Palaeowind estimation of cross-bedding within the aeolian Kurkar layers of the Gaza Formation , Gaza Strip , Palestine

Coastal plain deposits of the Gaza Strip have been distinguished in the Gaza Formation (Pleistocene) of the Kurkar Group. The Kurkar layers (medium to coarse-grained calcareous sandstone) are characterized by abundant crossbedding that has two sets of cross-beds. These Kurkar layers form the ridges along the Mediterranean coast. The dip, and dip direction of the cross-beds, were measured at fi ve sites to determine the palaeowind direction. The palaeowind directions were measured for the fi rst time in the study area. The Kurkar layers are interpreted as having been deposited in an aeolian environment. Results at Sheikh Ejlin Ridge show that the lower set is dipping eastward (19/099), while the upper set dips northeast (23/051). At the Al Montar Ridge however, the lower set dips southeast (18/146), while the upper set dips north (19/001). These ridges are not coeval, but the precise age relationship between them is as yet unknown. However, the results indicate that there was a change in the palaeowind direction during deposition of the Kurkar layers, from westerly to southwesterly at the Sheikh Ejlin Ridge and from northwesterly to southerly at Al Montar Ridge.


INTRODUCTION
The Gaza Strip is located on the southeastern coastal plain of the Mediterranean Sea (Fig. 1).The coastal zone of the Gaza strip is 42 km long, and varies between 6 and 12 km wide, covering an area of 365 km 2 .It is overlain by Pleistocene-Holocene sediments that have alternating stratifi ed calcareous sandstone (locally named as Kurkar) and red brown layers (locally named as Hamra).The coastline of the Gaza Strip forms only a small section of a larger concave system that extends from Alexandria at the west side of the Nile Delta, via Port Said, the Bardawil Lagoon, El Arish, Gaza, up to the Bay of Haifa (INMAN & JENKINS, 1984).
The topography of the coastal plain is determined by the exposure of two aeolian Kurkar ridges presumably of different ages (SIVAN & PORAT, 2004).The ridges are separated by a deep depression (20-40m above sea level) containing alluvial deposits.These ridges have been studied by several authors.PICARD (1943) noted that the Kurkar is mainly distributed in the western half of the coastal plain and forms ridges parallel to the coast.NEEV et al. (1987) recorded three onshore Kurkar ridges along coastal line, while GAZA EN-VIRONMENTAL PROFILE (GEP, 1994) mapped fi ve scattered ridges on the Gaza Strip.ANAN & ZAINELDEEN (2008) mapped just two ridges within the Gaza Strip running The exposures at the two ridges, are composed of alternating of Kurkar and Hamra layers, in the Gaza Formation of the Kurkar group (Fig. 2).The Kurkar Group describes coastal sediments from the Pliocene through to the Recent periods.
These Kurkar ridges extend further to the north, parallel to the Mediterranean coast as observed by YAALON & DAN (1967).
The sands of the Kurkar layers are wind blown deposits originating from the Nile delta (HOROWITZ, 1979).Since the beginning of the Pleistocene sediments transported into the Mediterranean Sea by the Nile River, have been spread along the Sinai and Palestinian coasts by longshore currents (POMERANCBLUM, 1966;GOLDSMITH & GOLIK, 1980;COLEMAN et al., 1981;INMAN & JENKINS, 1984;ROHRLICH & GOLDSMITH, 1984;CARMEL et al., 1985;STANLEY, 1989;FRIHY et al., 1991;FRIHY & LOTFY, 1997;PERLIN & KIT, 1999;ZVIELY et al., 2006;ZVIELY et al, 2007;ANAN & ZAINELDEEN, 2008).The aim of the present work was to 1) study the crossbedding within the Kurkar layers at the two ridges in order to determine the type of cross beds occurring at each ridge, 2) measure the dip and dip direction of the cross beds to estimate the palaeowind direction during deposition, and 3) compare the palaeowind directions of the two ridges with each other to give a better view of the circumstances during their respective formation.

OBJECT OF STUDY
The study area is located within the Gaza Strip of Palestine.Five sites (GS01, GS02, GS03, GS04 and GS05), as shown in Fig. 1, were examined over many fi eld campaigns.Sites were located on the best exposures of the Kurkar layers in the Gaza Strip.Three of these sites (GS01, GS02 and GS03) are located on the Sheikh Ejlin Ridge, and the other two (GS04 and GS05) are on the Al Montar Ridge.The coordinates of these fi ve sites are shown Table 1.All observation sites were located using a portable GPS.RODRIGEZ-LOPEZ et al. (2008) carried out a study of the aeolian sand at the mid-Cretaceous western Tethyan margin, Iberian Basin, Spain.They noted that sub-tropical climbing translatent strata, grain fl ow and soft sediment deformation are among some of the small scale structures and components observed in the aeolian dune sandstones.KOCUREK (1981) showed that surfaces and other manifestations of microtopography common in ancient aeolian strata are interpreted as representing damp or cemented surfaces.

