Paleoenvironmental Evaluation Using an Integrated Microfacies Evidence and Triangle Model Diagram: A Case Study from Khurmala Formation, Northeastern Iraq

Abstract

The sequence of the Khurmala Formation located in northeastern Iraq was measured and sampled to evaluate its paleoenvironmental features, including sedimentological and microfacies analyses. The studied formation was analyzed under an optical microscope and was dominated by three main types of microfacies: coralligenous–algal wackestone, foraminiferal–peloidal packstone, and foraminiferal–peloidal grainstone. These hosted microfacies in the Khurmala Formation rarely contain a non-geniculate algae that insufficient for complete reef-building as a crest, but among the common algae, there are calcareous geniculate and green algae associated with benthic foraminifera and a minor component of planktonic foraminifera in the basin due to high-energetic open shallow-water environmental conditions during the deposition of the Khurmala Formation. The relative percentages of foraminifera, including both benthic and planktonic, plotted on triangular diagrams revealed a graphic indicator of paleoenvironment analyses. Detailed examination and analyses for microfacies, new findings of calcareous green algae (Acicularia and Clypeina), and microfacies analyses based on the triangle method and standard facies zones, denote that the upper part of the Khurmala Formation was richer in fined grain and Acicularia green algae, reflecting lower energy conditions than during deposition in the lower part of the formation, which was represented by algal wackestone microfacies and dominated by Clypeina green algae. In summary, these fluctuations in facies/microfacies changes, the appearance of new green algae, and different percentages of foraminiferal content are linked to the global sea level fluctuation that occurred during the Paleocene–Eocene interval.

1. Introduction

The Khurmala Formation is a carbonate-dominated stratigraphic sequence. The formation crops out in north and northeastern Iraq and has a restricted distribution in a belt between Jabal Maqlub in the NW and the Chemchemal-Qizil Dagh area in the SE [1]. The authors reported that the formation was likely deposited in a relict basin confined to the SE part of Kolosh trough, and the formation belongs to the Paleogene stratigraphic units, which have a somewhat unknown stratigraphical position [2]. To track the evolution of carbonate rock and the alteration of pristine facies, several approaches were used to show precise data about the evolution of the paleoenvironment of the Khurmala Formation during the deposition interval; besides a detailed study that dealt with lithology, microfacies and standard triangle microfacies zones were considered in this study. Furthermore, this study, for the first time, adds new data to the depositional environment of the Khurmala Formation based on new genera of green algae. Additionally, the surface datasets from the studied region (Figure 1) were selected for evaluating the micro-properties under an optical microscope and analyzing the paleoenvironment of the Khurmala Formation to find out the logical reasons that influenced the marine carbonate rocks during depositional time and later the impact of diagenesis on the host limestone.

Integrating the analyses of microfacies, the new finding of the dasycladacean green algae species, and calculation of the percentages of foraminiferal content in each microfacies type utilizing the improved triangle model diagram and standard facies zones shows a supportive argument to evaluate the paleoenvironment from the sensitive facies alteration data during the depositional time of the Khurmala Formation.

2. Geological Setting

During Paleocene–Eocene times, the megasequence of the Arabian Plate was deposited as a period of renewed subduction and volcanic arc activity associated with the final closure of the Neo-Tethys [3]. This caused the uplifting along the northeastern margin of the Arabian Plate with the formation of ridges and basins, the general NW–SE trend in north and mid Iraq and E–W trend in west Iraq [1]. Tectonic features like anticlines and synclines displayed a lateral change across these features; uplift of the eastern margin of the Arabian Plate during the Early Paleocene and subsequent erosion revealed the absence of the Danian sequence from many parts of the High Folded Zone and the Foothill Zone in Iraq [1].

The formations of the Paleocene–Early Eocene sequence comprise the phosphatic facies of the Akashat Formation and Swab member of the Ratga Formation, and the carbonate evaporite facies of the Umm Er Radhuma and Rus Formations [1]. The Khurmala Formation is considered a part of the Paleogene stratigraphic units during the Paleocene–Early Eocene cycle [4].

Within the Unstable Shelf area, two separate longitudinal basins developed during the Mid Paleocene time: the Kolosh basin, which occupied the area of the High Folded Zone, was partly isolated from the Red Beds basin in the NE by a ridge along the Balambo-Tanjero Zone from Amadiya in the NW through Rawandus and Ranya towards Halabja in the SE.

