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美国西部红层地貌发育及与中国东南部丹霞地貌的对比
潘志新1,, 任舫2,, 彭华3
1. 海南大学旅游学院,海口 570228
2. 中国地质科学院地质力学研究所,北京 100081
3. 中山大学地理科学与规划学院,广州 510275

作者简介:潘志新(1986- ),男,江西婺源人,博士,讲师,研究方向为红层与丹霞地貌。 E-mail: panzhix@mail2.sysu.edu.cn

通讯作者:任舫(1980- ),女,江西玉山人,博士,助理研究员,研究方向为红层与丹霞地貌。 E-mail: 546615019@qq.com
摘要

美国西部和中国东南部都是世界上中生代红层的主要分布区,形成了以赤壁丹崖为特征的地貌景观,具有很高的对比研究价值。通过地质资料分析和野外考察,对美国西部红层的分布和形成年代,地质构造背景,岩性和地貌特征等进行梳理,并和中国东南部丹霞地貌进行对比。结果表明:美国西部红层地貌和中国东南部丹霞地貌都是发育在红层基础上的侵蚀地貌,都有显著的红色陡崖坡,但受控于区域地质构造、物质基础、及主导外营力等因素的差异,两地具体的地貌特征并不完全一致。美国西部红层主要形成于三叠纪和侏罗纪,红层堆积的构造环境为大型弧后盆地,沉积环境复杂多变,以陆相为主,尤以风沙沉积最为典型,岩性以细砂—粉砂岩和泥岩为主,砾岩很少,主导外营力为流水下切,红层发育的地貌类型相对单一,总体以高原—峡谷景观为主。中国东南部丹霞地貌区的红层多形成于白垩纪,构造环境为内陆中小型断陷/拗陷盆地,沉积环境全部为陆相,以粗碎屑混杂堆积为主,主导外营力以流水冲刷侧蚀为主,发育的地貌类型较为多样,整体以簇群式峰丛—峰林景观为主。

关键词: 红层; 丹霞地貌; 美国西部; 中国东南部; 对比研究;
Development of red bed landform in the western United States and a comparison with Danxia landform in southeast China
PAN Zhixin1,, REN Fang2,, PENG Hua3
1. School of Tourism, Hainan University, Haikou 570228, China
2. Institute of Geo-mechanics, Chinese Academy of Geological Sciences, Beijing 100081, China
3. School of Geography and Planning, Sun Yat-sen University, Guangzhou, Guangdong 510275, China
Abstract

The western United States and southeastern China, featured by landscapes of red cliffs, are major distribution regions of red beds in the world. It is of great value to make a comparative study on red beds and landform development in these two regions. Based on geologic literature analysis and field investigation, this study summarizes the distribution, age, geologic background, lithology, and geomorphic features of red beds in the western United States. Then, a comparison with Danxia landform in southeast China was conducted. It reveals that red bed landform in the western United States and Danxia landform in southeast China are erosional landform developed on red beds, with red cliffs as a remarkable geomorphic feature. However, due to differences in regional geologic background, material basis, and the dominant exogenic force, specific geomorphic features in these two areas are not exactly the same. Red beds in the western United States were mainly deposited in a huge back-arc basin during the period from Triassic to Jurassic. Although depositional environment for these red beds are complex, most of them are continental deposits, especially eolian deposit. In terms of lithology, the majority of red beds in the western United States are composed of fine-grained sandstones, siltstones or mudstones, with very few conglomerates. In terms of geomorphic development of red beds in this region, the downcutting process by flowing streams has been playing a dominant role, creating relatively simple red bed landform types that are characterized by a geomorphic combination of plateau and canyons. As for Danxia landform in southeast China, red beds were deposited in Cretaceous in small and medium-sized rift basins or depression basins. They are all continental deposits, mainly composed of coarse-grained clastic rocks. In aspect of landform development, the controlling exogenic factor is lateral erosion by streams, which produces various types of red bed landforms, with peak clusters and hoodoos as overall landscape feature.

Keyword: red beds; Danxia landform; western United States; southeastern China; comparative study;
1 引言

地质学上通常把红层认为是各个地质历史时期沉积的,因岩石颗粒、孔隙或基质中存在氧化铁而形成的以红色为主色调的碎屑沉积岩[1,2,3,4]。在国内地貌学界,发育在红层基础上的侵蚀地貌被定义为红层地貌[1,5]。其中,以陡崖坡为特征的红层地貌被称为丹霞地貌[6]。近年来,在查阅文献及与国外同行进行交流时发现,红层在国外很多地区都有分布,并且发育了很多与中国丹霞类似的地貌景观。但由于学科归属的差异,国外并没有红层地貌或丹霞地貌的专题研究,相关研究多分散在砂岩地貌[7],或红层的古地磁、古生物、古气候等基础地质研究中。与此同时,国内同行对国外红层的分布和地貌发育特征也缺乏系统深入的了解,缺乏一手数据资料的支撑,这使得在开展国际对比研究时存在难以对接的问题。在此背景下,国内学者走出国门去实地认识和了解国外的红层及其地貌发育便显得尤为必要。

美国西部的科罗拉多高原是世界上中生代红层的主要分布区,发育形成了很多著名的地貌景观,它们与中国的丹霞地貌具有诸多的相似性和差异性,有很高的对比研究价值。为增进国内同行对美国西部红层及其地貌发育的了解,本文基于对美国西部红层的两次系统考察和对采集岩样的理化性质分析,系统梳理美国西部红层的分布、形成年代、地质构造、岩性特征、地貌发育特征等。在此基础上,和以丹霞山为代表的中国东南部丹霞地貌进行对比,总结两地在红层(丹霞)地貌发育机制上的共性和差异。

2 美国西部红层的分布和形成年代

根据相关地质资料和野外考察得知,美国西部的红层广布于科罗拉多高原及其边缘的山前/山间断陷或拗陷盆地。这些红层的形成年代跨越了宾夕法尼亚纪(晚石炭世)到古近纪,但以三叠纪和侏罗纪为主(图1)。其中,宾夕法尼亚纪的红层主要分布在科罗拉多高原东部落基山脉南段的山间盆地和落基山脉东部的山麓地区,总体呈近南北向分布,如科罗拉多州首府——丹佛附近的红石公园(Red Rocks Park)出露的Fountain组[8];二叠纪红层主要在犹他州东南部、大峡谷国家公园(Grand Canyon National Park)以及科罗拉多高原与盆地和山脉区(Basin and Range)的过渡处,如犹他州峡谷地国家公园(Canyonlands National Park)出露的Cutler群[9],亚利桑那州大峡谷地区出露的Hermit组[10],塞多纳(Sedona)出露的Schnebly Hill组[11]等;三叠纪和侏罗纪红层分布最广,在犹他州东南部、亚利桑那州东北部和科罗拉多州西南部都有大面积出露,如Moenkopi组[12]、Chinle组[13]、Wingate组[14]、以及Navajo组[15]等,它们是红层地貌发育的主要成景地层;古近纪红层分布面积很小,只在犹他州西南部有少量出露,如犹他州布莱斯峡谷国家公园(Bryce Canyon National Park)出露的Claron组[16]

