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.

本部分为DS 781数据集提供了加利福尼亚州半月湾（Half Moon Bay）海域制图区的近海地质与地貌图相关数据。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。

加利福尼亚州领海范围内半月湾海域的大陆架水深较浅（0至约55米），地形平缓，向海倾斜坡度极缓（小于0.5°）。该大陆架的地貌与地质特征是区域构造作用、海平面上升、沉积过程与海洋动力过程共同作用的结果。构造作用影响与局部断层活动及抬升相关（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），导致海岸线与浪蚀平台逐步向东迁移（迁移距离约数十公里），并伴随海侵侵蚀与沉积作用（Catuneanu，2006）。如今半月湾海域为开阔大洋大陆架，承受着完整且有时较为强烈的波浪能量与强劲海流。

鉴于水深相对较浅且水动力能量较高，现代大陆架沉积物以砂质沉积（单元Qms）为主。更粗粒的砂与砾石沉积（单元Qmss与Qmsc）主要依据水深地形与高后向散射特征识别（水深地形数据；后向散射A[8101]与后向散射B[7125]——加利福尼亚州半月湾海域，DS 781）。单元Qmsc仅分布于柱角港（Pillar Point Harbor）防波堤南侧约10米水深的近岸沙坝中。单元Qmss形成于波纹冲刷洼地里的侵蚀残留沉积（例如Cacchione等，1984），分布范围更广且更分散，最大富集区位于柱角港外侧30至55米水深海域，以及柱角港南侧（水深5至15米的近岸区域）与柱角港北西北海域。

这类波纹冲刷洼地在加利福尼亚州沿岸该区域较为常见，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，导致近岸砂质沉积物相对较薄（无法完全填充洼地）。尽管单元Qmss冲刷洼地与单元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，会随季节与重大风暴事件发生变化。

大陆架沉积物以薄覆层（厚度小于2.5米或更薄）覆盖低起伏普里西马组（Purisima Formation，上中新统-上新统）或未分白垩系及（或）第三系基岩的区域，被划分为单元Qms/Tp与Qms/TKu。这类区域依据平坦地形、与中-高起伏基岩露头的连续性、高分辨率地震反射数据（详见野外活动S-15-10-NC），以及部分区域的中-高后向散射特征识别。上述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。在附近类似的高能环境中，Storlazzi等（2011）描述了蒙特雷湾北部水下基岩的季节性埋藏与抬升暴露过程。

离岸基岩露头被划分为上中新统-上新统普里西马组（单元Tp）、蒙塔拉山（Montara Mountain）白垩纪花岗质岩石（单元Kgr），以及未分白垩系及（或）第三系沉积岩（单元TKu）。上述单元通过陆上已填绘的地质露头及其延伸趋势，结合高分辨率水深地形数据揭示的独特表层纹理进行圈定（水深地形数据——加利福尼亚州半月湾海域，DS 781）。普里西马组露头形成独特的平直至弯曲的“脊状”构造，由抗侵蚀能力不同的岩性（例如砂岩与泥岩）差异侵蚀所致。与之相反，花岗质岩石具有密集的交叉裂隙表层纹理。

地图单元多边形基于多波束水深地形与后向散射数据生成的2米基础底图进行数字化勾绘。上述水深地形与后向散射数据采集于2006年至2010年间。

参考文献
Cacchione, D.A.、Drake, D.E.、Grant, W.D.与Tate, G.B.，1984。加利福尼亚州中部近岸内陆大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，爱思唯尔出版社，375页。
Gornitz, V.，2009，冰后期海平面变化，收录于Gornitz, V.主编的《古气候学与古环境百科全书》：《地球科学系列百科全书》，施普林格出版社，第887-893页。
Lambeck, K.与Chappell, J.，2001，末次冰期旋回中的海平面变化：《科学》，第292卷，第679-686页。