Cyanobacterial productivity, variations in the organic carbon, and facies of the Indidura Formation (Cenomanian-Turonian), Northeastern Mexico. Productividad cianobacteriana, variaciones del carbono orgánico y facies de la Formación Indidura (Cenomaniano-Turoniano), noreste de México Fabián Duque-Botero and Florentin J-M. R. Maurrasse Florida International University, Department of Earth Sciences, PC- 344, Miami, Florida 33199, USA. fduqu002@fiu.edu ; maurrasse@fiu.edu Received: 23/10/03 / Accepted: 01/06/04 Abstract Rock sequences of Cenomanian-Turonian age commonly assigned to the Indidura Formation in northeastern Mexico (Coahuila State) show distinct facies indicative of signifcant spatial variability over the carbonate platform. Three stratigraphic sections where selected to characterize these differences. Las Delicias section (stratotype) is composed of 10-30 cm thick beds of very-pale orange biocalcirudites, without internal primary structures. Allochems consist of abundant echinoderms, pelecypods, ammonites, and fewer planktonic foraminifera. Total carbonate (CaCO 3 ) varies between 48% and 94%, and total organic carbon (TOC) between 0.7 % and 1.5 %. La Casita Canyon section, southeast of Las Delicias, consists of 3-30 cm thick interbeds of pale yellowish brown biocalcilutites and olive gray shales. Hand specimens show no apparent depositional internal structures, whereas abundant bioturbation appears in thin sections. Allochems consist of sparse fragments of planktonic foraminifera and radiolaria concentrated in burrows. Total carbo- nate (CaCO 3 ) varies between 0.8 % and 59.3 %, whereas TOC fuctuates between 0.17 % and 5.8 %. In contrast, the Sierra de Parras section, south of Las Delicias and west of La Casita, includes a sequence with well defned rhythms. They consist of 8-200 cm thick beds of light olive gray and brownish black, to olive black shales; and 5-100 cm thick marly biocalcilutites. Both facies exhibit similar internal structures arranged in nearly even-parallel “varve-like” dual lamination (<3 mm thick). Microscopically, they include few planktonic foraminifera scattered in the dark laminae. Epifaunal remains include only spo- radic pelecypods (Inoceramus). Total carbonate (CaCO 3 ) content varies from 43% to 78.3%, while TOC is relatively high between 7.3% and 24.3%, more often higher than 20%. Microfacies in the Parras area reveal compositional differences in the laminae asso- ciated with varying abundance of microspheres or “micro-ooids” that we attribute to be of cyanobacterial origin. Laminae developed from fuctuating cycles of calcareous cyanobacteria blooms, which remained dominant throughout the sequence. C organic –rich black shales and limestones of the Parras region further document unique paleoceanographic conditions, which were also characterized by strong dysoxic/anoxic bottom conditions and rhythmical production of cyanobacteria. These conditions contrast sharply with prevailing paleoenvironments recorded at Las Delicias and La Casita where benthic epifauna, planktonic and nektonic organisms were able to thrive. Assuming that these facies are coeval, microfacies and TOC analyses of these rocks further demonstrate distinct spatial differences between these areas. Keywords: cyanobacteria, Cenomanian/Turonian, organic-rich sediments, Mexico, laminae, anoxia. ISSN: 0378-102X www.ucm.es\JIG Journal of Iberian Geology 31 (2004) 85-98