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Diagenetic significance of carbon, oxygen and strontium isotopic compositions in the Aptian-Albian Mural Formation in Cerro Pimas area, northern Sonora, Mexico/ Significado diagenetico de las composiciones isotopicas de carbono, oxigeno y estroncio en la Formacion Mural (Aptiense-Albiense, zona Cerro Pimas, norte de Sonora, Mexico).

1. Introduction

The stratigraphic correlation of marine carbonates using carbon isotope record has been successfully applied to Cretaceous marine carbonate sediments (Jenkyns, 1995; Weissert et al, 1998; Moullade et al, 1998). It was initially applied to pelagic succession, but has since been applied also to shallow marine carbonates (Jenkyns, 1995; Ferreri et al., 1997; Grotsch et al., 1998), with promising results, when compared to biostratigraphic (Masse et al., 1999; Erba et al., 1999) and magnetostratigraphic (Lini et al, 1992; Henning et al, 1999) data.

The mid-Cretaceous has been considered as one of the Earth's period of greenhouse climate (Barron and Washington, 1982; Berner et al., 1983). During the mid-Cretaceous, dysoxic and anoxic conditions (oceanic anoxic events: OAEs) developed in oxygen minimum zones along the continental margins of the tropical Tethys Sea, in restricted epicontinental seas, and in basins of the widening North and South Atlantic Ocean basins (Leckie et al, 2002). Carbon isotope records are mainly used to establish global organic carbon budget during these OAEs. The characteristic positive carbon isotope excursion corresponding to OAE2 (Scholle and Arthur, 1980) indicates that the volume of [C.sub.org] buried during this period was a sizable part of the global carbon budget. Carbon isotope shifts for the Aptian-Albian interval are somewhat complex (Pratt and King, 1986) and their relationship with dysoxic/anoxic conditions is not fully understood. [[delta].sup.13]C values increase above the Early Aptian (OAE1a) and remained high into the Albian. Major discrepancies exist in the chronostratigraphic correlation of the carbon isotope record in Mexico (Scholle and Arthur, 1980) and Europe (Weissert and Lini, 1991; Leckie et al., 2002; Herrle et al., 2004). Most of the information about the mid-Cretaceous global change derives from the investigations of the Tethyan sections exposed in France and Italy (Arthur and Premoli Silva, 1982; Premoli Silva et al, 1989; Coccioni et al, 1992; Herrle et al, 2004). Very little information on the mid-Cretaceous events is available from Mexico (Scholle and Arthur, 1980; Bralower et al, 1999). The isotopic study on mid-Cretaceous shallow marine carbonates have shown evidence for global-scale tectonics (Grocke et al, 2005; Maheshwari et al., 2005; Amodio et al, 2008), paleooceanographic processes (Kumar et al., 2002; Madhavaraju et al., 2004; Armstrong et al., 2009, 2011), climatic and biotic changes (Deshpande et al, 2003; Das Gupta et al, 2007; Mishra et al, 2010; Preat et al, 2010; Tewari et al, 2010).

Sedimentary rocks of Lower Cretaceous age are well exposed in northern and central Mexico and a thick sequence of clastic and carbonate rocks belonging to the Bisbee Group is widely distributed in Sonora, northwestern Mexico. Many studies have been carried out on the Aptian-Albian Mural Formation of this group to understand the stratigraphy and paleontology of the Bisbee basin (Bilodeau and Lindberg, 1983; Jacques-Ayala, 1995; Lawton et al., 2004; Gonzalez-Leon et al., 2008). Madhavaraju et al. (2010) carried out a preliminary goechemical study (major, trace and REEs) on certain sections of Mural Formation, but no detailed isotopic study on these sedimentary rocks. Hence aiming at exploring the existing gap of our understanding on the Early Cretaceous rocks and widening the existing database, carbon, oxygen and strontium isotope systematics on these carbonate has been applied in this study to unravel their depositional environments and palaeoceanography.

2. Geology and Stratigraphy

The Lower Cretaceous stratigraphic succession of the Mural Formation in northern Sonora that crops out at Cerro Pimas (Fig. 1) was deposited on shallow marine platform during Aptian-Albian time (Scott, 1987). Regionally, the Mural Formation overlies and underlies thick fluvial successions comprising of reddish brown sandstone, siltstone and conglomerate of Morita and Cintura Formations, respectively. These formations along with the Glance Conglomerate that underlies the Morita Formation compose the Bisbee Group and its outcrops extend from northern Sonora into southeastern Arizona and southwestern New Mexico in USA (Ransome, 1904; Bilodeau and Lindberg, 1983; Mack et al, 1986; Dickinson et al, 1989; Jacques-Ayala, 1995; Lawton et al, 2004).

