Tsunamiites and seismites
Records of some of nature's most catastrophically powerful, short-lived phenomena are preserved in sediments and ancient sedimentary rocks called tsunamiites and seismites, which are important not only for reconstructing ancient events but also for evaluating future hazards. Tsunamiites (also spelled tsunamites) are composed of predominantly marine or lake sediments that were transported by huge water waves called tsunami, which are set into motion by earthquakes, massive landslides, volcanic explosions, and asteroid impacts. Tsunamiites also include continental sediments that were transported when tsunamis suddenly dislodged offshore sediment and mixed it with water to form turbidity currents—dense, turbid masses that flow down the slopes of large lakes and oceans and then commonly travel for hundreds of kilometers over the deep sea floor before being deposited. A seismite is a sediment or sedimentary rock that contains textures and structures produced by earthquake shaking; unlike a tsunamiite, the sediments in seismites were already accumulating when the disturbance occurred. By comparing their pre- and postearthquake features, seismites can be used to calculate the timing, frequency, and local intensities of historic or prehistoric earthquakes, the orientation of their stresses, and even the basin slopes on which the sediments were accumulating. Various seismite features may even provide estimates of the magnitude of the earthquake that produced them, if the distance to the epicenter and the depth of focus are reasonably well known.
Adjacent to beaches where high wave energies have concentrated abundant coarse sediment, tsunamiites brought onshore can consist mainly of sand or gravel and even boulders (Fig. 1a). A tsunamiite may be indistinguishable from the deposits of a powerful storm; however, the seaward backwash of a storm surge is much more gradual and less violent than that of a tsunami. In addition, a tsunami may be propagated as more than one wave; thus the sediment fabric of a tsunamiite may document multiple reversals of flow (Fig. 1b,c).
Fig. 1 Tsunamiites. (a) Cobbly gravel deposited on the backshore of the Hirahama coast in Japan by a tsunami in 1993. (b) Schematic diagram showing tsunamiites deposited by an incoming wave and its backwash, which also may dislodge offshore material to generate turbidity current. (c) Stratigraphy left by two tsunami waves and their return flows during the Hirahama tsunami of 1993. (Parts a and c modified with permission from F. Nanayama et al., Sedimentary differences between the 1993 Hokkaido-nansei-oki tsunami and the 1959 Miyakojima typhoon at Taisei, southwestern Hokkaido, northern Japan, Sed. Geol., 135:255–264, 2000)
Tsunamiites may be remarkably thin compared to their hazard significance. Near Seattle, Washington, widespread marshes more than 2 km (1.2 mi) inland are draped with fine sand containing marine fossils only 5–15 cm (2–6 in.) thick, documenting a major earthquake and tsunami about 1000 years ago. Modern tsunamiites record how high above sea level the tsunami rose, from which the magnitude of the earthquake can be estimated. The Seattle tsunamiite is draped over terrain with a vertical relief of 2 m, indicating an earthquake of magnitude 7.5 or greater.
Volcanic explosions can set huge tsunamis into motion (Fig. 2). When the Santorini volcano in the Mediterranean violently collapsed 3500 years ago, tsunamis deposited raised beaches of pumice as far away as Israel, 1000 km (620 mi) to the east. Concurrently, 600 km (370 mi) west of the volcano, 65 km3 (15.6 mi3) of homogeneous mud was suddenly deposited in waters 4000 m (13,000 ft) deep, covering an area of about 5000 km2 (1930 mi2) and up to 24 m (79 ft) thick. The sediment, containing shallow-water carbonate minerals and fossils from the Gulf of Sirte off northern Africa, is interpreted as a tsunamiite or megaturbidite left by enormous turbidity currents that tsunamis triggered 800 km (500 mi) away from Santorini.
Fig. 2 Tsunamiites left in the deep Mediterranean by the Santorini explosion of 3500 years ago. (Data from M. B. Cita and G. Aloisi, Deep-sea tsunami deposits triggered by the explosion of Santorini (3500 y BP), eastern Mediterranean, Sed. Geol., 135:181–203, 2000; and W. Heicke and F. Werner, The Augias megaturbidite in the central Ionian Sea (central Mediterranean) and its relation to the Holocene Santorini event, Sed. Geol., 135:205–218, 2000)
Large as the Santorini tsunami must have been, it is dwarfed by the waves generated when asteroids hit the ocean at typical velocities of over 70,000 km/h (43,500 mi/h). The impact of an asteroid 10–14 km (6–9 mi) in size at the end of the Cretaceous Period 65 million years ago is blamed for killing off half of Earth's species, including the dinosaurs. It is believed to have made the Chicxulub crater, 180 km (112 mi) in diameter, which is centered on the Yucatan coast of Mexico (Fig. 3a). Large tsunamis left deposits at least 500 km (310 mi) inland, and as far from Chicxulub as northeastern Brazil. At numerous sites around the Gulf of Mexico and the Caribbean, sedimentary rocks 65 million years old with tsunamiite features are believed to be related to the end-Cretaceous asteroid because they contain microscopic glass droplets formed when rocks were melted by the heat of impact; quartz grains with their normal, invisible crystallographic structure cut across by microscopic “shock lamellae” from the enormous pressures of impact; and iridium, an element very rare on Earth but much more abundant in meteorites.
