The Upper Cretaceous Kometan and Shiranish formations of the Kurdistan region, NE Iraq, yield diverse planktonic foraminiferal assemblages, with a total of 93 species, which enable recognition of nine biozones and two subzones spanning the early Turonian to late early Maastrichtian. Sequential changes in planktonic foraminiferal assemblages map discrete intervals within the Kometan and Shiranish formations that suggest dominantly warm, nutrient-poor marine surface and near-surface conditions during the mid-Turonian to late Coniacian, latest Santonian, and late Campanian, and cooler more nutrient-rich surface and near-surface waters in the early Turonian, early to late Santonian, early Campanian and early Maastrichtian. These intervals appear to correlate with changes in water masses from other regions of the Cretaceous palaeotropics, and with a phase of global, early Maastrichtian climate cooling. The major intra-Campanian truncation surface between the Kometan and Shiranish formations, recognized from the foraminiferal biostratigraphy, represents a lowstand that appears to equate with regional tectonics and ophiolite obduction across the NE margin of the Arabian Plate.
The evolution of planktonic foraminifera from the Turonian to the early Maastrichtian is characterized by increasing species richness and morphological complexity (
To analyse these evolutionary patterns in an Arabian context, the early Turonian to early Maastrichtian planktonic foraminiferal assemblages from two localities in the Kurdistan region, NE Iraq (
Geological context for the Azmer (1) and Dokan (2) sections of the Kometan and Shiranish formations in the Kurdistan region of NE Iraq (map after
Two sections, at Dokan (35°56’15"N, 44°57’21"E) and Azmer (35°37’30"N, 45°31’45"E; see
Lithostratigraphy and chronostratigraphy for the Upper Cretaceous succession in Iraq and neighbouring Arabian countries. The lower and upper boundary and the distribution of the Kometan and Shiranish formations are indicated in light grey. The earliest Turonian and the mid to early late Campanian age unconformities are shown in thick zigzag lines (modified from
Some 411 samples, spaced
All samples from the Kometan Formation were thin sections. For the samples from the Shiranish Formation, both thin sections and a freeze–thaw method for disaggregating the rock have been used (
The Upper Cretaceous (early Turonian to early Maastrichtian age) strata of NE Iraq comprise two marine-deposited formations: the pelagic limestone of the Kometan Formation (early Turonian–early Campanian) and the marly limestones and marlstones of the Shiranish Formation (late Campanian–early Maastrichtian).
The type section of the Kometan Formation was first described in an unpublished report by H. V. Dunnington (1953,
The Kometan Formation has thicknesses of approximately 158 and 96.5 m in the Dokan and Azmer areas, respectively. In the Dokan area the formation is composed of well-bedded, light grey or white limestone with common chert nodules (
Field photos of the Kometan and Shiranish formations in the Dokan and the Azmer areas, NE Iraq. (
The Shiranish Formation was first defined in an unpublished report by F. R. S. Henson (1940,
The Shiranish Formation is well exposed in the localities studied and is about 260 and 144 m thick in the Dokan and the Azmer areas, respectively. In the Dokan area there is a glauconitic pebbly sandstone bed of around 0.5 m at the base of the ‘lower unit’ that may indicate a very slow rate of deposition or period of non-deposition (
Some 93 planktonic foraminiferal species belonging to 23 genera have been identified in the Kometan and the Shiranish formations during the present study (
This biozone is defined by the lowest and highest occurrences (LO, HO) of
Planktonic foraminiferal biozonation for the Kometan Formation in the Kurdistan region, NE Iraq.
Correlation of the Upper Cretaceous planktonic foraminiferal biozonation of the Kurdistan region, NE Iraq, with the biozonation for other regions of the Middle East, North Africa and the Mediterranean. The earlier definitions of some biozones (see text) led to significant differences in the chronostratigraphic position of the base of the H. helvetica, M. schneegansi, D. primitiva, D. concavata, D. asymetrica and G. gansseri biozones in some schemes. Therefore, the apparent diachroneity of these zonal boundaries is largely an artefact of the use of different correlations in different studies. The biozonation of this study is correlated with the standard biozonation of
Stratigraphic ranges of planktonic foraminifera for the lower Turonian to lower Campanian Kometan Formation in the Dokan section. For ranges in the overlying Shiranish Formation, see
Stratigraphic ranges of planktonic foraminifera for the lower Turonian to lower Campanian Kometan Formation in the Azmer section. For ranges in the overlying Shiranish Formation, see
This is a partial range zone between the HO of
This biozone is an interval zone between the LO of
In NE Iraq this biozone is defined as an interval zone between the LO of
The lower and upper boundaries of this biozone are marked by the LO and HO of
This is a partial range zone, recognized in NE Iraq from the HO of
Although some authors (e.g.
