Stratigraphic reassessment of the early Mesozoic Huayacocotla basin, eastern Mexico: new insights into the initiation of Pangea breakup and the source-to-sink system developed along its western equatorial margin

ABSTRACT The Huayacocotla basin is one of the most nearly complete stratigraphic records of Pangea breakup in Mexico. The lower strata are composed of several-hundred-meter-thick fluvial successions emplaced during the early stage of continental rift. These deposits were assigned to two units: the Huayacocotla Formation, the oldest unit marking the initiation of rifting in eastern Mexico, and the overlying Cahuasas Formation. These units were tentatively distinguished by color, lithologic assemblage, and sediment composition. However, these features are similar in both units, making it hard to construct a solid stratigraphic framework and an accurate geological map representative of the depositional architecture and tectono-sedimentary evolution of the basin. In this work, we document that, at least in the northern part of the Huayacocotla basin, the Huayacocotla and Cahuasas formations were emplaced in fluvial systems with distinct styles. Thus, they are composed of distinct fluvial elements, which permit their easy differentiation. The Huayacocotla Formation is composed of in-channel conglomerate to sandy bars, interpreted as the stratigraphic record of alluvial fans draining local uplifts. The Cahuasas Formation is dominated by floodplain, crevasse splay, and channel deposits, an association typical of anastomosing rivers. Reworked tuff deposits of ∼ 244 Ma in the lower part of the Huayacocotla Formation bracket to Middle Triassic time the initiation of rifting in eastern Mexico, like in other parts of Pangea. The occurrence of Triassic alluvial fans sourced from local basement uplifts in the Huayacocotla basin improves our understanding of the sedimentary systems developed along the western margin of Pangea and shows that the supercontinent was drained not only by transcontinental rivers with headwaters hundreds of kilometers inland in the supercontinent interior, but also by local fluvial systems developed across the supercontinent border.