The monogenetic Bayuda Volcanic Field, Sudan

Journal of Volcanology and Geothermal Research 356 (2018) 211–224

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The monogenetic Bayuda Volcanic Field, Sudan – New insights into geology and volcanic morphology Nils Lenhardt a,⁎, Suranjana B. Borah b, Sukanya Z. Lenhardt a, Adam J. Bumby a, Montasir A. Ibinoof a,c, Salih A. Salih c a b c

Department of Geology, University of Pretoria, Private Bag X20, Hatfield, 0028 Pretoria, South Africa North Eastern Space Applications Centre, Department of Space, Government of India, 793103 Umiam, India Department of Mineral Wealth, Faculty of Petroleum and Minerals, Al Neelain University, 11121 Khartoum, Sudan

a r t i c l e

i n f o

Article history: Received 21 September 2017 Received in revised form 23 February 2018 Accepted 16 March 2018 Available online 19 March 2018 Keywords: Bayuda Volcanic Field Quaternary Stratigraphy Volcanic geomorphology DEM analysis Sudan

a b s t r a c t The small monogenetic Bayuda Volcanic Field (BVF; 480 km2), comprising at least 53 cinder cones and 15 maar volcanoes in the Bayuda desert of northern Sudan is one of a few barely studied volcanic occurrences of Quaternary age in Sudan. The exact age of the BVF and the duration of volcanic activity has not yet been determined. Furthermore, not much is known about the eruptional mechanisms and the related magmatic and tectonic processes that led to the formation of the volcanic field. In the framework of a larger project focusing on these points it is the purpose of this contribution to provide a first account of the general geology of the BVF volcanoes as well as a first description of a general stratigraphy, including a first description of their morphological characteristics. This was done by means of fieldwork, including detailed rock descriptions, as well as the analysis of satellite images (SRTM dataset at 30 m spatial resolution). The BVF cinder cones are dominated by scoracious lapilli tephra units, emplaced mainly by pyroclastic fallout from Strombolian eruptions. Many cones are breached and are associated with lava flows. The subordinate phreatomagmatism represented by maar volcanoes suggests the presence of ground and/or shallow surface water during some of the eruptions. The deposits constituting the rims around the maar volcanoes are interpreted as having mostly formed due to pyroclastic surges. Many of the tephra rings around the maars are underlain by thick older lava flows. These are inferred to be the horizons where rising magma interacted with groundwater. The existence of phreatomagmatic deposits may point to a time of eruptive activity during a phase with wetter conditions and therefore higher groundwater levels than those encountered historically. This is supported by field observations as well as the morphological analysis, providing evidence for relatively high degrees of alteration of the BVF volcanoes and therefore older eruption ages as suggested by some researchers. A Lower Holocene to Upper Pleistocene age is proposed. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Small-volume volcanic eruptions are commonly associated with monogenetic constructional volcanic landforms such as cinder cones, tuff cones, tuff rings, or maar volcanoes consisting of bedded pyroclastic deposits that have been emplaced by fallout, density currents and/or by the down slope remobilization of tephra (Vespermann and Schmincke, 2000; Németh et al., 2007). Cinder cones, tuff cones, tuff rings and maars are varieties of constructional volcanic landforms formed by the variably explosive interaction of ascending magma with ground- and/or shallow surface water (Sheridan and Wohletz, 1983; Lorenz, 1986; Kano and Ohguchi, 2009). Eruptions occurring in well-drained areas ⁎ Corresponding author: Department of Geology, University of Pretoria, Private Bag X20, 0028 Pretoria, South Africa. E-mail address: [email protected] (N. Lenhardt).

https://doi.org/10.1016/j.jvolgeores.2018.03.010 0377-0273/© 2018 Elsevier B.V. All rights reserved.

generally produce cinder cones, whereas maars, tuff rings and tuff cones are produced by explosive phreatomagmatic eruptions that result from magma interaction with ground- and/or surface-water (Wood, 1980; Wohletz and Sheridan, 1983; White, 1991). A tuff cone is a small crater above the pre-eruptive surface that is surrounded by a steeply-dipping pyroclastic apron. A tuff ring is a maar volcano that has a crater floor at or slightly above the former ground surface and is rimmed by a well-bedded accumulation of quenched magma fragments (Heiken, 1971; Lorenz, 1973; Fisher and Schmincke, 1984). This volcanic feature is transitional to a maar (sensu strictu), which comprises a volcanic crater excavated into the ground by multiple shallow explosive eruptions. The study of the morphology of these volcanic landforms can give valuable insights into their formational processes and their underlying causes, i.e. structural setting, magma composition and flux, eruptive style and climate (e.g., Cotton, 1944; Francis, 1993; Thouret, 1999; Davidson and De Silva, 2000; Grosse et al., 2012) and allows the

