This paper presents new stratigraphic correlation and naming for younger basalt lavas (<24 Ma) and enclosed rhyolites near the intersection of the southern Red Sea rift and the Ethiopian rift valley, based upon three volcano-stratigraphic sections located over about 30 kilometres, and they bracket the timing of eruption of the upper sequence with existing geochronology. The correlations are based on the petrography (thin sections) of ca. 32 samples taken from these three sections, combined with outcrop description. The stratigraphy is proposed as 5 new formations, partially placed into an existing stratigraphic framework, based on the dominant unit-basalt or rhyolite in the package of sequences. These new stratigraphic columns also include non-volcanic units (interlayered sedimentary strata, paleosols and weathering surfaces), which are not usually described in the volcanic pile, showing that there was more complexity in this succession than previously known. This new stratigraphic framework will encourage more geochronology and geochemistry studies that are sorely needed.
Research Article
Stratigraphic framework of pulsed volcanism in central Ethiopian plateau
https://doi.org/10.21203/rs.3.rs-3090728/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 11 Mar, 2024
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Basalt
Ethiopia
formation
magmatic pulse
plateau
rhyolite
stratigraphy
Continental flood basalt (CFB) provinces form the largest volcanic features on Earth, typically both form in extensional environments and predate extension (Richards et al. 1989; Courtillot et al. 1999). There is a co-dependence between flood basalts and continental rifting but timing is complex. For instance, the Keweenawan LIP (large igneous province) of Mid-Continent rift was erupted into a developing rift (Elling et al. 2020), while the Deccan traps likely occurred prior to rifting (Sprain et al. 2019). On the other hand the flood basalts in East Africa seem to have erupted, at least partially, during localized phases of extension (Rooney 2017). Most CFB provinces appear to be characterized by vast thickness of laterally extensive lava flows of relatively evolved tholeiitic basalts (Hooper and Hawkesworth 1993; Hooper et al. 2007; Rooney 2017). High resolution geochronology has revealed that the peak of magmatism is quite a bit shorter than even <1 Ma (Bryan et al. 2010; Self et al. 2014).
Silicic volcanics are an important component of many flood basalt provinces (Halder et al. 2021). They usually occur in the upper parts of the flood basalt sequence. There is a problem or question (e.g., timing, magma plumbing system and origin) surrounding silicic volcanism in flood basalt provinces.
Stratigraphic studies are critical for understanding flood basalt provinces. Such studies highlight petrographic and geochemical variation in stratigraphy related to the magmatic differentiation system (Krans et al. 2018; Davis et al, 2021; Streck et al. 2023). Hence stratigraphic study is needed to document the temporal and spatial changes in magma chemistry and mineralogy, which are related to magma evolution.
Current review papers divide the Ethiopian flood basalts, related to plume impingement, into two sequences: the Oligocene traps phase (31-~28Ma; Rooney 2017) and the Eocene flood basalt province/LIP (Rooney 2017; Steiner et al. 2021). The study area (Fig. 1) is unrelated to the early Oligocene traps episode. Instead, it corresponds to younger lavas (<24 Ma), referred to as the early Miocene resurgence phase (Rooney 2017), in central Ethiopian plateau. Stratigraphic studies are lacking in the area, meaning there was not a detailed stratigraphic section. Much of the volcanic succession in this area is preserved due to the relatively young age compared to other flood basalts. Such a thing makes these lavas very useful in that much or all of their history is recorded. This study employs field observation and petrography in the context of stratigraphy, near the intersection of the southern Red Sea rift and the Ethiopian rift valley, with the aim to describe/define the formations and rheir stratigraphic units to help interprate the overall thickness, timescales, eruption evolution from basalt to silicic volcanism and pulsed eruption. It fully addresses the current state of knowledge regarding pulses of magmatism in East Africa (Rooney 2017; Krans et al. 2018; Rooney 2020a-d; Steiner et al. 2021).
