Carnets Geol. 15 (10)  

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Contents

[1. Introduction] [2. Geology of the study area] [3. Methodology] [4. Results]
[5. Discussion] [6. Chronostratigraphy and isotopic evolution]
[7. Conclusions] and ... [Bibliographic references]


Facies, biostratigraphy, diagenesis, and depositional environments
of Lower Cretaceous strata,
Sierra San José section, Sonora (Mexico)

Jayagopal Madhavaraju

Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Hermosillo, Sonora 83000 (México)

Robert W. Scott

Precision Stratigraphy Associates & University of Tulsa, 149 West Ridge Road, Cleveland Oklahoma 74020 (USA)

Yong Il Lee

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

Kunjukrishnan Sathy Bincy

Department of Geology, University of Madras, Chennai 600 025 (India)

Carlos M. González-León

Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Hermosillo, Sonora 83000 (México)

Sooriamuthu Ramasamy

Department of Geology, University of Madras, Chennai 600 025 (India)

Published online in final form (pdf) on July 14, 2015
[Editor: Bruno Granier]

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Abstract

We used petrofacies analysis, carbon, oxygen and strontium isotope data to interpret the isotopic variations in the carbonate rocks of the Mural Formation of Sonora (Sierra San José section), Mexico. The petrographic study reveals a range of lithofacies from wackestone to packstone. The analyzed limestones show significant negative δ18O values (-18.6 to -10.9 VPDB) and δ13C values ranging from negative to positive (-2.6 to +2.5‰ VPDB). The absence of correlation between δ13C and δ18O values suggests a primary marine origin for the δ13C values of limestones from the Sierra San José section. The limestones have large variations in 87Sr/86Sr values (0.707479 to 0.708790). Higher 87Sr/86Sr ratios in various levels of the studied section suggest that most of the sediments were derived from the Proterozoic basement of the Caborca block during Early Cretaceous time. A decrease in 87Sr/86Sr ratios at certain levels indicates an influx of lesser amounts of radiogenic Sr that could have been caused by contribution of sediments from the Triassic and Jurassic volcanic rocks.

Key-words

Aptian-Albian stages; Mexico; Mural Formation biostratigraphy; stable isotopes; strontium isotopes.

Citation

Madhavaraju J., Scott R.W., Lee Y.I., Bincy K.S., González-León C.M. & Ramasamy S. (2015).- Facies, biostratigraphy, diagenesis, and depositional environments of Lower Cretaceous strata, Sierra San José section, Sonora (Mexico).- Carnets Géol., Madrid, vol. 15, nº 10, p. 103-122.

Résumé

Faciès, biostratigraphie, diagenèse et environnements de dépôt des couches du Crétacé inférieur d'une coupe de la Sierra San José, Sonora (Mexique).- Nous avons utilisé l'analyse pétrofaciologique et les données isotopiques du carbone, de l'oxygène et du strontium pour interpréter les variations isotopiques enregistrées par les roches carbonatées de la Formation Mural (coupe de la Sierra San José) de Sonora, Mexique. Il ressort de l'étude pétrographique que les lithofaciès ont des textures soit de type wackestone, soit de type packstone. Les calcaires analysés présentent des valeurs fortement négatives du δ18O (comprises entre -18,6 et -10,9 VPDB) alors que celles du δ13C varient de négatives à positives (de -2,6 à +2,5‰ VPDB). L'absence de corrélation entre les valeurs du δ13C et celles du δ18O suggère une origine marine primaire pour les valeurs du δ13C des calcaires de cette coupe de la Sierra San José. Les calcaires analysés ont également enregistré de fortes variations des valeurs du rapport 87Sr/86Sr (de 0,707479 à 0,708790). Les rapports 87Sr/86Sr les plus forts rencontrés à différents niveaux de la coupe étudiée suggèrent qu'au Crétacé inférieur la plus grande partie des apports sédimentaires provienne du substratum protérozoïque du bloc Caborca. Une baisse dans les rapports 87Sr/86Sr de certains niveaux révèle des apports moindres en Sr radiogénique qui pourraient, cette fois-ci, être liés à une contribution de sédiments dérivés de roches volcaniques triasiques et jurassiques.

Mots-clefs

Étages Aptien-Albien ; Mexique ; biostratigraphie de la Formation Mural ; isotopes stables ; isotopes du strontium.


1. Introduction

The stable isotopic compositions along with petrographic information of carbonate rocks may prove to be an important tool in tracing the fluid origin and in reconstructing large-scale movements and evolution of fluids (Allan and Matthews, 1982). The carbon and oxygen isotopic composition of the carbonate sediments/rocks reflect the physicochemical properties of the waters, in which the sediment-contributing organisms grow (Morrison and Brand, 1986) and also provides information regarding the diagenetic processes and environments, which initiate the conversion of skeletal carbonates into limestones (Jenkyns et al., 1994). The carbon and oxygen isotopic composition of carbonate rocks also provides valuable information regarding the temperature of deposition (Ali, 1995; Coniglio et al., 2000), source of carbonate (Hudson, 1977; Gao et al., 1996; Kumar et al., 2002; Poulson and John, 2003), and/or palaeoclimate (Quade and Cerling, 1995; Srivastava, 2001; Scott, 2002).

