International Journal of Geosciences, 2013, 4, 985-992
http://dx.doi.org/10.4236/ijg.2013.46091 Published Online August 2013 (http://www.scirp.org/journal/ijg)
Late Maastrichtian Calcareous Nannofossil
Biostratigraphy and Paleoecology of the Tamera Well,
Siwa Area, Western Desert, Egypt
Esam Zahran
Geology Department, Faculty of Science, Damanhour University, Damanhour, Egypt
Received April 25, 2013; revised June 18, 2013; accepted July 15, 2013
Copyright © 2013 Esam Zahran. This is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
The upper Cretaceous interval of the Khoman Formation in the Tamera well, Siwa area, Western Desert of Egypt was
studied for the first time on the basis of calcareous nannofossils. Twenty-one nannofossil species were identified from
this interval. The study interval includes the Micula murus Zone, which is precisely dated as Late Maastrichtian age.
The Micula murus Zone includes besides the marker species: Micula decussata, Watznaueria barnesae, Arkhangelsktella
cymbiformis and relatively rare occurrences of Eiffellithus turrisieffellii, Cribro sphaerella ehrenbergii, Thoracosphaera
operculata and Braarudosphaera bigelowii. The latest Maastrichtian Micula prinsi Zone is missing, and an unconform-
ity surface is detected in this well. The high abundance of Micula decussata is indicative of very low surface productiv-
ity and high-stress environmental conditions. Several nannofossil species are recognized as cool water indicators such
as Micula decussata, and Arkhangelsktella cymbiformis.
Keywords: Formation; Nannofossils; Micula Prins; Late Maastrichtian
1. Introduction
The Siwa Oasis is one of the series of depressions that
lies in the shadow of the great Marmarica Plateau. It has
an east-west direction with its center about 300 km south
of the Mediterranean coast and approximately 65 km east
of the Libyan border. To the north it is bounded by an
escarpment which rises to about 100 m above the floor of
the depression. To the south of the depression a sand
dune belt exists trending in a northwest-southeast direc-
tion. The depression extends from Lake Zeitun in the east
to Lake Maraqi in the west by a total length of 76 km and
a maximum width of 20 km. The Siwa area is approxi-
mately 980 km2. The lower parts of the depression floor
average about 10 to 18 m below sea level.
The northern escarpment mainly trends in an east-west
direction, but between longitudes 26˚00' and 26˚30', it
extends southward for about 30 km into a promontory
that forms the dividing highland between the Siwa and
Qattara de pressions.
The Siwa area was subjected to serve tectonic events
due to its location in the Unstable Shelf area. From the
Paleozoic until the Cenozoic, the area was subjected to
faulting and many of the faults were being rejuvenated
from time to time [1].
The study well is located in northwestern of the Siwa
Oasis (Figure 1). The aim was to investigate and assign
the calcareous nannofossils of the upper part of the Kho-
man Formation (Maastrichtian) in the subsurface rocks of
the Western Desert of Egyp t.
2. Stratigraphy
The Cretaceous-Tertiary sedimentary succession, in the
studied well at Siwa Oasis, is generally subdivided into
seven lithostratigraphic units; three of th em are related to
the Neogene, namely Marmarica Formation (Middle Mio-
cene), Mamura Formation (Lower Miocene) and Apol-
lonia Formation (Paleocene-Middle Eocene); and the rest
to the Cretaceous, namely Khoman (Chalk) Formation
(Campanian-Maastrichtian), Abu Roash Formation (Tu-
ronian), Bahariya Formation (Cenomanian) and Burg El
Arab Formation (Early Cretaceous). The Khoman For ma-
tion consists of chalky limestone, partly argillaceous,
with few chert bands and containing few sand streaks at
the base. The chalky limestone points to open marine
sedimentation as a result to Upper Cretaceous marine
transgression [2].
C
opyright © 2013 SciRes. IJG

