The Palaeocene-early Oligocene Zacatecas conglomerate, Mexico: sedimentology, detrital zircon U–Pb ages, and sandstone provenance

This article presents detailed mapping results and the first U–Pb zircon dating and sedimentological characterization of the Zacatecas Conglomerate, which belongs to the Palaeogene red beds of central Mexico, deposited in fault-bounded basins during the Late Cretaceous to Eocene Laramide orogeny. The conglomerate was divided into five depositional facies associations according to their clast-type abundances and interlayered volcanic rocks. The lowermost member has a maximum depositional age based on young zircon grain ages varying from ca. 63 to 81 Ma. It is unconformably overlain by a continuous sequence characterized by a conglomerate rich in granite clasts at the bottom, with an interlayered tuff dated at 37.64 ± 0.36 Ma. Near the top, another tuff was dated at 30.84 ± 0.47 Ma, and a sandstone has a maximum depositional age of ca. 31.5 Ma. Normal grading, massive textures, channels, channel-form sandstone bodies, and upward-finning successions suggest that the Zacatecas Conglomerate is of fluvial origin. Late Jurassic to Early Cretaceous ages from zircons in plutonic rocks and sandstones bracket possible source regions for the Zacatecas Conglomerate. One possible source is Late Jurassic-Early Cretaceous granite derived from the Alisitos-Guerrero arc of western Mexico. Another possible source is the Tuna Manza Diorite, now exposed 250 km southeast of the study area. The lack of pre-Jurassic grains implies that possible sources such as the Nazas arc or the Potosí fan were not cropping out at that time, or at least that these areas were not affected by the fluvial system feeding the Zacatecas Conglomerate. It is possible that during the Palaeocene-early Oligocene the fluvial systems drained from west to east and from southeast to north, according to the above-mentioned constraints.