Carbonate rocks deposited in marine environments mainly record the carbon isotopic composition of the ocean water (Scholle and Arthur, 1980). Similarly, oxygen isotope studies from foraminifers and the paleobotanical record provide strong evidence that the Cretaceous Period was substantially warmer than today (Crowley and North, 1991; Steuber et al., 2005). Paleoclimatic conditions for a given region can be determined by studying temporal changes of meteoric diagenesis within a single lithology, particularly limestone, and the geochemical signature of the associated diagenetic products (James and Choquette, 1984). Isotopic studies on shallow marine Lower Cretaceous carbonate rocks have shown evidence of paleoceanographic processes (Kumar et al., 2002; Madhavaraju et al., 2004), climatic and biotic changes (Deshpande et al., 2003; Mishra et al., 2010; Préat et al., 2010; Tewari et al., 2010) and global-scale tectonics (Gröcke et al., 2005; Maheshwari et al., 2005; Amodio et al., 2008).

Lower Cretaceous Bisbee Group sedimentary rocks are well exposed in northern Sonora, Mexico. Extensive research activities have been undertaken on the Bisbee Group by various paleontologists and stratigraphers during the past decades, but detailed isotopic studies on the carbonate rocks are scanty and to date few studies have been undertaken on specific sections of the sedimentary rocks of the Mural Formation. Madhavaraju et al. (2013a, 2013b) carried out carbon, oxygen and strontium isotope studies on the limestones collected from the Cerro Pimas section (proximal part of the Bisbee basin) and Cerro El Caloso Pitaycachi section (distal part of the basin) to understand the paleoceanographic changes that occurred during the Early Cretaceous Epoch. Here we present carbon, oxygen and strontium isotope data from the northeastern part of the Bisbee basin exposed in the Sierra San José (Fig. 1 ). The objectives of the present study are: 1) To study the diagenetic changes in the carbon and oxygen isotopes; 2) to compare the carbon isotope variation of this section with that of the Cerro Pimas and Cerro El Caloso Pitaycachi sections in Sonora (Madhavaraju et al., 2013a, 2013b); 3) to identify strontium isotopic variations in these carbonate rocks; and 4) to assess the probable reasons for the fluctuations in 87Sr/86Sr ratios.

Fig. 1
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Figure 1: Location map of the Sierra San José section of the Mural Formation.

2. Geology of the study area

The Lower Cretaceous Bisbee Group is well exposed in the north-central part of Sonora, Mexico, and has similar stratigraphic characteristics and is correlative with similar rocks exposed in southern Arizona and New Mexico in the United States of America (Ransome, 1904; Cantu-Chapa, 1976; Bilodeau and Lindberg, 1983; Mack et al., 1986; Dickinson et al., 1989; Jacques Ayala, 1995; Lawton et al., 2004). The sedimentary rocks of the Bisbee Group consist of four formations: Glance Conglomerate, Morita Formation, Mural Formation, and Cintura Formation. The Glance Conglomerate mainly consists of cobbles and boulders of metamorphic and granitic rocks locally interbedded with volcanic flows and tuffs that represent syntectonic rift deposits (Bilodeau et al., 1987). The Morita and Cintura formations include reddish brown siltstone and lenticular beds of arkose and feldspathic arenite (Klute, 1991) deposited in fluvial environments. Fossiliferous clastic and carbonate units of the Mural Formation overlie the Morita Formation and represent major Aptian-Albian marine transgression (Scott, 1987). Lawton et al. (2004) defined six members in the Mural Formation of Sonora (Fig. 2 ): Cerro La Ceja, Tuape Shale, Los Coyotes, Cerro La Puerta, Cerro La Espina, and Mesa Quemada members. These members are laterally persistent from northeastern to northwestern Sonora, in a 300 km-long transect showing only minor facies changes through several measured sections (González-León et al., 2008).

Fig. 2
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Figure 2: Lithostratigraphic section of the Mural Formation in Sierra San José area (modified after González-León et al., 2008). Member designations: CLC - Cerro La Ceja; TS - Tuape Shale; LC - Los Coyotes; CLP - Cerro La Puerta; CLE - Cerro La Espina; MQ - Mesa Quemada. San José outcrop sample numbers are Sj, which for thin sections and geochemical samples were changed to SSJ.
Base of section at UTM 597021; 3455384; elevation 1761 m above SL.

The Mural Formation exposed in the Sierra San José was deposited in the Bisbee Basin, which extended southeastward into the Chihuahua Trough (Fig. 1 ) (Lawton et al., 2004). The Sierra San José section spans from the basal Cerro La Ceja Member of the Mural Formation to the Mesa Quemada Member at the top (Fig. 2 ) (González-León et al., 2008; Madhavaraju et al., 2010). Previous studies demonstrated that the formation ranges from upper Aptian to lower Albian. Prominent age-diagnostic fossils present in thin sections of the Sierra San José outcrop are benthic foraminifera; calcareous algae are rarely present but invariably long ranging. This low-diversity biota is consistent with the published late Aptian to early Albian age of strata in this section (Lawton et al., 2004; González-León et al., 2008).