图1 美国西部红层分布简图 Fig. 1 Sketch map of the distribution of red beds in the western United States

值得一提的是,在科罗拉多高原西南部,红层和其他岩层组合在一起,在地壳抬升过程中由于科罗拉多河的切割侵蚀作用下,形成了一个向北水平抬升的阶梯状地形,即著名的“大阶梯”(Grand Staircase)。该“大阶梯”底部始于亚利桑那州大峡谷国家公园(Grand Canyon National Park)的北部边缘,往北经犹他州南部的泽恩国家公园(Zion National Park)至布莱斯峡谷国家公园(Bryce Canyon National Park),总体向北水平延伸幅度达240 km,海拔跨度超过2000 m[17]。沿该“大阶梯”出露的地层沉积年代连续,Grand Canyon国家公园的地层最老,Zion国家公园居中,Bryce Canyon国家公园的地层最年轻。其中,Grand Canyon顶部的地层是Zion公园的底部地层,Zion公园的顶部地层则为Bryce Canyon公园的底部地层[17]图2)。

图2 美国科罗拉多高原西南部“大阶梯”构造简图和地层柱状图 Fig. 2 Sketch of Grand Staircase on the southwest of the Colorado Plateau, USA and the stratigraphic column

3 美国西部红层形成的地质背景

地质构造背景方面,中生代以前,科罗拉多高原地区处于泛大陆西部边缘的大陆架浅海或滨海平原环境。三叠纪末,由于泛大陆分裂解体,北美大陆开始向西北漂移。与此同时,太平洋板块向北美大陆板块的西部俯冲碰撞,在西部海岸火山链和古落基山脉之间形成大型的弧后盆地——西部内陆盆地(Western Interior Basin)[18],并开始接受大陆沉积。早侏罗世末至中侏罗世早期,北美大陆漂移至副热带地区,太平洋板块向北美板块进一步俯冲,形成内华达造山运动(Nevadan Orogeny)[19],使整个西部内陆盆地区处于副热带雨影区,形成广袤的沙漠环境和巨厚的风沙沉积[20]。中侏罗世末期至白垩纪期间,受太平洋板块俯冲加速的影响,北美大陆西部山脉随着塞维尔造山运动(Sevier Orogeny)隆起,近南北向延伸的西部内陆盆地进一步沉陷扩大,海水从北极和墨西哥湾相继侵入盆地,并最终贯通形成了西部内陆海道(Western Interior Seaway)(图3)[21,22],大陆沉积过程结束,整个科罗拉多高原地区又恢复到了中生代以前的滨/浅海沉积环境。到晚白垩世,太平洋板块向北美板块的俯冲碰撞引起北美大陆西部的深部岩浆上涌和地壳隆升,形成拉拉米造山运动(Laramide Orogeny),使西部内陆海道关闭,落基山脉开始隆起,西部内陆盆地被整体抬升,形成科罗拉多高原[23],在流水侵蚀的作用下,逐渐形成如今的各种红层地貌景观。

图3 白垩纪期间北美大陆形成的西部内陆海道 Fig. 3 The western interior seaway on the continent of North America in the Cretaceous Period

从沉积环境来看,美国西部红层以陆相沉积为主,兼具少数滨—浅海相和海陆过渡相沉积。陆相沉积方面,科罗拉多高原以东的宾夕法尼亚纪红层多为古落基山脉地区的冲—洪积扇相沉积,而科罗拉多高原面上的红层多为河流相、湖泊相和风成相。特别是风成相沉积,其典型特征是具大规模的板状交错层理,一般形成于海岸沙丘环境或广袤的内陆沙漠环境,前者如二叠纪Culter群的Cedar Mesa组[7]和White Rim 组[7];后者如早—中侏罗世的Wingate组[14]和Navajo组[15];海相沉积方面,主要是出露于大峡谷地区石炭纪Supai群[23]和早二叠世的Hermit组[15],以及犹他州南部的Moenkopi组[17],它们的共同特点是岩石表面具有波痕和泥裂构造,表明其沉积于潮滩或滨海平原环境。

4 美国西部红层的岩性特征

岩性方面,目前美国西部保存和出露的红层总体以细砂岩、粉砂岩和泥岩为主,砾岩等粗碎屑岩很少(只在盆地边缘偶有出露,或只是作为夹层的短期堆积)。而且由于沉积环境的变化,很多红层并不是单独出露,往往和其他岩层组合在一起,红层的上覆、下伏或夹层经常有石灰岩、火山岩和灰绿色泥岩。

在考察过程中,以犹他州西南部的Zion国家公园作为美国西部红层的案例地,沿高原面—崖壁—侵蚀峡谷底部这一垂直地层剖面采集了该区域出露的4个红层岩性组共10个岩性段(表1)的手标本岩样进行理化性质分析。这些红层岩样既有河流相、湖泊相、还有风成相,涵盖了科罗拉多高原主要的红层沉积类型(特别是形成于早—中侏罗世的Navajo组砂岩,它是地球上现存最大规模的风沙沉积,广泛分布于犹他州南部和亚利桑那州北部地区,出露的平均厚度600 m左右[24],也是形成该区域红色大崖壁的主要成景地层),可以较好地说明美国西部红层的主要特性。

表1 Zion国家公园Zion峡谷南段入口附近的地层剖面 Tab. 1 Stratigraphic profile near the South Entrance of Zion Canyon at Zion National Park