Fig. 3 Tsunamiites from the asteroid impact 65 million years ago. (a) Location of the Chicxulub impact site showing sites around and in the Gulf of Mexico and Caribbean Sea where tsunamiites from the impact have been recognized. Oceanic sites were cored by the Deep-Sea Drilling Project and Ocean Drilling Project. (b) Stratigraphy of the Peñalver Formation. (Modified from H. Takayama et al., Origin of the Peñalver Formation in northwest Cuba and its relation to K/T boundary impact event, Sed. Geol., 135:295–320, 2000)
One particularly well-studied end-Cretaceous tsunamiite is the Peñalver Formation of northwestern Cuba, a limestone 180 m (590 ft) thick (Fig. 3b). Its basal 25 m (62 ft) is a coarse conglomerate of angular fragments, including boulders a meter in diameter, that were ripped out of the underlying carbonate layer while it was still soft. Despite the coarseness of the basal conglomerate, its fabric—the fact that it is mixed randomly with smaller-grained sediment—documents that the debris was quickly mixed with water and flowed as a liquid. Above this base, the formation becomes finer upward, from carbonate sand interbedded with 14 thin layers of conglomerates probably brought in by successive tsunamis to calcareous mudstone. Abundant water-escape structures in the middle member of the formation suggest that the material settled rapidly from high-density suspensions, much like the Santorini tsunamiite. Remarkably, although most carbonate sediments are produced by organisms, the limestone was undisturbed by burrowing animals, and its fossils are mainly forms reworked from the underlying rocks. This suggests that the limestones were violently resuspended and then quickly redeposited during a period of major biological catastrophe.
The term “seismites” has been widely used since it was first proposed in 1969 to describe fault-graded beds (graded beds cut by faults) of mud that were accumulating in the quiet water of deep marine basins and were still relatively unconsolidated when earthquakes deformed them. Seismites centimeters to a meter (inches to several feet) thick also have been found in marsh and lake deposits. The term “seismite” refers only to sedimentary masses (such as fault-graded beds) that were deformed without being moved any significant lateral distance. Thus, seismoturbidites, the deposits of turbidity currents that were triggered by earthquakes, are not seismites; they contain no original and fault-induced structures that could be used for evaluating seismic stresses and strains.
Homogeneous mud gradually changes in consistency downward as its own accumulating weight squeezes out water, making it more compact. Thus, from top to bottom, it responds to seismic shaking in liquid, plastic, and brittle fashion. Depending on the intensity of shaking, fault-graded or mixed beds may have a surface zone in which liquefaction and shaking has obliterated all depositional structures. Beneath it is a zone of somewhat compacted sediment fragments encased in a soupy matrix that may also have been intruded from below by plumose structures (Fig. 4). Next is a zone of more compacted mud, commonly broken by miniature step faults that may indicate how the basin floor originally sloped. A basal, relatively undisturbed zone of yet more compacted sediment may be cut by larger faults spaced a few meters apart. Several such paleoseismograms may be preserved in a single stratigraphic sequence. The ages of the earthquakes and their recurrence periods can be dated using radioactive isotopes, fossils, and archeological artifacts in the seismites and in the undisturbed deposits between them.
Fig. 4 Seismite features. (Modified extensively from M. A. Rodriguez-Pascua et al., Soft-sediment sediment deformation structures in lacustrine sediments of the Prebetic Zone, SE Spain, and their potential use as indicators of earthquake magnitudes during the Late Miocene, Sed. Geol., 135:117–135, 2000)
Seismite structures produced by weak shocks may simply be minor disruptions of laminae, or loop bedding (that is, laminate bundles pulled apart by tensile stresses). Among the most common seismite structures are convolute folds, typically about a meter long and a few meters high, encased in undeformed beds. Clearly, they were produced while still soft and close to the sea or lake floor.
Additional seismite features form in interbedded muds and sands. Shocks from earthquakes with magnitudes greater than 5 commonly liquefy watersaturated sands by breaking their grain-to-grain contacts. For the minute or so that the shaking lasts, the mixture of sand and water behaves as a slurry without strength. Driven by the weight of overlying deposits, the slurry rises up through vertical fissures in overlying sequences and is emplaced as sand dikes. The dikes may remain connected to their source beds, or may themselves serve as sources for subordinate, orthogonally oriented dikes that have no direct contact with the mother bed. Liquefaction also may cause water-saturated gravels to intrude overlying beds as irregular-shaped dikes, but gravels are less susceptible to liquefaction than sands, and such dikes may be fractured and faulted by continuing shocks after they are emplaced. Rising silts and sands may also escape entirely from their sources to form isolated, concave-upward pillow structures in overlying layers. Other masses of disturbed sediment can rise or sink, depending on their densities, and be emplaced as isolated pseudonodules.
Along fault scarps, alluvial fans accumulate gravels that are faulted, commonly more than once (Fig. 5). For example, a fault along the front of the Helan Mountains in north-central China experienced an earthquake of magnitude 8 in 1739 that caused 88 km (55 mi) of surface rupture and displaced the famous Great Wall of the Ming dynasty almost a meter vertically and 1.5 m (3.3 ft) laterally. Trenches dug across the fault scarp revealed faulted gravels and colluvial deposits produced not only by the 1739 event but by three older earthquakes as well that recurred at intervals of 2300–3000 years, as determined by radiocarbon dating.
Fig. 5 Alluvial-fan seismite in China. (Modified from Q. Deng and Y. Liao, Paleoseismology along the range-front fault of Helan Mountains, north central China, J. Geophys. Res., 101(B3):5873–5893, 1996)
See also: Earthquake; Fault and fault structures; Sedimentology; Stratigraphy; Tsunami; Turbidity current; Volcano
Kelvin S. Rodolfo