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N. Lenhardt et al. / Journal of Volcanology and Geothermal Research 356 (2018) 211–224

calculation of the volume of erupted materials and other features such as the slope and orientation of the terrain after the eruption (Rodriguez-Gonzalez et al., 2010 and references therein). The latter are especially important parameters to determine the aggradation and/or degradation of volcanic terrains (Rodriguez-Gonzalez et al., 2010). Furthermore, the presence of subordinate phreatomagmatic volcanoes in a volcanic field can indicate variations in the physical conditions of the sub-surface stratigraphy of the volcanic field, or variation of the water saturation state of the sub-surface sediments or rock units over time (Aranda-Gómez and Luhr, 1996; Gutmann, 2002). The presence of extinct volcanoes in the Bayuda desert was first noted by Gregory (1920) and Grabham (1920). A preliminary account of the geology of the BVF was written by Almond et al. (1969). Basaltic volcanism in Sudan has continued intermittently since the Cretaceous (Almond, 1968; Almond et al., 1984) with the BVF volcanism forming part of the youngest phase of volcanism. Based on the preservation of primary volcanic landforms, Almond et al. (1969) proposed an eruption age of the Bayuda volcanoes of Pleistocene or younger but probably post-dating the last period of moist climate in Sudan ca. 5000 years ago (Almond et al., 1984). According to Vail (1988) radiocarbon dating indicates volcanic events in this area dating to 1100 years B.P. which would place the last known eruption at 850 CE ± 50. Comparable Holocene volcanoes can be found in the Darfur Province of western Sudan, notably in the Jebel Marra (Vail, 1972a, 1972b; Francis et al., 1973; Davidson and Wilson, 1989), Kutum (also known as the Tagabo Hills or Berti Hills) and Meidob fields (Almond, 1974; Franz et al., 1997, 1999), and a small volcanic field west of the Nile at Berber. The information gained through this contribution will form the basis for an ongoing project on the volcanic field that focuses on its magma plumbing system and eruption mechanisms in relation to the tectonic stress regime. Here, we present a first survey of the volcanic field, which can be considered as a first account of the general geology of the BVF volcanoes after more than 30 years and the first description of a general stratigraphy, including a first description of their morphological characteristics. The aims of this study were to record the characteristic deposits of selected volcanoes in the field, their degree of degradation and to interpret their predominant eruptional mechanisms. An additional morphometric analysis was carried out with the intention of quantifying the morphologic range of cinder cones and maar volcanoes within the BVF. This was done in order to provide statistical support for the interpretations with regards to degradation and age of the volcanic structures. 2. Geological setting The BVF (volcano number 225060; Global Volcanism Program, 2013) is located ca. 300 km north of Khartoum, the capital of Sudan, in the northern part of the Bayuda desert, situated inside the Great Bend of the River Nile (Fig. 1a). The Bayuda desert constitutes the southern part of the Bayuda terrane (Schandelmeier et al., 1994), which consists of Neoproterozoic [806 ± 19 Ma (Sm–Nd 11 WR isochron)]; Küster and Liégeois, 2001) metavolcanic and meta-sedimentary rocks of amphibolite facies that were originally formed in an island arc/marginal basin setting. The high-grade metamorphic rocks are bordered by the Halfa terrane in the northwest and by the Gabgaba terrane in the east (Küster and Liégeois, 2001). The Gabgaba terrane is part of the Arabian–Nubian Shield, being composed of low-grade metamorphic (mainly greenschist facies) juvenile Neoproterozoic island arc/back arc basin assemblages. The Halfa terrane contains both juvenile Neoproterozoic crust and high-grade gneisses with pre-Pan-African crustal heritage (Harms et al., 1994; Stern et al., 1994). The high-grade metamorphism and the existence of peraluminous granites in the Bayuda desert are related to a major frontal collision between the Bayuda and Halfa terranes at approximately 700 Ma (Harms et al., 1994; Abdelsalam and Stern, 1996) probably within the framework of a composite collision with the Saharan Metacraton (c.f., Abdelsalam et