在岩石颗粒组成和胶结特性方面,出露的红层均为细砂岩、粉砂岩,部分含泥质岩类夹层。其中,形成主崖壁的Navajo组砂岩为早—中侏罗世风沙沉积,具大型板状交错层理(图4)。因颜色差异,Navajo组砂岩自上而下又可分为White 段、Pink段、Brown段(表1),它们由纯度很高的中—细粒石英砂岩组成(根据岩样元素氧化物测定结果,其SiO2含量均在90%以上),颗粒均一、分选性和磨圆度都较好。但在偏光显微镜下的岩样薄片鉴定中却发现,不同岩性段在胶结方式和颗粒接触方式上存在差异。其中,崖壁上部的White段外观呈白色或淡黄色,粒间孔隙很多(孔隙率高达30%~40%),颗粒之间多呈点状接触,胶结物很少,为弱胶结型,颗粒非常容易剥落;而崖壁中下部的Pink段和Brown段外观呈褐红色,其粒间接触更紧密,且颗粒周围普遍有铁质胶结物包裹(图5)。关于Navajo砂岩崖壁上下颜色差异的成因,有学者认为Navajo组沉积于干燥富氧的沙漠环境,在被后期岩层覆盖埋藏以前,整个Navajo组应当都为红色[25]。但在后期地壳抬升过程中,地下埋藏的密度较小的碳氢化合物随断层和节理裂隙上升并释放出来,与Navajo组砂岩中的Fe3+发生还原反应,将其漂白。因而,Navajo砂岩崖壁从下往上,碳氢化合物含量越高,还原反应也越充分,颜色也越白[26]

图4 大规模风沙沉积形成的Navajo砂岩及交错层理 Fig. 4 Cross-bedded Navajo sandstone formed by great sand dunes

图5 正交偏光20倍镜下Navajo组砂岩White段(左)、Pink段(中)、Brown段(右)岩样的碎屑结构和胶结特性 Fig. 5 Clastic texture and cementation feature of rock samples taken from the White Member (Left), Pink Member (Middle), Brown Member (Right) of Navajo Sandstone under cross-polarized light with 20 times magnification

至于其他河流相和湖泊相沉积的岩性组/段,除Kayenta组Springdale段为细粒均一的粉砂岩形成低矮的崖壁外,其余都由细砂—粉砂岩和泥质岩类夹层组成。这些细砂—粉砂岩在碎屑物质、颗粒分选性、磨圆度、胶结方式和颗粒接触方式都较接近,都以铁质胶结为主。

此外,在野外使用N型施密特回弹应力仪(Schmidt Hammer, Type N)对该区域红层岩石强度进行现场测试,结果显示岩石的分选性和胶结特性对其强度和地貌形态发育的影响至关重要。主崖壁上部的Navajo组砂岩White段为弱胶结型,岩石结构松散,岩石强度最小,但其岩性均一,且有上覆盖层保护,即使岩石强度较低,仍能形成崖壁;而崖壁中下部的Navajo组Pink段和Brown段岩石胶结紧密,岩石强度要大得多。相比之下,崖壁下方的其他岩层岩性不均一,含有很多软弱的泥岩夹层,岩石强度要小很多,总体抗风化侵蚀能力弱,只能发育成缓坡。

5 美国西部红层地貌特征

总体而言,美国西部红层发育的地貌类型相对单一,整体上以高原—峡谷景观为主,多连续的方山和台地,岩层产状水平。但在部分构造复杂的高原边缘地区或山前/山间盆地,地貌形态相对多样。如犹他州国会礁国家公园(Capitol Reef National Park)受北西—南东向Waterpocket褶皱的影响,公园内红层整体向东倾斜(图6a);位于落基山脉东麓的科罗拉多州首府——丹佛的红石公园(Red Rocks Park)Fountain组红层受落基山脉隆起的影响被掀斜抬升形成单面山,倾角约为30°(图6b)。在有些红层盆地内部,受两组互相垂直的节理控制,在流水的切割侵蚀作用下还发育形成针状或柱状的峰丛—峰林景观,如犹他州的布莱斯峡谷国家公园(Bryce Canyon National Park)(图6c)和峡谷地国家公园(Canyonlands National Park)的The Needles区域(图6d)。

图6 美国西部红层地貌主要考察点典型地貌
a. 国会礁国家公园Waterpocket褶曲;b. 丹佛红石公园的红层单面山;c. 布莱斯峡谷国家公园的针状峰丛—峰林;d. 峡谷地国家公园The Needles区的岩柱群;e. 大峡谷国家公园的多级侵蚀面;f. Zion峡谷俯瞰;g. 羚羊峡Navajo砂岩被流水侵蚀切割形成的巷谷;h. 塞多纳的的红层方山和孤峰;i. 纪念碑谷地的孤峰;j. 地下水基部侵蚀形成的额状洞穴;k. 阿切斯国家公园的标志——精致拱;l. 泽恩国家公园Grotto山密集的垂直节理和尖锐的崖壁边缘棱角。
Fig. 6 Representative photos of red bed landforms in the western United States

从崖壁形态来看,形成美国西部红层崖壁的地层主要为巨厚的风成相砂岩,如Navajo组和Wingate组,它们形成的红色崖壁高耸陡直、体量巨大,往往延绵十几至几十公里。而且由于岩性均一,几乎不含泥岩夹层,这些红层崖壁很少有水平岩槽发育,同时也很少有竖向沟槽分布。有些地区的红层剖面上有多个岩层组交替出现,由于地壳间歇性抬升和不同岩性红层抗风化侵蚀能力的差异,其形成的谷壁并不是陡直均一,而是呈多级层状坡面,如亚利桑那州的大峡谷国家公园(Grand Canyon National Park)(图6e)。

从崖壁颜色上看,美国西部红层分布区气候干旱,崖壁表面上较少有生物附着,整体上呈鲜明的红色外观,但部分由风成相Navajo砂岩形成的崖壁经过后期的地质化学“漂白”作用,上下崖壁颜色呈现一定差异,上部为白色,中部为粉红色,下部为红褐色。而这种崖壁颜色变异也成为该区域红层地貌的一大特色。

从沟谷发育来看,美国西部红层分布区以强烈的流水下切侵蚀为主。由于降水少,河网密度小,侵蚀不易展开,科罗拉多高原面上很多地区的河谷侵蚀尚未达到区域侵蚀基准面,红层地貌发育总体处于仍青年晚期或壮年早期,多峡谷和巷谷。如犹他州Zion国家公园的Zion峡谷(图6f),它由科罗拉多河的支流——维京河(Virgin River)的北部支流(North Fork)切割侵蚀而成。Zion峡谷全长24 km,最大谷深达916 m,河道坡降为9.5~15.2 m/km[23],是整个北美地区坡降最大的河流之一。在Zion峡谷两侧,还发育有众多深窄的巷谷,它们是夏季暴雨洪流沿节理裂隙切割侵蚀而成。流水在下切过程中,携泥沙沿Navajo砂岩的交错层理进行冲刷磨蚀,形成波状起伏的谷壁,亚利桑那州北部的羚羊峡谷的成因也是如此(图6g)。此外,在峡谷源头地区,强烈的溯源侵蚀使谷地边缘的高原台地不断后退,多形成宽广的、港湾状的弧形崖壁(Amphitheatre),如犹他州的布莱斯峡谷国家公园(Bryce Canyon National Park),而在科罗拉多高原南部边缘的塞多纳(Sedona)(图6h)和纪念碑谷地(Monument Valley)等地区(图6i),红层地貌处于发育演化进程的老年期,以宽谷、方山、孤峰景观为主。

