白袋地區

白袋地區（英語：White Pocket）位於亞利桑那州大樓梯埃斯卡蘭特紀念碑(Grand Staircase Escalante Monuments)，地質上位於科羅拉多高原(Colorado Plateau)。納瓦霍砂岩 (Navajo Sandstone)以土丘和山脊的形式暴露在高原上。這些大多是淡紅色的侵蝕遺跡，但在白袋地區中的那些侵蝕遺跡卻被一層白色的岩石層覆蓋，看起來像花椰菜岩（CauliflowerRock）（見圖）。這白色層使該區域獨一無二，因此被稱為「白袋地區」。

成因

從表面上看，花椰菜岩似乎與底下地層交織在一起（見圖），因此被解釋為扭曲的地層，從而導致有關於白袋地區非凡地質過程的兩種思流派。一些地質學家宣稱，白袋地區的成因是由於軟沉積物變形(soft-sediment deformation) ^[1]，而另一派認為，成因是地震引起的大規模塌陷和山體滑坡^[2]。兩派都歸因扭曲地層於結構變形。小規模的軟沉積物變形在露頭中常見，並已在局部格倫峽谷（Glen Canyon)及錫安國家公園(Zion National Park)地區有報道^[3]^[4]. 但區域性的軟沉積物變形，例如在白袋地區中，卻很少有報道。因此需要一種地質機制來引發這種區域性變形。 Marc Deshowitz (2011) ^[2] 解釋說，地震形成了流動沙體，而構成現今地表上的毫無特色白色砂岩或「花椰菜岩」。他認爲扭曲地層是一種流體逃逸結構(fluid escape structure)，如同沙火山(sand volcano)。

Chen (2024)^[誰？]^[5]的最近的一項研究中提供了具信服力的證據，指出在亞利桑那州白色口袋地區出露的花椰菜岩是一種沉積構造，代表在乾旱地區形成的過度厚風化殼。它是由地下水上升所繫帶的的礦物質在地表沉積而形成的。花椰菜岩石的核心或下方都是未受干擾的地層層理（見圖），顯示白色地層形成過程中幾乎沒有構結幹擾。事實上，岩石表面出現的扭曲結構，是由於常規未受干擾的交錯層理, 出露在不同曲率和方向的露頭表面上而產生的。岩石表面也具有不同發育程度的風化殼（見圖）。這都是沉積過程的明顯證據（見圖）。

參考

^ Chan, M. A. and R. L. Bruhn, 2014, Dynamic liquefaction of Jurassic sand dunes: processes, Origins, and implications, Earth Surface Processes and Landforms, 39, pages 1478– 1491, doi: 10.1002/esp.3539.
^ ^2.0 ^2.1 Deshowitz, Marc, 2011, An Earthquake-Induced Flash Flood Event within the Early Jurassic Navajo Sandstone, White Pocket, Vermillion Cliffs National Monument, Arizona, AAPG Search and Discovery #90126, AAPG Rocky Mountain Section, Cheyene, Wyoming June 25-29 2011. (https://www.searchanddiscovery.com/pdfz/abstracts/pdf/2011/rocky/abstracts/ndx_deshowitz.pdf.html)
^ Bryant, Gerald, Giovanni Monegato, and Andrew Miall, 2013, An example of liquefaction-induced interdune sedimentation from the early Jurassic Navajo Sandstone, USA,Sedimentary Geology,Volume 297, Pages 50-62,ISSN 0037-0738, https://doi.org/10.1016/j.sedgeo.2013.09.001. (https://www.sciencedirect.com/science/article/pii/S0037073813001668)
^ Ford, Colby, Kevin Nick and Gerald Bryant, 2015, 3D visualization of liquefaction-induced dune collapse in the Navajo Sandstone, Utah, USA. EGU General Assembly 2015, held 12-17 April, 2015 in Vienna, Austria. id.8561 (https://ui.adsabs.harvard.edu/abs/2015EGUGA..17.8561F/abstract)
^ Percy PH Chen. Formation of 「White Pocket」 in The Surface Exposure of Jurassic Navajo Sandstone in Arizona. ESS Open Archive . August 19, 2024. DOI:10.22541/essoar.172405821.11678903/v1

[”Chan”-1] Chan, M. A. and R. L. Bruhn, 2014, Dynamic liquefaction of Jurassic sand dunes: processes, Origins, and implications, Earth Surface Processes and Landforms, 39, pages 1478– 1491, doi: 10.1002/esp.3539.

[”Deshowitz”-2] 2.0 ^2.1 Deshowitz, Marc, 2011, An Earthquake-Induced Flash Flood Event within the Early Jurassic Navajo Sandstone, White Pocket, Vermillion Cliffs National Monument, Arizona, AAPG Search and Discovery #90126, AAPG Rocky Mountain Section, Cheyene, Wyoming June 25-29 2011. (https://www.searchanddiscovery.com/pdfz/abstracts/pdf/2011/rocky/abstracts/ndx_deshowitz.pdf.html)

[”Bryant”-3] Bryant, Gerald, Giovanni Monegato, and Andrew Miall, 2013, An example of liquefaction-induced interdune sedimentation from the early Jurassic Navajo Sandstone, USA,Sedimentary Geology,Volume 297, Pages 50-62,ISSN 0037-0738, https://doi.org/10.1016/j.sedgeo.2013.09.001. (https://www.sciencedirect.com/science/article/pii/S0037073813001668)

[”Ford”-4] Ford, Colby, Kevin Nick and Gerald Bryant, 2015, 3D visualization of liquefaction-induced dune collapse in the Navajo Sandstone, Utah, USA. EGU General Assembly 2015, held 12-17 April, 2015 in Vienna, Austria. id.8561 (https://ui.adsabs.harvard.edu/abs/2015EGUGA..17.8561F/abstract)

[”Chen”-5] Percy PH Chen. Formation of 「White Pocket」 in The Surface Exposure of Jurassic Navajo Sandstone in Arizona. ESS Open Archive . August 19, 2024. DOI:10.22541/essoar.172405821.11678903/v1

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