Paleosols of the Upper Cretaceous–Lower Tertiary Maghra El-Bahari Formation in the northeastern portion of the Eastern Desert, Egypt: Their recognition and geological significance

Sedimentary Geology 183 (2006) 243 – 259 www.elsevier.com/locate/sedgeo

Paleosols of the Upper Cretaceous–Lower Tertiary Maghra El-Bahari Formation in the northeastern portion of the Eastern Desert, Egypt: Their recognition and geological significance H.A. Wanas *, M.M. Abu El-Hassan Geology Department, Faculty of Science, Menoufia University, Shebin El-Kom, Egypt Received 10 April 2005; received in revised form 19 September 2005; accepted 20 September 2005

Abstract The Upper Cretaceous/Lower Tertiary Maghra El-Bahari Formation at Gabal Ataqa and Gabal Shabrawet in the northeastern portion of the Eastern Desert of Egypt is subdivided into two informal lithostratigraphic parts: lower and upper. The lower part has common features of alluvial floodplain-dominated deposits with occasional occurrences of crevasse splay deposits. The upper part has sediments typical of marginal lacustrine environments. Both the floodplain and marginal lacustrine deposits exhibit pedogenic features comprising various types of paleosols. Among other soil-forming processes, diversity in the paleosols studied is mainly attributed to paleoclimatic and paleohydrologic changes. The paleosol criteria suggest two climatic regimes, a subhumid–semiarid one succeeded by a semiarid-arid one. The continental depositional environments recognized (floodplain and lacustrine) with their associated paleosols helped in the recognition of a marine regression in the area studied. In a regional perspective, comparison of the data presented in this study with paleosol data spanning the same time period in other localities suggests that the proposed paleoclimatic changes may have been of regional extent. D 2005 Elsevier B.V. All rights reserved. Keywords: Cretaceous/Tertiary; Paleosols; Maghra El-Bahari Formation; Egypt

1. Introduction A paleosol is a soil that formed on a landscape of the past and can form wherever lengthy episodes of landscape stability are present (Ruhe, 1965). Paleosols have been widely reported within ancient siliciclastic (Kraus, 1999) and carbonate (Wright, 1994) sequences in a variety of depositional settings. Paleosols in continental deposits have been interpreted to provide detailed insight into ancient landscape reconstruction (Bown and Kraus, 1987; Kraus, 1997; McCarthy and Plint, 1999), * Corresponding author. Tel.: +2 48 2238323; fax: +2 48 2235689. E-mail address: [email protected] (H.A. Wanas). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.09.013

paleoclimatic interpretation (Mack, 1992; Alonso Zarza et al., 1992; Driese and Mora, 2002; Lee et al., 2003; Demko et al., 2004), and sequence stratigraphic analysis (Wright and Marriott, 1993; Kraus, 1999; McCarthy and Plint, 1999; Atchley et al., 2004; Choi, 2005). The geology of Gabal Ataqa and Gabal Shabrawet has been studied by many authors since the beginning of the last century (see Wanas, 2002; El-Azabi, 1998 and references therein). There are only a few studies of paleosols and their geological implications through the Cretaceous/Tertiary transition (e.g., Fastovsky and McSweeney, 1987; Lehman, 1989; Cojan, 1993; Atchley et al., 2004), particularly so in Egypt. Also, the detailed study of paleosol development and its geolog-

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ical implications in the deposits of the Late Cretaceous/ Early Middle Eocene transition (Maghra El-Bahari Formation) are not clear. Therefore, the present work is focused on the pervasive pedogenic features developed in the Maghra El-Bahari Formation. As such, it is possible to clarify the main factors which controlled the pedogenic processes and their implications at both local and regional geologic perspectives. Depositional characteristics of the rock units studied are also described in order to provide information which may help in interpretations of the paleosols. 2. Geological setting Gabal Ataqa and Gabal Shabrawet form two topographic highs that resulted from tectonic activity related to the Syrian Arc System (Moustafa and Khalil, 1995). Gabal Ataqa overlooks the junction of the Gulf of Suez and the Suez Canal. It is located between latitudes 30800V and 29847VN and longitudes 32816V and 32828VE (Fig. 1A). Gabal Shabrawet lies close to the Suez Canal between Ismailia and Suez (about 40 km south of Ismailia, and about 137 km to the northeast of Cairo). It is located between latitudes 30813V and 30819VN and longitudes 32812V and 32820VE (Fig. 1B). Structurally, Gabal Ataqa is a tilted fault block bounded by normal faults dipping in a northwesterly direction (El-Akkad and Abdallah, 1971). In contrast, Gabal Shabrawet is a small asymmetrically plunging anticline, striking northeast–southwest and plunging to the southwest (Al-Ahwani, 1982). In the areas under study, the nearly flat-lying middle Eocene rocks sharply overlie tilted Cretaceous rocks with an obvious unconformable relationship that is mainly represented by the deposits of the Maghra ElBahari Formation that accumulated during the period from the end of the Late Cretaceous to the Early Middle Eocene (El-Akkad and Abdallah, 1971; Al-Ahwani, 1982). 3. Sampling and analytical techniques Two stratigraphic sections in the Maghra El-Bahari Formation at Gabal Ataqa and Gabal Shabrawet were described and measured in the field. Representative samples of the various kinds of rocks were collected and slabbed for inspection of macroscale features. In selected columnar sections, each coloured band (dry) was described according to the Munsell Colour chart (1975). Thin sections were prepared by dry cutting and polishing to avoid expansion of smectite if present. The thin sections were prepared for examination of micro-