除流水侵蚀外,风化对美国西部红层地貌发育的影响也很明显。如在砂岩崖壁底部不同岩层接触面,由于上下岩层的透水性存在差异,地下水沿砂岩崖壁孔隙或节理裂隙下渗,遇到崖壁底部下伏不透水的泥岩/页岩层,水与泥岩/页岩层中的铁质、钙质胶结物、以及粘土矿物等进行一系列微观风化过程,层间裂隙不断扩大,使上覆崖壁因失去支撑不断崩塌,形成额状洞穴(图6J)或天然拱桥,如犹他州阿切斯国家公园(Arches National Park),该公园内有超过2000座砂岩拱桥,是世界上天然拱桥数量最多的地方,其标志性的精致拱(Delicate Arch)净高达18 m(图6k)。由于该区域气温日较差和年较差较大,冻胀风化过程也较显著,它可能是导致该区域砂岩崖壁形成尖锐边缘棱角的重要原因。这些砂岩崖壁垂直节理密集,冬季垂直节理裂隙中的水分结冰使裂隙不断扩大,在风化作用还未来得及将原有的崖壁边缘棱角圆化之前,岩体就已崩塌,在崖壁上形成新的尖锐的边缘棱角(图6l)。

6 与中国东南部丹霞地貌的对比

丹霞地貌是中国地质、地貌学家命名的一种特殊地貌类型,红层是其发育的物质基础,红色陡崖坡是其最重要的形态特征。丹霞地貌在中国东南、西南、西北和青藏高原都有分布,但以东南部湿润区分布最为集中,形态特征也最为典型。东南部是中国丹霞地貌研究的起源地,也是研究最为深入的地区。从共性方面来看,美国西部的红层地貌和中国东南部丹霞地貌都是发育在红层基础上的侵蚀地貌,整体外观都呈红色,都有显著的陡崖坡,而且都经历了盆地形成—红层堆积—构造抬升—外力塑造这一基本的地质作用过程。但两地在区域地质构造、物质基础、及主导外营力方面存在差异,其具体的地貌景观特征并不完全一致。

6.1 区域地质构造的对比

美国西部红层堆积的构造环境为太平洋板块向北美板块俯冲碰撞形成的大型弧后盆地——西部内陆盆地,形成于三叠纪末。此后受造山运动影响,西部内陆盆地发生多次海侵和海退,导致其沉积环境复杂多变,但以广袤的内陆沙漠环境为主。白垩纪末,随落基山脉隆起,红层盆地被整体抬升形成科罗拉多高原。与此同时,受弧后扩张作用的影响,地壳的张应力使科罗拉多高原发生破裂断陷,形成很多断块山地和密集分布的节理群,控制了区内峡谷群的发育。岩层产状方面,除高原边缘和山麓地区外,大多数出露的红层基本保持近水平的产状。

中国东南部红层堆积的构造环境总体上以内陆中小型断陷/拗陷盆地为主,如丹霞山即为华南板块南岭加里东褶皱系中段的一个断陷盆地——丹霞盆地。中生代以前,中国广大的南方地区处于海平面以下,没有红层堆积。直到侏罗纪,中国古陆才基本拼接完成。随后的燕山运动期间,太平洋板块向中国东部俯冲碰撞,形成大量北北东或北东向褶皱断裂山地和中小型山间盆地。白垩纪期间,受中国大陆东部隆起带的影响,海洋水汽难以进入,中国东南部处于干旱—半干旱气候环境,红层开始在这些山间盆地中大量堆积。红层盆地形成后随喜马拉雅运动抬升,形成很多断层、节理,它们对丹霞地貌坡面发育具有控制作用。在盆地边缘和受断层带控制的区域,多发育顶斜的丹霞地貌;而在盆地内部受断层活动影响较小的地区,则发育顶平的丹霞地貌。

6.2 物质基础的对比

美国西部红层沉积年代跨度较大,从宾夕法尼亚纪(晚石炭世)到古近纪都有出露,但以三叠纪和侏罗纪为主,尤其是早—中侏罗世的Navajo组砂岩,它是形成红色大崖壁的主要地层。沉积环境方面,这些红层以陆相(尤其是沙漠沉积相)为主,兼具少量滨—浅海相和海陆过渡相沉积。岩性方面,美国西部红层以砂岩和泥质岩类为主,砾岩等粗碎屑岩很少。其中,形成崖壁的风成相砂岩为细砂—粉砂岩,颗粒分选很好,石英含量很高,以铁质胶结为主,钙质含量较低,岩石外观总体呈红色,但各段在岩石胶结特性和铁质胶结物含量有所差异,导致其颜色和岩体强度也有所不同。

中国东南部丹霞地貌区的红层较年轻,大多形成于白垩纪,沉积环境几乎全部为陆相,缺少海相沉积。岩性方面,中国的红层多为内陆山间盆地的混杂堆积,岩石颗粒分选差,多砾岩、砂砾岩和中—粗砂岩,并含大量泥岩夹层。其中,形成丹霞崖壁的红层多为粗碎屑堆积,胶结致密,岩性坚硬。矿物组成上,石英为主要矿物,长石和杂基占比较高,胶结物以铁质和钙质为主,兼有硅质和泥质。此外,因物源地多为石灰岩丘陵山地,在红层沉积过程中,大量石灰岩砾石和富钙溶液进入盆地沉积,导致红层岩体中的钙质含量普遍较高。

6.3 主导外营力的对比

美国西部红层分布区为干旱—半干旱气候条件,降水量少,河网密度小。如Zion国家公园的年均降水量仅为411 mm,不足丹霞山的四分之一,河流径流量和流量比丹霞山小几个数量级(表2)。不过,虽然总降水量较少,但该区域降水以夏季暴雨洪流为主,在塑造地貌时表现为强烈的流水下切,形成众多峡谷和巷谷。而在峡谷源头和台地边缘,流水溯源侵蚀作用显著,多形成弧形崖壁、孤峰等。此外,地下水对砂岩崖壁底部不透水泥岩层的侵蚀也较明显,使上覆崖壁崩塌形成额状洞穴。风化方面,因红层中钙质含量较低,溶蚀等化学风化作用不强烈,但冻胀风化和盐风化等物理风化过程较显著。