Geologic background
Recent reviews of magmatism in East Africa (Rooney, 2017; Krans et al. 2018; 2020a; 2020b; 2020c; 2020d; Steiner et al. 2021) detail pulses of magmatism in East Africa and contextualized those pulses into geologic process active during eruption. These review papers show that the flood basalt event (~45-28 Ma) is different tectonically and magmatically from the younger lavas (<24 Ma). Magmatism of this age is more closely linked to extensional events than to the impingement of the mantle plume that generated the flood basalts (while there is a plume component in the magma source).
Current review papers divide the Ethiopian flood basalts into two sequences: the Oligocene Traps phase (31-~28 Ma; Rooney 2017) and the Eocene flood basalt province/LIP (Rooney 2017; Steiner et al. 2021). The oldest flood basalt lavas are found in southern Ethiopia (Davidson and Rex 1980; Ebinger et al. 1993; George et al. 1998). The volume of flood lavas erupted in southern Ethiopia is significant (Ebinger et al. 1993; George and Rogers 2002; Steiner et al. 2021). There are also several thick silicic volcanics in the flood basalts of Ethiopia and northern Kenya (Ebinger et al. 1993; Steiner et al. 2021). Specifically, the regional Jimma volcanic unit, the Amaro tuff, and the Arba Minch tuff are some of the documented examples. The Oligocene traps phase was followed by volumetrically less significant lavas manifested as shield volcanoes or isolated flows, referred to as the early Miocene resurgence phase (Rooney 2017).
Rooney (2017; 2020a) and citations therein lay out that magma pulses in East Africa occur due to the interplay of (1) impingement of a mantle plume (the Oligocene flood basalts in northwest and central Ethiopia, and the Eocene flood basalts in southwestern Ethiopia, northern Kenya, and potentially in South Sudan); and (2) the interplay between extension, decompression melting, and elevated mantle temperatures beneath East Africa (the younger lavas <24 Ma).
The age of the lavas presented (20-10 Ma) would suggest that they are intimately associated with periods of extension. Though these younger lavas may take on the form of stratiform sheet flows, similar to the Oligocene traps phase from a morphological sense, their magmatic origin differs from the initial flood basalt pulses. Consequently, they are treated as a different sequence than those of the Oligocene flood basalts (Oligocene traps phase). Nevertheless, the main flood basalts nowhere show contact with the younger lavas in the area.
Traverse routes, made along road cuttings, were selected with the purpose of identifying all mappable units and constructing the local and composite stratigraphy. Our method entailed detailed field logging and regular sampling through the succession at each selected locality, followed by a geologic mapping. Three stratigraphic sections were examined, measured and sampled. We choose traverses that are bracketed by existing geochrolnology, but the bottom section of the sequences is undated. The 3 stratigraphic successions examined identify as mixed and layered sections. Basalts were sepatared out from rhyolites based upon the interpretation of hiatuses (non-volcanic units) such as sediments, or soils, or weathering surfaces. These non-volcanic units definitively mean that volcanism was not active at that spot for a certain amount of time. The larger units found below and above each hiatus are proposed as formations. The thickness of the volcanic pile was determined by measuring elevation variations recorded with a handheld GPS..
We sampled all identified flows (32 unique samples). Individual flows were identified in the field based upon lava texture (e.g., aphyric vs. porphyritic and modal mineralogy), used as a classification system (Renith 2014; Krans et al. 2018). The detailed petrography is given for each sample (Supp. Material). The details of the petrography for larger correlating units together with field observation are provided in the description of each formation. Figure 4 illustrates detailed sections that describe the stratigraphic units and locations of the studied samples. This eventually indicates the units and horizons (base, middle, top) of the samples where they are from.
