Geology and geomorphology--Offshore of Santa Barbara, California

This part of SIM 3281 presents data for the geologic and geomorphic map (see sheet 10, SIM 3281) of the Offshore of Santa Barbara map area, California. The vector data file is included in "Geology_OffshoreSantaBarbara.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreSantaBarbara/data_catalog_OffshoreSantaBarbara.html.
The offshore part of the map area largely consists of a relatively shallow (less than 75 m deep), gently offshore-dipping (less than 1 degree) shelf underlain by sediments derived primarily from relatively small coastal watersheds that drain the Santa Ynez Mountains. Shelf deposits are primarily sand (unit Qms) at water depths less than about 35 to 50 m and, at depths greater than about 35 to 50 m, are the more fine-grained sediments (very fine sand, silt, and clay) of unit Qmsf. The boundary between units Qms and Qmsf is based on observations and extrapolation from sediment sampling (see, for example, Reid and others, 2006) and camera ground-truth surveying (see sheet 6). It is important to note that the boundary between units Qms and Qmsf should be considered transitional and approximate and is expected to shift as a result of seasonal- to annual- to decadal-scale cycles in wave climate, sediment supply, and sediment transport.
Coarser grained deposits (coarse sand to boulders) of unit Qmsc, which are recognized on the basis of their high backscatter and, in some cases, their moderate seafloor relief (sheets 1, 2, 3), are found most prominently in a large (about 0.75 km2) lobe that is present from about 1,800 to 3,600 m offshore of the mouth of Arroyo Burro, in water depths of about 36 to 65 m. The lobe is inferred to consist of coarse-grained sediment (coarse sand to boulders) that is resistant to erosion. Although these coarse-grained deposits almost certainly are derived from Arroyo Burro, the lobe could represent either the underflow deposits of late Holocene floods or a relict geomorphologic feature, having been deposited in shallower marine deltaic (or even alluvial?) environments at lower sea levels in the latest Pleistocene and early Holocene. Unit Qmsc also is present in shallower water (depths of about 10 to 20 m), most notably in a small area (approximately 0.09 km2) that extends offshore from Montecito Creek, in the eastern part of the map area.
The presence of coarser grained sediment (coarse sand and possibly gravel) also is inferred in shallower water (depths of 10 to 20 m) offshore from Arroyo Burro, but these deposits are mapped as unit Qmss because they are found within arcuate scour depressions that have been referred to as "rippled scour depressions" (see, for example, Cacchione and others, 1984; Phillips, 2007) or "sorted bedforms" (see, for example, Murray and Thieler, 2004; Goff and others, 2005; Trembanis and Hume, 2011). Although the general area in which Qmss scour depressions are found is not likely to change substantially, the boundaries of the unit(s), as well as the locations of individual depressions and their intervening flat sand sheets, likely are ephemeral, changing during significant storm events.
Hydrocarbon-seep-induced topography, which is present most prominently along the axis of anticlines, includes many features (described by Keller and others, 2007) along the trend of the Mid-Channel Anticline, about 10 km south of the map area in the Santa Barbara Channel. Geologic map units associated with hydrocarbon emissions in the map area include grouped to solitary pockmarks (unit Qmp) and asphalt (tar) deposits (unit Qas), as well as areas of undifferentiated hydrocarbon-related features (unit Qhfu) that probably include a mix of mounds, mud volcanoes, pockmarks, carbonate mats, and other constructional and erosional "seabed forms" (see Keller and others, 2007), all of which are superimposed on consolidated, undivided Miocene and Pliocene bedrock (unit Tbu).
Offshore bedrock exposures are assigned to the Miocene Monterey Formation (unit Tm) and to the undivided Miocene and Pliocene bedrock unit (Tbu), primarily on the basis of extrapolation from the onland geologic mapping of Minor and others (2009), as well as the geologic cross sections of Redin (2005). These cross sections, which are constrained by industry seismic-reflection data and petroleum well logs, suggest that a considerable part of the undivided bedrock unit may belong to the Pliocene and Pleistocene Pico Formation. Bedrock is, in some places, overlain by a thin (less than 1 m?) veneer of sediment, recognized on the basis of high backscatter, flat relief, continuity with moderate- to high-relief bedrock outcrops, and (in some cases) high-resolution seismic-reflection data; these areas, which are mapped as composite units Qms/Tbu or Qms/Tm, are interpreted as ephemeral sediment layers that may or may not be continuously present, depending on storms, seasonal and (or) annual patterns of sediment movement, or longer term climate cycles.