中国东南部丹霞地貌区气候湿润(表2),水系很发育,很多地区(如丹霞山、龙虎山、江郎山等)主河谷已达到或接近区域侵蚀基准面,流水转为冲刷侧蚀为主,在重力作用配合下,多发育形成临溪的丹霞峰丛—峰林、岩墙、孤峰等景观。风化方面,各种物理、化学和生物风化作用均很显著。

表2 Zion国家公园和丹霞山气候和水文数据对比 Tab. 2 Comparison of climate and hydrology data of Zion National Park and Mt. Danxiashan
6.4 地貌特征的对比

总体上,美国西部红层发育的地貌类型相对单一,以高原—峡谷景观为主,崖壁高耸巨大。但在部分构造变动强烈的高原边缘地区或山前/山间盆地,地貌形态相对多样,也有单面山、峰丛—峰林、孤峰、石柱等。从崖壁形态上看,美国西部红层发育的崖壁边缘棱角尖锐,崖壁上很少有水平岩槽和竖向沟槽发育。从颜色上看,美国西部红层崖壁上较少有生物附着,整体呈鲜明的红色外观,但由风成相Navajo砂岩形成的崖壁经过后期的地质化学“漂白”作用,崖壁颜色有分层变异的现象。

中国东南部丹霞地貌多为簇群式峰丛—峰林景观,山块总体离散,局部相对集中,山崖体量相比美国西部要小很多,但形态更为多样,既有堡状方山,也有条带状的岩墙,还有孤峰和石柱,且崖壁形态浑圆。受软岩夹层风化凹进的影响,崖壁上普遍发育水平岩槽,部分崖壁受坡面水流影响,还形成平行分布的纵向沟槽;受盐风化等作用影响,很多丹霞崖壁或洞穴内还发育有蜂窝状洞穴。在崖壁的颜色外观方面,中国东南部丹霞崖壁大多呈红褐色,且表面普遍有生物附着,但岩体缺乏后期的颜色变异过程。

7 结论

(1)美国西部是世界中生代红层的主要分布区,集中出露于科罗拉多高原及其边缘的山前/山间断陷或拗陷盆地,形成年代以三叠纪和侏罗纪为主。红层堆积的构造环境为大型弧后盆地,受海侵海退影响,红层沉积环境复杂多变,以陆相为主,尤以风沙沉积最为典型,同时兼具少数滨—浅海相和海陆过渡相沉积。

(2)美国西部红层岩性以细砂岩、粉砂岩和泥岩为主,很少有砾岩等粗碎屑堆积。其中,广泛分布于科罗拉多高原并形成红色崖壁的主要是早—中侏罗世Navajo组砂岩。它由纯度很高的中—细粒石英砂岩组成,颗粒均一,分选性好,是世界上最大规模的风沙沉积,具大型板状交错层理。由于后期的地质化学“漂白”作用,Navajo砂岩崖壁的颜色出现变异。崖壁上部岩石颗粒结构松散,铁质胶结物很少,呈白色外观;中下部岩体以铁质胶结为主,岩石结构致密,呈红色外观。

(3)美国西部红层发育的地貌类型相对单一,总体上以高原—峡谷景观为主,在部分构造复杂的高原边缘地区或山前/山间盆地,地貌形态相对多样。地貌发育的主导外营力为强烈的流水下切,发育众多峡谷和巷谷。崖壁上很少有水平岩槽和纵向沟槽发育,但在崖壁底部由于地下水对下伏不透水泥岩层的侵蚀,也可以使上覆崖壁崩塌形成额状洞穴。化学风化作用不强烈,但冻胀风化等物理风化过程较显著,崖壁边缘棱角尖锐。

(4)美国西部的红层地貌和中国丹霞地貌都是发育在红层基础上的侵蚀地貌,都经历了盆地形成—红层堆积—构造抬升—外力塑造这一基本的地质作用过程。但受控于区域地质构造、物质基础、及主导外营力等因素的差异,两地具体的地貌景观特征并不完全一致。

The authors have declared that no competing interests exist.

