28-6 Geological Evolution of the San Ramón Sub-Basin, Eastern Margin of the San José del Cabo Basin, Southern Baja California Peninsula

Session: Recent Advances in the Gulf of California Oblique Rift: Offshore and Onshore Studies // Avances Recientes en el Rift Oblicuo del Golfo de California: Estudios Marinos y Terrestres (Part I)

Presenting Author:

Authors:

Schwennicke, Tobias¹, Plata Hernández, Elvia², Cota Ramírez, Daniel³, Walde, Michel Alain⁴, Picos Peraza, Emily⁵

(1) Departamento Académico de Ciencias de la Tierra, Universidad Autónoma de Baja California Sur, La Paz, Baja California Sur, Mexico, (2) Departamento Académico de Ciencias de la Tierra, Universidad Autónoma de Baja California Sur, La Paz, Baja California Sur, Mexico, (3) Departamento Académico de Ciencias de la Tierra, Universidad Autónoma de Baja California Sur, La Paz, Baja California Sur, Mexico, (4) Department Umweltwissenschaften, Universität Basel, Basel, Switzerland, (5) Departamento Académico de Ciencias de la Tierra, Universidad Autónoma de Baja California Sur, La Paz, Baja California Sur, Mexico,

Abstract:

The San José del Cabo Basin, located near the southern tip of the Baja California Peninsula, formed in the Late Miocene during the opening of the Gulf of California. Its sedimentary fill consists of marine and continental deposits. In the basin's early stage, during the Late Miocene, extension was initially distributed among several faults. Later, in the Pliocene, the San José del Cabo Fault became active, thus forming a new western boundary of the basin and rendering the other faults inactive, most of which remained buried within the San José del Cabo Basin. The activation of the San José del Cabo fault caused the rotation of the accumulated fill, and its eastern margin emerged. Consequently, several half-grabens formed during the early stage, today are exposed along its eastern margin. One of these is the San Ramón sub-basin. Its western and northern margins are bounded by the El Zapote fault, and the footwall consists of Cretaceous plutonic rocks. The sedimentary fill consists of alluvial and marine deposits and generally dips to the southwest, with a steeper dip in older beds, becoming almost horizontal in younger strata. The first deposits (La Calera Formation) are distributed in the eastern and northern marginal parts of the sub-basin and overlie the plutonic basement; they consist of conglomerate and sandstone formed in alluvial fans with a moderate to low gradient. To the west and southwest, these deposits interfinger with marine sediments (Trinidad Formation). In the transition zone, bioturbated sandy and muddy sediments with some macrofossils are exposed, formed under tidal flat and shallow bay conditions. Toward the El Zapote fault, bioturbated to laminated mudstone predominates, indicating its deposition below the influence of waves, partially under oxygen-deficient conditions related to marine upwelling. Calcareous nannofossils (Discoaster berggrenii and D. quinqueramus) indicate an age in biozone NN11 (late Tortonian - Messinian, approximately 8.3 – 5.4 Ma) for the marine rocks, while the onset of faulting and sedimentation likely occurred somewhat earlier during the late Tortonian. These data suggest that rapid subsidence of the San Ramón sub-basin occurred first, and subsequently, the El Zapote fault became inactive during the Messinian.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

doi: 10.1130/abs/2026CD-13823

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.