The Santa Barbara Channel region, including the map area, has a long history of petroleum production (Barnum, 1998). The Monterey Formation is the primary petroleum-source rock in the Santa Barbara Channel, and the Pico Formation is one of the primary petroleum reservoirs. The bedrock units typically are exposed in structural highs that include uplifts associated with the partly blind(?), south-dipping Rincon Creek Fault Zone and the outer shelf anticlinal uplift that developed above the south strand of the Red Mountain Fault in the southwestern part of the map area.
The Offshore of Santa Barbara map area is in the Ventura Basin, in the southern part of the Western Transverse Ranges geologic province, which is north of the California Continental Borderland (Fisher and others, 2009). This province has undergone significant north-south compression since the Miocene, and recent GPS data suggest north-south shortening of about 6 mm/yr (Larson and Webb, 1992). The active, east-west-striking Red Mountain and Rincon Creek Faults and their related folds are some of the structures on which this shortening occurs. This fault system, in aggregate, extends for about 100 km through the Ventura and Santa Barbara Basins and represents an important earthquake hazard (see, for example, Fisher and others, 2009). Very high uplift rates of onland marine terraces from More Mesa (2.2 mm/yr), in the western part of the map area, to Summerland (0.7 mm/yr), a few kilometers east of the map area, are further indication of rapid shortening in this region (Keller and Gurrola, 2000).
References Cited:
Barnum, H.P., 1998, Redevelopment of the western portion of the Rincon offshore oil field, Ventura, California, in Kunitomi, D.S., Hopps, T.E., and Galloway, J.M., eds., Structure and petroleum geology, Santa Barbara Channel, California: American Association of Petroleum Geologists, Pacific Section, and Coast Geological Society, Miscellaneous Publication 46, p. 201-215.
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. 1,280-1,291.
Dibblee, T.W., Jr., 1986a, Geologic map of the Carpinteria quadrangle, Santa Barbara County, California: Santa Barbara, Calif., Dibblee Geological Foundation Map DF-04, scale 1:24,000.
Dibblee, T.W., Jr., 1986b, Geologic map of the Santa Barbara quadrangle, Santa Barbara County, California: Santa Barbara, Calif., Dibblee Geological Foundation Map DF-06, scale 1:24,000.
Fisher, M.A., Sorlien, C.C., and Sliter, R.W., 2009, Potential earthquake faults offshore southern California from the eastern Santa Barbara channel to Dana Point, in Lee, H.J., and Normark, W.R., eds., Earth science in the urban ocean--The Southern California Continental Borderland: Geological Society of America Special Paper 454, p. 271-290.
Goff, J.A., Mayer, L.A., Traykovski, P., Buynevich, I., Wilkens, R., Raymond, R., Glang, G., Evans, R.L., Olson, H., and Jenkins, C., 2005, Detailed investigations of sorted bedforms or "rippled scour depressions," within the Martha's Vineyard Coastal Observatory, Massachusetts: Continental Shelf Research, v. 25, p. 461-484.
Keller, E.A., Duffy, M., Kennett, J.P., and Hill, T., 2007, Tectonic geomorphology and hydrocarbon potential of the Mid-Channel anticline, Santa Barbara Basin, California: Geomorphology, v. 89, p. 274-286.
Keller, E.A., and Gurrola, L.D., 2000, Final report, July, 2000--Earthquake hazard of the Santa Barbara fold belt, California: NEHRP Award #99HQGR0081, SCEC Award #572726, 78 p., available at http://www.scec.org/research/98research/98gurrolakeller.pdf.
Larson, K.M., and Webb, F.H., 1992, Deformation in the Santa Barbara Channel from GPS measurements 1987-1991: Geophysical News Letters, v. 19, p. 1,491-1,494.
Minor, S.A., Kellogg, K.S., Stanley, R.G., Gurrola, L.D., Keller, E.A., and Brandt, T.R., 2009, Geologic map of the Santa Barbara coastal plain area, Santa Barbara County, California: U.S. Geological Survey Scientific Investigations Map 3001, scale 1:25,000, 1 sheet, pamphlet 38 p., available at http://pubs.usgs.gov/sim/3001/.
Murray, B., and Thieler, E.R., 2004, A new hypothesis and exploratory model for the formation of large-scale inner-shelf sediment sorting and "rippled scour depressions": Continental Shelf Research, v. 24, no. 3, p. 295-315.
Phillips, E., 2007, Exploring rippled scour depressions offshore Huntington Beach, CA: Santa Cruz, University of California, M.S. thesis, 58 p.
Redin, T., 2005, Santa Barbara Channel structure and correlation sections--Correlation Section no. 34A, Summerland area, Santa Ynez Mountains, across the east central Santa Barbara Channel to the China Bay area, Santa Cruz Island: American Association of Petroleum Geologists, Pacific Section, Publication CS 34A, 1 sheet.
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, available at http://pubs.usgs.gov/ds/2006/182/.
Trembanis, A.C., and Hume, T.M., 2011, Sorted bedforms on the inner shelf off northeastern New Zealand--Spatiotemporal relationships and potential paleo-environmental implications: Geo-Marine Letters, v. 31, p. 203-214.
Weber, K.M., List, J.H., and Morgan, K.L., 2005, An operational Mean High Water datum for determination of shoreline position from topographic lidar data: U.S. Geological Survey Open-File Report 2005-1027, available at http://pubs.usgs.gov/of/2005/1027/.

