Geology and geomorphology--Offshore Pigeon Point, California

This part of DS 781 presents data for the geologic and geomorphic map of the Offshore Pigeon Point map area, California. The vector data file is included in "Geology_OffshorePigeonPoint.zip," which is accessible from http://dx.doi.org/10.5066/F7513W80 The continental shelf within California's State waters in the Pigeon Point map area is shallow (0 to ~55 m) and flat with a very gentle (less than 0.5 degrees) offshore dip. Shelf morphology and evolution result from the interplay between local tectonics and sedimentation as sea level rose about 125 to 130 m over the last ~ 21,000 years (Lambeck and Chappel, 2001). Shelf deposits are almost exclusively sand (unit Qms) at depths less than 60 m and transition to more fine grained, muddy sediment (unit Qmsf) at greater depths in the southwestern most part of the map area. The boundary between units Qms and Qmsf was determined based on seafloor sediment samples (Reid and others, 2006) and video observations from the Offshore of Pigeon Point and adjacent map area. This boundary likely shifts seaward or landward based on seasonal to decadal changes in sediment supply, sediment transport, and wave climate. More coarse-grained sands and gravels (units Qmss and Qmsc) are primarily recognized on the basis of high backscatter. Unit Qmsc occurs as a nearshore, shore-parallel bar at typical water depths between 5 and 10 meters. Unit Qmss forms erosional lags in rippled scour depressions (for example, Cacchione and others, 1984) at water depths of about 25 to 35 m, in contact with offshore bedrock uplifts and unit Qms. Although the general areas in which unit Qmsc and unit Qmss occur are not likely to change substantially, the boundaries of the unit(s) are likely ephemeral, changing seasonally and during significant storm events. Unit Qmss deposits are common along this stretch of the California coast where offshore sandy sediment can be relatively thin (thus unable to fill the depressions) due to both lack of river input and to significant sediment erosion and offshore sediment transport during large northwest winter swells. Areas where shelf sediments form thin (< 2.5 m or less) veneers over low relief, undivided Cretaceous and (or) Tertiary bedrock are mapped as units Qms/TKu and Qms/Tp. These areas are recognized based on the combination of flat relief, continuity with moderate to high relief bedrock outcrops, high-resolution seismic-reflection data, and in some cases moderate backscatter. These units are regarded as ephemeral and dynamic sediment layers that may or may not be present based on storms, seasonal/annual patterns of sediment movement, or climate cycles. Tertiary deposits mapped in the offshore include two units of the Purisima Formation (units Tp and Tpt). The Purisima units are characterized by high backscatter and distinct bedding recognized in multibeam imagery and/or seismic-reflection data. These Tertiary rocks are underlain by or in fault contact with Upper Cretaceous basement rocks, including sedimentary rocks of the Pigeon Point Formation (unit Kpp). The Pigeon Point Formation is mapped on the basis of high backscatter, massive and (or) rugged texture on multibeam imagery, and reflection-free character on seismic-reflection data. Offshore outcrops of the Pigeon Point Formation form the offshore Pigeon Point high, a major structural feature that extends ~30 km to the northwest and represents the northeast boundary of the Outer Santa Cruz Basin (McCulloch, 1987). Areas where bedrock is exposed on the seafloor but there is less certainty regarding age are mapped as Cretaceous and Tertiary, undivided (unit TKu). Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data (see "Bathymetry--Offshore of Pigeon Point Map Area, California" and "Backscatter--Offshore of Pigeon Point Map Area, California"). The bathymetry and backscatter data were collected between 2006 and 2010. References Cited Cacchione, D.A., Drake, D.E., Grant, W.D., and Tate, G.B., 1984. Rippled scour depressions of the inner continental shelf off central California: Journal of Sedimentary Petrology, v 54, p. 1280-1291. Lambeck, K., and Chappell, J., 2001, Sea level change through the last glacial cycle: Science, v. 292, p. 679-686. McCulloch, D.S., 1987, Regional geology and hydrocarbon potential of offshore Central California, in Scholl, D.W., Grantz, A., and Vedder, J.G., eds., Geology and resource potential of the continental margin of Western North America and adjacent ocean basins - Beaufort Sea to Baja California: Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, v. 6, p. 353-401. Reid, J.A., Reid, J.M., Jenkins, C.J., Zimmerman, M., Williams, S.J., and Field, M.E., 2006, usSEABED: Pacific Coast (California Oregon, Washington) offshore surficial-sediment data release: U.S. Geological Survey Data Series 182, http://pubs.usgs.gov/ds/2006/182/.本数据集为DS 781的一部分，提供了加利福尼亚州鸽子角（Pigeon Point）近海制图区的地质与地貌图数据。矢量数据文件存储于"Geology_OffshorePigeonPoint.zip"中，可通过链接http://dx.doi.org/10.5066/F7513W80获取。 鸽子角制图区内加利福尼亚州管辖海域内的大陆架（continental shelf）为浅水区（水深0~约55米），地势平坦，向海倾斜坡度极缓（小于0.5°）。大陆架的地貌形态与演化是区域构造作用与沉积作用共同耦合的结果，而在过去约21000年间，海平面上升了约125~130米（Lambeck与Chappel，2001）。 水深小于60米的区域，大陆架沉积物几乎全为砂质沉积（Qms单元，unit Qms）；在制图区最西南部的更深水域，沉积物逐渐过渡为粒度更细的泥质沉积（Qmsf单元，unit Qmsf）。Qms与Qmsf单元的边界是基于海底沉积物采样（Reid等，2006）以及鸽子角近海及邻区制图区的视频观测结果确定的。该边界会随沉积物供给、沉积物搬运以及波浪气候的季节至年代际变化，向海或向陆方向迁移。 粒度更粗的砂与砾石（Qmss单元，unit Qmss；Qmsc单元，unit Qmsc）主要依据高反向散射（backscatter）特征识别。Qmsc单元以近岸、平行于岸线的沙坝形式产出，常见产出水深为5~10米。Qmss单元形成于约25~35米水深的波纹冲刷洼地的侵蚀残留沉积（例如Cacchione等，1984），与近海基岩隆起区及Qms单元接触。尽管Qmsc与Qmss单元的大致分布区域不会发生显著变化，但单元边界具有暂时性，会随季节变化以及重大风暴事件发生改变。Qmss沉积在加利福尼亚海岸的该区域较为常见，由于缺乏河流输入，且冬季大型西北向涌浪期间发生了显著的沉积物侵蚀与离岸搬运，使得近海砂质沉积物相对较薄，因此无法填充洼地。 大陆架沉积物在低起伏、未分异的白垩纪及（或）第三纪基岩上形成薄覆层（厚度小于2.5米或更薄）的区域，被划分为Qms/TKu单元与Qms/Tp单元。这些区域的识别依据包括平坦的地形、与中至高起伏基岩露头的连续性、高分辨率地震反射数据（seismic-reflection data），以及部分区域的中等反向散射特征。上述单元被视为暂时性的动态沉积层，其是否存在取决于风暴事件、沉积物搬运的季节/年度模式以及气候周期。 近海制图的第三纪沉积包括普雷西马组（Purisima Formation）的两个单元（Tp单元与Tpt单元）。普雷西马组单元的特征是在多波束（multibeam）影像和（或）地震反射数据中呈现高反向散射与清晰的层理。这些第三纪岩石之上覆有上白垩纪基底岩石，或与上白垩纪基底岩石呈断层接触，其中包括鸽子角组（Pigeon Point Formation，单元Kpp）的沉积岩。鸽子角组单元依据高反向散射特征、多波束影像中的块状及（或）崎岖纹理，以及地震反射数据中的无反射特征进行制图。鸽子角组的近海露头构成了鸽子角近海隆起带——一个向西北延伸约30公里的主要构造特征，同时也是外圣克鲁斯盆地的东北边界（McCulloch，1987）。海底出露基岩但年龄确定性较低的区域被划分为未分异白垩纪与第三纪单元（TKu单元，unit TKu）。 制图单元多边形是基于多波束水深与反向散射数据生成的2米基础底图层进行数字化的（详见"Bathymetry--Offshore of Pigeon Point Map Area, California"与"Backscatter--Offshore of Pigeon Point Map Area, California"）。水深与反向散射数据的采集时间为2006年至2010年。 ### 引用文献 Cacchione, D.A., Drake, D.E., Grant, W.D., 及 Tate, G.B., 1984. 加利福尼亚中部近海内陆架的波纹冲刷洼地：《沉积岩石学杂志》，第54卷，第1280-1291页。 Lambeck, K., 及 Chappel, J., 2001. 末次冰期旋回中的海平面变化：《科学》，第292卷，第679-686页。 McCulloch, D.S., 1987. 加利福尼亚中部近海的区域地质与油气潜力，收录于Scholl, D.W., Grantz, A., 及 Vedder, J.G.编辑的《北美西部大陆边缘及邻近海盆的地质与资源潜力——从波弗特海至下加利福尼亚》：环太平洋能源与矿产资源学会地球科学系列，第6卷，第353-401页。 Reid, J.A., Reid, J.M., Jenkins, C.J., Zimmerman, M., Williams, S.J., 及 Field, M.E., 2006. usSEABED：太平洋海岸（加利福尼亚州、俄勒冈州、华盛顿州）近海表层沉积物数据发布：美国地质调查局数据系列182，http://pubs.usgs.gov/ds/2006/182/。