3. Methodology

Carbon and oxygen isotopic compositions were analyzed for eighteen samples using a Prism series II model mass spectrometer at Korea Basic Science Institute. The limestone samples were treated with H3PO4 in vacuum at 25°C and the resulted CO2 gas analyzed following the standard method of McCrea (1950). Results are reported in the standard per mil (‰) δ-notation relative to the Pee Dee Formation Belemnite (V-PDB) marine carbonate standard. Sample reproducibility is better than ±0.05‰ for carbon and ±0.1‰ for oxygen.

Eighteen whole rock samples were analyzed for Sr isotope composition using a VG 54-30 thermal ionization mass spectrometer equipped with nine Faraday cups at Korea Basic Science Institute. Several 10 mg of whole-rock powders were mixed with highly enriched 84Sr and 87Rb spikes and then dissolved with a HF/HClO4 acid (10:1) in Teflon vessels. Rb and Sr fractions were separated by conventional cation column chemistry (Dowex AG50W-X8, H+ form) in HCl medium. Instrumental fractionation was normalized to 86Sr/88Sr = 0.1194 and the measured 87Sr/86Sr ratios were further corrected for the contributions of the added spikes. Replicate analysis of NBS 987 gave a mean 87Sr/86Sr of 0.7102450 ± 0.000003 (n = 30, 2σ SE). Total procedural blank levels were below 100 pg for Sr. The 87Sr/86Sr ratios are presented after adjusting them to NBS 987 87Sr/86Sr ratio of 0.710230 (Verma, 1992; Verma and Hasenaka, 2004).

4. Results

4.a. Biostratigraphy

Few short-ranging taxa are identifiable in the thin sections from our measured section at Sierra San José (Fig. 3 ). The new data do not alter previously published Aptian-lower Albian correlations of these formations. A benthic foraminifer in the Cerro La Espina Member of the Mural Formation, Paracoskinolina sunnilandensis (Maync) (Fig. 3.6-7 ), is a lower Albian species, reported elsewhere in the Bisbee Basin (Lawton et al., 2004; González-León et al., 2008), which is common in the Trinity Group in Texas (Scott et al., 2003). Specimens of Mesorbitolina most likely are M. texana (Roemer), which in the Trinity Group ranges in age from 113.70-108.19 Ma (Scott, 2014). However, the key protoconch and deuteroconch structures necessary to identify the species were not present in the available thin sections. The benthic foraminifer, Buccicrenata subgoodlandensis (Vanderpool), is rare in the Los Coyotes Member and is characteristic of the Trinity and Fredericksburg groups.

A small, conical benthic foraminifera in the Cerro La Puerta and Cerro La Espina members is Novalesia producta (Magniez) (Fig. 3.1-5 ), which ranges from late Aptian to early Albian (Arnaud-Vanneau and Sliter, 1995). The biserial genus Novalesia differs from the biserial cuneolinid genus Vercorsella by its conical test, by its thin radial septa that do not join the median septum, and by the slit-shaped aperture (Loeblich and Tappan, 1988). Novalesia was widespread in the Tethys from Spain, France and the Pacific seamounts (Arnaud-Vanneau and Sliter, 1995). Caprinid fragments of partial valve margins in the Los Coyotes Member have elongate oval pallial canals that diverge at the outer shell layer. The canals of these fragments are similar to those of Coalcomana, which is the distinctive and common genus in early Albian strata (Scott and Filkorn, 2007). However complete specimens are needed to verify the identification.

A single planktic foraminifer specimen in the Cerro La Puerta Member is tentatively identified as Clavihedbergella sp., because of its axial profile (Fig. 3.8 ). This genus ranges from Barremian-Aptian to Coniacian (Loeblich and Tappan, 1988; Scott, 2014). To confirm the identification of the genus and species a transverse view showing the whorl expansion is needed. Clavihedbergella simplex (Morrow) is present in the upper Aptian-lower Albian interval (113.43-86.83 Ma; Scott, 2014).

Fig. 3
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Figure 3: Microfossils; scale bar = 0.1 mm/100 microns.
1-5. Novalesia producta (Magniez); 1-3, Cerro La Puerta Member, sample SSJ10 (9-15-1);
4-5, Cerro La Espina Member, 4, sample SSJ10; 5, sample SSJ19 (9-15-10);
6-7, Paracoskinolina sunnilandensis (Maync), Cerro La Espina Member, sample SSJ16 (9-15-7);
8, Clavihedbergella sp., Cerro La Puerta Member, sample SSJ10 (9-15-1), note aperture at base of final chamber.

4.b. Lithofacies and depositional environments

Petrographic data are from eighteen thin section samples of members of the Mural Formation (Table 1). The members of the Mural represent two longer-term depositional cycles: 1) the Cerro La Ceja, Tuape Shale and Los Coyotes cycle and 2) the Cerro La Puerta, Cerro La Espina and the Mesa Quemada cycle (Fig. 2 ) (Lawton et al., 2004; González-León et al., 2008). The overlying Cintura Formation represents a major shift in basin deposition from mixed carbonate and siliciclastic sediment to dominantly siliciclastic sediment.