[FIGURE 1 OMITTED]

The fossiliferous clastic and carbonate strata of the Mural Formation were deposited on a sharp ravinement surface developed on the Morita Formation during a major marine transgression. Lawton et al. (2004) defined six members in the Mural Formation in north-central Sonora, viz.: Cerro La Ceja, Tuape Shale, Los Coyotes, Cerro La Puerta, Cerro La Espina and Mesa Quemada Members. These members are laterally continuous from northeastern to northwestern Sonora, in a 300 km-long transect showing only minor facies changes (Gonzalez-Leon et al, 2008). East of this transect, in northeastern Sonora, however, members of the Mural Formation change facies laterally, grading to strata that represent deeper marine depositional environments. Lithofacies, fossils, and regional correlation of the Mural Formation members indicate that the depositional environments varied from restricted shelf with deltaic and fluvial influence to open shelf with coral and rudist buildups, and to offshore shelf (Gonzalez-Leon et al., 2008).

For the present study, we collected limestone samples from the Cerro Pimas section located in northwestern Sonora (Figs. 1 and 2). The 420-m-thick Mural Formation was deposited in shallow marine environments that ranged from nearshore with deltaic and fluvial influence to open marine environments (Gonzalez-Leon et al., 2008). The Cerro La Ceja (CLC) Member consists of interbedded bioclastic limestone, siltstone and calcareous sandstone. The limestone beds are composed of bivalve bioclastic grains, mostly with oyster and Trigonia shells, and are bioturbated and locally sandy. Siltstone beds are gray, green and reddish brown with calcareous nodules. The overlying Tuape Shale (TS) Member is mainly composed of gray to green and locally black mudstone to shale, shaly limestone containing bivalve shells with subordinate siltstone and fine grained sandstone intercalations. The Los Coyotes (LC) Member consists of thick to very thick, lenticular beds of gray, bioclastic wackestone-packstone and local thick beds with abundant rudist and colonial corals. The Cerro La Puerta (CLP) Member is composed of light green shale with occasional calcareous nodules in its lower and upper parts whereas in the middle part it is composed of reddish brown to light gray siltstone, green to light gray shale up to 5 m thick and subordinate medium-grained sandstone beds. The overlying Cerro La Espina (CLE) Member is composed of very thick-bedded bioclastic packstone-grainstone and thick- to medium-bedded grainstone and packstone. The uppermost Mesa Quemada (MQ) Member consists of reddish brown shale, siltstone and interbedded yellowish brown, bioturbated, shaly wackestone, locally dolomitic with oysters.

3. Methodology

Twenty nine limestone samples from the Cerro Pimas section were selected for petrographic and isotopic study (Fig. 2). A detailed study was carried out on these samples to find out the petrographic variations among different members of the Mural Formation and also to observe any diagenetic alteration in the limestone samples. The carbon and oxygen isotope composition of twenty nine samples was analyzed using a Prism series II model mass spectrometer at Korea Basic Science Institute. The limestone samples were treated with [H.sub.3]P[O.sub.4] in vacuum at 25[degrees]C and the resulted C[O.sub.2] gas was analyzed following the standard method of McCrea (1950). Normal corrections were applied and the results are reported in the standard per mil ([per thousand]) [delta]-notation relative to the Pee Dee Formation Belemnite (VPDB) marine carbonate standard. Sample reproducibility is better than [+ or -] 0.05[per thousand] for carbon and [+ or -] 0.1[per thousand] for oxygen.

Fifteen whole rock samples were analyzed for Sr isotope composition at Korea Basic Science Institute. Several 10 mg of whole-rock powders were mixed with highly enriched [sup.84]Sr and [sup.87]Rb spikes and then dissolved with a mixed acid (HF/HCl[O.sub.4] = 10:1) in Teflon vessels. Rb and Sr fractions were separated by conventional cation column chemistry (Dowex AG50W-X8, [H.sup.+] form) in HCl medium. Care was taken to avoid input from silicate impurities (Bailey et al., 2000). The [sup.87]Sr/[sup.86]Sr ratios were measured following the standard mass spectrometric procedures using a VG 54-30 thermal ionization mass spectrometer equipped with nine Faraday cups. Instrumental fractionation was normalized to [sup.86]Sr/[sup.88]Sr = 0.1194 and the measured [sup.87]Sr/[sup.86]Sr ratio were further corrected for the contributions of the added spikes. Replicate analysis of NBS 987 Sr standard gave a mean [sup.87]Sr/[sup.86]Sr of 0.710245 [+ or -] 0.000003 (n = 30, 2[sigma] SE). Total procedural blank levels were below 100 pg for Sr. The [sup.87]Sr/[sup.86]Sr ratios are presented after adjusting them to NBS 987 [sup.87]Sr/[sup.86]Sr ratio of 0.710230 (Verma, 1992; Verma and Hasenaka, 2004).

