hofuf formation al-saad et al

N. Jb. Geol. Palaont. Mh. 2002(7) 426-448 Stuttgart, Juli 2002

Stratigraphy and sedimentology of theHofuf Formation in the State of Qatar

in relation to the tectonic evolution of the

East Arabian Block

Hamad Al-Saad, Sobhi Nasir, Fadhil Sadooni, Doha,

and Abdulrahman S. Alsharhan, Al-Ain

With 10 figures and 3 tables

Al-Saad, H., Nasir, S., Sadooni, F. & Alsharhan A. S. (2002): Stratigraphyand sedimentology of the Hofuf Formation in the State of Qatar in relation to thetectonic evolution of the East Arabian Block. - N. Jb. Geol. Palaont. Mh., 2002:426-448; Stuttgart.

Abstract: Well-exposed, fine-grained to pebbly coarse-grained fluvial sandstones ofLate Miocene to Pleistocene age crop out in the south-central and south-westernparts of Qatar. These sandstones belong to the Hofuf Formation and were depositedlargely in stream channels along Wadi As-Sahba over a distance of 450 kms. Thegraben structure of Wadi As-Sahba reflects a strike-slip motion that took place inthe Plio-Quaternary. The evolution of the Hofuf Formation is related to the tectonicevolution of the East Arabian Block in Neogene time. The sandstones comprise threedistinct facies: clast-supported conglomerate, coarse-grained sandstone, and finegrained sandstone. Sandstone compositions show a uniform framework compositiondominated by monocrystalline quartz and feldspar with less abundant polycrystal linequartz, calcite, micas and igneous, metamorphic and sedimentary rock fragments.Quartz was derived mostly from plutonic source rocks. The heavy-mineral assemblage is characterized by abundant unstable minerals, particularly hornblende andpyroxene. The pebbles are granitic, metamorphic and sedimentary rocks derivedfrom the Arabian platform. Modal compositions of the sandstones indicate atransitional provenance that ranges from continental to a craton interior and recycledorogen.

0028-3630/02/2002-0426 $ 5.75© 2002 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart

Hofuf Formation in thestateofQatar in relation to the East Arabian Block 427

Zusammenfassung: Im zentralen und westlichen Teil des Siidens von Qatar sindfeinkornige bis grob-kornig konglomeratische fluviale Sandsteine von spatmio-zanem bis pleistozanem Alter der Hofuf Formation sehr gut aufgeschlossen. Siewurden in Flussrinnen entlang des Wadi As- Sahba iiber eine Distanz von 450 kmabgelagert. Die Graben-Struktur des Wadi As-Sahba entspricht einer 'strike-slip'Bewegung, die im Plio-Quartar stattgefunden hat.Die Entwicklung der Hofuf Formation ist mit der tektonischen Entwicklung desOstarabischen Blocks wahrend des Neogens verknupft. Die Sandsteine enthaltendrei verschiedene Fazies: komponenten-gestiitzte Konglomerate, grob- und feinkornige Sandsteine. Die Zusammensetzung der Sandsteine ist einheitlich: mono-kristalliner Quarz und Feldspate mit weniger haufigen polykristallinem Quarz,Kalzit, Glimmer und Bruchstiicken von magmatischen, metamorphen und sedi-mentaren Gesteinen. Der Quarz stammt meistens aus Plutoniten. Die Schwer-mineral-Vergesellschaftungen zeichnen sich durch haufige instabile Minerale,besonders Hornblenden und Pyroxene, aus. Die Komponenten der Konglomeratesind Granite, Metamorphite und Sedimente, die von der Arabischen Plattformstammen. Die modale Zusammensetzung der Sandsteine weist aufeine Herkunft hin,die vom kontinentalen Bereich bis zum Innern eines Kratons und einem rezykliertenOrogen reicht.

