34-4 Preliminary Geologic Map of Isla del Carmen, Gulf of California, Mexico

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 (Posters)

Poster Booth No.: 4

Presenting Author:

Scott Bennett

Authors:

Bennett, Scott E.K.¹, Dorsey, Rebecca J.², Gauer, Anne³, Olmos, Augusto⁴, Stelten, Mark Evan⁵

(1) Geology, Minerals, Energy, and Geophysics Science Center, United States Geological Survey, Portland, OR, USA; Portland State University, Portland, OR, USA, (2) University of Oregon, Eugene, OR, USA, (3) Maptek, Golden, CO, USA, (4) Conserva Loreto, Loreto, Baja California Sur, Mexico, (5) U.S. Geological Survey, California Volcano Observatory, Moffett Field, CA, USA,

Abstract:

Islands offshore eastern Baja California preserve a valuable record of faulting and marine sedimentation related to the early development of the Gulf of California oblique rift. We have initiated a geologic mapping and structural geology study of Isla del Carmen, a ~33 km-long, 3–10 km-wide island ~15 km offshore Loreto, Baja California Sur. The island consists mostly of volcanic rocks of the Late Oligocene-Middle Miocene Comondú Group, considered to be the extrusive record of the SW-migrating Comondú arc during Farallon-North America subduction. Pending Ar/Ar geochronology and geochemistry of these volcanic rocks will further characterize their age and composition. Marine deposits preserved in several locations unconformably overlie the Comondú Group and have been assigned to the San Marcos, Carmen, and Marquer formations. The oldest marine deposits in the Perico basin (northeast island) are ~750 m of Late Miocene(?)-Early Pliocene conglomerate that accumulated in the hanging wall of a major W-dipping normal fault. In the Arroyo Blanco basin (central island), the basal Tiombó conglomerate may correlate with the basal conglomerate in the Perico basin. Marine limestone and marlstone were deposited in these basins through Late Pliocene time and possibly until ~1 Ma in the Arroyo Blanco basin. By ~1 Ma, vertical uplift of the island had begun, recorded by at least 7 flights of marine terraces up to ~100 m asl. New evidence for marine terraces at ~180-300 m asl suggests a longer history and larger magnitude of vertical uplift. Major W-dipping normal faults occur at the salt flat north of Bahía Salinas, at Punta Perico, a newly discovered fault near Punta Colorado, and offshore faults in the Cochimie basin between Loreto and the island. Transtension, block rotation, and subsidence related to offshore faults likely separated the island from the peninsula sometime after deposition of the Late Miocene(?)-Early Pliocene Tiombó conglomerate. The NW-striking, Arroyo Blanco fault (dextral-oblique normal) separates the southern third of the island, which displays sub-horizontal dips and little to no paleomagnetic evidence of vertical-axis rotation, from the northern two-thirds of the island, where volcanic rocks dip 30-60°E and have experienced ≥40° of clockwise rotation. Future mapping, geochronology, and geochemistry will improve our geologic map of Isla del Carmen and provide additional insights into the early history of Gulf of California rifting.

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

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