本数据集隶属于SIM 3281系列，包含加利福尼亚州圣巴巴拉近海制图区的地质与地貌图数据（详见SIM 3281第10幅图幅）。矢量数据文件收录于"Geology_OffshoreSantaBarbara.zip"压缩包，可通过http://pubs.usgs.gov/ds/781/OffshoreSantaBarbara/data_catalog_OffshoreSantaBarbara.html获取。

该制图区的近海区域主体为水深不足75米的平缓陆架，整体向海缓倾（倾角小于1°），基底沉积物主要源自圣伊内斯山脉的小型沿海流域。陆架沉积在水深不足约35~50米时以砂质为主，对应地层单元Qms（unit Qms）；水深大于该范围的陆架沉积则以细粒沉积物（极细砂、粉砂及黏土）为主，对应地层单元Qmsf（unit Qmsf）。Qms与Qmsf地层单元的分界依据沉积物采样（例如Reid等，2006）与相机实地测量（详见第6幅图幅）的观测结果及外推数据划定。需特别说明的是，Qms与Qmsf的分界具有过渡性与近似性，其位置会随波浪气候、沉积物供给及输运的季节、年际至年代际周期变化而发生偏移。

地层单元Qmsc（unit Qmsc）的粗粒沉积（从粗砂至漂砾）可通过高后向散射特征及部分区域的中等海底地形起伏（第1、2、3幅图幅）识别。该类沉积最显著的分布区为阿罗约布鲁罗河口外约1800~3600米、水深36~65米处的一处大型沉积舌（面积约0.75平方千米）。该沉积舌被推断由抗侵蚀的粗粒沉积物（粗砂至漂砾）组成。尽管此类粗粒沉积几乎肯定源自阿罗约布鲁罗河，但该沉积舌可能是全新世晚期洪水的底流沉积，也可能是残留地貌：其形成于晚更新世末期至全新世早期海平面较低时期的浅海三角洲（甚至冲积）环境中。地层单元Qmsc也分布于较浅水区（水深约10~20米），其中最显著的一处位于制图区东部的蒙特西托溪近海区域，面积约0.09平方千米。