al., 2002). The oblique collision that affected the Gabgaba-Gebeit terrane was initiated in the same age range (before 700 Ma) but perhaps not with the Bayuda terrane (Küster and Liégeois, 2001). Their juxtaposition occurred later (640–580 Ma) through a large sinistral transpressive displacement along the Keraf mega-shear zone (Abdelsalam et al., 1998). A structural trend of ENE–WSW to NE-SW has been reported for the Precambrian rocks from the Bayuda desert (Vail, 1971, 1972b; Abdelsalam et al., 1998) which is similar to many other places in the Saharan Metacraton such as the Nubian desert (Schandelmeier et al., 1994; Stern et al., 1994; Abdelsalam et al., 1995), Kordofan and Darfur (Schandelmeier et al., 1987), Uweinat and the Jebel El Asr inlier (Sultan et al., 1994). This trend has been referred to as the Zalingei folded zone (Vail, 1976; Schandelmeier et al., 1987; Küster and Liégeois, 2001) and is thought to continue SW into Cameroon where similar structures are reported (Vail, 1976; Schandelmeier et al., 1987). This trend was interpreted to represent pre-Neoproterozoic structures in Central Africa (Grant, 1978; Annor and Freeth, 1985; Ngako, 1986; Ekwueme, 1987), although Toteu et al. (1987, 1994) obtained 630–620 Ma U/Pb zircon ages from granitoids deformed by these structures. Küster and Liégeois (2001) interpreted these structures as inherited from early Neoproterozoic terrane collisions. This interpretation is also supported by data from the Nuba Mountains, where a 778 ± 90 Ma (12 WR Nd isochron) oceanic arc/back-arc assemblages and older medium-grade gneisses were sheared and foliated along the NE-structural trend (Ibinoof et al., 2016). Following the emplacement of abundant postcollisional granitoids between ca. 620 and 560 Ma (e.g. Stern and Kröner, 1993; Küster and Harms, 1998) the region has been a stable cratonic area. More than 50% of the Precambrian rocks in the Saharan Metacraton are overlain by Cretaceous and younger unmetamorphosed and undeformed cover rocks or buried under sands of the Sahara desert and relatively few outcrops can be found in Aïr, Tibesti, Uweinat and the Bayuda desert (c.f., Abdelsalam et al., 2002). During the Neogene, at ca. 30 Ma, a major change in the tectonic regime triggered eruptions of flood basalts (e.g., Hofmann et al., 1997) in the Ethiopian Afar region to the SE and subsequent plate separation with ongoing rifting and ocean floor formation in the Gulf of Aden and the Red Sea (e.g., Bosworth et al., 2005). Mafic intraplate magmatism of small to moderate volume intensified in NE Africa before and after this flood basalt extrusion and the onset of rifting, whereas on the Arabian plate, many occurrences of intraplate magmatism postdate the onset of rifting (Lucassen et al., 2011). The Bayuda volcanism is compositionally related to the mainly alkaline province of intraplate magmatism in northern Sudan and southern Egypt (e.g., Franz et al., 1987, 1999; Lucassen et al., 2008). The age of some of these lavas is described as Mesozoic, but most are reported to be Paleogene to Holocene in age (Barth and Meinhold, 1979; Franz et al., 1987; Schandelmeier and Reynolds, 1997). The BVF covers an area of 476.4 km2 and contains about 70 small, monogenetic volcanoes with a predominance of cinder cones, some of them breached by lava flows, and subordinate maar volcanoes (Fig. 1b). Isolated volcanoes are scattered over the peneplain of the Precambrian basement surrounding the main field. “The extremely well-preserved primary morphology” of the volcanoes led Almond et al. (1984) to suggest that they were built up after the establishment of an arid climate in the area. The volcanic rocks vary in composition from melabasanite through basanite to nepheline hawaiite (Almond et al., 1984). 3. Methods For a first detailed account of the volcanological features of the BVF for more than 30 years (Almond, 1968, 1974; Almond et al., 1969, 1984), ten volcanoes were visited in the field to record their lithology and stratigraphy to find out more about the general eruptional

N. Lenhardt et al. / Journal of Volcanology and Geothermal Research 356 (2018) 211–224 Fig. 1. a) Tectonic map of Sudan showing the different geological terranes of the pre-Neoproterozoic Saharan Metacraton and the Neoproterozoic Arabian Nubian Shield. The distribution of Tertiary to Holocene volcanic rocks is shown in black with the age given for the Holocene occurrences. The location of the study area within the Bayuda Volcanic Field is highlighted. S. = suture, BCE = Before the Common Era, DTFTB = Dam et Tor fold and thrust belt; b) DEM of the Bayuda Volcanic Field with the location of all recorded cinder cones and maar volcanoes. Digits refer to their serial numbers in Electronic Supplements 1 and 2; c) Slope map of the BVF extracted from the DEM with the selected cinder cones and maars/tuff rings measured for this study. 213

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mechanisms of the BVF cinder cones and maar volcanoes. This, as well as observations of the degree of degradation of the volcanoes, was done in order to provide field-based support to the results and interpretations gained through the calculation of morphological parameters by means of satellite imagery analysis. Due to unrest of the local population at the time of fieldwork, however, a more in-depth field study was not possible and has to be postponed for a future contribution. Digital Elevation Models (DEMs) of the BVF extracted at 30 m spatial resolution from an SRTM dataset (USGS, https://glovis.usgs.gov/) were used to quantify the morphological parameters. A general break-inslope was used to demarcate the cinder cone and maar boundaries. For the cinder cones, cone width (Wco), cone height (Hco), crater width (Wcr), crater depth (Dcr), crater area and maximum height of the cone (Hcomax) were calculated (Fig. 2). The same was done for the cone height/width ratio (Hco/Wco), and crater width/cone width ratio (Wcr/Wco). The crater depth/cone height ratio (Dcr/Hco) was calculated for the cones, which had a measureable crater depth instead of a flat summit region. The measurements were done using the tools in the ArcGIS software following the approach of Settle (1979), Kervyn et al. (2012) and Grosse et al. (2012). The following morphometric parameters were calculated for the maars/tuff rings according to Grosse et al. (2012): Crater area, crater width (Wcr), crater depth (Dcr), basal width of the tuff ring (Wco), crater volume, crater width/basal width ratio (Wcr/Wco), crater depth/height ratio (Dcr/Hco). The heights (Hco) and (Hcomax) were calculated as the difference between the mean crater rim and mean cone base elevations. For breached walls the average heights of the existing crater rim was taken. The volume and angle of slope for both cinder cones and maars were directly measured using the 3D statistic and zonal statistic tool in the ArcGIS software.