This is an interval zone from the LO of the eponymous species to the LO of
Planktonic foraminiferal biozonation for the Shiranish Formation in the Kurdistan region, NE Iraq. The LO of
Stratigraphic ranges of planktonic foraminifera for the upper Campanian to upper lower Maastrichtian Shiranish Formation in the Dokan section.
Stratigraphic ranges and distribution of planktonic foraminifera for the upper Campanian to lower Maastrichtian Shiranish Formation in the Azmer section.
This is an interval zone from the LO of the eponymous species to the LO of
This is an interval zone defined by the LO of
The C. contusa biozone is represented through nearly 10 m of fossiliferous strata in the Dokan section, from sample numbers DSH-115 to DSH-120 (
Succeeding foraminiferal biozones cannot be recognized in NE Iraq, due to the absence of planktonic foraminifera in a rapidly shallowing marine succession. This is indicated by the presence of a massive bed of marly limestone about 1 m thick, bearing a mass of shallow-marine rudist bivalves near the contact with the overlying Tanjero Formation.
The unconformity between the Kometan and Shiranish formations is demarcated by
Planktonic foraminifera are widely used for palaeoceanographic reconstruction and to provide estimates of past sea surface temperatures for the calibration of General Circulation Models of palaeoclimate (e.g.
Based on the known palaeo-latitudinal and environmental distribution of Cretaceous planktonic foraminifera, and possible links with overall morphology, three major groups have been identified (following
Scheme showing suggested planktonic foraminifera reproductive strategy. The r-, k- or r/k-strategists are identified according to morphology (for methodology, see
Nutrient-rich/eutrophic Cretaceous marine environments, and also those subject to environmental instability have been interpreted to favour ‘r-selected opportunists’ (
Low-nutrient/oligotrophic Cretaceous marine environments that are also stable may be indicated by ‘k-selected specialists’ (
Between these end-members, foraminifera tolerant of Cretaceous mesotrophic environments exhibit a range of strategies and are termed ‘r/k intermediates’ and have been further subdivided into two subgroups: Subgroup 1 are the more k-selected r/k intermediates and include those with high trochospires, hemispherical chambers with marginal keel(s), flaring heterohelicids with more than two chambers per row, and medium-sized heterohelicids with chambers arranged from biserial to annular; these have been interpreted as occupying the oligotrophic portion of the mesotrophic spectrum (
The NE Iraqi sector of the Cretaceous Tethys Ocean represents tropical waters in an epicontinental sea setting. Interpreted water depths for the Kometan Formation are estimated at
Relative abundance and species diversity for planktonic foraminifera in the Kometan Formation. Numbers in the right-hand column indicate five assemblages that are distinguished on the basis of abundance of particular foraminifera displaying reproductive strategies interpreted as r-, k- or r/k-intermediate (for these groups, see
Relative abundance and species diversity for planktonic foraminifera in the Shiranish Formation. Numbers in the right-hand column idicate two assemblages that are distinguished on the basis of abundance of particular foraminifera interpreted as displaying r-, k- or r/k-intermediate strategies (for these groups, see
Planktonic foraminiferal Assemblage 1 is present through the basal part of the Kometan Formation, and occurs through an interval equivalent to the early Turonian Helvetoglobotruncana helvetica biozone. This assemblage is numerically dominated by simple test morphotypes, particularly those interpreted as r-strategists, such as species of
Based on analyses of stable oxygen and carbon isotopes from foraminiferal tests,
Species of whiteinellids are interpreted as taxa with a high tolerance toward eutrophic environments (
Planktonic foraminiferal Assemblage 2 is present in the Kometan Formation through the interval of the Marginotruncana schneegansi biozone, to the top of the Dicarinella concavata biozone, and this is equivalent to the mid-Turonian to late Coniacian time interval. Assemblage 2 suggests warmer, nutrient-poor waters relative to the preceding interval, with
Foraminiferal Assemblage 2 is also characterized by more k-selected r/k intermediates, such as species of