The Cerro La Ceja Member at the base of the section is composed of shale and thin-bedded sandstone and limestone (Table 1; Fig. 4.1 ). The samples are sandy bioclastic mudstone and wackestone with phosphate nodules and chert grains. Fine-grained quartz grains are subangular to subrounded. The biota consists of indeterminate bivalves, foraminifers, echinoderms, and ostracodes. Wavy stylolites suggest burial to moderate depths. These strata were deposited on a nearshore shallow shelf during transgression (Lawton et al., 2004). The Tuape Shale is dominantly shale with thin beds of limestone (Table 1; Fig. 4.2 ). The main limestone facies are silty lime wackestone and mudstone with chert nodules. Quartz grains are angular silt to fine sand. The biota consists of indeterminate bivalves, oyster, ostracodes, and foraminifera. Wavy stylolites are crosscut by fractures. This unit was deposited on a deep offshore shelf near local biotic buildups during maximum flooding. Farther southeast in the basin euxinic conditions prevailed (Lawton et al., 2004). Two samples of the Los Coyotes Member are composed of bioclastic calcareous sandstone and bioclastic packstone microfacies (Table 1; Fig. 4.3 ). The sparse biota is composed of indeterminate bivalves, foraminifera and echinoids. Wavy stylolites are cut by fractures. The depositional environment of the Cerro Los Coyotes Member was a shallow shelf complex with buildups and shoaling-up small-scale bed cycles during highstand conditions. Paleobathymetry was mainly within the local photic zone. This member records progradation and shoaling (Lawton et al., 2004).

The Cerro La Puerta Member is represented by orbitolinid wackestone-packstone (Table 1; Fig. 4.4 ). Biota is composed of indeterminate bivalves, gastropods, foraminifers including Mesorbitolina, Novalesia, Lenticulinids, the planktic foraminifera Clavihedbergella, and encrusting foraminifera, and ostracodes. Subparallel fracture sets are filled with calcite. Deposition was on offshore shallow shelf and represents flooding during a second depositional cycle (Lawton et al., 2004). The Cerro La Espina Member is composed of multiple facies in vertical succession from base to top: orbitolinid wackestone, caprinid wackestone-packstone, caprinid-algal boundstone, sandy coral-caprinid packstone, and capped by mollusk wackestone (Table 1; Fig. 4.5-10 ). Caprinids are common to abundant; colonial corals are common in one sample; calcareous algae and stromatoporids are rare; associated biota consists of echinoderms, benthic foraminifera, and ostracodes. Peloids are common in the Cerro La Espina Member; quartz sand is fine to medium, subangular to subrounded. Locally poikilotopic calcite surrounds echinoderm parts. Chert nodules and quartz overgrowths in optical continuity with chert are rare. Opaque grains of hematite and possible organic matter are rare in the matrix. Multiple sets of subparallel fractures are filled with calcite. The orbitolinids in the basal Cerro La Espina Formation are flat and the diameter-to-height ratio averages 4.56, which is greater than those in the Cerro Pimas section. This shape is consistent with deposition upon a deeper inner shelf environment. From base to top of the Cerro La Espina deposition shoaled during highstand of the second long-term depositional cycle (Lawton et al., 2004). A single thin section of the Mesa Quemada Member is silty mudstone with rare miliolids and ostracodes. Silt-sized to fine-grained quartz is present; secondary chert partly replaces some bioclasts. Deposition was on shallow nearshore shelf; in other parts of the basin this member represents complex environments (Lawton et al., 2004).

Table 1: Petrographic check-list of thin sections.

Fig. 4
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Figure 4: Mural Formation lithofacies and diagenetic features. Scale bar = 1 mm.
1.   Cerro La Ceja Member, sandy bioclastic wackestone, SSJ3 (9-14-3);
2.   Tuape Shale, silty wackestone, note recrystallized bivalve bioclasts, SSJ5 (9-14-5);
3.   Los coyotes Member, silty bioclastic packstone, note calcite-filled fracture and recrystallized bivalve bioclasts, SSJ7 (9-14-7);
4.   Cerro La Puerta Member, orbitolinid packstone, note calcite-filled fracture, SSJ11 (9-15-2);
5-10. Cerro La Espina Member, SSJ13-16, 19-20, 23-25 (9-15-4, -7, -10, -11, -14, -16);
5.   Orbitolinid wackestone, note fracture set filled with secondary calcite,SSJ13 (9-15-4);
6.   Caprinid packstone, oblique cross section of caprinid pallial canals, SSJ16 (9-15-7);
7.   Caprinid packstone, Paracoskinolina sunnilandensis (Maync) among bioclasts, SSJ19 (9-15-10);
8.   Caprinid-algal boundstone, colonial coral encrusted by multiple algal laminae, SSJ20 (9-15-11);
9.   Sandy coral-caprinid packstone; note encrusted colonial coral, SSJ23 (9-15-14);
10.  Mollusk wackestone, note gastropod, SSJ25 (9-15-16).

Fig. 5
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Figure 5: Carbon and oxygen isotope (‰ VPDB) curves of the Sierra San José section of the Mural Formation. CC is Cerro La Ceja Member (CLC) and CE is Cerro La Espina Member (CLE). Arrows point to excursions; see text for explanation.

Fig. 6
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Figure 6: δ13C-δ18O bivariate plot for limestones from the Sierra San José section of the Mural Formation. Carbon is X-axis, oxygen is Y-axis. Colored letters designate members.

