Earthquakes, sea levels, and coral reefs at the Gulf of Elat
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Publications:
Shaked, Y., Agnon, A., Cohen, C., Lazar, B., Marco, S., Sass, E., and Stein, M. 2002. Late Holocene shorelines at the Gulf of Elat: Migrating shorelines despite tectonic and sea level stability. EGS Stephan Mueller Special Publication Series 2:105-111. pdf file
Holocene coral reefs and coastal geomorphology are the basis of our reconstruction of the events shaping the coastline near Elat, at the northernmost end of the Red Sea (Figure). Widespread occurrence of fossil Holocene reefs at the Gulf of Elat serve as markers of relative sea level at the time of their growth. Elevation of these reefs relative to the present day sea level indicates changes in the relative sea level since the time they developed, changes that are due to fluctuation of the sea level itself or to tectonic displacement of the coastal region. We were able to differentiate these factors and establish the sea level curve for the Gulf of Elat in the past 6,000 years: Sea level rose throughout the early Holocene until a high stand of +1.2 m was reached some 6 ka. The highstand was sustained until about 4 ka, and present sea level was established about 2 ka. (Figure)
The regional significance and the global processes contributing to this pattern were analyzed by comparing this sea level pattern with the pattern observed at the nearby coast of the eastern Mediterranean Sea, and formulating the effect of water load on basin margins. We find the observed sea level pattern at the Gulf of Elat to be a regional feature, and thus the sea level can be used as a datum in investigations of local tectonics at the gulf.
Fossil reef flats that are discordant with the regional sea level pattern demonstrate the occurrence of repeated down-faulting earthquake events near Elat. At least two such events occurred in the past 6 ka. The first event, some 4 ka, downfaulted parts of the coast by nearly a meter and triggered catastrophic sediment influxes. A second larger event occurred about 2 ka with displacement reaching more than 3 m in some locations.
The recurring events that destroy reefs near Elat keep the modern fringing reef there at a constant “juvenile” state. This unique situation provides us with a glimpse into the stages and processes of fringing reef growth and development. Based on the morphology of the Elat fringing reefs we suggest a model of reef growth by which reefs initiate as isolated knolls on the sea floor that grow towards sea surface and then begin to spread laterally. Neighboring knolls will then coalesce to form an elongated reef structure along the coast. As the reef develops a lagoon forms between the active reef and the coast, and the initial knolls are sometimes still discernable as spurs jutting seaward.
A consequence of this growth model is that corals forming the reef flat may span an age range of several thousands of years. Indeed, we measured such a range on coral samples collected from fossil Holocene reef flats near Elat. While our findings stress the significance of obtaining in-situ samples from reef flats to gain accurate paleo-depth control, the inherent age range of reef flats should be considered when studying events in time.
We studied the downfaulting earthquakes near Elat by excavating the fossil reef from beneath its terrigenous sediment cover, by diving to sample submerged reefs, and by mapping the coastal morphology both on land and underwater. We found that displacement in both events varied along the coast, and that the shoreline morphology was altered in response to both events. About 9 km south of Elat we see that after the more recent event at ~2 ka the shoreline propagated about 100 m seaward, forming a wide beach terrace. Uplifted fossil reefs are found along the shores of the gulf in southern Sinai and near Aqaba, but downfaulting earthquakes seem to have shaped the coast near Elat throughout the Holocene.
גיאולוגיה של שוניות הולוקניות - מפלסי ים, וטקטוניקה במפרץ אילת
תזה לתואר דוקטור
המכון למדעי כדור הארץ, האוניברסיטה העברית
שוניות אלמוגים עשויות לשמש סמן מצוין למפלס הים בזמן גדילתן. לימוד המורפולוגיה של שוניות מאובנות במפרץ אילת ותיארוכן בעזרת פחמן 14 וסדרות האורניום אפשרו לנו לקבוע את דגם עלית מפלס הים בהולוקן במפרץ אילת, ולזהות מספר חריגות מדגם זה. במפרץ אילת עלה מפלס הים והגיע למקסימום לפני כ-6,500 שנה, ועד לפני כ-4,500 שנה ירד המפלס בשיעור של כמטר למיקומו הנוכחי. אנו מסבירים דגם זה על ידי כפיפה והתרוממות של קו החוף סביב מפרץ אילת כתוצאה מהעמסת מים בים התיכון בעקבות הפשרת הקרחונים . התרוממות קו החוף גורמת לירידת מפלס הים הנצפה במפרץ.
חריגות מקומיות מדגם זה ניכרות בשוניות אלמוגים מההולוקן המאוחר אשר מצויות תחת המים בעומק של מספר מטרים, או קבורות תחת סדימנטים באזור אילת. חריגות אלה מיוחסות לתנועות טקטוניות אשר פגעו באזור החוף של אילת ב-5,000 השנים האחרונות.
מצגת בעברית על הגיאולוגיה של מפרץ אילת יש כאן.
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A submerged archaeological site near the outer side of the reef, which was first noted by the archaeologist Uzi Avner, consists of circles and piles of stones. The flat surface below the site is made of fossil reef. |
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