features. A few small fresh chips of representative rock samples were prepared by gold-coating and investigated by Scanning Electron Microscope (type-Hitach S2500 SEM) equipped with Energy Dispersion Analysis of X-ray (EDAX). EDAX was carried out in a beam condition of 20 kV at a distance of 25 A8. Some clayrich samples were analysed by X-ray diffraction analysis (XED) using a Philips PV 1830 X-ray diffractometer employing Cu–K radiations. Prior to XRD analysis, the clay fractions were extracted from the selected samples (the selection of samples was performed after detailed thin sections invistigation), where 50 g of crushed samples were treated with 1 N sodium acetate–acetic acid buffer solution (pH = 5) to remove carbonate constituents. After that, the acid insoluble residue was repeatedly washed with distilled water. The silt (2–20 Am particle size) and clay (b 2 Am particle size) fractions were separated by centrifuge, and three-oriented clay aggregates (air-dried, glycolated and heated to 550 8C) were examined for each of selected samples. 4. Lithostratigraphy The Maghra El-Bahari Formation, named for Wadi Maghra El-Bahari at Gabal Ataqa, refers to the unfossiliferous red beds between the horizontally bedded fossiliferous chalky limestone of the Lower Middle Eocene Suez Formation above and the dark grey dolostone of the Campanian–Maastrichtian Adabiya Formation below (El-Akkad and Abdallah, 1971). In outcrop, the Maghra El-Bahari Formation appears as massive and featureless, but on close inspection it is seen to be comprised of a complex network of weathered surfaces. The Maghra El-Bahari Formation is subdivided into two informal lithostratigraphic parts (lower part and upper part, Fig. 2). The lower part was recorded at both Gabal Ataqa and Gabal Shabrawet whereas the upper part is absent at Gabal Shabrawet (Figs. 2 and 3A). The Maghra El-Bahari Formation varies in thickness in the Suez Canal district (Gabal Ataqa and Gabal Shabrawet) (Fig. 3A). The lower part at Wadi Maghra El-Hadida (Gabal Ataqa) is comprised of unfossiliferous fine-grained sandstones, siltstones and silty claystones intercalated with beds of medium-to coarse-grained sandstone and granule conglomerate (Fig. 2). These rocks exhibit different features such as colour horizons, mottling, vertical burrows and desiccation cracks. In addition, there are granules and pebbles which are mainly calcareous in composition, and occasionally ferruginous. At Gabal Shabrawet, the lower part consists of a thick-

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Fig. 1. Geological maps of Gabal Ataqa (A) (after El-Akkad and Abdallah, 1971) and Gabal Shabrawet (B) (after Al-Ahwani, 1982).

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Fig. 2. Lithostratigraphic columnar sections of the Maghra El-Bahari Formation at Gabal Shabrawet (Shabrawet Proper) and Gabal Ataqa (Wadi Maghra El-Hadida), western side of the Gulf of Suez, Eastern Desert, Egypt.

ness of 14 m of pebbly marly sandstones, siltstones and mudstones (Fig. 2). The pebbles and gravels occasionally form lens-shaped conglomeratic bands, and sometimes they form nodular and pisolitic structures. The marly mudstone and siltstones have vertically aligned purple columns or pipes (Fig. 2). The deposits of this

lower part are coarser than those of the lower part at Gabal Ataqa (Fig. 2). The upper part of the Maghra El-Bahari Formation consists of snow-white carbonates and evaporites that are intercalated with grey siltstones, silty claystones and greyish green marlstones (Fig. 2). The carbonate and

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Fig. 3. Lithologic correlation (A), sketch of depositional model (B) and block diagram of depositional environments (C) of the Maghra El-Bahari Formation at the studied areas and their neighbours.

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marlstone beds are poorly indurated, massive and have desiccation cracks (Fig. 2). 5. Depositional characteristics Sedimentological data indicate an existence of two different local depositional environments in the Maghra El-Bahari Formation. The lower part of the Formation has features common to alluvial floodplain-dominated deposits with the occasional occurrence of crevasse splay pebbly coarse-to medium-grained sandstones. The deposits of the upper part of the Maghra El-Bahari Formation have features characteristic of lacustrine environments (lake margin, in particular). Interpretation of the floodplain-dominated deposits is based on the occurrence of predominantly finegrained clastics (mudstones, siltstones and fine-grained sandstones) that are devoid of marine fossils and have pedogenic features such as colour mottling, root traces and desiccation cracks (Fig. 2, Reading, 1981; Kraus and Gwinn, 1997). Also, within the fine-grained deposits of Gabal Ataqa there are coarse-grained beds. These are represented by pebbly coarse to medium-grained sandstone beds. These beds show sharp contacts and non-erosional boundaries with their associated finegrained sediments (Figs. 2 and 3A). Such occurrences of coarse-grained deposits within the pedogenic finegrained sediments are thought to be a result of local influx of crevasse splay deposits (Fig. 3B, Kraus and Gwinn, 1997). This is also suggested by their limited thickness (about 0.5–1 m) within the fine-grained deposits (Fig. 3A, Reading, 1981). The abundance of crevasse splay deposits tends to increase from Gabal Ataqa to Gabal Shabrawet (Fig. 2). The colour mottling, root traces and iron nodules are more common in the fine-grained floodplain deposits than in the interbedded coarse-grained crevasse splay deposits (Fig. 2). In many cases, the coarse-grained crevasse splay deposits do not show pedofeatures. The grain size of the studied sediments varies laterally in the two areas of study: coarser grains dominate at Gabal Shabrawet (Fig. 3A, B and C). These differences between grain sizes in the two areas is thought to have been controlled by the proximity of the study area relative to the source areas (Reading, 1981, Alonso Zarza et al., 1992; Kraus and Gwinn, 1997), with Gabal Shabrawet being closer to the source area than Gabal Ataqa where finer sediments prevail (Fig. 3B and C). The coarse-grained sediments of Gabal Shabrawet contain pebbles of Cretaceous limestone and sandstone which are poorly sorted, randomly oriented in a matrix of very fine sand and mud. These rocks are brownish