Introduction

The Hofuf Formation, presumably of Late Miocene to Pleistocene age,represents the youngest Neogene deposits in the Qatar Peninsula (ArabianGulf region) and comprises about 3 % of the exposed rocks. The mainoutcropsof the Hofuf Formation in Qatarare in its south-central and southwestern parts (Fig. 1). It occurs in Qataras outliers associated with the DamFormation. The Hofuf Formation is named after its type locality, some 15 kmNNE of the Al-Hofuf town in the Eastern Province of Saudi Arabia. Thisname was formally used for the first time by Thralls & Hassan (1956).The regional stratigraphic sequence of the Neogene sediments in theArabianGulf region (East Arabian Block) indicates that they were deposited inan elongated NW-SE asymmetrical basin which has been named ZagrosSyncline (Powers 1968, Powers et al., 1966; Murris, 1980). The areaoccupied by this basin was decreasing during the Neogene, probably as aresultof global Mid-Oligocene emergence and the majorupliftof the regionthat began during the Oligocene and continued through the Miocene(Weijermars, 1998). The evolution of the Hofuf Formation is related tothe formation of the East Arabian Block area during the Neogene. The EastArabian Block is a relativelyrecently recognizedstructural element that wasfirst suggested by Hancock & Al-Kadhi (1978). New data presented by

428 H. Al-Saad et al.

Stfl3tf _SM_ sfptf Sfltllf _S2M

Fig. 1. Simplified geological map of Qatar showing location of the Al-Subaihaarea.

Weuermars (1998) support the existence of this tectonic block boundedby grabens and strike-slip faults and transpressive folds (Fig. 2). A Plio-Quaternary age for the central andeastern Arabian grabensystems has beensuggested by Vaslet et al. (1991) and recently supported by Weuermars(1998).

Hofuf Formation in the state of Qatar in relation to the East Arabian Block 429

Fig. 2. Tectonic compilation map highlighting the regional structural setting of theEast Arabian Block (modefied after Weuermars, 1998).

The Hofuf Formation has been generally dealt with in a few geologicalstudies of Qatar Peninsula, which are mostly concerned with its regionalgeologic setting (e.g., Cavelier, 1970, Blondeau & Cavelier, 1973;Standring & Sugden, 1978). The present study aims at elucidating thegeologic history, depositional environments and provenance of the HofufFormation in Qatar in the light of the geotectonic evolution of the ArabianPlate.

Regional Geology and Tectonics

Doming of the Arabian-African Shield began in Late Cretaceous-Eocenetime and was associated with a crack along the crest of this elongated dome.


In LateEocene-Oligocene time, NE-SW tension causednormal-step faultingand rifting of the crack and associated monoclinal down-warping of theedges. During this time, the northeast directed separation of Arabia fromAfrica began. In late Miocene-Pliocene time, tectonic activity renewedcausedby the breakingup of the basement crust and the start of the Red-Seafloorspreading. Drainage fromtheuplifted Arabian ShieldbrokethroughtheMesozoic escarpments of the interiorhomocline to deposit vast amounts ofquartzose clastic sediments, which now form the Miocene Hudrukh andMio-Pliocene Hofuf formations (Powers et al., 1966). The general slopeof the area was from west to east, causing eastward transport of erodedbasement rocks.

General Stratigraphy

In its type section in eastern Saudi Arabia, the Hofuf Formation attainsa thickness of about 95 m. In this locality, it unconformably overlies asequence of calcareous sedimentary rocks of the Dam Formation, whereasits top is eroded or covered by an overburden of recent deposits. The HofufFormation constitutes irregularly isolated hills. It consists of brown to lightyellow and grey to pale white friable rocks characterized by fining-upwardcycles. The type section is subdivided into four units: 1) a basal conglomerate formed ofpebbles and boulders, including igneous and metamorphicrocks and limestone; 2) calcarenitic limestone; 3) argillaceous sandstone;and 4) conglomerates.

In Qatar, the Al-Subaiha area represents the best single exposure of Hofufrocks. The Hofuf Formation has been considerably eroded and its thicknessranges between 2 and 18 m. This reduced thickness seems to representonly the lower part of the first unit defined in the reference section at its typelocality in Eastern Saudi Arabia (Thralls & Hassan, 1956). It consists ofthin beds of coarse-grained sandstones, gravels and conglomerates rich inpebbles of various igneous and metamorphic rocks (granite, basalts, gneiss,schist), quartzite, jasper, limestone and marl, in a white, coarsegrained sandymatrix. The Hofuf Formation in the study area overlies the early MiddleMiocene argillaceous limestones of the Dam Formation. The contact isconsidered to be a disconformity. A regional unconformity surface wasinferred between the Dam and the Hofuf formations in the eastern Arabian

Peninsula (Powers, 1968).

