The Occurrences and Formation of Ladderback Ripples on Barchans Dunes of Najaf Dunes Field, Iraq, in Aeolian Environments

This paper concerns the study of ripples that occur on the windward of Barchan dunes from the dunes field of Najaf governorate, Iraq. These dunes consist mainly of sand sediments with variable sizes, including medium, fine, and very fine sands. Quartz represents the major light mineral in the Najaf Dunes sand. The prevailing wind direction in the study area is NW-SE. The major ripple crest series of every pattern are oriented perpendicular to the NW-SE wind direction, whereas imbricated ripple groups within the troughs of the preexisting ripples are created by the WSW-ENE wind trend. These ripples tend to be formed by shortened ripples that occupy the troughs of the prolonged series. All crests of the ladderback ripples are oriented at right angles to asymmetry ripples. The ladderback ripples were noticed from fine to very finegrained sediments, which consist mainly of quartz. The wavelength of the ladderback ripples ranges from 2 – 4 cm, while they are 0.1 – 0.2 cm in height. The occurrence of ladderback ripples within an aeolian environment indicates a variety of wind directions, which influenced the arrangements of the crest ripples. Keyword: Ladderback ripples, interference ripples, Sand Dunes, Dunes Field. ملسلا مينلا تاملاع رههظو نيهكت ي ىلع ابثك لقح نم ناخربلا نابثك قا رعلا ,ةيئاههلا ةئيبلا يف فجنلا ن *يجانلا داهج دمحأ 1 مساج فهطك نسح 2 رمت نزام اغآ 2 سابع رماث يرمشلا 2 1 قا رعلا , فجشلا ,ةفهكلا ةعماج , مهمعلا ةيمك, ضرلاا مهمع مدق . 2 قا رعلا, دادغب ,دادغب ةعماج , مهمعلا ةيمك , ضرلاا مهمع مدق ةصلاخلا لاع ةسا ردب ثحبلا اذى متيي ةيممرلا نابثكمل حايرلا هاجتا ةيج ىمع ثدحت يتلا ميشلا تام نم يف نابثكلا لقح فجشلا ةعفاحم نهكتت .قا رعلا ةيممرلا نابثكلا بساور طسهتسلا مجحلا تاذ لامرلا نم ةعهشتم ةعهسجم نم و معاشلا و معاشلا زتراهكلا لثسي .ادًج لا س ةفيفخلا نداعسمل يديئرلا نهك لامر يف نابثك فجشلا . نأ دئادلا حايرلا هاجتا ةسا ردلا ةقطشم يف هى برغ لاسش – قرش بهشج . مسقلا ميشلا تاملاعل لكذب وييجهت متي ةلاح لك يف يديئرلا حايرلا هاجتا ىمع يدهسع NW-SE ضاهحأ يف ةبكا رتسلا جهستلا تاعهسجم ليكذت متي نيح يف ، ميشلا تاملاع ةدهجهسلا ب اقباس حايرلا هاجتا WSW-ENE ايل ةببدسلا هذى ليست . ميشلا تاملاعلا تاجهست نم نهكتت نأ ىلإ ISSN: 0067-2904 Al-Naji et al. Iraqi Journal of Science, 2021, Vol. 62, No. 5, pp: 1598-1604 1599 عيسجو لهطلأا ةعهسجسلا ضاهحأ لغذت ررقأ مسق ةيجهم ممدلا تاذ تاجهستلا ايدهسع ىمع تاجهستلا لا ريغ ةرظاشتم ةعحلام مت . عهن ميشلا تاملاع سمدلا ي نم نهكتت اينا تابيبح ةسعان ىلإ ادج ةسعان نم ايبمغاو دعم ن زتراهكلا و حوا رتي لا لهط ايل يجهسلا نم 2 4 مس ازياو نم تاجهستلا عافترا حوا رتي 1.1 1.2 دهجو نإ .مس ميشلا تاملاع ةئيب يف ممدلا ريظ ةيئاهى مسق تابيترت ىمع رثأ اسم حايرلا هاجتا نم ةعهشتم ةعهسجم ىمع ليلد تاجهستلا ازيا . Introduction Ripple marks are uniform ridge-like frameworks oriented cross to the current flow. They are grown and preserved at the interface across the space separating a fluid medium and a moveable sand sediment, by the interaction between these two environments [1]. The imbricated ripples are oriented at perpendicular straight lines to the existing crests, where major bed forms are oftentimes created on the wave-sheltered beds, when the level of water falls during the ebb period in intertidal environments. The consequent built structures have been described as ladder-back ripples [2]. The ladderback ripples are an interference form of ripple marks which have two groups of overlapped ripples that are especially oriented perpendicular to each other [3]. In intertidal environments, the ladderback ripples were described to reflect diagnostic features of the late-phase development of the run-off. [4]. Reddering, 1987, [5] provided an evidence that ladder-back ripples may also occur in sub-tidal environments. Ladderback and supercritically climbing ripples are typically interpreted to be formed in tidal conditions by bidirectional currents, either via tide or wave activity [6]. Ladderback ripples are widespread within the recent intertidal flat environments [713]. They are described as a form of deposition in intertidal environments that are often preserved in the rock lists in the literature [2, 14, 15, 22]. However, studies related to Ladderback ripples in aeolian environments are seldom. The present study aims to investigate the appearance and forming of ladderback ripples in aeolian conditions. The Najaf dunes field is sited in the west and southwest of Najaf City, Iraq. These dune fields extend parallel to the prevailing NW-SE wind orientation (Figure-1). The study area is located about 20 km west of Najaf City center, near the village of Al-Rahimiya, at latitude of (31o57′42′′N) and longitude of (44o08′16′′ E) (Figure-2). Barchan dunes type is present in the Najaf dunes field together with several morphological types of dunes, including the Barchanoid, Nabkha, and sand sheet. Fine sand size represents the main size of the fractions of Barchan dunes. Thier sorting average of 0.62 Ø reflects that they are moderately well sorted, where quartz represents the major light mineral of these dunes [24, 25]