[FIGURE 2 OMITTED]

4. Results

4.1. Petrography

Carbonate lithofacies were examined based on classification schemes of Dunham (1962) and Embry and Klovan (1971). A brief petrographic description of various members of the Mural Formation is given below:

Cerro La Ceja Member

The Sandy molluscan wackestone occurs in the lower and middle parts of the Cerro La Ceja Member contains algal and molluscan bioclasts and also encloses small amount (around 2%) of quartz and feldspar grains (Fig. 3A). Quartz grains are mostly monocrystalline exhibiting uniform extinction. The feldspars include orthoclase and plagioclase. Few echinoid plates are also floating on the micritic matrix. Pores are filled with sparry and poikilotopic calcite cement. The limestone exhibits bedding parallel irregular microstylolite seams. Clay and other insoluble mineral grains are concentrated along the microstylolite seams. The upper part of the Cerro La Ceja member is represented by Echinoidal molluscan wackestone having algal, echinoid and molluscan fragments (Fig. 3B) and few angular quartz and feldspar grains. Echinoid plates and spines are common in this lithofacies. This lithofacies also contains few reworked bioclasts having coatings. The pore spaces are filled with sparry calcite cement.

Tuape Shale Member

Limestone beds of the Tuape Shale Member are characterized by the Sandy algal molluscan wackestone lithofacies which contains algal and molluscan bioclasts in the micritic matrix. Also found are few echinoid plates. Few bioclasts are ferrugenized and they seem to be reworked grains derived from the older sequence. This lithofacies also contains some silt-sized quartz and feldspar grains. Quartz grains show well rounded shape. The limestone exhibits numerous stylolites in which many clastic grains are concentrated in the clay seams (Fig. 3C). The limestone contains some fractures filled with sparry calcite cement.

[FIGURE 3 OMITTED]

Los Coyotes Member

The Los Coyotes Member comprises molluscan foraminiferal oolitic packstone lithofacies in the lower part and oolitic grainstone in the upper part. The Molluscan foraminiferal oolitic packstone has framework grains of algal, molluscan and foraminifera (Fig. 3D) with few rudist fragments. It also contains some quartz and feldspar grains. This lithofacies contains several types of ooids: i) subrounded ooids, ii) oblate spheroidal ooids, iii) compound ooids, iv) broken ooids, v) deformed ooids, and vi) calcareous lumps. The nuclei of the ooids mainly consist of bioclasts and micritic grains. Ooid grains exhibit distinct compaction features such as flattened grains and parallel grain contacts. Oolitic grainstone comprises assorted size of ooids (Fig. 3E). They are subrounded (spherical) to oblate spheroidal in shape. Some ooid-like lumps are also seen. The nuclei of most of the ooids are dissolved. These ooids are cemented by microsparite and sparry calcite.

Cerro La Espina Member

The Algal rudist coral foraminiferal packstone represents the lower and middle parts of the Cerro La Espina Member. It contains algae, rudist, coral, and foraminifera (Fig. 3F). Besides, some molluscan fragments and echinoid plates are also seen in the micritic matrix. It also contains small quantity of quartz grains. Many bioclasts are coated with micritic layer. Because of micritic envelope, the internal structure of the shell fragments is not completely destroyed. Some algal lumps enclosing certain organic fragments are also seen along with hematite aggregates. Some bioclasts exhibits pressure solution effect around their boundaries and also the development of minor stylolitic seams. The inter- and intraskeletal pores are filled with microsparite and sparry calcite cement. The Algal molluscan foraminiferal grainstone represents the upper part of the Cerro La Espina Member and has algae, coral, mollusc and planktic foraminifer (Fig. 3G). Few echinoid and cranial plates are present in the sparry calcite cement. This lithofacies also contains some benthic foraminiferas. Most of the bioclasts are coated with micrite. In addition, few clastic grains are also seen.

Mesa Quemada Member

Limestone of the Mesa Quemada Member is represented by the sandy wackestone which contains considerable amount (around 10%) of quartz and feldspar grains (Fig. 3H). Most of the quartz grains are monocrytalline showing largely uniform extinction. Few polycrystalline quartz grains are also seen. Many quartz grains are angular in shape whereas some well rounded quartz grains are also seen. Orthoclase and plagioclase feldspars are present in small amount. Few ferrugenized reworked bioclasts derived from the older sequences are also seen in the micritic matrix.

4.2. Isotopic variations

The carbon and oxygen isotope variations are given in Figure 4. The [[delta].sup.18]O values range from -13.4 to -9.9[per thousand] VPDB for Cerro La Ceja Member (Table 1). The limestones of Tuape Shale and Los Coyotes Members show little variation in 518O values (-11.9 to -10.0[per thousand]; -10.8 to -8.9[per thousand] VPDB; respectively). The Cerro La Espina Member shows negative 518O values from -13.0 to -9.5[per thousand] VPDB. Likewise, the Mesa Quemada Member also shows significant negative oxygen isotope values (-13.3 to -11.5[per thousand] VPDB).

