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Of This Feature, to the Landward Fringe of Coastal Blue-Green Algal Mats, Are of Marginal Trafficability

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Of This Feature, to the Landward Fringe of Coastal Blue-Green Algal Mats, Are of Marginal Trafficability TRAFF1CABILITY OF SABKHAS by John E. Cromwell and David W. Valentine Tetra Tech, Inc. ABSTRACT The trafficability of a coastal sabkha in Saudi Arabia was examined. Surficial features which separate trafficable from non-trafficable sediments are presented. Sediments which are trafficable fall landward of a low relief scarp which we believe to be a relict shoreline. Sediments found seaward of this feature, to the landward fringe of coastal blue-green algal mats, are of marginal trafficability. Sediments under lying the algal mat are thixotropic saturated muds which are non-trafficable. This project was performed with the support of the Office of Naval Research under Contract N0OO14-80-C-0366. 293 J. Cromwell/Valentine INTRODDCTION Sabkhas are desert mud flats, whose elevation is controlled by the groundwater table. Sabkhas have been classified as con tinental (playas) and coastal (supra-tidal plains). In the former case, groundwaters are supplied by continental aquifers, while in the latter, both continental and seawater intrusion supply sufficient groundwater to maintain a supra-tidal equilibrium surface. Regardless of the water source, sabkhas contain substantial portions of evaporitic minerals, notably aragonite, gypsum and halite. The sabkhas of the Arabian Peninsula bordering the Arabian Gulf have been intensively studied over the past 15 years. The great majority of the studies were performed in the Trucial States, between the Qatar Peninsula on the northwest to Oman on the southeast. In this region, nearly continuous sabkha extends along the coast for 325 km and may, locally, extend inland for up to 30 km (Evans et al, 1964). These exten sive coastal sabkhas are frequently difficult to traverse. The shallow groundwater table provides moisture continually to the surface sediment which, on humid days, may be further dampened by the hygroscopic properties of surficial halite deposits. The purpose of our investigation was to examine a selected sabkha plain and ascertain whether there were sur ficial features or measureable parameters which might be use ful for determining or predicting sabkha trafficability. METHODS AMD MATERIALS The sabkha studied was the Sabkha al Fasl at Jubail, Saudi Arabia (Figure 1). This Sabkha is modest in size covering only some 250 km2. The area which we studied encompasses only a few square kilometers. Eighteen sites were sampled to a depth of 120 cm by coring and trenching. Each core sample was split and photographed, visual characteristics were described, sediment aliquots were sampled at contacts, and these samples were sieved using half phi mesh sizes from 4.0 to 0.063 mm. Identifiable shell retained on 2.0 mm and larger screen sizes were removed and weighed separately. Groundwater and seawater salinity, air and sediment temperature, barometric pressure, local tides, solar radiation, and ground water elevation were also measured. Penetrometer and shear vane measurements were taken but the results are not given here. The location and elevation of each of the 18 sites were determined with an accuracy of one centimeter and tied to the geodetic grid established at Jubail. The elevations given have been corrected to the Jubail port datum which is 44 cm below U.S.G.S. and British Admiralty chart datum for the Gulf and 31 cm below Indian SpringB Low Hater. Four other locations were sampled in addition to these IB sur veyed sites. Only qualitative assessments of sediment type 294 j. Cromwell /Valentine ETHIOPIA ARABIAN SBA: FIGURE t THE ARABIAN PENINSULA 295 J. Cromwell/ValentLne and water samples for salinity determinations were taken at these sites. RESULTS AND DISCUSSION We have presented below a combined RESULTS/DISCUSSION section highlighting the most significant findings. The seaward fringe of the Sabkha al Fasl is overlain by a blue-green algal mat which is approximately 1-2 cm thick. Under this mat are aragonitic muds. The algal mat/mud sequence was found at Sites 1-4 which are intertidal. Mean surface elevation of these sites was 160 cm, 30 cm below mean high water. These sediments are not trafficable. Site 5 was situated on a small crescentic berm, whose relief was 50 cm above that of the surrounding terrain. The berm sediments, to a depth of some 37.5 cm, were similar to dune sand except for a slightly higher shell content. Below 37.5 cm, a fine sand mud was encountered. The surface sediments at Sites 6-10 were muddy, fine to medium sands. These sediments appeared to be heavily influenced by aeolian processes at present. Interstitial crystalline inclusions (gypsum and anhydrite) were observed above the water table. Little shell was seen on the surface and none was found in the