Fault-related hydrothermal dolomites in Cretaceous slope carbonates (Cantabrian mountain chain, Northern Spain): Results of petrographic, geochemical and petrophysical studies

Mumtaz Muhammad SHAH*, Fadi Henri NADER*, Julie DEWIT†, Rudy SWENNEN†, Daniel GARCIA‡

* Institut Française du Pétrole (IFP), Av. Bois Préau, 92852, Rueil-Malmaison, France (mumtaz-muhammad.shah@ifp.fr)

† Geologie, Katholieke Universiteit Leuven, Celestijnenlaan 200E, B-3001 Heverlee, Belgium.

‡ Centre SPIN – ENSMSE, Departement GENERIC, 158, cours Fauriel, 42023 Saint-Etienne, France.

The present contribution documents NW-SE oriented fault and fracture related dolomites in Aptian-Albian

platform and slope to basin carbonates (Karrantza area; northern Spain). Petrographic and geochemical studies

demonstrate the superposition of different diagenetic events which were involved in multiphase dolomitization.

Three different types of dolomite textures are observed, including nonplanar, planar and zebra dolomites. The

formation of these dolomite textures was variably reworked by subsequent alterations, which resulted in

neomorphism and recrystallization, cataclastic deformation and calcite-filling of dolostones. Several phases of

hydrothermal calcite cement pre- and post-date dolomitization events. Non-planar and planar dolomites show

overlapping oxygen and carbon isotopic ratios ranging from -8.9 to +16.9‰ (δ18O V-PDB), and -2.6 to +3.1‰

(δ13C V-PDB). Zebra dolomite shows more depleted values of δ18O and δ13C as compared to non-planar and planar

dolomite (δ18O: -18.1 to -15.2 ‰V-PDB and δ13C: -8.1 to +1.6‰ V-PDB). All three dolomite textures are nearly

stoichiometric, with CaCO3values between 50 and 52 mole%. Limestones close to the dolomites also show

depleted δ18O values (similar to those of the dolomites), implying isotopic resetting during dolomitization.

Recrystallization (dissolution/precipitation) appears to have decreased the bulk porosity values in the interlocking

nonplanar dolomite (with negligible porosity), while late-stage cleavage twinned calcite cements occlude most of

the remaining porosity, and renders petrophysical measurements difficult to interpret. Fluid inclusion analyses

show homogenization temperature (Th) values from 120 to 200°C and estimated salinities ranges between 10 and

24 eq. wt. % NaCl.

The possible sources of dolomitizing fluids may include deeply buried Triassic evaporitic strata in the intra-

platform basin, Keuper salt diapers and/or Mg-bearing igneous rocks (e.g. gabbro, basalt).

Keywords: Aptian-Albian, HTD, Stable isotopes, Fluid inclusions, Helium porosity

INTRODUCTION

Various models are proposed by different workers to understand the mechanism of dolomite formation,

which include reflux, mixing zone, sabkha, sea water, microbial, burial, hydrothermal and others (Adams &

Rhodes, 1960; Badiozamani, 1973; Folk & Land, 1975; Machel & Burton, 1994; Muchez & Viaene, 1994;

Vasconcelos & McKenzie, 1997; Wright, 1997; Machel, 2006). Dolomite can occur in different forms and almost

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everywhere (Wilson et al., 1990; Swennen et al., 2003; Vandeginste et al., 2005; Smith & Davies., 2006; Nader et

al., 2006 & 2007).

There are various factors, which hinder the understanding of dolomite formation. Firstly, dolomites are

characterized by molecular structures, stoichiometry, isotope studies, fluid inclusion analyses and trace element

composition, that expresses different conditions of formation and evolution (Purser et al., 1994). Secondly,

recrystallization processes also partially or completely destroys the genetic characteristics of early dolomite

(Nielsen et al., 1994). Thirdly, physico-chemical conditions that determine dolomite nucleation and subsequent

growth in a particular environment are still poorly known. Lastly, geochemistry of the dolomites also remains

problematic due to neomorphic overprint, difficulty of intracrystalline microsampling, and interference of clay

content.

Fluid flow is important for mineral diagenesis in carbonate aquifers. Today, a general consensus invokes

that permeability and fluid flow is a key for pervasive dolomitization (Machel, 1999). Field studies have

characterized the link between brittle deformation and/or dolomite recrystallization during fracturing (Tarasewicz

et al., 2005), highlighting the intimate relationship between multiple phases of fracturing, fluid flow pattern and

dolomitization (Wilson et al., 1990; Mountjoy and Halim-Dihardja, 1991; Duggan et al., 2001).