The volcanic succession for the central Ethiopian plateau can be broadly separated into lower fissure-fed basalts and rhyolites overlain by upper point-sourced shield basalts. We used the formal stratigraphic term “Formation” to classify the different unit types which are mappable at this scale (Fig. 2). All units are formally named using a geographic name, followed by a lithic designation which is more descriptive than the generic term “Formation” (Reidel et al. 2013). The geographic designators were derived from the type locality/area where a complete sequence of each unit is exposed. Five formations are identified from base to top: Zaro Basalt, Gawna Rhyolite, Sasit Basalt, Alaji-Molale Rhyolite and Tarmaber-Guassa Basalt (Figs. 3 and 4), which are partially placed into an existing stratigraphic framework (Zanettin and Justin-Visentin 1973; Justin-Visentin et al. 1974). The formation ascriptions given in Table 1 are taken from the results of traverse and areal/local field mapping. The stratigraphic scheme constructed from this work is summarized as follows.
Zaro Basalt
The oldest in the stratigraphic sequence is here named the Zaro Basalt. The Zaro Basalt lies in contact with the underlying Mesozoic sandstone (Debre Libanos Formation, Asefa, 1991). The unconformity is described by an uneven (undulating) surface as indicated by frequent lava ponding. The Zaro Basalt occupies the lowest elevations (<2000 m above mean sea level) in the westward draining river valleys in the study area (Fig. 2). It has a total thickness of about 400 m (Fig. 4).
The Zaro Basalt forms subdued relief and the flows are poorly exposed. It consists dominantly of dark, basalt lava flows with a thin intercalation of rhyolite ignimbrite towards the top. Basalt flow thicknesses commonly vary between 10 and 40 m. The flow top of the lavas is typically scoriaceous. Individual flows are commonly columnar (Fig. 5). The flows are intensely fractured, typically with alteration surfaces.
The basalt flows exhibit variable textural features including aphyric and porphyritic varieties. The porphyritic varieties contain phenocrysts (up to 10 vol. %) of plagioclase (up to 2.8 mm in size) and olivine (1.5 mm in length) set in a microcrystalline groundmass consisting of plagioclase, olivine and opaque minerals (Fig. 6, Supp. Material). Olivine is often altered to iddingsite. The Zaro Basalt flows are variably altered and contain amygdales of quartz, calcite, chlorite and zeolite that are replacing phenocrysts and groundmass.
There is an intercalated sedimentary layer, dominantly sandstone with variable grain size and degree of weathering, in the middle of the succession (Fig. 3). A similar intercalation within basalts has been observed in the Jema River valley, farther west of the study area (Abbate et al. 2014; Passey et al. 2021). The presence of inter-lava sediments in these two localities and many other places suggests a considerable volcanic quiescence in the middle of the eruption of the Zaro Basalt. This also means that some areas were at lower elevation than others.
Gawna Rhyolite
The early Zaro Basalt is unconformably overlain by thick rhyolite lavas (Gawna Rhyolite) in the western side of the mapped area. This is the first reported occurrence of glassy rhyolite lava flows associated with the Ethiopian basalt pile. These silicic lavas appear to find no equivalent in the northern Ethiopian plateau. The Gawna Rhyolite forms prominent vertical cliffs (Fig. 7) and comprises a pile up to 170 m thick (Fig. 4).
The rhyolite lava sequence displays variable textural features (Fig. 8). In the lower and middle portions, the flows exhibit vertically oriented, regular colonnade-like structures (Fig. 8a). In contrast, towards the top the lava flows consist of randomly oriented joints forming entablature (Fig. 8b). The Gawna Rhyolite lavas exhibit flow banding and flow folding texture. Rhyolite flow thicknesses vary between 5 and 50 m. The rhyolite lavas frequently show perlitic cracks, implying hydration of obsidian.
The Gawna Rhyolite is dark in color and is typically porphyritic containing quartz and alkali feldspar phenocrysts set in a glassy matrix (Fig.6, Supp. Material). Towards the top, the Gawna Rhyolite is intensely altered and contains brown opals, white agate and green jasper/chert. The glassy (vitrophyre) texture of the Gawna Rhyolite possibly relates to rapid loss of dissolved gas by viscous lava fluxing at a higher rate to a cooler surface air condition (Hefferan and O’Brien 2010). The Gawna Rhyolite extends laterally for at least tens of kilometres, indicating that it was fluidal and mobile due to perhaps reduced viscosity as a result of compositional factor.