The formations of the Paleocene–Early Eocene sequences comprise the phosphatic facies of the Akashat Formation and Swamp member of the Ratga Formation, the carbonate–evaporite facies of the Umm Er Radhuma and Rus Formations, the outer shelf facies of the Aliji, the molasses of the Kolosh, the carbonate ramp facies of the Sinjar, and the inner shelf lagoonal carbonates of the Khurmala [1]. According to Buday [4], the cycle as a whole is marked and started with a widespread transgression on the shelf and ended by uplifting. The author also added that the next cycle is well separated from the first cycle by the regression, uplift, and erosion that occurred mostly within the Lower Eocene, and by the widespread transgression of the late Lower Eocene. The Lower Eocene break is well marked on the Unstable shelf and on the northeastern marginal parts of the Stable shelf as well. In other areas, only the transgressive character of the Late Lower Eocene was observed; there are some problems connected with the exact age of the beginning and mainly of the termination of the cycle itself. Finally, this cycle (the Mid-Late Eocene sequence) is represented by the Damam, Ratga, Avanah, Jaddala, Pila Spi, and Gercus Formations [1].

3. Material and Methods

Intense field work involved the description of strata of the lithological unit from the exposed rocks, and a total of 120 samples were collected from prepared thin sections in the North Oil Company lab, Kirkuk city. Standard thin sections were prepared for study under optical microscope to specify the intensive petrographic observation and analyze the sedimentologic part, including microfossils, microfacies, and paleodepositional environment of the Khurmala Formation. The whole samples were analyzed to measure a percentage of foraminiferal grains based on [5] triangle group components.

The microfacies type is described based on Dunham carbonate rock classification, 1962 [6], and the determination of paleoenvironmental analysis depends on Hallock and Glenn [5] for identifying the standard microfacies type (SMF) and facies zone (FZ).

4. Result

4.1. Fieldwork Observation

In the studied area, the exposed rock of the Khurmala Formation is around 19 m in thickness (Figure 2 and Figure 3). This formation is usually composed of limestone, thin- to medium-bedded, fossiliferous, recrystallized, partly dolomitized limestone. Furthermore, iron oxides and cross-laminated sandstones usually occur towards the top and enclose fine-grained sand sediments. The formation is underlain and overlain by the Kolosh and Gercus Formations, respectively.

Lithologically, the Khurmala Formation is divided into three parts:

• The lower part, composed of thin- to medium-bedded, gray- to brown-color limestone, in places recrystallized facies are observed, filling some voids with fine cements, occasionally fragmented and non-fragmented reefal limestone associated with benthic foraminifera intercalated, with thin-bedded sandy limestone also recognized. The dominant reef component is represented by calcareous red algae and coral forms. The former is followed by a thin- to medium-bedded, dolomitic marly limestone with dolomitic limestone containing red and green algae, plus red algae, echinoids, and bivalves.
• The middle part is exposed to a higher degree of recrystallization than the lower part, partly dolomitized, fossiliferous limestone rich in calcareous red algae, and a foraminiferal biocomponent.
• The upper part is thin to medium, well-bedded, and highly recrystallized compared to the middle and lower parts; dolomitic rock is ubiquitous; and fossiliferous limestone is less common than in other parts of the recent sequence.

4.2. Microfacies

Based on optical characteristics and the dominant bioclasts and other biogenic fauna, fragmented or preserved forms, and according to the classification of Dunham and Embry and Klovan [6,7], three main microfacies types were identified in the studied section, from the lower to upper part: (i) coralligenous–algal wackestone, (ii) foraminiferal–peloidal packstone, and (iii) foraminiferal–peloidal grainstone.

4.2.1. Coralligenous–Algal Wackestone (Figure 4a,b)

This microfacies represented the most dominated component of the lower part of this case, and is composed of corals, coralline algae (i.e., red algae), which characterized the moderate sorted grains. The sizes of these components ranged between 200 µm and several centimeters (up to 15 cm). In addition, brachiopod fragments and unknown bioclasts are considered part of this microfacies. Sparite filling of the pore spaces within the coral texture that floated in the micrite groundmass was identified. In places, the abraded biogenic texture of red algae, coral, etc., provide an alteration to the whole structures of these biogenic components. Other components are represented by benthic foraminifera, echinoids, bivalves. Coral star (Montastrea) floated within the micrite groundmass. The coral later diagenetically altered, abraded, and dissolved, then was filled by calcite cement.

Figure 4. (a,b) Non-geniculate red algae and (c,d) recrystallized coral wackestone floated in coralligenous–algal wackestone. Scale = 200 µm.

Figure 4. (a,b) Non-geniculate red algae and (c,d) recrystallized coral wackestone floated in coralligenous–algal wackestone. Scale = 200 µm.