Some dolomites show a broad range of superimposed diagenetic processes, which require multiphase

hydrothermal dolomitization (Nader et al., 2007). Present studies of the fracture-related dolomite bodies will help

in understanding the mechanism of dolomitising fluid flow through the host rock, which affects the geochemical

conditions as well as the porosity and permeability of the dolomitized rock.

This paper addresses the petrographic characteristics of fracture-related dolomites in the platform to basin

carbonate rocks as well as their geochemical attributes and results of fluid inclusions. Hence, a conceptual model

explaining the observed dolomitization is proposed and some insight given to its implication on reservoir

properties.

GEOLOGICAL SETTING AND STUDY AREA

The study area is part of the Basque-Cantabrian basin (BCB), which lies between the Pyrenees and the

Cantabrian mountains in the north of Spain. The Cretaceous sedimentary evolution of the region can be related to

the late tectonic relationship between the European and Iberian plates, and closely linked to the opening of the

North Atlantic Ocean and the Bay of Biscay (Malod & Mauffret, 1990; Olivet, 1996). Previous workers used

paleomagnetic signatures to demarcate the boundary between Iberian and European plates, which likely

corresponds to the fault zone of the Biscay synclinorium (Van der Voo, 1969; Vandenberg, 1980; Schott & Peres,

1987; Rat, 1988; Garcia-Mondéjar, 1996; Garcia-Mondéjar et al., 1996). Cretaceous tectono-sedimentary

evolution of the BCB is subdivided into multiple phase rifting stage (Early Cretaceous) and a post-rift stage (Late

Cretaceous) on the basis of surface and subsurface data (Azambre & Rossy, 1976; Lamolda et al., 1983; Meschede

1985; Cabanis & Le Fur-Balquet 1990; Castanares et al., 2001). The BCB underwent multidirectional stretching

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(NNE-SSW and then NW-SE) during early Cretaceous rifting (Montadert et al., 1979; Grimaud et al., 1982;

Boillot & Malod, 1988; Malod & Mauffret 1990; Olivet, 1996).

The Ranero area contains NW-SE, E-W, N-S, and NE-SW oriented faults, which are interpreted as related

to the extensional history of the study area (see Fig. 1). Major structures include NW-SE oriented (Güenes and

Ranero faults), N-S oriented (Ramales fault), E-W oriented (Arredondo and Hornijo faults) and other small scale

NE-SW directed faults and fault splays (Fig. 2). Güenes fault containing oblique-slip and horsetail splay features

was formed in Early-Mid Albian (Aranburu et al., 1994). Ranero fault is believed to be the off-shoot of the Güenes

fault with same characteristic features (Garcia-Mondejar et al., 1996). So, these NW-SE oriented faults are related

to early Cretaceous rifting in the BCB (Montadert et al., 1979; Grimaud et al., 1982; Boillot & Malod, 1988;

Malod & Mauffret 1990; Olivet, 1996). N-S faults (e.g., Ramales fault) are basement-related listric faults (Garcia-

Mondejar and Pujalte, 1975). E-W oriented Arredondo and Hornijo faults are deep seated and present along

carbonate platform margins (Garcia-Mondejar, 1985). E-W oriented faults in the study area post-date NW-SE

oriented faults and fractures, which presume that E-W oriented faults formed after Early-Mid Albian age (Fig. 4B).

The Ranero study area (Karrantza valley) is part of Ramales platform, which contain Aptian-Albian

carbonate buildups. At the foot of these reefal and back reef carbonate buildups, slope and basinal deposits also

occur in the study area (Fig. 1A). The short distance between the platform carbonates and the basinal deposits

suggests the formation of an intra-platform trough rather than a deep basin, which is also shown in the

paleogeographic map of the Early-Middle Albian age of Garcia-Mondéjar, (1996) (Fig. 1B). Fault-controlled as

well as irregular dolomite bodies (fronts) are in sharp contact with the host limestone. In the study area, these

dolomites are restricted to faults and fractures in the host limestone.

The study area is located near the Ranero village in Karrantza valley (northern Spain), at the border of the

Basque country and Province of Cantabria (see Fig. 1). Geographically, the study area is located to the south of

Laredo and north of Concha cities. The Aguera river and the city of Bilbao is situated in the eastern part, while the

Ason river and city of Ramales de la Victoria are located in the western side of the study area. Systematic sampling

of the dolomite bodies as well as the surrounding host limestone was performed across the main Ranero fault,

Ranero vein (dolomite body east of the main Ranero fault), dolomite front (irregular dolomite tongues on the

western side of Ranero fault) and Pozalagua quarry (Fig. 3).