Sasit Basalt
Whole rock Rb-Sr isochron on intercalated rhyolites from this formation gave an age of ca. 20.7 Ma (Ayalew 2011). The Sasit Basalt is composed of dense, dark basalt flows with numerous thin intercalations of rhyolite ignimbrite, especially towards the upper portion (Fig. 4). It is preserved over much of the plateau, making the Sasit Basalt the most voluminous of this region. The basalt succession reaches to 440 m thick (Fig. 4).
The Sasit Basalt comprises of at least nine basaltic flows that extend subhorizontally for at least tens or more kilometers (Fig. 9). The flows are between 15 and 50 m thick. Individual flows are very extensive, lithologically uniform, consistent with sheet flow geometry. The flow top of Sasit Basalt commonly consists of a vesicular to scoriaceous basalt or a ruby to brecciated basalt (Fig. 10). It typically occupies up to 50% of the entire thickness of a flow. Flow top breccias typically consists of angular, vesicular fragments of basaltic rubble that lie above a zone of nonfragmented, vuggy basalt displaying entablature-like cooling joints with fan-shaped base (Fig. 10). It is likely created by autobrecciation process. The flow bottom of the flows typically consists of sparsely vesicular, fine-grained basalt, suggesting lava flowage on relatively dry ground (Fig. 10). Although the great majority of the Sasit Basalt flows are sheet flow, there are a few compound flows with numerous small distinct lobes (Supp. Material). Individual lobes are affected by shpheroidal weathering.
The Sasit Basalt flows are variably porphyritic (up to 12 vol. %) with phenocrysts of dominantly plagioclase (up to 3 mm in size) with subordinate olivine (about 1.5 mm long) and pyroxene (<1 mm in length) set in a microcrystalline groundmass comprising of lath-shaped plagioclase microlites, olivine, clinopyroxene and opaque minerals (Fig. 6, Supp. Material). The phenocrysts often form glomerophyric aggregates. The basalts are locally altered and that an agate-bearing basaltic layer has been identified at an elevation of ca. 2585 m above base level.
It is important to note that the Sasit Basalt includes numerous thin (up to 20 m thick) intercalations of silicic tuff and ignimbrite (Fig. 4), but are not mappable at this scale. This unit is conspicuous for containing carbonized tree trunks/branches (Fig. 11), indicating wet environment during the deposition of this unit. Towards the top, there is a thin rhyolite level which bears precious opal (Ayalew et al. 2020). Inter-lava sediments (conglomerate, sandstone and mudstone) are common between basalt lava flows (Supp. Material). Their presence implies there was considerable volcanic quiescence between the eruptions of successive lava flows.
Alaji-Molale Rhyolite
The Alaji-MolaleRhyolite is dated at ca. 15.4 Ma (K-Ar dating, Zanettin et al., 1974 and Rb-Sr isochron, Ayalew et al. 2002). It comprises of silicic (rhyolite) ignimbrite and rests directly on the Sasit Basalt. The Alaji-MolaleRhyolite is widely distributed over the plateau, forming the flat terminal surface with typical pyramidal topography (Supp. Material). The thickness of the rhyolite succession attains about 120 m (Fig. 4) which pinches out westward away from the Afar rift margin. The Alaji-MolaleRhyolite serves as a marker horizon to separates the sheet flow sequence from the overlying shield-like pile.
The Alaji-MolaleRhyolite is dominated by pyroclastic deposits with a thin intercalation of basaltic lava flow. Individual flows are between 10 and 35 m thick. The ignimbrite sequence maintains a similar thickness over kilometers and includes both welded and unwelded units. The welded varieties show a well-developed eutaxitic texture, implying their emplacement at relatively high temperature. Individual prominent flows are identified over a distance of a few tens of kilometers which form typical landscape elements such as steep walls and pyramids (Supp. Material).