sediments. The mean elevation of these sta tions was 221 cm. These sediments are marginally trafficable when dry and non-trafficable when wet. These stations are subject to infrequent flooding when both high tides and high onshore winds combine to drive a thin sheet (-2 cm) of water across the normally supra-tidal plain. Underlying Sites 1-9 there exists a layer of porous, but very hard, cap rock at an elevation of approximately 139 cm above datum. This layer may well have underlain the remaining sample sites as well but, if so, it was below the limit to which we sampled. Sites 10 and 11 are 73 m apart and are separated by an abrupt 20 cm increase in elevation. This is a rather distinct relief since the mean slope between the two most separated sites, 4.3 km apart, is 18 cm/km. The zone between these two sites is marked by a swash line or windrow of foam and debris including unabraded shell. Site 11, 400 m inland, has a markedly different stratigraphy than the other sites, con taining significantly higher quantities of shell and sand. The remaining eleven sites (12 through 18 and four unsurveyed sites) are most easily categorized as landward of the eleva tion change previously noted between Stations 10 and 11, the 250 cm contour. Seaward of this contour, sediments were typically aragonitic muds which contained no (or few) shells. Landward of this contour, sediments were coarse grained. Some cores contained pure sand identical to that in present 296 J. Cromwell/Valentine day dunes. Others contained almost pure shell, characteristic of present day coastal shell berms. Several authors (Butler, 1969; Evans, 1970; Evans and Bush, 1967; Evans et al, 1969; Kendall and Skipwith, 1969) believe that present day sabkhas represent submerged and deflated relict shorelines, berms and sand dunes. The last major transgression in the Gulf of Arabia peaked around 6,000- 4,000 years B.P. This was followed by a rapid regression from 3,750 years B.P. which reached the present sea level some 1,000 years B.P. We propose that, in the Jubail region, the 250 cm contour roughly corresponds to the relict shoreline of 3,750 years B.P. Shoreward of this line, one finds palimpsest deposits of deflated shorelines, backshores, and dunes. The presumed fall in sea level would also change our study area from an open coast to the present protected bay environment. This transition was accomplished by a depositional regression dominated by aragonitic muds precipitated as the groundwater salinities here approach or may even exceed 70 °/00. Shore ward of the 2S0 cm contour, trafficable sand and shell sedi ments are encountered while to the seaward are marginally trafficable to untrafficable (in the algal mat region) muds. Thus, the hypothesized relict shoreline appears to represent a distinct dividing line in terms of sediment type as well as trafficability. Sediment trafficability, as well as the apparent absolute ele vation of the sabkha plain in general, appears to be mediated by groundwater levels. Previous authors, most notably Kinsman (1966) suggest that the mean sabkha water elevatio'i approximates that of mean sea level. While this may be true for Trucial States coastal sabkhas, it does not appear to be valid for the sabkha we studied. Mean sabkha groundwater elevations at Sites 1-4 were 178 cm in open pits and 185 cm in holes cased with a 1\ inch diameter plastic core liner. For the cased holes, the groundwater elevations increased to 192 cm at Sites 6-10, 197 cm at Sites 11-12 and was essentially stable at 221 cm for Sites 13-18. These data clearly show a trend of increasing water heights moving landward from the shoreline. The mean high tide level, as measured in the bay several kilometers north of Sites 1-4 was 179 cm for the period of the survey (May-June 1979). In other words, the shoreline sabkha groundwater levels more closely approximate the mean high tide level than the mean sea level. At the more distant, inland sites, the groundwater level increases to some 40 cm higher than even the mean high tide and remains remarkably constant between Sites 13 and 18 (a distance in excess of 4 km) at 221 cm (range 220-223 cm). The height of this water table, and the elevation to which it can moisten the sediment column, apparently determines the absolute height of the aeolian accretion-deflation equilibrium surface. Salinities were determined (argentometric titration) for the sabkha groundwaters. A generally increasing trend in salinities landward was noted. The bay of the Gulf adjacent 297 j. Cromwell/valentine to Site 2 had a salinity of 67.2 °/00, while that of Site 2 subsurface water was 112.8 G/oO. The most remote sites, 8 km from the bay, had salinities slightly exceeding 330 0/00. Similar patterns of sabkha groundwater salinity have been reported by several other investigators. The most interesting aspect of our data was not the confirmation of this macropat- tern, but rather micropattern deviations.
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