Calcareous Nannofossil Biostratigraphy and Stage Boundaries of the Santonian-Eocene Successions in Wadi El Mizeira Northeastern Sinai, Egypt

The stratigraphic successions exposed in Wadi El Mizeira have been dated through the analysis of the calcareous nannofossil assemblages. The results of this study indicate that the successions comprise the Santonian-Late Maastrichtian (Sudr Formation), the Paleocene (Esna Formation) and the Early Eocene (Thebes Formation). The following biozones were recognized: Late Santonian, CC16 Zone; Late Santonian/Early Campanian, CC17 Zone; Early Campanian, Aspidolithus parcus Zone (CC18) Zone; Late Maastrichtian, CC25c Zone; Early Paleocene (Late Danian), NP3 Zone and NP4 Zone; Late Paleocene (Thanethian-Selandian), NP5 Zone; Early Eocene, NP9b Zone, NP10a Zone, NP11 Zone, NP12 Zone and NP14 Zone. Several stratigraphic hiatus were recorded in the studied interval including the absence of Cretaceous nannofossil Zones CC19 to CC25b and CC26 as well as the early Paleocene Zones NP1 and NP2 and probably the basal part of Zone NP3, in addition to the absence of the Zones NP6 and NP7/8. These hiatus may be attributed to environmental conditions, structural activity and/or post depositional processes. This work repre-sents the first attempt to evaluate the nannofossil taxa of the Wadi El Mizeira, Northeastern Sinai.


Material and Methods
Calcareous nannofossil analyses have been performed for 129 samples taken on average every 1 m; with high resolution sampling adopted close to key biohorizons. Calcareous nannofossils are generally abundant and well diversified in the studied sequence, with a good degree of preservation.
Samples were processed by smear slide preparation from raw sediment samples [32]. All samples were prepared similarly to insure uniformity in the distribution and to minimize bias. Relative species abundances were determined by counting a population of at least 300 specimens along a random traverse with a light microscope at a magnification of about 1250×. Rare species were searched in two additional traverses across the slide to assure that no stratigraphic marker was overlooked. Total nannofossil abundance was calculated as the total number of specimens counted per number of fields of view (FOV) traversed.
Preservation was estimated as Good (G) for nannofossil specimens exhibiting little or no dissolution; Moderate (M) for moderate dissolution and/or overgrowth and Poor (P) for strong dissolution and/or over growth. A fully referenced taxonomic listing appears in [33]. The stratigraphic distribution, preservation and abundance of the nannofossils are provided in a distribution charts (Figures 3-5).

Lithostratigraphy
The Santonian-Eocene succession exposed at Wadi Mizeira is generally composed of carbonate sediments. Lithostratigraphically, the succession is subdivided from base to top into; the Sudr, the Esna and the Thebes Formations (Figure 2).

Sudr Formation
The term Sudr Chalk was originally introduced by [34] for the chalky limestone succession exposed at Wadi Sudr, west central Sinai. In the present study the Sudr Formation is exposed along the floor of Wadi El Mizeira and the foot-slope of Senaf El Mizeira Ridge. It is predominantly composed of chalk, chalky limestone with shale intercalations, siliceous limestone, chert and bioclastics.

Esna Shale
The term Esna Shale was first introduced by [35]. In the study area the Esna Formation (24 m thick) overlies the Upper Cretaceous Sudr Chalk and underlies the Lower Eocene Thebes Formation. At Wadi El Mizeira, the Esna Shale is predominantly composed of greenish gray shale, along Senaf El Mizeira Ridge the formation displays significant change in lithology and is made up of greenish hemipelagic marls, chalky with few grayish green shale intercalations in the middle part. The calcareous nannofossil content points to Late Maastrichtian to Early Eocene age for the Esna Shale.

Thebes Formation
The term Thebes Formation has been introduced by [36] to describe the limestone unit with chert bands overlying the Esna Shale in many parts of Egypt. At Wadi El Mizeira the Thebes Formation is well exposed forming the prominent cliff of Senaf El Mizeira Ridge. It is 65 m thick and composes of marls, marly chalk, and chalky limestone with chert bands and nodules. The formation exhibits lateral lithologic change and dominates with white chalk, siliceous limestone and some shale. The nannofossils content indicates an Early Eocene age for the Thebes Formation.

