SURVEY OF ROCKY SHORES BY LITHOLOGY IN THE GULF OF CALIFORNIA (MEXICO)

Regional quantification of rocky shores by lithotype is practical through processing of satellite images from the LANDSAT and ASTER programs. Bands 8,3,1 proved to be the best 3-band combination using ASTER data. For LANDSAT data, the combination of bands 7,4,1 was best. Landsat data was also processed to a resolution of 14.5 m/pixel using a Gram-Schmidt Spectral Sharpening technique that employed the ETM+ panchromatic band.

Google Earth provided high-resolution, 3D, topographic images that complemented our satellite data. Using the Google Earth mapping service is a convenient way to evaluate topographic expression on rocky shores and to distinguish adjoining beaches from mud flats. Our survey covers 33 islands in the Gulf of California and the peninsular coastline between the Colorado River delta and land's end at Cabo San Lucus. Rocky-shore types include granodiorite, andesite, volcanic rocks, metamorphic rocks, and limestone. Typically, metamorphic rocks are Paleozoic in origin, granodiorite is Cretaceous, andesite is mostly Miocene, other volcanic rocks are Pleistocene, and limestone is Pliocene-Pleistocene in age. In total, 1,829 km of gulf shores on the Baja California peninsula were categorized and measured to the nearest tenth of a kilometer. Gulf islands were treated, likewise, with a cumulative length of 1,111 km. Rocky shores were found to comprise 48% of the overall shoreline (1,415 km / 2,940 km). Andesite rocky shores are most frequent, at 24.3%. Limestone shores account for 7.5% of the total. Accuracy of this survey is considered high based on personal knowledge of selected islands and by comparison with geological maps available for some areas.

Visitation of the entire coastal zone in the Gulf of California would require years of fieldwork. However, virtual exploration of remote coastal sectors and islands is possible using a combination of Google Earth and programmed fly-overs using satellite images draped on digital elevation models.