Date of this Version
Pierson, N., and Burberry, C.M., 2014, Assessing Layer Parallel Shortening in the Eastern Colorado Front Range Using Thin Section Analysis and Analog Sandbox Models, [MS thesis], Lincoln, University of Nebraska-Lincoln, 45 p.
Layer parallel shortening (LPS) is an expression of compressive strain that occurs parallel to bedding surfaces. LPS is chiefly expressed by chemical changes to sediment volume, including stylolitization, porosity reduction and impingement of grains. This strain is typically omitted in the process of cross-section restoration due to a lack of understanding of spatial and temporal LPS accommodation. Bulk shortening calculated is considered to be a minimum shortening estimate. However, dismissing this inaccuracy due to shortening can lead to significant error in subsurface predictions and reconstructions.
To address this problem, a combination of thin section analysis and analog modeling assessed the amount of LPS across the central Colorado Front Range (CFR) system. A geologic cross-section of the eastern CFR was built using field data and a minimum bulk shortening was calculated. A scaled analog model was deformed incrementally in accordance with the cross-section restoration and Colorado’s well-constrained tectonic history. Cross-sections from the model were restored and compared to the cross-section of the CFR.
LPS results from the analog model and from thin sections show a complex relationship between LPS and geologic setting. LPS is variable with respect to depth in the stratigraphic column, with respect to divisions of mechanical strength in the stratigraphy, and in proximity to meso- and macroscale structures. Thus, there is no single rule for spatial and temporal LPS accommodation that can be applied to any deformed belt. LPS needs to be treated separately in each stratigraphic or mechanical unit and varies with increasing distance from major structural features such as faults. LPS is measurable, and can be addressed with strategic sampling using oriented core in any region dominated by sandstone and limestone.
Adviser: Caroline M. Burberry