DESCRIPTION OF THE EXPOSURES
The outcrops consist of alternating Kurkar and Hamra layers of Pleistocene age, (the Gaza Formation of the Kurkar Group).At each location, cross-bedding was recognized within the Kurkar layers (Fig. 3).The cross-bedded sediments were distinguished in the fi eld by layers of foresets, (the series of layers that form on the lee side of the bed form).They are also recognized by truncations in sets of foresets.Two sets of cross-bedding have been identifi ed within the Kurkar layer at the fi ve sites.
Kurkar layers in the study area consist of coarse-to medium-grained sandstone cemented by calcium carbonate

PALAEOWIND AND PALAEOWIND INDICATORS
Palaeowind indicators are oriented sedimentary structures interpreted to have been deposited by grain fl ow.Grain fl ow strata are slightly coarser than grain fall sands that occupy the upper middle, and upper parts of Aeolian slipface dunes (HUNTER, 1977).Cross-bed slip faces are one of the palaeowind indicators, and can be used to determine the direction of the wind that actually deposited the sediments.This corresponds to the mean value of the down-dip direction shown by the foreset beds (PETTIJOHN, 1975;KOCUREK, 1981).
Palaeowind indicator analysis is important because it provides direct information about the orientation of the sedimentary system.In the current study, cross beds have been used to estimate the palaeowind direction of the current that deposited the sandstones.
Several studies in aeolian processes have been seen in published papers (e.g.YAALON & LARONNE 1971;KO-CUREK, 1981, 1991).Studies dealing with sand dunes have also been published (e.g.YAALON & DAN, 1967;HUNTER, 1977;KOCUREK, 1981;CLEMMENSEN & ABRAHAM-SEN 1983;GREELY & IVERSEN, 1985;PYE, 1987;MOUNT-NEY & THOMPSON, 2002;RODRIGEZ-LOPEZ et al., 2008).The sand forms the bulk of the coastal sediments, together with additional calcareous materials and aeolian and alluvial silty and clayey materials.The sand is composed of well sorted medium to coarse quartz grains, with less than 5% of feldspar and heavy minerals.The sand supply was reduced northwards and less but fi ner grained sandy material deposited along the coast to Haifa (GAZA ENVIRONMEN-TAL PROFILE, 1994).The mineralogy of the aeolianites in the northern part of the coastal plain is dominated by quartz and a little feldspar (YAALON & LARONNE, 1971).FRECHEN et al. (2002) pointed out that neither the Kurkar nor Hamra layers correlate with glacial and interglacial periods of the Northern Hemisphere.SIVAN & PORAT (2004) showed that the Kurkar accumulated along the coastal plain at different time intervals throughout the Late Pleistocene, and that its deposition was At location GS01, at Deir ElBalah, a Kurkar layer overlies the Hamra layer.Tabular cross-bedding has been observed within the Kurkar layers at this site.Two sets of cross-beds have been recognized (Fig. 3A).The lower set dips east, while the upper set is dips northeast.70 measurements of dip and dip direction for the cross-beds of the lower and upper sets have been measured.The measurements plotted as rose diagram and as a stereographic projection (Fig. 4).The mean dip and dip direction have been calculated for the lower set as 16/098 (to the east) while it is 23/053 (to the northeast) for the upper set.At site GS02, south Sheikh Ejlin, the Kurkar layer also shows tabular cross-bedding (Fig. 3B) and overlies the Hamra layer.Fig. 5 shows the represented data collected at this location site.The mean dip and dip direction for the lower set of cross-beds is 20/096 (to the east) and for the upper set is 20/042 (to the north east).These mean values have been calculated from 41 measurements collected at this site.
At site GS03, at AnNawras, to the north of the Gaza Strip, only the Kurkar layer has been observed (Fig. 3C) and shows tabular cross-bedding with two sets of cross-beds.69 measurements of dip and dip direction were collected for the two sets of cross-beds (Fig. 6).The mean dip and dip direction for the lower set is 20/104 (to the east) and for the upper set is 27/055 (to the northeast).
Convolute cross-bedding, as observed here (Fig. 7A), is a distinctive kind of internal deformation seen within many sandy beds that have been laid down very rapidly.It takes an irregular form within the bed itself, which is otherwise not affected by the deformation.Although its origin is somewhat uncertain, it appears to affect sediments in a state of partial liquefaction caused by its very rapid deposition, so that this structure is most likely to be formed by the upwards escape of water during the very early stages of compaction (ROBERTS, 1996).
The three previous sites (GS01, GS02 and GS03) are located at Sheikh Ejlin Ridge where the lower set of crossbeds dips east and the upper one dips northeast.
At site GS04, Tal ErRayyes, two Kurkar layers, with Hamra layer in between, have been observed.Tal ErRayyes is located at the Al Montar Ridge where the dip directions of the two sets of the cross-beds are differ from those at the previous locations on the Sheikh Ejlin Ridge.Within each of Kurkar layer at this site (GS04), two sets of tabular crossbedding have been identifi ed (Fig. 3D).The lower set dips southeast while the upper one dips north.Fig 8 shows the results of 61 dip and dip direction measurements for the cross-beds, collected for the lower and upper sets within the two of Kurkar layers.The mean dip and dip direction for the lower set is 18/146 (to the southeast) and 21/359 (to the north) for the upper one.Convolute cross-bedding was again observed at Tal ErRayyes (Fig. 7B).
At site GS05, Tal ElKashef (Tal Zommo), a Kurkar layer with only one set of cross bedding was observed.The set dips north here (Fig. 9).Since sites GS04 and GS05 are located on the Al Montar Ridge, the set of cross-beds within the Kurkar layer observed at GS05, is the upper set of tabular cross-bedding that occur on the Al Montar Ridge.The mean dip and dip direction for this set is 18/003 (to the north).

