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下切谷

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下切谷(incised valley)是冲积体系在向盆地扩展其河道时和由于海平面相对下降时侵蚀下覆地层而形成的水道谷地。海平面下降时,侵蚀基准面(erosion base level)也跟着下降,导致河流向下切割形成河谷,海平面上升时,河谷被填。形成下切谷[1]。此后此地质名词也被广泛应用到地表岩层露头。下切谷是研究全球海平面升降的主要证据之一,亦是用来佐证全球古气候的及古地理变迁。它也是被用来鉴定在地震地层学的层序界面(sequence boundary)。它的下切幅度可用来推算全球海平面下降及古气候变迁。[2]

分布

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下切谷分布很广,在陆上可由岩层露头,钻井及浅层震测(shallow seismic survey)来判别。在海底必须根据多频道震测(multichannel seismic)。在全球大陆棚上,因为石油探勘而采集的震测网多而密,下切谷被鉴定出最多也最详细。 [3]

第四纪下切谷在墨西哥湾北部海岸平原及大陆棚被发现很多,几乎岸上河流都有下切谷通往大陆棚。在Mobile Bay的下切谷规模可比陆上科罗拉多大峡谷。 [4][5]

规模

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根据大陆冰盖体积,第四纪全球海平面升降的幅度为205米(升80米,降125米)。因此分布在全球大陆棚(continental shelf)上的第四纪的下切谷深度均为此幅度。[6]

在利比亚北部有一条晚中新世英语Late Miocene下切谷,150公里长,至少700米深,被河底切割的是中中世石灰岩,下切谷中的冲填物是上新世河流沉积物。同样年代的下切谷,也分布于利比亚东部及埃及西部。这似乎代表地中海的海面在晚中新世至少下降700米。超出第四纪的海平面升降幅度。[7]

参考资料

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  1. ^ Vail, P., et al. 1977 Seismic Stratigraphy: Applications to Hydrocarbon Exploration (AAPG Memoir 26), ISBN 0891813020
  2. ^ Slatt, R. M, 2013. Stratigraphic Reservoir Characterization for Petroleum Geologists, Geophysicists, and Engineers.in Developments in Petroleum Science, Vol. 61. Published by Elsevier ISBN 978-444-56365-1
  3. ^ Wang,Ru, Colombera, Luca, and Mountney, N.P., 2020. Quantitative analysis of the stratigraphic architecture of incised-valley fills: A global comparison of Quaternary systems, Earth-Science Reviews.Vol. 200
  4. ^ Kindinger, Jack, 2017. Evolution and History of Incised Valleys: The Mobile Bay Model , USGS Publication (https://pubs.usgs.gov/fs/incised-valleys/页面存档备份,存于互联网档案馆))
  5. ^ Anderson, John., et al. 2015. Recycling sediments between source and sink during a eustatic cycle: Systems of late Quaternary northwestern Gulf of Mexico Basin. Earth-Science Reviews, Vol. 153
  6. ^ Dalrymple, Robert, et al. 1994. Incised-Valley Systems: Origin and Sedimentary Sequences。SEPM SPECIAL PUBLICATION,Volume 51. ISBN electronic: 9781565760905
  7. ^ Barr, F. T. and Walker, B. R. 2004. LATE TERTIARY CHANNEL SYSTEM IN NORTHERN LIBYA AND ITS IMPLICATIONS ON MEDITERRANEAN SEA LEVEL CHANGES. in Petroleum Geology of Libya, ed. Hallett, Don. Publ. Elservier. ISBN 50525-3