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Although red beds are a widespread landscape in China, geosciences academia fail to pay much attention to it, and a lack of collaboration among related fields results in a trend that different disciplines have different interpretations of the same issue; specifically, much concern on red beds, the fragile geographical unit, is highly needed with regard to land degradation and environmental problems. Humid area in southern China is one of the places where red beds are densely distributed. However, most of red beds areas are suffering from serious geomorphological catastrophes and soil-water erosion; even worse, many places are witnessing huge patches of 'red beds desert'. Even if we might admit that irrational land use contributes to the land degradation in red beds, there is no doubt that land degradation is a natural result of rock landscape evolution. This paper first indicates that natural evolution serves as a process of multi-factors' effect, which can be regarded as a system of mutual causality. Also, this paper analyzes the current researches on slope erosion mechanism of red beds landforms and areas, and raises scientific problems in red beds researches are facing with. Furthermore, this paper claims that a new interdisciplinary thinking of System Dynamics should be introduced to conduct a comprehensive research on the interrelation between red beds and landform development, natural catastrophes, soil erosion, environment degradation, overall geographic environment as well as production-living. Finally, this paper will hopefully find a way to promote land management and environment optimization.
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[6] 彭华, 潘志新, 闫罗彬, . 国内外红层与丹霞地貌研究述评. 地理学报, 2013, 68(9): 1170-1181.
In the context that China Danxia landform went into the world and began the research of a new phase, it is necessary to make a review on the research progress of red beds and Danxia landform in and outside China. It is revealed that the domestic study was focused on the characteristics of red beds, engineering geology, features of Danxia landform and the basic problems of development. In general, in China there were no integrated studies of red beds concerning the field of geology and geomorphology, and the differences between lithologic characteristics of red beds and their related geomorphic development processes under the influence of exogenetic force were not well understood. However, research outside China were limited in the field of geology, and relevant studies of red beds and Danxia landform were conducted in the field of sandstone landform, with a microscopic, quantitative and experimental analysis. This paper suggests that most of the existing studies on red beds and Danxia landform were focused on single factor, and lack of subject system, and the integration of related subjects was also insufficient. Then, a new agenda for the future research was put forward, in which a clear discipline classification of red beds and Danxia landform and further basic research should be made, and the integration of related subjects, international communication and the global comparative research should also be strengthened.
DOI:10.11821/dlxb201309002      [本文引用:1]
[Peng Hua, Pan Zhixin, Yan Luobin, et al.A review of the research on red beds and Danxia landform. Acta Geographica Sinica, 2013, 68(9): 1170-1181.]
[7] Young R, Young W, Young A. Sandstone Landforms.London: Cambridge University Press, 2009.
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[8] Sweet D E, Soreghan G S.Late paleozoic tectonics and paleogeography of the ancestral front range: Structural, stratigraphic, and sedimentologic evidence from the fountain formation: Manitou Springs, Colorado. Geological Society of America Bulletin, 2010, 122(3/4): 575-594.
ABSTRACT Sedimentologic, stratigraphic, and struc-tural data indicate that the Fountain For-mation at Manitou Springs consists of three tectonostratigraphic units, herein termed the lower, middle, and upper Fountain Forma-tion. Both the lower and middle Fountain Formation were deposited in a fan-delta set-ting adjacent to the active ancestral Ute Pass fault. The lower is thin and predominantly characterized by depositional stability as re-corded by abundant well-developed, deeply rooted paleosols. The middle Fountain For-mation refl ects fan progradation and high-land rejuvenation as a result of increased activity of the ancestral Ute Pass fault. This activity was likely the result of a basinward splay of the ancestral Ute Pass fault with reverse dip-slip motion as indicated by (1) ex-humation of the lower Paleozoic Sawatch Formation and (2) fault-parallel folding. The upper Fountain Formation refl ects deposi-tion in a NW-SE rending, braided-river system and appears to postdate movement on the ancestral Ute Pass fault. Lithostrati-graphic correlation allows for a Latest Pennsylvanian arly Permian age for the upper Fountain Formation, thus constrain-ing cessation of the ancestral Ute Pass fault to Middle to Late Pennsylvanian time. The Fountain Formation was deposited within a NW-SE riented structural trough (i.e., Woodland Park trough) that separated the ancestral Front Range into a northern block (i.e., ancestral Front Range block) and a southern block (i.e., Ute Pass uplift). The Woodland Park trough was bounded on its southern margin by the ancestral Ute Pass fault, which was active throughout deposi-tion of the lower two Fountain tectonostrati-graphic units, during which time sediments were shed northward across the ancestral Ute Pass fault into a marine environment. By the time of deposition of the upper Foun-tain Formation, movement on the ancestral Ute Pass fault had ceased or dramatically decreased. During this time, the Fountain Formation records predominantly axially oriented, braided-stream deposition. Comparison of the timing and kinematic history of the ancestral Ute Pass fault with other documented reverse and strike-slip faults of the ancestral Rocky Mountains sug-gests that: (1) cessation of faulting within the ancestral Rocky Mountains displays a slight east-to-west younging, but the crude age reso-lution of adjacent basin fi ll also allows for the possibility of relatively synchronous cessation of faulting, and (2) kinematics of all faults examined are consistent with a NE-SW oriented maximum horizontal compres-sional stress fi eld. These inferences contrast with the proposed Laurentian ondwanan diach ro nous closure models for the ancestral Rocky Mountains, which require rotation of the maximum horizontal stress orientation through time and pronounced east-to-west younging of faulting. Rotation of the horizon-tal stress fi eld may be recorded in multistage, kinematic histories of faults, but multiple faults require study to assess this model. Age constraints on post-tectonic units of the an-cestral Rocky Mountains, such as the upper Fountain Formation tectonostratigraphic unit defi ned herein, are poor and further refi nement would greatly aid understanding of the kinematic timing and tectonic driving force of these ancient mountains.
DOI:10.1130/B26554.1      [本文引用:1]
[9] Jordan O D, Mountney N P.Styles of interaction between aeolian, fluvial and shallow marine environments in the Pennsylvanian to Permian lower Cutler beds, southeast Utah, USA. Sedimentology, 2010, 57(5): 1357-1385.