The Cerro La Ceja Member shows negative to positive [[delta].sup.13]C values (-1.7 to +1.3[per thousand] VPDB; Table 1). The Tuape Shale Member exhibits negative carbon isotope values (-2.4 to -0.4[per thousand] VPDB) except one sample which show zero value (0.0[per thousand] VPDB). The Los Coyotes Member show both negative and positive carbon isotope values (-2.5 to +1.5[per thousand] VPDB). Most of the limestones from Cerro La Espina Member show positive carbon isotope values (+0.4 to +2.2[per thousand] VPDB) except one sample (CP43) which show slight negative value (-0.1[per thousand] VPDB). The large carbon isotopic variations are observed in the Mesa Quemada Member (-4.1 to +2.2[per thousand] VPDB).

The strontium isotope compositions of limestones of Cerro Pimas section are given in Figure 5. The CLC Member shows least variations in [sup.87]Sr/[sup.86]Sr values (0.707241 to 0.707242; Table 2). The [sup.87]Sr/[sup.86]Sr value of TS member is 0.707336. The [sup.87]Sr/[sup.86]Sr values of LC Member vary from 0.707274 to 0.707325. The [sup.87]Sr/[sup.86]Sr values of CLE Member range from 0.707261 to 0.707340. The limestones of Mesa Quemada member show slight variations in [sup.87]Sr/[sup.86]Sr values (0.707221 to 0.707258).

5. Discussion

5.1. Petrography

The limestones from Cerro Pimas section are dominated by wackestone, packstone and grainstone lithofacies. The petrographic study indicates the presence of various cement types such as fibrous calcite, isopachous drusy calcite, equant calcite cement and isopachous blocky sparry calcite cement (Figs. 6A and B). Micritic cement is present in considerable amount suggesting the early diagenesis on the sea floor. Most of the bioclasts show micritic envelope which indicate the large scale micritisation. Many bioclast grains are partly or fully replaced by calcite cement. Some ooid grains exhibit flattened grains and parallel grain contacts which are characteristic features of compaction effect. In certain cases, the nuclei of ooids are partly dissolved that create secondary intraparticle porosity. Some bioclasts exhibits pressure solution features around their boundaries. The limestone also exhibits microstylolite set and the insoluble materials and mineral grains are often concentrated along the stylolite seams (Fig. 6A). The equant calcite cement, isopachous and blocky sparry calcite cement suggest that these limestones were undergone meteroric diagenesis.

[FIGURE 4 OMITTED]

5.2. Oxygen isotope composition

The limestones from the entire section show distinct negative oxygen isotope values (-13.4 to -8.9[per thousand] VPDB) which suggests that these limestones contain diagenetic overprint (Hudson, 1977; Price et al, 2008). The most negative values are observed in the lower part of the Cerro La Ceja and Cerro La Espina Members and upper part of the Mesa Quemada Member. The Tuape Shale and Los Coyotes Members show little fluctuations in the oxygen isotope values whereas the Cerro La Ceja, Cerro La Espina and Mesa Quemada Members exhibit more variations in the oxygen isotope values. The isotope values of limestones from the Cerro Pimas section undoubtedly contain diagenetic overprint. The fluid-rock alterations during metamorphism and diagenesis mainly result in the decreasing [[delta].sup.18][O.sub.carb] values whereas the diagenetic modifications are meager in [[delta].sup.13][C.sub.carb] values (Hudson, 1977; Dickson and Coleman, 1980).

The large variations in [[delta].sup.18]O compositions may be due to fluctuations, possibly related to climate, in local marine compositions. Pore waters in shallow burial environment generally communicate freely with overlying sea water and precipitate early diagenetic calcite which reveals a broad range of [[delta].sup.18]O values. The oxygen isotopic values of a carbonate precipitated from pore water mainly depends on the [[delta].sup.18]O composition and temperature of the water. Decreasing (more negative) [[delta].sup.18]O value is connected with decreasing salinity and increasing temperatures (Hudson, 1977). The range of depleted [[delta].sup.18]O values in many limestones is supportive of cementation under mainly burial and/or meteoric conditions

Diagenesis frequently results in more negative [[delta].sup.18]O values in marine carbonates (Land, 1970; Allan and Matthews, 1977). During diagenesis, the primary calcite may be replaced by calcite precipitated in equilibrium with the diagenetic environment, either during burial or on the sea floor (Fisher et al, 2005). The oxygen isotopes are more susceptible to diagenesis than the carbon isotopes, which is partly due to the temperature-related fractionation seen in oxygen isotopes (Morse and Mackenzie, 1990). The unaltered European chalks have average [[delta].sup.18]O values of -2.9[per thousand], ranging between -2 and -4[per thousand]. The diagenetic alteration of chalks can lead to more negative values, as low as -8[per thousand] (Jorgensen, 1987). The observed low isotopic values, and frequent and rapid fluctuation in the [[delta].sup.18]O profile for the Cerro Pimas section are perhaps consistent with diagenesis of the sediments. Hence, the observed large variations in [[delta].sup.18]O values together with lighter (negative) oxygen isotope values at the Cerro Pimas section were affected by diagenesis shifting the primary signal to more negative values.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