Planktonic foraminiferal Assemblage 3 of the Kometan Formation occurs through the interval of the lower and mid part of the Dicarinella asymetrica biozone, equivalent to the early to late Santonian time interval. The foraminiferal assemblages are characterized by a decrease in numbers of species interpreted as k-strategists, especially a rapid decline in the number of
Based on evaluation of stable isotope data from the carbonate test (
Assemblage 3 occupies an interval of time that is equivalent to the Santonian faunal turnover (
Planktonic foraminiferal Assemblage 4 of the Kometan Formation occurs through the interval of the top of the Dicarinella asymetrica biozone, and represents the latest Santonian time interval. The interval is associated with the disappearance of
The progressive diversification and increasing abundance of globotruncanids through the latest Santonian, seen also in the Kometan Formation, probably relates to an increase in tropical SSTs (
Planktonic foraminiferal Assemblage 5 occurs through the uppermost part of the Kometan Formation and spans the Globotruncanita elevata biozone, being time-equivalent to the early Campanian. The numerically dominant planktonic foraminifera are heterohelicids and globotruncanids (
This interval of the Kometan Formation is also characterized by abundant benthonic
Planktonic foraminiferal Assemblage 6 occurs through the lower part of the Shiranish Formation immediately post-dating the mid-Campanian unconformity, and represents the interval of the Globotruncana aegyptiaca biozone to the lower part of the Gansserina gansseri biozone (time-equivalent to the late and latest Campanian). The numerically dominant planktonic foraminiferal taxa are those interpreted as k-strategists (
Within those taxa interpreted as r/k intermediates, species of the genera
Based on stable isotopic analyses of a range of planktonic foraminifer tests, including globotruncanids (
Benthonic foraminiferal assemblages in this interval suggest gradual deepening of the marine basin throughout the late Campanian (Globotruncana aegyptiaca biozone). Common benthonic foraminifera are represented by calcareous species of
Planktonic foraminiferal Assemblage 7 occurs through the middle and upper part of the Shiranish Formation, being the interval of the upper part of the Gansserina gansseri biozone and the Contusotruncana contusa biozones (early Maastrichtian age).
Towards the top of this interval the percentage abundance of globotruncanids decreases sequentially until all foraminifera disappear (
Within heterohelicids from this interval of the Shiranish Formation the most abundant species are small biserial forms (
This interval is also characterized by abundant thin-walled
The temperature decrease signalled by the foraminifera of the Shiranish Formation in this interval is consistent with global models that suggest a marked phase of global cooling during the early Maastrichtian (
Benthonic foraminiferal assemblages in this interval suggest a shallowing trend for the Shiranish Formation in the early Maastrichtian. In the upper part of the
In this study 93 species of early Turonian to late early Maastrichtian planktonic foraminifera have been identified. The index planktonic foraminifera demarcate nine biozones and two subzones for the Kometan and Shiranish formations of the Kurdistan region, NE Iraq. Most previous studies suggested that the base of the Kometan Formation is late Turonian; however, based on the appearance of
Planktonic foraminifera of the Kometan Formation.
Planktonic foraminifera of the Kometan (figs 1–4) and Shiranish (figs 5–18) formations.
Planktonic foraminifera of the Shiranish Formation.
Planktonic foraminifera of the Shiranish Formation.
Planktonic foraminifera of the Shiranish Formation.
We are very grateful for the detailed and constructive comments of Chris Lowery, Francesca Falzoni and a third anonymous reviewer. We are also grateful for the thoughtful editing by Bridget Wade and Alan Lord. RBNJ thanks the Kurdistan Regional Government (KRG) for funding this work. We also thank Rob Wilson for his help during imaging of microfossils, and Colin Cunningham for thin section preparation at the University of Leicester. IPW publishes with permission of the Executive Director of the British Geological Survey (NERC).
Scientific Editing by Bridget Wade and Sigal Abramovich.