The Seismite Problem

G. Shanmugam

Department of Earth and Environmental Sciences,

The University of Texas at Arlington, Arlington, TX 76019, USA

Abstract During a period of 82 years (1931e2013), 39 genetic terms were introduced for various deposits. Of the 39 terms, only ten are meaningful in understanding the true depositional origin (e.g., turbidites), the remaining 29 are just jargons (e.g., seismites, tsunamites, etc.). The genetic term “seismites”, introduced by Seilacher (1969) for recognizing palaeoearthquakes in the sedimentary record, is a misnomer. The term was introduced in haste, based on an examination of a single exposure of the Miocene Monterey Formation (10 m) in California, without a rigorous scientific analysis. The fundamental problem is that earthquake is a triggering mechanism, not a depositional process. Type of triggers cannot be recognized in the ancient sedimentary record because evidence for triggers is not preserved by nature. Soft-sediment deformation structures (SSDS), commonly used as the criteria for interpreting seismites, are a product of liquefaction. However, liquefaction can be induced by any one of 21 triggers, which include earthquakes, meteorite impacts, tsunamis, sediment loading, among others. Brecciated clasts, typically associated with earthquake-induced deposits in the Dead Sea Basin, are also common depositional products of debris flows (i.e., synsedimentary product unrelated to earthquakes). Also, various types of SSDS, such as duplex-like structures and clastic injections, can be explained by synsedimentary processes unrelated to earthquakes. Case studies of sandstone petroleum reservoirs worldwide, which include Gulf of Mexico, North Sea, Norwegian Sea, Nigeria, Equatorial Guinea, Gabon, and Bay of Bengal, reveal that there is compelling empirical evidence for sediment loading being the primary cause of SSDS. The KrishnaeGodavari Basin, located on the eastern continental margin of India, is ideal for sediment failures by multiple triggering mechanisms where overpressure and liquefaction have led to multi-origin SSDS. Because tsunamis and meteorite impacts are important phenomena in developing extensive deposits, lateral extent of SSDS cannot be used as a unique distinguishing attribute of earthquakes.

Keywords Soft-sediment deformation structures (SSDS), Seismites, Earthquakes, Meteorite impacts, Liquefaction, Clastic injections

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