5.3. Carbon isotope composition

The carbonate sediments from the modern marine environment normally have [[delta].sup.13]C values ranging from 0[per thousand] to 4[per thousand] (Hudson, 1977; Moore, 2001). The variation in carbon isotope values in carbonate rocks are mainly influenced by the degree of alteration, and the amount of C[O.sub.2] derived from the oxidation of organic carbon included to the pore waters during limestone cementation as well as recrystallization (Marshall, 1992; Maliva et al., 1995; Maliva and Dickson, 1997). The carbon isotopic variations in pelagic and hemiplegic carbonates have been documented from different locations and time periods (Weissert, 1989; Follmi et al., 1994; Grotsch et al, 1998; Wendler et al, 2009). Short term variations in the [[delta].sup.13]C signals of shallow water carbonates are extensively used to identify the primary variations in the oceanic [[delta].sup.13]C signature of the Early Cretaceous (Jenkyns, 1995; Vahrenkamp, 1996; Grotsch et al. 1998). The carbon isotope curve shows three positive excursions in the lower part of the section whereas two distinct positive excursions are observed in the upper part of the section. Most of the samples from the upper part of the section show positive carbon isotope values. A decrease in [[delta].sup.13]C value is observed only in the top most part of the Cerro Pimas section (CP47: -4.1[per thousand] VPDB). The sample CP47 show more negative value than the limestones collected several meters below this samples. In general, the limestone collected below the subaerial exposure surface has more negative [[delta].sup.13]C values than the limestone deposited several meters below them (Allan and Matthews, 1982). The sample CP47 show more negative value than the limestones collected several meters below this samples (Fig. 4). It suggests that this sample had close contact with subaerial exposure which induced the meteoric diagenetic alterations in the limestone.

[FIGURE 7 OMITTED]

In general, the positive correlation between carbon and oxygen values mainly indicates the infiltration of fluids containing isotopically light carbon and oxygen, such as meteoric water (Hudson, 1977; Allen and Matthews, 1982; Fisher et al, 2005). The absence of such a positive correlation between carbon and oxygen values (r = -0.26; n = 29; the linear correlation coefficient (r) between these elements is not statistically significant, see Verma, 2005, for statistical significance of r values) is clearly seen in the limestones of the Cerro Pimas section (Fig. 7) which suggests that the carbon isotopic values remain unchanged during early or burial diagenesis (e.g., Jenkyns, 1974; 1996; Jenkyns and Clayton, 1986). In addition, Mn/Sr ratio is also useful for the evaluation of diagenetic changes in the carbonate rocks (Kaufman et al., 1993; Kaufman and Knoll, 1995; Jacobsen and Kaufman, 1999). The limestone deposited under marine environment with Mn/ Sr ratios less than 2 indicate that they were not subjected to significant diagenesis (Jacobsen and Kaufman, 1999; Sial et al., 2001; Marquillas et al., 2007; Nagarajan et al., 2008; Kakizaki and Kano, 2009). The observed low Mn/ Sr ratios (0.15 - 1.96; Table 2) in the limestones of Cerro Pimas section indicate that these limestones were least affected during diagenesis. The preservation of carbon isotope values during diagenesis is more common which is due to the buffering effect of carbonate carbon in the diagenetic system (Price et al., 2008).

The [[delta].sup.13]C records of the Cerro Pimas section suggest that the [[delta].sup.13]C values measured are primary in nature and they are suitable for [[delta].sup.13]C stratigraphy. The carbon isotope records of the Mural Formation show prominent positive [[delta].sup.13]C excursion in the Early Albian which show strong similarities to other European, Mexican and Pacific Aptian-Albian sections (Erbacher et al., 1996; Weissert et al, 1998; Bralower et al, 1999; Herrle et al, 2004). We have compared the carbon isotope curve of the Cerro Pimas with those of Scholle and Arthur (1980), Menegatti et al. (1998) and Bralower et al. (1999) to understand the similarities between them. Since, our study area fall between Late Aptian and the Early Albian we have compared our results with the middle part of their curves and identified three identical segments in our isotopic curve (segments C8, C12 and C14). Herrle et al. (2004) proposed a new terminology to describe the long and short term isotopic fluctuations, with the prefix "AP" for the Aptian stage and "Al" for the Albian stage. We have also compared our curve with carbon isotope curves of the Aptian to Lower Albian age of the Vocontian Basin (SE France) and Mazagan Plateau proposed by Herrle et al. (2004) and identified three similar segments i.e. Al1, Al2 and Al5 in our C-isotope curve. In the present study, a distinct positive isotopic excursion followed by the pronounced negative shift of [[delta].sup.13]C values are seen in the Los Coyotes Member (Early Albian age) suggest the presence of global OAE 1b in the Cerro Pimas section of the Mural Formation. The carbon isotope study is well suited for correlation of different marine and terrestrial environments than biostratigraphy because of the synchronicity of carbon isotope indicators in various sediment types (Herrle et al, 2004). Further study such as high-resolution carbon isotope stratigraphy on the carbonate rocks of the Mural Formation would be useful to establish the detailed isotopic curves and various OAEs that occurred during the Lower Cretaceous. The carbon isotope record yields a much higher temporal stratigraphic resolution than the biostratigraphy in the sedimentary rocks of Lower Cretaceous age and also allows more precise relative dating of the regional to global anoxic events which occurred during this period.

