert 5–7 cm in diameter occur in close association with the silicified evaporite nodules. These geodes were petrographically studied by Elorza and Rodrı´guez-La´zaro (1984), who described several microscopic concentric ‘bands’ (outside to inside) as follow. (a) The outermost band is composed of xenotopic megaquartz crystals with undulose extinction and sutured boundaries having a large number of anhydrite prismatic inclusions homogeneously spread throughout the quartz crystals. (b) The second band is composed of quartzinelutecite spherules, much more developed than in the previous band, and with a clear intergrowth of spherulites, Copyright # 2002 John Wiley & Sons, Ltd.
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Figure 3. (A) Half of a cauliflower-type quartz geode from the Tubilla del Agua section. White scale bar ¼ 4 cm. (B) Half of a fill-up quartz geode from the Lan˜o section. The upper part of the section contains megaspherules (s), (marked by arrowheads) produced by late remobilization of silica. White scale bar ¼ 0.5 cm. (C) Small quartz geodes (g) inside the cavities between Pycnodont valves (b) from the Lan˜o section. White scale bar ¼ 2.5 cm. (D) Detail of a small quartz geode (g) from the Lan˜o section adapted to the Pycnodont inner space (b). The two fractures (f), marked by arrowheads, permitted the infilling of the open space. White scale bar ¼ 1 cm. The coin is 2 cm in diameter.
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until they reach the inner band. The contact with the exterior megaquartz band is irregular and sutured at higher magnification. The quartzine-lutecite fibres run perpendicular to the contact surface. The absence of anhydrite inclusions clearly distinguishes the second band from the outer one. (c) A third innermost band of megaquartz could have been generated directly from the second band as a consequence of the continuous growth of the quartzine-lutecite fibres. Many petaloid megaquartz crystals bearing inclusions of anhydrite and calcified anhydrite laths were found. Finally, an occasional carbonate film about 0.5–1 mm thick partially covers the well-formed faces of the megaquartz. 4.2 Lan˜o geodes The morphology of quartz geodes from the Lan˜o section (1–20 cm in length) is essentially the same as that observed in samples from the Tubilla del Agua section, except for differences in size and development of the internal cavity. The geodes have a peculiar cauliflower shape with brain-like protuberances, small rose-like prominences, ‘beekite’ rings and the internal cavity is crowned by milky white quartz. Their external surface is covered by a thin patina of iron oxides, which sometimes enter through late radial fractures towards the central zones. They are scattered without mutual interference suggesting that they may have had an early sub-spherical to oblong growth. A diagnostic feature of these geodes, as sedimentary nodules with a drusy internal structure, is the small internal cavity which was preserved, and for this reason they show little available volume for later chemical precipitates (Figure 3B). Only large sparry calcite crystals 1–2 mm in size occupied the pores during later diagenetic stages. Smaller quartz geodes appear to fill fractures and bivalve internal cavities, and have a morphology which was controlled by the previously available open spaces (Figure 3C, D). The geodes from the Lan˜o section have been described by Go´mez-Alday et al. (1994). Several textures, from the external border to the inner hollow cavity, were produced during selective silicification. (a) An external band is mainly composed of microcrystalline quartz associated with fibrous spherulitic aggregates of quartzine-lutecite. The contact surfaces among crystals are irregular. Spherulites can be up to 1 mm in diameter and evolve into intergrowths of petaloid megaquartz (Figure 4A) that include opaque remains of organic matter (confirmed by ultraviolet light) together with small anhydrite inclusions dispersed inside the crystals. (b) An internal megaquartz band is related to the former through a continuous growth of fibrous quartzine-lutecite that evolves into prismatic megaquartz showing well-defined terminations (Figure 4B). Subhedral and euhedral megaquartz reveals mosaic textures having the highest density of anhydrite crystals. The prismatic sections of anhydrite (50–200 mm in size) are not regularly arranged, but tend to occupy just the centre of the megaquartz crystals, while they are absent at the borders (Figure 4C). We infer from radial fractures that the quartz geodes were brittle and so allowed the subsequent influx of fluids, which precipitated sparry calcite in the cavities and partially pseudomorphosed the anhydrite inclusions (Figure 4D). An additional texture of botryoidal megaspherulites of quartzine-lutecite megaquartz is indicative of subsequent remobilization of silica (Figure 3B). Such megaspherulites can be recognized at a glance (up to 1 cm in diameter). The fibrous quartzine, with radial or fan-like undulosity, is nucleated from the fracture walls and increases in size to megaquartz habits. The periphery is marked by dark inclusions which may be organic matter and small anhydrite inclusions remain regularly dispersed inside the crystals (Figure 4E). 4.3 Isotopic values The 34S and 18O values were measured in anhydrite inclusions within ten geodes from the Lan˜o section and in a gypsum sample from the Triassic (Keuper facies) of the Pen˜acerrada diapir. Eight samples of geodes with anhydrite inclusions from the Tubilla del Agua section were also analysed (Table 1). These values were compared with the Upper Cretaceous seawater sulphate curve (Claypool et al. 1980; Strauss 1997) and with several 34S values of evaporite samples studied by other authors in the Basque-Cantabrian and adjacent basins (Table 2 and Figure 5A). Measured 87Sr=86Sr ratios were obtained from anhydrite and calcite laths from the Tubilla del Agua (n ¼ 5 and 2, respectively) and Lan˜o (n ¼ 5) quartz geodes (Table 1 and Figure 6). The 18O and 13C values were determined in Copyright # 2002 John Wiley & Sons, Ltd.