METHODS

Dolomite bodies were delineated by means of satellite images and later on checked during fieldwork. 110

rock samples were taken from different sites. Petrographic observations included conventional (Nikon ECLIPSE

LV 100 POL) and cathodoluminescence microscopy (Cathodyne OPEA; operating conditions were 12 to 17 kV

gun potential, 350 to 600 μA beam current, 0.05 Torr vacuum) of thin sections. For dolomite and calcite

differentiation, thin sections were stained with Alizarin Red S and potassium ferricyanide (Dickson, 1966). Image

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analyses were done by means of JMicrovision software. XRD analyses (to determine bulk mineralogy,

stoichiometry and crystal ordering of the different dolomite phases) of the twenty-seven selected samples were

carried out using PANalytical X'Pert PRO XRD diffractometer (Cu-Kα radiation ~ 45kV, 40mA). The scan speed

was set at 0.2˚θ min-1 and sampling interval at 0.001˚θ per step. The dolomite stoichiometry mol% CaCO3 was

determined by applying Lumsden's equation (1979) to the measured d[104] spacing (M = 333.3 × d-spacing –

911.99).

Ninety four stable isotope (δ18O and δ13C) analyses of different dolomite types, calcite cements, matrix

limestone was carried out in the Laboratoire de Biominéralisation et Paléoenvironnement; Université Pierre &

Marie CURIE-Paris VI and Departement de Géologie, Université Jean Monnet, Saint-Etienne (Table. 1). All

stable isotope values are reported in per mil (‰) relative to Vienna Pee Dee Belemnite (V-PDB). Dolomite

isotopic composition values are corrected by fractionation factors given by Rosenbaum & Sheppard (1986).

Microthermometry was performed on two double-polished sections (each from zebra dolomite and late saddle

dolomite) using Linkam THMSG 600 heating cooling stage. This method is used to infer the nature of the

prevailing fluids and temperature during precipitation (or formation) of certain diagenetic phases. Twenty five

dolomite crystals containing primary, two-phase inclusions were analyzed.

Seventy samples were selected for petrophysical analyses (porosity, permeability). For porosity

determination, micromeritics GeoPycTM 1360 and micromeritics AccuPyc 1330 was used in the Reservoir

Characterization Department of IFP. For permeability analysis, the standard technique of vacuum generation using

Boyle’s Law was used.

FIELD OBSERVATIONS

Based on weathering colors, aerial photographs of the study area clearly differentiate the dolomite bodies

from the host limestone (Fig. 3). Dolomite has yellowish-brown to dark grey in color and display a crumbly

weathering, where as limestone is light grey colored and develop a karren karst (Fig. 4A). The dolomite-limestone

contact is very sharp and rarely concordant with the stratification. Field studies allow us to recognize different

dolomite bodies, namely (Fig. 3):

Fault-restricted dolomite body.

- Pozalagua Quarry

- Ranero corridor

Dolomite development around Ranero fault.

- Fracture filled, linear/elongated dolomite body (Ranero vein).

- Ranero front

Fault-restricted dolomite bodies are generally oriented N-S, NW-SE and NE-SW. Ranero fault (left lateral

strike slip) is believed to be the splay of the NW-SE oriented Early-Mid Albian Güenes fault, which contain

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dolomite bodies along its extension (Fig. 4B; Garcia-Mondejar et al., 1996). These dolomite bodies show variation

in their facies distribution across the fault. In the Pozalagua quarry site (see Fig. 3), various dolomite phases (dark

to light grey, brown to yellowish, pinkish and milky white to light pink) are observed, which could relate to

multiple influx of the dolomitizing fluids. Moreover, dolomite color is independent from the composition and/or

color of the host lithology. Zebra dolomite occurs as suspended blocks in the above stated dolomite and indicates

its early formation as compared to other dolomite types (Fig. 4C). Locally, dolomites in the Pozalagua quarry

contain brecciated limestone clasts in dolomite cement (Fig. 4D). In the Ranero fault section (from east to west i.e.,

upslope of the Pozalagua quarry site), various dolomite facies were sampled along with host limestone (see Fig. 3).

Beside these major structures, dolomite is also associated with minor faults and/or fractures (see Fig. 4B).

Horsetail splays containing dolomite were also observed, which indicate the strike-slip nature of the Ranero fault

(Fig. 4E; Harding, 1974). In fault-controlled bodies, massive dolomite facies are dominant and exhibit milky white

appearance, curved crystal faces interlocked with each other and contain negligible inter-crystalline porosity (Fig.

4F). Beside massive dolomite, zebra dolomite contains alternating bands of matrix (dark grey in color) and cement

(off-white color) dolomite phases (Fig. 5A).

Fracture filled, linear dolomite body (Ranero vein) trending NW-SE, is cut by almost E-W oriented right-

lateral strike-slip fault (Fig. 5B). Sampling was done along an east-west traject across the dolomite body in the host

limestone. In the Ranero vein section, calcite veins occur in both precursor limestones as well as dolostones and

generally have the same regional trend as that of the faults. In the host limestone, early calcite veins have their

central part filled by orange coloured dolomite (Fig. 5C). Macroscopic observations indicate that these partly

brecciated dolomite filled calcite veins are cross cut by late stage calcite veinlets (Fig. 5D).