4.c. Carbon and oxygen isotopic variations

The analyzed samples show large variations in carbon and oxygen isotope values (Table 2). The δ18O values range from -17.9 to -16.29‰ for the Cerro La Ceja member (CLC) (Table 2). The δ18O values of limestone in the Tuape Shale (TS) and Los Coyotes (LC) members vary little (-15.5 to -15.0‰; -14.7 to -13.8‰; respectively). The Cerro La Puerta (CLP) member has negative δ18O values from -15.5 to -13.8‰. The Cerro La Espina (CLE) member shows large variations in δ18O values (-18.6 to -10.9‰; Fig. 5 ). The Mesa Quemada (MQ) member also shows significant negative oxygen isotope values (-13.6‰).

Table 2: CaCO3 a, trace elements a (Sr, Mn) and carbon and oxygen isotope values for limestones of the Mural Formation.

Member/ Sample No   CaCO3 Mn Sr Mn/Sr δ13C b δ18O b Z value c
Sierra San Jose Section              
Mesa Quemada
SSJ27
87.61 3407 983 3.47 -1.3 -13.6 117.87
Cerro La Espina
SSJ25
SSJ23
SSJ21
SSJ20
SSJ19
SSJ18
SSJ16
SSJ13
 
92.36
90.41
93.80
94.15 95.80
97.46 93.71
95.20
 
541
150
74
80
86
77
77
94
 
313
390
439
410
425
407
434
395
 
1.73
0.38
0.17
0.20
0.20
0.19
0.18
0.24
 
2.5
1.2
1.3
1.5
2.2
2.3
2.0
1.7
 
-18.3
-10.9
-12.7
-14.8
-14.8
-15.4
-15.6
-18.6
 
123.31
124.33
123.64
123.00
124.44
124.34
123.63
121.52
Cerro La Puerta
SSJ11
SSJ10
SSJ9
 
92.28
93.82
93.28
 
542
154
155
 
827
849
673
 
0.66
0.18
0.23
 
2.4
3.2
2.9
 
-15.5
-15.4
-13.8
 
124.40
126.18
126.37
Los Coyotes
SSJ7
SSJ6
 
91.41
64.67
 
618
690
 
1183
753
 
0.52
0.92
 
1.1
-1.5
 
-13.8
-14.7
 
122.68
116.91
Tuape Shale
SSJ5
SSJ4
 
82.32
88.77
 
620
697
 
699
520
 
0.89
1.34
 
-2.5
0.4
 
-15.5
-15.0
 
114.46
120.65
Cerro La Ceja
SSJ3
SSJ2
 
66.53
58.51
 
2246
542
 
528
481
 
4.25
1.13
 
-2.6
-0.5
 
-16.2
-17.9
 
113.91
116.97

a Data from Madhavaraju et al. (2010)
b Present study
c Z = a(δ13C + 50) + b(δ18O + 50); a = 2.048, b = 0.498

The δ13C values in the CLC member are significantly negative to slightly positive (-2.6 to +0.5‰; Table 2). The TS and LC members exhibit both negative and positive carbon isotope values (-2.5 to +0.4‰; -1.5 to +1.1‰; respectively). The CLP member shows positive carbon isotope values (+2.4 to +3.2‰; Figs. 5 - 6 ). Likewise, limestone in the CLE member also has positive carbon isotope values (+1.2 to +2.5‰). The lone sample from the MQ member has a negative carbon isotope value (-1.3‰).

The strontium isotope composition of limestone of the Mural Formation is given in Table 3. The 87Sr/86Sr values of CLC Member vary from 0.708240 to 0.708320. The TS Member shows large variations in 87Sr/86Sr values (0.708196 to 0.708790; Table 3). The 87Sr/86Sr values of LC Member vary between 0.707853 and 0.708078. The 87Sr/86Sr values of CLP Member vary from 0.707634 to 0.707880. Limestone in the CLE Member has large variations in 87Sr/86Sr values (0.707479 to 0.708432). The MQ Member also has a higher 87Sr/86Sr value (0.707947) than the contemporary early Albian seawater values.

Fig. 7
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Figure 7: Chronostratigraphic interpretation of the Sierra San José section. Biostratigraphic data are from thin sections SSJ2 to SSJ27. The fossil ranges in the section are compared to their ages in the database CRETCSDB2, which is an earlier version of CRETCSDB4 (Scott, 2014). The FO and LO of Novalesia producta is projected into the LOC (dotted lines). The position of the Aptian/Albian boundary is uncertain but falls within the stratigraphic interval of the gray box.

Table 3: Strontium isotope values for limestones of the Mural Formation.