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Figure 4. Photomicrographs of Lan˜o quartz geodes. (A) Spherulitic texture of quartzine-lutecite with continuous growth into megaquartz (crossed polars; white scale bar ¼ 1 mm). (B) Polygonal megaquartz with ‘cubic’ sections and many inclusions of anhydrite and calcified anhydrite laths (crossed polars; white scale bar ¼ 0.26 mm). (C) Detail of a mosaic of megaquartz crystals (q) with a nucleus of oriented anhydrite inclusions (a) (crossed polars; white scale bar ¼ 0.13 mm). (D) Polygonal and euhedral megaquartz crystals (q) with nucleus of anhydrite inclusions (a). The open spaces were filled by large sparry calcite crystals (c) (crossed polars; white scale bar ¼ 1 mm). (E) Late quartzine-lutecite and megaquartz botryoidal megaspherule(s) included in a previous quartzine-lutecite petaloid matrix (crossed polars, white scale bar ¼ 0.2 mm). (F) Photomicrograph of chert nodule, with microcrystalline and fibrous chalcedonite including a considerable amount of siliceous sponge spicules (crossed polars, white scale bar ¼ 0.4 mm).
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Table 1. Isotopic (S, O, 87Sr=86Sr) composition of anhydrite and calcite inclusions from quartz geodes in the marine marl and calcarenite from the Tubilla del Agua and Lan˜o sections and from gypsum in the Pen˜acerrada diapir (Keuper) from the Basque Cantabrian Basin (northern Spain) 18O (% vs. SMOW)
Location/ age
Sample
Lithology* (host rock)
Lan˜o/ Campanian
ML-1 ML-2 ML-3 ML-4 ML-5 Lan˜o-A Lan˜o-B Lan˜o-C Lan˜o-D Lan˜o-E Mean value
A (calcarenite) A (calcarenite) A (calcarenite) A (calcarenite) A (calcarenite) A (calcarenite) A (calcarenite) A (calcarenite) A (calcarenite) A (calcarenite) (n ¼ 10)
þ13.7 þ12.3 þ15.2 þ13.6 þ14.7 þ14.7 þ14.1 þ11.4 þ13.0 þ13.0 þ13.6 1.2
Tubilla del Agua/ Campanian
TUB-2 TUB-3 TUB-4 TUB-5 TUB-6 TUB-7 TUB-9 TUB-10 Mean value
A (marly A (marly A (marly A (marly A (marly A (marly A (marly A (marly (n ¼ 8)
þ20.9 þ21.0 þ21.4 þ21.2 þ22.7 þ22.7 þ22.0 þ22.8 þ21.8 0.8
Pen˜acerrada/ MY-3 Keu¨per
clay) clay) clay) clay) clay) clay) clay) clay)
þ14.7
*G(marine evaporite)
34S (% vs. CDT)
87
þ18.34 þ18.12 þ18.89 þ18.53 þ18.69 þ19.50 þ19.15 þ19.12 þ19.15 þ18.88 þ18.8 0.4
Sr=86Sr *A
2 m
0.707707 0.707723 0.707758 0.707800 0.707839 — — — — — 0.70776 0.00005 (n ¼ 5) þ21.27 0.707680 þ21.31 0.707910 þ21.13 0.707668 þ20.98 0.707669 þ21.22 0.707790 þ21.43 0.707792 þ21.23 — þ21.39 — þ21.2 0.1 0.70779 0.00011 (n ¼ 6) þ13.2
87
Sr=86Sr 2 m *C
6 6 6 6 6
6 10 6 6 6 6
0.70851 0.70872
3 5
*A, anhydrite inclusion in quartz geode; *C, calcite in quartz geode; *G, gypsum.