Biostratigraphy
In the present study, nannofossil biostratigraphic framework is applied according to the biozonation scheme of [37] [38] for the late Cretaceous and Early Eocene, respectively. A total of 109 nannofossil species were identified from the Upper Maastrichtian, Paleocene and Lower Eocene rocks of the studied section. Their stratigraphic distributions are shown on Figures 3-5 and some representative calcareous nannofossil species are illustrated in Plates 1-3. Biostratigraphic subdivisions in the study area based on the calcareous nannofossils have been established and correlated with the standard biozones recognized in Egypt ( Table 1 and Table 2) and other parts of the world.
Twelve nannofossil zones are identified in the studied area (Santonian to Early Eocene), based on the first and last occurrences (FOs, LOs) of the marker species. The Upper Cretaceous rocks comprise CC16, CC17, CC18 and CC25c. The Lower Paleogene rocks comprise eight zones from NP3, NP4, NP5, NP9b, NP10a, NP11, NP12 and NP14. The recognized biozones and subzones and the correlation between them are discussed below.
Author: [37]. Age: Late Santonian. Thickness: 10 m; equivalent to the lower part of the Lower Chalk Member.  Assemblage: In general, the identified taxa in this zone are rare, but diversity is relatively high. In addition to the nominate taxon, this interval includes W. béarnaise and Q. Gartner among many other species.
Remarks: The first occurrence of L. cayeuxii is a very reliable event. Zone CC16 is equivalent to the upper part of Zone UC11 and the UC12-13 zones of [39].

Calculites obscurus Zone (CC17)
Definition: This zone covers the interval from the FO of Calculites obscurus to the FO of Aspidolithus parcus.
Author: [37]. Age: Late Santonian/Early Campanian. Thickness: 10 m; equivalent to the middle part of the Lower Chalk Member. Assemblage: The assemblage of this interval is similar to that of the underlying CC16 Zone, with the addition of rare occurrences of the nominate taxon.
Remarks: According to [39], the first occurrence of Arkhangelskiella cymbiformis defines the base of Subzone UC13a of Early Campanian age. In the present study, rare specimens of A. cymbiformis have been observed in the Late Santonian (CC17), indicating that the range of the A. cymbiformis extends down into the Santonian. The FO of Aspidolithus parcus is an event that has been used for zonation and coincides reasonably well with the Santonian/Campanian boundary.

Aspidolithus parcus Zone (CC18)
Definition: Interval from the FO of Aspidolithus parcus to the LO of Marthasterites furactus.
Author: [37]. Age: Early Campanian. Thickness: 15 m; equivalent to the upper part of the Lower Chalk Member. Assemblage: Similar to the nannofossil assemblage from the CC17 Zone, with the addition of the nominate taxon Aspidolithus parcus.
Remarks: The upper boundary of the CC18 not well defined due to the absence of M. furactus. The first occurrence of B. parcaparca is a reliable global stratigraphic datum. Numerous workers have indicated that this datum marks the base of the CC18 Zone and serves as an approximation to the Santonian/Campanian boundary [16] [39]- [46]. The current study maintains this approximation because it can be correlated globally at all palaeolatitudes and in all palaeobiogeographical provinces [39] [46].

Micula murus Zone (CC25c)
Definition: It is defined as the interval from the FO of Micula murus to the FO of N. frequens.

Definition: FO of Chiasmolithus danicus to FO of Ellipsolithus macellus.
Author: [51]. Age: Early Paleocene (Late Danian). Thickness: 2 m; equivalent to the lower part of the Esna Formation. Assemblage: It is dominated besides the marker species by Ericsonia cava and relatively rare to very rare occurrences of Cruciplacolithus primus, Coccolithus pelagicus and Thoracosphaera operculata.

