A FIRST GEOLOGICAL MAP OF ISLA MONSERRATE, BAHIA DE LORETO, BAJA CALIFORNIA SUR

Isla Monserrate is an island located about 18 km offshore in the Gulf of California and contains important outcrops and structures that reflect the obliquely rifted geologic history of Baja California south of Loreto. In 2022, our team began collaborative mapping and study of the island’s geology, augmented by drone surveys that yielded a high-resolution orthomosaic and, through structure-from-motion, a Digital Elevation Model (DEM) with an average pixel spacing of 16.1 cm. Isla Monserrate has five mappable geologic units: (1) volcanic and volcaniclastic rocks of the Comondú Formation, (2) mafic volcanics, (3) graben-filling silicic tuff, (4) fossiliferous orange sandstone, (5) littoral and shallow-marine carbonates and clastics (locally forming terrace caps). The Comondú consists of multiple sequences of fluvial, volcaniclastic, intermediate-composition lavas, and dikes. Silicic tuff exists together with sedimentary fill in fault-bounded extensional basins. Orange-colored sandtones rich in shell fragments unconformably overlies the Comondú basement. Isla Monserrate also has a sequence of coastal terraces and eroded platforms, ranging from lower elevations at ~15 m above sea level, to the highest points of the island towards the center, around 200 m elevation. The lower platforms are wave-cut and covered in round clastics, and higher platforms are stripped and recrystallized carbonate sheets. Sandstones and terrace-capping deposits contain well-preserved marine invertebrate fossils such as Argopecten, colonial corals, echinoids, and barnacles. Monserrate’s terrace/platform sequence documents Quaternary uplift of the island from below sea level.

The normal faults found throughout the island generally strike north to south, dipping west at 40~70˚. Pre-extensional Comondú strata dip 45-80° to the east. The syn-extensional silicic tuffs dip ~30˚ to the east. Interestingly, the much younger carbonate platforms/terrace surfaces also incline east, with much shallow dips, suggesting a progressive domino-style rotation of the island above a regional west-directed detachment system.

The research so far documents the geologic history of the region, from pre-lift convergent tectonics to syn-rift extension, then to significant and enigmatic Quaternary uplift during the post-rift phase.