Table 2. Comparative 18O% and 34S% values from Spanish evaporites (Upper Cretaceous and Triassic) and Upper Cretaceous seawater sulphate values Age Upper Cretaceous Triassic Triassic Triassic Upper Cretaceous
Number of samples
5 5 21 6 ?
18O (% vs. SMOW)
34S (% vs. CDT)
Reference
þ14.4 1.7 n.d. þ11.8 1.7 n.d.
þ18.2 0.5 þ14.7 0.5 þ13.4 1.4 þ15.2 0.8
Utrilla et al. (1992) Birnbaum & Coleman (1979) Utrilla et al. (1992) Boyce et al. (1990)
—
þ16 to 21%
Claypool et al. (1980)
11 calcite samples corresponding to sparry calcite infilling quartz geodes or calcite geodes collected from the Lan˜o and Tubilla del Agua sections (Table 3 and Figure 7) to detect the post-depositional evolution within the basins. 5. DISCUSSION An important component of diagenesis of the marine sediments at the Lan˜o and Tubilla del Agua sections is selective silicification of early anhydrite nodules, as well as the partial replacement of bivalve shells by silica (Figures 3 and 4). The formation of a quartz geode required: (a) a sulphate source for early growth of anhydrite nodules, and Copyright # 2002 John Wiley & Sons, Ltd.
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Figure 5. (A) Plot of oxygen versus sulphur isotope composition of the Lan˜o and Tubilla del Agua anhydrite inclusions together with other Triassic and Upper Cretaceous evaporite values from Spain. (B) Sulphur and strontium isotopic variability of Lan˜o and Tubilla del Agua anhydrite. The early diagenetic enrichment in 34S in the Tubilla del Agua samples was due to the removal of isotopically light bacterial sulphide.
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Figure 6. Plot of the 87Sr=86Sr composition of the anhydrite and calcite inclusions in quartz geodes from the Tubilla del Agua and Lan˜o sections versus age. The variation curve of seawater 87Sr=86Sr versus age is taken from Koepnick et al. (1985), with permission from Elsevier Science. Upper and lower lines define a band that encloses more than 98% of the Cretaceous data. The line drawn through the band is an estimation of seawater ratio versus time.
Table 3. Isotopic (C and O) composition of calcite geodes and calcite filling quartz geodes from the Tubilla del Agua and Lan˜o sections Location/ age
Sample
Lan˜o/ Campanian
FA-1 FA-2 FA-3 FA-4 FA-5 Mean value (n ¼ 5) UB-1 UB-2 UB-3 UB-4 UB-5 UB-6 Mean value (n ¼ 6)
Tubilla del Agua/ Campanian
Lithology* (host rock) CG CG CG CG CG
(calcarenite) (calcarenite) (calcarenite) (calcarenite) (calcarenite)
CG (carbonate) CG (carbonate) CG (carbonate) CF (marly clay) CF (marly clay) CF (marly clay)
18O (% vs. PDB) 8.64 10.43 10.04 10.88 10.65 10.13 0.89 6.81 6.65 7.19 6.75 6.86 6.86 6.85 0.18
13C (% vs. PDB) 3.42 0.13 0.54 þ0.48 þ0.38 0.65 1.60 5.92 5.06 5.61 þ0.89 þ0.79 þ1.09 2.30 3.55
CG, calcite geode; CF, calcite filling quartz geode.
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Figure 7. Plot of oxygen versus carbon isotope composition of the Lan˜o and Tubilla del Agua calcites. The Tubilla del Agua plotted values define a vertical trend with large and small variations characteristic of meteoric diagenesis, with a well-defined ‘meteoric calcite line’ (MCL). The lighter 13C values are characteristic of the vadose zone while the higher values seem to denote a phreatic zone. The lighter 18O values from the Lan˜o section were caused by an increase in temperature of the pore-waters during phreatic water percolation.