yellow in colour. Such rocks deposited in a floodplain close to the source area. The lacustrine environment of deposition of the upper part of the Maghra El-Bahari Formation is deduced on the basis of (1) the occurrence of alternating beds of dolostone and evaporite that lack evidence of tidal effects and their position directly overlying the floodplain clastic deposits (Fig. 3B). Similar successions have been described worldwide (e.g., Lowenstein and Hardie, 1985; Calvo et al., 1989; Alonso Zarza et al., 1992; Demko et al., 2004), (2) the carbonates are devoid of marine fauna which excludes the possibility of a marine origin (Reading, 1981). In these deposits, the admixture of evaporites and Mg-rich carbonates with terrigenous impurities (sands, silts and clays, Fig. 2) in addition to their bright white colour which indicates deposition near the margin of the lake (Alonso Zarza et al., 1992; Del Cura et al., 2001; Fig. 3B). In addition, their limited extent (present at Gabal Ataqa but wedging out laterally at Gabal Shabrawet, Figs. 2 and 3A) and lack of primary structures (e g., ripples and cross-stratification) indicates the ephemeral nature of the lake (Reading, 1981; Gustavson, 1991; Alonso Zarza et al., 1992). Furthermore, the occurrence of desiccation cracks in the carbonate and marl interbeds indicates that the lake-floor sediments periodically dried out after flood events (Gustavson, 1991). In addition, the occurrence of the lake deposits above the floodplain deposits may suggest that the lake was formed from an influx of water (e. g., rainfall or groundwater) without further floodplain deposition (Alonso Zarza et al., 1992; Del Cura et al., 2001). 6. Recognition of paleosols Although ancient soils may not be directly comparable to modern ones, several authors have discussed the criteria for recognition of paleosols in fluvial and lacustrine deposits during the Cretaceous/Tertiary transition (e.g., Fastovsky and McSweeney, 1987; Lehman, 1989; Cojan, 1993; Atchley et al., 2004). Consequently, in the deposits studied here, paleosols are recognized on the basis of their macroscopic and microscopic features. The macroscopic features were described in the field whereas the microscopic ones were obtained from thin sections. 6.1. Macroscopic features The red colouration and distinctive structure (massive and featureless in outcrop) in the lower part of the Maghra El-Bahari Formation (Fig. 4A) yielded the first

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Fig. 4. Field photographs of the Maghra El-Bahari Formation showing (A) general view of the Maghra El-Bahari Formation at Gabal Ataqa, (B) vertically aligned columns or pipes filled with greyish white silty claystone in yellowish brown parent siltstone at Gabal Ataqa, (C) vertically aligned columns or pipes filled with violet to purple silty claystone in yellow parent sandstone at Gabal Shabrawet, (D) root traces in yellowish brown siltstone at Gabal Ataqa, (E) dispersed nodules (see arrows) and vertically elongated column of greyish white carbonate-rich clays in yellow to yellowish brown siltstone at Gabal Ataqa, (F) colour mottling in siltstone at Gabal Ataqa, (G) slickensides in silty claystone at Gabal Ataqa, and (H) colour horizons in the siltstone and silty claystone at Gabal Ataqa. These colour horizons represent vertical colour segregation from reddish brown through yellow to greyish white. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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evidence for the probability of paleosol development (Kraus, 1997). Other characteristic macroscopic criteria of paleosol development in the studied floodplain deposits were also observed (e.g., vertical to oblique burrows, root traces, carbonate nodules and columns, colour mottling, desiccation cracks and colour horizons (Fig. 4B–H). Such criteria are similar to those recorded in many floodplain paleosols (e.g., Retallack, 1983; Bown and Kraus, 1987; Alonso Zarza et al., 1992; Kraus, 1997; Kraus and Gwinn, 1997; McCarthy and Plint, 1999; Kraus, 2002; Atchley et al., 2004). In the present study, the paleosol features are clear in the finegrained deposits (floodplain deposits), whereas in the lithologically heterogeneous deposits (coarse-grained crevasse splay deposits) they are not well developed and more localised (Fig. 2). These differences in paleosol development in the deposits of the lower part of the Maghra El-Bahari Formation are related to the grain size and rate of sediment accumulation. This is consistent with deposits discussed by Bown and Kraus (1987) and Kraus (2002) who concluded that the high rate of accumulation of the coarse-grained deposits diminishes pedogenesis. Vertically aligned columns or pipes ranging from 1 cm to 70 cm long are observed abundantly throughout the lower unit of the floodplain deposits (Fig. 2). They exhibit different colours and composition from their surroundings. They are commonly filled with violet siltstone and encased by greyish white clay drab-haloes (Fig. 4B and C). Such occurrence of greyish white clay drab-haloes may have resulted from the physical illuviation of fine clays during pedogenesis in a waterlogged soil (Retallack, 1983; Khademi and Mermut, 2003). The vertical pipes are deeply penetrating (30– 70 cm, Fig. 4B and C). It is not readily apparent whether the vertical pipes represent roots or burrows. However, the presence of such vertically aligned pipes infilled with material contrasting with their immediate matrix and surrounded by drab-haloes suggest that they resulted from roots (Retallack, 1983). Also, their deeply penetration, up to seventy centimeters, indicates prolonged time of pedogenesis (Retallack, 1983). Rootlet traces are also recognized by their downward branching (Fig. 4D). They are common at the topmost of the floodplain deposits (Fig. 2). The existence of these rootlet traces supports the inference of pedogenic influences (Lehman, 1989; Kraus and Gwinn, 1997). Carbonates which occur as rare nodules and vertically elongated columns (Fig. 4E) were noticed in the top of the lower part of the Maghra El-Bahari Formation (floodplain deposits) at Gabal Ataqa. The vertically elongated columns of carbonate are not