Hofuf Formation inthe state of Qatar in relation to the East Arabian Block 431

Methodology

Fifteen representative samples (-one kg each) were collected from thesandy/gravelly exposures of the Hofuf Formation at Al-Subaiha. A representative portion was taken from each sample and prepared for grain-sizeand petrographic analysis. Sieving of friable sandstones was carried out onbase of 1-<I> intervals, ranging from -4 to 4. Thin sections were preparedfrom gravels and consolidated samples. The heavy minerals were obtainedfrom a fraction of 62-250 urn (Friis, 1978), using bromoform. About 300non-opaque heavy-mineral grains were counted.

Sedimentology

In the Al Subaiha area, the studied section has a thickness of 18 m. At least 8sedimentary cycles are present (Fig. 3). Each cycle is composed mainly ofsandy conglomerate followed by coarse-grained sandstone, and ending withbioturbated fine-grained sandstone. The thickness of these cycles rangesbetween 1.2 and 3.3 m. The main sedimentary structures observed in thedifferent lithofacies are cross-bedding and bioturbation. Three sedimentaryfacies can be recognized within the formation.

Facies A: clast-supported sandy conglomerate

This facies represents the basal part of the cycle and ranges in thickness froma few centimeters to about one meter. The color is commonly white andbrown to dark brown. The sandy conglomerates are composed of igneous,metamorphic, limestone and quartzite pebbles up to 10 cm in diameter andsandstone lithoclasts. Most of these conglomerates are plane-bedded, someare grade-bedded.

Facies B: coarse-grained sandstones

This facies occurs at various levels in the Hofuf Formation and its color

varies from light brown to light yellow. The thicknesss ranges from 0.3 to1.2 m. Planar and tabular cross-bedding are the dominant sedimentarystructures.

Facies C: fine-grained sandstones

This facies always marks the upper part of a fining-upward cycles. Its thickness ranges from 0.3 to about 2.3 m. The color is light brown to creamy.The facies is characterized by abundant bioturbation and the occurrence ofplant fragments and small-scale planar cross-bedding.


Coarse-grainedsnudstonc

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Fig. 3. Composite section of the Hofuf Formation in the Al-Subaiha area, westernQatar.


Table 1. Grain-size parameters.