(b) a further silica contribution from adjacent sediments, allowing the selective replacement and partial dissolution of anhydrite nodules contained in both calcarenite and marly sediments (Elorza and Rodrı´guez-La´zaro 1984; Go´mez-Alday et al. 1994). Sulphate evaporites could have precipitated in two ways which are not necessarily exclusive: (a) by the leaching of (palaeo-outcropped) Keuper saline deposits in adjacent diapiric zones; and (b) by vertical percolation and/or lateral migration of brines that formed during the regressive period in a younger sabkha environment, just above the carbonate host-rock, with further precipitation as anhydrite nodules. With regard to the Lan˜o section, the synsedimentary activity of the Pen˜acerrada diapir, which lies close (c.7 km) to the studied area, was probably the cause of a local regressive period. The stratigraphic series became condensed and very shallow facies were installed (from tidal-flat to sabkha). Overlying evaporites, which formed subsequently, were further dissolved and are not preserved as such in the stratigraphic record (Go´mez-Alday et al. 1994). The 34S and 18O mean values (18.8 0.4% and 13.6 1.2% respectively), obtained from anhydrite inclusions within the megaquartz crystals of the Lan˜o geodes, are consistent with a unique seawater sulphate source of Upper Cretaceous age (Figure 5A, Tables 1 and 2). The 34S and 18O values of the Keuper gypsum taken from the Pen˜acerrada diapir are 13.2% and 14.7% respectively. These 34S values coincide with the global ones published by Claypool et al. (1980), namely from 16% to 21% for the Upper Cretaceous marine evaporitic sulphates, and from 13 to 16% for the Keuper salts, and are in good agreement with those mentioned for several Mesozoic evaporitic basins in Spain (Table 2). Birnbaum and Coleman (1979) obtained 34S values from 13.7% to 15.1% with a mean of 14.7 0.5% for the Keuper gypsum in the Ebro area (South-Pyrenean foreland, Spain). Utrilla et al. (1992) published 34S and 18O mean values of 18.2 0.5% and 14.4 1.7% respectively, for anhydrite and secondary gypsum samples (n ¼ 5) collected from several sedimentary basins of Upper Cretaceous age in Spain. The same authors (Utrilla et al. 1991) give a 34S mean value of 13.4 1.4% and a 18O mean value of 11.8 1.7% for Keuper gypsum samples. Boyce et al. (1990) obtained more diverse 34S values (from 13.8% to Copyright # 2002 John Wiley & Sons, Ltd.
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16.2%), with a mean of 15.2 0.8% for the Keuper gypsum from the distant Santander, Sopelana and Laredo salt diapirs (see Figure 1). These data indicate that the Lan˜o anhydrite nodules originated from the percolation of Upper Cretaceous marine brines through the calcarenites. It seems that Keuper sulphate evaporites from the Pen˜acerrada diapir did not contribute to the groundwater system. This interpretation is reinforced by the existence of a sedimentary break, now represented by erosional surface and/or abrupt changes of facies (continental sandstones), which should have enhanced precipitation of subsequently dissolved marine evaporites. The Tubilla del Agua quartz geodes exhibit different isotopic compositions of the anhydrite inclusions and demand a more complex explanation. In this area, marine Campanian sediments were deposited in a broad regressive shallow platform complex settled by rudists, and lack evidence of synsedimentary diapirism (Carreras Sua´rez and Ramı´rez del Pozo 1979; Floquet 1991), with a large distance (c. 24 km) from the Poza de la Sal diapir (Hempel 1967; Hernaiz Huerta et al. 1994; Klimovitz et al. 1999; Hernaiz Huerta and Sole´ Pont 2000). The anhydrite inclusions in the megaquartz crystals have mean values of 34S ¼ 21.2 0.1% and 18O ¼ 21.8 0.8% (Table 1), close to the