downward branching (Fig. 4E) and show no indication of having drab-haloes which are characteristic of former roots (Retallack, 1983). Therefore, they are more likely to result from the infilling of vertical animal burrows. In general, the occurrence of carbonate nodules and columns within the floodplain deposits studied indicate soil forming processes (Retallack, 1983; Gustavson, 1991; Mack, 1992). On the basis of colour mottling, the floodplain deposits comprise three main units: pale grey unit, brownish yellow unit and purple and/or red unit. The pale grey unit has purple mottles (Fig. 4F), and is common in the lowermost part of the floodplain deposits at Gabal Ataqa. The purple and or reddish brown unit possesses white mottles (Fig. 4E), and is ubiquitous at the top of the lower part of the Maghra El-Bahari Formation at Gabal Ataqa. The brownish yellow unit has purple mottles (Fig. 4C), and prevails in the floodplain deposits at Gabal Shabrawet. The white mottles are generally associated with the occurrence of carbonate nodules and columns (Fig. 4E). The brownish yellow unit has purple mottles and is found in the lower part of the Maghra ElBahari Formation at Gabal Ataqa (Fig. 4B) and Gabal Shabrawet (Fig. 4C). On the basis of the previously mentioned paleosol features (burrows, carbonate nodules and columns, Fig. 4B, C and E), it is clear that the colour mottlings commonly occur in the rocks that have root traces and burrows. Therefore, it can be suggested that the colour mottlings of the studied sediments are more likely to result from infilling of both root traces and/or burrows during soil-forming processes by sediments that have colours different from those of the surrounding rocks. This supports the inference that the co-occurrence of colour mottling within the floodplain beds would have been largely due to pedogenic features since burial alteration would be pervasive and not produce such a mixture (Kraus, 1997). Slickensides were also noticed in most areas of the floodplain deposits (Fig. 4G). These exhibit conjugated sets of oriented clay-filled fractures that are oriented roughly perpendicular to one another (Fig. 4G). They are 5–10 cm wide on the surface, and they are recognized by white colour within purple and red-coloured host rock (Fig. 4G). These slickensides are commonly recorded in the smectite-rich (about 70% of the bulk rock as revealed from XRD data) silty claystones of the floodplain deposits at both Gabal Ataqa and Gabal Shabrawet (Fig. 2). The pedogenic origin of these slickensides is indicated by their association with colour mottling in the host rock (Fig. 4G). In addition, burrow infillings and root traces in many horizons are not offset by slickensides, which in turn support the

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pedogenic origin of these slickensides. The occurrence of these slickensides indicate that the sediments underwent seasonal wetting and drying (Kraus, 2002). Colour horizons are indicated by the occurrence of vertical segregation of the deposits into distinctive greyish white, brownish yellow and purple and/or red-

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dish brown horizons (Fig. 4H). Such colour bands are considered to result from pedogenesis, because at present they form by groundwater fluctuation during soilforming processes, in which grey horizons form in respose to a high water table in a poorly drained soil, whereas the brownish yellow and purple and/or reddish

Fig. 5. Photomicrographs of thin-sectioned samples from the Maghra El-Bahari Formation showing (A) well-oriented clays (see arrow) filled the irregular fracture within the matrix of sandstone at Gabal Ataqa, (B) well-oriented clays coated the quartz grains in sandstone at Gabal Ataqa, (C, D) spar-filled root molds surrounded by dense micrite (see arrows) that was precipitated around rhizomes in the sandstones at Gabal Shabrawet and Gabal Ataqa, respectively, (E) spheroidal dolomite enclosing densely packed clotted micrite (see arrow), Locality, Gabal Ataqa, and (F) densely packed aggregates of clotted micrite (see arrows) in microsparry calcite groundmass, Locality, Gabal Ataqa.

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brown horizons result from well to moderate drainage conditions with a lack of groundwater influence (Kraus and Aslan, 1993; Kraus, 2002). In the lacustrine deposits (both carbonates and evaporites) paleosol macro-criteria are represented

by the loosely indurated and massive nature (lack of primary structures), combined with desiccation cracks in the carbonates and marlstones (Gustavson, 1991; Alonso Zarza et al., 1992; Del Cura et al., 2001).

Fig. 6. Photomicrographs of representative samples from the Maghra El-Bahari Formation showing (A) iron oxide micronodules within the limestone at Gabal Ataqa, (B) clays (yellow colour) in association with dolomite spheroids of the dolostone at Gabal, (C) fibers of palygorskite . This SEM image was taken to the yellow clays in (B), (D) EDAX of the fibers of the palygorkite of (C), (E) porphyrotopicgypsum that are represented by large euhedral crystals of gypsum (see arrow) scattering through the anhydrite groundmass, and (F) microsparry calcite aggregate (yellowish brown) filled the secondary pores of gypsum. Notice the occurrence of gypsum relic (see arrow) within the microsparry calcite aggregate, and also the sharp contact between the calcite aggregate and the gypsum groundmass. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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6.2. Microscopic features Because of problems of diageneesis (Retallack, 1991), identifications of paleosols in the field are often not sufficient for process-based interpretations of paleosols. However, study of micromorphology in this sections of sediments yields valuable information for recognizing paleosols (McCarthy et al., 1998). The micromorphological features of paleosols in the floodplain deposits include the occurrence of illuviated clays and rhizolith traces. Evidence of clay illuviation (pedogenic clay translocations) is recognized by the higher concentration of well-oriented clays that filled irregular fractures within the matrix (Fig. 5A), and/or coated the detrital grains (Fig. 5B). The illuviated clays are smectitic in composition and increase in abundance in muddominated floodplain deposits of the lower part of the studied succession. The illuviated clays are abundant in the floodplain deposits which have root traces and slickensides. Such occurrence of the illuviated clays indicate paleosol development, and represent microscopic expressions of slickensides and drab-haloes of the root traces (Retallack, 1983; Bown and Kraus, 1987; Lehman, 1989). The micritic rhizolith occurs as a rounded spar-filled mold having a dense micritic coat, and stands out in sharp contrast to the surrounding cement and grains (Fig. 5C and D). The rhizoliths were found in the uppermost of the lower part of the floodplain deposits. These rhizoliths could be a result of carbonate infilling molds left by decayed roots during pedogenesis (Driese and Mora, 2002). In the lacustrine sediments recognized, although the macroscopic paleosol criteria are not adequate and obvious, abundant microscopic features indicate soilforming processes in near-surface settings. This is inferred from the spherulitic-like microfabric present in the carbonate rocks (Fig. 5E), along with densely packed clotted micritic globules forming a clot-like texture (Fig. 5F), iron oxide-filled micropores (Fig. 6A) and palygorskite association (Fig. 6B–D). In addition to porphyrotopic secondary gypsum (Fig. 6E) that occasionally has clotted dolomite aggregates with minute gypsum inclusions (Fig. 6F). This clotted dolomite aggregate exhibits a sharp contact with the gypsum groundmass (Fig. 6F). This interpretation is based on (1) the spherulitic-like microfabric of lacustrine Mgrich carbonates (dolomite) which were interpreted to be formed by bacterial mats at the sediment–atmosphere interface in a lacustrine setting (Del Cura et al., 2001), which in turn supports the idea that the carbonates were formed during direct exposure to light, (2) clottedtextural carbonates were interpreted in the lacustrine