The Alaji-MolaleRhyolite consists of an alternance of alkaline to peralkaline large ignimbrites and tuffaceous levels. It is variably porphyritic, containing phenocrysts of alkali feldspar and quartz enclosed within a glassy matrix (Fig. 6, Supp. Material). The peralkaline varities include comendites; pantellerites are uncommon (Ayalew et al., 2002). The interbedded basalt flow is lithologically indistinguishable from underlying Sasit Basalt.
Tarmaber-Guassa Basalt
The Tarmaber-Guassa Basalt is dated at ca. 10.4 Ma (K-Ar technique, Chernet et al. 1998), terminating the central Ethiopian magmatic event. It overlies unconformably the Alaji-MolaleRhyolite ignimbrites, separated by about 30 m thick sediments with variegated conglomerates, sandstones and mudstones. The Tarmaber-Guassa Basalt crops out close to the rift margin in the more elevated places (their elevation can reach to 3315 m above mean sea level) of the mapped area. It locally fills ancient erosional channels. The thickness of this formation is highly variable, in places reaching up to 410 m (Fig. 4).
The Tarmaber-Guassa Basalt forms still-preserved edifices (shield volcanoes), made up of more localized, lenticular basalt flows with scoriaceous flow top. The flows are frequently separated by red paleosol. They dip 3-5° away from the summit. The lavas are commonly scoriaceous and that vesicles are often filled with amygdales. Pyroclastic layers are abundant in some sequences. Although basalt flows dominate the sequence, differentiated products are also frequent.
The Tarmaber-Guassa Basalt flows are distinctive by being porphyritic (phenocrysts comprise up to half, sometimes more, of the rock volume); plagioclase, pyroxene and olivine phenocrysts characterize the flows (Fig. 6, Supp. Material). Nevertheless, there are also aphyric varieties. The character of the basalts is decidedly alkaline. More evolved products include dominante plagioclase- or olivine-phyric hawaiite/mugearite and subordinate aphyric trachyte and tephrophonolite.
Overall thickness of the stratiform sheet flows
The basal flow of the sheet flowsequence is exposed in Jema gorge along the western side of the mapped area (Fig. 2). In this locality, the contact between the Mesozoic sandstone and overlying basalt is found at an altitude of ca. 1620 m above mean sea level (Fig. 4). The top of the stratiform sheet flows, away from plateau-Afar escarpment to minimize the effect at the rift margin where rift flank uplift is superposed on broader dynamic uplift, resides at an elevation of ca. 2910 m above base level (Fig. 4). Considering these elevations ca. 1620 m as the base and ca. 2910 m as the top of the sheet flowsequence, we estimated the present, preserved thickness of the basalt and enclosed rhyolite pile to be 1290 m.
Estimates of the thickness of the volcanic sequence are always subject to a certain degree of uncertainty due to the effects of erosion. It is worth mentioning that the calculated thickness should be considered as a minimum estimate because the calculation did not take into account the original stack before erosion. Hence, the total thickness of the basalt and rhyolite pile can exceed 1290 m and a somewhat larger figure applies. It is apparent that the sheet flow sequence in the area is far thinner than in the Oligocene flood basalts in the north (~2100 m thick, Ayalew et al. 2021). Nevertheless, it built up to the same order of thickness as other major flood basalt provinces (e.g., the Columbia River Basalt thickens 1500 m, Reidel et al. 2013).
Frequency of silicic volcanism
Silicic volcanics are an important component of many flood basalt provinces (Halder et al. 2021). Thick silicic volcanics exist both in the flood basalts of Ethiopia and northern Kenya, and subsequent younger lavas (Rooney 2017; Steiner et al. 2021). They occur in the volcanic stratigraphy as ash layers between basaltic lava flows or as individual eruptive units, comparable in volume to individual mafic lava units. Cooling units for these felsic ignimbrites and lavas extend laterally up to several tens of kilometers, indicating voluminous fissure eruptions.
