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

本项目的主要目标是使用最先进的高分辨率地震成像技术，描绘爱荷华州曼森撞击构造（MIS）内的浅层（上部300 m）地下构造配置。MIS是65.7 Ma的火山喷发特征，是白垩纪/第三纪界线铱异常源的重要候选者。因此，这一特征可能在地质历史上全球变化导致的大规模灭绝的最佳记录中发挥了核心作用。MIS的结构，地层学和岩石学的详细知识，需要作为基础数据的冲击动力学模型的建设。然而，由于30-90米厚的更新世冰碛物覆盖，结构内的确切几何形状知之甚少。地震反射线将提供撞击特征的结构和总体地层的直接图像，并将限制至少在地震线下方附近从钻探断面推断的地层和岩石学解释。主要地震线是沿着火山口的东西半径，从中央峰结构到火山口外的未受干扰的地层剖面。除了描绘中央隆起、护城河和环形地堑的总体结构外，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的冲击角砾岩和熔融区的内部几何形状。这项研究将提供基本的地质知识，有助于了解生物圈可能发生的全球变化，至少部分是由于在曼森的火流星影响的直接结果。地震数据将大大加强对曼森钻井计划所获得的所有数据集的解释，并将限制浅层地下结构和地层，从而能够建立更准确的撞击动力学模型。地震反射数据将有助于确定未来钻探的目标，以及其他方面随后进行的详细地表地球物理研究，如电磁、磁力和微重力调查。

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.