Spatial Discrimination of Complex, Low-Relief Quaternary Siliciclastic Strata Using Airborne Lidar and Near-Surface Geophysics： An Example from the Texas Coastal Plain, USA

Depositional units preserved on coastal plains worldwide control lithologic distribution in the shallow subsurface that is critical to infrastructure design and construction, and are also an important repository of information about the large-scale climate change that has occurred during many Quaternary glacial-interglacial cycles, The lateral and vertical lithologic and stratigraphic complexity of these depositional units and their response to climatic and sea-level change are poorly understood, making it difficult to pre-dict lithologic distribution and to place historical and future climate and sea-level change within a nat-ural geologic context, Mapping Quaternary siliciclastic depositional units on low-relief coastal plains traditionally has been based on their expression in aerial photographs and low-resolution topographic maps, Accuracy and detail have been hindered by low relief and lack of exposure, High-resolution air- borne lidar surveys, along with surface and borehole geophysical measurements, are being used to iden-tify subtle lateral and vertical boundaries of lithologic units on the Texas Coastal Plain within Quaternary strata, Ground and borehole conductivity measurements discriminate sandy barrier island and fluvial and deltaic channel deposits from muddy floodplain, delta-plain, and estuarine deposits, Borehole conductiv- ity and natural gamma logs similarly distinguish distinct lithologic units in the subsurface and identify erosional unconformities that likely separate units deposited during different glacial-interglacial stages, High-resolution digital elevation models obtained from airborne lidar surveys reveal previously unrecog- nized topographic detail that aids identification of surface features such as sandy channels, clay-rich interchannel deposits, and accretionary features on Pleistocene barrier islands, An optimal approach to identify lithologic and stratigraphic distribution in low-relief coastal-plain environments employs ① an initial lidar survey to produce a detailed elevation model; ② selective surface sampling and geophysical measurements based on preliminary mapping derived from lidar data and aerial imagery; and ③ borehole sampling, logging, and analysis at key sites selected after lidar and surface measure- ments are complete,

保存在全球滨海平原上的沉积单元控制着浅地层岩性分布,这对基础设施的设计和建设至关重要,同时也是许多第四纪冰期—间冰期循环中发生大规模气候变化的重要信息库。我们对这些沉积单元的横纵向岩性、地层复杂性及其对气候和海平面变化的响应知之甚少,因此很难预测岩性分布,以及将发展历史、未来气候和海平面变化置于同一自然地质环境中。传统上在低幅度滨海平原上描绘的第四纪硅质碎屑沉积单元建立在航拍图和低分辨率地形图的基础上。低幅度和低曝光率影响了观测的准确性和精确性。高分辨率机载激光雷达探测、地表地球物理勘探和地球物理测井正被用于识别得克萨斯州滨海平原上第四纪地层岩性单元的精确横纵向边界。地表和井中电导率测量能从泥质泛滥平原、三角洲平原和河道沉积中区分出砂质障壁岛、河流和三角洲河道沉积物。井中电导率和自然伽马测井记录同样可以区分地下不同的岩性单元,并识别出可能在不同冰期-间冰期阶段分离的沉积单元的侵蚀不整合面。从机载激光雷达探测获得的高分辨率数字高程模型揭示了以前不为人知的地形细节,有助于识别地表特征,如砂质通道、富含黏土的河间沉积和更新世障壁岛上的堆积特征。在低幅度滨海平原环境中识别岩性和地层分布的最佳方法是:(1)首先使用激光雷达探测生成详细的高程模型;(2)基于激光雷达数据和航拍图的初步定位,选择性进行地表采样和地球物理探测;(3)在激光雷达和地表探测完成后,选择关键位置进行钻孔取样、测井和分析。