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deposits as a result of pedogenic action (Alonso Zarza et al., 1992; Sanz et al., 1995; Del Cura et al., 2001), (3) the occurrence of iron oxide infilling micropores within the carbonate were interpreted to be a result of deposition in microscopic voids that were left during pedogenesis (Fastovsky and McSweeney, 1987), (4) the association of the Mg-rich carbonate (dolomite) and palygorskite mineral in the lacustrine deposits indicates soil development (Singer and Galan, 1984; Tateo et al., 2000; Del Cura et al., 2001), (5) the existence of porphyrotopic secondary gypsum in ancient lacustrine evaporite sequences was interpreted to be a result of near-surface influx of fresh water (Lowenstein and Hardie, 1985) and/or pedogenesis (Sanz et al., 1999), (6) the existence of clotted carbonate aggregate infilling the secondary porosity and making a sharp contact with the gypsum groundmass was listed among the characteristics of evaporites subjected to uplift (Lowenstein and Hardie, 1985). 7. Paleosol horizon characteristics Because many successive paleosols can be recognized through the studied lithologies, they are characterized by the dominance of four horizon, pale grey A horizon, purple or reddish brown B horizon, brownish yellow C horizon and white or greenish white D horizon. The pale grey A horizon, purple or reddish brown B horizon and brownish yellow C horizon dominate in the floodplain-dominated facies, whereas the white or greenish white D horizon characterizes the marginal lacustrine facies (Fig. 2). 7.1. A horizon It appears in exposure as grey in colour ranging from grey (7.5 Y 5/1) or light grey (2.5 GY 8/1) to olive grey (2.5 GY 6/1). Such units do not exhibit pronounced colour mottling, but they uncommonly have 5–20% mottles of dull reddish brown (7.5 R 5/3). Such mottles are faint, with irregular diffuse boundaries (Fig. 4F). The textures of sediment constituting the grey A horizon range from medium to fine-grained sandstones with occasional occurrence of pebbles. These grey horizons are almost, without exception, coarser than their underlying and overlying purple or red horizons. It is possible that these sand-rich grey units are crevasse splay deposits subsequently modified by pedogenesis. Grey units exhibit thicknesses ranging from 0.3 to 1.5 m, and are intercalated with purple or red units. Grey A horizon beds contain less smectitic clay (20–30%) than the purple and reddish brown beds. They are generally

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thinner than the associated purple and/or red units. Individual grey units may pinch out laterally. Apart from contrasts in colour and texture these grey units are difficult to distinguish from purple or reddish brown units. In some places, the grey units are unmodified by pedogenic features. In thin section, these sandstones have small amounts of illuviated clays (Fig. 5A and B). Paleosol development in these sandstones is weak, and in some cases, paleosol features are absent.

siltstones is weak. Brownish yellow C horizons have thicknesses ranging from 0.5 to 2.5 m, and are generally thicker than the associated grey units. Brownish yellow C horizons are sometimes seen above the grey A horizons and in other places of the succession, they are recorded below the purple and/or reddish brown B horizons. Individual brownish yellow units may be traced laterally in good exposures, and are more abundant in the floodplain deposits of Gabal Shabrawet than in those of Gabal Ataqa (Fig. 2).

7.2. B horizon 7.4. D horizon Both purple and reddish brown beds are described here together because the difference between them is one of degree and not of kind. Beds interpreted as B horizon are reddish brown (10 R 4/3), dull reddish brown (7.5 R 5/3), dark reddish brown (7.5 R 3/2 to 3/3, 10 R 3/2 to 3/3) and light purple (5 P 6/1). Purple and reddish brown beds contain more smectite (N60%) than the grey A horizon. Purple and/or red horizons have thicknesses ranging from 0.5 to 2.5 m, and are generally thicker than are associated grey A horizon. Beds of the B horizon are mainly constituted of mudstones and silty claystone facies of the floodplain deposits (Fig. 2). Some beds of the B horizon have greyish white carbonate nodules and columns and slickensides (Figs. 2 and 4E,G). Other beds of the B horizon have deep root traces (up to 70 cm long) delineated by greyish olive (7.5 Y 6/ 2) drab haloes (Fig. 4C), which reflect waterlogging in reduced conditions (Retallack, 1983). Beds having carbonate nodules and columns and root traces exhibit strong colour mottling. This suggests that such colour mottling is more likely to be a result of infilling of root molds and burrows. Beds having root traces, greyish white carbonate nodules and columns and slickensides occur abundantly in the mud-dominated floodplain deposits in both areas studied (Fig. 2). Microscopically, the beds of this horizon are characterized by features including illuviated clays and micritic rhizoliths (Fig. 5A–D). Individual purple and/or red units may be traced laterally in good exposures. In this horizon, the pedogenic features are widely developed. 7.3. C horizon Brown yellow (10 RY 5/2) beds are described here as C horizon. Beds of C horizon consist mainly of siltstones of the floodplain-dominated facies. Macroscopic criteria of beds of this horizon are represented by small iron oxide nodules (up to 6 cm) and colour mottling. In thin section, these siltstones have a little iron oxide pigmentation. Paleosol development in these

