Stratigraphy, Mapping, and Geomorphology

The second part of our Summer Geology Field Camp will provide experience with geologic mapping, field stratigraphy, and tectonic geomorphology. We will spend about a week mapping in the Frying Pan Gulch area (shown below), near Dillon, MT, and then move to the Red Rock fault.

1. Frying Pan Project

The Frying Pan area 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 record changing conditions through time. 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.

In this project students will create a geologic map, describe map units, interpret depositional environments, determine the thickness and stratigraphic order of the section, and construct a geologic cross section that reveals the major structures. There will be one day of office work at the end to finish drafting maps, cross sections, stratigraphic columns, and reports. Students will also be expected to work on the project in the evenings.

Work for this project will help students develop skills that are needed for all kinds of geologic field mapping and field study, with an emphasis on sedimentary rocks and stratigraphy. We will emphasize accurate field locations, placement of lithologic contacts on the map, thorough descriptions and interpretation of map units, and synthesis of map-scale structures. 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.

2. Red Rock Fault Project

There will also be a 3-day project to map and measure the Red Rock fault south of Dillon. This is an active Quaternary to Holocene normal fault produced by NE-SW extension related to Basin-and-Range tectonics and evolution of the Snake River plain. The fault is well expressed by a suite of classic geomorphic features produced by episodic slip, earthquakes, and related surficial processes of erosion, slope failure, and deposition. In this project students will learn to identify and map an active normal fault, and interpret related geomorphic features. You will measure the areas of footwall catchments and hanging-wall alluvial fans, calculate and plot fan:catchment ratios, and combine the data with field observations to analyze the dynamic controls on alluvial fan geometry.

The Red Rock fault forms a prominent scarp along the west side of the Red Rock River Valley. Ongoing motion on this fault produces uplift in the Tendoy Mountains to the west and subsidence in the valley to the east, generating young alluvial fans along the faulted basin margin.

Above: Mapping along the Red Rock fault. It's a hard job but someone has to do it!