阿罗约布鲁罗河近海的浅水区（水深10~20米）也被推断存在粗粒沉积物（粗砂，可能含砾石），但此类沉积被划归为地层单元Qmss（unit Qmss），因为它们分布于被称为“波纹状冲刷洼地”（例如Cacchione等，1984；Phillips，2007）或“分选沉积床形”（例如Murray与Thieler，2004；Goff等，2005；Trembanis与Hume，2011）的弧形冲刷洼地内。尽管Qmss地层单元对应的冲刷洼地分布的总体区域不会发生显著变化，但该地层的边界、单个洼地的位置及其间的平坦砂质床面，均可能具有暂时性，会在强风暴事件中发生改变。

烃渗漏诱导形成的地形最显著地分布于背斜轴部，其中在制图区以南约10千米的圣巴巴拉海峡中，沿中通道背斜走向分布有多处该类地貌（详见Keller等，2007）。该制图区内与烃类排放相关的地质填图单元包括单个或成群分布的麻坑（地层单元Qmp，unit Qmp）、沥青（焦油）沉积（地层单元Qas，unit Qas），以及未分化的烃类相关地貌区（地层单元Qhfu，unit Qhfu）：后者可能包含丘状体、泥火山、麻坑、碳酸盐岩垫及其他建造型与侵蚀型“海底形态”（详见Keller等，2007）。所有上述地貌均覆盖于固结的未划分中新世与上新世基岩（地层单元Tbu，unit Tbu）之上。

近海出露的基岩被划分为中新世蒙特雷组（地层单元Tm，unit Tm）与未划分的中新世-上新世基岩单元（Tbu），其划分依据主要为Minor等（2009）的陆上地质填图外推结果，以及Redin（2005）的地质剖面数据。这些以工业地震反射数据与石油测井曲线为约束的剖面显示，未划分基岩单元的相当一部分可能属于上新世-更新世皮科组。部分区域的基岩之上覆盖有厚度不足1米的沉积物薄覆层，其识别依据包括高后向散射特征、平坦地形、与中-高起伏基岩露头的连续性，以及部分区域的高分辨率地震反射数据；此类区域被划分为复合地层单元Qms/Tbu或Qms/Tm，被解释为暂时性沉积层：其是否持续存在取决于风暴事件、沉积物运移的季节/年际规律，或长期气候周期。

包括本制图区在内的圣巴巴拉海峡区域拥有悠久的石油开采历史（Barnum，1998）。蒙特雷组是圣巴巴拉海峡的主要烃源岩，而皮科组则是主要的油气储层之一。基岩单元通常出露于构造高点，包括与部分隐伏的向南倾伏林孔溪断裂带相关的抬升区，以及制图区西南部红山断裂南支上方发育的陆架外缘背斜抬升区。

圣巴巴拉近海制图区位于文图拉盆地内，该盆地隶属于加利福尼亚大陆边缘带以北的西横断山脉地质省（Fisher等，2009）。该地质省自中新世以来经历了显著的南北向挤压，最新GPS数据显示其南北向缩短速率约为6毫米/年（Larson与Webb，1992）。活动的东西向红山断裂与林孔溪断裂及其相关褶皱，便是该挤压缩短作用发生的部分构造载体。该断裂系统整体延伸约100千米，横穿文图拉与圣巴巴拉盆地，是重要的地震危险源（例如Fisher等，2009）。从制图区西部的莫尔台地（抬升速率2.2毫米/年）到制图区以东数千米的萨默兰（抬升速率0.7毫米/年），陆上海蚀阶地的抬升速率极高，进一步证实了该区域的快速挤压缩短作用（Keller与Gurrola，2000）。

