High-Resolution Seismic Reflection Surveys at the Manson Crater, Iowa

该项目的主要目标是利用最先进的高分辨率地震成像技术，描绘爱荷华州曼森冲击构造（MIS）内的浅层（300米以上）地下结构形态。MIS是一个年龄为65.7 Ma的陨石坑撞击特征，是白垩纪/第三纪边界铱异常的重要候选来源。因此，这个特征可能在地质历史上由于全球变化而导致的大规模灭绝的最佳记录之一中发挥了核心作用。MIS的结构、地层学和岩石学的详细知识是建立撞击动力学模型的基础数据。然而，由于30-90米厚的更新世覆盖层，结构内的确切几何形状鲜为人知。地震反射线将提供构造和冲击特征的总体地层的直接图像，并将约束从钻探断面推断的地层和岩石学解释，至少在地震线以下附近。主地震线沿火山口东西向半径，从中心峰构造到火山口外未受干扰的地层剖面。除了圈定中央隆起、护城河和环地堑的一般构造外，具体目标还包括确定断层的存在（或不存在），确定峰坑的深度和范围，以及解决中央隆起边缘冲击角砾岩和熔融带的内部几何形状。这项研究将提供基本的地质知识，有助于理解全球生物圈的变化，这种变化可能已经发生，至少部分是由于曼森撞击的直接结果。地震数据将大大提高对曼森钻井项目获得的所有数据集的解释，并将约束浅层地下结构和地层，从而建立更准确的冲击动力学模型。地震反射数据将有助于确定未来钻井的目标，以及后续详细的地面地球物理研究，如电磁、磁和微重力调查。

The major objective of this project is to delineate the shallow (upper 300 m) subsurface structural configuration within the Manson Impact Structure (MIS), Iowa, using state-of-the-art high resolution seismic imaging techniques. The MIS is a bollide impact feature 65.7 Ma in age, and is an important candidate for a source of the Cretaceous/Tertiary-boundary iridium anomaly. The feature may thus have played a central role in one of the best documented examples of mass extinction due to global change in geological history. A detailed knowledge of the structure, stratigraphy, and petrology of the MIS is needed as basic data for the construction of a model of the impact dynamics. However, the exact geometry within the structure is poorly known due to a 30-90 m thick Pleistocene till cover. Seismic reflection lines will provide a direct image of the structure and gross stratigraphy of the impact feature, and will constrain stratigraphic and petrological interpretations inferred from drilling transects, at least in the vicinity beneath the seismic lines. The major seismic line is along an east-west radius of the crater, from the central peak structure to the undisturbed stratigraphic section outside the crater. In addition to delineating the general structure of the central uplift, moat and ring graben, specific objectives include locating the presence (or absence) of faults, defining the depth and extent of the peak pit, and resolving the internal geometry of the impact breccia and melt zones at the edge of the central uplift. This study will provide fundamental geologic knowledge to contribute to an understanding of the global change in the biosphere that may have occurred, at least in part, as a direct result of the bollide impact at Manson. The seismic data will greatly enhance the interpretation of all data sets acquired as a result of the Manson drilling program, and will constrain the shallow subsurface structure and stratigraphy, allowing more accurate models of impact dynamics to be constructed. The seismic reflection data will help to identify targets for future drilling and for subsequent detailed surface geophysical studies such as electromagnetic, magnetic, and microgravity surveys by others.