Reconnaissance geologic map of part of the San Isidro Quadrangle, Baja California Sur, Mexico

Miscellaneous Field Studies Map 1799
Prepared in cooperation with The Consejo de Recursos Minerales de Patrimonio y Fomento Industrial de los Estados Unidos Mexicanos
By: , and 



The reconnaissance geology of part of the San Isidro 1:250,000-scale quadrangle of Baja California Sur, Mexico, was mapped in 1982, 1983, and 1984 to determine the age and stratigraphic relations of Tertiary sedimentary and volcanic rocks that comprise the bulk of the Sierra de la Giganta (southern Peninsular Ranges) and Pacific coastal plain. Focus was placed on determining the distribution and age of plateau-capping basalts, and on mapping the eastern limits of Paleogene marine strata and the western limits of the nonmarine Comondu Formation. The area was also examined for outcrops of pre-Tertiary crystalline rocks, however, none were found.

Mapping was done on aerial photographs and transferred, where possible, to 1:50,000-scale topographic base maps. Areas with roads were field checked; however, in the northeast part of the map area, lack of roads prevented field checks. Previous geologic surveys of parts of the map area were made by horseback in the early 1920's; reports were published by Darton (1921), Heim (1922), and Beal (1948). Subsurface data from petroleum exploration and a geologic map were incorporated in a regional study by Mina (1957). The first radiometric ages of rocks from the map area were published by Gastil and others (1979). Recently determined radiometric ages and chemical analysis of volcanic rocks were reported by Hausback (1984) and by Sawlan and Smith (1984). Our study incorporates geologic mapping with age control based on new radiometric ages as well as paleontology, Flows and tuffs were dated by the K-Ar method. Fossil ages are based on diatom and mollusk assemblages.

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Reconnaissance geologic map of part of the San Isidro Quadrangle, Baja California Sur, Mexico
Series title Miscellaneous Field Studies Map
Series number 1799
DOI 10.3133/mf1799
Year Published 1985
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Description Plate: 50.18 x 36.99 inches
Country Mexico
State Baja California Sur
Other Geospatial San Isidro Quadrangle
Scale 0
Online Only (Y/N) N
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details