RUI: Coupled Fold-fracture Evolution in the Stillwell Anticline, West Texas

断层、褶皱和裂缝系统的相互关联的形成和演化仍然是一个重要的研究领域，因为这些特征对地下流体的运动具有内在的重要性。对德克萨斯州西部暴露良好的斯蒂尔韦尔背斜进行的勘察调查发现，褶皱几何形状上的走向变化与褶皱演化的不同阶段有关；褶皱-S几何形状支持受下伏基底断裂系统传播控制的地层。过去的研究揭示了在这种背景下在裂缝系统发育中发挥重要作用的一系列因素，包括初始和过渡褶皱几何形状的影响，先前存在的裂缝的影响，层间滑动的存在或不存在，以及负责褶皱发育的下伏断层系统的影响。然而，很少有研究整合裂缝应变和宽度的定量描述，或者探索如何利用裂缝水泥结构来解开裂缝张开的时间或与裂缝网络相关的残留孔洞。这个项目的目标是整合一系列证据，试图回答四个主要的研究问题：1)背斜系统中的构造位置是否可以作为裂缝强度的预测指标？2)机械地层学能否与裂缝地层学相关联？3)裂缝组的分布、强度、方向和胶结物在褶皱形成之前、期间和之后如何变化？以及4)褶皱系统内不同位置的应变还有哪些(与断裂无关的)变形模式？背斜的良好暴露提供了直接观察褶皱层内的裂缝、断层和变形带的机会，这可以帮助约束褶皱的运动演化。此外，结合施密特锤法测试这些岩性，详细绘制横断面裂缝网络图，以提供关于变形岩层刚性的米级半定量信息，将生成可在褶皱和裂缝形成背景下评估的详细机械地层学。与力学行为分析密切相关的是，研究小组将基于褶皱系统内的层厚度和结构位置进行裂缝强度分析，并考虑到裂缝大小的广泛范围。这种方法可以评估裂缝强度的尺度依赖性，这可能是地下岩心研究中渗透率评估的一个严重问题。此外，标准的显微岩相学分析和基于扫描电子显微镜的阴极发光成像将有助于记录开放模式裂缝胶结物中的生长结构，以确定渗透率的同步运动变化。最后，将使用与断层相关的褶皱形成的计算机运动学模拟来约束整个褶皱系统的预测应变分布，以便将裂缝形成的时间与褶皱演化阶段联系起来。拟议的研究结果将为了解褶皱形成过程中渗透率的演变提供一个新的视角。有关地下水和石油勘探的重要决策通常是在数据不完整的情况下做出的。如果拟议的研究能够回答上述问题，那么对Stillwell背斜系统的褶皱-裂缝耦合调查将提供一个模型，应该可以减少全球范围内井约束较少的系统中渗透率评估的不确定性。除了研究的科学目标外，该项目还将本科生现场教学与现场研究相结合，确保本科生研究人员在他们自己的工作背景下对基本地质过程有一个透彻的全面了解。此外，三一大学本科生将有机会前往研究实验室，与较大研究机构的教职员工一起进行分析；将通过独立的研究论文、高级论文和在地区和国家会议上的陈述来传播他们的研究成果；所有项目研究成果和后续产品将通过与三一大学图书馆的合作项目向所有三一学生和公众开放。

The linked formation and evolution of faults, folds, and fracture systems continue to be an important area of research because of the inherent importance of these features to the movement of fluids in the subsurface. A reconnaissance investigation of the well-exposed Stillwell anticline in west Texas has revealed along strike variation in fold geometry tied to different stages of fold evolution; the fold?s geometry supports formation controlled by propagation of an underlying basement fault system. Past research has revealed a range of factors that play important roles in the development of fracture systems in this context, including the influence of initial and transitional fold geometries, the influence of pre-existing fractures, the presence or absence of interlayer slip, and the effects of the underlying fault systems that are responsible for fold development. However, few studies have integrated a quantitative description of fracture strain and width or explored how fracture cement textures can be used to unravel the timing of fracture opening or relict porosities associated with fracture networks. The goal of this project is to integrate an array of evidence in an attempt to answer four primary research questions including: 1) Can structural position within an anticline system be used as a predictor of fracture intensity?; 2) Can mechanical stratigraphy be correlated with fracture stratigraphy?; 3) How to do the distributions, intensities, orientations, and cements of fractures sets vary before, during, and after fold formation?; and 4) What other (non-fracture-related) modes of deformation accommodate strain at different positions within a fold system?The excellent exposure of the anticline provides the opportunity to directly observe fractures, faults, and deformation bands within the folded layers, which can be used to help constrain the kinematic evolution of the fold. In addition, detailed mapping of cross-sectional fracture networks in concert with Schmidt hammer testing of these lithologies, to provide meter-scale semi-quantitative information about the rigidity of deformed beds, will generate a detailed mechanical stratigraphy that can be evaluated in the context of fold and fracture formation. Closely linked with analysis of mechanical behavior, the research team will apply fracture intensity analysis based on both layer thicknesses and structural position within the fold system, taking the wide spectrum of fracture sizes into account. This approach permits evaluation of the scale-dependence of fracture intensity, which can be a serious problem in the evaluation of permeability in subsurface core-based studies. In addition, standard microscopic petrographic analysis and scanning-electron-microscope-based cathodoluminescence imaging will help document growth textures in opening-mode fracture cements to determine synkinematic changes in permeability. Finally, computer kinematic modeling of fault-related fold formation will be used to constrain the predicted distribution of strain across the fold system in order to relate the timing of fracture formation to stages of fold evolution. The results of the proposed research will provide a new view of how permeability evolves during fold formation. Important decisions regarding subsurface water and petroleum exploration are commonly made with incomplete data. If the proposed research can answer the questions posed above, then this coupled fold-fracture investigation of the Stillwell anticline system will provide a model that should reduce uncertainty in permeability evaluation in less well-constrained systems worldwide. In addition to the scientific objectives of the research, the project is integrating undergraduate field instruction with field research, ensuring that undergraduate researchers gain a thorough overall understanding of fundamental geological processes in the context of their own work. In addition, Trinity University undergraduate students will have the opportunity to travel to research labs to perform analyses with faculty at larger research institutions; will disseminate results of their research through independent research papers, senior theses, and presentations at both regional and national meetings; and all project research results and subsequent products will be made accessible to all Trinity students and to the general public through a collaborative project with the Trinity University library.