Widespread silicic volcanics occur in the study area. They are prolifically intercalated with and superimposed on the basalt flows, indicating periodic silicic eruption. This is supported by the first time thick rhyolite lava flows (Gawna Rhyolite) reported from the basalt succession of Ethiopia (Figs. 2 and 4). Neglecting the interlayered rhyolites, the larger felsic eruptives form about 30% by volume of the total volcanic pile (Fig. 4). This yields their value in labeling and identifying particular formations (Figs. 3 and 6). The presence at certain horizons and localities of prominent silicic ignimbrites and lavas capping the basalt pile leads to various petrogenetic models of silicic magma generation.
The dataset presented (20-10 Ma) suggest that these younger lavas are related to extension (Rooney 2017). This allows for the shallow ponding of mafic magma, and subsequent silicic magma generation possibly by fractional crystallization and ultimately periodic silicic eruptions. This is supported by the presence of extensive rhyolites in the lava sequence that illustrates the time-progressive evolution of mamga. This suggests that the frequency and volumetric importance of the silicic volcanism increased progressively in the younger lavas (<24 Ma).
Episodicity of Ethiopian volcanism
The current state of knowledge regarding magmatism in East Africa (Rooney, 2017; Krans et al. 2018; Rooney 2020a; 2020b; 2020c; 2020d; Steiner et al. 2021) detail pulses of magmatism in East Africa where the main flood basalt events (~45-28 Ma) end and subsequent shield building takes place afterward followed by rift-related volcanism. The age of the lavas presented (20-10 Ma) would suggest that they are ultimately linked with periods of extension. Though these younger lavas may take on the form of stratiform sheet flows, their magmatic origin (petrologically and geochemically) differ from the initial flood basalt pulses (Rooney 2017; 2020a and citations therein). As a result, they are treated as a different sequence than those of the Oligocene flood basalts (Oligocene traps phase).
The 3 stratigraphic sections logged (Fig. 4) bracket the timing of eruption of the upper sequence with existing geochronology, but the bottom section remains undated. Whole rock Rb-Sr isochron on intercalated rhyolites within the Sasit Basalt yield an age of ca. 20.7 Ma (Ayalew 2011), permitting an earlier start of volcanism. These geochronologic dates suggest that volcanism began in the area most likely in early Miocene, flowing the massive outpouring of flood basalts and enclosed rhyolites (the Oligocene traps phase 31-~28 Ma).
The volcanic succession in the area is characterized by significant basalt flows in pulses, each capped by silicic volcanics. Long intervals between successive flows within or between individual formations are attested to by the common development of inter-lava sediments, paleosols and alteration surfaces (Supp. Material), implying long time spans of eruption. This Miocene activity typically defines as a significant basaltic event followed by a period of quiescence or silicic activity, consistent with the breakdown and merging of regional units in Rooney (2020a). Yet, this long period, suggesting pulsed magma eruption, has resulted in the Ethiopian pile building up to the same order of thickness as other major flood basalt provinces (e.g., central Atlantic magmatic province, Marzoli et al. 1999).
Stratigraphy study, together with any published ages, on rift-related lavas (which may take on the form of stratiform sheet flows) in central Ethiopian plateau indicates that important episodes of magmatism have been active in the area, characterized by slow widespread basalts in pulses, each capped by silicic volcanic (ignimbrites and lavas). Five formations, separated by non-volcanic units (e.g., sediments, paleosols and weathering surfaces) are identified and mapped. These formations, partially placed into an existing stratigraphic framework, are named from base to top: Zaro Basalt, Gawna Rhyolite, Sasit Basalt, Alaji-Molale Rhyolite and Tarmaber-Guassa Basalt.
Acknowledgements
The authors would like to thank Addis Ababa University (“thematic research” grant scheme) and Amhara Mines Resource Development Promotion Agency (geologic mapping of five Woredas in north Shoa province project) for their financial support. We are indebted to anonymous journal reviewers and Associate Editor Loÿc Vanderkluysen who made useful comments on the manuscript.
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