4.2.2. Foraminiferal–Peloidal Packstone (Figure 4c,d)

This microfacies is the most abundant type in the middle part of the Khurmala Formation. The main components of this microfacies are rotaliids, bivalves, and some fragments of corals and red algae. The sources of rounded and irregular forms of peloids could be linked to these fragments, where the carbonate clasts, or clasts of calcareous algae grains, are formed by endolithic microorganisms. The lower part of this microfacies is diagenetically altered, intra- and inter-cementation is frequently recognized. The main components in the lower part are corals, reef, red/green algae, and echinoderm debris.

4.2.3. Foraminiferal–Peloidal Grainstone (Figure 5)

This microfacies is characteristic of the upper part of the studied section of the Khurmala Formation and represented by miliolids, rotaliids, and benthic foraminifera, like Textularina sp. This microfacies in the Khurmala Formation is characterized by intense recrystallization of fossiliferous limestone and is rich in calcareous red/green algae (Clypeina sp., Acicularia sp.; see Figure 6) and benthic foraminifera. The cement is mainly composed of blocky and drusy cements.

Figure 5. (a–c) Grainstone microfacies with significant grains of benthic foraminifera are observed; (d) dissolved vug filled by dolomitic blocky cement. Scale = 200 µm.

Figure 5. (a–c) Grainstone microfacies with significant grains of benthic foraminifera are observed; (d) dissolved vug filled by dolomitic blocky cement. Scale = 200 µm.

4.3. Triangle Model Diagram and Standard Facies Zone

Modern and ancient carbonate environments, diagenetic processes, and facies models, especially microfacies, have become an essential part of facies analysis and an evolutionary key to examining paleoenvironments hosted in carbonate rocks. Recently, recognition of the reef-associated foraminifera from optical characteristics has been extensively used as an indicator tool for analyzing optical characteristics. Foraminifera, dasycladacean green algae, and red algae are usually associated with reef zones (i.e., reef crest, fore-reef, and back-reef). The distribution of algae-associated foraminifera in the Khurmala Formation plotted using the triangle model method was a support in further paleoenvironmental analysis of shallow-water carbonate (e.g., [5]). This method provides information about the involvement of water energy, depth of seawater, and open marine affinities of the depositional environments during the accumulation of foraminiferal assemblages. Shallow-water and high-energy conditions were indicated for faunas plotted to the lower left, assemblages of protected environments plotted to the lower right, and deeper open marine conditions toward the apex [5]. Benthic foraminiferal assemblages have recently been focused on as a reliable proxy for the environmental characterization and sensitivity of these assemblages for marine habitats and paleoenvironmental analyses [8,9]. Benthic foraminiferal assemblages are also adequate for marine caves that are based on light, oxygen, salinity, temperature, etc.; a decreasing pattern of nutrient supply; and decrease in species richness and biomass of benthic organisms [10].

The triangle model method was used in this study to differentiate three environmentally characteristic groups based on morphologies, habitats of living foraminifera, especially benthic foraminifera including both large benthic foraminifera (LBF) and smaller ones, etc. [9]. This model is a significant method, where Group I includes larger rotaliine foraminifera (LBF); Group II includes mostly planktonic foraminifera with some larger rotaliine with known deeper water affinities (in the current study, we only have a few planktonic foraminifers); Group III includes smaller rotaliines and porcelaneous foraminifera (Figure 7).

One should differentiate between foraminiferal sizes, shapes, and groups, which represent one of the major contributors for paleoenvironmental evaluation, which could contain mostly benthic and occasionally planktonic forms. Khurmala Formation in the studied section is characterized by presence of rotaliids and miliolids. Benthic foraminifera have been successfully used as paleoenvironmental indicators in modern and ancient carbonates [11,12]. Consideration of the textural parameter in foraminiferal tests (grain size, sorting) allowed for conducting transport and resediment tests, and for autochthonous assemblages to be differentiated [13]. The second major component in the Khurmala Formation is algae—both red and green algae. The green algae were poorly sorted and distributed through the studied section, but mostly concentrated in the middle part. The following genus and species are considered as the new finding genera of the green algae: Clypeina sp. and Acicularia sp., with associated fragments of red algae, especially non-geniculate red algae. In addition to irregularly distributed coral that appeared in different forms like solitary, brain, and cup shapes, the Bryozoa appeared as fragments and the gastropod shells took a spirally coiled-type form, in addition different types of echinoid spine being observed.

