Geology and geomorphology--Offshore of Half Moon Bay Map Area, California

This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Half Moon Bay map area, California. The vector data file is included in "Geology_OffshoreHalfMoonBay.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreHalfMoonBay/data_catalog_OffshoreHalfMoonBay.html. The continental shelf within California's State waters in the Half Moon Bay area is shallow (0 to ~55 m) and flat with a very gentle (less than 0.5 degrees) offshore dip. The morphology and geology of this shelf result from the interplay between local tectonics, sea-level rise, sedimentary processes, and oceanography. Tectonic influences are related to local faulting and uplift (see below). Sea level has risen about 125 to 130 m over the last ~21,000 years (for example, Lambeck and Chappel, 2001; Gornitz, 2009), leading to progressive eastward migration (a few tens of km) of the shoreline and wave-cut platform and associated transgressive erosion and deposition (for example, Catuneanu, 2006). The Offshore of Half Moon Bay map area is now an open-ocean shelf that is subjected to full, and sometimes severe, wave energy and strong currents. Given the relatively shallow depths and high energy, modern shelf deposits are mostly sand (unit Qms). More coarse-grained sands and gravels (units Qmss and Qmsc) are primarily recognized on the basis of bathymetry and high backscatter (Bathymetry; Backscatter A [8101]; and Backscatter B [7125]--Offshore Half Moon Bay, California, DS 781). Unit Qmsc occurs only as a nearshore bar (~ 10 m water depth) just south of the Pillar Point Harbor jetty. Unit Qmss forms erosional lags in rippled scour depressions (see, for example, Cacchione and others, 1984) and is more extensive and distributed, with the largest concentrations occurring at water depths of 30 to 55 m offshore Pillar Point, and in the nearshore (depths of 5 to 15 m) south of Pillar Point Harbor and north-northwest of Pillar Point. Such rippled-scour depressions 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 sediment supply from rivers and to significant sediment erosion and offshore transport during large winter storms. Although the general areas in which both unit Qmss scour depressions and unit Qmsc bars occur are not likely to change substantially, the boundaries of the unit(s) are likely ephemeral, changing seasonally and during significant storm events. Areas where shelf sediments form thin (< 2.5 m or less) veneers over low relief Purisima Formation (upper Miocene and Pliocene) or undifferentiated Cretaceous and (or) Tertiary bedrock are mapped as units Qms/Tp and Qms/TKu. These areas are recognized based on the combination of flat relief, continuity with moderate to high relief bedrock outcrops, high-resolution seismic-reflection data (see field activity S-15-10-NC), and in some cases moderate to high backscatter. These units are regarded as ephemeral and dynamic sediment layers that may or may not be present at a specific location based on storms, seasonal/annual patterns of sediment movement, or longer-term climate cycles. In a nearby similarly high-energy setting, Storlazzi and others (2011) have described seasonal burial and exhumation of submerged bedrock in northern Monterey Bay. Offshore bedrock outcrops are mapped as the upper Miocene and Pliocene Purisima Formation (unit Tp), the Cretaceous granitic rocks of Montara