Spatial and temporal reconstruction of the late Quaternary Dead Sea sedimentary facies from geophysical properties
Introduction
The Dead Sea is a hypersaline terminal lake currently located at −432 m below mean sea level. During the late Quaternary series of lakes occupied the Dead Sea Basin (DSB). The sedimentary sequences that accumulated within these lakes recorded the climate and tectonic histories of the DSB and its vicinity (for comprehensive overviews of the subject see Stein, 2001, Stein, 2014 and references therein). Most of the information on the Dead Sea lakes was retrieved from exposures along the marginal terraces of the modern Dead Sea and shallow cores that were drilled along the retreating shorelines, e.g. the exposures of the last glacial Lake Lisan and the Holocene Dead Sea (e.g., Bartov et al., 2002; Haase-Schramm et al., 2004; Migowski et al., 2006; Stein et al., 2010; Litt et al., 2012 and others). The cores that were drilled during 2010–2011 by the International Continental Drilling Programs (ICDP) at the abyssal floor of the Dead Sea (at water depth of 300 m) and in the shallow waters (Fig. 1) open new opportunities to explore the history of the lake (e.g. Neugebauer et al., 2014; Lazar et al., 2014, Torfstein et al., 2015 and reference therein). These studies were mainly based on the information retrieved from the drilled cores. However, the drilled cores reveal several gaps in data possibly due to drilling issues (e.g. difficulty to recover sand or salt units) and the correlation between the deep and marginal settings remains unknown. These problems are resolved with geophysical logging data retrieved from the boreholes, which are used here to establish a regional correlation and map the sedimentary structure beneath the lake.
We present downhole logging data acquired from the deep floor boreholes: ICDP 5017-1-A, 5017-1-C and from the shallow 5017-3-C borehole (Fig. 1). We use a quantitative method to build a generic zonation and classification of the main lithologies from a combined set of logs and core observations. Our technique was also undertaken with the objective of understanding the response of petrophysical data in a mixed carbonate-siliciclastic environment and to examine its accuracy as a lithofacies predicator, especially where no cored material was available. Once calibrated, the outcome predicts a continuous vertical lithofacies distribution at each of these sites and thus enables to quantify the proportion of the different lithologies deposited in the Dead Sea Basin. The paper fills the gap of missing lithological information where core recovery was poor as well as providing a new map of the sedimentary structure beneath the lake with continuous stratigraphic information on the lacustrine deposits from the marginal to the deep area of the basin. As such, it sets the stage for a comprehensive framework for further understanding of the regional paleoclimate and tectonic histories and future sedimentary studies.
Section snippets
Geological background
The Dead Sea Basin (DSB) is a 150 km long and 15–17 km wide structure comprising an active pull-apart basin formed along one sector of the Dead Sea Transform (e.g., Garfunkel, 1981; Ben-Avraham and Lazar, 2006; Lazar et al., 2006). The modern Dead Sea (Fig. 1) whose surface waters currently lie at 432 m below mean sea level (bmsl), comprises a residual water-body that evolved from a series of lakes that filled the Dead Sea Basin during the late Quaternary (Stein, 2001, Stein, 2014). The
The ICDP Dead Sea Deep Drilling Project (DSDDP)
A multiple range of analyses were conducted on the cored sediments recovered from the main ICDP borehole located in the deep depocenter of the Dead Sea (5017-1-A). Neugebauer et al. (2014) produced a lithostratigraphic column for this borehole using sedimentary data from core analysis (available in the ICDP repository). A basic lithological description was also provided by the authors for the 5017-1-C borehole. The 5017-3-C site, located at the margin of the lake, remains undescribed.
Neugebauer
Methodology
Between November 2010 and March 2011, a set of wire-logging measurements were collected by the ICDP Operational Support Group (OSG) in three separate wells located in the northern Dead Sea basin (Fig. 1). These sites, which form a southwest-northeast transect are: 5017-1-A and 5017-1-C located in the central basin, and 5017-3-C in the near-shore of the lake. Available well logs include: Master γ-ray (GR) and spectral γ-ray with potassium (K), uranium (U) and thorium (Th), resistivity (R),
Data quality
Caliper log measurements provided a continuous record of the size and shape of the borehole with depth and were used to investigate the quality of the logs in holes 5017-1-A and 5017-3-C. Downhole caliper logs for 5017-3-C were limited to the upper 120 m. No information was available for 5017-1-C. Generally, only few intervals were characterized by an elliptical geometry recognized by larger values of the caliper log relative to bit size. These areas usually correspond to a zone of evaporitic
General
By incorporating core and logging data, three main logfacies (mudstone interbedded siltstones to sandstones, medium to thick-bedded mudstone to sandstones and evaporites) were shown to correspond to the three main core facies in the 5017-1-A, 5017-1-C and 5017-3-C boreholes. The recognition of some of these lithofacies appears to be limited by the resolution of the standard logging tool, the quality of the data and potentially by incorrect core-to-log depth correlations resulting from logging
Summary and conclusions
The examination of well logging data and core descriptions from three wells drilled during the ICDP campaign in the northern Dead Sea were combined to identify and correlate the main lithofacies that accumulated in the subsurface of the basin. Using the available petrophysical data (gamma ray, compressional velocity and resistivity logs) it was possible to construct a continuous lithostratigraphic profile for each well and overcome local gaps coincident with poor core recovery. On the basis of
Acknowledgements
This study uses results from the International Continental Scientific Drilling Program (ICDP) Dead Sea deep drilling project. The authors wish to thank DOSECC for performing the drilling operations and the Moti Gonen Marine Station in Ein Gedi for organizational support. We are very grateful to the ICDP Operational Support Group and we would like to thank Uli Harms, Ronald Conze, Jochem Kück, Matxalen Rey Abasolo, Martin Töpfer and Christian Carnein for downhole logging. In addition, we are
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