STRATIGRAPHIC ANALYSIS OF THE LATE MIOCENE BOLEO FORMATION, SANTA ROSALIA BASIN, BAJA CALIFORNIA SUR, MEXICO

The purpose of this study is to understand the stratigraphic architecture of the late Miocene Boleo Formation in the Santa Rosalia Basin, and reconstruct the depositional processes and environments for this unit. We documented sedimentary lithofacies and lateral thickness changes to better understand the depositional setting and tectonic controls on basin formation. This study focused on a parasequence bounded by two laterally extensive thin mudstone units that contain concentrated Cu-Zn-Mn-rich strataform ore deposits, Mantos 2 and 3. We measured 6 stratigraphic sections and constructed a 4-km long stratigraphic panel in which the sections are correlated using Mantos 2 and 3 and the 6.76 ± 0.9 Ma Cinta Colorada (Holt et al., 2000), a volcanic tuff in the middle of this parasequence. A model for depositional paleoenvironments and tectonically controlled subsidence is derived from facies analysis, thickness changes, and lateral correlations.

We document doubling of thickness of sediments toward the SE between Mantos 3 and 2, and a lateral change to conglomerate-dominated facies in the SE-most section (Arroyo Purgatorio). Inverse grading, general absence of cross bedding, and rare erosional scours in conglomeratic facies support an alluvial fan setting for proximal deposits in the SE. The coarse alluvial-fan facies pass laterally to the NW into fine-grained, rhythmically laminated mud, silt and sand which we tentatively interpret as tidal rhythmites, consistent with a marginal-marine model proposed by Conly et al. (2006). The southeast-ward increase of stratigraphic thickness can be attributed to an increase in the rate of production of accommodation space closer to the sediment source, which we infer was due to fault-controlled tilting and increased subsidence toward the SE during deposition. The observed pattern can be explained by deposition in a pull-apart basin that was bounded by NW-striking transform faults on the NE and SW margins of the basin, and tilted southeast toward a NW-dipping normal fault. The lack of marine fossils likely can likely be explained by a high rate of siliciclastic input, and/or chemical toxins produced by active volcanism and geothermal activity. Future study of micropaleontology and fault kinematics may provide more insight into the controls on deposition of the Boleo Formation.