BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 49, PP. 1651-1694. 7 PLS., 6 FIGS. NOVEM BER 1, 1938 STUDIES OF THE MEXICAN GEOSYNCLINE BY EALPH W. IMLAY CONTENTS Page Abstract....................................................................................................................................... 1652 Introduction............................................................................................................................... 1652 Acknowledgments..................................................................................................................... 1654 Earlier investigations............................................................................................................... 1654 Geography................................................................................................................................... 1654 Topography and drainage.............................................................................................. 1654 Culture................................................................................................................................ 1655 Sedimentary rocks..................................................................................................................... 1657 General discussion.......................................................................................................... 1657 Jurassic system................................................................................................................. 1657 Zuloaga limestone.......................................................................... „............ 1657 La Caja formation................................................................................................... 1659 Correlation............................................................; ................................................ 1662 Cretaceous system.................................................................; ......................................... 1662 Taraises formation................................................................................................... 1662 Cupido limestone..................................................................................................... 1662 La Pefia formation.................................................................................................. 1663 Cuesta del Cura limestone.................................................................................... 1664 Indidura formation.................................................................................................. 1665 Caracol formation.................................................................................................... 1666 Parras shale............................................................................................................... 1667 Quaternary system........................................................................................................... 1667 Mayrdn formation................................................................................................... 1667 Igneous and metamorphic rocks........................................................................................... 1670 Structural units in northern Zacatecas and southern Coahuila................................... 1672 General features................................................................................................................ 1672 Folds.................................................................................................................................... 1672 Atajo Anticline......................................................................................................... 1672 Yeso Anticline.......................................................................................................... 1673 Sombreretillo Anticline.......................................................................................... 1673 Zuloaga Anticline..................................................................................................... 1675 Canutillo, La Caja, and Santa Rosa Anticlines.............................................. 1675 Los Alamos Anticline............................................................................................. 1675 Ramirez Anticline.................................................................................................... 1675 Faults................................................................................................................................... 1676 Mineral Deposits....................................................................................................................... 1676 Comparison of facies in Mexican geosyncline................................................................... 1677 Upper Jurassic................................................................................................................... 1677 La Gloria formation and Zuloaga limestone.................................................... 1677 La Casita and La Caja formations..................................................................... 1683 Cretaceous.......................................................................................................................... 1687 Taraises formation................................................................................................... 1687 (1651) 1652 R. W. IMLAY— MEXICAN GEOSYNCLINE Page Las Vigas, Parritas, and Cupido formations.................................................... 1687 La Peña formation................................................................................................... 1689 Aurora and Cuesta del Cura limestones............................................................ 1690 Indidura, Cuesta del Cura, and Caracol formations..................................... 1691 Parras shale and Difunta formation................................................................... 1692 Works to which reference is made........................................................................................ 1692 ILLUSTRATIONS Figure Page 1. Index map of northern Mexico...................................................................................... 1653 2. Stratigraphic sections in the Mexican geosyncline.................................................. 1658 3. Principal structural features........................................................................................... 1674 4. Stratigraphic sections along northern part of the Mexican geosyncline........... 1678 5. Northern part of Mexican geosyncline at close of Cretaceous sedimentation.. 1686 6. Time relations of facies in the Mexican geosyncline............................................. 1689 Plate Facing page 1. Mountains........................................................................................................................... 1660 2. Formations and faults...................................................................................................... 1661 3. Formations.......................................................................................................................... 1666 4. Sierra Jimulco.................................................................................................................... 1667 5. Contacts............................................................................................................................... 1668 6. Mayrán conglomerates..................................................................................................... 1669 7. Geologic map of the Melchor Ocampo area.............................................................. 1672 ABSTRACT The Melchor (San Pedro) Ocampo region lies on the boundary of Coahuila and Zacatecas in the central part of a broad belt of eastward-trending folds characterized by strong asymmetry. In contrast to the region farther north, the Jurassic limestone is extensively developed and forms the highest parts of the mountain ranges; the Cretaceous rocks form the foothills. About 100 square miles adjacent to Melchor Ocampo was studied intensively as a foundation for reconnaissance studies in adjoin­ ing areas. During the Upper Jurassic and Lower Cretaceous epochs the distribution of marine facies of deposition was controlled by erosion from the Coahuila Peninsula to the north. The seaward margins of the clastic facies and the limestone reef facies ap­ proximately coincide with the southern side of the Sierras de Parras and Jimulco. Facies' of Upper Cretaceous marine sedimentation were controlled by rising land masses in central Mexico, as shown by increasing thickness of clastics toward the south.. Folding in northern Zacatecas involves both the Jurassic and the Cretaceous beds and apparently occurred at the same time as that farther north. The Jurassic forma­ tions become progressively more elevated toward the south and in general are as intensely folded as the Lower Cretaceous formations of the Sierras de Parras and Jimulco in Coahuila. The greater height of the ranges bordering the site of the Coahuila Peninsula (e.g., Sierras de Parras, Jimulco, Rosario) than that of the ranges farther south, is ascribed (1) to the presence of the thick rudistid-bearing Aurora limestone, and (2) perhaps to less intense orogeny with consequent less effective erosion. INTRODUCTION The Melchor (San Pedro) Ocampo area in north-central Mexico on the boundary of Coahuila and Zacatecas