Sample Graphic Graphic Graphic Graphic First Second Third Fourth

No. mean std.dev skewness kurtosis Moment Moment Moment Moment

Facies A

H3 -0.43 1.95 -0.42 0.68 0.08 1.92 -0.51 1.91

H6 -0.37 1.99 -0.45 0.82 0.32 1.84 -0.62 2.32

H12 -0.07 2.22 -0.44 0.78 0.51 2.06 -0.45 1.97

H14 -0.37 2.02 -0.39 0.58 0.09 1.98 -0.4 1.86

H16 -0.4 1.99 -0.41 0.69 0.15 1.92 -0.46 1.99

Facies B

HI 0.73 0.94 -0.14 1.08 1.18 0.96 -0.67 4.4

H7 0.75 0.88 -0.04 1.01 1.13 0.89 -0.38 4.5

H13 0.77 0.92 -0.4 1.06 1.22 0.97 -0.49 4.16

H17 0.77 0.89 0.0 1.08 1.22 0.98 -0.55 4.6

H20 0.7 0.94 -0.01 1.27 1.08 0.91 -0.8 4.4

Facies C

H2 1.57 0.91 -0.15 0.99 1.89 0.95 -0.76 4.5

H8 1.13 0.71 -0.24 0.98 1.61 0.84 -0.91 5.76

H9 1.07 0.79 0.09 1.19 1.57 0.89 -0.95 5.5

Hll 1.03 0.83 -0.07 1.12 1.43 0.97 -0.89 5.2

H21 1.27 1.04 -0.15 1.38 1.63 1.07 -0.92 4.65

Average 0.54 1.27 -0.18 0.98 1.02 1.27 -0.61 3.88

Grain-size analysis

Results of the grain-sizeanalyses are given in Table 1.The sieving data arerepresented by frequency curves (Fig. 4) and cumulative frequency curves(Fig. 5). The frequency distribution curves for values of the graphic parameters are plotted in Fig. 6. The statistical size parameters were calculatedusing the formulas of Folk & Ward (1957). Friedman's (1961) momenttextural parameters were also calculated (Table 1). However, suite statisticsmethodology offers greater variability than composite statistics (e.g.,Tanner 1991; Balsillie & Tanner, 1999). The suite mean is usuallyequivalent to the composite mean, whereas composite standard deviation,skewness and kurtosis values can depart significantly from the valuesobtained by suite statistics (Balsillie & Tanner, 1999). The size fractionpercentages of the investigated samples (Table 1, Fig. 4 ) indicate that thefacies A samples (H3, H6, H12, H14 andH16)are sandy gravel, the facies


B samples (HI, H7, HI3, H17 and H20) are gravelly sand, whereas thefacies C samples (H2, H8, H9, H11,andH21) are sand. Thegravel fractionis mainly composed of cobbles, pebbles and granules. It is present in considerable amounts mainly in the sandy gravel samples (27-34%), whereas itranges between 0.4 and 6 % in the gravelly sands and the sandy samples.Size measurements of the gravels according to the method of Wentworth(1922) indicate that the most abundant gravel fraction is the pebble sizewhich reaches up to 80% whereas thecobble fraction makes up to 20 %.Themud fraction (silt and clay) is less than 1 %. The cumulative curves of thesamples (Fig. 5) further illustrate that the suspension subpopulation (mud)constitutes a very small amount of the sediment, whereas the traction sub-populations of facies A (sandy gravel) is most prominent. This may suggestthat most of the material could have been deposited from a dense heterogeneous fluvial bed-load.

Mechanical analysis of the investigated samples has shown the followingresults:

Mean size

The inclusive graphic mean size of the studied samples ranges between-0.43 0 ( very fine pebbles) to 1.57 0 (medium sand) with an average of0.54 0 (coarse sand). Relatively similar results are also obtained by themoment method (0.08-1.89 0 range, 1.01 0 average). The frequency distribution of the mean values (Fig. 6) shows bimodal curves indicating thenon-uniformity of the fluviatile processes responsible for the Hofufsediments. The increase of grain size in the facies A samples suggests arather strong increase inriver energy ascompared tothe the group 3samples(finegrained sand) (Visher, 1969).

Standard deviation (sorting)

Sorting of the Hofufsandstones canbe described according to the formulasof Folk & Ward (1957) as moderately sorted (facies B and C) to verypoorly sorted (facies A), as reflected in the wide spread of the grain-sizedistribution curves (Fig. 6). The sorting values range from 0.53 to 2.22 0 andaverage at 1.25 0: Thisis in agreement withthose of mostriversand, thataregenerally above 0.5 0 (Friedman, 1961). Results obtained from the momentmethod range between 0.84 and2.06 0 andaverage at 1.27 0.


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Skewness

The values of inclusive graphic skewness fluctuate between -0.45 (stronglycoarse skewed) and 0.09 (near-symmetrical) and have an average of-0.19(coarse-skewed). This indicates a wide range of variation in depositionalenergy. The frequency distribution curve of the skewness values show twonegative modes located at -0.41 (facies A) and at 0.-0.08 (facies B and C).This negative skewness is due to an excess of coarse grains during earlyflood stages (e.g. Friedman, 1967).


Kurtosis

The graphic kurtosis values show also a wide range of fluctuation. They range from 0.58 (very platykurtic) to 1.38 (leptokurtic) and average at 0.94(mesukurtic). This result is also reflected by the frequency distribution curveof the kurtosis values (Fig. 6), which is also bimodal with two principalmodes at 0.9 and at 0.51.