Beds of D horizon are white to greenish white in colour (5 GY 7/1, 5 R 4/1, 10 Y 6/2). The white unit is characterized by the presence of faint purple mottling, microkarst and microfractures. Desiccation cracks are abundant. The beds of D horizon are mainly observed in the marginal lacustrine facies (Fig. 2). In thin section, these beds display scattered iron nodules, spheroliticlike microfabric carbonates, densely packed micritic globules forming clotted texture, palygorskite association with carbonates and porphyrotopic gypsum (Figs. 5E,F and 6A–F). In these beds, although the macroscopic pedogenic features are not obvious, the microscopic pedogenic ones are clearly developed. 8. Classification of paleosols According to the classification of paleosols by Mack et al. (1993), the studied floodplain paleosols appear to be related to the order of soils comprising the vertisols (argilli- and calcic-vertisols). On the other hand, calciosols and gypsisols can be attributed to the marginal lacustrine paleosols. The argilli-vertisols are clay-rich (smectitic) soils with high-shrink-swell and have pedogenic characteristics, including slickensides, desiccation cracks, mottling, root and burrow molds and others (Mack et al., 1993). The characteristic features of argilli-vertisol are preserved well in the paleosols of the lowermost part of the lower part of the floodplain deposits (as described in A and B horizons, Fig. 2), where there is an upward increase in the smectitic clayrich (30–70%) fraction, and the presence of well-developed slickensides in carbonate-poor fraction (Fig. 2). Calcic-vertisols are carbonate-rich vertisols, in which there are clay-rich beds with carbonates in the form of micritic rhizoliths, nodules and burrow-fill morphologies (Driese and Mora, 2002). The characteristic features of calcic-vertisols are recognized in the paleosols of the uppermost part of the lower part of the floodplain deposits (as described in B horizon, Fig. 2). Calciosols

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are carbonate-rich soils displaying pedogenic features such as vadose textures, brecciation, scattered nodules, pisoliths, poorly indurated massive carbonate beds and others (Mack et al., 1993). The characteristic features of calciosols were noticed in the marginal lacustrine paleosols (as described in D horizon, Fig. 2). Gypsisols are gypsum and/or anhydrite exhibiting evidences of near surface or vadose origin. Gypsisols were noticed in the marginal lacustrine paleosols (as described in D horizon, Fig. 2). According to the classification of Kraus and Aslan (1993) for floodplain paleosols, three types of paleosols are recognized in the floodplain-dominated and crevasse splay deposits (Fig. 2 ). Paleosol types 1 and 2 are recorded in the floodplain-dominated deposits, whereas the paleosol type 3 is developed in the crevasse splay deposits. The paleosol types 1 and 3 are similar to simple paleosols, whereas the paleosols type 2 can be attributed to cumulative paleosols. The cumulative paleosols (paleosol type 2) are recognized in the muddominated floodplain deposits at Gabal Ataqa and Gabal Shabrawet where great pedogenic development is noticed (as observed in B horizon, Fig. 2). These paleosols may be formed in response to slow sedimentation and long periods of pedogenic modification (Kraus and Aslan, 1993). The simple paleosols (paleosol types 1 and 3) are recognized in the sandstonedominated floodplain and crevasse splay deposits at Gabal Ataqa and Gabal Shabrawet, where the weakly developed paleosols and unmodified beds were deposited (as described in the A and C horizons, Fig. 2). These paleosols may be a result of rapid sedimentation and relatively little time for pedogenesis (Kraus and Aslan, 1993). 9. Significance of paleosols In order to understand the significance of the paleosols, it is necessary to evaluate the factors responsible for soil development (Mack, 1992). Differences among paleosols may be the result of topography, texture of sediments, parent material, time, vegetation, drainage conditions, basin position relative to source area and climate (Jenny, 1941; Retallack, 2001). Therefore we will discuss below the factors controlling development of the paleosols examined: 9.1. Time Although it is difficult to evaluate the span of time that the paleosols represent within the studied continental system due to the absence of fossils, previous

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authors (El-Akkad and Abdallah, 1971; Al-Ahwani, 1982) attributed similar ages to the sediments in the two areas (Late Cretaceous–Early Middle Eocene). Due to the similarity in age estimates, it seems unlikely that time was responsible for the differences between paleosols in the studied areas. In addition, palynological and chronological evidence required to obtain precise data are hard to obtain in the studied deposits. Therefore, it is has not been possible to obtain a more precise idea of the time duration represented by the paleosols, except for the prolonged time period of pedogenesis as it is indicated from the deeply penetration of root traces or burrows (Fig. 4B, C, Retallack, 1983). 9.2. Vegetation Although the observed paleosols show root traces and burrows (Fig. 4B–D), no direct evidence of vegetation can be detected. This is because these paleosols appear to have been too oxidized for the preservation of pollen, spores, wood, leaves and other organic materials. 9.3. Organisms Despite the occurrence of burrows in the studied deposits, no direct evidences of living chambers or fossilized remains of invertebrates and vertebrates were found. Therefore, the role of organisms in soil development can not be obtained. 9.4. Topography The deposits accumulated on a low-lying landmass in the form of alluvial floodplain and lacustrine facies (see sedimentological data). Consequently, topographic variability did not play a role in the formation of the paleosols in the two areas. 9.5. Tectonism On the basis of (1) the occurrence of the studied formation in the two areas of study as flat-lying strata sharply onlapping tilted Cretaceous rocks (El-Akkad and Abdallah, 1971; Al-Ahwani, 1982), which indicates a period of stabilization that followed late Cretaceous tectonism (Moustafa and Khalil, 1995), (2) there is no evidence of secondary structures, and (3) the depositional characteristics and pedofeatures of the studied formation are very similar to those described in tectonically stable areas elsewhere (Alonso Zarza et al., 1999), it can be suggested that the paleosols were developed in tectonically stable areas. Thus, the dif-

