Andrew P. Barth

Department of Earth Sciences, IUPUI

This field trip is designed for second or third year undergraduate geology and earth science students, as part of a required or elective petrology or earth materials course. The trip is designed to visit and explore a wide variety of rock types in their field context, and reinforce both rock identification skills and students' knowledge of the geologic time scale (typically introduced in an earlier survey course).

• Basic mineral and rock identification
• Exposure to the geologic time scale

• Field trip in petrology or earth materials course
• May also provide opportunity for sample collecting for individual term projects

The goals of this field trip are to reinforce mineral and rock identification skills, and to do this with samples collected in context. With the instructor providing geochronologic data at each stop, the students can combine rock identification and basic petrologic interpretations with the geologic time scale to construct links between petrologic processes and time.

• Formulation of petrologic hypotheses from the identity and diversity of rocks
• Synthesis of hypotheses at later stops as diversity of rocks continues to increase
• Development of a hand sample analog of the time scale integrated from all stops

The San Bernardino and San Gabriel Mountains provide an excellent setting for exploring the evolution and diversity of crystalline rocks in California. The oldest rock-forming events which can be explored in these ranges involved episodic Paleoproterozoic magmatism and orogenesis extending from 1.81 to 1.65 Ga. Rock units of this age are widespread both east and west of the San Andreas fault. This Paleoproterozoic tectonism was followed by intrusion of younger Mesoproterozoic anorogenic igneous rocks that are areally limited, but well exposed in the San Gabriel Mountains as 1.19 Ga gabbro, anorthosite, and syenite. Proterozoic igneous activity and tectonism in southwest North America was followed by rifting during the Neoproterozoic, which led to development of the Cordilleran geosynclinal belts. Belts of rocks within the geosyncline in southern California trend northeast-southwest, with deeper water rocks to the northwest, and Neoproterozoic and Paleozoic metasedimentary rocks in the San Bernardino Mountains belong to the transition zone between the cratonal and deeper water miogeoclinal sequences. Passive margin sedimentation ended with initiation of arc magmatism oriented along a northwest to southeast trend in Late Permian time. A diverse group of Mesozoic plutons and dike swarms as young as Late Cretaceous in age characterize the crystalline terranes of both the San Bernardino and San Gabriel Mountains, culminating in emplacement of large calc-alkalic intrusive suites in both ranges about 78 Ma.

The diversity of ages and types of crystalline rocks makes a field trip through either or both of these ranges a great opportunity to engage students in active learning while linking petrology and historical geology course content in a field context. Students can utilize rock identification skills learned in the laboratory, and with knowledge of available geochronologic data, can construct a more detailed geologic time scale for the region.

Here we will provide an example of a one-day trip to examine Proterozoic metamorphic and Mesozoic intrusive igneous rocks that are easily accessible in roadcuts and on short field traverses along National Forest roads. The trip is adapted from more detailed field guides and road logs for this region (principally Barth et al., 2001), with a focus on undergraduate learning.

For this type of field trip, I use a pre- and post-trip test of the overall construction of the geologic time scale and major events in the region of interest, in this case in terms of California geology.

I also integrate samples from selected stops in later laboratory practice tests

Weigand, P.W., Barth, A.P., and Ehlig, P.L., 1989, Field trip guide to a portion of the western San Gabriel Mountains, Los Angeles County, California, in Collins, L.G. (ed.), Geologic Excursions in the Greater Los Angeles Area: National Association of Geology Teachers, pp. 38-44.

Barth, A.P., 1990, Mid-crustal emplacement of Mesozoic plutons, San Gabriel Mountains, California, and implications for the geologic history of the San Gabriel terrane, in Anderson, J.L. (ed.), The Nature and Origin of Cordilleran Magmatism: Geological Society of America Memoir 174, pp. 33-45.

Barth, A.P., Jacobson, C.E., and May, D.J., 1991, Mesozoic evolution of basement terranes of the San Gabriel Mountains, southern California: Summary and field guide, in Walawender, M.J., and Hanan, B.B. (eds.), Geological Excursions in Southern California and Mexico: Geological Society of America, pp. 186-198.

Barth, A.P., Jacobson, C.E., Coleman, D.S., and Wooden, J.L., 2001, Construction and tectonic evolution of Cordilleran continental crust: Examples from the San Gabriel and San Bernardino Mountains, in Dunne, G., and Cooper, J. (eds.), Geologic Excursions in the California Desert and Adjacent Transverse Ranges: Pacific Section, Society of Economic Paleontologists and Mineralogists, Book 88, 17-53.

Barth, A.P., Anderson, J.L., Jacobson, C.E., Paterson, S.R., and Wooden, J.L., 2008, Magmatism and tectonics in a tilted crustal section through a continental arc, eastern Transverse Ranges and southern Mojave Desert, in Duebendorfer, E.M., and Smith, E.I. (eds.), Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in Selected Areas of Arizona, California, and Nevada: Geological Society of America Field Trip Guidebook 11, 101-117.