参考文献
Barnum, H.P., 1998, Redevelopment of the western portion of the Rincon offshore oil field, Ventura, California, in Kunitomi, D.S., Hopps, T.E., and Galloway, J.M., eds., Structure and petroleum geology, Santa Barbara Channel, California: American Association of Petroleum Geologists, Pacific Section, and Coast Geological Society, Miscellaneous Publication 46, p. 201-215.
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.
Dibblee, T.W., Jr., 1986a, Geologic map of the Carpinteria quadrangle, Santa Barbara County, California: Santa Barbara, Calif., Dibblee Geological Foundation Map DF-04, scale 1:24,000.
Dibblee, T.W., Jr., 1986b, Geologic map of the Santa Barbara quadrangle, Santa Barbara County, California: Santa Barbara, Calif., Dibblee Geological Foundation Map DF-06, scale 1:24,000.
Fisher, M.A., Sorlien, C.C., and Sliter, R.W., 2009, Potential earthquake faults offshore southern California from the eastern Santa Barbara channel to Dana Point, in Lee, H.J., and Normark, W.R., eds., Earth science in the urban ocean--The Southern California Continental Borderland: Geological Society of America Special Paper 454, p. 271-290.
Goff, J.A., Mayer, L.A., Traykovski, P., Buynevich, I., Wilkens, R., Raymond, R., Glang, G., Evans, R.L., Olson, H., and Jenkins, C., 2005, Detailed investigations of sorted bedforms or "rippled scour depressions," within the Martha's Vineyard Coastal Observatory, Massachusetts: Continental Shelf Research, v. 25, p. 461-484.
Keller, E.A., Duffy, M., Kennett, J.P., and Hill, T., 2007, Tectonic geomorphology and hydrocarbon potential of the Mid-Channel anticline, Santa Barbara Basin, California: Geomorphology, v. 89, p. 274-286.
Keller, E.A., and Gurrola, L.D., 2000, Final report, July, 2000--Earthquake hazard of the Santa Barbara fold belt, California: NEHRP Award #99HQGR0081, SCEC Award #572726, 78 p., available at http://www.scec.org/research/98research/98gurrolakeller.pdf.
Larson, K.M., and Webb, F.H., 1992, Deformation in the Santa Barbara Channel from GPS measurements 1987-1991: Geophysical News Letters, v. 19, p. 1491-1494.
Minor, S.A., Kellogg, K.S., Stanley, R.G., Gurrola, L.D., Keller, E.A., and Brandt, T.R., 2009, Geologic map of the Santa Barbara coastal plain area, Santa Barbara County, California: U.S. Geological Survey Scientific Investigations Map 3001, scale 1:25,000, 1 sheet, pamphlet 38 p., available at http://pubs.usgs.gov/sim/3001/.
Murray, B., and Thieler, E.R., 2004, A new hypothesis and exploratory model for the formation of large-scale inner-shelf sediment sorting and "rippled scour depressions": Continental Shelf Research, v. 24, no. 3, p. 295-315.
Phillips, E., 2007, Exploring rippled scour depressions offshore Huntington Beach, CA: Santa Cruz, University of California, M.S. thesis, 58 p.
Redin, T., 2005, Santa Barbara Channel structure and correlation sections--Correlation Section no. 34A, Summerland area, Santa Ynez Mountains, across the east central Santa Barbara Channel to the China Bay area, Santa Cruz Island: American Association of Petroleum Geologists, Pacific Section, Publication CS 34A, 1 sheet.
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, available at http://pubs.usgs.gov/ds/2006/182/.
Trembanis, A.C., and Hume, T.M., 2011, Sorted bedforms on the inner shelf off northeastern New Zealand--Spatiotemporal relationships and potential paleo-environmental implications: Geo-Marine Letters, v. 31, p. 203-214.
Weber, K.M., List, J.H., and Morgan, K.L., 2005, An operational Mean High Water datum for determination of shoreline position from topographic lidar data: U.S. Geological Survey Open-File Report 2005-1027, available at http://pubs.usgs.gov/of/2005/1027/.