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ferences between the paleosols are not the result of tectonism. 9.6. Parent materials The parent materials are similar in both of the two areas of study (represented by detrital mud, silt, sand), except for clay-rich and carbonate-rich sediments in the floodplain and marginal lake facies, respectively. The original clay mineral composition of the parent material can not be estimated confidently because of the possibilty of diagenetic changes in the clay mineral suites, and because of the difficulty in separating original, detrital material from translocated clay. On the other hand, the carbonate-rich material in the marginal lake facies is authigenic in origin and is a good indicator of climatic conditions (Alonso Zarza et al., 1992). It is deduced that the above mentioned factors (e. g., topography, time, tectonism, parent materials and vegetation) do not explain the paleodevelopment. On this basis paleosol variability in the studied rock unit seems to be related more likely to paleoclimatic and paleohydrologic changes as well as to distance from the sediment source area. 9.7. Sediment texture and basin position In the relationship between sediment texture and/or basin position, it is clear that there are various sets of paleosol development, with marked differences, not only between the two areas of study but also within the vertical succession of sediments of each area. This is particularly true with regard to the rate of soil development. The proximal coarse-grained alluvial floodplain facies are more abundant in areas adjacent to a local source area (Gabal Shabrawet, Fig. 3) produced relatively weakly developed paleosols. In contrast, the distal fine-grained floodplain and marginal lake deposits lie further from the source area (Gabal Ataqa, Fig. 3) exhibit relatively well-developed paleosols. Furthermore, within the vertical succession, there is variation in the rate of soil development, in which the muddominated floodplain sediments exhibit well-developed paleosols whereas the crevasse splay sediments show weakly developed paleosols (Fig. 2). 9.8. Paleosols and drainage conditions Paleosols exhibiting colour nodules support the influence gleying and indicate diverse floodplain sediments (Mack,

horizons and iron-oxide of gleying and psuedodrainage conditions in 1992; Retallack, 1983;

Kraus and Gwinn, 1997; Kraus, 2002). Consequently, grey horizons indicate poorly drained paleosols and a high water table. In contrast, the yellow and yellowish brown horizons suggest well to moderately drained paleosols not affected by groundwater. In our study, it is found that the grey A horizons which have weak soil development characterize the coarser floodplain deposits (Fig. 2), whereas yellow, purple and yellow brown B and C horizons exhibit well-developed pedogenic features and occur in a relatively fine-grained floodplain deposits (Fig. 2). Therefore, when our findings are compared with other studies (e. g., Mack, 1992; Retallack, 1983; Kraus and Gwinn, 1997; Kraus, 2002), it can be suggested that an increase in development of pedogenic features in the floodplain deposits led to an increase of soil drainage and red colouration (rubification) of sediments. Furthermore, the degree of soil drainage and its colouration are not related to sediment textures. 9.9. Paleosols and paleoclimate Paleosols have been used as a proxy for paleoenvironment, especially paleoclimate (Mack, 1992; Alonso Zarza et al., 1992; Driese and Mora, 2002; Lee et al., 2003; Demko et al., 2004). Some of the paleosols which show the influence of gleying and pseudogleying processes (e. g., the paleosols exhibiting grey A horizon colours and iron-oxide nodules as discussed above) are probably not good paleoclimatic indicators (Mack, 1992). In this study, throughout the floodplain succession there is an abundance of pedofeatures such as red colouration, carbonate nodules, illuviation clays, slickensides and deep root traces and burrows (0.5–1 m depth). Such features provide evidence of their formation above the water table (Mack, 1992). Consequently, such features are the most suitable for paleoclimate interpretation (Mack, 1992). Also, the abundance of carbonate, Mg-rich clays and evaporites in the marginal lacustrine facies is more suitable for paleoclimate interpretation (Alonzo Zarza et al., 1992). On the basis of the paleosol features suitable for climate interpretation, certain aspects of paleoclimate can be deduced. Illuviation clay in the base of the lower part of the Maghra El-Bahari Formation (vertisols) suggests a prevailing humid to semi-humid climate during paleosol development (Mack, 1992; Lee et al., 2003; Demko et al., 2004). The absence of illuviation clays was noticed in carbonate and evaporite-cemented clastics as in the lithofacies of the upper part of the studied rock unit (marginal lacustrine deposits). Such absence may be related to the masking of illuviated

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clays during carbonate and gypsum crystallization in arid conditions such as those described by Khademi and Mermut (2003). The occurrence of carbonate nodules and columns in the top of the lower part of the studied rock unit can be cited as evidence of a semi-arid climate (Retallack, 1983; Gustavson, 1991; Mack, 1992; Lee et al., 2003). The association of the Mg-rich carbonate (dolocrete) and palygorskite mineral in the lacustrine deposits indicates soil development under arid to semiarid climatic conditions (Singer and Galan, 1984; Tateo et al., 2000; Del Cura et al., 2001). The presence of slickensides and desiccation cracks in the floodplain deposits indicates seasonal wetting and drying following periods of flooding and desiccation (Yaalon and Kalmar, 1978; Gustavson, 1991; Paik and Lee, 2003). In conclusion, the deduced paleoclimate in the Maghra El-Bahari Formation can be subdivided into two time successive climatic regimes (from sub-humid through semi-arid to arid climate). A sub-humid to semi-arid regime prevailed during the development of vertisols on the basal part of the floodplain deposits, whereas a semi-arid to arid regime prevailed during the soil-forming processes in the uppermost part of the floodplain deposits and the lacustrine deposits. 10. Geological implications The inferred depositional environments and paleosol development contribute to the geological history of the study areas. The depositional environments indicate non-marine (continental) deposition (floodplain and lacustrine), which in turn indicate a marine regression from the area under study during the deposition of the Maghra El-Bahari Formation. Furthermore, the repeated development of paleosols indicates that the climate changed from subhumid through semiarid to arid. When the criteria of depositional system and paleosols are used together, they suggest that during the Late Cretaceous/Early Tertiary, Gabal Ataqa and Gabal Shabrawet comprised an internally drained basin that was influenced by repeated episodes of soil formation. In that time period, proximal and distal floodplain sediments of the Maghra El-Bahari Formation were derived from low-lying landmass at the extreme north of Gabal Shabrawet (Fig. 3C) which explains why proximal floodplain sediments prevail in Gabal Shabrawet (basin), whereas distal floodplain sediments were laid down in the Gabal Ataqa area (basin) (Fig. 3C). Later, Gabal Shabrawet was higher than the Gabal Ataqa area in which the ephemeral lake was formed from rainfall and/or ground water and floodplain deposition ceased. In addition, the clear