4. Results 4.1. Stratigraphy of the cinder cones From aerial photographs and verification on the ground, 53 cinder cones could be identified (Fig. 1b; Table 1). Only a few cones do not show any signs of erosion and many of them are breached. Sub-circular lava fields slope gently away from the breached cones. The majority of the visited cinder cones such as Jebel Mazrub (Fig. 3a; 32°26′41.29″E, 18°29′14.19″N) are built of thick beds of coarse-grained and commonly clast-supported black to red scoriaceous lapilli with ballistic bombs and blocks, minor intercalated lava flows and thin beds of ash-sized tephra. The beds are planar bedded, well-sorted, internally massive and exhibit uniform thicknesses. Large vesicular spindle-shaped lava bombs and blocks as well as breadcrust bombs and blocks up to 0.5 m in diameter are common. The materials are largely of alkaline basaltic composition with abundant mantle-derived xenoliths (c.f. Lucassen et al., 2011) and minor basement-derived fragments. The lack of horizontal transportation indicators suggests that the majority of these beds were formed by pyroclastic fallout. One exception to this description is the cinder cone Bayt al-nur. Bayt al-nur (Figs. 3b, 4a, e; 32°49′42.4″E, 18°16′43.1″N) is a breached cone whose center is easily accessible. Within the center of the cone, dense basalt can be observed that exhibits a reaction zone with the surrounding sand deposits, characterized by a peperitic breccia (Fig. 4b). The majority of the cone is made up of deposits of loose, clast-supported, 1–2 cm big, vesicular scoria lapilli (Fig. 4c). Some bomb-sized fragments reach sizes of 10 × 15 cm. The larger clasts show spindle-shapes, signs for plastic deformation during flight. Intercalated lava-flows within this part are of pahoehoe-type, shown by their ropy surface texture (Fig. 4d). Of particular interest at this cone are the matrix-supported, yellow-beige, fine-grained cross-

Fig. 2. Sketch of the calculated morphometric parameters of (a) cinder cones and (b) maar volcanoes/tuff rings (after Kervyn et al., 2012). Wco: cone base average diameter; Wcr: crater average diameter; Hco: cone average height; Hcomax: maximum cone height between average basal elevation and highest point of the crater; Dcr: average crater depth; α: outer slope repose angle.


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Table 1 Mean, median and standard deviation for the key morphometric parameters and morphometric ratios for the BVF cinder cones. Cinder cones

Hco (m)

Wco (m)

Wcr (m)

Hcomax (m)

Hco/Wco

Hcomax/Wco

Wcr/Wco

Angle of slope (°)

Vol (106 m3)

Mean Standard deviation Median

123.4 58.9 116.0

834.8 300.1 811.5

275.5 127.2 272.9

148.1 66.4 133.0

0.15 0.04 0.14

0.18 0.05 0.17

0.33 0.11 0.34

16.7 3.7 16.9

55.6 55.8 36.9

Fig. 3. Examples for cinder cones and maar volcanoes of the BVF: a) Jebel Mazrub (32°26′41.29″E, 18°29′14.19″N); b) the extensively breached Bayt al-nur (32°49′42.4″E, 18°16′43.1″N); c) El Muweilih (32°30′55.9″E, 18°24′13.7″N); d) Jebel Hebeish (32°28′47.5″E, 18°29′0.1″N); e) Zein Umm Araysh (32°44′43.5″E, 18°30′09.0″N).

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Fig. 4. a) The inner side of cinder cone Bayt al-nur (32°49′42.4″E, 18°16′43.1″N), showing a change from grey, coarse, vesicular scoria particles to yellow-beige, fine-grained, cross-bedded layers, characterizing a transition from strombolian to phreatomagmatic eruptive activity; b) peperitic breccia grading into massive lava at the base of the breached cone; c) clastsupported scoria deposits that predominate the cone; d) folded upper surface of a pahoehoe lava flow in the lower part of the cone; e) stratigraphic section of Bayt al-nur.


