Biology
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmpp24a..04b&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #PP24A-04
Biology
0448 Geomicrobiology, 0460 Marine Systems (4800), 3030 Micropaleontology (0459, 4944), 4243 Marginal And Semi-Enclosed Seas, 4999 General Or Miscellaneous
Scientific paper
The Agua Nueva Formation in the vicinity of Xilitla, State of San Luis Potosí, Central Mexico, consists of interbedded brown shale (Grayish orange 10YR 7/4 to Moderate yellowish brown 10YR 5/4) and dark-gray fossiliferous limestone (Bluish gray 5B 6/1 to Dark bluish gray 5B 4/1), varying between 10 and 20 cm in thickness. The sequence also includes 2 to 4 cm- thick intermittent bentonite layers (Moderate greenish yellow 10Y 7/4, to dark greenish yellow 10Y 6/6 and Light olive 10Y 5/4). At the field scale, shaly intervals show no apparent internal structures, whereas most limestone beds show primary lamination at the millimeter scale (1-2 mm), and intermittent layers of black chert of about 5 cm thick. Pyrite is present as disseminated crystals and as 2 cm-thick layers. Bioturbation or macrobenthic organisms other than inoceramids do not occur in the Agua Nueva Formation at Xilitla. Unusual macrofossils are present only in limestone strata, and consist of well- preserved diverse genera of fishes such as sharks, Ptychodus sp. and teleosteans, Rhynchodercetis sp., Tselfatia sp., Goulmimichthys sp., and scales of Ichtyodectiformes, as well as ammonites and inoceramids (Blanco et al., 2006). The presence of Inoceramus (Mytyloides) labiatus (Maldonado-Koederll, 1956) indicates an Early Turonian age for the sequence. Total carbonate content (CaCO3 = TIC) varies between 62 and 94% in the Limestone beds, which yield Total Organic Carbon (TOC) from 0.4% to 2.5%; the shale intervals contain TIC values consistently lower than 33% and TOC lower than 0.8% Microscopically the limestone beds vary from mudstone to packstone composed essentially of coccoid cyanobacteria similar to coeval deposits in northeastern Mexico, Coahuila State, at Parras de La Fuente (Duque- Botero 2006). Similarly, the microspheroids are spherical to sub-spherical, and occur as isolated elements or aggregates forming series of chains of parallel-packed light lamina 1-2 mm thick. Filamentous cyanobacteria (> 1 mm in length) are also present oriented parallel to stratification. In addition to filamentous and coccoid cyanobacteria, the limestone beds contain rare benthic foraminifera, common planktic foraminifera, heterohelicids, Rotalipora spp., Rotalipora cf cushmani, Whiteinella spp, W. praehelvetica, which indicate a time interval from the latest Cenomanian to the earliest Turonian. Lithological, paleontological and microfacies data thus indicate that the sediments accumulated in open-marine to semi-restricted platform environments, under low-energy conditions. Primary lamination, pyrite and excellent degree of preservation of fishes, suggest that low oxygen concentration lead to the formation of anoxic/dysoxic conditions during the accumulation of these exceptional deposits, which are coeval with the worldwide development of OAE-2. Planktonic foraminifera and fishes indicate oxygenated conditions in the photic zone, but dysoxic/anoxic conditions near the bottom, which is consistent with the presence of inoceramids and the absence of bioturbation in the sediment.
Blanco-Piñón A.
Duque-Botero F.
Maurrasse F. J.
Rojas-León Antonio
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