Four sections of the Galala and Maghra El Hadida formations on the footwalls of the slopes of the northern and southern Galala plateaus in Wadi Araba (Eastern Desert) have been measured and sampled in great detail. The Galala Formation is ranging in thickness from 55 to 95 meters. It unconformably overlies the Malha Formation which forms the base of the studied sections. The upper boundary of the Galala Formation is characterized by a major unconformity which separates it from the overlying the Maghra El Hadida Formation. The Galala Formation can be subdivided into five shallowing-upward cycles, each cycle starting with deep-lagoonal, marly-silty deposits at the base and grading into highly fossiliferous shallow-lagoonal limestones at the top. Only the basal part of the Galala Formation consists of unfossiliferous, greenish sandy siltstones intercalated with thin cross-bedded, bioturbated, fine- to medium-grained sandstones. Despite the lack of biostratigraphic markers in that lower part, its age can be assigned to the late Middle Cenomanian, since the conformably overlying strata contain the ammonite Neolobites vibrayeanus (D’ORBIGNY), the index marker of the early Upper Cenomanian which extends into the top of the formation. The measured thickness of the overlying Maghra El Hadida Formation is ranging from 59 to 118 meters. This formation starts with the Ghonima Member, introduced in this work to distinguish a brown, fine- to medium-grained calcareous sandstone unit in its lower part. The Ghonima Member is erosionally incised into the Galala Formation, explaining its strong lateral variability in thickness, ranging from 3 to 21 meters. It is mostly unfossiliferous except for irregular bioturbation in its upper part. The Ghonima Member is assigned to the middle Upper Cenomanian, based on its stratigraphic position between the lower Upper Cenomanian Neolobites vibrayeanus Zone and the overlying upper Upper Cenomanian Metoicoceras geslinianum and Vascoceras cauvini zones. This means that the lower part of the Maghra El Hadida Formation, about 20 – 30 m thick, accumulated during the latest Cenomanian and that the base of the formation does not coincide with the base of the Turonian as commonly believed. The overlying succession of the Maghra El Hadida Formation is characterized by an increase of carbonate content, represented by yellow, soft marls intercalated with fine-grained wacke- to packstones containing a highly fossiliferous ammonite assemblage of the upper Upper Cenomanian and Lower Turonian (zones of Vascoceras proprium, Choffaticeras spp., and Wrightoceras munieri). The Middle Turonian part of the Maghra El Hadida Formation consists of poorly fossiliferous, thick-bedded yellowish marls with upward-increasing silt content, showing occasional intercalations of medium- to coarse-grained sandstones with hummocky cross-stratification. The topmost part of the Maghra El Hadida Formation consists of brownish, medium-grained sandstones topped by fossiliferous marly limestones yielding the Upper Turonian zonal ammonite Coilopoceras requienianum (D’ORBIGNY). Based on sequence stratigraphic analyses, four complete 3rd order depositional sequences and the lower part of a fifth one, each bounded by major unconformities, can be recognized: depositional sequence DS WA 1 (upper Middle – lower Upper Cenomanian) includes the entire Galala Formation, while the Maghra El Hadida Formation comprises all the overlying depositional sequences: DS WA 2 (upper Upper Cenomanian – Lower Turonian) reaches from the base of the Metoicoceras geslinianum Zone to the top of Wrightoceras munieri Zone, DS WA 3 and DS WA 4 comprise the Middle Turonian, while Upper Turonian sequence DS WA 5 is not complete. The stratigraphic positions of the recognized sequence 2 boundaries SB WA 1 to SB WA 5 match well with contemporaneous sequence boundaries known from Europe and elsewhere. The stacking pattern of the basic cycles and bundles of the Galala Formation (5:1) and the Maghra El Hadida Formation (4:1) strongly suggest an orbital forcing by MILANKOVITCH periodicities. The Galala Formation is composed of five 5th-order bundles which equal to ~500 kyr, each bundle equals to ~100 kyr (short eccentricity). Every bundle has five basic (6th-order) cycles, each one representing ~20 kyr (precession). Based on this precession-short eccentricity syndrome, the accumulation rate of the Galala Formation therefore accounts for about 19 cm/kyr. The rate of sea-level fall at sequence boundary SB WA 2 (equivalent to the quasi-global mid-Late Cenomanian SB Ce V) estimated is with 35 cm/kyr which can be explained only by glacio-eustasy. The Upper Cenomanian and Lower Turonian part of the Maghra El Hadida Formation is considered to equal to ~1200 kyr, based on the existence of three 4th-order bundles with an inferred duration of ~400 kyr for each bundle (long eccentricity of the MILANKOVITCH Band). Every bundle consists of four basic cycles with a duration of ~100 kyr. This means that the upper Cenomanian part of the Maghra El Hadida Formation is equivalent to ~400 kyr, while the Lower Turonian (consisting of the two upper bundles) lasted 800 kyr. This matches well with the recently proposed 785 kyr duration of the Early Turonian (SAGEMAN et al., 2006; VOIGT et al., 2008) and contradicts the 1300 kyr according to the standard time scale of GRADSTEIN et al. (2004). According to this temporal constrains, the accumulation rate of the Maghra El Hadida Formation is about 4.25 cm/kyr. In addition, based on the cyclostratigraphic analysis, the range of the Early Turonian genus Choffaticeras (HYATT) is equivalent to ~325 kyr and morphological changes within its lineage can be quantified. The macrobenthos (bivalves, gastropods, echinoids) and cephalopods of the Galala and Maghra El Hadida formations were identified and illustrated in 24 figures. The ammonite taxonomy and palaeobiogeographic distribution is discussed in detail. Four genera and eight ammonite species are recorded from Egypt for the first time. The microfloral and -faunal assemblage identified in thin sections revealed two species of dasycladalean algae, two species of udoteacean algae, five species of benthic foraminifera, and two species of crustacean microcoprolites. The six facies types of the upper Middle – Upper Cenomanian Galala Formation document largely open-lagoonal, warm water conditions, while the depositional environment of the Upper Cenomanian – Turonian Maghra El Hadida Formation (16 facies types) is suggested to range from a deep-subtidal to intertidal.
| Identifer | oai:union.ndltd.org:uni-wuerzburg.de/oai:opus.bibliothek.uni-wuerzburg.de:3309 |
| Date | January 2009 |
| Creators | Nagm, Emad Hamdy Mahmoud |
| Source Sets | University of Würzburg |
| Language | English |
| Detected Language | English |
| Type | doctoralthesis, doc-type:doctoralThesis |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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