Remarks:
The upper boundary is defined by the FO of Ellipsolithus macellus, which is easy to recognize in well preserved material. In poorly preserved assemblages and/or in high latitudes, this species is missing but the zonal boundary can be identified by substitute evidence such as the absence of species of Neochiastozygus (N. saepes, N. perfectus), Chiasmolithus bidens and Prinsius martinii, all forms which are used to subdivide the high latitude Danian sections. Chiasmolithus danicus Zone is correlated to CP2 of [52].
Author: [51]. Remarks: NP10a has not been found in several sequences where it might be expected to present, probably due to the absence of the genus Tribrachiatus in certain areas for ecological reasons. When present, however, it is very easy to recognize NP 10 by the presence of T. bramlettei and/or T. contortus. Detailed studies of T. bramlettei, T. contortus and T. orthostylus were published by [50] [68]. In overgrown material both species have a tendency to look like ordinary calcite rhombohedron. Many species continue from the Paleocene into the Eocene, including some species, the FO of which is used as zonal markers in the Paleocene, i.e. Ellipsolithus macellus and Discoaster multiradiatus. Fasciculithus is only found in the lowermost part of NP 10 according to [50] and it's LO is used by other authors to define the Paleocene/Eocene boundary in sections where Tribrachiatus is very rare. It is correlated with Tribrachiatus contortus Subzone, CP9a of [52].
[69] subdivided the NP10 Zone into four subzones: NP10a (biostratigraphic interval from FO of T. bramlettei to the FO of T. digitalis); NP10b (total range of T. digitalis); NP10c (from LO of T. digitalis to the FO of T. contortus) and NP10d (total range of T. contortus). None of these subdivisions can be differentiated in the current study. This may be attributed to large space of sampling and/or small hiatus.
Authors: [53]. Age: Early Eocene (Ypresian). Thickness: 17 m; equivalent to the middle part of the Thebes Formation. Assemblage: Discoaster distinctus, Sphenolithus editus, S. radians and S. conspicus appear within NP11. Remarks: Tribrachiatus orthostylus, a species which evolved from T. contortus appears near the NP10/NP11 boundary and evolves from a form with a slight bifurcation at the end of three arms to a form with three simple arms [50] [68] within NP11.
The top of NP10 is generally defined by the LO of Tribrachiatus contortus [32] [50] [56]. Alternatively, the FO of Tribrachiatus orthostylus takes place shortly before the LO of Tribrachiatus contortus and can be used as an approximation of the NP10/NP11 boundary [32]. We have tentatively placed that boundary at the FO of Tribrachiatus orthostylus so that our Zone NP11 may be a bit more reduced in reality than it appears. In this context, Sphenolithus radians record its FO at the base of NP11. It is correlated with Discoaster binodosus subzone, CP9b of [52].
In several sections in Egypt [70]- [72], the T. orthostylus sometimes co-occurs with T. contortus at the extreme top part of zone NP10 or appears directly above the last occurrence of the latter, i.e., at the very base of Zone NP11. T. orthostylus was originally described from the Lower Eocene rocks of California and Austria [73]. In Egypt this species is predominate and characterize the Lower Eocene sediments. The stratigraphic value and occurrence of Tribrachiatus orthostylus as a guide fossil have already been proved [50]. Remarks: D. lodoensis overlaps with D. sublodoensis in the lower part of the zone and the FO of R. inflata is used to subdivide the zone into subzones CP12a and CP12b. The top of the zone has sometimes been approximated by using the FO of any species of Nannotetrina instead of N. fulgens, which can be very rare in some sections where other species of Nannotetrina are found consistently to be present. In these cases, R. inflate has been found overlap with Nannotetrina, Sphenolithus furcatolithoides and S. spiniger appear in the upper part of NP14. It is equivalent to Discoaster sublodoensis Zone, CP12 of [52], with a different definition for the top of the zone, when the LO of R. inflata is used as a substitute marker.

Stage Boundaries
The boundaries between stages are delineated based on the calcareous nannofossil datum events as follow: 1) Santonian/Campanian boundary In the present samples the Santonian-Campanian boundary is marked by the last occurrences of Calculites obscures and the first occurrences of Aspidolithus parcus.
2) Campanian/Maastrichtian boundary The Campanian/Maastrichtian boundary is not clear due to the absence of nannofossil assemblages as a result of post depositional changes.
3) The Cretaceous/Paleogene (K/Pg) boundary The Cretaceous/Tertiary boundary is marked by the absence of the Late Cretaceous nannofossil Zone; CC26, as well as the Early Paleocene nannofossil biozones, NP1 and NP2 and probably the lower part of NP3. 4) Selandian/Thanethian boundary A major hiatus is suggested at the Selandian/Thanethian boundary as indicated by absence of the nannofossil biozones; NP6, NP7 and NP8. On the other hand, the latest Paleocene calcareous nannofossil subzone NP9a is missed. 5) Paleocene/Eocene boundary The Paleocene/Eocene boundary in the studied area is correlative to the base of the Discoaster multiradiatus NP9b Subzone. On the other hand, the latest Paleocene calcareous nannofossil subzone NP9a is missed, suggesting a minor hiatus at the P/E boundary.
Several stratigraphic hiatus were recorded in the studied section. The Cretaceous/Paleogene boundary is marked by the absence of the Late Cretaceous nannofossil Zone; CC26, as well as the Early Paleocene nannofossil biozones, NP1 and NP2 and probably the lower part of NP3. A major hiatus is suggested at the Selandian/ Thanethian boundary as indicated by absence of the nannofossil biozones; NP6, NP7 and NP8. On the other hand, the latest Paleocene calcareous nannofossil subzone NP9a is missed, suggesting a minor hiatus at the P/E boundary.