Member/
Sample No
86Sr/87Sr
± 2 s.d
(x 10-6)
86Sr/87Sr
mean
Mesa Quemada
SSJ27
 
0.707947
 
19
0.707947
Cerro La Espina
SSJ25
SSJ23
SSJ21
 
0.707479
0.707653
0.707743
 
11
11
14
0.707625
 SSJ20
SSJ19
SSJ18
SSJ16
SSJ13
0.708432
0.708166
0.708429
0.708105
0.707805
14
14
12
14
12
0.708187
Cerro La Espina
SSJ11
SSJ10
SSJ9

0.707880
0.707652
0.707634
 
11
14
15
0.707722
Los Coyotes
SSJ7
SSJ6

0.707853
0.708078

10
15
0.707966
Tuape Shale
SSJ5
SSJ4

0.708196
0.708790

11
18
0.708493
Cerro La Ceja
SSJ3
SSJ2

0.708240
0.708320
 
11
11
0.708280

5. Discussion

5.a. Carbon and oxygen isotope composition

The limestones from the Sierra San José section show negative oxygen isotopic values (-18.6‰ to -10.9‰ VPDB) (Fig. 6 ). The lower part of the section shows slight variations in isotopic values whereas the middle part of the section shows large fluctuations. Likewise, the upper part of the section also exhibits more variations in the oxygen isotope values. The most negative values are observed in the lower part of the Cerro La Ceja and Cerro La Espina members and middle part of the Mesa Quemada Member. The lowest δ18O value (-18.6‰) occurs in the lower part of the CLE Member probably related to a sudden change in the sedimentation conditions and also due to the effects of early meteoric diagenesis (e.g., -2‰ to -15‰, Dickson, 1992). Marine limestones affected by diagenesis in general possess more negative δ18O values (Morse & Mackenzie, 1990; Land, 1970) because cementation and/or re-crystallization commonly takes place in fluids depleted in δ18O with respect to sea water (e.g., meteoric water) or at elevated temperatures (burial conditions).

Carbon isotopes are less affected by diagenetic alteration than oxygen isotopes (Hudson, 1977; Banner and Hanson, 1990; Marshall, 1992; Frank et al., 1999), because of the buffering effect of carbonate carbon in the diagenetic system (Price et al., 2008). The correlation of δ13C and δ18O values of Mural Limestone in the Sierra San José section is not statistically significant (r = 0.01, n = 18; lack of statistically significant correlation (Verma, 2005) (Fig. 5 ) indicates a lack of diagenetic influence on the carbon isotopic signatures (e.g., Jenkyns, 1974; Jenkyns and Clayton, 1986)

Another test of the diagenetic alteration of limestone is by the following equation: where Z = a (δ13C + 50) + b (δ18O + 50), in which a and b are 2.048 and 0.498 respectively (Keith and Weber, 1964). In this study of Mural Limestone the Z value discriminates between marine and freshwater limestone. Limestones with Z values above 120 are considered marine, whereas those with Z values below 120 would be classified as freshwater type. In the present study, the majority of limestones have Z values above 120, whereas few exhibit Z values below 120. This measure further supports that these limestones were least altered during diagenesis.

The carbon isotope curve shows two negative excursions in the lower part and one negative isotopic excursion in the upper part of the Sierra San José section. Most of the samples from the middle and upper part of the section show positive carbon isotope values (Fig. 6 ). An abrupt decrease in δ13C value is observed in the uppermost part of the section (SSJ27: -1.3‰ VPDB). Sample SSJ27 has a more negative value than limestone collected several meters below this samples (Fig. 6 ). Negative values of δ13C are mainly due to biogenic production of CO2 in the soil (Cerling and Hay, 1986) and indicate subaerial exposure, because of incorporation of lighter carbon isotope from soil-borne carbon dioxide and decay of terrestrial matter (Hudson, 1977).

The positive isotopic excursion observed in the middle and upper parts of the Sierra San José section indicates the increasing impact of primary production in the photic zone, with associated organic burial rates exceeding those of its oxidative mineralization of organic matter (Kump and Arthur, 1999). Variations in the δ13C signatures of shallow marine carbonates are widely used to interpret the primary variations in seawater δ13C during the Early Cretaceous (Jenkyns, 1995; Vahrenkamp, 1996; Grötsch et al., 1998; Granier, 2012, 2014). The δ13C values, of the present study suggests that the δ13C values measured from the Sierra San José section correspond to original seawater composition (mainly above 0 and below +3‰; Föllmi et al., 1994, 2006; Menegatti et al., 1998; Bralower et al., 1999; Herrle et al., 2004; Wissler et al., 2004). In addition, lack of correlation between δ13C and δ18O and the environmental significance of Z values also suggest that Mural limestones from the Sierra San José section exhibit primary carbon isotope signatures. Hence, most carbon isotope data of bulk rocks from the Sierra San José section of the Mural Formation are comparable with the published values of late Aptian - early Albian age.

5.b. Strontium Isotopes

The strontium isotopic record provides possible constraints on the importance of various factors that affect global weathering, rates and rock types being weathered, such as orogenic events (Edmond, 1992) and glacial activity (Hodell et al., 1989), and the relative significance of postulated changes in mid-ocean ridge hydrothermal output (Rea, 1992). The 87Sr/86Sr composition of seawater served as an important tool for stratigraphic correlations and indirect age assignment, reconstruction of global tectonics, and understanding the diagenetic processes (Burke et al., 1982; Veizer, 1989; Veizer et al., 1997, 1999; McArthur et al., 1990; 1992a, 1992b, 1994, 2000; Howarth & McArthur, 1997; Halverson et al., 2007). The 87Sr/86Sr ratio of modern oceans (0.7092) is mainly a combination of detrital input from continental weathering (0.7120) and hydrothermal alteration of the oceanic crust (0.7035; Edmond, 1992).