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Table 2 Mean, median and standard deviation for the key morphometric parameters and morphometric ratios for the BVF maar volcanoes. Maar volcanoes

Hco (m)

Wco (m)

Wcr (m)

Hcomax (m)

Dcr (m)

Hco/Wco

Hcomax/Wco

Wcr/Wco

Dcr/Wcr

Angle of slope (°)

Vol (106 m3)

Mean Standard deviation Median

67.5 32.6 65.0

705.3 168.8 682.0

415.2 154.9 409.6

91.5 46.1 80.0

114.6 84.6 97.0

0.10 0.05 0.09

0.13 0.06 0.12

0.58 0.08 0.58

0.27 0.12 0.24

11.6 5.2 10.8

26.2 30.5 18.8

bedded layers, overlying the clast-supported scoriaceous deposits that may be interpreted as pyroclastic surge deposits related to a shortlived phreatomagmatic pulse during the eruption. These layers are in turn overlain by another yellow-beige, fine-grained, massive and matrix-supported deposit (Fig. 4a, e) that may be interpreted as phreatomagmatic fall deposit. 4.2. Stratigraphy of the maar volcanoes Within the BVF, 15 volcanic structures could be identified as maar volcanoes (Fig. 1b; Table 2). They are cut into either the crystalline pre-eruption surface rocks or the unconsolidated sand of the Bayuda desert and generally occur in elevations of between 398–620 m. All maars of the BVF are surrounded by relatively low circular rims of pyroclastic ejecta beds which accumulated on the surface and decrease in thickness very rapidly outwards (Fig. 5a). The thickness of beds varies between a few cm and several tens of meters. From the foot of the tephra ring, very thinly bedded tephra extends outwards in a thin veneer. The distal tephra deposits in the near vicinity of the tephra ring still contain base-surge material and ballistic bombs but rapidly decrease in thickness and grain size. Further away, the material is dominated by ash fall deposits and is ultimately covered by recent sand dunes. None of the maars contain perennial lakes, although El Muweilih (Fig. 3c; 32°30′55.9″E, 18°24′13.7″N) exhibits a small saline well in the centre of the crater floor from which natron-rich clay is dug out and dried by local nomads. Most of the maars of the BVF are filled-up with debris up to the surrounding surface, by collapse of their crater walls and drifting sand from the desert, as can be seen at Jebel Hebeish. At Jebel Hebeish (Figs. 3d, 6; 32°28′47.5″E, 18°29′0.1″N), the first rim deposits that can be observed are yellowish-beige, stratified layers of matrixsupported, ash-rich material (Fig. 5b). The material consists of a mixture of juvenile clasts, and large amounts of accidental lithics (of granitic composition, derived from the basement). Bedding is often disturbed by embedded volcanic bombs causing impact sag structures. Due to the composition, clast characteristics and texture of these deposits, they are interpreted to originate from dilute pyroclastic density currents or surges, related to phreatomagmatic eruptions (c.f. Murcia et al., 2015). The impact sag structures can be a sign of wet phreatomagmatic tephra deposits (Johnson, 1989), however, the absence of accretionary lapilli suggests rather relatively dry surges (Weinstein, 2007). Similar to other maar volcanoes of the BVF, at Jebel Hebeish the matrix-supported beds are intercalated by thicker, massive clast-supported beds of lapilli- and bomb-rich scoriaceous material (Fig. 5c). The beds show uniform thickness and are interpreted as fall deposits. The clasts within these deposits are usually rimmed by palagonite, suggesting post-depositional wet conditions during early diagenesis. In maar M13 (32°52′48.2″E, 18°17′27.6″N) most of the observed deposits consist of fine-grained, yellowish-brown, strongly palagonized beds showing low angle crossbedding and dune structures that can be interpreted as base surge deposits (Fig. 5d; c.f. Carmona et al., 2011). The bed thicknesses are usually a few centimetres in thickness, yet can be up to 1 m. At Zein Umm Araysh (Fig. 3e; 32°44′43.5″E, 18°30′09.0″N), the tephra ring (floor level with the surface surrounding the volcanic structure) is asymmetrical, being higher in the SW (maximum height of 532 m), where the internal and external slopes are also steeper, than in the north (maximum high of 470 m). The deposits at the south-