Bivariate relationships of grain-size parameters

Plots of sorting, skewness, and mean size are considered by several authorsto be the most sensitive parameters to discriminate among depositionalenvironments, (e.g., Inman, 1949; Folk & Ward, 1957; Friedman, 1961,Moiola & Weiser, 1968; Friedman & Sanders, 1978). Friedman (1967)used a scatter diagram of sorting versus skewness to discriminate betweenfluvial and marine depositional environments. Fig. 7 shows that most of thesamples fall within Friedman's (1967) "river field". Similar results are alsoobtained when constructing the diagram of graphic mean versus graphicsorting (Moiola & Weiser, 1968) (see Fig. 8).

Petrography

The microscopic study shows that the Hofuf sandstones are arkosic to sub-arkosic according to the classification of Pettijohn et al. (1972).The mainconstituents are quartz, feldspar, calcite, mica, and a few rock fragments ofmafic igneous rocks, chert, gypsum, and carbonates (Table 2). Most grainsare subangular to subrounded, and poorly sorted. Quartz occurs mainly asmonocrystalline grains, some of which (-15 %) have undulatory extinction.Others (~5 %) are polycrystalline (2-3 units per grain). Feldspars are mainlyalbite and orthoclase, with some sandstones containing minor amounts ofmicrocline. Most of the albite shows multiple-twins. Limestone occursas rounded micritic grains. The sandstones contain a wide range of rockfragments, including metamorphic (quartz-muscovite schist and gneiss),volcanic (basalt and rhyolite), granitic (quartz-orthoclase) and sedimentary(limestone and chert). Granitic and rhyolitic fragments are most abundant.Mica, mainly muscovite, occurs only in minor amounts.

The sand fraction is immature arkosic to subarkosic, mainly composed ofquartz (64-87 %) and feldspars (5-30 %). Mica, calcite and heavy mineralsare minor constituents. Quartz and feldspar occur as subangular to sub-rounded grains. Dickinson et al. (1983) suggested several QFL (quartz-feld-spar-lithics) diagrams to identify the tectonic setting of the source areas of


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the detrital grains. The QFL compositions of the investigated sandstones arein the transitional, craton interior, and recycled orogen fields (Dickinsonetal., 1983) (Fig. 9).

Petrographic studies of the gravels indicate that they include igneous,metamorphic, sedimentary rocks and quartz (Fig. 10). All types occur inaboutequal abundance. Igneous rockspebbles consistmainly of rhyolite andgranite whereas basalt is subordinate. The igneous rock pebbles are similarpetrographically to those of the Arabian Shield (e.g., Stoeser & Elliot1980; Duyverman, 1984; Drysdall et al., 1984, Jackson et al., 1984;Stoeser, 1986). Pebbles of metamorphic rocks are biotite gneiss and meta-greywacke, similar to the metamorphic rocks that outcrop in the ArabianShield (Church, 1979; Reischmann et al., 1984). Sedimentary pebblesconsist mainly of micritic and biomicritic limestones similar to Mesozoicand Tertiary carbonates of the Arabian Peninsula.


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Fig. 8. Biavariant discrimination plot of graphicsorting versusgraphic mean.

The shape of the pebbles is very variable.According to the classificationof Zingg (1935), disc, spherical, blade androd shapes were recognized. Thespherical anddisc-shaped pebbles arethemostabundant whereas rod-shapedpebbles are the leastabundant ones. Theblade-shaped pebbles, mainly composedof basaltand limestone, come third in abundance. Granitic, rhyolitic,quartzite and some limestone pebbles occur mainly in spherical, disc androd-shape.

Theheavy minerals range between 0.1 and0.9wt. % of the samples. Themajority of opaque grains are identified as magnetite and ilmenite, with


Table 2. Proportion of light minerals in the sandstones from the Hofuf Formation.