Overall, 87Sr/86Sr values of limestone in the Mural Formation at the Sierra San José section vary greatly from 0.707479 to 0.708790 (Table 3). Such large variations in 87Sr/86Sr values in limestone are largely controlled by diagenesis, hydrothermal input and riverine sources (relative proportion of young vs old silicate rocks undergoing weathering, Taylor and Lasaga, 1999).

5.c. Implication for Diagenesis

The trace elements variations have been considered to be an important tool to identify diagenetic alteration of ancient carbonate rocks (e.g., Brand and Veizer, 1980; 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; Madhavaraju et al., 2013a, 2013b). Mn may be incorporated and Sr may be expelled from the carbonate system during diagenesis (Brand and Veizer, 1980; Veizer, 1983). The Mn/Sr ratio is useful to understand the diagenetic changes in the carbonate rocks (Kaufman et al., 1993; Kaufman and Knoll, 1995; Jacobsen and Kaufman, 1999). Marine limestones with Mn/Sr ratios less than 2 indicate that those limestones were least altered by diagenesis (Jacobsen and Kaufman, 1999; Sial et al., 2001; Marquillas et al., 2007; Nagarajan et al., 2008; Kakizaki and Kano, 2009).

In the present study, the limestones of the Mural Formation have higher 87Sr/86Sr ratios than the contemporary Aptian-Albian seawater. Such elevated ratios in carbonate rocks may be influenced by diagenetic modifications. However, most of the studied samples have low Mn/Sr ratios less than 2 (Table 2) suggesting that the higher isotopic ratios have not been diagenetically altered.

5.d. Implications for Hydrothermal Input

The 87Sr/86Sr composition of seawater serves as an important tool for stratigraphic correlations and indirect numerical age assignments, reconstruction of global tectonics, and understanding diagenetic processes (Burke et al., 1982; Veizer, 1989; McArthur et al., 1990; 1992a, 1992b, 1994; Halverson et al., 2007). In addition, a significant amount of seawater-oceanic crust interaction takes place at low temperatures that contribute third components such as palagonite, smectite and/or carbonates (Jochum and Verma, 1996). Detailed studies of 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).

Sr that enters the ocean from hydrothermal systems along mid-ocean ridges has an initial 87Sr/86Sr ratio of 0.7027 (e.g., Allegre et al., 1983). The exact Sr-isotope evolution of MORB-source mantle is unknown, but hydrothermal alteration of oceanic crust results in a ratio of 0.7035 (Davis et al., 2003). The average composition of continental crust has not changed greatly since 3.7 Ga (e.g., Condie, 1993). The phenomenon is better interpreted to reflect a change in the ratio between hydrothermal and continental flux to the oceans, that is, the flux ratio (Derry and Jacobsen, 1988).

The 87Sr/86Sr ratio of modern oceans (0.7092) is mainly a mixture of detrital input from continental weathering (0.7120) and hydrothermal alteration of the oceanic crust (0.7035; Davis et al., 2003). The lower limit of the 87Sr/86Sr ratio is higher than the contemporary Early Albian (0.7073) seawater as well hydrothermal flux. Hydrothermal solutions mainly originate in the deep marine environments; however, such source is doubtful for the limestones of the Formation which were deposited in shallow marine environments (Lawton et al., 2004; González-León et al., 2008). Hence, the observed 87Sr/86Sr ratio variations may be influenced by some other sources than the hydrothermal input.

5.e. Implications for continental weathering

Sedimentologic observations suggest that the Mural Formation formed on a large, shallow marine shelf covering southern Arizona and northern Sonora. Our samples represent shallow marine depositional environments, including open marine settings. Hence, it is not surprising that the trace element and isotopic signatures of the samples reflect an environment where open marine waters (those with the most juvenile isotope signatures) mixed with estuarine waters that were more influenced by relatively local continental hinterland. However, the present study found a stronger distinction between open marine and continent-dominated water chemistries, which clearly requires that relatively local sources of terrestrial input from the hinterland. Mural Formation limestones show remarkable variations in 87Sr/86Sr ratios among various members. Higher 87Sr/86Sr ratios are observed in the CLC, TS and CLE members than in other members, a pattern which suggests that 87Sr/86Sr ratio fluctuations are related to the decrease in the riverine inputs. Units at the base of the section (CLC and TS members) have high 87Sr/86Sr ratios followed by a gradual decrease in the Sr isotopic values in the LC and CLP members. Above in lower and middle parts of the CLE member the 87Sr/86Sr ratio gradually increases, and in upper part of the CLE member the ratio abruptly falls, followed by an abrupt increase in 87Sr/86Sr values in the MQ member. Such short term reduction could have been caused by weathering rates of older crystalline rocks vs. younger volcanics and/or rising sea levels that reduced the area of continents exposed to weathering.

Probst et al. (2000) noted that during the weathering process Sr composition initially spikes and that 87Sr/86Sr ratios range up to 0.7420. The relatively high 87Sr/86Sr ratio in the Sierra San José section of the Mural Formation suggests significant weathering of a granitic provenance. The 87Sr/86Sr ratio of granites from Caborca block of Sonora is up to 0.7090 (Valenica-Moreno et al., 2001, 2003). The significant fluctuations in the 87Sr/86Sr values in the studied section may be related to variations in the episodic/periodic influx of siliciclastics from the provenance area.