western side have a fluid-like “spattered” appearance, i.e. more rounded spindle-shaped lapilli and bombs occur and a high degree of agglutination can be observed (Fig. 5e–f), which are proposed to originate from a Hawaiian-type lava fountaining (c.f., Sánchez et al., 2014). According to Lorenz (1986) this could possibly be due to a sealing off of the groundwater supply, changing the eruptive style from phreatomagmatic to lava fountaining. Dense lava flows, some of them showing columnar jointing, observed in the walls of several maars, are assumed to represent either the pre-eruption surface topography or an early effusive eruption phase before the onset of explosive/phreatomagmatic activity. In the El Muweilih crater the approximately 15–20 m thick lava flow is covered by ca. 10 m of highly vesicular, reddish scoria lapilli with minor amounts of ash that have been interpreted as pyroclastic fallout typical for Strombolian ejecta (c.f., Houghton and Schmincke, 1986; Martin and Németh, 2006; Clarke et al., 2009) (Fig. 5g). This is in accordance with Almond et al. (1969) who described a first non-phreatomagmatic phase for the same crater resulting in formation of a cinder cone. At El Muweilih, the scoria lapilli beds are followed by yellowishbeige, stratified layers of matrix-supported, ash-rich material, similar to what has been observed at Jebel Hebeish. Similar to the latter maar, these deposits are also intercalated with layers of massive clast-supported beds of lapilli- and bomb-rich scoriaceous material, implying frequent changes between deposition from fall and dilute pyroclastic density currents. 4.3. Morphometric parameters of the cinder cones After excluding cinder cones with complex morphologies such as multiple cone growth, 53 cones of the BVF were selected for morphological analysis of which 21 cones have a flat summit region and are unbreached. The remaining 31 cones are breached. Fig. 2a shows the basic morphological parameters calculated for the cinder cones. Almost all of the larger cones (those with a cone width of more than 1000 m) are breached. Table 1 provides the mean, median and standard deviation values for the calculated morphometric parameters. The complete table showing the values for all the cinder cones can be found in the Electronic Supplement 1. Fig. 7a–c show the morphometric relationship between Wcr, Hco and Hcomax against the cone diameter (Wco). The demarcation between breached and unbreached cones is shown, as well as the best fit linear regression in each of these cases. The Hco/Wco and Hcomax/Wco ratios are compared to the average ratios of Porter (1972), Wood (1980) (Avg. Hco/Wco = 0.18) and the maximum ratio of Settle (1979) (Hco/Wco = 0.2). The average Hco/Wco and Hcomax/Wco ratios are 0.15 (±0.04) and 0.18 (±0.05) with median values of 0.14 and 0.17, respectively. The mean Wcr/Wco ratio is 0.33 (±0.11) with a median value of 0.34. The Hco/Wco ratio for breached cones is lower (0.14) than for unbreached cones (0.16) (Fig. 7a). The Hcomax/Wco ratio also follows a similar pattern, with the breached cones having a lower ratio than the unbreached cones. The breached cones have a very weak correlation between the two values (R2 = 0.16) (Fig. 7b). There is a great scatter in the Wcr/Wco ratio with the lowest value from 0.08 to 0.58 and also a very weak correlation for both breached (R2 = 0.37) and unbreached cones (R2 = 0.26) (Fig. 7c). There might have been slight discrepancies in the calculation of the Wcr due to the highly breached nature of the summit regions of the cinder cones.

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Fig. 5. a) Example (from Jebel Hebeish) of a low circular rim of pyroclastic ejecta surrounding the maar volcanoes of the BVF; b) stratified, heterolithologic deposits that have been interpreted as originating from dilute pyroclastic density currents; c) massive, scoriaceous pyroclastic fallout deposits; d) dune and pinch-and-swell structures of a base surge deposit at maar M13 (32°52′48.2″E, 18°17′27.6″N); e) and f) agglutinate of lava spatter with spindle-shaped and round lava bombs at Zein Umm Araysh (32°44′43.5″E, 18°30′09.0″N); g) internal stratigraphy of the El Muweilih (32°30′55.9″E, 18°24′13.7″N) maar with a lava flow on top of the palaeosurface, followed by massive layers of scoria and phreatomagmatic deposits on top.


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are 0.1 (±0.05) and 0.09, respectively. The average and median ratio between crater width and maar width is 0.58 (±0.12). The mean and median crater depth – crater width ratio are 0.27 (±0.12) and 0.24, respectively. The Hco/Wco graph (Fig. 7d) shows a large scatter in the values with an R2 value of 0.05. A better correlation can be seen between the Wcr and Wco with an R2 value = 0.82 (Fig. 7f). A positive correlation (R2 = 0.52) can also be seen between the Dcr and Wcr values (Fig. 7e). The total ejecta volume of the maars is ca. 393.3 × 106 m3. Fig. 8b shows the relationship between the slope and the Hco/Wco ratio of the maars. No evident correlation can be seen in case of the three unbreached maars but there is a positive correlation in case of the breached maars. 5. Discussion 5.1. Geology of the Bayuda Volcanic Field

Fig. 6. Lithostratigraphic section of Jebel Hebeish.

Fig. 8a shows the relationship between the angle of slope and the Hco/Wco ratio of the cinder cones. A linear positive relationship is seen for both breached (R2 = 0.25) and unbreached (R2 = 0.26) cones, which indicates that the angle of slope generally increases with the increasing Hco/Wco ratio. The total volume of the cinder cones amount to ca. 2945.6 × 106 m3. 4.4. Morphometric parameters of the maar volcanoes The BVF maars are fewer in number compared to the cinder cones and are mainly concentrated to the north and south of the central BVF with a few outlying maars (Zein Umm Araysh (M2) and Jebel Hebeish (M8)) in the northern region (Fig. 1b). Fig. 2b displays the parameters used after Grosse et al. (2012). The mean, median and standard deviation of the calculated parameters are presented in the Table 2. The complete table recording all the parameters has been attached to the Electronic Supplement 2. Out of the 15 maars and tuff rings, only three seem to have an unbreached crater wall. The remaining 12 are in various stages of degradation. The crater depth, on the other hand, could be measured for all of them. The biggest maar in the region is the Hosh Ea Dalam (32°35′38,32″E, 18°24′29,07″N) with a crater diameter of 880 m and a depth of 386 m. The average and median ratio between maar height and maar width