The Pennsylvanian to Permian lower Cutler beds comprise a 200 m thick mixed continental and shallow marine succession that forms part of the Paradox foreland basin fill exposed in and around the Canyonlands region of south-east Utah. Aeolian facies comprise: (i) sets and compound cosets of trough cross-bedded dune sandstone dominated by grain flow and translatent wind-ripple strata; (ii) interdune strata characterized by sandstone, siltstone and mudstone interbeds with wind-ripple, wavy and horizontal planar-laminated strata resulting from accumulation on a range of dry, damp or wet substrate-types in the flats and hollows between migrating dunes; and (iii) extensive, near-flat lying wind-rippled sandsheet strata. Fluvial facies comprise channel-fill sandstones, lag conglomerates and finer-grained overbank sheet-flood deposits. Shallow marine facies comprise carbonate ramp limestones, tidal sand ridges and bioturbated marine mudstones. During episodes of sand sea construction and accumulation, compound transverse dunes migrated primarily to the south and south-east, whereas south-westerly flowing fluvial systems periodically punctuated the dune fields from the north-east. Several vertically stacked aeolian sequences are each truncated at their top by regionally extensive surfaces that are associated with abundant calcified rhizoliths and bleaching of the underlying beds. These surfaces record the periodic shutdown and deflation of the dune fields to the level of the palaeo-water-table. During episodes of aeolian quiescence, fluvial systems became more widespread, forming unconfined braid-plains that fed sediment to a coastline that lay to the south-west and which ran approximately north-west to south-east for at least 200 km. Shallow marine systems repeatedly transgressed across the broad, low-relief coastal plain on at least 10 separate occasions, resulting in the systematic preservation of units of marine limestone and calcarenite between units of non-marine aeolian and fluvial strata, to form a series of depositional cycles. The top of the lower Cutler beds is defined by a prominent and laterally extensive marine limestone that represents the last major north-eastward directed marine transgression into the basin prior to the onset of exclusively non-marine sedimentation of the overlying Cedar Mesa Sandstone. Styles of interaction between aeolian, fluvial and marine facies associations occur on two distinct scales and represent the preserved expression of both small-scale autocyclic behaviour of competing, coeval depositional systems and larger-scale allocyclic changes that record system response to longer-term interdependent variations in climatic and eustatic controlling mechanisms. The architectural relationships and system interactions observed in the lower Cutler beds demonstrate that the succession was generated by several cyclical changes in both climate and relative sea-level, and that these two external controls probably underwent cyclical change in harmony with each other in the Paradox Basin during late Pennsylvanian and Permian times. This observation supports the hypothesis that both climate and eustasy were interdependent at this time and were probably responding to a glacio-eustatic driving mechanism.
DOI:10.1111/j.1365-3091.2010.01148.x      [本文引用:1]
[10] Duffield J A.Depositional environments of the Hermit Formation, Central Arizona. Flagstaff: Master Dissertation of Northern Arizona University, 1985.
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[11] Blakey R C, Middleton L T.Permian shoreline eolian complex in Central Arizona: Dune changes in response to cyclic sealevel changes. Developments in Sedimentology, 1983, 38: 551-581.
DOI:10.1016/S0070-4571(08)70813-6      [本文引用:1]
[12] Walker T R, Larson E E, Hoblitt R P.Nature and origin of hematite in the Moenkopi Formation (Triassic), Colorado Plateau: A contribution to the origin of magnetism in red beds. Journal of Geophysical Research Solid Earth, 1981, 86(B1): 317-333.
Petrographic studies show that hematite is present in the Moenkopi Formation in at least five and possibly six forms: (1) microcrystalline hematite, (2) crystals of specular hematite, (3) polycrystalline and monocrystalline grains, (4) grains of partly hematitized ilmenite, (5) grains composed of primary ilmenite-hematite intergrowths, and (6) ultrafine pigment. The microcrystalline hematite and crystals of specular hematite are unequivocally authigenic. They form cement in interstitial and secondary voids, and they have replaced detrital iron-bearing silicate minerals. Furthermore, microcrystalline hematite is superimposed on other authigenic cementing minerals such as potassium feldspar, calcite, dolomite, and quartz, and in some cases it has replaced authigenic pyrite. In addition, both microcrystalline and specularite crystals are common daughter products of intrastratally altered biotite grains. Thermodynamic considerations coupled with studies of hematite-magnetite relationships in modern sediments indicate that most of the hematite in the polycrystalline grains, and probably the monocrystalline grains as well, was formed authigenically by post-depositional replacement of detrital grains of magnetite. The ilmenite probably has similarly altered in situ to hematite. The only hematite of unquestionable detrital origin in the red beds is the hematite in the ilmenite-hematite intergrowths (tiger striped grains) and that in monocrystalline detrital grains containing rutile exsolution platelets, both of which are products of high-temperature processes. With the exception of the ultrafine pigment, each of the above forms is coarser grained than the superparamagnetic threshold for hematite, and therefore each contributes components of remanent magnetism to the rocks. Inasmuch as most of the hematite varieties represent authigenic products of intrastratal alterations that require considerable geologic time, we conclude that the red bed remanence is largely chemical remanent magnetization (CRM) acquired over long time intervals. The pigment in the Moenkopi red beds consists partly of authigenic ultrafine red iron oxide and partly of translucent microcrystalline hematite. The ultrafine red iron oxide may or may not be hematite, but even if it is, the grain size probably lies below the paramagnetic threshold for hematite. Much of the pigment, therefore, may not contribute greatly to the remanent magnetism in the rocks.
DOI:10.1029/JB086iB01p00317      [本文引用:1]
[13] Blakey R C, Gubitosa R.Controls of sandstone body geometry and architecture in the Chinle Formation (Upper Triassic), Colorado Plateau. Sedimentary Geology, 1984, 38(1-4): 51-86.
The Upper Triassic Chinle Formation forms a widespread continental deposit across the Colorado Plateau. Varying sandstone/mudstone ratios and resulting architectural differences throughout the formation provide the basis for recognition of six formal members and one informal unit in southeastern Utah and adjacent northern Arizona. The Shinarump and Moss Back Members consist of very broad thin sheets of interconnected sandstone bodies that formed in braided stream environments. The Monitor Butte and Petrified Forest Members consist of disconnected ribbon and narrow sheet sandstone bodies enclosed in thick mudstone sequences. The sandstone bodies formed in chiefly meandering streams. The Owl Rock and Churck Rock Members and Kane Springs strata are more variable packages of sandstone, mudstone and limestone that formed by various fluvial and lacustrine processes. Analysis of sandstone-body geometry and interconnectedness of the Shinarump, Moss Back, Monitor Butte, and Petrified Forest Members suggests that changing regional subsidence rates were major causes for their architectural patterns. These conclusions are supported by a comparison of Chinle sandstone-body geometry with sandstone-body geometry produced by quantitative models in the literature. The alternating sheet and ribbon geometry of the calcarenite and calcirudite channel deposits in the Kane Springs strata is related to diapiric salt activity in the Salt Anticline region. Controls that affected the sandstone-body geometry in the Church Rock Member are not fully understood but are probably related to increased aridity at the close of Chinle deposition.
DOI:10.1016/0037-0738(84)90074-5      [本文引用:1]
[14] Clemmense L B, Olsen H, Blakey R C.Erg-margin deposits in the Lower Jurassic Moenave Formation and Wingate Sandstone, southern Utah. Geological Society of America Bulletin, 1989, 101(6): 759-773.
Abstract The erg-margin deposits, which extend across a 50- to 100-km-wide region, are composed of dune, eolian sand-sheet, mud-flanked eolian, sabkha, ephemeral-stream and lacustrine facies. The succession is divided into a basal erg-margin association (8-27 m) dominated by eolian facies and an overlying erg-margin association (as much as 90 m thick) composed mainly of fluvial and lacustrine facies. -from Authors
DOI:10.1130/0016-7606(1989)1012.3.CO;2      [本文引用:2]
[15] Fossen H, Schultz R A, Torabi A.Conditions and implications for compaction band formation in the Navajo Sandstone, Utah. Journal of Structural Geology, 2011, 33(10): 1477-1490.