The dolomite distribution across the fault/fracture is generally dependent upon the nature of the host rock.

Finger like, irregular dolomite bodies (Ranero fronts) in the host limestone, occur next to the faults (Fig. 5E).

Various dolomite facies are observed in the Ranero front section. Dominant facies includes dirty grey colored,

sucrosic dolomite, which contains considerable intercrystalline pore spaces (Fig. 5F).

PETROGRAPHY

Pozalagua Quarry

Limestone ranges from wackestone to packstone, with partially replaced fossils and fossil fragments

(Dunham, 1962; Fig. 6A&B). These fossils include biomolds, tabular corals, debris of rudists and forams. Early

calcite cement is observed in the core of many of the biomold (see Fig. 6A).

Sucrosic (Planar dolomite) consisted of rhombic, subhedral to euhedral medium crystalline dolomite (Fig.

6C&D). Inter-crystalline pore spaces are filled with calcite and locally also with pyrite (see Fig. 6C). CL

examination highlights the rombic shape of these planar dolomite crystals with locally well-developed

intercrystalline porosity (see Fig. 6D).

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Zebra dolomite exhibit alternating bands of fine-to-medium crystalline and coarse crystalline dolomite

(Fig. 6F). Fine to medium crystalline dolomite shows straight extinction while coarse crystalline cement dolomite

shows undulose extinction. In the cross-polarized light, aggradational neomorphism (crystal's over-dimensioned

growth) is observed.

Coarse-crystalline, nonplanar dolomite showed undulose extinction (Fig. 6E), which is typical of saddle

dolomite (Radke & Mathis, 1980). Nonplanar dolomite cement displays primary crystal growth in CL (Fig. 7A).

Ranero Fault Section

Petrographic studies show host limestone, calcite cements and different dolomite facies along with their

alteration products. Texturally, limestone (about 35% of the whole section) ranges from wackestone to packstone

and contains fossils and fossil fragments, which include tabulate corals and foraminifera. Host limestone contains

veins filled with twinned calcite and dolomite cements.

Planar dolomites (about 6% in the section; Fig. 9) are subhedral to euhedral fine to medium crystalline

dolomites, ranging in size between 200-600µm. These planar dolomites contained considerable intercrystalline

pore spaces, which is often filled by calcite. Fine to medium crystalline (300 to 700µm), polymodal, nonplanar

dolomite (about 51% of the section) often occurs adjacent to the planar dolomite and exhibits an interlocking

pattern. Under CL, nonplanar dolomite cement shows different zonations of dull to bright red colours with red

rims. In the Ranero fault section, the size of these nonplanar dolomite crystals increased from east to west as very

coarsely crystalline dolomites were observed at the western extremity. Zebra and/or zebra-like dolomite (about 5%

of the section) are composed of coarse crystalline dolomite cement (400 to 700µm) alternating with fine crystalline

dolomite matrix (20 to 100µm). Coarse-crystalline dolomite have undulose extinction typical of saddle dolomite,

while fine crystalline dolomite exhibit straight extinction. Intercrystalline pore spaces in various dolomite facies

are commonly filled by calcite but some dolomite veins post-dating the calcite veins as well (Fig. 7B). Besides

dolomite cement phases, two types of calcite cements are recognized in the dolomite body, which include brown to

dull orange luminescence and non luminescence (sometimes with thin yellow luminescent zones) in CL analyses

(Fig. 7C&D).

Ranero Vein Section

Limestone (about 38% of the section; Fig. 9) contains uniserial benthic foraminifera, gastropods, pellets

and intraclasts with considerable amounts of dissolutional porosity, which might be due to surface weathering (Fig.

13A). Generally, the porosity is destroyed by calcite cement filling. Fracture-filled calcite is also present, which

shows abundance from host limestone towards the dolomite body.

Medium crystalline, polymodal, nonplanar dolomite (about 39% of the section; Fig. 9) shows interlocking

pattern, characteristic undulose extinction and exhibits dull to light red colours with bright red rim in CL.

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Nonplanar dolomite increases in their crystal size from eastern to western part in the studied section as very

coarsely crystalline nonplanar dolomite is observed in the extreme west of the studied section. Zebra and/or zebra-

like dolomite (about 16% of the section; Fig. 9) are composed of coarse crystalline dolomite cement (300 to

500µm) alternating with fine crystalline dolomite matrix (50 to 100µm)

Diagenetic alterations include recrystallization (dissolution/precipitation) phenomenon typical of nonplanar

dolomite, which partially or completely destroy the intercrystalline porosity as well as dissolutional porosity (Fig.