The predominance of coarse, clast-supported scoria related to pyroclastic fallout and minor intercalated pahoehoe lava flows building up the cinder cones, implies that volcanic activity in the BVF was primarily of Strombolian and Hawaiian type. In at least one cinder cone (Bayt alnur) a change from Strombolian to phreatomagmatic activity was noticed at a later stage of cone formation as shown by yellow-beige, fine grained, cross-bedded layers representing surge deposits. In the Eifel Volcanic Field, Germany, ca. 2/3 of the cinder cones show an initial maar phase or some phreatomagmatic phase (Schmincke, 1977; Lorenz, 2003), mostly, however, at an early stage of volcanism. In the BVF the phreatomagmatic eruptions during cone building were shortlived, only depositing material of a few metres thickness. Thus, access of groundwater did not lead to sustained phreatomagmatic eruptions. Evidently groundwater supplies generally were inadequate to support phreatomagmatism for long. The finer-grained deposits at the top of Bayt al-nur may have led to a higher amount of degradation of this cone (and possibly other cone structures of the BVF) by wind erosion. Previous research has shown that wind has significant abrading power of volcanic structures when persistent (e.g., Greeley and Iversen, 1987; Hooper, 1999; Holness, 2004). Typically, eruptions at maars, tuff rings and tuff cones are initially phreatic or phreatomagmatic (e.g., Schmincke, 1977; Cas and Wright, 1987; Lorenz, 2003). In contrast, phreatomagmatic activity of at least two maars of the BVF (El Muweilih, maar M11) did not commence when magma first rose through the metamorphic basement rocks and overlying sediments to the surface. Instead, basal flows followed by cinder production occurred and were only later followed by phreatomagmatic activity. Gutmann (1979, 2002) ascribed eruption of unusual basal flows in maar volcanoes from the Pinacate volcanic field in Sonora (Mexico) to the rapid rise of magma, thus preventing bubble growth and coalescence from occurring before extrusion. The ensuing Strombolian activity probably developed as the rate of magma rise slowed (Wilson and Head, 1981). Sustained phreatomagmatic volcanism can only occur with effective water/magma ratios, preferably in excess of a minimum value of 0.1 (Sheridan and Wohletz, 1983). According to Houghton et al. (1999) these effective water/magma ratios are usually attained during low magma flux. Therefore, initial magma flux in the BVF conduits may have overwhelmed the available groundwater flux. The onset of phreatomagmatic activity may have been caused by the preceding Strombolian eruptions themselves. Bursting of large bubbles in the conduit may have produced mild decompression waves that were transmitted downwards, thus promoting local failure of conduit walls and of insulating vapor films separating magma and groundwater (e.g., Gutmann, 2002) and thus increasing groundwater flow towards the desiccated zone around the conduit. Furthermore, pyroclastic emptying of the uppermost conduit could have lowered the magma column height from time to time and thus afforded direct access of groundwater to the top of the magma column. According to Houghton et al. (1999)

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Fig. 7. The morphometric relations for the BVF showing (a) mean cone height (Hco), (b) maximum cone height (Hcomax), and (c) crater diameter (Wcr) against the cone diameter (Wco). The best fit linear regression values are given for both breached and unbreached cones and compared to the average values of Porter (1972) and the maximum values of Settle (1979). Morphometric relationship for the BVF maars and tuff rings, (d) mean cone height (Hco), (f) crater diameter (Wcr) measured against the cone diameter (Wco) and (e) the crater depth (Dcr) against the crater diameter (Wcr) have been plotted. The best fit linear regression values are given for all the measured maars and tuff rings.


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Fig. 8. (a) Regression relationship between angle of slope and Hco/Wco ratio for the BVF cinder cones. Data are discriminated between simple breached and simple unbreached cones. The positive linear regression line suggests that there is a general increase in the slope angle with the increase in the Hco/Wco ratio. (b) Regression relationship between angle of slope and Hco/ Wco ratio measured for the BVF maars and tuff rings. The positive correlation is very much evident in the breached cones but no correlation as such could be seen in the unbreached cones.

phreatomagmatism is inhibited if the magma is ponded at levels above the water table. Almond et al. (1969) describe one BVF maar that can be associated with a cinder cone which partly occupies the maar crater. Unfortunately, the authors do not provide coordinates for this volcano. Nevertheless, this feature has been observed elsewhere (e.g. Lorenz and Büchel, 1980; Boivin et al., 1991; Carn, 2000; Lorenz, 2003) and is due to the onset of (or return to) magmatic activity following the initial phreatomagmatic explosions during maar formation (e.g., Kienle et al., 1980).

