SPECTACULAR FIELD-TRIP LOCALITIES IN THE MARIA FOLD AND THRUST BELT, WESTERN ARIZONA AND SOUTHEASTERN CALIFORNIA

The Maria fold and thrust belt (MFTB) in west-central AZ and southeastern CA has some of the most spectacular field-trip sites in the Cordillera. The MFTB displays ductile thrusts that mostly emplace crystalline rocks over folded, sheared, and metamorphosed Paleozoic, Triassic, and Jurassic supracrustal rocks. The Paleozoic, Triassic, and lower Jurassic rocks represent metamorphosed and structurally attenuated equivalents of well-known units on the Colorado Plateau and are succeeded by Jurassic arc-related volcanic and sedimentary rocks. In the Big Maria Mountains, north of Blythe, a km-plus sequence of Paleozoic rocks is overturned and attenuated in a spectacularly exposed regional syncline on the west side of the range, locally to less than 30 meters thick, with each unit still preserved (UTM 713796 E, 3743961 S). At Plomosa Pass, southwest of Bouse, an unconformity between Triassic and Permian metasedimentary units is isoclinally folded into a thin fin of rock, with stretched-pebble conglomerate (basal Triassic) on both sides of the fin (UTM 769755 E, 3744861 N). In the moderately accessible northwestern Granite Wash Mountains, northeast of Vicksburg, upturned section-parallel and section-repeating thrusts in Paleozoic-Triassic rocks are discordantly overridden by thrusts that carry Jurassic volcanic rocks, attenuated Paleozoic sections, and Proterozoic crystalline rocks. In places, entire Paleozoic formations are meters thick, less than several percent of normal (UTM 244204 E, 3747360 N). On the easily accessed west side of Martin Peak in the Little Harquahala Mountains, south of Salome, a quartz vein cutting Proterozoic granite can be followed uphill to where it is depositionally overlain by near-vertical Cambrian quartzite, an exposure of the Great Unconformity (UTM 259123 E, 3727937 N). The basal quartzite contains large angular blocks of the quartz vein, which probably formed a wall-like island in the early Cambrian seas. Cunningham Pass (UTM 262063 E; 3762860 N), north of Wenden, contains incredible exposures of banded gneiss, migmatite, and superimposed fabrics, representing an exposure of the Cretaceous middle crust. Together, these sites nicely represent the geology and complex structural history of the Maria Fold and Thrust Belt and are great geologic sites to visit with students!