8A).

Ranero Front Section

An irregularly distributed dolomite tongue-shaped dolomite front on the western side of the main Ranero

fault was studied (Fig. 3). Matrix limestone (about 21% of the section; Fig. 9) contains calcite vein, which are not

observed in the dolomite. Besides matrix limestone, different dolomite facies include nonplanar, planar and

cataclastic dolomites are observed. In contrast to the other studied sections, this section contains relatively more

planar dolomite. Planar dolomite (about 45% of the section; Fig. 9) crystals are subhedral to euhedral and range in

size from 30 to 200μm. These planar dolomites are surrounded by late stage reddish calcite cement (stained

colour). Nonplanar dolomite (23% of the section; Fig. 9) crystal faces are subhedral to anhedral and crystal size

ranges from 50 to 400μm. Some of the dolomite cements show planar appearance and straight extinction in cross-

polarized light and exhibit zonations of dull and bright red coloured luminescence under CL, which indicate

different generations of dolomite growth (Fig. 8 B&C).

Diagenetic alterations include cataclastic deformation, which resulted in the formation of broken crystals of

various sizes cemented by coarse crystalline calcite (Fig. 8D). Angular rock fragments are observed in certain

zones, which is due to small scale fracture and fault passing through the dolomite front.

Dolomite Stoichiometry

All analysed dolomite samples from different sections show nearly non-stoichiometric dolomite, ranging from 50.1

to 51.3 mol% CaCO3.

GEOCHEMISTRY

Oxygen and carbon isotopes

Oxygen and carbon isotopic signature of the Early Cretaceous marine carbonate ranges from -3 to +1 to ‰

δ18O and 0 to +4‰ δ13C (Veizer et al., 1999). In the Pozalagua quarry, different dolomite facies show a wide range

of δ18O depleted values. These include zebra and/or zebroid dolomite with values ranging from -18.0 to -13.0‰ for

δ18O and -0.7 to +1.5‰ for δ13C (Fig. 10). Massive (nonplanar) dolomite exhibits δ18O values ranges from -17.0 to

-12.0 ‰ and δ13C values ranges from -0.7 to +1.0‰. Sucrosic (planar) dolomite shows wide range of depleted

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values in δ18O values lies between -13.8 to -9.7‰ and δ13C as these values range from -2.7 to -0.7‰ (Fig. 10).

Cataclastic dolomite show δ18O values range from -16.7 to -14.0‰ and δ13C value between -1.6 and +1.1‰.

In the Ranero vein section, early calcite cement shows values of -4.2‰ for δ18O and +1.8‰ for δ13C, which

are close to the original marine signatures of Early Cretaceous carbonates (Fig. 10). δ18O values of matrix

limestone ranges from -8.0 to -7.0‰ and δ13C values ranges from +2.0 to +2.5‰. Nonplanar dolomite values

varies from -16.7 to -11.7‰ for δ18O and +0.8 to +1.8‰ for δ13C, while planar dolomite shows cluster as δ18O

values range from -14.2 to -13.0‰ and δ13C values vary from +1.2 to +1.3‰ (Fig. 10). Early calcite cement shows

highly depleted δ18O values (-18.1‰).

The Ranero front section contains two dolomite facies, which show highly depleted δ18O values. Nonplanar

dolomite varies from -18.3 to -17.5‰ with respect to δ18O, while δ13C values range from +0.4 to +1.15‰ (Fig. 10).

Zebra dolomite show narrow range between -18.2 to -18.1‰ for δ18O and +1.1 for δ13C. Late calcite cement shows

values from -10.1 to -6.1‰ for δ18O and -5.6 to -3.1‰ for δ13C (Fig. 10).

Fluid Inclusions analyses

Analyzed fluid inclusions from zebra dolomite and massive, nonplanar dolomite were two-phase, primary

in nature as most of these are isolated and located along the border of the crystals. The crystals display uniform

cloudiness due to the presence of small fluid inclusions. The size of the investigated inclusions was variable (up to

5µm) and their shape is irregular.

Homogenization temperatures (TH) for the fluid inclusions of zebra dolomite ranged from 131 to 196ºC

(average TH = 167.1±16.4ºC), most values lying between 140 and 170°C (Fig. 11). For late saddle dolomite, the TH

varies from 125 to 185ºC (average temperature range = 158.9±13.6ºC; see Fig. 11).

