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摘要
Shell beds are sedimentary features that can potentially provide significant palaeoenvironmental and sequence stratigraphic information. High-energy events, such as storms/hurricanes and tsunamis, might originate sedimentary shell beds due to either landward-incoming waves or basinward backwash flows. Many papers have dealt with the taphonomic characterization of storm shell beds (=tempestites) along palaeobathymetric gradients. On the contrary, only a few taphonomic studies have examined the skeletal remains deposited in tsunami shell beds (=tsunamiites). These studies seek to differentiate taphonomic traits of skeletal remains in tempestites and tsunamiites. These assessments are restricted to tsunamiites deposited inland in connection with historically well-known tsunami events. To date, no studies of tsunami shell beds deposited offshore are available in the literature and, consequently, whether taphonomic attributes can be used to separate storm and tsunami shell beds deposited along the shelf remain to be demonstrated. In the Sorbas Basin (SE Spain), uppermost Tortonian-lowermost Messinian temperate-water carbonate ramp deposits record shell beds attributed to storm events and one thick shell bed interpreted as a tsunamiite. A quantitative taphonomic study allows the differentiation of bioclastic tempestites and tsunamiites deposited on the ramp: 1) tsunamiites are characterized by a mixture of organisms coming from different parts along the ramp (from inner to outer ramp), while tempestites include closely spaced organisms; 2) fragmentation is higher in tempestites than in tsunamiites; 3) shell fragments are predominantly rounded in tempestites but preferentially sharp in tsunamiites; 4) skeletal remains are preferentially oriented horizontally (<30 degrees with respect to stratification) in tempestites, while obliquely (between 30 degrees and 60 degrees) and perpendicularly (>60 degrees) in tsunamiites; 5) concave shells tend to be concave-up stacked in tempestites but chaotically arranged in tsunamiites; and 6) encrustation and borings are more abundant in tempestites than in tsunamiites. The intensity and scale of the depositional mechanisms associated with high-energy events, storms vs. tsunamis, account for the different preservation styles of shells. The key taphonomic features identified in this study help to discriminate offshore shell beds originated either by storms or tsunamis in other geological contexts regardless of the age and the geography, and contributes to the understanding of the scarce examples of fully marine shelly tsunami deposits.
- 出版日期2017-4-1
