Bedrock Mapping and Stratigraphy (Becky Dorsey)

The second part of the UO Summer Geology Field Camp will provide an introduction to the basics of geologic bedrock mapping, field stratigraphy and related skills of observation, description, and interpretation. The Frying Pan locality lies about 15 miles northwest of Dillon, MT. The photo below is an aerial view looking north across this unique map area.

The Frying Pan contains abundant bedrock exposures, lithologic diversity, and relatively simple fold structures. The rocks comprise a thick succession of Triassic- to Cretaceous-age fluvial and marine sedimentary rocks that include conglomerate, sandstone, shale and limestone of the Dinwoody, Morrison and Kootenai Formations. These sediments were deposited under varying tectonic and climatic conditions. The Cretaceous deposits accumulated near the western margin of a large interior seaway that flooded the continent during mountain building in the Sevier fold-thrust belt. The basal conglomerate of the Kootenai Formation (on right) is a regionally widespread unit that records Cretaceous uplift and erosion in the mountain belt to the west.

We will map about 1 square mile in the Frying Pan area. In addition to creating a geologic map, students will describe map units and interpret their environments of deposition, determine the thickness and stratigraphic order of the section, and construct a geologic cross section that reveals the structures that deform the rocks. There will be one day of office work at the end to finish drafting maps, cross sections, stratigraphic columns, and written reports. Students will also be expected to work on these things in the evenings during the course of the field work.

The objective of this project is to teach students critical skills that are needed for all kinds of geologic field mapping and field study, with an emphasis on sedimentary rocks and stratigraphy. For the mapping part we will emphasize the importance of accurate field locations, correct placement of lithologic contacts on the map, accurate descriptions of map units, and synthesis of map-scale structures. In the stratigraphy part we will begin with primary observations of lithologies, sedimentary structures, and vertical transitions, and we will then work systematically toward interpretations of depositional processes and environments. Ultimately, we will see that these rocks record the regional evolution of the Rocky Mountains region prior to and during deposition in a large Mesozoic foreland basin.