5. Discussion

The reef-associated foraminiferal distribution in the Shira Swar area was influenced mainly by environmental conditions that are associated with depth and distance from the shoreline; additionally, the water energy and restriction of the shoreline again had a significant role in distribution of biocomponents in the Khurmala Formation.

The fossil clusters from the recent studied samples, including foraminiferal and algae assemblages show three main principal microfacies: (i) coralligenous–algal wackestone microfacies, (ii) foraminiferal–peloidal packstone microfacies, and (iii) foraminiferal–peloidal grainstone microfacies. The dominant components of these microfacies are corals, coralline (red algae), green algae, rotaliid, brachiopod fragments, miliolids, Textularia sp., and peloids with glauconite minerals.

The occurrence of algae in temperate, tropical, and subtropical waters range in depth from few meters up to 100 m, and is controlled by light intensity, current regime, and sediment input [14]. Specifically, green algae indicate quiet water below the wave base or in protected lagoonal environments under the tide zone (ranging between 30 and 90 m; see [15]). However, this study corresponds to the consistency of algae, foraminifera, and other minor biocomponents in coralligenous–algal floated in micritic groundmass (Figure 4). The mechanism of micrite formation usually requires a calm condition to precipitate. Again, the dominant biocomponents in the coralligenous–algal wackestone and foraminiferal–peloidal packstone microfacies are represented by diversity in green algae and non-geniculate red algae associated with benthic and some planktonic forams. These observations likely refer to a lagoonal, and, to some extent, open platform environment setting, where washing of wackestone was displayed by packstone microfacies and dominant existence of peloids in the same microfacies. Also, the absence of protected interstices and washing and cementation of previous micrite was probably linked to discontinuous tide and wind-generated waves washing across the shallow platform, which apparently removed the smaller species and the juveniles of the larger species (e.g., [5]).

The diversity in foraminiferal assemblages, especially miliolids and rotalids in grainstone microfacies and the coarse-grain sediment-like sand-sized sediments that washed out the micritic groundmass due to high water energy conditions in the platform environment, are key indicators for open-platform and nonrestricted back-reef environments, and consequently reflect standard facies zone 7 based on analyses from Hallock and Glenn [5]. Furthermore, the triangle methods in foraminiferal grainstone microfacies recorded a higher percentage of large foraminiferal assemblages than those from algal wackestone microfacies. The latter facies suggests more restricted environment conditions than the grainstone microfacies, and likely coincide with standard facies 8 (Figure 8).

The middle and lower parts mostly enclose the reef-associated foraminifera and are represented by coralligenous–algal wackestone and foraminiferal–peloidal packstone, including coral colonization; usually, corals colonize the mesophotic zone (water depth exceeding 40 m) and calcareous green and red algae. Glauconite is also observed in this facies, represented by hydrous potassium iron aluminosilicate mineral with a high Fe³⁺/Fe²⁺ ratio; the iron content depends on its concentration in the environment of formation [14]. Terry and Williams [15] reported that “the coral biomicrite patches coalesced into one massive coral reef body”. This interpretation gives the impression that the coral reef body represents a frame reef sensu Riding [16] with skeletons in contact. Moreover, a rimmed carbonate platform with a barrier reef and a steep slope surrounding the Intisar embayment was interpreted [17,18] as frame reef sensu Riding [16] with significant topographic relief. Fieldwork integrated with microfacies analysis enabled the recognition of three microfacies from the Tertiary carbonate rocks in the Khurmala Formation.

Coralligenous–algal wackestone and foraminiferal–peloidal packstone occur randomly in lower and upper parts of the Shira Swart. The diversity in foraminiferal assemblages, especially miliolids and rotalids in grainstone microfacies, and the coarse-grain sediment-like sand-sized sediments that washed out the finer grains due to high water energy conditions in the platform environment, are key indicators for open-platform and nonrestricted back-reef environments, and consequently reflect standard facies zone 7 based on analyses from Hallock and Glenn [5]. Furthermore, the triangle methods in foraminiferal grainstone microfacies recorded a higher percentage of large foraminiferal assemblages than those from algal wackestone microfacies. The latter facies suggests more restricted environment conditions than grainstone microfacies, and likely coincides with standard facies 8 (Figure 7, Figure 8 and Figure 9).