DISCUSSION
The current study presents palaeowind directions obtained from the cross-bedding data from within the aeolian Kurkar layers of the Gaza Strip, Palestine.
Wind has much less competence than water to pick up particles.The aeolian environment is characterized by spectacular cross-beds at high angles of 20-30 degrees.Most of these beds are tabular (planar), as shown in the Kurkar layers in the study area.Moreover, sediments deposited by water are more consistent than those deposited by the wind.The Kurkar of the Gaza Strip is less consistent.Therefore, the Kurkar of the study area is interpreted as having been deposited in an aeolian environment.Kurkar layers are soft and reliable.YAALON (1967) proposed that the main factors controlling the transformation of the calcareous dune sand into either a consolidated aeolianite or a Hamra layer are the original content of CaCO 3 in the dune and the intensity of leaching as affected by the relief and micro-climatic conditions.He concluded that all the sedimentary deposits and associated soils could have formed under similar climatic conditions.PICARD (1943) noted that Kurkar is defi ned as calcareous sandstone derived from littoral as well as coastal dunesand and later diagenetically hardened by calcareous solutions.NEEV et al. (1987) noted that the Kurkar seems to have been deposited in a continental environment from aeolian sands that accumulated during both high stands and regressions of sea level.Increased sand is related to increased  FRECHEN et al. (2002) and PORAT et al. (2004) noted that the Kurkar ridges at the Levant are designated to represent typical longitudinal sand dunes formed synchronously par-allel to the coastline by a dominant wind direction perpendicular to the coast.The different facies are related to changes in the environmental setting.They dated these sediments by luminescence methods as 65 ka to 5 ka.
Careful study of cross beds can often determine ancient wind directions as the dip direction of the cross-bedded sediments provides insight into wind direction.The fi eld observations show that there are two sets of cross-beds within the   As sites GS04 and GS05 are located on the Al Montar Ridge, the mean values of the dip and dip direction of the two sets were calculated for the two observation sites.Since this ridge is presumably not age equivalent with Sheikh Ejlin, it is not surprising that the dip and dip directions are different.The mean dip and dip direction of the lower set of the cross-beds at GS04 site (the lower set observed just in this site) is 18/146 (to southeast).For the upper sets, the mean dip and dip direction at the two locations is 19/001 (to the north) as shown in Table 2 and Fig. 10.It can be inferred that the palaeowind direction was northwesterly and southerly at Al Montar Ridge.The mean direction of the palaeowind at each site was plotted at a map as shown in Fig. 11.
The results show that the direction of the palaeowind was westerly for the bottom (lower set) of the Kurkar layer and southwesterly for the upper set of the layer at Sheikh Ejlin Ridge, while it was northwesterly at the bottom of the layer and southerly at the top of it at Al Montar Ridge.This result was clear for all fi ve sites within the study area, suggesting that there was a change in wind direction during deposition of the Kurkar layers from a westerly to southwesterly direction at Sheikh Ejlin Ridge, and from a northwesterly to southerly direction at the Al Montar Ridge.

