--> --> Mapping Israel’s Western Paleo-Coastlines From 3-D Seismic Data

AAPG Geoscience Technology Workshop

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Mapping Israel’s Western Paleo-Coastlines From 3-D Seismic Data

Abstract

The location of the western coastline of Israel during the last glacial period had been inferred by fitting the global sea-level curve to Mediterranean bathymetry, assuming it has not changed. Thus, the coastline was determined by the eustatic depth of the last glacial maximum. Accounting for sediments deposited since then, the coastline is actually buried; direct evidence for its location has not, to date, been produced. Global glaciation resulted in a global sea level drop of ~120 m, and westward migration of a few kilometers of Israel’s western coastline. This sea level drop occurred twice in the last 130 kyr: the first occurrence at 130 kyr ago (MIS6), and the second between 26-33 kyr ago (MIS2), which is the last glacial maximum. The sea level changed little for millennia, until deglaciation caused an abrupt rise. This research aims to map the coastline from the glacial periods along the western shores of Israel and to reconstruct Mediterranean relative sea level during the last glacial periods. In order to do so, we interpreted the “Gabriela” seismic volume, a high resolution 3D seismic survey (time-depth migrated) located 12 km off the Netanya shoreline, so as to identify ancient coastlines, horizons and faults in the Eastern Mediterranean. The coastlines traverse pinch-out points per the stratigraphic sequence. A coastline found in this manner is likely from one of the aforementioned glacial peaks and located 14-16 km west of the modern shoreline. This coastline was determined at an absolute depth of ~130 m below sea level, is buried under up to 45 m of sediment, and up to 100 m water depth. This coastline is compatible with the eustatic sea level of the last glacial periods. The interpretation of the seismic volume was facilitated by Schlumberger’s PetrelTM . Through the use of unique attributes in the program, it is possible to emphasize changes in the nature of the seismic reflector, or to locate properties that are not clearly visible in a standard display of the data. For example, the “RMS Amplitude” attribute calculates the mean root squares of the amplitude on a particular surface at a given depth. As a result of changes in the reflector density typical to shorelines, this well emphasizes in particular areas suspected to be occupied by ancient coastlines. Thus, this amplitude emphasizes the coastline that has earlier been hand-mapped. In order to more accurately assess the age of the coastline, future work will include the comparison of our findings with sparker high resolution cross sections and samples from a geotechnical borehole off the Dor shoreline. These samples yield ages of sedimentation at two depths. Specifically, the samples at a water depth of around 90 m, and depth beneath sea floor of 28.1 m and 33.7 m yielded ages of 14 kyr and 35 kyr respectively. The sedimentation rate deduced from the borehole is estimated to be ~ 2 mm/year over the course of the last 14 kyr (the end of the last glacial period). If the shoreline dates to the last glacial maximum (~ 26 Ka), the sedimentation rate at the eastern Mediterranean sea is estimated to be ~1.3-2.3 mm/year. Future assessment of additional 3D seismic volumes may enable more extensive mapping of the coastline. This mapping will provide more depth to the current information. Reconstructing ancient coastlines constrains a reliable model, predicting conditions and processes that occurred during sea level lowstands. By assessing the coastline, and comparing with other data mentioned above, it will be possible to construct a high-resolution model of the Mediterranean sea level in the late quartenary and to compare it with the global sea level.