maximum values obtained by Claypool et al. (1980) for Upper Cretaceous marine evaporitic sulphates. The Tubilla del Agua anhydrite was significantly enriched in heavy isotopes relative to normal seawater sulphate solutions and is distinct from that at Lan˜o. The 34S and 18O values of anhydrite in both areas have been plotted and compared with the values of five marine anhydrite samples and secondary gypsum samples from the Upper Cretaceous in Spain (Utrilla et al. 1992) (Figure 5A). The isotopic values in the Lan˜o section are similar to the anhydrite/gypsum samples from the Upper Cretaceous. This relationship is indicative of the same mechanism of evaporitic precipitation. The high 34S values of the Tubilla del Agua sulphate could have been produced by two different mechanisms. (a) The isotopic composition of seawater sulphate in this shallow restricted platform may have been naturally higher than in the less restricted platform of Lan˜o. However, sulphur isotope fractionation between parental seawater and the salt precipitate depends on the degree of brine concentration, but is generally less than 1.5% (Strauss 1997; Worden et al. 1997), which can be considered as insignificant or negligible in ancient evaporite deposits (Holser and Kaplan 1966). (b) The trend toward heavier 34S values most likely reflects fractionation processes associated with partial bacterial sulphate reduction (BSR), during progressive stages of brine concentration by evaporation (Pierre and Rouchy 1986; Utrilla et al. 1992). This mechanism is viable when brines are in contact with organic-rich deposits or with hydrocarbons. Early BSR is common in hypersaline settings when it can generate substantial amounts of H2S through the fol lowing reaction: SO2 4 þ 2CH2 O ! 2HCO3 þ H2 S þ 2H2 O. This gas can be lost into the atmosphere. Alterna2þ tively, if Fe is available in sediment, pyrite can be formed through the following reaction: Fe2þ þ H2 S ! FeS þ 2Hþ . During the first reaction, the resulting H2S is strongly depleted in 34S, with more or less negative 34S values, due to the preferential utilization of the light sulphur by the sulphate reducers. In the case of Tubilla del Agua nodules, anhydrite has heavy 34S values, which indicates that the residual sulphate of the parent brine was 34S-enriched. At the top of the regressive parasequences with lignitiferous beds, sometimes roots and stumps in living position are present in the Quintanaloma Formation, as already observed by Floquet (1992). These might have been a potential organic nutrient source for bacterial colonies, which give rise to BSR. In addition, under restricted shallow conditions, BSR removes the light oxygen isotope leaving residual oxygen values (21.8 0.8% SMOW), which are higher than the normal Upper Cretaceous mean value (14.4 1.66% SMOW) obtained by Utrilla et al. (1992) in Spain. Finally, the trend toward heavy S isotope values is not paralleled by changes in the 87Sr=86Sr ratio (Figure 5B), which is consistent with the fact that the latter is unaffected by redox reactions, as mentioned by Spo¨tl and Park (1996). Reference curves for the global evolution of marine strontium isotopic stratigraphy (SIS) during the Late Cretaceous are now well-defined for precise correlation to be attempted. Crame et al. (1999) through 87Sr=86Sr analysis (n ¼ 17) of six macrofossils (pycnodontid, trigoniid), permitted the establishment of a correlation between a Maastrichtian horizon basal marker in Vega Island (Antarctica) and reference sections in Europe and the USA. In addition, the strontium isotopic data of taxon-specific foraminifers have been successfully used to indicate extensive and pervasive reworking of up to 30% of the mass of foraminifers in the Maastrichtian and lower Danian Copyright # 2002 John Wiley & Sons, Ltd.