CONCLUSION
The study shows that the Kurkar layers are characterized by their tabular (planar) and convolute cross-bedding.Two sets of cross-beds observed within the Kurkar layers are documented.The dip direction of these sets indicates the direction of wind during deposition of the Kurkar layers.The lower set dips east at the Sheikh Ejlin Ridge and southeast at the Al Montar Ridge, while the upper set dips northeast at Sheikh Ejlin Ridge and north at the Al Montar Ridge.
It can be concluded that a change of palaeowind direction occurred during deposition of the Kurkar layers in the Gaza Strip in both analysed ridges.This appears to have been a change from a westerly to southwesterly wind at Sheikh Ejlin Ridge, as already reported by YAALON & LARONNE (1971), and from a northwesterly to southerly wind at the Al Montar Ridge.

Fi gu re 1 :
A Simplifi ed map of the Gaza Strip showing the distribution of sites within the study area (after ANAN & ZAINELDEEN, 2008).Fi gu re 2: Kurkar and Hamra layers at site GS01 (hammer scale is 32 cm long).NE-SW, parallel to the Mediterranean coastline.They named the two ridges as: Sheikh Ejlin Ridge, which extends up to the current coastline in the west, and the Al Montar Ridge running near the armistice line in the east.HASSAN et al. (1986) recorded a series of eight limestone ridges that run parallel to the coast, west of Alexandria, Egypt.STANLEY (2005) also traced eight Kurkar ridges to the west of Alexandria.
).Most of the Kurkar layers are cross-bedded, laminated and massive sandstone.Tabular types of crossbedding have been observed in the study area.GVIRTZMAN et al. (1984GVIRTZMAN et al. ( , 1998) ) pointed out that Kurkar and Hamra sequences are very similar along the coastal plain.The sand dunes occur along the shoreline especially near Deir ElBalah, and vary in thickness up to 15 m.The dunes are oriented east-northeast to west-southwest.Further to the north, the dunes become irregular and the sand accumulations are scattered in a 2 to 3 km zone along the coast(GAZA  ENVIRONMENTAL PROFILE, 1994).Aeolian deposits are dominant in the Gaza Strip.Those deposited especially in the southern Gaza Strip, have more loess and are mixed with windblown sands, but are much thinner.Fi gu re 4: Rose diagram and stereographic projection for the cross-beds at the Deir ElBalah site (GS01).Column (A) for the lower set and (B) for the upper one.A: Lower set B: Upper set

Figure 5 :
Figure 5: Rose diagram and stereographic projection for cross-beds at the south Sheik Ejlin site (GS02).Columns A and B are same as for Figure 4.

Figure 6 :
Figure 6: Rose diagram and stereographic projection for the cross-beds at the AnNawras site (GS03).Columns A and B as Figure 4.

Figure 7 :
Figure 7: Convolute cross-bedding observed within the Kurkar layers.A) at AnNawras, B) at Tal ErRayyes.Hammer and pencil scale are 32 and 14 cm respectively.

Figure 8 :
Figure 8: Rose diagram and stereographic projection for the cross-beds at the Tal ErRayyes site (GS04).Columns A and B as for Figure 4.

A
Kurkar layers in the study area.As sites GS01, GS02 and GS03 are located on the same, Sheikh Ejlin Ridge, the mean values for dip and dip direction for each set were calculated for the three observation sites.The result shows that the mean dip and dip direction of the lower set at Sheikh Ejlin Ridge is 19/099 (to the east) while it is 23/051 (to the northeast) for the upper one (Table2and Fig.10).Since the dip direction of the cross-bedded sediments provides insight into wind direction, the palaeowind direction at the Sheikh Ejlin Ridge was westerly and southwesterly during deposition of the Kurkar layers.The same direction was obtained byYAALON & LARONNE (1971) at the aeolianite coastal ridge a few tens of kilometres north of the study area.They indicated that a westerly and southwesterly wind regime prevailed in the past.This ridge is the extension of Sheikh Ejlin Ridge of the study area.

Figure 9 :Figure 10 :
Figure 9: Rose diagram and stereographic projection for the upper set of cross-beds at the Tal ElKashef site (GS05).

Table 1 :
Coordinates of the observation sites.

Table 2 :
Summary of the mean dip and dip direction of the cross-beds within the Kurkar layer of the study area.
Mean18/146 19/001Fi gu re 11: Rose of palaeowind directions at each site within the study area.