Abstract
A carbon, oxygen and strontium isotopic study on the predominantly carbonate succession of the shallow marine Mural Formation in the Cerro Pimas locality of northern Sonora (Mexico) was undertaken to understand isotopic variations in seawater during Aptian- Albian time. Petrographic study has been carried out on these limestones to support the isotopic study. The limestones show low content of TOC, both negative and positive δ13C values (-4.1 to +2.2‰ VPDB) and d18O values varying from -13.4 to -8.9‰ VPDB. The carbon isotope composition, δ13C vs δ18O plot and Mn/Sr ratio suggest that the δ13C measured values are primary in nature and remain unaltered during diagenesis. The carbon isotope curve shows a positive δ13C excursion followed by a negative event in the lower part of the Los Coyotes Member (Lower Albian) of the Mural Formation and confirm the global nature of the episode OAE 1b in the Cerro Pimas section. The 87Sr/86Sr ratios for limestones of the Mural Formation (0.707221 to 0.707340) are similar to the ratios for Late Aptian and Early Albian seawater (0.70726 to 0.70740). In addition, 87Sr/86Sr age of the Cerro La Ceja member (113.4 Ma; Late Aptian age) is comparable to that of published zircon radiometric age. The numerical age of Los Coyotes (112.0 Ma and 111.1 Ma) and Cerro La Espina (110.9 Ma and 110.7 Ma) members indicate also Early Albian age. The whole-rock Sr isotope ages in this study are consistent with the previously published radiometric and biostratigraphic ages.
Highlights
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)
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 δ18O values compared with the carbonates precipitated in equilibrium with contemporaneous seawater, which indicates that the studied limestones were subjected to shallow burial diagenesis
Summary
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). 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). The isotopic study on mid-Cretaceous shallow marine carbonates have shown evidence for global-scale tectonics (Gröcke 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; Préat et al, 2010; Tewari et al, 2010)
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