STRATIGRAPHIC ANALYSIS OF THE UPPER MIOCENE BOLEO FORMATION AT ARROYO PROVIDENCIA, BAJA CALIFORNIA SUR, SOUTHWEST MARGIN OF THE PALEO-GULF OF CALIFORNIA

The upper Miocene Boleo Formation is the oldest unit of the Santa Rosalia Basin (SRB) located in northeast Baja California Sur, Mexico. The SRB formed on the SW margin of the Gulf of California during early development of the transtensional plate boundary, but the structural controls on basin development remain poorly understood in this area. Wilson and Rocha (1955) proposed that spatial variations in thickness of the Boleo Fm record onlap of sediment onto pre-existing paleotopography. We studied sedimentary lithofacies and stratal architecture in Arroyo Providencia, southern SRB, to test this hypothesis and understand mechanisms of basin formation. Cerro Juanita, one of the presumed paleo-hills, is a ca. 300 m high, 2.5 km long hill of volcanic rocks including basalt and basaltic andesite dated at 9.66 ± 0.21 Ma (Busby et al., 2020) and exposed in the uplifted footwall of the NW-striking, SW-dipping Juanita normal fault. The Boleo Fm unconformably overlies the volcanic rocks and consists of interbedded alluvial conglomerate and sandstone, fine-grained tidal rhythmites, and thin shale units rich in Cu and Mn ore (mantos). Rhythmites and shale contain marine calcareous nannofossils belonging to genera Sphenolithus and Ceratolithus. Drone imagery, measured sections, and geologic map of Wilson and Rocha (1955) reveal stratal wedge geometries in the Boleo Fm that display systematic thickening toward the north, NE, and ESE, away from Cerro Juanita. These geometries represent a growth structure that formed by domal uplift in the footwall of the Juanita fault and tilting radially away from the uplift during deposition of the Boleo Fm. We also observe a SW-to-NE change from conglomerate to sandstone in the lower Boleo Fm; stratal downlap in upper Boleo conglomerate that records Gilbert delta progradation to the NE; and channel geometries and cross bedding that indicate transport to both the NE and NW. These results are consistent with data from other parts of the SRB that reveal 3D basin geometries, small- to large-scale fault-controlled tilting, and absence of a discrete boundary fault on the SW margin of the basin. Collectively, stratal and structural relationships in the SRB record subsidence by growth of a large monocline that tilted the crust down to the NE toward the opening paleo-Gulf of California during latest Miocene time.