Southfork Mapping Project
Overview
The South Fork slide (SFS) is exposed over an area of 800 km2 in northwest Wyoming, and its evolution is connected in space and time to the adjacent and overlying 49.19 Ma Heart Mountain slide (HMS). The youngest rocks deformed as part of the SFS are the Eocene Willwood Formation sands and mudstones, which have a U/Pb detrital zircon youngest depositional age (Tuftzirc calculation) of 51.80 Ma as well as a spectrum of older zircons that were eroded from the Sevier Highlands to the west and the Archean Beartooth uplift to the north (n=379 U-Pb zircon ages). The SFS is overlain by allochthonous Paleozoic carbonate and Eocene volcanic rocks ofthe HMS. The SFS deformation involves shallowly plunging thin-skinned folds that trend northeast-southwest, where the slip surface could have been the top ofthe Jurassic Gypsum Springs Formation. Detailed mapping reveals the absence of any cleavage or joints and a plethora ofminor structures unreported in any thin-skinned belt as well as the fact that nearly a third of the exposed allochthonous sediments are overturned. Calcite in Sundance Formation limestones is mechanically twinned, and the resultant strain analysis reveals that the predetachment Sevier layer-parallel shortening strain axes (31/31 samples; n p 1231 twins) are now in a random orientation. Synemplacement calcite veins record a horizontal shortening strain parallel to the veins. Neither calcite strain data set has any record of a strain overprint (low negative expected values). Rare slip indicators in the basal Sundance suggest northwest southeast motion. Poorly constrained cross sections and palinspastic restorations indicate 125 km of shortening ( and uncertain transport distances), and the above evidence suggests that the SFS formed as an Eocene landslide slightly earlier than the HMS. The SFS, then, represents the largest single allochthon (terrestrial or marine) that maintained its internal stratigraphy and did not disaggregate while in motion.
Purpose and Introduction
This is a new mapping are for 2017. Like with the Mickelson project that we will do in a few days, this area is in the vicinity ofthe Heart Mountain slide. While not as famous or well studied as its younger cousin, the geology of this area is rich in complexity and will be a nice challenge for you. This project is entirely on private land, and the only road access is from the Carter Mountain road. It will be best to start high each day, and then walk down hill in a series of strike parallel traverses. The stratigraphy will include Mesozoic rocks and include the section from the Sundance through the Frontier. The most notable difference here as compared to where you have seen these rocks before is that the Morrison Formation is much thicker, and contains chert pebble conglomerate in the middle of the section. Eocene rocks of the Will wood Formation also are present here.
The structure here is rich in complexity. You will find both folds and faults. The exposure is excellent as well. So be careful that you spend the time needed to solve the structure.
Project Components and Layout
Follow the Guidelines for Map and Cross Section construction when preparing your project. The map, explanation and cross section should be constructed on the paper you are provided. You are free to design the layout of these items any way you like, but they all must fit on the same sheet of paper. You are free to design the layout of these items any way you like, but they all must fit on the same sheet of paper.
When mapping in the field or drawing cross sections, observe the following guidelines: Draw your cross section deep enough to show all of the structure. This will be the most challenging cross section that you will draw at camp.
You Will Turn In
- A final colored and inked map
- the map explanation with brief descriptions and formal names for each map unit,
- a geologic cross section assigned by the instructor(s),
- your field notebook