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Investigating Layer Parallel Diagenetic Shortening Using Field Data, Thin Section Analysis, and Analog Models

Abstract

Layer parallel shortening (LPS) is deformation that occurs in a compressive stress regime parallel to bedding, including both mechanical and chemical changes. Typically this deformation is not incorporated into cross-section restoration, although an amount of bulk shortening can be calculated, because the amount and timing of increments of shortening is unknown. Previous research indicates that omitting this deformation can lead to error in palinspastic reconstructions and subsurface predictions up to 20%. A better understanding of when and where LPS deformation is accommodated can improve predictions important to the hydrocarbon industry such as fluid flow trajectories, trap volume, trap location, porosity and permeability. To address this issue, a combination of field measurements and thin section analysis calculated the amount of LPS across the central Colorado Front Range (CFR) system. Though the overarching goal of the research is to locate where and when LPS is accommodated during a deformation sequence, this contribution focuses on how much LPS is taken up by processes such as compaction and pressure solution. A transect of measurements across the central CFR was recorded, including attitude of foliations, bedding, fault planes, and stylolitic surfaces. Trend and plunge measurements of fold hinges and mineral lineations along fault planes were also recorded. Relative stylolite frequencies and fracture patterns were studied to assess variation in mechanical and chemical properties. Layer parallel structural diagenesis was assessed from these data as well as on the micro scale by examining directional grain dissolution and diagenetic histories in thin section. Analog sandbox models, scaled to be representative of the CFR, were deformed incrementally in accordance with Colorado's well-constrained tectonic history. Field data were compared to the model to test the robustness and applicability of the field measurements. Preliminary results indicate that layer parallel deformation occurs at the onset of each tectonic event before discrete structures are formed. LPS can be estimated at outcrop and precise calculations from thin section measurements corroborate the field data. These calculated LPS amounts fall within previously reported ranges. Whilst these results are specific to the CFR, a set of best practice recommendations, applicable to the general case, can be applied to future geomechanical models and cross-sections.