Many phase changes at low temperature (i.e. melting of salt hydrates) could not be identified and in most

fluid inclusions only the melting temperatures of ice (TM), often corresponding to the final melting temperatures

(TM final), were measured. Melting temperature (TM) for zebra dolomite varies between -2 to -21ºC, while it ranges

from -18 to -8ºC for massive, nonplanar dolomite. The estimation of salinity was done according to Bodnar’s

equation (Bodnar, 1993). The salinity of fluid inclusions in the selected zebra dolomite has a broad range from 4.5

to 21.3 eq. wt % NaCl (Fig. 12). The mode value (major occurrence) of salinity is 15.2±4.6 eq. wt. % NaCl as only

few TM were above -10ºC, most are between -20 and -15ºC. The salinity of fluid inclusions in massive, nonplanar

dolomite has a broad range from 11.7 to 21.0 eq. wt. % NaCl (Fig. 12). The mode value for these fluid inclusions

is 16.3±4.2 eq. wt. % NaCl. Salinity of the massive dolomite shows no considerable variation with the change in

the homogenization temperature (TH).

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POROSITY AND PERMEABILITY ANALYSIS

Selected samples from different sites were chosen for petrophysical studies to assess any petrophysical

variations as a function of dolomite textures (Table. 2).

In the Pozalagua quarry site, nonplanar dolomite is characterized by low porosity and permeability values

respectively (1 to 2.5% and less than 0.5mD). Zebra dolomite show relatively higher porosity (2.1 to 11%) but

similar permeability values. Relatively high porosity values might be due to matrix phase in the zebra dolomite

(Fig. 13B). Cataclastic dolomite contains porosity values ranging from 1.6 to 3.8% and permeability values up to

0.2mD.

In the Ranero fault section, porosity/permeability values for nonplanar dolomite ranges up to 7.3% and

1.6mD respectively (Table. 2). The porosity/permeability values greatly (up to 5%) differ from one side of the

section to the other side, in agreement with obvious differences in textures (fine to medium crystalline nonplanar

dolomite show considerable pore spaces in the eastern part as compared to very coarsely crystalline, interlocked

dolomite exhibit negligible porosity in the western part of the studied section). Zebra dolomite show relatively high

porosity (up to 4%) and permeability (up to 1.5mD), which is due to the intercrystalline pore spaces in the matrix

dolomite phase as cement dolomite phase contain negligible porosity. Planar dolomite contains high porosity but

low permeability values, which range upto 6.0% and 0.7mD respectively. High porosity values are due to

intercrystalline porosity in these planar dolomites (Fig. 13C&D). Cataclastic deformation resulted in the increase

in the porosity (if not occluded by late calcite cement) but show no effect on the permeability, which is evident by

high porosity values (up to 6%) and low permeability values (0.3mD).

In the Ranero Vein section, limestone contains relatively high dissolutional porosity in some places (Fig.

13A). In the dolomite facies, nonplanar dolomite show low porosity (1 to 4%) and permeability (0.1 to 1mD)

values throughout the section. Planar dolomite show intercrystalline porosity ranges upto 3.5% and permeability

upto 0.5mD. Cataclastic dolomite contain high porosity values (up to 4%), which is due to the breakage of pre-

existing saddle dolomite.

The nonplanar and planar dolomite in the Ranero front section shows almost same values of porosity (2-

7%) and permeability (0.1-0.3mD). Cataclastic dolomite exhibits comparatively low porosity (up to 4%) and

permeability (0.1mD) values (Table. 2).

DISCUSSIONS

The study area underwent three rift phases, which include: 1) Early Triassic rifting resulted in the formation

of Cantabrian graben; 2) Late Jurassic rifting that caused the deposition of Cretaceous successions and the creation

of Bay of Biscay and 3) Early Cretaceous rifting resulted in NNE-SSW direction of simple stretching

perpendicular to the axes of the main NW-SE structural trend (Rat, 1959; Pujalte, 1977; Garcia-Mondejar, 1989).

These rift phases resulted in major faults and fractures oriented in NW- SE, E-W and NE-SW directions. Two

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periods of fault-controlled regional subsidence (Middle Triassic to Middle Jurassic and Late Cretaceous) resulted

in thick sedimentary successions of upper Jurassic – lower Cretaceous age (Rat, 1959 & 1988; Voort, 1963;

Feuillee and Rat, 1971; Garcia-Mondejar, 1989; Rosales, 1995). These sedimentary successions comprise of upper

Jurassic – Barremian continental clastics, lower Aptian – upper Albian platform, rudist limestone, basinal marls

and mudstone, and upper Albian – lower Cenomanian fluvial siliciclastics to turbidites (Garcia-Mondejar, 1989).

In the study area, Aptian-Albian massive and micritic limestone with orbitolinas, corals, oyster-like chondrodonta

and rudist debris occur (Ramales Formation), which confirmably overlies the Rio Yera Formation, while the upper

contact with the Valmaseda Formation is unconformable. Transitional marine shales and sandstones (lower

Hauterivian to upper Barremian) of the Villaro Formation are present in the intraplatform depression (Gibbons and

Moreno, 2002; Fig. 14).

