Coastal Geomorphology of Andesite from the Cretaceous Alisitos Formation in Baja California (Mexico)
Coastal Geomorphology of Andesite from the Cretaceous Alisitos Formation in Baja California (Mexico)
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下加利福尼亚州（墨西哥）白垩纪 Alisitos 组安山岩海岸地貌
DOI：
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
Markes E. Johnson
中科院分区：
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作者：
Hovey C. Clark;Markes E. Johnson
沿着墨西哥下加利福尼亚北部的太平洋海岸，阿利西托斯组包括大量的安山岩，这些安山岩形成于中白垩纪时期。在白垩纪晚期，在更新世晚期，以及现在，这些被挖掘出来的火成岩一直是崎岖海岸的守护者。这条崎岖海岸线的长期存在与传统的海岸线演变模型不符。这里所称的“锁孔入口”是安山岩岬角上的联合控制的开口，在开阔的海岸上受到强烈海浪冲击的侵蚀。它们很窄，长宽比通常在3:1到7:1之间。I型锁孔发生在结构一致的岩石中，只有很少的节理或大量的无序节理，没有稳定的主导面。这些陡壁特征的成熟与气孔的扩大有关。II型锁孔通常表现出较低的岩壁起伏，形成于具有垂直节理面的岩石中，频率在每米4 ~ 45个节理之间。二级甚至三级节理组可以在与第一个节理组亚垂直的角度上观察到，结果是两个锁孔相交形成一个孤立的海蚀柱。这一海岸侵蚀过程在白垩纪晚期大规模发生，安山岩小岛从大陆上分离出来。
Exposed along the Pacific shores of northern Baja California, Mexico, the Alisitos Formation includes a significant amount of andesite which formed during Middle Cretaceous time. In Late Cretaceous time, again in Late Pleistocene time, and now at present, these exhumed igneous rocks were and continue to be the resistant guardians of a rugged shore. The longevity of this rough coast line is at odds with traditional models of shoreline evolution. Features named herein " keyhole inlets " are joint- controlled openings in andesite headlands, eroded by intense wave shock on an open coast. They are narrow with a length to width ratio commonly between 3:1 and 7:1.Type I keyholes occur in rocks which are structurally consistent with very few joints or a multitude of chaotically oriented joints with no dominant plane of instability. Maturity of these steep-walled features is associated with the enlargement blowholes. Type II keyholes typically express much lower wall relief, forming in rocks having vertical joint planes with variable frequencies ranging between 4 and 45 joints per meter. Secondary or even tertiary joint sets may be observed at an angle subperpendicular to the first , with the result that two keyholes intersect to form an isolated sea stack. This process of coastal erosion operated on a grand scale during late Cretaceous times, when small andesite islands were separated from the mainland.