Lizárraga, L. Hernández, J. Ángeles, S.
The present paper documents the presence of elements such as isolated crystals, framboids and teeth and socket structures in samples of iron oxides collected from macrofossils from the Upper Cretaceous (Turonian) Eagle Ford Formation, cropping out in the vicinity of Muzquiz county, at Coahuila state, northeastern Mexico. At this area, the inner shelf Boquillas Formation consists of at least 6 m of interbedded brown-yellowish marlstone of about 7-15 cm thick and occasional decimetric (10-25 cm) beds of grey limestone. Marlstone exhibits a parallel millimetric- scale lamination associated to microbial activity, and occasionally shows iron oxide minerals exposed on nodules with a diameter of about 3 cm and millimetric layers arranged parallel to the stratiphicationplane resembling the distribution of sedimentary pyritein dysoxic/anoxic sediments. Fossils of macroorganismsconsist mainly of bivalves, ammonites and fish,and they present several degrees of preservation. Ocassionally, fossils are completely replaced by iron oxides.