Biostratigraphy of Upper Valanginian-Upper Aptian Balambo Formation Near Barsarin Village in Rawanduz Area, Northeastern Iraq

ABSTRACT The biostratigraphy of Balambo Formation has been investigated within a well-exposed section at the northeastern limb of Zozik anticline, near Barsarin Village, northeastern Iraq. The studied section consists mainly of marl, marly limestone and limestone. These rocks yielded moderately diversified planktonic foraminiferal fauna and radiolarian skeletons, in addition to some benthonic foraminifera and bioclast. The study is focusing on planktonic foraminiferal taxa, which are classified to ten species belonging to three genera. the stratigraphic distribution of the recorded species permits the recognition of five well defined biozones. These are in descending order: . Globigerinelloides algeriana Taxon range Zone. (part) . Globigerinelloides ferreolensis Interval Zone. . Globigerinelloides blowi Interval Zone. . Hedbergella sigali Interval Zone. . Globuligerina hoterivica Interval Zone. The planktonic foraminiferal biozones are correlated with other zonal schemes. They are considered to be extending from Late Valanginian to Late Aptian age.


INTRODUCTION
The studied section is located in northeastern limb of Zozik anticline, near Barsarin village (Fig. 1), near Rawanduz town. The midpoint of the section is at ( 36° 32ˊ 00ʺ) Latitude and (44° 40ˊ 00ʺ) Longitude within the imbricated zone of the unstable shelf according to Jassim and Buday (2006).
The Balambo Formation (Valanginian -Turonian) was first described by Wetzel in 1947 in (Bellen et al., 1959) from Sirwan Valley, near Halabja, northeastern Iraq. The type section is about 762m thick and embraces two divisions: Upper division composed of 503m of thin-bedded globigerinal limestone, passing downwards to radiolarian limestone. Lower division of 259m thin-bedded, blue ammonitiferous limestone with intercalation of olive green marls and dark blue shale.
The studied section is 188m thick and consists of alternation of thin bedded pale brownish limestone, marly limestone and pale bluish marl. The lower boundary is recognized by the sharp contact between thick dolostone bed and thinly bedded limestone and marl of Sarmord and Balambo formations respectively. The upper boundary is gradational with the Qamchuqa Formation, determinate,may be determined by the alternation beds of thinly bedded marly limestone (Balambo Formation) and massive rocks of Qamchuqa Formation.
The goal of the present research is to identify the planktonic foraminiferal speciesand establishing their biostratigraphic zones, which provided the basis for age determined of the studied section. Other microfossils such as radiolaria and benthonic foraminifera are recognized, and tabulated for general biostratigraphic distribution.

BIOSTRATIGRAPHY
The investigation of foraminiferal assemblages within the studied section revealed that planktonic foraminifera have been used for biostratigraphic analysis. This study enabled us to identify ten planktonic foraminiferal species (Fig. 2). Based on the geological time scale, which recorded by Ogg et al. (2016), the absolute age of the planktonic foraminiferal zones have been determined.
The identified planktonic foraminiferal biozones are correlated with their equivalent standard biozones in other regions of the world (Fig. 3). The studied section can be subdivided into five biozones, these biozones are described below in ascending order.

Correlation and Age Determination:
The present zone is equivalent to the Globuligerina hoterivica zone of Caron (1985), Sliter (1989), Altiner (1991) and Rojay and Altiner (1998)   Definition: The present zone was first defined by Moullade (1966) from the Barremian to Early Aptian. It represents the stratigraphic range of the index species Hedbergella sigali Moullade, from the first occurrence of Hedbergella sigali Moullade, to the first occurrence of Globigerinelloides blowi (Bolli).

Boundaries:
The lower boundary of this zone is marked by the first appearance of index species Hedbergella sigali Moullade, while the upper boundary is indicated by the first appearance of the species Globigerinelloides blowi (Bolli). Thickness: 50 m represent by samples (10 -23).

Correlation and Age Determination:
This zone is correlative to the Hedbergella sigali zone of Caron (1985), Hedbergella sigali, Hedbergella similis and Globigerinelloides duboisi zones of Sliter (1989) of Early Barremian -Early Aptian age. Also it is equivalent to the Hedbergella sigali zone of Altiner (1991) and Rojay and Altiner (1998) in same age. While it is correlative with the Hedbergella sigali/ delrioensis and Hedbergella similis zones of Hardenbol et al., (1998) and Hedbergella sigali/ delrioensis and lower part of Hedbergella similis zones of Ogg et al. (2016) in Latest Valanginian -Early late Barremian age.

3-Globigerinelloides blowi Interval Zone.
Definition: This zone was first defined by Moullade (1974) from Early Aptian, representing the interval from the first occurrence of Globigerinelloides blowi (Bolli), to the first occurrence of Globigerinelloides ferreolensis (Moullade).

Boundaries:
The lower boundary of this zone is marked by the first appearance of Globigerinelloides blowi (Bolli), whereas the upper boundary is determined by the first appearance of the species Globigerinelloides ferreolensis (Moullade).

Correlation and Age Determination:
This zone is correlated with the Globigerinelloides blowi and lower part of Schakoina cabri zones of Caron (1985),