<p>The Santa Rosalía basin on the central Baja California peninsula offers a unique opportunity to investigate the Late Miocene early transform-rift deposits of the Gulf of California that are recorded in the Boleo Formation. This research contributes new insights into the gypsum deposits of the Boleo Formation to better understand early marine incursions into the incipient gulf and their relationship to copper mineralization within the formation. Detailed sedimentological analysis of gypsum outcrops along Arroyo Boleo reveals six distinct lithofacies: (1) laminated gypsum (Facies A), (2) laminated gypsum with bottom-nucleated selenite crystals (Facies B), (3) wavy laminated gypsum (Facies C), (4) mud-gypsum laminae with nodular gypsum (Facies D), (5) gypsrudite (Facies E), and (6) siliciclastic sediments (Facies F). The Boleo Formation records four asymmetrical, shallowing-upward cycles that begin with gypsum facies (A, B, or C) and are capped by one or more siliciclastic facies (F1–F3). Copper mineralization at the base of each cycle indicates the coeval nature of the Boleo copper ore deposits. We infer that the sedimentary cycles developed in response to pulses of fault motion that drove basin subsidence, renewed hydrothermal flow, rapid inflow and seepage of marine waters, and subsequent progressive shallowing and subaerial exposure due to gradual evaporative drawdown. Repeated cycles of subsidence and marine incursion likely were controlled by pulses of syn-basinal, oblique-transtensional faulting during early development of the Gulf of California. Deposition of sulfur-rich gypsum and siliclastic sediment over young volcanic rocks in the Boleo Formation provides a model for syngenetic, stratiform-copper ore formation.</p>