5.4 Strontium isotopes

The [sup.87]Sr/[sup.86]Sr ratio of seawater is nearly constant at one particular period in geologic time in the entire ocean because of the long residence time of strontium in the ocean (Burke et al, 1982; Hodell et al, 1989). The strontium isotopes are not fractionated in nature (Faure, 1986), such as during the precipitation of carbonate minerals from aqueous medium. The strontium isotopic composition of past seawater is identified using marine carbonate shells and rocks which served as a reliable proxy for tectonic evolution of the Earth System, because the variations in 87Sr/86Sr ratios suggest the waxing and waning of Sr input from rivers (continental flux) vs. the input from the hydrothermal systems (mantle flux) (Faure, 1986; Taylor and Lasaga, 1999). The [sup.87]Sr/[sup.86]Sr composition of seawater served as a commanding tool for stratigraphic correlations and indirect age assignment, reconstruction of global tectonics, and understanding the diagenetic processes (Burke et al, 1982; Veizer, 1989; McArthur et al, 1990; 1992a,b; 1994; Banner, 2004; Halverson et al, 2007). In addition, a significant amount of seawater-oceanic crust interaction takes at low temperatures that contribute third components like palagonite, smectite and/or carbonates (Jochum and Verma, 1996). The detailed studies on the hydrothermal fluids provide important information regarding the seawater-oceanic crust interaction (Michard and Albarede, 1986; Piepgras and Wasserburg, 1986; Hinkley and Tatsumoto, 1987; Klinkhammer et al., 1994). The [sup.87]Sr/[sup.86]Sr ratio of modern ocean (0.7092) mainly point out a combination of detrital input from continental weathering (0.7120) and hydrothermal alteration of the oceanic crust (0.7035; Davis et al., 2003).

The strontium isotope stratigraphy has been considered as a reliable tool for the dating and stratigraphic correlation (Veizer, 1989; McArthur, 1994; Howarth and McArthur, 1997; McArthur et al, 2000). The strontium isotope compositions of seawater for several periods of the Phanerozoic are well established by many researchers (McArthur et al, 1994; 2000; Howarth and McArthur, 1997; Veizer et al., 1997; 1999). Thus, the strontium isotope stratigraphy plays a vital role in Phanerozoic rocks where the limited availability of biostratigraphic and radiometric dating. It is necessary to find out that the observed [sup.87]Sr/[sup.86]Sr ratios in the ancient limestones are free from diagenetic effects. The variations in trace elements have been considered as one of the useful techniques to identify diagenetic alteration (Brand and Veizer, 1980; Ditchfield et al, 1994; Jones et al, 1994a, 1994b; Price and Sellwood, 1997; Podlaha et al., 1998; Hesselbo et al, 2000; Price et al, 2000; Jenkyns et al, 2002; Grocke et al., 2003). During diagenesis, Mn may be incorporated and Sr may be expelled from the carbonate system (Brand and Veizer, 1980; Veizer, 1983). Thus, the diagenetic alteration of low-Mg calcite generally show low Sr content and high Mn content (Veizer, 1983). However, no significant relationship is observed between Mn and Sr in the limestones of the Mural Formation (r = 0.27; n = 15; Table 2) and the linear correlation coefficient (r) between these elements is not statistically significant (see Verma, 2005, for statistical significance of r values). Hence, the results of [sup.87]Sr/[sup.86]Sr ratios in the present study support the preservation of original strontium isotope compositions.