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intervals of the Ocean Drilling Program (ODP) Hole 738 C. The presence of many redeposited Cretaceous taxa in the basal few metres of Danian strata were thus detected by their anomalous 87Sr=86Sr ratios (MacLeod and Huber 1996). The 87Sr=86Sr ratios of the anhydrite from the Tubilla del Agua and Lan˜o sections are represented in Figure 6, which also shows the variation curve of 87Sr=86Sr in seawater through Cretaceous time (from Burke et al. 1982; Koepnick et al. 1985). All anhydrite samples have initial 87Sr=86Sr ratios close to the seawater value at their real and later time of deposition around the Campanian–Maastrichtian limit, with mean values of 0.70774 0.00011 in Tubilla del Agua samples and 0.70776 0.00005 in Lan˜o samples, more in accordance with the curve values at the time of host-rock deposition (middle–upper Campanian). The C and O isotopic compositions of calcite from the Lan˜o and Tubilla del Agua sections (Table 3) indicate two clear trends (Figure 7). The isotopic compositions of the calcite geodes replacing anhydrite nodules and sparry calcite that fills the void spaces of the quartz geodes of Tubilla del Agua indicate that calcite was precipitated in bicarbonate-rich meteoric waters under shallow conditions. These values indicate significant 13C changes (5.92 to þ1.09) and negligible 18O variations (7.19 to 6.65), both characteristic of meteoric diagenesis, and demonstrate the trend called ‘meteoric calcite line’ (MCL), as defined and applied by Allen and Matthews (1982), Meyers and Lohmann (1985) and Lohmann (1988). These 13C values of the precipitated carbonate depend on the origin of the carbon (organic or inorganic), with depleted values indicating a major organic control of the bicarbonate. In addition, the Sr isotopic ratios obtained from calcite inclusions inside the quartz crystals of Tubilla del Agua geodes (0.70851 and 0.70872) confirm their meteoric origin (Figure 6 and Table 1). For calcite of the Lan˜o section, the lower 18O values (between 10.88 and 8.64) indicate that later diagenetic conditions were different from those defined at Tubilla del Agua (Figure 7). These variations could be due to temperature increase under depth conditions (but not to deep burial) that modified the isotopic values to lighter ones. This burial meteoric mechanism is justified by Pierre (1986), Choquette and James (1987), Swennen et al. (1990), Ulmer-Scholle and Scholle (1994) and Dejonghe et al. (1998) for calcite pseudomorphs after anhydrite in their respective case histories. In the two areas, the source of sulphate is marine (normal for Lan˜o and modified by BSR in Tubilla del Agua) and the precipitation of anhydrite was due to brine formation by seawater evaporation in shallow depth conditions. Thus, the origin of anhydrite nodules is unique, from evaporated seawater. The causes for shallowing were different at the Tubilla del Agua and Lan˜o sites. Figure 8 illustrates a first attempt at reconstructing the
Figure 8. Simplified diagram showing a partial configuration of the Basque-Cantabrian Basin during the middle–upper Campanian with eustatic shallowing which affected the Tubilla del Agua sediments versus eustatic and diapiric shallowing which affected the Lan˜o sediments. The mean isotopic values of the anhydrite samples from the Tubilla del Agua and Lan˜o series together with the mean values of Upper Cretaceous marine evaporite and Keuper gypsum from the Pen˜acerrada diapir are included.
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tectonosedimentary factors that influenced the palaeogeography at both sectors during evaporite nodule formation. Whereas eustatic change can be regarded as the main reason for shallowing in the Tubilla del Agua sediments (North Castilian Platform), the combination of eustasy and local diapiric uplift produced morphologically similar nodules in the Lan˜o section (Navarro Cantabrian Domain). Only differences in oxygen and sulphur isotopic values testify to the distinct evolution of the nodules. Opaline sponge spicules are considered the main source of silica due to the high degree of solubility of this type of silica and the surrounding anhydrite nodules may have been subjected to silicification, as was admitted by authors such as Chowns and Elkins (1974), Geeslin and Chafetz (1982) and Ulmer-Scholle and Scholle (1994). The amorphous silica solubility in the carapaces of the micro-organisms (210 ppm in pH < 9 at 25 C) allows easy dissolution and precipitation of the quartz in general (6 ppm in pH < 9 at 25 C) almost at the same time (Krauskopf 1979). The source of silica for the silicification of anhydrite nodules at both the Lan˜o and Tubilla del Agua sections seems to be biogenic, because the chert nodules at the same stratigraphic horizon as the quartz geodes preserve a number of siliceous sponge spicule remains (Figure 4F). This is not an uncommon feature in chert nodules associated with quartz geodes from other sites of the Basque-Cantabrian Basin and suggests that silica was biogenic in origin (Elorza and Rodrı´guez-La´zaro 1984; Go´mez-Alday et al. 1994). Other possible origins for silica, such as shale modification with the smectite to illite transformation during intermediate stages of burial