In the study area, dolomite bodies associated with Ranero sinistral strike-slip fault is related to early

Cretaceous rifting (Early-Mid Albian; Aranburu et al., 1994). Dolomite bodies show different geometries nearby

Ranero fault, which include linear (fracture filled), irregular (fronts) and fault constrained. The presence of linear

(fracture filled) dolomite bodies support the idea that the dolomitising fluids migrated and spread through the

sedimentary pile along these discontinuities (Fig. 5B). Irregular, tongue-like dolomite bodies (fronts) suggest that

the permeability and the primary mineralogy of the precursor limestone were the important factors controlling the

distribution of dolomite bodies as rock type in contact with the dolomitising fluids may also control the

dolomitisation process (Fig. 5E; Murray and Lucia, 1967; Bullen and Sibley, 1984). In such a way, physical and

chemical characteristics of the host limestone control the dolomite distribution (e.g. Nader et al., 2007). Physical

characteristics include permeability (influences volume of fluid flowing through the pores), and particle size

(determines the amount of surface area available for fluid-rock interaction). Chemical characteristics, such as the

solubility of carbonate minerals, may play a significant role as well. Fault constrained dolomite bodies (corridor)

strictly indicate tectonic control on the dolomite distribution (Fig. 4B; Garcia-Mondejar, 1996; Lopez-Hogue et al,

2005). In the Pozalagua quarry (on the main Ranero fault), various generations of dolomite facies indicate multiple

pulses of the dolomitising fluids (Fig. 4C). The sharp limestone-dolomite contacts mostly cutting stratification and

sedimentary structures suggest a late origin for the dolomites (Fig. 4A). Various dolomite facies including

sucrosic, massive and zebra dolomite were observed in the study area. Zebra dolomite existed as broken clasts in

the nonplanar dolomite, indicate its early formation in the Pozalagua quarry (Fig. 4C).

Three principle dolomite textures were identified, which include planar, nonplanar, and zebra (Gregg and

Sibley, 1984). Planar dolomite represents subhedral to anhedral sub-rhombic dolomite crystals, ranging in size

from 200 to 700µm and exhibit straight extinction. Nonplanar dolomite contains subhedral to euhedral crystals,

showing undulose extinction (typical of saddle dolomite) and size ranges from 100 to 700µm (Fig. 6E). Dull and

bright red coloured zonation in nonplanar dolomite might relate to episodic stages of dolomitization of the host

rock. Planar and nonplanar dolomite were also affected by alteration in the later stages, resulted in

10

recrystallization, precipitation and cataclastic deformation (see figs. 8A&D). Zebra dolomite consisted of

alternating bands of replacive phase and void-filling cement phase (Fig. 6F). Several authors (e.g., Beales and

Hardy, 1980; Martin et al., 1987; Tompkin et al., 1994) believed that the development of zebra dolomite is strictly

controlled by sedimentary facies. Polymodal, nonplanar dolomite consisted of interlocking anhedral crystals,

which underwent aggradational neomorphism demonstrating the evolution into planar dolomite crystals as was

described by Nielsen et al., 1994 and Vendeginste et al., 2005 (Fig. 7A). In the study area, zebra structures are

often unconformably oriented relative to the bedding of the host limestone which may exclude the possibility of

facies-controlled formation of zebra dolomites (Fig. 4C).

Replacive and void-filling dolomite cement were recognized. Replacive dolomite cement originated by the

replacement of precursor limestone, while void-filling dolomitisation consisted in the precipitation of sparry

dolomite, which filled cavities and fractures created during the dolomitisation process (Figs. 6D, 7A-B). Replacive

and void-filling dolomite phases have similar geochemical signatures (Fig. 10-12). This suggests rather uniform

physico-chemical conditions during their formation. It is assumed that continuous dolomitisation process which

evolved from a replacive stage towards a void-filling stage in a nearly isochemical system (Gasparrini, 2003).

Two calcite cement generations (early and late phases) have been identified in the studied samples.

Microscopically, early calcite cement shows brown orange CL. The second type of calcite exhibits non-

luminescence (Fig. 7C-D), which is similar to calcite formed near to meteoric recharge (Choquette and James,

1988; Niemann and Read, 1988; Meyers, 1991; Reeder, 1991).

The paragenetic sequence of the most important events recognised in the studied dolomites was

reconstructed as follows (Fig. 15). In the host limestone, faulting and fracture development occurred, which was

filled by early calcite cement. Later on, another episode of shearing resulted in the dolomitization process. This

dolomitization event is also evident in early calcite, where dolomite filled calcite vein in their central part (Fig. 5C-

D). Different dolomite facies are recognized, which include planar, nonplanar and zebra dolomite (Figs. 6C-F).