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986
0200 400Km
Kharga
Dakhla
Farafra Qena
CAIRO
MEDITERRANEAN SEA
SINAI
RED
SEA
N
25 2729 31 3335 37
23
25
27
29
31
Tamera Well
Figure 1. Location map of Tamera Well, Siwa Basin, Western Desert, Egypt.
In the southern part of the Siwa area, the Khoman
Chalk is unconformably underlain by the Turonian rocks
of the Abu Roash Formation that composed of coarse
grained, subangular, fairly sorted sandstone. At Siwa Oa-
sis, the formation is built up typically of limestone and
dolomite. The limestone is finely crystalline, highly frac-
tured, partly vuggy, and occasio nally with calcite crystals
in cavities.
In the north and northeastern parts of the Siwa area,
the Khoman Formation is composed of soft and slightly
calcareous shale that intercalated with argillaceous lime-
stone. In such areas, the lithology reflects a near-shore to
very shallow shelf environment, with a subsequent effi-
cient fresh-water circulation [2].
In the study well, the upper part of the Khoman For-
mation consists mainly of shales, shaly limestone, and
sandstone with shale intercalations (Figure 2).
3. Material and Methodology
A total of 56 samples were obtained from the upper part
of the Khoman Formation of the Tamera well that lo-
cated in the northwestern part of the Siwa area, Western
Desert, Egypt. The samples were studied for their cal-
careous nannofossils. The relative abundances of the cal-
careous nannofossils were estimated from simple smear-
slides following the methodology described in [3]. These
slides were viewed at 1250× magnification, using an oil
immersion objective lens on an Olympus light-micro-
scope that equipped with automatic camera.
The relative abundances of the species were estimated
over three traverses of each slide. The abbreviations used
in this study are: few (F = 20 - 40 specimens), rare (R =
10 - 20 specimens), and very rare (VR = <10 specimens).
Preservation varies between moderate to poor: moder-
ate (M = virtually all specimens are easily identifiable
without secondary calcite overgrowth and/or calcite dis-
solution) and poor (P = depleted assemblage due to cal-
cite dissolution and/or an appreciable proportion of speci-
mens are difficult to id entify due to calcite dissolution or
secondary overgrowth.

E. ZAHRAN 987
1200
1180
1160
1140
1120
1100
1080
1060
1040
1020
1000
980
960
940
920
900
880
860
840
820
800
00 m
10 m
20 m
30 m
5
1
10
25
30
35
40
45
50
56
Sandstone,
white with
intercalations
ofshales and
sandy shales
Grey Shale
Shaly
Limestone
DescriptionL ithology
Thick-
ness
Sample
No.
Rock Units
Age
15
20
Figure 2. Stratigraphic section of Tamera Well, Siwa Basin, Western Desert, Egypt.
4. Nannofossil Biostratigraphy
In the present study, the biozonation scheme of [4], as
modified by [5] was applied for the Maastrichtian sedi-
ments in the present stud y.
The distribution of the identified nannofossil taxa is
shown in Figure 3, some representative nannofossil taxa
are illustrated in Plates 1 and 2.
In the present study only one calcareous nannofossil
biozone (Micula murus Zone) is identified. Details de-
scription of that zone is given in Figure 3.
Micula murus Zone:
The Micula murus Zone was proposed by [6] and modi-
fied by [7]. It is identified from the lowest occurrence
(LO) of Micula murus to the LO of Micula prinsi.
It is worth mentioning that the latest Maastrichtian in-
terval is missing due to the absence of the calcareous
nannofossil Micula prinsi Zone. The Micula murus Zone
correlates with the lower part of the Nephro lithus fre-
quens Zone [8] modified by [9 ] and th e CC25c of [4]. [4 ]
suggested a subdivision of CC25 Zone by the lowest

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988
Figure 3. Distribution chart of the identified calcareous nannofossils in Tamera Well, Western Desert.
occurrence of A. cymbifo rmis and lowest occurrence of L.
quadratus. Figure 4 shows a comparison between the
used zonal scheme and that applied by [4]. This zone is
dominated besides the marker species by Micula decus-
sata, Watznaueria barnesae, Arkhangelsktella cymbifor mi s
and relatively rare occurrences of Eiffellithus turrisieffel-
lii, Cribrosphaerella ehrenbergii, Thoracosphaera op-
erculata and Braarudosphaera bigelowii.
5. Paleoecology
The Micula decussata is a major component of the Late
Cretaceous nannofossil assemblage. Few authors reported
very high abundances of this species in well-preserved
taxa from different paleoecological conditions from dif-
ferent parts of the world. Several authors suggested that
this taxon might have preferred cooler temperatures [10-
12]. However, biogeographic studies of [10,13-16] s how ed
that this taxon is clearly cosmopolitan and can reach as
far as 80% in both tropical and sub-tropical assemblages.
[17,18] interpreted the high abundances of Micula de-
cussata as indicative of very low surface productivity
and high-stress environmental cond itions.
Several nannofossil species are recognized as cool wa-
ter indicators. These are mainly Arkhangelsktella cymbi-
formis, [10,13] referred Arkhangelsktella cymbiformis as
a high-latitude taxon. However, [16] shows that this spe-
cies is common down into tropical paleolatitudes, al-
though it prefers high-latitudes.
Micula murus is clearly restricted to warm tropical
waters and is totally absent from the high-latitude areas,
all along its biostratigraphical range [13,16,19,20]. Thus,
it could be considered as a good warm-water indicator.
Some nannofossil species are good indicators of sur-
face water fertility. Watznaueria barnesiae is a cosmo-
politan species which is generally dominant in tropical
latitudes and only common in high-latitude sites. Thus,
several authors used it as a warm-water indicator [11,
12,20]. However, several studies showed that Watznau-
eria barnesiae is mainly a low-nutrient indicator [21-25].
Other common taxa of Maastrichtian assemblages such
as Cribrosphaerella ehrenbergii do not show any latitu-
dinal preferences nor seem to be related to surface water
fertility.
A summary of nannofossil temperature and fertility
indices is presented in Table 1.
In the investigated samples of the present well, the
dominance of cool water nannofossil assemblages (Micula
decussata, and Arkhangelsktella cymbiformis) may indi-
cate cooling conditions that prevailed during the deposi-
tion of the Khom an Formation.