Introduction
Ripple marks are uniform ridge-like frameworks oriented cross to the current flow. They are grown and preserved at the interface across the space separating a fluid medium and a moveable sand sediment, by the interaction between these two environments [1]. The imbricated ripples are oriented at perpendicular straight lines to the existing crests, where major bed forms are oftentimes created on the wave-sheltered beds, when the level of water falls during the ebb period in intertidal environments. The consequent built structures have been described as ladder-back ripples [2]. The ladderback ripples are an interference form of ripple marks which have two groups of overlapped ripples that are especially oriented perpendicular to each other [3]. In intertidal environments, the ladderback ripples were described to reflect diagnostic features of the late-phase development of the run-off. [4]. Reddering, 1987, [5] provided an evidence that ladder-back ripples may also occur in sub-tidal environments. Ladderback and supercritically climbing ripples are typically interpreted to be formed in tidal conditions by bidirectional currents, either via tide or wave activity [6].
Ladderback ripples are widespread within the recent intertidal flat environments [7][8][9][10][11][12][13]. They are described as a form of deposition in intertidal environments that are often preserved in the rock lists in the literature [2,14,15,22]. However, studies related to Ladderback ripples in aeolian environments are seldom. The present study aims to investigate the appearance and forming of ladderback ripples in aeolian conditions. The Najaf dunes field is sited in the west and southwest of Najaf City, Iraq. These dune fields extend parallel to the prevailing NW-SE wind orientation (Figure-1). The study area is located about 20 km west of Najaf City center, near the village of Al-Rahimiya, at latitude of (31º57′42″N) and longitude of (44º08′16″ E) (Figure-2). Barchan dunes type is present in the Najaf dunes field together with several morphological types of dunes, including the Barchanoid, Nabkha, and sand sheet. Fine sand size represents the main size of the fractions of Barchan dunes. Thier sorting average of 0.62 Ø reflects that they are moderately well sorted, where quartz represents the major light mineral of these dunes [24,25] . Figure 1-Contour map of wind direction in Iraq showing the study area at NW contour direction [23].

Methodology
The methods applied in this study involve a basically detailed fieldwork to study the ladderback ripples. The orientation was measured by a Brunton compass, the wavelength and height were measured by a tape measure and ruler, and a GPS device was utilized to indicate the exact points. The readings were conducted in September and October 2019, with sampling of sand ripples from many positions on Barchans dunes. The grain size analysis was performed by a Ritsch Shaker in the laboratory of the geology department of the University of Baghdad.

Ladderback ripples occurrence on sand dunes
Ladderback ripples are observed on the windward side of Barchan dunes at Najaf Dune Field. The orientations of these ripples are WSW-ENE, while the main ripples orientation is perpendicular to the prevailing wind direction, i.e. NW-SE. The asymmetrical ripple marks are observed in the study area because of the effects of wind direction. The ladder-back ripples of the present dunes field consist of superimposed asymmetrical ripples that are extended at 90º to the original crested line ripple series. Ladderback ripples were characterized to have medium to very fine-grained size (Figure-3). The wavelength of the ladderback ripples ranges from 2 -4 cm and their height ranges from 0.1 -0.2 cm (Table-1). The evidence of a late stage of wind action is provided throughout the ladderback ripples. The minimal ripple series was perhaps created by wind influx laterally over the troughs basin of the main ripples, possibly due to local wind action as well as short periods of wind current impacts (Figure-5).   Figure 3-Histogram of the average grain size of ladderback ripples sand. .

Discussion
The ripple marks might represent one of the most frequent phenomena described in the sedimentology records. Based on modern methodologies; ripples are recognized readily via their measurement, setting, and morphology [26]. Ripple marks are divided into two broad portions: water formed and aeolian ripples [27]. Wind ripples are widespread on the entire sand flats, unless these afford highly fast sedimentation and provide a good model of itself-predominant of aeolian regime [28].
The ripple marks are perfect indicators of wind current orientations [29]. The prevailing wind direction in the study area is northwest-southeast (Fig.1). Thus, any change in local wind direction and velocity are reflected in the ripples marks. The parallel crests are orientated vertically to the wind direction that affects the formation of superimposed ladderback ripples in the troughs of older asymmetrical ripples. Ladderback ripples have low wavelengths, which is a function of wind velocity, grain size of sand, and separation and sorting processes. Thus, ripples of coarse sands have a greater interval than those of fine sands [30]. Ladderback ripples can occur in other depositional environments, created by various techniques and diverse scales, such as the intertidal and subtidal ones [5]. Ripples and ladderback ripples in aeolian environments have a low preservation probability because the sand grains do not contain cement materials between them. They also lack the moisture required for the consolidated sample. Thus, they are intractable during geological recording. Conclusions 1. This study re-assessed the occurrence and formation mechanisms of ladderback ripples in aeolian environments. We observed that these sedimentary structures are associated with asymmetrical ripples on the windward side of Barchans dunes in Najaf dunes field. 2. The formation mechanisms of the ladderback ripples involve late-stage wind action on the sand surface between the main ripples. The minimal ripple series was perhaps created by wind influx laterally over the troughs basin of the main ripples, maybe due to local wind action and current impacts of short periods of wind.