Geologic map and digital database of the Yucaipa 7.5' quadrangle, San Bernardino and Riverside Counties, California

The Yucaipa 7.5' quadrangle is located at the southeastern margin of the San Bernardino Basin, an extensional region situated within a right-step-over zone between the San Jacinto and San Andreas Fault zones. The quadrangle is traversed by several faults of the San Andreas system, including (from oldest to youngest) the Banning Fault and the Wilson Creek, Mission Creek, Mill Creek, and San Bernardino Strands of the San Andreas Fault. The Mill Creek Strand of the San Andreas Fault is the easternmost strand of the San Andreas in the Yucaipa quadrangle. It separates granitic and metamorphic rocks of the San Bernardino Mountains block from a thin slice of similar rocks on Yucaipa Ridge, and thus has only a small amount of strike-slip displacement. The Wilson Creek Strand traverses Yucaipa Ridge and converges toward the Mlll Creek Strand in the Santa Ana river Canyon. The fault has juxtaposed an igneous and metamorphic complex (Wilson Creek block) and overlying nonmarine sedimentary rocks (Mill Creek Formation of Gibson, 1971) against rocks of San Bernardino Mountains-type, and thus has significant strike-slip displacement. The Mission Creek Strand is inferred to lie beneath Quaternary surficial deposits along the southwestern base of the San Bernardino Mountains. This fault is the major strand of the San Andreas Fault zone, and has juxtaposed crystalline rocks of San Gabriel Mountains-type (including Pelona Schist overlain by the Vincent Thrust and associated upper-plate crystalline rocks) against the Wilson Creek block and the San Bernardino Mountains. The San Bernardino Strand defines the modern trace of the San Andreas Fault. The strand forms primary fault features in all but the youngest Quaternary surficial units, and is thought to have evolved in the last 125,000 years or so based on regional fault relations. Complications within the San Andreas Fault system over the last several hundred thousand years have created a landscape setting in which Quaternary surficial materials of the Yucaipa quadrangle have accumulated. Crustal extension throughout the San Bernardino Basin region led to uplift of the Crafton Hills block and down-dropping of the Yucaipa Valley region on faults of the Crafton Hills and Chicken Hill complex. Subsequent middle and late Quaternary streamflows deposited several generations of axial-valley and alluvial-fan sediment in the down-dropped lowlands. These deposits and the older San Timoteo beds they overlie record the history of Quaternary fault movements, and form reservoirs for ground water in the Yucaipa quadrangle. Digital Data: The geologic database of the Yucaipa 1:24,000-scale 7.5' quadrangle, San Bernardino and Riverside Counties, California, was prepared by the Southern California Areal Mapping Project (SCAMP), a regional geologic-mapping project sponsored jointly by the U.S. Geological Survey and the California Geological Survey. The database was created in ARC/INFO (Environmental Systems Research Institute, ESRI), and includes the following files: (1) a readme.txt file, (2) this metadata file, (3) coverages containing geologic data and station-location data, (4) associated INFO attribute data files, (5) a browse graphic (.pdf) of the geologic-map plot and map-marginal explanatory information, (6) a PostScript graphics file of the geologic-map plot with map-marginal explanatory information, and (7) .pdf text files describing the map units of the Yucaipa quadrangle (Description of Map Units) and their geologic age and correlation (Correlation of Map Units).尤凯帕7.5′分幅图幅（Yucaipa 7.5' quadrangle）位于圣贝纳迪诺盆地（San Bernardino Basin）东南缘，该盆地是发育于圣哈辛托断裂带（San Jacinto Fault Zone）与圣安德烈亚斯断裂带（San Andreas Fault Zone）之间右阶跃带（right-step-over zone）内的伸展构造区。圣安德烈亚斯断裂系（San Andreas system）的多条断裂贯穿该图幅，按形成时代从老至新依次为班宁断裂（Banning Fault），以及圣安德烈亚斯断裂的威尔逊溪段（Wilson Creek Strand）、使命溪段（Mission Creek Strand）、米尔溪段（Mill Creek Strand）与圣贝纳迪诺段（San Bernardino Strand）。 圣安德烈亚斯断裂的米尔溪段是尤凯帕图幅内圣安德烈亚斯断裂最东侧的分支，它将圣贝纳迪诺山地块（San Bernardino Mountains block）的花岗岩（granitic rocks）与变质岩（metamorphic rocks）岩体，与尤凯帕岭（Yucaipa Ridge）上的一套薄层同类岩石分隔开，因此仅具有微弱的走滑位移（strike-slip displacement）。 威尔逊溪段横穿尤凯帕岭，并在圣安娜河峡谷（Santa Ana River Canyon）处向米尔溪段汇聚。该断裂将火成变质杂岩（igneous and metamorphic complex）与威尔逊溪地块（Wilson Creek block），以及上覆的非海相沉积岩（nonmarine sedimentary rocks，即1971年Gibson提出的米尔溪组（Mill Creek Formation）），与圣贝纳迪诺山型岩体并置，因此具有显著的走滑位移。 使命溪段被推断埋藏于圣贝纳迪诺山脉西南麓的第四纪（Quaternary）地表沉积之下。该断裂是圣安德烈亚斯断裂带的主要分支，它将圣盖博山型（San Gabriel Mountains-type）结晶岩体（包括被文森特逆冲断层（Vincent Thrust）覆盖的佩洛纳片岩（Pelona Schist）及其相关上盘结晶岩（upper-plate crystalline rocks）），与威尔逊溪地块及圣贝纳迪诺山脉并置。 圣贝纳迪诺段构成了圣安德烈亚斯断裂现今的地表迹线。除最新的第四纪地表沉积单元外，该段在所有其他地层中均形成典型断裂构造；根据区域断裂构造关系推断，该段形成于约12.5万年以来的时段。 近数十万年来圣安德烈亚斯断裂系的构造活动复杂化，造就了尤凯帕图幅内第四纪地表沉积物的赋存环境。圣贝纳迪诺盆地全域的地壳伸展作用，使得克拉夫顿山地块（Crafton Hills block）抬升，而尤凯帕谷（Yucaipa Valley）地区在克拉夫顿山与奇肯山断裂杂岩（Chicken Hill complex）的作用下发生断陷。随后的中、晚第四纪河流径流在断陷低地沉积了多期轴向河谷与冲积扇沉积物。这些沉积物及其上覆的更老的圣蒂莫托地层（San Timoteo beds）记录了第四纪断裂活动历史，同时构成了尤凯帕图幅内的地下水储层（ground water reservoir）。 数字数据： 加利福尼亚州圣贝纳迪诺县与里弗赛德县范围内的尤凯帕1:24000比例尺7.5′分幅图幅地质数据库，由南加州区域填图项目（Southern California Areal Mapping Project, SCAMP）编制完成。该项目是由美国地质调查局（U.S. Geological Survey, USGS）与加州地质调查局（California Geological Survey）联合资助的区域地质填图项目。数据库基于ARC/INFO（环境系统研究所，ESRI）平台构建，包含以下文件：(1) readme.txt说明文件；(2) 本元数据文件（metadata file）；(3) 存储地质数据与测站位置数据的地理覆盖层（coverages）；(4) 配套的INFO属性数据文件（INFO attribute data files）；(5) 地质图件与图廓注记信息的浏览用PDF文件；(6) 包含地质图件与图廓注记信息的PostScript格式图形文件；(7) 描述尤凯帕图幅图单元的PDF文本文件《图单元描述（Description of Map Units）》及其地质时代与对比关系文件《图单元对比（Correlation of Map Units）》。