The [sup.87]Sr/[sup.86]Sr ratio of seawater during the Late Aptian and Early Albian vary between 0.70726 and 0.70740 (Howarth and McArthur, 1997; McArthur et al, 2001). Hence the limestones of the Mural Formation deposited during this period should record the strontium isotope composition of Late Aptian and Early Albian seawater. The [sup.87]Sr/[sup.86]Sr ratio of the limestones from the present study shows similar isotope ratio of the contemporary seawater (0.707221 to 0.707340). Based on the [sup.87]Sr/[sup.86]Sr values, numerical ages were derived using a "look-up" table (Version 4:08/04) provided by Howarth and McArthur (1997) and McArthur et al. (2001). The Cerro La Ceja Member is the basal member of the Mural Formation and its [sup.87]Sr/[sup.86]Sr value of 0.707241 is interpreted to indicate 113.4 Ma (Table 2). Zircon radiometric dating from a tuff bed on the top of the Morita Formation and a few meters below the base of the Cerro La Ceja Member at the Rancho Santa Marta area yielded an age of 115.5 [+ or -] 0.7 Ma (Peryam et al., 2005); hence, the available information suggests that the base of the Cerro La Ceja Member of the Mural Formation in central Sonora is no older than Late Aptian (Gonzalez et al, 2008). The numerical age derived from the whole rocks [sup.87]Sr/[sup.86]Sr value is consistent with the previously published zircon radiometric age. In addition, two numerical ages were derived from the [sup.87]Sr/[sup.86]Sr values of LC Member, i.e. 112.0 Ma and 111.1 Ma indicate the Early Albian age. Likewise, CLE Member also reveals two numerical ages such as 110.9 Ma and 110.7 Ma which also suggest the Early Albian age. The numerical ages derived from the strontium isotope composition on the whole rocks samples of the present study are consistent with the previously published palaeontological and radiometric ages. Furthermore, the strontium isotope study on the limestone samples collected at close interval between the uppermost part of the Tuape Shale Member and the lower most part of the Los Coyotes Member would indicate the Aptian/Albian stage boundary in the Mural Formation of the Bisbee Basin.

6. Conclusions

The limestones collected from Cerro Pimas section of the Mural Formation comprise wackstone, packstone and grainstone lithofacies. The petrographic study reveals the presence of fibrous calcite cement, isopachous drusy calcite, equant calcite cement, isopachous blocky sparry calcite cement and small scale stylolites which indicate that the limestones of the Mural Foramtion were subjected to shallow burial diagenesis. The limestones of the Mural Formation are significantly depleted in [[delta].sup.18]O values compared with the carbonates precipitated in equilibrium with contemporaneous seawater, which indicates that the studied limestones were subjected to shallow burial diagenesis. The oxygen isotope data are consistent with the petrographic information. Even though, the limestones of the Mural Formation were affected by diagenesis, the carbon isotope composition indicate the pristine character that was least affected by diagenesis. Our study provides new information about the C-isotope stratigraphy of the Late Aptian and Early Albian interval in northern Mexico. The shape of the carbon isotope curve of the Cerro Pimas section is comparable with the isotopic curves in Mexico and also other part of the world. The abrupt positive peak in the top most part of the Late Aptian followed by decrease in C-isotope values of similar magnitude in the Early Albian is the characteristic feature of OAE1b. The sedimentary patterns and C-isotope fluctuations in response to oceanic anoxic event in the Early Albian suggest that the observed OAE1b in the Mural Formation confirms the global nature of the event. The [sup.87]Sr/[sup.86]Sr ratio of limestones of the Mural Formation ranges from 0.707221 to 0.707340, which is similar to Late Aptian and Early Albian seawater (0.70726 to 0.70740). The whole rock Sr isotope ages of the Mural Formation is consistent with the previously reported radiometric and biostratigraphic ages.

http://dx.doi.org/10.5209/rev_JIGE.2013.v39.n1.41749

Acknowledgements

The first author thanks Dr. Thierry Calmus, ERNO, Instituto de Geologia, Universidad Nacional Autonoma de Mexico for his support during this work. We would like to thank Dr. Hugh C. Jenkyns, University of Oxford, UK for his useful suggestions and innovative ideas on general problems of isotope studies of Aptian-Albian age. Comments on the earlier version of the manuscript by Dr. Surendra P. Verma are gratefully acknowledged. We would like to thank Dr. A.N. Sial and Prof. P.K. Saraswati for their critical comments which helped us to improve our presentation. We acknowledge the support rendered by Universidad Nacional Autonoma de Mexico through PAPIIT Project No.IN121506-3. We thank Mr. Pablo Penaflor for powdering of limestone samples for isotope analyses. We also thank Ms. Adriana Aime Orci Romero for preparing thin sections for petrographic study. This research was partly supported by Korea Research Foundation (grant 2010-0009765 to YIL).

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J. Madhavaraju (1) *, Yong IL Lee (2), C.M. Gonzalez-Leon (1)

(1) Estacion Regional del Noroeste, Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Apartado Postal 1039 Hermosillo, Sonora 83000, Mexico.

(2) School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Korea

Corresponding author: mj@geologia.unam.mx

Received: 16/05/2011 / Accepted: 12/04/2013
Table 1.-Carbon and oxygen isotopic values for whole rock limestone
samples of Mural Formation.

Tabla 1.- Valores isotopicos de carbon y oxigeno para roca total
de las calizas de la Formacion Mural.