Sample Quartz Feldspar Calcite Mica Rock

fragments

HI 80 15 1 3

H2 81 12 1 5

H3 86 11 1 0 2

H6 85 10 1 3

H7 82 13 1 3

H8 85 10 0 4

H9 83 10 1 0 6

Hll 82 12 0 5

H12 71 25 0 3

H13 64 30 2 3

H14 72 23 1 3

H16 65 25 3 3 4

H17 76 15 4 2 3

H20 84 10 2 1 3

H21 87 5 1 2 5

lesser amounts ofhematite and limonite. The opaques occur as subangular tosubrounded grains. They vary between 20-28 vol. % of the heavy minerals.Major transparent minerals are zircon (10-33 %), tourmaline (10-25 %),hornblende (13-38 %), pyroxene (18-40 %) and epidote (0-5 %). Minorminerals include rutile (0-3 %) and apatite (0-2 %). Trace minerals aregarnet, kyanite, andalusite, sphene and staurolite. Amphiboles and pyroxenes(ortho- and clino-) are the dominant non-stable heavy minerals. Pyroxenesare represented by irregular to subrounded prismatic augite, diopside andenstatite in almost equal amounts. The grains are colorless, light green andsubhedral. Amphibole occurs in two varieties: green and brown basaltic.Green hornblende is the most frequent variety. Amphiboles occur as prismatic and subhedral corroded grains. Epidote is represented by subroundedgreenish yellow grains. Zircon is found in all samples as subhedral, prismatic, oval and rounded colorless grains. The tourmaline grains are brownand/or green, subrounded, and pleochroic. Garnet in traces occurs as angularto subangular pink grains. Apatite occurs in traces ( < 2 %) as colorless,elongate prisms or angular to sub-angular grains. Rutile grains are reddish-brown and oval.


Fig. 9. Q-F-L plot of the Hofuf samples. CI: craton interior, TC: transitionalcontinental, BU: basement uplift, RO: recycled orogen, DA: dissected arc, TA:transitional arc, UA: undissected arc. Provenance fields after Dickinson et al(1983).

Discussion

The light and heavy minerals in the Hofuf Formation reveal a main mine-ralogical province dominated by quartz, feldspar and zircon, tourmaline andsilicates (pyroxene, amphibole, epidote, garnet). The parent rocks providedthe initial control on the compositionof the heavy mineral suite. If the heavyminerals in the source area are sufficiently distinctive, the grains can betraced into the adjacent sandstones.The mineralogicalassemblage is characteristic of mixed sediments from two types of source rocks: acidic plutonicrocks (zircon, tourmaline and rutile) and basic igneous and metamorphicrocks (amphiboles, pyroxenes, epidotes). The unstable heavy minerals, with


Fig. 10. Types of pebbles and cobbles. A: andesite, 13: biotite schist. C: micagneiss. D: granite, E: porphyretic rhyolite. F: rliyolite. G: chert. II: limestone, andI: quartzite.

much hornblende, pyroxene and epidote. indicate an elevated metamorphicand magmatic terrain. Transport was fast and without intensive weathering,thus preserving much of the composition of the source rocks. However, thepredominance of quartz in the fine-grained sandstone facies may indicatesevere chemical weathering, and possibly long transport distance. Penological data indicate variable composition of the pebbles. Igneous and metamorphic pebbles were eroded from the Arabian Shield, whereas chert andcarbonates could have been derived from the Phancrozoic succession ofthe Arabian Platform. Quartz-types (mostly monocrystalline, and non-undulatory) indicate that the Hofuf sandstone was derived from plutonicsource rocks (e.g., Basu et al., 1975).


Table 3. Proportion of heavy minerals in the sandstones of the Hofuf Formation.

Sample HM. Opaques Non-•opaques

No. wt.% vol.% Amph Pyrx Epdi Gr Zr Tour Rut Apat others

HI 0.8 25 25 38 3 _ 21 12 _ 1 t

H2 0.1 25 27 40 3 t 15 10 3 2 t

H3 0.9 28 33 35 3 - 15 11 1 1 -

H6 0.9 20 35 30 2 - 20 10 1 2 -

H7 0.2 20 20 40 4 t 25 9 1 1 -

H8 0.3 25 35 20 5 t 30 10 - - -

H9 0.2 20 38 18 2 - 28 12 2 - t

Hll 0.9 25 20 40 5 - 20 13 1 1 t

H12 0.7 25 15 35 3 - 25 12 2 3 -

H13 0.3 26 29 33 t t 22 15 - 1 -

H14 0.6 25 22 25 2 - 30 19 1 1 -

H16 0.5 25 15 30 1 - 30 20 3 1 -

H17 0.8 20 16 25 1 t 35 20 2 1 t

H20 0.5 25 10 30 - - 30 30 - t t

H21 0.3 25 13 27 - - 35 25 - t t

HM: Heavy minerals, Amph: amphiboles, Pyrx: pyroxenes, Epid: epidotes, Gr:garnet, Zr: zircon, Tour: tourmaline, Rut: rutile, Apat: apatite, and others- silli-manite, kyanite, topaz, monazite.