Observations from quartz-rich eolian Navajo Sandstone in the Buckskin Gulch site in southernmost Utah show that pure compaction bands only occur in sandstones where current porosity > 0.29 ± 3, permeability > 10 ± 7 darcy, and grain size > 0.4 mm – properties restricted to the lower and most coarse-grained and well-sorted parts of grain flow units within the dune units. Hence a direct correlation between stratigraphy and band occurrence has been established that can be used to predict deformation band occurrences in similar sandstone reservoirs. We show that the pure compaction bands formed perpendicular to a subhorizontal σ 1, bisecting conjugate sets of shear-enhanced compaction bands. The latter bands locally developed into shear-dominated bands that transect entire dune units, suggesting that an increase in the amount of simple shear promotes band propagation into less porous and permeable lithologies. Stress considerations indicate that, as a continuous and overlapping sequence of events, pure compaction bands in quartz-rich Navajo Sandstone initiated at 10–20 MPa (651 km depth), followed by shear-enhanced compaction bands that locally developed into more stratigraphically extensive shear-dominated bands. The rare combination of special lithologic and stress conditions may explain why pure compaction bands are rarely observed in naturally deformed sandstones.
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[16] Bown T M, Hasiotis S T, Genise J F, et al.Trace fossils of hymenoptera and other insects and paleoinveronments of the Claron Formation (Paleocene and Eocene), Southwestern Utah US. Geological Survey Bulletin, 1997, 2153: 41-58.
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[17] Morris T H, Ritter S M, Laycock D P.Geology unfolded: An Illustrated Guide to the Geology of Utah's National Parks. Provo: Brigham Young University Press, 2010.
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[18] Beaumont C, Quinlan G M, Stockmal G S.The evolution of the Western Interior Basin: Causes, consequences and unsolved problems. Evolution of the Western Interior Basin: Geological Association of Canada Special Paper, 1993, 39: 97-117.
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[19] Schweickert R A, Bogen N L, Girty G H, et al.Timing and structural expression of the Nevadan orogeny, Sierra Nevada, California. Geological Society of America Bulletin, 1984, 95(8), 967-979.
Nevadan structures show great variation in style but relatively constant orientations. Slaty cleavages and tight folds are the characteristic main-phase structures in the western belt of Jurassic island-arc volcanic rocks and flysch-type sedimentary rocks. The central belt of Paleozoic metasedimentary and metavolcanic rocks shows the most extreme variation in style of main-phase structures, from weak, spaced to crenulation cleavages in the south, where polyphase deformed rocks formed a structural basement, to slaty and phyllitic cleavages and asymmetric to isoclinal folds in the north, where most of the Paleozoic basement rocks lack penetrative pre-Nevadan fabrics. Eastward-directed thrust faulting apparently was important only in the northern part of the range, where main-phase deformation was most intense. The eastern belt of Jurassic and Triassic magmatic arc-volcanic and sedimentary rocks defines the core of a major synclinorium, and the rocks contain penetrative slaty cleavages and asymmetric, tight to isoclinal folds. Nevadan Orogeny involved underthrusting of island-arc rocks on the west and significant crustal shortening in the central and eastern belts; features suggest that the orogeny resulted from the collision of the island-arc (western belt) with an andean-type arc (eastern belt) situated at the western edge of North America.--Modified journal abstract.
DOI:10.1130/0016-7606(1984)952.0.CO;2      [本文引用:1]
[20] Blakey R C, Peterson F, Kocurek G.Synthesis of late Paleozoic and Mesozoic eolian deposits of the Western Interior of the United States. Sedimentary Geology, 1988, 56(1): 3-125.
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[21] Wright E K.Stratification and paleocirculation of the late cretaceous western interior seaway of North America. Geological Society of America Bulletin, 1987, 99(4): 480-490.
Paleotemperature and salinity profiles for the Western interior Seaway are reconstructed by use of oxygen and carbon isotopic signatures of shell material. Data have been gathered on different groups of molluscs distributed in three distinct habitat types that reflect different positions in the water column: infaunal benthonic, epifaunal benthonic, and nektonic. Isotopic variations and inferred environmental tolerances of the organisms were used to determine upper and lower temperature boundaries for each habitat group and therefore water depth zones. Water density for each temperature-salinity combination within a habitat range was calculated and used to develop a physically stable stratification model. The results clearly show that the layers in the water column were decoupled, had different isotopic histories, and consisted of denser, more saline warm bottom layers overlain by cooler less saline intermediate layers. It is proposed that this profile originated due to a surface current transporting runoff water from the western highlands towards the eastern shallow border of the seaway where evaporation and heating of surface water in a warm humid atmosphere caused increased density and sinking, followed by westward return flow of the warm saline bottom water. This model is compatible with known climatic and oceanographic information for the Cretaceous.
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[22] Rice D D, Shurr G W.Patterns of sedimentation and paleogeography across the Western Interior Seaway during time of deposition of Upper Cretaceous Eagle Sandstone and equivalent roks, northern Great Plains. In: Reynolds M W, Dolly E D. Mesozoic Paleogeography of the West-Central United States: Rocky Mountain Section, SEPM, 1983: 337-358.
East of the Black Hills, the lower part of the Gammon grades into the upper part of Niobrara Formation which contains widespread chalk tongues. The chalk tongues were probably deposited on a northwest sloping carbonate ramp which was essentially free of western-derived detritus. In central South Dakota, an unconformity, developed at the stratigraphic position of Shannon and younger Gammon rocks, occurs along a northeast-southwest trend and represents submarine erosion along the Transcontinental Arch. In southeastern South Dakota and southwestern Minnesota, the unconformity is present on a flat surface cut into the Precambrian basement on the Sioux Ridge which was an island near the eastern shoreline.
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[23] English J M, Johnston S T.The laramide orogeny: What were the driving forces?. International Geology Review, 2004, 46(9): 833-838.
The Laramide orogeny is the Late Cretaceous to Paleocene (80 to 55 Ma) orogenic event that gave rise to the Laramide block uplifts in the United States, the Rocky Mountain fold-and-thrust belt in Canada and the United States, and the Sierra Madre Oriental fold-and-thrust belt in east-central Mexico. The Laramide orogeny is believed to post-date the Jurassic and late Early Cretaceous accretion of the terranes that make up much of the North American Cordillera, precluding a collisional origin for Laramide orogenesis. Instead, the deformation belt along much of its length likely developed 700-1500 km inboard of the nearest convergent margin. The purpose of this paper is to show, through a review of proposed mechanisms for producing this inboard deformation (retroarc thrusting, "orogenic float" tectonics, flat-slab subduction and Cordilleran transpressional collision), that the processes responsible for orogeny remain enigmatic.
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[24] Biek R F, Willis G C, Hylland M D, et al.Geology of Zion National Park, Utah. In: Sprinkel D A, Chidsey T C, Anderson P B. Geology of Utah's Parks and Monuments. Utah Geological Association Publication, 2000, 28: 107-138.
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[25] Surdam R C, Jiao Z S, MacGowan D B. Redox reactions involving hydrocarbons and mineral oxidants: A mechanism for significant porosity enhancement in sandstones. American Association of Petroleum Geologists Bulletin, 1993, 77(9): 1509-1518.
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[26] Beitler B, Chan M A, Parry W T.Bleaching of Jurassic Navajo Sandstone on Colorado Plateau Laramide highs: Evidence of exhumed hydrocarbon supergiants?. Geology, 2003, 31(12): 1041-1044.
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