The shape of the maar craters seems to be strongly dependent on the nature of the geological substrate on which they form. In the field, it can be clearly seen that the unconsolidated sand of the Bayuda desert caused the vast majority of the maar craters to have evolved with very low wall angles and with collapse structures and sand flows towards the crater centre. In contrast, in areas of exposed basement rocks, such as at the Hosh Ea Dalam, the underlying Precambrian rocks gave rise to steep-sided diatremes. This phenomenon has been observed elsewhere in volcanic fields, for instance in the Miocene

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Ellendale Volcanic Field of Australia (c.f. Smith and Lorenz, 1989; Stachel et al., 1991). Today, only the maar at the lowest elevation (El Muweilih; 422 m) within the main volcanic field is water-bearing. Only Jebel Hebeish (398 m) is lower (and dry) but is already situated outside the main part of the volcanic field. All other maars at higher elevations are dry. This may be attributed to a closer proximity to the groundwater during the time of eruption when the groundwater table was higher than today, possibly during a brief period of wet conditions and groundwater recharge. This, and the fact that even some cinder cones exhibit the deposits of phreatomagmatic eruptions may contradict Almond et al.'s (1984) hypothesis that the majority of Bayuda's volcanic features were built up after the establishment of an arid climate in the area. In fact, the wettest phase in the eastern Sahara during the Holocene occurred between approximately 9000–4000 years B.P. (c.f., Ritchie et al., 1985; Haynes Jr., 1987; Kröpelin, 1987; Abell and Hoelzmann, 2000). During this time rising water tables were sufficiently high to support lakes at many localities in Sudan and southern Egypt (e.g., Haynes Jr. et al., 1989; Pachur et al., 1990; Franz et al., 1997; Nicoll, 2004). The famous rock art on cliffs along the Gilf Kebir, Jebel Uweinat, Wadi Hussein and the Libyan Plateau near Dakhla depicting animals and swimming people are evidence for this period of wetter conditions (Bagnold, 1933; De Almásy, 1936; Shaw, 1936; Winkler, 1938; Peel, 1939; Rhotert, 1952; Gauthier, 1987; Krzyzaniak, 1990). Rainfall during the Late Holocene (after 2000 B.P.) through the late historic period was not sufficient to cause substantial groundwater recharge across southern Egypt and northern Sudan (Haynes Jr., 1987). Apart from these wetter phases during Holocene time, the high degree of degradation of the majority of volcanic structures may also point to an older, possibly Pleistocene age where a range of wet phases have been described (e.g. Abotalib et al., 2016). However, with only a single age for the last known eruption (1100 years B.P.; Vail, 1988) at hand it is hard to speculate about the timing of the maar eruptions. 5.2. Morphometry of the Bayuda Volcanic Field The average angle of slope of the cones (~17° ± 3.7°) is less than the pristine cone slope of 33° as proposed by Wood (1980) and Settle (1979) for young, Holocene volcanoes. Tables 1 and 2 show that the maars have a gentler slope (mean = 11.6° ± 5.2°) than the cinder cones and a lower height as compared to the cones. The Hco, Hcomax and Wcr have a positive linear relationship to the Wco (Fig. 7a–c). The Hco/Wco ratio shows a weak positive correlation between parameters as opposed to the cinder cones. The near perfect correlation of the crater width and the cone width demonstrates that the maars with a bigger base also have a larger crater. The crater depth also shows a positive correlation to the crater diameter. The relatively high degree of degradation of the BVF volcanoes as seen in this study lends further support against Almond et al.'s (1984) hypothesis that volcanism in the BVF is younger than 5000 years. Instead, the results of this study provide evidence that volcanism may have been much older, possibly of Pleistocene age. Further studies will test this new hypothesis. 6. Conclusions The studied cinder cones of the Bayuda Volcanic Field are characterized primarily by deposits that have been interpreted to originate from eruptions of Strombolian and Hawaiian type. Nevertheless, deposits from short-lived phreatomagmatic eruptions, pointing to a short-term access of groundwater during an eruption have been encountered as well. The maar volcanoes are predominantly related to phreatomagmatic eruptions, although some of them seem to have started to form with initial effusive eruptions, followed by cinder production from more explosive eruptions. The existence of the BVF maar volcanoes and their related phreatomagmatic deposits (also within the cinder cones) may point to eruptive activity during a time of wet conditions and higher

groundwater levels than those encountered during recorded historical times, possibly the most prominent wet phase in the eastern Sahara between 9000–4000 B.P. or during the Pleistocene, when a variety of wetter phases occurred. The degree of degradation of cinder cones and maar volcanoes as observed in the field and statistically shown by means of a morphological analysis may support this hypothesis. Supplementary data to this article can be found online at https://doi. org/10.1016/j.jvolgeores.2018.03.010. Acknowledgements The National Research Foundation of South Africa (NRF) (N.L., grant no. 90800; A.J.B., grant no. 85815) and the University of Pretoria are thanked for their financial support. The Geological Research Authority of Sudan is thanked for providing logistical support. Furthermore, we thank our editor Joan Marti, and Christoph Breitkreutz and an anonymous reviewer for their help and constructive criticism. References Abdelsalam, M.G., Stern, R.J., 1996. 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