Soft-sediment deformation produced by tides in a meizoseismic area, Turnagain Arm, Alaska
Soft-sediment deformation produced by tides in a meizoseismic area, Turnagain Arm, Alaska
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DOI：
10.1130/g23209a.1
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发表时间：
2007-05
期刊：
影响因子：
5.8
通讯作者：
S. Greb;A. W. Archer
中科院分区：
文献类型：
--
作者：
S. Greb;A. W. Archer
Turnagain Arm是阿拉斯加东南部的一个半日潮高潮河口，记录的潮差为9米。在河口潮汐沉积物中保存的扭曲层理和流动卷先前被解释为1964年阿拉斯加大地震的结果。在沉积物和岩层中未变形层之间的流动卷层位已被用来推断其他地区的古地震。虽然许多类型的软沉积物变形结构可以由地震形成，但2003年和2004年夏季对Turnagain Arm内部潮滩沉积的观测表明，在没有地震冲击的情况下，大潮汐河口可以形成广泛的软沉积物变形结构，类似于推断已由地震形成的结构。在2004年的沉积速率测量中，记录了在一天的潮汐中形成的软沉积物变形结构，这是由于快速潮汐下降过程中滩涂过压或1.8米涌潮通过对河床施加的剪切应力。结构由流动辊、盘结构、火焰和17 cm厚的床中的小脱水管组成。将来，如果在已知受到大规模地震影响的河口，在11公里长的区域内的孤立露头中发现Turnagain Arm的流动卷，它们会被解释为震积岩吗？这些例子表明，当使用流卷的层位来推断河口沉积物中的古地震活动性时，需要谨慎，因为许多机制（潮汐通量、潮汐孔、滑塌、洪水）可能导致快速沉积、松散的粉砂和砂土变形，其数量级比大地震更常见。
Turnagain Arm is a semidiurnal hypertidal estuary in southeastern Alaska with a recorded tidal range of 9 m. Contorted bedding and flow rolls preserved in tidal sediments within the estuary have previously been interpreted as resulting from the Mw 9.2 Great Alaskan earthquake of 1964. Horizons of flow rolls between undeformed beds in sediments and rock strata have been used to infer ancient earthquakes in other areas. Although many types of soft-sediment deformation structures can be formed by earthquakes, observations of sedimentation on tidal flats in the inner parts of Turnagain Arm in the summers of 2003 and 2004 show that a wide range of soft-sediment deformation structures, similar to those inferred to have been formed by earthquakes, can form in macrotidal estuaries in the absence of seismic shock. During sedimentation rate measurements in 2004, soft-sediment deformation structures were recorded that formed during one day's tide, either in response to overpressurization of tidal flats during rapid tidal drawdown or by shear stress exerted on the bed by the passage of a 1.8 m tidal bore. Structures consisted of flow rolls, dish structures, flames, and small dewatering pipes in a bed 17 cm thick. In the future, if the flow rolls in Turnagain Arm were found in isolated outcrops across an area 11 km in length, in an estuary known to have been influenced by large-magnitude earthquakes, would they be interpreted as seismites? These examples show that caution is needed when using horizons of flow rolls to infer paleoseismicity in estuarine deposits because many of the mechanisms (tidal flux, tidal bores, slumping, flooding) that can cause deformation in rapidly deposited, unconsolidated silts and sands, are orders of magnitude more common than great earthquakes.