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Age [4] Bioevents Present study
CC26 LO. M. pri nsii
LO. N. frequens, C. kamptneri Hiatus
Hiatus
Late CC25 c-
b-
a-
LO. M. mur us
LO. L. quadratus
HO. R. Levis Hiatus
Maastrichtian
Middle CC24 HO. T. phacelosus, Q. trifidum Hiatus
Figure 4. Maastrichtian calcareous nannofossil events in the studied well and that of [4]. (HO. Highest Occurrences, LO.
Lowest Occurrences).
Table 1. Calcareous nannofossil paleoecological indices considered in this study (compiled from different sources).
Fertility indices Temperature indices
High-fertili ty taxa not detected Cool water taxa Arkhangelsktella cymbiformis
Mid-fertility taxa not detected
Low-fertility taxa Watznaueria barnesiae Warm- water taxa Micula murus & Watznaueria barnesiae
6. Discussion and Conclusions
This study is considered the first attempt to study the
nannofossil taxa of the Kho man Formation in th e Tamera
well, Western Desert of Egypt. Twenty-one nannofossil
species have been identified in this study. The study in-
terval (the upper part of the Khoman Formation) includes
the Micula murus Zone, which is precisely dated by means
of its calcareous nannofossils as Late Maastrichtian age.
The Micula prinsi Zone of the latest Maastrichtian age is
missed in the stud y well.
The Watznaueria barnesiae and Micula decussata are
considered as two high solution-resistant species and are
generally used to test the preservation degree of the as-
semblage [26,27].
Throughout the Cretaceous, the Watznaueria barne-
siae is rare to frequent in the study samples and consid-
ered to be a eurytopic cosmopolitan species as previously
mentioned by [28 - 30] .
In the present core samples, the dissolution resistsnt
Watznaueria barnesae is rare to frequent and it is con-
sidered to be a good index taxon to indicate alteration of
the assemblages.
Watznaueria barnesae seems to also be more charac-
teristic of low-mid latitude areas, being rare in high-lati-
tude sites.
It is probable that differing paleoceanographic condi-
tions (i.e. paleotemperature, paleosalinity, fertility fluc-
tuations) affected the relative abundant of Watznaueria
barnesae.
On the other hand, the distribution of Watznaueria ba-
rnesiae seems to be a good indicator of surface water
fertility changes and recently have been used as nanno-
fossil fertility index [31,32]. During the Maastrichtian,
this species is generally rare or absent from high-latitude
assemblages [33-35]. Moreover, it seems that tempera-
ture might also have played a significant role in the dis-
tribution of this taxon (Watznaueria barnesiae).
The Micula murus is a rare component of the Maas-
trichtian calcareous nannofossil assemblages in th e study
well. The dominance of cold nannossil assemblages in
the studied upper Maastrichtian interval may suggest cool
surface water paleotemperature during this time.
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Plate 1. All figures ×1250. (1, 2, 3): Watznaueria barnesae [36], 1 sample No. 1, 2 sample No. 34, 3 sample No. 43; (4):
Cyclagelosphaera reinhardtii [37], sample No. 48; (5, 6, 7): Lucianorhabdus cayeuxii [38], 5 sample No. 21, 6 sample No. 22, 7
sample No. 23; (8, 9, 10): Micula decussata [39], 8 sample No. 34, 9 sample No. 43, 10 sample No. 48; (11): Microrhabdulus
decoratus [38], sample No. 22; (12, 13, 14): Arkhangelskiella cymbiformis [39], 12 sample No. 19, 13 sample No. 22, 14 sample
No. 50; (15): Calculites obscurus [38], sample No. 22; (16): Cribrosphaerella ehrenbergii [40], sample No. 48.

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Plate 2. All figures ×1250. (1): Cyclagelosphaera reinhardtii [37], sample No. 1; (2, 3): Watznaueria barnesae [36], 2 sample No.
21, 3 sample No. 48; (4, 5): Arkhangelskiella cymbiformis [39], 4 sample No. 43, 5 sample No. 50; (6): Eiffellithus turrisiefelii
[41], sample No. 37; (7, 8): Micula murus [42], 7 sample No. 1, 8 sample No. 37; (9, 13): Micula decussata [39], 9 sample No. 21,
13 sample No. 34; (10): Glaukolithus diplogrammus [41], sample No. 19; (11): Rhagodiscus angustus [43], sample No. 22; (12):
Micrantholithus vesper [41], sample No. 43; (14): Lucianorhabdus cayeuxii [38], sample No. 21; (15): Calculites obscurus [38],
sample No. 22; (16): Zeugrhabdotus pseudanthophorus [44], sample No. 23.