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Geology Club Seminar

Thursday, October 3, 2019
4:00pm to 5:00pm
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Arms 151 (Buwalda Room)
"Modern halite deposition dynamics in the Dead Sea: Lessons for the deposition of thick halite sequences" & "Fluvial incision and coarse gravel redistribution across the modern Dead Sea shelf as a result of base-level fall"
Ido Sirota, Graduate Student, Institute of Earth Sciences, Hebrew University of Jerusalem,
Haggai Eyal, Graduate Student, Institute of Earth Sciences, Hebrew University of Jerusalem,

Modern halite deposition dynamics in the Dead Sea: Lesson for the deposition of thick halite sequences

Layered halite sequences were deposited in deep hypersaline basins throughout the geological record. However, analogues of such sequences are commonly studied in shallow environments. Here we study active precipitation of halite layers from the only modern analog for deep, halite-precipitating basin, the hypersaline Dead Sea. The link between spatiotemporal evolution of halite precipitation and the seasonal thermohaline stratification in the Dead Sea was characterized by means of monthly observations of the i) lake thermohaline stratification (temperature, salinity, density and degree of halite saturation), ii) textural evolution of the active halite deposits. We present the observed relationships between textural characteristics of layered halite deposits (i.e. grain size, consolidation, and roughness) and the degree of saturation, which in turn reflected the limnology and hydro-climatology. The lakefloor is divided into two principle environments: A deep hypolimnetic lakefloor in which halite continuously precipitates with seasonal variations, forming distinct summer and winter halite layers. A shallow epilimnetic lake floor is highly influenced by the seasonal temperature variations, and by intensive summer halite dissolution which results in thin sequences with annual unconformities. This emphasizes the control of temperature seasonality on the precipitated halite layers characteristics. In addition, precipitation of halite in the hypolimnetic floor, on the expense of the dissolution of the epilimnetic floor, results in lateral focusing and thickening of halite deposit in the deeper part of the basin and thinning of the deposits in shallow marginal basins. We termed this syndepositional thickening of depocentral halite - 'halite focusing'. Global distribution of halite units displays significant thickness variations over the basin and commonly show thick and massive halite in basins' depocenter vs. absence of halite in the basins' margins (e.g. MSC). These observations have been an enigma in evaporites study for decades and we suggest that depth dependence of halite deposition and the resulted 'halite focusing' to explain basinal halite distribution.

Fluvial incision and coarse gravel redistribution across the modern Dead Sea shelf as a result of base-level fall

Global eustatic lowstands can expose vast areas of continental shelves, and occasionally the shelf edge and the continental slope. The degree of fluvial connectivity to receding shores influences the redistribution of sediments across these emerging landscapes. Shelf and slope emergence in the Dead Sea since the middle of the 20th century, offers a rare opportunity to examine evolution of stream connectivity in response to continuous base-level decline and coarse sediment redistribution. We characterize the connectivity evolution of two streams, using high-resolution time series of aerial imagery and elevation models, field mapping, and grain-size analyses. Our rich spatiotemporal dataset of evolving channel geomorphology, sediment transport conditions, and sediment redistribution, allows calculating potential coarse sediment mobility in response to base level decline. Following shelf emergence, alluvial fans first prograde onto the low-gradient shelf under unfavourable conditions for transporting coarse sediment to the regressing shoreline. Then, with shelf and slope emergence, the two adjacent streams evolved differently. The smaller, more arid watershed still maintains its highstand delta progradation on the shelf and is practically disconnected from the receding lake. The larger catchment, heading in wetter environments and having a narrower shelf, has incised the shelf and renewed and gradually intensified the sediment transport from the highstand to the lowstand delta. Sediment mobilization to lowstand shorelines is controlled by the evolution of the channel profile and by the average speed of gravel transport (10s-100s m yr-1). These findings from the Dead Sea are relevant to fluvial processes operating on continental shelves during glacial maxima. Streams would have commonly stored high proportions of their coarse sediment on the continental shelves rather than efficiently connecting with the lowstand level. Additionally, differences in sediment routing patterns should exist among nearby streams, primarily due to continental margin geometry and watershed hydrology.