EXAMINING LAYER PARALLEL SHORTENING USING FIELD MEASUREMENTS, THIN SECTION ANALYSIS, AND ANALOG SANDBOX MODELS

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 omitted in the process of cross-section restoration due to a lack of understanding where and when it occurs. Bulk shortening calculated is considered to be an inherent restoration error. However, dismissing this inaccuracy could lead to error in subsurface predictions and reconstructions up to 20%, indicated by prior research. It follows that better understanding LPS location and timing can improve subsurface predictions such as porosity, permeability and fluid flow trajectories.

Field measurements and thin section analysis were utilized to calculate the amount of bulk LPS across the Colorado Front Range (CFR) to investigate this problem. Field measurements, taken across the central CFR in Boulder, Clear Creek, Jefferson, Park and Summit counties, were used in constructing a cross-section of the study area, and in assessing differences in mechanical processes known to vary during diagenesis. At outcrop, LPS can be estimated; thin section LPS calculations offer more precision, corroborate the field data and are within published ranges. The study area thin sections provide a window into how layer parallel diagenetic processes such as stylolitization, compaction, and pressure solution account for volume loss by examining grain recrystallization and directional grain dissolution.

Field data and thin section measurements were compared to analog sandbox models representatively scaled to the CFR, which has well-defined discrete tectonic events. Research questions involving the timing and amount of LPS during deformation sequences were addressed using the analog models.

Initial results indicate that layer parallel strain pulses at the onset of each tectonic event before faults and folds form, and is enhanced nearer to the base of the deforming system. General rules and best practices applicable to compressional regimes can be pulled from this study. Though results are specific to the central CFR, they can be applied to future geomechanical models and cross-section restorations in order to avoid errors propagated through the deformation sequence.