diagenesis and/or pressure solution of terrigenous quartz (Hesse 1989), should be discarded because illite and dissolution features were not observed and it is improbable that all crystalline terrigenous silica could have dissolved away leaving no trace. Once the Lan˜o quartz geodes formed, silicification was reactivated and new botryoidal macrospherulites grew along radial fractures (Figures 3B and 4E). Such a process has not been detected in other previously studied Upper Cretaceous geodes from the El Ribero site at Burgos (Elorza and Garcı´a-Garmilla 1993) and the Langre site at Santander (Elorza and Rodrı´guez-La´zaro 1987). The progression of different silica polymorphs records changes in the character of the reactive solutions through time; for equant megaquartz, this occurs with a low silica concentration, whereas for the fibrous morphology of quartzine-lutecite, a higher silica concentration, almost exclusively in association with sulphates and evaporates, is required (Folk and Pittman 1971). While the Lan˜o quartz geodes are almost completely filled with quartz, those from Tubilla del Agua are almost empty (Figure 3A, B). This difference can be interpreted as the predominance of a continuous volume-for-volume replacement of anhydrite by silica during the earliest silicification phase. Later, when the anhydrite solution exceeds quartz replacement, the anhydrite is removed in advance of replacement and a secondary porosity develops. The quartz may now begin to assume euhedral faces where voids are available (Chowns and Elkins 1974). The diagenetic volume-for-volume replacement of one mineral by another through a film located between them is explained by the concept of force of crystallization driven by overpressure created by the growth of the authigenic crystal or by the decrease of pressure caused by the dissolution of the host phase. The whole process can be driven by this depression, for it reduces the solubility of the authigenic crystal and thus causes its precipitation (see Minguez and Elorza 1994). Thus, the Lan˜o section is rich in quartz geodes with a continuous volume-for-volume replacement, while the Tubilla del Agua quartz geodes represent a small period with volume-for-volume replacement and a dilated period where anhydrite solution exceeds quartz replacement.
6. CONCLUSIONS In the overall light of our data, we conclude that the quartz geodes found in middle–upper Campanian sediments at both the Lan˜o and Tubilla del Agua sections were produced by the replacement of earlier anhydrite nodules. Quartz geodes contain different types of quartz: micro- and megaquartz, together with fibrous quartz (quartzine-lutecite). The early silicification at the Lan˜o section was subsequently reactivated as quartzinelutecite-megaquartz megaspherulites which occupied radial fractures produced during compaction/fracturation. Silica was biogenic in origin at both sections. The predominance of a continuous volume-for-volume replacement of anhydrite by silica generated more compact quartz geodes with scarce void spaces. Copyright # 2002 John Wiley & Sons, Ltd.
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The previous growth of anhydrite nodules in internal platform sediments suggests that the brines percolated from overlying marine evaporites, which have since disappeared, with an associated sedimentary gap. At the Lan˜o section, the 34S and 18O values clearly indicate a unique marine source for sulphates, without any influence from recycled Keuper salts. Bacterial sulphate reduction processes are evidenced at the Tubilla del Agua section by the significant isotopic enrichment in both 34S and 18O. The anhydrites have 87Sr=86Sr ratios close to the seawater value during the Campanian–Maastrichtian limit. The 13C and 18O values of the later calcite suggest that the circulation of bicarbonate-rich groundwaters at greater burial depth in the Lan˜o section allowed the complete replacement of evaporitic remains, producing calcite geodes or infilling the spaces left within the quartz geodes by sparry calcite. The local shallowing of the Lan˜o section was mainly caused by the proximity of the Pen˜acerrada diapir. This was active during the Campanian and favoured environmental conditions which were suitable for the development of anhydrite nodules. The shallowing at the Tubilla del Agua section was mainly eustatic, apparently not being influenced by the penecontemporaneous Poza de la Sal diapiric influence.
ACKNOWLEDGEMENTS This study was financed by Research Projects UPV/EHU 130.310-EB177/96 and 130-310-EB34/99 sponsored by the University of the Basque Country. Our thanks are due to Drs J.J. Pueyo and R. Utrilla (Universidad de Barcelona) for critically reading the draft manuscript. We would also like to thank the journal reviewers Drs C. Pierre (Universite´ Pierre et Marie Curie), S.N. Ehrenberg (Statoil, Norway), and I.D. Somerville (Editor-in-Chief) for their helpful comments on an early version of the manuscript. Several geodes from Tubilla del Agua were contributed by Mr A. Franco (Ente Vasco de la Energı´a, Gobierno Vasco). Finally, we would also like to thank E. O’Broin MA and D.J. Fogarty PhD for correcting the English version of the manuscript.
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