The QFL composition of the Hofuf sandstones (Fig. 9) indicates that thesediments were derived from craton interior through transitional continentaland recycled orogene sources. In such provinces, granites and high-grademetamorphic rocks crop out in an area of low topographic relief subjected todeep weathering and might be associated with wrench tectonism (Dickinsonet al., 1983; Lee & Sheen, 1998; Anani, 1999). This is consistent with thesteep change in curvature of the wrench fault in the basement beneath theNisah - Sahba Graben (Fig. 2). Field studies have revealed the presenceof large, shallow-marine gravel fan-delta deposits, dated as late Pliocene toearly Pleistocene,emanatingfrom the Wadi As-Sahba (Weuermars, 1998).This implies that the Nisah - Sahba fault, which controls the course ofWadi As-Sahba, probably had already been initiated in the late Pliocene. Alength of 450 km has been established for the Nisha - Sahba Fault Zone(Weuermars, 1998). The eastern segment of the Nisha - Sahba fault hasbeen interpreted as a sinistral strik-slip zone, with minimum lateral displacement of 5 to 8 km, accompanied by a minor up-throw of the northern


wall of only a few meters (Weuermars, 1998). The Alpine movements ofthe Zagros mountain range during the Cenozoic have raised the gradient oftheArabian Shield and Platform from west to east and large areas were subjected to denudation. Wadi Sahba is a witness for the tectonic movementof the eastern Arabian Block which contributed to the sedimentation ofthe Hofuf Formation. The Arabian Shield includes granitic, rhyolitic, sedimentary andmetamorphic rockassemblages thatmayhave provided the rockfragments of the HofufFormation. In latePliocenetime, tectonicmovementscaused the formation of the Salwa Syncline and the Dukhan Anticline inQatar. Continuous wind-erosion created mesa structures and deflation ofthe upper horizon of the Hofuf deposits. In late Pleistocene time, a majorpluvial phase occurred (Mc Clure, 1984), coinciding with the last glacialmaximum. During this pluvial phase, the whole Hofuf area was an inlandlake anda greatriverwasflowing mainly in southern Qatarfrom theArabianShield to the Arabian Gulf (Holm, 1969; McClure, 1984; Edgell, 1989).This river was also noted by the Greek geographers as the Aftan River(Golden, 1984). The drainagechannelof this riverwas fault-controlled. Thesediment structures indicate an enormous volume of fluvial transport. Theabundant pebbles suggest transportation by vigorous floods and the cyclicsedimentation indicate variation in the transport energy of the river duringwet and dry periods. The volume of Neogene sediments deposited suggeststhat a pluvial and humid climate prevailed during the Miocene andPliocene(Whybrow & McClure, 1981; Edgell, 1989). Stream erosion was activeduring this pluvial time, and sediment transport was from west to east. Thesediments eroded upstream were deposited in a delta-plain environment inHofuf andin theWadi As-Sahba in southern Qatar.

Acknowledgements

The authors would like to express there thanks to Dr. I. Kassas for his suggestionsand interest in this work and to Y. I. Lee for his valuable review of the manuscript.

References

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Received: December 8,2000.Revised manuscript accepted by the Tubingen editors: April 12,2001.

Addresses of the authors:

Dr. HamadAl-Saad , Prof. Sobhi Nasir, Dr. Fadil Saadoni, Department of Geology, University of Qatar, P.O.Box 2713, Doha, Qatar; Prof. Abdel-RahmanAl-Sharhan, Department of Geology, U.A.E.University, Al-Ain, United ArabEmirates.

hofuf formation al-saad et al

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