Abstract
San Basilio basin in Baja California Sur (Mexico) exhibits distinct styles of volcanism that interrupted phases of normal sedimentation correlated with the Zanclean Stage (Lower Pliocene). Sea cliffs around a 4-km2 bay opening onto the Gulf of California are dominated by rhyolite, mudstone, sandstone, and limestone. Volcanism associated with re-sedimented hyaloclastite is regionally uncommon and the goal was to investigate interactions between volcanic events and intervals of stability represented by fossil-rich strata. Methods of study involved a combination of microfossil and macrofossil analyses. Relating the basin’s faults to Pliocene development in the greater Gulf of California was a secondary goal. Microfossils Bolivina bicostata and B. interjuncta recovered from mudstone indicate an initial water column of 150 m. An abrupt hydromagmatic explosion ruptured the mudstone cover, followed by banded rhyolite flows inter-bedded with sandstone. Outlying limestone beds with the index fossil Clypeaster bowersi are separated from rhyolite by conglomerate eroded under intertidal conditions. A renewed phase of activity saw eruption of smaller volcanoes in the basin center semi-contemporaneous with pecten limestone deposited on unstable slopes. Normal faults conform to a pattern of extensional rifting in the proto-gulf, followed by cross-cutting faults indicating the onset of transtensional tectonics beginning about 3.5 Ma.
Highlights
Examples of volcanic edifices with associated marine sedimentary deposits are rare from within the Gulf of California or along the adjacent peninsular Baja California shore
The methods applied to the present study of San Basilio geology entailed the recording of stratigraphic descriptions and graphic logs to provide details on the intercalation of sedimentary and volcanic rocks at key localities
Discerning the overall relationship of igneous rocks, volcanoclastic sediments, and sedimentary strata is complicated by the rugged topography around San Basilio (Figure 2A), but geological mapping confirms the basic outline of key events (Figure 2B)
Summary
Examples of volcanic edifices with associated marine sedimentary deposits are rare from within the Gulf of California or along the adjacent peninsular Baja California shore. In the upper gulf region that features a large, half-eroded volcanic crater and a pair of intact rhyolite domes [2]. In the central gulf region, Isla Tortuga (11.4 km2 ) is a basaltic shield volcano that retains a central crater [3]. A third example is Isla Coronados (7.5 km2 ), which projects the classic shape of a stratovolcano covering 70% of the island with andesite flows.
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