Seismites

 The Dead Sea region has experienced six destructive earthquakes during the last 1000 years, with an average recurrence interval of around 200 years (188 to be exact) beginning with the 1060 AD earthquake. This cluster of events ended with the 1927 earthquake which had a magnitude of 6.2.

The Dead Sea Fault is a Left lateral transform plate boundary, separating the Arabian plate and the Sinai sub-plate and has been active since the Miocene, with movement still occurring in present day. (Fig. 1). (Garfunkel, 1981). This fault zone lies within the Dead Sea graben of which this post will focus upon. More specifically, depositional attributes of the laminated layers that comprise the sedimentary record of the Dead Sea and how those layers can serve as a portal into the past regarding paleo-seismic events.

With the recent drop in lake levels, the banks are quite accessable. By scraping away the halite/sand encrusted sides of these banks, layers of mud and sand/silt are exposed. Instead of being undisturbed laminated layers of sediment, you see beautful swirls and designs where the layers have intermixed. These disturbances, termed siesmites, are not only quite stunning to behold but also have an important significance in regards to providing a geologic record on ancient earthquakes along the Dead Sea transform.

 The lacustrine sediments of the Dead Sea are comprised of alternating layers of aragonite and detritus sediments. The latter of which are composed of dark, silt-clayey size detritus derived from flooding (fluvial events) as suspended material and range in thickness from a couple of centimeters to almost 20 (can vary; these are my measurements). The aragonite layers are intermingled white and dark laminae of silt-clayey sized detritus, and are much thinner in comparison, being in the thicknesses of millimeters. (Bookman, et al., 2004). These layers were originally continuous alternating laminae of aragonite and fine detritus, lying flat on the bottom of the Dead Sea undisturbed. They were later fluidized (brought on by seismic events), disturbing the top of the sediment and causing it to be drawn back into suspension. Deformation of the laminae occurs when the sediment comes to rest after resettling. The event is encompassed by undisturbed sediments above and below.  This mixed layer indicates a disturbance due to a seismic event, and its timing is constrained by the first overlying undisturbed lamina.

Syndepositional faulting in the Dead Sea sediments has been interpreted as when (Marco et al, 2004) a fault offsets a surface creating subaqueous scarp. The top of the sediment is deformed due to liquefaction and suspension during a seismic event, and a mixed layer forms on both sides of fault scarp. After the suspended sediments resettle, the mixed layer in down-thrown block is slightly thicker. As further sedimentation ensues, a thicker sequence accumulates on down-thrown block. The lower mixed layer in the downthrown block is also bent and overlain by folded layers.

The occurrence of seismites and their correlation to historically documented earthquakes has been determined by radiocarbon dating organic material found within the layers of sediment, solidifying the association of fluidizations of sediment and seismic events.







Bookman (Ken-Tor), R., Enzel, Y., Agnon, A. and Stein, M. 2004: Late Holocene lake levels of the Dead Sea. GSA Bulletin 116, 555 71.
Garfunkel Z (1981) Internal structure of the Dead Sea leaky transform (rift) in relation to plate kinematics. Tectonophysics 80:81-108
 Marco, S., and Agnon, A., 1995. Prehistoric earthquake deformations near Masada, Dead Sea graben. Geology, 23: 695-698.


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