According to Bullen et al. (1997), significant quantities of radiogenic Sr may be leached from K-feldspar during weathering of granitoid provenances. The source area of the Bisbee Basin in Sonora was mainly composed of granitic rocks that released significant amount of radiogenic Sr to these limestones through riverine Sr flux. So, high 87Sr/86Sr ratios at various levels in the Sierra San José section of the Mural Formation suggest that a considerable amount of sediments was contributed by Proterozoic basement of the Caborca block during Aptian-Albian age. The decrease in 87Sr/86Sr ratios at certain levels of the studied section indicates a decreased influx of radiogenic Sr to the Mural Formation.

6. Chronostratigraphy and isotopic evolution

The Mural Formation members were correlated with the upper Aptian and lower Albian substages by ammonites, bivalves, and foraminifera (González-León et al., 2008). The stage boundary was correlated approximately with the Tuape Shale/Los Coyotes Member contact. In the Sierra San José section the Cerro La Espina and uppermost Cerro La Puerta members yield age-diagnostic fossils, but other members in the Sierra San José section have no diagnostic fossils as yet. A graphic plot of the Sierra San José section projects the Aptian/Albian boundary at approximately 200 m in the lower part of the Cerro La Puerta (Fig. 7 ). The age model (CRETCSDB) is defined by ranges of more than 3500 events in more than 200 sections (Scott, 2014). The line of correlation (LOC) is constrained by the first occurrence (FO) of Buccicrenata subgoodlandensis and the last occurrence (LO) of Mesorbitolina texana. In other sections in the region the lower Albian rudist, Coalcomana ramosa (Boehm), is reported in the Los Coyotes (González-León et al., 2008). Therefore the stage boundary is close to the base of the Los Coyotes and the LOC would be steeper than in Fig. 7 , which would project events below 240 m slightly older than shown in Fig. 7 .

Based on the graphic plot (Fig. 7 ) the bases of carbon isotope events 11, 12 and 13 are projected into the upper part of the Cerro La Puerta and the lower part of the Cerro La Espina members. The ages of carbon isotope events are projected from the Santa Rosa Canyon section (Bralower et al., 1999), where the base of C11 is a steep negative shift similar to the data at Sierra San José. The base of C12, which is a steep positive shift at Santa Rosa, is projected high in the Sierra San José section in a relatively flat interval (Fig. 7 ) above a distinct positive shift at 240 m. The position of Oceanic Anoxic Event 1b is projected from a number of deep oceanic sections and here spans from upper Cerro La Puerta to lower Cerro La Espina (Fig. 7 ). The range of carbon isotope data in the Sierra San José section is from -2.6 to 3.2 ppm, which is somewhat greater than the range of 1.8 to 3.0 ppm in upper Aptian-Albian data from Mid-Pacific seamounts (Jones and Jenkyns, 2001, their Fig. 4) suggesting that Albian oceanic concentrations are preserved in parts of this section. However the 87Sr/86Sr ratios in this section range from 0.70748 to 0.70879, and are heavier than the Pacific data that range from 0.70722 to 0.70747 (Jones and Jenkyns, 2001). Similarly in the upper Aptian-lower Albian Glen Rose Formation in the East Texas Basin the strontium ratios range from 0.70723 to 0.70746 (Denison et al., 2003). The heavier ratios in the Mexican section may be the result of greater terrestrial flux into the Bisbee Basin.

7. Conclusions

Limestones collected from the Sierra San José section of the Mural Formation are wackestone and packstone lithofacies. Mural Formation limestones consist mainly of both micrite and coarse grained carbonate. The negative δ18O isotopic values and common rapid fluctuation in the δ18O profile the Mural Formation suggest diagenetic changes affected oxygen isotope values. The lack of correlation between δ13C and δ18O suggests that limestones exhibit primary carbon isotope signatures. The carbon isotope curve shows two negative excursions in the lower part and one negative isotopic excursion in the upper part of the Sierra San José section. Most of the samples from the middle and upper parts of the section have positive carbon isotope values. The δ13C values of limestone in the Sierra San José section suggest that the δ13C values represent original Albian seawater composition and in combination with chronostratigraphic data, that OAE 1b extended into the Bisbee Basin and infringed onto the carbonate shelf.

The 87Sr/86Sr ratio of Mural Formation limestones varies widely from 0.707479 to 0.708790. Higher 87Sr/86Sr ratios at various levels in the Sierra San José section suggest that a considerable amount of sediment was derived from the Proterozoic basement of the Caborca block during Early Cretaceous. The decrease in 87Sr/86Sr ratios recorded at certain levels of the studied section indicates a decrease in influx of radiogenic Sr to the Mural Formation. 

Acknowledgments

We would like to acknowledge the support provided by the Universidad Nacional Autónoma de Mexico through PAPIIT Project No.IN121506-3. We would like to thank Dr. Timothy Lawton for his critical review and constructive comments. We thank Mr. Pablo Peñaflor for powdering limestone samples for isotopic studies. This research was partly supported by the Korea Research Foundation (grant 2010-0009765 to YIL). We also thank Mrs. Adriana Aime Orci Romero for preparing thin sections for petrographic study. Timothy Lawton and Hannes Löser shared valuable observations from their long experience with the Mural Formation.

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