Therefore, the integrated methods for analyzing the paleoenvironment of the Khurmala Formation reveal a semi-restricted back-reef environment, which represent the main characteristic of the depositional environment of the Khurmala Formation. This analysis is based on the diversity in bio-contents, fluctuating in microfacies and lithology (cf. [17,18,19]); from the fore-reef environment, the flooding encrustation of calcareous red algae is absent, which represents the major components of organic reef that occasionally occurs in this study, and this probably suggests a sub-environment close to the main body of the reefal crest in the back-reef area. Based on the percentage of each group (I, II, III) (See Figure 7 and Figure 9;

Table 1. Percentages of foraminifera based on the first, second, and third groups in wackestone, packstone, and grainstone microfacies.

SN	Microfacies	% of Foraminifera in Groups
		GI	GII	GIII
1	Coralligenous–algal wackestone	13	8	79
2		8	16	76
3		23	9	68
4		4	16	80
5		6	8	86
6		12	17	71
7		18	10	72
8			14	86
9			24	76
10		22	11	67
11		23	7	70
12	Foraminiferal–peloidal packstone	15	8	77
13		13	12	75
14	Foraminiferal–peloidal grainstone	52	10	38
15		22	16	62
16		20	12	68
17		39	5	56
18		22	10	68
19		30	11	59
20		46	5	49

) from the coralligenous–algal wackestone microfacies, the upper part characterizes a lagoonal environment equivalent to standard microfacies type (SMF) 8, which reflects a tropical lagoon surrounded by coral reefs (FZ8).

The foraminiferal–peloidal packstone and grainstone microfacies are usually characterized by open back-reef environmental setting assemblages with abundant peloids, large miliolids and benthic rotaliids, and dasycladacean green algae. These fluctuations in standard facies zone probably belong to short episodes of more open marine influence-affected lagoon conditions within this interval; the appearance of Acicularia in algal wackestone microfacies indicates low energy and protected environmental conditions. The same cases have been reported in Middle Miocene deposits, which contained a calcareous green alga of Acicularia in mainly shallow marine conditions in tropical–subtropical waters within coastal lagoon settings [20,21]; this could probably be linked to a relative sea level rise during the Paleocene–Eocene interval [22,23]. Therefore, the triangle diagram revealed a relatively high percentage of diverse of bio-components from miliolids and small benthic foraminifera, and dasycladacean green algae (Figure 9 and Table 1). These bio-components are linked to FZ 7, which reflects a shallow marine environment with moderate water circulation (FZ 8), while there is a shifting in this diagram, mostly toward an open-platform environment (FZ 7). Another open platform indicator is described in Flügel [14], in which modern calcareous green algae and the occurrence of Clypeina in grainstones is considered a satisfactory indicator for an open platform [24,25,26,27,28,29,30].

6. Conclusions

This study revealed new insights in understanding the role of large benthic foraminifera (LBF) and dasycladacean green algae in paleoenvironmental analyses. The integration of field/optical observations and laboratory/calculation-based methods were used to examine the role of LBF in paleoenvironmental analyses, distribution of and variability in carbonate production across back-reef settings, and their ecologic sensitivity and importance. The following are the main highlighted points of this research:

• The Khurmala Formation consists mostly of thin- to medium-bedded, fossiliferous, partly recrystallized limestone.
• Diverse benthic foraminiferal and agal bio-components were identified using optical observation, showing abundant dasycladacean green algae, calcareous geniculate red algae, and benthic foraminifera.
• Three main microfacies types were recognized: (i) coralligenous–algal wackestone, (ii) foraminiferal–peloidal packstone, and (iii) foraminiferal–peloidal grainstone. New findings of dasycladacean green algae, Acicularia and Clypeina, together with abundant geniculate red algae, suggest an open-platform environment setting.
• The abrupt and discontinuous changes in microfossil and microfacies belts indicate a change in relative sea level during the Paleocene–Eocene cycle, marked by rapid and significant climatic disturbances during this interval.
• Foraminiferal–peloidal grainstone shifted toward the open platform; in this case, washing of mud needs high water energy, leading to transport of the grains along wider areas. Consequently, all these reasons are related to environmental conditions due to the sea level fluctuations that affected appearance, lifestyle of organisms, and depositional textures of the facies.
• Using a triangle model diagram, this study postulated that most of the Khurmala Formation deposited in shallow marine environments—both open-platform and semi-restricted back-reef environments.
• Shallow-water, high-energy water conditions indicated that bio-components populated the lower-left part of triangle diagram faunas, denoting an open energetic, back-reef environment (FZ 7).
• The bio-assemblages of protected environments populated the lower right, where most of the bio-components of wackestone facies were included, and finer-grain bounded material denoted the protected environment (FZ 8). The appearance of Acicularia in wackestone floating in fine-grain micritic components indicated low-turbulence lagoonal environmental conditions.