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basal conglomerate above the well-developed paleosols (Fig. 2) marks an important regional unconformity at the base of the Lower Middle Eocene rocks, and can share in explaining why there was a time gap of sedimentation during Late Cretaceous–Early Tertiary transition in the area studied. These foregoing results support the idea that the regressive phases of the sea at the Late Cretaceous/Early Tertiary transition were more common in the North Egypt (Issawi and Osman, 2000), and more particularly in the northern portion of the Eastern Desert (Abdel-Azeam and Metwally, 1998). In terms of a regional perspective, the climatic change to more arid conditions at the Early Tertiary passing through subhumid at the Late Cretaceous in the recognized paleosols at Gabal Ataqa and Gabal Shabrawet is consistent with climatic changes that have been recorded in the paleosols of the same time period elsewhere (e. g., Lehman, 1989; Cojan, 1993; Atchley et al., 2004). 11. Conclusions The Upper Cretaceous–Lower Tertiary Maghra ElBahari Formation at Gabal Ataqa and Gabal Shabrawet is differentiated into lower and upper parts. The lower part comprises mainly clastic deposits, whereas the upper part consists mainly of carbonate and evaporite sediments. Sedimentological data of the Maghra ElBahari Formation indicate its deposition in alluvial floodplain and lacustrine environments. The Maghra El-Bahari Formation preserves evidence of paleosol development. The recognized features include both macro- and micromorphological criteria. The macromorphological criteria are represented by colour horizons, slickensides, desiccation cracks, carbonate nodules, colour mottling, burrows and root traces. The micromorphological criteria include illuviated clays, micritic rhizoliths, spheruliticand clotted-like microfabric of carbonates. The paleosols indicate the presence of two climatic regimes in the studied areas during the Late Cretaceous/Early Tertiary transition. These regimes are superimposed vertically with sub-humid to semi-arid below to semi-arid–arid climates later. The paleosol evidence indicates the presence of three types of paleosol (vertisol, calcisol and gypsisol). The vertisol characterizes the lowermost part of the floodplain deposits, whereas both calcisol and gypsisol are recorded in the upper part of the floodplain and lacustrine deposits. In addition, in terms of degree of paleosol development, two types of paleosols are recognized: simple and cumulative.

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In terms of their geological implications, the inferred depositional environments and paleosols indicate that the studied area was a part of a low-lying landmass in northern Egypt where alluvial floodplain and lacustrine deposits accumulated that were influenced by repeated episodes of soil formation. During this period of pedogenesis, the climate changed from sub-humid to semiarid. In regional perspective, the inferred paleoclimatic changes agree with contemporaneous paleoclimatic data from other parts of the world. Therefore, the paleoclimatic data presented in this study may have been of regional significance. Acknowledgements The authors would like to thank Prof. Dr. M.A. Khalifa and Dr. H.E. Soliman (Menoufia University, Egypt) for reading an earlier version of this paper. We would like to appreciate Prof. Dr. Mary J. Kraus (Colorado University, USA) and an anonymous reviewer for their critical reviews which improved the manuscript. We also thank Prof. Dr. Bruce W. Sellwood (Reading University, UK) for his editorial work, constructive suggestions and improvement of English. References Abdel-Azeam, S., Metwally, M.H., 1998. Stratigraphy of Abu Darag Upper Cretaceous, with some regional aspects on the related sediments in the western side of the Gulf of Suez, Egypt. M. E. R. C. Ain Shams Univ.. Earth Sci. Ser. 12, 90 – 105. Al-Ahwani, M.M., 1982. Geological and sedimentological studies of Gabal Shabrawet area, Suez Canal district Egypt.. Ann. Geol. Surv. Egypt 12, 305 – 381. Alonso Zarza, A.M., Wright, V.P., Calvo, J.P., Garcia, del Cura, M.A., 1992. Soil-landscape and climatic relationships in the middle Miocene of the Madrid Basin. Sedimentology 39, 17 – 35. Alonso Zarza, A.M., Sopena, A., Sanchez Moya, Y., 1999. Constracting paleosol development in two different tectonic settings, Central Spain. Terra Nova 11, 23 – 29. Atchley, S.C., Nordt, L.C., Dworkin, S.I., 2004. Eustatic control on alluvial sequence stratigraphy: a possible example from the Cretaceous–Tertiary transition of the Tornillo Basin, west Texas, USA. J. Sediment. Res. 74, 391 – 404. Bown, T.M., Kraus, M.J., 1987. Integration of channel and floodplain suites: I. Development sequence and lateral relations of alluvial paleosols. J. Sediment. Petrol. 57, 587 – 601. Calvo, J.P., Alonso-Zarza, A.M., Carcia del Cura, M.A., 1989. Models of marginal lacustrine sedimentation in response to varied source areas in the Madrid Basin (Central Spain). Paleogeogr. Paleoclim. Paleoecol. 70, 199 – 214. Choi, K., 2005. Pedogenesis of late Quaternary deposits, northern Kyonggi Bay, Korea: implications for relative sea-level change and regional stratigraphic correlation. Paleogeogr. Paleoclim. Paleoecol. 220, 387 – 404. Cojan, I., 1993. Alternating fluvial and lacustrine sedimentation tectonic and climatic controls (Provence Basin, S. France,

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