Mountain (unit Kgr), and undivided sedimentary rocks of Cretaceous and (or) Tertiary age (unit TKu). These units are delineated through extending outcrops and trends from mapped onshore geology and from their distinctive surface textures as revealed by high-resolution bathymetry (Bathymetry--Offshore Half Moon Bay, California, DS 781). Purisima Formation outcrops form distinctive straight to curved "ribs," caused by differential erosion of more- and less-resistant lithologies (for example, sandstone and mudstone). In contrast, granitic rocks have a densely cross-fractured surface texture. Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data. 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. Catuneanu, O., 2006, Principles of Sequence Stratigraphy: Amsterdam, Elsevier, 375 p. Gornitz, V., 2009, Sea level change, post-glacial, in Gornitz, V., ed., Encyclopedia of Paleoclimatology and Ancient Environments: Encyclopedia of Earth Sciences Series. Springer, pp. 887-893. Lambeck, K., and Chappell, J., 2001, Sea level change through the last glacial cycle: Science, v. 292, p. 679-686.本数据集为DS 781的组成部分，提供加利福尼亚州半月湾近海区域的地质与地貌图数据。矢量数据文件收录于"Geology_OffshoreHalfMoonBay.zip"，可通过http://pubs.usgs.gov/ds/781/OffshoreHalfMoonBay/data_catalog_OffshoreHalfMoonBay.html获取。 半月湾区域加州州属水域内的大陆架（continental shelf）水深较浅（0至约55米），地势平坦，向海倾斜坡度极缓（小于0.5°）。该陆架的地貌与地质特征是区域构造作用、海平面上升（sea-level rise）、沉积作用（sedimentary processes）与海洋动力相互作用的结果。构造影响与局部断层作用（faulting）和抬升（uplift）相关（详见下文）。过去约21000年间，海平面上升了约125至130米（Lambeck与Chappel，2001；Gornitz，2009），导致海岸线（shoreline）及浪蚀台地（wave-cut platform）向东渐进式迁移（数十公里），伴随海侵侵蚀与沉积（transgressive erosion and deposition）作用（Catuneanu，2006）。如今半月湾近海区域为开阔海洋陆架（open-ocean shelf），承受着全强度甚至高强度的波浪能（wave energy）与强劲海流（current）。 鉴于水深较浅且水动力条件较强，现代陆架沉积物（shelf deposits）以砂质为主（地层单元Qms）。较粗粒的砂与砾石（coarse-grained sands and gravels，地层单元Qmss与Qmsc）主要依据水深测绘（bathymetry）与高反向散射（backscatter）数据识别（水深测绘；反向散射A[8101]及反向散射B[7125]——加利福尼亚州半月湾近海，DS 781）。地层单元Qmsc仅分布在皮拉尔角港防波堤以南约10米水深的近岸沙坝（nearshore bar）区域。地层单元Qmss则形成于波纹冲刷洼地（rippled scour depressions）的侵蚀残留沉积（erosional lags）（Cacchione等，1984），分布范围更广，最大富集区位于皮拉尔角港外30至55米水深海域，以及皮拉尔角港以南、皮拉尔角港北北西方向的近岸（水深5至15米）区域。这类波纹冲刷洼地在加州沿岸该段较为常见，由于河流沉积物供给不足，加之冬季大型风暴期间发生显著的沉积物侵蚀与离岸搬运，近岸砂质沉积物相对较薄，无法填充洼地。尽管Qmss冲刷洼地与Qmsc沙坝的大致分布区域不会发生显著变化，但地层单元的边界具有暂时性，会随季节变化与重大风暴事件发生改变。 陆架沉积物以薄覆层（veneer，厚度小于2.5米或更薄）覆盖于低起伏的普里西马组（Purisima Formation，上新统至上新统upper Miocene and Pliocene）或未分白垩纪（Cretaceous）及（或）第三纪（Tertiary）基岩（bedrock）之上的区域，被划分为Qms/Tp与Qms/TKu地层单元。这类区域通过平坦地形、与中至高起伏基岩露头的连续性、高分辨率地震反射数据（high-resolution seismic-reflection data，野外活动S-15-10-NC），以及部分区域的中至高反向散射特征综合识别。这些地层单元被认为是暂时性的动态沉积层，其在特定位置是否存在取决于风暴事件、沉积物运移的季节/年度模式，或长期气候周期。在邻近的类似高能环境中，Storlazzi等（2011）曾描述了蒙特雷湾北部水下基岩的季节性埋藏与出露过程。 近海基岩露头被划分为上新统至上新统普里西马组（地层单元Tp）、蒙塔拉山的白垩纪花岗岩类岩石（granitic rocks，地层单元Kgr），以及未分白垩纪及（或）第三纪沉积岩（地层单元TKu）。这些地层单元通过陆上已填绘的地质露头及其延伸趋势，结合高分辨率水深测绘揭示的独特表面纹理进行圈定（水深测绘——加利福尼亚州半月湾近海，DS 781）。普里西马组露头呈现独特的平直至弯曲的“脊状”形态，由抗侵蚀性不同的岩性（lithologies，如砂岩sandstone与泥岩mudstone）差异侵蚀（differential erosion）形成。与之相反，花岗岩类岩石表面呈现密集的交叉裂隙（cross-fractured）纹理。 地图单元多边形是基于多波束水深测绘（multibeam bathymetry）与反向散射数据生成的2米基础底图进行数字化勾绘的。水深与反向散射数据采集于2006至2010年间。 参考文献 Cacchione, D.A., Drake, D.E., Grant, W.D., 与 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。 Catuneanu, O., 2006, 《层序地层学原理》：阿姆斯特丹，爱思唯尔（Elsevier），375页。 Gornitz, V., 2009, 冰后期海平面变化，载于Gornitz, V. 主编，《古气候学与古环境百科全书：地球科学系列百科全书》，施普林格（Springer），第887-893页。 Lambeck, K., 与 Chappell, J., 2001, 末次冰期旋回的海平面变化：《科学》（Science），第292卷，第679-686页。