Member/             [[delta].sup.13]C       [[delta].sup.18]O
Sample No         ([per thousand] VPDB)   ([per thousand] VPDB)
Mesa Quemada
CP47                      -4.1                    -11.5
CP46                       1.4                    -13.3
CP45                       2.2                    -12.7

Cerro La Espina
CP43                      -0.1                    -9.6
CP42                       1.4                    -10.0
CP41                       0.4                    -9.5
CP40                       1.4                    -10.6
CP38                       0.6                    -10.5
CP36                       1.2                    -11.1
CP35                       2.2                    -11.3
CP33                       1.8                    -13.0

Los Coyotes
CP29                       0.6                    -10.1
CP28                       1.3                    -9.5
CP26                       1.5                    -9.9
CP25                      -1.5                    -9.4
CP24                      -0.9                    -9.3
CP22                      -2.5                    -8.9
CP20                      -2.3                    -9.4
CP18                      -0.3                    -9.3
CP17                       1.2                    -10.8
CP15                      -1.6                    -9.7
CP14                      -1.5                    -9.3

Tuape Shale
CP12                      -0.4                    -10.0
CP10                      -1.1                    -11.9
CP9                       -2.4                    -10.2
CP6                        0.0                    -11.3

Cerro La Ceja
CP4                        1.3                    -11.2
CP3                        0.2                    -9.9
CP1                       -2.1                    -13.4

Table 2.-Mn, Sr and strontium isotope values for limestones of Mural
Formation. Numerical ages derived after Howrath and McArthur, 1997
and McArthur et al., 2001 (SIS Look/up Table Version 4: 08/04). The
analytical uncertainties mentioned for individual measurements  are
two times the standard error of the mean ([2s.sub.E]). (1) Numerical
ages reported in the table include lower age limit (>), mean age and
upper age limit (<) and the limiting ages are presented at 95%
confidence interval. (2) Data from Madhavaraju et al. (2010).

Tabla 2.-Valores de Mn, Sr e isotopos de Sr para las calizas de la
Formacion Mural. Las edades numericas estan derivadas de Howrath and
McArthur (1997) y McArthur et al. (2001) (SIS Look/up Table Version
4: 08/04). Las incertidumbres analiticas mencionadas para las
mediciones individuales son el doble del error estandar de la media
([2s.sub.E]). (1) Las edades numericas reportadas en la tabla
incluyen el limite de la edad minima (>), la edad media y el limite
de la edad maxima (<). El intervalo entre edad minima y maxima tiene
una certidumbre de 95%. (2) Datos tomados de Madhavaraju et al.
(2010).

Member/ Sample No   [sup.86]Sr/[sup.87]Sr          Age (Ma) (1)
                    [+ or -] [2s.sub.E]

Mesa Quemada
CP47                0.707258 [+ or -] 12
CP45                0.707221 [+ or -] 12
Cerro La Espina
CP42                0.707326 [+ or -] 12     > 110.80   111.06
CP40                0.707261 [+ or -] 11     > 111.00   111.25
CP38                0.707340 [+ or -] 11     > 110.38   110.67
CP36                0.707307 [+ or -] 11     > 111.32   111.56
CP35                0.707333 [+ or -] 11     > 110.58   110.87
CP33                0.707323 [+ or -] 12     > 110.89   111.14
Los Coyotes
CP29                0.707314 [+ or -] 11     > 111.14   111.38
CP28                0.707274 [+ or -] 10     > 112.12   112.37
CP26                0.707325 [+ or -] 11     > 110.83   111.09
CP17                0.707290 [+ or -] 14     > 111.75   111.98
Tuape Shale
CP10                0.707336 [+ or -] 12     > 110.49   110.79
Cerro La Ceja
CP4                 0.707242 [+ or -] 12     > 113.04   113.40
CP3                 0.707241 [+ or -] 11     > 113.07   113.43

Member/ Sample No   Age (Ma) (1)   Mn (ppm) (2)   Sr (ppm) (2)   Mn/Sr

Mesa Quemada
CP47                               618            425            1.45
CP45                               387            631            0.61
Cerro La Espina
CP42                < 111.30       1060           540            1.96
CP40                < 111.48       752            401            1.88
CP38                < 110.94       931            544            1.71
CP36                < 111.79       697            529            1.32
CP35                < 111.12       640            511            1.25
CP33                < 111.38       612            746            0.82
Los Coyotes
CP29                < 111.61       644            486            1.33
CP28                < 112.62       457            540            0.85
CP26                < 111.32       620            428            1.45
CP17                < 112.23       492            518            0.95
Tuape Shale
CP10                < 111.05       929            1116           0.83
Cerro La Ceja
CP4                 < 113.81       75             510            0.15
CP3                 < 113.84       542            391            1.39
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Title Annotation:articulo en ingles
Author:Madhavaraju, J.; Lee, Yong I.L.; Gonzalez-Leon, C.M.
Publication:Journal of Iberian Geology
Date:Jan 1, 2013
Words:9874
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