These dolomite facies underwent alterations, which include neomorphism, filling and in the end cataclastic

deformation (Fig. 8A&D). Late stage calcite cement filling in the open spaces (fracture filled or intercrystalline)

cemented the dolomite (Fig. 7D). Besides this, sulfides (pyrite) are also associated with these dolomites.

The stable isotope analyses reveal a broad range of δ18O values mainly between -18 and -10‰ (Fig. 10). The

highly depleted and broad ranged δ18O values may indicate multiphase dolomitization and dolomite

recrystallization, as already observed in the study area (Fig. 4C; cf. Nielsen et al., 1994). Limestone and dolomite

phases indicate a clear shift from the Early Cretaceous marine carbonate isotope signatures, while early marine

calcite shows close resemblance with it (Fig. 10). All the observed facies show deviation towards negative oxygen-

isotope signatures. Therefore, high temperature conditions may be invoked to explain such negative δ18O values

(Swennen et al., 2003; Vandeginste et al., 2005). The reported δ13C values of such dolomites are not far from the

original marine signature of the host rocks (i.e. early Cretaceous), thus carbon isotopic composition of the

11

dolomitizing fluids may have been buffered by the host rock. The limestones close to the dolomites also show

depleted δ18O values (similar values to those of the dolomites), implying recrystallization during dolomitization

(Fig. 10). Other depleted δ13C values may be interpreted as resulting from late telogenic calcite phases (Fig. 10).

Fluid inclusions analyses show homogenization temperatures range between 120 and 200°C (Fig. 11).

Stable isotope analyses and fluid inclusions studies confirm very hot dolomitizing fluids and hence the

hydrothermal origin for the investigated dolomites (Fig. 11). Besides, presence of saddle dolomite may suggest

temperature of formation between 60 to 150 ºC (Radke and Mathis, 1980), although this temperature may vary

from 90 to 160°C (Machel & Mountjoy., 1987; Spotl and Pitman, 1998). Considering the resulted TH distribution

range together with δ18O values of the zebra dolomites (between -15 to -18‰ V-PDB), the δ18O values (‰ V-

SMOW) of the parent fluid can be calculated following Land's (1985) dolomite-water fractionation equation (cf.

Adams et al., 2000; Fig. 11). The δ18O values approximately range between -4 and +2.1‰ V-SMOW, indicating

fluids had low δ18O values than those representing normal seawater. Dolomitizing fluids may come from deeper

basinal formations (intra- platform basin) or from basement rocks along Bilbao-Villaro Fault zone, which provided

pathways to dolomitize Aptian-Albian carbonates (Fig. 14).

Dolomitisation may generate, preserve or destroy porosity depending on the textures of the replaced

carbonates, rate and composition of fluids and duration of the process (Purser et al., 1994). Porosity and

permeability values depend upon type of dolomite facies, crystal size distribution and nature of porosity

development. Dissolution during dolomitization and precipitation of dolomite cement are considered responsible

for positive or negative effect of dolomitisation on porosity development. In the analyzed dolomite samples,

dissolution/precipitation decreased the porosity, while cataclastic deformation present in some samples increased

the porosity. Planar and zebra dolomite shows good intercrystalline porosity (3 to 5%; Figs. 13B-D).

CONCLUSIONS

The petrographic, geochemical, fluid inclusion and porosity/permeability studies resulted in the following

conclusions.

Fieldwork helped in assessing the geometric and dimension pattern of the dolomite exposures and their

relationship with the Ranero Fault.

Petrographic studies resulted in distinguishing three major dolomite facies: planar-s to surcosic, nonplanar

(interlocking) and zebra. Moreover, textural studies helped in identifying three different dolomite alteration

types (i.e. cataclastic deformation, recrystallization – dissolution/precipitation, and hydrothermal calcite

cementation), which occurred during the multiphase hydrothermal dolomitization.

Geochemical studies (O & C stable isotopes) show a broad range of δ18O from -18 to -11‰ V-PDB,

confirms the multiphase hydrothermal origin. XRD data show that the dolomites are nearly non-

stoichiometric (around 50 M% CaCO3) for most of the samples.

12

Porosity/permeability data represents the effect of different dolomite facies on the porosity/permeability

values. Besides this, the dolomite alteration products show increase in porosity (cataclastic dolomite).

Highly depleted δ18O values (-18.5‰ V-PDB) and high temperature range (up to 200ºC) of fluid inclusions

indicate a hot and deep source of dolomitizing fluids, which correspond to the upwelling of these fluids

along major faults (Guenes fault and Bilbao-Villaro Fault zone).

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